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Large areas of nuclear emulsions and plastic detectors were exposed to the primary cosmic radiation during high-altitude balloon flights. From an analysis of 141 particle tracks recorded during a total exposure of 13,000,000 sq m-ster-sec, a charge spectrum of the VVH particles has been derived.
For comparisons between theoretical and observed charge spectra of VVH particles to be meaningful, at least two conditions must be met. First, charge resolution must be adequate to separate important groups of nuclei, and there should be no significant systematic errors in the charge scale developed. Second, there must be adequate rejection of slower particles of smaller Z, which have been observed in several flights. Within these conditions, it has been shown that observed features of the charge spectrum are not accidents of the analysis but reflect real variations in the relative abundances that must be explained by any successful model.
Very rapid measurements, ranging from a few microsecond to milliseconds in duration, characterize studies made in shock regions or behind them. A system to measure gas density under such conditions in a 15.24-cm (6-in.) expansion tube is described. The basic elements are an electron beam of moderate energy and high current capability, an optical detector, and the associated electronics and data readout equipment. A heated-cathode electron gun, capable of pulsed operation and delivering up to 200 milliamperes current, provides the source of electrons. Optics include a simple collector lens, aperture, collimator lens, filters, and a photomultiplier tube. The photomultiplier output signal was recorded by means of photographed oscilloscope traces for pulsed beam operation.
Cosmic-ray particles with kinetic energies well below cutoff values were detected during a high-altitude balloon flight at 41 deg N geomagnetic latitude. These particles had kinetic energies up to 400 MeV/amu and charges in the range from 6 to 30. They are probably reentrant albedo particles and are of interest primarily because they can be confused with fast ultraheavy primary particles in some experiments.
Measurements of the mean static free-stream gas density have been made in two Langley Research Center helium facilities, the 3-inch leg of the high-Reynolds-number helium complex and the 22-inch hypersonic helium tunnel. Rayleigh scattering of a CW argon ion laser beam at 514.5 nm provided the basic physical mechanism. The behavior of the scattered signal was linear, confirmed by a preliminary laboratory study. That study also revealed the need to introduce baffles to reduce stray light. A relatively simple optical system and associated photon-counting electronics were utilized to obtain data for densities from 10 to the 23rd to 10 to the 25th per cu m. The major purpose, to confirm the applicability of this technique in the hypersonic helium flow, was accomplished.
It is noted that as a result of the ISEE 1 and 2 missions, it was found that return ions, that is, ions streaming into the solar wind away from the bow shock, are comprised of separate classes having distinguishable energy and directional distributions. In addition, each of these classes of return ions, which are essentially all protons, is associated with its own level of upstream magnetic wave activity. It is shown that each class is also associated with a particular section of the foreshock determined by the IMF and the energies of the particles themselves. A series of schematic figures depicting the IMF-shock geometry for selected instants during the first two hours of ISEE particle data of November 8, 1977, is presented.
Measurements of the velocity of discrete whistler-mode wave packets in the region upstream of the earth's bow shock are presented. Data from the dual magnetometers aboard the ISEE 1 and 2 spacecraft was used to determine the time delay between the appearance of a wavefront at each spacecraft in order to obtain the wave velocity in the spacecraft frame, and from it the intrinsic wave phase velocity. Results from the 11 events characterized by sufficiently large velocities reveal the wave packets to exhibit a nearly perfectly circular polarization with the packet field rotating in the left-hand sense about the ambient field direction in the spacecraft frame. In the plasma rest frame, these waves are found to be right-handed polarized waves with frequencies several times the proton gyrofrequency which are attempting to propagate upstream against the solar wind but are in fact being carried towards the earth by the solar wind flow.
The ULF electromagnetic waves associated with the earth's foreshock appear in two discrete frequency ranges, designated the low frequency waves at 0.01 - .05 Hz and the high frequency waves at 0.4 - 1.0 Hz. Falling within this second class are both the 0.4 Hz discrete wave packets and the slightly higher frequency wave trains commonly found just preceding the bow shock. Similar waves have now also been observed upstream of, but clearly associated with, the bow shocks of Mercury, Venus and Jupiter. Those observations are reviewed along with original recent work using the two point measurements made possible by the launch of ISEE 1 and 2 to further characterize the terrestrial waves.
It has long been known that the periods of Pc 3, 4 pulsations on the ground correlate with the magnitude of the interplanetary magnetic field. This fact has been used to argue for an exogenic source for these pulsations. Particularly attractive candidates for the source of pulsations in this frequency range are the upstream waves of similar frequencies which are associated with populations of ions reflected from the bow shock. However, the dependence of the period of these waves on the strength of the interplanetary magnetic field has never been checked. This paper performs such a check and confirms that the upstream waves have the proper functional relationship.
Measurements from the Lepedea plasma instruments and the flux gate magnetometers on ISEE 1 and 2 are used to examine the nature of the hydromagnetic waves associated with the various classes of ions backstreaming from the earth's bow shock. The reflected ions, which are confined to a narrow energy and angular range, are accompanied by small amplitude (less than approximately 1/2 gamma peak to peak) left-handed waves at frequencies close to 1 Hz in the spacecraft frame. Diffuse backstreaming particles with a broad energy spectrum are associated with low frequency (approximately 30-s period), large amplitude (approximately 5 gamma peak to peak) waves. Intermediate particles are associated with a mixture of these two wave types. Often the waves associated with the diffuse beams steepen as if they were minishocks. The leading edge (trailing edge in the spacecraft frame) frequently appears to break up into a whistler mode wave packet. These discrete wave packets are right-hand polarized and have frequencies from below the proton gyrofrequency to well above it in the plasma frame and are blown back towards the earth by the solar wind.
One property of the collisionless shocks that may be studied through a comparison of their behavior in a variety of plasma conditions at several different planets is the occurrence of MHD waves, associated with particle beams accelerated at these shocks and flowing backward to the sun. Mercury, Venus, earth and Jupiter observations of one of these wave classes show that (1) the empirical relationship between interplanetary field strength and wave frequency in the observer's rest frame is approximately true at all the planets considered, and (2) the observed frequencies are consistent with resonance with beams of ions of the same energy at each of the planets. This is, in turn, in keeping with the Sonnerup (1969) geometrical model of ion reflection at collisionless shocks. It is suggested that this ion acceleration mechanism may occur in astrophysical systems similar to the solar system, providing a source of acceleration for cosmic rays.
A new class of upstream wave is reported with relatively high frequencies of about 1 Hz and small amplitudes compared to the more common larger amplitude, low-frequency (0.03 Hz) upstream wave. The waves were first noted in association with beams of ions reflected back upstream at the bowshock, and although beam presence appears to be a necessary condition for the observation of the waves, it is not a sufficient condition for the existence of the waves. Magnetometer measurements are used to determine intrinsic properties of the waves, and simultaneous two point measurements are used to calculate and eliminate Doppler shifting effects. Results indicate that the waves are right-hand elliptically polarized whistler mode waves with plasma rest frame frequencies of about 20-100 times the proton gyrofrequency and wavelengths of about 100 km.
The parametric variation with solar wind conditions in the overshoot in magnetic field strength observed in planetary bow shocks and believed to be associated with the ion reflection process is examined based on both terrestrial and planetary bow shock data. The combined data from Venus, earth, Jupiter and Saturn cover a wider range of solar wind densities, magnetic field strength, electron and proton temperatures, Mach numbers, beta, spiral angle, and scale lengths than observable from earth alone. The bow shock structure, particularly the magnitude of the post-shock field strength overshoot, is found to depend principally on plasma beta and the magnetosonic Mach number with a continuously increasing overshoot strength associated with increasing beta and Mach number.
Plasma waves associated with the magnetosphere from the magnetosheath to the outer magnetosphere are investigated to obtain a clear definition of the boundaries and regions, to characterize the waves observed in these regions, to determine which wave modes are present, and to determine their origin. Emphasis is on high time resolution data and a comparison between measurements by different antenna systems. It is shown that the magnetosheath flux transfer events, the magnetopause current layer, the outer magnetosphere, and the boundary layer can be identified by their magnetic field and plasma wave characteristics, as well as by their plasma and energetic particle signatures. The plasma wave characteristics in the current layer and in the boundary layer are very similar to the features in the flux transfer events, and upon entry into their outer magnetosphere, the plasma wave spectra are dominated by intense electromagnetic chorus bursts and electrostatic emissions.
High-temporal-resolution measurements of electron velocity distributions have been obtained for many transits through the earth's bow shock. Within all oblique shocks studied, the maximum of the electron velocity distribution is offset with respect to the ion rest frame parallel to the magnetic field vector and directed downstream. These observations indicate that electron thermalization within the bow shock consists first of a downstream acceleration parallel to the magnetic field vector by the macroscopic shock electric field, followed by beam-driven plasma instabilities.
Wide separations up to more than 1 earth radius between ISEE 1 and 2 during the second half of 1978 have been used to measure the correlation length of magnetic pulsations in quasiparallel shocks. When the two spacecraft were less than a few hundred km apart, magnetic oscillations measured by magnetometers on both spacecraft exhibited virtually identical waveforms, but at distances of several thousand km, the two time series of field variation showed no detailed similarily at all. The correlation coefficients of the pulsations dropped from close to 1.0 for spacecraft separations of less than 100 km to 0.2 for separations of greater than 800 km. A correlation length of several hundred km may be related to the gyroradius of return protons with energy typical of the peaks of diffuse and beam ion distributions.
The ISEE-1 and -2 spacecraft have provided the opportunity for studying the thickness and velocity of the bow shock on a regular basis. There are two models for the thickness of quasi-perpendicular laminar shocks. Galeev (1976) has calculated the thickness of the nearly perpendicular laminar bow shock by means of a weak ion sound turbulence (WIST) approach. Morse and Greenstadt (1976), on the other hand, assume that the shock thickness adjust itself to stay close to the drift velocity for marginal stability of current driven electrostatic waves. The present investigation is concerned with an evaluation of the competing theories, taking into account ISEE-1 and -2 observations of laminar bow shocks. It is found that the formula used by Morse and Greenstadt does not predict the observed thicknesses as well as the formula given by Galeev.
New information is presented on the general characteristics of electron distribution functions upstream, within, and downstream of the earth's bow shock, thereby providing new insights into the instabilities in collisionless shocks. The results presented are from a survey of electron velocity distributions measured near the earth's bow shock between October 1977 and December 1978 using the Los Alamos/Garching plasma instrumentation aboard ISEE 2. A wide variety of distribution shapes is found within the different plasma regions in close proximity to the bow shock. It is found that these shapes can be classified into general types that are characteristic of three different plasma regions, namely the upstream region or electron foreshock, the shock proper where most of the heating occurs, and the downstream region or the magnetosheath. Evidence is provided that field-aligned, rather than cross-field, instabilities are the major source of electron dissipation in the earth's bow shock.