This meeting was dedicated to Jean-Louis Leroy, who pioneered polarimetry in French astronomy, and who is one of the leading experts in the field. He applied this diagnostics tool to better understand various astrophysical objects: the Sun, comets, stars, the interstellar medium. Some of his major achievements will be recalled here.

In this paper the present status of theoretical understanding - mostly relying on mean-field dynamo models - of global magnetic activity of late-type stars with convective envelopes is reviewed. The principles of the mean-field dynamo theory are introduced, and a discussion on some of the present-day problems and challenges is presented. The remaining sections are devoted to discuss the current status of solar and stellar dynamo models.

Fossil magnetic fields (i.e. fields that survive from one stage of stellar evolution to a significantly later epoch without being regenerated) have been invoked to explain phenomena in the Sun, magnetic CP stars, white dwarfs and neutron stars. The feasibility of such processes is discussed critically. Estimates are made that suggest that flux survival through pre-main sequence evolution is more likely to occur in stars with M > 1 M⊙. Any link between CP star and white dwarf magnetism is argued to favour the fossil field origin of CP star fields.

Because matter is most often in its plasma state, a strong coupling generally exists in the universe between flow dynamics and electromagnetic field. This is the reason why magnetic field plays such an important role in many astrophysical contexts, especially in sun and stars. Ideal MagnetoHydroDynamics can, to a certain extent, allow describing the magnetic field effects, but this elementary theory is unfortunately not of infinite validity: the understanding of phenomena as crucial as reconnection, dynamo effect, or plasma heating and acceleration for instance lies out of the scope of ideal MHD. For media sufficiently dense and for phenomena of sufficiently large scale with respect to the collision scales, resistive (and/or viscous) MHD has to be considered, which does not introduce much larger complexity. But for more dilute plasmas, or for shorter scale phenomena, resistive MHD is not valid either, and any interpretation must ground on more refined theories: Hall! -MHD, bi-fluid, multi-fluid, or fully kinetic. When applied, all these different theories do lead to quite different consequences. Because the observations from sun and stars can only rely on remote sensing techniques, it is generally quasi-impossible to discriminate between the different situations and decide which kind of theory is justified for interpreting correctly the phenomena. On the contrary, in the solar wind and the magnetospheres, in-situ measurements, from various experiments onboard many spacecraft, had provided a wealth of experimental results, often ruling out the pre-existing simplest interpretations and shedding some light on the reality of some phenomena like collisionless reconnection. The paper will try to display a few of such examples where solar wind and magnetosphere play a role of plasma laboratory for universal phenomena.

In the current paradigm of star formation magnetic fields play a very central role. Indeed, they probably help or even channel the initial gravitational collapse of the parent molecular cloud. But their most spectacular effect is certainly the production of ubiquitous, powerful, bipolar self-collimated jets. Also, in the last decade, evidence has come that in such systems a magnetosphere links the protostar to its surrounding accretion disc. I will first briefly review the magnetohydrodynamic models of jets from young stars. Then, I will discuss some constraints on the magnetospheric interaction since accretion must proceed despite strong stellar magnetic fields. Finally, I will introduce the 3Reconnection "X-wind" scenario that leads to episodic jet formation with efficient removal of protostellar angular momentum.

The dynamo equation is solved for the solar convection zone with the given ("observed") rotation law and positive α-effect. If the latter exists in the entire convection zone the resulting dynamo shows strong toroidal field belts in the polar region migrating equatorwards. The same happens for α concentrated at the bottom of the convection zone but then we get too many belts with higher amplitude. The cycle period is always too short. Including meridional circulation which is directed equatorwards at the bottom of the convection zone (where the eddy diffusivity is reduced), the amplitude of the toroidal field grows and the butterfly diagram reaches low-latitudes. The cycle time approaches the solar value. The dynamo regime is highly sensitive to the interplay between flow and diffusivity at the bottom of the convection zone. Stationary solutions are not very seldom. For less active stars a slight increase of the cycle period with the rotation period is observed in agreement with the decrease of the meridional flow for faster rotation.

During recent years, solar seismology comes into a new era due to the quantity and the quality of data. In particular, data sets spanning a solar cycle or an important fraction of this cycle become available, allowing a seismic investigation of this magnetic cycle. However, despite some interesting results, this investigation has to face serious limitations. On the other hand, some new analysis techniques in helioseismology provides unexpected and spectacular results on magnetic regions, maybe opening a new way to the understanding of the magnetic cycle.

Recent results on the properties of stellar oscillations in presence of rotation and magnetic field are presented. Their implication on the diagnostics of magnetic field in pulsating magnetic stars are given and illustrated with results on roAp stars.

A review of broad-band polarimetric observations is presented, from the first systematic observations of Jean-Louis Leroy in sunpots and active regions to the more recent applications concerning the polarization diagrams of chemically peculiar stars. The historical evolution of the interpretation is also discussed, with particular emphasis on the differential saturation mechanism and on its physical meaning.

The Hanle effect was first used in solar physics to determine weak magnetic fields in prominences (see Leroy 1981). More recently the development of a new generation of very accurate spectro-polarimeters has open a new window for the diagnostics of weak magnetic fields in the solar photosphere and chromosphere, based on the Hanle effect. I present here such recent research programs performed with the solar telescope THEMIS and with other polarimeters such as ZIMPOL and the LOCARNO polarimeter.

We report spectropolarimetric observations with THEMIS telescope multi-lines operating mode (MTR) during the last campaigns. We demonstrate the very favorable performances of THEMIS for investigations of the magnetic field and we report also observations devoted to the measurement of the scattering polarization. In that case, the signals are weak and are not "a priori" included in the THEMIS objectives but results show that THEMIS is a suitable instrument not only for strong field at high spatial resolution but also for weak signal at high polarimetric sensitivity. We describe briefly the multi-line spectropolarimetry mode (MTR) available at the franco-italian solar telescope THEMIS.

ESPaDOnS is a new-generation cross-dispersed échelle spectropolarimeter, the commissioning phase of which is scheduled at the Canada-France-Hawaii Telescope (CFHT) for autumn 2003. This instrument will provide full coverage of the optical domain (370 nm to 1000 nm) in all polarization states (circular and linear) at a resolving power of about 70 000, with a peak efficiency of 20% (telescope and detector included). It includes a bench-mounted spectrograph, fiber-fed from a Cassegrain-mounted module including all polarimetric and calibration facilities. ESPaDOnS should be the most powerful tool dedicated to stellar spectropolarimetry, therefore opening unprecedented perspectives for major issues of stellar physics, from studies of stellar interiors to investigations of stellar atmospheres, stellar surfaces, stellar magnetic fields, and to observations of circumstellar environments and extra-solar planets.

NARVAL is a new generation spectro-polarimeter for the Télescope Bernard Lyot (TBL) at Observatoire du Pic du Midi. It is an adapted copy of ESPADONS and should be 10 to 20 times more efficient than MUSICOS currently in use at TBL.

The diagnosis of stellar magnetic fields must include the determination of the transverse field component, thus the measurement of linear polarization across spectral lines. While the neighbouring continuum is often considered as a zero reference level, it can be polarized by various scattering mechanisms. I discuss some consequences of such a superposition.

Modern spectro-polarimeters produce huge amounts of data, and the computing time needed to achieve data inversion is then becoming an important bottleneck for the modelling activity. Recently, an alternative to the traditional non-linear least squares technique has been proposed; it relies on an old technique in multivariate analysis, principal component analysis (PCA), and proves to be much faster than traditional techniques (a gain of two orders of magnitude in computation time is not unusual). We will briefly recall this technique and discuss some means of making it still more robust.

We propose to take benefit of optical aperture synthesis arrays to resolve local magnetic structures and patchy stellar surfaces. This requires to be able to polarimetrically resolve magnetic lines and thus to add a spectro-polarimetric device at the combined focus of an interferometric array. Within this instrumental context, it becomes possible to map magnetic fields thanks to fringe visibility and phase measurements in circularly polarized light and to map abundance inhomogeneities thanks to "classical" interferometric measurements (i.e. without the polarimeter). This appears to be of great interest to better understand the key role of magnetism in atmosphere structuration, in ion migration across the stellar surface, in chemical stratification... In this talk we show how the interference fringe phase is the suitable observable for polarimetric measurements and for mapping patchy surfaces. We illustrate that on various typical cases of magnetic topology and abundance distribution of Chemically Peculiar (CP) stars. Finally we give some instrumental perspectives within the context of the optical interferometric arrays such as the VLTI.

The aim of this paper is to present some recent advances in the diagnostic of solar prominences, with an emphasis put on the results obtained with SOHO through its six years of observations. We show the progress made since the Hvar Reference Model in the determination of basic thermodynamic parameters, especially in the Prominence-Corona Transtion Region (PCTR). We address the important contribution of ``prominence seismology" for determining e.g. density and magnetic field. We summarize the most recent comparisons between non-LTE modelling predictions and spectra of optically thick lines. Finally, we mention the most recent magnetic measurements which seem to challenge the historic Hanle measurements of Leroy (1988) as summarized in Bommier et al.  (1994).

In this paper we present a review of the past, current and future space missions relevant for observing stellar activity from space. In the past, missions were focused on UV and X-ray fluxes, variability and spectroscopy. Currently coronal spectroscopy diagnostics are developed. HST high resolution imaging in the visible and near IR give access to circumstellar disks and outflows. The future space missions will concentrate on space photometry (for asteroseismology, exoplanets search and microvariability due to activity) and interferometry (for measuring stellar sizes and environments).

This review about solar photospheric magnetic fields will follow the two traditionnal approaches to study these fields. I will first review some recent results about magnetic fields observed at small scales, from sunspots and active regions down to network and intranetwork magnetic fields. The second part of the talk will be devoted to the global approach, mostly concerning the solar cycle: large spatial and temporal scales as well as global motions including differential rotation. I will finish by a discussion about studies attempting to link the two approaches.