Ionospheric F 2 region: Variability and sudden stratospheric warmings

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH: SPACE PHYSICS, VOL. 118, , doi: /jgra.50570, 2013 Ionospheric F 2 region: Variability and sudden stratospheric warmings A. K. Upadhayaya 1 and K. K. Mahajan 1 Received 2 August 2013; revised 13 September 2013; accepted 13 September 2013; published 15 October [1] The ionospheric F 2 region is known to show a large day-to-day and hour-to-hour variability. Some of this variability has recently being linked to sudden stratospheric warmings (SSWs). We therefore investigate the extent of ionospheric changes following SSWs of 2007, 2008, and 2009 using ionosonde data from six different stations in the Asian zone, thus covering a broad latitudinal range from 23.2 N to 45.1 N. We find that ionospheric F 2 region shows some significant perturbations soon after the start of the warming. However, characteristics of these perturbations vary from event to event and from station to station. We also examine the data on equatorial electrojet strength (EEJ) during these warmings and find that there are significant changes in the EEJ strength during the SSW events. A counter electrojet coincident with the start of warming was observed for the SSW event of We then compare this SSW-linked variability observed by us to the normal day-to-day and hour-to-hour variability seen in the ionospheric data. We find that even during times when there are no SSWs and solar and magnetic indices are quite stable and close to their minimum values, the ionospheric variability is comparable to the variability attributed to these warmings. Further, it seems to us that it is difficult to quantify with precision the changes in f o F 2, as well as in the ionospheric response times involved, following these events. Citation: Upadhayaya, A. K., and K. K. Mahajan (2013), Ionospheric F 2 region: Variability and sudden stratospheric warmings, J. Geophys. Res. Space Physics, 118, , doi: /jgra Introduction [2] The possibility of links between the meteorological phenomena and the upper atmosphere have been discussed very profoundly during the last two decades [e.g., Forbes and Leveroni, 1992; Hagan and Roble, 2001; Abdu et al., 2006; Lastovicka, 2006; Fuller-Rowell et al., 2008]. One of the well-known meteorological phenomena which could be an important agent in this link is the large meteorological variation in the wintertime polar stratosphere, called the sudden stratospheric warming (SSW). During a SSW, there is a sudden increase in stratospheric temperature, T (which could be as large as 60 K), the polar vortex shifts off the pole, and the zonal wind (U) become weak. This type of warming is designated as minor. However, if the vortex breaks up and the zonal wind (U) changes direction, then the event is designated as a major SSW. [3] The SSWs have been reported to affect the temperature, wind, chemistry, and wave activity in the middle atmosphere [Shepherd et al., 2007; Vineeth et al., 2007; Sridharan et al., 2009, 2012]. And according to Forbes et al. [2000], the 1 CSIR, National Physical Laboratory, Radio & Atmospheric Sciences Division, New Delhi, India. Corresponding author: A. K. Upadhayaya, CSIR, National Physical Laboratory, Radio & Atmospheric Sciences Division, New Delhi, India. (upadhayayaak@nplindia.org) American Geophysical Union. All Rights Reserved /13/ /jgra SSW can affect the vertical thermodynamical coupling in a large range of altitudes and latitudes resulting in modulating and affecting the ionosphere and exosphere. In addition, several authors have observed an association between the occurrence of equatorial counter electrojet (CEJ) and SSWs [e.g., Stening, 1977; Stening et al., 1996, 1997; Sridharan et al., 2009; Vineeth et al., 2009]. [4] Although there have been some early attempts for identifying any ionospheric response to meteorological events like the sudden stratospheric warmings from theory, as well as from measurements [e.g., Liu and Roble, 2002; Kazimirovsky and Kokourov, 1991;Danilov and Vanina, 2003], the field has seen relatively a vigorous activity only recently. This renewed vigor has mostly been due to the Incoherent Scatter World Day (ISWD) campaign of January 2008 [Goncharenko and Zhang, 2008]. This campaign was basically organized to examine ionospheric effect of SSWs at altitudes above 100 km. In this campaign, incoherent scatter radars at Jicamarca (11.95 S, W), Arecibo (18.3 N, W), and Millstone Hill (42.6 N, 71.5 W) were brought into operation to study the temporal and altitudinal variations of electron density, electron temperature, ion temperature, and ionospheric drifts during the SSW of [5] The first result reported from this ISWD campaign was by Goncharenko and Zhang [2008] and dealt with the response of the ionospheric ion temperature at Millstone Hill (a midlatitude station) for altitudes between 100 and 300 km. These authors observed an alternating region of 6736

2 cooling between 150 and 200 km, while a warming was observed in the region between 120 and 140 km. The cooling was observed 4 days prior to the peak of the event and subsided 8 days after the peak. The maximum cooling in the ionospheric F region was 75 K and was prominent in the morning as well as in the afternoon hours. [6] The equatorial vertical E B drift, measured at Jicamarca (11.95 S, W) for this event by Chau et al. [2009], showed an anomalous temporal variation. A large difference from the long-term mean values in the E B drifts, characterized by a semidiurnal wave, was identified by these authors throughout the warming period. This difference was found to be positive in the mornings and negative in the evenings. In addition to this large variability in the E B drift, Chau et al. [2009] further found out a good relationship between the magnitude of E B variability and the difference of SSW parameters ΔT and ΔU, from their 30 year median values. However, when the largest SSW event of 2009 occurred and the Jicamarca radar was in operation, this relationship was not seen at the peak of the event but occurred 4 to 5 days later. [7] In a subsequent work, Goncharenko et al. [2010a] examined ionospheric response to stratospheric warmings with the GPS-total electron content (TEC) data for the events of 2008 and These authors obtained global TEC from the network of worldwide GPS receivers in 1 1 bins of latitude/longitude with temporal resolution of 5 min. The data from the low-latitude ionospheric sensor network were used. The study revealed that the TEC variation was most pronounced in daytime hours and had a clear semidiurnal characteristic seen 3 to 6 days after the peak warming. According to these authors, this variation in TEC was consistent with the observed semidiurnal changes in the vertical ion drift, supposed to be caused by the enhanced semidiurnal tide in the lower thermosphere [Forbes and Leveroni, 1992; Pedatella and Forbes, 2010]. In a further work, Goncharenko et al. [2010b] noted a large variation of % in the low-latitude ionosphere (200 to 1000 km) from the GPS-TEC data several days after the January 2009 SSW event. They, in addition, showed a reasonably good agreement of the large increase observed in TEC with that predicted by the Thermosphere-Ionosphere- Mesosphere Electrodynamics-General Circulation model. These authors attributed these changes to the nonlinear interaction of atmospheric tides with planetary waves, which are known to amplify before the warming and their amplitude decreases during and after the warming. [8] A similar study with incoherent scatter radar measurements at Arecibo (18.24 N, W) and Jicamarca (11.95 S,76.8 W) stations and total electron content (TEC) from dual frequency GPS measurements at Arecibo was carried out by Chau et al. [2010] for understanding F region dynamics for the SSW events of 2008 and They found that the ionosphere over the low-latitude station, Arecibo, exhibited significant perturbations in electron density and electron temperature after the SSWs and the effect was prominent during the daytime when the plasma line observations were possible. Large enhancements of TEC were observed in both events. These results were also in accordance with the enhanced daytime vertical drift observed at Jicamarca. However, the local time dependence of the enhancements in TEC differed between the two events. Further, there were no reports on any changes in the ion temperature at Arecibo, though significant variations in the ion temperature were observed by Goncharenko and Zhang [2008] at the midlatitude stations, Millstone Hill, during the January 2008 warming. [9] By using vertical plasma drift measurements from the Jicamarca radar and the magnetic field measurements from CHAMP satellite and ground-based magnetometer in the American, Indian, and Pacific equatorial regions, Fejer et al. [2010] showed the presence of strong global equatorial electrodynamic perturbations during the warmings. These perturbations, which lasted for several days after the warmings, were found to be highly longitudinal dependent and got enhanced with the onsets of full moon [Fejer et al., 2010, 2011]. [10] Similarly, Anderson and Araujo-Pradere [2010], by using drift velocities derived from magnetometer data in the Peruvian and the Philippine longitudes, reported the semidiurnal-type behavior in drift velocities during SSWs of January 2003 and January This feature first appeared in the Peruvian sector and then moved to the Philippine sector 3 days later. [11] Ionospheric response to SSW was also examined with Formosa Satellite mission-3/constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) data by Yue et al. [2010] and Pancheva and Mukhtarov [2011]. Yue et al. [2010] studied the January 2009 SSW event by examining electron density profiles over the American (60 W) and Atlantic (0 ) sectors at eight different locations. A pronounced enhancement in electron density was seen between 10 and 12 LT on the SSW days in comparison with the non-ssw days. However, the electron density showed a decrease between 16 and 18 LT at the same locations. Further, the peak height of the F 2 layer (h m F 2 ), the peak density (N m F 2 ), and ionospheric total electron content increased in the morning and decreased in the afternoon. The COSMIC observations also indicated a more complex response at high latitudes in comparison to that at low latitudes and midlatitudes. [12] Pancheva and Mukhtarov [2011] found some evidence for changes in zonal mean electron density and in the diurnal component of electron density from the analysis of COSMIC data around the December solstices of and , following the SSW event of 2008 and In addition, they observed a decrease in both the zonal mean f o F 2 and mean h m F 2. This change was strongest at low latitudes and extended up to middle latitudes. The decrease in N m F 2 and h m F 2 was observed 2 4 days after the peak in stratospheric temperature. Further, their study indicated that the ionospheric response to SSW was mainly in the Northern Hemisphere and was confined mostly to altitudes above the F 2 peak. [13] Liu et al. [2011a] examined the TEC measurements at three stations in the Asian sector (100 E) following the SSW of One station was located at the magnetic equator and the other two at 12.7 N and 10.1 S dip latitudes, respectively. They found a strong latitude-dependent response of perturbations in TEC. While TEC decreased at the magnetic equator, the low latitudes (the stations around the equator) exhibited a semidiurnal perturbation. They also detected a hemispherical dependence on the start time of the semidiurnal perturbation on the same day of the peak warming in the Northern Hemisphere but 2 days later in the Southern Hemisphere. 6737

3 Table 1. Locations of Ionosonde Stations Station Geographic Latitude Geographic Longitude Geomagnetic Latitude Trivandrum 8.36 N 76.6 E 0.63 S 0.5 Bhopal N E 14.2 N 33.2 Okinawa 26.6 N E 17.0 N 36.8 Delhi 28.2 N 77.6 E 19.2 N 42.4 Yamagawa 31.2 N E 21.7 N 43.8 Kokubunji N E 26.8 N 49 Wakkanai 45.1 N E 36.4 N 59.3 [14] Sripathi and Bhattacharyya [2012] presented observations of TEC from several stations in India, along with simultaneous measurements of equatorial electrojet (EEJ) strength, for the SSW event of January They observed a morning increase and an evening decrease in TEC as well as in EEJ strength following this event. In addition, they reported a large-scale wavelike structure in TEC near the crest of equatorial ionization anomaly. This structure had a periodicity of quasi 16 day wave in TEC which was quite similar to the one seen in the EEJ strength. [15] There are also reports about changes in the neutral atmosphere during SSWs. Liu et al., 2011b, from the measurements of the thermospheric densities by CHAMP and Gravity Recovery and Climate Experiment satellites, observed a decrease in their values during the SSW of January This decrease was larger in the Southern Dip Hemisphere as compared to the Northern Hemisphere. In addition, these authors found that the electron densities in the topside also decreased. [16] In a more recent work, Goncharenko et al. [2012] have reported a correlation between the degree of enhanced variability in the total electron content and stratospheric ozone density, following the SSW of January They have suggested that ozone variations affect the ionosphere through a modified tidal forcing. [17] Further, a recent study by Sridharan et al. [2012] has shown that there is a relationship between the high-latitude Northern Hemispheric major SSWs and low-latitude mesospheric tidal variability in zonal winds as observed by the MF radar at the Indian station Tirunelveli. It was also found that the ozone-mixing ratio increased at low latitudes during the SSWs. [18] The above studies have shown that the sudden stratospheric warmings leave a global signature on ionospheric F region. A major feature observed with the Jicamarca radar is the semidiurnal variation in the equatorial vertical plasma drifts which lasts for several days after the warming. These plasma drifts result in notable changes in the plasma densities especially at the equatorial and low-latitude stations. Further, strong global equatorial electrodynamical perturbations occur during SSWs, and these perturbations are highly longitude dependent. There is also some evidence of relationship between the zonal mean ionospheric parameter (like h m F 2 and N m F 2 ) and stratospheric temperatures during these warmings. Figure 1. (A) Summary of stratospheric and geomagnetic conditions for the winter of (a) Stratospheric temperature at 90 N and 10 hpa (~32 km), (b) zonal mean stratospheric temperature at 60 N 90 N, (c) mean zonal wind at 60 N, (d) planetary wave 1 activity at 60 N and 10 hpa, (e) planetary wave 2 activity at 60 N and 10 hpa, (f) F 10.7 index, and (g) Kp index. Lines indicate 30 year means of stratospheric parameters, and solid circles indicate data for the winter of [from Goncharenko et al., 2010b]. (B) Same as Figure 1A but for the winter of Four stratospheric warmings occurred, with peaks on 24 January, 6 February, 16 February, and 23 February 2008 [from Goncharenko et al., 2010b]. (C) Same as Figure 1A but for the winter of Two stratospheric warmings occurred, first during February and the second during 2 4 March

4 UPADHAYAYA AND MAHAJAN: IONOSPHERIC VARIABILITY AND SSW Figure 2. Plots of deviation (ΔfoF2) in critical F2 layer frequency from average of prewarming period (3 12 January 2009) at (a) Okinawa, (b) Yamagawa, and (c) Kokubunji. Plots of (d) F10.7 and (e) Kp indices with days of the year Dark black lines denote the SSW period, and the dotted lines denote the dates of peak warming. [19] It is to be noted that most of these studies have been confined to the Western Hemisphere, particularly the 75 W meridian. In view of the large longitudinal dependence [Fejer et al., 2010] of the equatorial electrodynamics perturbations during SSWs, we have attempted to examine ionospheric effects following SSW events of 2007, 2008, and 2009 in the Asian zone by using ionosonde data from six different stations. These stations cover a broad geographic latitude range from 23 N to 45 N and the longitude range from 76.6 E to E. We find that there are some perceptible changes in the ionosphere following these warmings at these stations. We then compare the magnitude of these changes with the normal day-to-day and hour-to-hour variability which exists in the ionospheric F2 region even at times when there are no SSWs and solar and magnetic indices are quite stable and close to their lowest values. We also use the horizontal component of the geomagnetic field measured at the equatorial station Tirunelveli (8.7 N, 77.8 E; dip latitude: 0.4 N) and a near-equatorial station Alibag (18.5 N, 72.9 E; dip latitude: 13.0 N) to derive equatorial electrojet strength to examine and compare the day-to-day changes at the magnetic equator station Trivandrum and equatorial anomaly station Bhopal during these events. This paper presents the results of this study. 2. Observations and Analysis [20] In our study, to examine the ionospheric response to SSWs, we have used the hourly data of F layer critical frequency (fof2) from four Japanese stations, namely, Kokubunji, Okinawa, Yamagawa, and Wakkanai. Geographic and geomagnetic coordinates of these stations along 6739

5 used hourly ground-based data of the horizontal component H of the geomagnetic field at Tirunelveli (a station close to the dip equator) and Alibag (another station away from the influence of EEJ) by the technique suggested by Rastogi and Klobuchar [1990]. 3. Analysis and Results [22] We have examined ionospheric response to the following SSW events. [23] 1. Winter of [24] 2. Winter of [25] 3. Winter of Figure 3. Plot of average F 2 layer critical frequency (f o F 2 ) with days of the year 2009 for forenoon (10 to 12 LT) and afternoon (16 to 18 LT) at Okinawa (17.0 N geomagnetic latitude). Forenoon f o F 2 values remain higher than the afternoon values for several days after the warming. On other days, forenoon f o F 2 values are generally lower than the afternoon values. with other relevant parameters are given in Table 1. The f o F 2 data are based upon vertical sounding with ionosondes and have been downloaded from the Space Physics Interactive Data Resource web, managed by National Geophysical Data Center, Boulder, and from the National Institute of Information and Communications Technology (NICT), Japan World Data Center. The ionosondes at these stations produce ionograms which are recorded digitally on a computer storage medium. The digitally recorded ionograms are collected from each station by a central computer, and these are reduced to numerical values and to summary plots by an automatic processing system. The ionograms obtained are scaled by the pattern recognition method except at Kokubunji where these are manually scaled in accordance with the URSI Hand Book of Ionogram Interpretation and Reduction (UAG-23A, 1972). In addition to Japanese stations, we have also used the ionospheric data from the low-midlatitude station Delhi, the equatorial anomaly crest station Bhopal, and the equatorial station Trivandrum. These data are obtained by using digital ionosonde system IPS-71 of Kel Aerospace Ltd. at Delhi and Bhopal and Lowell digisonde at Trivandrum. Geographic and geomagnetic coordinates for these stations along with other relevant parameters are also given in Table 1. IPS-71 is fully computer controlled, operates in vertical incidence mode, and has additional features like HF spectrum, phase, and Doppler ionograms. The regular vertical sounding is carried out every 15 min round the clock, with a frequency resolution of 0.1 MHz. [21] Most of the variability in the ionospheric F region in the equatorial ionization belt is believed to be caused by the equatorial vertical ion drifts. These drifts have been found to be related to equatorial electrojet (EEJ) strength [Anderson et al., 2002]. To derive EEJ strength, we have 3.1. SSW Event of 2009 [26] The winter is one period during which there was an exceptionally strong and long-lasting stratospheric warming. An excellent summary of this event has been given in a paper by Goncharenko et al. [2010a], and to recapitulate the stratospheric and geomagnetic conditions during this event, we reproduce this summary in Figure 1A. During this event, the stratospheric temperatures at 90 N brusquely increased by about 60 K, and the zonal mean zonal wind at 60 N got reversed from westerly to easterly. While the stratospheric warming at the 10 hpa level was seen from 20 January to 9 February 2009, the peak warming occurred on January The solar activity and geomagnetic activity remained at a very low and steady level during this period, with Kp 2 and F 10.7 index around units. The extremely low solar and geomagnetic activity thus presented the ideal scenario to look into any changes in the ionosphere following this warming. To examine the response of low-latitude ionosphere to this event, we present in Figure 2a the difference of F 2 layer critical frequency (Δf o F 2 ) from that of normal quiet time behavior at the lowlatitude station Okinawa for the first 60 days of the year This 60 day period covers about 20 days before the start of the warming, 20 days during the warming, and 20 days after the end of the warming. To characterize the normal behavior of the quiet time ionosphere, we selected the quiet period of 3 12 January 2009, which was prior to the stratospheric warming. It is to be pointed out that we have concentrated our analysis to daytime measurements alone, as has been done by several other researchers [e.g., Goncharenko et al., 2010a, 2010b; Chau et al., 2010; Yue et al., 2010; Pancheva and Mukhtarov, 2011; Liu et al., 2011a; Sripathi and Bhattacharyya, 2012]. One can note in Figure 2a that there is a long period of enhanced Δf o F 2 which starts soon after the warming (identified by a thick dark line in figures). This enhancement, which is located around noon (10 14 LT), continues throughout the warming period and beyond, thus covering a period of about 35 days. Maximum enhancement in Δf o F 2 is seen 8 days after the start of the warming and 4 days after the peak of the warming. There is some evidence of a 3 4 day periodicity in Δf o F 2 during the warming period. [27] The average quiet time value of f o F 2 around noon is about 7 MHz. The 6 MHz increase in f o F 2 means an enhancement of more than 200% in peak electron density at these times. A lot of variability in Δf o F 2 can also be noted during and after the warming. There are periods of depressed Δf o F 2 too, which are generally coincident with periods of enhanced Δf o F 2 and start at local times soon after when 6740

6 Figure 4. Plots of F 2 layer critical frequency (f o F 2 ) at 12:00 and 17:00 h several days before and after the SSW event of 2009 at different stations. enhancements end. These depressions, however, are not as conspicuous as the enhancements seen in Δf o F 2. Another short period of enhanced Δf o F 2 located at LT is also seen 14 days (day no. 38) after the peak warming. [28] The geomagnetic index Kp was 2 and the 10.7 cm solar radio flux was ~ 75 units during this period. Solar and geomagnetic activities are the two important variants that are responsible for ionospheric variability. We have therefore plotted the 10.7 cm solar radio (representing solar-ionizing flux) and the index Kp (representing the geomagnetic activity) in Figures 2d and 2e, respectively. It can be noted that these indices were not only near the minimum of their values but quite stable also during and after the SSW event. The periods of enhanced and depressed Δf o F 2 as observed soon after and during the SSW could thus point toward the influence of the stratospheric warming on the ionosphere and are consistent with similar response in the low-latitude total electron content reported by other authors [e.g., Goncharenko et al., 2010a; Chau et al., 2010; Liu et al., 2011a; Sripathi and Bhattacharyya, 2012] during similar events. [29] To examine any latitudinal effect of SSW on the ionosphere, we show the variation of Δf o F 2 for Yamagawa and Kokubunji (two low-midlatitude stations) in Figures 2b and 2c. As observed in the case of low-latitude station Okinawa, there is a long period of enhanced Δf o F 2 at Yamagawa around noon (10 to 14 LT), which occurs soon after the warming and continues for several days after the end of the warming. The maximum noontime (10 14 LT) at Yamagawa occurs on day 35, 15 days after the start of the warming. However, afternoon depressions in Δf o F 2 are not seen. The enhancements in Δf o F 2 at Yamagawa are less prominent in comparison to those observed at Okinawa, as can be noted from Figure 2b. The short period of enhanced Δf o F 2 (17 18 LT), observed at Okinawa 14 days after the peak event, is also present at Yamagawa and Kokubunji (Figure 2c). There is some evidence of prenoon enhancement in Δf o F 2 at Kokubunji as can be noted from Figure 2c. [30] A noticeable ionospheric feature following the stratospheric warming was that the f o F 2 values at Okinawa (magnetic latitude: 17.0 N; dip: 36.80) were contrastive around 11 LT and 17 LT a feature not seen at other times. This is demonstrated in Figure 3, which shows a plot of f o F 2 at 11 LT (averaged over LT) and 17 LT (averaged over LT) for this station for the entire period of the study. A diverse variation of the enhanced effects at 11 and 17 LT can be noted on a large block of days after the warming (days 24 to 47), where the enhancement in f o F 2 at 11 LT dominates over that at 17 LT. The trend is somewhat opposite for the rest of the entire period the afternoon values generally dominate over the prenoon values. A similar feature has been reported by Goncharenko et al. [2010b], Liu et al. [2011a], Anderson and Araujo-Pradere [2010], Yue et al. [2010], and Sripathi and Bhattacharyya [2012] in the total electron content behavior at the low latitudes during this event. This observation indicates a semidiurnal influence which, as we shall find later, could be of lunar origin [Fejer et al., 2010, 2011]. [31] As pointed out earlier, within periods of enhanced and depressed Δf o F 2, which extend over several days, there are 6741

7 Figure 5. Plots of deviation in critical F 2 layer frequency (Δf o F 2 ) from average of prewarming period (2 11 January 2008) at (a) Okinawa (low latitude), (b) Yamagawa (midlatitude), (c) Kokubunji (midlatitude), (d) Wakkanai (far midlatitude), (e) Bhopal (EIA Crest), and (f) Delhi (low latitude). Plots of (g) F 10.7 and (h) Kp indices with days of the year Dark black lines denote the SSW period, and the dotted lines denote the dates of peak warmings. large day-to-day variations in the upper ionosphere. This is demonstrated in Figure 4, which shows f o F 2 plots at 12:00 and 17:00 h for several days before and several days after the warming at all three stations. Figures are drawn for the local times when enhancements and depressions were observed. Because of the large day-to-day variability in f o F 2 within these periods, it is difficult to track any day-to-day shift in the local time of maximum enhancement. However, one observes quite high values of f o F 2 even before the warming, and these values are comparable to those seen during the SSW. Some of these enhancements are generally associated with equatorial vertical ion drifts, as we shall demonstrate in a later section. [32] From above, it is clear that the SSW event of 2009 affected the low-latitude as well as midlatitude ionospheres. The times of the effect were nearly the same at all the stations, although the intensity of the effect decreased as latitude increased SSW Event of 2008 [33] During the winter of , the SSW peak was observed on four occasions, namely, 24 January, 6 February, 16 February, and 23 February A summary of this event has been given by Goncharenko et al. [2010a], and we reproduce this summary in Figure 1B. The first three warmings 6742

8 UPADHAYAYA AND MAHAJAN: IONOSPHERIC VARIABILITY AND SSW Figure 6. Plots of deviation in critical F2 layer frequency (ΔfoF2) from average of prewarming period of the year 2007 at (a) Okinawa, (b) Yamagawa, (c) Kokubunji, (d) Wakkanai, and (e) Bhopal. Plots of (f) F10.7 and (g) Kp indices with days of the year Dark black lines denote the SSW period, and the dotted lines denote the dates of peak warmings. were considered as minor, whereas the fourth one was a major stratospheric warming because in this case the zonal wind at 60 N turned negative, and the temperature at 90 N increased by ~ 40 K within 3 days. Solar activity remained at a very low level after 20 January 2008, with F10.7 index around 70 to 75 units. The geomagnetic activity was low too, though somewhat marginally higher than that which was observed in This period was largely quiet, with geomagnetic activity Kp index 4 as can be noted from Figure 1Bg. The solar-ionizing flux index, F10.7, was near its lowest level and was quite stable as can be seen in Figure 1Bf. [34] To examine the response of ionosphere to this event at low latitude, we plot in Figure 5a the variation of ΔfoF2 at Okinawa for the first 80 days (20 days before start of the warming, 40 days during the warming, and 20 days after the end of warming) of the year The mean behavior of the quiet time ionosphere was obtained by selecting the period of 2 11 January 2008 prior to the stratospheric warming. Days of peak warming are marked in black dotted lines in the figure. A close examination of Figure 5a reveals that there was a long period of depressed ΔfoF2 which started on day 22, soon after the warming, and was located between 10 to 14 LT. The local time of the depression in ΔfoF2 shifted from h to 9 15 h as the warming progressed. Some fluctuations in ΔfoF2 were also seen during this period. The period of depressed ΔfoF2 was then followed by intermittent periods (days 29, 35, 39, 42, and 43) of enhanced ΔfoF2 in the afternoon hours, starting after the first warming peak and continuing a few days after the fourth warming peak. Within this long period of intermittently enhanced ΔfoF2, 6743

9 UPADHAYAYA AND MAHAJAN: IONOSPHERIC VARIABILITY AND SSW Figure 7. Plots of variation of fof2 (ΔfoF2) from normal quiet period at (a) Bhopal and (b) Trivandrum along with (c) EEJ strength for 60 days, days 30 to 90 of the year large fluctuations in the ionosphere (i.e., ΔfoF2) can be seen as also observed for the SSW event of There is some evidence of a 4 5 day periodicity in ΔfoF2 behavior during this warming period. The maximum enhancement in the peak electron density was more than 200%, as in this case. It may also be noted that the local times for the enhanced ΔfoF2 shifted from 18 LT to 12 LT as the warming progressed. A long period of enhanced ΔfoF2 for about 25 days was also seen soon after the third warming peak, but the latter could as well be due to the well-known semiannual variations in the ionosphere [e.g., Mayr and Mahajan, 1971 and Rishbeth et al., 2000 for a review]. [35] There is, thus, enough evidence to support that the ionosphere at the low-latitude station Okinawa was disturbed following this stratospheric warming too. The response times (meaning the difference between times of start of SSWs and fof2 disturbance) as well as the local times of enhancement, however, were somewhat different. This result is contrary to what has been observed during the 2009 SSW event: in 2009, an enhancement followed the warming, while in 2008, a depression followed the warming. [36] At the low-midlatitude stations Yamagawa and Kokubunji, as can be seen in Figures 5b and 5c, similar ionospheric changes as observed at Okinawa were present, although the intensity of depressions and enhancements in ΔfoF2 was not that large. The midlatitude station Wakkanai also showed depression in ΔfoF2, but there was not much evidence for the enhancement in ΔfoF2, except for the one at LT, observed after the fourth warming peak (Figure 5d). [37] A comment about ionospheric changes seen after the SSW of 2008 at the low-midlatitude stations, Yamagawa and Kokubunji, and the midlatitude station, Wakkanai, is in order. Although the changes are, perhaps, within the day-to-day fluctuations present in the average quiet time ionosphere used to study the ionospheric response during the SSW, these are significant because they are coincident in time with those that were seen at the low-latitude station Okinawa. [38] The ionospheric response to this SSW event at Bhopal, an equatorial ionospheric anomaly (EIA) station, is examined in Figure 5e. Intermittent depressions in ΔfoF2 from 12 to 14 LT were seen throughout the warming period. Enhancements in ΔfoF2 were also observed during the warming period, showing a strange behavior of depressions and enhancements (10 14 LT) as the warming progressed. Prominent enhancement around the fourth peak can be noticed from 10 to 12 LT at this station, which increased the peak electron density by more than 200%. [39] Ionospheric response to this event at Delhi (another low-midlatitude station) is examined in Figure 5f. The ionospheric changes observed are generally similar to those seen at low-midlatitude stations Yamagawa and Kokubunji. Similar enhancement in ΔfoF2 was seen at LT and between 10 and 12 LT around the fourth warming peak. However, there were depressions and enhancements even before and after the end of warming. [40] It appears from the above observations that ionospheric variations seen at various stations during the 2008 warming are quite different from those observed for the 2009 warming. There was not any evidence for the semidiurnal perturbation which was seen in the 2009 warming SSW Event of 2007 [41] Figure 1C presents the stratospheric and geomagnetic conditions during this SSW period which lasted from 15 February 2007 to 15 March After staying at low levels in January and for the first three weeks of February 2007, the peaks of warming at the 10 hpa level showed up during February and 2 4 March Figure 1Ca illustrates the atmospheric conditions at 10 hpa level. We have used stratospheric data from the National Center for Environmental Predictions to generate this figure. While Figure 1Ca shows stratospheric temperature at 10 hpa for 90 N, Figure 1Cb shows zonally averaged temperature for N in February March Comparison with their 30 years median temperatures (1979 to 2009) is also shown in the figure. Stratospheric temperature at 90 N abruptly increased by 40 K, and the zonal mean zonal wind at 60 N got reversed from westerly to easterly (see Figure 1Cc). Planetary wave 1 and wave 2 activities at 60 N and 10 hpa are shown in Figures 1Cd and 1Ce. As can be seen from these figures, the circulation is dominated by planetary wave 1, which exceeds the long-term median level by a factor of about 2.5, while the activity of planetary wave 2 remains low. The solar activity and geomagnetic activity indices during this period have been depicted in Figures 1Cf and 1Cg, respectively. Low and stable values of solar and geomagnetic indices can be noted during this period. 6744

10 Figure 8. Plot of five-point running average of f o F 2 at Bhopal and Trivandrum along with EEJ strength at 11:00 and 14:00 h Indian Standard Time (IST) for 70 days, days 20 to 90 of the year Full moon is represented by white circles and the new moon by dark circles. [42] The ionospheric response of this warming event is examined at the low-latitude station Okinawa in Figure 6a. The mean behavior of the quiet time ionosphere was obtained by selecting the 10 magnetically quietest days of January 2007 (7 8, 13 14, and January, respectively) prior to the stratospheric warming. Enhancements in Δf o F 2 start soon after the warming (12 16 LT) and continue for 5 days (days 45 50). Another long period of enhanced Δf o F 2 (days 55 68) starts soon after the first warming peak which is located between 12 and 16 LT. These enhancements could result in a 200% increase in the peak electron density. The enhancement in Δf o F 2 continues for several days after the end of the warming too, but this could be a mix from the well-known semiannual ionospheric variations. There were enhancements and depression in Δf o F 2 even before the start of the warming. [43] Figures 6b and 6c show variations of Δf o F 2 for the low-midlatitude stations Yamagawa and Kokubunji. Similar enhancements in Δf o F 2, as observed at Okinawa, are also seen at these stations for similar duration and time. There was not any evidence of any depression in Δf o F 2. Minor enhancements in Δf o F 2 at the midlatitude station Wakkanai (Figure 6d) are also seen. [44] Ionospheric response to this SSW event at an equatorial anomaly station Bhopal is examined in Figure 6e. It can be noted that there are periods of depression and enhancements throughout this SSW. Depressions are in the early mornings and soon after the noons, whereas enhancements are observed during prenoons and evenings, providing some evidence of about 6 h periodicity in the SSW-related ionospheric perturbations. Further, there is a lot of variability in the ionosphere during the warming period. The change (increase, as well as decrease) could be more than 200% in the peak electron density during the warming as also seen at the low-latitude station Okinawa. [45] As noted for the SSWs of January 2008 and 2009, there were periods of enhancements and depressions in Δf o F 2 even before and after the end of the warming, as for this event. There was not any activity in the solar and geomagnetic indices though. [46] From the above observations, we find that the SSW of 2007 affected ionospheres of low as well as midlatitude stations, and there was some uniformity in terms of times of effect at the Japanese stations. At the Indian station Bhopal, the ionospheric changes were somewhat different in terms of times and intensity, thus pointing toward the role of geographic longitude. Again, there was not any evidence for the warming-related semidiurnal perturbation at any of the station for this event too. On the other hand, there was some evidence of a 6 h periodicity at EIA Crest station Bhopal. 6745

11 Figure 9. Same as that of Figure 8 but for days 1 to 70 for the year Figure 10. (a) EEJ, (b) f o F 2 over Bhopal during January to March 2007, and (c d) their Lomb-Scargle spectral analysis. 6746

12 Figure 11. Plot of daily variation of maximum EEJ strength (EEJ max ) at Trivandrum for the first 6 months of the years 2009, 2008, and Dark black lines represent the SSW period, and the dotted black lines represent the dates of peak warmings. 4. F Region Variability [47] The variability in low and equatorial ionosphere is strongly influenced by the equatorial ionization anomaly which is, by and large, related to the equatorial electrojet (EEJ) [e.g., Sethia et al., 1980; Balan and Iyer, 1983; Rastogi and Klobuchar, 1990; Bagiya et al., 2009; Sripathi and Bhattacharyya, 2012]. [48] To examine whether any correlation exists between the variabilities observed in f o F 2 at different latitudes and EEJ strength during SSW events, we analyzed data from two Indian stations, Trivandrum (an equatorial station) and Bhopal (the equatorial anomaly, EA, station) and then compared it with strength of equatorial electrojet (EEJ). The EEJ strength was derived by subtracting variation in horizontal component of magnetic field at Alibag (18.5 N, 72.9 E; dip latitude: 13.0 N) and was obtained by eliminating the contribution of the Earth s main field from data from that at Tirunelveli (8.7 N, 77.8 E; dip latitude: 0.4 N). [49] In Figure 7, we show the variations (change from normal quiet period) of Δf o F 2 at (Figure 7a) Bhopal and at (Figure 7b) Trivandrum along with the EEJ strength (Figure 7c) during a period of 60 days, about 15 days before and 15 days after the end of the warming of It can be noted from the figures that the EEJ strength increases during the warming period especially around the second warming peak. Further, the variation in Δf o F 2 at Bhopal generally follows the variation in EEJ strength. As expected, prenoon Δf o F 2 enhancements at Bhopal are generally accompanied by depression at Trivandrum. Counterelectrojets in the afternoon soon after the peak warming and throughout most of the daytime later are also seen during the warming period, as reported previously by several researchers [e.g., Sridharan et al., 2009; Vineeth et al., 2009; Liu et al., 2011a]. [50] The large increase in f o F 2 at Bhopal following the SSW event can thus be interpreted as an increase in the equatorial ion drift, in agreement with observations of Chau et al. [2009, 2010], Goncharenko et al. [2010a, 2010b], and Liu et al. [2011a], who reported similar increases in TEC at low latitudes during the SSW events of 2008 and We demonstrate this further in Figures 8 and 9 which show plots of the five-point running average of f o F 2 and EEJ strength at 11:00 and 14:00 (when abrupt enhancements in Δf o F 2 were noted) for a period of 70 days for the years 2007 and 2008, respectively. It can be seen from this figure that for the 2007 event, EEJ strength increases soon after the warming and so does f o F 2 at Bhopal. The average f o F 2 remains high, and a similar feature is seen in EEJ strength. [51] The variations in f o F 2 generally follow the EEJ strength even before the warming, thus relating the f o F 2 variability to vertical ion drifts at the equator. The f o F 2 values at Trivandrum, as expected, show a negative relationship with EEJaswellaswithf o F 2 at Bhopal. There is a steep fall in the EEJ strength (days and 60 65) indicating a possible association with the f o F 2 depressions seen during these periods. [52] For the 2008 event, there was a long period of CEJ soon after the start of the warming, but the f o F 2 did not show any decrease. This was followed by a high strength EEJ. Consequently, f o F 2 showed an enormous increase. This is reflected in Δf o F 2 plot (Figure 5e) at Bhopal around the fourth warming peak (day 55) for 1100 LT. However, f o F 2 decreased during the CEJ at 1400 LT. This was followed by a dramatic increase in f o F 2 (day 31) in response to increase in EEJ strength. [53] An important feature to be noted in Figure 8 is that there is some kind of periodicity in f o F 2, as well as in EEJ strength, especially for periods before and during the SSW. This periodicity gets perturbed after the end of the warming. We estimate a period of about days from these data. However, such periodicity is not as apparent in the 2008 SSW event. Dates for full (white circles) and new moon (dark circles) are marked in the figures. So there is some evidence for a possible role of lunar tide in f o F 2 changes during SSWs [e.g., Fejer et al., 2010, 2011]. [54] To examine whether any periodicity in precipitable water time scale (12 16 days) exists in EEJ strength or F 2 layer critical frequency (f o F 2 ) during SSW event, we have performed Lomb-Scargle spectral analysis [Lomb, 1976; Scargle, 1982]. This analysis was carried out on daily f o F 2 values at EA station Bhopal and the EEJ strength for the first 3 months of year 2007 for 10:00,12:00, and 14:00 LT. We show the normalized power spectral density of EEJ strength along with F 2 layer critical frequency (f o F 2 ) for one of the local times, that is, 10:00 IST. The Lomb-Scargle periodogram for EEJ strength is shown in Figure 10c, and that of F 2 layer critical frequency (f o F 2 ) is shown in Figure 10d. This can be 6747

13 Figure 12. Plot of f o F 2 values at various local times for different stations during the first 6 months of the year The red dots show the median f o F 2 values for the respective local times. The green dots indicate the maximum and the minimum value observed during the warming period. noted from the spectral analysis that a dominant period close to days is predominantly seen both in EEJ strength and in F 2 layer critical frequency at Bhopal at 10:00 h. However, apart from day periodicity, periods of 10, 17.5, 30 36, and 48 days were also observed in the EEJ strength and f o F 2 at different times. However, analysis of EEJ strength and f o F 2 data for the 2008 SSW event did not show this day periodicity. [55] In Figure 11, we demonstrate the day-to-day variations in the maximum EEJ strength (EEJ max ) at Trivandrum for the first 6 months of the years 2007, 2008, and The duration periods for the three warming events are also shown. It can be observed from Figure 11 that the highest values of this parameter are generally seen during the stratospheric warmings. 5. Discussion [56] Several studies, as mentioned in section 1 and also recently reviewed by Chau et al. [2012], have shown that there are significant ionospheric variations, especially at the equatorial and low latitudes during SSW events. While the physical processes which generate these variations are not yet fully understood, it is generally believed that there is a vertical coupling between the lower and upper atmosphere during these events. This coupling is not direct and is thought to be due to nonlinear interaction of quasi-stationary planetary waves (QSPW) and migrating tides [e.g., Chau et al., 2009; Sridharan et al., 2009; Goncharenko et al., 2010a, 2010b; Fejer et al., 2010; Yue et al., 2010; Pedatella and Forbes, 2010; Liu et al., 2011a]. It is known that there is a growth of QSPWs during SSWs, and this interaction between QSPWs and tides generates increased variability in the amplitudes and phases of resulting atmospheric tides. These, in turn, control the E region ionospheric dynamo in the equatorial regions. Any modulation in the wind field would then change the zonal electric field at the magnetic equator, thereby affecting the low-latitude ionosphere through the well-known fountain effect. The ionospheric current can also get modulated due to superposition of the lunar semidiurnal 6748

14 tides on the solar quiet current system [Vial and Forbes, 1994]. These lunar tides can get strongly enhanced during SSWs as proposed by [Fejer et al., 2010]. [57] In our study on the response of ionospheric F region to the SSWs of January 2009, January 2008, and February 2007 in the Asian zone (covering a broad latitudinal range from 23.2 N to 45.1 N), we found that while there were perceptible variations in the F 2 layer critical frequency at all stations soon after the commencement of SSWs, there was no uniformity in terms of other characteristics. For the SSW of 2009, for example, there was some evidence of semidiurnal perturbation, but no such activity was noticed for the SSWs of 2008 and Similarly, while the perturbations in Δf o F 2 in terms of magnitude and time for the events of 2009 were similar to those for the event of 2008 at the various stations, these were quite different at the two low-latitude stations Okinawa and Bhopal for the 2007 SSW, thus pointing toward the longitudinal differences in the F region changes following an SSW. Further, we would like to mention that the event of 2007 gave some evidence for the day lunar-related periodicity in EEJ and f o F 2 variations at the EIA Crest station Bhopal but not the other two events. Shorter periodicities of 3 4 daysand 4 5 days were noted for the SSW event of 2009 and 2008, respectively. [58] From our analysis, we find that there are no well-set or organized kind of ionospheric perturbations which follow an SSW. The most uncontroversial statement would be that the ionospheric F region is highly perturbed during SSWs. [59] The complexity and thus the elusiveness of the F 2 region is well known [e.g., Rishbeth, 2000, 2006; Rishbeth et al., 2000; Rishbeth and Mendillo, 2001; Forbes et al., 2000; Mendillo et al., 2002; Mayr and Mahajan, 1971]. We had therefore, while identifying the possible effects of the three SSW events in section 3, been pointing out the other periods of enhanced and depleted f o F 2 in the data presented. We also pointed out some of these anomalous cases even before the start of warming. Most of these features are linked to the large and often unexpected variability in the ionospheric F 2 region even at times when the solar and geomagnetic indices are low and quite stable. [60] We demonstrate the extent of this variability in Figure 12 where we have plotted the observed f o F 2 values for6to18ltforthefirst 6 months of the year 2008 for the stations analyzed by us. Variation in f o F 2 F 2 values (minimum and maximum) during the warming periods is shown by green dots. Red dots represent the median values of f o F 2 F 2 for respective local times. Although each station shows a very good diurnal pattern, there are large day-to-day and hour-to-hour fluctuations at each of the stations. It can be seen from this figure that during the SSW event of 2008, the variations in f o F 2 F 2 values were close to the maximum and minimum excursions during the 6 month period. Similar results were seen for the 2009 and 2007 SSW events. We would like to mention that the data cover the first 6 months of each year and thus include all the well-known diurnal and seasonal variations. It can also be noted from the figure that the f o F 2 variations are comparable to and sometimes much larger than those seen by us following the three SSWs events. The solar and magnetic indices were low and generally quiet stable. [61] The well-known physical processes that control the ionosphere F 2 region, in addition to solar-ionizing radiation and magnetospheric activity, depend upon neutral atmosphere and electrodynamics. While there are daily indices like F 10.7 and Ap for solar-ionizing radiation and magnetospheric activity, respectively, there are none to represent daily changes in neutral atmosphere and electrodynamics. This, indeed, is the major reason for our inability to explain the day-to-day and hour-to-hour variability of the ionosphere F 2 layer. A good part of this variability at low latitudes, as is well known, is related to the changes in vertical ion drifts at the equator and to the neutral composition. 6. Summary and Conclusions [62] Based on our investigation on the extent of ionospheric changes observed following the three SSW events of 2007, 2008, and 2009 using ionosonde data from six different stations in the Asian zone, the following conclusion are drawn from the analysis. [63] 1. There are perceptible ionospheric perturbations which can be linked to these warmings. [64] 2. These perturbations are in the form of enhancements and depressions in f o F 2 resulting in peak electron density variations which could be larger than 200% when compared with the normal magnetically quiet time ionosphere. [65] 3. The low-latitude station Okinawa (26.6 N, E) showed semidiurnal ionospheric perturbations during the SSW event of 2009, a feature previously reported from the total electron content measurements [e.g., Goncharenko et al., 2010b; Liu et al., 2011a; Anderson and Araujo-Pradere, 2010; Yue et al., 2010; Sripathi and Bhattacharyya, 2012]. This feature was not found during the SSW event of On the other hand, the EIA Crest station Bhopal (23.29 N, E) showed some evidence of 6 h periodicity in ionospheric changes during the SSW event of [66] 4. During the SSW event of 2008, Δf o F 2 was predominantly depressed soon after the first warming. In the other two cases, Δf o F 2 was enhanced. [67] 5.A day periodicity in ionospheric perturbations was seen for the SSW event of Similar periodicity in ionospheric perturbations has earlier been reported by Fejer et al. [2010, 2011] and Sripathi and Bhattacharyya [2012] during such SSW events. However, we found some evidence of 3 4 and 4 5 day periodicities in ionospheric perturbation following the SSW events of 2009 and 2008, respectively. [68] 6. The highest and lowest values of f o F 2 during the 6 month period, January to June for the years 2007, 2008, and 2009, generally occurred during the SSW events at almost all the stations. [69] 7. In view of the large variability within the ionosphere, it is difficult to quantify the changes as well as the response times in the ionosphere to these stratospheric warmings. [70] Acknowledgments. The authors are thankful to NICT Japan World Data Center for making the ionospheric data available on its web site. K.K. Mahajan is thankful to the Indian National Science Academy for the award of INSA Honorary Scientist Scheme. This work was also supported by ISRO s PLANEX program, coordinated by PRL, Ahmedabad, and A.K. Upadhayaya is grateful to CSIR for providing the facilities. The authors are thankful to Raj Kumar Choudhary (VSSC) for providing Trivandrum Ionosonde data. Thanks also go to B.M. Pathan for providing processed geomagnetic data. The authors thank the reviewers of this paper for their comments and helpful suggestions. [71] Robert Lysak thanks the reviewers for their assistance in evaluating this paper. 6749