He<sup>+</sup> dominance in the plasmasphere during geomagnetically disturbed periods: 1. Observational results

Abstract

Abstract.Observations made by the DMSP F10 satellite during the recovery phase from geomagnetic disturbances in June 1991 show regions of He + dominance around 830 km altitude at 09:00 MLT. These regions are co-located with a trough in ionisation observed around 55 • in the winter hemisphere. Plasma temperature and concentration observations made during the severe geomagnetic storm of 24 March 1991 are used as a case study to determine the effects of geomagnetic disturbances along the orbit of the F10 satellite. Previous explanations for He + dominance in this trough region relate to the part of the respective flux tubes that is in darkness. Such conditions are not relevant for this study, since the whole of the respective flux tubes are sunlit. A new mechanism is proposed to explain the He + dominance in the trough region. This mechanism is based on plasma transport and chemical reaction effects in the F-region and topside ionosphere, and on the time scales for such chemical reactions. Flux tubes previously depleted by geomagnetic storm effects refill during the recovery phase from the ionosphere as a result of pressure differences along the flux tubes. Following a geomagnetic disturbance, the He + ion recovers quickly via the rapid photoionisation of neutral helium, in the F-region and the topside. The recovery of the O + and H + ions is less rapid. This is proposed as a result of the respective charge exchange reactions with neutral atomic hydrogen and oxygen. Preliminary model calculations support the proposed mechanism.

Simulation of field-aligned H<sup>+</sup> and He<sup>+</sup> dynamics during late-stage plasmasphere refilling

“…Comparisons of the H + and He + plasmaspheres are of interest because the extreme ultraviolet (EUV) imager on board the IMAGE satellite shows the He + plasmasphere and its structure, which is generally assumed to correspond to that of the higher density H + plasmasphere. However, observations show that He + densities can exceed H + densities during early-stage refilling (Horwitz et al, 1990a;Denton et al, 2002).…”

“…This diurnal variation is largely absent from the H + curves. To obtain a comparison between these simulations and the results of Denton et al (2002), who found He + /H + density ratios exceeding unity in the winter-hemisphere topside ionosphere during refilling, we plotted (not shown) H + and He + concentrations at an altitude of 900 km, 11-50 h after the storm for the run shown in Figs. 3 and 4.…”

The outer radiation belt injection, transport, acceleration and loss satellite (ORBITALS): A canadian small satellite mission for ILWS

“…Strong Injection CRRES dawn-dusk electric field vs. Dst and L when -10 < dDst/dt < -1 nT/hr Average CRRES dawn-dusk electric field as a function of L and of Dst; panels show the L-dependence as a function of dDst/dt for (a) weak injection (À10 < dDst/dt < À1 nT/hr); (b) strong injection (dDst/dt < À10 nT/h); (c) weak decay (+1 < dDst/dt < + 10 nT/h) and (d) strong decay (dDst/ dt > + 10 nT/h) (Rowland and Wygant, Personal Communication 2005; see also Rowland and Wygant, 1998). refill more quickly than other species (e.g., Denton et al, 2002). Indeed, measurements from the DE1 RIMS instrument show that the He + population can vary between $2% and 50% of that of H + by number (Horwitz et al, 1984;Moldwin, 1997).…”

“…Denton et al, 2002). However, the micro-physics responsible for the progression of the refilling process is poorly understood.…”

Comparison of topside equatorial parameters derived from DMSP, Jicamarca, and Another Model of the Ionosphere (SAMI2)

“…It has been noted that the topside ionosphere is a sensitive indicator of the composition and dynamics of the thermosphere. Most recently, layers of He + at middle latitudes have been investigated by Gonzalez and Sulzer [1996] and Denton et al [2002]. Seasonal and solar cycle variations in the ion composition have been investigated by West and Heelis [1996], and the relationships between ion temperature and composition have been investigated by Venkatraman and Heelis [1999].…”