Auroral proton precipitation and hydrogen emissions
Authors: Eather R. H. 
Published: February 2008

Protons in the one to hundreds of kev energy range precipitate into the atmosphere in the auroral zone. Limited rocket and satellite measurements suggest total nighttime fluxes of the order of 107 cm−2 sec−1sterad−1 for energies ≳10 kev, with about 1% of this flux above 100 kev, and perhaps 10−3% above 500 kev, as being reasonable but not necessarily average. The proton flux below 10 kev may exceed the flux above 10 kev by as much as a factor of 10 or even 100. Daytime fluxes appear to be less than nighttime fluxes. Various pitch‐angle distributions have been reported, and there is some suggestion that the proton flux at high pitch angles increases with decreasing proton energies. The region of precipitation is normally a broad (3°–7° of latitude) diffuse zone that locates on the equatorward side of the region of electron precipitation. Protons undergo charge‐exchange interactions in the atmosphere, forming neutral hydrogen atoms in excited states in sufficient quantity to give readily detectable Balmer series emissions (Hα, Hβ, and Hγ) on the ground. The Hβ intensity is typically less than 100 R, and the Balmer decrement Hα/Hβ is about 3. This proton precipitation also results in the excitation of various oxygen and nitrogen emission, such as λ3914, λ4709 N2+, and λ5577 O I, and theoretical ratios of the intensities of these emissions to the Hβ intensity in an aurora excited entirely by protons are 5‐20, 0.3‐1.0, and 4‐12, respectively. Strong visual auroras cannot be excited entirely by protons.

The Balmer emissions are radiated by moving hydrogen atoms, and thus the radiation is Doppler‐shifted. The magnetic‐zenith profile commonly shows about a 6‐A Doppler shift of the profile peak, whereas the magnetic‐horizon profile is unshifted; the proton pitch‐angle distribution results in a Doppler broadening of both profiles. The shape of the hydrogen line profiles reflects the energy and pitch‐angle distributions of the incident protons, as well as the energy dependence of the charge‐exchange cross sections involved. Theoretical interpretation of measured line profiles in terms of simplified models of proton precipitation has led to dubious conclusions. There is little or no dependence of the occurrence of hydrogen auroras on the presence of stronger, visual (electron‐excited) auroras, though visual auroras may frequently be superimposed on the broader, diffuse zone of hydrogen emission. There is little evidence for rapid variations in hydrogen intensities similar to the pulsations often observed in electron‐excited emissions, though longer period (∼minutes) variations have been reported. The zone of hydrogen emission locates equatorward of quiet visual arcs and moves to lower latitudes before midnight and back poleward again after midnight, and it often expands poleward in association with auroral breakup events. The zone appears to widen and move equatorward during increased magnetic activity.

Other effects of proton precipitation include ‘r’ type Es ionization and possibly certain types of radio auroras. Polar‐glow auroras are associated with higher‐energy proton events (though the main excitation could be due to lower‐energy particles). Low‐energy proton precipitation may cause appreciable heating of the upper atmosphere and may excite certain high‐altitude red arcs at both midlatitudes and high latitudes. Auroral protons may also represent a significant source of neutral hydrogen at auroral latitudes.