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ACE News #45 - May 9, 2000

ACE News Archives

What is driving CME expansion in the low corona ?

Coronal Mass Ejections (CMEs) are often observed to expand after leaving the solar surface. Expansion is an important aspect of the dynamics of these ejections and needs to be understood in order to accurately predict when the CME will arrive at the Earth. Using data from ACE/SWICS, ACE/MAG, and ACE/SWEPAM, we have investigated CMEs detected during 1998, using the presence of counter-streaming halo electrons as the identifying signature. Some of these CMEs have gradually decreasing speed profiles such as that shown in the first panel of the plot above. This signature suggests radial expansion of the CME, as depicted in the sketch above: the leading edge travels with the sum of the bulk velocity away from the Sun plus the expansion velocity, while the trailing edge travels slower, at the bulk velocity minus the expansion velocity. Assuming the CME expands adiabatically, the ratio of the plasma temperature to the magnetic field strength (T/B) and the quantity T(B/n)2, where n is the plasma density, are expected to be constant so that the values of T, B, and n measured at the spacecraft can be directly related to the same parameters close to the Sun. The initial plasma temperature can be determined from the measured O7+/O6+ ratio (second panel), thus allowing us to estimate the magnetic field of a CME near the solar surface, which is a quantity that cannot be easily measured. In addition, the ratio of the particle pressure to the magnetic pressure (known as the beta of the plasma) can also be derived close to the Sun (last panel). In this particular CME, the plasma beta is clearly less than unity, indicating the dominance of magnetic pressure during expansion. Applying this technique to several CME events, we have found that some CMEs have a plasma beta near unity, indicating a balance between kinetic and magnetic pressures. Our current results suggest that the plasma betas of CMEs may follow a bimodal distribution.

Contributed by Alysha Reinard and Thomas Zurbuchen of the University of Michigan.

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Last modified 9 May 2000, by Steve Sears