PUBLICATIONS Journal of Geophysical Research: Space Physics RESEARCH ARTICLE 10.1002/2014JA020392 Special Section: New perspectives on Earth's radiation belt regions from the prime mission of the Van Allen Probes Key Points: • The Kennel-Petschek (KP) limit standardizes energetic ion spectra comparisons • Only Earth and Jupiter spectra are limited-sculpted by the differential KP limit • The efficacy of the KP limit for ions needs to be tested by the Van Allen Probes
Correspondence to: B. H. Mauk,
[email protected]
Citation: Mauk, B. H. (2014), Comparative investigation of the energetic ion spectra comprising the magnetospheric ring currents of the solar system, J. Geophys. Res. Space Physics, 119, 9729–9746, doi:10.1002/2014JA020392. Received 14 JUL 2014 Accepted 8 NOV 2014 Accepted article online 12 NOV 2014 Published online 9 DEC 2014
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MAUK
Comparative investigation of the energetic ion spectra comprising the magnetospheric ring currents of the solar system B. H. Mauk1 1
Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland, USA
Abstract Investigated here are factors that control the intensities and shapes of energetic ion spectra that make up the ring current populations of the strongly magnetized planets of the solar system, specifically those of Earth, Jupiter, Saturn, Uranus, and Neptune. Following a previous and similar comparative investigation of radiation belt electrons, we here turn our attention to ions. Specifically, we examine the possible role of the differential ion Kennel-Petschek limit, as moderated by Electromagnetic Ion Cyclotron (EMIC) waves, as a standard for comparing the most intense ion spectra within the strongly magnetized planetary magnetospheres. In carrying out this investigation, the substantial complexities engendered by the very different ion composition distributions of these diverse magnetospheres must be addressed, given that the dispersion properties of the EMIC waves are strongly determined by the ion composition of the plasmas within which the waves propagate. Chosen for comparison are the ion spectra within these systems that are the most intense observed, specifically at 100 keV and 1 MeV. We find that Earth and Jupiter are unique in having their most intense ion spectra likely limited and sculpted by the Kennel-Petschek process. The ion spectra of Saturn, Uranus, and Neptune reside far below their respective limits and are likely limited by interactions with gas and dust (Saturn) and by the absence of robust ion acceleration processes (Uranus and Neptune). Suggestions are provided for further testing the efficacy of the differential Kennel-Petschek limit for ions using the Van Allen Probes.
1. Introduction What criterion or standard do we use when comparing the particle populations within the highly diverse magnetospheres of the solar system? Should a planet that is 10 times bigger or that has a magnetic field that is 10 times stronger have energetic particle populations that are 10 times more intense or 10 times more energetic? For relativistic electron spectra of the strongly magnetized planets of the solar system, one answer has been suggested and supported. Specifically, following the pioneering work of Kennel and Petschek [1966], Schulz and Davidson [1988], Davidson et al. [1988], and Summers et al. [2009], Mauk and Fox [2010] developed a general approach to evaluating a relativistic differential Kennel-Petschek (KP) limit for energetic magnetospheric electrons that successfully provides a standard for comparing the relativistic electron radiation belts of all of the strongly magnetized planets of the solar system. The electron KP limit makes use of the expected runaway growth of whistler waves under certain conditions and the associated strong scattering of electrons into the magnetic loss cone. This work also confirmed for the relativistic regime the nonrelativistic Schulz and Davidson [1988] prediction of a characteristic E1 shape of the spectra for energies 1), LC to assume propagation parallel to the “left-handed compressional (|ε| < 1), and R to “right handed.” The ion magnetic field. Table 2 shows the composition used is that provided in Table 2. The theoretical expressions used to generate this plot are reproduced from Stix [1992] in Appendix A. compositions that were assumed for the five planets. Much experimentation has demonstrated that the Kennel-Petschek results that we report here are quite insensitive to the exact numerical fractions used and to the addition of very minor species with contribution fractions
