ORIGIN OF ENERGETIC IONS IN THE POLAR CUSP INFERRED FROM ION COMPOSITION MEASUREMENTS BY THE VIKING SATELLITE

The magnetospheric ion composition spectrometer MICS on the Swedish Vi king satellite provided measurements of the ion composition in the ene rgy range 10.1 keV/e less than or equal to E/Q less than or equal to 3 26.0 keV/e. Data obtained during orbit 842 were used to investigate th e ion distribution in the northern polar cusp and its vicinity. The sa tellite traversed the outer ring current, boundary region, cusp proper and plasma mantle during its poleward movement. H+ and He++ ions were encountered in all of these regions. He+ ions were present only in th e ring current. The number of O+ and O++ ions was very small. Heavy hi gh-charge state ions typical for the solar wind were observed for the first time, most of them in the poleward part of the boundary region a nd in the cusp proper. The H+ ions exhibited two periods with high int ensities. One of them, called the BR/CP event, appeared at energies up to 50 keV. It started at the equatorward limit of the boundary region and continued into the cusp proper. Energy spectra indicate a ring cu rrent origin for the BR/CP event. Pitch angle distributions show downw ard streaming of H+ ions at its equatorward limit and upward streaming on the poleward side. This event is interpreted as the result of pitc h angle scattering of ring current ions by fluctuations in the magneto pause current layer in combination with poleward convection. The other of the two periods with high H+ ion intensities, called the accelerat ed ion event, was superimposed on the BR/CP event. It was restricted t o energies less than or equal to 15 keV and occurred in the poleward p art of the boundary region. This event is regarded as the high-energy tail of magnetosheath ions that were accelerated while penetrating int o the magnetosphere. The cusp region thus contains ions of magnetosphe ric as well as of magnetosheath origin. The appearance of the ions dep ends, in addition to the ion source, on the magnetic field configurati on an dynamic processes inside and close to the cusp.