The combined effect of a subsurface steady homogeneous flow and a chromospheric uniform magnetic field on the solar p- and f-modes is evaluated theoretically for a simple model of the Sun. The derived dispersion relation is solved analytically in limit of the long wavelength approximation and is evaluated numerically for arbitrary wavelengths. The influence of an equilibrium flow is more dominant in limit of small wavenumbers. For arbitrary wavelengths the effect of a magnetic field might be stronger than frequency shifts caused by a steady homogeneous flow.

Ring-diagram analyses have become a powerful local helioseismic tool for studying large-scale flows in the upper solar convection zone. Using this technique on a dense-pack mosaic of many small regions extracted from the full-disk Doppler velocity data taken with the Michelson Doppler Imager (MDI) on SOHO, we study how the mean meridional and zonal flows vary with depth and latitude over the course of the advancing solar cycle from 1996 to 2000.

Using full-disk Doppler velocity data from MDI on SOHO, we have determined the central frequencies of high-degree p modes and f modes over a dense mosaic of localized regions on the sun using ring-diagram analyses (Haber et al. 2000). The motion of active regions as they rotate across the solar disk is well traced by changes in the central frequencies of the acoustic wave modes. Active regions appear as locations of large positive frequency shifts.

Numerical experiments on three-dimensional magnetoconvection in a stratified compressible layer reveal a range of different patterns, depending on the strength of the imposed magnetic field. As the field is decreased there is a transition from small-scale plumes, in the magnetically dominated regime, to large-scale vigorous plumes when the field is dominated by the motion. In the intermediate regime magnetic flux separates from the motion, so that there are almost field-free regions, with clusters of vigorous plumes, surrounded by regions where the Lorentz force is strong enough to control the dynamics. There is a range of field strengths where either small-scale plumes or flux-separated solutions can persist, depending on initial conditions for the computation. These results can be related to magnetic features at the surface of the Sun.

Over the past decade, “thin flux tube” models have proven successful in explaining many properties of active regions in terms of magnetic flux tube dynamics in the solar interior. On the other hand, recent 2-D MHD simulations of the emergence of magnetic flux have shown that many of the assumptions adopted in the thin flux tube approximation are invalid. For example, unless the flux tubes exhibit a large degree of initial field line twist — and observations of emerging active regions suggest they do not — they will fragment (break apart) before they are able to emerge through the surface. We attempt to resolve this paradox using a number of 3-D MHD simulations (in the anelastic approximation) that describe the rise and fragmentation of twisted magnetic flux tubes. We find that the degree of fragmentation of an evolving Ω-loop depends strongly on the 3-D geometry of the loop, and that the Coriolis force plays a dynamically important role in the evolution and emergence of magnetic flux.

I review recent observational and theoretical progress in the study of condensations of cool material trapped in the coronae of rapidly rotating stars. Such condensations were discovered over a decade ago, producing transient moving absorption features in the optical Balmerseries lines. These condensations appear to be in a physical state similar to that of solar prominence material, in the sense that they are sufficiently cool and dense to contain substantial amounts of neutral hydrogen. The strong coronal magnetic fields of the stars in which they occur serve both to confine and to insulate the cool material. The coronal condensations (sometimes dubbed “slingshot prominences”) form at a variety of distances from the star, but are observed to favour formation sites near the Keplerian co-rotation radius. Simple support models involving purely dipolar fields can explain the existence of stable mechanical equilibria for sheet-like prominences in the equatorial plane. More complex field configurations show a wider variety of stable formation sites, both inside the co-rotation radius and out of the stellar equatorial plane.

In this paper, we analyze the soft X-ray flares and the relationship with photospheric vector magnetograms and current helicity in solar active regions.

Upflows of several tens of km/s have been observed by SOHO-CDS in the late gradual phase of the M6.8 two-ribbon flare on April 29, 1998. These upflows observed in EUV lines formed at coronal temperatures are interpreted as chromospheric evaporation which fills the post-flare loops with hot plasma. In order to achieve chromospheric evaporation, the chromospheric plasma has to be heated to coronal temperatures. The energy for this heating process is assumed to be provided by magnetic reconnection. The mechanism which transports the energy from the reconnection site to the chromosphere must be either thermal or non-thermal. We compare the observed upflow velocities with the velocities derived by different chromospheric heating models in order to decide which mechanism might account for the chromospheric heating. From non-thermal models we take the electron energy flux necessary to achieve the observed velocities and calculate the expected hard X-ray counts in Yohkoh/HXT for non-thermal thick-target Bremsstrahlung generated by this electron flux. We conclude that energetic (> 15keV) non-thermal electrons are unlikely to cause the chromospheric heating since a significant number of HXT counts are expected from the resulting electron energy flux but not observed. Recent thermal conduction models seem to be more appropriate for explaining the observations.

We examine time series of the transition region spectral line O v 629 Å, observed with the Coronal Diagnostic Spectrometer (CDS) onboard the SoHO spacecraft. Primarily we were looking for intensity and velocity oscillations in polar plumes, however by chance we detected a giant macro-spicule at the limb and were able to follow its dynamical structure. We have reported the dynamics of the giant macro-spicule in Banerjee et al. (2000a) and have also reported the existence of long period oscillations in the polar plumes as observed in the O v 629 Å line in Banerjee et al. (2000a,b). In this short contribution we revisit one of these dataset, namely s16834r01 to test the reliability of the observed periods. The statistical significance of the oscillations is estimated by using a randomisation method. Our analysis re-confirms the presence of compressional waves with periods of 20-25 minutes.

Based on the Huairou Solar Observing Station dataset, we computed the current helicity for several hundreds of active regions and found that: (1) Active regions that do not follow the hemispheric helicity sign rule show more flare activity than normal active regions. (2) The relative number of active regions with reversed helicity sign is higher near sunspot maximum. (3) It appears that during solar cycle 22 the southern hemisphere has more the reversed-sign active regions and stronger flare activity than the northern hemisphere.

We present results of investigation of the large-scale structure of the solar corona during the transition period between solar cycles 22 and 23 and at the beginning of the current cycle 23 using the SOHO/EIT EUV data obtained in 171 Å, 195 Å, 284 Å and 304 Å lines. For this analysis the data were transformed into synoptic maps for each of the spectral lines, and for the 195/171 line ratio which is an index of the coronal temperature. The synoptic maps reveal stable longitudinal structures in the coronal intensities and temperature, which are related to large-scale magnetic field structures. We discuss the relation between the coronal and photospheric magnetic structures obtained from the SOHO/MDI data, and compare the rotation rates of these structures with the rotation profile of the solar interior in order to determine the possible origin of the coronal structures.

We investigate the magnetic sensitivity of molecular lines observed in sunspot umbrae and briefly discuss their major differences compared to atomic lines.

This paper reports the first extremely near infrared observation of two prominence events on Jan. 8, 1999 and Feb. 9, 1999 at H Pfund β (46525Å) and H Bracket α (40512Å) using the McMath Telescope. We scanned the spectrograph slit across the prominences parallel to the limb to obtain three-dimensional data cubes (two spatial dimensions and one spectral dimension). By fitting these observed lines, we can directly determine some important physical parameters such as Doppler width (ΔλD), the optical thickness at line center (τΔ0), and the line displacement (Δλ0). Combined with simultaneous observation of Balmer Hα, Hβ, and Ca K lines, the new results of structure and dynamics of limb prominence is presented. These results imply that these infrared lines can penetrate through the prominence and become a potential probe and tool, while Hα and Hβ sense the prominence skin only.

A flare of size M8 occurred while SUMER was recording a spectral scan above the active region NOAA 8537 at the west limb. We recorded spectra during the pre-flare phase, at flare onset, and during the decay phase of this main flare in a series of events. More than 60 flare lines were identified during this observation, which include Fe XVIII - Fe XXIII lines that provide evidence of 107 K plasmas. We also recorded lines from He-like ions, such as Ne IX, Na x, Mg XI, and Si XIII. Accurate wavelength measurements of such lines are of interest in basic atomic physics studies. Using plasma diagnostic techniques, we investigated the temporal evolution of the electron densities and temperatures during the event.

We report on EUV observations of a solar limb flare obtained by the SUMER spectrometer. A time series was taken with fixed slit position and several spectral windows that covered a wide temperature range (104-107 K), preceded and followed by contextual raster scans in a He I line. During the time series, a C4.6 flare occurred in the region, also imaged in the EUV by SOHO/EIT and in soft X rays by YOHKOH/SXT. The temporal evolution seen in the SUMER spectra reveals a close spatial relationship and a correlated dynamical behaviour of the hot (T ≈ 107 K) and cool (T ≈ 104 K) material, which are difficult to reconcile with the notion (based on the Kopp-Pneuman flare model) that cool loops form at a lower height than the hot flare loops.

Using a 1995-1998 data set of vector magnetograms, the magnetic field flux, shear angle of the transverse field and nonpotential energy of active regions were calculated. The evolution of these parameters were analyzed together with time series of the solar monthly sunspot relative number and area to study their relationships in the ascending phase of solar cycle 23. We find the magnetic flux and nonpotential energy have a good correlation with sunspot relative number and area. But the magnetic shear angle does not develop as above indices.

A new solution of the force free field equation is presented in cylindrical coordinates.

Solar active regions are believed to correspond to the topmost portions of Ω-shaped arching flux tubes that have risen buoyantly from the base of the solar convection zone, where strong toroidal magnetic fields are being generated by the dynamo process. The development of such emerging Ω-loops is likely a result of the buoyant instability associated with the submerged toroidal magnetic field. Using an anelastic MHD code, we simulate the formation of buoyant, arching flux tube structures as a result of the non-linear growth of the undular instability of a neutrally buoyant layer of horizontal, unidirectional magnetic field at the base of the solar convection zone.

The Coronal Diagnostic Spectrometer (CDS) on SOHO carries out daily synoptic observations of the Sun in four EUV (extreme ultraviolet) spectra: He I 584 Å, O V 630 Å, Mg IX 368 Å and Fe XVI 360 Å, over a 4 arcmin-wide strip along the solar central meridian. Using 53 active regions observed in this data set along with co-temporally observed SOHO-MDI (Michelson Doppler Imager) magnetograms we study the correlation of the chromospheric, transition region and coronal emission with the photospheric magnetic field for meridional active regions, probing the relation between the radiative output and magnetic observables. We also establish empirical, quantitative relations among intensities of different lines, and between intensities and the magnetic field flux.

We studied the evolution of an active region and its relation with flare activities in order to solve the physical mechanism of solar flare occurrence. We investigated the evolutionary characteristics of the active region NOAA 8948 (April 2000) and found that the newly flux emergence caused the flares in this active region.