Observations of density fluctuations in earth's magnetosheath with geotailand wind spacecraft

Large fluctuations of plasma densities in the earth's flank-side magnetoshe ath (local times 1500-1700) are examined. The characteristics of the fluctu ations are large amplitudes (Delta N/N similar to 1), long oscillation peri ods (T similar to 30 m) and anti- correlation between the plasma densities and magnetic field strengths. The variations of the solar wind plasma densi ties are negligible, whereas considerable fluctuations are found for the so lar wind magnetic fields. Two possibilities are considered for the density fluctuations: 1) repeated traversals of the plasma depletion layer and the standing slow-mode waves near the magnetopause and 2) convected slow-mode w aves generated at the earth's bow shock. Temperature anisotropy and density profile as expected from the repeated traversals are not confirmed with th e observations. Based on the consideration that the present observations oc cur in the flanks where plasma flows are largely tangential to the magnetop ause, we propose that the convected slow-mode waves play a role in the gene ration of the density fluctuations.