A survey of dayside flux transfer events observed by ISEE 1 and 2 magnetometers

Abstract

A survey is made of magnetic flux transfer events (FTE's) observed both exterior and interior to the dayside magnetopause region by the ISEE 1 and 2 spacecraft during the first two years of operation. FTE's are found to be a common feature of the magnetopause region covered by the spacecraft when the magnetic field in the magnetosheath has a southward component but not when it is northward. The most commonly observed signature of an FTE in the field component normal to the magnetopause is an outward (positive) followed by an inward (negative) bipolar perturbation and is usually seen at northern latitudes. A reversed normal component signature is seen most frequently at low or southerly latitudes. Only very rarely are both signatures seen on the same pass. On occasion, FTE's are observed with a more complex normal field signature. During periods of southward magnetosheath field the average number (˜5) and recurrence time (˜8 min) of FTE signatures observed per magnetopause crossing in the magnetosheath is similar to the average number (˜4) and recurrence time (˜7 min) of FTE signatures observed interior to the magnetopause. We conclude that the magnetosheath and magnetosphere FTE's are part of the same physical phenomenon. The dimension of FTE flux tubes normal to and along the magnetopause is estimated as ˜1 RE and ˜2 RE, respectively. We derive a lower limit of ˜6 kV for the voltage associated with FTE magnetic flux removal during southward field conditions. However we argue that this value actually may be up to an order of magnitude larger. Thus FTE's could provide the dominant means of flux transfer required to drive geomagnetic disturbances.