Explosive events in the solar transition zone

Abstract

The properties of explosive events in the solar transition zone are presented by means of detailed examples and statistical analyses. These events are observed as regions of exceptionally high velocity (∼ 100 km s⁻¹) in profiles of Civ, formed at 10⁵ K, observed with the High Resolution Telescope and Spectrograph (HRTS). The following average properties have been determined from observations obtained during the third rocket flight of the HRTS: full width at half maximum extent along the slit - 1.6 × 10³ km; maximum velocity - 110 km s⁻¹; peak emission measure - 4 × 10⁴¹ cm⁻³; lifetime - 60 s; birthrate - 4 × 10⁻²¹ cm⁻² s⁻¹ in a coronal hole and 1 × 10⁻²⁰ cm⁻² s⁻¹ in the quiet Sun; mass - 6 × 10⁸ g; and, kinetic energy - 6 × 10²² erg. The 6 examples show that there are considerable variations from these average parameters in individual events. Although small, the events show considerable spatial structure and are not point-like objects. A spatial separation is often detected between the positions of the red and blue shifted components and consequently the profile cannot be explained by turbulence alone. Mass motions in the events appear to be isotropic because the maximum observed velocity does not show any correlation with heliographic latitude. Apparent motions of the 100 km s⁻¹ plasmas during their 60 s lifetime should be detected but none are seen. The spatial frequency of occurrence shows a maximum near latitudes of 40–50°, but otherwise their sites seem to be randomly distributed. There is enough mass in the explosive events that they could make a substantial contribution to the solar wind. It is hard to explain the heating of typical quiet structures by the release of energy in explosive events.