FORMOSAT‐3/COSMIC observations of seasonal and longitudinal variations of equatorial ionization anomaly and its interhemispheric asymmetry during the solar minimum period

Abstract

Using an extremely valuable global data set from Formosa Satellite (FORMOSAT‐3)/Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) radio occultation experiment, a comprehensive study has been carried out on the seasonal and longitudinal variations of equatorial ionization anomaly (EIA) and the temporal variation in the hemispheric asymmetry of EIA during the low solar activity period from November 2006 to October 2007. The interesting result observed from this investigation is the local‐time‐dependent variation in the hemispheric asymmetry of EIA. During the solstices, it has been consistently observed that the EIA crest in the winter hemisphere appears stronger than that in the summer hemisphere during morning to noon hours. In contrast to this, during noon to early afternoon hours, the ionization in the winter EIA crest decreases rapidly, and the crest in the summer hemisphere becomes more intensified than that in the winter hemisphere. Further, this transition of stronger EIA crest from winter hemisphere to summer hemisphere occurs around 1200–1300 LT during the December solstice months and is delayed by a couple of hours (seen around ∼1400 LT) during June solstice months. The causative neutral and electrodynamical mechanisms are discussed in light of relative contributions from the field‐aligned plasma transport due to transequatorial interhemispheric neutral wind, the strength of the equatorial fountain process, and the ion drag effects during different local times and seasons. The results from the Sami2 is Another Model of the Ionosphere (SAMI2) model simulation also exhibit similar local‐time‐dependent variation in the hemispheric asymmetry of EIA, which further supports our argument. Also, it was observed that the large magnetic declination of the field lines and the four‐peaked longitudinal structure of EIA can significantly modulate the interhemispheric asymmetry of EIA even during the equinoxes.

Latitude and local time variations of topside magnetic field-aligned ion drifts at solar minimum

“…This allows field‐aligned plasma drifts to occur readily as a result of plasma pressure gradient, gravitational, or collisional forces that align with the magnetic field. The existence and behavior of field‐aligned plasma transport has been inferred from observations of ion composition [ West et al , 1997; West and Heelis , 1996], ion temperature [ Venkatraman and Heelis , 1999; Chao et al , 2004; Su et al , 2004], and electron density [ Tulasi Ram et al , 2009; Chen et al , 2009] in the topside ionosphere. These studies have described the solar cycle, temporal, and spatial variations of field‐aligned plasma transport that are consistent with the measured parameters, but had not been directly observed in the plasma.…”

Electron Density Comparison Between IRI 2007 and DEMETER Satellite Data in Solar Minimum Year

“…The single peak was observed at altitudes of 600 km from Hinotori during 09:00 -11:00 LT and 670 km in the case of DEMETER around 10:30 LT. The single EIA peak has been detected as low as 500 km altitude from the FORMOSAT-3/COSMIC observation from 07:00 -15:00 LT Tulasi Ram et al 2009). …”

“…If the neutral wind effect prevails the electron density concentrates at locations closer to the magnetic equator or to the southern/winter hemisphere. Otherwise, if the fountain effect is dominant, the EIA crest forms in a pole ward location Tulasi Ram et al 2009). At the topside ionosphere the DEMETER altitude the neutral wind effect will be prominent rather than the fountain feature in a solar minimum year.…”

Electron density retrieval from occulting GNSS signals using a gradient-aided inversion technique

“…RO is currently performed in receiver constellations, providing the advantages of global coverage. Several studies using the FORMOSAT-3/COSMIC constellation observing system have provided new discoveries about the electron density distribution in the ionosphere (Wang et al, 2004;Arras et al, 2008;Burns et al, 2008;Liang et al, 2008;Luan and Solomon, 2008;Zeng et al, 2008;Aragon-Angel et al, 2009;Arras et al, 2009;Lin et al, 2009;Liu et al, 2009;Mayer and Jakowski, 2009;Pedatella et al, 2009;Ram et al, 2009;Thampi et al, 2009;Tsai et al, 2009). The FORMOSAT-3/COSMIC (referred as COSMIC hereafter) is a joined Taiwan and the United States mission that includes six LEO satellites carrying ionospheric and atmospheric remote sensing instruments and flying in circular orbits at about 800 km altitude.…”