Heating of proton conics by resonant absorption in a multicomponent plasma: 1. Experimental evidence

ELF emissions observed on the low‐altitude AUREOL 3 satellite are seen in association with H+ ions at large pitch angle. The flux in the upward direction for ∼120° pitch angle is found to be equal to or larger than the flux at ∼60° pitch angle, which provides evidence for transverse acceleration at or below the spacecraft. These emissions have a sharp lower‐frequency cutoff of the transverse components of the electric field and a narrow peak at, or, more precisely, just below the proton gyrofrequency fH+. This narrow peak is more easily seen on the parallel component and appears as a narrow line on the spectrogram of this component. A statistical study of the occurrence of this line at f ∼ fH+ is presented. It is shown that this line is observed at relatively high invariant latitude within the light ion trough where a strong depletion of thermal H+ ions occurs. Detailed analysis of ELF waves observed just below fH+ demonstrates that they propagate in the left‐hand mode. These observations are interpreted as a signature of mode conversion from a fast magnetosonic mode into a slow proton cyclotron mode. It is suggested that this slow proton cyclotron wave can accelerate protons up to a few hundreds electron volts in the transverse direction. This mode conversion process can operate over a much broader of large altitude range than covered by AUREOL 3; it is a likely candidate for explaining the formation of H+ conics. Theoretical calculations that support the above conclusions are given in a companion paper by Le Quéau et al. (this issue).