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Recent optical/UV data of Geminga (Mignani et al. 1998) have strengthen the significance of the emission feature present over the thermal continuum best fitting the EUV/soft X-ray data. Here we present a phenomenological model interpreting the data in terms of an ion cyclotron feature, arising from a thin outer layer of hot plasma covering Geminga's polar caps. The width of the feature favours for Geminga the strongly oblique rotator configuration, as already deduced from the soft X-ray data (Halpern & Ruderman 1993). Our interpretation implies a magnetic field of 3 - 5 10d11 G, consistent with the value deduced from the dynamical parameters of the pulsar. This would represent the first case of an independent measurement of the surface magnetic field of an isolated neutron star. The expected time modulation of Geminga's feature is also discussed., Comment: 7 pages, LaTex, 7 Postscript figures. Astronomy&Astrophysics-in press

Recent optical/UV data of Geminga (Mignani et al. 1998) have strengthen the significance of the emission feature present over the thermal continuum best fitting the EUV/soft X-ray data. Here we present a phenomenological model interpreting the data in terms of an ion cyclotron feature, arising from a thin outer layer of hot plasma covering Geminga's polar caps. The width of the feature favours for Geminga the strongly oblique rotator configuration, as already deduced from the soft X-ray data (Halpern & Ruderman 1993). Our interpretation implies a magnetic field of 3 - 5 10d11 G, consistent with the value deduced from the dynamical parameters of the pulsar. This would represent the first case of an independent measurement of the surface magnetic field of an isolated neutron star. The expected time modulation of Geminga's feature is also discussed., Comment: 7 pages, LaTex, 7 Postscript figures. Astronomy&Astrophysics-in press

Experiments with strong localized electron cyclotron heating (ECH) in the RTP tokamak show that electron heat transport is governed by alternating layers of good and bad thermal conduction. For central deposition hot filaments are observed inside the q = 1 radius. Moving the ECH resonance from the centre to the edge of the plasma results in discrete steps of the central electron temperature. The transitions occur when the minimum q value crosses q = 1,2,5/2 or 3, and correspond to the loss of a transport barrier situated close to the rational q value. Close to the transitions a new type of sawtooth activity is observed, characterized by the formation of sharp off-axis maxima on the profile, which collapse abruptly. The formation of the off-axis maxima is attributed to heat deposition precisely `on top of' a transport barrier.

The problem of the propagation of a harmonic temperature perturbation in a plasma with both diffusive and nondiffusive energy transport is addressed. The energy flux is modeled by two (radially varying) effective coefficients for the diffusive and nondiffusive transport, and the effects of perturbed energy sinks and of cylindrical geometry are taken into account. A simple, local relationship is found between the two transport coefficients and the gradients of the phase and amplitude of the temperature perturbation. This relationship can be used for the interpretation of heating modulation experiments, provided data at different modulation frequencies are available. Since a harmonic density perturbation in a plasma follows a similar linearized transport equation, a similar model can be applied also to density modulation experiments.

This work was directed toward the ultimate goal of developing an improved model of the temperature, density, and composition of that region of the earth's neutral atmosphere above the turbopause. A wealth of data has become available, mainly in the form of total density and composition from satellite-borne mass-spectrometers, in recent years. Most of the work performed under the contract consisted of the analysis of data from the ESRO 4 mass spectrometer with a view towards improvement of the model of the geomagnetic variation in the thermosphere and exosphere.

Prepared in cooperation with Harvard College Observatory, Contract N5ori-07647.

The conversion of upper-atmosphere densities to temperatures by means of atmospheric models, whether time-dependent or static, is subject to limitations, which are critically examined. Nicolet's model is used to obtain temperatures from the drag of five artificial satellites in the time interval from 1958 to 1963. The temperature variations with solar and geomagnetic activity are reviewed. The relation between the day and night temperatures averaged over one solar rotation and the corresponding decimetric solar flux shows a definite departure from linearity, which is just about the same for the 8 cm and the 10.7 cm flux. The diurnal variation is a very stable feature; its 2 PM maximum and 4 AM minimum do not show any appreciable shift with solar activity, nor does the ratio of the maximum to the minimum temperature, which is close to 1.30. An analytical model of the diurnal variation is presented as a function of solar time and latitude. This model is used to eliminate the day-and-night effect in a study of the semi-annual temperature oscillation, whose amplitude is found to be related to solar activity in the same manner as the atmospheric temperature itself. The original explanation based on the solar wind now appears unlikely. A practical method for the computation of exospheric temperatures as a function of geographic, solar, and geomagnetic parameters is schematically presented. (Author), Pub. in Space Research (Netherlands) v5 p1152-74 1964 (Copies available only to DDC users). Proceedings of the International Space Science Symposium (no. 5) held at Florence, Italy, 8-20 May 64.

Seven satellites with perigee heights between 206 and 1121 km showed atmospheric-drag perturbations during the geomagnetic disturbances of November 1960. These perturbations closely paralleled the planetary geomagnetic-index curves with no detectable time lag. The amplitudes of the perturbations ranged from a factor of 2 to a factor of 8; they showed a general tendency to increase with height, but little correlation with latitude or position relative to the sun. These results are confirmed by the accelerations of two satellites during the magnetic storm of October 6 and 7, 1960. (Author)

Piero Jacchia

Piero Jacchia

Piero Jacchia