Energy transport in the solar transition layer

• Published in: Proceedings of the Royal Society. A, Mathematical, physical, and engineering sciences Mathematical, physical, and engineering sciences, 2001-07, Vol.457 (2012), p.1873-1888
• Main Authors: Ashbourn, J. M. A., Woods, L. C.
• Format: Article
• Language: English
• Subjects: Chromosphere; Differential Emission Measure; Electric current; Electrons; Energy Transport; Heat flux; Ion-Acoustic Instability; Ions; Magnetic fields; Research Article; Resistance heating; Solar Transition Layer; Temperature gradients; Thermal conductivity; Transition layers
• ISSN: 1364-5021; 1471-2946
• DOI: 10.1098/rspa.2001.0791

We have developed a theory for the differential emission measure in the solar transition layer, which has a temperature in the range 104K < T < 106K. On comparing the resulting log versus log T plot with curves derived from observations, we find good agreement over the whole range. As heat flows down from the coronal reservoir, the vertical temperature gradient T′ increases until it reaches a critical value, T′m, beyond which any further increase takes the heat flux in the electron gas parallel to the magnetic field (qe||) above the value that could be convected by the electrons drifting at the ion sound speed Cs. This gradient is reached at about the midpoint of the temperature range and we infer that as a consequence the ionacoustic instability generates turbulence in the ion gas; this substantially increases the effective collision frequencies in the ion fluid with the result that at about T = 105K, where T′ = T′m, the parallel electron heat flux and the perpendicular ion heat flux (qi⊥) are comparable. Below T = 105K, qi⊥ is dominant and gives rise to a slope in our plot of ca. -3.5, as observed, whereas above this temperature, qe|| dominates and the slope increases to ca. 1.5, as is also observed. Ohmic dissipation by the ion-sound limited current leads to a heating rate of the form AT−5/2, where the constant A depends on the unknown size of the flux tubes involved in the heat transport.