LASCO and EIT Observations of Coronal Mass Ejections

The LASCO and EIT instruments on the SOHO spacecraft have provided an unprecedented set of observations for studying the physics of coronal mass ejections (CMEs). They provide the ability to view the pre-event corona, the initiation of the CME and its evolution from the surface of the Sun through 30 R ⊙. An example of the capability of these instruments is provided in a description of a single event (Dere et al., 1997). During the first 2 years of operation of LASCO and EIT on SOHO, a substantial fraction, on the order of 25 to 50%, of the CMEs observed exhibited structure consistent with the ejection of a helical magnetic flux rope. An examples of these has been reported by Chen et al. (1997) and Dere et al. (1999). These events may be the coronal counterpart of magnetic clouds discussed by Burlaga et al.(1981) and Klein and Burlaga (1982). They analyzed observations of magnetic fields behind interplanetary shocks and deduced that the field topology was that of a helical flux rope. Recently, we have explored a number of the consequences of the helical flux rope description of these types of CMEs. Vourlidas et al. (1999) examined the energetics of CMEs with data from the LASCO coronagraphs on SOHO. The LASCO observations provide fairly direct measurements of the mass, velocity and dimensions of CMEs. Using these basic measurements, we determined the potential and kinetic energies and their evolution for several CMEs that exhibited a flux-rope morphology. Assuming magnetic flux conservation ('frozen-in' fields), we used observations of the magnetic flux in a variety of magnetic clouds near the Earth to determine the magnetic flux and magnetic energy in CMEs near the Sun. Figure 1 shows these quantities for a few representative flux rope CMEs. In general, we find that the potential and kinetic energies increase at the expense of the magnetic energy as the CME moves out, keeping the total energy roughly constant. This demonstrates that flux rope CMEs are magnetically driven. Furthermore, since their total energy is constant, the flux rope parts of the CMEs can be considered to be a closed system above ∼ 2 R ⊙. Subramanian et al. (1999) examined images from LASCO to study the relationship of coronal mass ejections (CMEs) to coronal streamers. We wished to test the