Experimental evidence is presented that the low-energy proton population, which is the signature of the polar cleft region, has a high-energy tail of above 100-keV proton fluxes. The precipitation zone of these higher-energy protons covers the same latitude band as the low-energy particles do and also experiences the same latitudinal displacements during changes in the geomagnetic conditions. The precipitation has an isotropic pitch angle distribution and the measured higher-energy intensities compare favorably with values extrapolated from the spectrum at lower energies. The statistical invariant latitude pattern described by above 100-keV precipitating protons in the magnetic local time span 0006-1800 hours is presented. The measurements by Heikkila and Winningham [1971], Frank [ 1971 ], and Frank and A ckerson [ 1971 ] established that soft particles penetrated to low altitudes through the day side magnetospheric cusps. They also identified these fluxes with the magnetosheath plasma [Hundhausen et at., 1969; Frank, 1970]. More recently, Winningham [1972], Burch [1973], and Yasuhara et al. [1973] have studied and described the latitudinal extent and motion of these low-energy particle fluxes during changing geomagnetic, solar wind, and interplanetary magnetic field conditions. The region is also correlated with light emissions fEather and Mende, 1971;Hoffman and Berko, 1971; Heikkila et al., 1972] and is considered important as an entry for particles into the magnetosphere. Particles of higher energies have also been recorded in these latitude ranges. Electrons above about 50 keV have been observed [Russell et al., 1971; Burrows et al., 1972] but are interpreted to have gradient-drifted from the night hemisphere. Magnetosheath intensities of electrons at these energies are poorly known. Analyses of data obtained by the Esro 1 and Isis 1 satellites have shown that the cusp region also contains significant fluxes of above 100-keV protons [Lindalen et al., 1971; Amundsen et at., 1972; S4raas, 1972; Amundsen, 1973; Wedde et al., 1973; Burrows et al., 1972]. S4raas [1972] has shown this precipitation to be a conjugate phenomenon. Contrary to the electrons, the protons are observed to have an isotropic pitch angle distribution at these latitudes. Lindaten et al. [1971] suggested a local day side source for these highlatitude protons. In the following, attention will be focused on what looks like a clear cusp signature in these above 100-keV proton fluxes. EXPERIMENT The data to be presented in this respect were obtained by a proton spectrometer on board the Esro 1A satellite. The satellite was launched on October 3, 1968, into a near-polar orbit with inclination of 94 ø, initial apogee of 1533 km, and perigee of 258 km. The satellite orbit precessed 0.5 ø per day. The satellite was magnetically stabilized by means of permanent magnets. The proton spectrometer is a joint experiment between the University of Bergen in Norway and the Danish Space Research Institute in Denmark and employs three totally depleted surface b...
