The dayside zone of soft precipitation can be divided into four distinct types of plasma regimes, each corresponding to the respective magnetospheric source region: the cusp, the mantle, the low‐latitude boundary layer (LLBL), and the dayside extension of the BPS. Based on a detailed spectral study, including comparisons with nonsimultaneous ISEE 1 satellite LLBL data, we identify regions of LLBL‐type plasma in the DMSP data set and compare these plasma boundaries with convection reversal boundaries (CRBs) as determined by either Sondrestrom or the drift meter instrument on board the DMSP F9 spacecraft. The nine cases considered are all in the prenoon local time sector. We find that in eight of the nine cases the CRB occurs within the LLBL as expected, generally near to, but not coincident with, the equatorward edge of the LLBL‐type plasma. In our sample set, chosen for cases with latitudinally wide, easily identifiable LLBL signatures, the average latitudinal width was 1.85° magnetic latitude. The CRB, defined as the onset of steady antisunward convection, occurred about 30% of this width beyond the equatorward onset of LLBL‐type particles. The most equatorward portion of the region with LLBL‐type plasma usually had near‐zero or erratic convection and may correspond to the “stagnation region” reported from ISEE observations. The potential drop observed across the low‐altitude LLBL is roughly estimated to be typically ∼5 keV. A summary is given on how the various high‐altitude sources can be identified when plasma regions are observed at low altitude in the dayside auroral oval.

Transient or patchy magnetic field line merging on the dayside magnetopause, giving rise to flux transfer events (FTEs), is thought to play a significant role in energizing high‐latitude ionospheric convection during periods of southward interplanetary magnetic field. Several transient velocity patterns in the cusp ionosphere have been presented as candidate FTE signatures. Instrument limitations, combined with uncertainties about the magnetopause processes causing individual velocity transients, mean that definitive observations of the ionospheric signature of FTEs have yet to be presented. This paper describes combined observations by the PACE HF backscatter radar and the DMSP F9 polar‐orbiting satellite of a transient velocity signature in the southern hemisphere ionospheric cusp. The prevailing solar wind conditions suggest that it is the result of enhanced magnetic merging at the magnetopause. The satellite particle precipitation data associated with the transient are typically cusplike in nature. The presence of spatially discrete patches of accelerated ions at the equatorward edge of the cusp is consistent with the ion acceleration that could occur with merging. The combined radar line‐of‐sight velocity data and the satellite transverse plasma drift data are consistent with a channel of enhanced convection superposed on the ambient cusp plasma flow. This channel is at least 900 km in longitudinal extent but only 100 km wide. It is zonally aligned for most of its extent, except at the western limit where it rotates sharply poleward. Weak return flow is observed outside the channel. These observations are compared with and contrasted to similar events seen by the EISCAT radar and by optical instruments.

The development of the ring current ions in the inner magnetosphere during the main phase of a magnetic storm is studied. The temporal and spatial evolution of the ion phase space densities in a dipole field are calculated using a three dimensional ring current model, considering charge exchange and Coulomb losses along drift paths. The simulation starts with a quiet time distribution. The model is tested by comparing calculated ion fluxes with Active Magnetospheric Particle Tracer Explorers/CCE measurement during the storm main phase on May 2, 1986. Most of the calculated omnidirectional fluxes are in good agreement with the data except on the dayside inner edge (L < 2.5) of the ring current, where the ion fluxes are underestimated. The model also reproduces the measured pitch angle distributions of ions with energies below 10 keV. At higher energy, an additional diffusion in pitch angle is necessary in order to fit the data. The role of the induced electric field on the ring current dynamics is also examined by simulating a series of substorm activities represented by stretching and collapsing the magnetic field lines. In response to the impulsively changing fields, the calculated ion energy content fluctuates about a mean value that grows steadily with the enhanced quiescent field.

Early on March 14, 1989, a thermal plasma probe on the Defense Meteorological Satellite Program (DMSP) F9 spacecraft detected extensive and dramatic decreases in the ion density at 840 km, near 2130 LT, during two consecutive transequatorial passes over South America. The order of magnitude decreases in the ion density extended more than 4000 km along the satellite track. The depletions were accompanied by upward and westward plasma drifts, both in excess of 100 m/s. Their onsets and terminations were marked by extremely sharp density gradients. DMSP F9 observed no similar depletions over the Atlantic during preceding orbits. A partial depletion was detected over the eastern Pacific during the following orbit. The DMSP F9 ground track passed slightly west of a Brazilian total electron content (TEC) station and two Brazilian ionosondes during the first depletion encounter. The TEC fell far below normal during the night of March 13-14. The ionosonde measurements indicate that, in the hour after sunset, before DMSP passed through the depletions, the F2 layer rose rapidly and disappeared, but at the time of the first depletion encounter, hmF2 was decreasing over one of the stations. The DMSP F8 satellite, which orbits in the dawn-dusk meridian, made related measurements on March 13 and 14. Crossing the equator at dust on March 13, at the same longitude where DMSP F9 encountered the first depletion, DMSP F8 detected upward and westward drifts, but it measured extremely large rather then depleted ion densities. During two dawn passes over the eastern Pacific on March 14, DMSP F8 observed depletions somewhat similar to those detected by DMSP F9. Large westward drifts accompanied the depletions detected by DMSP F8. It is quite probable that the morningside depletions detected on March 14 are remnants of those detected earlier by DMSP F9 in the evening sector.We develop a phenomenological model reconciling DMSP F8, F9, and groundbased measurements. Our calculations show that rapid upward drifts sustained for several hours can produce depletions in the equatorial ion density with sharp gradients at their high-latitude boundaries, consistent with the data. We discuss possible contributing mechanisms for generating these upward drifts. These include direct penetration of the magnetospheric electric field to low latitudes, the electric fields generated by the disturbance dynamo, and the effects of conductivity gradients near the dusk terminator and the South Atlantic anomaly. 1989. Previous WorkThe use of ground instrumentation to study the effects of magnetic storms and sub storms on the ionosphere has a long history [see Matsushita, 1959, and references therein]. The DMSP F9 satellite measurements described in this paper were accompanied by decreases in the peak densityfoF2 and increases in altitude hmF2 of the F layer, as well as by a large drop in the total electron content (TEC). These changes,

Abstract. The Dessler-Parker-Sckopke relation (DPS) predicts a linear dependence of the perturbation magnetic field at the surface of the Earth on the total ring current kinetic energy. In this paper, we test DPS by using measurements of the major ring current ion species made by the charge-energy-mass spectrometer on the Active Magnetospheric Particle Tracer Explorers CCE spacecraft. We use spectra from passes through the equatorial storm time ring current near the maximum phase of 80 magnetic storms between 1984 and 1989 to estimate the global ring current energy content ERe and compare it with the average value for Dst during each pass. Our work shows that DPS holds well on average. In particular, there is a strong linear correlation between ring current energy estimated from nightside ion measurements and the Dst index, and the slope of the least squares fit line giving Dst as a function of nightside ERe is in good agreement with the prediction of DPS. In contrast, dayside measurements of E}•½ do not yield a robust correlation with Dst. Although we cannot rule out the possibility that currents other than the ring current (for example, tail currents and the magnetopause current) may cause large magnetic perturbations, we conclude that these perturbations, if they exist, must be largely compensating. By examining how the ratio of Dst to E•½ varies with the local time sector of the in situ ion measurements, we obtain statistical information on the anisotropy of the storm time ring current. We find that the largest values of E•c/Dst result from nightside measurements and the smallest values result from measurements in the 0600 to 1200 LT region, as would be expected for an ion population injected on the nightside that must drift westward around the Earth, undergoing losses, to reach the dayside morning sector.