Acceleration Of Solar Cosmic Rays Research Articles

Solar Cosmic Ray Acceleration by a Shock Wave in the Lower Solar Corona on May 7, 1978

Based on the theory of diffusive shock acceleration of charged particles, we have theoretically studied the spectra of protons produced by a shock wave driven by a coronal mass ejection (CME) in the lower solar corona with known solar plasma parameters for the solar cosmic ray (SCR) event recorded near the Earth’s orbit on May 7, 1978 (ground level enhancement no. 31, GLE31) using numerical methods. The proton flux data from the CPME instrument installed on the IMP-8 spacecraft and the worldwide network of ground-based neutron monitors combined with the particle measurements by the telescopes on the IMP-7 and IMP-8 satellites have been used to analyze the event. In our calculations, to satisfactorily reproduce the proton spectrum measured at the Earth’s orbit, the CME velocity was assumed to be constant and equal to 600 km s–1. The shock acceleration of SCRs proceeded up to 1.4 R⊙ solar radii for 256 s.

Solar Cosmic Ray Acceleration by a Shock Wave in the Lower Solar Corona on May 7, 1978

Solar Cosmic Ray Acceleration by a Shock Wave in the Lower Solar Corona on May 7, 1978

Influence of a Coronal Mass Ejection on the Solar Cosmic Ray Acceleration by a Shock Wave in the Lower Solar Corona

Influence of a Coronal Mass Ejection on the Solar Cosmic Ray Acceleration by a Shock Wave in the Lower Solar Corona

Investigation of the mechanism of solar flare and acceleration of solar cosmic rays in real conditions of the solar corona

The generation of solar cosmic rays occurs during explosive energy release in a solar flare, so in order to understand this phenomenon it is necessary to study both the mechanism of the solar flare and the process of particle acceleration by the generated electric field. During a solar flare in the solar corona above the active region (AR), the energy stored in the magnetic field of the current sheet is released. Using the results of numerical simulation and observations, I.M. Podgorny proposed an electrodynamic model of a solar flare, explaining its main observational manifestations, in particular, the appearance of X-ray emission on the surface of the Sun. The acceleration of protons occurs along a singular line of the magnetic field of the current sheet by the electric field E = -V×B/c. To obtain accurate results it is necessary to carry out MHD simulation in the real scale of time, which is only be done using parallel calculations. The parallelization of the program PERESVET carried out using graphics card (GPU), the calculations were accelerated by ~ 120 times. As a result of optimization of the approximation of the boundary conditions of free exit at the non-photospheric boundary, instability near the boundary was stabilized. The first results of MHD simulation in the real scale of time above the AR 10365 showed the appearance of a plasma flow near singular X-type lines, which have to cause to the formation of a current sheet.

Solar Cosmic Ray Acceleration at the Front of a Fast Shock in the Lower Solar Corona

Based on the theory of diffusive shock acceleration of charged particles, we have numerically studied the spectra of protons produced by a fast shock with a speed of 5000 km s–1 in the lower solar corona with known solar plasma parameters. We show that protons with energies $$ \gtrsim $$ 105 MeV can be produced at a distance up to 3 $${{R}_{ \odot }}$$ ( $${{R}_{ \odot }}$$ is the solar radius) within 274 s.

Physical Mechanism of a Solar Flare Based on the Accumulation of the Energy in the Magnetic Field of the Current Sheet in the Solar Corona

During a solar flare, the magnetic energy of ~1032 erg is released in a few dozens of minutes. The invariability of the magnetic field on the solar surface during flares proves the appearance of a flare in the corona. An analysis of the dynamics of the electron temperature of the solar atmosphere provides independent evidence of the coronal origin of the flare. The appearance of the flare in the corona is explained by the release of the energy accumulated in the magnetic field of the current sheet. To study the flare situation, numerical MHD simulation was performed for a real active region. The simulation results showed the appearance of a current sheet, which position coincides with the position of the observed source of thermal X-ray emission. A methodology for calculations in real time scale has been developed. Based on the mechanism of energy release in the current sheet, an electrodynamic model of a solar flare is proposed, which explains its main observed manifestations. Sources of hard X-ray emission appear due to deceleration in the lower dense layers of the solar atmosphere of electron beams accelerated in field-aligned currents. The acceleration of solar cosmic rays occurs in the current sheet by the electric field E = –V × B/c, and not in shock waves.

Solar Cosmic Ray Acceleration by a Shock Wave in the Lower Solar Corona on November 22, 1977

Based on the theory of diffusive shock acceleration of charged particles, we have investigated the spectra of protons recorded in the solar cosmic ray event near the Earth’s orbit on November 22, 1977 (ground level enhancement no. 30, GLE30). The proton flux data from the CPME instrument installed on the IMP-8 spacecraft and the worldwide network of neutron monitors have been used to analyze the event. Using GLE30 as an example, we have shown for the first time that solar cosmic rays of relativistic energies can be produced by a shock wave with a relatively low speed of 560 km s–1 in the lower solar corona at a distance up to 1.6 $${{R}_{ \odot }}$$ ( $${{R}_{ \odot }}$$ is the solar radius) within 615 s. The calculated proton spectra satisfactorily reproduce the measurements in the Earth’s orbit.

Ground-level enhancement of solar cosmic rays on October 28, 2003: A mechanism of the generation of particles in the sun

In order to reveal mechanisms of the generation of solar cosmic rays, the spectrum of an event of the groundlevel enhancement on October 28, 2003 (the GLE65 event) in a maximally wide energy range has been analyzed using direct measurements of solar particle fluxes on the АСЕ, GOES, and WIND spacecrafts, as well as measurements on the worldwide network of neutron monitors. The spectrum in the relativistic energy range has been estimated within the previously proposed “effective-energy method.” In this method, each ground-based instrument is assigned the corresponding effective momentum (or energy) of primary particles at which the flux of solar cosmic rays is determined. The effective momentum is chosen such that errors in the determination of the solar-particle spectrum are minimized. It has been shown that the error of the estimate of the effective momentum within the proposed method for the determination of solar-particle fluxes is no more than 20 MeV/c. It has been found that the spectrum of solar cosmic rays from the event under study measured in the orbit of the Earth extends from ≈40 keV to ≈5 GeV and is described by a power law with an exponential cutoff at relativistic energies. A quasilinear theory of the regular acceleration of charged particles by shock waves in the lower corona of the Sun, which was developed at the Shafer Institute of Cosmophysical Research and Aeronomy, Siberian Branch, Russian Academy of Sciences, has been used to reveal the nature of solar cosmic rays. It has been shown that the acceleration of solar cosmic rays on the front of a coronal shock wave in the event under study ended at a distance of no longer than four radii of the Sun.

Acceleration of solar cosmic rays in a flare current sheet and their propagation in interplanetary space

Analyses of GOES spacecraft data show that the prompt component of high-energy protons arrive at the Earth after a time corresponding to their generation in flares in the western part of the solar disk, while the delayed component is detected several hours later. All protons in flares are accelerated by a single mechanism. The particles of the prompt component propagate along magnetic lines of the Archimedean spiral connectng the flare with the Earth. The prompt component generated by flares in the eastern part of the solar disk is not observed at the Earth, since particles accelerated by these flares do not intersect magnetic-field lines connecting the flare with the Earth. These particles arrive at the Earth via their motion across the interplanetary magnetic field. These particles are trapped by the magnetic field and transported by the solar wind, since the interplanetary magnetic field is frozen in the wind plasma, and these particles also diffuse across the field. The duration of the delay reaches several days.

Shock acceleration of solar cosmic rays

We investigated the acceleration of solar cosmic rays (SCRs) by the shock waves produced by coronal mass ejections. We performed detailed numerical calculations of the SCR spectra produced during the shock propagation in the solar corona in terms of a model based on the diffusive transport equation using a realistic set of physical parameters for the corona. The resulting SCR energy spectrum N(e) ∝ e−γ exp [− (e/emax)α] is shown to include a power-law portion with an index γ≃2 that ends with an exponential tail with α ≃ 2.5 − β, where β is the spectral index of the background Alfven turbulence. The maximum SCR energy lies within the range emax = 1–300 MeV, depending on the shock velocity. Because of the steep spectrum of the SCRs, their backreaction on the shock structure is negligible. The decrease in the Alfven Mach number of the shock due to the increase in the Alfven velocity with heliocentric distance r causes the efficient SCR acceleration to terminate when the shock reaches a distance of r = 2–3R⊙. Since the diffusive SCR propagation in this case is faster than the shock expansion, SCR particles intensively escape from the shock vicinity. A comparison of the calculated SCR fluxes expected near the Earth’s orbit with available experimental data indicates that the theory satisfactorily explains all of the main observed features.

Combined mechanisms of solar cosmic ray acceleration

Spectra of solar cosmic rays on the Sun’s surface at a flare site and near Earth are modeled using the Monte Carlo method. Two of the most important mechanisms of energy accumulation by the particles are considered simultaneously: the regular acceleration of ions by the impulsive electric field of the current sheet and stochastic acceleration by the Alfvenic turbulence. This leads to substantial variations in the particle spectra in the low-energy region.

Acceleration of solar cosmic rays and the fine spectral structure of type II radio bursts

On the basis of data, obtained by means of the ground-based solar service RSTN (Radio Solar Telescope Network) and the geostationary satellite system GOES, the relationship between the solar cosmic rays (SCR) intensity I p with the proton energy E p > 1 MeV and parameters of meter-decameter type II radio bursts in the frequency range of 25–180 MHz is studied. The process of proton acceleration by shock waves was characterized by the frequency drift velocity of radio bursts V mII and the relative difference between radio emission frequencies at the first two harmonics b. It is shown that the coefficient of correlation between I p and b increases with E p growing from 0.40 to 0.70, while a similar coefficient between I p and V mII does not exceed 0.30. Indications in favor of the two-stage SCR acceleration model are obtained.

Two-stage acceleration of solar cosmic rays

On the basis of data from the Radio Solar Telescope Network (RSTN), as well as the Geostationary Operational Environmental Satellite (GOES) and the WIND spacecraft, for the period from 1989 to 2006 covering 107 flare events, we investigated the relationship between the intensity of solar cosmic rays and parameters of continuum radio bursts (25–15400 MHz), as well as type II radio bursts in the meter and decahectometer wavelength ranges. Proton fluxes with energies Ep > 1−100 MeV were calculated with regard to a reduced heliolongitude. The maximum correlation between solar cosmic rays and solar parameters of microwave bursts was 0.80. Its value was no more than 0.40 for the drift rate of type II bursts and 0.70 for the compression rate of coronal shock waves. Based on linear regression equations, we estimated the contribution of coronal shock waves to the acceleration of protons. We found that major acceleration processes occur in the area of burst energy release and complimentary processes occur at the fronts of coronal shock waves. The contribution of the latter to the acceleration process increases significantly with proton energy.

On the threshold effect of proton acceleraton in solar flares

Based on the reconnection theory of a flare and on recent observational and statistical findings, the problem of the initial acceleration of solar cosmic rays (SCR) is discussed. Simple estimates of the electric fields required to start the electron acceleration are obtained and the problem of proton ionization losses for overcoming the Coulomb barrier is considered. We take into account also the possible differences between proton and electron spectra from the very beginning of the acceleration process. Special attention is paid to the distribution functions of solar flare events in various parameters (peak fluxes and/or energy fluences in X-ray and radio wave bursts, in proton and electron emissions, etc.). It is shown that the distribution functions allow the interpretation of some scale and time flare parameters in terms of expected threshold effects. However, these functions are still insufficient to evaluate the relative share of different emissions in the global energy budget of a flare. In this context, a more promising approach is to derive the direct ratio between the number of accelerated protons,Np, and total flare energy,Wf, within the frame of a certain acceleration model. It is argued that an absolute threshold for proton production (in Hudson's formulation) does not exist. Meanwhile, the flux and threshold energy of accelerated protons overcoming the Coulomb loss maximum, in fact, may depend heavily on the global output of flare energy.

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.

On the Acceleration of Energetic Cosmic Particles by Electrostatic Double Layers

The capability of electrostatic double layers of accelerating charged particles to high energies is investigated. Starting from a one-dimensional relativistic double-layer model a two-dimensional relativistic double layer in a current filament is studied. It is found that the filamentary double layer has a maximum potential drop that depends both on the magnitude of the filamentary current and on the composition of the layer. The results are applied to two cosmic double layers-one in a solar electric circuit and another in a galactic circuit. If the layers are composed of protons and electrons, these particles may be accelerated to 1011 eV in the solar layer and to 1014 eV in the galactic layer. It is suggested that the solar double layer may account for the acceleration of solar cosmic rays while the galactic layer may contribute to the generation of cosmic radiation.

A transient MHD model applicable for the source of solar cosmic ray acceleration

A two-dimensional, time-dependent magnetohydrodynamic (MHD) model is used to describe the possible mechanisms for the source of solar cosmic ray acceleration following a solar flare. The hypothesis is based on the propagation of fast mode MHD shocks following a sudden release of energy. This model has already been used with some success for simulation of some major features of type II shocks and white light coronal transients. In this presentation, we have studied the effects of initial magnetic topology and strength on the formation of MHD shocks. We consider the plasma beta (thermal pressure/magnetic pressure) as a measure of the initial , relative strength of the field. During dynamic mass motion, the Alfvén Mach number is the more appropriate measure of the magnetic field's ability to control the outward motion. We suggest that this model (computed self-consistently) provides the shock wave and the disturbed mass motion behind it as likely sources for solar cosmic ray acceleration.

Stochastic acceleration of solar cosmic rays in an expanding coronal magnetic bottle

Several key features of the coronal propagation of solar cosmic rays have previously been explained by a ''magnetic bottle'' model proposed by Schatten and Mullan. The major apparent difficulty with that model is that expansion of the closed bottle might have a severe cooling effect on the cosmic rays trapped inside. In the present paper, we examine this difficulty by applying the equation for stochastic acceleration to an expanding bottle. Following our earlier suggestion, the scattering centers are taken to be small-scale magnetic inhomogeneities which are present in the corona prior to the flare, and which are set into turbulent motion when a flare-induced shock passes by. We identify the inhomogeneities with the collapsing magnetic neutral sheets discussed by Levine in the context of normal coronal heating. We find that the acceleration efficiencies can indeed be high enough to offset expansive cooling: within the time intervals that are typically available for closed bottle evolution (1000--3000 s), protons can be accelerated from 1 keV to 100 MeV and more. Our results indicate that the flux of particles which are accelerated to (say) 100 MeV is very sensitive to shock speed if this speed is less than about 10/sup 3/ km s/sup more » -1/. « less

Propagation of an MHD Shock in the Vicinity of a Magnetic Neutral Sheet

The acceleration of solar cosmic rays in association with certain solar flares is known to be highly correlated with the propagation of an MHD shock through the solar corona (Svestka, 1976). The spatial structure of the sources of solar cosmic rays will be determined by those regions of the corona which are accessible to the flare-induced shock. The regions to which the flare shock is permitted to propagate are determined by the large scale magnetic field structure in the corona. McIntosh (1972, 1979) has demonstrated that quiescent filaments form a single continuous feature (a “baseball stitch”) around the surface of the sun. It is known that helmet streamers overlie quiescent filaments (Pneuman, 1975), and these helmet streamers contain large magnetic neutral sheets which are oriented essentially radially. Hence the magnetic field structure in the low solar corona is characterized by a large-scale radial neutral sheet which weaves around the entire sun following the “baseball stitch”. There is therefore a high probability that as a shock propagates away from a flare, it will eventually encounter this large neutral sheet.

Interplanetary acceleration of solar cosmic rays to relativistic energy

Although the August 1972 cosmic ray storm (i.e., a rapid succession of Forbush decreases) was the greatest in more than 3 decades of continuous observations, halving the flux of galactic cosmic rays above 1 GeV, its basic characteristics were not unusual despite its unprecedented magnitude and complexity. However, the associated ground level event of August 4, representing the arrival of relativistic solar particles, displayed abnormal features. It is suggested that the observed nucleonic intensity enhancement was a consequence of the acceleration of ambient lower-energy solar protons to relativistic energies by reflection between two shocks moving with respect to each other in the interplanetary medium. A reexamination of the only other recorded disturbance that approached the magnitude of the recent cosmic ray storm shows that the ground level event of July 19, 1959, may have been similarly produced by this interplanetary acceleration mechanism.