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.

In an attempt to place short-lived, high-speed magnetotail flows termed bursty bulk flow events (BBFs) in the context of substorm phenomenology we analyze one such event that took place on April 11, 1985, using data from several spacecraft and many ground stations. The substorm onset, which took place at 0127 UT, had a meridian 2 hours of local time east of AMPTE/IRM. The satellite did not detect high-speed flows at that time. A high-latitude (--•70 ø corrected geomagnetic) substorm intensification took place at 0202 UT centered --•0.5 hour of local time west of the AMPTE/IRM meridian. The ISEE 2 satellite at the magnetotail lobe and the LANL 019 satellite at geosynchronous altitude were both at the same meridian as AMPTE/IRM at the time. The 0202 UT substorm intensification was associated with (1) a dipolarization at the ISEE 2 satellite at 0200:30 UT, (2) a BBF onset at AMPTE/IRM at 0202 UT accompanied by an intense dipolarization consistent with current wedge formation, (3) an energetic particle injection at geosynchronous altitude that took place at 0204 UT. The plasma acceleration region associated with this substorm intensification was estimated to be --• 8 R E tailward of AMPTE/IRM. Thus, during this activity the BBF event was due to an observed tail collapse Earthward of X --• -26 RE. The Earthward energy transport measured at AMPTE/IRM can account for the expected magnetospheric power consumption if the BBF has a cross-sectional area of only 1-2 R2e in the Y-Z direction. Similarly, the Earthward magnetic flux transport rate measured at AMPTE/IRM during the BBF event can result in a potential drop comparable to the expected transpolar cap potential if the BBF event has a size of 1-2 R E in the Y direction. The large amounts of flux transport measured past the satellite necessitate the existence of lobe flux reconnection tailward of AMPTE/IRM. The above results assume the validity of the frozen-in condition over the --•10-min duration of the BBF event.Although activity continued in the ionosphere and the ring current for well over 1.5 hours after the 0202 UT substorm intensification, most of the earthward energy and magnetic flux transport past IRM had ceased --•10 min after the BBF onset. We propose that the fast flows transport and pile up magnetic flux through a very narrow (a few R E in Y extent) flow channel in the midtail to the edge of an expanding dipolarization front in the near-Earth region. After the plasma sheet dipolarizes at a given location enhanced flux transport ceases, resulting in an apparent short (10-min timescale) duration of the fast flows. Unlike the near-Earth plasma sheet, which dipolarizes across many hours of local time, the midtail plasma sheet may exhibit longitudinally localized dipolarization. This may explain the often observed lack of one-to-one correlation between midtail activity and substorms.

The direct injection of magnetosheath plasma into the cusp produces at low altitude a precipitation regime with an energy‐latitude dispersion—the more poleward portion of which we herein term the “cusp plume.” An extensive survey of the Defense Meteorological Satellite Program (DMSP) F7 and F9 32 eV to 30 keV precipitating particle data shows that similar dispersive signatures exist over much of the dayside, just poleward of the auroral oval. Away from noon (or more precisely, anywhere not immediately poleward of the cusp) the fluxes are reduced by a factor of about 10 as compared to the cusp plume, but other characteristics are quite similar. For example, the inferred temperatures and flow velocities, and the characteristic decline of energy and number flux with increasing latitude is essentially the same in a longitudinally broad ring of precipitation a few degrees thick in latitude over much of the dayside. We conclude that the field lines on which such precipitation occurs thread the magnetospheric plasma mantle over the entire longitudinally extended ring. Besides the location of occurrence (i.e., immediately poleward of the dayside oval), the identification is based especially on the associated very soft ion spectra, which have densities from a few times 10−2 to a few times 10−1/cm³; on the temperature range, which is from a few tens of eV up to about 200 eV; and on the characteristic gradients with latitude. Further corroborating evidence that the precipitation is associated with field lines which thread the plasma mantle includes drift meter observations which show that regions so identified based on the particle data consistently lie on antisunward convecting field lines. Our observations indicate that some dayside high‐latitude auroral features just poleward of the auroral oval are embedded in the plasma mantle.

Abstract. The possible role of precipitation losses in eroding stormtime ring current is subject to debate. To explore this controversy, the recovery phase of the February 6-10, 1986, great magnetic storm is examined, when intense ion precipitation was observed at midlatitudes by NOAA-6 and DMSP satellites. This storm period is particularly interesting because the ring current exhibits distinctive two-phase decay as seen in the Dst index, the early rapid timescale decay corresponding to the intense ion precipitation period described above. Hamilton et al. [1988]

Auroral zone observations often show significant ULF power. We have analyzed auroral and plasma sheet observations during two prolonged periods of strongly southward and relatively steady interplanetary magnetic field (IMF). We find evidence that auroral poleward boundary intensifications (PBIs), which have large intensity and occur repetitively throughout such periods, may be a manifestation of a large‐scale ULF oscillation mode that strongly perturbs the plasma sheet and the auroral ionosphere. If this is correct, then ULF modes would be a major component of tail dynamics, of magnetosphere coupling to the ionosphere, and of auroral zone disturbances during periods of enhanced convection. They would simultaneously affect a large region of the nightside, extending along auroral zone field lines from the ionosphere to the equatorial plasma sheet and extending from field lines that lie near the magnetic separatrix to, at times, as close to the Earth as synchronous orbit. They would also occasionally have amplitudes as large as the changes that occur in association with other auroral zone disturbances such as substorms. Here we have found peak‐to‐peak amplitudes as high as several hundred nanoteslas in ground X, an order of magnitude in synchronous energetic proton fluxes, ∼20–40 nT in synchronous magnetic field components, ∼20 nT in tail magnetic field components, ∼1000 km/s in tail flow speeds, and ∼400 m/s in ionospheric flow speed. We find evidence for significant power at 0.5–0.7 mHz (∼25–30 min period), significant power at a possible second harmonic (∼1.1–1.3 mHz), and power at frequencies that could be higher harmonics simultaneously within the auroral ionosphere and within the nightside plasma sheet.