3. Coronal Heating

(a) X-ray Bright Points
A major puzzle in astrophysics that has challenged scientists for over fifty years is to determine the mechanisms that are heating the solar corona.

The solar corona has a three-fold structure of x-ray bright points, coronal holes and coronal loops, and part of the coronal heating problem has recently been solved when it was demonstrated convincingly that the Converging Flux Model (Priest, Parnell and Martin; 1994, Astrophysical J. 427, 459) is a likely explanation for the majority of x-ray bright points. According to the model, magnetic fragments in the photosphere approach one another and drive reconnection in the overlying corona. As well as setting up a simple model and conducting a numerical experiment, we also compared with observations of Leon Golub from NIXT (see above Figure) which revealed the internal structure of bright points, one of them as shown having the shape of the wings and body of a beautiful bird. The underlying photospheric sources consist of four magnetic fragments, and, as they move together, the overlying reconnection heats loops that have the correct shape, as shown in Fig 3.1 (Parnell, Golub and Priest, 1994, Solar Phys. 151, 57).

(b) Heating of Coronal Loops and Holes
How are coronal loops and coronal holes heated? The main theories that have been proposed are:

1. Alven waves, which may dissipate either by phase mixing (Heyvaerts and Priest, 1993, Astron. Astrophys. 117, 220) or by resonant absorption (e.g. Goedbloed, 1983; Goossens, 1991; Davila, 1987).
2. Reconnection in many small current sheets scattered through the volume that form in response to braiding (e.g. Parker, 1994). This may in turn lead to an MHD turbulent state which may be analysed in a self-consistent way (Heyvaerts and Priest, 1993, Astrophys. J. 390, 297).

In a recent paper (Priest, Foley, Heyvaerts, Arber, Culhane and Acton, 1997a, Nature, in press; 1997b, Ap J, submitted) we have proposed a new two-part approach to trying to solve the coronal heating problem, namely:

1. Use observed temperature profiles to deduce the form of heating in loops or arcades;
2. Use that heating form to deduce the likely heating mechanism.

We have applied this approach to large-scale diffuse loops observed with the SXT instrument on Yohkoh. It implies that the loops are not heated preferentially near the loop base or near the loop summit, but instead rather uniformly along the loop. In turn this suggests that the most likely mechanism is turbulent magnetic reconnection in many current sheets distributed throughout the loop.

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