Broad plasma decreases in the equatorial ionosphere

Abstract

During June 2008 broad plasma density decreases (BPDs) were detected repeatedly by the Planar Langmuir Probe (PLP) on board the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. These density minima, not to be confused with Equatorial Plasma Bubbles (EPBs), occurred within 15° of the equator, consisted of reductions in plasma density up to an order of magnitude and extended across several degrees in azimuth along the orbit. Analysis revealed that the BPDs occurred nearly daily from May through July 2008 on C/NOFS, and that the widest BPDs were observed in the vicinity of the South Atlantic Anomaly (SAA). Similar BPDs simultaneous with the C/NOFS measurements were observed by instruments on the CHAllenging Minisatellite Payload (CHAMP) and Defense Meteorological Satellite Program (DMSP) satellites. An examination of plasma densities observed by the DMSP satellites over several years revealed that these phenomena were a frequent occurrence during (1) the period around June solstices; during (2) solar minimum years; (3) in the vicinity of the SAA. Neutral densities were examined during periods when BPDs were detected, and at times there are simultaneous neutral depletions. One possible explanation is a decrease in temperature of both ions and neutrals in the equatorial region at these times, consistent with downwelling in the ionosphere and thermosphere. Measurements of plasma temperatures on DMSP support this hypothesis.

Ion-neutral coupling during deep solar minimum

“…We suggest that these large depletions are a persistent feature of the tidal pattern and are not restricted only to dawn local times. This was confirmed by Huang et al (2009) in which BPDs on DMSP at around 20 LT were reported as a regular occurrence during solar minimum. Further evidence that BPDs are a regular occurrence comes from the IMAGE and GUVI UV images shown in Fig.…”

“…Results from the C/NOFS mission have confirmed that tidal structures are dominant in the ionosphere, with reports from a number of separate investigations over the entire altitude range of the satellite (Araujo-Pradere et al, 2012;Dao et al, 2011;Pfaff et al, 2010a). In this paper we present an analysis of longitudinal structures found in C/NOFS plasma densities and velocities, and focus on one particular phenomenon called a Broad Plasma Decrease (BPD), first reported by Huang et al (2009).…”

Observations and simulations of formation of broad plasma depletions through merging process

“…Broad plasma depletions detected by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite near dawn are significant decreases of plasma density in the low‐latitude ionosphere over a very large longitudinal range (1000–4000 km) [ Burke et al , 2009; de La Beaujardière et al , 2009; Huang et al , 2009; Kelley et al , 2009; Su et al , 2009; Huang et al , 2011]. Different mechanisms have been proposed to explain the generation of the broad region of reduced plasma density.…”

“…Different mechanisms have been proposed to explain the generation of the broad region of reduced plasma density. Huang et al [2009] suggested that the formation of broad plasma decreases could be related to a cooling or downwelling of the ionosphere and thermosphere in the equatorial region and ruled out the possibility of plasma bubbles as the cause of broad plasma decreases. The ion vertical velocity is small or weakly downward in the ionospheric downwelling process.…”

“… Huang et al [2011] reported the observations that upward plasma drift of 200–300 m s −1 occurs in the broad depletion region. The new type of broad plasma depletions is different from those caused by ionospheric downwelling [ Huang et al , 2009] in two aspects. The first major difference is the presence of large upward plasma drift, and the second is the role of equatorial plasma bubbles in the formation of broad plasma depletions.…”