PHOTOSPHERIC AND CHROMOSPHERIC OSCILLATIONS IN THE BASE OF CORONAL HOLES


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Abstract

In this paper we carry out an analysis of the spatial–temporal line-of-sight velocity variations measured in the chromospheric (Hα, Hβ) and photospheric (Fe i 6569 Å, Fe i 4864 Å, Ni i 4857 Å) lines at the base of 17 coronal holes. Time series of a duration from 43 to 120 min were recorded with the CCD line-array and the CCD matrix. Rather frequently we observed quasi-stationary upward flows with a measured velocity of up to 1 km s−1 in the photosphere and up to 4–5 km s−1 in the chromosphere (equivalent radial velocity of up to 3 km s−1 and up to 12–15 km s−1 accordingly) near dark points on the chromospheric network boundary inside polar CH. Line-of-sight velocity fluctuation spectra contain meaningful maxima in the low-frequency region clustering around the values 0.4, 0.75, and 1 mHz. Usually, the spatial localization of these maxima mutually coincides and, in our opinion, coincides with the chromospheric network boundary. Acoustic 3- and 5-min oscillations are enhanced in the coronal hole region and reach 1 km s−1 in the photosphere and 3–4 km s−1 in the chromosphere. These oscillations are not localized spatially and are distinguished throughout the entire region observed.

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