Fluid and particle signatures of dayside reconnection

Using measurements of the AMPTE/IRM spacecraft, we study reconnection signa tures at the dayside magnetopause. If the magnetopause is open, it should h ave the properties of a rotational discontinuity. Applying the fluid concep t of a rotational discontinuity, we check for the existence of a de Hoffman n-Teller frame and the tangential stress balance (Walen relation). For 13 o ut of 40 magnetopause crossings in a statistical survey we find a reasonabl e agreement between observed plasma flows and those predicted by the Walen 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, counterstream ing of solar wind and cold ionospheric ions, two-beam distributions of sola r wind ions, and distributions of solar wind particles associated with fiel d-aligned heat flux. While a particular type of particle signature is obser ved 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 Walen relation can be used to infer the sign of the normal magne tic field, B-n. We find that the two ways of inferring the sign of B-n lead primarily to the same result. Thus, both the particle signatures and a rea sonable agreement with the Walen relation can, in a statistical sense, be c onsidered as a useful indicator of open field lines. On the other hand, man y crossings do not show any reconnection signatures. We discuss the possibl e reasons for their absence.