Observations of the solar wind speed near the sun

Details Observations of the solar wind speed near the sun Observations of the solar wind speed near the sun

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996aipc..382..108g&link_type=abstract

Proceedings of the eigth international solar wind conference: Solar wind eight. AIP Conference Proceedings, Volume 382, pp. 108

Computer Science

3

Particle Emission, Solar Wind, Particle Measurements

Scientific paper

Two-antenna scintillation (IPS) observations can provide accurate measurements of the velocity with which electron density fluctuations drift past the line of sight. These fluctuations can be used as tracers for the solar plasma and allow us to estimate the solar wind velocity near the Sun where spacecraft have not yet penetrated. We present recent IPS measurements made with the EISCAT and VLBA arrays. We have found that by using baselines which are several times the scale size of the diffraction pattern we are able to partially deconvolve the line of sight integration which affects remote sensing data. The long baselines allow the fast and slow components of the solar wind to be separated and their velocities estimated individually. In modeling IPS it is important that the scattering be ``weak'' because the model then requires only 1 spatial parameter instead of 3. EISCAT can only operate near 933MHz which limits the observation to outside of 18Rsolar, however the VLBA has higher frequency receivers which allow it to observe inside of 15Rsolar. The density variance δNe2 in the fast wind is a factor of 10-15 less than in the slow (Coles et al., 1995) making it necessary to consider the entire line of sight, particularly when the fast wind occupies the center portion. Using the point of closest approach and the average velocity to characterize the observation can lead to an incorrect interpretation of the data. We have compared our IPS observations with maps made from the Yohkoh soft X ray, HAO's white-light electron density, and Stanford magnetic field measurements as well as with the IMP8 and Ulysses spacecraft data to assist in placing the fast and slow wind. Here we have selected those observation from 1994 which were dominated by the southern coronal hole and have estimated a velocity acceleration profile for the fast solar wind between 7 and 100Rsolar which is presented in Figure 1. The observations suggest that the fast solar wind is fully developed by ~7Rsolar and shows no acceleration over the distance range probed. It is also inconsistent with wave driven acceleration models which continue to show significant acceleration out past 20Rsolar. This talk has been submitted to Nature (Grall et al., 1995).

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