Bulk flow velocities in the solar corona at solar maximum

Abstract

We present velocity estimates of bulk motions seen in the solar corona by the Large Angle Spectrometric Coronagraph (LASCO) onboard the Solar and Heliospheric Observatory (SOHO) spacecraft. These estimates reflect the mass-weighted mean outflow of all coronal mass ejections (CMEs) seen during a 1-yr period on the approach to solar maximum. The particular period of interest spans 1999 March 6 to 2000 March 5 and velocities are compared with those obtained previously during an epoch of minimum activity. We use a correlation analysis and we address issues concerned with qualifying this method with regard to its possible limitations. Our method allows us to determine objectively velocity estimates at altitudes from 2 to 30 R⊙ at all latitudes. This represents an increased range over what was possible at solar minimum, and the high number of events has also allowed for much improved statistical accuracy in our estimates. At all latitudes we find outflow speed profiles that are similar in form to those seen in the equatorial region at solar minimum, although overall they are slightly faster. We do not see the large-scale latitudinal dependence of velocity evolution that was apparent at solar minimum and we associate this with the much reduced strength of the solar magnetic dipole relative to higher-order multipoles. We find that a typical kilogram of ejected plasma enters the C2 field at a speed of around 50 km s−1. The typical kilogram of plasma accelerates at a reasonably uniform rate of about 5 m s−2 and eventually reaches the edge of the C3 field with a velocity of ∼450 km s−1. To maintain this trajectory we find that the ejected plasma requires a remarkably constant input of energy, at a rate of around 2.2×106 W kg-1, throughout its entire evolution. In total, we find that a typical kilogram of plasma takes approximately 24 h to traverse the combined field of the LASCO white-light coronagraphs. This is somewhat slower than typically quoted CME speeds and highlights how the velocity associated with the leading edge of an event does not always accurately reflect the motion associated with the bulk of the mass of an event. We compare our velocity profiles with a basic model describing the expansion of an isothermal solar wind and find strong similarities in the flow speed. We suggest that at altitudes as low as 5 R⊙ in the corona the motion of the mass-dominant class of CMEs is governed by their immersion in the surrounding wind flow. We discuss the relaxing of the internal energy requirements of the CME phenomenon associated with their being carried through the lower corona by the solar wind and provide what we believe to be the most reasonably parametrized velocity profiles to be met by models of CME eruption.