SXR Coronal Polar Jets and Recurrent Flashes

Abstract

We give evidence for the existence of a new type of soft X-ray (SXR) brightening event that we call coronal polar jets and flashes. The phenomenon was observed on a long-exposure time series taken with the SXT of Yohkoh in a north polar coronal hole, near the activity minimum. Events last as little as 1.5 minutes and the detected SXR photon fluxes span a broad range, from small scale brightenings, at the minimum detectable level near 10²⁴ erg, to more smeared multiple pixel brightenings still an order of magnitude fainter than the known small SXR bright points and/or transient brightenings that have been observed. The typical observed occurrence rate of flashes is one event per super-granule per 10 mn period. At least some of the coronal flashes are recurrent and some of them have associated SXR jets. It is not clear what, if any, is their optical counterpart. Polar jetlets are observed extending radially without showing a significant radial gradient up to the 50 Mm heights. Their occurence rate is typically one order of magnitude lower than flashes and their lifetime seems to be longer when they are broader. The average upward proper motion observed in polar jets is 200 to 400 km s^-1.