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LOWER ENERGY COSMIC RAYS AND THE SOLAR CYCLE P. Meyer Enrico Fermi Institute for Nuclear Studies University of Chicago The low energy portion of cosmic radiation (from 100 Mev up to several Bev), which may produce spallation nuclides in meteorites, is strongly influenced by solar phenomena. On the one hand one does observe striking changes in the flux and energy spectrum of the galactic cosmic ra- diation which are correlated with the 11-year solar activity cycle or with single solar flares (Forbush decreases), and on the other hand one ob- served occasional solar emission of high energy particles with an energy spectrum quite different from that of the galactic component. We wish to discuss briefly these two phenomena, namely (1) solar modulation of galactic cosmic radiation, and (2) solar emission of high energy particles. Let us consider first the general changes occurring during the 11- year solar cycle. During years of solar minimum the differential spec- trum of the isotropic galactic proton flux closely follows a power law, with N(E)dE = const. E-2. 7 dE. (1) The range of validity of this relation extends from about 1 Bev up to 20 to 30 Bev at least; no low-energy cutoff seems to be present but the exact shape of the spectrum is not known for the lower energies. During the active phase of the Sun, however, the galactic proton flux is reduced by a factor of about two for energies above 1. 5 Bev, and follows a flatter power law distribution, namely N(E)dE = const. E-2.0 dE. (2) At solar maximum there is a distinct low-energy cutoff: the number den- sity of protons drops rapidly below 1 Bev. The two cases are illustrated in Figure 1, necessarily somewhat approximately since the information at hand really applies only to the present solar cycle. It is widely believed today that during the active phase of the solar cycle a steady stream of ionized particles of density near 100 particles/ cm3 travels outward from the Sun at velocities the order of 500-1000 km/ sec and is responsible for this cosmic ray modulation. The kinetic energy of the particles in this "solar wind" would be only the order of 103 ev, far 58
At Solar Cycle Minimum At Solar Cycle Maximum Cosmic Ray Proton Energy E Figure 1. too small to produce nuclear effects, but nonetheless capable of producing important modulation effects on the interplanetary magnetic fields, and through this agent affecting the trajectories of all ionized particles (includ- ing cosmic rays) in the solar system. We shall return to this point. The cosmic ray phenomena associated with solar flares are of two different types, production and modulation. The Sun emits clouds or bursts of particles at the time of a flare. Most small flares (Class 1), which may appear at the rate of several per day at peak activity, probably confine themselves to ejecting relatively low energy streams of particles. Somewhat larger flares (Classes 2 and 2+) may rarely emit particles with energies extending up to perhaps 300-400 Mev. The energy spectrum of these particles appears to follow approximately an E-4 law. This spectrum was observed by balloon experiments. The much rarer flares of intensity Classes 3 and 3+ are known to occasionally emit particles up to about 20 Bev. During one such event (February 1956) the energy spectrum of the high energy particles was measured to go roughly as E-". On the basis of radionoise absorption data and several months of satellite and space probe observations one may estimate the frequency of the occurrence of solar particle emission during the past maximum of solar activity. It appears that, in this phase of the solar cycle, the Sun emits particles with energies up to 400 Mev once or twice a month. The emission of relativistic protons seems to occur about once every four years and no obvious correlation with the phase of the solar cycle has been found for such events. 59
The bulk of low energy particles emitted in a solar flare usually has a velocity of 1000-2000 km/sec, which results in a transit time from Sun to the Earth of about a day or two. This solar plasma is assumed to be the cause of the Forbush decreases in galactic cosmic ray intensity and of geomagnetic effects which often follow an intense flare. The low-energy galactic cosmic ray flux during a Forbush decrease will show a sharp drop which in some cases may amount to as much as 25-50 percent. The re- covery is much slower, and appears to take place approximately expo- nentially over the next few days or weeks. A number of theories have been proposed to account for various of these effects which we have noted. One which is unusually comprehensive in scope is that of Eugene Parker at the University of Chicago. Parker notes that the high temperature of the solar corona may extend several solar radii into space, with the result that the corona expands hydro- dynamically with supersonic velocity into space. The radially-directed gas can therefore extend the magnetic lines-of-force of the general solar field in the radial direction. These radially-directed fields can be ex- pected to extend for many astronomical units outward from the Sun, and with the rotating motion of the Sun imposed on them they should appear as spirals. Parker shows that this hydromagnetic system leads to the well known nose instability beyond the orbit of Earth. In the instability region the lines-of-force are highly disordered and form a diffuse barrier that shields the inner portions of the solar system from some of the lower energy galactic cosmic radiation. In other words, Parker's theory sug- gests a reason for the reduction in the galactic cosmic ray flux during the active phase of the 11-year solar cycle, and it explains the return of the galactic flux to normal during the quiet phase by the absence (or very great reduction) of the solar wind in the inactive part of the solar cycle. The role of a solar flare in producing the Forbush decrease, in Parker's view, is to eject a stream of ionized particles into the spiral lines-of-force which wind outwards from the Sun. Since the ejection is purely radial, the ions impart some transverse energy to the spiral mag- netic field which is propagated along the lines-of-force as a shock wave. This shock wave has a radial velocity of about 2000 km/sec. It reaches the Earth about a day after the onset of the flare and continues beyond. As it passes, a sharp decrease in galactic cosmic ray intensity occurs (the Forbush decrease), due to the fact that incoming charged particles in the Mev to several Bev range cannot penetrate the shock barrier, and the galactic cosmic rays continue to be excluded until the shock waves have finally been damped out by particle collisions over a period of days or weeks. A slight increase in galactic cosmic radiation prior to the onset of the Forbush decrease is expected in this model and has been noted several times. Note that the stationary barrier which affects the galactic cosmic radiation throughout the active phase of the solar cycle acts 60
efficiently only on particles with less than~5 Bev energy. The shock barrier in Parker's model has quite different characteristics which may explain the energy range affected by the Forbush decreases. There are a few points in Parker's theory which are of special importance in relating the cosmic ray flux to nucleogenesis in the meteor- ites. First, the meteorites would be expected to receive a full flux of galactic cosmic radiation during whatever time their orbits were outside the instability barrier, regardless of the phase of the solar cycle. When the meteorites are inside this barrier they receive a normal flux of gal- actic radiation during the quiet phase of the Sun and a reduced flux of radiation below 3-4 Bev during the active phase. The more energetic galactic flux is not affected by the solar cycle (see Olbert's paper). Second, the "brightness" of the solar particle emission after flares does not depend on the distance of the meteorite from the Sun, so long as it is within the instability zone (the solid angle which the Sun subtends does depend on the distance, of course). The instability barrier probably re- flects flare particles and accounts for the observed storage in the inner solar system which may last several days. It is probably premature to do too much speculation about the exact nature of these effects. It will be very nice, however, to try to correlate the theory with observations when we have a space probe travelling in or beyond the zone of instability. Arnold: In estimating the effects of the solar modulation of the cosmic ray primaries on the production of nuclides in a meteorite, it is important to know how much material has been removed from the surface. If very much ablation has occurred, then perhaps none of the cosmogenic nu- clides we see were produced by primaries below about 1 Bev. Olbert: There is no doubt about the existence of the sort of instability which Parker postulates, provided a stream of ionized particles does flow from the Sun during the active phase. Any time we have a dynam- ical motion of ionized gases with magnetic fields frozen in, the pressure is anisotropic and instabilities can arise. Yet the analysis is essen- tially dimensional in character, and the position of the instability (pre- dicted to be near the orbit of Mars) is probably uncertain by a factor of two. Kohman: Where does the acceleration of the "solar wind" arise? Meyer: This is based on the experimental temperature distribution of the corona â essentially the solar wind is a steady-state dynamical expan- sion of a hot gas. The problem is hydrodynamic in nature and does not depend on magnetic forces. Nonetheless, there are some ambiguities 61
in the solutions of the hydrodynamic equations, so that the solar wind concept is by no means universally accepted. Fish: Are the lines-of-force which extend radially in Parker's model somehow connected back to the Sun in order to satisfy div B = 0? If so, any shock waves propagating outward would also eventually present a shock front directed inward, toward the Sun, and should result in anisotropies in the galactic cosmic ray flux some days after a flare. Van Allen: It is not strictly necessary that the lines-of-force be closed for the divergence of ^to vanish, provided the lines-of-force consti- tute a sufficiently complicated network. Fish: It might be possible to apply a critical test to Parker's model for cosmic-ray modulation by studies of the occasional large solar flares which occur during the quiet period in the solar cycle. If Parker's theory is correct there should be much smaller Forbush decreases following these flares than would be the case for flares during the active period. 62
