1. Introduction
The Earth's ionosphere, the charged part of the upper atmosphere with maximum ionization at an altitude of about 300 km, is very variable. For the most part, the state of the ionosphere depends on the level of solar and geomagnetic activity, but part of its variability is also associated with processes in the lower atmosphere [
1]. It means that there is a large class of disturbances in the F region of the ionosphere, which can be superimposed on regular diurnal variations in the electron concentration. Moreover, different types of disturbances are characterized by different physical mechanisms of their generation.
The main mechanisms for the formation of ionospheric storms acting during the geomagnetic disturbances are well known and presented in detail in numerous works [
2,
3,
4,
5,
6,
7,
8,
9,
10]. It has been generally accepted that a large amount of energy deposited into the thermosphere at high latitudes during the geomagnetic disturbances leads to heating of the lower part of the thermosphere in the auroral zone, to an increase of the neutral gas temperature T and variations of the neutral composition (depletion of the atom-to-molecular ratio, [O]/[N
2]). These both factors are a main reason for the decrease in electron concentration Ne (the negative phase of the ionospheric storm) in the high-latitude ionosphere. The heating, in turn, causes its own circulation, which tends to move the air towards the equator to lower latitudes. However, this circulation can coincide in direction or be reversed with respect to the background circulation, which is determined by the time of the day and the season. As a result, an interference of the storm induced and regular circulations determines the spatial distribution of the electron concentration in various seasons with the manifestation of Ne variations in a wide range of heights, according to ionosonde observations, from 100 km to the maximum of the F region [
11,
12].
The response of the F-region of the ionosphere to a geomagnetic storm (an ionospheric disturbance) manifested by the deviation of the critical frequencies of the F2 layer from a certain norm (usually from the monthly median) of both signs, ± ΔfoF2. The ionospheric disturbance with ΔfoF2 > |20%|, which corresponds to a change in the electron concentration at the maximum of the F2 layer ΔNmF2 ≈ 40%, is classified (depending on the sign of ΔfoF2) as a positive or negative ionospheric storm [
13]. The response of the ionosphere to a geomagnetic storm can be different depending on the coordinates of the observation site, season and time of day. Thus, for the summer season and middle latitudes during a geomagnetic storm, the most likely negative phase of the ionospheric storm both day and night. This is due to the nature of the background circulation system in the summer thermosphere, which is directed toward the equator for most part of the day coinciding with the storm-induced circulation [
9]. For the winter ionosphere, this pattern of thermospheric circulation is different; it is different for day and night conditions. During the daytime, the background and storm-induced circulations are opposite and the negative phase of the storm is limited to high latitudes. However, at night, both circulations coincide in direction (both equatorward), which leads to the appearance of a negative phase of the ionospheric storm at mid-latitudes at night in winter.
Another type of ionospheric disturbances is unusual variations in the electron concentration of the ionospheric F region during sudden stratospheric warmings (SSWs). Sudden stratospheric warmings are large-scale meteorological events caused by the dissipation of planetary waves propagating upward from the troposphere [
14]. The SSW events are identified based on changes occurring in the stratosphere, including rapidly increasing polar temperatures and slowing zonal mean zonal winds and classified as major SSW if the mean zonal winds at 60°N and 10 hPa vary from east to west, and are considered minor if the winds remain easterly throughout the event [
15]. Although SSWs were originally identified and characterized by changes in stratospheric dynamics, they are now recognized to lead to disturbances throughout the atmosphere. This includes changes in tropospheric weather conditions [
16], stratosphere-mesosphere chemistry and dynamics, and ionosphere-thermosphere composition, dynamics and electrodynamics [
17,
18] which can subsequently lead to a change in the average state of the ionosphere, affecting ion temperatures, vertical drift, total electron content [
19,
20,
21].
Experimental and presented by various models results have shown many effects in the ionosphere attributed to SSWs
[19,22,23,24,25,26,27,28,29,30] where, along with the morphology of the phenomenon, the drivers of the influence of SSW on the ionosphere are considered. All these studies clearly showed an obvious SSW induced forcing in the ionosphere-thermosphere system. For example, Yue et al. [
22] using global COSMIC (Constellation Observing System for Meteorology, Ionosphere, and Climate) data showed that NmF2, height of the F2 layer maximum (hmF2), and total electron content (TEC) can increase during SSW days by up to 19%, 12 km and 17% in the morning and decrease up to 23%, 19 km and 25% in the evening, respectively, in comparison with those during non-SSW days.
All these studies are focused on either the ionosphere/thermosphere response to the geomagnetic disturbances or the SSW events. However, simultaneous effects of geomagnetic storms and SSW events were studied in a few papers only (e.g. [
31,
32,
33]).
In [
31] the authors used TIE-GCM (Thermosphere-Ionosphere-Electrodynamics General Circulation Model) simulations to investigate how the ionosphere would respond to a super geomagnetic storm (“Halloween Storm” October 2003) if it occurred in January coinciding with a major stratospheric warming event (SSW 2013). Modeling results showed that the total electron content response to a geomagnetic storm can vary up to 100% relative to storm-induced change, and temporal variability of the TEC values during the storm recovery phase is also considerably different if SSW events are considered in the model.
Three major SSWs observed over Europe were studied in [
32], one occurred during quiet geomagnetic conditions (SSW 2009) and two during minor-to-moderate geomagnetic storms (SSW 2019 and 2018/2019). In all three studied SSW events, an increase in the foF2 values (by about 20-30% compared to reference days) around the peak height of the F2 region, in TEC, and presence of wave activity were found on days maxima of stratospheric temperature (or very close to them) and in coincidence with the reversal zonal wind that defines the occurrence of major SSW. Moreover, if during SSW 2009 the increase in foF2 was about 1 MHz (
Figure 4 in [
32]), then for SSW2018/2019 this increase was more significant, approximately 2 MHz (
Figure 10 in [
32]). The authors presume that the ionospheric changes observed during 2018 and 2018/2019 SSWs are a combination of both geomagnetic and SSW forcing.
Siddiqui et al. [
33] investigated the nature of the Total Electron Content (TEC) variability during the same SSW 2009 and SSW 2019 events using the GNSS (Global Navigation Satellite System) based TEC observations and the TIE-GCM (Thermosphere-Ionosphere-Electrodynamics General Circulation Model) simulations. The authors reported that the TIE-GCM simulation reproduces the observed TEC variations during both SSWs, and the comparison of the TIE-GCM simulation results with and without geomagnetic forcing showed that the dominant TEC enhancement during the 2019 SSW event was geomagnetically forced and may be a result of large-scale traveling ionospheric disturbances (LSTIDs) while for the 2009 SSW event the TEC enhancement was mainly due to lower atmosphere forcing.
In general, the global picture of ionospheric disturbances is quite complex, especially if the ionosphere is under the influence of various physical processes and results of their interaction. The purpose of this work is to study the features of the formation of the ionospheric response over Almaty [43.25N, 76.92E] to two minor (-100 < Dst < -50nT) geomagnetic storm events of January 2013 which occurred under conditions of strong sudden stratospheric warming. Ionosonde measurements of critical frequency foF2 and virtual heights h’F combined with independent Spectral Airglow Temperature Imager (SATI) observations of the OH emission rate were used in the study.
2. Materials and Methods
Ionosonde observations: The ionospheric response to the January 2013 geomagnetic storms was studied using ionosonde observations from the mid-latitude station Alma-Ata (geographic latitude 43.25°N, geographic longitude 76.92°E, and geomagnetic latitude 34.11°N). Ionograms have been recorded at 60 min interval using a Russian advanced digital ionosonde PARUS (e.g. [
34] and references therein; for a description, please, also see
http://www.izmiran.rssi.ru). The ionospheric data are obtained by semiautomatic ionogram scaling with the participation of an operator using a program designed at the Institute of Ionosphere (Almaty, Kazakhstan). The ionosonde provides accuracy of 0.05MHz for the critical frequency of F2 (foF2) and ~2.5 km for h'(t).
To describe the F-layer behavior the observed hourly F region critical frequency (foF2) and the minimum virtual height of the ordinary wave F trace (minimum heights of the Fl layer (h’F1) in daytime; in nighttime - h'F) measured at the station were analyzed to look for the time ionospheric changes over Almaty. The fluctuating components of the critical frequency (ΔfoF2) and height (Δh) obtained as a relative deviation of the hourly foF2 values from their corresponding background level are analyzed to estimate the ionospheric state and describe the day-to-day ionospheric variability:
Optical Observations: The Spectral Airglow Temperature Imager (SATI) is a spatial scanning Fabry-Perot spectrometer (described in more detail by Sargoytchev et al. [
36]) that observes two airglow emissions, for which the unperturbed (6,2) Meinel band of the hydroxyl radical is emitted from about 87 km altitude and the O
2 (0,1) Atmospheric band is emitted from about 94 km. The instrument resolves the integrated emission rates (photon s
-1 from a vertical 1 cm
2 column) of the rotational lines in the Meinel and Atmospheric bands observed. The ratios of the rotational lines give the rotational temperature because the population of the rotational levels depends on temperature. The absolute column integrated emission rate of the band-integrated emission is a measure of the column integrated atomic oxygen concentration over the altitudes of the OH and O
2 airglow layers. These airglow emissions are observable from the Earth’s surface only at night, since the light of the daytime sky is many orders of magnitude larger. The ground-based SATI instrument is operated at the experimental station “Orbita” (43°03N, 76°58E) near Almaty at 2730 m altitude above sea level in the absolute absence of the Almaty city light. An observational run is about 8-10 hrs, the temporal resolution is 2-min, the observational time period is January – February 2013.
Information about space weather events was used from the “WEEKLY HIGHLIGHTS” prepared by the Space Weather Prediction Center (CWPC),
http://www.swpc.noaa.gov/. The IMF parameters and SYM-H data were downloaded from OMNIWeb (
https://omniweb.gsfc.nasa.gov/). The Dst data were obtained at the
http://wdc.kugi.kyoto-u.ac.jp/ site, the daily F10.7 solar flux was used as a measure of solar activity provided by Solar Influences Data analysis Center (SIDC) of the Royal Observatory of Belgium at
http://www.sidc.be/products/bul. The geomagnetic planetary Ap-index provided by the GFZ German Research Centre for Geosciences at ftp://ftp.gfz-potsdam.de/pub/home/obs/kp-ap/tab/ was used to estimate level of the geomagnetic field activity in January 2013. An Ap-index less than 8 indicates quiet, the Ap greater or equal to 8 and less than or equal to 15 – unsettled, Ap greater than 15 and less than 30 – active geomagnetic conditions, and the Ap index greater than 29 indicates geomagnetic storms (
http://www.sidc.oma.be/).
4. Discussion
As shown in
Section 3.1, the main phases of the January 2013 geomagnetic storms observed at the Alma-Ata station started at 13:30UT and 23:00UT, that is at 18:00LT and 04:00LT on 17 January and 26 January 2013. The peak SYM-H values occurred at 17:00–20:00UT and 22:00UT on 17 January and 26 January what does it mean at 22:00–04:00LT and 03:00LT on 17/18 January and 27 January respectively. This corresponds to winter conditions and night time for this region. According to the Danilov and Lăstovička’s [
5] scheme, in winter the daytime background circulation is poleward. This leads to the effect of stopping the movement of the negative phase towards the equator, and the negative phase region is “closed to high latitudes”. But at night, these two circulations (background and storm-induced) coincide (both are directed towards the equator) and thus air with a reduced [O]/[N2] ratio spreads to low latitudes, which should lead to the appearance of a negative phase at mid-latitudes at night in winter. In our case,
Figure 8 shows, that a significant decrease in the foF2 values in ionospheric response to the geomagnetic storms of January 2013 was not observed. This does not mean that the above scheme does not work; perhaps under the influence of another source of disturbance the response of the ionosphere to geomagnetic storms was masked. In this case, such a masking source could be the effect on the ionosphere “from below” caused by processes associated with the major sudden stratospheric warming (SSW) event in January 2013 (see section 3.2).
In addition, it should be noted the appearance of large (up to 60% and more) foF2 increases relative to the background level at the local night/morning hours and quasi-periodic (~6-day) variations in foF2, h'F over Almaty (see
Section 3.3) during periods of increased stratospheric temperatures and easterly winds during the SSW effects in January 2013 (see
Section 3.2). These results are in good agreement with the
Chen’s et al. [45], Mosna’s et al. [32] and Siddyqui’s et al. [33] studies. In [32,33] an increase in foF2 and presence of wave activity were found on days of maxima of stratospheric temperature and in coincidence with the reversal zonal wind
. Chen et al. [45], who studied the 2013 SSW on the meridional chain from 30.5N to 42.8N in northern China, showed the foF2 enhancements dominating in 8-12LT and after sunset during the 2013 SSW, and a strong ~7-day oscillation that was observed at 39.5N and 42.8N. The prominent “quasy-6-day wave” variations in the dayside low-latitude region, including 20-40% variations in the topside electron density and 5-10% variations in the topside total electron content (TEC), were observed by Yamazaki et al. [
46] and references therein] in the ionospheric response to the September 2019 SSW occurred in the Southern Hemisphere. These wave-like fluctuations of ionospheric parameters with a period of less than 10 days, observed during stratospheric warmings, interpreted as planetary waves ([
30]
, [
45]
) or large-scale waves [46], which lead to ionospheric variability, should definitely be investigated, but this issue is beyond the scope of our present work. which can be interpreted as a manifestation of wave activity in the lower atmosphere in December 2012-January 2013.
It is difficult to say what led to compensation for the expected response of the ionosphere to geomagnetic storms (changes in stratosphere-mesosphere chemistry, ionosphere-thermosphere composition or dynamics and electrodynamics), but we can definitely say that the mechanisms accompanying SSWs influenced the development of ionospheric storms.