Kaichi Maeda

Energy deposition by auroral electrons in the atmosphere

Abstract The spatial distribution of the energy deposited by electrons in the atmosphere has been calculated by the Monte Carlo method. The distribution has been obtained as a function of the altitude and of the radial distance from the axis of the incident electron beam. The calculations take into account the deflection of the electrons by the geomagnetic field, and the scattering and slowing down due to multiple Coulomb interactions with atomic nuclei and orbital electrons. The assumed conditions were: 1. (1) a semi-infinite air medium, extending downwards from a height of 300 km, with a composition and density corresponding to that of the CIRA (1965) Mean Atmosphere 2. (2) a vertical magnetic field with a strength of 0.6 G 3. (3) monoenergetic incident electron beams that are symmetric about a chosen field line 4. (4) incident electron energies between 20 keV and 2 keV 5. (5) various incident pitch-angles between 0° and 90°, or a pitch-angle distribution corresponding to an incident flux isotropic over the downward hemisphere. Calculations made with the same program, for a constant-density medium and no magnetic field, give good agreement with the results of laboratory experiments. The calculations are also in agreement with recent observations on an artificial aurora produced in the atmosphere with 8.7-keV electrons.

Some new results on electron transport in the atmosphere

Abstract The penetration, diffusion and slowing down of electrons in a semi-infinite air medium has been studied by the Monte Carlo method. The results are applicable in the atmosphere at altitudes up to ~ 300 km. Most of the results pertain to monoenergetic electron beams, with energies between 2 keV and 2 MeV, injected into the atmosphere at a height of 300 km, either vertically downwards or with a pitch-angle distribution isotropic over the downward hemisphere. Some results were also obtained for various initial pitch angles between 0° and 90°. Information has been generated concerning the following topics: 1. (a) the backscattering of electrons from the atmosphere, expressed in terms of backscattering coefficients, angular distributions and energy spectra; 2. (b) the altitude dependence of energy deposition by electrons and by secondary bremsstrahlung, for incident electron beams that are monoenergetic or have exponential spectra with e-folding energies between 5 and 200 keV; 3. (c) the evolution of electron flux spectra as function of the atmospheric depth, for incident beam energies between 2 and 20 keV.

Variations of polar mesospheric oxygen and ozone during auroral events

Abstract Temporal solutions are given of the photochemical equations describing the distribution of ozone and atomic oxygen in an oxygen atmosphere for the polar regions. Similar calculations are applied to auroral events, and indicate a strong dependence of mesospheric atomic oxygen on the intensity and energy spectrum of auroral electrons, which dissociate molecular oxygen. It is shown that there can be no significant atmospheric ozone and atomic oxygen modifications due to soft spectrum electron events which appear to characterize most bright auroras. On the other hand, the hard spectrum auroral electrons which appear in most weak, quiet auroras should cause significant increases in the atomic oxygen and ozone concentration below 80 km, provided their flux is more than 106/cm2 sec.

Diffusion of low energy auroral electrons in the atmosphere

Abstract By means of the Monte Carlo method, Spencer s work on the energy dissipation of electrons in air is extended below 25 keV, and the results are applied to the diffusion of monoenergetic electrons in the polar atmosphere. The results show significant effects of straggling, not only on the distribution of the most penetrating flux but also on the height of maximum dissipation and on the back-scattering. The dependence of back-scattering on the angular distributions as well as the energies of incident electrons are calculated, indicating that it decreases rapidly below 10 keV and that a maximum of back-scattering seems to occur at energies around 20 keV, with values of the order of 7 and 20 per cent for vertical and isotropic incidence, respectively for intensity. Corresponding energy back-scattering coefficients (albedos) are approximately half of these values.

A calculation of auroral hiss with improved models for geoplasma and magnetic field

Abstract Intensities of auroral hiss generated by the Cerenkov radiation process by electrons in the lower magnetosphere are calculated with respect to a realistic model of the Earths magnetosphere. In this calculation, the magnetic field is expressed by the “Mead-Fairfield Model” (1975), and a static model of the iono-magnetospheric plasma distribution is constructed with data accumulated by recent satellites (Alouette-I, -II, ISIS-I, OGO-4, -6 and Explorer 22). The energy range of hiss producing electrons and the frequency range of the calculated VLF are 100–200 keV, and 2–200 kHz, respectively. Intensities with a maximum around 20 kHz, of the order of 10 −14 W/m 2 /Hz 1 at the ground seem to be ascribable to the incoherent Cerenkov emission from soft electrons with a differential energy spectrum E −2 having an intensity of the order of 10 8 cm −2 /sec/sr/eV at 100 eV. It is shown that the frequency of the maximum hiss spectral density at geomagnetic latitudes 80° on the day-side and 70° on the night-side is around 20 kHz for the soft spectrum (∼ E −2 ) electrons, which shifts toward lower frequency (∼10 kHz ) for a hard spectrum (∼ E −1·2 ) electrons. The maximum hiss intensity produced by soft electrons is more than one order higher than that of hard electron produced hiss. The higher rate of hiss occurrence in the daytime side, particularly in the soft electron precipitation zone in the morning sector, and the lesser occurrence of auroral hiss in night-time sectors must be, therefore, due to the local time dependence of the energy spectra of precipiating electrons rather than the difference in the geomagnetic field and in the geoplasma distributions.

Cyclotron side-band emissions from ring-current electrons

Abstract VLF-emissions with subharmonic cyclotron frequency from magnetospheric electrons have been detected by the S3-A satellite (Explorer 45) whose orbit is close to the magnetic equatorial plane where the wave-particle interaction is most efficient. These emissions are observed during the main phase of a geomagnetic storm in the nightside of the magnetosphere outside of the plasmasphere around L = 3–5. The emissions consist essentially of two frequency regimes, one below the equatorial electron gyro-frequency, ƒ H 0 , and the other above ƒ H 0 . The emissions below ƒ H 0 are whistler mode and there is a sharp band of “missing emissions” along ƒ= ƒ H 0 2 . The emissions above ƒ H 0 are electrostatic mode and the frequency ranges up to 3ƒ H 0 2 . It is concluded that these emissions are generated by the enhanced relativity low energy (1–5 keV) ring current electrons, penetrating into the nightside magnetosphere during the main phase of a magneto storm. Although the high energy (50–350 keV) electrons showed remarkable changes of pitch angle distribution, their associations with VLF-emissions are not so significant as those of low energy electrons.

Stratospheric ozone response to a solar proton event - Hemispheric asymmetries

Ozone depression in the polar stratosphere during the energetic solar proton event on 4 August 1972 was observed by the backscattered ultraviolet (BUV) experiment on the Nimbus 4 satellite. Distinct asymmetries in the columnar ozone content, the amount of ozone depressions and their temporal variations above 4 mb level (∼38 km) were observed between the two hemispheres. The ozone destroying solar particles precipitate rather symmetrically into the two polar atmospheres due to the geomagnetic dipole field These asymmetries can be therefore ascribed to the differences mainly in dynamics and partly in the solar illumination and the vertical temperature structure between the summer and the winter polar atmospheres. The polar stratosphere is less disturbed and warmer in the summer hemisphere than the winter hemisphere since the propagation of planetary wave from the troposphere is inhibited by the wind system in the upper troposphere, and the air is heated by the prolonged solar insolation. Correspondingly, the temporal variations of stratospheric ozone depletion and its recovery appear to be smooth functions of time in the (northern) summer hemisphere and the undisturbed ozone amount is slighily, less than that of its counterpart. On the other hand, the tempotal variation of the upper stratospheric ozone in the winter polar atmosphere (southern hemisphere) indicates large amplitudes and irregularities due to the disturbances produced by upward propagating waves which prevail in the polar winter atmosphere. These characteristic differences between the two polar atmospheres are also evident in the vertical distributions of temperature and wind observed by balloons and rocker soundings.

Asymmetries of the Upper Stratospheric Ozone Distribution Between Two Hemispheres

Abstract Based on the Nimbus-7 solar backscattered UV-radiation (SBUV) data which are free from the instrumental background noise (dark-current) produced by magnetospheric particles, it is found that the southern winter hemispheric ozone densities in the upper stratosphere are nearly 20% higher than their counterparts in the Northern Hemisphere; i.e., the ozone mixing ratios at the 1.5 mb (∼45 km) level are 10.2 μg g−1 at 60°S in July 1979 versus 8.5 μg g−1 at 60°N in December 1979. This is in significant contrast to the well-known hemispheric asymmetry of the total ozone content which is higher in the northern hemispheric winter than in the southern hemispheric winter. Comparisons of those findings with the previously obtained similar results from the Nimbus-4 backscattered UV radiation (BUV) experiment have manifested that the dark-current effect on the latter was negligible. Therefore, using the stratospheric temperature which was observed by means of the selective chopper radiometer (SCR) simultaneous...

Ion cyclotron bands in VLF saucers

Abstract In the wideband VLF data obtained by the polar orbiting DE -1 satellite over the polar night ion trough region of the upper ionosphere, conspicuous frequency-band structures are found to occur both in absorption and emission, particularly associating with VLF saucers. These proton cyclotron harmonic bands are sometimes observable up to the 10th harmonic. The attenuation bands, which appear in both the magnetic and electric data from DE -1, presumably indicate that the ions of atomic hydrogen from the polar ionosphere are accelerated by the ac electric fields of VLF waves oscillating normal to the static magnetic field, analogous to a cyclotron accelerator. The observed frequencies of the cyclotron harmonics are generally somewhat higher than the local cyclotron frequencies computed from the onboard magnetometer data, suggesting that the acceleration is taking place in the layer below the satellite at a geocentric distance of less than about 1.5 Earth radii. This example indicates the existence of upward propagating hiss at those altitudes inside the auroral zone. On the other hand, the frequency shifts of the emission bands are found to be space and time dependent, with the harmonic frequencies inside the V-shaped saucers being somewhat higher than those outside. These frequency shifts are attributed to a combination of two different types of Doppler shift, one due to the orbital motion of the satellite and the other due to the upward motion of the medium at the emission source. This indicates the existence of an upward plasma flow at the source, with a velocity of the order of 20 km s −1 inside the saucer. The amount of this frequency shift decreases with increasing harmonic order, indicating a higher phase velocity for the electrostatic waves of higher harmonic order.

North-south asymmetries of solar particle events in upper stratospheric ozone

Abstract Stratospheric ozone depressions, following intense solar particle events (SPE) observed by the backscattered ultraviolet (BUV) experiment on the Nimbus-4 satellite, indicate the existence of distinct asymmetries between the Northern and Southern Hemispheres. These asymmetries are observed in the magnitude of the depressions above the 5-mb level, their temporal variations, and the spatial (i.e., latitude and longitude) dependence of these variations. Possible causes of asymmetries, shown by two events on 4 August 1972 and 25 January 1971, can be attributed to: (1) tilt of the interplanetary magnetic field (IMF) with respect to the Earths dipole magnetic field which influences the precipitation of energetic solar particles into the polar atmospheres; (2) differences in ozone chemistry caused by the large change in atmospheric temperature between summer and winter hemispheres; (3) seasonal differences of the stratospheres dynamic states which are affected by upward propagating planetary waves in winter in contrast to the relatively undisturbed zonal flow in summer; (4) topographic asymmetry between Northern and Southern Hemispheres. These effects are shown by three-dimensional plots of the events in geographic coordinates and by color contour plots of the stratospheric ozone distributions in geomagnetic and geographic polar coordinates, respectively.