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Tests of Dynamical Flux Emergence as a Mechanism for CME Initiation  

james leake   Submitted: 2010-07-30 09:41

Current coronal mass ejection (CME) models set their lower boundary to be in the lower corona. They do not calculate accurately the transfer of free magnetic energy from the convection zone to the magnetically dominated corona because they model the effects of flux emergence using kinematic boundary conditions or simply assume the appearance of flux at these heights. We test the importance of including dynamical flux emergence in CME modeling by simulating, in 2.5D, the emergence of sub-surface flux tubes into different coronal magnetic field configurations. We investigate how much free magnetic energy, in the form of shear magnetic field, is transported from the convection zone to the corona, and whether dynamical flux emergence can drive CMEs. We find that multiple coronal flux ropes can be formed during flux emergence, and although they carry some shear field into the corona, the majority of shear field is confined to the lower atmosphere. Less than 10% of the magnetic energy in the corona is in the shear field, and this, combined with the fact that the coronal flux ropes bring up significant dense material, means that they do not erupt. Our results have significant implications for all CME models which rely on the transfer of free magnetic energy from the lower atmosphere into the corona but which do not explicitly model this transfer. Such studies of flux emergence and CMEs are timely, as we have new capabilities to observe this with Hinode and SDO, and therefore to test the models against observations.

Authors: James E Leake, Mark G Linton, Spiro K Antiochos
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Publication Status: Accepted in ApJ
Last Modified: 2010-08-04 06:38
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The emergence of magnetic flux through a partially ionised solar atmosphere  

james leake   Submitted: 2006-01-25 03:59

We present results from 2.5D numerical simulations of the emergence of magnetic flux from the upper convection zone through the photosphere and chromosphere into the corona. Certain regions of the solar atmosphere are at sufficiently low temperatures to be only partially ionised, in particular the lower chromosphere. This leads to Cowling resistivities orders of magnitude larger than the Coulomb values, and thus to anisotropic dissipation in Ohm's law. This also leads to localised low magnetic Reynolds numbers (Rm<1). We find that the rates of emergence of magnetic field are greatly increased by the partially ionised regions of the model atmosphere, and the resultant magnetic field is more diffuse. More importantly, the only currents associated with the magnetic field to emerge into the corona are aligned with the field, and thus the newly formed coronal field is force-free.

Authors: Leake, J.E., Arber, T.D.
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Publication Status: Accepted in Astronomy and Astrophysics
Last Modified: 2006-01-26 07:33
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Collisional dissipation of Alfvén waves in a partially ionised solar chromosphere  

james leake   Submitted: 2005-09-19 08:26

Certain regions of the solar atmosphere are at sufficiently low temperatures to be only partially ionised. The lower chromosphere contains neutral atoms, the existence of which greatly increases the efficiency of the damping of waves due to collisional friction momentum transfer. More specifically the Cowling conductivity can be up to 12 orders of magnitude smaller than the Spitzer value, so that the main damping mechanism in this region is due to the collisions between neutrals and positive ions (cite{khudachenko}). Using values for the gas density and temperature as functions of height taken from the VAL C model of the quiet Sun (cite{vernazza}), an estimate is made for the dependance of the Cowling conductivity on height and strength of magnetic field. Using both analytic and numerical approaches the passage of Alfvén waves over a wide spectrum through this partially ionised region is investigated. Estimates of the efficiency of this region in the damping of Alfvén waves are made and compared for both approaches. We find that Alfvén waves with frequencies above {change 0.6 Hz} are completely damped and frequencies below 0.01 Hz unaffected.

Authors: Leake, J.E., Arber, T.D., Khodachenko, M.L.
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Publication Status: accepted 19th july
Last Modified: 2005-09-19 08:30
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Tests of Dynamical Flux Emergence as a Mechanism for CME Initiation
The emergence of magnetic flux through a partially ionised solar atmosphere
Collisional dissipation of Alfven waves in a partially ionised solar chromosphere

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