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  • 1
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    Radio Emissions Observed by Galileo near Io (1998)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The Galileo spacecraft observed spin modulation of radio wave emissions near Io in the frequency range from about 600 kHz to about 1.2 MHz. Assuming transverse EM radiation, we have used the modulation of the high-frequency sweep-frequency receiver signals of the electric dipole antenna over many spins to estimate the plane through the source. The emission has a range of frequencies close to the local upper hybrid frequency of the plasma near Io. We conclude that the emission may be associated with either the plasma torus or magnetic flux tubes in the wake of Io (the Alfven current system). We postulate this emission may be associated with a free-energy source such as density gradients, energetic plasma beams and/or an electron distribution with a temperature anisotropy. All of these free-energy sources are observed or expected in the torus near Io. The observations are the first in the hectometric frequency range that have a source associated with Io or in the Io torus.
    Keywords: Space Radiation
    Type: Paper-97GL03442 , Geophysical Research Letters (ISSN 0094-8534); 25; 1; 25-28
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  • 2
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    Auroral Kilometric Radiation and the Auroral Electrojet Index for the January 1997 Magnetic Cloud Event (1998)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Auroral kilometric radiation (AKR) observations by Polar and Geotail are compared with the auroral electro jet index for the January 1997 magnetic cloud event. These two-spacecraft measurements are complementary in covering the AKR emission cones throughout the event and,together,reasonably represent the auroral electrojet AE index. We point out, however, limitations of both the AKR index and the AE index in providing truly global measurements of substorm activity.
    Keywords: Space Radiation
    Type: Paper-98GL00404 , Geophysical Research Letters (ISSN 0094-8534); 25; 15; 3027-3030
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  • 3
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    Local Time Dependence of Jovian Radio Emissions Observed by Galileo (1999)
    In:  Other Sources
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    Publication Date: 2019-07-13
    Description: Galileo has been in orbit around Jupiter since December 1995. All the orbits are equatorial and elliptical, with apogees between 60 R(sub J) - 142 R(sub J) and perigees from 8 - 12 R(sub J). Since orbit injection, the plasma wave instrument (PWS) has been collecting data over specific intervals of each of the orbits at all local times and a range of different radial distances. We present the results of a survey of the data for the frequency range 300 kHz to 5.6 MHz, which includes the hectometric (HOM) and low-frequency decametric (DAM) emissions. The results indicate that both the HOM and DAM emission are more intense and occur more frequently in the midnight sector of Jupiter. This is in analogy to Earth and consistent with a magnetic substorm source for a portion of the radio emissions in this frequency range. Another peak in the power levels is observed on the Jovian dayside in the local time range 11 hrs 〈 LT 〈 12 hrs. This peak does not have a terrestrial counterpart. We speculate that this dayside peak may be a result of sampling near perigee, but we cannot rule out the possibility that this is not the case.
    Keywords: Space Radiation
    Type: Paper-1999GL900047 , Geophysical Research Letters (ISSN 0094-8276); 26; 5; 569-572
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  • 4
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    The Influence of The Galilean Satellites on Radio Emissions From The Jovian System (2000)
    In:  Other Sources
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    Publication Date: 2019-07-10
    Description: The Galilean satellites influence radio emissions from the Jovian system in a variety of ways. The best and most familiar example of these is the Io control of decametric radiation discovered in 1964 by Bigg. Voyager observations of broadband kilometric radiation revealed a low-latitude shadow zone cast by the Io torus at frequencies between a few tens of kHz and about 1 MHz. Voyager also discovered narrowband kilometric radio emissions emanating from the outer edge of the torus. In this paper we will discuss expansions in the suite of satellite influences based on new observations by Galileo. These include the discovery of Ganymede's magnetosphere and evidence of radio emissions generated via mode conversion from upper hybrid waves in the frequency range of about 20 - 100 kHz. There is evidence that Ganymede may control some of the hectometric or low-frequency decametric radio emissions based on occultation measurements and statistical studies of radio emission occurrence as a function of Ganymede phase. Direction-finding measurements in the vicinity of Io suggest that a portion of the hectometric emissions may be generated near the lo L-shell. A rotationally modulated attenuation band in the hectometric emission appears to be the result of scattering at or near the Io L-shell where the waves propagate nearly parallel to the magnetic field. There is even a tantalizing hint of a Europa connection to the source of narrowband kilometric radiation.
    Keywords: Space Radiation
    Type: Geophysical-Monograph-119 , Radio Astronomy at Long Wavelenghts; 213-225
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  • 5
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    Radio Emissions from the Outer Heliosphere (1996)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: For nearly fifteen years the Voyager 1 and 2 spacecraft have been detecting an unusual radio emission in the outer heliosphere in the frequency range from about 2 to 3 kHz. Two major events have been observed, the first in 1983-84 and the second in 1992-93. In both cases the onset of the radio emission occurred about 400 days after a period of intense solar activity, the first in mid-July 1982, and the second in May-June 1991. These two periods of solar activity produced the two deepest cosmic ray Forbush decreases ever observed. Forbush decreases are indicative of a system of strong shocks and associated disturbances propagating outward through the heliosphere. The radio emission is believed to have been produced when this system of shocks and disturbances interacted with one of the outer boundaries of the heliosphere, most likely in the vicinity of the the heliopause. The emission is believed to be generated by the shock-driven Langmuir-wave mode conversion mechanism, which produces radiation at the plasma frequency (f(sub p)) and at twice the plasma frequency (2f(sub p)). From the 400-day travel time and the known speed of the shocks, the distance to the interaction region can be computed, and is estimated to be in the range from about 110 to 160 AU.
    Keywords: Space Radiation
    Type: NASA-CR-204670 , NAS 1.26:204670 , Space Science Reviews; 78; 53-66
    Format: application/pdf
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