Extremely cold electrons in the January 1997 magnetic cloud

Abstract

The 3D Plasma and Energetic Particle (3DP) instrument on the WIND spacecraft detected extremely cold solar wind electrons in the January 10–11, 1997 magnetic cloud, ∼4 times lower temperature than any previously reported measurement. Detailed fits to the electron distributions show that the core electron temperature, Tec, generally ranged from ∼1 to 4.5 eV through the cloud, dropping to a low of ∼0.7 eV in an unusually dense (n>∼150 cm−3) region in the trailing portion of the cloud. Due to the extremely low average electron temperature Te and high density, the ions and electrons are collisionally coupled as confirmed by Te=Tp, the first such observation in the solar wind. For most of the cloud the halo density is very low (neh<∼0.1 cm−3), implying magnetic disconnection from the Sun. Correlations in Te and Tp are observed throughout the cloud and are particularly evident when neh is low. We suggest the diminished halo density has reduced a significant heat source to the core population, thus allowing the electrons to cool more thoroughly. Furthermore, we show that Coulomb collisions are a significant mechanism of energy transfer between the halo and core distributions under normal solar wind conditions.