WHY DOES THE APPARENT MASS OF A CORONAL MASS EJECTION INCREASE?

Mass is one of the most fundamental parameters characterizing the dynamics of a coronal mass ejection (CME). It has been found that CME apparent mass measured from the brightness enhancement in coronagraphs increases during its evolution in the corona. However, the physics behind it is not clear. Does the apparent mass gain come from the outflow from the dimming regions in the low corona, or from the pileup of the solar wind plasma around the CME? Here we analyze the mass evolution of six CME events. Based on the coronagraph observations from the Solar Terrestrial Relations Observatory, we find that their masses increased by a factor of 1.3-1.7 from 7 to 15 R-S, where the occulting effect is negligible. We then adopt the "snow-plow" model to calculate the mass contribution of the piled-up solar wind. The result gives evidence that the solar wind pileup probably makes a non-negligible contribution to the mass increase. In the height range from about 7 to 15 R-S, the ratio of the modeled to the measured mass increase is roughly larger than 0.55 though the ratios are believed to be overestimated. It is not clear yet whether the solar wind pileup is a major contributor to the final mass derived from coronagraph observations, but it does play an increasingly important role in the mass increase as a CME moves further away from the Sun.