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PHYSICAL CONDITIONS IN CORONAL HOLES

O. S. Gopasyuk, E. A. Baranovskii, V. P. Tarashchuk, and N. I. Shtertser

The physical conditions in the chromosphere of polar and equatorial coronal holes in the decay phase of the 24-th solar activity cycle are studied. Models of the chromosphere are constructed based on the intensities of the HeI 10830 Å and HD lines and using a non-LTE program. The calculations showed that the temperature in equatorial coronal holes is similar to that of polar coronal holes. The temperature in coronal holes is lower than that of the unperturbed chromosphere and increases depending on height in the chromosphere. The difference between the atmospheres of coronal holes and the quiet Sun begins at the level of the upper photosphere- lower chromosphere. Keywords: Sun: chromosphere: coronal holes: physical conditions

1. Introduction

Coronal holes (CH) are large-scale structures in the solar corona. Because of their considerably lower density and temperature compared to the surrounding corona, coronal holes show up as dark regions in the X-ray and extreme ultraviolet images. The magnetic lines of force are mainly closed and in the quiet Sun they have a loop structure, while in CH the lines of force have an open configuration. The open magnetic flux expands rapidly above the base of the CH. The plasma accelerates along the open magnetic field lines to form high-velocity flows of solar wind (> 600 km/s) [1,2]. CH are observed at all latitudes from the poles [3,4] to the equator [5-7]. Based on their lifetime

Crimean Astrophysical Observatory (CrAO), Russian Academy of Sciences, Crimea, Russia; e-mail: [email protected]

Original article submitted November 19, 2019; accepted for publication June 24, 2020. Translated from Astrofizika, Vol. 63, No. 3, pp. 479-488 (August 2020) 0571-7256/19/6303-0421 ©2020 Springer Science+Business Media, LLC

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and position, three main categories of CH can be distinguished [8,9]: polar, nonpolar (low latitude, isolated), and transient (coronal dimmings), associated with eruptive events such as the eruption of filaments and coronal mass ejections (e.g., Refs. 8, 10,11). In this study we discuss the first two categories: polar and equatorial. Polar CH lie at high latitudes near the poles, occupy a substantial portion of the solar surface, and dominate during a minimum of solar activity. Their lifetime reaches seven-eight years [8,12]. Equatorial CH are usually rarely observed during a minimum of solar activity. At that time the global magnetic structure of the Sun is approximately a magnetic dipole field with open fields of opposite polarity at both poles and a curved current layer, minimally distant from the equator compared to other periods of the solar cycle. There are, however, also some exceptions. Thus, during the minimum between the 22-nd and 23-rd cycles, in 1996, a large CH was observed at low latitudes which extended from the north pole to a large active region in the southern hemisphere [5]. During the minimum of solar activity between cycles 23 and 24,