420 ISSN 0010-9525, Cosmic Research, 2018, Vol. 56, No. 6, pp. 420–425. © Pleiades Publishing, Ltd., 2018. Original Russian Text © Yu.T. Tsap, I.N. Myagkova, Yu.G. Kopylova, G.G. Motorina, A.V. Bogomolov, T.B. Gol’dvarg, M.I. Panasyuk, S.I. Svertilov, V.V. Bogomolov, I.V. Yashin, V.L. Petrov, 2018, published in Kosmicheskie Issledovaniya, 2018, Vol. 56, No. 6. Electron Acceleration and Subsecond Time Delays of Hard X-Rays of Solar Flares According to Lomonosov Satellite Data Yu. T. Tsap a, b, *, I. N. Myagkova c , Yu. G. Kopylova b , G. G. Motorina b , A. V. Bogomolov c , T. B. Gol’dvarg d , M. I. Panasyuk c , S. I. Svertilov c , V. V. Bogomolov c , I. V. Yashin c , and V. L. Petrov c a Crimean Astrophysical Observatory, Russian Academy of Sciences, Nauchnyi, 298409 Russia b Central Astronomical Observatory, Russian Academy of Sciences, St. Petersburg, 196140 Russia c Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, 119899 Russia d Gorodovikov Kalmyk State University, Elista, 358000 Russia *e-mail: yur_crao@mail.ru Received March 17, 2018 Abstract—On the basis of satellite observations obtained with the help of X-ray and gamma-ray detector units (BDRG/Lomonosov in Russian), using digital filtering methods, the analysis of time delays between the sec- ond pulsations of time profiles of hard X-ray radiation of different frequency bands from the solar flare that occurred on July 21, 2016 has been made. It is shown that the count rate in energy channels of 10–20, 20–35, and 35–60 keV detected by BDRG/Lomonosov are correlated with an accuracy up to 0.1 s. The result agrees well with the observations of the Gamma-ray Burst Monitor on board the Fermi satellite and supposes an effective acceleration of charged particles over the entire length of the flare magnetic arc. DOI: 10.1134/S0010952518060096 1. INTRODUCTION Nowadays, it is beyond question that a significant part of solar flare energy accounts for accelerated charged particles [1]. However, when interpreting dif- ferent flare phenomena, the question about the accel- eration and propagation mechanisms of these particles remains open. In this connection, the study of pecu- liarities of time profiles of nonthermal flare radiation over a broad range of wavelengths acquires a special value. A significant contribution to understanding of nonthermal phenomena can be made by studying delays τ between time profiles of hard X-ray emission of different frequency bands generated by accelerated electrons by means of bremsstrahlung mechanism. Their existence was reported for the first time by Bai and Ramaty [2]. They analyzed two powerful flare events that occurred on August 4 and 7, 1972, which were observed by the ESRO TD 1A satellite in four low-energy (30–107 keV) and three high-energy (107–430 keV) channels with a time resolution of 1.2 and 4.8 s, respectively. They came to the conclusion that, at first, the time delays slowly increase from the first to the fifth channel, reaching 5 s, then, starting from the channel of 122–168 keV, they jump up to 15 s. The authors have related the second delays to the dependence of frequency v of collisions with the back- ground plasma particles; the decasecond delays, to the two-stage charged particle acceleration mechanism. Later, the most detailed investigations of the time delays of hard X-ray emission were conducted by Aschwanden et al. [3–6], using 16 channels (20– 200 keV) of the Burst and Transient Source Experi- ment (BATSE) setup aboard the Compton Gamma Ray Observatory (CGRO) satellite for 108 events with a time resolution of 0.064 s. For the overwhelming number of events (80%) with a clearly pronounced fine time structure that has a shape of second and sub- second pulses, the positive time delays with a duration of about 0.1 s and shorter were discovered [3, 4] (the profiles of higher-energy channels are lagging with respect to the low-energy channels). This result, as well as images of sources obtained by YOHKOH [4], formed the basis for the hypothesis of the connection of delays with the time of flight of electrons from the region of the top of the arch to its bases [6]. At that, the time delays from 1 to 10 s for the so-called slow profile component have proved to be negative; they demon- strate a regular increase according to the ε 3/2 law, where ε is photon energy [5]. According to the authors, these results support indirectly the coronal magnetic trap model in which the Coulomb diffusion of trapped electrons to the loss cone dominates. Cheng et al. [7], used the observational data obtained by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite with a time resolution of 0.125, owing to the demodulation mech- anism. To minimize the influence of noises, they ana-