The aim of JOP042 is to find the origin of coronal heating through studies of line profiles and line broadening of transition region lines. Preparatory work on JOP042 started after a discussion between Eric Priest and Per Maltby, where Eric argued in favour of phase-mixing of Alfv{\'e}n waves (see Heyvaerts \& Priest, A\&A 117,220), and Per argued in favour of nanoflares in the corona causing MHD disturbances travelling towards the chromosphere (see Hansteen ApJ 402, 741 and Hansteen, Maltby \& Malagoli 1996). In order to prove/disprove one or both options, we decided for a joint St.Andrews (Ireland, Priest)/Oslo (Brekke, Maltby) proposal JOP042.
In order for the phase-mixing hypothesis to work, gardients in the magnetic field in the horizontal plane at photospheric heights must be present. Hence, high resolution magnetic field- and velocity field-- observations by MDI are essential for deciding whether or not Alfv{\'e}n waves coming from below are essential for explaining the line profiles observed in the transition region.
We plan to observe at Sun center and request that MDI observe high resolution magnetograms, dopplergrams and line depth at a 30s (60s) cadence. The JOP has a scheduled length of ca. 8hr but MDI support for the full length of the experiment is not essential.
A fuller description of JOP 42 can be found on the WWW at http://sohowww.nascom.nasa.gov/operations/JOPs/jop042.txt.