Chapter 5 Summary
The goal of this thesis has been to study how asymmetries in the magnetosphere, in-duced by an IMFBy component, evolves during intervals with enhanced reconnection in the near-Earth tail. Based on the conjugate auroral observations reported in Paper I and Paper III, it seems clear that the asymmetry of corresponding features are reduced during the substorm expansion phase, which means that the relative displacement of the footpoint of field lines in the closed magnetotail is reduced. Further, by estimating the nightside reconnection rates in Paper I, we found a positive dynamical correlation be-tween the reconnection rate and the rate of change in∆MLT. In Paper II, we showed that also the lobe convection is less asymmetric for strong near-Earth tail reconnection. The reported results all point in the same direction: Increased near-Earth tail reconnection reduces asymmetries in the magnetosphere.
We have also proposed a possible interpretation of the observed reduction, namely that a reduction in lobe pressure during intervals with strong tail reconnection could reduce asymmetries. This seems like a plausible explanation since it is now well docu-mented that pressure gradients play a major role when the asymmetries are introduced, and since numerous studies from the past decades have shown that the lobe pressure is reduced during the substorm expansion phase.
47
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Paper I
Evolution of asymmetrically displaced footpoints during substorms
A. Ohma, N. Østgaard, J. P. Reistad, P. Tenfjord, K. M. Laundal, K. Snekvik, S. E. Haa-land, M. O. Fillingim
Journal of Geophysical Research: Space Physics, 123, doi:10.1029/2018JA025869 (2018)
63
Journal of Geophysical Research: Space Physics
Evolution of Asymmetrically Displaced Footpoints During Substorms
A. Ohma1 , N. Østgaard1 , J. P. Reistad1 , P. Tenfjord1 , K. M. Laundal1 , K. Snekvik1 , S. E. Haaland1,2 , 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
AbstractIt is well established that a transverse (y) component in the interplanetary magnetic field (IMF) induces aBycomponent 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 IMFBy≠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 aBycomponent in the interplanetary magnetic field (IMF) leads to the presence of aBycomponent with the same polarity inside the closed magnetosphere (e.g., Cowley & Hughes, 1983; Petrukovich, 2011; Wing et al., 1995). The presence of a large-scaleBycomponent 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 fea-tures are displaced longitudinally when IMFBy≠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 IMFBy, 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 IMFBycomponent leads to a localBycomponent in the closed magnetosphere. Some have explained it terms of a simple penetration of the IMF, where theBy
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 withBybeing introduced into the closed magnetosphere by asymmetric pressure gradients
Correspondence to:
A. Ohma, anders.ohma@uib.no
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
©2018. The Authors.
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 recon-nection and that theBycomponent 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 IMFBy≠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 IMFBy>0. Depending on the sign of IMFBz, 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 asym-metry enters the closed magnetosphere when these field lines reconnect in the tail. An alternative description of how aBycomponent 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 trans-verse 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 aBycomponent. SinceByis introduced in the closed magneto-sphere in direct response to the asymmetric pressure distribution, a more prompt response to changes in the IMFByis expected from this mechanism compared toBybeing introduced by tail reconnection. By using mag-netohydrodynamics (MHD) simulations, Tenfjord et al. (2015, 2018) found that aBycomponent appeared in the closed magnetosphere just a few minutes after introducing aBycomponent in the IMF. Superposed epoch analysis of theByresponse to IMFByreversals 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 Byintroduced by this mechanism as inducedBy. We will also use the term “induced” when we refer to theBy introduced in the closed magnetosphere by the IMFBy, although the term “penetrated” is often used in the literature.
There are also other factors that can produce aBycomponent in the equatorial plane. One source ofByis 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 aBycomponent 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 mag-netic 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 aBy component, which is positively correlated with tilt angle at allYGSMpositions (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 ofByis the rotation of the magne-totail around the tail axis, driven by the IMFBy(Cowley, 1981; Fairfield, 1979). This reduces the inducedBy, but the contribution is considered to be of minor importance and since it is solely dependent on the IMFByit can be considered as part of the inducedBy(Petrukovich, 2011). For completeness, we note that there is also a forth source ofByin the magnetosphere; since the field lines are connected to the ionosphere, there is a flar-ing component of theByaway 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 B2 2𝜇0
, 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 sub-sequent 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 andBy 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 andBydominated 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 Exploraconjunc-tion (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 cam-eras, 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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