M.H. Rees

AURORAL IONIZATION AND EXCITATION BY INCIDENT ENERGETIC ELECTRONS

Abstract Ionization in the Earths atmosphere produced by energetic primary auroral electrons in the energy range 0.4 to 300 keV is computed. Three angular distributions for the incident stream are considered: (1) a monodirectional beam, (2) a distribution varying as the cosine of the pitch angle, and (3) an isotropic distribution. The luminosity profiles for the N 2 + 1 N.G. (0,0) band are predicted for various energy distributions of auroral primaries. Power laws and exponential laws with a range of exponents are investigated: the shape of the luminosity profile is highly sensitive to the energy distribution function adopted. Profiles for a number of electron energy distributions obtained from rocket measurements through aurorae are computed showing satisfactory agreement with luminosity variations registered by the rocket borne photometers.

Auroral heating and the composition of the neutral atmosphere.

Abstract Heating of the neutral atmosphere by auroral particle fluxes and by orthogonal electric fields is responsible for large changes in the thermospheric composition that have been observed by satellite mass spectrometers. Vertical winds of a few meters per second are produced in the region subject to auroral heating; this vertical upwelling drives circulation cells that extend the effects of heating in the auroral region on a global scale. Our analysis focuses on the initial phase of a magnetic storm within the auroral region.

Time dependent studies of the aurora—II. Spectroscopic morphology

Abstract The temporal morphology of auroral spectral emission features is investigated. For a given energy distribution of bombarding electrons but a time varying flux magnitude, the emission rates of various auroral radiations exhibit a nonlinear time response due to the variety of reactions that contribute to excitation. Absolute intensities and intensity ratios of various spectroscopic features, therefore, can vary solely with atmospheric interaction effects, indicating the importance of considering the time history of a precipitation event when attempting to infer the characteristics of auroral electrons from optical measurements.

Time dependent studies of the aurora. I. Ion density and composition

Abstract Ion densities and composition are investigated in a time varying model aurora. There is a time lag between turning on the source of ionization and the resulting increase in ion densities that depends on the species and the height level in the ionsophere, so that altitude profiles of auroral electron densities evolve with time. Characteristic buildup times for the ionization are a few seconds at the altitude of maximum energy deposition, increasing to tens of seconds above and below this level. A wide range of composition ratios, n (NO + / n (O 2 + and n (NO + / n (O + ), can be expected, depending on the time an observation is made during buildup or decay of ionization. The concentrations of atomic nitrogen and nitric oxide increase as a result of auroral ionization, but the associated characteristic times are long compared to the average duration of ‘auroral event’. Thus, intermittent auroral bombardment could result in a gradual buildup of these minor neutral constituents in the auroral atmosphere. Variations in the electron density during pulsating, fluctuating or coruscating aurora lag the source function variations by a few seconds in a typical aurora.

Auroral excitation of the forbidden lines of atomic oxygen

Abstract Excitation of the forbidden lines of atomic oxygen, λ 5577 and λ 6300, has been investigated in detail. The calculations are based on a well-documented auroral are observed in Alaska, for which height-luminosity profiles have been obtained in λ 3914 from photometric triangulation. The O ( 1 S) term is excited principally by secondary (non-ionizing) electrons produced by auroral bombardment; dissociative recombination of O 2 + contributes about 30 per cent to the photon emission rate. The O ( 1 D) term is excited by several processes: secondary electron impact, dissociative recombination of O 2 + , cascading from O ( 1 S) and thermal electron impact. Secondary electron excitation predominates up to about 250 km, but quenching climates much λ 6300 radiation at these altitudes. Excitation by the thermal electron gas, which reaches a temperature of about 4500°K at high altitudes, dominates above 250 km. Diffusion of the excited o ( 1 D) atoms broadens the region of emission of λ 6300 compared to the region of the atmosphere under bombardment. An electron density in excess of 10 6 cm −3 is reached in this aurora at 120 km, the altitude of maximum ionization.

Ionospheric electron densities and temperatures in aurora.

Abstract Electron density profiles corresponding to chemical equilibrium, and electron temperatures corresponding to heating of ambient electrons by auroral secondaries have been computed for five stable auroral arc systems with published height profiles of λ 3914 volume emission rates. The maximum electron density varies approximately as the three-quarters power of the λ 3914 intensity while the maximum temperature varies remarkably little. For these five auroras the temperature maxima lie between 2920°K and 3511°K and occur at altitudes between 330 km and 390 km.

The energy spectrum of primary auroral electrons determined from auroral luminosity profiles

Abstract The vertical luminosity profiles of sixteen auroral arcs have been measured in the light of λ3914 (N 2 + ) by a technique of photometric triangulation. A comparison between these observed profiles and predicted profiles calculated using two model atmospheres and several electron spectral shapes yields information on the flux and energy distribution of the primary auroral electrons. In the energy range of about 0.1–30 keV the primary electron stream may be characterized by a differential number flux of the form N ( E ) ∼ E γ exp (− E / α ) where γ is usually 1 and α varies between 1 and 4 keV. The function has a maximum at energy E = γα which may be interpreted as the characteristic energy of the stream. Thus, the electron stream which pro- duces the luminous aurora has a characteristic energy between 1 and 4 keV, although extreme values of 0.7–6.0 were found in the sixteen cases studied. The energy distribution above 30 keV is not well determined from luminosity data and plays a small part in the auroral luminescence. The total electron flux associated with the sixteen arcs ranged between 2.9 × 10 9 and 1.1 × 10 11 electrons/cm 2 sec. The total energy flux carried by the incident electrons ranged between 28 and 590 ergs/cm 2 sec.

Secondary electrons in aurora

Abstract The Bethe approximation is used with measured and theoretical values of ionization cross sections and measured values of differential oscillator strengths to derive the initial energy spectrum of auroral secondary electrons. The differential flux of the auroral secondaries is then calculated, using the approximation of continuous energy loss. The calculations are applied to a particular aurora for which rocket data have been published. There is substantial disagreement between theoretical and measured electron spectra. The theoretical spectra show structure at energies less than 20 eV, associated primarily with vibrational and electronic excitation of molecular nitrogen. This structure is largely absent in the measured spectrum. Substantially more high energy electrons were measured than theory predicts. In addition, there are disagreements in the altitude profiles of the total number of non-thermal secondary electrons. Calculated values of OI green line photon emission rates which result from excitation by secondary electrons and dissociative recombination of O 2 + fall short of the measured values. The effect on the excitation rate of varying several parameters is investigated, and it is found that the results are particularly sensitive to competing inelastic processes in N 2 .

The effect of oxygen cooling on ionospheric electron temperatures

Abstract There are discrepancies between ionospheric electron temperatures derived from Thomson scatter data and electron temperatures predicted from the solar ultra-violet heat source. The inclusion of electron cooling by excitation of the fine-structure levels of atomic oxygen removes the discrepancy throughout the day at all altitudes above 320km. Below this altitude a discrepancy persists, but it probably lies within the uncertainties arising from the basic atomic and molecular data employed in the theoretical analysis and from the solar-flux data.

Ionization in the Earth's atmosphere by aurorally associated Bremsstrahlung X-rays

Abstract The production of bremsstrahlung X-rays associated with energetic auroral electron streams is computed, and the ionization rate due to the X-rays is deduced. The incident electron stream is assumed to have an isotropic angular distribution over the downward hemisphere; energy distribution functions which closely predict luminosity profiles of N 2 + radiation are adopted. Bremsstrahlung ionization production peaks at altitudes between 40 and 74 km for electron streams yielding auroral height-luminosity profiles which have a maximum in the range 100 to 120 km, but the ionization rate due to bremsstrahlung is several powers of ten below that obtained from direct ionization by electron impact. The spectral distribution and flux of X-rays associated with auroral bombardment are predicted at various altitudes, particularly in the height range reached by balloons.