ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-36-509-2018The effect of subauroral polarization streams on the mid-latitude thermospheric disturbance neutral winds: a universal time effectSAPS UT effectWangHuih.wang@whu.edu.cnhttps://orcid.org/0000-0001-8459-5213ZhangKedengZhengZhichaoRidleyAaron JamesDept. of Space Physics, School of Electronic Information, Wuhan University, Hubei, 430072, ChinaNational Center for Atmospheric Research, Boulder, CO 80301, USAClimate and Space Sciences and Engineering, University of Michigan, Ann Arbor, USAHui Wang (h.wang@whu.edu.cn)29March201836250952524October201719February201812February2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://angeo.copernicus.org/articles/36/509/2018/angeo-36-509-2018.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/36/509/2018/angeo-36-509-2018.pdf
The temporal and spatial variations in thermospheric neutral winds at an altitude of 400 km in response to
subauroral polarization streams (SAPS) are investigated using global ionosphere and thermosphere model simulations under
the southward interplanetary magnetic field (IMF) condition. During SAPS periods the westward neutral winds in the
subauroral latitudes are greatly strengthened at dusk. This is due to the ion drag effect, through which SAPS can
accelerate neutral winds in the westward direction. The new findings are that for SAPS commencing at different universal
times, the strongest westward neutral winds exhibit large variations in amplitudes. The ion drag and Joule heating effects
are dependent on the solar illumination, which exhibit UT variations due to the displacement of the geomagnetic and
geographic poles. With more sunlight, stronger westward neutral winds can be generated, and the center of these neutral
winds shifts to a later magnetic local time than neutral winds with less
solar illumination. In the Northern Hemisphere and Southern Hemisphere, the
disturbance neutral wind reaches a maximum at 18:00 and 04:00 UT, and
a minimum at 04:00 and 16:00 UT,
respectively. There is a good correlation between the neutral wind velocity and cos0.5(SZA) (solar zenith
angle). The reduction in the electron density and enhancement in the air mass
density at an altitude of
400 km are strongest when the maximum solar illumination collocates with the SAPS. The correlation between the
neutral wind velocity and cos0.5(SZA) is also good during the northward IMF period. The effect of a sine-wave
oscillation of SAPS on the neutral wind also exhibits UT variations in association with the solar illumination.
Neutral winds are known to be important in the
ionosphere–thermosphere–magnetosphere coupling system due to the neutral
drag effect , the dynamo effect , as well as the
flywheel effect
. Therefore, many studies have been conducted concerning neutral winds using both observations and
simulations
.
The global neutral wind in the F region is driven by a combination of pressure gradients induced by local or global
heating, ion drag, Coriolis and centrifugal forces, and upward-propagating atmospheric tides . At high
latitudes the ion drag force is considered to be the primary factor during quiet times . The
neutral wind patterns generally follow the direction of the plasma convection, exhibiting anti-sunward flow over the polar
cap and sunward flow along the auroral oval . In addition, the neutral flow pattern can
be modulated by Coriolis and viscous forces at high latitudes . At mid-latitudes the quiet time neutral winds
are mainly driven by the pressure gradient caused by the solar heating, resulting in the eastward flow from post-noon to
early morning and the westward flow at all other times. During magnetic disturbed periods, the thermospheric neutral winds
are highly disturbed due to Joule and particle precipitation heating and enhanced plasma convection
. Interhemispheric comparisons
of zonal neutral winds at mid-latitudes
have been performed recently by using neutral wind observations at Palmer (54∘ S magnetic latitude, MLat,
64∘ W geographic longitude, GLon) and Millstone (53∘ N MLat, 71.5∘ W GLon)
e.g.,. found more conjugacy during
geomagnetically active periods when high-latitude
effects became dominant. reported that the local time variation of neutral wind showed a very good conjugacy
at equinox but not at June solstice, which was explained by the seasonal asymmetry of hemispheric polar
activity. reported that the solar heating can explain the observed longitudinal variation of zonal neutral
winds during quiet periods, based on Challenging Minisatellite Payload (CHAMP) observations and Global
Ionosphere–Thermosphere Model (GITM) simulation. conducted a comprehensive study on the global
thermospheric zonal neutral winds' dependences on solar flux, geomagnetic
activity, and hemisphere by using 7 years of
CHAMP observations. They reported that the westward zonal neutral winds at subauroral latitudes were stronger under more
sunlight conditions during both quiet and storm periods. In particular,
previous work has disclosed that subauroral polarization streams (SAPS) can
account for the westward disturbance neutral wind in the dusk–pre-midnight
sectors at
mid-latitudes, which have a flow direction opposite to the anti-sunward neutral winds induced by the pressure gradient
caused by the solar heating, as has been previously reported .
SAPS are one of the interesting and important features observed at mid-latitudes, and they have been investigated
extensively by using both observations and simulations
e.g.,. SAPS represent rapid westward plasma flow in
the mid-latitudes in the dusk and pre-midnight sectors. The term SAPS was
first introduced by to encompass
two different types of plasma flow: (1) the more intense and latitudinally narrower plasma flow, also known as subauroral
ion drifts (SAID) or polarization jets (PJ) ; and (2) broader plasma flow with longer duration
. SAPS are driven by a strong poleward electric field located equatorward of the auroral oval, which is
generated when the R2 field-aligned currents flow into the region of low conductance in the subauroral regions
. Other SAPS formation models have also been proposed to explain the rapid development of SAPS after
substorm onset . SAPS can alter the composition of the ionosphere, create an electron density trough, cause
storm-enhanced density, and are related to plasmaspheric plumes and erosions .
presented the first two-dimensional observations of the variability of SAPS using mid-latitude Super Dual Auroral Radar
Network (SuperDARN) radar located at Wallops Island. They found SAPS electric
fields were highly variable on timescales
of a few minutes. reported SAPS evolution with substorm as observed by the Hokkaido East radar and compared
it with theoretical simulations. found that the electric fields
and latitudinal widths of SAPS showed
hemispheric asymmetries by using mid-latitude SuperDARN radars with conjugate fields of view. The hemispheric differences
were attributed to seasonal variations in the ionospheric conductivity and magnetic distortion effects in the inner
magnetosphere. presented the first comprehensive statistical study of occurrence, location, and intensity of
SAPS during both storm and non-storm periods by using measurements from the US mid-latitude SuperDARN radars. They found
that SAPS occurred 15 % of the time during quiet periods and 87 % of
the time during moderately disturbed
conditions. They developed a new empirical model with Dst as input to estimate the occurrence probability of SAPS at
a given magnetic latitude and magnetic local time (MLT). They also found a linear relationship between SAPS velocity and
MLT during low and moderate levels of geomagnetic activities, but this
relationship became nonlinear as the geomagnetic activity increased. They
interpreted this behavior as the active feedback of the ionosphere and
thermosphere
playing
an important role in modulating SAPS speeds. investigated the occurrence frequency, magnetic latitude, and
width of SAID during different solar activity, season, and local time by using Defense Meteorological Satellite Program
(DMSP) observations during 1987–2012. compared the time lag and lifetime of SAPS during substorm and storm
periods. simulated SAPS by using a state-of-the-art magnetosphere
model during the 17 March 2013 SAID event and compared it with observations.
They found that the model showed an underestimate of the SAPS. The simulated
SAPS
penetrated deeper than observations, implying that the shielding from the Region 2 field-aligned currents in the model was
not well represented. investigated SAPS during the 29 June 2013 geomagnetic storm in the equatorial plane
with Van Allen probes. The relationships between plasma sheet ion and
electron boundaries, and the spatial location of the SAPS, were investigated.
They suggested that earthward-propagating injections were driving the
observed strong electric
fields at low L levels in the equatorial magnetosphere. conducted a detailed study during the great
geomagnetic storm on 17–18 March 2015 by using incoherent scatter radar observations at Millstone Hill and Arecibo and
DMSP observations. They reported enhanced density plume, penetrating electric
field, large ion temperature enhancements, as
well as upward ion drifts during the SAPS periods.
SAPS can drive westward neutral winds at the subauroral latitudes in the dusk
sector due to the strong ion drag effect. By
using coordinated CHAMP and DMSP satellite observations, studied the relationship between the SAPS and the
upper thermospheric zonal neutral winds in the subauroral regions in the dusk
and pre-midnight sectors. They found that
both neutral wind and plasma velocities minimized at approximately the same latitude during SAPS periods. By applying the
SAPS model to the Thermospheric Ionosphere Electrodynamics General
Circulation Model (TIEGCM), modeled the
effect of the SAPS on the thermosphere (temperature and neutral winds) and ionosphere (electron density), and confirmed
that the strong ion drag effect caused by the SAPS could increase the neutral temperature in the subauroral and auroral
regions and drive the large westward disturbance neutral winds in the SAPS
region. They also found that the heating to
the thermosphere by SAPS caused deeper and extended ionospheric electron density depletion. have not
examined the universal time effect of SAPS on the thermosphere and ionosphere (neutral winds, air mass and electron
density), which is the focus of the present work. found that
the SAPS' ion fluxes, a product of the SAPS
ion velocity and ion density, exhibited similar seasonal variation to those of the zonal neutral winds.
found that the SAPS had the largest effect on neutral winds around 19:00 MLT, and the least effect around
23:00–24:00 MLT. They believed that this was related to the longer duration of SAPS events around 19:00 MLT than at
other times. Based on observations from the Millstone Hill incoherent scatter radar and Fabry–Perot interferometers,
reported that during SAPS periods the westward neutral winds could establish a poleward neutral wind surge
due to the poleward Coriolis force. found subauroral westward jets during storm periods based on CHAMP
observations.
The ion drag force and Joule heating depend on the electron density (Ne) and ionospheric conductivity. The
electron density at fixed magnetic latitude bands varies with the universal time (UT) due to the displacement of the
geographic and geomagnetic poles. Such UT effects during magnetic storms and substorms have been reported in the
literature. found that both meridional and zonal disturbed neutral winds attained peaks around 18:00 and
06:00 UT in the Northern Hemisphere and Southern Hemisphere, respectively,
during storm periods. determined that
atmospheric disturbances traveling from polar latitudes to lower latitudes reached their maximum propagation speeds in the
near magnetic pole longitudinal region on the nightside. concluded that the air mass density and temperature
were highest when the maximum solar ionization occurred in the same location as the high-latitude potential
pattern. showed that during substorms the disturbed neutral
winds at the high latitudes exhibited large
variations with UT. Due to the presence of more solar illumination in the auroral oval, stronger disturbed neutral winds
could be generated.
In this study we investigate the thermospheric disturbance neutral winds in response to the presence of SAPS occurring at
different universal times by using a sophisticated global
ionosphere–thermosphere model. Instead of focusing on the
magnetosphere–ionosphere coupling, this study focuses on the
ionosphere–thermosphere coupling during SAPS periods.
Model
The GITM was developed at the University of Michigan. The GITM is
a three-dimensional model that simulates ionospheric and thermospheric dynamics . The continuity, momentum,
and energy equations are solved for the thermosphere and ionosphere using realistic source terms. The GITM solves the
neutral momentum equation by considering the pressure gradients, ion drag, viscous force, and Coriolis force. It is driven
by the solar extreme ultraviolet (EUV) radiation, high-latitude electric field , precipitation of auroral
particles , and the tides at the lower boundary . The magnetic topology is described by the
international geomagnetic reference field (IGRF) . However, we have to note that the ionosphere and
thermosphere model used in the present work does not take into account the potential effect of the ionospheric
conductivity feedback on the inner magnetospheric electric fields. Future work will use a coupled magnetosphere,
ionosphere and thermosphere model that includes SAPS to investigate the
electrodynamic effect on the neutral winds in
a self-consistent way.
Geomagnetic latitude and magnetic local time variation of the
ionospheric electrical potential. The outer ring represents
50∘ MLat. The substorm epoch time runs at 20 min intervals
from -1 h to 2 h 40 min. Noon is at the top of each
panel. Midnight is at the bottom. Dawn is to the right and dusk to the left.
SAPS potential is located 7∘ equatorward of the boundary of the
auroral oval. SAPS potential increases linearly from 00:00 to 00:30 UT, and
then persists from 00:30 to 02:00 UT. SAPS potential reaches a minimum of
-24 kV around 19:00 MLT, and decreases towards noon and midnight.
SAPS potential is superimposed on the background potential in the subauroral
region. The maximum and minimum potentials in the auroral and subauroral
regions are shown at the bottom of each plot.
Several studies have used the GITM for ionosphere–thermosphere physical
studies, and good agreements were achieved when model
outputs were validated with observations and other modeling results
e.g.,.
validated the neutral winds within the model by using the Wind
Imaging Interferometer instrument observation, and showed that the simulated
neutral winds were
consistent with the observations. investigated longitudinal differences in thermospheric zonal neutral winds
in the subauroral region for different seasons and under solar maximum and medium conditions by using CHAMP observations
and GITM simulations. They found that the model results confirmed observed results in large-scale structures, and the
local time and hemispheric asymmetries of the peak differences were consistent with CHAMP observations. However, the model
underestimated the observed longitudinal differences of the zonal neutral wind. studied the temporal and
spatial variation in thermospheric neutral winds at 400 km altitude in response to substorms by using CHAMP
observations and GITM simulations. They found that the substorm time neutral wind amplitudes from the model were
underestimated, especially at dawn when compared to observations. They proposed several reasons for explanation of the
model–observation differences. The cross-track neutral winds observed by
CHAMP were predominantly in the geographic zonal
direction due to its high-inclination orbit, not in the magnetic zonal direction. This could somewhat affect the
comparison with GITM results. Model resolution could affect the magnitudes of outputs. found that when the
latitudinal resolution changed from 5∘ to 1.5∘, the neutral gas heating rate could increase by 20 %,
because the model could better capture small-scale electric field and particle precipitation.
The GITM was run with a resolution of 2∘ latitude and 10∘ longitude for altitudes of 100 to
600 km. A quasi-steady state was reached after the model was run for 48 h. After the steady state was reached,
the simulations were run for another 24 h, the data of which were used for
our study. The input parameters for the March 2000 equinox were as follows:
IMF Bxgse= 0 nT, Bygse= 0 nT,
Bzgse=-2 nT, solar wind velocity, Vxgse=-400 kms-1,
F10.7 = 100 sfu, and hemispheric power (HP) = 20 GW. The tropospheric tides were turned off in order
to focus on the in situ physical processes.
Geomagnetic latitude and magnetic local time variation of the plasma flow in the Northern Hemisphere from 00:20 to 02:00 epoch times. Each panel is the difference between the plasma velocity at the panel UT and the plasma velocity 24 h earlier. Vectors are shown with black arrows, and the zonal velocity is shown in color. Negative values represent westward plasma flow.
Figure shows the polar maps of the ionospheric potential for an ideal run, which was used as input to the GITM
in the magnetic latitude and local time coordinate system. The same
potentials were applied to both the Northern Hemisphere and Southern
Hemisphere, because showed that the SAPS electric field was
almost conjugate in both
hemispheres. The SAPS for this model run occurred at 00:00 UT. The potential maps shown are for 00:00 to 02:40 UT with
a 20 min time interval. The typical positive and negative potential cells can be seen in the polar cap region
during the southward IMF period. SAPS are latitudinal narrow regions of
intense westward flows and are considered to result
from poleward directed subauroral electric fields . In the subauroral region, we
artificially add a negative potential to the background potential. It
results in a poleward electric field in a subauroral region and a westward
plasma flow, i.e., SAPS. The SAPS potential is set to minimize around
19:00 MLT and gradually
approached zero towards noon and midnight, which is consistent with the most probable local time of SAPS from previous
observational work . In the simulation during the southward IMF period, the open closed boundary of the
auroral oval is around 72∘ MLat at 19:00 MLT. The SAPS potential is set to minimize at 7∘ MLat
equatorward of the open closed boundary of the auroral oval, and
a latitudinal width of 2∘ (see the maps from 00:20 to
02:00 UT). This makes SAPS minimize around 65∘ MLat at 19:00 MLT, which is equatorward of the auroral oval
(the electron density pattern around 150 km altitude is not shown).
The SAPS MLat is within the range of previous statistical results
. By using 2 years of DMSP ion drift meter measurements,
found that SAPS peaked between 62∘ and
66∘ MLat when Kp<3, depending on the ionospheric
conductivity
condition. The SAPS potential varied linearly from 0 to -24 kV within half an hour, and remained at
-24 kV for another 1.5 h (from 00:30 to 02:00 UT), after which
the SAPS potential was set to zero.
Geomagnetic latitude and magnetic local time variation of the disturbance neutral zonal wind in the Northern Hemisphere from 00:20 to 03:00 epoch time. The wind velocity at the same UT but 24 h earlier has been subtracted in each panel. Vectors are shown with black arrows, and the zonal velocity is shown in color. Negative values represent westward neutral wind flow.
Figure shows the plasma velocity of the disturbance on the polar map at an altitude of 400 km. The SAPS
for this event occurred at 00:00 UT, and 00:00 epoch time marks the onset time of the SAPS. The plasma velocity of the
disturbance is defined as ΔVi=Visaps-Vinosaps, where
Visaps is the plasma velocity during SAPS periods, and Vinosaps is the pattern
during no-SAPS periods at the same UT but 24 h earlier. The horizontal plasma velocity vectors are shown as black
arrows. The zonal disturbance velocity is depicted by colored contours. Negative values represent westward
deflection. In Fig. , the strong westward plasma flow is present as a result of the imposed SAPS potential
equatorward of the auroral oval boundary in the dusk sector. The latitude of the SAPS decreases with increasing MLT. The
MLT dependence of the SAPS have been reported from observations in the literature . The SAPS
amplitudes decrease from 00:00 UT to 00:30 UT, and remain at their minimum (-720 kms-1) for another
1.5 h. The SAPS disappeared after 02:00 UT. There is another eastward drift channel located poleward of the SAPS, which
is caused by the imposed negative potential. This eastward flow could weaken the regular sunward or westward plasma flow
in the auroral oval.
Simulation resultsDisturbance neutral winds
Figure shows the disturbance neutral winds in the Northern Hemisphere at an altitude of 400 km
simulated by the GITM, which has the same format as Fig. . The disturbance neutral winds are the differences
between the SAPS runs and the no-SAPS runs. The neutral wind velocities are
shown in the quasi-dipole (QD) coordinates
system : Umn=Ugn⋅cosD+Uge⋅sinD,Ume=-Ugn⋅sinD+Uge⋅cosD, where Ugn,Uge
are the meridional and zonal velocities in the geographic coordinate system, Umn,Ume are the
meridional and zonal velocities in the QD coordinate system, and D is the angle between the magnetic meridian and
geographic north.
Time variation of the strongest disturbance neutral wind in the dusk
sector (18:00–24:00 MLT). Different colors represent SAPS onset times at
2 h intervals from 00:00 to 22:00 UT. The SAPS potential attains
extreme values around 00:30 epoch time, as indicated by the black vertical
dashed line. SAPS become zero around 02:00 epoch time, as indicated by the
blue vertical line. The top panel is for the Northern Hemisphere, and the
bottom panel is the Southern Hemisphere.
It can be seen that strong westward neutral winds develop from noon to midnight in the subauroral region. The westward
neutral winds are aligned with the SAPS channel with larger latitudinal extent, and are driven by the SAPS due to the
strong ion drag effect. The SAPS effect on the thermospheric neutral winds
has been examined by using
coordinated CHAMP and DMSP observations. It was found that SAPS could cause strong westward neutral winds with larger
latitudinal extent than SAPS. CHAMP observed westward neutral winds with a velocity of about -215 ms-1
inside the zone of SAPS with a velocity of -710 ms-1 when Kp<4 (see Table 1 in
). Our results are qualitatively in agreement with the CHAMP observations, but the neutral wind velocity seems
to be underestimated in the model. The strongest neutral winds can be found at 01:40 epoch time with a speed of
-163 ms-1 at 15:00 MLT and 65∘ MLat, which is 50 min after the SAPS reached their minimum
value around -710 ms-1. The modeled neutral wind speed is about 50 ms-1 weaker than the
observation. The disturbance neutral winds did not recover as quickly as SAPS (see the 02:20, 02:40 and 03:00 epoch
times). There were eastward disturbance neutral winds located poleward of the westward neutral winds. These eastward
neutral winds could decrease the traditional westward neutral winds in the auroral oval in relation to the plasma
convection flow. Such eastward neutral winds in the auroral oval are also reproduced by .
UT variations
In the model, the SAPS were artificially set to commence at different universal times, at an interval of 2 h from
00:00 to 22:00 UT on 21 March 2000. Twelve isolated SAPS runs were conducted for different UT times, and they all lasted
for 5 h. Polar maps of the disturbance neutral winds were produced for each SAPS event using the same format as
Fig. . The peak westward neutral winds at each SAPS epoch time were identified and recorded.
The variations of the peak westward neutral winds with epoch time are illustrated in Fig. , where 00:00 epoch
time is the onset time of the SAPS. Different colors denote SAPS events that occur at different UT times. As shown in
Fig. , the SAPS minimize at 00:30 epoch time (as indicated by the vertical black line), and disappear at
02:00 epoch time (as indicated by the vertical blue line). For all SAPS events, the westward neutral winds attain their
minimum approximately 50–90 min later, and they recover more slowly than the SAPS due to the neutral inertia
effect. found the timescale of approximately an hour following
the onset of strong ion flow relative to the
neutrals.
We conclude that the velocities of the disturbance neutral winds differ greatly when the SAPS occur at different UTs. In
the Northern Hemisphere, a strongest disturbance neutral wind velocity of about -222 ms-1 is achieved when
the SAPS start at 18:00 UT, while the weakest neutral wind velocity of -147 ms-1 occurs at
04:00 UT. However, in the Southern Hemisphere, the strongest velocity of -214 ms-1 tends to occur at
04:00 UT, and the weakest value of -104 ms-1 occurs at 16:00 UT. Note that the imposed SAPS potential is
the same for all events. Such UT variations may be related to the solar illumination effect in the SAPS regions, which
will be addressed in more detail in the Discussion section.
The MLT variation of the strongest disturbance westward neutral
winds driven by SAPS. Different colors represent SAPS onset times from 00:00
to 22:00 UT at 2 h intervals. The top panel is in the Northern
Hemisphere, and the bottom one in the Southern Hemisphere.
MLT distribution
As introduced in Sect. , the SAPS potential was artificially forced to minimize at 19:00 MLT and to decrease
towards both noon and midnight. The strongest westward disturbance neutral winds in each MLT bin at 01:20 epoch time were
selected out. Figure shows the MLT variations of these strongest westward neutral wind velocities. The different
colors represent SAPS events that occur at different UTs. We conclude that the neutral winds tend to locate at an earlier
local time for weaker disturbance neutral wind velocity, and vice versa. For example, in the Northern Hemisphere, the
strongest neutral winds locate at approximately 16:00 MLT for SAPS occurring at 18:00 UT, while the weakest neutral
winds are found around 14:00 MLT for SAPS starting at 04:00 UT.
DiscussionSAPS effects on neutral winds
In the previous section we discuss the variations in the zonal neutral winds when SAPS are included in the model. In the
SAPS–GITM run, the westward neutral winds in the subauroral region are
reproduced in the dusk sector. This can be
attributed to the ion drag effect through which the SAPS drive the neutral winds in the westward direction. Our
simulation results are consistent with previous observations and simulation results. Based on observations made by
Dynamics Explorer 2 during the 24 November 1982 magnetic storm, concluded that there were strong SAPS and
enhanced westward neutral winds during that event. Based on 2 years of coordinated CHAMP and DMSP observations,
investigated the relationship between the SAPS, the electrojet, the zonal neutral wind and the mass
density. They determined that the zonal neutral winds streamed westward in the same direction as the SAPS equatorward of
the high-conductivity channel. Both the neutral winds and the plasma flows minimized at the same latitude.
reported stronger subauroral westward jets during storm time from CHAMP observations, which was not shown in the empirical
global geomagnetic disturbance wind model (DWM07).
incorporated an empirical model of SAPS into the TIEGCM and simulated the
effect of SAPS on the global thermosphere and ionosphere during a moderately
geomagnetically active period. They found that the SAPS caused large westward
neutral winds at higher altitudes in the afternoon–midnight sector. Our
model work is different from that of , mainly in two ways:
(1) imposed the SAPS velocity on the TIEGCM, while we used the
GITM imposed by the SAPS potential. (2) were concerned about
the effect of SAPS on the zonal neutral winds, while we focused on the UT
variations of such an effect.
In this study, it is quite interesting to note that although the SAPS potential is fixed in all cases, the resulting
westward neutral wind velocities vary greatly in amplitude and are concluded to depend on the universal time when SAPS
start to occur. The strength of the ion drag is proportional to the magnitude of the ionospheric electron density.
concluded that there was a good linear relationship between
the SAPS' westward ion flux and the zonal
neutral wind, which highlighted the important role of the background electron density. Thus, it is speculated that the
background electron density in the SAPS region might exhibit UT variations, which would account for the UT variations of
the ion drag force.
The top row shows the correlation between the strongest westward
disturbance neutral wind with peak electron density in both hemispheres. The
bottom row shows the correlation between the strongest westward disturbance
neutral wind with cos1/2(SZA) (solar zenith angle, SZA) in both
hemispheres. The correlation coefficient (R) and linear relationship
between the parameters are shown in each plot.
The electron density of the subauroral region depends on the solar ionization, because the particle precipitation is
mainly confined to the auroral region. Other factors that can affect the
spatial distribution of the electron density
include the transport from the electric fields and neutral winds and neutral composition ratios . The solar
illumination at fixed local times on the dayside is related to the solar zenith angle (SZA), which is related to
the geographic latitude (GLat). The GLat of the fixed subauroral magnetic latitude can exhibit obvious variation with
longitude or UT, and thus different levels of sunlight. For example, in the Northern Hemisphere, at 60∘ MLat
and 15:00 MLT, the SZA attains a minimum of 59.6∘ at 18:00 UT, and a maximum of 75.6∘ at
04:00 UT. This means that the subauroral region receives the largest amount of solar illumination at 18:00 UT and the
smallest amount at 04:00 UT. Therefore, the ion drag force is strongest at 18:00 UT and weakest at 04:00 UT in the
Northern Hemisphere for the same SAPS potential at both UTs. This explains the stronger zonal neutral winds that occur
when the SAPS start at 18:00 UT and the weaker zonal neutral wind when SAPS initiate at 04:00 UT. In the Southern
Hemisphere, at -60∘ MLat and 15:00 MLT, the SZA attains a minimum of 57.6∘ at 04:00 UT,
and a maximum of 82.9∘ at 16:00 UT. This corresponds to the SAPS onset times when the maximum and minimum zonal
neutral winds are generated. found that the TIEGCM with a SAPS
model reproduced stronger westward neutral winds
between 00:00 and 07:00 UT in the Southern Hemisphere but lesser westward neutral wind in the Northern Hemisphere. This
is consistent with our model result that the zonal neutral wind is strongest at 04:00 UT in the Southern Hemisphere
and
weakest at 04:00 UT in the Northern Hemisphere. Thus, we think that the solar illumination effect might explain the
hemispheric asymmetry of the zonal wind in response to SAPS as shown in . Our conclusions that the solar
illumination can affect the strength of the zonal winds are consistent with previous observation and model studies
e.g.,. found that solar illumination
could explain the longitudinal UT
variation of zonal winds at subauroral regions during quiet periods by using CHAMP observations and GITM
simulations. reported that the quiet and storm time zonal winds at subauroral latitudes were influenced by
solar illumination conditions by using several years' CHAMP observations. They found that both the quiet time and
storm-induced subauroral westward neutral winds were stronger in the summer
hemisphere. As outlined in Sect. , the
local time periods in which the disturbance neutral winds attain their extreme values exhibit UT variations. This may be
related to the variation in the area of the solar illumination with UT. For example, the illumination region will shift
more towards the dayside at 04:00 than at 18:00 UT in the Northern Hemisphere, and more toward the dayside at 16:00 UT
than at 04:00 UT in the Southern Hemisphere. Thus, the strongest disturbance neutral winds tend to locate at an earlier
local time at 04:00 than at 18:00 UT in the Northern Hemisphere, while they tend to locate earlier at 16:00 than at
04:00 UT in the Southern Hemisphere.
The strongest westward neutral winds are identified from Fig. for
12 SAPS simulation cases that occur at 12 different UT times. According to
Chapman's theory , the Ne due to solar illumination is
proportional to cos1/2(SZA). Fig. shows the correlation between the strongest westward neutral wind
velocities and the Ne (top panel), and cos1/2(SZA) (bottom panel). The SZA is calculated at
±60∘ MLat and 15:00 MLT (representing the subauroral region
in the dusk sector), in the Northern Hemisphere and Southern Hemisphere,
respectively. The quantitative relationships between the strongest disturbed
neutral winds and the
Ne, and cos1/2(SZA) for all of the SAPS events are shown in Fig. . The disturbed neutral
winds correlate well with the Ne and cos1/2(SZA) with correlation coefficients of > 0.6. The
excellent correlation between cos1/2(SZA) and the westward neutral wind velocities in both hemispheres
highlights the importance of solar illumination in explaining the UT effect of SAPS on the disturbance zonal neutral
winds; that is, with more solar illumination and a subsequently larger
Ne in the subauroral region, stronger
disturbance neutral winds can be generated.
One may notice that the correlation coefficient between the disturbance neutral wind velocity and the electron density is
stronger (R=-0.9) in the Southern Hemisphere than that (R=-0.6) in the Northern Hemisphere, which
might be due to the larger displacement between the geomagnetic and geographic poles in the Southern Hemisphere than in
the Northern Hemisphere. Thus, the UT variation of the solar illumination in the Southern Hemisphere is stronger than that
in the Northern Hemisphere. The correlation between the disturbance neutral wind and Ne is moderate
(R=-0.6) in the Northern Hemisphere, which might be for the
following reasons. The Ne used for the correlation study is the peak
density in the subauroral region during SAPS periods, which might not fully
represent
the variation in the ion drag force. The ion drag force in the zonal direction is Neνin(V-U), where
νin is collision frequency and V and U are ion and neutral
velocity in the zonal direction. The correlation
between the disturbance neutral wind velocity and the disturbance Ne(V-U) can be improved to 0.7 in the Northern
Hemisphere. Another reason might be that there are other factors that can affect the zonal neutral wind. SAPS can affect
the neutral winds in two ways: (1) a direct ion drag effect that drives the
neutrals westward; (2) frictional heating that changes the air pressure and
global wind circulation. The frictional heating can heat the neutrals and
ions
locally. Through thermal expansion and upwelling of the thermosphere, the heat can be transported globally through
nonlinear dynamic processes. This is consistent with in that
the neutral temperature was enhanced inside the
polar cap by about 40 K through compressional heating caused by SAPS. The equatorward neutral wind turns westward
at subauroral regions as a result of the Coriolis force. The additional role of the global circulation effect caused by
SAPS might explain partly the moderate linear correlation between the zonal neutral wind and electron density.
The top row shows a correlation analysis of the largest disturbance
electron density at subauroral latitude with cos1/2(SZA) in both
hemispheres. The bottom row shows a correlation analysis between the peak
disturbance air mass density with cos1/2(SZA) in both
hemispheres. The correlation coefficient (R) and linear relationship
between the parameters are shown in each plot.
A correlation analysis of the strongest westward disturbance neutral
wind with cos1/2(SZA) in both hemispheres during northward IMF
periods. The correlation coefficient (R) and linear relationship between
the parameters are shown in each plot.
Time variation of the strongest disturbance neutral wind in the dusk
sector (18:00–24:00 MLT). A sine-wave oscillation with a 20 min time
period and 8 kV amplitude is added to the SAPS peak potential in the
steady case. Different colors represent SAPS onset times at 2 h
intervals from 00:00 to 22:00 UT. The SAPS potential in the steady case
attains extreme values around 00:30 epoch time, as indicated by the black
vertical dashed line. SAPS become zero around 02:00 epoch time, as indicated
by the blue vertical line. The top panel is for the Northern Hemisphere, and
the bottom panel is the Southern Hemisphere.
A correlation analysis of the strongest disturbance westward neutral
wind at subauroral latitude with cos1/2(SZA) in both hemispheres
for the sine-wave oscillation case. The correlation coefficient (R) and
linear relationship between the parameters are shown in each plot.
SAPS effects on Ne and ρ
The strong SAPS-related plasma drifts and frictional heating can enhance the neutral and ion temperatures. Due to the
upwelling of the thermosphere the mass density of the air can be enhanced at higher altitudes. The upwelling of
molecular-rich air can enhance the plasma recombination rate. The plasma
transportation due to SAPS can bring plasma from nighttime
to daytime. Both the increased recombination rates and plasma transportation can result in a depletion in the ionospheric
plasma density in the mid-latitude trough . Previous studies have reported that SAPS can deepen the
mid-latitude density trough and form an air mass density bulge at 400 km altitudes
. By imposing an intense westward ion drift on a closed subauroral plasma tube,
found that the ion-neutral frictional heating increased the ion temperature and caused F region plasma
density to decrease due to the enhanced loss by chemical reactions and
westward transportation. By implying a 2 h
poleward electric field in the dusk sector at subauroral region in the Sheffield Coupled
Thermosphere–Ionosphere–Plasmasphere (CTIP), showed that SAPS caused a decrease in the F region peak
density and an increased O+ loss through chemical reactions. Based on CHAMP observations, found that
large SAPS-induced neutral winds could heat the upper thermosphere and cause a 10 % increase in the mass density (with
respect to periods without SAPS) at an altitude of 400 km. The density peak occurred in the same location as the
SAPS peak, which indicated efficient thermospheric heating from the ion and neutral friction. Through simulations,
also found that SAPS contributed to deeper and more extended
ionospheric electron density depletions at the
subauroral latitudes. The interesting result in the present work is that there are UT variations in the SAPS' effect on
the disturbance of the electron density and air mass density, consistent with that of the disturbance neutral winds.
Figure shows the correlations between cos1/2(SZA) and ΔNe and Δρ at
subauroral regions. Figure has the same format as Fig. . Here, ΔNe and Δρ
are the peak disturbance electron and air mass densities for each SAPS event. There are correlations among these
parameters, with correlation coefficients from 0.5 and upward. This indicates that with more sunlight, a larger decrease
in Ne and a larger increase in the air mass density are produced in
the SAPS region.
The correlations are moderate for Δρ and cos0.5(SZA) in the Northern Hemisphere (R= 0.5)
and for both ΔNe and Δρ in the Southern Hemisphere (R=-0.5 and -0.6). This implies
that there are other factors in addition to solar illumination that can have effects on the depletion of electron
densities and enhancement of neutral densities. The electron density can be reduced by a combination of several processes:
SAPS-related westward transportation, recombination due to enhanced
O+, and vertical transport from disturbance
neutral winds. The disturbance neutral winds can cause vertical motion of the plasma at subauroral latitudes under
non-zero geomagnetic declination and inclination angle conditions at
mid-latitudes. The F region Ne can be
increased due to upward motion as the recombination rate is reduced at higher altitudes. Conversely the F region
densities can be decreased by downward motion of plasma into regions of higher recombination rates. showed
that the neutral wind-geomagnetic declination mechanism could cause larger longitudinal or UT variations in the ΔNe in the Southern Hemisphere than in the Northern Hemisphere, as the longitudinal differences in the geomagnetic
declination were larger in the Southern Hemisphere, where the vertical transport from the neutral winds was more
effective. We believe this might explain the weaker correlation between ΔNe and cos1/2(SZA) in
the Southern Hemisphere than in the Northern Hemisphere. The frictional heating is proportional to the ionospheric
conductivity, which can cause the enhancement of ρ at higher altitudes. There is an inverse relationship between the
ionospheric conductivity and Earth's dipole moment. It is known that the
magnetic field strength shows obvious longitudinal
differences in the two hemispheres. There are two peaks, which are located around 120∘ W and
120∘ E GLon, respectively, in the Northern Hemisphere. And there is only one peak, which is located around
140∘ E GLon, in the Southern Hemisphere. The longitudinal variation
of geomagnetic field strength can complicate
the UT and longitudinal effect of frictional heating, and make the correlation between the solar illumination and the air
mass density less excellent than expected.
Northward IMF conditions
In this section, we check the UT variations of SAPS effects on zonal neutral winds during a northward IMF period (IMF
Bz= 2 nT). The regular high-latitude two-cell pattern
during southward IMF periods turned into a four-cell pattern under the
northward IMF condition . The SAPS potentials were added in the
same way as performed during the southward IMF period. The correlations
between the disturbance zonal neutral winds and cos1/2(SZA)
are shown in Fig. in the two hemispheres. Good correlation can be found between these two parameters,
indicating that the UT effect of SAPS on the disturbance zonal neutral wind velocities in relation to the solar
illumination also exists under the northward IMF condition. However, the
disturbance zonal neutral winds and the peak electron densities during SAPS
periods are not well correlated in both hemispheres (figures not shown). The
background
convection at high latitude might be an important component in modulating the F layer electron density at mid-latitudes,
which needs to be verified by observational studies. However, there are quite
a few observational studies concerning SAPS
effects on the ionosphere and thermosphere system during northward IMF periods. investigated SAPS' impacts on
the mid-latitude trough and auroral regions during the 21–22 January 2005 geomagnetic storm during southward and
northward IMF periods in the American sector. They found that the polar cap became a low plasma density region under the
sunward convection condition, and the mid-latitude trough was more unstructured during northward IMF. applied
the SAPS electric field to the Utah State University time-dependent
ionospheric model in order to assess the effect of
the SAPS on the mid-latitude nighttime F region ionosphere during southward IMF periods. They tested for different
background convection patterns: one is the Volland symmetric two-cell pattern, the other is the H-M pattern
. They found different density features with the Volland
convection pattern from those with an H-M
background. Thoroughly examining SAPS effects on the thermosphere and ionosphere under northward IMF conditions will be
conducted in the future by using both observations and simulations.
Temporal variation of SAPS
In the above section, the SAPS potential is changed linearly and held constant throughout its period of
existence. However, previous work has reported the temporal variation of
SAPS. reported that SAPS fluctuated on timescales of minutes
between 500 ms-1 and 2 kms-1, for example, the
increase from
0.4 kms-1 at 03:10 UT to 1.4 kms-1 at 05:20 UT, the increase from 1.3 kms-1 at
02:50 UT to 2.5 kms-1 at 03:10 UT, or the increase from
0.8 kms-1 at 03:50 UT to
1.8 kms-1 at 04:00 UT. Rapidly changing electric fields have been observed by incoherent scatter radars at
Millstone Hill and Irkutsk research radar . The GITM can
update its electric field approximately every 2 s, but typically does it
every minute . Thus, the GITM can be run using idealized
conditions to study the response
of the system to the input temporal fluctuations.
In this section, the SAPS potential changes such that the average potential is the same as in the previous section, but
it oscillates in time. A simple sine-wave variation with a 20 min time
period and 8 kV amplitude was added to
the SAPS peak potential: A⋅sin(2πt/T), where A= 8 kV, and
T= 20 min. All input parameters were the same as the southward IMF
case, except for the SAPS potential. This causes wave-like oscillations in
the SAPS velocity with the amplitude changing by about 500 ms-1
in the time range
of 10 min (figure not shown). Figure shows the temporal variation of the westward neutral winds with SAPS
epoch time, which is in the same format as Fig. . Different colors denote SAPS events that occur at different UT
times. There are obvious wave oscillations in the disturbance zonal neutral wind velocities with oscillation periods of
20 min. There are phase differences of about 5 min between the plasma and neutral wind peak velocities. The peak
disturbance zonal neutral winds are recorded for all SAPS runs that occur at different UTs. Figure shows the
correlation between the disturbed zonal neutral winds and cos1/2(SZA) in both hemispheres. Generally good
correlations can be found between the disturbance zonal neutral wind velocities and cos1/2(SZA) in both
hemispheres. This implies that for the sine-wave case, the SAPS effects on the neutral winds also exhibit UT variations in
association with the solar illumination condition.
In order to quantitatively differentiate the steady and sine-wave cases, we have calculated the averages and SDs of these
strongest disturbance zonal neutral wind velocities (as shown in
Figs. and ). The average neutral
wind velocities are -183.9 and -190.2 ms-1 for the steady and sine-wave cases in the Northern Hemisphere,
and -165.6 and -167.1 ms-1 in the Southern Hemisphere. The SDs are 28.8 and 35.7 ms-1 in the
Northern Hemisphere, and 37.7 and 37.1 ms-1 in the Southern Hemisphere. The ratios of the deviations to the
average velocities are thus 16 and 19 % for the steady and
sine-wave cases in the Northern Hemisphere, and 23
and 22 %
in the Southern Hemisphere. It can be seen that the average velocities are larger for the sine-wave case in both
hemispheres than those for the steady case. Both the deviations and the
deviation percentage of the average velocities are
slightly larger for the sine-wave cases in the Northern Hemispheres, but comparable in the Southern Hemispheres. Thus, the
UT effect is slightly larger for the sine-wave SAPS than the steady case in the Northern Hemisphere.
In this study, a simple sine-wave oscillation has been added to represent the temporal variation of SAPS, which might be
different from real conditions. provided observation of 10 min
quasi-periodic variation of the SAPS electric field with
10 mVm-1 amplitude. reported large-scale
(30 mVm-1) peak-to-peak wave-like oscillations in the SAPS
electric field magnitude with 200 and 300 s periodicity. These small-scale
structures of
SAPS electric fields might derive from processes internal to the magnetosphere and ionosphere system, i.e., the subauroral
ionospheric feedback instability . further reported
that SAPS wave structures might originate from Alfvén waves. Our future
modeling work will investigate the effect of wave oscillations of SAPS on the
thermosphere and ionosphere by including different frequencies and amplitude components.
Conclusions
By using a global ionosphere and thermosphere model, the temporal and spatial variations in the thermospheric zonal
neutral winds in response to SAPS that start at different universal times (UT) are studied at 400 km altitude
under a southward interplanetary magnetic field (IMF) condition. SAPS can
drive westward neutral winds in the dusk sector in
the SAPS region. These disturbance neutral winds exhibit large variations with UT during equinox conditions. With more
solar illumination, stronger westward disturbance neutral winds are generated. The largest disturbance neutral winds occur
at 18:00 UT in the Northern Hemisphere and at 04:00 UT in the Southern Hemisphere. The SAPS driven neutral winds tend to
peak at earlier local times when there is less solar illumination, and at later local times when there is more solar
illumination. The extent of the SAPS' effect on the deepening of the mid-latitude ionospheric trough and the enhancement
of the air mass density shows UT variations in relation to the solar illumination. Obvious UT effects of SAPS on the
neutral winds exist under northward IMF conditions and for a sine-wave oscillation case.
We acknowledge the use of the GITM developed by the
University of Michigan (Aaron Ridley, ridley@umich.edu). Simulation
data are available from the authors upon request.
The authors declare that they have no conflict of interest.
Acknowledgements
This work is supported by the National Nature Science Foundation of China
(nos. 41674153, 41521063, and 41431073). The authors
thank the reviewers for valuable suggestions regarding this work. The topical editor, Keisuke Hosokawa, thanks three anonymous referees for help in evaluating this paper.
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