H. C. Dara

2D MHD modelling of compressible and heated coronal loops obtained via nonlinear separation of variables and compared to TRACE and SoHO observations

An analytical MHD model of coronal loops with compressible flows and including heating is compared to obser- vational data. The model is constructed via a systematic nonlinear separation of the variables method used to calculate several classes of exact MHD equilibria in Cartesian geometry and uniform gravity. By choosing a particularly versatile solution class with a large parameter space we are able to calculate models whose loop length, shape, plasma density, temperature and ve- locity profiles are fitted to loops observed with TRACE, SoHO/CDS and SoHO/SUMER. Synthetic emission profiles are also calculated and fitted to the observed emission patterns. An analytical discussion is given of the two-dimenional balance of the Lorentz force and the gas pressure gradient, gravity and inertial forces acting along and across the loop. These models are the first to include a fully consistent description of the magnetic field, 2D geometry, plasma density and temperature, flow velocity and thermodynamics of loops. The consistently calculated heating profiles which are largely dominated by radiative losses and concentrated at the footpoints are influenced by the flow and are asymmetric, being biased towards the upflow footpoint.

Distribution of coronal heating in a solar active region

Aims. We investigate the distribution of heating of coronal loops in a non-flaring solar active region, using a simple electrodynamic model: the random displacements of the loop footpoints, caused by photospheric plasma motions, generate electric potential differences between the footpoints and, as a result, electric currents flow along the loops, producing Ohmic heating. Methods. We implement our model on the closed magnetic field lines in the potential magnetic field extrapolation of an MDI active region magnetogram. For each one of the magnetic field lines, we compute the heating function and obtain the hydrostatic distribution of temperature and pressure. We find that coronal heating is stronger close to the footpoints of the loops and asymmetric along them. We obtain scaling laws that relate both the mean volumetric heating to the loop length, and the heating flux through the loop footpoints to the magnetic field strength at the footpoints. Our results agree with observational data. Results. According to our model, we attribute the observed small coronal-loop width expansion to both the preferential heating of coronal loops of small cross-section variation, and the cross-section confinement due to the random electric currents flowing along the loops. Conclusions. We conclude that our model can be used as a simple working tool in the study of solar active regions.

A solar active region loop compared with a 2D MHD model

We analyzed a coronal loop observed with the Normal Incidence Spectrometer (NIS), which is part of the Coronal Diagnostic Spectrometer (CDS) on board the Solar and Heliospheric Observatory (SOHO). The measured Doppler shifts and proper motions along the selected loop strongly indicate unidirectional flows. Analysing the Emission Measure Curves of the observed spectral lines, we estimated that the temperature along the loop was about 380 000 K. We adapted a solution of the ideal MHD steady equations to our set of measurements. The derived energy balance along the loop, as well as the advantages/disadvantages of this MHD model for understanding the characteristics of solar coronal loops are discussed.

Dopplershifts in the solar transition region

We study the dynamics of the quiet sun transition region, using observations obtained with the SOHO CDS/NIS and SUMER spectrographs. We examine the morphology of the network as a function of temperature and we compare the intensity features with those of the dopplergrams. The velocity distributions have a dierent behaviour for the bright features which outline the network and the dark ones, located in the internetwork. A redshift and a smaller standard deviation are observed for the bright feature distributions relative to the dark ones. It should be mentioned that the internetwork is also statistically redshifted, with the exception of the He i line. Velocity distributions from dierent lines are compared.

Small-scale motions over concentrated magnetic regions of the quiet Sun

We have used a 5.5 min time-sequence of spectra in the Fe i lines λ5576 (magnetically insensitive), λ6301.5 and λ6302.5 (magnetically sensitive) to study the association of concentrated magnetic regions and velocity in the quiet Sun. After the elimination of photospheric oscillations we found downflows of 100–300 m s −1, displaced by about 2″ from the peaks of the magnetic field; this velocity is comparable to downflow velocity associated with the granulation and of the same order or smaller than the oscillation amplitude. Quasi-periodic time variations of the vertical component of the magnetic field up to ± 40% were also found with a period near 250 s, close to the values found for the velocity field. Finally we report a possible association of intensity maxima at the line center with peaks of the oscillation amplitude.

Coronal oscillation above a supergranular cell of the quiet Sun chromospheric network

We have detected an oscillation in the low corona, using a raster of the SUMER EUV spectrograph in the Ne VIII, 770 A line formed at about 700 000 K. The oscillation was found in the Ne VIII Doppler map above the interior of a supergranular cell of the chromospheric network in the quiet Sun, while it is absent in line radiance. The photospheric magnetic field, extrapolated to coronal levels, was used to relate this phenomenon to the magnetic structure. This oscillation phenomenon, reported here for the first time, seems to occur only above the darkest cells of the chromospheric network. We interpret our findings as a collective non-compressible oscillation of the corona above the whole cell interior. This oscillation may originate at the chromosphere and the driving wave may undergo a mode conversion at the top chromosphere, where the magnetic pressure equals the gas pressure. Our interpretation cannot be definitive and should be verified with more data.