The Defense Metereological Satellite Program (DMSP) series of satellites are in a circular, 830-840 km sun-synchronous orbit at an inclination of . The orbital period is about 101 minutes. Although the satellites are in a sun-synchronous orbit, they cover a wide range in corrected geomagnetic latitude (CGL) and magnetic local time (MLT) due to the offset between the geographic and geomagnetic poles. All satellites are three-axis stabilized so that all sensors are in a known direction with respect to an intertial reference frame at all times .
The DMSP SSJ/4 instruments monitor the energy flux of electrons and ions that precipitate from the earth's magnetosphere. On each hourly orbit, DMSP satellites cross four auroral zones. The SSJ/4 sensors are configured on the spacecraft to look at the local zenith. Each SSJ/4 detector consists of an array of four cylindrical, curved plate, electrostatic analyzers configured into two pairs; one pair each to measure electrons and ions. Each pair covers the energy range from 30 eV to 30 keV in 20 channels spaced at equal intervals in energy on a logarithmic scale. The two sensors in each pair are stepped together such that a complete 20-point spectrum is returned once per second. The two low energy detectors consist of 10 channels centered at 34, 49, 71, 101, 150, 218, 320, 460, 670 and 960 eV. The high energy detector measures particles in 10 channels centered at 1.0, 1.4, 2.1, 3.0, 4.4, 6.5, 9.5, 14.0, 20.5 and 29.5 keV. The ratio of is approximately 0.2 for both sensors and for both electrons and ions with a nominal response efficiency of 50% within that channel. Processing of the SSJ/4 data occurs on board the satellite and at the ground station. The measured counts are converted to a differential number flux in units of electrons per per second per steradian per eV [HGB89]. Figure 4.1 shows two electron spectra as measured by the satellite which differ in characteristic energy .
Figure 4.1: Electron spectra measured by the J4 instrument for a low and a high characteristic energy
DMSP satellite data have been used in many studies involving particle precipitation and ionospheric responses. For example, field-aligned current systems have been measured during magnetic substorms by Lopez et al. [LSM91] and Yeh et al. [YFH 90], for quiet conditions by Ohtani et al. [OPN 95] and in the dayside cusp region by Lu et al. [LLR 95] and Candidi et al. [CMOM89]. As well, ionospheric conductances have been estimated [HGRM87, RVM 87, WdN92]. Combined radar and satellite measurements have generated a wealth of information, such as greater than normal electric field turbulence in the cusp [BGR 90], and a longitudinal spread of the precipitation from the tail region of more than one hour [SDC 94].