Investigation of the outer and inner low-latitude boundary layers

We analyze 22 AMPTE/IRM crossings of the dayside low-latitude boundary laye r for which a dense outer part can be distinguished from a dilute inner par t. Whereas the plasma in the outer boundary layer (OBL) is dominated by sol ar wind particles, the partial densities of solar wind and magnetospheric p articles are comparable in the inner boundary layer (IBL). For 11 events we find a reasonable agreement between observed plasma flows and those predic ted by the tangential stress balance of an open magnetopause. Thus, we conc lude that, at least in these cases, the OBL is formed by a local magnetic r econnection. The disagreement with the tangential stress balance in the oth er 11 cases might be due to reconnection being time-dependent and patchy. T he north-south component of the proton bulk velocity in the boundary layer is, on average, directed toward high latitudes for both low and high magnet ic shear across the magnetopause. This argues clearly against the possibili ty that the dayside low-latitude boundary layer is populated with solar win d plasma primarily from the cusps. "Warm", counterstreaming electrons that originate primarily from the magnetosheath and have a field-aligned tempera ture that is higher than the electron temperature in the magnetosheath by a factor of 1-5, are a characteristic feature of the IBL. Profiles of the pr oton bulk velocity and the density of hot ting current electrons provide ev idence that the IBL is on closed field lines. Part of the IBL may be on new ly opened field lines. Using the average spectra of electric and magnetic f luctuations in the boundary layer, we estimate the diffusion caused by lowe r hybrid drift instability, gyroresonant pitch angle scattering, or kinetic Alfven wave turbulence. We find that cross-field diffusion cannot transpor t solar wind plasma into the OBL or IBL at a rate that would account for th e thickness (similar to 1000 km) of these sublayers. On the duskside, the d awn-dusk component of the proton bulk velocity in the IBL and magnetosphere is, on average, directed from the nightside toward local noon. Formation o f the IBL may also be due to mechanisms operating in the magnetotail.