Dynamics Explorer 2 satellite observations and satellite track model calculations in the cusp/cleft region
Authors: Wu Q. , Kilelen T. L. , Deng W. , Burns A. G. , Winningham J. D. , Spencer N. W. , Heelis R. A. , Hanson W. B. 
Published: February 2004

We present two detailed case studies of the ionospheric and thermospheric response to soft particle precipitation in the cusp/cleft region using multi‐instrument observations from the Dynamics Explorer 2 (DE 2) satellite during orbits 688 and 748, together with supporting model calculations. The experimental data set is among the most comprehensive reported to date and includes a specification of both the soft particle input to the atmosphere and the detailed local atmospheric response to these inputs. Specifically, observations are presented of precipitating 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 thermospheric signature of soft electron precipitation. The two orbits were chosen to provide representative cases for active (orbit 688; Kp=5; interplanetary magnetic field (IMF) Bz=−0.5 nT) and quiet (orbit 748; Kp=1; IMF Bz=+7.8 nT) geomagnetic conditions. Some of the data were used to provide boundary conditions and inputs to a satellite track code that was used to calculate the ionospheric structure below and along the orbital track. The satellite track code enables us to calculate the Joule‐ and particle‐heating altitude profiles and the ionization rate altitude profile. During each of the two orbits the region of soft electron precipitation associated with the cusp/cleft region was clearly identifiable. The principal findings of this study are as follows: (1) The maxima in the cusp/cleft soft electron (<100 eV) precipitating flux were ∼3 × 10−3 and ∼5×10−4 ergs/(cm2s sr eV) for orbits 688 and 748, respectively, and these fluxes produced oxygen redline volume emission rate maxima of 580 ph/(cm3 s) for the active orbit (688) and ∼80–90 ph/(cm3 s) for the quiet orbit (748). For both orbital passes the red‐line enhancements were evident above the background day glow, peaking at altitudes near 325 km. (2) The calculated ionization rate peaked at an altitude near 265 km, or 60 km lower than the altitude of the enhancement in the 630.0‐nm volume emission rate; the difference is ascribed to low‐altitude quenching of the emission. (3) Large changes in magnitude and direction for both the observed cusp/cleft neutral winds and ion drifts were observed, particularly to the poleward side of the region, with the ion drifts exhibiting more shearlike reversals and the neutral winds undergoing more rotationallike reversals. (4) Despite the negative IMF Bz (−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 track code results showed maxima in particle and Joule heating rates at −260 and 120 km altitude, respectively. The Joule heating rate was greater than the particle heating rate at all altitudes below the satellite, and the Joule heating peaked on the poleward side of the cusp/cleft precipitation region. (6) Both ion and relative neutral vertical velocities exhibited perturbations in the upward sense on the poleward side of the cusp/cleft, consistent with intense local heating and up welling.