Energy transport in the solar transition layer

We have developed a theory for the differential emission measure in the sol ar transition layer, which has a temperature in the range 10(4) K < T < 10( 6) K. On comparing the resulting log versus log T plot with curves derived from observations, we find good agreement over the whole range. As heat flo ws down from the coronal reservoir, the vertical temperature gradient T ' i ncreases until it reaches a critical value, Tm, beyond which any further in crease takes the heat flux in the electron gas parallel to the magnetic fie ld (q(e)//) above the value that could be convected by the electrons drifti ng at the ion sound speed Cs. This gradient is reached at about the midpoin t of the temperature range and we infer that as a consequence the ion-acous tic instability generates turbulence in the ion gas; this substantially inc reases the effective collision frequencies in the ion fluid with the result that at about T = 10(5) X, where T ' = T ' (m), the parallel electron heat flux and the perpendicular ion heat flux (qi(perpendicular to)) are compar able. Below T = 10(5) K, qi(perpendicular to) is dominant and gives rise to a slope in our plot of ca. -3.5, as observed, whereas above this temperatu re, q(e)// dominates and the slope increases to ca. 1.5, as is also observe d. Ohmic dissipation by the ion-sound limited current leads to a heating ra te of the form AT(-5/2), where the constant A depends on the unknown size o f the flux tubes involved in the heat transport.