AMPTE-IRM observations of particles and fields at the dayside low-latitude magnetopause

Using measurements of the AMPTE-IRM spacecraft, we study the structure of the low-latitude magnetopause and its boundary layer on the dayside (08:00-16:00 LT). We analyze three magnetopause (MP) passes in detail and present an overview of the entire data set of IRM passes. In about half of the MP passes the boundary layer is divided into two distinct parts. Whereas the outer boundary layer (OBL) is clearly dominated by solar wind plasma, the densities of solar wind and magnetospheric particles are comparable to one another in the inner boundary layer (IBL). The distinctive feature of the IBL are ''warm'' counterstreaming electrons that originate mainly from the magnetosheath and have a field-aligned temperature that is a factor of 1-5 higher than the electron temperature in the sheath. There is evidence that the IBL is on closed field lines: The bulk flow changes from tailward in the OBL to stagnant (often even directed toward the subsolar point) in the IBL, and the density of ring current electrons increases sharply at the interface between OBL and IBL. For two of the three passes analyzed in detail, the shear of the magnetic field across the MP is high, i.e., greater than 45 . In these cases we find reconnection signatures in the OBL and the magnetosheath close to the MP: proton bulk flows in reasonable agreement with the tangential stress balance of a rotational discontinuity, field-aligned streaming of solar wind and ring current electrons, ''D-shaped'' distributions of solar wind ions, solar wind ions reflected off the MP, and counterstreaming of solar wind and ionospheric ions. Thus we conclude that the MP is open during those passes. The third MP pass analyzed in detail has low (<30 ) magnetic shear. (orig.)Available from TIB Hannover: RN 9303(407) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman