Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011apj...731...82h&link_type=abstract
The Astrophysical Journal, Volume 731, Issue 2, article id. 82 (2011).
Astronomy and Astrophysics
Astronomy
Magnetic Reconnection, Plasmas, Sun: Filaments, Prominences
Scientific paper
Observations of quiescent prominences with the Solar Optical Telescope on the Hinode satellite have revealed the ubiquitous existence of downflows forming coherent thin and highly structured vertically oriented threads with velocities between 10 and 20 km s-1. Their widths range between 300 and 500 km. They are often initiated at the top of the visible prominence, but sometimes also at intermediate level. We propose that the downflows are made of plasma packets that squeeze themselves through the dominantly horizontal field under the action of gravity. Their origin is assumed to be hot plasma supplied from either inside or the immediate vicinity of the prominence and condensing at its top. Under compression and further cooling, the matter overflows to the flanks of the prominence dragging its magnetic field with it. Under the increasing action of gravity, vertical structures are forming which eventually disconnect from the field of the inflow channel thus forming finite plasma packets. This process is reminiscent of water flowing over a mountain ridge and breaking up into a multitude of droplets. Like water droplets being subject to air drag, the falling plasma droplets experience a drag force by the horizontal prominence field and assume a steady vertical velocity. This happens via the excitation of Alfvén waves. Lateral confinement by the prominence field determines their spatial extent. The small scales of the droplets and the directional balance of their internal tangled magnetic fields can explain the absence of appreciable vertical components in magnetic field measurements. On the basis of the observed width and vertical speed of the downflows and by adopting a prominence field of about 8 G, we derive central density and temperature of the droplets, which turn out to be quite consistent with known prominence characteristics. In the formulation of the drag force a dimensionless "magnetic drag coefficient" has been introduced with a value well below unity.
Berger Thierry
Haerendel Gerhard
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