Formation of a plasma depletion shell in the equatorial ionosphere

Abstract

[1] An accurate description of the irregularity region defined by a plasma bubble is critically important in understanding the dynamics of the region and its effects on radio scintillation. Here we describe a plasma depletion region as a ''depletion shell'' and show how two-dimensional optical images from space can be used to define the shape of the depletion shell. Our simple model calculation demonstrates that the space-based optical observation can detect the plasma-depleted magnetic flux tubes only near the F-peak height. The backward C-shape in bubble images from optical observations is the trace of the plasma depletion shell near the F-peak height. The westward tilt of bubbles at the magnetic equator can also be explained by this shell structure. The in situ measurement of the ion velocity at night in the topside shows the decrease of the eastward plasma drift with an increase of latitude. The formation of the plasma depletion shell is consistent with the latitudinal/altitudinal shear in the zonal plasma flow.

Three-dimensional morphology of equatorial plasma bubbles deduced from measurements onboard CHAMP

“…The observed TEC gradient along the CHAMP track is strongest when the corresponding GNSS satellite is located equatorward and westward of CHAMP with elevation angles of about 40-60 • . These elevation and azimuth angles are in agreement with the angles expected from the morphology of the plasma depletion shell proposed by Kil et al (2009). …”

“…As ambient ionospheric currents make a detour along EPB surfaces (due to low conductivity inside EPBs), the resultant current loops are expected to generate magnetic field deflections in space pointing along the EPB surface. In Park et al (2009) the average magnetic field deflection in the plane perpendicular to the ambient B field exhibits elongation towards a westward/outward or eastward/inward direction, which is as expected from the morphology of the plasma depletion shell proposed by Kil et al (2009). The 3-D shell structure was also demonstrated in first-principle simulations by Huba et al (2009) and Retterer (2010).…”

“…inverted-C on the horizontal plane and westward tilt on the equatorial/vertical plane) and the field-aligned nature of EPBs (e.g. Sultan, 1996), Kil et al (2009) suggested that the three-dimensional (3-D) morphology of EPBs has a shell-like structure. According to their model: (1) the highest-altitude point of the shell structure is located westward/equatorward of any other points on the shell, and (2) shell cross-sections perpendicular to the ambient B field exhibit elongation towards westward/outward (outward = toward higher L shell) or eastward/inward directions.…”

The Global-Scale Observations of the Limb and Disk (GOLD) Mission

“…Space-based optical detection of ionospheric bubbles using FUV observations has been demonstrated from LEO, most prominently by the GUVI instrument on TIMED (Kil et al 2004(Kil et al , 2009. Since the GUVI measurements are from a constant local time on any given night, they contain an inherent spatial-temporal ambiguity as the bubbles form and move.…”

Climatology of GPS signal loss observed by Swarm satellites

“…This orientation dependence is possibly related to the shell structure of EPIs. For EPIs with westward-tilted ("inverted-C" shell structure, proposed by Kil et al, 2009), the LOS directions of GPS satellites from southwest and northwest direction are mostly aligned with the EPIs extension with respect to the Swarm receiver, causing the signal loss events highlighted at orientation with azimuth angle around 225 and 315 • . However, as pointed out by Huba et al (2009), the shell structure of EPIs can be tilted with different angles, depending on the background zonal wind.…”