Context. Waves offer a unique opportunity to understand and diagnose the properties of the solar coronal plasma. Aims. Hinode/EIS observations are analysed to detect wave and oscillatory motions in the solar corona. Methods. The EIS observations were carried out using a selection of EUV lines. Hinode/XRT images are taken concurrently. Results. Two examples of oscillations in active regions are presented. We analysed the evolution of the intensities and Doppler shifts. The Hinode/XRT images suggest that both events occurred along loop-like structures. The first event is interpreted as a slow sausage (acoustic) type wave with a period of 1.2 mHz. The second example is associated with a transverse, most likely kink type, wave with a period of 3 mHz. The EUV line ratios were then used to determine the value of the coronal magnetic field.

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.

Taroyan, Y., Erd?lyi, R. (2009). Heating Diagnostics with MHD Waves. Space Science Reviews, 149 (1-4), 229-254.The heating of the solar atmosphere is a fundamental problem of modern solar and astrophysics. A review of the seismological aspects of magnetohydrodynamic (MHD) waves with an emphasis on standing longitudinal waves in the context of coronal heating is presented. Efforts made recently may be split into two categories: forward modelling and data inversion. Forward modelling can be applied to predict the observational footprints of various heating scenarios. A new diagnostic method based on the analysis of Doppler shift time series is outlined with specific application to solar coronal conditions. The power of the method is demonstrated and tested using synthetic data and comparing them with actual high-resolution (e.g. SoHO/SUMER) observations. Further, related recent examples of standing longitudinal oscillations in coronal loop structures observed with the new Hinode/EIS instrument are also presented. These latter observations provide an advanced ground for MHD seismology as a tool for plasma heating diagnostics in the atmosphere of the Sun.Peer reviewe

An example of hot active region loop oscillations observed by SUMER and SXT is presented. The hypothesis that a fundamental mode standing slow sausage (acoustic) wave is initiated by a footpoint microflare is tested and confirmed using a forward modeling approach. The oscillation is set up immediately after the heating pulse. The duration, temporal behavior, and total heat input of the microflare are estimated using the oscillation parameters. The rapid energy release is followed by cooling. The time-distance profile of the heating rate along the loop is recovered using the intensity and Doppler-shift time series. Hot loop oscillations are mainly observed in the Doppler shift. The absence of intensity oscillations in this and similar cases is explained. It is also found that the intensity oscillation, unlike the Doppler shift oscillation, undergoes half a period phase variation when the background intensity passes through its maximum, thus making it more difficult to detect.

Analysis of a huge data set reveals global solar cycle variation of the solar coronal temperature and other properties.

The Kelvin–Helmholtz instability (KHI) of magnetohydrodynamic surface waves on the magnetopause is examined and the resonant flow instability (RFI) of these waves is studied under the assumption that the plasma density decreases in the direction from the magnetosheath towards the magnetosphere. It is found that when a finite plasma temperature is taken into account, three different kinds of surface modes (labeled as “primary” modes and “β” modes on sides 1 and 2) may exist. The β modes remain almost unaffected by the flow in the corresponding rest frames and can be excited in certain velocity intervals which are determined. These modes do not interact with each other, but their interaction with primary modes leads to KHIs confined to narrow velocity intervals when the mode energies are of opposite signs. Only primary mode can be resonantly unstable with growth rates smaller than those typical for KHIs. Starting with the slow resonant instability the negative energy primary and β modes are subject to several RF and KH instabilities which are separated from each other by narrow velocity intervals of stability. The growth rates of these instabilities tend to increase with increasing velocity shear. Possible applications to, e.g., observations in the terrestrial magnetosphere, are discussed.

Context. The nature of loops is still a matter of debate with several explanations having been put forward. Simultaneous spectral and imaging data have the capacity to provide a new insight into mass motions, dynamics and energetics of loops. Aims. We report on spectral data taken with the Solar Ultraviolet Measurements of Emitted Radiation spectrograph (SUMER) and imaging data from the Transition Region and Coronal Explorer (TRACE) of a transient event which occurred in a cold loop, lasting a few minutes. Methods. A sequence of TRACE images in the 1550 Å and 171 Å filters show a disturbance which originated at one foot-point and propagates along the loop. The SUMER slit was placed at the other foot-point of the loop. In order to interpret the results, numerical simulations were performed with the results then converted into observable quantities and compared with the data. Results. During the event a radiance increase and a relative red shift of ≈20 km s Conclusions. The results suggest that the observations could be interpreted in terms of a short-lived siphon flow reaching a speed of 120 km s −1 and driven by a nonlinear heating pulse. The energies required to drive the observed red-shifts are estimated to be about 10 25 erg. The absence of a significant blue-shift caused by the return flow is explained.