Solar activity and heliosphere‐wide cosmic ray modulation in mid‐1982

Abstract

A major episode of flare activity in June and July of 1982 was accompanied by a pair of heliosphere‐wide cosmic ray modulation events. In each case, a large Forbush decrease (FD) at earth was followed in turn by apparently related decreases at Pioneer 11 (P11) and Pioneer 10 (P10). The Pioneer spacecraft were separated by ∼155° in ecliptic longitude. We reviewed white light coronagraph and near‐sun (≤1 AU) satellite data to identify plausible solar origins of these modulation events. The first widespread intensity decrease (FD 1) can be attributed to the combined effects of a backside flare on June 3 from solar active region 18382/18383, located 23° in ecliptic longitude from Pioneer 10, and a visible disk flare from 18405 on June 6, when this region was 9° from Pioneer 11. The second widespread modulation event during this period (FD 2) may be linked to flares from active region 18474 on July 12 and 22. The July 12 flare was located 34° in azimuth from Pioneer 11, and the July 22 flare was 24° from Pioneer 10. Since even fast shocks would take ∼1 month to propagate to Pioneer 11 (12 AU) and ∼2 months to reach Pioneer 10 (28 AU) in mid‐1982, these “one‐to‐one” associations must be regarded with caution. The processes of entrainment and coalescence can cause a given traveling interplanetary disturbance to lose its identity enroute to the outer heliosphere. The fact that we were able to identify plausible solar flare candidates for each of the four Forbushlike decreases observed at the Pioneer satellites (two each at P10 and P11), however, removes the need to invoke a shock from a single flare as the sole cause of either FD 1 (at both P10 and P11) or FD 2. Such single‐flare scenarios have recently been suggested by several investigators to account for the widespread intensity decreases in mid‐1982. Instead, the heliosphere‐wide modulation during this period appears to result primarily from a sustained episode of powerful flares from a relatively narrow range of active solar longitude. A significant fraction (1/2 to 3/4) of the major coronal mass ejections (CMEs) and near‐sun shocks observed during June and July 1982 originated in flares occurring in a 45° band of Carrington longitude. Because of solar rotation these flares occur over the full range of ecliptic longitude and can generate an outward propagating shell of CMEs and shocks that encompasses the sun to produce the observed azimuthal symmetry in the cosmic ray modulation. The prolonged high‐speed wind stream at P10 in the second half of 1982 may have resulted, at least in part, from the coalescence of a series of fast transient streams directed toward that distant spacecraft into an extended compound stream.