Coronal hole boundaries at small scales: III. EIS and SUMER views
Maria Madjarska Submitted: 2012-07-03 03:43
We report on the plasma properties of small-scale transient events identified in the quiet Sun, coronal holes and their boundaries. We aim at deriving the physical characteristics of events which were identified as small-scale transient brightenings in XRT images. 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. 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 log10Ne ≈8.76 cm−3 while in the flaring site it is log10Ne ≈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. 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 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.
Authors: M.S. Madjarska, Z. Huang, J.G. Doyle and S. Subramanian
Publication Status: accepted for publication in A&A
Last Modified: 2012-07-03 10:47