Abstract.We report the analysis of the high-resolution soft X-ray spectrum of the nearby F-type star Procyon in the wavelength range from 5 to 175Å obtained with the Low Energy Transmission Grating Spectrometer (LETGS) on board Chandra and with the Reflection Grating Spectrometers (RGS) and the EPIC-MOS CCD spectrometers on board XMM-Newton. Line fluxes have been measured separately for the RGS and LETGS. Spectra have been fitted globally to obtain self-consistent temperatures, emission measures, and abundances. The total volume emission measure is ∼4.1 × 10 50 cm −3 with a peak between 1 and 3 MK. No indications for a dominant hot component (T ∼ > 4 MK) were found. We present additional evidence for the lack of a solar-type FIP-effect, confirming earlier EUVE results.

Abstract. Using multi-instrument and multi-wavelength observations (SOHO/MDI and EIT, TRACE and Yohkoh/SXT), as well as computing the coronal magnetic field of a tiny bipole combined with modelling of Wind in situ data, we provide evidences for the smallest event ever observed which links a sigmoid eruption to an interplanetary magnetic cloud (MC). The tiny bipole, which was observed very close to the solar disc centre, had a factor one hundred less flux than a classical active region (AR). In the corona it had a sigmoidal structure, observed mainly in EUV, and we found a very high level of nonpotentiality in the modelled magnetic field, 10 times higher than we have ever found in any AR. From May 11, 1998, and until its disappearance, the sigmoid underwent three intense impulsive events. The largest of these events had extended EUV dimmings and a cusp. The Wind spacecraft detected 4.5 days later one of the smallest MC ever identified (about a factor one hundred times less magnetic flux in the axial component than that of an average MC). The link between this last eruption and the interplanetary magnetic cloud is supported by several pieces of evidence: good timing, same coronal loop and MC orientation, same magnetic field direction and magnetic helicity sign in the coronal loops and in the MC. We further quantify this link by estimating the magnetic flux (measured in the dimming regions and in the MC) and the magnetic helicity (pre-to post-event change in the solar corona and helicity content of the MC). Within the uncertainties, both magnetic fluxes and helicities are in reasonable agreement, which brings further evidences of their link. These observations show that the ejections of tiny magnetic flux ropes are indeed possible and put new constraints on CME models.

The temperature of the Sun's outer atmosphere (the corona) exceeds that of the solar surface by about two orders of magnitude, but the nature of the coronal heating mechanisms has long been a mystery 1 . The corona is a magnetically dominated environment, consisting of a variety of plasma structures including X-ray bright points, coronal holes and coronal loops. The latter are closed magnetic structures that occur over a range of scales and are anchored at each end in the solar surface. Large-scale regions of diffuse emission are made up of many long coronal loops 2 . Here we present X-ray observations of the diffuse corona from which we deduce its likely heating mechanism. We find that the observed variation in temperature along a loop is highly sensitive to the spatial distribution of the heating. From a comparison of the observations and models we conclude that uniform heating gives the best fit to the loop temperature distribution, enabling us to eliminate previously suggested mechanisms of low-lying heating near the footpoints of a loop. Our findings favour turbulent breaking and reconnection of magnetic field lines as the heating mechanism of the diffuse solar corona.Part of the coronal heating question, namely what heats X-ray bright points, has already been answered because the converging flux model 3 has explained their observational properties in a natural way in terms of reconnection of magnetic field lines in the corona. This is driven by solar surface motions of the footpoints of such field lines and accounts for the internal structure of particular bright points 4 . However, the mechanisms for heating coronal loops and coronal holes have not yet been identified 5 .Observations from the Soft X-ray Telescope on board the Japanese/US/UK Yohkoh satellite have built on previous rocket and Skylab observations and revealed that the corona is a complex magnetohydrodynamic system that is highly time-dependent and three-dimensional 2 , with myriads of magnetic loops continually evolving and interacting. These observations have also given important clues as to how the corona is heated. In particular, a distinction has been revealed between local, time-dependent impulsive components to the heating and global, steadier components on a much larger scale 6 . For example, in active regions there are tiny transient brightenings 7 , which are likely to be driven by reconnection (like X-ray bright points), but they fall short by a factor of 5-10 of being able to heat active-region loops. Furthermore, the hottest loops in active regions (Ͼ6 MK) appear to be multiple structures that are interacting by reconnection or tiny cusp-like features 8 that are closing down by reconnection, as in large eruptive solar flares 9 . The cooler coronal loops (3-5 MK) are much steadier in their emission 10 .Sturrock et al. 11 studied a large magnetically closed region of the diffuse corona and deduced from filter ratios that the temperature increases with radius from 1.6 MK at the limb to 2.3 MK at 1.5 R ᭪ . They modelled this temperatu...

One of the paradigms about coronal heating has been the belief that the mean or summit temperature of a coronal loop is completely insensitive to the nature of the heating mechanisms. However, we point out that the temperature proÐle along a coronal loop is highly sensitive to the form of the heating. For example, when a steady state heating is balanced by thermal conduction, a uniform heating function makes the heat Ñux a linear function of distance along the loop, while T 7@2 increases quadratically from the coronal footpoints ; when the heating is concentrated near the coronal base, the heat Ñux is small and the T 7@2 proÐle is Ñat above the base ; when the heat is focused near the summit of a loop, the heat Ñux is constant and T 7@2 is a linear function of distance below the summit. It is therefore important to determine how the heat deposition from particular heating mechanisms varies spatially within coronal structures such as loops or arcades and to compare it to high-quality measurements of the temperature proÐles.We propose a new two-part approach to try and solve the coronal heating problem, namely, Ðrst of all to use observed temperature proÐles to deduce the form of the heating, and second to use that heating form to deduce the likely heating mechanism. In particular, we apply this philosophy to a preliminary analysis of Y ohkoh observations of the large-scale solar corona. This gives strong evidence against heating concentrated near the loop base for such loops and suggests that heating uniformly distributed along the loop is slightly more likely than heating concentrated at the summit. The implication is that large-scale loops are heated in situ throughout their length, rather than being a steady response to low-lying heating near their feet or at their summits. Unless waves can be shown to produce a heating close enough to uniform, the evidence is therefore at present for these large loops more in favor of turbulent reconnection at many small randomly distributed current sheets, which is likely to be able to do so. In addition, we suggest that the decline in coronal intensity by a factor of 100 from solar maximum to solar minimum is a natural consequence of the observed ratio of magnetic Ðeld strength in active regions and the quiet Sun ; the altitude of the maximum temperature in coronal holes may represent the dissipation height of waves by turbulent phase mixing ; and the di †erence in maximum temperature in Alfve n closed and open regimes may be understood in terms of the roles of the conductive Ñux there.

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