Characteristics and sources of ionization in the continuous aurora

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Abstract
The continuous aurora is defined as E region ionization that is steady in time and uniformly distributed over a large portion of the auroral oval. We have examined the characteristics and sources of ionization in the continuous aurora using data from the Chatanika incoherent scatter radar and several polar orbiting satellites. The altitude profile of ionization in the continuous aurora usually has an E region maximum of 0.5 to 5×105 cm−3 at altitudes between 110 km and 140 km. The profile is very similar to that expected from precipitating electrons with a Maxwellian energy distribution of 0.5–4 keV characteristic energy and an energy flux of 0.2 to 16 erg/cm2 s. We have used simultaneous electron flux data from the DMSP satellites to assess the extent to which precipitating electrons are responsible for the continuous aurora. We show two cases in which the simultaneous incoherent scatter radar data revealed substantial ionization equatorward of the region of electron precipitation. We attribute this ionization to precipitating protons. The presence of significant ionization produced by proton precipitation is confirmed by a third data set in which measurements of precipitating proton fluxes made by the NOAA 6 satellite over the diffuse aurora near Chatanika indicated a proton energy flux of about 0.5 ergs/cm2 s. The energy flux carried by precipitating electrons was more than an order of magnitude less. Ionization produced by proton precipitation can have an altitude distribution very similar to electron aurora. In addition, the proton aurora has the same latitudinal profile as the electron aurora, so that distinction between the two cannot be made on the basis of electron density measurements alone.