In the solar atmosphere the initial decrease of temperature down to 4300 K at the temperature minimum is followed by an enormous increase with increasing distance from the photosphere. This increase is gradual in the chromosphere but very steep in the lower part of the solar corona where the temperature reaches up to K. This temperature increase is one of the most challenging observational facts for theorists in plasma-astrophysics. The high temperature of the solar corona is only possible if there is a non-thermal heating mechanism that is continuously working. Biermann (1946) and Schwarzschild (1948) suggested that the corona may be heated by sound waves. Until the mid 70's this acoustic wave heating model was accepted by most theorists. However, this model was largely abandoned when it became clear from UV observations, that the energy flux of the acoustic waves is too small in the upper part of the solar atmosphere to account for the observed radiative losses.
The X-ray observations from Skylab showed that more than 90% of the X-ray flux originates from closed loops with the heating being directly linked to the solar magnetic field. The source of this heating has to be the convection zone which is a huge reservoir of kinetic energy. The real problem is to explain how a fraction of this kinetic energy is transported to the coronal layers and how it is dissipated and converted into heat there.
There are three broad classical branches of theories for plasma heating that have been considered during the last two decades: Alfvén wave heating, resistive dissipation of direct electric currents and selective decay of a turbulent cascade of magnetic fields. The literature on these theories has undergone a recent expansion (see, e.g., Browning, 1991; Cargill, 1993; Forbes, 1991; Gomez, 1990; Heyvaerts, 1990; Hollweg, 1991; Narain & Ulmschneider, 1990,1996; Melrose, 1990; Priest, 1990 and Zirker, 1993).
In this paper we consider observations obtained by the SUMER (Wilhelm et al. 1995) instrument on the Solar and Heliospheric Observatory spacecraft (SOHO) to look for signatures of wave heating. Practically there are two possibilities for detecting wave activity in the solar atmosphere: (i) the first is via measuring directly the individual intensity oscillations if these oscillations can be resolved spatially (e.g., Doyle et al. 1997, 1998); or (ii) measure the averaged effect of oscillations in the broadening of optically thin emission lines. The latter being the subject of the present paper.
There have been numerous measurements of line widths, e.g., Boland et al. (1975), Moe & Nicholas (1977), Cheng et al. (1979), and Mariska et al. (1978, 1979). All the different estimations involving wave heating have assumed that the observed non-thermal broadening is isotropic in nature, i.e., it has the same magnitude at disk center and at the limb. Previous results have tended to support this assumption (Mariska et al., 1978), although Roussel-Dupré et al. (1979) did suggest a center-to-limb variation. Here we look again at this question analysing observational data taken at different locations on the solar disk and comparing these to calculated widths assuming different types of wave heating.
Note that although MHD (Alfvén) waves are good candidates to explain the center-to-limb line broadening, there can also be other physical processes responsible for this, e.g., turbulent line broadening produced by nanoflares (e.g., Cargill 1996), explosive events (e.g., Innes et al. 1997, Erdélyi et al. 1997, 1998, Sarro et al. 1997).
The high spectral and spatial resolution capabilities of SUMER can enable a search for Alfvén and/or magneto-acoustic waves in the solar atmosphere via an observational sequence as suggested by McClements et al. (1991). The original idea was to select an active region loop at disk center, measuring the widths in different high temperature lines as the loop moved towards the limb. If Alfvén waves are dominant in the coronal loop than there should be a broadening of the line widths from disk center to limb. However, if the magneto-acoustic waves have the dominant contribution, the width of the lines should decrease. Before attempting this type of observation we choose to observe an average quiet Sun region at disk center and at the limb, measuring the widths of lines formed at chromospheric and coronal temperatures.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998