Electron distribution functions associated with electrostatic emissions in the dayside magnetosphere

Electron distribution functions constructed from ISEE‐1 dayside observations of low energy magnetospheric electrons at low latitudes are presented. We choose examples corresponding to three distinct types of odd half harmonic electrostatic emissions observed by the plasma wave experiment on ISEE‐1and occurring between the electron cyclotron frequency f ce and the upper hybrid frequency f UH . These are (a) intense (3/2)f ce emissions; (b) weak (n + 1/2)f ce noise; and (c) intense f UH waves at (9/2)f ce . No loss cone‐like features are discernable in any of the distribution functions at the level of the instrument angular resolution. Howevera weak ringlike featureconsisting of a small localized region in velocity space near 4 × 10 9 cm s −1 and a = 90° where ∂f/∂v ⊥ > 0is found in the distribution function corresponding to the occurrence of the intense f UH noise. This ringlike feature may provide a source of free energy to drive the f UH instability. The apparent absence of detectable loss cones in the cases of the intense (3/2)f ce and f UH emissions is consistent with the expectation that these plasmas are roughly at the level of strong pitch angle diffusion computed from the observed wave intensities at these times. Although each of the distribution functions displays a temperature anisotropy T ⊥ > T ∥ the anisotropies are too small to excite any of the odd half harmonic emissions.