Coronal hole boundaries evolution at small scales

III. EIS and SUMER views^⋆

¹ Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
² UCL-Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
e-mail: madj@arm.ac.uk

Received: 1 May 2012
Accepted: 29 June 2012

Abstract

Context. We report on the plasma properties of small-scale transient events identified in the quiet Sun, coronal holes and their boundaries.

Aims. We aim at deriving the physical characteristics of events that were identified as small-scale transient brightenings in XRT images.

Methods. We used spectroscopic co-observations from SUMER/SoHO and EIS/Hinode combined with high-cadence imaging data from XRT/Hinode. We measured Doppler shifts using single and multiple Gaussian fits of the transition region and coronal lines as well as electron densities and temperatures. We combined co-temporal imaging and spectroscopy to separate brightening expansions from plasma flows.

Results. The transient brightening events in coronal holes and their boundaries were found to be very dynamical, producing high-density outflows at high speeds. Most of these events represent X-ray jets from pre-existing or newly emerging coronal bright points at X-ray temperatures. The average electron density of the jets is log₁₀ N_e ≈ 8.76 cm^-3 while in the flaring site it is log₁₀ N_e ≈ 9.51 cm^-3. The jet temperatures reach a maximum of 2.5 MK but in the majority of the cases the temperatures do not exceed 1.6 MK. The footpoints of jets have maximum temperatures of 2.5 MK, though in a single event scanned a minute after the flaring the measured temperature was 12 MK. The jets are produced by multiple microflaring in the transition region and corona. Chromospheric emission was only detected in their footpoints and was only associated with downflows. The Doppler shift measurements in the quiet Sun transient brightenings confirmed that these events do not produce jet-like phenomena. The plasma flows in these phenomena remain trapped in closed loops.

Conclusions. We can conclude that the dynamic day-by-day and even hour-by-hour small-scale evolution of coronal hole boundaries reported in Paper I is indeed related to coronal bright points. The XRT observations reported in Paper II revealed that these changes are associated with the dynamic evolution of coronal bright points producing multiple jets during their lifetime until their full disappearance. We demonstrate here through spectroscopic EIS and SUMER co-observations combined with high-cadence imaging information that the co-existence of open and closed magnetic fields results in multiple energy depositions, which propel high-density plasma along open magnetic field lines. We conclude from the physical characteristics obtained in this study that X-ray jets are important candidates for the source of the slow solar wind. This, however, does not exclude the possibility that these jets are also the microstreams observed in the fast solar wind, as recently suggested.