ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-1113-2017Ionospheric F-region response to the 26 September 2011 geomagnetic storm in the Antarctica American and Australian sectorsCorreiaEmiliaecorreia@craam.mackenzie.brhttps://orcid.org/0000-0003-4778-3834SpogliLucahttps://orcid.org/0000-0003-2310-0306AlfonsiLucillaCesaroniClaudioGulisanoAdriana M.ThomasEvan G.https://orcid.org/0000-0001-8036-8793RamirezRay F. HidalgoRodelAlexandre A.Instituto Nacional de Pesquisas Espaciais, INPE, São José dos Campos, BrazilCentro de Rádio Astronomia e Astrofísica Mackenzie, Universidade
Presbiteriana Mackenzie, 01302-907 São Paulo, BrazilIstituto Nazionale di Geofisica e Vulcanologia, Rome, ItalySpacEarth Technology s.r.l., Rome, ItalyInstituto Antártico Argentino/Dirección Nacional del Antártico, Buenos Aires, ArgentinaInstituto de Astronomía y Física del Espacio (UBA-CONICET), Buenos Aires, ArgentinaDepartamento de Física FCEyN Universidad de Buenos Aires, Buenos Aires, ArgentinaThayer School of Engineering, Dartmouth College, Hanover, New Hampshire, USAEmilia Correia (ecorreia@craam.mackenzie.br)5October20173551113112919June20172September201728August2017This 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/35/1113/2017/angeo-35-1113-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/1113/2017/angeo-35-1113-2017.pdf
The ionospheric response at middle and
high latitudes in the Antarctica American and Australian sectors to the
26–27 September 2011 moderately intense geomagnetic storm was investigated
using instruments including an ionosonde, riometer, and GNSS receivers. The
multi-instrument observations permitted us to characterize the ionospheric
storm-enhanced density (SED) and tongues of ionization (TOIs) as a function
of storm time and location, considering the effect of prompt penetration
electric fields (PPEFs). During the main phase of the geomagnetic storm,
dayside SEDs were observed at middle latitudes, and in the nightside only
density depletions were observed from middle to high latitudes. Both the
increase and decrease in ionospheric density at middle latitudes can be
attributed to a combination of processes, including the PPEF effect just
after the storm onset, dominated by disturbance dynamo processes during the
evolution of the main phase. Two SEDs–TOIs were identified in the Southern
Hemisphere, but only the first episode had a counterpart in the Northern
Hemisphere. This difference can be explained by the interhemispheric
asymmetry caused by the high-latitude coupling between solar wind and the
magnetosphere, which drives the dawn-to-dusk component of the interplanetary
magnetic field. The formation of polar TOI is a function of the SED plume
location that might be near the dayside cusp from which it can enter the
polar cap, which was the case in the Southern Hemisphere. Strong GNSS
scintillations were observed at stations collocated with SED plumes at middle
latitudes and cusp on the dayside and at polar cap TOIs on the nightside.
Ionosphere (Ionospheric disturbances)Introduction
The magnetosphere–ionosphere–thermosphere system is strongly disturbed during
geomagnetic storms. The ionospheric response to a geomagnetic storm has been
studied for decades, but many open questions about its dynamics at regional
and global scales still exist (e.g., Prolss, 1995; Buonsanto, 1999; Mendillo,
2006; Danilov, 2013, and references therein). Particularly at high latitudes,
ionospheric dynamics are strongly driven by coupling processes involving
the solar wind, the interplanetary magnetic field (IMF), and the
magnetosphere. During geomagnetic storms, the magnetosphere is compressed,
inducing intense electric fields and an increase in magnetospheric
convection. The interplanetary electric field (IEF) is mapped along the
magnetic field lines to the high-latitude ionosphere but can also propagate
across the magnetic field lines and appear in the midlatitude and low-latitude
ionosphere; in this case it is called a prompt penetration electric field
(PPEF). The PPEF effect at the equatorial ionosphere was first identified during
substorms (Nishida, 1968), and it was believed that it could last for only
∼30min, which is the magnetospheric shielding time constant (e.g.,
Tanaka and Hirao, 1973; Senior and Blanc, 1984; Spiro et al., 1988; Fejer
et al., 1990). But the effect of PPEF has been observed for hours during the
main phase of strong geomagnetic storms, evidencing a long-duration
penetration of interplanetary electric field to the low-latitude ionosphere
without shielding (Tsurutani et al., 2004, 2008; Huang et al., 2005; Mannucci
et al., 2008). High-latitude ionosphere electrodynamics are strongly
affected by the mapped IEF that induces various electric fields and also by
the polar plasma convection (e.g., Gonzalez et al., 1999). The induced
ionospheric electric fields refer to the near-equatorial PPEF (Nishida,
1968), the disturbance dynamo (Blanc and Richmond, 1980), the equatorial
polarization (Balan and Bailey, 1995), and the subauroral polarization stream
(SAPS; Foster and Burke, 2002). PPEFs are often observed in the
equatorial latitudes (e.g., Sastri, 1988), convecting the ionosphere upward in
the dayside and downward in the nightside. Particularly during major
geomagnetic storms, PPEFs are substantially larger than the
fields associated with the normal fountain effect (Tsurutani et al., 2004),
lifting the dayside equatorial plasma to higher altitudes and latitudes than
normal with the crests of the equatorial ionospheric anomaly (EIA) reaching
the middle latitudes. This has been called the dayside ionospheric
superfountain (DIS) effect (Tsurutani et al., 2004, 2008). At high
latitudes, the precipitation of energetic particles into the thermosphere
enhances ionospheric conductivities and generates intense electrical
currents (Buonsanto, 1999). The dissipation of these currents by the Joule effect
heats the auroral zone, which expands, changing the lower thermospheric
composition and driving large-scale neutral winds (Fuller-Rowell et al.,
1994; Buonsanto, 1999; Danilov and Lastovicka, 2001). The combination of
these ionospheric processes during major geomagnetic storms results in
a large-scale thermal plasma redistribution involving the equatorial through
the polar latitude regions.
The response of the distinct ionospheric regions to geomagnetic storms is
different because the electron density changes are controlled by different
physical mechanisms. The lower ionosphere, regions E and D, shows
a significant enhancement of electron density in the auroral zone produced by
increased precipitation of energetic particles (Lastovicka, 1996). In
contrast, F2-region response to geomagnetic storms shows very complicated
spatial and temporal behavior (e.g., Danilov, 2013). They are called
ionospheric storms and could present an increase in (positive phase) or
a depletion (negative phase) of electron density, which is produced by
different mechanisms associated with electrodynamic processes and neutral
composition changes (e.g., Danilov and Lastovicka, 2001).
Ionospheric storm morphology is a function of the energy input in the
high-latitude upper atmosphere, which is maximized during the main phase of
the geomagnetic storms (Gonzalez et al., 1994), and its behavior during the
same storm could be very different depending on the station latitude and
longitude, local time of storm onset, storm time, and season. The F2-region
response to geomagnetic storms is very complex, but a general
morphology and physical processes have been established, as described in
many review papers (e.g., Prolss, 1995, 2008; Buonsanto, 1999; Mendillo, 2006;
Danilov and Lastovicka, 2001; Danilov, 2013).
The negative phase of the ionospheric storms are thought to be well
understood: they are mostly observed at high and middle latitudes and occur
in all seasons but winter. One of the most significant characteristics is
equatorward drift from auroral to middle latitudes, which shows seasonal
behavior and is more developed in the summer hemisphere where it penetrates to
lower latitudes than in the winter hemisphere (Prolss, 1995, 2008; Buonsanto,
1999). Due to the differences between the background thermospheric and
storm-induced circulation, the negative phase occurs rather frequently at
middle latitudes during winter nighttimes, while in summer it is frequently
observed both in the daytime and nighttime (Danilov, 2013). Its physical
mechanism was first suggested by Seaton (1956), who attributed the negative
phase at high latitudes to changes in the thermosphere produced by the
heating of its lower part in the auroral zone. The main source of this
heating is the Joule effect, but it could also have some contribution from the
direct precipitation of particles (Prolss, 1995). The temperature increase in
the F2-region also affects the linear recombination coefficient, which
reduces the electron concentration. Thus, the negative phase is formed by
composition changes and a temperature increase in the thermosphere (Mikhailov
and Foster, 1997; Mansilla, 2008). The equatorward drift of the negative
phase occurs preferentially in the post-midnight sector during the main phase
of geomagnetic storms (Prolss, 1995). At the equatorial region, the nighttime
negative phase observed just after the onset of the main phase storm is due to
the dawn-to-dusk electric field associated with the PPEF, which is westward
and causes a strong downward drift of ionospheric plasma, increasing the
recombination and reducing the electron density.
The morphology of the positive phase is more complicated due to more
complex physical processes. It mostly occurs at middle and low latitudes in
the winter season. There are various possible mechanisms responsible for
the positive phase, namely the F2-region uplifting due to vertical drift, plasma
fluxes from the plasmasphere, and downwelling produced by storm-induced
thermosphere circulation at low latitudes (e.g., Buonsanto, 1999; Mendillo,
2006; Danilov, 2013). The vertical drift is an important factor that affects
the F2-region conditions, and it is controlled by the equatorward winds,
particularly at middle latitudes where the magnetic field lines are inclined,
and by PPEF at equatorial latitudes, which can be strong during major
geomagnetic storms. The thermospheric wind circulation favors upward
vertical drift during daytime in the winter season at middle latitudes, and at
near-equatorial latitudes the upward drift is due to electrodynamic processes
due to the EIA anomaly in association with the PPEF effect, which increases the
electron concentration because the production is still occurring (e.g.,
Prolss, 1978; Buonsanto, 1999).
Total electron content (TEC) enhancements observed in the dusk sector at
middle latitudes during the main phase of major geomagnetic storms have been shown
to be associated with the sunward convection of high-density plasma
originating
from lower latitudes, which shows a plasma drift toward noon and poleward at
ionospheric heights; this is called storm-enhanced density (SED; Foster, 1993). The observations have shown a large-scale redistribution of
ionospheric plasma during major geomagnetic storms, covering the
equatorial to the polar latitude regions (Foster, 2008). In a first step the PPEF
enhances the fountain effect at the equatorial region, increasing the EIA peaks
(Tsurutani et al., 2004), which under the effect of the polarization electric
field at dusk redistribute the low-latitude TEC in both longitude and
latitude, resulting in plumes of SED (Sandel et al., 2001). These SED
plumes under the influence of the subauroral polarization stream (SAPS)
electric fields are transported into the dayside cusp from which they enter the
polar cap, forming the tongue of ionization (TOI; Foster, 2008). The
pronounced enhancement of ionospheric density near dusk at middle latitudes
observed during the main phase of geomagnetic storms is the called dusk
effect (Mendillo et al., 1970; Mendillo, 2006).
The impact of solar wind disturbances on the magnetosphere–ionosphere
system results in a highly inhomogeneous ionosphere, producing steep electron
density gradients and irregularities. These structures vary on a wide range
of scale sizes from centimeters to hundreds of kilometers and affect the
performance of radio communication and navigation systems. These density
irregularities can produce rapid fluctuations in the amplitude and phase of
GNSS (global navigation satellite system) signals. At L-band, amplitude
scintillations are due to irregularities with a scale size from hundreds of
meters down to tens of meters (according to Fresnel's filtering
mechanism), while phase scintillations are caused by structures from a few
hundred meters to several kilometers (see, e.g., Kintner et al., 2007). The
phase fluctuation is also estimated from rate of TEC (ROT), which gives
information about structures with a scale size bigger than the Fresnel scale
(on L1 signal about 250 m). The occurrence of ionospheric
scintillations depends on magnetic local time, season, magnetic activity,
solar cycle, and geographic location (Spogli et al., 2009; Li et al., 2010;
Alfonsi et al., 2011; Prikryl et al., 2011). The ionospheric regions strongly
affected by scintillation are the nightside auroral oval, the cusp on the
dayside, and the polar cap at high latitudes, as well the equatorial regions
affected by the EIA anomaly. At high and middle latitudes, GPS phase
scintillation observations have shown that ionospheric irregularities are
primarily enhanced in the cusp in association with storm-enhanced plasma
density (SED). A tongue of ionization (TOI) can be formed and broken
into patches that are transported into the polar cap (e.g., Aarons et al.,
2000; De Franceschi et al., 2008; Spogli et al., 2009, 2013a; Prikryl et al.,
2011, 2015a, b, 2016; Thomas et al., 2013; Horvarth and Lovell, 2015). In
the auroral oval, GPS scintillation has been observed during energetic particle
precipitation events (Skone et al., 2008; Kinrade et al., 2013; Prikryl
et al., 2013a, b, 2016).
Most of the work done on specific geomagnetic storms refers to effects in the
ionosphere at different sectors in the Northern Hemisphere (e.g., Yizengaw
et al., 2005; De Franceschi et al., 2008; Spogli et al., 2009; Prikryl
et al., 2011, 2015a, b, 2016; Danilov, 2013; Thomas et al., 2013; Hovart and
Lovell, 2015; and references therein) where there is a dense network
of instrumentation for ionospheric studies. In contrast, in the Southern Hemisphere
where the network of instrumentation is sparse, there is less work and most ionospheric studies have been done in the Australian and African
sectors.
The purpose of this paper is to investigate the high-latitude and midlatitude
ionospheric response to the 26–27 September 2011 moderately intense
geomagnetic storm in the Southern Hemisphere, focusing on the American sector
in Antarctica. The ionospheric features associated with this geomagnetic
storm were already studied at high latitudes in the Northern Hemisphere by
Thomas et al. (2013) and at low and equatorial latitudes by Hairston
et al. (2014), so this paper will investigate the features observed at middle
and high latitudes in the Southern Hemisphere. The goal is to complete the
picture of such a storm by characterizing the dynamics of the disturbed
ionosphere and its irregularities during the geomagnetic storm main phase. We use ionosonde, GPS-TEC, and GNSS scintillation measurements
taken at the Brazilian station Comandante Ferraz (EACF) on King George Island at
middle latitude and at Mario Zucchelli Station (BTN) and
Concordia Station (DMC) located in the cusp/cap and polar cap regions,
respectively. The results permit us to evaluate the ionospheric dynamics during
this storm in Antarctica in comparison with other work and discuss its
interhemispheric peculiarities.
Geomagnetic conditions on 26–27 September 2011. Solar wind
(a) velocity (Vsw) and dynamic pressure (Psw),
IMF (b)Bz, (c)By, and (d) Bt components
(GSM). The (e) auroral electrojet (AE) and
Sym-H, (f) merging electrical field (Em), and interplanetary
electric field (IEF). (g) The Southern
Hemispheric power (HP POES) index and the polar cap index for the Southern Hemisphere
(PCS). The ACE data for IMF components and solar
wind parameters were time shifted.
The paper proceeds as follows. In Sect. 2 the space weather and geomagnetic
conditions are given. In Sect. 3 are the observations and data analysis. The
results are in Sect. 4, and the discussion and conclusions are in Sect. 5.
Space weather and geomagnetic conditions
The magnetosphere–ionosphere system was disturbed on 26 September 2011 by the
arrival of an interplanetary shock produced by a coronal mass ejection (CME),
which occurred in association with an M7 long-duration X-ray event as observed
by the GOES satellite. This disturbance resulted in a moderately strong
geomagnetic storm (G2 level) on 26–27 September with Kp =6 and
Sym-H ∼-130nT.
The interplanetary shock arrived on 26 September at ∼ 12:40 UT when
a sudden change in parameters is observed and leads to a sudden impulse (SI).
Sudden variations are observed in the solar wind and interplanetary magnetic
field (IMF) parameters (Fig. 1) with increases in the solar wind speed
(Vsw) from 350 to 550 kms-1, pressure from 5 to ∼25nPa, total IMF (Bt) from ∼10 to 30 nT, and the
beginning of fast fluctuations in the Bz and By components of IMF. The main
phase onset of the geomagnetic storm was at ∼15:15 UT on 26 September
when IMF Bz turned southward and slowly reached the minimum value of
-30nT at ∼17:00 UT, then turned northward. After 18:00 UT,
Bz turned southward again and remained at ∼-30nT until
∼19:00 UT when it started to oscillate with decreasing amplitude between
north and south until the end of the day in association with an increase in
the solar wind dynamic pressure that reaches ∼25nPa. The IMF
By component is positive from the main phase storm onset until ∼20:00 UT when it also starts to oscillate with decreasing amplitude until the
end of the day, similarly to Bz. The Sym-H index shows a long initial phase
(∼3 h) storm and a complex variation during its main phase reaching
two local minima of -70 and -100 nT at 17:00 and 18:30 UT,
respectively. The minimum of the Sym-H index of ∼-130nT
occurred at 23:00 UT on 26 September.
The auroral activity (AE) and polar cap for the Southern Hemisphere (PCS)
indices (Fig. 1e and f) show intensification peaks at ∼12:40, 17:00,
and 19:00 UT in association with SSC and the two Sym-H minima, respectively.
The merging electric field (Em; e.g., Kan and Lee, 1979) is used to estimate
the energy input into the magnetosphere–ionosphere system (Fig. 1f). The
solar wind motional zonal electric field (IEF; Fig. 1f) is considered to
evaluate the PPEF, which occurs during the periods that IMF Bz is negative
when about 5–12 % of the associated eastward IEF can penetrate into the
ionosphere (Gonzalez et al., 1989; Kelley et al., 2003). The IEF is
calculated as -Vx×Bz (Manucci et al., 2005) with Vx being the
x component of the solar wind velocity
(http://omniweb.gsfc.nasa.gov), which means that the northward positive
Bz is associated with the occurrence of westward electric fields on the
dayside and eastward electric fields on the nightside. The hemispheric power
index from NOAA POES (HP POES) for the Southern Hemisphere (Fig. 1g) is used
to estimate the power (GW) deposited in the polar region by energetic
particle precipitation in the aurora oval. Both these indices also show
strong increases in close association with the AE index.
The recovery phase of the geomagnetic storm extended until late on 27 September
with IMF Bt at ∼5nT, Bz and By oscillating with ∼5nT of amplitude around zero, and the AE index showing some increases
reaching ∼1000nT followed by the PCS index.
The solar wind and interplanetary magnetic field (IMF) data were obtained
from the ACE/SWEPAM and OMNIWeb data services
(http://www.srl.caltech.edu/ACE/ASC/level2/lvl2DATA_MAG-SWEPAM.html,
http://omniweb.gsfc.nasa.gov). The IMF components are in geocentric
solar magnetospheric (GSM) coordinates. Data from ACE take into account the
propagation delays from the spacecraft to the nose of the Earth's bow shock.
The geomagnetic indices used here are the 1 min auroral electrojet (AE) and
Sym-H and the 3 h Kp, which were obtained from the World Data Center for
Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html).
The polar cap index for the Southern Hemisphere (PCS) gives a quantitative estimate
of geomagnetic activity at polar southern latitudes and also serves as
a proxy for energy input into the magnetosphere. Here we use the polar cap
index (PCS) that is derived by the magnetic data of Vostok
(http://pcindex.org/about-3). NOAA POES hemispheric power (HP POES) for
the Southern Hemisphere was obtained from the Space Weather Prediction Center of NOAA
(http://legacy-www.swpc.noaa.gov/ftpdir/lists/hpi).
Map showing the station locations and respective instrumentation.
The colors and symbols at each station indicate data
from ionosondes (red), GNSS receivers (green), magnetometers (black), and riometers (*). The dashed lines indicate
isoclinic lines of geomagnetic latitudes. The shaded areas show the night regions at 16:00 UT.
Geographic and geomagnetic coordinates of stations with their
respective code and instrumentation.
Station nameGeogra. coordinates Geomag. coordinates LTMLT midaInstrument listbLat (∘ N)Long (∘ E)Lat (∘ N)Long (∘ E)UTAmerican SectorPort Stanley (PST)-51.70-57.89-37.6310.55UT - 44IonoComandante Ferraz (EACF)-62.08-58.39-47.1111.73UT - 44GNSS, IonoAkademik Vernadsky (AIA)-65.25-64.25-49.899.12UT - 44MagSan Martin Base (SMA)-68.13-67.10-52.668.32UT - 44IonoAustralian SectorHobart (HOB)-42.88147.35-54.12-133.45UT + 1013IonoMacquarie Island (MCI)-54.50158.95-64.54-111.90UT + 1012IonoCasey Station (CAS)-66.30110.50-80.85155.62UT + 818RiomMario Zucchelli Station (BTN)-74.70164.12-80.00-52.45UT + 128GNSSConcordia Station (DMC)-75.25124.17-88.6843.26UT + 81GNSSScott Base (SBA)-77.85166.76-79.92-32.89UT + 127Mag
a Mid is midnight;
b iono is ionosonde, mag is magnetometer, GNSS is GNSS receiver, riom is riometer
Observations and data analysis
Figure 2 shows the location of the stations considered in this study. Table 1
summarizes the geographic and geomagnetic coordinates of the stations,
including the instrumentation list. The geomagnetic coordinates were computed
using the on-line conversion tool available at
http://www.ukssdc.ac.uk/cgi-bin/wdcc1/coordcnv.pl.
Here we use data from three GNSS ionospheric scintillation and total electron
content (TEC) monitor (GISTM) receivers, one operating at the Brazilian Antarctic
station Comandante Ferraz (EACF), one at the Italian station Mario Zucchelli
(BTN), and one at the Italian-French station Concordia (DMC). The receivers
are GISTM GSV 4004B systems (Van Dierendonck et al., 1993), which are Novatel
OEM4 dual-frequency systems with special firmware to compute and record the
60 s amplitude (S4′) and phase (σϕ′) scintillation indices of
the GPS L1 signal and the 15 s ionospheric TEC and its changes (rate of
TEC, ROT) from the GPS L1 and L2 carrier-phase signals. The system also
records the receiver independent exchange format (RINEX) data. The calibrated
TEC is obtained from RINEX data using the methodology described by Ciarolo
et al. (2007) considering only satellite measurements taken at an elevation angle
above 30∘.
In order to account for the satellite measurements taken at different
elevation angles, TEC and the scintillation indices S4′ and σϕ′ are projected to the vertical as follows:
S4=S4′(F(e))b,σϕ=σϕ′(F(e))a,VTEC=sTECF(e),
where S4, σϕ, and VTEC are the vertical values of the respective
parameters, sTEC is the slant TEC, e is the satellite elevation angle, and
F(e) is the mapping function (Mannucci et al., 1993):
F(e)=1-cos(e)1+h/RE2-1/2,
where h is the height of the ionospheric piercing point (assumed to be
350 km), and RE is the Earth's radius. The exponents a
and b in the expressions of the scintillation indices are assumed to be a=0.5 and b=0.9 in agreement with Spogli et al. (2009). The sTEC and VTEC
are in TEC units (1 TECU=1016elm-2). A critical
discussion about advantages and drawbacks of projecting the scintillation
indices to the vertical can be found in Spogli et al. (2013b).
The ionosonde parameters foF2 and h′F2 are obtained from ionograms
and refer to the F2-layer vertical incidence critical frequency and the
F2-layer bottom virtual height, respectively. At EACF they were obtained from
ionograms performed every 5 min with a CADI ionosonde using the software
UNIVAP Ionosonde Digital Data Analysis (UDIDA) developed at the University of
Paraíba Valley (Fagundes et al., 2005). At San Martin Base, the
ionosonde data were obtained from the ionograms performed each hour with an
ionospheric sonde (IPS42 Mca; KEL Aerospace). The parameters were obtained
from the ionograms interpreted manually one by one by a trained technician at
the station making the conversion from the logarithmic scales of the ionogram
and using the ordinary rays as usual. These data are complemented by
foF2 from ionosondes operating at the Port Stanley (PST), Hobart
(HOB), and Macquarie Island (MCI) stations obtained from the Space Physics
Interactive Data Resource (SPIDR; http://spidr.ionosonde.net/spidr).
The analysis considers the NmF2 (1011elm-3)
calculated from values of foF2 by using the formula
NmF2=1.24×(foF2inMHz)2×1010.
The ionospheric absorption in the auroral oval was estimated from cosmic noise
absorption (CNA) measured at Casey Station, which was obtained from the World
Data Centre Space Weather Services (SWS, formally known as IPS Radio and
Space Services or IPS; http://www.sws.bom.gov.au).
The surface magnetic field conditions at or near the station locations
with GNSS observations are evaluated using the H (horizontal) component
obtained from the INTERMAGNET database (www.intermagnet.org; St-Louis
et al., 2012). There are representative data only for the stations Vernadsky
(AIA) at middle latitude in the American sector and Scott Base (SBA) in the
auroral oval. At high latitudes, the H component gives the horizontal
direction of the auroral electrojet with positive values indicating an electrojet
in the eastward direction. Fast decreases in the H component at high latitudes
are associated with changes in the ionospheric currents produced by
energetic particle precipitation events into the upper atmosphere.
The energy flux and mean energy of energetic particles precipitating in the
southern high latitudes are obtained using the data from the 140 to
150 nm band (LBHS; e.g., Zhang and Paxton, 2008) from the Special
Sensor Ultraviolet Scanning Imager (SSUSI). The sensors are onboard the
Defense Meteorological Satellite Program (DMSP) satellites
(http://sd-www.jhuapl.edu/Aurora/) and measures the auroral and airglow
emissions in the far-ultraviolet bands, providing partial global auroral
images (Paxton et al., 2002).
Using the plotting tools developed at Virginia Tech, which are available
online at the Space@VT SuperDARN website (http://vt.superdarn.org), it
is possible to make side-by-side comparisons of the ionospheric convection
map using Super Dual Auroral Radar Network (SuperDARN) data with GPS-TEC
maps (Thomas et al., 2013) using TEC data from the Madrigal database
(http://madrigal.haystack.mit.edu/madrigal/). The line-of-sight (LOS)
velocities measured with SuperDARN radars give information about
decameter-scale plasma irregularities in the ionosphere (Greenwald et al.,
1995; Chisham et al., 2007). The global TEC maps are processed using the MIT
Automated Processing of GPS (MAPGPS) software package (Rideout and Coster,
2006). These combined maps are used to spatially characterize ionospheric
convection and plasma irregularities.
Variations in NmF2 and VTEC measured by instruments in the
(a) Antarctica American and (b)
Antarctica Australian sectors during the geomagnetic storm
that occurred on 26–27 September 2011 (dotted line, darker curves)
compared to the quiet day curve (light curves with error bars).
Station names and geomagnetic coordinates are shown on the
panels. NmF2 is in units of 1011elm-3. The
error bars on QDC curves refer to the SD (±σ) of the
respective parameter.
Results
To characterize the ionospheric response to the geomagnetic storm that
occurred on 26–27 September 2011 as a function of storm time, local time, and
station geomagnetic location, we use GNSS and ionosonde data from receivers
operating at stations located from middle to high latitudes in the
Antarctica American and Australian sectors (Fig. 2), which are
representative of daytime and nighttime sectors, respectively. The data
coverage is not the same in both sectors because in the American sector the
instrumentation used for the present study is only over the Antarctic
Peninsula at middle-latitude regions, while in the Australian
sector there are stations from middle to high latitudes.
The VTEC, NmF2, and h′F2 variations are compared with a quiet day
curve (QDC) obtained from averaging four nearby geomagnetically quiet days
with Kp <2 and AE <200nT. The SD (σ) of VTEC,
NmF2, and h′F2 parameters is about 1 TECU, 0.4×1011elm-3, and 25 km, respectively, and they are
shown as error bars in the QDC curves (Fig. 3).
Ionospheric response in the Antarctica American sector
The main phase of the geomagnetic storm started at ∼ 15:00 UT
(11:00 LT) and peaked at ∼23:00 UT (21:00 LT) on 26 September, which
means from noon to night in this sector. The NmF2 and VTEC
parameters have values always above the quiet day level, showing that the
positive phase of the ionospheric storm is dominant during the main phase
storm at middle latitudes (Fig. 3a) where it started to develop at the
geomagnetic storm onset.
The ionosonde data show three NmF2 enhancements. The first one is
∼40 % at PST and EACF and 10 % at SMA with a peak at ∼16:00 UT (∼12:00 LT, local noon time) at the beginning of the main
phase storm. The second one is ∼130 % at PST, 280 % at EACF,
and 100 % at SMA with a peak at ∼20:00 UT (16:00 LT, local
afternoon). The third one is ∼240 % at PST and 250 % at EACF
with no definition at SMA and a peak at ∼23:00 UT (19:00 LT, dusk).
Afterwards it presents an abrupt drop, achieving values below the quiet day
level after ∼24:00 UT (20:00 LT). Each NmF2 enhancement
occurred just after a rise of ∼40, 100, and 50 km in the h′F2
parameter as observed at the EACF and SMA stations.
VTEC data from the EACF station also show three enhancements above the quiet day
level in close association with the ones observed in NmF2. The VTEC
enhancements were ∼100, 300, and 180 % (Fig. 3a bottom). The
comparison between NmF2 and VTEC enhancements at EACF shows that the
first density increase was much more pronounced in VTEC and the second one was
of the same intensity in both parameters, while the third one is more intense
in the NmF2. The first enhancement at ∼17:00 UT shows an
increase in VTEC ∼2 times stronger than in NmF2, suggesting
that the electron concentration changes occurred at altitudes above the F2-layer maximum at which the electrodynamical processes (PPEF) might be dominant
(e.g., Tsurutani et al., 2008; Danilov, 2013) during a period of slow
intensification of IMF Bz in southern direction. The second one at ∼20:00 UT occurred after a sudden southern turning of IMF Bz and shows
a similar increase in both parameters, suggesting that the density increase
was in the height of the layer maximum and might be mostly due to the
influence of meridional neutral winds generated by the dynamo disturbance
(e.g., Prolss, 1995; Buonsanto, 1999; Mendillo, 2006). The third enhancement
occurred at ∼23:00 UT during the evening hours and can be
attributed to the dusk effect in the F-region, which is a combination of
mechanisms including neutral winds and neutral composition changes
(Buonsanto, 1999).
During the recovery phase that started early on 27 September, the
NmF2 parameter values are below the QDC at all stations. Between
00:00 and 14:00 UT, NmF2 shows a density depletion of ∼30 % at EACF, ∼60 % at SMA, and only a small depletion at PST.
Afterwards and until the end of the recovery phase late on 27 September,
it practically returned to quiet conditions showing no significant departure from
QDC. Only at the PST station during the afternoon does NmF2 show three
strong enhancements superimposed on the slow depletion variation. VTEC
parameter at EACF shows variation very similar to NmF2.
Ionospheric response in the Antarctica Australian sector
The geomagnetic storm in this sector started near local midnight.
NmF2 and VTEC parameters predominantly show values below the QDC
level during the entire geomagnetic storm (Fig. 3b) at all stations.
(a) Variability in ROT values along satellite passes
over the midlatitude EACF station in the American sector (a.1)
and the polar cap/cusp BTN station and polar cap DMC station, both in
the Australian sector (a.2). (b) Time profiles of
ROT compared with the H component of surface magnetic field measured at
sites near the GNSS station in the American sector (b.1) and in the Australian sector (b.2). The vertical thick line
marks the time of the SSC. The open and filled circles refer to local
noon and midnight, respectively. ROT is in units of
TECUmin-1.
During the main phase storm the NmF2 parameter suggests that the density
depletion started earlier in the auroral oval, as seen in the measurements
taken at the MCI station. Here the values dropped below the QDC level after ∼10:00 UT (20:00 LT) on 26 September, even before the time of the SSC, and
reached a depletion of ∼60 % in the maximum of the main phase at
the end of the day (morning hours). At the middle-latitude HOB station, the
NmF2 values show a small depletion between 15:00 and 20:00 UT
(after midnight), which increases afterwards also reaching ∼60 % at the end of the day. The VTEC parameter from the BTN and
DMC stations also shows a similar slow increasing depletion in the polar cap
region that started near the geomagnetic storm onset at ∼15:00 UT
(near local midnight), reaching ∼60 % at the maximum of the
geomagnetic storm. Superposed to this VTEC negative slow variation, there are
fast increases of ∼10TECU. At BTN there is one around ∼22:00 UT (∼10:00 LT, pre-afternoon sector, near cusp), and at DMC
there are two around ∼17:00 and 19:00 UT (01:00 and 04:00 LT, respectively, in
the midnight sector).
During the recovery phase, NmF2 and VTEC parameters show that the
ionospheric response is very similar at all stations. The parameters stayed
at same level of depletion reached at the end of the storm main phase late on
26 September until 12:00 UT (late afternoon at all stations) on
27 September when they slowly returned to QDC values later in the day following
the decreasing geomagnetic activity.
Variability in scintillation indices S4 (a) and σϕ (Phi60) (b) at the EACF station in the
American sector (top panel a.1 and b.1) and at
the BTN and DMC stations in the Australian sector (bottom
panels a.2 and b.2). Black curves refer to the H
component of magnetic field measured at nearby magnetic stations.
The red curve refers to cosmic noise absorption measured
at the CAS riometer station, which like the BTN station is at the poleward edge of the auroral oval.
Ionospheric irregularities
To analyze the dynamics of high-latitude ionospheric irregularities produced
by this geomagnetic storm, we used three GNSS-derived parameters: the rate of
TEC (ROT) and the amplitude (S4) and phase (σϕ) scintillation
indices, which, when combined, give information about the size scales of
irregularities. The analysis is based on the GNSS receivers operating at the EACF
(middle-latitude) station located in the Antarctic American sector and at the
BTN (cusp/cap) and DMC (polar cap) stations located in the Antarctic
Australian sector.
SSUSI (DMSP) southern auroral image scans as a function of magnetic
latitude and MLT
(http://sd-www.jhuapl.edu/Aurora/) at the (a) initial
phase and (b–d) main phase of the
geomagnetic storm. The stars mark the locations of the EACF, BTN, and DMC
stations.
The presence of ionospheric irregularities started to be recorded at ∼12:00 UT on 26 September, near the time of the SSC, and persisted during
the main phase storm until ∼23:00 UT. The ROT analysis (Fig. 4) shows that the
most intense fluctuations (ROT >1.0TECUmin-1) of the GNSS phase
signal occurred all night at the BTN and DMC stations (cusp/cap and
polar cap regions), while at EACF (middle latitude) it only suggests a slight
rise between 19:00 and 22:00 UT (local evening). The scintillation analysis
shows that the amplitude of scintillation index S4 (Fig. 5a) has no significant
values above the 0.25 level at EACF and BTN with a significant enhancement
only at the DMC station. In contrast, the phase scintillation index σϕ
(Phi60 in Fig. 5b) shows strong enhancements, reaching up to ∼1.0rad at the BTN and DMC stations and only 0.2 rad at EACF,
which occurred in close association with ROT enhancements. The periods of
scintillation enhancements at the BTN station were accompanied by cosmic noise
absorptions of ∼0.5dB measured at the CAS riometer station
(Fig. 5b red curve) and fast decreases in the H component at the nearby
SBA geomagnetic station; both are located at the poleward edge of the auroral
oval in the Australian sector.
The scintillations observed at the DMC station appeared in four main groups:
the first from 12:00 to 15:00 UT during the initial
phase of the geomagnetic storm and the other three during the main phase
storm from 15:00 to 18:00 UT, 19:00 to 21:00 UT, and
21:00 to 23:00 UT. The three groups of scintillations occurring during the main
phase storm have a good association with the density enhancements observed at
the middle-latitude stations PST, EACF, and SMA located in the dayside American
sector.
The close association of phase scintillation intensification and ROT
fluctuations with peaks in HP POES, PCS, and AE indices, as well as with
ionospheric absorptions and fast H-component decreases observed near the BTN
station indicates that they were produced by ionospheric irregularities caused by
energetic particle precipitation in the auroral oval. The stronger
enhancements in the phase scintillation rather than in the amplitude index might
indicate they are produced by structures with scale sizes higher than one to
a few hundred meters (Fresnel scale for L1 signal).
Figure 6 shows the SSUSI image scans mapped in magnetic latitude and MLT
at ∼13:53, 17:55, 19:36, and 20:34 UT, which are representative of
four energetic precipitation events that occurred in close association with
the phase scintillation enhancements observed at the BTN station. They show the
expansion of the auroral oval with the geomagnetic activity since the initial
phase (13:53 UT) of the geomagnetic storm until the end of the main phase.
The image scan at the initial phase suggests Sun-aligned arcs in the
nightside polar cap that might be caused by electron precipitation fluxes
with energies of ∼3keV (Newell et al., 2009), which can
explain the scintillations observed at the BTN and DMC stations. The other image
scans suggest that the strong phase scintillations observed at BTN were produced
by irregularities in the auroral oval during its poleward expansion.
During the recovery phase of the geomagnetic storm on 27 September, the
auroral activity persisted but at lower levels (Fig. 1). The scintillation
indices show one significant enhancement at the BTN station between 15:00 and
18:00 UT (post-midnight sector), which occurred in close association with
auroral particle precipitation as evidenced by an increase in the HP POES index
and SSUSI image scans (not shown).
Discussion and conclusions
The ionospheric response to the moderately intense geomagnetic storm of
26–27 September 2011 was analyzed at high and middle latitudes in the American
and Australian sectors in Antarctica. The ionospheric response was observed on
the ground by multiple instruments, such as GNSS receivers, ionosonde,
riometers,
and magnetometers, which were complemented by an auroral imager on satellites
and ground high-frequency radars. The overall results show a complex ionospheric response
as a function of the local and storm time.
During the main phase of the geomagnetic storm on 26 September, the
observations show a strong positive phase of the ionospheric storm at middle
latitudes in the American sector where the geomagnetic storm onset occurred
near local noon. NmF2 shows a complex evolution with three
enhancements accompanied by F2 uplifts. The density enhancements observed at
∼17:00 and 20:00 UT occurred in association with peaks in the AE and
HP-POES indices (Fig. 1), which means close to episodes of energy input into
the polar region by energetic particle precipitation in the auroral oval. At
the EACF station, the enhancements were seen simultaneously in NmF2
and VTEC but with different amplitudes, which suggests that they were
produced by a combination of mechanisms. The positive phase of the
ionospheric storm, or storm-enhanced density (SED), observed during the
afternoon at middle latitudes can be attributed to a superposition of two
mechanisms. The first positive storm occurred just after geomagnetic storm
onset (∼16:00 UT, near local noon time), presenting a stronger
electron density increase above the F2-layer maximum. Thus it could be
attributed to the predominance of the PPEF effect at middle latitudes. The
analysis of the equatorial meridional flows observed on the dayside by the
CINDI instrument on the C/NOFS spacecraft showed
only excess downward flows at the
equatorial region after
17:00 UT, which were attributed to a combined effect of the overshielding
and disturbance dynamo processes (Hairston et al., 2014). This behavior could
be explained by the effect of the strong IMF By on the polar cap potential
pattern until 19:50 UT along with strong southward Bz (Thomas et al.,
2013), which reduces the influence of the PPEF at the Equator as suggested by
Mannucci et al. (2014). The SEDs observed after 19:00 UT (16:00 LT) might
be a combination of PPEF and storm-time convection effects that generate
equatorward winds, which lifted the maximum density layer to greater heights
at a time when electron production was still operating (e.g., Prolss, 1997;
Buonsanto, 1999). These equatorward winds might be associated with traveling
atmospheric disturbances (TADs; Prolss and Jung, 1978) generated by impulsive
auroral heating. After the dusk effect observed at middle latitude, the
ionospheric storm shows an abrupt electron density decrease that is probably
associated with the passage of a trough of ionization.
(a–d) GPS TEC maps of storm-enhanced density (SED)
episodes during the main phase of the geomagnetic storm with
SuperDARN convection patterns overlaid. (e–h) SuperDARN LOS
velocity measurements from ionospheric scatter for
selected radars. Figures are in magnetic latitude and MLT with
magnetic noon at the top for times 13:00, 16:40, 19:25,
and 20:50 UT. The stars mark the BTN (black) and DMC (red) station
locations.
At middle and high latitudes in the Australian sector where the main phase
of the geomagnetic storm started during the night and the dawn-to-dusk
electric fields are westward, the ionosphere shows only the negative phase
(density depletion). The density decrease started just at the onset of the main
phase storm almost simultaneously at stations in the auroral oval and polar
cap and about 3 h later at middle-latitude stations. The nighttime negative
phase at high and middle latitudes might be dominated by disturbance dynamo
processes due to thermosphere heating in the auroral region mainly produced by
the Joule dissipation of electric currents (e.g., Seaton, 1956; Prolss, 1995;
Buonsanto, 1999), which causes neutral composition changes. The negative phase
shows an intensification in the morning sector, probably due to an additional
influence of convection effects (Prolss, 1995). Superposed to the slow
negative phase observed at the BTN station is a fast VTEC increase at ∼23:00 UT, the time at which this station enters the dayside cusp region. At the polar
cap DMC station there are two fast VTEC increases at ∼17:00 and
19:00 UT. These times correspond to periods during which the IMF Bz is strongly
southward with IMF By>0 (duskward), which are favorable conditions for
polar TOI formation (Hosokawa et al., 2010; Thomas et al., 2013). So,
these fast VTEC increases might be TOIs originating from SED plumes formed at
middle latitudes in the dayside sector (Foster et al., 2004, 2005) that
entered the polar cap through the cusp.
To investigate SED–TOI formation over Antarctica, we considered GPS TEC
maps with SuperDARN convection patterns overlaid (Fig. 7). Despite the low
GPS network coverage in the Southern Hemisphere, the GPS TEC maps at ∼16:40, 19:30 and 20:50 UT (Fig. 7b–d) suggest SED plumes at middle
latitudes in the American sector (dayside). The SED plumes have a good
association with the middle-latitude TEC enhancements observed at EACF and
the fast ones observed at the polar cap DMC station (nightside), suggesting that they
enter the polar cap region trough the dayside cusp as a TOI. The strong
phase scintillations observed at the cusp/cap BTN and polar cap DMC stations
occurred in close association with the fast TEC enhancements and ionospheric
backscatter observed in the SuperDARN LOS velocity measurements (Fig. 7e–h)
taken in the polar cap region. The results show that during the main phase storm
the scintillations were collocated with antisunward convection and TOI
originating from dayside SED (Fig. 7f and g). Strong scintillations were also
observed at the BTN and DMC stations between ∼21:00 and 23:00 UT when the
IMF almost returned to quiet conditions with IMF Bz and By nearly
zero. They occurred in close association with TEC enhancement at the BTN station,
which enters the dayside cusp from which a TOI was drawn into the
polar cap, as confirmed by ionospheric backscatter observed in the polar cap
region by SuperDARN radars (not shown).
The ionospheric response in the Northern Hemisphere to this geomagnetic storm
was reported by Thomas et al. (2013), who show that a linked SED–TOI event
occurred between 18:30 and 19:40 UT and a lack of TOI associated with the
SED period from 20:30 to 24:00 UT. The SED–TOI event has a counterpart in
the Southern Hemisphere, but contrary to their SED results with no TOI, in Antarctica
we have a TOI formation. This difference in the ionospheric response can be
explained by interhemispheric asymmetry due to the high-latitude coupling
between solar wind and the magnetosphere. This asymmetry drives the dawn–dusk
component of the interplanetary magnetic field that defines the cusp location
and thus determines whether the storm-enhanced density plasma will enter the polar cap
and the orientation of the antisunward convection in the polar cap
relative to the noon–midnight meridian (Cherniak et al., 2015; Horvarth and
Lovell, 2015; Prikryl et al., 2013, 2015b).
During the initial phase of the geomagnetic storm, a group of GNSS
scintillations was observed at the polar cap DMC station between ∼12:40 and
15:00 UT, which also has an ionospheric backscatter counterpart in polar cap
region (Fig. 7e). Since they occurred under northward IMF Bz conditions,
these scintillations might be produced by polar cap irregularities associated
with transpolar Sun-aligned arcs, as evidenced by the SSUSI (DMSP F18) auroral
image over Antarctica obtained at ∼13:53 UT (Fig. 6a), which is in
agreement with Newell et al. (2009) and Prikryl et al. (2015a, b).
A schematic of the ionospheric processes associated with this moderately
strong geomagnetic storm at middle and high latitudes in the Southern
Hemisphere is as follows: during the main phase of the geomagnetic storm, the PPEF
penetrates both the dayside and nightside ionosphere. On the dayside, the
normal eastward dawn-to-dusk electric field is reinforced by the PPEF
lifting the equatorial ionospheric plasma to higher altitudes
and latitudes, forming the so-called dayside ionospheric superfountain (DIS)
effect (Tsurutani et al.,
2004). The DIS results in an overall dayside ionospheric electron density
increase with EIA crests reaching midlatitudes, which might account for the
stronger TEC enhancement compared to the NmF2 density increase
observed at the middle-latitude stations PST, EACF, and SMA during the
first 2
hours of the main phase geomagnetic storm. In the following, the TEC and
NmF2 show similar increases, suggesting that the positive storm at
midlatitudes was dominated by the disturbance dynamo processes up to the end of
the main phase storm. On the nightside during the main phase storm, from middle to
high latitudes the westward dawn-to-dusk electric fields are dominated by the
dynamo disturbance, causing a downward plasma drift, which increases the
recombination and decreases the electron density.
In conclusion, this paper shows the middle- and high-latitude ionospheric
response during the 26–27 September 2011 moderately intense geomagnetic
storm in Antarctica. The multi-instrument observations permitted us to
characterize the ionospheric response as a function of storm time and
location in the American and Australian sectors in Antarctica and compare
these results with the ones reported in the Northern Hemisphere to
complete the picture of this storm in the interhemispheric context. The
analysis shows that the ionosphere was highly structured and dynamic as
a consequence of solar wind coupling with the magnetosphere–ionospheric
system, suggesting a combination of effects associated with PPEFs
and disturbance dynamo processes. Storm-density enhancements (SEDs) are
observed at middle latitudes in the dayside sector just after the onset of the
main phase storm, suggesting that they are due to the influence of electrodynamical
processes associated with PPEF; however, after a couple of hours they might be
dominated by storm-time disturbance dynamo processes. The depletion density
observed from middle to high latitudes in the nightside sector are due to the
westward direction of the dawn-to-dusk electric fields, which cause
the ionospheric plasma to drift downward, increasing the recombination processes. The
ionospheric irregularities that are responsible for the strong GNSS
scintillations and ionospheric backscatter were observed (a) during the
initial phase of the geomagnetic storm in association with transpolar Sun-aligned
arcs and (b) during the main phase storm collocated with SED plumes at middle
latitudes and cusp on the dayside, with the auroral oval during energetic
particle precipitation events, and with the polar cap TOI features observed on the
nightside under different periods of IMF conditions. Two SED–TOI formations
were observed during the main phase storm in association with disturbance dynamo
processes at high latitudes in Antarctica. The second one, which occurred at
the end of the main phase storm, has no counterpart in the geomagnetically
conjugate location in the Northern Hemisphere, which can be attributed to
interhemispheric asymmetry due to the IMF dawn–dusk component. In the Southern
Hemisphere the IMF was in the dawnward direction, which drives the SED plasma in
the cusp direction, forming the TOI in the polar cap. In the Northern
Hemisphere it was in the duskward direction, which drives the SED away from the
cusp and no polar TOI is formed. Thus, the formation of polar TOI is
a function of the SED plume location and local electric field action, which
might be nearby and in the direction of the dayside cusp from which it can
enter the polar cap.
The solar wind and interplanetary magnetic field (IMF) data
were obtained from the ACE/SWEPAM and OMNIWeb data services
(http://www.srl.caltech.edu/ACE/ASC/level2/lvl2DATA_MAG-SWEPAM.html,
http://omniweb.gsfc.nasa.gov). The geomagnetic indices were obtained
from the World Data Center for Geomagnetism, Kyoto
(http://wdc.kugi.kyoto-u.ac.jp/wdc/Sec3.html). The polar cap index is
derived using the magnetic data of Vostok (http://pcindex.org/about-3).
NOAA POES Hemispheric Power data was obtained from the Space Weather
Prediction Center of NOAA
(http://legacy-www.swpc.noaa.gov/ftpdir/lists/hpi). The ionospheric
parameters from ionosondes operating at the PST, HOB, and MCI stations were
obtained from the Space Physics Interactive Data Resource (SPIDR;
http://spidr.ionosonde.net/spidr). The cosmic noise absorption (CNA)
measured at Casey Station was obtained from the World Data Centre Space
Weather Services (SWS, formally known as IPS Radio and Space Services or IPS;
http://www.sws.bom.gov.au). The surface magnetic field was obtained
from the INTERMAGNET database (http://www.intermagnet.org; St-Louis et
al., 2012). The energy flux and mean energy of precipitated energetic
particles were obtained from the Special Sensor Ultraviolet Scanning Imager
(SSUSI) onboard the Defense Meteorological Satellite Program (DMSP)
satellites (https://ssusi.jhuapl.edu). The TEC maps were obtained using
the plotting tools available online at the Space@VT SuperDARN website
(http://vt.superdarn.org). The TEC data were downloaded through the
Madrigal database at Haystack Observatory. The GNSS and ionosonde data from
EACF station are available upon request from the corresponding author at
ecorreia@craam.mackenzie.br. The GNSS data from the DMC and BTN stations are
available upon request from the author Luca Spogli at luca.spogli@ingv.it,
and the ionosonde data from San Martin Base from the author Adriana Gulisano
are available at adrianagulisano@gmail.com.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Space weather
connections to near-Earth space and the atmosphere”. It is a result of the
6∘ Simpósio Brasileiro de Geofísica Espacial e Aeronomia
(SBGEA), Jataí, Brazil, 26–30 September 2016.
Acknowledgements
Emilia Correia thanks the National Council for Research and Development
(CNPq) for individual research support (process nos. 556872/2009-6, 406690/2013-8, and 306142/2013-9) and the National
Institute for Space Research (INPE/MCTI). The authors also
acknowledge the support of the Brazilian Ministry of Science,
Technology and Innovation (MCTI), the Ministry of the Environment (MMA), and
the Inter-Ministry Commission for Sea Resources (CIRM). This work
integrates the National Institute of Science and Technology
Antarctic Environmental Research (INCT-APA) under scientific
and financial support from the CNPq (process no. 574018/2008-5) and the Carlos Chagas Research Support Foundation
of the State of Rio de Janeiro (FAPERJ no. E-16/170.023/2008).
DMC and BTN data have been acquired in the framework of the projects
PNRA14_00133 and PNRA14_00110. The authors acknowledge
the GRAPE Expert Group endorsed by SCAR to facilitate the
collaboration (http://www.grape.scar.org/). The DMSP
particle detectors were designed by Dave Hardy of the Air Force
Research Laboratory, and the data were obtained from the Johns
Hopkins University Applied Research Laboratory (http://ssusi.jhuapl.edu). The
TEC maps were obtained using the
plotting tools available online at the Space@VT SuperDARN
website (http://vt.superdarn.org). The TEC data were
downloaded through the Madrigal database at Haystack Observatory.
The authors acknowledge the use of SuperDARN
data. SuperDARN is a collection of radars funded by the national scientific
funding agencies of Australia, Canada, China,
France, Japan, South Africa, the UK, and the United States of America. The authors
also thank the two anonymous referees for
their valuable comments and suggestions. The topical editor, Jean-Pierre Raulin, thanks two
anonymous referees for help in evaluating this paper.
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