Fast Ejecta During the Ascending Phase of Solar Cycle 23: ACE Observations, 1998 -- 1999

L. F. Burlaga, R. M. Skoug, C. W. Smith and D. Webb

Journal of Geophysical Research, submitted 2000.

Abstract:

We discuss fast ejecta observed at 1 AU during a period of increasing solar activity from day 36, 1998 to day 333, 1999. “Fast ejecta” are transient, non-corotating flows that move past the earth during a day or more, with a maximum speed > 600 km/s. We identify two classes of fast ejecta at 1 AU: 1) magnetic clouds, whose local magnetic structure is that of a flux rope; and 2) “complex ejecta”, which are not flux ropes and have disordered magnetic fields. The complex ejecta have a variety of structures and origins, but no attempt is made to classify them because our sample size is small. Nearly equal numbers of magnetic clouds and complex ejecta were found: 4 and 5, respectively. The magnetic clouds had stronger magnetic fields and lower proton temperatures than the complex ejecta on average. The average beta for the complex ejecta (0.25 +/- 0.09) was larger than that for the magnetic clouds (0.06 +/- 0.04). The magnetic clouds and complex ejecta had comparable speeds on average, viz., 558 +/- 80 km/s and 500 +/- 63 km/s, respectively. Using the duration of the stream and that of the counterstreaming electrons to measure the ejecta, the average time for the complex ejecta to move past ACE was 3 days, which is approximately twice that for the magnetic clouds. We discuss several hypotheses concerning the structures and origins of complex ejecta, including the possibility that they are formed by a series of CMEs of various sizes. All of the magnetic clouds caused geomagnetic storms. Three complex ejecta produced no geomagnetic storms. The other two complex ejecta produced geomagnetic storms indirectly: one by driving a shock into the rear of a magnetic cloud, and the other by amplifying southward fields in its leading edge and interaction region.