ANGEOAnnales GeophysicaeANGEOAnn. Geophys.1432-0576Copernicus PublicationsGöttingen, Germany10.5194/angeo-35-923-2017Similarity and differences in morphology and mechanisms of the foF2 and TEC
disturbances during the geomagnetic storms on 26–30 September 2011KlimenkoMaxim V.maksim.klimenko@mail.ruhttps://orcid.org/0000-0002-7103-6612KlimenkoVladimir V.ZakharenkovaIrina E.RatovskyKonstantin G.KorenkovaNina A.YasyukevichYury V.https://orcid.org/0000-0002-3098-224XMylnikovaAnna A.CherniakIurii V.West Department of Pushkov IZMIRAN, RAS, Kaliningrad, RussiaImmanuel Kant Baltic Federal University, Kaliningrad, RussiaInstitute of Solar-Terrestrial Physics, SB RAS, Irkutsk, RussiaUniversity of Warmia and Mazury, Olsztyn, PolandMaxim V. Klimenko (maksim.klimenko@mail.ru)9August201735492393829July201618June201726June2017This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/This article is available from https://angeo.copernicus.org/articles/35/923/2017/angeo-35-923-2017.htmlThe full text article is available as a PDF file from https://angeo.copernicus.org/articles/35/923/2017/angeo-35-923-2017.pdf
This study presents an analysis of the ground-based observations and model
simulations of ionospheric electron density disturbances at three
longitudinal sectors (eastern European, Siberian and American) during
geomagnetic storms that occurred on 26–30 September 2011. We use the Global
Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM
TIP) to reveal the main mechanisms influencing the storm-time behavior of the
total electron content (TEC) and the ionospheric F2 peak critical frequency
(foF2) during different phases of geomagnetic storms. During the
storm's main phase the long-lasting positive disturbances in TEC and
foF2 at sunlit mid-latitudes are mainly explained by the storm-time
equatorward neutral wind. The effects of eastward electric field can only
explain the positive ionospheric storm in the first few hours of the initial
storm phase. During the main phase the ionosphere was more changeable than
the plasmasphere. The positive disturbances in the electron content at the
plasmaspheric heights (800–20 000 km) at high latitudes can appear
simultaneously with the negative disturbances in TEC and foF2. The
daytime positive disturbances in foF2 and TEC occurred at middle and
low latitudes and at the Equator due to n(O) /n(N2) enhancement
during later stage of the main phase and during the recovery phase of the
geomagnetic storm. The plasma tube diffusional depletion and negative
disturbances in electron and neutral temperature were the main formation
mechanisms of the simultaneous formation of the positive disturbances in
foF2 and negative disturbances in TEC at low latitudes during the
storm's recovery phase.
Ionosphere (ionospheric disturbances; modelling and forecasting; general or miscellaneous)Introduction
The way the ionosphere F region and total electron content (TEC)
respond to geomagnetic storms is one of the most essential and unresolved
issues in ionospheric physics and has been widely discussed for decades (Mayr and
Volland, 1973; Mayr et al., 1978; Prölss, 1995, 2013; Schunk and Sojka,
1996; Buonsanto, 1999; Schunk and Nagy, 2000; Mendillo, 2006). Theoretical studies have been performed using different numerical models of
the ionosphere (Pirog et al., 2006; Balan et al., 2009, 2010; Huba et al.,
2000; Pavlov and Pavlova, 2011), as well as more complex models of the
Earth's upper atmosphere (Sojka et al., 1994; Namgaladze et al., 2000;
Fuller-Rowell et al., 2007; Lei et al., 2008; Lu et al., 2008; Pawlowski et
al., 2008; Klimenko et al., 2011a–c). These studies have greatly increased
our understanding of the formation mechanisms of the ionospheric response to
geomagnetic storms. Despite undeniable progress in this research direction,
there are still many open questions and contradictions, in particular
understanding the mechanisms of the positive ionospheric storm formation in
the ionospheric F region and total electron content (TEC). During
geomagnetic storms, there occurs heating of the thermosphere caused by the
strengthening of the high-latitude ionospheric electric fields, currents and
auroral particle precipitation. This heating leads to the appearance of
additional equatorward neutral wind (Mayr et al., 1978). The equatorward
neutral wind moves the plasma upward (due to ion–neutral collisions) along
the inclined geomagnetic field lines at low and mid-latitudes into the
regions with lower chemical loss rates in ion–molecular reactions; this
results in an increase in F-region electron density (Rishbeth and Garriott,
1969; Mayr et al., 1978; Prölss, 1995). In the vicinity of the Equatorial
Ionization Anomaly (EIA), the equatorward neutral wind reduces the downward
plasma diffusion along the geomagnetic field lines that also produces
positive disturbances in the electron density at the vicinity of the
geomagnetic equator. As shown by Rishbeth and Garriott (1969) the occurrence
of the additional eastward electric field leads to the additional
electromagnetic drift with poleward direction in the plane of the geomagnetic
meridian and to the upward plasma transport into the region of lesser
chemical loss rate, which leads to the positive effects in the ionospheric
F-region electron density at low and middle latitudes. Daytime eastward
electric field in the vicinity of the geomagnetic equator produces an
increase in the electron density at the EIA crests, their shift to higher
latitudes, and the deepening of the EIA trough at the geomagnetic equator.
Mannucci et al. (2005) demonstrated that an increase in TEC at
middle latitudes is accompanied by the EIA intensification, and suggested
that this is due to the vertical and horizontal transport of ionospheric
plasma. Heelis et al. (2009) concluded that an equatorward expansion of the
high-latitude convection pattern resulted in the TEC enhancements at
middle and low latitudes.
Using model simulation results Namgaladze et al. (2000), Lu et al. (2008),
Balan et al. (2009, 2010) and Klimenko et al. (2011c, 2015a) confirmed the
Mayr et al. (1978) theory, reporting that the equatorward neutral wind is
required to produce positive ionospheric storms at middle and low latitudes
during daytime hours. Considerable debate on mechanisms of the positive
ionospheric storms (Rishbeth et al., 2010; Heelis et al., 2013; Tsurutani et
al., 2013) has not ended with a single point of view on this issue. According
to Tsurutani et al. (2013), in order to explain the positive ionospheric storms, it is necessary to have the combined action of prompt penetration magnetospheric convection
electric fields to low latitudes, equatorward expansion of the affected area
of magnetospheric convection to lower latitudes and neutral wind
disturbances. It is obvious that such a formulation requires further
clarification. It is necessary to determine the spatial and temporal
boundaries of the discussed mechanisms' impact on the formation of positive
ionospheric storms, as well as other mechanisms such as neutral composition
change and ionosphere–plasmasphere interaction.
Behavior of the Dst, Kp, AE and
AL indices of geomagnetic activity during disturbed periods from
24 to 30 September 2011.
Only few studies were published on another important issue – correlations
between storm-time disturbances of the F2 layer peak critical frequency,
foF2 (which is related to the F2 layer peak electron density,
NmF2 m-3= 1.24 × 1010
(foF2 MHz)2), and TEC (e.g., Maruyama et al., 2004; Wang et
al., 2013; Liu et al., 2016). TEC and foF2 are the most important
and useful parameters in the ionospheric variability studies and various
applications. Nowadays the dense networks of the Global Positioning System
(GPS) receivers and ionosondes allow the simultaneous coverage of TEC and
foF2 values at different sites. One of the main limitations of the
GPS technique is that the value of GPS TEC has an integral character, and it
is difficult to determine precisely the ionospheric contribution to the GPS
TEC based on the GPS measurements only. Ground-based ionosondes provide
observations of the electron density for altitudes below the F2 layer peak.
Generally, it is assumed that TEC variability can be represented by
foF2 (NmF2) variability. This assumption is based on the
arguments that TEC and foF2 are highly correlated (Liu et al., 1996)
and the electron density at plasmasphere is several orders of magnitude lower
than the F-region electron density (Gallagher et al., 2000). However, recent
studies demonstrate that (1) NmF2 and TEC behavior can be
significantly different during a geomagnetic storm especially at a recovery
phase (Cherniak et al., 2014), (2) the contribution of the topside ionosphere
and plasmasphere to TEC results in a shift to earlier hours and weakening of
the Mid-latitude Summer Evening Anomaly in TEC as compared to one in
NmF2 (Klimenko et al., 2015b), and (3) sometimes the regions
above the F2 layer peak height provide the largest contribution to TEC
(Afraimovich et al., 2011; Klimenko et al., 2015c). This effect is even more
pronounced during nighttime at the solar activity minimum, where the
plasmaspheric contribution to TEC can exceed the ionospheric one (Lunt et
al., 1999a, b; Cherniak et al., 2012; Klimenko et al., 2015c). In fact, the
TEC variability depends on the lower and topside ionosphere as well as the
plasmasphere (Balan et al., 2002; Gulyaeva and Gallagher, 2007; Yizengaw et
al., 2008; Cherniak et al., 2012; Lee et al., 2013; Zakharenkova et al.,
2013; Klimenko et al., 2015b, c; Lei et al., 2015). Previous studies have
demonstrated both the positive correlation between TEC and foF2 at
high and middle latitudes during many storm events and no evident correlation
between TEC and foF2 at low and middle latitudes during some storms
(Maruyama et al., 2004; Liu et al., 2012). Recent studies (Astafyeva et al.,
2015; Lei et al., 2015; Liu et al. 2015) demonstrate that the topside and
bottomside ionosphere can react with opposite sign to a geomagnetic storm,
which complicates the understanding of how these parts can finally contribute
to the storm-time TEC variation. Several important issues regarding the
relationship between foF2 and TEC disturbances are still unresolved:
(1) what is the global relationship between foF2 and TEC response to
geomagnetic storms, and (2) how does this relationship vary with time, storm
phase, longitude and latitude? It is important to note that only a limited
number of first-principle models allow for this problem to be investigated,
because the majority of the developed models have an upper boundary much
lower than the GPS satellite orbits (Roble and Ridley, 1994; Jin et al.,
2012).
The thermospheric heating at high latitudes during geomagnetic storms
increases the scale height of all neutral species including molecular
nitrogen, which in turn leads to a decrease in the n(O) /n(N2)
ratio at heights of the ionospheric F region (Mayr et al., 1978). As atomic
oxygen is the primary source of ionization at the F-region heights, and
molecular nitrogen is the primary source of recombination, the change of the
n(O) /n(N2) ratio controls the daytime electron density. The
daytime reduction of this ratio leads to the negative disturbances in
electron density at the ionospheric F-region heights (Rishbeth and Garriott,
1969). However, at night, when the main ionization source (solar radiation) is
almost absent, molecular nitrogen plays a dominant role in the electron
density control by neutral composition changes. The additional equatorward
wind formed by the same thermospheric heating also leads to the transport of atomic oxygen
toward the middle and equatorial latitudes with velocity much
greater than the transport velocity of the molecular thermospheric species
(Mayr et al., 1978). This brings us to the following question: what happens in the
thermosphere–ionosphere system during 1–2 days after the action of the
storm-time high-latitudinal thermospheric heating source? The ionospheric
F-region disturbances during the recovery phase of geomagnetic storms are one
of the most unexamined issues on the topic of the ionosphere's response to a
geomagnetic storm. This issue has only been broadly discussed in recent years
(Balan et al., 2013; Suvorova et al., 2013; Klimenko et al., 2015a). The
problem is very important in terms of understanding the interrelated
processes in the upper atmosphere, and it is the key problem for the selection of background
values to study the ionospheric disturbance effects of different
origin. Therefore, it is necessary to carry out a study of the temporal
development of spatial distribution of the ionospheric disturbances during
the recovery phase of geomagnetic storms and to clarify the impact of the
neutral composition changes and ionosphere–plasmasphere connections to the
formation of such ionospheric effects. Here, by making use of both
observation and model simulation results, we present a comprehensive study of
the ionospheric disturbances in the ionospheric F2 peak and TEC
during the main and recovery phases of the geomagnetic storms on
26–30 September 2011.
Geomagnetic storm description
We analyze the ionospheric response to geomagnetic storms occurred on
26–29 September 2011. Figure 1 illustrates the geomagnetic conditions in
late September 2011. The intense geomagnetic storm started with a storm
sudden commencement (SSC) at 12:35 UT on 26 September 2011 and reached a
minimum Dst of -100 nT at 24:00 UT on 26 September. A rapid decrease in
the Dst index occurred during the main storm phase. Further, a moderate
geomagnetic storm occurred with SSC at 21:00 UT on 27 September, while the
Dst index reached its minimum of -60 nT at 08:00 UT on 28 September 2011.
The Kp and auroral electrojet (AE) indices showed high values for the whole
period of 26–29 September 2011. The AE index reached the maximum value of
2000 nT at ∼ 19:30 UT on 26 September and Kp did not exceed 6. This
event occurred during the ascending phase of the 24th solar cycle with the
F10.7 index, varying from 133.4 up to 148.2. Ionospheric response to this
geomagnetic storm event was studied by Hairston et al. (2013), Wang et
al. (2013), Kotova et al. (2015), Klimenko et al. (2015a), Solomentsev et
al. (2015), and Chen et al. (2016).
Observation data
We analyze the database constructed of the F2 peak critical frequency
(foF2) and the total electron content values (TEC) derived
from the Irkutsk (52.3∘ N, 104.3∘ E) and Kaliningrad
(54.6∘ N, 20.0∘ E) ionosondes and co-located GPS receivers.
The foF2 values were derived from the manually scaled ionograms
using the interactive ionogram scaling software SAO Explorer (Khmyrov et
al., 2008) in the case of the Irkutsk ionosonde and the PARUS software
(Karpenko and Manaenkova, 1996) for the Kaliningrad ionosonde. The
foF2 sampling rate was 15 min for Irkutsk and 1 h for Kaliningrad.
As a reference of the ionospheric parameters over these ionospheric stations
we calculated their median values and interquartile range from all
geomagnetically quiet days in the range of ±13 days with respect to
26 September 2011.
The diurnal GPS TEC variations over Irkutsk and Kaliningrad were derived from
the raw observations provided by the ground-based GPS stations. Also, in the
analysis we involved the GPS TEC observations derived from the IGS global
ionospheric maps (GIMs) generated on the basis of the worldwide network of
ground-based GNSS receivers (e.g., Hernández-Pajares et al., 1999). GIMs
are produced and released independently by several IGS centers (e.g., CODE,
ESAG, JPLG, EMRG) with different set of stations and algorithms. Here we used
the IGSG final product, which is a combined map generated from all GIMs. The
spatial range of GIMs in standard IONEX format
(ftp://cddis.gsfc.nasa.gov/pub/gps/products/ionex/) is from 0 to
360∘ in longitude and from -87.5 to 87.5∘ in latitude;
dimensions of the elementary GIM cell are 5∘ in longitude and
2.5∘ in latitude. We extract TEC data from GIMs and generate the
daily files of TEC latitudinal profiles for three specific geographic
longitudes (105, 15∘ E and 75∘ W) that represent the
closest longitudes to Irkutsk (the east Siberian sector) and Kaliningrad (the
European sector) and the American longitudinal sector. Here, we chose the TEC
variation for the previous quiet day of 24 September 2011 as a reference
value. Comparison of the TEC variability during the quiet day of
24 September 2011 and disturbed period of 26–30 September 2011 revealed the
TEC disturbance evolution with time over the selected longitudes.
Brief description of GSM TIP and statement of the problem
The Global Self-consistent Model of the Thermosphere, Ionosphere, and
Protonosphere (GSM TIP) (Namgaladze et al., 1988; Korenkov et al., 1998) was
developed in the WD IZMIRAN (West Department of Pushkov Institute of
Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Russian
Academy of Sciences). This model calculates time-dependent global
three-dimensional distributions of temperature, composition and velocity
vector of neutral gas; density, temperature, and velocity vectors of atomic
and molecular ions and electrons; and two-dimensional distribution of
electric potential, both of a dynamo and magnetospheric origin. All model
equations are solved by the finite-difference method. The Earth's magnetic
field is approximated by the tilted dipole. Thus, the discrepancy between the
geographical and geomagnetic axes is taken into account. Klimenko et
al. (2006, 2007) have recently modified the calculation of electric fields.
This modification to the GSM TIP model allow us to investigate more correctly
the equatorial ionosphere (Klimenko et al., 2011b, c, 2012). The modified GSM
TIP model has been already used to study the ionospheric behavior during
geomagnetic storms from 2000 till 2011 (Klimenko et al., 2011a–c, 2015a;
Klimenko and Klimenko, 2012), and we obtained the following most important
results: (1) we explained the F3 layer formation mechanism and multi-layer
structure in the equatorial F region during geomagnetic storms; (2) we
correctly revealed the ionospheric effects of the disturbance dynamo electric
field, the prompt penetration magnetospheric convection electric field to
mid- and low latitudes and overshielding effects; and (3) we investigated the
formation mechanisms of the positive and negative ionospheric storms. The
comparison of the model results of various ionospheric parameters with
observations in high-, mid- and low-latitude ionosphere presented in these
articles revealed satisfactory qualitative and sometimes quantitative
agreement.
To calculate the cross-polar cap potential difference ΔΦ we set
ΔΦ at geomagnetic latitudes ±75∘ according to
(Feshchenko and Maltsev, 2003) we used the expression ΔΦ= 38 + 0.089 × AE (kV). The changes of the polar cap sizes
were not taken into account. Using experimental results of (Snekvik et al.,
2007; Cheng et al., 2008), we have constructed the empirical dependences of
the R2 FAC (region 2 field-aligned current) amplitudes from the AE index of
geomagnetic activity:j2 (A m-2)=3× 10-8+
1.2 × 10-10× AE. We have also included the 30 min
time delay of the R2 FAC variations with respect to the variations in
cross-polar cap potential difference (Kikuchi et al., 2008). In addition,
according to Sojka et al. (1994) we varied the position of the R2 FAC maximum
depending on changes of a cross-polar cap potential difference:
±65∘ for ΔΦ≤ 40 kV; ±60∘ for
40 kV < ΔΦ≤ 50 kV; ±55∘ for
50 kV < ΔΦ≤ 88.5 kV; ±50∘ for
88.5 kV < ΔΦ≤ 127 kV; ±45∘ for
127 kV < ΔΦ≤ 165.4 kV; ±40∘ for
165.4 kV < ΔΦ≤ 200 kV; ±35∘ for
ΔΦ>200 kV.
Variations in the foF2 (red) above Irkutsk and
Kaliningrad during 26–30 September 2011: (a) the GSM TIP
calculation results and (b) ionosonde data. Blue shows the
quiet geomagnetic conditions, with 27-day median and interquartile range
bars for ionosonde observations.
In the given simulation we also used the Vorobjev and Yagodkina (2008)
empirical model for high-energy particle precipitation that was developed in
the Polar Geophysical Institute, Apatity, Russia. In this model, the energy
and energy flux of precipitating electrons depend on a 1 min AL index. The GSM TIP model input parameters, such as cross-polar
cap potential, location of the polar cap, values and position of the Region 2
field aligned currents and using the climatological model of particle
precipitation are not simple variables. These model input parameters have
complex spatial and temporal resolution. Also, the thermosphere–ionosphere system
is a very complex coupled medium. Therefore, it is a very challenging task to
select the spatial and temporal variations in the model input parameters for
any particular storm, and such a specific selection could provide improved model
simulation results only for this particular storm event. One of the main
goals of our paper is to demonstrate the possibility of general statement of
the problem of thermosphere–ionosphere response to geomagnetic storm using
the coupled thermosphere–ionosphere–electrodynamics model (GSM TIP) with
input parameters from different empirical models. Such a statement of the
problem has been continuously improved by our group over the last 6 years (Klimenko et
al., 2011a, c, 2015a; Bessarab et al., 2015; Suvorova et al., 2016; Dmitriev
et al., 2017) and may be used for different geomagnetic storm cases.
In this paper we used the described statement of the problem for
interpretation of the observed ionospheric disturbances during geomagnetic
storms on 26–30 September 2011. As a reference level for all thermospheric
and ionospheric parameters we selected calculation results for the quiet day
of 24 September 2011, when low geomagnetic activity was observed. Solar
activity during that day was the same as for the entire period.
The same as in Fig. 2, but for the TEC calculated from the GSM TIP
and derived from the GPS receiver.
Results
We have analyzed the ionosphere response to the geomagnetic storms of
26–30 September 2011 using the GSM TIP simulations with the same model input
parameters (Klimenko et al., 2015a; Kotova et al., 2015). Using the Irkutsk
ionosonde data, Klimenko et al. (2015a) reported an occurrence of the
positive effects in electron density at midlatitudes during the storm
recovery phase. These effects were caused by an increase in the
n(O) /n(N2) ratio. We have also demonstrated that the model
simulation results can also be applied for the medium description in the
radio wave propagation tasks. Kotova et al. (2015) present model results of
ray tracing changes during this storm event using the GSM TIP and IRI models.
Here, we consider the latitudinal distribution of the storm-time
foF2 and TEC effects along three different longitudes.
Special attention is paid to the latitudinal extent and temporal evolution
of the positive effects in foF2 and TEC during the recovery phase of
these geomagnetic storms.
Behavior of the GPS TEC disturbances, and the GSM TIP
model-derived disturbances in TEC and foF2 during
geomagnetic storms on 26–30 September 2011 at the longitude of
105∘ E (a), 15∘ E (b), and 75∘ W
(c).
Figure 2 shows the foF2 variations over Irkutsk and Kaliningrad
stations during 26–30 September 2011 as deduced from the vertical sounding
data and the GSM TIP model simulation. Over Irkutsk during the main phase of
the geomagnetic storm on 26 September 2011, the positive foF2
disturbances occurred in the evening sector. These effects were further
replaced by the negative disturbances at night. The negative effects in
foF2 occurred on the recovery phase of geomagnetic disturbances, on
28 September, whereas on 29–30 September they are replaced by the daytime
positive and nighttime negative disturbances in foF2. It is clear
that the largest daytime positive disturbance on 29 September exceeds
∼ 3 times the ordinary day-to-day variation. Based on the satisfactory
qualitative agreement between the simulation results and observations, we are
able to explore the formation mechanisms of the ionospheric disturbances
during the main phase of the geomagnetic storm on 26 September and recovery
phases of geomagnetic disturbances on 29–30 September. Based on the results
over Irkutsk, Klimenko et al. (2015a) concluded that (1) the main mechanism of
the positive disturbances in foF2 in sunlit hours during the main
phase is the storm-time equatorward wind that pushes plasma upward to the
higher altitudes of slow molecular recombination (this result confirmed the
theory of Mayr et al., 1978); (2) after the positive effects in foF2
during the storm's main phase, the plasmaspheric flux tubes are depleted at
the middle latitudes due to changes in the neutral composition of the
thermosphere and expansion of the magnetospheric convection to middle
latitudes, which is a common feature for all storms (Schunk and Nagy, 2000);
(3) the positive daytime disturbances in foF2 during the storm's
recovery phase are formed by increasing n(O) /n(N2) (this is
new result that has never been discussed); and (4) negative nighttime effects in
foF2 are associated with underfilling flux tubes depleted during the
main phase (Carpenter and Park, 1973; Krinberg and Tashchilin, 1982), which
leads to a decrease in plasma flows from the plasmasphere to support the
nighttime ionosphere. If, in general, the behavior of the foF2
disturbances in model simulations and observational data are in rather good
agreement, the disturbance magnitude in observations will be much larger than in
the model, particularly for the case of the negative perturbation on
27 September 2011. According to the model calculations for this day, there is
no sharp decrease in the critical frequency during the main phase on
27 September, which was recorded in observations. In our opinion, significant
negative disturbances in foF2 observed at middle latitudes during
daytime hours are related mainly to ion losses due to recombination caused
by the neutral atmosphere composition changes associated with heating of the
high-latitude thermosphere during geomagnetic storms. It is very important to
highlight that the differences in foF2 disturbances over Kaliningrad
and Irkutsk are qualitatively reproduced by GSM TIP model results: the
presence of only negative foF2 disturbances over Kaliningrad during
28–30 September in comparison to daytime positive and nighttime negative
disturbances in foF2 over Irkutsk during this period. Such a
longitudinal difference may be explained by several factors. Firstly, there
is a difference in local time between Kaliningrad and Irkutsk during
different storm phases. Secondly, Kaliningrad is located at a higher
geomagnetic latitude than Irkutsk, which means a closer position of Kaliningrad
to a heating source at the auroral region during geomagnetic storms.
Figure 3 shows the diurnal TEC variations over Irkutsk and
Kaliningrad in quiet conditions and during the geomagnetic storm. These
results were derived from the GSM TIP model simulations and from the GPS
signal measurements of the GPS stations in Irkutsk and Kaliningrad.
Comparison of Figs. 2 and 3 leads to the following conclusions: (1) the
disturbances in foF2 and TEC have the same sign during the
main phases on 26 and 27 September and at nighttime during the recovery
phase, and (2) the largest differences between the disturbances in foF2
and TEC were observed in daytime during the recovery phase. During
the main phase of the storm on 26 September 2011, the behavior of the critical
frequency foF2 over Irkutsk, as well as TEC, demonstrated a
clear negative effect, which was more pronounced in foF2 than in
TEC – i.e., during the main phase the midlatitude ionosphere was more
changeable than the plasmasphere. In contrast the behavior of foF2
and TEC over Kaliningrad demonstrated a clear positive effect during
the main phase of the storm on 26 September 2011. These disturbances were
more pronounced in TEC than in foF2 – i.e., the sub-auroral
ionosphere is less changeable in comparison with the plasmasphere during the
main phase. During the recovery phase the TEC perturbations were
always negative, even in the daytime, while the daytime positive disturbances
appear in foF2 over Irkutsk. However, from day to day this negative
effect in the TEC values decreases with an increase in the positive
effect in foF2.
As seen from Figs. 2 and 3, the GSM TIP simulations agree qualitatively with
the observations both for Irkutsk and Kaliningrad. At the same time, the GSM
TIP noticeably underestimates the magnitude of negative disturbances. This
discrepancy is more pronounced for Kaliningrad, where the disturbances are
negative throughout the 27–30 September interval, in contrast to Irkutsk.
The greater model–data mismatch for Kaliningrad than for Irkutsk can be
explained by the higher geomagnetic latitude of Kaliningrad and therefore its
closer position to the high-latitudinal energy inputs from the magnetosphere.
Obviously, the model–data discrepancy is associated with an underestimation
of changes in the neutral composition of the thermosphere and smaller
expansion of the magnetospheric convection to middle latitudes. One of the
possible reasons of such underestimation may be insufficient heating of the
high-latitude thermosphere due to particle precipitation and Joule heating in
the model calculations. This could be due to the low precision and simplified
nature of the input parameters for the electric field calculation in the GSM
TIP model (cross-polar cap potential, location of the polar cap, values and
position of the region 2 field-aligned currents), using the climatological
model of particle precipitation and dipole geomagnetic field approximation in
the GSM TIP model.
Behavior of the GSM TIP model-derived disturbances in zonal electric
field, neutral temperature and meridional velocity of the thermospheric wind
(a), n(O), n(N2) and n(O) /n(N2)(b)
at the height of 300 km during geomagnetic storms on 26–30 September 2011
at the longitude of 105∘ E.
The same as in Fig. 5, but for 15∘ E longitude.
The same as in Fig. 5, but for the longitude of 75∘ W.
Figure 4 presents evolution of the storm-time disturbances in observations
and simulated results as a function of geographical latitude and time during
the geomagnetic storms on 26–30 September 2011. Here, we analyze the
meridional slices (latitudinal profiles) of the TEC disturbances
derived from the GPS observations (GIMs) and together with disturbances in
TEC and foF2 calculated by the GSM TIP model. The results
are shown at three specific longitudes of 105, 15∘ E, and
75∘ W, which represent the east Siberian, European and American
sectors, respectively. The model-calculated TEC disturbances in
general are qualitatively consistent with the GPS TEC observation.
Both model simulation results and observations reveal the negative TEC
disturbances propagating equatorward from high latitudes. This effect was
observed at all considered longitudes. The main difference between model
results and observation is the smaller values of negative TEC
disturbances in the model results, especially on 27 September 2011. The
reason for these discrepancies was discussed above. During the main phase of
the geomagnetic storm on 26 September we observed and reproduced the following using the GSM
TIP model: (1) the essential positive daytime disturbances in
TEC at the American longitudinal sector, (2) the pre-dusk
mid-latitude positive effects at 105∘ E are replaced by the strong
negative TEC disturbances almost at all latitudes, and (3) a number
of non-essential positive and negative disturbances at the European sector.
During 27–30 September 2011 the negative TEC disturbances occurred
at practically all longitudes and latitudes except for some areas of positive
daytime TEC disturbances near the Equator. These negative
TEC disturbances were greater at the American longitudinal sector
and were much smaller at 105∘ E, where the most pronounced
positive disturbances were observed. We found that the disturbances in
TEC and foF2 are similar, especially during the main phase
of geomagnetic storms. However, they are not correlated – at the same
latitude and time, the disturbances in TEC and foF2 may have
the different sign, especially at daytime during the recovery storm phase. We
should note the formation of the negative disturbances in TEC and
foF2 at high latitudes and the positive disturbances at lower
latitudes and at the Equator. An interesting feature of the storm-time
ionospheric effects in the vicinity of the geomagnetic equator is the
formation of positive daytime disturbances simultaneously in foF2
and TEC on 26 and 27 September. The positive daytime disturbances
were still observed in foF2 during 27–30 September, whereas the
negative disturbances occurred in TEC. In this case, the low- and
mid-latitude positive disturbances in foF2 span to a broader spatial
region in comparison to the positive disturbances in TEC. The
largest latitudinal extent of the positive disturbances in foF2
occurred at 105∘ E, and the smallest one at 75∘ W. It
should be emphasized that, during the recovery phase of the geomagnetic storms,
the positive disturbances in foF2 occur in the daytime mid-latitude
ionosphere, but at night the formation of the negative effects appears.
Figures 5, 6 and 7 present the latitudinal profiles of the GSM TIP simulation
results at three selected longitudes of 105, 15∘ E, and
75∘ W, respectively. They are disturbances in zonal electric field,
neutral temperature, meridional neutral wind, and thermospheric composition
(n(O), n(N2), n(O) /n(N2)) calculated for height of
∼ 300 km. The effects of eastward electric field can explain the
positive ionospheric disturbances in TEC and foF2 at the initial
storm phase on 26 September (during the first few hours). The maximal effects
of the positive ionospheric storm in foF2 and TEC occurred at
∼ 20:00 UT (8 h after the SSC) in the American sector, 75∘ W.
We explain these effects by means of the well-known chain of phenomena in the
thermosphere during storms (Mayr et al., 1978) and confirm model results
using the GSM TIP (Figs. 6 and 7). During geomagnetic storms the
strengthening of the high-latitude ionospheric electric fields, currents and
auroral particle precipitation causes thermospheric heating. This heating
drives an additional equatorward neutral wind that supports an increase in
the F-region electron density at middle and low latitudes. These effects of
the neutral wind at different latitudes can explain the formation of the
long-lasting positive ionospheric storm. At the same time, the significant in
the neutral atmosphere composition change inhibit the formation of the
positive disturbances in foF2 and TEC.
Behavior of the GSM TIP model-derived PEC (electron content
in the altitudinal range 800–20 000 km) disturbances during geomagnetic
storms on 26–30 September 2011 at the longitude of 105∘ E
(a), 15∘ E (b), and 75∘ W (c).
Behavior of the GSM TIP model-derived disturbances in the electron
temperature, Te, at the height of 300 km (b) and
10 000 km (a) during geomagnetic storms on 26–30 September 2011
at the longitude of 75∘ W.
The thermospheric heating at high latitudes increases the scale height of all
neutral species including molecular nitrogen, which in turn leads to a
decrease in the n(O) /n(N2) ratio and consequently electron
density at the ionospheric F-region heights. The additional equatorward
wind driven by the same thermospheric heating also leads to the atomic oxygen
transport toward the middle and equatorial latitudes with greater velocity
than the transport velocity of the molecular thermospheric species. During
the equinox conditions, this atomic oxygen transport happens in a similar
manner in the Northern and Southern Hemisphere. At the F-region heights,
atomic oxygen is the dominant neutral component. Therefore, this process
leads not only to an increase in n(O) at middle and low latitudes but also
to an increase in the total neutral density in these spatial areas. This in
turn, in the presence of weakly varying daytime source of the neutral
heating, should lead to a neutral cooling at middle and low latitudes, and
consequently to the reduction of the scale height of the neutral species,
including molecular nitrogen. This is the main reason for the decrease in
n(N2) at low latitudes during the recovery phase of geomagnetic storm
on 28, 29, and 30 September. The increase in n(O) and decrease in
n(N2) lead to a significant enhancement in n(O) /n(N2)
ratio at low and equatorial latitudes. These results contradict the mechanism
proposed by Lynn et al. (2004), in which the negative disturbances in
n(O) /n(N2) ratio are transported from the high latitudes to
the Equator. Such changes in the neutral atmosphere composition, in our
opinion, are the main source for the daytime positive disturbances in
foF2 at low latitudes during the recovery phase of this storm
(27–30 September). These results are confirmed by the comparison of the
disturbances in n(O) /n(N2) and foF2 at different
longitudes. It is evident that the largest area of positive
n(O) /n(N2) occurs at 105∘ E, which is in agreement
with the largest positive disturbances in foF2. For this particular
storm we did not find the essential electric field disturbances during the
recovery storm phases. Such results may be explained by the moderate level of
geomagnetic activity during considered geomagnetic storms, which leads to the
non-essential disturbances in wind velocity at the lower thermosphere, which
is the main process in formation of disturbance dynamo electric field.
The mechanical effects of neutral wind lead to (1) reduction (or termination)
in the downward plasma diffusion along the geomagnetic field lines and
(2) uplifting of the ionosphere to higher altitudes with reduced chemical loss
rates and, hence, plasma accumulation at heights near and above the
ionospheric F2 peak centered at around ±30∘ magnetic latitude.
According to the GSM TIP model results, the prompt penetration of eastward
electric field dominates in producing the positive ionospheric storm only
after storm onset, and equatorward neutral wind dominates during the main
phase and several hours after that. However, we conclude that during the
recovery phase the neutral composition changes play the most important role
in the daytime electron density increase at low latitudes.
Under nighttime conditions, the main source (solar radiation) of heating and
ionization at low latitudes is absent; therefore, (1) an increase in n(O)
at the F-region heights by itself does not lead to an increase in electron
density, and (2) an increase in total neutral density due to increase in n(O)
at the F-region heights at low latitudes does not lead to additional
cooling, hence, it does not change n(N2) and, as a consequence, to the
chemical loss rates in ion–molecular reactions. This can explain the absence
of the storm-time positive perturbations in foF2 and TEC at
nighttime due to changes in the neutral atmosphere composition at the
ionospheric F-region heights. Also, at night the heating in auroral region
is transported equatorward from high latitudes. This heating leads to an
increase in the N2 scale height, and hence to an increase in
n(N2) at heights of the ionospheric F region, which leads to the
nighttime negative disturbances in foF2 and TEC.
The simultaneous formation of the positive disturbances in foF2 and
negative disturbances in TEC at low latitudes on
27–30 September 2011 could be explained in the following way. It is known
that geomagnetic storms lead to the depletion of the plasma tubes (tubes that
are formed by geomagnetic field lines, filled by thermal plasma) (Carpenter
and Park, 1973). When geomagnetic activity increases, the plasma tubes
deplete. During the recovery phase of the geomagnetic storm, the refilling of
plasma tubes originates from the ionospheric source (Carpenter and Park,
1973; Bailey and Moffett, 1978; Krinberg and Tashchilin, 1982). The level of
depletion depends on the geomagnetic storms intensity. The time of plasma
tubes refilling depends on their volume (Bailey and Moffett, 1978; Krinberg
and Tashchilin, 1982). The time for the tube refilling by plasma increases
with tube volume and can take several days or even tens of days (Krinberg
and Tashchilin, 1982; Rasmussen et al., 1993; Krall and Huba, 2013). The
near-equatorial plasma tubes refilled much faster due to their smaller
volume. With an increase in the distance from the Equator the degree of
filling of the plasma tubes decreases. The vertical TEC is an
integral/sum of the electron density for all altitudes up to 20 000 km.
Plasma tubes with bases at different geomagnetic latitudes contribute to
the electron density at different altitudes above the considered location.
Plasma tubes with the base at a higher geomagnetic latitude cross higher
altitudes. Thus, during the recovery phase of geomagnetic storms the main
contribution to TEC at lower altitudes is provided by more filled
plasma tubes with bases at lower latitudes. With a height increase the plasma
tubes filling level is reduced, which leads to a reduction in the contribution
of these tubes into TEC as compared to the values of the quiet-time
conditions. During recovery phases, the mechanism of the plasma tube
diffusional depletion becomes the main mechanism in the formation of the
negative perturbations in TEC. As shown above, there are three
mechanisms – equatorward wind, eastward electric field, and an increase in
the n(O) /n(N2) ratio – which lead to the positive
disturbances in foF2. At the same time, the perturbations in
TEC are caused by the same mechanisms that counteract the mechanism
of the plasma tubes diffusional depletion caused by the decrease in the
n(O) /n(N2) ratio at high and middle latitudes. This is the
reason for the differences between foF2 and TEC disturbances
during the geomagnetic storm.
Figure 8 shows the behavior of the model-derived disturbances in vertical
electron content at heights of plasmasphere (from 800 to 20 000 km),
PEC (plasmaspheric electron content), as a function of geographic latitude
and time during geomagnetic storms on 26–30 September 2011. Meridional
slices are constructed for three specific longitudes: 15, 105∘ E and
75∘ W. Immediately after the geomagnetic storm onset the negative
PEC perturbation occurred at all considered longitudes and latitudes, except
high latitudes, pointing to the processes of the plasma tubes' depletion. The
model simulation results support the assumption made above that the negative
perturbations in foF2 and TEC at night during the recovery
phase are related to depletion of the thermal plasma at the plasmasphere
altitudes. In addition, we should note two other interesting results obtained in
the model simulation. The first one is the formation of the negative
PEC disturbances at middle and low latitudes and positive
PEC perturbations in the areas from subauroral latitudes to the
poles, where the concurrent negative foF2 perturbations occurred.
This finding is consistent with the recent results of Liu et al. (2016),
who analyzed behavior of the vertical profiles of electron density over the
Millstone Hill ISR during the 17 March 2015 great storm and reported a drop
in the electron density at the F2 layer peak and its significant increase
at altitudes of the topside ionosphere. The negative effects in electron
density within the plasma tubes occurred due to the tubes' depletion as a
response to the geomagnetic disturbances. The latitudinal distribution of the
PEC disturbances depends on the latitudinal distribution of the
electron temperature disturbances, which causes the scale height changes of
the ionosphere and plasmasphere. Figure 9 demonstrates an example of the GSM
TIP model-derived electron temperature disturbances at 300 and 10 000 km
altitude near 75∘ W. We obtained the electron heating at the
300 km altitude at all areas apart from the cooling areas close to the
Equator. This electron heating source is produced through the neutral heating
by Joule heating and increasing of the energy impact caused by auroral
particles precipitations during geomagnetic disturbances. Figures 8–9
demonstrate that the predominate mechanism of the positive PEC
disturbances at high latitudes is the electron heating due to rise of the
neutral temperature. The neutral heating leads to the negative disturbances
in foF2 through the n(N2) increase. Such explanation of
opposite disturbances in PEC and foF2 is consistent with
the Millstone Hill ISR observations during the March 2015 storm (Liu et al.
2016). The electron cooling at the 300 km altitude near the Equator can be
explained by an increase in n(O) and the neutral temperature decrease
(Fig. 7). The resulted electron temperature disbalance at the F-layer
heights through thermal conductivity leads to the cooling effects at the
plasmaspheric heights (Fig. 9a). The electron temperature changes lead to an
increase in the ionospheric electron scale height globally but excluding the
equatorial area. A decrease in the electron scale height in the topside
ionosphere and plasmasphere occurs in the equatorial region (Fig. 8), which
leads to negative effects in PEC. The second important result is a
significant difference in magnitude and duration of the negative PEC
disturbances between the three considered longitudinal sectors. The
largest are ones in the American longitudinal sector and the smallest ones in the
east Siberian longitudinal sector. This can be explained by the fact that, at
the American longitudinal sector, the points with the same geographical
latitudes in the Northern Hemisphere are found at larger L shells than in the
east Siberian sector; thus, here, the plasma tubes have a larger volume and it
takes more time to refill them after their depletion at the initial
stage of the geomagnetic storms. Therefore, the plasma tubes in the east
Siberian sector have a smaller volume and are filled very quickly
(Fig. 8).
Conclusions
This study, based on both observational and model simulation results,
presents a comprehensive analysis of the ionospheric disturbances during
the main and recovery phases of the geomagnetic storms that occurred on
26–30 September 2011. The main phase of the geomagnetic storm leads to the
essential long-lasting positive disturbances in TEC and
foF2 at the daytime American longitudinal sector and pre-dusk
mid-latitude east Siberian region mainly due to storm-time equatorward
neutral wind. Effects of eastward electric field can explain the short-term
positive ionospheric storm in TEC and foF2 only during the
first few hours at the initial storm stage. The negative disturbances in
TEC and foF2 at high latitudes are formed due to a decrease
in the n(O) /n(N2) ratio during practically the whole
storm-time period. At night, in spite of the decrease in n(N2), the
negative effects in foF2 at middle and low latitudes are formed due
to underfilling of the plasma tubes as a result of their depletion during
the main phase of the geomagnetic storm.
The GSM TIP simulations agree qualitatively with the observations both for
Irkutsk and Kaliningrad, but at the same time there are noticeable
quantitative differences manifested in the underestimated magnitudes of
negative disturbances. An improved agreement between the model and
observations for the given storm case could potentially be achieved through
an appropriate selection of the spatial and temporal variation in the model
input parameters, but this was not the goal of the present paper.
Nevertheless, we have demonstrated the possibility of a general statement
regarding the problem of the thermosphere–ionosphere system response to geomagnetic
storm using a coupled thermosphere–ionosphere–electrodynamics model (GSM TIP)
with input parameters from different empirical models, which was one of the
main goals of our paper. The mentioned statement of the problem has been
continuously improved by our group over the last 6 years (Klimenko et al.,
2011a, c, 2015a; Bessarab et al., 2015; Suvorova et al., 2016; Dmitriev et
al., 2017) and may be used for different geomagnetic storm cases. We
should also note that first-principle model results are intended to illustrate
how the different sources operate and produce the ionospheric response to
geomagnetic storms.
We emphasize the new results – formation of the positive daytime disturbances
in foF2 and TEC at middle and low latitudes and at the
Equator due to n(O) /n(N2) enhancement during the latest stage
of the main phase and during the recovery phase (at that, the latitudinal
extent and duration of foF2 disturbances are greater than
TEC). The spatial extent of this new phenomenon depends on the
longitudinal sector: for the geomagnetic storm on 26–30 September 2011 this
effect was more significant at the east Siberian longitudinal sector and it
was practically absent at the American sector.
The comparison of the model-derived foF2, PEC and TEC disturbances allowed us to make
the following conclusions:
During the main phase of the geomagnetic storm the foF2 behavior
demonstrates a more pronounced negative effect than TEC; the ionosphere is more
changeable in comparison with the plasmasphere during the main phase of the
geomagnetic storm.
The largest differences between the storm-time disturbances in foF2 and
TEC were found in both observations and simulation results at middle and low
latitudes during the storm recovery phase, when the negative TEC perturbations
can occur simultaneously with the positive disturbances in foF2.
The simultaneous formation of the positive disturbances in foF2 and
negative disturbances in TEC at low latitudes during the recovery
phase is explained by the counteraction between mechanisms that lead to the
positive disturbances in foF2 (namely, an increase in the
n(O) /n(N2) ratio) and to the negative perturbations in
PEC and TEC (the plasma tube diffusional depletion and
electron cooling).
The positive disturbances in the electron content at plasmaspheric
heights (800–20 000 km) at high latitudes can appear simultaneously with
the negative disturbances in TEC and foF2 due to electron
and neutral temperature heating.
GSM TIP model results and different kind of presented
observation data are available from the authors upon request.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “The 14th
International Symposium on Equatorial Aeronomy”. It is a result of the 14th
International Symposium on Equatorial Aeronomy, Bahir Dar, Ethiopia, 19–23
October 2015.
Acknowledgements
The geomagnetic indices were obtained from the World Data Center for
Geomagnetism, Kyoto (http://wdc.kugi.kyoto-u.ac.jp/). We are grateful
to the International GNSS Service (IGS) for raw GPS data and GIM products.
This investigation was performed with the financial support of the Russian
Science Foundation grant (no. 17-17-01060). Experimental data over Irkutsk
were recorded by the Angara Multiaccess Center facilities at ISTP SB RAS. The
Kaliningrad ionospheric data analysis and interpretation during geomagnetic
storm were supported by the program “5–100” to improve competitiveness of
I. Kant Baltic Federal University. The
topical editor, Duggirala Pallamraju, thanks Nirvikar Dashora, Ana G. Elias,
and three anonymous referees for help in evaluating this paper.
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