[en] Three important observations recorded in the energetic particle data secured at Halley's comet during March 1986 are reviewed. These include (a) quasi periodic variations of cometary ion fluxes observed inbound and outbound by both the EPONA instrument aboard Giotto and by the Tunde-M instrument aboard Vega 1. A possible explanation of the results in terms of a spin modulation of the outgassing rate of the nucleus is discussed; (b) by combining the EPONA data with JPA-IIS data it is possible to infer that the ion fluxes measured at encounter by EPONA were of the water group. These particles displayed energies in excess of those attained by the pick-up process acting alone. Comparisons between energy spectra prepared using the composite observational data and, corresponding, theoretically derived plots suggest that, downstream of the shock (inbound), stochastic (second-order-Fermi) acceleration may have contributed to energizing the particles; (c) large fluxes of electrons (E>300keV) and ions (E>3.5 MeV) were unexpectedly recorded by EPONA in the magnetic cavity. The observed enhancements (up to approximately three orders of magnitude) appear to be cometary in origin

[en] Scintillation and ionosonde data from several stations ranging from 50⁰-71⁰ corrected geomagnetic latitude in both hemispheres are used to show that the energy from magnetic storms stored most probably in the ring current is a primary source of energy for the formation of F-layer irregularities at sub-auroral latitudes in the region of the plasmapause during the recovery phase of a geomagnetic storm

[en] In the ionosphere, the EUV solar flux ionizes the neutral gas, producing a primary distribution of electrons. These primary electrons may have sufficient energy to produce new secondary electrons, by inelastic collisions with neutrals. We define the efficiency of this production by the ratio between the secondary and the primary production. Computation of this efficiency for different solar conditions shows that it is approximately constant above 200 km, varying from 12 % with a quiet sun to 22 % with a disturbed sun at 500 km. It increases drastically in the E-region reaching a peak value of 2 to 3, (between 120 and 150 km, depending on the intensity of the solar flux, and the solar zenith angle). Influences of solar zenith angle, altitude and latitude on the ionization efficiency are studied. A simple model is described that allows easy computation of the secondary production

[en] Limited range ionosonde recordings have shown that in mid-latitude regions the daytime E-region is often disturbed and small-scale stratifications can be observed regularly on ionograms. The true range separations between stratifications vary from 1 to 8 km with a well-pronounced peak for separations in the range 2 to 2.5 km. Also, fixed-frequency recordings near the daytime penetration frequency of the F₂-layer have shown the continuity of the virtual - height changes which occur to the quasi-horizontal-trace (QHT) segments of the F₂-layer traces. These segments are often observed when travelling ionospheric disturbances (TIDs)are present. On the fixed-frequency recordings particular traces (associated with the QHT segments) are seen to change virtual range at a much greater rate than the background layer

[en] In order to simulate the thermal plasma of the polar cap F-region, four differential equations corresponding to O⁺, O₂⁺, N₂⁺, NO⁺ ions together with the momentum equation are solved using a spectral method. Solar production, chemical recombinations and transportation of the ionization due to diffusion, neutral wind and electric field are taken into account. The intense electric field existing at high latitude accelerates the ions and produces an anisotropy of the plasma which necessitates the introduction of the gradient of a stress tensor in the diffusion operator. Also, recombination of 0⁺ and N₂⁺ depends on an effective temperature which is a function of the square of the electric field. The ions and electrons drift along the equipotentials of the convection field which is derived from the 16-17 June 1982 EISCAT measurements of the three velocity components. The antenna scans the ionosphere in a magnetic meridian from 71 to 61⁰ invariant latitude. A method is proposed to extrapolate the convection field deduced from the radar to the inner polar cap. The results of the simulation show that EISCAT observations are reproduced with 25% of inaccuracy due to several parameters that are discussed. In the morning sector, chemical recombination seems insufficient to explain a strong depletion of the ionization density. An additional mechanism is invoked

[en] A Fourier analysis of about 7000 observations of the horizontal solar diameter made by a single observer using the same equipment has been performed. These observations span about 36 years during a period of reduced magnetic activity. There is a clear signature of the solar cycle corresponding to a periodicity of 9.4 years

[en] Using the heliospheric neutral sheet determinations of the Stanford University Wilcox Solar Observatory group, and assuming a radial expansion of the solar wind, the heliomagnetic latitude of the earth over the years 1976-1984 has been determined. Correlations of this latitude with the cosmic ray flux, as measured by the Deep River neutron monitor, have been calculated for two selected periods, before and after the last reversal of the solar magnetic field. These periods correspond to May 1976 to June 1978 and January 1982 to Decembrer 1984. The results confirm the existence of negative latitudinal gradients: for the first period the gradient was found to be symmetric across the current sheet and of a magnitude of ∼ 2.7% AU^-1, while for the second period the existing gradients were asymmetric: ∼ 3.6% AU^-1 in the southern hemisphere and ∼ 2.8% AU^-1 in the northen hemisphere. The differences observed during the two periods are explained in terms of a north-south asymmetry in solar activity

[en] The spread-F echoes on the vertical incidence ionograms at a temperate latitude station outside the equatorial F-region anomaly belt are shown to start with additional p'-f traces near the penetration frequencies due to off-vertical echoes. With time, these additional p'-f traces increase in intensity and in number, and extend to lower frequencies giving rise to range type of spread-F. The temperate latitude spread-F is suggested to arise mostly from the reflection of radio waves from the ripples and undulations in the iso-ionic surfaces in the ionosphere rather than from the scattering of radio waves from plasma instabilities. These characteristics are thus very different from the development of spread-F at equatorial stations

[en] In the environment of comet P/Halley, a sharp transition, named the cometopause, was detected by the VEGA and GIOTTO spacecraft around 170000km from the nucleus. This boundary separates the outer region of the cometosheath controlled by the solar wind from the inner region of cometary plasma. In electron measurements provided by the GIOTTO-RPA experiment, a clear discontinuity was detected 20 % closer to the nucleus than the cometopause as seen in the ion measurements and coincided with an abrupt increase of the magnetic field magnitude when the field lines began to pile up. Significant changes in the electron distribution function through this discontinuity and within the following region down to 45000 km from the nucleus are presented. The main changes noted at the electron plasma discontinuity were a decrease of the density above 10 eV and a modification of the distribution function which turned from isotropic to anisotropic. Following this discontinuity structures of different plasma characteristics were found in regions of different magnetic field directions which are throught to result from convected interplanetary magnetic field features. It is noted some resemblance between the Venus mantle and the region around the comet which is comprised between the cometopause and the ionopause

[en] The OH^* (7-5) mesospheric hydroxyl emission was monitored at 1.89 micron by the near infrared spectrometer on board of the Solar Mesosphere Explorer satellite (SME). The vertical distributions of monthly averaged dayglow and nightglow radiances are in general agreement with theoretical calculations. Averages over one or two months display recurrent variations from one year to the next at low and mid-latitudes, indicating a seasonal cycle. These seasonal variations are compared with the theoretical predictions of a two-dimensional dynamical/chemical model wherein the seasonal and latitudinal variations in diffusion induced by breaking gravity waves are explicitly considered. The agreement is good at low latitude for both dayglow and nightglow, providing support for theories suggesting that breaking gravity waves play an important role in determining the chemical composition of the mesosphere and lower thermosphere. At middle latitudes, only dayglow data revealed systematic seasonal fluctuations, and these were not in good agreement with model predictions. Seasonal behavior of atomic hydrogen in the upper mesosphere and/or uncertainties the OH Meinel band mechanism parameters are suggested as possible explanations for this behavior