A correlative study of ssc's, interplanetary shocks, and solar activity

Abstract

We have examined 93 ssc's during the 4‐year period from 1968 to 1971 at and near the peak of the solar activity cycle. Of the 93 ssc's, 81 could be associated with solar activity such as solar flares and type 2 and type 4 radio bursts. The mean propagation speeds of these flare‐associated events ranged from 400 to 1000 km/s, the average speed being 600–700 km/s. Disturbances associated with 48 of the ssc's have been studied in detail by using the corresponding interplanetary (IP) magnetic field and plasma data when they were available. We found that 41 of the 48 disturbances corresponded to IP shock waves, and the remaining 7 events were tangential discontinuities. Thirty percent of the IP shocks had thick structure (i.e., the magnetic field jump across the shock occurred over a distance much greater than 50 proton Larmor radii). Also given is a statistical study of the gross geometry of a ‘typical’ or average shock surface based on multiple spacecraft sightings and their relative orientation with respect to the solar flare and/or solar activity. By considering the orientations of 22 well‐determined shock normals in relation to the positions of the parent flares on the solar disk, a method that is a modification of one given by Taylor (1969), it is suggested that a typical shock front propagating out from the sun at 1 AU has a radius of curvature of the order of 1 AU. Also given are some general properties of oblique IP. flare shocks.

Solar wind interaction with Comet Bennett (1969i)

“…). In accordance with the results of Burlaga and Ogilvie (1970) and Chao and Lepping (1972), which show that sse's are generally caused by shocks, we infer that the impulse on April 1 probably signaled the arrival of an interplanetary shock. Unfortunately, no direct interplanetary measurements of the shock are available.…”

“…The spacecraft observations are not sufficiently complete toallDw computation of the shock orientation and speed from the Rankine-Hugoniot equations. However, if it is assumed that the shock was moving nearly radially away from the sun as is usually the case (Burlaga, 1972;Hundhausen, 1972;Chao and Lepping, 1972), then the speed can be calculated from the fact that the discontinuity moved from OGO-5 at 0659 UT to ALSEP, which was 43~downstream, 12 min later, at 0711 UT (Neugebauer et al, 1972).…”

“…The most probable shock direction is radial, away from the sun, although there is a large scatter (~300) in the direction of the shock normals (e.g. Chao and Lepping, 1972) and occasionally the shock normal may deviate as much as 55 0 from the radial direction . If we assume a nearly spherical shock front, we find…”

Heliocentric distance dependence of the interplanetary magnetic field

“…The majority of the shocks observed at I AU have been associated with solar flare events (e.g. Chao and Lepping, 1974). They are seen much less frequently (roughly one per month) than directidnal discontinuities.…”

“…Flare-associated shocks are predicted to propagate outward with a thickness of the order of a few proton Larmor radii during most of their passage through interplanetary space. From a study of the orientations of 22 well-determined shock normals in relation to the positions of the parent flares on the solar disk, Chao and Lepping (1974) suggested that a typical shock front propagating out from the sun has a radius of curvature of 1 AU at I AU, although any single case may vary considerably from this average.…”