Context. We diagnose the properties of the plume and interplume regions in a polar coronal hole and the role of waves in the acceleration of the solar wind. Aims. We attempt to detect whether Alfvén waves are present in the polar coronal holes through variations in EUV line widths. Methods. Using spectral observations performed over a polar coronal hole region with the EIS spectrometer on Hinode, we study the variation in the line width and electron density as a function of height. We use the density sensitive line pairs of Fe xii 186.88 Å and 195.119 Å and Fe xiii 203.82 Å and 202.04 Å.Results. For the polar region, the line width data show that the nonthermal line-of-sight velocity increases from 26 km s −1 at 10 above the limb to 42 km s −1 some 150 (i.e. ∼110 000 km) above the limb. The electron density shows a decrease from 3.3 × 10 9 cm −3 to 1.9 × 10 8 cm −3 over the same distance. Conclusions. These results imply that the nonthermal velocity is inversely proportional to the quadratic root of the electron density, in excellent agreement with what is predicted for undamped radially propagating linear Alfvén waves. Our data provide signatures of Alfvén waves in the polar coronal hole regions, which could be important for the acceleration of the solar wind.

Abstract. This paper presents the formation, evolution and decay of a coronal bright point via a spectroscopic analysis of its transition region counterpart and the evolution of the underlying magnetic bipole during 3 days of almost continuous observations. The data were obtained with various instruments on-board SoHO, including the SUMER spectrograph in the transition region line S  933.40 Å, CDS in the He  584.33, O  629.73 and Mg  368.06 Å lines, plus MDI and EIT. The existence of the coronal feature is strongly correlated with the evolution of the underlying bipolar region. The lifetime of the bright point from the moment when it was first visible in the EIT images until its complete disappearance was ∼18 hrs. Furthermore, the bright point only became visible at coronal temperatures when the two converging opposite magnetic polarities were ∼7000 km apart. As far as the temporal coverage of the data permits, we found that the bright point disappeared at coronal temperatures after a full cancellation of one of the magnetic polarities. The spectroscopic analysis reveals the presence of small-scale (∼6 ) transient brightenings within the bright point with a periodicity of ∼6 min. The Doppler shift in the bright point was found to be in the range of −10 to 10 km s −1 although it is dominated by a red-shifted emission which is associated with regions characterized by stronger "quiet" Sun photospheric magnetic flux. Small-scale brightenings within the bright point show velocity variations in the range 3-6 km s −1 . In general the bright point has a radiance ∼4 times higher than that of the network. No relation was found between the bright point and the UV explosive event phenomena.

Abstract. We report here on an investigation of high frequency oscillations in active regions, carried out using high cadence observations of O v 629Å, Mg ix 368Å and Fe xvi 335Å with the Coronal Diagnostic Spectrometer (cds) on soho. Using the techniques of wavelet analysis on various temporal series datasets, we find that certain oscillation frequencies are favoured for each line. We find furthermore that a ∼5 min oscillation signature is commonly present in all lines, suggesting a coupling of the photospheric driver with the transition region and coronal loop modes. We report on the tendency for higher frequency oscillations to be present at lower intensity values, suggesting that higher frequency oscillations occur in interloop regions or at loop boundaries, possibly as a result of some resonant absorption process. In addition, we find that the coronal lines of Fe xvi and Mg ix show more significant oscillations in the velocity than in the intensity, which suggests that in the velocity we measure additional non-compressive wave modes not visible in the intensity. As this effect is not seen in the transition region line of O v it would seem that these additional non-compressive modes are produced in and limited to the corona. We suggest that there are two main mechanisms responsible for the observed oscillations; either resonant Alfvén and/or fast kink waves or propagating slow magnetoacoustic waves, both present in coronal loops.

Aims. We present near ultraviolet (NUV: 1750-2800 Å) and far ultraviolet (FUV: 1350-1750 Å) light-curves for flares on 4 nearby dMe-type stars (GJ 3685A, CR Dra, AF Psc and SDSS J084425.9+513830.5) observed with the GALEX satellite. Methods. Taking advantage of the time-tagged events recorded with the GALEX photon counting detectors, we present high temporal resolution (<0.01 s) analysis of these UV flare data. Results. A statistical analysis of 700 s of pre-flare quiescence data for both CR Dra and SDSS J084425.9+513830.5 failed to reveal the presence of significant micro-flare activity in time bins of 0.2, 1 and 10 s intervals. Using an appropriate differential emission measure for both the quiescent and flaring state, it is possible to reproduce the observed FUV:NUV flux ratios. A major determinant in reproducing this flux ratio is found to be the value of plasma electron density during the flare. We also searched the count rate data recorded during each of the four flare events for periodicity associated with magneto-hydrodynamic oscillations in the active region coronal loops. Significant oscillations were detected during the flare events observed on all 4 stars, with periodicities found in the 30 to 40 s range. Flare oscillations with this periodicity can be explained as acoustic waves in a coronal loop of length of ≈10 9 cm for an assumed plasma temperature of 5−20 × 10 6 K. This suggests a loop length for these M-dwarf flares of less than 1/10th of the stellar radii. We believe that this is the first detection of non-solar coronal loop flare oscillations observed at ultraviolet wavelengths.

Abstract.A new theoretical model for the study of slow standing sausage mode oscillations in hot (T > 6 MK) active region coronal loops is presented. These oscillations are observed by the SUMER spectrometer on board the SoHO satellite. The model contains the transition region and the upper chromosphere which enables us to study the entire process of hot loop oscillationsfrom the impulsive footpoint excitation phase to the rapid damping phase. It is shown that standing acoustic waves can be excited by an impulsive heat deposition at the chromospheric footpoint of a loop if the duration of the pulse matches the fundamental mode period. The pulse is immediately followed by a standing wave consistent with the SUMER observations in hot loops. The amount of released energy determines the oscillation amplitude. The combined effects of thermal conduction and radiation on the behaviour of the standing acoustic waves in hot gravitationally stratified loops are investigated. In addition to damping, these effects lead to downflows which are superimposed on the oscillations. The implications of the results in coronal seismology are discussed.