D. V. Bisikalo

A kinetic model of the formation of the hot oxygen geocorona. 1: Quiet geomagnetic conditions

A model of the hot oxygen geocorona in the transition region near the exobase is described. It is based on a Monte Carlo solution of the nonlinear Boltzmann equation for hot oxygen atoms produced by chemical processes usually considered as a source of hot oxygen (photodissociation of O2 and dissociative recombination of O2+ and NO+ ions). The evolution of the system is described stochastically as a series of random Markovian processes. The energy distribution function of the thermal and non-thermal O(³P) atoms and of the nonthermal O(¹D) atoms is calculated from the thermospheric collision-dominated region to the exosphere where the gas flow is virtually collisionless. The model is applied to equatorial latitudes for conditions of low solar and geomagnetic activity. Numerical simulations show that the distribution function of thermal oxygen is increasingly perturbed by collisions with the hot oxygen population at high altitudes and departs significantly from a Maxwellian distribution at all altitudes. The number density and temperature of the nonthermal oxygen atoms are derived from their microscopic distribution function and found to be in qualitative agreement with previous theoretical and experimental estimates.

Observation of the proton aurora with IMAGE FUV imager and simultaneous ion flux in situ measurements

The far ultraviolet cameras on board the IMAGE satellite images the aurora in three different spectral regions. One of the channels of the spectrographic imager SI12 observes the Doppler-shifted Lyman α emission of precipitating protons. It makes it possible to spectrally discriminate between the proton and electron FUV aurora and to globally map the energetic protons. Its response depends on the auroral Lyman α line shape which reflects the characteristics of the proton pitch angle and energy distributions. We illustrate the dependence of the SI12 count rate on the characteristic energy of the proton precipitation and the viewing geometry. Simultaneous in situ observations of the precipitated protons have been collected during a FAST satellite pass when IMAGE was observing the global north polar region. The premidnight region located at the equatorward boundary of the oval is dominated by proton precipitation with a mean energy Ē = 7 keV which is separated from the electron component. The prenoon crossing exhibits a softer proton energy spectrum with Ē = 0.9 keV. The measured proton energy distribution is used as an input to a Monte Carlo model to calculate the expected SI12 signal along the magnetic footprint of the satellite orbit. If the different spatial resolution of the two types of measurements is accounted for, a good quantitative agreement is found with the IMAGE observations. Similarly, ion flux measurements collected on board the Defense Meteorological Satellite Program Fl5 satellite during an overflight in the postmidnight sector provide good agreement with the SI12 observations at the footprint aurora. The comparisons confirm the reliability of the FUV IMAGE cameras to remotely discriminate between the electron and the proton precipitations. The vertical emission rate profiles of the N2 Lyman-Birge-Hopfield and OI(1356A) emissions are calculated in the proton-dominated premidnight region. It is shown that the protons and the electrons produce FUV emissions with contributions peaking at different altitudes. Excitation by secondary electrons dominates the production of both emissions.

The role of proton precipitation in the excitation of auroral FUV emissions

Far ultraviolet remote sensing from a high-altitude satellite is extensively used to image the global aurora, derive its energetics, and follow its dynamical morphology. It is generally assumed that the observed emissions are dominated by the interaction of the precipitated electrons with the thermospheric constituents. A model to calculate far ultraviolet emissions excited by auroral electrons and protons and the secondary electrons they generate has been used to calculate the volume excitation rate of the H I Ly-α, O I 1304 and 1356 A, N 11493 A multiplets, and the N2 Lyman-Birge-Hopfield (LBH) bands. The characteristic energy and the energy flux are derived from the observed statistical distribution of precipitated protons and electrons. This model is applied to the midnight aurora, the noon cusp, and a proton-dominated aurora for moderately disturbed conditions. We show that in the first two cases, direct electron impact dominates the vertically integrated emission rate over the proton component, although proton excitation plays an important role at some altitudes in the daytime cusp. In afternoon regions of the auroral zone near the auroral boundary, secondary electrons due to proton ionization are the main source of FUV emissions. The energy dependence of the efficiency of LBH band emission viewed from high altitude is calculated for electron and proton precipitations. Maps of the N2 LBH emission excited by both components are obtained, and regions of proton-dominated auroral emission are identified. It is found that the distribution of the ratio of proton-induced to electron-induced brightness resembles maps of the ratio of the respective precipitated energy fluxes. Proton-dominated FUV emissions are thus located in a C-shaped sector extending from prenoon to midnight magnetic local times with a maximum proton contribution near the equatorward boundary of the statistical electron oval. The distribution of the Ly-α/LBH intensity ratio is found to mimic the ratio of the proton flux/total energy flux, although it is insufficient by itself to accurately determine the relative fraction of auroral energy carried by the protons.

An updated model of the hot nitrogen atom kinetics and thermospheric nitric oxide

New observations and reanalysis of previous measurements suggest an upward revision of the measured thermospheric nitric oxide density. Our previous model of NO production by fast N(4S) atom collisions with O2 has been updated. It includes the effect of soft solar X rays, Auger electrons, and a detailed treatment of N2 dissociative ionization channels. In addition, new calculations indicate that the transfer of translational energy in N + N2 collisions is less efficient than in the hard sphere approximation. This result leads to reevaluation of the energy dependent relaxation cross section and to an upward revision of the reacting efficiency of collisions of N with O2 to form nitric oxide. The calculated peak NO density increases by a factor of ∼ 2 when the effect of superthermal nitrogen atoms is included. The model response of the N(4S) energy distribution function and NO density to solar cycle variations is presented. The NO density at 110 km changes from 5.4 × 107 to 1.3 × 108 cm−3 when the solar F10.7 index varies from 70 to 245, but its response depends on the magnitude of the soft X ray increase with solar activity.

Morphology of the interaction between the stream and cool accretion disk in a semidetached binary system

AbstractWe analyze heating and cooling processes in accretion disks in binaries. For realistic parameters of the accretion disks in close binaries (

The possible nature of dips in the light curves of semidetached binaries with stationary disks

\dot M \simeq 10^{ - 12} - 10^7 M_ \odot /yr

Non thermal nitrogen atoms in the Earth's thermosphere 1. Kinetics of hot N(4S)

and α⋍10−1–10−2), the gas temperature in the outer parts of the disk is from ∼104 to ∼106 K. Our previous gas-dynamical studies of mass transfer in close binaries indicate that, for hot disks (with temperatures for the outer parts of the disk of several hundred thousand K), the interaction between the stream from the inner Lagrange point and the disk is shockless. To study the morphology of the interaction between the stream and a cool accretion disk, we carried out three-dimensional modeling of the flow structure in a binary for the case when the gas temperature in the outer parts of the forming disk does not exceed 13 600 K. The flow pattern indicates that the interaction is again shockless. The computations provide evidence that, as is the case for hot disks, the zone of enhanced energy release (the “hot line”) is located beyond the disk and originates due to the interaction between the circumdisk halo and the stream.

The importance of new chemical sources for the hot oxygen geocorona

The role of the non maxwellian nitrogen atoms in the odd nitrogen thermospheric chemistry is investigated with a numerical model. This one-dimensional model solves the continuity equation, including molecular and turbulent transport for the coupled NO-N(4S) system. It is shown that the very fast reaction between hot N(4S) atoms and O2 provides an additional source of nitric oxide. At equatorial latitudes, this contribution amounts to 6–30% of the other classical production sources of NO for solar minimum activity conditions. The cross section for elastic collisions with ambient gas introduces the largest uncertainty on this estimate. It is concluded that, although it does not drastically alter the conclusions of previous models of NO, the effect of hot N atoms must be considered in future models of the NO-N system chemistry.