Interplanetary Alfvén waves and auroral (substorm) activity: IMP 8

Almost 1 year of IMP 8 interplanetary magnetic field and plasma data (days 1–312, 1979) have been examined to determine the interplanetary causes of geomagnetic AE activity. The nature of the interplanetary medium (Alfvénic or non‐Alfvénic) and the B s correlation with AE were examined over 12‐hour increments throughout the study. It is found that Alfvénic wave intervals (defined as V x −B x cross‐correlation coefficients of >0.6) are present over 60% of the time and the southward component of the Alfvén waves is well correlated with AE (average peak correlation coefficient 0.62), with a median lag of 43 min. The most probable delay of AE from B s is considerably shorter, about 20–25 min. Southward magnetic fields during non‐Alfvénic intervals ( V x −B x cross‐correlation coefficients of <0.4) are equally effective in producing geomagnetic activity. Peak correlation coefficients and lags are similar to those of Alfvénic intervals. From this statistical study, no major differences in the magnetospheric response to Alfvénic and non‐Alfvénic intervals were obvious. The high‐intensity long‐duration continuous AE activity (HILDCAA) events discussed previously by Tsurutani and Gonzalez (1987) are demonstrated to be caused by the southward components of the Alfvén waves, presumably through the process of magnetic reconnection. The lag times of AE from B s were variable from event to event (and at different times within the Alfvén wave train), ranging from 45 min to as little as 0 min. The cause of this variable delay is somewhat surprising and is not presently well understood.