Abstract:

We present a study of nine months of data from the solar wind plasma electron instrument on the ACE spacecraft. Electron pitch angle distributions were used to identify intervals of counterstreaming halo electrons, which were observed about 16% of the time that the spacecraft was not connected to the bow shock. Counterstreaming electrons presumably indicate a closed magnetic field topology, and thus indicate the passage of coronal mass ejections (CMEs) in the solar wind. In this study, we separately examine electron moments at times with and without counterstreaming electrons, including both magnetic clouds and non-cloud CMEs. The properties of both the core and halo electron populations were nearly identical at times with and without counterstreaming electrons. Both low and high electron densities and temperatures were observed in either type of event. Magnetic clouds, on the other hand, on average showed increased densities and reduced temperatures. The core/halo density ratio and the total electron density were anticorrelated in all cases, indicating that the halo contributes more to the total electron density at low density times, regardless of magnetic topology. Total electron temperature was inversely proportional to electron density in all types of events. This consistent anticorrelation implies that such single-spacecraft measurements of temperature and density cannot be used to determine a polytropic index for solar wind electrons. Instead, the anticorrelation may be due to pressure balance of the solar wind plasma, or may be a remnant of coronal conditions.