A STRONG DAWN DUSK ASYMMETRY IN PC5 PULSATION OCCURRENCE OBSERVED BY THE DE-1 SATELLITE

Using the magnetic field data obtained by the DE-1 polar orbiting sate llite, statistical characteristics of transverse Pc5 pulsations in the inner magnetosphere are examined. The occurrence distribution is foun d to have a strong dawn/dusk asymmetry. The occurrence is most frequen t in the region around 72 degrees invariant latitude (ILAT) between 08 00 and 1000MLT. The distribution shifts to lower ILAT both in the earl y morning and in the afternoon sectors. Our results are generally cons istent with the occurrence distributions obtained by previous studies except for appreciable differences in the local time of the peak occur rence. The dependence of the transverse Pc5 occurrence on solar wind v elocity is also investigated. Results show that the higher the solar w ind velocity, the more frequent the Pc5 occurrence. This fact suggests that the energy source of these Pc5's is in the Kelvin-Helmholtz inst ability (KHI) on the magnetopause. To explain the strong dawn/dusk asy mmetry, the dependence of the Pc5 occurrence on the angle between the solar wind velocity and the IMP in the ecliptic plane is investigated. It is found that this angle controls the magnetic local time of Pc5 a ppearance when the solar wind velocity is small. This dependence may b e explained by the low threshold of KHI due to an influence of a quasi -parallel bow shock.