G. Martelli

GEOS-1 observations of electrostatic waves, and their relationship with plasma parameters

In this paper we describe and discuss the occurrence of natural wave emissions detected by GEOS-1 at frequencies above the electron gyrofrequency. The bulk of the data presented comes from the first six months of satellite operation and thus concerns mainly dayside phenomena. The paper is arranged as follows:After some general remarks, a classification of the wave phenomena is developed in Section 2, and experimental evidence and morphological information relevant to this classification are contained in Section 3. Section 4 includes some preliminary comments on nightside observations. The results are discussed in Section 5, where it is argued that they can be understood as manifestations of electron cyclotron harmonic (Bernstein) wave emission in a plasma parameter range which has only very recently received any theoretical examinations. This theme is further developed in a comparison paper (Ronnmark et al., 1978).

Catastrophic disruption experiments: recent results

Abstract This paper presents a review of the progress in the field of catastrophic disruption experiments over the past 4 years, since the publication of the review paper by Fujiwara et al. (Asteroids II, pp. 240–265, University of Arizona Press, Tucson, 1989). We describe the development of new techniques to produce shattering impacts relevant to the study of the collisional evolution of the asteroids, and summarise the results from numerous experiments which have been performed to date, using a variety of materials for both the impactor and the targets. Some of these, such as ice-on-ice, loose aggregates and pressurised targets, are quite new and have provided novel and exciting results. Some of the gaps existing previously in the data on fragment ejection-angle distributions, as well as translational and rotational velocity fields (including fine fragments) have been filled, and these new results will be surveyed.

Optical detection of electrostatic ion cyclotron waves in the auroral plasma

Abstract Spectral analysis of recent photometric observations has revealed the existence of narrow frequency band pulsations in the brightness of some auroras. Peaks in the power spectra, between 25 and 32 Hz have been observed. We suggest that these observations are associated with electrostatic ion cyclotron waves, which are excited when the relative drift between ions and electrons due to field aligned (Birkeland) currents, exceeds a certain critical value.

Simulation of hypervelocity impacts using a contact charge

Abstract Two sets of hypervelocity impact experiments have been performed in the open using a contact charge technique and recorded using fast-framing cameras. It has been possible to record the uninterrupted ballistic trajectories of fragments from the catastrophically disrupted targets, together with their velocity and rotational properties directly after the impact, as well as their size. By performing these experiments in the open and on fairly soft ground, secondary fragmentation normally caused by impact onto the walls or floor of a test chamber has been minimised. A total of 10 experiments have been performed using targets of artificial rock which were either homogeneous, cored or carefully pre-fractured. We report here on the analysis of some of these data using a computer and special software written and developed by our group, with an indication of the results obtained.

Magnification of pre-existing magnetic fields in impact-produced plasmas, with reference to impact craters

Abstract Magnetic fields generated by the electrical currents associated with thermal forces in an impact-produced plasma cloud are proposed as a possible explanation of the magnetic perturbation observed during hypervelocity impact events. Order of magnitude estimates for this effect show that this is compatible with experimental findings. We suggest that this effect may contribute to the magnetisation observed in the neighbourhood of lunar craters (see, e.g. Anderson and Wilhelms, Earth Planet. Sci. Letts.46, 107).

Jets of Fragments from Catastrophic Break-Up and their Astrophysical Implications

We present some preliminary results of a series of catastrophic break-up experiments carried out in the open, against targets of natural and artificial rock, with and without a harder core. These experiments were aimed at investigating the outcomes of hypervelocity impact disruption phenomena, designed to understand the influence of large-scale collisions on the evolution of asteroids and other small solar system bodies. For the first time in this kind of experiments, evidence was found of collimated jets, i.e. the ejection of a statistically significant number of fragments all closely aligned about some preferential planes. Moreover, the presence of some groups of fragments lying close to each other on the ground was also detected.

Drift and diffusion of filament ion clouds in the earth's ionosphere

Abstract The behaviour of a needle shaped perturbation (‘filament ion cloud’) in the plasma of the ionospheric F 1-layer in the presence of a neutral wind and a uniform electric field having a component perpendicular to the geomagnetic field has been investigated. Expressions have been obtained for the plasma density, the diffusion coefficients and the polarisation field induced in the ambient plasma by the perturbation, and its drift motion has been examined.

Non-linear resonant excitation of whistler mode waves in the Earth's ionosphere

We examine the resonant non-linear interaction in the Earths ionosphere of two powerful high frequency radio beams with frequencies f1 and f2 (both larger than the plasma frequency at F2max) and wave numbers k1 and k2 such that a whistler mode wave can be excited with a frequency f3 = f1 — f2 and a wave number k3 = k1 − k2. The feasibility of an effective ground based installation, sited at low latitudes, is discussed and the field strength of the wave emerging from a 10 km wide ionospheric region illuminated by the beams is evaluated for a range of transmitted frequencies, beam orientations and plasma frequencies in the interaction region. It is suggested that the longitude dependence of the enhancement of VLF noise bands detected by the Ariel 3 satellite may be due to a non-linear interaction of this type between any two or more medium wavelength signals from areas where there is a high concentration of commercial broadcasting stations, such as the NE region of the U.S.A.

Determination of electric fields and neutral wind velocities in the ionospheric E- and F-regions from observations of artificial ion clouds

Abstract A new method of interpreting the behaviour of artificial ion clouds released in the Earths ionosphere is presented. It is shown that values for the ionospheric electric field, neutral wind velocities and, in some circumstances, ion collision frequency, can be deduced from a study of the motion and deformation of the ion clouds, including those released in the E -region.

Non-linear resonant excitation of whistler mode waves in the earth's ionosphere using two high frequency transmitters

Abstract We discuss the possibility of exciting whistler mode waves (WMWs) in the Earths ionosphere, by using two high frequency beams of electromagnetic waves ( f 1 ∼ f 2 ) suitably orientated to the geomagnetic field H o , so that a non-linear resonant interaction can take place in the natural ionospheric plasma, approximately at the altitude of the F 2 maximum electron density. Within the limitations imposed by ionospheric inhomogeneities in the interaction region, it should be possible to excite a WMW which propagates along a predetermined direction, e.g. parallel to H o . If we assume f 1 and f 2 to be approx 30 MHz (i.e. well above the ionospheric plasma frequency), this method would make it possible to select and vary the frequency range of the excited WMW up to a few hundreds kHz without substantial alterations to the high frequency transmitting system. Since the two beams should form an angle close to 90° to the direction of propagation of the WMW, this technique may prove particularly suitable for active wave experiments at low geomagnetic latitudes, where the geometry of the geomagnetic field limits the feasibility of direct wave injection experiments. Using the results of theoretical calculations of the three wave coupling coefficients, it will be shown that the transmitters required to produce WMWs with field strengths comparable to that of naturally occurring strong whistlers are substantial, but feasible.