Study of the 28 October 2003 and 20 January 2005 solar flares by means of 2.223 MeV gamma-emission line

By the data on intensity-time profiles of the neutron capture line of 2.223 MeV we have studied some characteristics of two solar flares, 28 October 2003 and 20 January 2005 (INTEGRAL and CORONAS-F observations, respectively). The SINP code was applied making allowance for the main processes of neutron interactions and deceleration in the solar plasma, character of neutron source, losses of neutrons and density model of the solar atmosphere. Comparison of the computed time profiles of 2.223 MeV line with observed ones for the flare of 28 October 2003 confirms the results obtained earlier for three other flares. Namely, the effect of density enhancement (EDE) in the sub-flare region, as well as the variations (hardening) of accelerated particle spectrum in the course of the event have been confirmed. The usual modeling procedure by the SINP code, however, seems to be inapplicable to the event of 20 January 2005. Possible causes of density enhancements during some flares and peculiarities of the 20 January 2005 flare are discussed.     

Relativistic solar protons in the ground level event of 23 February 1956: New study

In this study we applied again to the outstanding solar particle event of 23 February 1956, the largest one in the entire history of observations of solar cosmic rays. Due to significant improvement of the analysis/modeling techniques and new understanding of physical processes in the solar atmosphere and interplanetary space, a possibility arises to interpret the old data in the light of modern concept of multiple particle acceleration at/near the Sun. In our new analysis the data of available then neutron monitors and muon telescopes are used. The technique of the analysis includes: (a) calculation of asymptotic cones of ground-based detectors; (b) modeling of cosmic ray detector responses at variable parameters of the flux of solar relativistic protons; (c) determination of primary solar proton parameters outside magnetosphere by comparison of computed responses with observations. Certain evidence was obtained that the flux of relativistic solar protons consisted of two distinct components: prompt and delayed ones. The prompt component with exponential energy spectrum caused a giant impulse-like increase at a number of European cosmic ray stations. The delayed component had a power-law spectrum and was a cause of gradual increase at cosmic ray stations in the North American region. A numerical simulation of the proton acceleration in the vicinity of the magnetic reconnection region brings to the proton spectrum with exponential dependence on energy. This agrees with observational data for the prompt component. It is also shown that the huge increase in ∼5000% on neutron monitors was due to the prompt component only with the exponential proton spectrum. The power-law spectrum of comparable intensity gave considerably smaller effect.     

Factors influencing the acoustic characteristics of the reverberation chamber

Main factors influencing the characteristics of the acoustic reverberation chamber are considered. Studies of mode density and calculation of required acoustic power are carried out from the relations of the RK-600 chamber dimensions depending on technical requirements for the ground-based experimental development of communication space vehicles (SV).     

On the motions of RHESSI flare footpoints

The footpoint motions of flare hard X-ray (HXR) sources are directly related to the reconnection scenario of a solar flare. In this work, we tried to extract the information of footpoint motions for a number of flares observed with RHESSI. We found that the RHESSI flare results of the footpoint motions strongly support the classification proposed from the observations of YOHKOH/HXT. Furthermore, it is found that a flare can consist of two types of footpoint motions. We discussed the connections of the footpoint motions with the two-dimensional reconnection models.     

The investigation of powerful solar flares characteristics by analysis of excited states of C and various neutrons capture lines

Accelerated energetic particles in solar flares produced nuclear γ-lines in interactions with ambient solar atmosphere. Analysis of intensity of ratios between various γ-lines allows us to make estimations of abundance of elements, parameters of surrounding media and other solar characteristics. In this article we discuss the flux ratio between two lines from excited states of ¹²C (f_15.11/f_4.44) and our results of preliminary calculation of intensity ratio between two neutron capture lines at ³He and ¹H (f_20.58/f_2.223). In particular we consider the opportunity to obtain n(³He)/n(¹H) ratio during solar flares and using high-energy gamma-emission studying, based on the satellite data. Possible interpretation of spectral features observed during the January 20, 2005 solar flare is discussed. Preliminary analysis of energy spectrum in the band of 2–21 MeV gives n(³He)/n(¹H) ∼ 8 × 10⁻⁴ for January 20, 2005 solar flare.     

Large solar event of September 29, 1989: ten years after

Out of the 56 Ground Level Enhancements (GLEs) of solar cosmic rays (SCRs) observed since 1942 until the present, 15 events were recorded in the 22nd cycle of solar activity (1986–1996). Solar proton events (SPEs) in that cycle displayed some peculiarities, which may need an interpretation on a new concept base. The event of September 29, 1989 is of special interest. Since the well-known event of February 23, 1956, it proved to be the most intense in the relativistic range of proton energies. This GLE affords a unique opportunity to study the propagation of SCRs over a wide range of rigidity.In spite of its occurrence behind the western solar limb, the originating major flare could be observed over a wide range of the wavelengths and particle energy spectra – from gamma rays to decametric radio waves, from >2 MeV electrons to multi-GeV protons; there were also measurements of the energy spectra and charge states of solar heavy nuclei. The flare was followed by some energetic solar phenomena (large magnetic loops, coronal eruptions and mass ejections, shocks, etc.). Due to the very hard rigidity spectrum, this was the first GLE recorded by underground muon detectors. The event also has a number of other unusual features, for example, an extended component of gamma-ray emission and the change in direction of the probable particle source during the event's initial stage. In addition, the intensity-time profile of the GLE is notable for its non-classic shape, showing a two-peak structure. The latter implies the possibility of a two-component (or two-source) ejection of accelerated particles from the Sun.The available observational data for the event is described in detail, the main focus of this paper is concentrated on different attempts to interpret the data within the framework of traditional and non-traditional concepts: shock and/or post-eruption acceleration, two-component (dual) ejection, two-source model of particle acceleration in large (extended) coronal structures, etc. None of the models put forward for explaining this event is exhaustive. The rigidity spectrum of ejected protons is estimated and the problem of the maximum rigidity, R _m, of the accelerated particles is discussed. In the relativistic range, this event proved to be by 1–2 orders less intense than the event of February 23, 1956. It is also shown that the event of September 29, 1989 could not have been recorded with the present-day neutrino detectors.     

Relativistic proton production at the Sun in the 20 January 2005 solar event

Based on the concept of multiple acceleration of solar energetic particles (SEP) we analyzed the super-event of 20 January 2005 by the data of ground level, balloon and spacecraft observations. The main characteristics of relativistic solar protons (energy spectra, anisotropy directions and pitch-angle distributions) are derived and their dynamics during the event is studied. It is shown that the flux of relativistic solar protons may consist of two distinct components, the so-called prompt and delayed ones. Within a two-source model of particle generation, one of which is associated with an expanding magnetic loop, we solved the transport equation in energy phase space, including adiabatic losses simultaneously with the stochastic acceleration process, and calculate the expected spectra of the delayed component at the source. The confrontation of experimental spectra with theoretical ones shows that the delayed component may be correctly described by stochastic acceleration, but not the prompt component. The required acceleration efficiencies turned out to be rather high, so that, for this particular event, adiabatic cooling is practically negligible. Our results provide a new support to the existence of two populations of relativistic solar protons in some SEP events.     

Particle acceleration and gamma rays in solar flares: Recent observations and new modeling

Experiments on SMM, GAMMA, Yohkoh, GRANAT, Compton GRO, INTEGRAL, RHESSI and CORONAS-F satellites over the past three decades have provided copious data for fundamental research relating to particle acceleration, transport and energetics of flares and to the ambient abundance of the solar corona, chromosphere and photosphere. We summarize main results of solar gamma-astronomy (including some results of several joint Russian–Chinese projects) and try to appraise critically a real contribution of those results into modern understanding of solar flares, particle acceleration at the Sun and some properties of the solar atmosphere. Recent findings based on the RHESSI, INTEGRAL and CORONAS-F measurements (source locations, spectrum peculiarities, ³He abundance etc.) are especially discussed. Some unusual features of extreme solar events (e.g., 28 October 2003 and 20 January 2005) have been found in gamma-ray production and generation of relativistic particles (solar cosmic rays, or SCR). A number of different plausible assumptions are considered concerning the details of underlying physical processes during large flares: (1) existence of a steeper distribution of surrounding medium density as compared to a standard astrophysical model (HSRA) for the solar atmosphere; (2) enhanced content of the ³He isotope; (3) formation of magnetic trap with specific properties; (4) prevailing non-uniform (e.g., fan-like) velocity (angular) distributions of secondary neutrons, etc. It is emphasized that real progress in this field may be achieved only by combination of gamma-ray data in different energy ranges with multi-wave and energetic particle observations during the same event. We especially note several promising lines for the further studies: (1) resonant acceleration of the ³He ions in the corona; (2) timing of the flare evolution by gamma-ray fluxes in energy range above 90 MeV; (3) separation of gamma-ray fluxes from different sources at/near the Sun (e.g., different acceleration sources/episodes during the same flare, contribution of energetic particles accelerated by the CME-driven shocks etc.); (4) asymmetric magnetic geometry and new magnetic topology models of the near-limb flares; (5) modeling of self-consistent time scenario of the event.     

Large ground level events in solar cycle 22 and some peculiarities of relativistic proton acceleration.

Some of the Ground Level Enhancements of Solar Cosmic Rays (SCR) recorded by neutron monitors during solar cycle 22 are analyzed. The events appeared in series, and their occurrence rate in 1989-1991 was a factor of 4 larger than the average value (approximately 1/yr) for the total observation interval (since 1942). The events of Sept. 29 and Oct. 24, 1989 proved to be the more intensive. The analyzed events show no peculiarities as to the distribution of the T1/2 parameter, whereas the shape of the profile of some events is notable for a two peak structure. The latter implies the possibility of a two component SCR ejection from two different sources in the solar atmosphere. For the event of Sep. 29 we have estimated the ejection rigidity spectrum of protons to be Dsolar (R) = (1-2) 10(32)R(-2.9) Gv-1 at R > or = 1 Gv. As to its proton flux, this event proved to be by 1-2 orders less intensive than the well known event of Feb. 23 1956.