Fluid and particle signatures of dayside reconnection

Using measurements of the AMPTE/IRM spacecraft, we study reconnection signatures at the dayside magnetopause. If the magnetopause is open, it should have the properties of a rotational discontinuity. Applying the fluid concept of a rotational discontinuity, we check for the existence of a de Hoffmann-Teller frame and the tangential stress balance (Walén relation). For 13 out of 40 magnetopause crossings in a statistical survey we find a reasonable agreement between observed plasma flows and those predicted by the Walén relation. In addition, we check if the measured distribution functions show single particle signatures which are expected on open field lines. We find the following types of signatures: field-aligned streaming of ring current particles, “D-shaped” distributions of solar wind particles, counterstreaming of solar wind and cold ionospheric ions, two-beam distributions of solar wind ions, and distributions of solar wind particles associated with field-aligned heat flux. While a particular type of particle signature is observed only for the minority of magnetopause crossings, 24 of the 40 crossings show at least one type of signature. Both the particle signatures and the fit to the Walén relation can be used to infer the sign of the normal magnetic field, Bn. We find that the two ways of inferring the sign of Bn lead primarily to the same result. Thus, both the particle signatures and a reasonable agreement with the Walén relation can, in a statistical sense, be considered as a useful indicator of open field lines. On the other hand, many crossings do not show any reconnection signatures. We discuss the possible reasons for their absence.