Presentation #300.01 in the session “Solar Physics Division (SPD) George Ellery Hale Prize Lecture”.
The unexpected observation in 1971 of a sudden expulsion of mass through the solar corona generated a new field of interest in solar and stellar physics. The discovery was with a white light coronagraph, creating an artificial eclipse of the Sun, enabling the viewing of the faint glow from the corona. Although unexpected, there were previous observations which we now interpret as the coronal mass ejection or CME, such as during the1860 eclipse and the 1955 discovery of moving Type IV radio emission. Erupting prominences were generally thought to drain back to the Sun, and therefore were not leaving the Sun. although we now know that many do erupt as part of the CME process. Our understanding of the CME has evolved over the last five decades, as the observations of CMEs and their global context have greatly improved. As more details of the process were revealed, the theoretical interpretation also evolved. Very importantly, the counterpart of the near-Sun event, became identified with in-situ observations in the solar wind. Because the in-situ event appeared to be so different from the coronal event that it was called an Interplanetary Coronal Mass Ejection (ICME). The distinguishing characteristic of the CME in the coronagraph is the excess mass (the coronagraph does not measure the magnetic field), whereas for the ICME it is the low mass density and strong, uniform field. The interpretation of the CME has evolved from the MHD response to an impulse (temperature or mass increase) at the base of the corona to an instability releasing a magnetic flux rope. In this talk, we will provide an historical review of the evolution of observations and theories.