Dayside energetic electron precipitation over the south pole (λ = 750)

Balloon bremsstrahlung measurements of electron precipitation between 25 and 500 keV were made near South Pole station, 750 invariant latitude, between December 15, 1985 and January 15, 1986. On January 2 and 3, 1986, a period of low to moderate geomagnetic activity, precipitation with characteristic (average) energy E0 in the range 20--200 keV was observed on the dayside, i.e., between ∼0500 and ∼1500 local magnetic time. In one event, with E0 intially 150 keV, 35-keV electrons arrived 9 min later than 350-keV ones; here the trapped energetic electron population may have been enhanced during an injection in the midnight sector and drifted eastward to undergo precipitation in the morning sector. In another event, E0 was 200 keV, but the least energetic electrons arrived within <0.5 min of the most energetic, indicating a scattering process close to the station. These are the first published reports known to use of precipitating electrons at L>10 with fluxes measured in the entire energy range from 20 keV to over 250 keV. Near noon on both days, effects were seen in the data that suggest passage of the cusp or polar cap boundary into the field of view of the balloon sensors. copyright American Geophysical Union 1988