A brief overview is given of some of the current outstanding problems in solar physics with greatest emphasis on high energy phenomena in the atmosphere. The importance of plasma kinetic effects, as well as MHD, in understanding the complex finely structured and dynamic solar atmospheric plasma is stressed. Key results from the RHESSI Mission on energetic flare particle acceleration, propagation, and flare energy budgets are presented as are recent findings concerning the solar and stellar flare Neupert effect and the possible role of energetic particles in micro-events in the ‘non-flaring’ sun. Finally, evidence showing that magnetic fields are also important in hot star phenomena is mentioned.

The study of stellar activity is now an almost classical astronomical topic. The first Ca ii-H&K observations were made a hundred years ago by Eberhard & Schwarzschild1 and many thousand papers were published after its rediscovery some three decades ago by O. C. Wilson. The complexity of the atmospheric and interior magnetic activity as observed on the Sun is hard, if not impossible, to extrapolate to solar-type stars. So far there is no solar twin found, despite that it appears that just a single process acts as the driving mechanism for activity in all atmospheric layers and partially even in the convective envelope: the dynamodriven magnetic field. In this paper, I will try to give examples where the solar analogy holds and where it is clearly not appropriate, putting some emphases on differential surface rotation and meridional circulation. I stress the importance of mapping stellar surfaces as fingerprints of the underlying dynamo action and directly measure surface magnetic fields.

We present spectroscopic analysis of ∼1000 stars on the Lick, Keck and AAT planet search projects. This analysis provides a quantitative, and unbiased correlation between metallicity and the rate of occurrence of detected gas giant planets with orbital periods shorter than three years. As stellar metallicity increases, the occurrence of planets increases. Stars with [Fe/H] that is one third of solar only have gas giants detected ∼ 3% of the time. Stars with solar metallicity have a planet occurrence rate of 5 − 10%. The occurrence of gas giant planets rises to 20% in stars with a metallicity that is three times solar.

At issue is whether the quantitative dependence of planet occurrence on metallicity is primarily an initial condition, or a by-product of accretion of gas-depleted material onto the convective zone of the star. Accretion could be distinguished as the underlying mechanism for enhanced metallicity if: 1) planet-bearing F-type stars with thinner convective envelopes show a higher mean metallicity than planet-bearing G- or K-type stars, or 2) planet-bearing sub-giants with diluted convective zones showed statistically lower metallicity than their main sequence counterparts.

The primary scientific objective of RHESSI Small Explorer mission is to investigate the physics of particle acceleration and energy release in solar flares, through imaging and spectroscopy of X-ray/gamma-ray continuum and gamma-ray lines emitted by accelerated electrons and ions, respectively. Here we briefly describe the mission and summarize the new solar observations, including the first hard X-ray imaging spectroscopy, the first high resolution spectroscopy of solar gamma-ray lines, and the first imaging of solar gamma-ray lines and continuum.

The European γ-ray satellite INTEGRAL, launched on October 17, 2002, is the successor to the highly successful American satellite Compton-GRO. Even though its main observational program focuses on “classical” high-energy sources like compact X-ray binaries or AGNs, some time is being devoted to γ-ray studies of massive stars and magnetically active late-type stars. We briefly describe here the four instruments of INTEGRAL, and summarize the ongoing stellar programs.

The Submillimeter Array (SMA) is nearing completion in “millimeter” valley just below the summit of Mauna Kea in Hawaii. Numerous astronomical observations have been undertaken in the partially transparent bands between 0.45 and 1.3 mm. The status of the instrument is described in this paper, and examples of images that have been made with the partially completed Array of the dust and gas envelopes of a high mass protostellar object (IRAS 18089—1732), a low mass young star (LkCa15), and three evolved stars (CRL 2688, V Hydrae, and IRC+10216) are presented.

We have conducted K band interferometric observations of four nearby main-sequence Vega-like stars at the VLTI with very long baselines. The very high resolution allowed us to probe the innermost region of the disks, where planets are supposed to be formed. The diameters of three bright and nearby prototypes β Pictoris, Fomalhaut (α PsA) and ∊ Eridani as well as τ Ceti have been measured with VINCI, the VLTI commissioning instrument, with a high accuracy. The derived diameters were used to constrain their age with help of the evolution code CESAM. The precision achieved with VINCI allowed us to discuss the shape of their photosphere and the possible detection of warm circumstellar material within the narrow interferometric field of view.

Main Sequence (MS) stars are by far the most numerous class in the Universe. They are often somewhat neglected as they are relatively quiet objects (but exceptions exist), though they bear testimony of the past and future of our Sun. An important characteristic of the MS stars, particularly the solar-type ones, is that they host the large majority of the known extrasolar planets. Moreover, at the bottom of the MS, the red M dwarfs pave the way to understanding the physics of brown dwarfs and giant planets. We have measured very precise angular diameters from recent VINCI/VLTI interferometric observations of a number of MS stars in the K band, with spectral types between A1V and M5.5V. They already cover a wide range of effective temperatures and radii. Combined with precise Hipparcos parallaxes, photometry, spectroscopy as well as the asteroseismic information available for some of these stars, the angular diameters put strong constraints on the detailed models of these stars, and therefore on the physical processes at play.

In this short paper, I summarize Chandra observations of various objects in the Solar System. In addition to the Moon and several comets, Chandra has observed all the planets between Venus and Uranus. With one exception, all have been clearly detected.

The ability of the Transition Region and Coronal Explorer (TRACE) to image the Sun at high spatial resolution and high cadence over a very broad range of temperatures makes it a unique instrument for observing solar flare plasma. TRACE observations have confirmed the reconnection model for solar flares, at least qualitatively. TRACE flare observations show impulsive footpoint brightenings that are followed by the formation of high-temperature loops in the corona. These loops then cool to lower temperatures, forming post-flare loop arcades. Comparisons between TRACE and lower spatial resolution Yohkoh Soft X-Ray Telescope (SXT) observations have revealed that solar flares are composed of a multitude of fine coronal loops. Detailed hydrodynamic modeling of flare light curves shows that this fine scale structuring is crucial to understanding the evolution of the observed emission. Models based on single, isothermal loops are not consistent with the TRACE observations. Models based on the sequential heating of small-scale loops, in contrast, are able to reproduce many of the salient features of the observed light curves. We will discuss the implication of these results for more energetic stellar flares as well as smaller-scale events that may be responsible for the heating of solar active region loops.

The solar chromosphere is a very dynamic and highly structured environment. In contrast to the higher density hydrodynamically controlled photosphere, this environment is controlled by magnetohydrodynamic forces. The lower density chromosphere encompasses the transition from optically thick to thin so that Non-LTE radiative transfer applies. While the omni-presence of emission in UV lines indicates that a chromospheric rise in temperature from the cool photosphere to the hot corona should occur everywhere on the solar surface, the presence of cool material at chromospheric heights observed in infrared CO lines has made us reconsider the validity of using this interpretation of the observed spectrum based on average one-dimensional models for chromospheric thermal structure. It can be shown that the mean thermal properties of spatially inhomogeneous and/or time-dependent atmospheres in fact are not well represented by an average one-dimensional semi-empirical model based on the mean spectrum. Observations of CO lines show that the coolest locations in the upper photosphere/lower chromosphere are associated with the strong adiabatic expansion that occurs over granules. Simulations that try to model chromospheric dynamics should therefore probably include the effects of solar convection, wave dynamics, as well as the effect of radiative cooling in the multitude of infrared CO lines on the thermal balance of the atmosphere.

Heating and dynamics of the solar as well as stellar chromospheres are separate phenomena, judging by their spatial and temporal characteristics in the Sun. Simulations of chromospheric heating in late-type stars as well as simulations of the dynamics of the large-amplitude oscillations of internetwork calcium bright points in the solar atmosphere reproduce the main features of the respective phenomena. Differences between models and observations imply (1) that the Sun has a permanent, hot chromosphere and that its temperature structure cannot be obtained by averaging the time-dependent temperature profile of a model that describes only the oscillations, and (2) that the acoustic waves causing the dynamics and the heating propagate upward as spherical waves.

We investigate the role of chromospheric fine structures, e.g. mottles (spicules), in the mass balance and heating of the solar atmosphere by studying two-dimensional high-resolution Hα observations. The temporal and spatial variations of the line-of-sight (LOS) velocity, obtained with an inversion technique based on a cloud model, provide strong indications that the mechanism responsible for the driving of the observed flows is magnetic reconnection. Apart from the LOS velocity, application of the cloud model enables the derivation of several other physical parameters, like pressure, temperature, density etc. Mean values of these parameters permit the estimation of the role of these structures in the mass balance of the solar atmosphere. They, furthermore, permit a reasonable estimate of the energy provided by magnetic reconnection which is available for the heating of the solar corona.

Using a new Sun-as-a-star approach we analyze sunspot group data for the past 120 years and reveal that sunspots are formed preferably in two persistent migrating active longitudes 180° apart. Their migration is determined by changes of the mean latitude of sunspots and the surface differential rotation. The two active regions periodically alternate being the dominant region with a period of about 3.7 years similar to the “flip-flop” phenomenon known in starspot activity. The fact that the Sun shows the same pattern of magnetic activity as highly active stars strengthens the solar paradigm for magnetic activity on cool stars.

We discuss how gamma-ray spectroscopy provides information about the acceleration and transport of electrons and ions in solar flares, and their interaction with the solar atmosphere. Temporal studies illuminate differences in the acceleration and transport of electrons and ions. Nuclear line studies reveal the elemental abundance, density, and temperature of the ambient solar atmosphere; and the spectrum, composition, and directionality of the accelerated ions.

Energetic phenomena on the Sun and late-type stars pose a number of fascinating puzzles. These include coronal heating, flares, and coronal mass ejections, all believed to be manifestations of magnetic energy release. Radio radiation is a sensitive tracer of energetic phenomena on both the Sun and stars. Radio observations of the Sun over the past decade have produced new insights into the physics of magnetic energy release in flares and coronal mass ejections. Radio observations of late-type stars have exploited sensitive imaging and spectroscopic techniques to further constrain the nature of the relevant emission mechanisms. A surprise has been the recent discovery of radio emission from brown dwarf stars, implying the existence of substantial magnetic fields and a means of dissipating magnetic energy, neither of which are understood.

Coronal structure and coronal heating are intimately related in magnetically active stars. Coronal structure is commonly inferred from radio interferometry and from eclipse and rotational modulation studies. We discuss to what extent flares may be responsible for coronal structure and global observable properties in magnetically active stars.

Particle beam bombardment on the solar chromosphere produces non-thermal ionization and excitation. The effect on hydrogen lines is investigated by using non-LTE theory and semi-empirical flare models. It has been found that in the case of electron bombardment, the Hα line is widely broadened and enhanced. Significant enhancements at the wings of Lyα and Lyβ lines are also predicted. In the case of proton bombardment, less strong broadening and less central reversal are expected. We found that the total energy flux of the particle beam and the atmospheric condition give much influence on the line profiles, which, however, are less sensitive to the power index. Based on the Hα line profile measurement, a method to deduce the total energy flux of the particle beam is proposed.