Electron demagnetization and collisionless magnetic reconnection in βe << 1 plasmas -: art. no. 092903

Abrupt, intense electric field enhancements (EFEs) with E>100 mV/m surveyed over 3 years of NASA's Polar spacecraft data are used to illustrate the occurrence and locales of nonguiding center demagnetization of thermal electrons in strongly inhomogeneous electric fields. A lower bound E-*(a) on the perpendicular electric strength sufficient to cause nongyrotropic effects on the electron pressure tensor is determined for EFE thickness Delta x=a rho(e). Minimum E-*(a) occurs when a approximate to 1. Of 258 observed EFEs, 15.3% (39) are demagnetizing (DEFEs) with E >= E-*(1). DEFEs occur within 3x10(-5)<=beta(e)<= 3x10(-1), while EFEs are found as low as beta(e)=10(-8). While E-*(1) does not depend on the ambient density, the DEFEs are organized by the density-dependent inequality lambda(De)/rho(e)< 1 and are consistently understood as sites where the electron pressure tensor could become agyrotropic, enabling collisionless magnetic reconnection. The geophysical locales of the demagnetizing EFEs are not random, always occurring within magnetic cusp invariant latitudes, strongly concentrated at noon magnetic local times and at orbit apogee near the nominal magnetopause. (C) 2005 American Institute of Physics.