Margarita P. Ryutova

On the Transition Region Explosive Events

We describe the properties of high-velocity and explosive events in the solar transition region determined from time series of data taken by the Transition Region and Coronal Explorer and the SUMER instrument on the Solar and Heliospheric Observatory (SOHO) simultaneously in several chromospheric and transition region lines co-aligned with high-resolution Michelson Doppler Imager (SOHO) magnetograms. We outline what the various features of these events can tell us about the heating mechanisms and formation of plasma flows. Our results strongly support the mechanism of hydrodynamic cumulation of energy associated with the cascade of shock waves produced by colliding and reconnecting flux tubes in the photospheric network (as recently discussed by T. D. Tarbell et al.). We find that the majority of the explosive events are caused by the explosive instability occurring in the presence of the behind-shock downflows, and less than 10% can be explained by the direct collision of shock fronts.

Electro-Mechanical Coupling Between the Photosphere and Transition Region

We study the response of the chromosphere and transition region to dynamic changes in the photospheric network magnetic fields. We present results from simultaneous measurements taken by TRACE in chromospheric and transition region (CIV) images, high-resolution magnetograms taken by MDI, and spectra of chromospheric (CII) and transition region lines (Ovi) obtained with the SUMER instrument on SOHO. Enhanced emission in the CIV line is generally co-spatial with the magnetic pattern in the photosphere. We propose a mechanism of electro-mechanical coupling between the photosphere and upper layers of atmosphere based on hydrodynamic cumulation of energy produced by reconnecting flux tubes in the photosphere/chromosphere region (Tarbell et al., 1999). We believe that a basic process causing energetic events is the cascade of shock waves produced by colliding and reconnecting flux tubes. The continuous supply of flux tubes in the ‘magnetic carpet’ ensures the ubiquitous nature of this process and its imprint on the upper atmosphere. The appearance of bright transients often, but not always, correlates with canceling mixed polarity magnetic elements in the photosphere. In other cases, transients occur in regions of unipolar flux tubes, suggesting reconnection of oblique components. Transients arc also seen in regions with no fields detected with the MDI sensitivity; these may be reconnections of tiny features with diameters less than 100 km. Blinkers and other bright transients are often accompanied by two directional plasma jets. These may be generated by cylindrical self-focusing of shock fronts or by collision of shocks produced by neighboring reconnection processes. The observations suggest that stronger emissions correspond to lower velocity jets, and vice versa; this property is a natural consequence of the proposed mechanism. Plasma flows are always seen whenever the slit crosses strong magnetic flux tubes or vertices of converging flows in the supergranular network. The overall energy distribution between heating and plasma flows is an intrinsic feature of our mechanism.

Magnetic Coupling Between the Solar Surface and Corona: Theory and Observations

Multi‐wavelength observations taken simultaneously by several instruments on the Solar and Heliospheric Observatory (SOHO) and Transition Region and Coronal Explorer (TRACE) revealed a clear connection between the photospheric magnetic fields and the energetic events in the overlying atmosphere. We find that the EUV coronal emission above the photosphere dominated by single polarity magnetic elements is spongy in space and has coherent braid‐like structures in time. Contrary to these long living structures, corona above the regions with mixed polarity magnetic elements is highly discrete and consists of sporadic microflares, supersonic jets and their combinations. We believe that in the unipolar magnetic regions the energy flow from the surface to corona is associated with the nonlinear collective phenomena in the ensemble of oscillating magnetic flux tubes. These phenomena lead to formation of hot coronal “clouds” that have the properties of energetically open turbulence with tendency to self‐organizatio...

FINE STRUCTURE AND DYNAMICS OF SUNSPOT PENUMBRA

A mature sunspot is usually surrounded by a penumbra: strong vertical magnetic field in the umbra, the dark central region of sunspot, becomes more and more horizontal toward the periphery forming an ensemble of a thin magnetic filaments of varying inclinations. Recent high resolution observations with the 1-meter Swedish Solar Telescope (SST) on La Palma revealed a fine substructure of penumbral filaments and new regularities in their dynamics. These findings provide both the basis and constraints for an adequate model of the penumbra whose origin still remains enigmatic. We present results of recent observations obtained with the SST. Our data, taken simultaneously in 4305 A Gband and 4396 A continuum bandpasses and compiled in high cadence movies, confirm previous results and reveal new features of the penumbra. We find e.g. that individual filaments are cylindrical helices with a pitch/radius ratio providing their dynamic stability. We propose a mechanism that may explain the fine structure of penumbral filaments, the observed regularities, and their togetherness with sunspot formation. The mechanism is based on the anatomy of sunspots in which not only penumbra has a filamentary structure but umbra itself is a dense conglomerate of twisted interlaced flux tubes.