Biqiang Zhao

A study of the Weddell Sea Anomaly observed by FORMOSAT‐3/COSMIC

[1] More than two years of COSMIC electron density profiles at low solar activities are collected to study the evolution of the Weddell Sea Anomaly (WSA), which appears as an evening enhancement in electron density during local summer. Observations show that the change in NmF2 (the F2 peak electron density) is associated with the change in hmF2 (the F2 peak height), while the latter is correlated closely with the components of the geomagnetic field. We find that (1) in the afternoon, hmF2 is more liable to rise drastically in regions with a larger jsin(2I)j value, which would occur early at certain declinations, eastward in the southern hemisphere and westward in the northern hemisphere; (2) subsequently, a larger increment of hmF2 is coincidentally followed by a stronger enhancement of NmF2 and the enhancement ends just around the local sunset; and (3) in midlatitudes, the evolution pattern of hmF2 in the evening of equinoxes and winter is similar to that in summer, albeit without a lasting NmF2 enhancement as that in summer. These features suggest that the NmF2 enhancement and the hmF2 increase could arise from the thermospheric wind effect, and solar photoionization plays a crucial role in the enhancement as well. The general midlatitude F2 layer enhancement in local summer evening is consistent with the WSA on the above features, indicating that the WSA is a manifestation, with a particular geometry of the magnetic field, of the evening enhancement induced by the winds.

Longitudinal variations of electron temperature and total ion density in the sunset equatorial topside ionosphere

Based on the DMSP F13 Satellite observations from 1995 to 2005, the longitudinal distributions of the electron temperature (T-e) and total ion density (N-i) in the sunset equatorial topside ionosphere are examined. The results suggest that the longitudinal variations of both T-e and N-i exhibit obvious seasonal dependence as follows: (1) wavenumber-four longitudinal structure in equinox, (2) three peaks structure in June solstice, and (3) two peaks structure in December solstice. Moreover, the longitudinal variations of T-e and N-i show significant anti-correlation, and we speculate that the longitudinal variation of T-e may result from that of N-i which can control T-e through the electron cooling rate. The wavenumber-four longitudinal structures of both T-e and N-i in equinox may relate to the eastward propagating zonal wavenumber-3 diurnal tide (DE3), which has effect on the amplitude of the daytime zonal electric field. The longitudinal variation of T-e and N-i in the two solstices may be caused both by longitudinal variation of geomagnetic declination and DE3.

Data assimilation retrieval of electron density profiles from radio occultation measurements

[1] In this paper, the Kalman filter is used to retrieve the electron density profile along the tangent points by assimilating the slant total electron content data observed during a radio occultation (RO) event into an empirical background model. The RO data observed by COSMIC satellites on day of year 266 in 2009 are selected to do both the simulation work and the real data retrieval test. The results show that the data assimilation technique can improve the electron density retrieval in comparison with the Abel inversion. It is less influenced by the ionospheric inhomogeneity than the Abel method. Some pseudo‐large‐scale features made by the Abel retrieval, such as the plasma cave underneath the equatorial ionization anomaly region and the three peaks along the latitude direction in the E layer, disappear in the data assimilation retrieval results. Independent validation by ground‐based ionosonde observations confirms the improvement of data assimilation retrieval below the F2 peak. In addition, some potential research on RO data assimilation is also discussed.

A study on the nighttime midlatitude ionospheric trough

Constellation Observing System for Meteorology, Ionosphere, and Climate(COSMIC) electron density profiles are used to investigate the nighttime midlatitudeionospheric trough (MIT). We find that at midnight the longitudinally deepest MIT occursto the west of the geomagnetic pole in both the Northern and Southern Hemispheres duringthe equinox seasons and local summer. The deepest MIT could be ascribable to theenhanced depletion caused by horizontal neutral wind. In the early evening, the eastwardneutral wind prevails in the midlatitude F region, which blows the plasma downwardwhere the declination is eastward in the Northern Hemisphere but westward in theSouthern Hemisphere, both lying to the west of the geomagnetic pole. The downward driftwould enhance the plasma depletion for more molecular composition at lower altitude.In addition, we find for the first time that the location of nighttime MIT minimumoscillated with a periodicity of 9 days and an amplitude of about 1°–1.5° geomagneticlatitude during 2007–2008, associated with the recurrent high‐speed solar wind. Ourresults shed new light on the empirical description and numerical simulation of MIT.

Anomalous enhancement of ionospheric electron content in the Asian-Australian region during a geomagnetically quiet day

National Natural Science Foundation of China[40725014]; National Important Basic Research Project[2006CB806306]; Knowledge Innovation Program of the Chinese Academy of Sciences

Applying artificial neural network to derive long-term foF2 trends in the Asia/Pacific sector from ionosonde observations

[ 1] An artificial neural network ( ANN) method is first used for deriving long-term trends of the F2-layer critical frequency (foF2) at 19 ionospheric stations in the Asia/Pacific sector. It is found that the ANN method can eliminate the geomagnetic activity effect on foF2 more effectively than usual regression methods. Of the selected 19 stations, there are significant long-term trends corresponding to a confidence level >= 90% at 14 stations and 12 of these stations present negative trends. An average trend of - 0.05% per year in the selected area can be obtained if the 12 stations with significant negative long-term trends be considered. No pronounced diurnal and latitudinal effects in trends and no uniform pattern of seasonal variation in most stations are detected. The long-term trends for low latitude and equatorial stations differ from other stations suggest that some special dynamical processes may take effects in the equatorial anomaly region. Many factors which can influence ionosphere, such as the greenhouse effect, solar and geomagnetic activity, and neutral background gas, might contribute to the trend.

East-west differences in F-region electron density at midlatitude: Evidence from the Far East region

The global configuration of the geomagnetic field shows that the maximum east-west difference in geomagnetic declination of northern middle latitude lies in the US region (similar to 32 degrees), which produces the significant ionospheric east-west coast difference in terms of total electron content first revealed by Zhang et al. (2011). For verification, it is valuable to investigate this feature over the Far East area, which also shows significant geomagnetic declination east-west gradient but smaller (similar to 15 degrees) than that of the US. The current study provides evidence of the longitudinal change supporting the thermospheric zonal wind mechanism by examining the climatology of peak electron density (NmF2), electron density (Ne) of different altitudes in the Far East regions with a longitude separation of up to 40-60 degrees based on ground ionosonde and space-based measurements. Although the east-west difference (R-ew) over the Far East area displays a clear diurnal variation similar to the US feature, that is negative R-ew (West Ne > East Ne) in the noon and positive at evening-night, the observational results reveal more differences including: (1) The noontime negative R-ew is most pronounced in April-June while in the US during February-March. Thus, for the late spring and summer period negative R-ew over the Far East region is more significant than that of the US. (2) The positive R-ew at night is much less evident than in the US, especially without winter enhancement. (3) The magnitude of negative R-ew tends to enhance toward solar maximum while in the US showing anticorrelation with the solar activity. The altitude distribution of pronounced negative difference (300-400 km) moves upward as the solar flux increases and hence produces the different solar activity dependence at different altitude. The result in the paper is not simply a comparison corresponding to the US results but raises some new features that are worth further studying and improve our current understanding of ionospheric longitude difference at midlatitude. Citation: Zhao, B., M. Wang, Y. Wang, Z. Ren, X. Yue, J. Zhu, W. Wan, B. Ning, J. Liu, and B. Xiong (2013), East-west differences in F-region electron density at midlatitude: Evidence from the Far East region, J. Geophys. Res. Space Physics, 118, 542-553, doi:10.1029/2012JA018235.

Influences of geomagnetic fields on longitudinal variations of vertical plasma drifts in the presunset equatorial topside ionosphere

On the basis of the measurements from the ion drift meter on the Defense Meteorological Satellite Program F13 Satellite from 2000 to 2002, the relative longitudinal variations of E x B vertical plasma drifts in the presunset equatorial topside ionosphere at 1745 LT are examined. Obvious influences of geomagnetic fields on longitudinal variations of E x B vertical plasma drifts, which also show a seasonal dependence, have been found. During the June solstice, the relative vertical plasma drifts is most significantly influenced by the geomagnetic field declination. However, the relative vertical plasma drifts during the December solstice more strongly correlates with the geomagnetic field strength. Although the relative vertical plasma drifts during equinox are also influenced by the geomagnetic field declination, an underlying dependence on magnetic field magnitude, as seen in December, is also present.

Global characteristics of occurrence of an additional layer in the ionosphere observed by COSMIC/FORMOSAT-3

Global observations of electron density profile (EDP) from the COSMIC/FORMOSAT-3 satellites were used to investigate, for the first time, the additional stratification of the F2 layer over the equatorial ionosphere on a global scale, which is called F3 layer. The F3 layer in EDP was recognized through the altitude differential profile featured by two maxima existing from 220 km to the peak height of the electron density. There were similar to 9,400 cases of F3 layer selected out of similar to 448, 000 occultation events at low and equatorial areas during the period of April 2006-September 2010. Statistical results show that the highest occurrence of F3 layer appears at dip latitude 7 similar to 8 degrees/-7 similar to-8 degrees for Northern/Southern Hemisphere and is more pronounced during summer months at 10:00-14:00 LT. The occurrence also has a clear longitude dependence during boreal summer, with relatively higher occurrence at -80 similar to-100 degrees, -20 similar to 20 degrees, 80 similar to 120 degrees and -160 similar to-170 degrees longitudes, that is possibly associated with the wavenumber-3 diurnal tide (DE3). The results support the principle of the F3 layer proposed by Balan et al. (1998), which in turn validate the accuracy of the retrieval of the COSMIC EDP data. Citation: Zhao, B., W. Wan, X. Yue, L. Liu, Z. Ren, M. He, and J. Liu (2011), Global characteristics of occurrence of an additional layer in the ionosphere observed by COSMIC/FORMOSAT-3, Geophys. Res. Lett., 38, L02101, doi:10.1029/2010GL045744.

The transition to overshielding after sharp and gradual interplanetary magnetic field northward turning

Overshielding is referred to a shielding status, during which the dawnward shielding electric field dominates over the duskward penetration electric field in the inner magnetosphere, typically appearing when the interplanetary magnetic field (IMF) suddenly turns northward after a prolonged southward orientation. It is expected that the transition to overshielding after IMF northward turning can be affected by the shape of northward turning (sharp or gradual). Moreover, the initial shielding status (undershielding or goodshielding) prior to the transition may also have influence on the transition. Here we analyze two groups of cases, in which the transitions appear after sharp (duration less than 5 min) and gradual (duration more than 30 min) northward turning. Each group includes two cases, in which the transition initiated from undershielding and goodshielding. These cases show that (1) the beginning of the transition to overshielding coincides with sharp IMF northward turning but appears in the midst of gradual IMF northward turning; (2) the transition from goodshielding to overshielding is always associated with convection electric field drop and/or polar cap shrinkage, regardless of the shape of IMF northward turning; and (3) the typical solar wind condition in which the IMF suddenly turns northward after a prolonged southward orientation is neither a necessary condition nor a sufficient condition for overshielding. Furthermore, we will discuss the effect of substorm processes on overshielding.