The famous extreme solar and particle event of 20 January 2005 is analyzed from two perspectives. Firstly, using multi-spectral data, we study temporal, spectral, and spatial features of the main phase of the flare, when the strongest emissions from microwaves up to 200 MeV gamma-rays were observed. Secondly, we relate our results to a long-standing controversy on the origin of solar energetic particles (SEP) arriving at Earth, i.e., acceleration in flares, or shocks ahead of coronal mass ejections (CMEs). Our analysis shows that all electromagnetic emissions from microwaves up to 2.22 MeV line gamma-rays during the main flare phase originated within a compact structure located just above sunspot umbrae. In particular, a huge (≈ 10 5 sfu) radio burst with a high frequency maximum at 30 GHz was observed, indicating the presence of a large number of energetic electrons in very strong magnetic fields. Thus, protons and electrons responsible for various flare emissions during its main phase were accelerated within the magnetic field of the active region. The leading, impulsive parts of the ground-level enhancement (GLE), and highest-energy gamma-rays identified with π 0 -decay emission, are similar and closely correspond in time. The origin of the π 0 -decay gamma-rays is argued to be the same as that of lower-energy emissions, although this is not proven. On the other hand, we estimate the sky-plane speed of the CME S.N. Kuznetsov deceased 17 May 2007. 150 V.V. Grechnev et al.to be 2 000 -2 600 km s −1 , i.e., high, but of the same order as preceding non-GLE-related CMEs from the same active region. Hence, the flare itself rather than the CME appears to determine the extreme nature of this event. We therefore conclude that the acceleration, at least, to sub-relativistic energies, of electrons and protons, responsible for both the major flare emissions and the leading spike of SEP/GLE by 07 UT, are likely to have occurred nearly simultaneously within the flare region. However, our analysis does not rule out a probable contribution from particles accelerated in the CME-driven shock for the leading GLE spike, which seemed to dominate at later stages of the SEP event.

The origin of relativistic solar protons during large flare/CME events has not been uniquely identified so far. We perform a detailed comparative analysis of the time profiles of relativistic protons detected by the worldwide network of neutron monitors at Earth with electromagnetic signatures of particle acceleration in the solar corona during the large particle event of 20 January 2005. The intensity-time profile of the relativistic protons derived from the neutron monitor data indicates two successive peaks. We show that microwave, hard X-ray and γ-ray emissions display several episodes of particle acceleration within the impulsive flare phase. The first relativistic protons detected at Earth are accelerated together with relativistic electrons and with protons that produce pion decay γ-rays during the second episode. The second peak in the relativistic proton profile at Earth is accompanied by new signatures of particle acceleration in the corona within ≈ 1 R ⊙ above the photosphere, revealed by hard X-ray and microwave emissions of low intensity, and by the renewed radio emission of electron beams and of a coronal shock wave. We discuss the observations in terms of different scenarios of particle acceleration in the corona.

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The solar flare of 28 October 2003 (X17.2/4B) was recorded by the SONG instrument onboard the CORONAS-F satellite. A description of the SONG instrument, its in-orbit operation and the principal data reduction methods used to derive the flare gamma-ray properties are presented. Appreciable gamma-ray emission was observed in the 0.2 -300 MeV energy range. Several time intervals were identified which showed major changes in the intensity and spectral shape of the flare gamma-ray emission. The primary bremsstrahlung proves to be extended to 90 MeV and dominates during 11:02:11 -11:03:50 UT time interval, i.e. at the beginning of the flare impulsive phase. Afterwards, the SONG response was consistent with detection of the pion-decay gamma emission. A sharp increase in the piondecay-generated gamma-ray emission was observed at 11 : 03 : 51 ± 2 s UT, implying a substantial change in the spectrum of accelerated ions, which testified the appearance of protons with energies of > 300 MeV on the Sun. This emission lasted at least 8 -9 min until the end of our measurements. The ion acceleration to high energies was also proved by the detection of neutrons with energies > 500 MeV. It was found that the most efficient acceleration of high-energy protons coincides in time with the highest rate of the magnetic-flux change rate. The maximum gamma-ray flux at 100 MeV was 1.1 × 10 −2 photons cm −2 s −1 MeV −1 , exceeding all the fluxes that have ever been recorded.

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