Energetic Particles In Solar Flares Research Articles

Injection of particles into the diffusive shock acceleration process: Monte Carlo simulation of heliospheric shocks

We compare a Monte Carlo simulation of diffusive shock acceleration to observations of the nearly radial bow shock an interplanetary shock and solar flare energetic particle events. The simulation assumes that the same scattering operator that gives rise to shock acceleration can also describe a viscous shock governed by hydrodynamic turbulence. This implies that accelerated ions can be drawn directly from the thermal plasma with no separate superthermal seed population. Good agreement between the simulation and observations lends support to thermal leakage of downstream, shock heated ions as an important mode of injection.

Temporal variations of nucleonic abundances in solar flare energetic particle events. I - Well-connected events

We have surveyed the temporal variations of the relative abundances of 0.6--2.4 MeV per nucleon ions accelerated in solar particle events whose parent flares were located in the so-called well-connected region between W20/sup 0/ and W80/sup 0/ solar longitude. The survey was carried out under solar minimum conditions from 1973 November to 1977 December using the University of Maryland/Max-Planck-Institut ULET telescope on the IMP 8 spacecraft. Several particle events were found which originated in the well-connected region, and of these, two events appeared remarkably free of contamination from other solar particle events or interplanetary disturbances. During the rise phase, the abundances measured at 1 AU were characterized by large (factor of approx.5) decreases in the Fe/O ratios, of the type reported by others, as well as significant decreases in the O/He ratio. After examination of several mechanisms which might cause the abundance variations, it is concluded that the rise phase variations may be understood in terms of interplanetary propagation effects. A detailed comparison of model calculations with the rise phase observations leads to the conclusion that the radial interplanetary scattering mean-free-path varies with rigidity as lambda/sub r/proportionalP/sup 0.61plus-or-minus0.18/, in agreement with calculations from the quasi-linear theory. However, the size ofmore » lambda/sub r/ approx.0.1--0.2 AU is larger than predicted by the theory.« less

Solar activity and solar neutrino flux

Among discussions of the implications of the Davis experiment1 to detect solar neutrinos, there have been several suggestions that manifestations of solar activity are related to the flux of the neutrinos2–4. For example, it has been implied that (1) the physical processes in the deep interior of the Sun are related to, or even produce, the solar activity (exemplified by solar flares and sunspots) observed in the photosphere2,4; or that (2) some of the observed neutrinos are produced in nuclear reaction processes caused by energetic particles in solar flares and solar active regions3. These suggestions have been speculative because the available solar neutrino data covered only a fraction of a solar cycle. We now re-examine the proposals using data over a significantly longer time base and find no evidence for either of these possibilities.

On 3-He Rich Solar Particle Events

Extensive measurements of the composition of energetic solar flare particles have revealed the frequent occurrence of solar flares with large enrichments of 3He and heavy ions compared to the composition of the solar atmosphere. The basic characteristics of these events can be summarized as follows: –3He/4He-ratios up to more than 1 are observed–the 3He/4He-ratio increases with energy–no comparable enhancement of 2H or 3H is found–there is a strong correlation of 3He and heavy ion enrichment.

The isotopic composition of solar flare accelerated neon

The individual isotopes of neon in energetic solar flare particles have been clearly resolved with arms mass resolution of 0.20 amu. We find ^(20)Ne/^(22)Ne = 7.6 (+2.0, -1.8) and ^(21)Ne/^(22)Ne ≾ 0.11 in the 11-26 MeV per nucleon interval. This isotopic composition is essentially the same as that of meteoritic planetary neon-A and is significantly different from that of the solar wind.

Plasma convection in force-free magnetic fields as a mechanism for chemical separation in cosmical plasmas

DURING the past two decades, progress in space research has forced us to abandon the earlier idea of a cosmical plasma as being a homogeneous medium. Filament formation and chemical separation are two important phenomena that are often observed in cosmical plasmas and illustrate the importance of inhomogeneity. An average composition from the Sun's atmosphere used to be taken as a standard when expressing cosmic abundancies in general. Numerous observations, however, of the chemical composition in the solar wind from in situ measurements and from lunar soil and meteorite samples, indicate a great variability in these ratios1. The proton to α-particle flux ratio for solar flare energetic particle events varies widely between and within events2. Recent observations show that this variability is connected with the flare process itself and the earlier history of the flare, rather than propagation effects between the flare site and the observer. Some local chemical differentiation mechanism in the preflare history of the specific active region is probably responsible for the composition of the emitted, energetic particles. The phenomena of filamentation and chemical separation may be coupled, as in the presence of a temperature gradient, the plasma convection associated with filamentary structures provides an effective means of selective transport. The general principles of this mechanism are described here. These principles may be important not only in solar flares but in many other cosmical plasmas where force-free fields and thermal gradients occur.

Criticism of reconnection models of the magnetosphere

Reconnection involves singular lines called X-lines on the day and night sides of the magnetosphere, and the reconnection rate is proportional to the component of the electric field along the X-line. Although there is some indirect support for this model, nevertheless direct support is totally lacking. However, there are two distinct pieces of clearly contradictory observational evidence on the dayside. First is the failure to account for the implied energy dissipation by the magnetopause current, over 10 11 W, which should be easily observable as heating or enhanced flow of the plasma near the magnetopause. In marked contrast to this prediction, HEOS-2 satellite data reveal a plasma with decreased energy density and reduced flow. Second, the boundary of closed magnetic field lines is in the wrong location. In the reconnection process the plasma outflow would cut across open field lines toward higher latitudes; there should be a band of open field lines equatorward of the cleft. Observations of trapped energetic particles indicate closed field lines within the entry layer and cleft. Either one of these pieces of evidence is sufficient by itself to require drastic revision, even rejection, of the reconnection model. There is also contradictory evidence on the night side. The last closed field line capable of trapping energetic particles is poleward of auroral arcs. The implication is that the X-line is at the distant magnetopause, and not in the plasma sheet. Consequently, even if the reconnection process were operative at the nightside X-line, it would be isolated from steady state plasma sheet and auroral processes. On the other hand, substorm phenomena, in which stored magnetic energy is converted into particle kinetic energy, necessarily involve an induced electric field; that is excluded in theories of the reconnection process in which it is assumed that curl E = 0. Nevertheless, the observed easy access of energetic solar flare particles to the polar caps, and especially the preservation of interplanetary anisotropies as differences between the two polar caps, argues strongly for an open magnetosphere, with interconnection between geomagnetic and inter-planetary magnetic field lines. It is suggested that the resolution of this apparent paradox involves electric fields parallel to the magnetic field lines somewhere on the dawn and dusk sides of the magnetosphere, with an equipotential dayside magnetopause.

The radial transport of energetic solar flare particles from 1 to 6 AU

Data from the Pioneer 10 and 11 missions to Jupiter and beyond are used in a study of solar flare accelerated particles at distances of several astronomical units from the sun. The intensity-time profiles out to at least 6 AU are consistent with mainly diffusive interplanetary propagation. The outer boundary of the diffusion region is thought to be beyond 10 AU, since no obvious effects of a free escape boundary were observed. Four solar particle events are discussed in detail. The events are interpreted in terms of a spherically symmetric propagation model which includes the effects of diffusion, convection, and adiabatic deceleration and which assumes that the particles are impulsively injected at the sun.

Plasma turbulence in the vicinity of a magnetic neutral line

The behavior of a collisionless plasma in the vicinity of a magnetic neutral line has been investigated. An electric field externally applied along the neutral line results in a redistribution of plasma density which is consistent with E×B flows. The development of a complex spectrum of low frequency waves, ωci<ω<ωpi, is observed. These waves are spatially localized to within 3.5 cm of the neutral line and propagate antiparallel to the applied electric field. The power spectrum varies as P∝ω−5 and increases monotonically with the applied electric field, saturating just prior to electric field saturation. Heating of both the bulk and the tail of the electron energy distribution is observed and increases with increasing intensity of the density fluctuation spectrum. Typical measured values for the density, electron temperature, and electric field indicate an anomalously large resistivity. The Sagdeev and Galeev effective collision frequency for a current-driven ion acoustic mode with Vde≃Vs leads to electric fields consistent with those experimentally measured. These observations indicate the presence of a variant of the current-driven ion acoustic mode, which gives rise to significant heating of the electron distribution. Such a process may likely play an important role in the generation of energetic particles in solar flares and the geomagnetic tail neutral sheet.

Theories of Small-Scale Magnetic Fields

This review is concerned with the origin of the fine structure of the fields and their relationship to the heating of the solar chromosphere and corona, the structure of prominences and the production of energetic particles in solar flares. The dynamics of sunspot formation, and the large-scale structure of individual sunspots have not been dealt with, although the evolution of AR fields has been considered insofar as it affects the flare problem.

Solar wind and solar energetic particles: Properties and interactions

This paper reviews findings since 1966 about the properties of the interplanetary magnetic field; the solar wind plasma; solar wind interactions with the earth, the moon, Mars, and Venus; and the properties and propagation characteristics of energetic solar flare particles. For most purposes the solar wind can be characterized as a fluid that retains some of the properties of its source as it flows outward and that contains an anisotropic temperature distribution and embedded magnetic field. The magnetic field has a sector structure which slowly varies in a manner that resembles the evolution of magnetic features on the solar surface. As the solar wind encounters planetary bodies in its path, the nature of its interaction depends on the size and magnetic properties of the bodies. The solar wind creates a bow shock similar to that at earth as it interacts with Venus, and probably also with Mars. At Venus the plasma is apparently not diverted by a magnetic field, but by the planetary, ionosphere. At Mars the plasma is diverted either by a weak magnetic field or by an ionosphere. Further studies near earth have led to an explanation of the earth's bow shock in terms of hydromagnetic and ion waves. Energetic electrons (>400 kev) have been found in the neutral sheet of the geomagnetotail. The absence of thermal plasma in the earth's outer magnetosphere, beyond the plasmapause, has been explained in terms of a large‐scale convection field. The solar wind interaction with the moon is of an entirely different character. No bow shock is found, and the interplanetary magnetic field appears to convect through the moon. A single‐particle model of the interaction appears adequate to explain the observed phenomena, including the region behind the moon that is void of plasma but contains an enhanced magnetic field. Energetic electrons (20 kev to several Mev), protons (500 kev to 200 Mev), and α particles produced in solar flares have been studied in conjunction with solar X rays and radio bursts. It is now believed that these particles are not necessarily produced in an impulsive event during the flash phase of the flare; proton precursors have been found in some events, and it appears that energetic particles continue to leave the vicinity of the flare for hours or even days. These particles propagate preferentially along interplanetary magnetic field lines. Thus, their velocity distribution is generally anisotropic as long as the injection continues.