Solar Flare Particles Research Articles

Particle Acceleration in Impulsive Solar Flares

We present the major observationally-derived requirements for a solar flare particle acceleration mechanism, briefly discuss some general electrodynamic constraints that also need to be considered, and suggest a unified electron and ion acceleration theory. This theory consists of two elements: cascading MHD turbulence generated at large scales during the primary flare energy release, which is responsible for the energization of electrons and all ions except 3He, and an electron beam, which excites the waves necessary for 3He acceleration. An issue of special importance for understanding ion acceleration is the convincing measurement of the charge state of Fe, which can be accomplished by the Advanced Composition Explorer in the upcoming solar maximum.

The debate on protons and electrons in solar flares

The series of articles in Eos (September 10, 1996) on particles in solar flares caught my eye. As I am not directly working in, but certainly am interested in, the field of particle acceleration in solar flares, I expected comprehensive, easy to understand summaries of two conflicting points of view I had read about before. The introduction by Peter Cargill set the stage perfectly for such a discussion.Too bad the debate did not fulfill my expectations. It became very clear as I read the articles that [the authors of the debate] were not dealing with the subject on an equal basis. Clearly, the article by George M. Simnett about protons had been written without prior insight into the opposing view. Unfortunately, the case for the electrons by A. Gordon Emslie was basically a reply to the proton case, sometimes specifically citing parts of the article. This would have been appropriate if Simnett had had the chance to reply, but unfortunately no such reply was added to the series.

THE AMOUNT OF LITHIUM PRODUCED DURING IMPULSIVE FLARES

Recent observations have provided much real information about the acceleration of particles in solar flares. High-reliability data about accelerated particles have been obtained for an impulsive phase of some flares of the activity cycle XXII. Therefore, it seems reasonable to re-estimate the amount of Li atoms produced in the upper photospheric layers by α − α reactions. A value of 5 × 10 29 nuclei during the largest impulsive solar events has been found from calculations for the thick-target model. This agrees with observations of the γ line of lithium. In conclusion, the probability of enhanced Li absorption observed after large impulsive flares in the sunspot penumbra is discussed.

Diffusion analysis of the GLE (ground-level enhancement) of September 29, 1989

On September 29, 1989, neutron monitors and muon telescopes around the world recorded a significant increase in their count rate. This ground-level enhancement affords a unique opportunity to study the rigidity dependence of the interplanetary scattering mean free path, [Formula: see text]. By fitting the well-known diffusion model to worldwide neutron monitor and muon telescope data, we calculate the value of [Formula: see text] observed by each station. A plot of [Formula: see text] observed by each station versus the mean rigidity of solar flare particles observed by each station (which ranges from 3–16 GV) clearly indicates that [Formula: see text] is rigidity dependent. If we assume that [Formula: see text] for P > 3 GV, where the power-law index q is a measure of the deviation from isotropic scattering (q = 1), then we find that q = 0.86 ± 0.03 at the time of this ground-level enhancement.

Decay rate of the second radiation belt

Variations in the Earth's trapped (Van Allen) belts produced by solar flare particle events are not well understood. Few observations of increases in particle populations have been reported. This is particularly true for effects in low Earth orbit, where manned spaceflights are conducted. This paper reports the existence of a second proton belt and it's subsequent decay as measured by a tissue-equivalent proportional counter and a particle spectrometer on five Space Shuttle flights covering an eighteen-month period. The creation of this second belt is attributed to the injection of particles from a solar particle event which occurred at 2246 UT, March 22, 1991. Comparisons with observations onboard the Russian Mir space station and other unmanned satellites are made. Shuttle measurements and data from other spacecraft are used to determine that the e-folding time of the peak of the second proton belt. It was ten months. Proton populations in the second belt returned to values of quiescent times within eighteen months. The increase in absorbed dose attributed to protons in the second belt was approximately 20%. Passive dosimeter measurements were in good agreement with this value.

High-Energy Particles in Solar Flares

Accelerated particles appear to coexist inseparably with most forms of energy release in solar flares and coronal mass ejections. We identify at least five different populations of high-energy electrons and ions. High-energy particles, accelerated efficiently in the flare in great numbers, transport a large fraction of the flare energy to other sites. This behavior makes them an integral part of the flare process. Much new data has come from two spacecraft launched in 1991: the Compton Gamma­Ray Observatory and Yohkoh. This review concentrates on particles in flares, mainly using X-ray and gamma-ray data rather than measurements of “escaping” particles observed in interplanetary space.

Relativistic solar proton events

Energetic solar flare particles contain rich information concerning mechanisms of particle acceleration on the Sun and subsequent transport through turbulent interplanetary space. Even the most energetic particles, in particular protons with kinetic energy above ∼500 MeV, may undergo coronal and interplanetary propagation effects, disturbing their accelerated injection spectrum after release from the solar flare. Relativistic solar proton events are recorded by neutron monitors at ground level. A detailed knowledge of the response of these ground-based detectors to the impact by a beam of protons on the top of the atmosphere is required to analyze these observations. The spectral index of arriving protons can be obtained from the response of the world-wide network of neutron monitors provided their directional anisotropy is known. The spectral index may also by determined from the relative enhancements in count rates of two similar detectors at different altitudes but similar asymptotic cones of acceptances, or from the relative enhancements of two detectors with different spectral sensitivities but at the same location of high latitude. Ground level enhancements from solar flare protons have been recorded at Sanae, Antarctica, since 1971 by two neutron monitors with different sensitivities to primary protons in the rigidity range from ≲1 GV to ∼5 GV. Spectral indexes of about 20 of these more energetic solar flare proton events have been determined from the two detector enhancements recorded at Sanae. These indexes do not show any increase (softening of the relativistic proton spectra) with increasing heliolongitude away from the preferred IMF connection region as was obtained for 20–80 MeV protons. Furthermore, most of the enhanced count rates show fluctuations larger than statistical, indicative of propagation in a mostly turbulent interplanetary magnetic field.

High-Energy Particles in Solar Flares

Accelerated particles appear to coexist inseparably with most forms of energy release in solar flares and coronal mass ejections. We identify at least five different populations of high-energy electrons and ions. High-energy particles, accelerated efficiently in the flare in great numbers, transport a large fraction of the flare energy to other sites. This behavior makes them an integral part of the flare process. Much new data has come from two spacecraft launched in 1991: the Compton Gamma­Ray Observatory and Yohkoh. This review concentrates on particles in flares, mainly using X-ray and gamma-ray data rather than measurements of “escaping” particles observed in interplanetary space.

Solar Flare Particle Effects and Seasonal Radiocarbon Variations in Tree Rings of the Northern and Southern Hemispheres

Measurements of ∆14C and δ13C, separately in early wood and late wood of two Mediterranean pine species, with a nearly year-long growth season, were made to better define possible solar flare particle effects. The measurements were made for suitable periods around the years of big flares (e.g., 1942, 1946, 1989) in Pinus pinea of the Adriatic area and Pinus halepensis from South Australia. We found that the 14C concentration is consistently greater in early wood than in late wood of the same growth ring and considerably higher in the Australian samples than in the European samples. The data so far reveal no evidence of solar-flare effect.

Currents in astrophysics and cosmology: papers in honor of Maurice M. Shapiro

Part I. Cosmic Rays: 1. Galactic cosmic rays and galactic halo X-ray emission 2. Highly luminous radiogalaxies as sources of cosmic rays 3. Where is the cosmic-ray modulation boundary of the heliosphere? 4. Origin of primary cosmic rays above 1 TeV 5. Astrophysical discoveries derived from transient cosmic ray and solar flare particle earth-based investigations: the early years Part II. X-Ray, Gamma-Ray and Neutrino Astronomy: 6. How X-ray astronomy has extended the horizon of physics 7. Selected topics in high energy gamma-ray astrophysics 8. Cosmic gamma rays, the origin of cosmic rays, and the structure of the galaxy: the IRAS connection: reminiscences 9. Ultra-high-energy gamma-ray astronomy 10. Recent gamma-ray burst studies by Ginga 11. High-energy astrophysics in solar flares 12. High-energy gamma radiation from supernova 13. IMB detection of neutrinos from SN1987A: history and results 14. High-energy neutrino astronomy: project DUMAND Part III. Cosmology: 15. The cosmic quark-hadron phase transition 16. Ponderable soliton stars from early universe 17. Dark matter and the generation of cosmological structure Part IV. Reminiscences and Poetic Black Holes: 18. Contributions of Maurice M. Shapiro and his group to cosmic-ray astrophysics 19. From Jerusalem to heavenly realms - memoir of a cosmic journey 20. Maury Shapiro and ERICE 21. Black holes.

The Energetic Particle Spectrometer HEP onboard the GEOTAIL Spacecraft.

The instruments for the HEP (High Energy Particle) experiments of the GEOTAIL mission, launched in July 1992, consist of 5 spectrometers (LD, BD, MI-1, MI-2 and HI). The LD (Low energy particle Detector) and the BD (Burst Detector) are mainly used to measure electrons, protons, helium and oxygen ions which reflect plasma dynamics in the magnetotail region. On the other hand, MI-1, MI-2 (Medium energy Isotope telescope-1, -2) and HI (High energy Isotope telescope) are used to measure the isotope abundance of solar flare particles and cosmic ray particles which reflect physical conditions of interplanetary space and origin of these particles

Acceleration in solar flares: Interacting particles versus interplanetary particles

We study the properties of flare accelerated particles using gamma ray observations. We derive ion and electron energy spectra, electron to proton ratios, and numbers of interacting particles. We investigate the effects of the abundance variations implied by the gamma ray data on these parameters. We compare the results with interplanetary observations of solar flare particles.

A fast mechanism for the acceleration of solar cosmic rays and solar energetic particles in solar flares

It is pointed out that the mechanism of collective ion acceleration by means of electron beams may be realized in solar flares. It is also shown that this mechanism provides the following in agreement with the observational data: (1) proton acceleration up to GeV energies in times 10 −1−10 −2 s, (2) the similarity of the proton and electron spectrum spectra and the existence of the scaling law that relates the number of the accelarated protons and electron, (3) nuclei acceleration and identity of their spectra per nucleon.

A study of an expanding interplanetary magnetic cloud and its interaction with the Earth's magnetosphere: The interplanetary aspect

In a series of three interlinked papers we present a study of an interplanetary magnetic cloud and its interaction with the Earth's magnetosphere on January 14/15, 1988. This first paper is divided into three parts describing the principal results concerning the magnetic cloud. First, by applying the cylindrically symmetric, magnetic flux rope model to the high time resolution magnetic field and plasma data obtained by the IMP‐8 spacecraft, we show that the axis of the magnetic cloud in question is approximately in the ecliptic and orthogonal to the Earth‐Sun line. We note the presence of pulsations of ∼5‐hour period in the bulk flow speed which are superimposed on an otherwise monotonically falling bulk speed profile. Second, we apply ideal MHD to model the self‐similar, radial expansion of a magnetic cloud of cylindrical geometry. As initial condition for the magnetic field we choose a constant‐α, force‐free magnetic configuration. We demonstrate that the theoretical velocity profile for the free expansion of a magnetic cloud is consistent with observations made during the January 14/15, 1988, magnetic cloud encounter. Comparing model with data, we infer that prior to the start of observations at 1 AU the magnetic cloud had been expanding for 65.4 hours; the radius of the magnetic cloud at the time it arrived at Earth was 0.18 AU; and its expansion speed at 1 AU was ∼114 km/s. Third, we discuss energetic (∼1 MeV) ion data, also from instrumentation on IMP‐8. We highlight the appearance of a sharp enhancement in the intensity of ∼0.5‐MeV ions while IMP‐8 was inside the cloud. These ions travel as a collimated, field‐aligned beam from the west of the Sun. This is an “impulsive” solar event in which particles accelerated at a magnetically well‐connected solar flare arrive promptly at the spacecraft. The observation of solar flare particles inside the cloud suggests that field lines within the magnetic cloud remained connected to the Sun. The observation is, however, inconsistent with the supposition that the cloud is formed of closed magnetic field loops disconnected from the Sun.

Lunar magnetic fields: Implications for utilization and resource extraction

Although the Moon possesses no large‐scale intrinsic magnetic field, locally strong crustal fields exist which must be considered in assessing lunar utilization scenarios. On the basis of available data, orbital anomalies most probably imply the presence of subsurface concentrations of metallic iron in the single‐domain size range. Such concentrations may indicate larger iron volume fractions than are characteristic of sampled materials, allowing more efficient extraction of metal than is now considered likely using nominal regolith compositions. Most other implications relate to the deflection of interplanetary and magnetospheric ions by crustal fields. Numerical simulations are employed to show that solar wind ion deflection by strong lunar magnetic anomalies can produce local increases, as well as decreases, in the implantation rate of solar wind hydrogen. The largest concentrations of implanted solar wind volatiles may therefore be found in local areas near strong anomalies. If water ice exists in permanently shadowed regions of the lunar poles, locally strong magnetic fields can shield these volatiles from sputter erosion losses by interplanetary and magnetospheric ion fluxes. Model simulations indicate that the ability of magnetic anomalies to shield the surface from incident ions increases with the angle of incidence and hence for most particle sources, with selenographic latitude. Finally, the possibility that relatively strong anomalies can provide significant protection of men and materials against major solar flare particle events is examined and found to be unlikely.

Gamma rays from pion decay - Evidence for long-term trapping of particles in solar flares

The energy spectrum and time dependence of the 50 MeV to 2 GeV gamma rays observed from the 1991 June 11 solar flare are analyzed. It is shown that the emission detected at the late phase of this flare with EGRET on the Compton Gamma-Ray Observatory can be explained by a model in which the bulk of the particles were accelerated during the impulsive phase and subsequently trapped in coronal magnetic loops. The observed spectrum was fit with a combination of pion decay radiation and primary electron bremsstrahlung. The 1991 June 11 data are compared with data for the 1982 June 3 and 1991 June 15 flares from which pion decay emission was also observed. The fact that the fluxes from these three flares are ordered in time in accordance with the predicted time dependence of emission produced by trapped particles provides support for the model.

Energetic particles in solar flares: theory and diagnostics

Recent progress and future prospects in diagnostics of energetic electrons and ions in the flares are reviewed, together with the roles they play in the flare as a whole. Most of the discussion centres on hard X-ray and gamma-ray and thermal plasma emission data, rather than on radio sources. SinceSolar Maximum MissionandHinotorithere has been major progress in all areas of flare electron diagnostics. Electron spectra are now recoverable with some precision, electrons with energies above 10 MeV are known to be highly anisotropic, and indications are available of the spatial distribution of electrons at 20 keV. Timescales of electron acceleration are now known to be shorter than 0.1 s. Energetic electrons are believed to carry much of the flare power. Ion diagnostics are more limited. For greater than 1 MeV ions the flux, spectrum and acceleration timescale are now quite well known. Low energy ions are hard to diagnose but have been invoked as a flare heating mechanism alternative to electron beams. The problems with beam heating models are discussed with special attention to the problems of the low energy proton model and its only direct diagnostic, Hα impact polarization. Finally, theoretical problems associated with return currents and with accelerator requirements are discussed and attention is drawn to the possible importance of entropy as well as energy considerations.

Consequences of strong pitch-angle diffusion of particles in solar flares

The consequences of strong pitch-angle diffusion of energetic electrons and ions during the impulsive phase of a solar flare have been analyzed. Strong diffusion suggests that the regions with high levels of small-scale turbulence (turbulent mirrors) are formed in the magnetic trap. These additional mirrors scatter the energetic particles, and therefore, the propagation velocity of the particles toward the footpoints decreases. It has been shown that the strong diffusion of energetic particles by waves is realized very easily in a flare loop. The main consequences of strong pitch-angle diffusion are as follows: (1) the turbulent propagation time for greater than about 10 MeV ions is about one order of magnitude longer than for greater than about 30 keV electrons, (2) the energetic particle spectra in a flare loop reflect the source particle spectrum, and (3) the energetic particle density is constant along the flare loop axis. The strong diffusion approach makes it possible to explain, for example, the time delays in hard X-ray and gamma-ray emission and the properties of particle fluxes in the interplanetary space, and it offers new possibilities for flare plasma diagnostics. 17 refs.

On the transport and acceleration of solar flare particles in a coronal loop

The turbulent environment of a flaring solar coronal loop directly affects the population of particles to be accelerated or already accelerated. Under the assumption of a uniform turbulent MHD wave field within the loop, the behavior of a particle distribution as it interacts with the turbulence is discussed, including particle precipitation to the footpoints of the loop and the evolution of the energy distribution as the particles undergo second-order stochastic acceleration. Two cases are discussed in detail: (1) particles spatially diffusing within the loop and precipitating with minimal acceleration in the short time scale of an impulsive event and (2) particles diffusing in both real and momentum space in a long duration event. Collisional losses due to ambient electrons are included. The gamma-ray flare of June 3, 1982 is modeled, and good agreement is obtained between predicted and observed time profiles if the loop length is 100,000 km with an intrinsic spatial diffusion time of 100-450 s. It follows that the production of high-energy neutrons and pi mesons extends over a time scale of 1000 s as observed.

Energetic particle composition measurements from Phobos 2: Results of the LET experiment

The Low Energy Telescope (LET) experiment carried on board the Phobos spacecraft measured the flux, spectra and elemental composition of nuclei from hydrogen up to iron, in the energy range ∼ 1–75 MeV nucleon −1. Isotope separation for helium was also achieved. We present the results of a study of solar energetic particle (SEP) composition using LET data acquired during the period July 1988 to March 1989. The set of particle events selected for study comprises six large solar flare events, two 3He-rich events and two energetic storm particle increases associated with interplanetary shocks. Three of the six large flare events occurred during a period of unusually high solar activity in March 1989. In two of these events, large Fe/O ratios were measured (0.44 ± 0.05 and 0.95 ± 0.24). The Fe/O ratios determined for the complete set of large flare events show an inverse correlation with the spectral index of oxygen, suggesting that the acceleration mechanism that produces events showing enhanced heavy-ion abundances is different from that responsible for lower Fe/O ratios. Our results in the case of the 3He-rich events are in agreement with earlier work, showing an enrichment in heavy ions relative to the average SEP composition. The abundances measured for the shock-associated increases are consistent with the acceleration of the ambient population of solar flare particles.