The Spartan ultraviolet coronal spectrometer observed a helmet streamer in 1993. The measured HI Ly- intensities are given in Table 10. The streamer intensities are about 8 to 10 times that of the coronal holes observed by the same instrument. Notice that the streamer intensity is about 50 times higher than that of the coronal hole observed in 1982.
Table 10. Streamer Intensities
* (ph s cm sr) The OVI intensities provided in Table 10 were derived from the calculated OVI values for a coronal hole, which are in Table 3, and Ly- intensities and PB values from Spartan. The prescription was to scale the resonantly scattered component of OVI in the hole at 1.5 solar radii by the streamer to hole Ly- ratio to obtain the streamer OVI resonant component at 1.5 solar radii, and scale the collisional component by the square of Ly-. Then scale the resonant component by the PB ratio to obtain the values for other heights. The collisional components are scaled by PB squared. The above approach is based on an assumption that the resonantly scattered Ly- and O VI intensities are proportional to the electron density in regions of negligible Doppler dimming, and that the collisional component of O VI is proportional to the square of the electron density. It is also assumed that PB is proportional to the electron density. The coronal hole values at larger heights were not used in the above because Doppler dimming is expected to have a large effect on the resonantly scattered components. (An alternative would have been to scale from the calculated O VI collisional components at larger heights.) The above method ignores changes in ionization balance between the hole and the streamer and between the base of the streamer and larger heights. It also ignores changes in the H and O abundances. Obviously, the above estimates are only a rough approximation.