# Global asymmetry in $\Delta$X variations during the 06 April 2000
geomagnetic storm: Relative roles of IMF $B_{z}$ and $B_{y}$

###### Abstract

This investigation is directed to understand the asymmetry in $\Delta$X
variations caused due to the relative roles played by IMF $B_{z}$ and IMF
$B_{y}$ in a particular interval (22:22 - 22:55 UT), during the main phase of
a strong geomagnetic storm event of April 06, 2000 ($A_{p}$ = 236). Two pairs
of antipodal stations, being part of the SuperMAG network, are considered
here. Ionospheric convection maps from SuperDARN network are used to
understand spatio-temporal evolution of the DP2 ionospheric convection
patterns over high-latitudes. The two-dimensional maps of equivalent currents
are used to show signatures of global DP2 currents associated with the
interplay effect between the two IMF components. Observations show increases
in the difference in $\Delta$X variations between nearly antipodal stations
from the Japanese-European/African sector with respect to the same between the
nearly antipodal stations from the Pacific/American-Indian sector. This
asymmetry is observed during the period when the absolute magnitude of IMF
$B_{y}$ is larger than that of IMF $B_{z}$ resulting in a significant and
conspicuous enhancement in IMF $|By/Bz|$. It is suggested that the distortions
in DP2 cells and associated rotation of electrodynamic day-night divider,
bring one pair of stations under the same DP2 cell and one station of the
other pair under a different DP2 cell and throat flow region leading to the
asymmetry in $\Delta$X variations between the antipodal stations. Therefore,
the work highlights the importance of the interplay between IMF $B_{z}$ and
IMF $B_{y}$ in determining the ionospheric impact over low latitudes during
strong geomagnetic conditions.

JGR: Space Physics

Space and Atmospheric Sciences Division, Physical Research Laboratory,
Ahmedabad 380009, Gujarat, India

Sumanjit<EMAIL_ADDRESS>
First observations of asymmetry in $\Delta$X variations between two pairs of
nearly antipodal locations over low latitudes.

Increase in the ratio of IMF $B_{y}$ to $B_{z}$ is suggested to cause
differences of $\Delta$X variations in one pair with respect to the other.

Distortions in DP2 cells, rotation of electrodynamical divider and associated
throat flows are believed to determine the asymmetry observed.

##  1 Introduction

The interaction of the Earth’s Magnetosphere-Ionosphere (MI) system with the
continuously emanating solar wind carrying the frozen-in Interplanetary
Magnetic Field (IMF) forms the basis of solar-terrestrial coupling and related
physical mechanisms. The solar wind energy gets transferred to the
magnetosphere by the process of magnetic reconnection. The amount of this
energy being transferred depends on the orientation of the north-south
component ($B_{z}$) of the IMF [Dungey (1961)]. The coupling between the solar
wind and the MI system is strongest, setting conditions favorable for the
occurrence of strong geomagnetic storms: Dst$<=$-100 nT [Loewe Prölss (1997)],
when the IMF $B_{z}$ orientation becomes completely southward and stays in
this configuration for a sufficient interval of time (at least 3 hours)
[Gonzalez Tsurutani (1987), Echer . (2008), Tsurutani . (1992), Gonzalez .
(1994)]. Due to this interaction, in addition to changes over the polar and
high-latitude ionosphere, the quiet-time electrodynamics of the ionosphere
over the low-to-equatorial latitudes gets perturbed, via the process of Prompt
Penetration of interplanetary Electric Field (PPEF), causing performance
degradation of space-based global navigation satellite systems like the GPS
over these geosensitive locations (see sc:41,sc:49 and references therein).

Large-scale convection patterns over the polar and the high-latitudes, as a
function of the IMF, have been portrayed by researchers (sc:28,sc:20 and
references therein) utilizing ionospheric measurements. Furthermore, during
the 1980s, the 1990s, and the beginning of the 2000s, the IMF dependencies of
global convection have been addressed and consolidated by several researchers
[Friis-Christensen . (1985), Papitashvili . (1994), Rich Hairston (1994),
Weimer (1995), Ruohoniemi Greenwald (1996), Ridley . (2000)]. Their studies
involved less direct measurements utilizing ground-based magnetometers and the
development of statistical models using High-Frequency (HF) radar networks
that are ground-based. It is well known that the $B_{y}$ component of IMF
plays a crucial role in the rotation of the two-cell or DP2 plasma convection
patterns produced by the $B_{z}$ component of IMF in the polar and high-
latitude regions of the ionosphere (sc:5 and references therein). Several
other studies [Sonnerup (1974), Iijima . (1978), Cowley . (1991), Weimer
(2001), Park . (2006), Green . (2009), Trattner . (2012), Tenfjord . (2015)]
show that the reconnection geometry in the dayside is affected by IMF $B_{y}$
while the dawn-dusk asymmetries, of the magnetospheric convection, are caused
due to the stress exerted by the $B_{y}$ component of IMF on the open magnetic
field lines in addition to the modulation of the distribution of Field-
Aligned-Currents (FACs), in the dayside, by IMF $B_{y}$. Additionally, under
the influence of IMF $B_{y}$, modulation in the spatial and amplitude
variations of the ionospheric electric field is observed [Heppner Maynard
(1987), Weimer (1995)]. Under the influence of varying IMF $B_{y}$, few
studies [Khurana . (1996), Immel . (2006), Yamazaki . (2015)], utilizing
simulations and observations, show changes in the thermospheric densities,
high-latitude hemispheric asymmetries, and wind patterns.

Several studies [Vasyliunas (1970), Kikuchi . (2008), Kikuchi . (2010),
Chakrabarty . (2015)] have well established the crucial role played by IMF
$B_{z}$ in terms of the PPEF from the high- to the equatorial and the low-
latitude ionosphere. During geomagnetically active periods, the study by
[Nopper Jr. Carovillano (1978)] shows PPEF to have an eastward polarity up to
22:00 LT and become westward thereafter. However, as the overshielding
electric field perturbation has an opposite polarity (westward/eastward during
daytime/nighttime) over the equatorial ionosphere [Simi . (2012), Chakrabarty
. (2015)], opposite polarities of PPEF are expected over these regions during
daytime and nighttime is expected, being consistent with the curl-free
condition of the ionospheric electric field. This theory has been explained
thoroughly in the studies by [Gonzales . (1979), Kelley . (2007)] where they
observe opposite polarities of the PPEF over nearly antipodal locations, one
at Jicamarca in the Peruvian sector and the other at Thumba in the Indian
sector. Due to such dominant effects of IMF $B_{z}$, it becomes difficult to
properly understand the effect of IMF $B_{y}$ over these regions. Studies by
[Kelley Makela (2002), Chakrabarty . (2017), Kumar . (2020)] show
modifications in the PPEF polarity during the geomagnetic storm period, the
same polarity of PPEF over nearly two antipodal locations: Jicamarca and
Thumba, and asymmetry in the ring current respectively as a result of the role
played by the $B_{y}$ component of IMF. In the study by [Chakrabarty .
(2017)], they show the evolution of the $\Delta$X component (see Figure 3 of
that paper) of magnetic field (obtained from SuperMAG) from complete anti-
correlation to becoming strongly correlated as one goes from high-latitude to
low-latitude antipodal locations, due to varying orientation of IMF $B_{y}$,
under southward IMF $B_{z}$, which thereby caused alterations of the
equipotential contour distribution over the low-latitude ionosphere, thus
bringing the two nearly antipodal locations under the same convection cell.

Although there are several studies present in the literature that addresses
the dominant effect of IMF $B_{z}$ over the low-latitude regions during the
geomagnetic storm period, there are very few studies that address the relative
roles played by both IMF $B_{z}$ and IMF $B_{y}$ to affect the magnetic field
perturbations over nearly antipodal locations of these regions. The present
study, for the first time, investigates how the variation of the $\Delta$X
component of the magnetic field would be affected under the combined effects
of IMF $B_{z}$ and IMF $B_{y}$ in the main phase of a strong geomagnetic
event. For this investigation, the strong geomagnetic storm of April 06, 2000,
has been taken up as a case study. Observations of $\Delta$X variations, from
SuperMAG, for two pairs of nearly antipodal locations (details of which are
given in Table LABEL:sc00) distributed across the globe have been utilized for
this study. The motivation of this work comes from the fact that earlier
studies have showed similar variations of magnetic field over the antipodal
locations due to the role of IMF $B_{y}$ under southward IMF $B_{z}$, however
we wanted to investigate that whether the same would be true for any choice of
pairs of antipodal stations under the effects of interplay between the two IMF
components. The novelty of this study is in bringing forward the observation
of an asymmetry in the magnetic field variations, under geomagnetically
disturbed conditions, due to the relative roles played by both IMF $B_{y}$ and
IMF $B_{z}$. This study is crucial for understanding the changes in global
electrodynamics during strong space weather events and would be vital for
developing a reliable space weather forecast system.

The manuscript is divided as follows: Section 2 presents the descriptions of
the locations of observation and the data used for the analyses. Section 3
describes the results obtained. Section 4 describes the discussions of these
results and observations. Finally, Section 5 presents the summary of the work.

##  2 Data and Observation Sites

In the present study, the locations of the four stations from where the
$\Delta$X component of the magnetic field are obtained (post nighttime base
level subtraction) for analysis along with the corresponding Magnetic Latitude
(MLAT) and the Local Time (LT) with respect to the Universal Time (UT) are
given in Table LABEL:sc00. The SuperMAG [Gjerloev (2009), Gjerloev (2012)], is
a global collaboration of national agencies and organizations, that present;y
operates more than 300 ground-based magnetometers. They provide ground
magnetic field variations from all the available stations in a common
coordinate system, with a common baseline removal approach and same time
resolution. It utilizes three-dimensional vector measurements of the magnetic
field from these ground-based magnetometers. SuperMAG resamples raw data into
one-minute temporal resolution and converts all the units into nT. This data
is then rotated into a local magnetic field coordinate system and the baseline
is subtracted by an automated technique. These magnetic data are available at
http://supermag.jhuapl.edu/.

Table 1: Locations of the magnetometer stations (from SuperMAG) used for the analysis along with the corresponding MLAT and LT with respect to the UT Station Name | Station Code | Sector | MLAT (deg) | LT (h)
---|---|---|---|---
Chichijima | CBI | Japanese | 20.00 | UT+09.49
Alibag | ABG | Indian | 12.05 | UT+04.86
Guimar | GUI | European/African | 14.33 | UT-01.09
Ewa Beach | EWA | Pacific/American | 21.44 | UT-10.53

Furthermore, the ionospheric convection maps used in the present work are
obtained from the Super Dual Auroral Radar Network [Greenwald (2021)] or
SuperDARN which consists of more than 30 low-power high-frequency radars. They
observe Earth’s upper atmosphere from the mid-latitudes to the polar regions.
These radars continuously operate to provide valuable information on the near-
earth space environment by observing the plasma dynamics occurring in the
ionosphere. These convection maps are available at http://vt.superdarn.org/.

Additionally, for observations of the interplanetary, auroral and geomagnetic
conditions, the high-resolution (1 minute) IMF $B_{z}$ and IMF $B_{y}$ (nT),
the solar wind flow pressure (nPa) along with the Polar Cap index (PCN), the
Auroral Upper and Lower (AU and AL, nT) and the SYM-H and the ASYM-H (nT) data
are all obtained from the NASA OMNIWEB portal at
https://omniweb.gsfc.nasa.gov/form/omni_min.html. The SuperMAG AU and AL
indices are obtained from http://supermag.jhuapl.edu/.

##  3 Geomagnetic storm of April 06, 2000: Results

A CME erupted near the western limb of the Sun on April 04, 2000, and hit the
magnetosphere of Earth on April 06, 2000, causing one of the strongest
geomagnetic storms of the solar cycle 23, being reported by several
researchers [Lee . (2002), Huttunen . (2002), Pulkkinen . (2003), Liu .
(2004), Rastogi Chandra (2016), Chakraborty . (2020c)]. Figures 1 and 2 show
the interplanetary and geomagnetic variations during the period from 18:00 UT
of April 06, 2000, to 24:00 UT (or 00:00 UT) of April 07, 2000. The selected
period of observation is in the main phase of the strong geomagnetic storm as
can be observed in panel 1(a). Figure 1(b) shows the corresponding IMF $B_{y}$
(in blue) variation and IMF $B_{z}$ (in red) variation which can be observed
to be completely southward throughout the period. Figure 1(c) shows the ratio
between IMF $|B_{y}|$ and IMF $|B_{z}|$. Figure 1(d) shows the $\Delta$X
component of the magnetic field as measured by the magnetometers over nearly
antipodal locations: EWA (in blue) and ABG (in red) while Figure 1(e) shows
the same over another set of nearly antipodal locations: GUI (in blue) and CBI
(in red). The Local Times (LT) of these locations with respect to the
Universal Time (UT) are presented in Table LABEL:sc00 as well as marked in
Figure panels 1(d) and (e). Figure 1(f) shows the difference of $\Delta$X
between the locations EWA and ABG while Figure 1(g) shows the same between the
locations CBI and GUI. This difference has been calculated by subtracting the
$\Delta$X variation over the location at the nightside from the $\Delta$X
variation over the location at the dayside. For the same period, Figure 2:
panels (a) to (d) show the Auroral Upper (AU, in red) and the Auroral Lower
(AL, in blue), the ASYM-H, the Polar Cap index (PCN) and the solar wind flow
pressure respectively.

The region of interest has been shaded in yellow in both Figures 1 and 2. The
duration of this shaded region is from 22:22 UT to 22:55 UT on April 06, 2000.
The motivation for selecting this region is due to the fact that it is the
only region where the magnitude of IMF $B_{y}$ can be observed to be
significantly higher than that of IMF $B_{z}$. During this interval, the SYM-H
(Figure 1(a)) value ranged from -266 nT to -287 nT. Corresponding IMF $B_{y}$
values were -11.08 nT and -10.05 nT (both negative) while the IMF $B_{z}$
values (both southward) were -27.94 nT and -29.08 nT (Figure 1(b)). To
understand the relative roles of the two IMF components, the ratio of the
absolute values of IMF $B_{y}$ to IMF $B_{z}$ has been calculated and plotted
(Figure 1(c)). During this particular interval, both the IMF components were
negative thus suggesting calculation of absolute values of the ratio and the
calculation of a simple ratio between these two IMF components would bring out
the same result. The two peaks observed in this panel (c) occur at 22:26 UT
and 22:40 UT with values of 1.52 and 2.45 which suggests that during this
interval, although both were negative, the IMF $B_{y}$ magnitude had been
1.5-2.5 times that of the magnitude of IMF $B_{z}$. The $\Delta$X variations
between the pair of nearly antipodal locations (EWA and ABG) and (CBI and GUI)
can be observed to be correlated, which is in agreement with that reported in
[Chakrabarty . (2017)] for nearly antipodal locations over these latitudinal
regions due to the effect of IMF $B_{y}$ under southward IMF $B_{z}$. It is to
be noted that during this interval of time, the LTs of these four stations
were as follows: EWA (11:50-12:23 of April 06, 2000), ABG (03:14-03:47 of
April 07, 2000), CBI (07:52-08:15 of April 07, 2000) and GUI (21:17-21:50 of
April 06, 2000) thus pointing to the fact that EWA and CBI were in dayside
while ABG and GUI were in the nightside. Taking reference to these LTs, the
difference between the $\Delta$X (dayside-nightside) variations:
Diff($\Delta$X), between the two pairs of nearly antipodal stations are
plotted in the panels (f) and (g) of the same figure where it can be observed
that during this interval, the difference is almost constant (around 90 nT)
for the EWA and ABG pair while an increase from about 90 nT to 125 nT is
observed for the CBI and GUI pair of stations. Further, it is important to
note that during the first peak of the ratio between IMF $|B_{y}|$ and IMF
$|B_{z}|$, which should have a value of 1.52, the Diff($\Delta$X) between EWA
and ABG was 86.8 nT and that between CBI and GUI was 96.7 nT, however, when
the second peak had occurred (value of the ratio: 2.45), the Diff($\Delta$X)
between CBI and GUI increased to 125.5 nT although the same between EWA and
ABG was 89.0 nT. Thus an asymmetry in the magnetic field variations between
these two pairs of nearly antipodal locations can be observed due to the
combined effects of IMF $B_{z}$ and IMF $B_{y}$.

Figure 1: Variations of (a) SYM-H (nT), (b) IMF $B_{y}$ (in blue) and IMF
$B_{z}$ (in red), (c) ratio between the absolute values of IMF $B_{y}$ and IMF
$B_{z}$, $\Delta$X (nT) variations between (d): EWA (in cyan) and ABG (in
orange) and (e): CBI (in orange) and GUI (in cyan) and differences in the
$\Delta$X variations (Diff($\Delta$X, nT) between (f): EWA and ABG and (g):
CBI and GUI during 18:00 UT of April 06, 2000, and 00:00 UT of April 07, 2000.
The yellow-shaded region of interest lies between 22:22 UT and 22:55 UT on
April 06, 2000.

Figure 2 has been presented to observe for any substorm and/or pressure-
related effect during the period of interest. Based on the criteria defined by
[Newell Gjerloev (2011), Forsyth . (2015)], using SML index (Figure 2(b)) we
can infer occurrence of a substorm around 22:40 UT. However, we do not see any
magnetic bay-like disturbances [Chakrabarty . (2008), Hui . (2017)] at this
time in the $\Delta$X variation (Figure 1(d)) over the nightside stations.
These suggest that the $\Delta$X variations at this time over these stations
are not significantly affected by substorms. In addition, we also do not
observe any sharp changes in the ram-pressure (Figure 2(e)) at this time which
will cause transient electric field perturbations and associated changes in
the $\Delta$X variations.

Figure 2: Variations of (a) AU (nT, in red) and AL (nT, in blue), (b) SMU (nT,
in red) and SML (nT, in blue), (c) ASYM-H (nT), (d) PCN and (e) solar wind
flow pressure during 18:00 UT of April 06, 2000, and 00:00 UT of April 07,
2000. The yellow-shaded region of interest lies between 22:22 UT and 22:55 UT
on April 06, 2000.

To validate the fact that there had been a relative role played by both IMF
$B_{z}$ and IMF $B_{y}$ during this particular interval that had caused such
asymmetry in the $\Delta$X variations, Figure 3 has been shown which consists
of the SuperDARN ionospheric convection maps at time stamps: (a) 22:00-22:02
UT and (b) 23:00-23:02 UT of April 06, 2000. Anti-clockwise rotation,
distortions of both the dawn (in red) and the dusk (in blue) cells, as well as
the electrodynamic divider (due to southward IMF $B_{z}$ and dawnward IMF
$B_{y}$), can be observed on the map of Figure 3(b) with respect to the map of
Figure 3(a). Further, in 3(b) one can observe distinctly enhanced throat flows
in between the two cells (see sc:22 and references therein).

It is important to note that the SuperDARN maps available during the study
period are of low (1 hour) resolution and further, these maps give the
velocity vectors which are not suitable for the present study which is focused
on the observations of the $\Delta$X variations and associated equivalent
currents. Therefore, for performing global studies of the terrestrial MI
coupling and the associated signatures of the global DP2 currents related to
the interplay effect between the two components of IMF, one has to look into
the two-dimensional maps of equivalent currents that are derived using the
global magnetometer data. These maps, which represent the global perturbation
magnetic field, are obtained by performing a spherical cap harmonic expansion
of the SuperMAG magnetic field data. The procedure to obtain these two-
dimensional maps are explained in detail in [Waters . (2015)].

The maps of equivalent currents used for this study are shown in Figures 4 and
5, during intervals between 22:00 UT to 23:00 UT of April 06, 2000. In all the
panels, the locations of the four stations (following their MLT distributions)
have been marked. The grey-shaded regions in all the maps mark the nightside.
The colored circles show the region where changes are observed. The maps in
the panels at the time instant of 22:00 UT and 22:10 UT in Figure 4 show a
green circle marked inside which the usual orientation of the current vectors
can be observed. Coming to the time instant of 22:22 UT, a slight distortion
(inside the light red circle) in these current vectors can be observed.
Further, at the time instant of 22:26 UT, there is a clear observation of a
distortion feature in the current vectors that become radially outward (inside
the dark red circle) over the station GUI which is different compared to the
current vectors over stations EWA, ABG and CBI. During 22:40 UT (first map of
Figure 5 and inside the dark red circular region) the distortions of the
current vectors over GUI increase with respect to that observed during the
previous time instant (22:26 UT map in Figure 4). Interestingly, at 22:26 UT
and 22:40 UT, there were occurrences of the first and the second (higher in
magnitude) peaks in the ratio of IMF $B_{y}$ and IMF $B_{z}$. The map at the
time instant of 22:50 UT shows a continuation of these distortions in the
current vectors. The vectors then get restored to their usual orientations in
the maps during the time instances 22:55 UT and 23:00 UT. Further explanations
of these results will be discussed in the following section.

Figure 3: SuperDARN ionospheric convection maps along with DP2 contours during
(a) 22:00–22:02 UT and (b) 23:00–23:02 UT of April 06, 2000.

Figure 4: The two-dimensional maps of equivalent current as obtained from the
global magnetometer data during 22:00 UT, 22:10 UT, 22:22 UT, and 22:26 UT of
April 06, 2000. Anomalous orientations of the current vectors and associated
throat flow regions are designated in light and dark shades of red circles,
while the same for usual orientations are designated in green circles.

Figure 5: Same as Figure 4 but during 22:40 UT, 22:50 UT, 22:55 UT, and 23:00
UT of April 06, 2000.

##  4 Discussion

Before discussing the probable mechanism to have caused the observed asymmetry
in the present study, it is important to rule out the effects of ring current
asymmetry during the main phase of the geomagnetic storm as well as the
effects of the disturbance dynamo electric fields on the magnetic field
variations.

There could be effects of asymmetry of the ring current on the magnetic field
perturbations observed during the main phase of geomagnetic storms over the
low-latitudes as shown by [Tsuji . (2012)], however, the asymmetry would occur
near the dawn and the dusk sector. The distribution of the stations selected
is away from the dawn-dusk sector. Furthermore, during the geomagnetic storm
main phase, [Kumar . (2020)] have shown that there would be changes (away from
the dawn-dusk sector in the MLT distribution of the ring current as observed
at the ground as a result of the influence of IMF $B_{y}$ under southward IMF
$B_{z}$, however under the condition that IMF $B_{y}$ has to be of the same
orientation for longer timescales (at least 12 h), which is not the case in
the present study. It can also be observed from Figure 2(c) that there was a
steady decrease of the ASYM-H during the period of interest, which supports
the fact that the observations are due to changes in the ionospheric currents
and not due to the intensification of partial ring current.

Additionally, as the effects of the disturbance dynamo electric fields are
longitude dependent, there would be some contribution of the same in the
nighttime stations’ (one in the post-sunset sector and the other in the post-
midnight sector) $\Delta$X variations. However, we believe that these would
not be able to produce the changes observed in the present study in the
differences of the $\Delta$X variations of one pair with respect to the other
pair of nearly antipodal locations as the event under consideration occurs for
a short period of time and that too, rather abruptly. Therefore, it is
unlikely that disturbance dynamo produces such transient and sharp changes in
the $\Delta$X variations reported in the present case.

In the context of the evolution of the DP2 cells, it is well-known [Ruohoniemi
Greenwald (2005), Tanaka (2007), Chakrabarty . (2017)] that under southward
$B_{z}$, the dawn and the dusk cells take the circular/orange and the
crescent/banana-like shapes having a distinct anti-sunward throat flows in
between these two cells. When the IMF $B_{y}$ is negative (or turns dawnward),
there is an overall anti-clockwise rotation of the electrodynamical divider
with respect to the noon-midnight characteristic orientation when IMF $B_{y}$
= 0, further resulting in a clockwise rotation of the throat flows. In the
present study, the occurrence of enhanced throat flows can be observed in
Figure 3. However, the SuperDARN maps during this period are of low cadence
and they only represent the velocity vectors which are not appropriate for
studying the variations in the global current system under the effects of the
relative roles of IMF $B_{z}$ and $B_{y}$. As a result, two-dimensional maps
are used in the present study. Interestingly, on all the maps in Figures 4 and
5, the orientation of the current vectors near EWA, CBI, and ABG are nearly
similar whereas, over GUI, the current vectors become distorted/radially
outward during the period of interest, especially more distorted when there
was the observation of sharp peaks in the ratio of IMF $B_{y}$ to IMF $B_{z}$.
This suggests that as the station GUI was under the region where there were
radially outward currents during these intervals, differences in the
corresponding ground magnetic field variation were observed which resulted in
an asymmetry when observing Diff($\Delta$X) in the pair of stations: CBI and
GUI with respect to the pair of stations: EWA and ABG.

Finally, to understand the probable orientation of the electrodynamic divider,
the shapes of the cells and the location of the stations, Figure 6 shows the
schematic of the DP2 equipotential contour for the conditions under southward
IMF $B_{z}$ and negative IMF $B_{y}$ (range of values highlighted on top of in
the figure for ready reference) during the region of interest (22:22 UT to
22:55 UT of April 06, 2022). The grey-shaded hemisphere here represents
nighttime, the solid black line represents the midnight-noon meridian while
the dashed line in black indicates how the electrodynamic divider had rotated
during this particular period. The most important point of this schematic is
the locations of the four stations with respect to the electrodynamic divider,
wherein although both the pairs are at nearly antipodal locations (CBI and
GUI: marked as violet squares and EWA and ABG: marked as red stars and placed
according to their MLT), asymmetry in the Diff($\Delta$X) variations of CBI
and GUI is observed with respect to EWA and ABG since GUI is the only station
falling in the throat flow region. Therefore, an important observation comes
out from this schematic which suggests that antipodal locations (day-night
separator) are not a sufficient factor but the electrodynamical day-night or
the electrodynamic divider and associated throat flow regions, are the factor
that would decide how the changes in IMF $B_{y}$ and IMF $B_{z}$ and their
relative roles that had distorted as well as rotated the DP2 cells in such a
manner to present asymmetry between the differences in the pairs of nearly
antipodal stations over these regions throughout the globe. It is crucial to
note that the role of IMF $B_{y}$ over these locations comes as a secondary
effect and sparse studies of such cases can be explained by the fact that one
has to rule out the other drivers like substorm, solar wind ram pressure, ring
current asymmetry, disturbance dynamo, etc, thus making statistical studies of
such to be highly non-trivial. However, identifying more such cases, as in the
present study, would be helpful for the community to perform a comprehensive
statistical study of the global MI coupling in the near future.

Figure 6: A schematic of the DP2 equipotential contour under southward IMF Bz
and negative IMF By condition during the region of interest. The shaded (grey)
hemisphere represents the nightside. The solid black line represents the
midnight-noon meridian while the dashed black line represents the
electrodynamical divider between the dawn (red) and dusk (blue) cells. The
nearly antipodal station pairs are marked in violet squares for CBI and GUI
and red stars for EWA and ABG.

##  5 Summary

The present work was directed to understand whether there would be
similarities or asymmetries in the magnetic field variations of any given
pairs of antipodal stations under the relative roles of IMF $B_{z}$ and IMF
$B_{y}$. The results showed that it is the changes in the spatial distribution
of the equivalent currents and the subsequent distortions in the DP2 cells in
addition to the rotation of the electrodynamical divider, due to the effects
of the interplay between the two IMF components, that determines the
asymmetries observed in the $\Delta$X variations in one pair of nearly
antipodal stations with respect to the other pair. This study would be
pertinent for developing a reliable space weather forecast system as it
highlights the importance of the interplay between the two IMF components in
determining the ionospheric impact over low latitudes during strong
geomagnetic conditions.

###### Acknowledgements.

The authors acknowledge the use of magnetic data, the two-dimensional maps of
equivalent currents and the SMU/L indices openly available from the SuperMAG
network website (http://supermag.jhuapl.edu/). The authors further thank the
PIs of the magnetic observatories and the national institutes that support the
observatories. The authors also acknowledge the use of SuperDARN data openly
available from the Virginia Tech SuperDARN website (http://vt.superdarn.org/).
SuperDARN is a collection of radars funded by national scientific funding
agencies in Australia, Canada, China, France, Italy, Japan, Norway, South
Africa, the United Kingdom, and the United States of America. Further
acknowledgments go to the NASA OMNIWEB Data Explorer website
(https://omniweb.gsfc. nasa.gov/form/omni_min.html) for the high resolution (1
minute) IMF $B_{y}$, IMF $B_{z}$, AU, AL, SYM-H, ASYM-H, PCN and solar wind
flow pressure data. The authors thank both reviewers for their valuable
comments and suggestions that have enhanced the quality of the manuscript.
This research work is supported by the Department of Space, Government of
India.

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