Estimation of solar wind speed within 20Rs of the Sun by using limb CMEs

The speeds of propagation of CMEs in interplanetary space are less distributed than their initial speeds measured on their departure from the limb of the Sun. Gopalswamy et al.(2000, 2001) presented a linear relationship between initial speeds of limb CMEs and their average acceleration during their travel time in interplanetary space. The linear relationship suggests that some dragging force is acting on CMEs, depending on difference in speed between the CME and their ambient plasmas. The ambient speed obtained from the coeficcients of the linear relationship was 406 km/s, which is nearly the same as the real solar wind speed. If similar relationship holds within 20 solar radii from the Sun, it would give information on the initial speed of 'ambient' solar wind in the vicinity of the Sun. The relationship between the initial acceleration and the initial speeds of limb CMEs was examined by using SOHO/LASCO CME Catalogue (http://cdaw.gsfc.nasa.gov/CME_list/). Coefficients of correlation between the initial acceleration and the initial speeds of low-latitude CMEs were calculated by sliding 27-day windows in 1999. Although there were many cases where linear relationship was not clear, we found significant number of periods for which correlation coefficient was fairly good (from -0.6 to -1). For such cases, the 'ambient' solar wind speed within 20 solar radii was estimated to be 150 - 570 km/s. It is somewhat slower than but close to the speeds of real solar wind measured in interplanetary space. It suggests that low-latitude solar wind plasma was accelerated within a short distance. It may also indicate that coronal holes are not the only source of the solar wind. The 'ambient' speeds thus obtained did not always agreed with simultaneous, in-situ measurements by NOZOMI and ACE. Estimation of 'ambient' speed was also carried out by using CMEs that appeared in higher latitude, but no latitudinal dependence was found. Acknowledgments:This CME catalog is generated and maintained by NASA and The Catholic University of America in cooperation with the Naval Research Laboratory. SOHO is a project of international cooperation between ESA and NASA. Reference: Gopalswamy et al., GRL, 27, p145, 2000. Gopalswamy et al., JGR, 106, p29207, 2001.