Variations in characteristics of coronal holes in the process of their evolution
The variations in characteristics of long-lived (several months) and short-lived (several days) coronal holes (CH) have been studied in the process of their evolution from the origination to extinction. In the paper we used solar observations in the НеI 1083 nm line (CrAO RAS), SDO data (AIA, channel at 19.3 nm, HMI, magnetic measurements) and observations with the SOLIS telescope. The results are as follows.
- For individual CH the velocities of their origination and decay differ significantly. For the short-lived CH the time from the origination to maximum is about several hours. For the long-lived CH this process can take several rotations of the Sun. The variation in brightness of CH over their development delays compared to variations in the area. The maximum in the area begins earlier than the brightness minimum. Against the background of the development, the short-term growth in the area of CH sometimes occurs on account of its enlargement at the boundary of a coronal hole.
- The openness of lines of force cannot be considered as an undoubted characteristic of CH. At the birth of a coronal hole the closed lines of force occur which enclose in the neighborhood at a distance of 10°–60°. Open lines of force fill more fully CH near the maximum of development. At the decay of a coronal hole the closed lines of force become predominant again. The indicated directions in which they extend are a characteristic feature of lines of force.
- The magnetic field in CH varies slightly with time. As an example, for one long-lived CH the average over 4 months value of module of the longitudinal magnetic field component was ≈103 Gauss. The other magnetic field characteristics (module of magnetic induction, radial and transverse field components and inclination angle of the field lines of force toward the radial from the center of the Sun direction) also proved to vary slightly with time.
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