M. Mathioudakis

Evidence for Non-Maxwellian Electron Energy Distributions in the Solar Transition Region: Si III Line Ratios from SUMER

Recent calculations of Si III emission-line strengths are compared with SUMER observations from a quiet solar region, a coronal hole, and an active region. Diagnostic line ratios are used to derive Te and Ne for the emitting plasma in each region, and good agreement between theory and observations is found. A major enhancement in the intensity of the 1313 A emission line is observed, as well as some evidence of a small decrease in the temperature of maximum ionization fraction, as one moves from the coronal hole to the active region. Possible explanations for these effects are discussed, and it is concluded that they may be caused by the presence of nonthermal electrons in the transition region.

A Comparative Study of Flaring Loops in Active Stars

Dynamo activity in stars of different types is expected to generate magnetic fields with different characteristics. As a result, a differential study of the characteristics of magnetic loops in a broad sample of stars may yield information about dynamo systematics. In the absence of direct imaging, certain physical parameters of a stellar magnetic loop can be extracted if a flare occurs in that loop. In this paper we employ a simple nonhydrodynamic approach introduced by Haisch, to analyze a homogeneous sample of all of the flares we could identify in the EUVE DS database: a total of 134 flares that occurred on 44 stars ranging in spectral type from F to M and in luminosity class from V to III. All of the flare light curves that have been used in the present study were obtained by a single instrument (EUVE DS). For each flare, we have applied Haischs simplified approach (HSA) in order to determine loop length, temperature, electron density, and magnetic field. For each of our target stars, a literature survey has been performed to determine quantitatively the extent to which our results are consistent with independent studies. The results obtained by HSA are found to be well supported by results obtained by other methods. Our survey suggests that, on the main sequence, short loops (with lengths ≤0.5R*) may be found in stars of all classes, while the largest loops (with lengths up to 2R*) appear to be confined to M dwarfs. Based on EUVE data, the transition from small to large loops on the main sequence appears to occur between spectral types K2 and M0. We discuss the implications of this result for dynamo theories.

Limits on detectability of mass loss from cool dwarfs

Recent spectroscopic evidence supports the theoretical expectation that certain cool dwarfs may have stellar winds with M-dot values several orders of magnitude larger than the solar rate. For large enough values of M-dot, the emission from the wind is expected to have a spectrum which, at low enough frequencies, becomes a power law, S(v) about v exp alpha with alpha about 0.7. Data from IRAS and VLA suggest that such a spectrum may in fact occur in certain M dwarfs: a key test of the wind spectrum would be provided if the stars could be detected at lambda about 1 mm. We show that the M-dot required to ensure power-law emission is a few times 10 exp -10 solar mass/yr. With M-dot of this order, fluxes at lambda about 1 mm would be tens of mJy. Using the James Clerk Maxwell Telescope, we have tested this prediction on several stars: the data are suggestive but are near the limits of detection. Confirmation of our estimates will be important for evolution and for interstellar medium (ISM) physics: if even a few percent of all M dwarfs are losing mass at the above rates, the mass balance of the ISM will be dominated by M dwarfs.

The Radial and Angular Variation of the Electron Density in the Solar Corona

We derive, for the first time, electron densities as a function of both radius (R) and position angle (θ) for the southwest quadrant of the off-limb corona, using the density-sensitive Si IX λ349.9/λ341.9 and Si X λ356.0/λ347.7 extreme-ultraviolet line ratios. The observations were made with the coronal diagnostic spectrometer on board the Solar and Heliospheric Observatory over the ranges of 1.00 R☉ < R < 1.20 R☉ and 180° < θ < 270°. Within the south polar coronal hole, the density varies from 2.3 × 108 cm-3 at 1.0 R☉ to 8.3 × 107 cm-3 at 1.20 R☉, while at the equator, the density varies from 6.3 × 108 cm-3 at 1.0 R☉ to 1.6 × 108 cm-3 at 1.20 R☉. The density falloff with height is therefore faster in the equatorial region. We also find that electron densities are, on average, a factor of 2.7 larger in the equatorial regions than in the polar coronal hole at a given radial distance. Finally, we find remarkable agreement between our measured densities as a function of radius and position angle and those predicted by a recent analytic MHD model of the solar wind, strongly supporting its basic premises.

The Extreme-Ultraviolet Continuum of a Strong Stellar Flare

We present the serendipitous detection of an extreme-ultraviolet flare on EUVE J0613-23.9B. The flare showed over a 200-fold increase above the quiescent emission in the DS/Lexan 60-200 ? wavelength band. Optical spectroscopy revealed that the event was associated with an active dM3.5e star. The EUVE spectra are dominated by emission lines formed at temperatures in excess of 107 K. The observation is unique as we have detected, for the first time, a strong Lyman continuum in the EUVE long-wavelength range (320-650 ?). The flare in the continuum (T ? 20,000-30,000 K) was extremely short, lasting for less than 500 s, while in the DS (T ? 107 K) its duration was ?28 ks. The total energy of the flare in the DS is ~3 ? 1034 ergs. We have made a fit to the continuum using semiempirical model atmospheres and derived the time-averaged temperature and density structures.

Opacity in the upper atmosphere of AU Mic

In this paper we investigate the validity of the optically thin assumption in the transition region of the late-type star AU Mic. We use Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of the Ciii multiplet and Ovi resonance lines, hence yielding information at two dierent levels within the atmosphere. Significant deviations from the optically thin fluxes are found for Ciii in both quiescent and flare spectra, where only 60% of the flux is actually observed. This could explain the apparent deviation of Ciii observed in emission measure distributions. We utilize escape probabilities for both homogeneous and inhomogeneous geometries and calculate optical depths as high as 10 for the Ciii 1175.71 A component of the multiplet. Using a lower limit to the electron density (10 11 cm 3 )w e derive an eective thickness of<100 km for the scattering layer. The emission originates from very small and compact regions, consistent with a filling factor of 10 5 derived for the flare plasma.

Flare X-Ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity

X-ray spectra of the late-type star AB Dor obtained with the XMM-Newton satellite are analyzed. AB Dor was particularly active during the observations. An emission measure reconstruction technique is employed to analyze flare and quiescent spectra, with emphasis on the Fe XVII 15-17 A wavelength region. The Fe XVII 16.78 A/15.01 A line ratio increases significantly in the hotter flare plasma. This change in the ratio is opposite to the theoretical predictions and is attributed to the scattering of 15.01 A line photons from the line of sight. The escape probability technique indicates an optical depth of ≈0.4 for the 15.01 A line. During the flare, the electron density is 4.4 × 1010 cm-3, and the fractional Fe abundance is 0.5 ± 0.1 of the solar photospheric value. Using these parameters, a path length of ≈8000 km is derived. There is no evidence of opacity in the quiescent X-ray spectrum of the star.

Kelvin-Helmholtz instability in solar chromospheric jets: theory and observation

Using data obtained by the high resolution CRisp Imaging SpectroPolarimeter instrument on the Swedish 1-m Solar Telescope, we investigate the dynamics and stability of quiet-Sun chromospheric jets observed at disk center. Small-scale features, such as Rapid Redshifted and Blueshifted Excursions, appearing as high speed jets in the wings of the H

Fe x Emission Lines in Solar and Stellar Spectra

\alpha

Extreme-Ultraviolet Flares in an F2 Star

line, are characterized by short lifetimes and rapid fading without any descending behavior. To study the theoretical aspects of their stability without considering their formation mechanism, we model chromospheric jets as twisted magnetic flux tubes moving along their axis, and use the ideal linear incompressible magnetohydrodynamic approximation to derive the governing dispersion equation. Analytical solutions of the dispersion equation indicate that this type of jet is unstable to Kelvin-Helmholtz instability (KHI), with a very short (few seconds) instability growth time at high upflow speeds. The generated vortices and unresolved turbulent flows associated with the KHI could be observed as broadening of chromospheric spectral lines. Analysis of the H