The propagation of Jovian electrons to Earth

Abstract

Jovian electron flux increases are observed by the University of Chicago experiment on the earth-orbiting satellite Imp 8 throughout five -• 13-month Jovian synodic years during the period from launch of the satellite in late 1973 to 1979. A detailed analysis of these data is presented to define the characteristics of Jovian electron propagation to earth. Corotating interaction regions (CIR) that form at the leading edges of fast solar wind streams continue to modulate the propagation of MeV electrons from Jupiter to the orbit of earth to produce -•27-day recurrent variations in the Jovian etectron flux intensity. Between these CIR's, both the spatial distribution of Jovian electrons around earth's orbit and the time-intensity profiles of the Jovian electron flux induced by the CIR modulation effect are shown to be consistent with the predictions of a simple diffusion model calculation, assuming that Jupiter's magnetosphere is a 'point' source of electrons. A new and significant result of this study is that these time-intensity profiles are more accurately described not by the assumption that Jupiter is a constant source of electrons, but rather by assuming that electron emission is initiated with each passage of a CIR by Jupiter with the emission continuing for several days only. A measurement of the spatial gradient of these electrons demonstrates that there is a significant reduction in the Jovian electron flux between Jupiter and earth even when earth is aligned with Jupiter along the average direction of the interplanetary magnetic field. This observation is inconsistent with 'scatter-free' propagation. telescope on the earth-orbitin• satellite Imp 8 is presented. The purpose of this study is to define the characteristics of Jovian electron propagation to each and to test the propagation model used by Conlon [1978] in his analysis of Jovian electron data obtained on the Pioneer 10 and 11 spacecraft. Conlon [1978] showed that one of the primary mechanisms affecting the propagation of Jovian electrons was the modulation of these electrons by corotating interaction regions (CIR). These CIR's are regions of compression in the solar wind that are formed as a fast solar wind stream overtakes slower plasma [Smith and Wolfe, 1976; Hundhausen and Gosling, 1976]. As Conlon and Simpson [1977] have shown, the effect of a CIR is to inhibit the propagation of Jovian electrons through the CIR, across the average direction of the interplanetary magnetic field (IMF). Conlon [1978] demonstrated that between CIR's, a diffusion model, including the effect of convection by the solar wind and assuming that Jupiter was a constantly emitting 'point' source of electrons, could account for the Jovian electron flux intensity observed at Pioneer 11. ½onlon [1978] concentrated his analysis on data obtained at Pioneer 11 when that spacecraft was within 2 AU of Jupiter, upstream in the solar wind, and across the average direction of the IMF from the planet. At earth, however, every ,-•13 months (the length of the Jovian synodic year) eart...

Modulation of Jovian electrons at 1 AU during solar cycles 22–23

“…Jovian electrons have been identified by their characteristic signature of 13-month periodicity [Chenette, 1980]. Our observations show a puzzling decrease in Jovian fluxes near the solar cycle minimum.…”

“…The figure also shows that the fluxes exhibit a 27-day periodicity in addition to the 13-month periodicity. This is due to modulation of electron intensity by corotating interaction regions (CIR) [Conlon, 1978;Chenette, 1980]. CIRs, which are due to quasi-stationary high speed solar wind streams interacting with low speed plasma, are well established during the declining part of the solar cycle (the first part of each panel) and less so during the ascending part of the cycle (the last part of each panel) when sporadic CMEs are more common.…”

“…The discovery of Jovian electrons by Pioneer 10 led to the recognition of quiet time electron increases as being of Jovian origin [Teegarden et al, 1974;Mewaldt et al, 1976]. Subsequent observations, ranging from 0.5 -11 AU, up to 16°helio-spheric latitude and over full 360°longitude, established that Jupiter is a strong and persistent source of energetic electrons in the heliosphere [Chenette, 1980].…”

“…A low energy electron channel, ID2, of the CRNC instrument which measures electrons in the range 0.7-to 2.0-MeV is used to identify solar electrons. As in prior analyses [Chenette, 1980], we use the ratio of ID2 electron rate to the ID3 rate to select quiet times. Days during which this ratio has a value of less than 2.5 and greater than 0.5 are termed as quiet times.…”

“…We use the ELO channel from PET which measures electrons in the range of 2-to 6-MeV to detect the Jovian signal. The Cosmic Ray Nuclear Composition (CRNC) instrument [Chenette, 1980] and the Goddard Cosmic Ray Experiment (GCRE) [McDonald, 1972] onboard IMP8 measure high energy electrons at 1 AU in the near-Earth region. The 2-to 12-MeV channel, ID3, from CRNC and the 3.5-to 18.5-MeV channel from the GCRE are used to observe Jovian electrons.…”