re e fir udd e e ude eigh ons ly t onse The sudden stratospheric warming (SSW) is a violent large- scale thermo-dynamical phenomenon in the winter polar region, which strongly affects the middle atmosphere, causing significant the b ve acti h latit osed b growt their teract inds s a do eating indicated that although the planetary wave activity related to Recent studies have clearly identified the large perturbations of ion temperature, ion drift and total electron density during SSW events (Goncharenko and Zhang, 2008; Chau et al., 2009; s the . The stem to SSWs is mainly due to the electrodynamic effect of the Contents lists available at ScienceDirect .e Journal of Atmospheric and Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1697–1702 response to the SSWs in 2008 and 2009. It utilizes the ionosphericE-mail address:
[email protected] (D. Pancheva). SSW is concentrated at high latitudes, the nonlinear interaction disturbed lower thermospheric wind system (composed of chan- ged circulation and enhanced migrating and nonmigrating tides), which produces atypical dynamo electric fields during SSWs. This study presents new evidence of the global ionospheric 1364-6826/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2011.03.006 n Corresponding author. and migrating tides seen in the SABER temperatures during the 2003/2004 major SSW. Modeling results by Liu et al. (2010) driving the currents arises largely from tides propagating upward from the lower atmosphere. Therefore, the ionospheric response coupling by tides has just recently been attracted. Pancheva et al. (2009a) reported enhanced nonmigrating tides associated with nonlinear interaction between stationary planetary waves (SPWs) generates the equatorial ionization anomaly (EIA), which i most dominant feature in the global ionosphere structure E-region dynamo is strong during the day; the wind sy lation in the mesosphere resulting in adiabatic cooling (Liu and Roble, 2002). While the vertical coupling from stratosphere to lower ther- mosphere by planetary waves during SSWs is well studied (Shepherd et al., 2007 and references therein), the attention to fields are driven by a combination of the E- and F-region dynamo processes. The net result at the magnetic equator is eastward electric fields (upward plasma drift) during the day and westward electric fields (downward drift) at night. The upward plasma transport induced by the electrodynamic drift on the dayside variations in the mesosphere–lower It involves considerable changes in perature, planetary and gravity wa of ozone and other chemicals at hig ism behind the SSW, initially prop now widely accepted, relates to the ing transient planetary waves and with the zonal mean flow. The in reverses the eastward winter w mesosphere system and also induce the stratosphere causing adiabatic h osphere (MLT) as well. ackground wind, tem- vity and redistribution udes. The key mechan- y Matsuno (1971) and h of upward propagat- nonlinear interaction ion decelerates and/or in the stratosphere– wnward circulation in and an upward circu- Goncharenko et al., 2010a,b). These observational results have been facilitated by the appearance of strong SSW events in 2008 and 2009 and very low level of geomagnetic activity since 2008 because of the extreme solar minimum. These are ideal conditions for studying vertical coupling between different atmospheric regions and they allow unambiguous determination of the ionospheric effects related to forcing from below. Because the SSWs are related to rapid enhancement of SPWs, migrating and nonmigrating tides as well as the circulation changes, the observed ionospheric variability could be related to these dynamical processes. It is known that during quiet times, the low-latitude electric therm Stratospheric warmings: The atmosphe Dora Pancheva n, Plamen Mukhtarov Geophysical Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria a r t i c l e i n f o Article history: Received 14 January 2011 Received in revised form 1 March 2011 Accepted 9 March 2011 Available online 21 March 2011 Keywords: Stratospheric warming Vertical plasma drift Atmosphere–ionosphere coupling a b s t r a c t The paper presents for th ionospheric response to s 2008/2009. To elucidate th density data at fixed altit electron density at fixed h high latitudes. Similar resp response is confined main negative ionospheric resp 1. Introduction journal homepage: www –ionosphere coupling paradigm st time the global spatial (latitude and altitude) structure of the mean en stratospheric warming (SSW) events in winters of 2007/2008 and ffect of the SSWs on the ionosphere the COSMIC foF2, hmF2, and electron s are analyzed. Both the mean foF2 and hmF2 parameters and the mean ts indicate regular negative responses to the SSW temperature pulses at e is found for the diurnal variability of the COSMIC electron density. The o low and middle latitudes. A possible mechanism causing the observed is suggested. & 2011 Elsevier Ltd. All rights reserved. between tides and SPWs enhances migrating and nonmigrating tides globally. lsevier.com/locate/jastp Solar-Terrestrial Physics X4 2p 2p� � 60, 90, 115 and 140. The foF2 and hmF2 response in the second D. Pancheva, P. Mukhtarov / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1697–17021698 Yðt,lÞ ¼Y0þ s ¼ 1 Bs cos 360 sl� 360 cs þ X3 k ¼ 1 X4 s ¼ �4 Cks cos k 2p 24 t� 2p 360 sl� 2p 360 gks � � ð1Þ where t is time in hours counted from 0 UT on 1 October, l is longitude in degrees, s- is wavenumber and k is the number of diurnal harmonics. The eastward (westward) propagation corre- measurements of the Constellation Observing System for Meteor- ology, Ionosphere and Climate (COSMIC), as the COSMIC electron density profiles are used to determine the ionospheric F2 layer critical frequency (foF2) and peak height (hmF2) as well as the electron density at fixed heights. The zonal mean temperature at �10 hPa pressure level and at latitude of 601N is used from the UK Met Office assimilated data (Swinbank and Ortland, 2003). 2. Observations and method for data analysis The electron density profiles employed in this study were retrieved by the COSMIC satellites using the radio occultation inversion technique. The advantages of making use of the COS- MIC-measured data to study the F-region electron density varia- bility are nearly uniform global distribution of the electron density profiles with sufficiently fine height and horizontal resolutions, which cannot be achieved for the data taken from the conventional ground-based ionosonde networks. The electron density data were downloaded from the web site http://cosmi c-io.cosmic.ucar.edu/cdaac/. The electron density data were sorted out according to (i) latitude, at each 101 between 801S and 801N; (ii) longitude, at each 151; (iii) altitude, at each 25 km between 100 and 800 km (this height resolution is used only when we consider the electron density at fixed height); and (iv) time, for each UT hour. The F-region is characterized by its foF2 and hmF2. These parameters have their real values obtained from the profiles. It has been already mentioned that the ionospheric response to SSWs can be caused mainly by the variability of the electrody- namic drifts due to the disturbed wind system. As the wind system in the lower thermosphere is shaped largely by tides forced from below, the ionospheric response has to be connected with the enhanced migrating and nonmigrating tides during the SSWs. In order to extract the ionospheric tidal responses we use similar approach to that described in Pancheva et al. (2009b). This means that we have to separate from the COSMIC foF2, hmF2 and electron densities the waves with periods of 24, 12 and 8 h with zonal wavenumbers up to 4, as well as planetary waves (sta- tionary and propagating) with zonal wavenumbers up to 4. The preliminary spectral analysis of both winters revealed relatively weak traveling planetary waves. Only the zonally symmetric waves (zonal wavenumber zero) indicated large amplitudes, but it was found that they are due to external forcing (Lei et al., 2008). It is known that the SSWs can last for several days or weeks. Goncharenko et al. (2010a) found that the perturbations in the EIA were observed for up to 3 weeks after the peak in high- latitude stratospheric temperatures. Therefore, the ionospheric tidal response has to be determined using as small window as possible. In this case the smallest possible window was found to be 16 days. Then the 16-day window is moved through the time series with steps of 1 day in order to obtain the daily values of the wave characteristics for both winters (the data for intervals 1 September–30 April are analyzed and results for 1 October–30 March are considered here). The waves at given latitude and height are extracted using the following expression: sponds to s40 (so0). The three terms on the right hand side of winter is presented only by one strong negative response cen- tered near day number 120, i.e. again 3–4 days delay with respect to the temperature peak from Fig. 1a (right plot). The reduction of foF2 in both winters is on the average of �0.7–0.8 MHz, which means �10% change of the foF2, while the reduction of the hmF2 is on the average of �10–12 km, which is �4% change of the hmF2. It is important to note that while the zonal mean tempera- tures from Fig. 1a are daily data, the foF2 and hmF2 response data from Fig. 1b are obtained using 16-day window, i.e. they are 16- day averaged data. That is why the ionospheric responses to temperature peaks at a distance less than 16 days cannot be separated. Such a case is observed in winter 2007/2008, where the close temperature peaks are marked by oval; their iono- spheric response is centered just between them. Fig. 2a presents the latitude structure of the COSMIC mean electron density response at altitudes of 400 km (upper row of the above expression are as follows: (i) the mean ionospheric parameter Y0 (zonal and time mean), (ii) stationary planetary waves with amplitudes Bs and phases cs and (iii) 24,12 and 8 h tides with amplitudes Cks and phases gks. In the above expression the phases of the tides are defined as the time of the wave maxima at zero longitude and are presented in degrees, while the phase of the stationary wave—as the longitude of the wave maximum. It is important to note that the results for the COSMIC foF2, hmF2 and electron density sun-synchronized 24 h (DW1), 12 h (SW2) and 8 h (TW3) waves are produced by a combination of the diurnal variability of photoionization, which depends on the solar zenith angle and the effect of the migrating diurnal, semidiurnal and terdiurnal tides forced from below. The results in this communication are presented in the modified dip latitude (modip) frame introduced by Rawer (1984). 3. Results In this communication we will show only the mean (zonal and time) and migrating 24 h response of the ionosphere related to the SSWs in the winters of 2007/2008 and 2008/2009. The results of the full analysis will be presented in a next study. The SSW events as well as the solar and geomagnetic activity of the considered winters are described in detail by Goncharenko et al. (2010a,b). Here only the UKMO temperature at �10 hPa pressure level and 601N latitude is used to characterize the SSWs. The ionospheric response to SSWs in both winters will be considered together. We will start with the response of the ionospheric parameters foF2 and hmF2 as they are defined by ground-based ionosonde as well. Fig. 1a presents the zonal mean daily temperature at �10 hPa pressure level and 601N latitude for winters 2007/2008 (left plot) and 2008/2009 (right plot). It is seen that both winters are very different; while the first winter is composed of a sequence of minor warming events and a major final warming (around day number 146), the second winter presents a unique event that was particularly strong and long-lasting; the peak warming at �10 hPa level was reached on 24 Jan 2009 (day number 116). Fig. 1b shows the latitude structure of the COSMIC mean (zonal and time) foF2 (upper row of plots) and hmF2 (bottom row of plots) parameters for winters 2007/2008 (left column of plots) and 2008/2009 (right column of plots). It is seen that the mean responses of foF2 and hmF2 are also very different. The iono- spheric response in the first winter is composed of several foF2 and hmF2 decreases, which indicate slight delay (�2–3 days) with respect to the temperature pulses from Fig. 1a (left plot), i.e. the negative ionospheric responses are centered near day numbers plots) and 300 km (bottom row of plots) for winters 2007/2008 D. Pancheva, P. Mukhtarov / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1697–1702 1699 2007/2008 Day Number (start 1 Oct 2007) 200 210 220 230 240 Te m pe ra tu re (K ) UKMO, ZM Temp (60N, ~10 hPa) -50 -30 -10 10 30 50 70 od ip L at itu de (d eg re e) COSMIC ZM foF2 3.2 4.4 5.6 6.8 8.0 0 30 60 90 120 150 180 (left column of plots) and 2008/2009 (right column of plots). Again the ionospheric response is different for both winters. While the response in 2007/2008 is composed of several decreases of the electron density centered near days 60, 90, 115 and 140 (responses to the temperature peaks from Fig. 1a—left plot), the winter 2008/2009 consisted of mainly one electron density reduction centered near day 120, which is a few days delayed response to the temperature peak from Fig. 1a (right plot). Fig. 2a shows two main features of the latitude structure of the mean electron density response: (i) the ionospheric response amplifies toward the equator and (ii) it is not symmetrical with respect to the modip latitude; the response is mainly in the Northern Hemisphere (NH). Fig. 2b shows the altitude structure of the mean electron density response for the winters 2007/2008 (left plot) and 2008/ 2009 (right plot). Again different temporal variabilities of the ionospheric responses in both winters can be distinguished; several decreases in 2007/2008 versus only one reduction in 2008/2009. Again all ionospheric perturbations happen with 2–3 days delay with respect to the stratospheric temperature peaks as in Fig. 1a. The altitude structure of the mean electron density shows that the response is observed mainly above the F-region maximum (above �300 km height). -70 M 2.0 0 Day Number (start 1 October 2007) -70 -50 -30 -10 10 30 50 70 M od ip L at itu de (d eg re e) COSMIC ZM hmF2 215 235 255 275 295 315 30 60 90 120 150 180 0 30 60 90 120 150 180 Fig. 1. (a) Zonal mean daily temperature in Kelvin at �10 hPa pressure level and 601N oval peaks are situated at a distance shorter than 16 days; (b) latitude structure of the (bottom row of plots) parameters for winters 2007/2008 (left column of plots) and 20 2008/2009 Day Number (start 1 Oct 2008) 200 210 220 230 240 250 Te m pe ra tu re (K ) UKMO, ZM Temp (60N, ~10 hPa) -50 -30 -10 10 30 50 70 od ip L at itu de (d eg re e) COSMIC ZM foF2 3.2 4.1 5.1 6.0 7.0 0 30 60 90 120 150 180 We mentioned before that the DW1 electron density oscilla- tion is produced by a combination of the diurnal variability of the photoionization and the effect of the migrating diurnal tide forced from below and that the used data analysis method cannot separate the two effects. However, Mukhtarov and Pancheva (2011) presented evidence indicating that the DW1 electron density oscillation is shaped mainly by the photoionization, i.e. is due to a solar zenith angle effect. This is the reason for this oscillation to be called diurnal variability of the ionosphere. Fig. 3a shows the latitude structure of the COSMIC electron density diurnal variability at an altitude of 400 km in winters 2007/2008 (left plot) and 2008/2009 (right plot). Again different temporal variabilities of the ionospheric response in both winters is detected. Several falls of the diurnal amplitude in the first winter, centered near days 60, 90, 120 and 145 (very weak), can be seen. A single strong reduction of the diurnal amplitude in the second winter, centered near day 120, can be distinguished; it is preceded by an increase of the diurnal amplitude centered near day 105. The average reduction of the electron density at 400 km height during both winters is �0.3 MHz, which means �11% change of the electron density. The latitude structure of the COSMIC diurnal variability clearly indicates a response only in the NH. -70 M 2.2 Day Number (start 1 October 2008) -70 -50 -30 -10 10 30 50 70 M od ip L at itu de (d eg re e) COSMIC ZM hmF2 210 222 234 246 258 270 282 294 0 30 60 90 120 150 180 0 30 60 90 120 150 180 latitude for winters 2007/2008 (left plot) and 2008/2009 (right plot); the marked COSMIC mean (zonal and time) foF2 in MHz (upper row of plots) and hmF2 in km 08/2009 (right column of plots). 30eg re 3.9 D. Pancheva, P. Mukhtarov / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1697–17021700 -70 -50 -30 -10 10 M od ip L at itu de (d 1.0 1.7 2.4 3.2 0 30 60 90 120 150 180 2007/2008 50 70 e) COSMIC ZM (h=400 km) 4.6 Fig. 3b displays the altitude structure of the COSMIC electron density diurnal variability at modip latitude of 20oN in winters 2007/2008 (left plot) and 2008/2009 (right plot). The temporal variability of the diurnal response shows the same feature as its latitude structure. The altitude structure of the diurnal response, particularly that in the winter of 2008/2009, indicates that the response happens mainly above the F-region electron density maximum, e.g. above 300–350 km height. 4. Discussion and conclusions In this paper we have presented the global spatial (latitude and altitude) structure and temporal variability of the mean ionospheric response to SSW events in winters of 2007/2008 Day Number (start 1 October 2007) -70 -50 -30 -10 10 30 50 70 M od ip L at itu de (d eg re e) COSMIC ZM (h=300 km) 1.5 2.1 2.7 3.3 3.9 4.5 5.1 0 Day Number (start 1 October 2007) 100 150 200 250 300 350 400 450 500 550 600 650 700 H ei gh t (k m ) COSMIC ZM (equator) 0.0 0.8 1.7 2.5 3.4 4.2 5.0 30 60 90 120 150 180 0 30 60 90 120 150 180 Fig. 2. (a) Latitude structure of the COSMIC mean (zonal and time) electron density (in M for winters 2007/2008 (left plot) and 2008/2009 (right plot); (b) altitude structure of the and 2008/2009 (right plot). 2008/2009 -70 -50 -30 -10 10 30 50 70 M od ip L at itu de (d eg re e) COSMIC ZM (h=400 km) 0.9 1.6 2.3 2.9 3.6 4.3 0 30 60 90 120 150 180 and 2008/2009. To elucidate the effect of the SSWs on the ionosphere the COSMIC foF2, hmF2 and electron density data at fixed altitudes are analyzed in terms of their mean (zonal and time) values, various migrating and nonmigrating tidal and SPW components (see formula (1)). The results only for the zonal and time means (Y0 from formula (1)) and diurnal (migrating) variability have been reported here. As presented in Fig. 1b, the negative response to the SSW events of the mean foF2 and hmF2 parameters indicates that such ionospheric variability can be detected by ionosonde measurements as well. Many years ago Pancheva and Spassov (1989) investigated the effect of the SSWs on the ionospheric D- and F- region variabilities by analyzing the radio-wave absorption and ionosonde foF2 data, respectively. The authors found negative responses of both regions; usually the F-region response lags �1–2 days behind that of the D-region. Day Number (start 1 October 2008) -70 -50 -30 -10 10 30 50 70 M od ip L at itu de (d eg re e) COSMIC ZM (h=300 km) 1.5 2.2 2.9 3.6 4.3 5.0 Day Number (start 1 October 2008) 100 150 200 250 300 350 400 450 500 550 600 650 700 H ei gh t (k m ) COSMIC ZM (equator) 1.2 1.8 2.4 3.0 3.6 4.2 4.8 0 30 60 90 120 150 180 0 30 60 90 120 150 180 Hz) at altitudes of 400 km (upper row of plots) and 300 km (bottom row of plots) COSMIC mean electron density at modip equator for winters 2007/2008 (left plot) D. Pancheva, P. Mukhtarov / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1697–1702 1701 2007/08 Day Number (start 1 October 2007) -70 -50 -30 -10 10 30 50 70 M od ip L at itu de (d eg re e) COSMIC Diurnal Var (h=400 km) 0.0 0.7 1.4 2.0 2.7 300 350 400 450 500 550 600 H ei gh t (k m ) COSMIC Diurnal Var (20N) 1.2 1.6 2.0 2.4 2.8 0 30 60 90 120 150 180 This study presents regular negative response of the mean and diurnal variability of the COSMIC electron density to the SSW temperature peaks at high-latitude stratosphere. The ionospheric response is confined mainly to low and middle latitudes. The typical timescales of the ionospheric responses are 2–3 weeks, therefore by no means can they be a result of the well known low- latitude semiannual ionospheric variability of �26 weeks. Two main features characterize the latitude structure of the response: (i) it amplifies toward the equator and (ii) it is not symmetrical with respect to the modip latitude; the response is mainly in the NH. The main feature of the altitude structure of the response is that it can be seen mainly above the F-region maximum, i.e. above 300–350 km altitude. The above mentioned features of the considered ionospheric response indicate that it is related to the changes of the vertical plasma drifts driven by the disturbed lower thermospheric wind system. Liu and Roble (2002) found that during the stratospheric warming and mesospheric cooling, also a lower thermospheric warming is detected in the model simulation. Prediction of such heating is also discussed in the WAM model (Fuller-Rowell et al., 2010). Recently by using temperature data from the MIPAS limb emission spectrometer on board ESA’s Envisat satellite Funke et al. (2010) found clear signatures of a mesospheric cooling and a thermospheric warming, as the latter peaking at 120–140 km in agreement with model predictions. Experimental evidences of heating in the lower thermosphere during SSW events are reported also by Goncharenko and Zhang (2008) and Kurihara et al. (2010). 0 Day Number (start 1 October 2007) 100 150 200 250 0.0 0.4 0.8 30 60 90 120 150 180 Fig. 3. (a) Latitude structure of the COSMIC migrating diurnal (DW1) electron density v 2009 (right plot); (b) altitude structure of the COSMIC DW1 electron density variab (right plot). 2008/2009 Day Number (start 1 October 2008) -70 -50 -30 -10 10 30 50 70 M od ip L at itu de (d eg re e) COSMIC Diurnal Var (h=400 km) 0.0 0.5 1.1 1.6 2.2 2.7 300 350 400 450 500 550 600 H ei gh t (k m ) COSMIC Diurnal Var (20N) 0.8 1.1 1.5 1.9 2.3 0 30 60 90 120 150 180 The increase of the zonal mean lower thermospheric temperature at high latitudes (at latitude of 70–901N shown by Funke et al., 2010) can change the observed plasma drifts at low latitudes in a manner similar to the so called ‘‘disturbed dynamo’’ (Blanc and Richmond, 1980). In this case however the disturbed dynamo is a current system driven by the equatorward lower thermospheric winds forced at high latitudes because of thermospheric heating. As the wind move toward the equator it must also move to the west (in NH) to preserve the original angular momentum. In the low and middle latitudes the wind surge has to have a significant zonal (westward) component that would affect the daily mean vertical plasma drift; it has to become predominantly negative (downward) or less positive. In this way the mean ionospheric plasma will be moved generally to the lower altitudes, where the recombination is stronger, i.e. the mean ionospheric response to SSW events will be negative. To find evidence for the above mentioned idea we plan to analyze the ROCSAT vertical plasma drifts by applying the same method as was used here. In conclusion, we note that by a detailed analyses of the derived global mean and diurnal (migrating) variability of the COSMIC electron density at fixed altitudes we for the first time presented experimental evidence that the low and middle latitude ionosphere regularly responds to almost all SSW stratospheric temperature pulses at high latitudes. This result highlights the importance of understanding the temporal variability of the lower atmospheric weather systems and their effect and possible predictability of the ionosphere development. Day Number (start 1 October 2008) 100 150 200 250 0.0 0.4 0 30 60 90 120 150 180 ariability (in MHz) at 400 km altitude for winters 2007/2008 (left plot) and 2008/ ility at modip latitude of 201N for winters 2007/2008 (left plot) and 2008/2009 Acknowledgement We are grateful to the UKMO and the BADC for access to the data on http://www.badc.rl.ac.uk/data/assim and to the COSMIC teams for the access to the data on http://cosmic-io.cosmic.ucar. edu/cdaac/. References Blanc, M., Richmond, A., 1980. The ionospheric disturbance dynamo. Journal of Geophysical Research 85, 1669. Chau, J.L., Fejer, B.G., Goncharenko, L.P., 2009. Quiet variability of equatorial ExB drifts during sudden stratospheric warming event. Geophysical Research Letters 36, L05101. doi:10.1029/2008GL036785. Fuller-Rowell, T., Wu, F., Akmaev, R., Fang, T.-W., Araujo-Pradere, E., 2010. A whole atmosphere model simulation of the impact of a sudden stratospheric warming on thermosphere dynamics and electrodynamics. 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