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last change 2006 August 14, R. Arlt
Solar Physics

New instability drives solar eruptions

The magnetic field in solar active regions or in quiescent prominences carries large electrical currents which store huge amounts of energy. When this storage stresses the field too much, the configuration is destabilized and erupts, suddenly transforming magnetic energy into kinetic energy of coronal mass ejections, eruptive prominences and flares. These most violent energy releases in the solar system also perturb the terrestrial environment, which has become known as the space weather. The strongest effects occur if plasma clouds are ejected with high velocities of about 1000 km/s and above. Then the X-ray emissions are hardest and most intense, relativistic particles fill interplanetary space, and the electric currents induced upon impact at the terrestrial magnetosphere and ionosphere are strongest. Understanding the trigger and drive mechanisms of solar eruptions is a prerequisite in the quest to achieve forecasting capability of space weather events and it is also of general astrophysical interest, since similar eruptions occur on active stars and possibly at galactic scales as well. However, several models compete in explaining the triggering, and none of them has so far been able to establish the conditions that lead to fast ejections.

Scientists at the AIP have recently discovered that a plasma instability which is known and easily suppressed in the tokamak device used in fusion research may trigger and drive solar eruptions. The instability causes expansion of current rings that find their equilibrium in an externally produced magnetic field and occurs if the field decreases sufficiently rapidly with increasing distance from the ring. The current ring is of toroidal shape in the tokamak, hence the name torus instability. Solar coronal magnetic loops and prominence arches represent partial tori, whose missing part may be thought of as being submerged below the photosphere. While the shape of the external field is easily controlled in the tokamak to ensure stability, the Sun may be unable to provide an external field of the required shape, especially if the highly concentrated field of active regions and in prominence environments is strongly twisted or sheared as a result of the energy storage.

The instability not only represents a further model for the triggering of the eruptions, it also has very interesting properties regarding the acceleration and final velocity of the resulting ejecta. A strong concentration of the acceleration low in the corona and high resulting speed are predicted if the external field decreases steeply with height above the original current ring – just as mass ejections from active regions typically show. Gradual acceleration in a wide height range and lower resulting speeds are predicted for prominence eruptions in the more diffuse field outside of active regions, again as is typically observed. A unifying mechanism for the apparently disparate classes of fast and slow mass ejections has thus been found.

Velocity profiles
Velocity profiles
Figure 2: Original observation of two apparently separate classes of coronal mass ejections (data from MacQueen and Fisher 1983): fast flare-associated ejections from active regions (red lines) acquire speeds of order 1000 km/s already very low in the corona (behind the occulting disk indicated in grey) and then propagate with nearly constant speed, while slow ejections (blue lines) are accelerated only gradually to reach the gravitational escape speed (dashed line). The torus instability yields similar velocity profiles, plotted here for a range of characteristic decay indices n of the external field B, where n = 3 to 4 is characteristic of active regions and n ≈ 3/2 is characteristic of the quiet Sun.



Figure 1: Erupting prominence that evolved into a coronal mass ejection observed by instruments on the SOHO satellite on 27 Feb 2000. The Sun is occulted by a disk in the image of the mass ejection, with its size indicated by the white circle.


Moreover, a direct link between the properties of the coronal magnetic field and the velocity of mass ejections has been established for the first time. The coronal field cannot be measured directly with sufficient accuracy, but a combination of photospheric measurements, such as those planned with AIP instrumentation on the new telescope GREGOR on Tenerife, with numerical extrapolation of the field into the corona will permit further investigations of the link. The authors expect that this research will open a path to predicting the occurrence and the velocity of coronal mass ejections successfully.

Bernhard Kliem, Tibor Török, Gottfried Mann


Dr. Bernhard Kliem
Astrophysikalisches Institut Potsdam
An der Sternwarte 16
D-14482 Potsdam
(0331) 7499 527

Bernhard Kliem & Tibor Török: Torus Instability. Physical Review Letters 96, 255002 (2006); arXiv: physics/0605217



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