Coronal hole boundaries at small scales: III. EIS and SUMER views

M.S. Madjarska, Z. Huang, J.G. Doyle and S. Subramanian

Fig. 5 Event No. 1 on January 10. The raster was taken in the Fe XII 195 Å line between 12:35 UT and 12:59 UT. The left and right panels represent the Fe XII 195Å intensity and Doppler shift images, respectively. The intensity image is plotted in negative logarithmic scale.

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 which were identified as small-scale transient brightenings in XRT images.

Methods We use spectroscopic co-observations from SUMER/SoHO and EIS/Hinode combined with high cadence imaging data from XRT/Hinode. We measure Doppler shifts using single and multiple Gauss fits of transition region and coronal lines as well as electron densities and temperatures. We combine 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 large 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 Ne ≈8.76 cm−3 while in the flaring site it is log10 Ne ≈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 temperatures of a maximum 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 dis- appearance. We demonstrated 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 an important candidate 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.

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Last Revised: 2012 July 2nd