Explorer 12 observations of solar cosmic rays and energetic storm particles after the solar flare of September 28, 1961

Abstract

A full description of the cosmic ray experiment on Explorer 12 is given and cosmic ray measurements made during the solar event of September 28, 1961, are reported and discussed. Galactic cosmic ray measurements are also reported. A few hours before the class 3 flare of September 28, two short counting rate increases were observed and are interpreted as electron bursts. The anisotropy of the medium-and low-energy solar protons early in the event and their intensity throughout the event are described. It is found that the history of the intensity of the solar protons is consistent, once isotropy is established, with their having diffused through interplanetary space with an effective mean free path of 0.04 AU. This result is discussed and is shown to be not obviously in disagreement with the generally accepted views regarding the configuration of the interplanetary magnetic field. An estimate of the distance from the sun at which diffusion becomes unimportant and particles escape gives 2 to 3 AU. It is pointed out that simple diffusion (where the particles are scattered from discrete scattering centers and the influence of a general magnetic field is negligible) does not account for the behavior of the anisotropy before isotropy is reached. Two days after the flare, and beginning just before the sudden commencement of a magnetic storm, there was a large increase in the intensity of protons between 2 and 15 Mev, the lower energy limit being determined by the sensitivity of the detectors. As most of these particles, which we have called 'energetic storm particles,' arrived after the sudden commencement occurred, we suggest that they were solar protons trapped within the plasma cloud which caused the magnetic storm. The outline of a possible trapping mechanism is given. Explorer 12 measurements of the Forbush decrease of September 30, 1961, are compared with neutron monitor measurements at I)eep River. The decrease is larger at Explorer 12 by a factor of 1.7 _ 0.3. This paper describes the detectors and discusses the measurements made during the solar cosmic ray event initiated by a class 3 flare on September 28, 1961. Fortunately, the satellite was at apogee both at the time of the arrival of the solar cosmic rays and when the magnetic storm began two days later. The rise and decay of the solar proton intensity were recorded as a function of energy from 2 to 600 Mev. A plasma cloud, apparently emitted at the time of the flare, produced a large magnetic storm and a moderate Forbush decrease about 46 hours later. At the time of the sudden commencement of the magnetic storm, an increase of intensity of low-energy (( 15 Mev) protons was observed. The other four solar cosmic ray 4983 4984 BRYANT, CLINE, DESAI, AND McDONALD

Interplanetary fast forward shocks and energetic storm particle events above 1.5 MeV

“…Investigation of IP shocks is important because of their role in particle acceleration (Reames, 1999) and as a cause of geomagnetic storms (Richardson et al, 2001;Zhang et al, 2007a,b). Shock passages are usually identified as abrupt changes in the solar wind plasma parameters, but sometimes they are also associated with energetic storm particles (ESPs) (Bryant et al, 1962;Rao et al, 1967). Acceleration of ESPs in ICME-driven (transient) interplanetary shocks has been studied by many authors, e.g.…”

Transient Shocks and Associated Energetic Particle Distributions Observed by ACE during Cycle 23

“…We describe the assemblage and analysis of such a base of statistical samples using data from NASA's Advanced Composition Explorer (ACE) spacecraft. Intensity increases of energetic ions associated with the passage of IP shocks are called energetic storm particle (ESP) events [5]. During a typical ESP event, lowenergy ion intensities may increase by orders of magnitude.…”

Post‐shock spikes: A new feature of proton and alpha enhancements associated with an interplanetary shock wave

“…(a) Energetic Storm Particle (ESP) events, having time scales on the order of hours and normally observed in association with an interplanetary shock front or tangential discontinuity in the interplanetary magnetic field (Bryant, et al, 1962;Lanzerotti, 1969;Kahler, 1960;McCracken and Rao, 1970;Kunstmann and Wibberenz, 1973), and (b) "Shock-Spike" events with time scales of 5 to 30 minutes and typically occuring within 5 to 10 minutes of a shock front passage (Axford and Reid, 1963;Armstrong, et al, 1970;Ogilvie and Arens, 1971;Singer and Montgomery, 1971). Simultaneous measurements of protons and alpha particles at equal velocities, equal rigidities and equal energy per charge as a function of time have been recognized to be of fundamental importance in determining the probable causes for these enhancements (Lanzerotti and Robbins, 1969;Scholer et al, 1972).…”