Abstract. We present the first X-ray observation of an oscillation during a stellar flare. The flare occurred on the active M-type dwarf AT Mic and was observed with XMM-Newton. The soft X-ray light curve (0.2-12 keV) is investigated with wavelet analysis. The flare's extended, flat peak shows clear evidence for a damped oscillation with a period of around 750 s, an exponential damping time of around 2000 s, and an initial, relative peak-to-peak amplitude of around 15%. We suggest that the oscillation is a standing magneto-acoustic wave tied to the flare loop, and find that the most likely interpretation is a longitudinal, slow-mode wave, with a resulting loop length of (2.5 ± 0.2) × 10 10 cm. The local magnetic field strength is found to be 105 ± 50 G. These values are consistent with (oscillation-independent) flare cooling time models and pressure balance scaling laws. Such a flare oscillation provides an excellent opportunity to obtain coronal properties like the size of a flare loop or the local magnetic field strength for the otherwise spatially-unresolved star.Key words. stars: coronae -stars: flare -stars: magnetic fields -stars: oscillations -X-rays: stars -stars: individual: AT Mic IntroductionOscillations in the solar corona have by now been observed for many years. Wavelength regimes ranging from radio to hard X-rays have been investigated in the search for evidence of waves. Table 1 in Aschwanden et al. (1999) provides a summary and description of the different periods found (0.02 to 1000 s). Most of these waves have been explained by magneto-hydrodynamic (MHD) oscillations in coronal loops, see Roberts (2000) for a detailed review of waves and oscillations in the corona.One of the most exciting aspects of "coronal seismology" is that it potentially provides us with the capability for determining the magnetic field strength in the corona (Roberts et al. 1984), as well as, in the stellar case, with information on otherwise unresolved spatial scales, e.g., flare loop lengths. It is notoriously difficult to measure the magnetic field strength in the corona because of the very high speeds of coronal electrons, which broaden spectral lines far beyond the width of Zeeman splitting. Techniques using both near-infrared emission lines and radio observations are successful, but have poor spatial resolution. Indirect methods are commonly used, such as the extrapolations of the coronal magnetic field from the photospheric magnetic field, which in turn can be measured using the Zeeman effect.Many of the observations of waves have been determined from variations in intensity. However, a huge step forward was achieved in solar coronal physics due to the high spatial resolution available with the Transition Region and Coronal Explorer (TRACE, Handy et al. 1999). Aschwanden et al. (1999) observed the first spatial displacement oscillations of coronal loops. It was suggested that these oscillations were triggered by a fast-mode shock from a flare site, and they were interpreted as standing fast kink-mode waves. Nakari...

Abstract. We present simultaneous ultraviolet and X-ray observations of the dMe-type flaring stars AT Mic, AU Mic, EV Lac, UV Cet and YZ CMi obtained with the XMM-Newton observatory. During 40 h of simultaneous observation we identify 13 flares which occurred in both wave bands. For the first time, a correlation between X-ray and ultraviolet flux for stellar flares has been observed. We find power-law relationships between these two wavelength bands for the flare luminosity increase, as well as for flare energies, with power-law exponents between 1 and 2. We also observe a correlation between the ultraviolet flare energy and the X-ray luminosity increase, which is in agreement with the Neupert effect and demonstrates that chromospheric evaporation is taking place.

We present multi-wavelength observations of a small B-class flare which occurred on the Sun on 2007 May 22. The observations include data from Hinode, GOES, TRACE and the Nobeyama Radioheliograph. We obtained spatially and spectrally-resolved information from the Hinode EUV Imaging Spectrometer (EIS) during this event. The temporal and temperature coverage of the EIS observations provides new insights into our understanding of chromospheric evaporation and cooling. The flare showed many "typical" features, such as brightenings in the ribbons, hot (10 MK) loop emission and subsequent cooling. We also observed a new feature, strong (up to 170 km s −1 ) blue-shifted emission in lines formed around 2-3 MK, located at the footpoints of the 10 MK coronal emission and within the ribbons. Electron densities at 2 MK in the kernels are high, of the order of 10 11 cm −3 , suggesting a very narrow layer where the chromospheric evaporation occurs. We have run a non-equilibrium hydrodynamic numerical simulation using the HYDRAD code to study the cooling of the 10 MK plasma, finding good agreement between the predicted and observed temperatures, densities and ion populations. Line blending for some potentially useful diagnostic lines for flares, which are observed with Hinode/EIS, is also discussed.

The Solar X-ray Monitor (XSM) payload on board Chandrayaan-2 provides disk-integrated solar spectra in the 1–15 keV energy range with an energy resolution of 180 eV (at 5.9 keV) and a cadence of 1 s. During the period from 2019 September to 2020 May, covering the minimum of Solar Cycle 24, it observed nine B-class flares ranging from B1.3 to B4.5. Using time-resolved spectroscopic analysis during these flares, we examined the evolution of temperature, emission measure, and absolute elemental abundances of four elements–Mg, Al, Si, and S. These are the first measurements of absolute abundances during such small flares and this study offers a unique insight into the evolution of absolute abundances as the flares evolve. Our results demonstrate that the abundances of these four elements decrease toward their photospheric values during the peak phase of the flares. During the decay phase, the abundances are observed to quickly return to their preflare coronal values. The depletion of elemental abundances during the flares is consistent with the standard flare model, suggesting the injection of fresh material into coronal loops as a result of chromospheric evaporation. To explain the quick recovery of the so-called coronal “First Ionization Potential bias” we propose two scenarios based on the Ponderomotive force model.

We present an analysis of the X-ray emission of the rapidly rotating giant star YY Mensae observed by Chandra HETGS and XMM-Newton. The high-resolution spectra display numerous emission lines of highly ionized species; Fe XVII to Fe XXV lines are detected, together with H-like and He-like transitions of lower Z elements. Although no obvious flare was detected, the X-ray luminosity changed by a factor of two between the XMM-Newton and Chandra observations taken 4 months apart (from log L X ≈ 32.2 to 32.5 erg s −1 , respectively). The coronal abundances and the emission measure distribution have been derived from three different methods using optically thin collisional ionization equilibrium models, which is justified by the absence of opacity effects in YY Men as measured from line ratios of Fe XVII transitions. The abundances show a distinct pattern as a function of the first ionization potential (FIP) suggestive of an inverse FIP effect as seen in several active RS CVn binaries. The low-FIP elements (< 10 eV) are depleted relative to the high-FIP elements; when compared to its photospheric abundance, the coronal Fe abundance also appears depleted. We find a high N abundance in YY Men's corona which we interpret as a signature of material processed in the CNO cycle and dredged-up in the giant phase. The corona is dominated by a very high temperature (20 − 40 MK) plasma, which places YY Men among the magnetically active stars with the hottest coronae. Lower temperature plasma also coexists, albeit with much lower emission measure. Line broadening is reported in some lines, with a particularly strong significance in Ne X Lyα. We interpret such a broadening as Doppler thermal broadening, although rotational broadening due to X-ray emitting material high above the surface could be present as well. We use two different formalisms to discuss the shape of the emission measure distribution. The first one infers the properties of coronal loops, whereas the second formalism uses flares as a statistical ensemble. We find that most of the loops in the corona of YY Men have their maximum temperature equal to or slightly larger than about 30 MK. We also find that small flares could contribute significantly to the coronal heating in YY Men. Although there is no evidence of flare variability in the X-ray light curves, we argue that YY Men's distance and X-ray brightness does not allow us to detect flares with peak luminosities L X ≤ 10 31 erg s −1 with current detectors.

We present meridional flow measurements of the Sun using a novel helioseismic approach for analyzing SOHO/MDI data in order to push the current limits in radial depth. Analyzing three consecutive months of data during solar minimum, we find that the meridional flow is as expected poleward in the upper convection zone, turns equatorward at a depth of around 40 Mm (~0.95 Rsol), and possibly changes direction again in the lower convection zone. This may indicate two meridional circulation cells in each hemisphere, one beneath the other.Comment: 4 pages, 3 figure

Abstract. The energy input into the lower solar corona by flare evaporation events has been modeled according to the available observations for quiet regions. The question is addressed whether such heating events can provide the observed average level of the coronal emission measure and thus of the observed flux of extreme ultraviolet (EUV) and X-ray emission without contradicting the observed average power spectrum of the emission measure, the typical emission measure variations observed for individual pixels and the observed flare energy distribution. As the assumed flare height influences the derived flare energy, the mathematical foundations of nanoflare distributions and their conversion to different height assumptions are studied first. This also allows a comparison with various published energy distributions differing in height assumptions and to relate the observations to the input parameters of the heating model. An analytic evaluation of the power spectrum yields the relationship between the average time profile of nanoflares (or microflares), assumed to be self-similar in energy, and the power spectrum. We find that the power spectrum is very sensitive to the chosen time profile of the flares. Models are found by numerical simulation that fit all available observations. They are not unique but severely constrained. We concentrate on a model with a flare height proportional to the square root of the flare area. The existence of a fitting model demonstrates that nanoflare heating of the corona is a viable and attractive mechanism.

Aims. With the Solar Optical Telescope on Hinode, we investigate the basic properties of high-degree solar oscillations observed at two levels in the solar atmosphere, in the G-band (formed in the photosphere) and in the Ca ii H line (chromospheric emission).Methods. We analyzed the data by calculating the individual power spectra as well as the cross-spectral properties, i.e., coherence and phase shift. The observational properties are compared with a simple theoretical model, which includes the effects of correlated noise.Results. The results reveal significant frequency shifts between the Ca ii H and G-band spectra, in particular above the acoustic cutoff frequency for pseudo-modes. The cross-spectrum phase shows peaks associated with the acoustic oscillation (p-mode) lines, and begins to increase with frequency around the acoustic cut-off. However, we find no phase shift for the (surface gravity wave) f-mode. The observed properties for the p-modes are qualitatively reproduced in a simple model with a correlated background if the correlated noise level in the Ca ii H data is higher than in the G-band data. These results suggest that multi-wavelength observations of solar oscillations, in combination with the traditional intensity-velocity observations, may help to determine the level of the correlated background noise and to determine the type of wave excitation sources on the Sun.