Abstract
Abstract. Structured dispersion patterns of the ion precipitation in low- and mid-altitude cusp regions have been reported by many authors. These patterns are interpreted either as temporal features in terms of the pulsed reconnection model or as spatial changes caused by a combination of the particle velocity with the convection of magnetic field lines. It is generally expected that the spatial dispersion is predominantly observed in lower altitudes where the spacecraft crosses a wide range of geomagnetic coordinates in a short time, whereas the high-altitude spacecraft observes temporal changes because it stays nearly on the same field line for a long time. We have analyzed one pass of the INTERBALL-1/MAGION-4 satellite pair through the high-altitude cusp and found that both temporal and spatial dispersion effects are important even in the magnetopause vicinity. The analysis of the present event shows a spatial nature of the observed dispersion in the LLBL and in the plasma mantle. We have identified two sources of a mantle precipitation operating simultaneously. Our investigations suggest that besides already reported latitudinal dispersion, the longitudinal dispersion can be observed during intervals of sufficiently high east-west interplanetary magnetic field component.
Highlights
Two cusps are spatially narrow regions where magnetosheath plasma of a solar wind origin can directly enter into the magnetosphere and the dayside ionosphere (Heikkila and Winningham, 1971; Smith and Lockwood, 1996)
Our study of one pass of the INTERBALL-1 and MAGION4 spacecraft across the lowlatitude boundary layer (LLBL)/mantle region reveals a surprising stability of sources of the cusp precipitation during a long interval of a changing interplanetary magnetic field (IMF) orientation (Sandahl et al, 2000)
– The LLBL population was trapped on closed field lines and its energy decreased gradually as the satellite moved from dawn toward the local noon
Summary
Two cusps are spatially narrow regions where magnetosheath plasma of a solar wind origin can directly enter into the magnetosphere and the dayside ionosphere (Heikkila and Winningham, 1971; Smith and Lockwood, 1996). A unique feature of reconnection is that it requires the relevant physical processes to take place only in a narrow diffusion region, while its consequences are global: Once the interplanetary and magnetospheric field lines become interconnected, they remain connected while being convected with the solar wind, and plasma continues to enter the magnetosphere This is in contrast to all other mechanisms that operate only locally, and their occurrence at different locations is essentially uncorrelated. Variations of the IMF orientation may changed the degree of acceleration of the ions as they cross the dayside magnetopause or a satellite could pass from flow stream line on one X-line to stream lines from a second X-line, and this motion would appear as a step in the ion energy dispersion due to the different time history of reconnection (Lockwood et al, 1995). The detectors are oriented toward the local zenith and provide a complete energy spectrogram once per second (Hardy et al, 1984)
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