Outer Solar System A Program for Exploration, Report of a Study (1969) / Chapter Skim
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PARTICLES, FIELDS, AND RADIO PHYSICS
PARTICLES, FIELDS, AND RADIO PHYSICS
Pages 46-66
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From page 46... ...
SOLAR WIND Near the earth's orbit, typical parameters of the solar wind are a radial velocity of 400 km/sec, a density of 5 protons per cnr } an ion temperature of l00,000 K, an electron temperature several times as high, a flow direction (corrected for aberration) that is a couple of degrees east of the sunearth line, and noticeable temperature anisotropies with the random ion velocity being greatest parallel to the magnetic field.
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From page 47... ...
The physical basis for the high ion temperature is presumably partially the thermal conductivity of the electrons and also a substantial conversion to thermal energy of the mechanical energy by wave-particle interactions. It seems likely that somewhere beyond 3 to l0 AU most of the velocity fluctuations would have been smoothed out and the waves damped out except as regenerated by plasma instabilities perhaps associated with the anisotropic expansion.
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From page 48... ...
There is no reason to think in terms of a smooth, symmetrical model; there should be irregularities in plasma properties, magnetic fields, and fluxes of energetic particles over a vast range of scales. The region near the ecliptic, where the large-scale interplanetary field is mainly in the azimuthal direction and normal to the flow, may be very different from the sun's polar region, where even at very large distances the interplanetary field and the flow are expected to be nearly parallel except for the magnetic fluctuations generated by the probably inevitable instabilities.
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From page 49... ...
Dispersion and polarization measurements conducted simultaneously will make it possible to separate effects of the plasma density of the solar corona from magnetic field effects, so that one may study space and time variations of the coronal plasma density and of longitudinal components of the coronal magnetic field. Range measurements, corrected by the results on dispersions, would provide accurate determinations of the 30-km apparent change in range due to the general relativistic effect of the solar gravity field, perhaps making it possible to test alternative theories.
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From page 50... ...
INTERPLANETARY MAGNETIC FIELD The solar wind convectively transports the solar magnetic field into interplanetary space. Here its presence controls the motion of more energetic charged particles originating from transient events on the sun, such as solar flares, as well as the continual flux of extrasolar origin.
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From page 51... ...
Evidence for the existence of a sectored structure of the interplanetary magnetic field should be detected at distances > l.5 AU, although the terminology of outward- and inward-directed fields obviously is no longer appropriate. Instead, the positive polarity will refer to fields directed
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From page 52... ...
The correlations of these measurements with those conducted by earth-orbiting satellites will be an important method of studying the largescale structure and dynamics of the interplanetary medium. As the sector and filamentary magnetic structures in the solar wind are swept far out from the sun, the spiral direction becomes nearly the azimuthal direction at low solar latitudes and there must be many interfaces between tubes in which the field runs in opposite directions.
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From page 53... ...
Accurate vector measurements of the interplanetary magnetic field and its variations for frequencies less than l Hz are of fundamental significance in a study of the interplanetary medium and the dynamics of the solar-system plasma. GALACTIC AND SOLAR COSMIC RAYS Knowledge of the composition and energy spectrum of galactic cosmic rays is of considerable astrophysical importance.
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From page 54... ...
Measurements near the earth do not provide the necessary information, since low-energy cosmic rays are partially screened out of our part of the solar system by the irregular magnetic fields convected outward by the solar wind. This effect is called modulation, because the magnitude of the effect is a function of solar activity.
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From page 55... ...
, the unmodulated spectrum can be estimated directly from the spectra of the primary particles, and hence observations of the behavior of these particles at various heliocentric distances should provide a very good direct test of modulation theories. There have been as yet no in situ measurements of the interplanetary magnetic field or plasma at high heliographic latitudes.
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From page 56... ...
The time delay must, of course, be very much greater, because the particles have to travel l0 to 20 AU (depending on solar-wind velocity) along the spiraled interplanetary magnetic field lines, the anisotropies are likely to be correspondingly less pronounced, the direction of arrival must be typically from 80° west of the sun, and the relationship of the event to observed solar flares might not be clear.
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From page 57... ...
In the case of the moon, which has essentially no intrinsic field, no ionosphere, and a very low conductivity, the solar wind flows unimpeded into the surface, leaving a nearly empty cavity behind. The interplanetary magnetic field passes through the moon and through the cavity with small and sometimes undetectable perturbations.
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From page 58... ...
This conclusion is based on the discovery, study, and analysis of radio emission in various frequency ranges. This permits estimates to be made of the topology and magnitude of the planetary magnetic field.
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From page 59... ...
for perhaps intermittent radio emission from Saturn, but it is clear that it is not nearly so spectacular a radio source as Jupiter and hence that its radiation belts must be far less significant even though, of all the planets, it most resembles Jupiter in size, rotation rate, and composition. However, if the rings of Saturn are an effective absorber of charged particles, it could still possess a significant magnetic field.
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From page 60... ...
for r exceeding RM, RV, and R^on, respectively. Tentatively, it is thought that the existence and intensity of Jovian radiation belts certify that the directed flow of solar plasma (the solar wind)
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From page 61... ...
Detailed study of the charged-particle populations in Jupiter's magnetosphere (c) Search for magnetic fields and radiation belts of Saturn, Uranus, Neptune, and Pluto and detailed investigation if positive findings occur in exploratory studies (d)
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From page 62... ...
Its relation to other planetary parameters, such as the magnetic field, the thermal and nonthermal particle populations, and the different forms of wave motion in Jupiter's magnetosphere and ionosphere will surely clarify the at present still schematic suggestions about its physical origin. That novel plasma physical effects are involved appears to be an implication of the strong modulation of decametric emission by the first Galilean satellite lo (similar effects have not been detected for Europa, Ganymede, or Callisto)
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From page 63... ...
The result of these radio astronomical studies, carried out synoptically over a range of orbiter positions with respect to the major satellites and to the rotational aspect of Jupiter, should be an adequate physical explanation of one of the most intense emissions known to astronomy. Galactic Low-Frequency Radio Noise The local plasma density of the solar wind near the earth limits observations of galactic radio waves to frequencies above ~30 kHz.
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From page 64... ...
It is therefore important that they be studied from orbiters as well as flybys. Planetary The same radio links between the spacecraft and earth discussed earlier in this chapter as a means of determining some of the plasma parameters of the corona and solar wind can also be used to provide accurate plasma measurements near the planets, for study of their magnetospheres and ionospheres and their interactions with the solar wind.
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From page 65... ...
A detailed study of the external magnetic field and of the charged particle population in the magnetosphere of Jupiter should be undertaken. In particular, the nature of Jupiter's nonthermal radio emissions should be studied both from spacecraft and from the earth.
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From page 66... ...
We recommend that first priority be given to a balanced program that combines planetary and interplanetary objectives and that smaller purely interplanetary missions to the outer solar system be used only if their scientific objectives cannot otherwise be met. To give a reasonable balance between the first exploration of new regions and extensive investigation of the most significant problems, we recommend the following order of importance of the missions (which is not the recommended chronological order)
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