Journal of Geophysical Research: Space Physics

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Evolution of Asymmetrically Displaced Footpoints During Substorms

A. Ohma¹ , N. Østgaard¹ , J. P. Reistad¹ , P. Tenfjord¹ , K. M. Laundal¹ , K. Snekvik¹ , S. E. Haaland^1,2 , and M. O. Fillingim³

1Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, Bergen, Norway,

2Max-Planck Institute for Solar Systems Research, Göttingen, Germany,³Space Sciences Laboratory, University of California, Berkeley, CA, USA

Abstract

It is well established that a transverse (y) component in the interplanetary magnetic field (IMF) induces aB_ycomponent in the closed magnetosphere through asymmetric loading and/or redistribution of magnetic flux. Simultaneous images of the aurora in the two hemispheres have revealed that conjugate auroral features are displaced longitudinally during such conditions. Although the direction and magnitude of this displacement show correlations with IMF clock angle and dipole tilt, single events show large temporal and spatial variability of this displacement. For instance, we know little about how the displacement changes during a substorm. A previous case study demonstrated that displaced auroral forms, associated with the prevailing IMF orientation, returned to a more symmetric configuration during the expansion phase of two substorms. Using the far ultraviolet cameras on board the Imager for Magnetopause-to-Aurora Global Exploration and Polar satellites, we have identified multiple events where conjugate auroral images are available during periods with substorm activity and IMFB_y ≠ 0. We identify conjugate auroral features and investigate how the asymmetry evolves during the expansion phase. We find that the system returns to a more symmetric state in the events with a clear increase in the nightside reconnection rate and that the displacement remains unchanged in the events with little or no net closure of open magnetic flux. The return to a more symmetric state can therefore be interpreted as the result of increased reconnection rate in the magnetotail during the expansion phase, which reduces the asymmetric lobe pressure.

1. Introduction

Research from the past decades has shown that aB_ycomponent in the interplanetary magnetic field (IMF) leads to the presence of aB_y component with the same polarity inside the closed magnetosphere (e.g., Cowley & Hughes, 1983; Petrukovich, 2011; Wing et al., 1995). The presence of a large-scaleB_ycomponent in the equatorial plane means that the magnetosphere has departed from its symmetric quiet day configuration and that the footpoints of magnetic field lines are displaced longitudinally between the hemispheres. Studies using simultaneous images of the global aurora in both hemispheres have shown that distinct auroral features are displaced longitudinally when IMFB_y≠0(Frank & Sigwarth, 2003; Østgaard et al., 2004, Østgaard, Tsyganenko, et al., 2005; Reistad et al., 2013, 2016; Stubbs et al., 2005). By assuming that the auroral features in the northern and southern hemisphere are produced by particles accelerated along the same field line, the displacement of the auroral features describe how displaced the field line itself is. Statistical studies have found that the displacement depends on the IMFB_y, the IMF clock angle and the dipole tilt angle (e.g., Liou

& Newell, 2010; Liou et al., 2001; Østgaard, Laundal, et al., 2011; Wang et al., 2007). Østgaard, Laundal, et al.

(2011) found a maximal average longitudinal displacement at substorm onset of 0.5 hr of magnetic local time (MLT) between the two hemispheres, but event studies (e.g., Reistad et al., 2016) have shown that the relative displacement (ΔMLT) can be as large as 3 hr. It is currently unknown how large this asymmetry can get. The asymmetry is also manifested in the global convection pattern (e.g., Haaland et al., 2007; Heppner & Maynard, 1987; Pettigrew et al., 2010) and in the ionospheric and ﬁeld-aligned current systems (e.g., Anderson et al., 2008; Green et al., 2009; Laundal et al., 2016, 2018).

There is still a debate about how the presence of an IMFB_ycomponent leads to a localB_ycomponent in the closed magnetosphere. Some have explained it terms of a simple penetration of the IMF, where theB_y

RESEARCH ARTICLE

10.1029/2018JA025869

Key Points:

• Longitudinally displaced auroral features become more symmetric during substorm expansion phase

• There is a relationship between the change in asymmetry and the nightside reconnection rate

• The observed reduction in asymmetry is consistent withB_ybeing introduced into the closed magnetosphere by asymmetric pressure gradients

Correspondence to:

A. Ohma,

[email protected]

Citation:

Ohma, A., Østgaard, N., Reistad, J. P., Tenfjord, P., Laundal, K. M., Snekvik, K., et al. (2018). Evolution of

asymmetrically displaced footpoints during substorms.Journal of Geophysical Research:

Space Physics,123.

https://doi.org/10.1029/2018JA025869

Received 6 JUL 2018 Accepted 25 OCT 2018

Accepted article online 5 NOV 2018

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modiﬁcations or adaptations are made.

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component in the magnetosphere is considered as a superposition of the IMF and terrestrial magnetic field (e.g., Kozlovsky et al., 2003; Rong et al., 2015). Others have suggested that it is a consequence of tail reconnection and that theB_y component enters the closed magnetosphere as open field lines with displaced footpoint are closed in the magnetotail (e.g., Browett et al., 2017; Cowley, 1981; Fear & Milan, 2012; Motoba et al., 2010; Østgaard et al., 2004; Stenbaek-Nielsen & Otto, 1997). Cowley (1981) and Cowley and Lockwood (1992) describe how this situation could arise as the consequence of asymmetric loading and/or rearranging of magnetic flux due to tension forces acting on newly reconnected field lines when IMFB_y≠0. In this con- text, loading refers to opening of magnetic flux at the dayside magnetosphere and the subsequent transport to the lobes, while rearranging refers to the circulation of open flux associated with lobe reconnection. Let us consider the case when IMFB_y>0. Depending on the sign of IMFB_z, there might be dayside and/or lobe reconnection. The tension force acting on these newly reconnected field lines will pull them toward dawn in the northern lobe, and toward dusk in the southern lobe, increasing the magnetic pressure in these regions.

In response to this increased pressure, the plasma will flow towards dusk in the northern lobe, and towards dawn in the southern lobe. Since the field can safely be considered as frozen-in in this region, the magnetic field moves along with the flowing plasma. The footpoint of the open field lines on the nightside are therefore displaced duskward in the northern hemisphere and dawnward in the southern hemisphere, and the asymmetry enters the closed magnetosphere when these field lines reconnect in the tail. An alternative description of how aB_ycomponent could arise in the closed magnetosphere without invoking tail reconnection was first suggested by Khurana et al. (1996) and further developed by Tenfjord et al. (2015). They argue that the transverse flows set up by the asymmetric pressure distribution will also affect field lines deep within the closed magnetosphere directly. This represents a shear flow along the closed field lines, which effectively rotates the closed magnetospheric field lines, resulting in aB_ycomponent. SinceB_yis introduced in the closed magnetosphere in direct response to the asymmetric pressure distribution, a more prompt response to changes in the IMFB_yis expected from this mechanism compared toB_ybeing introduced by tail reconnection. By using mag- netohydrodynamics (MHD) simulations, Tenfjord et al. (2015, 2018) found that aB_ycomponent appeared in the closed magnetosphere just a few minutes after introducing aB_ycomponent in the IMF. Superposed epoch analysis of theB_yresponse to IMFB_yreversals observed at geosynchronous orbit during both southward and northward IMF confirms this prompt response (Tenfjord et al., 2017, 2018). Tenfjord et al. (2015) referred to B_yintroduced by this mechanism as inducedB_y. We will also use the term “induced” when we refer to theB_y introduced in the closed magnetosphere by the IMFB_y, although the term “penetrated” is often used in the literature.

There are also other factors that can produce aB_y component in the equatorial plane. One source ofB_y is related to the orientation of Earth’s dipole axis relative to the solar wind velocity. The dipole tilt angle is defined as the angle between the dipole axis and theZaxis in the geocentric solar magnetospheric (GSM) coordinate system, and it has been shown that the neutral sheet gets warped when the tilt angle is nonzero (Fairfield, 1980; Russell & Brody, 1967). Liou and Newell (2010) discuss how the warping leads to aB_ycomponent in the tail. When there is a positive tilt angle, the center of the neutral sheet moves above the equatorial plane, while the flanks are less affected and stay close to the equatorial plane. This lead to aycomponent in the magnetic field, positive in dusk, and negative in dawn (see Liou & Newell, 2010, Figure 3). The result is opposite for negative tilt angles. There is also observational evidence of another dipole tilt effect, as there exists aB_y component, which is positively correlated with tilt angle at allY_GSMpositions (Petrukovich, 2011). This effect has been termed the even tilt effect. For positive tilt angles, this effect adds to the warping effect in the dusk sector, and opposes the warping effect in the dawn sector. A third source ofB_yis the rotation of the magnetotail around the tail axis, driven by the IMFB_y(Cowley, 1981; Fairfield, 1979). This reduces the inducedB_y, but the contribution is considered to be of minor importance and since it is solely dependent on the IMFB_yit can be considered as part of the inducedB_y(Petrukovich, 2011). For completeness, we note that there is also a forth source ofB_yin the magnetosphere; since the field lines are connected to the ionosphere, there is a flar- ing component of theB_yaway from the tail axis. This effect is equal and opposite in the two hemispheres and is therefore not responsible for a displacement between the hemispheres.

If the asymmetry is caused by an asymmetric pressure distribution in the lobes, it is reasonable to assume that the system will return to a more symmetric state if the pressure imbalance is reduced or removed.

In this region, the pressure is in essence the magnetic pressure, given as _2𝜇^B²

0, as the thermal and dynamic pressure are negligible here. Observations have shown that the near-Earth lobe pressure varies signiﬁ- cantly during substorms. The pressure increases for several hours before substorm onset, and then quickly

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decreases in less than 1 hr (Caan et al., 1975, 1978; Yamaguchi et al., 2004). In a previous case study, Østgaard, Humberset, et al. (2011) studied the evolution of longitudinally displaced auroral features during two subsequent substorms. They found that the conjugate features became more symmetric during the expansion phase of both substorms. Throughout the ﬁrst substorm the authors analyzed, the IMF was stable andB_y dominated, so they concluded that the longitudinal displacements was removed by processes related to the magnetospheric substorm.

In the present paper we present 10 events where simultaneous images of the aurora in both hemispheres are available during periods with both substorm activity andB_ydominated solar wind. The goal is to find out if the reduction of asymmetry is a prominent signature of the global magnetospheric change during a substorm, and on what time scales the system reconfigures. Furthermore, we investigate how the change from an asymmetric state to a more symmetric state relates to the enhanced reconnection during the expansion phase. The paper is organized as follows: In section 2, we describe the different cameras used to obtain the global auroral images and how we have handled the image data. In section 3, we present the results of the analysis, before we discuss these results in section 4. Our findings are summarized in section 5. In Appendix A, we describe in detail the method used to remove the sunlight induced contribution from the auroral images.

In Appendix B, we present supplementary auroral images from four of the events presented in section 3, when images from a second camera pair is available in the two hemispheres.

2. Data and Methods

2.1. The Conjugate Data Set

The largest existing data set of simultaneous global auroral images from both hemispheres are the conjunc- tion of the images obtained by the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) (Burch, 2000) and Polar (Acuña et al., 1995) missions. IMAGE was launched in 2000, and Polar was launched in 1996.

Both spacecraft had highly elliptical polar orbits, with apogee over the northern hemisphere at the start of the missions. Due to orbital precession, the apogee of the orbits moved gradually southward. The apogee of Polar’s orbit crossed the equatorial plane in 2001, whereas the apogee of IMAGE’s orbit did not cross until a few years later. This conﬁguration, with Polar’s apogee near the equatorial plane, allowed Polar to view the southern aurora for a few hours in each pass, while IMAGE was still mainly viewing the northern hemisphere, enabling conjugate monitoring of the global aurora.

The images from the northern hemisphere are obtained using the Far Ultraviolet (FUV) instrument suite (Mende, Heetderks, Frey, Lampton, Geller, Habraken, et al., 2000). The instrument suite consists of three cameras, of which two are used in this study: the Wideband Imaging Camera (WIC; Mende, Heetderks, Frey, Lampton, Geller, Abiad, et al., 2000) and the Spectrographic Imager 13 (SI13; Mende, Heetderks, Frey, Stock, et al., 2000). WIC is sensitive to the Lyman-Birge-Hopfield (LBH) band from N₂and a few atomic N lines in the 140- to 190-nm range, with the 130.4-nm oxygen line almost entirely excluded. SI13 is sensitive to the atomic oxygen line at 135.6 nm, which is spectrally separated from the 130.4-nm line. Both cameras have a cadence of 123 s, which is equal to the spin period of IMAGE. Flat-field correction has been applied to the images, cor- recting for differences across the detectors. Also, a normalization for voltage and temperature changes has been applied to be able to compare intensities from different times throughout the mission.

There are also two FUV cameras available on board the Polar spacecraft: the Visible Imaging System Earth Cam- era (VIS-EC, hereby VIS; Frank et al., 1995) and the Ultraviolet Imager (UVI; Torr et al., 1995). VIS was intended as a pilot camera for two cameras monitoring the aurora at visual wavelengths, but it has also been used on its own due to its large field of view, which has enabled VIS to see the entire auroral zone from rather low alti- tudes. The camera is sensitive to emissions in the 124- to 149-nm range. The dominating FUV emission line in this interval is the 130.4-nm line (Frey et al., 2003), but there are contributions also from the LBH band and a Nitrogen line (Frank & Sigwarth, 2003). UVI has a narrow field of view and therefore covers only parts of the auroral oval except when Polar is near apogee. The camera has four filters; 130.4-nm oxygen line, 135.6-nm oxygen line, LBH short (∼150 nm), and LBH long (∼170 nm). In the events considered in this study, UVI has two modes of operation: In the first mode, it uses only the LBH-long filter and in the second mode, it cycles through the filters, taking a pair of images with each filter in each cycle. It turns out that UVI is only viewing the region of interest in four of our events and that it is operating in the first mode in three of those events.

In the last event, where it cycles through all ﬁlters, the signal is very weak in the 130.4- and 135.6-nm ﬁlters.

For that reason, we have only considered the images obtained with the LBH ﬁlters in this study. The relevant attributes of all the cameras used in this study are summarized in Table 1.

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Table 1

Camera Speciﬁcations

Hemisphere Spacecraft Camera Sensitivity (nm) Cadence (s) Integration time (s) Resolution

North IMAGE WIC 140–190 123 10 256×256

SI13 135.6 123 5 128×128

South Polar VIS 124–149 54 32.5 256×256

UVI 146–154/165–175^a 36.8 9.2/18.4/36.8 228×200

aUVI has four ﬁlters: 130.4 nm, 135.6 nm, LBH-short (∼150 nm), and LBH-long (∼170 nm). Only the LBH ﬁlters are used in this study.

Neither of the four cameras available are sensitive to the exact same emissions. Both the 130.4 nm and the 135.6 nm emission line are produced by electrons colliding with atomic oxygen. The former line is four times brighter and more scattered than the latter, but the intensity of the two emission lines respond linearly to diﬀerent electron energies (Frey et al., 2003). Since the dominating emission observed by VIS is the 130.4-nm line, VIS and SI13 should have a similar energy response. The fact that VIS is also sensitive to the lower part of the LBH band will cause deviation from a linear dependence between the intensities in the two cameras. Both WIC and UVI are sensitive to emissions from the LBH band, which are produced by electron impact on N₂and the N line. WIC covers the entire range observed by UVI, and any feature observed by UVI should be observed by WIC if it is there. The intensities are not directly comparable, as WIC has a wider spectral range than UVI.

However, among the cameras on Polar, UVI has the most similar response to the WIC camera on IMAGE.

The solar wind data used in this study are from the OMNI 1-min data set (King & Papitashvili, 2005), which is time shifted to Earth’s bow shock nose. We note that there is an additional travel time of a few minutes before it reaches the magnetopause. Substorm onset time is determined by examining the images and by consulting the substorm list presented by Frey et al. (2004).

2.2. Data Analysis

In addition to the auroral emissions, which is produced by precipitating particles, Earth’s atmosphere emits a dayglow induced by sunlight. In the FUV range, the radiance comprises of molecular, atomic, and ionic line emissions, where the excited states are caused by photoelectrons produced by sunlight at much shorter wavelengths (Meier, 1991). Since we are interested in the auroral contribution, the emissions associated with dayglow must be subtracted from the images. To ﬁnd the dayglow contribution, we have constructed a dayglow model based on all the images in each event. The novel method is described in the appendix. With the contribution from dayglow emissions subtracted, we can compare the images from the two hemispheres to identify conjugate features and determine their longitudinal displacement in each image pair. We have used three diﬀerent approaches to do this: 1-D correlation, 2-D correlation, and visual inspection, which we will describe in detail in the following sections.

2.2.1. The 1-D Correlation Analysis

The 1-D correlation is performed using a similar scheme as described by Østgaard, Humberset, et al. (2011).

We consider a region of each image spanning 15∘in magnetic latitude (mlat) and up to 8 hr in MLT, covering the auroral substorm. The selected region remains fixed in each event. Each region is divided into MLT sectors of 0.2 hr, and we calculate the latitudinal integrated intensities by taking the mean intensity in each 0.2-hr bin. The intensity profile in the northern hemisphere is cross-correlated with the corresponding intensity profile from the image the southern hemisphere, using steps of 0.2 hr. The shift yielding the highest correlation coefficientris selected as the relative displacement between the images. The value ofris a metric of how similar the images are, where a higher value correspond to higher similarity. This metric does not, however, tell us how unique this solution is compared to other shifts. If we for instance consider diffuse aurora, where the intensity in each sector is approximately the same, we would get a high correlation for any shift. In addition to considering the value ofr, we have therefore constructed another metric to determine the uncertainty of the estimated displacement. By applying Fisher’s z-transformation, we transformrinto an approximate normal distribution with mean

𝜇= 1

2ln(1+r 1−r )

(1) and standard error

𝜎= 1

√N−3

(2)

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Figure 1.The 1-D correlation scheme on an image pair from the 15 November 2002 event. (a) Image of a distinct auroral feature in the northern hemisphere. (b) Image of a distinct auroral feature in the southern hemisphere. (c) Mean intensity of the two images before shift is applied. (d) Scatter plot of the intensities in corresponding bins before shift is applied. (e) The correlation between the northern and southern hemisphere for different shifts. The shaded region indicates the confidence interval of the correlation. The orange dot indicates the shift yielding the highest correlation, and the orange line indicates the confidence interval of this shift. (f ) Same as (a), but shifted−1.8 hr. (g) Same as (b).

(h) Same as (c), but with northern hemisphere shifted−1.8 hr. (i) Same as (d), but with northern hemisphere shifted−1.8 hr.

whereNis the number of points in the correlation. We can then find the confidence interval of𝜇using the normal distribution, and transform this to the confidence interval ofrusing the inverse of equation 1. The error of the estimated displacement can now be considered as any shift yieldingrwithin the confidence interval of the “best” shift. For the 1-D correlation, we have used a 70% confidence interval for𝜇. An example of how the 1-D correlation performs are shown in Figure 1, to be discussed when introducing the first event.

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2.2.2. The 2-D Correlation Analysis

The 2-D correlation is done by transforming the images into a rectangular grid with a resolution of 1∘in the latitudinal direction and 0.2 hr in the longitudinal direction. At auroral latitudes, this corresponds to an area of about 100×100km. We again consider a region of the transformed images, spanning 15∘ in mlat and up to 8 hr in MLT. The selected region remains fixed in each event. The image from the northern hemisphere is then cross-correlated with the corresponding image from the southern hemisphere, using steps of 1∘ and 0.2 hr in the latitudinal and longitudinal direction, respectively. The shift yielding the highest correlation is selected as the relative displacement between the hemispheres. In addition to the value ofr, we estimate the error in the same way as for the 1-D correlation, only this time using a 90% confidence interval to account for the increased number of points in the correlation. The 1-D and 2-D correlation schemes applied in this study is similar to the techniques used by Østgaard, Humberset, et al. (2011) and Reistad et al. (2013), with the addition of the new metric for the uncertainty. A detailed example of how the 2-D correlation performs are shown in Figure 2 and will be further described when introducing the first event.

2.2.3. Visual Inspection

In order to verify that the longitudinal shifts found by the two automatic methods are reasonable, we have also performed a visual inspection to determine the relative displacement. This comparison is performed by plotting the images from the northern hemisphere (SI13 and WIC) above the corresponding images from the southern hemisphere (VIS and UVI when available) on a MLT versus mlat grid. We use the intensities at onset to get an approximate scaling of the intensities from the diﬀerent cameras. The images from the northern hemisphere are then moved in steps of 0.1 hr, until matching features align as closely as possible. The onset locations in the two hemispheres serve as a starting point in the analysis, as substorms are known to have a common source in the plasma sheet, and are therefore expected to have conjugate sig- natures in the ionosphere. When we compare the images, we focus on large auroral features with similar shape and size. In addition, we have considered the temporal evolution of the identiﬁed features, as the same dynamical behavior combined with similar shape and size are strong indications of a common source in the magnetosphere.

2.2.4. Estimating the Open-Closed Boundary and Nightside Reconnection Rate

In order to establish whether there is a connection between changes in the asymmetry and the nightside reconnection rate, we estimate the latter quantitatively. This can be achieved since the amount of open magnetic fluxF_PCin the polar cap is related to the opening of flux on the dayside and closing on flux on the nightside through the equation

dF_PC

dt = Φ_D− Φ_N (3)

whereΦ_DandΦ_Nare the dayside and nightside reconnection rates, respectively (e.g., Siscoe & Huang, 1985;

Milan et al., 2007). The amount of open ﬂux can be estimated from the images by ﬁnding the open-closed boundary (OCB), andΦ_Dcan be estimated from the solar wind data using the coupling function by Milan et al. (2012). We assume that the OCB is located at the poleward edge of the auroral oval (Boakes et al., 2008;

Laundal et al., 2010; Milan et al., 2003, 2007, 2008). We use the images from the northern hemisphere (WIC), since the entire oval is only ever seen by IMAGE in all events. Our technique to estimate this poleward edge is as follows: We first bin the image into 48 overlapping MLT sectors, each bin with the size of 1 hr. The boundary in each sector is set when the number of counts passes a threshold value and stays above this for at least 2∘magnetic latitude. The threshold value is kept fixed in each event. We have used values of∼100 counts (about 250 R) to identify the boundaries, which is in good agreement with the value used by Østgaard, Moen, et al. (2005). Identified boundaries that are obviously inside the polar cap are ignored. We have also checked that the positions are consistent with publicly available DMSP passes. If the boundary is not found in a sector, we interpolate between the adjacent sectors. When we have found the boundary, the total open magnetic flux inside the OCB is calculated by integrating equation 4.15 in Richmond (1995), using the International Geomagnetic Reference Field (IGRF) (Thébault et al., 2015) as the the magnetic field. Doing this for each image in an event, we getF_PCas a function of time. The identification of the OCB can be uncertain in some MLT sectors, mainly due to weak auroral emissions leading to a high signal to noise ratio, especially in the dayside aurora. This can lead to sudden jumps in the identified OCB between the images in those regions. We are interested in the time derivative ofF_PC, a quantity that is sensitive to variations. We have therefore smoothed the curve, using a running mean with a window of five images.

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Figure 2.The 2-D correlation scheme on an image pair from the 15 November 2002 event. (a) Image of a distinct auroral feature in the northern hemisphere, binned in a rectangular grid. (b) Image of a distinct auroral feature in the southern hemisphere, binned in a rectangular grid. (c) Scatter plot of the intensities in corresponding bins before shift is applied.

(d) The correlation between the northern and southern hemisphere for different shifts. (e) Maximum correlation for each ΔMLT shift. The shaded region indicates the confidence interval of the correlation. The orange dot indicates the shift yielding the highest correlation, and the orange line indicates the confidence interval of this shift. (f ) Same as (a), but shifted−1.4 hr in the longitudinal direction and1∘in the latitudinal direction. (g) Same as (b). (h) Same as (c), but with the northern hemisphere shifted−1.4 hr and1∘.

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Φ_Dwas estimated using the coupling function found by Milan et al. (2012), as this provides the appropriate units. The coupling function is given as

Φ_D=3.3×10⁵V

4 3

xB_yzsin⁹² 1

2𝜃 (4)

whereV_xis the radial component of the solar wind,B_yzthe transverse component of the IMF and𝜃the IMF clock angle. Using the 1 min OMNI data as input values givesΦ_Dwith 1-min resolution. To account for uncertainties in propagation time, we average the 1-min resolutionΦ_Dover the previous 15-min interval at each image. The nightside reconnection rate between imageiandi+1is then given as

Φ_N,i= Φ_D,i−F_i+1−F_i

Δt (5)

whereΔtis the time between the images.

Using this scheme to determine the nightside reconnection rate, we sometimes getΦ_N<0between individ- ual images. Since the nightside reconnection rate cannot be negative, this just reﬂects the uncertainty in the identiﬁcation of the OCB, as the boundary can have sudden jumps between the images in regions with weak aurora. Also, since the coupling function used to determineΦ_Dis found statistically, it will not necessarily represent the exact dayside reconnection rate in single events.

3. Observations

In the following section we will present auroral images from several substorm events, all occurring during periods with a prominentycomponent in the IMF and with conjugate coverage of the two hemispheres. The auroral images are displayed in modified Apex coordinates (Richmond, 1995), with the longitudinal coordinate displayed in MLT. A property of Apex coordinates is that an IGRF field line has the same coordinates in both hemispheres, only with the sign of the latitudinal component reversed. This means that any displacement in this coordinate system is a deviation from the IGRF field. From this we can define the relative displacementΔMLT as the longitudinal differences between two conjugate regions. PositiveΔMLT means that the region in the northern hemisphere is located duskward of the region in the southern hemisphere, and vice versa for negativeΔMLT. This is similar to the convention used by Østgaard, Humberset, et al. (2011) and Reistad et al. (2013). From this definition, a positive IMFB_ywill usually lead to a positiveΔMLT. We will first describe in detail how our methodology is applied to the 15 November 2002 event, and then go through the other events.

3.1. The 15 November 2002 Event

The 1-D correlation scheme for one image pair in the 15 November 2002 event is shown in Figure 1. Figure 1a displays the aurora in the northern hemisphere, and Figure 1b displays the aurora in the southern hemisphere.

The red box shows the part of the images used in the correlation analysis. The region in the southern hemisphere is wider, to ensure that the same numbers of bins are compared for each shift in the cross-correlation.

Figure 1c shows the integrated intensities from both hemispheres, where the northern and southern hemisphere are plotted in blue and red, respectively. From these three figures it is clearly seen that the bright auroral feature is located at a later local hour in the northern hemisphere compared to the southern hemisphere. Figure 1d is a scatter plot of the mean intensities in the same MLT sectors in the two hemispheres, before the northern hemisphere is shifted. There is no clear relationship between the two. Figure 1e displays the correlation coefficient for each shift of the northern hemisphere. The shaded region indicates the confidence interval of each shift. There is a peak inrwhen the northern hemisphere is shifted−1.8 hr, indicated by the orange dot. The confidence interval of this shift is indicated by the orange line. In Figure 1f, the aurora in the northern hemisphere is shifted by−1.8 hr. Figure 1g shows the southern hemisphere for reference, and Figure 1h shows the integrated intensities from both hemispheres with the northern hemisphere shifted−1.8 hr. From these figures it is clear that the aurora features align better after the shift. Figure 1i displays the scatter plot of corresponding MLT sectors after the northern hemisphere has been shifted, showing a near linear response.

The 2-D correlation scheme of the image pair is shown in Figure 2. Figures 2a and 2b displays the same auroral feature as displayed in Figures 1a and 1b but binned in a rectangular grid. The red box shows the part of the image in the northern hemisphere that is cross-correlated with the southern hemisphere. Figure 2c is a scatter plot of the intensities in corresponding bins before the northern hemisphere is shifted. Figure 2d displays the

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correlation coeﬃcientrfor diﬀerent shifts in both the latitudinal and longitudinal direction. Here we see a peak whenΔMLT= −1.4hr andΔmlat=1∘. To estimate the error, we select the maximum correlation in the latitudinal direction of eachΔMLT shift. This is displayed in Figure 2e, where again the peak is indicated by the orange dot and the error by the orange line. In Figure 2f, the aurora in the northern hemisphere is shifted−1.4 hr in the longitudinal direction and 1∘in the latitudinal direction. The auroral feature is now better aligned with its counterpart in the southern hemisphere, displayed in Figure 2g. Figure 2h shows the intensities in corresponding bins after the northern hemisphere has been shifted. The response is now more linear.

The survey of the entire event occurring on 15 November 2002 is presented in Figure 3. The format of this figure will be the same in all the events presented in this study. Figures 3a1 and 3a2 display the substorm onset in the northern and southern hemisphere, respectively. The red ellipses indicate identified conjugate features, and the blue line in Figure 3a2 is the sunlight terminator. Figures 3b1 and 3b2 display the conjugate aurora later in the expansion phase. The evolution of the OCB is shown in Figure 3c. Figures 3d and 3e display the IMF and the SML index before and during the expansion phase, where the IMF is time shifted to the bow shock and displayed in GSM coordinates. The SML index is equivalent to the AL index, but based on observations from over 100 stations (Gjerloev, 2012). The result of the 1-D correlation, the 2-D correlation, and the visual comparison is presented in Figures 3f, 3g, and 3h, respectively. For the correlation analysis, green signifiesr≥0.6and orange signifiesr<0.6. The error bars correspond to the horizontal orange lines in Figures 1 and 2. Figure 3i displays the amount of open flux, based on the boundaries displayed in Figure 3c and smoothed using a running mean. The estimated dayside (blue) and nightside (orange) reconnection rates are shown in Figure 3j.

Figure 3k displays the evolution of the substorm expansion phase, with the northern hemisphere in the left column and the southern hemisphere in the right column. To reduce the noise and emphasize large-scale structures, the images have been smoothed using a gaussian ﬁlter. The vertical blue lines have been manually added to the images to highlight prominent auroral features in the two hemispheres, identiﬁed as conjugate.

We emphasize that the analysis is based on the 1-D and 2-D correlation, and that the blue lines are only shown for visual guidance. The displacement of the lines between the hemispheres correspond to the displacement determined by the visual inspection.

As seen in the figure, the displayed time interval has a negative and stable IMFB_y. The IMFB_zcomponent is initially northward, turning southward around 13:20 and back northward 20 min later. The dipole tilt angle𝜓 is also negative, with𝜓= −10∘at onset. Substorm onset is first observed at 13:55:56 and is clearly displaced longitudinally between the hemispheres. Since onset is located at dusk, both the negativeB_yand negative tilt angle contribute to the asymmetry. After onset, the asymmetry is quickly reduced, seen consistently by the visual comparison, 1-D correlation, and 2-D correlation. The analysis shows that the asymmetry is removed in less than 10 min. Both the initial asymmetry and the more symmetric configuration later in the expansion phase are also seen by WIC/UVI, displayed in Figure A3. The reduction of the asymmetry is also associated with a significant increase in the nightside reconnection rate. The dynamical evolution of the expansion phase aurora is displayed in Figure 3k. Here we see the same dynamical behavior in both hemispheres, with the auroral intensity peaking in K4, then decreasing at the western edge (K5) and finally at the eastern edge (K6).

The similarities in both the dynamical behavior and in spatial extent supports the idea that the regions have a common source in the magnetosphere.

3.2. The 28 May 2001 Event

Figure 4 displays the 28 May 2001 event. The IMF is dominated by a positive and stableycomponent, andB_zis mostly northward before and during the substorm. The IMF is very stable between 02:50 and 03:45. Substorm onset is first observed at 03:29:46, with𝜓 = 12∘. The onset location is clearly displaced longitudinally, with both the IMFB_yand the tilt effects contributing to the asymmetry. The asymmetry is significantly reduced during the expansion phase, seen consistently by the 1-D correlation, the 2-D correlation, and the visual comparison. There is also a clear increase in the nightside reconnection rate in the same time interval. We note, especially when considering the travel time of the IMF from the bow shock to the magnetosphere, that the the entire reduction of the asymmetry occurs in the interval with very stable IMF conditions. It therefore seems clear that the change in displacement is related to internal processes in the magnetosphere.

The evolution of the aurora during the expansion phase is shown in Figure 4k. Substorm onset is shown in K1, withΔMLT=2.0hr. In K3, the aurora intensiﬁes on the eastern side of the onset location in both hemispheres. The eastern part of this feature shows the same dynamic behavior in both hemispheres. Starting in K6, the aurora located between 21 and 24 MLT also becomes very similar in shape and size. The combi-

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Figure 3.The 15 November 2002 event. (a) Image of the aurora in the northern (A1) and southern (A2) hemisphere at onset. The red ellipse indicate conjugate features, the blue line is the sunlight terminator. (b) Image of the aurora in the northern (B1) and southern (B2) hemisphere later in the substorm. (c) Evolution of the OCB during the expansion phase. (d) The three components of the IMF in GSM coordinates;B_x(black),B_y(green), andB_z(red). (e) The SML index. (f )ΔMLT determined by 1-D correlation. (g)ΔMLT determined by 2-D correlation. The green dots in (f ) and (g) indicater≥0.6; orange dots indicater<0.6. (h)ΔMLT determined by visual inspection. (i) EstimatedF_PCduring the expansion phase. (j)Φ_D(blue) andΦ_N(orange). The vertical blue line in (d) and (e) indicates the onset time according to Frey et al. (2004). The vertical grey lines in (d)–(j) indicate the times of the image displayed in (a) and (b). (k) Evolution of the aurora in the northern (left) and southern (right) hemisphere during the expansion phase, with MLT on the horizontal axis and mlat on the vertical axis. The vertical blue lines highlight prominent auroral features in the two hemispheres, identiﬁed as conjugate. The displacement of the lines between the hemispheres correspond to the displacement determined by the visual inspection.

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Figure 4.The 28 May 2001 event, same format as Figure 3.

nation of similar shape, size, and dynamic behavior gives strong support for these regions being conjugate.

Thus, the displacement is signiﬁcantly reduced compared to the initial displacement. The conjugate features later in the expansion phase are closer to midnight compared to the initial brightening, which means that the contribution from dipole tilt is also reduced in the latter images.

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Figure 5.The 13 August 2001 event, same format as Figure 3.

3.3. The 13 August 2001 Event

Figure 5 displays the 13 August 2001 event. The IMFB_ycomponent is stable and positive, whereas the IMFB_z component changes polarity several times before the substorm. Substorm onset is ﬁrst observed at 22:33:45, with𝜓=15∘and is clearly displaced longitudinally. Onset is located in the dawn sector, and the displacement is therefore expected to be reduced by the warping of the magnetotail. About 8 min later, the aurora is near symmetric in the dawn sector. A clear increase in the nightside reconnection rate is also seen in this 8-min interval. This event shows that the asymmetry is also reduced when the substorm occurs in the dawn sector.

The evolution of the expansion phase is displayed in Figure 3k. The ﬁgure clearly shows how the auroral feature in the northern hemisphere moves faster towards dawn than its counterpart in the southern hemisphere, thus reducing the asymmetry. The evolution of the intensities are equal in the two hemispheres, peaking in K3 before gradually decreasing, supporting the interpretation that the indicated features are conjugate.

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The 5 November 2002 event is displayed in Figure 6. The IMFB_yis positive, except for an approximately 10-min interval around 10:10. TheB_ycomponent is also positive before the time interval shown in the ﬁgure. Accord- ing to the the substorm list by Frey et al. (2004), onset occurs at 11:12:56, but from the images, it is clear that it occurs as early as 10:58:23. The dipole tilt angle is negative in this event, with𝜓= −14∘at onset. The location of the onset is clearly displaced longitudinally in the two hemispheres. As seen in the ﬁgure, the pixel size is quite large around the onset in the northern hemisphere. The reason is that IMAGE is viewing the onset location at a large satellite zenith angle. This large viewing angle leads to an uncertainty in the exact location of the onset, but due to IMAGE position at around 11 MLT, the uncertainty is mainly in the latitudinal direction. The second pair of auroral images (Figures 6b1 and 6b2) clearly shows how the aurora has become near symmetric, which is also seen by the 1-D correlation, the 2-D correlation, and the visual comparison. This event shows that an asymmetry in the dusk sector is also reduced when the IMFB_yand the dipole tilt have opposite signs.

Figure 6k displays the evolution of the expansion phase. K1 and K2 show how the onset is displaced longitudinally. The dynamical evolution is equal in the two hemispheres: first, a bright onset feature (K1 and K2), then weaker aurora (K3 to K6), an increase at the westward edge (K7) and finally an increase in the intensity at the eastward edge (K11). The shape of the aurora is also quite similar at the end of the expansion phase. The spatial extent is the same in the longitudinal direction but different in the latitudinal direction. This difference is possibly due to the different viewing angle in the two hemispheres. The similarity of the dynamical behavior and the overall shape in the two hemisphere, support these highlighted regions as being conjugate. Thus, the aurora is symmetric at the end of the expansion phase in this event.

3.5. The 2 July 2001 Event

Figure 7 shows the 2 July 2001 event. As seen in the figure, the IMF is very stable and dominated by a neg- ativeB_y. Onset is first observed at 04:29:18, with a clear longitudinal displacement. The dipole tilt is 13∘at onset. The pair of images from later in the substorm (Figure 7b) shows that the displacement have persisted throughout the substorm. This is also seen clearly in the visual comparison and the 2-D correlation, where the displacement is only slightly reduced. The same result is also seen in the 1-D correlation but with more variations. The result is further supported by images obtained by WIC/UVI, presented in Figure A4. This event is also included in the papers by Østgaard et al. (2004) and Reistad et al. (2013). Østgaard et al. (2004) only considered the onset location, whereas Reistad et al. (2013) performed a 2-D correlation similar to the technique used in this study. They also found that the longitudinal displacement remained relatively constant in this event, but the authors did not comment in particular on this, as the focus of the study was nonconjugate aurora features observed in concurrent images, and not on the time evolution of the IMFB_y-induced asymmetry. A plausible explanation of the small change in the displacement can be related to the small increase in the overall nightside reconnection rate found in this event. The evolution of the OCB does not show any clear poleward expansion, and the change of open flux is small compared to the events presented above.

The evolution of the aurora in the event is shown in Figure 7k. The aurora in both hemispheres have very similar shape and size, and the dynamical behavior of the onset feature is the same in both hemispheres, so there is little doubt that the indicates regions are conjugate. The displacement is therefore not reduced signiﬁcantly in this event.

3.6. The 23 October 2002 Event

The 23 October 2002 event is displayed in Figure 8. The IMFB_ycomponent is negative, but the magnitude is reduced 10 min prior to the onset, which occurs at 10:58:06. The dipole tilt angle is−10∘at onset. From the visual analysis and the 1-D and 2-D correlation, it is clear that the asymmetry is unaﬀected by the auroral breakup in this event. This is similar to the observations in the previous event, but for opposite dipole tilt angle.

In Figure 8c we see that there is no signature of a poleward expansion, and from the flux estimations we find that the average nightside reconnection rate is 0 kV. The low tail reconnection offers a plausible explanation of why the asymmetry remains unchanged. This will be further discussed in section 4. This event was also studied by Stubbs et al. (2005), Laundal et al. (2010), and Reistad et al. (2013), and our result is consistent with the displacement found by Reistad et al. (2013).

From Figure 8k, we see that the aurora has similar shape and spatial extent in both hemispheres throughout the substorm and that the dynamical behavior is the same. It therefore seems clear that the indicated regions are conjugate.

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Figure 6.The 5 November 2002 event, same format as Figure 3.

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Figure 7.The 2 July 2001 event, same format as Figure 3.

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Figure 8.The 23 October 2002 event, same format as Figure 3.

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An overview of the 3 August 2001 event is displayed in Figure 9. As seen in the figure, the IMF was very stable and the transverse component completelyB_ydominated. Substorm onset was first observed at at 23:09:40, where a tilt angle of𝜓=16∘acts to reduce the IMFB_yinduced asymmetry. The SML index in Figure 9 has been modified in this event, as the original index is dominated by single station (Zyryanka) suffering from a baseline problem in the time interval considered. The contribution from this station has therefore been removed in the calculation of the SML index displayed in the figure. At onset, we see only a small perturbation in the SML index, which does not decrease significantly before 23:40. Regardless, it is evident from the images of the aurora in the northern hemisphere and the estimated change in the OCB that the substorm starts around 23:09:40. The first image pair that unambiguously shows the displacement is from 4 min after the onset, and is the starting point of our analysis (Figure 9a). Here we see a clear longitudinal displacement between the hemispheres. Figure 9b shows that the aurora is near symmetric 27 min later. Both the initial asymmetry and the more symmetric aurora later in the expansion phase is also observed by WIC/UVI, shown in Figure A5.

The evolution of the substorm is displayed in Figure 9k. Here we also see that the FOV in the southern hemisphere is limited prior to 23:38. Distinct auroral features are visible in K1 to K4, and show a clear displacement.

The comparison is more uncertain in K5 to K11, due to the limited FOV in the southern hemisphere. In K12, global images are available in the southern hemisphere as well, revealing a much more symmetric aurora.

The data coverage is not optimal in this event, but it still adds valuable information; The identified features are very similar in both shape and size, so the identification is quite certain. The reduction is also observed by WIC/UVI, showing that sensitivity differences do not apply in this event. This means that the aurora have become more symmetric, with no noticeable changes in the solar wind driving. Due to the limited field of view, it is possible that the time to a symmetric state is overestimated in this event.

A small breakup occurs in the recovery phase, starting just before 00:30. The breakup is associated with small drop in the SML index. Figure 10 displays the auroral feature, which is observed by all four cameras. The new feature have the same longitudinal displacement as the initial onset displacement, indicating that the system has again returned to a more asymmetric state.

Figure 11 shows the 4 August 2000 event. As seen in the ﬁgure,B_yis positive and quite stable, andB_zis negative throughout the event. According to the substorm list by Frey et al. (2004), onset occurs at 00:54:14, but we believe that the onset occurs near 01:04:27, as a brightening at this time is associated with a drop in the SML index and change in open ﬂux. This feature is clearly displaced between the hemispheres. The dipole tilt is 11∘at onset, making the conditions similar to the 28 May 2001 event, but with negative IMFB_z. Also, images from WIC/UVI are available in this event. The image pair from later in the expansion phase shows that the aurora have become more symmetric. This is also seen by the visual comparison and both the 1-D and 2-D correlations. We also see an increase in the reconnection rate in the same time interval. The same reduction is also observed by the camera pair WIC/UVI, shown in Figure A6.

The evolution of the expansion phase is shown in Figure 11k. The initial brightening (K1) is located at 20.5 MLT in the northern hemisphere and at about 22 MLT in the southern hemisphere. The exact displacement is difficult to determine, since the feature in the southern hemisphere is observed at a slant viewing angle, but there is a clear displacement between the hemispheres. The visual and 1-D correlation suggest that the displacement remains unchanged in the first 10 min after the first brightening, but from the images it is clear that these results should be treated with caution as only parts of the feature is seen in the southern hemisphere in K2 to K5. The 2-D correlation is not able to get a reliable result in K1 to K5. Starting in K6, the feature located at the eastern side of the initial brightening intensifies in both hemispheres. Since these features show the same dynamical evolution in both hemispheres and are located at an approximate equal distance from the initial brightening, they are most likely conjugate. The initial asymmetry is therefore about the same in K6 as it was 10 min earlier. From K6 to K12 this feature grows in intensity and moves eastward in the northern hemisphere, while its counterpart in the southern hemisphere shows the same dynamical behavior, but moves slightly westward. The relative displacement is thus reduced. The exact mapping is uncertain in K9 and K10, as the auroral forms have different shape in the two hemispheres. In K11 and K12, the highlighted feature is most intense in both hemispheres, has the same spatial extent and is detached from the western part of the aurora. These features are clearly more symmetric than the initial onset locations.

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Figure 10.A sudden brightening observed in the recovery phase of the 3 August event, with the northern and southern hemisphere shown in (a) and (b), respectively. The displacement of this feature is−1.3 hr, approximately the same as the initial asymmetry.

An overview of the 15 November 2001 event is presented in Figure 12. There is no time shifted OMNI data available in the time interval considered in this event, so we have time shifted the IMF data from ACE to 17 R_Eusing simple planar propagation. Since the more sophisticated techniques applied in the OMNI data set discards the time shift in this time interval, we have compared the result of the planar shift with Geotail for validation. Geotail is located at approximatelyX_GSM=5R_Ein the magnetosheath, and observe the negative turning in the IMFB_yat 16:57, which is reasonable compared to negative turning estimated to occur at 16:51 at 17R_E. Substorm onset is ﬁrst observed at 17:20:31, with𝜓= −8∘and positive IMFB_y. The conditions are quite similar to the conditions in the 15 November 2002 event, but with IMFB_z<0. The onset location is displaced longitudinally. The aurora seems to become more symmetric during the expansion phase. This event was also included in Østgaard et al. (2004). They considered the evolution of the centroid and 50 percentile of the auroral intensities during the expansion phase and found that the two hemisphere became more symmetric, in agreement with our result.

An overview of the 3 November 2002 event is presented in Figure 13. IMFB_yis positive and IMFB_zis negative in this event. Onset is ﬁrst observed at 04:10:28, with𝜓 = −25∘. The conditions in this event are similar to the conditions in the 5 November 2002 event, but with much larger tilt angle. The images from substorm onset reveal a clear longitudinal displacement. Since onset is located in the dusk sector, the tilt eﬀects reduce the displacement caused by the IMFB_ycomponent. The pair of auroral images taken 18 min later shows that the displacement is reduced, a result also found by the visual comparison and both correlation analyses. All methods consistently show that the asymmetry is reduced in approximately 20 min. This event was also studied by Reistad et al. (2013), and the result of their 2-D correlation is in agreement with our result, although not commented by the authors.

The evolution of the substorm is displayed in Figure 13k. Both hemispheres show a similar dynamical behavior, with the most intense aurora in K3 to K5, and then a gradual decrease. The eastward expansion of the aurora is faster in the northern hemisphere than in the southern hemisphere, which gradually makes the aurora more symmetric. In K8 and K9, the conjugate aurora in both hemisphere have similar shape with two arcs.

4. Discussion

4.1. Are Asymmetries Reduced During Substorm Expansion Phase?

We have presented 10 events showing the evolution of conjugate auroral features during substorm expansion phase. Key values from the events are summarized in Table 2. As described in the introduction, an IMFB_y component is expected to give a longitudinal displacementΔMLT with the same sign as theB_ycomponent, whereas the neutral sheet warping, associated with the dipole tilt angle, is expected to give a displacement that is in opposite directions at dusk and dawn. The largest asymmetries are therefore expected when the two eﬀects act in the same direction. From Table 2, we see thatΔMLT is always consistent with the IMFB_y polarity. Although some events suggest larger asymmetries when the two eﬀects act together, this relation is not systematic. This suggests that other factors are also important to determine the displacement, for example the onset magnetic latitude and the time history of the IMFB_yand dipole tilt angle.

In eight of the events, the initial asymmetry is signiﬁcantly reduced during the expansion phase. The IMFB_y is quite stable in the majority of these events, and the most plausible explanation of the the observations is therefore that the asymmetry is reduced by processes related to the magnetospheric substorms. The asym-

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metry was not signiﬁcantly reduced in the 2 July 2001 and 23 October 2002 events. Our analysis shows that these events are also associated with the smallest increase in the estimated nightside reconnection rate. In Figure 14, we show the relationship between the change in asymmetry and the estimated average nightside reconnection. This average is calculated in the interval between the highlighted images in each event. The ﬁgure clearly shows that the most rapid reduction occurs for the events with the highest estimated reconnection rates and that there appears to be a relation between the reconnection rate and the rate of change in

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Table 2

Key Parameters From the Events

IMFB_y 𝜓 ΔMLT_onset Reduction ΔF_PC Δt ⟨Φ_N⟩ ⟨Φ_D⟩

Date UVI (nT) (^∘) (hr) (hr) (MWb) (min) (kWb/s) (kWb/s)

4 Aug 2000 yes 5.8 11 1.6 1.2 100 10 132 34

28 May 2001 no 7.5 12 2.0 1.7 60 16 70 10

2 Jul 2001 yes −8.4 13 −1.3 0.1 10 23 24 14

3 Aug 2001 yes −3.9 16 −1.0 0.8 100 29 71 11

13 Aug 2001 no 7.9 15 1.1 0.9 40 8 99 30

15 Nov 2001 no −8.4 −8 −0.6 0.6 −30 19 72 96

23 Oct 2002 no −2.2 −10 −0.8 0.1 −70 31 0 39

3 Nov 2002 no 7.6 −25 1.0 0.6 −40 17 44 85

5 Nov 2002 no 3.2 −14 1.2 1.1 −40 23 31 63

15 Nov 2002 yes −6.3 −10 −1.6 1.6 70 8 138 5

Note.The table shows the following, in columns from left to right: The date of each event, whether or not UVI is available, the IMFB_yat onset, the dipole tilt angle_𝜓 at onset, the longitudinal displacement at onset, the change in longitudinal displacement, the change in open ﬂuxΔF, the time interval, and the average nightside and dayside reconnection rates. Each parameter in the ﬁve latter columns refers to the time interval between the top images in Figures 3 to 13. Additional images from the events where UVI is available are shown in Appendix B.

ΔMLT. Our results therefore indicate that a reduction of the longitudinal displacement is indeed a common signature of magnetospheric change during the substorm expansion phase and that the rate of change is related to the reconnection rate.

The fastest change of the relative displacement is observed in the 15 November 2002 event, where the displacement changes 1.6 hr in 8 min. Using 65∘mlat as a reference, this corresponds to a flow speed of 1,175 m/s in the ionosphere, assuming that the footprints of the field lines move with the same velocity in both hemispheres. This is a fast flow, but it is not unreasonable. The corresponding speeds for all the other events are well within reasonable flow velocities in the ionosphere, ranging from 20 to 700 m/s.

Our observations are consistent with Østgaard, Humberset, et al. (2011), who were the ﬁrst to report that conjugate auroral features become more symmetric during substorm expansion phase. The same could also be seen in Østgaard et al. (2004), where the displacement of the centroid is reduced during expansion phase in the presented events. More recently, Østgaard et al. (2018) studied the longitudinal displacement of the conjugate aurora during a geomagnetic storm. Two substorms occurred in the time interval considered, and Østgaard et al. (2018) found that the relative displacement between the hemispheres was smallest after each substorm, and build up in the interval between the substorms. Statistical support of the reduction is seen in Milan et al. (2010), who did a superposed epoch analysis of substorms using auroral images from the northern hemisphere obtained by IMAGE. The authors sorted the data into early, typical and late onsets, determined by the MLT location of each onset. They found that the centroid of the early onsets, associated with positive

Figure 14.(a) The time to reach a more symmetric state in the eight events with a signiﬁcant change in the displacement versus average nightside reconnection rate. (b) The rate of change inΔMLT for all 10 events versus average nightside reconnection rate.

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IMFB_y, moved faster towards midnight compared to the centroid of the late onsets, thus making the auroral more symmetric during the expansion phase. Recent observations of the ionospheric convection also suggest that increased tail reconnection makes the magnetospheric system more symmetric. It is well established by statistical studies that the two cell convection pattern changes into a crescent “banana” cell and a round

“orange” cell when IMFB_y≠0, where the pattern mirror across the noon-midnight meridian in the two hemispheres (e.g., Heppner & Maynard, 1987). In a recent study, Reistad et al. (2018) found that the convection pattern becomes more symmetric and that the ﬂow speeds at dawn and dusk become more similar as tail reconnection increases. Also, Grocott et al. (2010, 2017) have shown that substorms produce a similar convection pattern in the nightside ionosphere, independent on the polarity of IMFB_y. These results support the claim that tail reconnection reduce asymmetries.

4.2. Uncertainties in the Identiﬁcation of Conjugate Auroral Features

When the images from the two hemispheres are compared, it is important to be aware that both sensitivity diﬀerences and physical processes can lead to observed intensity diﬀerences in the two hemispheres. We emphasize that the goal of this study is not to compare intensities between the hemispheres, but to identify conjugate auroral features and investigate how the longitudinal displacement of these features evolve.

Regardless, it is appropriate to discuss sources of intensity differences between the hemispheres, as these differences introduce an uncertainty in the identification of conjugate features.

One factor causing intensity differences between the hemispheres are different exposure to sunlight. Statis- tically, increased exposure should decrease the mean energy and energy flux of the precipitating particles, and hence produce less intense aurora (Newell et al., 1996; Ohtani et al., 2009). In the 10 events presented in this study, eight have the conjugate features in darkness in both hemispheres and should therefore not be affected by this. Two events have the entire conjugate region in darkness in one hemisphere and partly in darkness in the other hemisphere, and only one event has the entire conjugate area in the sunlit region in one hemisphere. In the event occurring on the 28 May 2001, the onset in the northern hemisphere appears at around 20 MLT, which is a region exposed to sunlight. Since the conductivity differences between the hemispheres should decrease the intensity of a feature in the sunlit hemisphere, there is no reason to believe that there is a matching feature at 20 MLT in the southern hemisphere which is not seen due to conductivity differences. It therefore seems clear that the matching feature is the feature appearing at around 22 MLT (as indicated in Figure 4), an interpretation further supported by the fact that they appear simultaneous and have roughly the same size. The features which we related later in the substorm both appear in darkness, so sunlight illumination differences do not apply. In the 3 August 2001 event, parts of the aurora in the northern hemisphere is exposed to sunlight. The different exposure to sunlight offers an explanation of the more intense aurora observed at the poleward edge in the southern hemisphere at the end of the expansion phase. Since the overall shape, spatial extent, and dynamical behavior of the identified conjugate regions are so similar, we are confident that the identified region is conjugate despite this intensity difference. For the 5 November 2002 event, the entire conjugate region in the southern hemisphere is in daylight. But the features are of similar shape and size as the counterparts in the northern hemisphere and have the same dynamical evolution.

We therefore believe that conductivity diﬀerences between the hemispheres do not play a crucial role in our identiﬁcation of conjugate auroral features.

The strength of the terrestrial magnetic field is different in the two hemispheres and can therefore be different at conjugate points (Laundal et al., 2017). Theoretically, this can alter the intensities in the two hemispheres significantly, but this is only relevant in special cases with weak particle precipitation and weak diffusion (Laundal et al., 2017; Stenbaek-Nielsen et al., 1973) and should therefore not be important in the events studied here.

There are two other sources of uncertainty in the identification of conjugate features, which to some degree are related. The first is introduced by the different cadence and integration times in the different cameras, and the second is related to how similar in shape and size the conjugate auroral features in the two hemispheres are. VIS has an integration time of 32.5 s, compared to 5 s in SI13 (and 10 s in WIC). This means that the the auroral features in the southern hemisphere are more smeared out compared to the images in the northern hemisphere and could therefore alter their shape and size. In two events (2 July 2001 and 4 August 2000), UVI uses only the LBH-long filter and an exposure time of 9.2 s, which is practically identical to the exposure time in WIC. The UVI and WIC images from these events are displayed in Appendix B and show the same displacements as seen by SI13 and VIS. For these events, differences in exposure do not apply. The longer integration