On the temperature profile and heat flux in the solar corona: Kinetic simulations

In the solar corona the collisional mean free path lambda for a thermal par ticle (electrons or protons) is of the order of 10(-2) to 10(-4) times the typical scale of variation H of macroscopic quantities like the density or the temperature. Despite the relative smallness of the ratio lambda /H, an increasingly large number of authors have become convinced that the heat fl ux in such a plasma cannot be described satisfactorily by theories which su ppose that the local particle velocity distribution functions are close to Maxwellian. We address this question through kinetic simulations of the low solar corona by assuming that non thermal velocity distribution functions are present at the base of the corona. In particular, we show that if one a ssumes that the electron velocity distribution functions at the base of the corona have sufficiently strong suprathermal power law tails, the heat flu x may ow upwards, i.e. in the direction of increasing temperature. Using ka ppa velocity distribution functions as prototypes for non thermal velocity distributions, we find that the heat conduction can be properly described b y the classical Spitzer & Harm (1953) law provided the kappa index is great er than or similar to 5. This value is much smaller than the value previous ly found by Dorelli & Scudder (1999). In addition we show that, unless extr emely strong power law tails are assumed near the base of the corona (i.e. k <4), a local heating mechanism (e.g. waves) is needed to sustain the temp erature gradient between the base of the corona and the coronal temperature maximum.