Flux Transfer Events at the Dayside Magnetopause: Transient Reconnection or Magnetosheath Dynamic Pressure Pulses?

The suggestion isdiscussed that characteristic particle and field signatures at the dayside magnetopause, termed "flux transfer events" (FTEs), are, in at least some cases, due to transient solar wind and/or magnetosheath dynamic pressure increases, rather than timedependent magnetic reconnection. It is found that most individual cases of FTEs observed by a single spacecraft can, at least qualitatively, be explained by the pressure pulse model, provided a few rather unsatisfactory features of the predictions are explained in terms of measurement uncertainties. The most notable exceptions to this are some "two-regime" observations made by two satellites imultaneously, one on either side of the magnetopause. However, this configuration has not been frequently achieved for sufficient ime, such observations are rare, and the relevant ests are still not conclusive. The strongest evidence that FI'Es are produced by magnetic reconnection is the dependence of their occurrence on the north-south component of the interplanetary magnetic field (IMF) or of the magnetosheath field. The pressure pulse model provides an explanation for this dependence (albeit qualitative) in the case of magnetosheath FI'Es, but this does not apply to magnetosphere FI'Es. The only surveys of magnetosphere FTEs have not employed the simultaneous IMF, but have shown that their occurrence is strongly dependent on the north-south component of the magnetosheath field, as observed earlier/later on the same magnetopause crossing (for inbound/outbound passes, respectively). This paper employs tatistics on the variability of the IMF orientation to investigate the effects of IMF changes between the times of the magnetosheath and FI'E observations. It is shown that the previously published results are consistent with magnetospheric FTEs being entirely absent when the magnetosheath field is northward: all crossings with magnetosphere FTEs and a northward field can be attributed to the field changing sense while the satellite was within the magnetosphere (but dose enough to the magnetopause to detect an FTE). Allowance for the IMF variability also makes the occurrence frequency of magnetosphere FTEs during southward magnetosheath fields very similar to that observed for magnetosheath FTEs. Conversely, the probability of attaining the observed occurrence frequencies forthe pressure pulse model is 10 '14. In addition, it is argued that some magnetosheath FTEs should, for the pressure pulse model, have been observed for northward IMF: the probability hat the number is as low as actually observed is estimated tobe 10 4ø. It is concluded that although the pressure model can be invoked to qualitatively explain a large number of individual FTE observations, the observed occurrence statistics are in gross disagreement with this