[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] 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] 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] We have used Monte Carlo simulations of ion velocity distributions in the high latitude F region to improve the calculation of incoherent radar spectra in the auroral ionosphere. The new calculations confirm that when the ion temperature becomes large due to frictional heating in the presence of collisions with the neutral background constituent, F region spectra evolve from a normal double-hump to a triple hump to a spectrum with a single maximum at the mean Doppler shifted frequency produced by the ions' E X B drift. The features obtained with the present simulations are qualitatively similar to those obtained earlier with an empirical model of the ion velocity distribution. In fact, for radar aspect angles of the order of 70⁰ or more, the new computations give results that are in quantitative agreement with earlier calculations, even for 100 mV/m perpendicular electric fields. For aspect angles of the order of 30⁰ or less, the present computations further reveal that, except for changes in the line-of-sight ion temperature, the spectrum should be indistinguishable from that which would be observed if the plasma were Maxwellian. However, for angles between 30 and 70⁰, the Monte Carlo results indicate that an analysis based either on a Maxwellian assumption or on earlier empirical or analytical procedures will be inadequate at times. To remedy this problem, we propose a new empirical approach based on the analysis of non Maxwellian spectra simulated with our Monte Carlo program. The new method, while not being 100 percent accurate, should be very economical and should help experimentalists retrieve proper values for various plasma parameters (for instance the ratio T_e/T_i with a minimum of effort

[en] Measurements of low (E = 250-1000 eV) and high (E = 60 to > 300 keV) energy particles as well as the magnetic field obtained by 3 different instruments (Reme Plasma Analyser, Energetic Particle Analyser, MAGnetometer) during the Giotto-Halley encounter on 13/14 March 1986 are used to study the field line merging process. Spikes of 5-15 min duration in the high energy particle flux which are superimposed on the general intensity time profile are correlated with minima in the low energy particle flux and time periods of oppositely directed magnetic field lines. Strong changes in the pitch angle distribution of energetic ions are observed simultaneously. The observations are considered as evidence for sporadic field line merging processes in the front side of Halley's cometosheath which can accelerate ions and electrons up to E ∼ 300 keV

[en] A large ion composition data set consisting of 3-h averages has been assembled for the energy per charge range 0.9-15.9 KeV/e. It includes all data collected by the Ion Composition Experiment on the ESA/GEOS-2 satellite from August 1, 1978 through August 24, 1985. Five ion species have been routinely identified: H⁺, He⁺⁺, He⁺, O⁺⁺ and O⁺. Ions believed to be of ionospheric origin (He⁺, O⁺⁺ and O⁺) exhibit a very striking statistical correlation with solar EUV flux. The 0⁺ number density varies by a factor of 14 over the solar cycle. The He⁺ and O⁺⁺ densities increase by a factor of 5 each. A small (factor of 1.3), but statistically still significant increase is found for the H⁺ number density. It is also shown that the ion number density exhibits variation over a time scale of one solar rotation, 27 days. These observations are interpreted in terms of the composition and dynamics of two sources of magnetospheric plasma: the solar wind and the high-latitude topside ionosphere

[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