Are there double layers in solar coronal transition region that accelerate ions originating solar wind?
Abstract
Drawing from the physics of laser plasma expansion, laboratory experiments, magnetospheric observations on double layers (DLs), theory and simulations, we suggest that solar coronal transition region may contain randomly distributed numerous patches of double layers, which accelerate the ions from the upper chromosphere into the low corona. We expect the DLs to be distributed in a vertically thin layer while horizontally they are distributed widely. Such DLs provide a novel mechanism for the origin of the solar wind. The DL-based mechanism is based on expansion of dense cold chromospheric plasma consisting of cold ions, cold electrons with temperature Tc and a minor hot electron population with temperature Teh. Such plasma expansion naturally involves electric double layers, which are thin structures with vertical widths L much less than, say, about 100 plasma Debye lengths d. We envision that such double layers form in the upper chromosphere where charged particles collisions with neutral particles become rare at heights h > 1500 km above the photosphere and where ion-electron collision mean-free-path length S > L~ 10d. Such DLs are likely to form in magnetic flux tubes (coronal funnels) undergoing magnetic reconnection with magnetic fields in the chromospheric magnetic networks. Such reconnections produce the prerequisite hot electron population in the flux tube. The apparent vertical width of the coronal transition region (CTR) of the order of a few hundred kilometers is the consequence of the DLs vertically displaced in the plasma volume where the condition S > L ~ 10d is met. Since the magnetic reconnection and the DLs are dynamically evolving events, the CTR must be a highly dynamic region giving the observed zigzag boundary. The existence of the DLs in the CTR can be inferred from the outflow of ions from the chromosphere as beams, consisting of heavy and light ions, having the same energy before they are affected by other plasma processes and the solar gravity. The CTR double layers are analogous to the terrestrial auroral double layers that form at the zigzag transition between the dense ionospheric cold plasma and the hot plasma in the auroral density cavities. In the latter case, the hot electrons in the ionosphere result from backscattered hot electrons mirroring in the Earth's magnetic field. The mirroring of hot coronal electrons penetrating into the chromosphere may provide an additional source of the minor hot electron population in the upper chromosphere, in addition to the reconnection.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMSH33A2055S
- Keywords:
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- 7546 SOLAR PHYSICS;
- ASTROPHYSICS;
- AND ASTRONOMY Transition region;
- 7800 SPACE PLASMA PHYSICS;
- 7807 SPACE PLASMA PHYSICS Charged particle motion and acceleration