Auroral electron fluxes parallel to the geomagnetic field lines

Auroral electron flux characteristics from ∼300 eV to 30 keV were measured with twelve detectors launched on the ESRO Skylark rocket S46/1 from Kiruna, Sweden, on November 10, 1969, at ∼2346 magnetic local time (MLT) into a breakup aurora. The energetic electrons (E ≳ 5 keV) displayed isotropic angular distributions during the entire flight, whereas the lowest-energy electrons (E ≲ 5 keV) were characterized by very anisotropic angular distributions, maximum along geomagnetic field lines when the precipitation was enhanced, ≳108 (cm² s sr keV)−1. The highest value of the ratio between the fluxes measured at 0° and 30° pitch angles was about 40. The energetic and angular distributions thus observed must result from the action of a selective energy mechanism acting mainly on the parallel component of the electron velocity. Experimental results are in agreement with the presence of a parallel electric field along the field lines in the upper ionosphere.