Small-Scale Structure of Thermal X-Ray Background of the Solar Corona and Microflares in the Photon Energy Range of 3-16
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Small-Scale Structure of Thermal X-Ray Background of the Solar Corona and Microflares in the Photon Energy Range of 3−16 keV I. K. Mirzoeva* Space Research Institute, Russian Academy of Sciences, Moscow, 117997 Russia *e-mail: [email protected] Received April 3, 2017; in final form, May 10, 2017
Abstract—The small-scale structure of the thermal X-ray background of the solar corona and microflares in the photon energy range from 3 to 16 keV according to the RHESSI data obtained on March−April 2003 is analyzed. The study of the reduction in the X-ray intensity in this spectral range is continued. A reduction and, in some cases, an increase in the X-ray intensity of the thermal background of the solar corona and microflares in narrow subranges of the X-ray spectrum of the quiet Sun are revealed in the photon energy range of 3−11 keV. DOI: 10.1134/S1063780X18010117
in the emission intensity. In most cases, it was the 3to 4-keV component, in some cases accompanied by the 4- to 5-keV component. In this work, we continue analysis of the reduction in the X-ray intensity by using data of the RHESSI project obtained on March–April 2003. Let us clarify some terms used in this work. At present, solar flares are commonly classified by the peak value of the X-ray intensity of the flare recorded on the Earth’s orbit. The X-ray intensity of the flare is measured in watts per square meter (W/m2). According to this classification, solar flares are divided into the following classes: the X class with a peak intensity of 10–4–10–3 W/m2 (flares of this class are the most powerful); the M class with an intensity of 10–5– 10‒4 W/m2 (also large flares but weaker than those of the X-class); the C class with an intensity of 10–6– 10‒5 W/m2 (middle flares); and small solar events with intensities of 10–7–10–6, 10–8–10–7, and 10–8– 10–9 W/m2 (the B, A, and 0 classes, respectively). All flares with intensities of less than 10–6 W/m2 are commonly called microflares. The weakest solar events with an X-ray intensity of 10–8–10–9 W/m2 are sometimes denoted as “nanoflares.” The term “quiet Sun” stands for periods of X-ray emission without flare events (i.e., the thermal background emission of the solar corona) or with very small solar flares in the absence of larger ones. The term “small-scale” structure refers to detailed analysis of data in different scales (temporal and spectral). The temporal small-scale structure implies anal-
1. INTRODUCTION The development of the present-day observational means, in particular, the implementation of the RHESSI project, provides wide opportunities to study the quiet Sun. This research area was the subject of study in [1–4]. In those works, however, the thermal coronal background was not analyzed and the mechanism for the appearance and time evolution of microflares was studied in insufficient detail. It can be expected that studies of the thermal background of the solar corona will throw light on the physical mechanism of solar corona heating on the whole. This problem was earlier considered in [5
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