The winter anomaly (or seasonal anomaly) at middle latitudes is a phenomenon during which the daytime plasma density at the F‐peak height (NmF2) is greater in winter than in summer. Radio occultation measurements from the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites provide a new data source for study of the winter anomaly on a global scale. In this study we investigate the altitude, local time, latitude, longitude, and hemispheric variations of the electron density in the middle‐latitude ionosphere by analyzing the COSMIC data measured in 2007 during a magnetically quiet period (Kp ≤ 3). The seasonal mean behavior of the NmF2 obtained from COSMIC data shows the occurrence of the winter anomaly feature during 0800–1600 LT in the Northern Hemisphere but not in the Southern Hemisphere. The intensity of the winter anomaly is variable with longitude, and a more intense winter anomaly is likely to occur at longitudes closer to the magnetic pole. At northern middle latitudes, a greater electron density in the winter than in the summer occurs in the narrow altitude range near the F‐peak height. Except for the winter anomaly feature at northern middle latitudes, the electron density at middle latitudes is greater during the summer than during the winter in both hemispheres.

We present first report on the periodic wave-like signatures (WLS) in the D region ionosphere during 22 July 2009 total solar eclipse using JJI, Japan, very low frequency (VLF) navigational transmitter signal (22.2 kHz) observations at stations, Allahabad, Varanasi and Nainital in Indian Sector, Busan in Korea, and Suva in Fiji. The signal amplitude increased on 22 July by about 6 and 7 dB at Allahabad and Varanasi and decreased by about 2.7, 3.5, and 0.5 dB at Nainital, Busan, and Suva, respectively, as compared to 24 July 2009 (normal day). The increase/decrease in the amplitude can be understood in terms of modal interference at the sites of modes converted at the discontinuity created by the eclipse intercepting the different transmitter-receiver great circle paths. The wavelet analysis shows the presence of WLS of period~16-40 min at stations under total eclipse and of period~30-80 min at stations under partial eclipse (~85-54% totality) with delay times between~50 and 100 min at different stations. The intensity of WLS was maximum for paths in the partially eclipsed region and minimum in the fully eclipsed region. The features of WLS on eclipse day seem almost similar to WLS observed in the nighttime of normal days (e.g., 24 July 2009). The WLS could be generated by sudden cutoff of the photo-ionization creating nighttime like conditions in the D region ionosphere and solar eclipse induced gravity waves coming to ionosphere from below and above. The present observations shed additional light on the current understanding of gravity waves induced D region ionospheric perturbations.

The ratio of height‐integrated atomic oxygen number density, [O], to molecular nitrogen number density, [N2], defined as Σ[O]/[N2], has been used as an indicator of the thermosphere‐ionosphere coupling during geomagnetic storms. However, the disturbance in [O]/[N2] in the F region can be different from the disturbance in Σ[O]/[N2], and knowledge of the storm‐time behaviors of [O] and [N2] is necessary to precisely describe the thermospheric neutral composition disturbance and its effect on the ionosphere. In this study, we examine the separate roles of [O] and [N2] in modifying the F region [O]/[N2] and Σ[O]/[N2] by analyzing far ultraviolet images made by Thermosphere, Ionosphere, Mesosphere Energetics and Dynamics (TIMED)/Global Ultraviolet Imager (GUVI) during the severe 20 November 2003 geomagnetic storm. The GUVI observation shows that the fractional storm‐time change in [N2] is greater than that in [O] in the F region in areas where Σ[O]/[N2] is both decreased and increased. Therefore the disturbance in [O]/[N2] in the F region is primarily determined by the change in [N2]. The reduction in [O]/[N2] in the F region during the storm period is consistent with the reduction in Σ[O]/[N2]. However, only a minor change of the F region [O]/[N2] is observed in the region where the increase in Σ[O]/[N2] is observed. The TEC increase observed at the locations of the Σ[O]/[N2] increase in the American‐Atlantic sectors is associated with the poleward extension of the equatorial ionization anomaly rather than with the neutral composition change in the F region.

No abstract

No abstract