Session 4 - SOHO Corona II.
Oral session, Friday, June 27
Ballroom A, Chair: Therese Kucera

## [4.03] An Observational Test of Transition Region Lines as a Pressure Gauge

G. H. Fisher (UC Berkeley), N. W. Griffiths (UC Berkeley), O. H. W. Siegmund (UC Berkeley)

The simplest models of the chromosphere-corona transition region assume a balance between the divergence of conductive heat flux and optically thin radiative losses. Hawley and Fisher (1992, Ap. Suppl. 78, 565) used this assumption to derive numerical proportionalities between the plasma pressure at 10^5K and the surface flux of transition region lines such as CIV (1548Åand NV (1242ÅWhile it is generally thought that this model is too simple to explain observed emission measure distributions below 10^5K, it would be interesting to see if the model applies at any transition region temperature. The capabilities of the SUMER instrument on SOHO allow us to test such pressure gauge'' relationships directly. We have designed an observing sequence which measures fluxes in N II,III,IV, and NV spectral lines, formed at temperatures ranging from \simeq 3 \times 10^4K to \simeq 3 \times 10^5K. We also observe lines of O II,III,IV, and OV which are formed in a similar temperature range. We isolate the role of possible abundance variations by examining lines from each element separately. Surface fluxes measured from the SUMER observations can be used to derive plasma pressure values, assuming the model is valid. We also measure the density sensitive line pair OIV (1401Åand 1407Åwhich provides an independent estimate of the plasma density and hence pressure, since the line formation temperature is known.

Our observing sequence has been run on both the quiet Sun and on an active region. We will discuss the results of our study. This research was supported by NASA.