Corresponding Author: Giulia D’Angelo
Mail: [email protected] Phone: +393479307634
The response of high latitude ionosphere to the 2015 St. Patrick’s Day storm from in situ and ground based observations
Giulia D’Angeloa, Mirko Piersantib,c, Lucilla Alfonsid, Luca Spoglid,e, Lasse Boy Novock Clausenf, Igino Cocod, Guozhu Lig, Ning Baiqig
aDipartimento di Matematica e Fisica, Università degli Studi “Roma Tre”, Via della Vasca Navale, 84, 00146, Rome, Italy. [email protected]
bIstituto di Astrofisica e Planetologia Spaziali, Rome, Italy. [email protected]
cConsorzio Area di Ricerca in Astrogeofisica, Università di L’Aquila, Via Vetoio, 67100, Località Coppito, L`Aquila, Italy.
dIstituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143, Rome, Italy. [email protected] [email protected], [email protected]
eSpacEarth Technology, Via di Vigna Murata 605, 00143, Rome, Italy.
fDepartment of Physics, University of Oslo, Oslo, Norway. [email protected]
gInstitute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China.
[email protected] [email protected]
Corresponding Author: Giulia D’Angelo [email protected]
Abstract
The storm onset of the so-called “St. Patrick’s day geomagnetic storm”, on March 17th, 2015 triggered several fluctuations of the electron density in the ionosphere causing severe scintillations at high latitudes of both hemispheres. Leveraging on ground-based Global Navigation Satellite Systems (GNSS) receivers we investigate the ionospheric response to the main phase of the most intense storm of the current solar cycle, in terms of phase scintillations on L-band signals recorded simultaneously in Antarctica and in the Arctic. In detail, we analyse phase scintillation index () data from Eureka (79.99 °N, 274.10 °E), Concordia (75.10 °S, 123.35 °E), Resolute Bay (74.75 °N, 265.00 °E), Mario Zucchelli (74.41 °S, 164.10 °E), Ny-Ålesund (78.92 °N, 11.98 °E) and Zhongshan (69.37 °S, 76.37 °E) stations. Furthermore, by using ancillary data obtained from in-situ and ground-based observations, we investigate the origin and the evolution of the ionospheric irregularities causing scintillations, reconstructing the ionospheric background in which such irregularities formed and moved. The multi- instrumental approach used in this work allows identifying the Antarctic ionosphere as the most responsive to the solar perturbation driving the storm. Our study reveals how the in-situ electron density data can be used to reconstruct the picture of the ionospheric dynamics, both locally and globally. Finally, our results identify the important role played by particles precipitation in triggering the observed scintillations.
Keywords
High-latitude ionosphere; Ionospheric irregularities; GNSS scintillations; Multi-instrumental observations; Ionospheric dynamics; Interhemispheric study
2 1. Introduction
In the last two decades, the interest in scintillation on trans-ionospheric L-band signals was growing fast, because of the considerable effects on the performance of satellite communication and navigation (see, e.g., Fisher & Kunches 2011; Board 2008; Aquino et al. 2007; Alfonsi et al. 2006).
In the case of GNSS signals, scintillation may drastically reduce the accuracy of the pseudorange and phase measurements. Consequently, the positioning errors increase and, in extreme cases, the service can become unavailable. In fact, during intense scintillation events, the signal power can drop below the threshold limit, the receiver loses lock to the satellite and the GNSS positioning is not possible (see, e.g., Kintner et al. 2009).
At high latitudes, the ionospheric impact on GNSS signal propagation is due to the presence of fast moving small-scale plasma irregularities (see, e.g., Basu et al. 2002; Wernik et al. 2004; De Franceschi et al. 2008). The formation and the dynamics of such irregularities are closely linked to the physical processes that originate in the interplanetary medium (Fejer & Kelley 1980; Tsunoda 1988; Kelley 1989; Hunsucker & Hargreaves 2002). The coupling between the Interplanetary Magnetic Field (IMF) and the Earth’s magnetic field directly exposes the high latitude ionosphere to the solar wind forcing and variability (Baumjohann & Treumann 1996) resulting in complex plasma dynamics that lead to the formation of irregularities which can cause scintillations. The knowledge about the ionospheric conditions in which such irregularities form and move is crucial to understand the underlying physical mechanism lying above and then develop reliable prediction models and mitigation techniques.
In this paper, we investigate the irregularities causing scintillation, reconstructing the ionospheric conditions in the polar regions of both hemispheres, during the main phase of 2015 St. Patrick’s day geomagnetic storm. This storm constituted a powerful test bench for probing the physical conditions in which irregularities appear and evolve. The Solar Wind (SW) and the IMF conditions related to the storm, indeed, produced a strong disturbance of the Earth’s magnetic field causing an intense particle precipitation and an enhancement in substorm activity with a subsequent and conspicuous development of plasma density irregularities (Wu et al. 2016, Cherniak et al. 2015, Cherniak &
Zakharenkova 2015).
In this paper, we report an investigation of the disturbed polar ionosphere based on a multi-instrument observation helpful through retrieving a broad spectrum of information to characterize the plasma dynamics, which produces fluctuations of the electron density distribution, which in turn produces scintillations. Being the GPS-based technique, a proven method to detect and monitor ionospheric irregularities occurrence and dynamic (Aarons 1997; Pi et al. 1997; Jakowski et al. 2005; Coster and Komjathy 2008; Tiwari et al. 2013), our analysis starts from the detection of the ionospheric irregularities by using scintillation observations. For this scope, we analyse scintillations recorded at six sitesequally distributed both in the Arctic and in the Antarctica. Specifically, Eureka, Resolute Bay and Ny-Ålesund stations are located in the Arctic, while Concordia, Mario Zucchelli and Zhongshan stations are located in Antarctica. Under quiet geomagnetic conditions, these stations look respectively at cap, cusp and auroral regions; this allows us to investigate the ionospheric response to the storm in both hemispheres and in different geomagnetic sectors.
To characterize the physical background causing the observed scintillation, we combine information coming from Total Electron Content (TEC) and scintillation parameters with ancillary data obtained from in-situ and ground-based observations. Specifically, we use measurements acquired by Super Dual Auroral Radar Network (SuperDARN), Swarm and Polar-orbiting Operational Environmental
3 Satellites (POES) constellations, which are able to characterise the ionospheric plasma conditions both globally and locally. Furthermore, we use measurements acquired by WIND spacecraft to characterise the disturbance level of the interplanetary medium and to relate the irregularities triggering scintillation dynamics to the storm drivers.
This paper is organized as follows: it starts with a brief description of the period under investigation and the introduction of the experimental observations used in the data analysis; then it illustrates the method adopted in the study (section 2); finally, it discusses the results, providing the concluding remarks (sections 3 and 4, respectively).
2. Data and methods
The selected period covers the main phase of the St. Patrick’s geomagnetic storm, which occurred on 2015-03-17. The storm, studied by several authors (see e.g. Cherniak et al. 2015; Kamide & Kusano 2015; Liu et al. 2016 and references therein), was caused by two interacting Interplanetary Coronal Mass Ejections (ICMEs) (Liu et al. 2015) preceded by an interplanetary (IP) shock and characterised by a long lasting period with negative values of the IMF Bz component.
Figure 1 reports, from top to bottom, the IMF amplitude |B|IMF, the IMF By component, the IMF Bz component, the SW density, the SW proton temperature, the SW velocity, the SYM-H index and the AU (black) and AL (red) indices on 2015-03-17. The shaded regions indicate the two ICME intervals and the black dashed vertical line marks the corresponding shock arrival at WIND.
The WIND spacecraft, located at the firstLagrangian point, acquired SW and IMF measurements, while the World Data Center for Geomagnetism of Kyoto provided the SYM-H, the AU and the AL geomagnetic indices.
As one can see in panel d, during the ICMEs intervals, the North-South component of the IMF (Bz,IMF) was consistently southward (at about -20 nT), except for a brief interruption between 09:00 and 11:00 UT. This resulted in an intense particle precipitation and an enhancement in substorm activity producing a severe geomagnetic storm lasting for most of the second half of the day. In fact, the SYM- H index (panel h) shows a short drop to a minimum of ~-245 nT observed at ~23:00 UT. The upper and lower auroral electrojet current indices (AU and AL: black and red line in panel i) also show, from ~06:00 UT until the end of the day, a strong level of current intensity of the eastward and westward northern auroral electrojets, respectively.
To detect ionospheric irregularities produced by the highly disturbed conditions of the geomagnetic field we use GNSS data, acquired by selected GPS Ionospheric Scintillation and TEC Monitor (GISTM, Van Dierendonk et al. 1993) receivers located in Arctic and Antarctica.
Specifically, Arctic receivers are located at Eureka, Resolute Bay and at Ny-Ålesund stations, while Antarctic receivers are located at Concordia, Mario Zucchelli (Terra Nova Bay) and Zhongshan stations. They are all Novatel GSV4004 receivers, and are managed by the Canadian network CHAIN (Canadian High Arctic Ionospheric Network, Jayachandran et al. 2009), by INGV (Istituto Nazionale di Geofisica e Vulcanologia, De Franceschi et al., 2006) and by IGGCAS (Institute of Geology and Geophysics, Chinese Academy of Sciences).
Table 1: Locations, identifiers, geographic and geomagnetic coordinates of the GISTM receivers used in this study.
Location Station ID Owner Latitude Longitude CGLat CGLon
4
Arctic
Eureka EURC CHAIN 79.99 °N 274.10 °E 87.41 °N 342.32 °E
Resolute Bay RESC CHAIN 74.75 °N 265.00 °E 82.45 °N 326.10 °E Ny- Ålesund 0 NYA0 INGV 78.92 °N 11.98 °E 76.54 °N 108.79 °E
Antarctica
Concordia DMC0 INGV 75.10 °S 123.35 °E 88.02 °S 225.55 °E Mario Zucchelli BTN0 INGV 74.41 °S 164.10 °E 77.12 °S 274.75 °E
Zhongshan ZSGN CAA 69.37 °S 76.37 °E 75.59 °S 102.53 °E
5 Figure 1:The SW observations (WIND spacecraft) and the geomagnetic response at low and high latitudes on 2015-03-17. From the top to the bottom: (a) the IMF amplitude |B|IMF; (b) the IMF Bx,IMF
component; (c) the IMF By,IMF component; (d) the IMF Bz,IMF component; (e) the SW density ρSW; (f) the SW proton temperature TSW; (g) the SW velocity VSW; (h) the SYM-H index; (i) the AU (black line) and the AL (red line) indices. The shaded regions indicate the two ICME intervals (respectively:
ICME1 and ICME2) and the black dashed vertical line marks the corresponding shock arrival at WIND.
6 Table 1 gives the geographic and geomagnetic coordinates of the sites, together with the receivers’
identifiers, the locations and the stations owners. Figure 2 shows the geographic locations of the receivers.
Each receiver collects GPS data from up to 10 visible satellites at L1 (1575.42 MHz) and L2 (1227.6 MHz) frequencies, and it measures the phase and the amplitude (at 50 Hz rate), and the code/carrier divergence (at 1 Hz rate) of the signal for each satellite being tracked on L1. The firmware of the receiver provides both amplitude and phase scintillation by computing the S4 (Yeh & Liu 1982) and the (Van Dierendonck et al. 1993) indices over different time windows (1, 3, 10, 30 and 60 seconds). It also provides TEC and the rate of change of TEC (ROT) values computed every 15 seconds from combined L1 and L2 pseudorange and carrier phase measurements.
Figure 2: Geographic locations of the selected receivers. On the top the locations of Eureka (EURC), Resolute Bay (RESC) and Ny-Ålesund (NYA0) stations. On the bottom the location of Concordia (DMC0), Mario Zucchelli (BTN0) and Zhongshan (ZSGN) stations.
In this work, we consider 1-minute data of the phase scintillation index, directly provided by the receiver, and of the ROT, being the sum of the ROT values calculated every 15 seconds.
We analyse index as it is the most reactive indicator of the presence of ionospheric irregularities at high latitudes (Doherty et al. 2003; Alfonsi et al. 2011). Following the recommendations given by Spogli et al. (2009, 2013b), the index was mapped to the vertical to compensate for the impact of longer paths through the ionosphere of signals from low-elevation satellites on the measurement according to the following formula:
7 / ( ( ))
vert slant a
F elev
(1.1)
where slant is the index directly provided by the receiver at a given elevation angle along the slant path; F(elev) is the obliquity factor that is defined as (Mannucci et al. 1993):
2
( ) 1
1 cos
elev
e elev
e IPP
F
R R H
. (1.2)
In equation (1.2), Re is the Earth’s radius and HIPP is the height of the Ionospheric Piercing Point, here assumed to be located at 350 km of altitude. According to equation (19) of Rino (1979a, 1979b) and as described by Spogli et al. (2009), we can reasonably assume the exponent a = 0.5 in (1.1). In using equation (1.1), we also take into account the caveat reported in Spogli et al. (2013b) for strong scattering regimes. To prevent the inclusion of data affected by multipath that could mimic scintillation events, a mask of 20° on the elevation angle of the satellites is applied. Moreover, to minimize possible mismeasurements of the scintillation indices and ROT following a loss-of-lock event, only data characterized by lock time greater than 240 s are included in our analysis (Smith et al., 2008).
To support the identification of the electron density gradients leading to the observed ionospheric scintillation, we investigate the ROT. Since the ROT is related to the spatial Nyquist period (Zou &
Wang 2009) it allows retrieving information about the scale length of the irregularities involved in scintillations. The scale length of the irregularities, which corresponds to the Nyquist period, are given by the vector sum of the components of the ionospheric projection of the satellite motion and the drift velocity of the irregularities in the direction perpendicular to the GPS ray path. Since experimental evidences from high latitudes show that plasma convection velocities span the range 100 m/s to 1 km/s (Ruohoniemi & Greenwald 2005), the scale length of the irregularities, sampled by ROT, varies from a few to tens of kilometres (Basu et al. 1999). Hence, by a comparison between the ROT excursions and the scintillations occurrence, it is possible to retrieve information about the scale sizes of the irregularities into which the ionospheric plasma is fragmented (Alfonsi et al., 2011).
In order to characterize plasma conditions producing ionospheric irregularities detected by ROT and scintillation parameters, we integrate ionospheric information retrieved by GNSS measurements with in-situ and ground-based observations. To consider ionospheric density, we analyse in-situ measurements of ionospheric plasma density provided by Langmuir probes on board of Swarm constellation (Friis-Christensen et al. 2006). This constellation consists of three satellites (A, B and C) placed in two different polar orbits, two flying side by side (A and C) at an altitude of 450 km and a third (B) at an altitude of 530 km. Each satellite is able to provide high precision and high-resolution measurements of the strength, direction and variations of the Earth's magnetic field and plasma data (such as electron density and temperature). This allows characterising the ionospheric conditions before, during and after the solar perturbations. Furthermore, the peculiar geometry of the Swarm constellation allows understanding the role played by the ionospheric dynamics in scintillation production both at different altitudes and in different geomagnetic sectors.
To obtain a local characterization of the ionospheric plasma condition triggering scintillations, we compare the electron density variation recorded along the orbits of the Swarm satellites over an area covering 50° to 90° of magnetic latitude in both hemispheres with the scintillations simultaneously
8 recorded by the GNSS stations. In addition, we compute and analyse the electron density root mean square (rms) to describe the ionospheric response to the storm's drivers in terms of plasma turbulence.
The rms, being an indicator of the electron density variability, could give a measure of the ionospheric turbulence level.
Finally, we analyse the spectral widths associated with the line of sight velocity measurements and the ionospheric electric potential, provided by SuperDARN (Greenwald et al. 1995). Specifically, we select spectral widths greater than 200 m/s, since this threshold facilitates discrimination on the presence of plasma turbulences (Woodfield et al. 2002). In order to summarize the ionospheric conditions in terms of formation and dynamics of plasma irregularities, we report such measurements in polar view maps, represented in Altitude Adjusted Corrected GeoMagnetic (AACGM) coordinates covering the ranges 00–24 Magnetic Local Time (MLT) and 50°–90° Magnetic Latitudes (MLat). In each map and for each hemisphere, we also compare the intensity profile of electron density recorded along the orbit of one of the Swarm satellites to the projection of ionospheric scintillation simultaneously recorded by all GNSS receivers, which lie in the considered hemisphere.
3. Results
Figure 3 shows the time profile of recorded at EURC and DMC0 (panel a), RESC and BTN0 (panel b), NYA0 and ZSGN (panel c) on 2015-03-17. The black horizontal lines indicate the 0.25 radians
threshold, reasonably chosen to identify moderate to strong levels of phase scintillations (see e.g., Spogli et al. 2009). Different colours refer to different satellites in view. The shaded regions indicate the intervals of two ICMEs and highlight the scintillation events analysed in this work. It is worth noticing that scintillation events principally occur during the first and second ICME arrivals (Liu et al. 2015). Furthermore, during the entire day a highly intense and long lasting scintillation activity affects the Antarctic ionosphere (bottom figures in panels a, b and c). The intensity of the ionospheric response at the different stations revealed by the scintillations appears closely related to the different geomagnetic sectors. Polar cap stations, DMC0 and BTN0, recorded intense scintillation events between 08:00 and 11:00 UT, with a maximum intensity respectively of ~0.95 radians and of ~1.25 radians at ~09:30 UT (bottom plots in panels a and b, respectively). They also recorded a second, much more intense scintillation series starting from 13:30 and up to 24:00 UT, with a maximum intensity of ~1.8 radians between 16:00 and 17:00 UT. The station DMC0 (bottom plot in panel a) recorded, at ~19:00 UT, another very intense scintillation event with a maximum intensity of ~1.8 radians. On the other hand, ZSGN (bottom plot in panel c) recorded scintillations roughly at the same hour of the day, but the maximum intensity level is ~0.55 radians, reached at ~08:30, 10:00 and 18:30 UT. In the Northern Hemisphere, the polar cap stations, EURC and RESC (top plots in panels a and b respectively), recorded a weak scintillation event at ~08:30 UT on only one satellite in view, with an intensity of about 0.3 radians and 0.25 radians, respectively. The station RESC (top plot in panel b) recorded another weak scintillation event at ~11:50 UT with an intensity of about 0.3 radians, which involved several satellites in view. The station EURC (top plot in panel a) also recorded scintillations between 15:30 and 20:45 UT, with a maximum intensity of ~0.55 radians at ~18:00 UT;
while RESC recorded scintillations mainly concentrated between 15:30 and 21:00 UT, with a maximum intensity of ~0.75 radians at ~20:30 UT. The NYA0 receiver, observing mainly the cusp region, recorded two scintillation series during the day (top plot in panel c). The first occurred between 08:00 and 12:45 UT, with a maximum intensity of ~0.55 radians at ~12:00 UT with a data gap roughly between 08:30 and 09:00 UT; the second occurred between 17:15 and 21:30 UT, with a
9 maximum intensity of ~0.50 radians few minutes before 20:00 UT. The NYA0 receiver recorded another weak scintillation event of ~0.35 radians at ~14:15 UT.
Figure 4shows the ROT for the same stations and in the same time interval as above. As in Figure 3, the shaded regions identify the ROT behaviour recorded during the ICME1 and the ICME2 intervals respectively. It is interesting to note that, in all panels of Figure 4, the most intense and sudden ROT excursions occur during the two ICME intervals. Furthermore, they occur in correspondence with the scintillation events shown in Figure 3. The ROT excursions recorded during the day by southern receivers (bottom plots in panel a, b, c of Figure 4) appear to be stronger and lasting for a longer time, as compared to those recorded by the northern receivers. The intensity of ROT is less dependent on the different geomagnetic sectors.
10 Figure 3: Time profiles of , recorded by EURC and DMC0 (a), RESC and BTN0 (b), NYA0 and ZSGN (c) receivers during 2015-03-17. Different colours refers to different satellites in view. The shaded regions indicate the two ICME intervals (respectively: ICME1 and ICME2). The black horizontal lines characterize the 0.25 radians threshold, which defines the transition from weak (below the line) to strong (above the line) phase scintillation levels.
11 Figure 4: Time profiles of Rate of TEC (ROT), recorded by EURC and DMC0 (a), RESC and BTN0 (b), NYA0 and ZSGN (c) receivers during 2015-03-17. Different colours refer to different satellites in view. The shaded regions indicate the two ICME intervals (respectively: ICME1 and ICME2).
12 Figure 5 shows polar view maps, covering 00:00–24:00 MLT and 50°–90° MLat for Northern (top) and Southern (bottom) Hemispheres. These maps portray the snap-shots of the mean ionospheric conditions in the time interval between 08:00-10:00 UT, when the first significant scintillation peak occurs (shown in Figure 3).
Figure 5: Polar view maps in AACGM coordinates for the Northern (top) and Southern Hemispheres (bottom) in the time interval between 08:00-10:00 UT. Each map reports: SuperDARN measurements of spectral widths (black squares) greater than 200 m/s and isocontours of ionospheric electric potential (red=positive, blue=negative potential), recorded in the two minutes following the time shown at the top of each map; the electron density (black line) recorded along the traces of Swarm A (SWA) and B (SWB) satellites; phase scintillations greater than 0.25 rads (colored dots) recorded simultaneously along the Swarm tracks. In each map, which covers 00:00-24:00 MLT and 50°–90°
MLat, the magnetic noon/midnight is at the top/bottom.
Each map in Figure 5 shows the intensity profile of the electron density (black line) recorded along the track of Swarm A (SWA) or B (SWB) satellite. The Swarm data are compared to the SuperDARN measurements of spectral widths greater than 200 m/s (black squares) and to the ionospheric electric potential (red/blue isocontours). The SuperDARN data were collected in the two minutes following the time shown at the top of each map. Moreover, we report the projection of ionospheric scintillation greater than 0.25 rads (coloured dots) simultaneously recorded by Eureka, Resolute Bay and Ny- Ålesund stations in the Northern Hemisphere (top panels) and by Concordia, Mario Zucchelli and Zhongshan stations in the Southern Hemisphere (bottom panels). In accordance with Figure 3, different colours refer to different satellites in view from each GNSS station; moreover, such coloured dots refer to scintillations recorded during the time taken by the Swarm satellite to cross the selected area (indicated in the figure).
13 It is interesting to note (Figure 5) that scintillations mainly originate in the ionospheric region between the two convection cells. This region is known to be characterized by plasma turbulence, as confirmed by the high spectral width values provided by SuperDARN network (Woodfield et al. 2002) (see e.g.
Figure 5 panels d and f). The same region has been often identified to host the ionospheric irregularities causing the observed scintillations (see, e.g., De Franceschi et al 2008; Mitchell et al 2005; Moen et al. 2013)
In the Southern Hemisphere (panels b, d and f, respectively) scintillations seem to spread on wider area of the ionosphere with respect to the Northern Hemisphere (panels a, c and e, respectively). In the Southern Hemisphere scintillations appear both in correspondence with steep gradients of electron density (identified by the enhanced thickness of the Swarm trace) and of high spectral width values (black squares). The latter are mainly concentrated in the polar cap and in the cusp regions of the Southern Hemisphere, and in the night side of the auroral oval of the Northern Hemisphere.
Figure 6: Polar view maps in AACGM coordinates for the Northern (top) and Southern Hemispheres (bottom) in the time interval 16:00 to 20:00 UT. Each map reports: SuperDARN measurements of spectral widths (black squares) greater than 200 m/s and isocontours of ionospheric electric potential (red=positive, blue=negative), recorded in the two minutes following the time shown at the top of each map; the electron density (black line) recorded along the traces of Swarm A (SWA) and B (SWB) satellites; phase scintillations greater than 0.25 rads (colored dots) recorded simultaneously along the Swarm tracks. In each map, which covers 00:00–24:00 MLT and 50°–90° MLat, the magnetic noon/midnight is at the top/bottom.
As for Figure 5, Figure 6 shows the polar maps for Northern (top) and Southern (bottom) Hemispheres in the time interval 16:00 to 20:00 UT (i.e. when the second significant scintillation peak occurs).
Figure 6 shows results very similar to those described in Figure 5. In fact, the scintillation that affects the Northern Hemisphere (panels a, c, e, g) appears in correspondence with high spectral width values and with electron density gradients. The latter, however, are steeper in the Southern Hemisphere than in the Northern Hemisphere (panels b, d, f, h).
Moreover, high values of spectral widths are observed mainly in the cusp in the Northern Hemisphere and at the polar cap and at lower latitudes in the evening sector in the Southern one.
14 Figure 7 shows the time profile of electron density root mean square, rms, measured by Swarm A (SWA, circles) and B (SWB, stars) on both Polar Regions between 50° and 90° of magnetic latitude, during 2015-03-17.
Figure 7: Electron density root mean square measured by Swarm A (circles) and Swarm B (stars) during 2015-03-17. Blue colour refers to measurements in the Northern Hemisphere, while red colour refers to measurements in the Southern Hemisphere. The Magnetic Latitude ranges between 50° and 90°. Dashed vertical lines mark the interplanetary shock arrivals and the shaded areas evidence the ICMEs intervals.
Larger rms values occur in the Southern Hemisphere (red) than in the Northern Hemisphere (blue), especially in correspondence with the arrival of the IP shock (black dashed line) and of the ICMEs (shaded areas). However, rms values in the Southern (red) and in the Northern Hemisphere (blue) are comparable before the arrival of the solar perturbations, between 00:00 and 04:50 UT, and in conjunction with the positive switch of the z component of the IMF (Bz, Figure 1d), between 10:00 and 16:00 UT. Furthermore, between 00:00 and 13:00 UT, Swarm A measures, in the Southern Hemisphere, rms values (red circles) larger than those measured by Swarm B (red stars), except between 08:00 and 10:00 UT when the opposite is observed. Starting from 15:00 UT, the Swarm B rms values are larger than those measured by Swarm A.
In the Northern Hemisphere, even though before 04:00 UT the Swarm A rms values (blue circles) are larger than the Swarm B values (blue stars), the two satellites record roughly comparable values until the end of the day.
4. Discussions
During 2015-03-17, the selected GNSS receivers (EURC, RESC, NYA0, DMC0, BTN0 and ZSGN) show the occurrence of strong and long lasting phase scintillation events. They occur mainly in correspondence with the first and the second ICME intervals (shaded regions in Figure 3). This is in accordance with the IMF conditions related to the two plasma clouds hitting the Earth (Figure 1). The North-South component of the IMF (Bz,IMF), indeed, was consistently southward (Figure 1d), except
15 between 09:00 and 11:00 UT. These conditions led to remarkable particles precipitation and to an enhancement of the substorm activity, as visible from the auroral indices variations shown in Figure 1, panel i. The large variation of the AL index (panel i, red line) compared to the AU index variation (panel i, black line), suggests that the number of particles that come into the ionosphere is significantly greater compared to the number of particles that leave the ionosphere (Turner et al. 2000; Kallio et al. 2000; Feldstein et al. 1996; Lui 2011). In such environment, ionospheric irregularities form and accelerate inducing the observed phase fluctuations on the GNSS signals received at ground. The ROT time profiles, recorded by the GNSS receivers, show intense and highly variable ROT gradients (Figure 4) in correspondence with the observed scintillations (Figure 3). According to Wernik et al.
(2004) and Alfonsi et al. (2011) it is possible to infer that the investigated regions are populated by irregularities of largely varying scale-sizes. This is confirmed by the work of Cherniak et al. (2015), who reported the presence of very large scale-size irregularities from middle to high latitudes over both hemispheres. The plasma fragmentation is further confirmed by the turbulent regime shown both by the SuperDARN high spectral width values and by the Swarm rms electron density data.
Furthermore, particle precipitation on the one hand favours the formation of localized irregularities but, on the other hand, increases the conductivity of the ionosphere causing an increase in its dynamics. Scintillations appear both in correspondence of steep gradients of electron density and of high spectral width values (Figure 5 and Figure 6). It is worth noticing that the comparison between the phase scintillation measurements and the Swarm electron density measurements at different heights allows the altitudes of the irregularities causing scintillations to be determined. Moreover, as the appearance of scintillations coincide with steep variations of the rms measurements (Figure 7) this confirms the correlation between scintillation and abrupt changes in plasma dynamics.
The intensity of the ionospheric response as shown by the scintillations (Figure 3) appears closely related to the different geomagnetic sectors included in the field of view of the GNSS receivers and characterized by a marked hemispheric asymmetry, especially in the responses of the polar caps. A more intense and long lasting scintillation activity affected the Antarctic polar cap as compared with the Arctic one. Such asymmetry might be related to the different amount of particles precipitating in the two hemispheres, as reported by Cherniak et al. (2015) and Cherniak and Zakharenkova (2016).
To confirm the different behaviour in the two hemispheres we have computed the Electron and Proton total atmospheric integral energy flux at 120 km (as a function of the magnetic latitude) measured on 17 March by the Polar-orbiting Operational Environmental Satellites (POES, Evans and Greer 2004).
Our results (Figure 8) show a more intense particle precipitation in the Southern Hemisphere. Indeed, only the MetOp2 satellite measured a higher particle precipitation flux in the Northern Hemisphere, while all the other satellites observed a larger amount of particles entering in the Southern Hemisphere. A further confirmation of the asymmetry comes from the variation of the Polar Cap Indices (Troshichev et al., 1979) shown in Figure 9.
16 Figure 8: Electron and Proton total atmospheric integral energy flux at 120 km as a function of the magnetic latitude, measured by the TED (Total Energy Detector) on board of MetOp1 (panel a), MetOp2 (panel b), NOAA15 (panel c), NOAA18 (panel d), NOAA19 (panel e) satellites of the POES constellation, during 2015-03-17.
Figure 9: The northern (blue line) and southern (red line) Polar Cap indices derived by magnetic data of Thule and Vostok stations respectively during 2015-03-17.
This observed hemispheric asymmetry in particle precipitation might be linked to the interplanetary shock’s geometry hitting the Earth’s magnetopause on 17 March. According to the shock parameters, provided by the Database of interplanetary shocks of the University of Helsinki (http://ipshocks.fi/), and to the results of Wu et al. (2016), the shock mainly hit the Southern Hemisphere. This condition, together with the negative value of the dipole tilt angle (~ -6.19°), caused the Southern Hemisphere to be more exposed to the impact of the solar perturbations than the Northern Hemisphere (Villante et. al, 2008; Alberti et al. 2016; Piersanti et al. 2012). A further corroboration comes from the rms
17 variation of the electron density measured by the Swarm satellites in both hemispheres (Figure 7).
The measurements over the two hemispheres are comparable before the arrival of the solar perturbations and in conjunction with the positive switch of the z component of the IMF (Bz, Figure 1d). The difference between the two hemispheres happens at the arrival of the solar perturbations, when the rms variation of the electron density shows a higher variability of the plasma density in the south.
We exclude a significant role of seasonal effect in the observed asymmetry because the overall scenario derived by in-situ and ground-based records appear symmetric until the arrival of the solar perturbations. Additionally, the St. Patrick’s Day storm of March 2015 occurred close to the spring equinox. Hence it would not be justified to propose any physical interpretation of the observed asymmetries based on pronounced seasonal (summer/winter) effects in the ionospheric background conditions in both hemispheres (see e.g., Emery et al. 2008).
The different exposure of the two hemispheres to the storm drivers well explains the response of the ionosphere over Antarctica in terms of phase scintillations.
5. Summary and conclusions
This paper presents an interhemispheric, multi-instrument study of the response of the high latitude ionosphere to the main phase of the most intense storm of the 24th solar cycle in terms of phase scintillation on L-band signals. Namely, we combine information from TEC spatial-temporal variation (ROT) and scintillation parameters derived from GPS data with measurements acquired by Super Dual Auroral Radar Network (SuperDARN), Swarm and Polar-orbiting Operational Environmental Satellites (POES) constellations to investigate the origin and the evolution of the ionospheric irregularities causing scintillations.
We can summarise our findings as follows:
The comparison between ROT, phase scintillation parameters and Swarm measurements allows concluding that the scintillation events recorded on 17 March 2015 at high latitudes in both hemispheres is due to the presence of fast-moving irregularities with several scale-sizes.
The comparison between the spectral width and the phase scintillations provides insights about the link between the GPS phase scintillation and the turbulence of the ionospheric plasma. In particular, this comparison suggests that the ionospheric regions characterized by high spectral width are more likely to give rise to phase scintillation.
The Swarm electron density measurements, compared with the phase scintillation measurements, allow confirmation of the cause-effect mechanisms between the presence of localized plasma irregularities and the scintillation. This comparison also allows an estimation of the heights of the irregularities causing scintillations and of the relations between the irregularities and the storm drivers.
This study also highlights how a detailed reconstruction of the ionospheric scenario in which irregularities causing scintillation form and move can be achieved through a multi-observation approach. The use of ancillary data obtained from in-situ and ground-based observations allows characterising the ionospheric dynamics both locally and globally during a geomagnetic storm, and to investigate the different impacts of the storm in the two hemispheres. As the ionospheric scintillation is a complex effect, hard to predict, this work exemplifies how the study of the ionosphere by using ground and space-based measurements, which allow observing the ionosphere at different
18 heights and with different sampling frequencies, can provide useful information about the scintillation occurrence in the high latitude ionosphere. This can open the door to new approaches for scintillations prediction, especially in terms of identification of the areas most likely to be affected by the scintillation that results from irregularities.
Acknowledgments
The authors thank PNRA (Programma Nazionale di Ricerche in Antartide) for supporting the upper atmosphere observations at Mario Zucchelli Station and Concordia Station (Antarctica), and CNR (Consiglio Nazionale delle Ricerche) for supporting the upper atmosphere observations at Dirigibile Italia Station at Ny Alesund (Svalbard). The authors thank Dr. Giorgiana De Franceschi, Dr.
Vincenzo Romano and Dr. Ingrid Hunstad for management of the INGV (Istituto Nazionale di Geofisica e Vulcanologia) stations.
The authors acknowledge the use of SuperDARN data. SuperDARN is a collection of radars funded by national scientific funding agencies of Australia, Canada, China, France, Italy, Japan, Norway, South Africa, United Kingdom and the United States of America.
Eureka GNSS data are provided by Canadian High Arctic Ionospheric Network (CHAIN, http://chain.physics.unb.ca). Infrastructure funding for CHAIN was provided by the Canada Foundation for Innovation and the New Brunswick Innovation Foundation. CHAIN and CGSM operation is conducted in collaboration with the Canadian Space Agency (CSA).
Zhongshan GNSS data are provided by Beijing National Observatory of Space Environment IGGCAS through the Data Center for Geophysics, National Earth System Science Data Sharing Infrastructure
The solar wind plasma and magnetic field data of WIND were obtained from the NASA's cdaweb site (http://cdaweb.gsfc.nasa.gov/istp_public/).
Swarm data are provided by the European Space Agency upon registration (https://earth.esa.int/web/guest/swarm/data-access/). We kindly thank Dr. Stephan Buchert from IRFU, Sweden, for providing the Swarm Langmuir Probe Extended Dataset.
The GOES magnetic field data were provided by Dr. H. Singer (National Oceanic and Atmospheric Administration Space Environment Center, Asheville, N.C.) through the NOAA swpc web site (http://www.swpc.noaa.gov/products/goes-magnetometer).
This research work is supported by the Italian MIUR-PRIN grant 2012P2HRCR on The active Sun and its effects on Space and Earth climate.
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