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Zanetti et al. 1981
Zanetti, L.J., Potemra, T.A., Doering, J.P., Lee, J.S. and Hoffman, R.A. (1981). Magnetic field-aligned electron distributions in the dayside cusp. Journal of Geophysical Research 86: doi: 10.1029/JA080i011p08957. issn: 0148-0227.

Observations of low-energy electron fluxes made over a 6-year period with the photoelectron spectrometer on board the AE-C and AE-D spacecraft have been used to compile the first detailed survey of electron pitch angle in the low-altitude cusp. One 16-point energy spectrum from 2 to 500 eV was obtained every 1/4 s with the photoelectron spectrometer instrument (with energy resolution ΔE/E = 2.5%) providing a pitch angles resolution of about 6¿ at the 4-rpm spacecraft spin rate. The location of the cusp was verified by the presence of protons detected with the low-energy electron instrument. Observations were generally acquired below a 300-km altitude with a few cases near 1000-km altitude. Isotropic fluxes of precipitating electrons with Maxwellian energy spectra and characteristic energies of a few tens of electron volts were observed in the low-altitude cusp as expected. However, the outstanding result that has emerged from this study is the presence of low-energy electrons with pitch angles less than 15¿ streaming down into the cusp ionosphere. The energies of these streaming electrons sometimes appear as a peak superimposed upon the 'normal' cusp Maxwellian background of isotropic electrons. The streaming electron dluxes appear in about half the cusp observations, so we believe them to be stable features (i.e., they are not short bursts of particles). Using the velocity distribution contours computed from these photoelectron spectrometer measurements, we conclude that the streaming electrons cannot be explained in terms of a cusp plasma falling through an electric potential above the Atmosphere Explorer spacecraft.

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Journal of Geophysical Research
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