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The best seen visible auroral forms are due to precipitating electrons. The hydrogen emission lines were first detected in auroral spectra by Vegard (1939), and the classic review paper on the early work is by Eather (1967). Rocket and satellite measurements have shown later that both energetic H+ and O+ ions can precipitate into the atmosphere (e.g., Shelley et al., 1972). O+ precipitation has been discussed more by Kozyra et al. (1982) and Ishimoto et al. (1986). For information about neutral H and O precipitation, see low latitude aurorae.
Precipitating charged particles gyrate around the magnetic field line they are on. However, protons suffer from charge exchange collisions with the neutral gas, after which they proceed in the direction acquired on the collision as neutral atoms (until the next ionization-stripping collision). At this stage auroral emissions are observable, typical measured line being H(beta) from Balmer series (486.1 nm). As the initial beam spreads out, a diffuse type of emission is created. As typical for diffuse aurora, the proton precipitation is confined to be most often on the equatorward part of the oval.
Auroral substorm studies have shown that the onset arc is within or near the poleward edge of the proton precipitation region (Fukunishi, 1975; Vallance Jones et al., 1982; see also Samson et al., 1992).
When proton aurora occurs at the same local time sector as pulsating (electron) aurora, they are typically located northward from pulsations (e.g., Creutzberg et al., 1981). In addition, Viereck and Stenbaek-Nielsen (1985) reported that when the emissions occur together, increasing proton precipitation leads to increasing off phase in auroral pulsations. The proton emission show no pulsation themselves (e.g., Eather, 1968; Miller and Zeitz, 1970), although in one case rocket measurements have shown weak modulation in the precipitating flux (Smith et al., 1980).