Photospheric Active Regions Research Articles

Generation and annihilation of three dimensional magnetic nulls in extrapolated solar coronal magnetic field: data-based Implicit Large Eddy simulation

Three-dimensional (3D) magnetic nulls are the points where magnetic field vanishes and are preferential sites for magnetic reconnection: a fundamental process which converts magnetic energy into kinetic energy, heat, and energy of non-thermal particles along with a rearrangement of magnetic field lines. Reconnection is ubiquitous in nature and plays a major role in various magnetically confined laboratory and space/astrophysical plasmas. In the solar corona, the reconnection manifests as coronal transients including solar flares, coronal mass ejections and coronal jets—often associated with 3D nulls. The nulls are generally found to be collocated with complex active regions on the solar photosphere and merits further attention, particularly in terms of their generation. A recent idealized magnetohydrodynamics simulation initiated with an analytically constructed preexisting proper radial null has identified magnetic reconnection to be responsible for spontaneous generation of these 3D nulls. It is then imperative to further explore the plausibility of spontaneous generation of nulls in naturally occurring plasmas, identify the mechanism and verify the outcome vis-à-vis observations. An apt test bed for such an initiative is the solar atmosphere, as abundant space and ground-based observations are available. In the above backdrop, the paper attempts to investigate 3D null generation by carrying out a data-based simulation of a C6.6 class flare associated with the photospheric active region NOAA 11 977. The simulation confirms spontaneous pairwise generation of 3D nulls with magnetic reconnections as the underlying cause. Importantly, magnetic field lines associated with the spontaneously generated nulls are found to trace observed chromospheric bright points—highlighting their observational relevance. Overall, such spontaneous generation and annihilation of nulls through magnetic reconnections opens up a new avenue for solar coronal and chromospheric heating.

A Stacking Search for Gamma-Ray Emission from Nearby Flare Stars and the Periodic Source TVLM 513-46546

Abstract So far, the Sun is the only isolated main-sequence star detected in gamma-rays, particularly during powerful flares. Young ultracool dwarfs are far more active so they are also plausible gamma-ray sources. We performed a spatial stack of 97 of the nearest X-ray and radio flare stars to search for GeV emission using nearly 12 yr of data from the Fermi Gamma-ray Space Telescope. The stacked residual maps showed no significant signal. Modeling the upper limits indicates a peak stellar flux at least a factor of 7 below the noise level. We also analyze the phase-folded light curve of the rapidly rotating radio star TVLM 513-46546, report a tentative (TS = 30) pulsed signal, and refine its period. We examine the possibility of a false-positive signal by analyzing nearby Fermi catalog sources and test fields, and by repeating the analysis using different periods. No other periodic signals are found, despite clear detections of the catalog sources, and the TS value for TVLM 513 increases systematically to the optimal period. The putative gamma-ray signal is nearly in phase with the optical peak, and out of phase with the radio pulses by 0.4 ± 0.05 rotations. These results argue for emission from relativistic protons streaming down flux tubes toward the photospheric active regions. The protons colliding with the atmosphere create neutral pions that decay into gamma-ray photons. This would be the first detection of a normal, isolated star in gamma-rays, and the strongest evidence yet for proton acceleration in stellar magnetospheres.

CSI 2264: Simultaneous optical and X-ray variability in the pre-main sequence stars of NGC 2264

Context. Pre-main sequence stars are variable sources. In diskless stars this variability is mainly due to the rotational modulation of dark photospheric spots and active regions, as in main sequence stars even if associated with a stronger magnetic activity. Aims. We aim at analyzing the simultaneous optical and X-ray variability in these stars to unveil how the activity in the photosphere is connected with that in the corona, to identify the dominant surface magnetic activity, and to correlate our results with stellar properties, such as rotation and mass. Methods. We analyzed the simultaneous optical and X-ray variability in stars without inner disks (e.g., class III objects and stars with transition disks) in NGC 2264 from observations obtained with Chandra/ACIS-I and CoRoT as part of the Coordinated Synoptic Investigation of NGC 2264. We searched for those stars whose optical and X-ray variability is correlated, anti-correlated, or not correlated by sampling their optical and X-ray light curves in suitable time intervals and studying the correlation between the flux observed in optical and in X-rays. We then studied how this classification is related with stellar properties. Results. Starting from a sample of 74 class III/transition disk (TD) stars observed with CoRoT and detected with Chandra with more than 60 counts, we selected 16 stars whose optical and X-ray variability is anti-correlated, 11 correlated, and 17 where there is no correlation. The remaining stars did not fall in any of these groups. We interpreted the anti-correlated optical and X-ray variability as typical of spot-dominated sources, due to the rotational modulation of photospheric spots spatially coincident to coronal active regions, and correlated variability typical of faculae-dominated sources, where the brightening due to faculae is dominant over the darkening due to spots. Conclusions. Stars with “anti-correlated” variability rotate slower and are less massive than those with “correlated” variability. Furthermore, cool stars in our sample have larger u − r variability than hot stars. This suggests that there is a connection between stellar rotation, mass, and the dominant surface magnetic activity, which may be related with the topology of the large-scale magnetic field. We thus discuss this scenario in the framework of the complex magnetic properties of weak-line T Tauri stars observed as part of recent projects.

Simulations of 3D Magnetic Merging: Resistive Scalings for Null Point and QSL Reconnection

Starting from an exact, steady-state, force-free solution of the magnetohydrodynamic (MHD) equations, we investigate how resistive current layers are induced by perturbing line-tied three-dimensional magnetic equilibria. This is achieved by the superposition of a weak perturbation field in the domain, in contrast to studies where the boundary is driven by slow motions, like those present in photospheric active regions. Our aim is to quantify how the current structures are altered by the contribution of so called quasi-separatrix layers (QSLs) as the null point is shifted outside the computational domain. Previous studies based on magneto-frictional relaxation have indicated that, despite the severe field line gradients of the QSL, the presence of a null is vital in maintaining fast reconnection. Here, we explore this notion using highly resolved simulations of the full MHD evolution. We show that for the null-point configuration, the resistive scaling of the peak current density is close to $J\sim\eta^{-1}$, while the scaling is much weaker, i.e. $J\sim\eta^{-0.4}$, when only the QSL connectivity gradients provide a site for the current accumulation.

Polar Field Reversals and Active Region Decay

We study the relationship between polar field reversals and decayed active region magnetic flux. Photospheric active region flux is dispersed by differential rotation and turbulent diffusion, and is transported poleward by meridional flows and diffusion. We summarize the published evidence from observation and modeling of the influence of meridional flow variations and decaying active region flux’s spatial distribution, such as the Joy’s law tilt angle. Using NSO Kitt Peak synoptic magnetograms covering cycles 21–24, we investigate in detail the relationship between the transport of decayed active region flux to high latitudes and changes in the polar field strength, including reversals in the magnetic polarity at the poles. By means of stack plots of low- and high-latitude slices of the synoptic magnetograms, the dispersal of flux from low to high latitudes is tracked, and the timing of this dispersal is compared to the polar field changes. In the most abrupt cases of polar field reversal, a few activity complexes (systems of active regions) are identified as the main cause. The poleward transport of large quantities of decayed trailing-polarity flux from these complexes is found to correlate well in time with the abrupt polar field changes. In each case, significant latitudinal displacements were found between the positive and negative flux centroids of the complexes, consistent with Joy’s law bipole tilt with trailing-polarity flux located poleward of leading-polarity flux. The activity complexes of the cycle 21 and 22 maxima were larger and longer-lived than those of the cycle 23 and 24 maxima, and the poleward surges were stronger and more unipolar and the polar field changes larger and faster. The cycle 21 and 22 polar reversals were dominated by only a few long-lived complexes whereas the cycle 23 and 24 reversals were the cumulative effects of more numerous, shorter-lived regions. We conclude that sizes and lifetimes of activity complexes are key to understanding the diversity of polar reversals.

Measuring stellar differential rotation with high-precision space-borne photometry

We introduce a method of measuring a lower limit to the amplitude of surface differential rotation from high-precision, evenly sampled photometric time series. It is applied to main-sequence late-type stars whose optical flux modulation is dominated by starspots. An autocorrelation of the time series was used to select stars that allow an accurate determination of starspot rotation periods. A simple two-spot model was applied together with a Bayesian information criterion to preliminarily select intervals of the time series showing evidence of differential rotation with starspots of almost constant area. Finally, the significance of the differential rotation detection and a measurement of its amplitude and uncertainty were obtained by an a posteriori Bayesian analysis based on a Monte Carlo Markov Chain approach. We applied our method to the Sun and eight other stars for which previous spot modelling had been performed to compare our results with previous ones. We find that autocorrelation is a simple method for selecting stars with a coherent rotational signal that is a prerequisite for successfully measuring differential rotation through spot modelling. For a proper Monte Carlo Markov Chain analysis, it is necessary to take the strong correlations among different parameters that exist in spot modelling into account. For the planet-hosting star Kepler-30, we derive a lower limit to the relative amplitude of the differential rotation of \Delta P / P = 0.0523 \pm 0.0016. We confirm that the Sun as a star in the optical passband is not suitable for measuring differential rotation owing to the rapid evolution of its photospheric active regions. In general, our method performs well in comparison to more sophisticated and time-consuming approaches.

Doppler-beaming in theKeplerlight curve of LHS 6343 A

Context. Kepler observations revealed a brown dwarf eclipsing the M-type star LHS 6343 A with a period of 12.71 days. In addition, an out-of-eclipse light modulation with the same period and a relative semi-amplitude of 2 x 10^-4 was observed showing an almost constant phase lag to the eclipses produced by the brown dwarf. In a previous work, we concluded that this was due to the light modulation induced by photospheric active regions in LHS 6343 A. Aims. In the present work, we prove that most of the out-of-eclipse light modulation is caused by the Doppler-beaming induced by the orbital motion of the primary star. Methods. We introduce a model of the Doppler-beaming for an eccentric orbit and also considered the ellipsoidal effect. The data were fitted using a Bayesian approach implemented through a Monte Carlo Markov chain method. Model residuals were analysed by searching for periodicities using a Lomb-Scargle periodogram. Results. For the first seven quarters of Kepler observations and the orbit previously derived from the radial velocity measurements, we show that the light modulation of the system outside eclipses is dominated by the Doppler-beaming effect. A period search performed on the residuals shows a significant periodicity of 42.5 +- 3.2 days with a false-alarm probability of 5 x 10^-4, probably associated with the rotational modulation of the primary component.

MAGNETIC NONPOTENTIALITY IN PHOTOSPHERIC ACTIVE REGIONS AS A PREDICTOR OF SOLAR FLARES

Based on several magnetic nonpotentiality parameters obtained from the vector photospheric active region magnetograms obtained with the Solar Magnetic Field Telescope at the Huairou Solar Observing Station over two solar cycles, a machine learning model has been constructed to predict the occurrence of flares in the corresponding active region within a certain time window. The Support Vector Classifier, a widely used general classifier, is applied to build and test the prediction models. Several classical verification measures are adopted to assess the quality of the predictions. We investigate different flare levels within various time windows, and thus it is possible to estimate the rough classes and erupting times of flares for particular active regions. Several combinations of predictors have been tested in the experiments. The True Skill Statistics are higher than 0.36 in 97% of cases and the Heidke Skill Scores range from 0.23 to 0.48. The predictors derived from longitudinal magnetic fields do perform well, however they are less sensitive in predicting large flares. Employing the nonpotentiality predictors from vector fields improves the performance of predicting large flares of magnitude $\geq$M5.0 and $\geq$X1.0.

The Heliophysics Feature Catalogue, a tool for the study of solar features

AbstractThe behavior of filaments and prominences during the Solar Cycle is a signature of Sun's activity. It is therefore important to follow their evolution during the cycle, in order to be able to associate it with the various phases of the Solar Cycle as well as with other Solar features or events. The virtual observatory HELIO provides information that can be used for such studies, especially its Heliophysics Feature Catalogue gives a unique access to the description of various features during around one cycle. Features available are: filaments, prominences, photospheric and coronal active regions, coronal radio emission, type III radio bursts, coronal holes and sunspots. Web interfaces allow the user to query data for these features. Useful information can also be shared with other HELIO services, such as Heliophysics Event Catalogue, which provides access to dozens of tables of events such as flares, CMEs, . . .

Starspot activity and rotation of the planet-hosting star Kepler-17

Context. Kepler-17 is a G2V sun-like star accompanied by a transiting planet with a mass of ~2.5 Jupiter masses and an orbital period of 1.486 d, recently discovered by the Kepler space telescope. This star is highly interesting as a young solar analogue. Aims. We used about 500 days of high-precision, high-duty-cycle optical photometry collected by Kepler to study the rotation of the star and the evolution of its photospheric active regions. Methods. We applied a maximum-entropy light curve inversion technique to model the flux rotational modulation induced by active regions that consist of dark spots and bright solar-like faculae with a fixed area ratio. Their configuration was varied after a fixed time interval to take their evolution into account. Active regions were used as tracers to study stellar differential rotation, and planetary occultations were used to constrain the latitude of some spots. Results. Our modelling approach reproduces the light variations of Kepler-17 with a standard deviation of the residuals comparable with the precision of Kepler photometry. We find several active longitudes where individual active regions appear, evolve, and decay with lifetimes comparable to those observed in the Sun, although the star has a spotted area ~10-15 times larger than the Sun at the maximum of the 11-yr cycle. Kepler-17 shows a solar-like latitudinal differential rotation, but the fast spot evolution prevents a precise determination of its amplitude. Moveover, the star shows a cyclic variation of the starspot area with a period of 47.1 \pm 4.5 d, particularly evident during the last 200 days of the observations, similar to the solar Rieger cycles. Possible effects of the close-in massive planet on stellar photospheric activity cannot be excluded, but require a long-term monitoring to be unambiguously detected.

Photospheric activity, rotation, and star-planet interaction of the planet-hosting star CoRoT-6

The CoRoT satellite has recently discovered a hot Jupiter that transits across the disc of a F9V star called CoRoT-6 with a period of 8.886 days. We model the photospheric activity of the star and use the maps of the active regions to study stellar differential rotation and the star-planet interaction. We apply a maximum entropy spot model to fit the optical modulation as observed by CoRoT during a uninterrupted interval of about 140 days. Photospheric active regions are assumed to consist of spots and faculae in a fixed proportion with solar-like contrasts. Individual active regions have lifetimes up to 30-40 days. Most of them form and decay within five active longitudes whose different migration rates are attributed to the stellar differential rotation for which a lower limit of \Delta \Omega / \Omega = 0.12 \pm 0.02 is obtained. Several active regions show a maximum of activity at a longitude lagging the subplanetary point by about 200 degrees with the probability of a chance occurrence being smaller than 1 percent. Our spot modelling indicates that the photospheric activity of CoRoT-6 could be partially modulated by some kind of star-planet magnetic interaction, while an interaction related to tides is highly unlikely because of the weakness of the tidal force.

Photometric and spectroscopic observations of three rapidly rotating late‐type stars: EY Dra, V374 Peg, and GSC 02038‐00293

AbstractHere, BV (RI)C broad band photometry and intermediate resolution spectroscopy in Hα region are presented for two rapidly rotating late‐type stars: EY Dra and V374 Peg. For a third rapid rotator, GSC 02038‐00293, intermediate resolution Hα spectroscopy and low resolution spectroscopy are used for spectral classification and stellar parameter investigation of this poorly known object. The low resolution spectrum of GSC 02038‐00293 clearly indicates that it is a K‐type star. Its intermediate resolution spectrum can be best fitted with a model with Teff = 4750 K and v sin i = 90 km s–1, indicating a very rapidly rotating mid‐K star. The Hα line strength is variable, indicating changing chromospheric emission on GSC 02038‐00293. In the case of EY Dra and V374 Peg, the stellar activity in the photosphere is investigated from the photometric observations, and in the chromosphere from the Hα line. The enhanced chromospheric emission in EY Dra correlates well with the location of the photospheric active regions, indicating that these features are spatially collocated. Hints of this behaviour are also seen in V374 Peg, but it cannot be confirmed from the current data. The photospheric activity patterns in EY Dra are stable during one observing run lasting several nights, whereas in V374 Peg large night‐tonight variations are seen. Two large flares, one in the Hα observations and one from the broadband photometry, and twelve smaller ones were detected in V374 Peg during the observations spanning nine nights. The energy of the photometrically detected largest flare is estimated to be 4.25 × 1031– 4.3 × 1032 erg, depending on the waveband. Comparing the activity patterns in these two stars, which are just below and above the mass limit of full convection, is crucial for understanding dynamo operation in stars with different internal structures (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The chromospherically active binary CF Tuc revisited

This paper presents results derived from analysis of new spectroscopic and photometric observations of the chromospherically active binary system CF Tuc. New high-resolution spectra, taken at the Mt. John University Observatory in 2007, were analysed using two methods: cross-correlation and Fourier-based disentangling. As a result, new radial velocity curves of both components were obtained. The resulting orbital elements of CF Tuc are a1 sin i= 0.0254 ± 0.0001 au, a2 sin i= 0.0228 ± 0.0001 au, M1 sin i= 0.902 ± 0.005 M⊙ and M2 sin i= 1.008 ± 0.006 M⊙. The cooler component of the system shows Hα and Ca ii H&K emissions. Using simultaneous spectroscopic and photometric observations, an anticorrelation between the Hα emission and the BV light curve maculation effects was found. This behaviour indicates a close spatial association between photospheric and chromospheric active regions. Our spectroscopic data and recent BV light curves were solved simultaneously using the Wilson–Devinney code. A dark spot on the surface of the cooler component was assumed to explain large asymmetries observed in the light curves. The following absolute parameters of the components were determined: M1= 1.11 ± 0.01 M⊙, M2= 1.23 ± 0.01 M⊙, R1= 1.63 ± 0.02 R⊙, R2= 3.60 ± 0.02 R⊙, L1= 3.32 ± 0.51 L⊙ and L2= 3.91 ± 0.84 L⊙. The primary component has an age of about 5 Gyr and is approaching its main-sequence terminal age. The distance to CF Tuc was calculated to be 89 ± 6 pc from the dynamic parallax, neglecting interstellar absorption, in agreement with the Hipparcos value. The orbital period of the system was studied using the O−C analysis. The O−C diagram could be interpreted in terms of either two abrupt changes or a quasi-sinusoidal form superimposed on a downward parabola. These variations are discussed by reference to the combined effect of mass transfer and mass loss, the Applegate mechanism and also a light-time effect due to the existence of a third body in the system.

Spots, plages, and flares onλAndromedae and II Pegasi

Aims. We present the results of a contemporaneous photometric and spectroscopic monitoring of two RS CVn binaries, namely λ And and II Peg. The aim of this work is to investigate the behavior of surface inhomogeneities in the atmospheres of the active components of these systems that have nearly the same temperatures but different gravities. Methods. The light curves and the modulation of the surface temperature, as recovered from line-depth ratios (LDRs), were used to map the photospheric spots, while the Hα emission was used as an indicator of chromospheric inhomogeneities. The spot temperatures and sizes were derived from a spot model applied to the contemporaneous light and temperature curves. Results. We find larger and cooler spots on II Peg (Tsp � 3600 K) than on λ And (Tsp � 3900 K); this could be the result of both the difference in gravity and the higher activity level of the former. Moreover, we find a clear anti-correlation between the Hα emission and the photospheric diagnostics (temperature and light curves). We have detected a modulation in the intensity of the He i D3 line with the star rotation, suggesting surface features also in the upper chromosphere of these stars. A rough reconstruction of the 3D structure of their atmospheres was also performed by applying a spot/plage model to the light and temperature curves and to the Hα flux modulation. In addition, a strong flare affecting the Hα ,t he Hei D3, and the cores of Na i D1,2 lines has been observed on II Peg. Conclusions. The spot/plage configuration has been reconstructed in the visible component of λ And and II Peg, which have nearly the same temperature but very different gravities and rotation periods. A close spatial association of photospheric and chromospheric active regions, at the time of our observations, was found in both stars. Larger and cooler spots were found on II Peg, the system with the active component of higher gravity and a higher activity level. The area ratio of plages to spots seems to decrease when the spots get bigger. Moreover, with both this and literature data, a correlation between the temperature difference ∆T = Tph −Tsp and the surface gravity is also suggested.

Photospheric and Chromospheric Active Regions in Four Young Solar‐Type Stars

We present a photometric and spectroscopic study of four G-K dwarfs, namely HD 166, Eri, ?1 Ori, and ?1 Cet. In three cases, we find a clear spatial association between photospheric and chromospheric active regions. For ?1 Ori we do not find appreciable variations of photospheric temperature or chromospheric H? emission. We applied a spot/plage model to the observed rotational modulation of temperature and flux to derive spot/plage parameters and to reconstruct a rough three-dimensional map of the outer atmosphere of ?1 Cet, HD 166, and Eri.

Comparing different approaches to model the rotational modulation of the Sun as a star

Context. The space missions MOST, COROT and Kepler are going to provide us with high-precision optical photometry of solar-like stars with time series extending from tens of days to several years. They can be modelled to obtain information on stellar magnetic activity by fitting the rotational modulation of the stellar flux produced by the brightness inhomogeneities associated with photospheric active regions. Aims. The variation of the total solar irradiance provides a good proxy for those photometric time series and can be used to test the performance of different spot modelling approaches. Methods. We test discrete spot models as well as maximum entropy and Tikhonov regularized spot models by comparing the reconstructed total sunspot area variation and longitudinal distributions of sunspot groups with those actually observed in the Sun along activity cycle 23. Appropriate statistical methods are introduced to measure model performance versus the timescale of variation. Results. The maximum entropy regularized spot models show the best agreement with solar observations reproducing the total sunspot area variation on time scales ranging from a few months to the activity cycle, although the model amplitudes are affected by systematic errors during the minimum and the maximum activity phases. The longitudinal distributions derived from the models compare well with the observed sunspot group distributions except during the minimum of activity, when faculae dominate the rotational modulation. The resolution in longitude attainable through the spot modelling is ~$60^{\circ}$, on average. Conclusions. The application of the maximum entropy modelling to solar analogues will provide us with a quite detailed picture of their photospheric magnetic activity that can be the base for comparative and evolutionary studies of solar-like magnetic activity and irradiance variations.

Magnetic helicity and flux tube dynamics in the solar convection zone: Comparisons between observation and theory

Magnetic helicity, a conserved topological parameter in ideal MHD systems, conditions close to which are realized in the solar plasma, is intimately connected to the creation and subsequent dynamics of magnetic flux tubes in the solar interior. It can therefore be used as a tool to probe such dynamics. In this paper we show how photospheric observations of magnetic helicity of isolated magnetic flux tubes, manifested as the twist and writhe of solar active regions, can constrain the creation and dynamics of flux tubes in the solar convection zone and the nature of convective turbulence itself. We analyze the observed latitudinal distribution of twists in photospheric active regions, derived from solar vector magnetograms, in the largest such sample studied till‐date. We confirm and put additional constraints on the hemispheric twist helicity trend and find that the dispersion in the active region twist distribution is latitude‐independent, implying that the amplitude of turbulent fluctuations does not vary with latitude in the convection zone. Our data set also shows that the amplitude and dispersion of twist decreases with increasing magnetic size of active regions, supporting the conclusion that larger flux tubes are less affected by turbulence. Among the various theoretical models that have been proposed till‐date to explain the origin of twist, our observations best match the Σ effect model, which invokes helical turbulent buffeting of rising flux tubes as the mechanism for twist creation. Finally, we complement our analysis of twists with past observations of tilts in solar active regions and tie them in with theoretical modeling studies, to build up a comprehensive picture of the dynamics of twisted magnetic flux tubes throughout the solar convection zone. This general framework, binding together theory and observations, suggests that flux tubes have a wide range of twists in the solar convection zone, with some as high as to make them susceptible to the kink instability mechanism that results in the formation of δ spot or non‐Hale active regions.

Photospheric and chromospheric active regions on three single-lined RS CVn binaries

A monitoring of three active RS CVn binaries has been performed with medium resolution spectroscopy with the aim of investigating the behavior of chromospheric and photospheric inhomogeneities. Surface temperature, as recovered from line-depth ratios (LDRs), allowed us to map the photospheric spots, while the H α emission has been used as an indicator of chromospheric inhomogeneities. We have found that the rotational modulation of the H α emission is always in anti-phase with the temperature wave, i.e. at the time of our observations active regions at chromospheric and photospheric levels are closely spatially associated in these active stars. The residual H α profiles, obtained as the difference between the observed spectra and non-active templates, are well reproduced by a two Gaussian fitting. The broad emission component, responsible for the wide emission wings in near all the spectra, is often blue-shifted with respect to the center of the stellar disk. The narrow H α emission displays a phase-dependent variation in all stars and is anti-correlated with the photospheric diagnostics, while the broad one displays no or little rotational modulation. We suggest that the broad emission component is mainly related to physical phenomena, like micro-flaring or strong chromospheric velocity fields, occurring all over the star disk, while the central narrow emission is more affected by chromospheric plages. We have also detected a modulation of the intensity of the $\ion{He}{i}$ D 3 line with the star rotation, suggesting surface features also in the upper chromosphere of these stars.

Vertical Lorentz Force and Cross‐Field Currents in the Photospheric Magnetic Fields of Solar Active Regions

We demonstrate that the vertical Lorentz force and a corresponding lower limit of the cross-field electric current density can be calculated from vector magnetograms of solar active regions obtained at a single height in the solar atmosphere, provided that the vertical gradient of the magnetic field strength is known at this height. We use a predicted vertical magnetic field gradient derived from a previous analysis. By testing various force-free solutions, we find that the numerical accuracy of our method is satisfactory. Applying the method to active region photospheric vector magnetograms, we find vertical Lorentz forces ranging from several hundredths to a few tenths of the typical photospheric gravitational force, and typical cross-field current densities up to several times 10 mA m-2. The typical vertical current density is found to be 2-3 times smaller, on the order of 10-15 mA m-2. These differences are above the associated uncertainties. The values of the cross-field currents decrease in an averaged vector magnetogram, but the ratio of the cross-field to the vertical current density increases, also above the uncertainties. We conclude that the photospheric active region magnetic fields are not force-free, contrary to the conjectures of some recent studies.

Relations between photospheric structure and X-ray emission

The relation between photospheric and coronal active regions in late–type stars is studied from two different points of departure. First, I report on 5 years of ROSAT X–ray monitoring of the active young K–star AB Dor. I compare the X–ray data with 16 years of V–band brightness monitoring showing a 10–year decline between 1978 and 1989 and a subsequent rise phase. Quite differently, the X–ray flux of AB Dor (while exhibiting strong variability on time scales of minutes to weeks) reveals no pronounced long–term trend over the 5 years of the program. This supports the concept of a saturated corona. Second, I present rotationally modulated ROSAT X–ray light curves of three active stars (AB Dor, CF Tuc, YY Men) and compare them with contemporaneous Doppler images. I demonstrate that it is possible to explain the X–ray light curves by coronal emission regions that are spatially related with photospheric active regions. I discuss the concept of X–ray bright loops connecting the major star spot complexes.