<title>Calculations and observations of proton precipitation in the dayside aurora</title>

Abstract

ABSTRACT A simple procedure to evaluate the Balmer excitation rates of H0 and H to produce the corresponding volume emission rates vs. height, using semi-empirical range relations of protons in air is used. The calculations are carried out with identified ion—energy particle spectra of the dayside aurora obtained by low altitude satellites. Itis found, that the calculated emission intensities of H, and H are observable for ground—based optical detection.Measurements of the dayside aurora at Longyearbyen, Svalbard, are discussed in relation to these calculations.Furthermore it is observed, pulsed proton events on the dayside, which for decreasing time between each event shows increase in the emission intensity and a fluctuating behavior in the primary initial proton energy. Each pulsed proton event was associated with an enhancement in the green 5577 A emission, and some corresponded to poleward—moving auroral forms. 1. INTRODUCTION Around the vicinity of the subsolar point, magnetic reconnection is most likely to occur (Cowley, 1982). During thereconnection process magnetosheath ions are accelerated to about twice the Alfven velocity. Satellite measurementshave shown step like patterns in the ion energy—time profiles (Newell and Meng, 1991; Smith and Lockwood, 1990;Lockwood and Smith, 1992), which are common cusp precipitation signatures for ions (Lockwood et al., 1993).Each step in the ion energy—time plot is thought to be indicative of magnetic reconnection taking place. This isbecause for each subsequent reconnection ions are accelerated at the reconnection site and move along the fluxtube, precipitating down into the ionosphere. Precipitation of magnetosheath electrons into the ionosphere alsooccurs during magnetic reconnection, but the electrons are not accelerated by the reconnection process because theirthermal velocities exceed the antisunward convection speed. The electrons interact with atomic oxygen producingboth red [01] 6300 A and green [01] 5577 A emissions. These newly opened flux tubes are thought to be the sourceof the cusp precipitation. Ground based optical observations (Sandholt et al., 1986, 1989; Rairden and Mende,1989; Fasel et al., 1992, 1993) have seen poleward—moving auroral arcs, in both the red and green line emissions,which may be ionospheric signatures of the newly reconnected flux tubes. The diffuse red band produced by lowenergetic electrons colliding with atomic oxygen seen near magnetic noon was defined as the cusp by Heikkila (1985)and was later redefined by Sandholt et al. (1989) to also contain poleward—moving auroral forms having green lineemissions. This region corresponds to the gap of discrete auroras seen by the DMSP satellites in the dayside auroraloval (Meng, 1981).