M. H. Rees

Time-dependent studies of the aurora: Effects of particle precipitation on the dynamic morphology of ionospheric and atmospheric properties

Abstract A self-consistent, time-dependent numerical model of the aurora and high-latitude ionos-phere has been developed. It is used to study the response of ionospheric and atmospheric properties in regions subjected to electron bombardment. The time history of precipitation events is arbitrarily specified and computations are made for a variety of electron spectral energy distributions and flux magnitudes. These include soft electron precipitation, such as might occur on the poleward edge of the auroral oval and within the magnetospheric cleft, and harder spectra representative of particle precipitation commonly observed within and on the equatorward edge of the auroral oval. Both daytime and night-time aurorae are considered. The results of the calculations show that the response of various ionospheric and atmospheric parameters depends upon the spectral energy distribution and flux magnitudes of the precipitating electrons during the auroral event. Various properties respond with different time constants that are influenced by coupling processes described by the interactive model. The soft spectrum aurora affects mainly the ionospheric F region, where it causes increases in the electron density, electron temperature and the 6300 A red line intensity from normal quiet background levels during both daytime and night-time aurora. The fractional variation is greater for the night-time aurora. The hard spectrum aurorae, in general, do not greatly affect the F-2 region of the ionosphere; however, in the F-1 and E regions, large increases from background conditions are shown to occur in the electron and ion temperatures, electron and ion densities, airglow emission rates and minor neutral constituent densities during the build-up phase of the auroral event. During the decay phase of the aurora, most of these properties decrease at nearly the same rate as the specified particle precipitation flux. However, some ionospheric and atmospheric species have a long memory of the auroral event. The odd nitrogen species N(4S) and NO probably do not ever reach steady-state densities between auroral storms.

On the interaction of auroral protons with the earth's atmosphere

Abstract The interaction of energetic auroral protons with the atmosphere is investigated. The results of a random number algorithm that describes the proton-hydrogen interconversion reactions as the beam loses energy are adopted to construct an energy deposition curve applicable over a wide range of initial proton energies. lonization rates and production rates of ejected electrons are computed and emission rates of hydrogen Balmer alpha and beta lines are evaluated using recently available low energy cross-sections.

Angular dependent transport of auroral electrons in the upper atmosphere

Abstract The transport of auroral electrons through the upper atmosphere is analyzed. The transport equation is solved using a discrete ordinate method including elastic and inelastic scattering of electrons resulting in changes of pitch angle, and degradation in energy as the electrons penetrate into the atmosphere. The transport equation is solved numerically for the electron intensity as a function of altitude, pitch angle, and energy. In situ measurements of the pitch angle and energy distribution of precipitating electrons over an auroral arc provide boundary conditions for the calculation. The electron spectra from various locations over the aurora represent a variety of anisotropic pitch angle distributions and energy spectra. Good agreement is found between the observed backscattered electron energy spectra and model predictions. Differences occur at low energies (below 500 eV) in the structure of the pitch angle distribution. Model calculations were carried out with various different phase functions for elastic and inelastic collisions to attempt changing the angular scattering, but the observed pitch angle distributions remain unexplained. We suggest that mechanisms other than collisional scattering influence the angular distribution of auroral electrons at or below 300 km altitude in the low energy domain.

The systematic behaviour of hydrogen emission in the aurora—I

Abstract Analysis of short duration spectra taken in rapid succession reveals a systematic behaviour of the aurorally associated hydrogen emissions. Two principal effects are noted. Firstly, there is a daily motion along the geomagnetic meridian in which the spectral line progresses from the northern horizon southward, followed by a return to the north some time during the night. Secondly, the limit of the southern extent of the hydrogen light correlates well with the magnetic activity, i.e. the latitude at which the protons penetrate to auroral heights is a function of the magnetic disturbance.

Ohmic heating as evidence for strong field-aligned currents in filamentary aurora

A large increase in electron temperature measured in filamentary aurora with the European incoherent scatter radar has been modeled with a one-dimensional electron transport and ion chemistry code. To account for the observed changes in electron temperature, while also reproducing the measured E region electron density profiles, a source of electron heating is required in addition to local heating from energy degradation of the precipitating electrons. We show that ohmic heating in a strong field-aligned current can account for the required heat source.

The morphology of n and no in auroral substorms

Abstract The effects of a typical auroral electron precipitation substorm sequence on odd nitrogen species in the thermosphere have been investigated. The analysis makes use of the time dependent model of the aurora developed by Roble and Rees (1977), which couples the thermal properties to the ionospheric chemistry and transport self-consistently and includes diffusive transport of NO, N(2D) and N(4S). A substantial increase in the E-region density of NO or of N(4S) is predicted, with the result depending on the production ratio of N(2D) to N(4S) in the aurorally dominant source mechanism, electron impact dissociation of N2. A production ratio that favors N(2D) by a factor of one half or larger leads to enhancement of NO, while a ratio of 1 4 N ( 2 D )+ 3 4 N ( 4 S ) results in a buildup of N(4S). The cyclical behaviour of the substorm, i.e. alternate intervals of electron precipitation and quiet periods, accentuates the scavenging effect of the initially dominant odd nitrogen species upon the less abundant one.

Large fluxes of auroral electrons in filaments of 100 m width

The combination of high time and space resolution measurements in the magnetic zenith (optical and radar) and detailed modeling of the ionospheric response to auroral particle precipitation has produced the new result that most of the energy density in a bright arc resides in an extremely narrow filament, embedded in a broader feature. We are able to show that the narrow filaments, of the order of 100 m width, are produced by monoenergetic beams of electrons, whereas the surrounding emissions of lower energy density are produced by broad spectral distributions. The new result is achieved by combining different energy spectra as input to an auroral model and comparing the resulting electron density profiles with those measured with the European incoherent scatter radar at 0.2 s time resolution. The result is dependent on the optical evidence that the narrow energetic beams do not fill the fields of view of the detectors, i.e. both photometer and radar. Agreement between the observed structure, scale lengths, and evolution of the aurora and modeled predictions suggests that transient parallel electric fields are important acceleration mechanisms above the ionosphere.

The kinetics of “hot” nitrogen atoms in upper atmosphere neutral chemistry

Abstract The fate of suprathermal ground state nitrogen atoms produced by auroral electron bombardment of the thermosphere is investigated using a kinetic theory approach. The processes considered are thermaliiation, reactive collisions and transport. At neutral gas temperatures and densities encountered at altitudes below 150 km, thermalication is virtually complete in a fraction of a second, and N(4S) atoms acquire a Maxwellian distribution. Transport is unimportant during thermalication due to the short time constant and small scattering length. The reactive collision rate of N(4S) with O2 to produce NO molecules is very sensitive to the relative velocity of the reactants, and therefore to the form of the non-thermal velocity distribution fonction. Adopting a hard-sphere collision model to describe elastic collisions, and a best estimate for the reactive cross section with O2, it is found that the departure from a Mazwellian distribution is of little consequence for the odd nitrogen chemistry in the lower thermosphere.

Ionosphere‐magnetosphere simulation of small‐scale structure and dynamics

This work presents first results and the numerical methods of a highly improved two-dimensional three-fluid simulation model of the ionosphere-magnetosphere system. The model considers ionization and recombination, ion-neutral friction, the Hall term in Ohms law, and various heat sources in the energy equations. The electrodynamic response and the evolution of the collision frequencies are treated self-consistently in the height-resolved ionosphere. This model is the first and to our knowledge the only simulation model that can resolve the dynamic and nonlinear electromagnetic interaction between the ionosphere and the magnetosphere. The simulation is aimed at modeling fast temporal and small spatial scale ionospheric structures associated, for instance, with filamentary aurora and ionospheric heating experiments. The results presented in this paper focus on ion and electron heating by different sources, i.e., ion heating due to plasma-neutral friction, electron heating resulting from energetic particle precipitation and by ohmic dissipation in strong field-aligned currents. This work is motivated by a specific auroral event that was observed simultaneously with optical and radar instruments. A consistent explanation of this event is possible in the presence of ohmic heating of electrons in a strong field-aligned electric current layer.

The altitude region sampled by ground-based Doppler temperature measurements of the OI 15867 K emission line in aurorae

Abstract Measurements of atmospheric optical emissions with ground-based spectrometers give columnintegrated line profiles. Therefore, measurements from a single station are insufficient to infer the height of emission and, thus, the height of temperature and wind determinations. In aurorae the temperature measured by a ground-based spectrometer can be lower than similar measurements in the nightglow because the 15867 K (630.0 nm ; 1 K = 1 cm −1 ) emitting region may occur at lower altitudes. Temperature measurements obtained on an individual night from College, Alaska, illustrate this effect.