Nobeyama radio heliograph observations of RHESSI microflares

¹ Astronomy Dept., University of Maryland, College Park, MD 20742, USA e-mail: ed@astro.umd.edu
² Lab for Solar and Space Physics, NASA/GSFC, Code 612.1, Greenbelt, MD 20771, USA
³ Institute of Astronomy, ETH Zurich, Switzerland
⁴ Special Astrophysical Observatory, St. Petersburg, Russia
⁵ Nobeyama Radio Observatory, Minamimaki, Nagano, Japan

Received: 4 August 2005
Accepted: 19 January 2005

Abstract

Aims.We present a summary of the analysis of thirty microflares, observed simultaneously by the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in hard X-rays and by Nobeyama RadioHeliograph (NoRH) in microwaves (17 GHz).

Methods.We used microflares observed by RHESSI in the energy range 3-25 keV, and for larger events, up to 35 keV. The observations were made 2002, May 2-6.

Results.We describe the imaging characteristics of these microflares including their locations in hard X-rays and microwaves and the relative positions of the micro-flaring sources. We discuss the brightness temperatures, emission measures and their hard X-ray spectral properties. We see small (mini) flaring loops clearly in NoRH and RHESSI images. The microwave emission often seems to come from the RHESSI foot points (for higher energies), and from the entire small (mini) flaring loop (for lower energies). Sometimes the two (microwave and hard X-ray) sources coincide, at other times they are at opposite ends of a mini flaring loop. Typically, the hard X-ray spectrum of the microwave associated RHESSI microflares can be fit by an isothermal component at low energies (below 10 or 12 keV) and a nonthermal component at higher energies (above 12 keV).

Conclusions.Microflares in hard X-rays and in microwaves behave like normal flares in many respects. They can have both thermal and nonthermal components appearing in bremsstrahlung and gyrosynchrotron radiation.