[1] Cold plasma mass density profiles in the plasmasphere have been determined for the geomagnetically quiet day of 19th August 2000 using the cross-phase technique applied to ground-based magnetometer data from the SAMNET, IMAGE and BGS magnetometer arrays. Cross-phase derived mass densities have been compared to electron densities derived from both ground-based VLF receiver measurements, and the IMAGE satellite RPI. The crossphase results are in excellent agreement with both the VLF and IMAGE observational results, thus validating the crossphase technique during quiet times. This is the first such coordinated multi-instrument study, and has enabled very few heavy ions to be inferred in the plasmasphere for L > 3.45 on this day. The observational results were compared to plasma mass densities from the SUPIM model and were found to be in excellent agreement. IMAGE EUV data also verified the existence of azimuthal structure in the outer quiettime plasmasphere.

Two independent ground‐based experiments and two satellite‐borne experiments are used to interpret the changes in plasmaspheric composition at the same point in space during moderate geomagnetic activity on 22 January and 14 February 2001. Mass density at L = 2.5 was determined from an array of magnetometers on the Antarctic Peninsula, while the electron number density along the same flux tube was determined from analysis of the group delay of man‐made VLF transmissions from north‐east America. The IMAGE satellite RPI experiment provided in situ measurements of the electron number density in passing the equatorial region of the same field line, while the EUV Imager experiment was able to resolve the He+ abundance by looking back toward the same place a few hours later. On 22 January 2001 all measurements were consistent with a moderately disturbed plasmasphere. On 14 February 2001 there appeared to be a significant response of the plasmasphere to the moderate (Kp = 5) activity levels. Both the electron number density and the mass density determined from the ground‐based experiments were markedly higher than on 22 January 2001. Also, the IMAGE RPI gave a markedly lower electron number density than did the ground‐based data; this is explained by differences in the longitude at which the measurements were made and the presence of localized plasmaspheric structures. At Antarctic Peninsula longitudes a He+ column abundance value of 6 × 1010 cm−2 is found to be equivalent to plasmaspheric electron density levels of 3000 cm−3 at L = 2.5. For these conditions the He+ mass abundance was about 12–16% compared with H+. Both decreases and increases in the He+ column abundance measured by the EUV Imager appear to be linearly correlated to changes in the percentage occurrence of He+ as determined from a combination of ground‐based VLF and ULF observations.

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.

[1] Light ion concentration observations from the Arecibo incoherent scatter radar have shown regions of He + layering in the topside ionosphere in the postmidnight hours. The He + layering phenomenon has been observed during both solar maximum and solar minimum conditions. During solar maximum the layer is well defined, giving maximum relative abundances of over 50%. The observational results also show regions of He + dominance in the topside ionosphere during solar maximum in the height range 750-1200 km. During solar minimum the magnitude of the layer is less, giving a maximum relative abundance of only $20%; however, the layer is still a well-defined feature. The coupled thermosphere-ionosphere-plasmasphere (CTIP) model is shown to model accurately the He + layer and the regions of He + dominance, provided that the modeled temperature profiles are normalized to the observed values. This study serves to further highlight the importance of helium ions in the topside ionosphere, particularly during solar maximum nighttime conditions.

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