DYNAMICS OF THE LOW LATITUDE TERRESTRIAL MAGNETOPAUSE FROM THEMIS MEASUREMENTS

Multi-spacecraft THEMIS observations in temporal orbit during March - September 1997 provide a unique opportunity to collect material on the dayside magnetopause surface dynamics. During this period THEMIS crossed the magnetopause surface more than 300 times. About half of observed crossings were found to be a multicrossing (several inward and outward crossing of a single spacecraft during short-term interval). Taking into account the magnetopause surface normal velocity magnitude (averaged normal velocity is about 40 km/s), we speculate what periodic oscillations of the magnetopause position were observed. Two types of the magnetopause motion, namely, "flapping" (one-dimensional change of surface position) and "waving" (two-dimensional structure propagation) were found. Amplitudes of observed oscillations were estimated to be in the range of (0.1-2.2)R-E. The algorithm based on minimum variance analysis for surface normal defmition, the Hoffman-Theller and timing surface velocity definition technique is proposed to distinguish these two types of the magnetopause surface dynamics. Two-dimensional wavelike travelling disturbances were found mainly on the flanks of the magnetosphere. The magnetopause motion normal velocity was found to lie in the range of 50 to 150 km/s. One-dimensional flapping of the surface (the magnetopause motion normal velocity was found to be in the range from 10 to 70 km/s) was observed predominantly in the midday magnetosphere sector. We speculate that such quasiperiodic motion is the manifestation of the same phenomena in different magnetosphere regions. The generation can be initiated by dayside magnetosphere cavity modes (such events were associated with the magnetopause surface oscillations in several cases) or quasiperiodic changes of solar wind parameters. Surface disturbances are driven by magnetosheath plasma flow tailward with velocity about 140 180 km/s. In the case of periodic oscillations of the subsolar magnetopause the wavelike structures can be observed on the flanks of the magnetopause. The amplitude of surface disturbances can be increased by the Kelvin - Helmholtz instability.