Magnetospheric plasma pressures in the midnight meridian: Observations from 2.5 to 35 RE

Abstract

Plasma pressure data from the ISEE 2 fast plasma experiment (FPE) were statistically analyzed to determine the plasma sheet pressure versus distance in the midnight local time sector of the near‐earth (12–35 RE) magnetotail plasma sheet. The observed plasma pressure, assumed isotropic, was mapped along model magnetic field flux tubes (obtained from the Tsyganenko and Usmanov [1982] model) to the magnetic equator, sorted according to magnetic activity, and binned according to the mapped equatorial location. In regions (L ≳ 12 RE) where the bulk of the plasma pressure was contributed by particles in the energy range of the FPE (70 eV to 40 keV for ions), the statistically determined peak plasma pressures vary with distance similarly to previously determined lobe magnetic pressures (i.e., in a time‐averaged sense, pressure balance normal to the magnetotail magnetic equator in the midnight meridian is maintained between lobe magnetic and plasma sheet plasma pressures). Additional plasma pressure data obtained in the inner magnetosphere (2.5 < L < 7) by the Explorer 45, ATS 5, and AMPTE CCE spacecraft supplement the ISEE 2 data. Estimates of plasma pressures in the “transition” region (7–12 RE), where the magnetic field topology changes rapidly from a dipolar to a tail‐like configuration, are compared with the observed pressure profiles. The quiet time “transition” region pressure estimates, obtained previously from inversions of empirical magnetic field models, bridge observations both interior to and exterior to the “transition” region in a reasonable manner. Quiet time observations and estimates are combined to provide profiles of the equatorial plasma pressure along the midnight meridian between 2.5 and 35 RE.