Protons in the one to hundreds of key 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 -• sec -• sterad -• for energies >•10 key, with about 1% of this flux above 100 key, and perhaps 10-3% above 500 key, as being reasonable but not necessarily average. The proton flux below 10 key may exceed the flux above 10 key 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 (3ø-7 ø 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 (I-Ia, It•, and I-I•) on the ground. The I-I• intensity is typically less than 100 R, and the Balmer decrement I-Ia/ttfi is about 3. This proton precipitation also results in the excitation of various oxygen and nitrogen emission, such as ?,3914, ?,4709 Ns +, and ?,5577 O I, and theoretical ratios of the intensities of these emissions to the I!• 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 mo•lels 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 electronexcited emissions, though longer period (•minutes) variations have been reported. The 207 208 R.H. EATHER 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 E, ionization and possibly c...

A new instrument for all-sky spectrophotometric imaging of aurora and airglow has been installed in the Air Force Geophysics Laboratory's Airborne Ionospheric Observatory. Initial observations of equatorial and near-equatorial 6300-• O I airglow show the existence of north-south aligned regions of airglow depletion. These dark bands often extend more than 1200 km in the north-south direction and 50-200 km in the east-west direction. They are observed to drift toward the east during the eveningmidnight hours, with one observation of westward drift after local midnight. Airglow fine structure associated with the boundaries of the dark bands has been observed down to the 2.5-kin resolution limit of the instrument. Simultaneous airborne ionospheric soundings indicate that these regions of airglow depletion are characterized by an increase in the virtual height of the F layer. A simple model of fieldaligned electron density depletion in the bottomside of the F layer explains both the airglow observations and the ionospheric soundings.

A series of recent studies of Pc 3 magnetic pulsations in the dayside outer magnetosphere has given new insights into the possible mechanisms of entry of ULF wave power into the magnetosphere from a bow shock related upstream source. In this paper we first review many of these new observational results by presenting a comparison of data from two 10‐hour intervals on successive days in April 1986 and then present a possible model for transmission of pulsation signals from the magnetosheath into the dayside magnetosphere. Simultaneous multi‐instrument observations at South Pole Station, located below the cusp/cleft ionosphere near local noon, magnetic field observations by the AMPTE CCE satellite in the dayside outer magnetosphere, and upstream magnetic field observations by the IMP 8 satellite show clear interplanetary magnetic field field magnitude control of dayside resonant harmonic pulsations and band‐limited very high latitude pulsations, as well as pulsation‐modulated precipitation of what appear to be magnetosheath/boundary layer electrons. We believe that this modulated precipitation may be responsible for the propagation of upstream wave power in the Pc 3 frequency band into the high‐latitude ionosphere, from whence it may be transported throughout the dayside outer magnetosphere by means of an “ionospheric transistor.” In this model, modulations in ionospheric conductivity caused by cusp/cleft precipitation cause varying ionospheric currents with frequency spectra determined by the upstream waves; these modulations will be superimposed on the Birkeland currents, which close via these ionospheric currents. Modulated region 2 Birkeland currents will in turn provide a narrow‐band source of wave energy to a wide range of dayside local times in the outer magnetosphere.

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There are at least four distinct regions of auroral particle precipitation. The auroral oval and nightside proton aurora are well known; they are well separated in the evening sector and overlap in the morning sector. There is a zone of soft‐electron precipitation at latitudes above the auroral oval where ∼0.5‐kev electrons precipitate; no protons are observed in this region. Electron energies in this soft zone decrease systematically with latitude (for a given Kp). There is a zone of soft‐electron (∼100–200 ev) and soft‐proton precipitation at latitudes above the auroral oval near the midday meridian, and these particles probably come directly from the magnetosheath. No higher energy protons (as in the nightside proton aurora) were observed near midday, during quiet magnetic conditions.