The angular extents of solar/interplanetary disturbances and modulation of galactic cosmic rays

Abstract

From comparisons of solar activity with galactic cosmic ray (GCR) modulation events at 1 AU and in the outer heliosphere, we argue against the suggestion that individual solar eruptions can give rise to major cosmic ray modulation events that span the heliosphere. For the inner heliosphere, we use a sample, covering a ∼30‐year period, of eruptive flares and confidently associated interplanetary shocks to show that the following three parameters, determined at 1 AU, all decrease with increasing longitudinal distance from the flare: probability of detecting a shock, shock transit speed, and magnitude of any associated Forbush decrease (FD). In addition, we associate each of the 18 large (≥10%) FDs observed at Earth from 1958 to 1991 with one or more eruptive flares located near solar central meridian (E47 – W31). For each of the six heliosphere‐wide modulation events that have been associated thus far by other investigators with individual flares we are able to identify multiple powerful flares, with one or more of these eruptive events generally occurring close in ecliptic longitude to each affected spacecraft. Supporting evidence for the limited longitudinal extent of major interplanetary effects from single eruptions is provided by a remarkable outer heliosphere event in late 1989. A large shock (ΔV – 200 km/s) and Forbushlike decrease (18%) at Pioneer 10 at ∼50 AU was not preceded by a commensurate disturbance at Voyager 1/Voyager 2/Pioneer 11 located at ∼30–40 AU on the opposite side of the heliosphere. We conclude that wide‐spread modulation events in the outer heliosphere result from systems of solar/interplanetary disturbances that, only in aggregate, encompass 360° of ecliptic longitude, rather than from azimuthally symmetric shocks associated with single eruptive flares. We point out that the documented preference for coronal mass ejections to occur at low and middle solar latitudes will naturally form the wedge‐shaped barrier proposed by Van Allen and Mihalov to account for the “inside‐out” pattern of recovery of individual modulation events observed at progressively greater distances in the heliosphere.