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If we accept the hypothesis, that cometary nuclei may essentially be aggregates of sub-nuclei [Weissman, P.R., Are cometary nuclei primordial rubble piles? Nature 320, 242–244, 1986.], several independent kinds of observational facts point to the possibility, that there are at least two populations of sub-nuclei, and they can be mixed in the nucleus of a comet [Illes-Almar, E., On two different populations of cometary sub-nuclei, Antarctic Meteorites XX, June 6–8, Tokyo. Abstracts, pp. 93–94, 1995]. One kind of the sub-nuclei could be loose aggregates from fluffy material, while the other one may be more compact and cohesive.

Utilizing the opportunity that our previously analyzed drag material covers the whole latitude interval between 84°N and 84°S, we have reanalyzed our 16 thousand observed total density data comparing them with model densities as a function of different parameters. A significant asymmetric dependence has been found on geographic latitudes, namely the residuals decrease continuously and evenly from the North Pole to the South Pole. The effect is significant also using data derived from the decay of several subgroups and of single satellites. Finally simple formulae are suggested for the CIRA’72, for the CIRA’86 and for the NRLMSISE-00 models explaining about 10% of the variances in all three cases.

In this paper, we demonstrate that, besides the well-known corpuscular heating of auroral origin, there is another heat source in the equatorial region of the neutral upper atmosphere acting during and after geomagnetic storms as a consequence of the precipitation of ring current ions. The effect of the two sources has been separated on the basis of their induced diurnal variations using measurements from the CACTUS micro-accelerometer. In Almar and Illes-Almar [Adv. Space Res., this issue, 2004, doi:10.1016/j.asr.2003.04.060 ], the observational facts and our suggestion for the improvement of the CIRA'86 model are summarized to draw the attention of the constructors of the new CIRA model on two effects that are not yet built into the earlier CIRA models. In the present paper, we try to outline the possible physics behind the observational facts necessitating the improvement of the model.

The paper intends to draw the attention of future authors of the planned CIRA model to two effects discovered by us previously. They should be taken into account both in the description of the total density and in the physical connection between the magnetosphere and the neutral upper atmosphere. The first is the nighttime density maximum – even in geomagnetically quiet periods – which is probably connected with compressional heating due to global winds originating from the subsolar point. The second is the dependence of the total density residuals (with respect to the CIRA '86 model values) on D st , which might be the consequence of heating by particles precipitating from the ring current [Nabl. ISZ, vol. 24 (1986) pp. 121–127]. This effect hints at an additional magnetospheric heating (besides the well-known auroral heating in the main phase of geomagnetic storms) which acts during the recovery phase of the storms as well with a maximum near the equator. The equatorial source may be connected with ions, neutralized by charge exchange (Energetic Neutral Atoms, ENA), precipitating from the ring current and having a velocity vector directed towards the Earth. A new multiplicating function to the CIRA '86 model (ddMSIS model) describes both additional heating effects.

A morphological analysis of the linear features on asteroids 951 Gaspra and 243 Ida as well as on the Martian moon Phobos is presented. On all of these small bodies there are different groove systems. In every case the most numerous grooves constitute a system with a characteristic direction almost perpendicular to the long axis of the body, and a coverage of the visible surface. It is supposed that these are indications of their layered inner structure. Together with some kind of magnetism discovered at Gaspra and Ida they suggests that these small bodies of the Solar System may originate from the near surface of some ancient, already differentiated planetary bodies of larger dimension.

The fracture pattern around some gray bands on Europa has been investigated by edge and color enhancement techniques applied to some of the new Galileo images. The analysis supported the theoretical explanation on the origin of gray bands, namely that they took their origin from fracturing of the icy lithosphere, later the two margins moved away from each other leaving place for a newly formed strip of icy crust. The processed pictures reveal an older population of gray bands more clearly than the original images. Interestingly enough the newer gray band is cutting the old one at quite large angle of ∼70°. The measurement of ridge-width carried out inside the bands gave the same results of about 0.9 km independently of age.

Neutral density measurements by the Drag Balance Instrument (DBI) on board the Italian San Marco V satellite are made with high time resolution. These measurements permit the direct investigation of the thermospheric wave pattern variation in the equatorial zone. The investigation has been carried out not only statistically, but also through case studies. One of the interesting results is that, although the wave amplitude generally increases continuously with altitude, there are some cases when it changes abruptly. These abrupt changes can occur at different altitudes. The coupling processes such as ion drag and spatial resonance have also been studied as a function of altitude (h), local solar time (LST) and other parameters.

Alba Patera is a unique volcanic structure on Mars. It is suggested that it might be an ancient coronal structure buried under the thick volcanic material of the Tharsis Bulge (Illes, 1996).

Based on CACTUS (Capteur Accelerometrique Capacitif Triaxial Ultra Sensible) microaccelerometer measurements it has been demonstrated that — after taking into account all effects included in the MSIS'86=CIRA'86 (COSPAR, 1988) model - there are residual fluctuations in the density of the upper atmosphere much larger than that the accuracy of the measurements can account for. These fluctuations are attributed to some kind of wave activity (Illes-Almar, 1993, Illes-Almar et al. 1996a). The average deviations from a model are considered as a measure of the amplitude of the waves in the atmosphere and are analysed as a function of geomagnetic coordinates, altitude and local solar time, in order to identify possible wave sources either in the lower lying atmosphere or in the thermosphere/ionosphere system. As a first step, the present investigation intends to make a map of the wave pattern by this method.

Total density data based on CACTUS microaccelerometer measurements — carried out between 400 and 600 km altitudes at low latitudes in a quiet solar activity period (1975–1977) — have been compared to corresponding MSIS'86=CIRA'86 model values. The differences proved to be a double valued function of Kp but a single valued one of Dst. The residuals are a function of local solar time (LST), of geomagnetic latitude (ϕgeom) and of the intensity of geomagnetic activity respectively. A new geomagnetic term is presented to replace the Kp dependent term in the otherwise unchanged model. The improved MSIS (iMSIS) model represents the observations better and hints more directly at the physical background of the geomagnetic phenomenon in the equatorial zone.

Total density data based on CACTUS microaccelerometer measurements — carried out between 230 and 700 km altitudes at low latitudes in the period 1975–1979 — have been compared to corresponding MSIS'86 = CIRA'86 model values. A Dst dependent geomagnetic term is presented with height dependent coefficients (hMSIS) to replace the inadequate Kp dependent term in the otherwise unchanged model. This new geomagnetic term is the generalisation of the previously published function to altitudes below 400 km.

Total density data based on CACTUS microaccelerometer measurements have been analysed between 220 and 700 km altitudes at low latitudes in the time interval 1975–79. Instantaneous residual density values with respect to atmospheric models for quiet as well as for disturbed days have been investigated as a function of local solar time (LST). It has been found that the density residuals present conspicuous increases of the variance in several LST intervals as well as a general broadening which can be in connection with various coupling mechanisms between the thermosphere and the ionosphere/magnetosphere on the one hand, and the middle atmosphere on the other. An attempt is made to select those mechanisms responsible for the observed features.

In the first 15 years of the Space Age satellite position measurements derived from visual and photographic observations helped to determine the decay rate of satellites, by means of which the first models of the upper-atmosphere have been calculated. Our group concentrated on the study of the geomagnetic effect and collected several ten thousand observations from Europe and from Asia before and after geomagnetic storms. It was already conspicuous during data gathering that there are considerably less satellite observations in the time periods following almost every geomagnetic storm than in the preceding ones – indicating that the cloudiness might increase considerably after geomagnetic storms. The paper presents the statistical treatment of this phenomenon.

The excess thermospheric density at low latitudes during the recovery phase of geomagnetic disturbances found in earlier studies has been related to the ring current belt. This would mean that the geomagnetic effect is due not only to an auroral but also to an equatorial source. The low latitude excess density could be separated into a storm time dependent and a local time dependent component. Thus, the morphology of this effect is similar to that of the geomagnetic disturbance field. The heating can be attributed mainly to the precipitation of energetic neutral particles produced by charge exchange, as well as to the dumping of energetic charged particles due to wave-particle interaction. The local time dependent component could be connected also with the asymmetry of the composition of the ring current and with the irregular shape of the plasmasphere.

Based on measurements of the CACTUS accelerometer it has been found that during the recovery phase of geomagnetic disturbances the models are unable to describe properly the total density changes in the equatorial thermosphere. Introducing the geomagnetic activity index Dst instead of Kp as a model input parameter gives a much better description of the measurements. The residuals show a diurnal dependence, hinting partly at model errors in the diurnal effect (though MSIS 86 is much better than DTM in this respect), partly at a diurnal term in the geomagnetic effect.

The neutral density excess as compared to the MSIS 86 model indicates a double valued dependence on the Kp geomagnetic activity index; a very similar dependence was found in some ionospheric parameters. The similarity hints at coupling between the ionosphere and the neutral upper-atmosphere during geomagnetic disturbances. Relations between the changes of these parameters and the neutral density are considered.

L-grooves are the consequence of layered structure of Phobos, which are made up of parallel layers of different composition or hardness.

Previous investigations of the authors based on the decay rates of many satellites have demonstrated the existence of a post-storm effect in the neutral atmosphere after geomagnetic storms. Its maximum appears 4–6 days after the storm onset. It generally lasts 8–10 days, but if there is also an ionospheric post-storm effect, then it is about twice as long at mid-latitudes and in the evening hours. The observed characteristics of the post-storm effect seem to indicate that it is related to the precipitation of ring current particles due to charge exchange and wave-particle interactions.

The neutral post-storm effect is reconsidered by means of accelerometric data. Since Δρ has proved to be different function of Kp during and outside recovery phases, but a unique function of Dst, the latter is considered as a better index for correcting the effect of geomagnetic activity in models, i.e. it seems that the ring current plays an important role in the geomagnetic effect of the equatorial thermosphere.