Streamers, although observed for decades, are still poorly known features. This situation is due to spectroscopic instruments being, so far, unable to operate at the large heliocentric distances reached by streamers. On the other hand, these features are ideal structures to investigate the differences in temperature, density, flow velocity and magnetic field structure between closed and open coronal regions. There are three sub-topics to this program. All will be greatly enhanced by coordinated LASCO and in situ observations.
The first sub-topic, streamer structure, requires synoptic observations of streamers (low temporal resolution) to define their 3-D shape, growth, and ultimate fate. The purpose of this part of the observation program would be to make a detailed comparison between UVCS streamer observations and predictions from a 3-D numerical model of streamers (Noci et al. 1993). The model is flexible enough to allow us to change a number of parameters to represent a variety of different situations. Our goal is to achieve detailed agreement between data and model predictions, thus reaching a physical understanding of the streamer 3-D structure. Observationally, the 3-D behavior of temperature and density in the streamer can be derived via tomographic techniques.
The second sub-topic, cusps, requires an observing campaign focused on a small number of streamer cusps (three to six). We wish to analyze the wind flows above the cusp regions of coronal streamers. The properties of the solar wind at the edge of coronal streamers are very poorly understood. The contribution of the streamer to the overall solar wind flow is also unknown, although some believe coronal streamers to be the sources of the slow solar wind streams at 1 A.U. According to an existing 2-D MHD model of streamers in which magnetic reconnection should occur more-or-less continuously, enhanced plasma outflow velocities should characterize the high-density region of the streamer axis for some distance beyond the magnetic cusp in the low corona. So far there have been no direct observations of plasma velocities in the vicinity of the streamer cusp. The use of the direct velocity sensing capabilities of UVCS allows us to detect the predicted flow enhancements near the streamer axis and to determine the time scale over which they persist.
The third sub-topic concerns the transition layer. Little observational data covering the transition layer between the open field lines forming the coronal holes surrounding the streamers, and the closed field lines within streamers, are currently available. Our goal is to characterize the physical properties, such as plasma density, temperature and flow speed, in this transition layer. The program requires a series of coordinated space and ground-based observations to map these structures, and study their phenomenological and physical properties. UVCS and LASCO data will be complemented by coordinated ground-based observations of their origin at the solar surface, mainly via cm radio observations. Coordinated interplanetary scintillation measurements will be made with EISCAT in Norway to infer flow speeds beyond heliocentric distances covered by SOHO. Observations with UVCS and LASCO should be carried out with the best spatial resolution possible in both the radial and azimuthal directions.
The first column in the tables refers to the analysis of wind flows out of coronal streamers, and the second refers to the phenomenological structure of a coronal streamer and the edge of a coronal hole.
Stationary Coronal Streamers and Cusps