Aims. We aim to further explore the small-scale evolution of coronal hole boundaries using X-ray high-resolution and high-cadence images. We intend to determine the fine structure and dynamics of the events causing changes of coronal hole boundaries and to explore the possibility that these events are the source of the slow solar wind. Methods. We developed an automated procedure for the identification of transient brightenings in images from the X-ray telescope on-board Hinode taken with an Al Poly filter in the equatorial coronal holes, polar coronal holes, and the quiet Sun with and without transient coronal holes. Results. We found that in comparison to the quiet Sun, the boundaries of coronal holes are abundant with brightening events including areas inside the coronal holes where closed magnetic field structures are present. The visual analysis of these brightenings revealed that around 70% of them in equatorial, polar and transient coronal holes and their boundaries show expanding loop structures and/or collimated outflows. In the quiet Sun only 30% of the brightenings show flows with most of them appearing to be contained in the solar corona by closed magnetic field lines. This strongly suggests that magnetic reconnection of co-spatial open and closed magnetic field lines creates the necessary conditions for plasma outflows to large distances. The ejected plasma always originates from preexisting or newly emerging (at X-ray temperatures) bright points. Conclusions. The present study confirms our findings that the evolution of loop structures known as coronal bright points is associated with the small-scale changes of coronal hole boundaries. The loop structures show an expansion and eruption with the trapped plasma consequently escaping along the "quasi" open magnetic field lines. These ejections appear to be triggered by magnetic reconnection, e.g. the so-called interchange reconnection between the closed magnetic field lines (BPs) and the open magnetic field lines of the coronal holes. We suggest that these plasma outflows are possibly one of the sources of the slow solar wind.

We study the spatial distribution and evolution of the slope of the emission measure (EM) between 1 MK and 3 MK in the core of the active region (AR) NOAA 11193, first when it appeared near the central meridian and then again when it reappeared after a solar rotation. We use observations recorded by the Extreme-ultraviolet Imaging Spectrometer (EIS) aboard Hinode, with a new radiometric calibration. We also use observations from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO). We present the first spatially resolved maps of the EM slope in the 1-3 MK range within the core of the AR using several methods, either from approximations or from the differential emission measure (DEM). A significant variation of the slope is found at different spatial locations within the active region. We selected two regions that were not greatly affected by lower temperature emission along the line of sight. We found that the EM had a power law of the form EM ∝ T b , with b = 4.4 ± 0.4, and 4.6 ± 0.4, during the first and second appearance of the active region, respectively. During the second rotation, line-of-sight effects become more important, although difficult to estimate. We found that the use of the ground calibration for Hinode/EIS and the approximate method to derive the EM, used in previous publications, produce an underestimation of the slopes. The EM distribution in active region cores is generally found to be consistent with high frequency heating, and does not change much during the evolution of the active region.

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 highdensity 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 log 10 N e ≈ 8.76 cm −3 while in the flaring site it is log 10 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.

Our knowledge of the diffuse emission that encompasses active regions is very limited. In the present paper we investigate two off-limb active regions, namely AR10939 and AR10961, to probe the underlying heating mechanisms. For this purpose we have used spectral observations from Hinode/EIS and employed the emission measure (EM) technique to obtain the thermal structure of these diffuse regions. Our results show that the characteristic EM distributions of the diffuse emission regions peak at log T = 6.25 and the cool-ward slopes are in the range 1.4 -3.3. This suggests that both low as well as high frequency nanoflare heating events are at work. Our results provide additional constraints on the properties of these diffuse emission regions and their contribution to the background/foreground when active region cores are observed on-disk.

We study the thermal structure and energetics of the point-like EUV brightenings within a system of fan loops observed in the active region AR 11520. These brightenings were simultaneously observed on 2012 July 11 by the HIgh-resolution Coronal (Hi-C) imager and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We identified 27 brightenings by automatically determining intensity enhancements in both Hi-C and AIA 193 Å light curves. The energetics of these brightenings were studied by using the Differential Emission Measure (DEM) diagnostics. The DEM weighted temperatures of these transients are in the range log T (K) = 6.2 − 6.6 with radiative energies ≈10 24−25 ergs and densities ≈ a few times 10 9 cm −3 . To the best of our knowledge, these are the smallest brightenings in EUV ever detected. We used these results to determine the mechanism of energy loss in these brightenings. Our analysis reveals that the dominant mechanism of energy loss for all the identified brightenings is conduction rather than radiation.A&A proofs: manuscript no. hic_final_2C_final spheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamic Observatory (SDO). These tiny dots are the smallest EUV brightenings ever observed or reported in the literature and have energies a few orders of magnitude more than the nanoflares energy budget.Using a potential field extrapolation of line-of-sight magnetograms obtained with Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012) on board SDO, Régnier et al. (2014) found that these events are predominantly located at the footpoints of large scale trans-equatorial coronal loops with energy content of 10 26 ergs per brightening. Régnier et al. (2014) concluded that these bright dots are multi-thermal in nature with temperatures ranging between log T (K) = 5.3 − 6.5 using the EM loci analysis method (see e.g., Jordan et al. 1987;Tripathi et al. 2010).Since direct observations of the heating mechanisms responsible for these brightenings are still infeasible, a comprehensive understanding of such small scale brightenings is crucial. A quantitative analysis of the energetics involved in these transients is of paramount importance to establish their role in heating the fan loops and the solar corona in general. Here, we study the brightenings as a statistical ensemble, i.e., by analysing all the individual brightenings that occurred near the footpoint of the trans-equatorial fan loop system during the course of the Hi-C observations. Our objective is to identify such brightenings in Hi-C observations and study their energetics by employing Differential Emission Measure (DEM) diagnostics. For such diagnostics, it is mandatory to have multi-filter observations. Therefore, we searched for the same brightening in the AIA 193 Å observations. The DEMs are then derived using AIA filters. Even though the AIA 193 Å channel's effective area and thermal response are similar to that of Hi-C, these events are readily observed in the latter but not in the former...