DYNAMICS-EXPLORER-2 SATELLITE-OBSERVATIONS AND SATELLITE TRACK MODEL-CALCULATIONS IN THE CUSP CLEFT REGION

We present two detailed case studies of the ionospheric and thermosphe ric response to soft particle precipitation in the cusp/cleft region u sing multi-instrument observations from the Dynamics Explorer 2 (DE 2) satellite during orbits 688 and 748, together with supporting model c alculations. The experimental data set is among the most comprehensive reported to date and includes a specification of both the soft partic le input to the atmosphere and the detailed local atmospheric response to these inputs. Specifically, observations are presented of precipit ating ion and electron fluxes, ion and neutral horizontal and vertical wind speeds, neutral temperatures and densities, and volume emission rates for the OI 630.0-nm red line, which is often used as a thermosph eric signature of soft electron precipitation. The two orbits were cho sen to provide representative cases for active (orbit 688; Kp=5; inter planetary magnetic field (IMF) B-Z=-O.5 nT) and quiet (orbit 748; Kp=1 ; IMF B-Z=+7.8 nT) geomagnetic conditions. Some of the data were used to provide boundary conditions and inputs to a satellite track code th at was used to calculate the ionospheric structure below and along the orbital track. The satellite track code enables us to calculate the J oule- and particle-heating altitude profiles and the ionization rate a ltitude profile. During each of the two orbits the region of soft elec tron precipitation associated with the cusp/cleft region was clearly i dentifiable. The principal findings of this study are as follows: (1) The maxima in the cusp/cleft soft electron (<100 eV) precipitating flu x were similar to 3x10(-3) and similar to 5x10(-4) ergs/(cm(2) s sr eV ) for orbits 688 and 748, respectively, and these fluxes produced oxyg en red-line volume emission rate maxima of 580 ph/(cm(3) s) for the ac tive orbit (688) and similar to 80-90 ph/(cm(3) s) for the quiet orbit (748). For both orbital passes the red line enhancements were evident above the background dayglow, peaking at altitudes near 325 km. (2) T he calculated ionization rate peaked at an altitude near 265 km, or 60 lan lower than the altitude of the enhancement in the 630.0-nm volume emission rate; the difference is ascribed to low-altitude quenching o f the emission. (3) Large changes in magnitude and direction for both the observed cusp/cleft neutral winds and ion drifts were observed, pa rticularly to the poleward side of the region, with the ion drifts exh ibiting more shearlike reversals and the neutral winds undergoing more rotationallike reversals. (4); Despite the negative IMF B-Z (-0.5 nT) during orbit 688, the convection characteristic of the ion drifts was consistent with a northward IMF for both orbits. (5) The satellite tr ack code results showed maxima in particle and Joule heating rates at similar to 260 and 120 km altitude, respectively. The Joule heating ra te was greater than the particle heating rate at all altitudes below t he satellite, and the Joule heating peaked on the poleward side of the cusp/cleft precipitation region. (6) Both ion and relative neutral ve rtical velocities exhibited perturbations in the upward sense on the p oleward side of the cusp/cleft, consistent with intense local heating and upwelling.