Geonhwa Jee

A climatology of medium‐scale gravity wave activity in the midlatitude/low‐latitude daytime upper thermosphere as observed by CHAMP

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.

Global ionospheric total electron contents (TECs) during the last two solar minimum periods

The last solar minimum period was anomalously extended and low in EUV irradiance compared with previous solar minima. It can readily be expected that the thermosphere and the ionosphere must be correspondingly affected by this low solar activity. While there have been unanimous reports on the thermospheric changes, being cooler and lower in its density as expected, the ionospheric responses to low solar activity in previous studies were not consistent with each other, probably due to the limited ionospheric observations used for them. In this study, we utilized the measurements of total electron content (TEC) from TOPEX and JASON-1 satellites during the periods of 1992 to 2010, which includes both the last two solar minimum periods, in order to investigate how the ionosphere responded to the extremely low solar activity during the last solar minimum compared with previous solar minimum. Although the global daily mean TECs show negligible differences between the two solar minimum periods, the global TEC maps reveal that there are significant systematic differences ranging from about −30% to +50% depending on local time, latitude, and season. The systematic variations of the ionospheric responses seem to mainly result from the relative effects of reduced solar EUV production and reduced recombination rate due to thermospheric changes during the last solar minimum period.

Equatorial broad plasma depletions associated with the evening prereversal enhancement and plasma bubbles during the 17 March 2015 storm

Broad plasma depletions (BPDs) in the equatorial F region represent plasma depletions whose longitudinal and latitudinal scales are much greater than those of normal plasma bubbles. This study investigates the characteristics and origin of BPDs using the coincident ionospheric observations by the Communication/Navigation Outage Forecasting System, Defense Meteorological Satellite Program, and Swarm satellites during the 2015 St. Patricks Day (17 March) storm. Two types of BPDs were detected before midnight during the main phase of the storm. One type of BPDs showed a gradual plasma density variation (Type 1), and the other type of BPDs showed a steep density gradient (Type 2) at the walls of BPDs. The Type 1 BPDs were detected with no signature of plasma bubbles nearby, whereas the Type 2 BPDs were accompanied by bubbles. The formation of the Type 1 BPDs is attributed to the uplift of the bottomside of the F region above the satellite altitude by the action of storm-induced electric fields. The steep walls of Type 2 BPDs are associated with the ionospheric uplift and the spatial discontinuity of the ionosphere produced by bubbles. The detection of BPDs that are more than 15° wide in latitude by the polar orbit Swarm satellites arises from the elongation of bubbles along the magnetic field lines and the alignment of the elongation with the plane of the orbit.

Seasonal Characteristics of the Longitudinal Wavenumber-4 Structure in the Equatorial Ionospheric Anomaly

Using the global total electron contents (TEC) measured by the TOPEX satellite from Aug. 1992 to Oct. 2005, we investigate the variations of the longitudinal wavenumber-4 (LW-4) structure in the equatorial anomaly (EA) crests with season, local time, and solar activity. Our study shows that the LW-4 structure in the EA crests ( MLAT in both hemispheres) has clear four peaks at fixed longitude sectors during the daytime for both equinoxes and June solstice. In spite of being called a wavelike structure, however, the magnitudes and spatial intervals of the four peaks are far from being the same or regular. After sunset, the four-peak structure begins to move eastward with gradual weakening in its amplitude during equinoxes and this weakening proceeds much faster during June solstice. Interestingly, the longitudinal variations during December solstice do not show clear four-peak structure. All these features of the LW-4 structure are almost the same for both low and high solar activity conditions although the ion densities are greatly enhanced from low to high solar activities. With the irrelevancy of the magnetic activity in the LW-4, this implies that the large changes of the upper atmospheric ion densities, one of the important factors for ion-neutral interactions, have little effect on the formation of the LW-4 structure. On the other hand, we found that the monthly variation of the LW-4 is remarkably similar to that of the zonal component of wavenumber-3 diurnal tides at low latitudes, which implies that the lower atmospheric tidal forcing, transferred to the upper atmosphere, seems to have a dominant role in producing the LW-4 structure in the EA crests via the E-region dynamo.

Climatology of Plasmaspheric Total Electron Content Obtained From Jason 1 Satellite

We used more than 40 million Total Electron Content (TEC) measurements obtained from the GPS TRSR (TurboRogue Space Receiver) receiver onboard the Jason-1 satellite in order to investigate the global morphology of the plasmaspheric TEC (pTEC) including the variations with local time, latitude, longitude, season, solar cycle, and geomagnetic activity. The pTEC corresponds to the total electron content between Jason-1 (1336 km) and GPS (20,200 km) satellite altitudes. The pTEC data were collected during the seven-year period from January 2002 to December 2008. It was found that pTEC increases by about 10 - 30 % from low to high solar flux conditions with the largest variations occurring at low latitudes for equinox. During low solar flux condition, pTEC is largely independent of geomagnetic activity. However, it slightly decreases with increasing geomagnetic activity at low latitudes during high solar flux. The seasonal variations such as the annual and semiannual anomalies in the ionosphere also exist in the low-latitude plasmasphere. In particular, the American sector (around 300°E) shows strong annual asymmetry in the plasmaspheric density, being larger in December than in June solstice.

New method of estimating temperatures near the mesopause region using meteor radar observations

We present a novel method of estimating temperatures near the mesopause region using meteor radar observations. The method utilizes the linear relationship between the full width at half maximum (FWHM) of the meteor height distribution and the temperature at the meteor peak height. Once the proportionality constant of the linear relationship is determined from independent temperature measurements performed over a specific period of time by the Microwave Limb Sounder (MLS) instrument on board the Aura satellite, the temperature can be estimated continuously according to the measurements of the FWHM alone without additional information. The temperatures estimated from the FWHM are consistent with the MLS temperatures throughout the study period within a margin of 3.0%. Although previous methods are based on temperature gradient or pressure assumptions, the new method does not require such assumptions, which allows us to estimate the temperature at approximately 90 km with better precision.

First Simultaneous Multi‐station Observations of the Polar Cap Thermospheric Winds

Based on two northern (Eureka 80.0 N, 85.9 W, MLAT 88, and Resolute, 74.7 N, 94.8 W, MLAT 83) and one southern (Jang Bogo, 74.7S, 164.2E, MLAT 77) polar cap stations, simultaneous thermospheric wind data during the northern (December) and southern (June) winter month were obtained and compared with the NCAR TIEGCM model simulations driven by the Weimer ion convection model. The TIEGCM simulation overestimates the thermospheric wind by 30 to 60 %. The thermospheric winds at Eureka were larger than those at Resolute probably because Eureka is at high geomagnetic latitude. The observed ion drifts observed by RISR-N (Resolute Incoherent Scatter Radar North face) at Resolute also show the Weimer ion convection model overestimate the ion drift. The observation and simulation also appear to suggest the ion convection pattern in the Weimer model may be too large. The observed thermospheric winds at Jang Bogo are smaller than the TIEGCM but mostly in the zonal component. That the thermospheric winds are smaller at Jang Bogo than the two northern stations is likely due to its lower magnetic latitude. The results indicate the presence of internal structure in the thermospheric winds inside the polar cap and call for more observations in the polar cap to be undertaken.

A case study on occurrence of an unusual structure in the sodium layer over Gadanki, India

The height-time-concentration map of neutral sodium (Na) atoms measured by a Na lidar during the night of 18 to 19 March 2007 over Gadanki, India (13.5° N, 79.2° E) reveals an unusual structure in the Na layer for around 30 min in the altitude range of 92 to 98 km which is similar to the usual ‘C’ type structures observed at other locations. In order to understand the physical mechanism behind the generation of this unusual event, an investigation is carried out combining the data from multiple instruments that include the meteor wind radar over Thiruvananthapuram, India (8.5° N, 77° E) and the SABER instrument onboard the TIMED satellite. The temperature and wind profiles from the data set provided by these instruments allow us to infer the Richardson number which is found to be noticeably less than the canonical threshold of 0.25 above 92 km over Thiruvananthapuram suggesting the plausible generation of Kelvin-Helmholtz (KH) billows over southwestern part of the Indian subcontinent. Based on the average wind speed and direction over Thiruvananthapuram, it is proposed that the KH-billow structure was modified due to the background wind and was advected with it in nearly ‘frozen-in’ condition (without significant decay) in the northeastward direction reaching the Na lidar location (Gadanki). This case study, therefore, presents a scenario wherein the initially deformed KH-billow structure survived for a few hours (instead of a few minutes or tens of minutes as reported in earlier works) in an apparently ‘frozen-in’ condition under favorable background conditions. In this communication, we suggest a hypothesis where this deformed KH-billow structure plays crucial role in creating the abovementioned unusual structure observed in the Na layer over Gadanki.

Comment on “The night when the auroral and equatorial ionospheres converged” by Martinis, C., J. Baumgardner, M. Mendillo, J. Wroten, A. Coster, and L. Paxton

Intense OI 630.0 nm emission depletions were detected over Mexico by an all sky imager during the main phase of the geomagnetic storm on 1 June 2013 (minimum Dst index: –119 nT). Those emission depletions were interpreted to be associated with equatorial plasma bubbles [Martinis et al., 2015]. If bubbles were responsible for those middle-latitude emission depletions, they would have been extreme bubbles which extended over 40° magnetic latitudes and 7000 km in altitude at the magnetic equator. However, a few factors challenge this interpretation. First, the emission depletions detected over Mexico showed westward drift, whereas the equatorial ionosphere including bubbles drifted eastward on that night. Second, the middle-latitude emission depletions were tilted westward with respect to the geographic meridian, but the westward tilt of bubbles was not identified. Third, the growth of bubbles was not evident when the middle-latitude emission depletions grew. The westward tilt and westward propagation of the middle-latitude emission depletions are consistent with the characteristics of medium-scale traveling ionospheric disturbances (MSTIDs) observed over the United States on that night. Thus, the emission depletions over Mexico can be interpreted to be the signature of MSTIDs.

Ground-based Observations for the Upper Atmosphere at King Sejong Station, Antarctica

【Since the operation of the King Sejong Station (KSS) started in Antarctic Peninsula in 1989, there have been continuous efforts to perform the observation for the upper atmosphere. The observations during the initial period of the station include Fabry-Perot Interferometer (FPI) and Michelson Interferometer for the mesosphere and thermosphere, which are no longer in operation. In 2002, in collaboration with York University, Canada, the Spectral Airglow Temperature Imager (SATI) was installed to observe the temperature in the mesosphere and lower thermosphere (MLT) region and it has still been producing the mesopause temperature data until present. The observation was extended by installing the meteor radar in 2007 to observe the neutral winds and temperature in the MLT region during the day and night in collaboration with Chungnam National University. We also installed the all sky camera in 2008 to observe the wave structures in the MLT region. All these observations are utilized to study on the physical characteristics of the MLT region and also on the wave phenomena such as the tide and gravity wave in the upper atmosphere over KSS that is well known for the strong gravity wave activity. In this article, brief introductions for the currently operating instruments at KSS will be presented with their applications for the study of the upper atmosphere.】