Theories of solar eruptions: a review

The large-scale curvature of a flux rope can help propel it outward from the Sun. Here we extend previous two-dimensional flux-rope models of coronal mass ejections to include the curvature force. To obtain analytical results, we assume axial symmetry and model the flux rope as a torus that encircles the Sun. Initially, the flux rope is suspended in the corona by a balance between magnetic tension, compression, and curvature forces, but this balance is lost if the photospheric sources of the coronal field slowly decay with time. The evolution of the system shows catastrophic behavior as occurred in previous models, but, unlike the previous models, flux ropes with large radii are more likely to erupt than ones with small radii. The maximum total magnetic energy that can be stored before equilibrium is lost is 1.53 times the energy of the potential field, and this value is less than the limiting value of 1.662 for the fully opened field. As a consequence, the loss of ideal MHD equilibrium that occurs in the model cannot completely open the magnetic field. However, the loss of equilibrium does lead to the sudden formation of a current sheet, and if rapid reconnection occurs in this sheet, then the flux rope can escape from the Sun. We also find that the held can gradually become opened without suffering any loss of equilibrium if the photospheric field strength falls below a critical value. This behavior is analogous to the opening of a spherically symmetric arcade in response to a finite amount of shear.

https://iopscience.iop.org/article/10.1086/306108/pdf

The Effect of Curvature on Flux-Rope Models of Coronal Mass Ejections

This paper presents the results of a "blind test" for modeling the structure of an observed filament using the three-dimensional magnetohydrostatic model recently developed by Aulanier et al. in 1999. The model uses a constant shear α, and it takes into account the effects of pressure and gravity. The test consisted of predicting the structure of a filament (observed in the southern hemisphere) with a minimum observational input: only a line-of-sight magnetogram, with a straight line drawn on it to show the location of the filament, was provided. The filament was chosen by the observers (N. S. and S. F. M.) because it had a definite overall left-handed structural pattern known as sinistral, but the direction of component of the magnetic field along the filament axis was uncertain from the combination of Hα data and magnetograms. The modeler (G. A.) evaluated and fixed the values of some of the free parameters of the model while some others were varied in reasonable ranges. The Hα image of the filament was revealed only after the modeling. For α > 0, the three-dimensional distribution of magnetic dips computed by the model fairly well reproduces the structure of the filament and its barbs. Moreover, the models for which α < 0 do not match well the observations. This study then shows the first successful theoretical prediction for the magnetic field of an observed filament. It shows that the method based on the Aulanier et al. model is a powerful tool, not only for purposes of modeling, but also for prediction of the chirality, helicity, and morphology of observed filaments.

/pdf/model-prediction-for-an-observed-filament-18norz0pz7.pdf

Model Prediction for an Observed Filament

We discuss the results stemming from observations of the white-light and [Fe XIV] emission corona during the total eclipse of the Sun of 2008 August 1, in Mongolia (Altaj region) and in Russia (Akademgorodok, Novosibirsk, Siberia). Corresponding to the current extreme solar minimum, the white-light corona, visible up to 20 solar radii, was of a transient type with well pronounced helmet streamers situated above a chain of prominences at position angles 48°, 130°, 241°, and 322°. A variety of coronal holes, filled with a number of thin polar plumes, were seen around the poles. Furthering an original method of image processing, stars up to 12 mag, a Kreutz-group comet (C/2008 O1) and a coronal mass ejection (CME) were also detected, with the smallest resolvable structures being of, and at some places even less than, 1 arcsec. Differences, presumably motions, in the corona and prominences are seen even with the 19 minutes time difference between our sites. In addition to the high-resolution coronal images, which show the continuum corona (K-corona) that results from electron scattering of photospheric light, images of the overlapping green-emission-line (530.3 nm, [Fe XIV]) corona were obtained with the help of two narrow-passband filters (centered on the line itself and for the continuum in the vicinity of 529.1 nm, respectively), each with an FWHM of 0.15 nm. Through solar observations, on whose scheduling and details we consulted, with the Solar and Heliospheric Observatory, Hinode's XRT and SOT, Transition Region and Coronal Explorer, and STEREO, as well as Wilcox Solar Observatory and Solar and Heliospheric Observatory/Michelson Doppler Imager magnetograms, we set our eclipse observations in the context of the current unusually low and prolonged solar minimum.

https://iopscience.iop.org/article/10.1088/0004-637X/702/2/1297/pdf

The 2008 August 1 Eclipse Solar-Minimum Corona Unraveled

The Solar Eclipse Coronal Imaging System (SECIS) is an instrument designed to search for short-period modulations in the solar corona seen either during a total eclipse or with a coronagraph. The CCD cameras used in SECIS have the capability of imaging the corona at a rate of up to 70 frames a second, with the intensities in each pixel digitised in 12-bit levels. The data are captured and stored on a modified PC. With suitable optics it is thus possible to search for fast changes or short-period wave motions in the corona that will have important implications for the coronal heating mechanism. The equipment has been successfully tested using the Evans Solar Facility coronagraph at National Solar Observatory/Sacramento Peak and during the 11 August 1999 eclipse at a site in north-eastern Bulgaria. The instrument is described and preliminary results are outlined.

SECIS: The Solar Eclipse Coronal Eclipse Imaging System

Session 1: Principal Scientific Results from the Past Eclipse Observations. Session 2: Small and Large Scale Models of Coronal Structures. Session 3: Low Temperature Structures in Coronal Environment. Session 4: Specific Problems of Solar Eclipse Observations. Session 5: Tasks for Total Solar Eclipse of 11 August 1999. Session 6: Instrumental Improvement for Future Observations. Session 7: Public Education at Eclipse and Eye Safety. List of Poster Papers. Notes on General Discussion.

Magda Stavinschi

Papers

Theoretical and observational problems related to solar eclipses