Coronal heating: analogous processes in stellar and galactic media

Coronal plasmas of high temperature (106 K and more) have been confirmed now to exist not only around stars like the Sun, but also around the planes of galaxies including our Milky Way and even extragalactic systems. Structural similarities exist between stellar atmospheres comprising coronae and transition zones and the dilute plasma around galaxies. In this paper the heating of these plasmas is proposed to be partly caused by collisional dissipation of fast particles, which are able to overcome the lower layers of the solar chromosphere or, correspondingly, those near a galactic plane. The production of fast particles heating the coronal plasmas may have a multicausal origin which is not considered here in detail. In the case of the Sun, the transition zone between the photosphere (T < 104 K) and the corona (T > 106 K) is well investigated by earlier observations, e.g. in the visible and UV spectral ranges. On the average, the strong gradients of temperature (positive) and density (negative) are stationary. They can be explained by the assumption of a balance between, on the one hand, a steady flow of suprathermal particles (kinetic energy Ek) outward with a spectral energy distribution density N(Ek) ∝ E−(3÷4) k , and, on the other hand, an energy loss which is mainly due to radiation. As to our Galaxy, the transition between the neutral hydrogen (HI) layer near the plane (scale height about 130 pc) and the outer “halo” region f is not well known. But the existence of fast particles, even relativistic ones, is proved by their gyro-synchrotron radiation. Moreover, by the observations of ROSAT, an X-ray emitting Galactic corona is indicated (similarly “patchy” as the solar corona). The assumption is justified that, in larger dimensions and in a plane geometry, gradients in temperature and density onto this Galactic corona exist. Using the observed scale heights of neutral HI and of free electrons, a rough calculation for the slope of temperature and electron density is possible with the aid of Saha’s equation.