Observations of post-eruptive arcades on the Sun with the radio telescope RATAN-600

V.N. Borovik$^1$, V.V. Grechnev$^2$, V.E. Abramov-Maximov$^1$, I.Y. Grigorieva$^1$, V.M. Bogod$^3$, T.I. Kaltman$^3$, A.N. Korzhavin$^3$

$^1$Pulkovo Astronomical Observatory, Russian Academy of Sciences, St.-Petersburg 196140, Russia
$^2$Institute of Solar-Terrestrial Physics Siberian Branch of the Russian Academy of Sciences, Lermontov St. 126, Irkutsk 664033, Russia
$^3$St.-Petersburg Branch of the Special Astrophysical Observatory, Russian Academy of Sciences, St.-Petersburg 196140, Russia

We consider long-lived post-eruptive arcades observed on 22 October 2001, 2 November 1992, and 28-30 December 2001 and demonstrate, that the high-density hot regions in their top parts (thus, high $\beta$ regions) existed for a long time, and their radio emission contained non-thermal component, which is indicative of the presence of accelerated particles.

Authors of some studies of post-eruptive arcades, during two last decades, repeatedly came to the following unexpected conclusions: microwave emission of arcades was excessively polarized, presumably due to contribution of non-thermal electrons; their lifetime was much longer than the estimated cooling times, presumably due to post-eruptive energy release; the plasma pressure exceeded the magnetic pressure ($\beta \geq 1$) in their hot top parts. CORONAS-F/SPIRIT observations in a high-temperature ($\sim$ 10 MK) MgXII line and multi-wave RATAN-600 observations along with data from other spectral domains of the solar emission provided important information to verify these conclusions and assumptions. All the above facts were confirmed in analyses of this data set. They were explained in terms of the ``standard'' flare model (CSHKP) elaborated by Shibata and Yokoyama to qualitative account for the chromospheric evaporation, but applied to late post-eruptive phase. In this case, high $\beta$ conditions indicate magnetic reconnection processes, which are responsible for the prolonged heating and particle acceleration. This approach allows to reconcile the listed facts with known estimates of parameters of the coronal plasma in post-eruptive arcades, and to remove seeming contradictions with habitual conceptions.