L. I. Miroshnichenko

ASTROPHYSICAL ASPECTS IN THE STUDIES OF SOLAR COSMIC RAYS

This review paper comprises main concepts, available observational data and recent theoretical results related to astrophysical aspects of particle acceleration at/near the Sun and extreme capacities of the solar accelerator(s). We summarize underground and ground-based observations of solar cosmic rays (SCR) accumulated since 1942, direct spacecraft measurements of solar energetic particles (SEP) near the Earths orbit, indirect information on the SCR variations in the past, and other relevant astrophysical, solar and geophysical data. The list of the problems under discussion includes: upper limit spectrum (ULS) for solar cosmic rays; maximum energy (rigidity), Em(Rm), of particles accelerated at/near the Sun; production of the flare neutrinos; energetics of SCR and solar flares; production of flare neutrons and gamma rays; charge states and elemental abundances of accelerated solar ions; coronal mass ejections (CMEs) and extended coronal structures in acceleration models; magnetic reconnection in acceleration scenarios; size (frequency) distributions of solar proton events (SPE) and stellar flares; occurrence probability of giant flares; archaeology of solar cosmic rays. The discussion allows us to outline a series of interesting conceptual and physical associations of SCR generation with the high-energy processes at other stars. The most reliable estimates of various parameters are given in each of research fields mentioned above; a set of promising lines of future studies is highlighted. A great importance of SCR data for resolving some general astrophysical problems is emphasized.

Solar cosmic rays: 70 years of ground-based observations

The main data have been summarized, and the results, achieved using data from the worldwide network during the entire period of ground-based observations of solar cosmic rays (SCRs) from February 28, 1942, when they were discovered, have been generalized. The methods and equipment for registering SCRs have been described. The physical, methodical, and applied aspects, related to the SCR generation, as well as the SCR interaction with the solar atmosphere, transport in the IMF, motion in the Earth’s magnetosphere, and the affect on the Earth’s atmosphere, have been discussed. It has been indicated that the fundamental results were achieved in this field of space physics during 70 years of studies. Special attention has been paid to up-to-date models and concepts of ground-level enhancement (GLE). The most promising tendencies in the development and application of this effective method of solar-terrestrial physics have been outlined.

Oscillations of galactic cosmic rays and solar indices before the arrival of relativistic solar protons

Using modern wavelet analysis techniques, we have made an attempt to search for oscillations of intensity of galactic cosmic rays (GCR), sunspot numbers (SS) and magnitudes of coronal index (CI) implying that the time evolution of those oscillations may serve as a precursor of Ground Level Enhancements (GLEs) of solar cosmic rays (SCR). From total number of 70 GLEs registered in 1942–2006, the four large events — 23 February 1956, 14 July 2000, 28 October 2003, and 20 January 2005 — have been chosen for our study. By the results of our analysis, it was shown that a frequency of oscillations of GCR decreases as time approaches to the event day. We have also studied a behaviour of common periodicities of GCR and SCR within the time interval of individual GLE. The oscillations of GLE occurrence rate (OR) at different stages of the solar activity (SA) cycle is of special interest. We have found some common periodicities of SS and CI in the range of short (2.8, 5.2, 27 and 60 days), medium (0.3, 0.5, 0.7, 1.3, 1.8 and 3.2 years) and long (4.6 and 11.0 years) periods. Short and medium periodicities, in general, are rather concentrated around the maxima of solar cycles and display the complex phase relations. When comparing these results with the behaviour of OR oscillations we found that the period of 11 years is dominating (controlling); it is continuous over the entire time interval of 1942–2006, and during all this time it displays high synchronization and clear linear ratios between the phases of oscillations of η, SS and CI. It implies that SCR generation is not isolated stochastic phenomena characteristic exclusively for chromospheric and/or coronal structures. In fact, this process may have global features and involve large regions in the Sun’s atmosphere.

Proton activity of the Sun in current solar cycle 24

We present a study of seven large solar proton events in the current solar cycle 24 (from 2009 January up to the current date). They were recorded by the GOES spacecraft with the highest proton fluxes being over 200 pfu for energies >10 MeV. In situ particle measurements show that: (1) The profiles of the proton fluxes are highly dependent on the locations of their solar sources, namely flares or coronal mass ejections (CMEs), which confirms the “heliolongitude rules” associated with solar energetic particle fluxes; (2) The solar particle release (SPR) times fall in the decay phase of the flare emission, and are in accordance with the times when the CMEs travel to an average height of 7.9 solar radii; and (3) The time differences between the SPR and the flare peak are also dependent on the locations of the solar active regions. The results tend to support the scenario of proton acceleration by the CME-driven shock, even though there exists a possibility of particle acceleration at the flare site, with subsequent perpendicular diffusion of accelerated particles in the interplanetary magnetic field. We derive the integral time-of-maximum spectra of solar protons in two forms: a single power-law distribution and a power law roll-over with an exponential tail. It is found that the unique ground level enhancement that occurred in the event on 2012 May 17 displays the hardest spectrum and the largest roll-over energy which may explain why this event could extend to relativistic energies.

Solar Proton Events in Solar Activity Cycles 21–24

It is shown that the number of solar proton events (SPEs) with proton energies (E) higher than 10 and 100 MeV in the current solar cycle (cycle 24) differs slightly from the number of the same events in earlier cycles (cycles 21–23), even though solar activity was low during the growth and maximum phase in cycle 24. A deficit was in this case observed for the most powerful GLE events, which are characterized by high proton fluxes with E > 100 MeV. The ratio of the number of SPEs with E > 10 and 100 MeV to the number of sunspots in cycle 24 doubled, compared to the same ratio in cycles 21–23, and the relative number of GLEs fell by more than half. The characteristics of flares and coronal mass ejections associated with proton events with E > 100 MeV in cycle 24 were virtually the same as the analogous parameters in cycle 23.

Search for periodicities in galactic cosmic rays, sunspots and coronal index before arrival of relativistic protons from the sun

Using different approaches and techniques of wavelet analysis we analyze variations (oscillations) of galactic cosmic rays, solar spot number, and coronal index of solar activity before ground level enhancements of solar cosmic rays. Obtained results are discussed in frames of recent ideas about periodicity phenomena in the photosphere, and corona of the Sun, interplanetary medium, and cosmic rays.

Near-Earth Radiation Environment for Extreme Solar and Geomagnetic Conditions

Abstract The near-Earth environment can be considered a unique place where different space radiation fields exist and can play a significant role in the estimation of radiation risks both for robotic and manned space missions. One cannot exclude the cumulative effect for particular spacecraft orbits the result of simultaneous impact of the different radiation fields. Among the types of space radiation, we consider: trapped radiation at low Earth orbits (LEO): solar energetic particles (SEP) penetration in the magnetosphere, including ground level enhancement events (GLE); and variations of galactic cosmic rays (GCR) during the different extreme levels of geomagnetic and solar activity. Results of empirical modeling of these different radiation fields and some results of measurements of their physical parameters (energy spectra, space and time variations) for different solar (extreme solar maxima and solar minima) and geomagnetic (the major magnetic storms) conditions are analyzed. The models are based on quantitative relationships between the particle fluxes, taking into account the solar activity (sunspot numbers). Examples of using these models to estimate changes of radiation hazards in interplanetary space for the possible case of solar activity reduction during the nearest solar cycles 25 and 26 are analyzed.

Source Energy Spectrum of the 17 May 2012 GLE

Among the several GLE (Ground Level Enhancements) that have presumptuously occurred in the period 2012-2015 the 17th May 2012 is that which is more widely accepted to be a GLE, in view of the high number of high latitude Neutron Monitor (NM) stations that have registered it. In spite of the small amplitude, it was the more prominent of the predicted GLE´s of the present decade (Perez-Peraza & Juarez-Zuniga, 2015). However, the lack of latitude effect makes it difficult to study the characteristics of this event in the high energy extreme of the spectrum. Nevertheless, several outstanding works have been able to derive observational spectra at the top of the earth atmosphere for this peculiar GLE. Some of these works find that the flow of protons is characterized by two components. Quite a great number of works have been published in relation with observational features obtained with different instrumentation, but the source phenomena, regarding the generation processes and source physical parameters have not been scrutinized. The main goal of this work is to look at such aspects by means of the confrontation of the different approaches of the observational spectra with our analytical theoretical spectra based on stochastic acceleration and Electric field acceleration from reconnection processes. In this way, we derive a set of parameters which characterize the sources of these two GLE components, leading us to propose possible scenarios for the generation of particles in this particular GLE event.

Skin-layer of the eruptive magnetic flux rope in large solar flares

The analysis of observations of large solar flares made it possible to propose a hypothesis on existence of a skin-layer in magnetic flux ropes of coronal mass ejections. On the assumption that the Bohm coefficient determines the diffusion of magnetic field, an estimate of the skin-layer thickness of ~106 cm is obtained. According to the hypothesis, the electric field of ~0.01–0.1 V/cm, having the nonzero component along the magnetic field of flux rope, arises for ~5 min in the surface layer of the eruptive flux rope during its ejection into the upper corona. The particle acceleration by the electric field to the energies of ~100 MeV/nucleon in the skin-layer of the flux rope leads to their precipitation along field lines to footpoints of the flux rope. The skin-layer presence induces helical or oval chromospheric emission at the ends of flare ribbons. The emission may be accompanied by hard X-ray radiation and by the production of gamma-ray line at the energy of 2.223 MeV (neutron capture line in the photosphere). The magnetic reconnection in the corona leads to a shift of the skin-layer of flux rope across the magnetic field. The area of precipitation of accelerated particles at the flux-rope footpoints expands in this case from the inside outward. This effect is traced in the chromosphere and in the transient region as the expanding helical emission structures. If the emission extends to the spot, a certain fraction of accelerated particles may be reflected from the magnetic barrier (in the magnetic field of the spot). In the case of exit into the interplanetary space, these particles may be recorded in the Earth’s orbit as solar proton events.

A comparison of proton activity in cycles 20–23

The solar proton singularities in cycles 20–23 were compared. For such a comparison, it is of special importance to present information about solar proton events (SPEs) uniformly. The Katalog working group, including representatives of different institutions, created SPE catalogs for 1970–2010. These catalogs include solar events with E ≥ 10 MeV proton fluxes exceeding 1 cm−2 s−1 sr−1 near the Earth. The various conditions that accompany SPEs result in a considerable scatter of the flux parameters and energy spectra and in a difference in time profiles. Regularities in the distribution of these parameters during four solar activity cycles can be specifically revealed only based on a statistical approach that is used in this work. In this case a comparison can be successful only when the analyzed data series are homogeneous. Solar activity singularities during the entire period of SPE measurements (from cycle 19 to the end of cycle 23) are described. The methods for adjusting solar events in particles that are registered near the Earth to solar sources are of special importance. These statistical data suggest that the number of SPEs in the cycles and the event distribution within a solar cycle may indicate that the SPE generation character is different in cycles 20–21 and 22–23.