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 coronagraph images shows an increasing trend during its evolution in the corona .
However , the physics behind it is not clear .
Does the apparent mass gain come from the mass outflow from the dimming regions in the low corona , or from the pileup of the solar wind plasma around the CME when it propagates outwards from the Sun ?
We analyzed the mass evolution of six CME events .
Their mass can increase by a factor of 1.6 to 3.2 from 4 to 15 Rs in the field of view ( FOV ) of the coronagraph on board the Solar Terrestrial Relations Observatory ( STEREO ) .
Over the distance about 7 to 15 Rs , where the coronagraph occulting effect can be negligible , the mass can increase by a factor of 1.3 to 1.7 .
We adopted the ‘ snow-plough ’ model to calculate the mass contribution of the piled-up solar wind in the height range from about 7 to 15 Rs .
For 2/3 of the events , the solar wind pileup is not sufficient to explain the measured mass increase .
In the height range from about 7 to 15 \mathrm { R _ { S } } , the ratio of the modelled to the measured mass increase is roughly larger than 0.55 .
Although the ratios are believed to be overestimated , the result gives evidence that the solar wind pileup probably makes a non-negligible contribution to the mass increase .
It is not clear yet whether the solar wind pileup is a major contributor to the final mass derived from coronagraph observations .
However , our study suggests that the solar wind pileup plays increasingly important role in the mass increase as a CME moves further away from the Sun .