THE POLAR CUSP FROM A PARTICLE POINT-OF-VIEW - A STATISTICAL STUDY BASED ON VIKING DATA

We present results from the particle measurements made on board the Viking satellite. For the period of interest the Viking orbits covered at high latitudes the whole dayside sector. Data from the Viking V-3 particle experiment acquired during the Polar Region Outer Magnetospheric International Study period (March 15 to June 15, 1986) have been used to study the extension of the cusp and cleft in magnetic local time and invariant latitude, and furthermore, their dependence on solar wind and interplanetary magnetic field parameters. The study is limited to the MLT range from 0900 to 1500 and to invariant latitudes (ILAT) from 74-degrees to 82-degrees. This region is divided into bins of size (15 min MLT x 0.2-degrees ILAT). We concentrated on the region where magnetosheath solar wind plasma penetrates more directly into the magnetosphere and is measured at Viking altitudes (almost-equal-to 13,000 km). This region is called the "cusp proper" (Lundin et al., 1988; Newell and Meng, 1989), to be distinguished from a broader region denoted the cleft, where more energetic particles are observed. Statistically, we find the cusp proper to extend from invariant latitudes of 75-degrees to 82-degrees and magnetic local times from 0930 to 1400 MLT. The width in ILAT is found to be on average almost-equal-to 2-degrees and in MLT almost-equal-to 2 hours. It is shown that a clear correlation exists between the densities in the cusp proper calculated from the Viking V-3 experiment in the cusp proper and those in the solar wind calculated from IMP 8 measurements. It is also shown that the position of the cusp proper in MLT depends on the sense of the By component of the interplanetary magnetic field (IMF By), giving a well-defined displacement of the region of maximum occurrence toward earlier MLTs for IMF By < 0 and a less defined displacement toward later MLTs for IMF By > 0.