2.4 Velocity and energy

Without special declaration, the CME velocity general means the radial propagation speed of the top part of a CME frontal loop. However, it should be noted that this velocity measures the motion of the CME frontal loop projected in the plane of the sky, therefore, it can be called projected velocity. There are continuous attempts trying to correct the propagation velocity for the projection effects. The CME projected velocity ranges from ∼ 20 km s–1 to > 2000 km s–1, occasionally reaching 3500 km s–1. The averaged velocity increases from 300 km s–1 near solar minimum to 500 km s–1 near solar maximum (Yashiro et al., 2004Jump To The Next Citation Point).

Two issues should be emphasized here. First, it is often taken for granted that the observed CME propagation in the coronagraph images is a mass motion, which was not judged based on spectroscopic observations. On the contrary, there is accumulating evidence to reveal that the propagation of the frontal loop in many CME events, especially those fast events, is not a mass motion. This issue is actually strongly related to the nature of CMEs (see discussions in Section 4.4). Second, all the plasmas within the CME cavity are also moving outward, not just the material along the frontal loop, as implied by spectral observations in the dimming region under the CME (Harra and Sterling, 2001Jump To The Next Citation Point).

It is found that the kinetic and potential energies of a typical CME amount to 1022 – 1025 J, which is similar to that of solar flares (Vourlidas et al., 2002; Emslie et al., 2004).


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