[1] The Weddell Sea Anomaly (WSA) in the ionosphere is characterized by higher plasma densities at night than during the day in the region near the Weddell Sea. According to previous studies on the WSA, it is known to occur mostly in southern summer and has not been reported in other seasons. We have utilized more than 13 years of TOPEX total electron content (TEC) measurements in order to study how the WSA varies with seasons and how it changes with solar activity. The TOPEX TEC data have been extensively utilized for climatological studies of the ionosphere because of their excellent spatial and temporal coverage. We investigate the seasonal and solar activity variations of the WSA using four seasonal cases (March equinox, June solstice, September equinox, and December solstice) and two solar activity conditions (F10.7 < 120 for solar minimum and F10.7 > 120 for solar maximum conditions) for geomagnetically quiet periods. Our analysis shows that (1) the WSA occurs only in the southern summer hemisphere for low F10.7, as in previous studies, but (2) the WSA occurs in all seasons except for winter when F10.7 is high; it is most prominent during the December solstice (southern summer) and still strong during both equinoxes. The TOPEX TEC maps in the midlatitude and high-latitude ionosphere display significant global longitudinal variations for a given local time in the Southern Hemisphere, which varies with season and solar activity. The observed WSA appears to be an extreme manifestation of the longitudinal variations.

[1] We compared the global plasmaspheric total electron content (pTEC) with the ionospheric TEC (iTEC) simultaneously measured by Jason-1 satellite during the declining phase of solar cycle 23 (2002-2009) to investigate the global morphology of the plasmaspheric density in relation to the ionosphere. Our study showed that the plasmaspheric density structures fundamentally follow the ionosphere, but there are also significant differences between them. Although the diurnal variations are very similar to each region, the plasmasphere shows much weaker variations, only approximately 1 TECU day-night difference. By analyzing the day-night differences in the plasmasphere, we found that the plasmaspheric contribution to the nighttime ionosphere does not increase with solar activity and the largest contribution occurs during June solstice. The plasmasphere shows similar seasonal variations to the ionosphere, except for the semiannual variation, which is essentially absent in the plasmasphere. There is also an important difference in the annual variation: although the annual variation in the ionosphere exists regardless of longitude, it occurs only at American sector in the plasmasphere. As solar activity increases to moderate level, the pTEC substantially enhances from approximately 2 to 4 TECU at the initial increase of solar activity below F10.7p = 100 and then quickly slows down while the iTEC almost linearly enhances. Although it is well known that magnetic storms are the major source of plasmaspheric density depletion, pTEC does not show this aspect of the plasmasphere probably due to the relatively small K p values for high magnetic activity

[1] TOPEX/Poseidon mission has provided an extensive database of vertical TEC over the ocean since August 1992. Data from nearly 10 years of TOPEX TEC observations were analyzed to study the TEC climatology. First, TEC data were binned by season, geomagnetic activity, and solar activity to create longitudinally averaged TEC maps in magnetic latitude and local time. These maps show the annual and semiannual anomalies well known from climatological studies of N m F 2 but lack the seasonal anomaly because of the longitudinally averaged binning. The equatorial anomaly is the most prominent feature in the maps, and they show strong TEC variations with solar activity but relatively weak variations with geomagnetic activity in our three K p bins. Compared with the low solar activity conditions (F10.7 < 120), the TEC values for F10.7 ! 120 are much larger and the equatorial anomaly lasts longer into the night, up to midnight. During geomagnetically active periods, the TEC maps generally show a noticeable increase at low latitudes for F10.7 < 120, but this effect is barely detectable for F10.7 ! 120. Finally, three longitudinal bins (Indian, Pacific, and Atlantic) were added in order to see how the TEC morphology varies with longitude. The TEC measurements display strong longitudinal variations that closely follow the longitudinal variation of the magnetic declination. In the southern Pacific, where the declination is positive and large, the diurnal TEC variations significantly differ from those in the other longitude sectors, where the magnetic declination is negative in the Southern Hemisphere. Also, at noon, the phase of the longitudinal TEC variation is typically opposite to that at midnight.

We report on a detailed global climatology of medium-scale (150-600 km) thermospheric gravity wave (GW) activity using mass density observations onboard the CHAMP satellite from 2001 to 2010. Our study focuses mainly on daytime (09-18 h in local time) and midlatitude/low-latitude upper thermosphere between 300 km and 400 km altitudes. Midlatitude GW activity is strongest in the winter hemisphere. GW activity during June solstice adjacent to the Andes and Antarctic Peninsula is stronger than in any other season or location. GW activity in the low-latitude summer hemisphere is stronger above continents than above oceans: especially during December solstice and equinoxes. In terms of relative density variation, GW activity is stronger during solar minimum than solar maximum. These results agree well with the characteristics of stratospheric GWs, implying that the CHAMP GWs are mainly caused by GWs from tropospheric/stratospheric processes. Using mesosphere/lower thermosphere wind observations at a Korean Antarctic station, we investigated at which altitudes the upper thermospheric GW climatology becomes visible. While the correlation is insignificant at z = 82-88 km, it becomes significant for most cases at z = 90-98 km, suggesting that the upper thermospheric GW climatology may start to emerge at z ≥ 90 km.

We statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosynchronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to identify the geosynchronous EMIC waves, we use high time resolution magnetic field data acquired from GOES 10, 11, and 12 over a 2 year period from 2007 and 2008 and examine the local time distribution of EMIC wave events. Unlike previous studies, which reported high EMIC wave occurrence in the postnoon sector with a peak around 1500–1600 magnetic local time (MLT) during magnetically disturbed times (i.e., storm and/or substorm), we observed that quiet time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100–1200 MLT. To investigate whether the quiet time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure variations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and interplanetary magnetic field Bz) and geomagnetic indices (AE and SYM‐H). From the superposed epoch analysis we found that solar wind dynamic pressure variation is a more important parameter than AE and SYM‐H for quiet time EMIC wave occurrence.