Transition zone flows observed in a coronal hole on the solar disk

Abstract

A rocket flight of the NRL High Resolution Telescope and Spectrograph (HRTS) was performed to obtain ultraviolet spectra of a well-defined coronal hole on the solar system. Observed emission lines included strong transition zone lines of C IV at 1548 and 1550 A, strong chromospheric lines of C I near 1560 A, and chromospheric lines of Fe (II) at 1563 A. Continuum radiation formed in the temperature minimum was also recorded. The cumulative histogram of the C IV velocities in the coronal hole shows an average Doppler shift of 2 km/sec to the red (downflow) with 26 percent of the profiles having blueshifts (outflows) compared to 7 percent in the quiet sun. The present observations have the advantages of higher spatial resolution and a nearly absolute wavelength scale determined from narrow chromospheric lines of Si I with an accuracy of about 1 km/sec. The nonthermal velocities, derived from the C IV line widths, are only marginally larger inside the coronal hole than those in the quiet sun, thus indicating that the magnitude of small-scale motions, which may represent energy available for coronal heating and solar wind acceleration, is equal in the two regions.