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ACE News #43 - March 31, 2000

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A coronal mass ejection (CME) observed in the solar wind by both ACE (1 AU) and Ulysses (5 AU)

In March, 1998, the ACE and Ulysses spacecraft were radially aligned with respect to the Sun. Fortuitously, a magnetic cloud was observed at both spacecraft during this lineup. Magnetic clouds are a subset of coronal mass ejections (CMEs) observed in the solar wind, characterized by a low proton temperature, a strong magnetic field, and a smooth rotation in the magnetic field direction. The event was first observed as a halo CME at the Sun on February 28, 1998. The cloud then passed ACE at 1 AU on March 4-6, and Ulysses at 5.4 AU on March 24-28. This spacecraft alignment provided a unique opportunity to observe the same CME at two very different distances from the Sun and to examine the expansion behavior of plasma within a CME.

The figure shows the relationship between temperature and density for the solar wind electrons (E<1 keV) measured by the Los Alamos ACE/SWEPAM and Ulysses/SWOOPS instruments. The black line gives a least-squares fit to the ACE data: the observed anti-correlation between temperature and density suggests that temperature might increase as the CME expands and the density drops. However, combined ACE and Ulysses observations show a different result. A fit to the combined data (red line) clearly shows that both the electron temperature and density decreased as the CME expanded from 1 to 5.4 AU. The blue line shows the temperature decrease that would be expected for an adiabatic expansion, with no interaction between the particles. The observed temperature decrease was less than that for an adiabatic expansion, implying that there is some interaction between solar wind particles and allowing quantification of the interactions. The solar wind proton temperature decrease between 1 and 5.4 AU was similar to that of the electrons. However, the solar wind alpha particle (He++) temperature decrease was smaller, suggesting that there is a process such as a plasma instability that removes less energy from the alphas than from the protons or electrons as the CME expands.

Contributed by Ruth Skoug, Dave McComas and Dan Reisenfeld of Los Alamos National Labs.

For more info on ACE/SWEPAM and Ulysses/SWOOPS, see and

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Last modified 31 March 2000, by Andrew Davis