1. Effects of January 2010 stratospheric sudden warming in the low-latitude ionosphere L. Goncharenko, A. Coster, W. Rideout, MIT Haystack Observatory, USA J. Chau, Jicamarca Radio Observatory, Peru K. Hocke, University of Bern, Switzerland C. Valladares, Boston College, USA Abstract. The winter of 2009-2010 was marked by a significant stratospheric warming event peaking in the end of January 2010. Although this warming was not as strong as SSW events of 2008 and 2009, it presents a perfect opportunity to study the coupling between different atmospheric regions under less extreme circumstances. We use GPS TEC and Jicamarca ISR data to study effects of this SSW event on low-latitude ionosphere. Our results indicate that disturbances in the upper atmosphere during the SSW event of 2010 are consistent with disturbances reported for other SSW cases. We discuss different properties of the observed phenomena and demonstrate unique features of this time interval. Figure 1. Summary of stratospheric and geomagnetic conditions. Peak stratospheric temperature at 90 o N and 10hPa level was reached on 22 January, and temperature remained above the long- term mean for over two-week period. Strong abatement in the zonal mean zonal wind at 60 o N was observed starting around January 10 and continuing well into March 2010. The stratospheric circulation was determined mostly by a planetary wave 1. 1. Stratospheric and geomagnetic conditions Strong PW1 activity, weak PW2 activity SSW peaks on Jan 22, 2010 at 10 hPa level and 90 o N Increase in geomagnetic activity on Feb 1-3 to Kp=3-…4 complicates interpretation 2. Vertical drift over Jicamarca Typical quite-time behavior prior to SSW Variations on Jan 19-20 likely due to increase in Kp Generally lower daytime drift during first days of SSW event Increase in the pre-reversal enhancement during SSW Stronger upward vertical drift in the morning, downward drift in the afternoon during SSW, persisting for several days (Jan 31 – Feb 3) Figure 2. Observations of vertical drift over Jicamarca (red line) in comparison with Fejer- Sherliess drift model (black line) 3. GPS TEC data TEC SSW 21 UT TEC change, %, 15 UT TEC change, %, 21 UT TEC increase in the morning TEC suppression in the afternoon 4. Planetary wave-type oscillations in GPS TEC data Characteristics of the ionospheric oscillations with periods between 2 and 20 days are derived from GPS TEC data at single longitude, 75 o W, range of geographic latitudes from 40 o S to 40 o N, and for the two-month period, January – February, 2010. The GPS TEC data is binned in 1-hour bins for this analysis. We use finite impulse response filter with Hamming window and forward/backward running. Figure 5. Example of temporal variation in the 2-day wave activity at 30 o S and 75 o W for January – February 2010. Blue line shows time series of TEC data in 1-hour bins. Black line presents daily mean TEC (16-day running average) and characterizes seasonal change in TEC. Red line describes 2-day wave, and the green line shows amplitude series (upper envelope). Amplitude of the 2-day wave reaches ~2 TECu prior to the SSW event and decreases during the SSW event. It is not clear if decrease in the 2-day wave amplitude is related to the stratospheric warming. Figure 6. Daily mean TEC (16-day running average) from 40 o S to 40 o N and 75 o W for the January – February, 2010 period. Increase in daily mean TEC after day 40 reflects combined effects of increased solar flux (see F10.7 index in Figure 1) and seasonal change in TEC. Decrease in daily mean TEC is observed concurrently with the SSW event at all latitudes. Analysis of longer time series is planned for the future to examine the effects of varying solar flux and to determine if the decrease in daily mean TEC is related to the SSW event or to the seasonal change. Figure 7. Amplitude of planetary wave-type oscillations in TEC as function of time and wave period for several latitudes (27 o S, 12 o S, 3 o N, 18 o N – southern and northern crests of equatorial ionization anomaly, magnetic equator, and latitude above northern crest). Strongest oscillations are found for the 2- day and 5-6 day periods, though 8-10 day and 16-day waves are also present. Figure 8. Amplitude of planetary wave-type oscillations in TEC as function of time and geographic latitude for selected wave periods (2-day, 5-day, 10-day, 16-day). The 2-day wave activity is dominant and reaches 4 TECu in the EIA crests. Decrease in the 2-day wave is observed during the SSW event at different latitudes. Complex patterns are observed for other wave periods. Figure 4. TEC variations at 75 o W in local time and latitude during the January 2010 SSW event. Top panel – multi-day mean TEC prior to SSW, determined from data on January 3-9, 2010. Lower panels – difference in TEC from the mean state during the SSW. Starting on January 25, semidiurnal signature appears in differential TEC as increase in TEC in the morning and decrease in TEC in the afternoon, with secondary weaker increase at 20-22 LT. The change in TEC reaches maximum values on February 2, 2010. Though the geomagnetic activity increases on February 1-3, essential features of the TEC variation remain the same. SSW signature in TEC data appears as 12-hour wave, with increase in TEC at ~8-10 LT, decrease in TEC at 16-18 LT, and secondary increase at 20- 22 LT. Figure 3. Observations of ionospheric behavior during stratospheric warming. Top panels: Typical distribution of total electron content (TEC) in the western hemisphere at 15 UT (left) and 21 UT (right). Black line shows magnetic equator. Middle panels: TEC in the morning sector (15 UT) and afternoon sector (21 UT) on Jan 31, 2010, during SSW. Increase in TEC is observed in the extended range of longitudes and latitudes in the morning. In contrast to observations during morning hours, in the afternoon TEC is decreased. Bottom panels: TEC change expressed in percents. The observed features are similar to those reported for SSW events of 2008 and 2009, but smaller in magnitude. TEC SSW 15 UT Mean TEC no SSW 15 UT morning Mean TEC no SSW 21 UT afternoon Same as other SSW events Same as other SSW events Same as other SSW events New Conclusions 1.We report perturbations in vertical drifts at the magnetic equator and 50-100% variations in GPS TEC in the American sector during the January 2010 SSW event. 2.Tidal variation in TEC becomes prominent on January 25-26, 2010, after the peak in stratospheric temperature, and lasts for several days. The amplitude of variation is smaller than in SSW of 2009, likely due to the weaker SSW event. 3.We analyze TEC oscillations with periods between 2 and 20 days and report strong 2-day activity subsiding during SSW event. Acknowledgments. Work at the MIT Haystack Observatory has been supported by NSF cooperative agreement with the MIT.