Jovian Cosmic-Ray Protons in the Heliosphere: Constraints by Cassini Observations

Abstract

Measurements of >82 MeV Galactic cosmic-ray (GCR) protons at Earth indicate that they may be mixed with protons that leak into the heliosphere from Jupiter's magnetosphere (Jovian cosmic-ray protons (JCRPs)). A ∼400 day periodicity in these proton fluxes, which is similar to the synodic period between Jupiter and Earth, and an excess proton flux observed when Jupiter and Earth can be connected through the interplanetary magnetic field were the basis for this claim. Using nearly 13 yr of GCR measurements at Saturn with Cassini's Magnetosphere Imaging Instrument, we show that the ∼400 day periodicity is also present in 100 MeV protons at ∼9.6 au, although the synodic period between Saturn and Jupiter is ∼20 yr. We also find that the features responsible for this periodicity were convected from 1 au to Saturn's distance with the solar wind velocity. Their origin is therefore heliospheric, not Jovian. We attribute these features to quasi-biennial oscillations, observed in the solar magnetic field and various heliospheric indices. This finding indicates that fluxes of JCRPs at 1 au, if present, are considerably overestimated, because the signal originally attributed to them represents the amplitude of the ∼400 day periodic GCR oscillation. This oscillation has to be subtracted before the resulting proton GCR flux residuals are analyzed in the context of a possible Jovian source. A confirmation of the presence of JCRPs over extended regions in the heliosphere and a constraint on their fractional abundance in GCR spectra may therefore require further validation and analysis, and several options are proposed for this purpose.

Saturn's Nightside Dynamics During Cassini's F Ring and Proximal Orbits: Response to Solar Wind and Planetary Period Oscillation Modulations

“…These data represent a suitable starting point since they correspond to the later part of the interval when both the modeled heliospheric parameters and the GCR flux exhibit a relatively simple periodicity with one clear compression event occurring each solar rotation. Data from these four Revs are shown in Figure 5, spanning ~25.9 days from 7.2 hr UT on Day 158 to 3.8 hr UT on Day 184 of 2017, close to the effective solar rotation period at Saturn of ~26 days (as determined empirically from recurrent GCR fluxes by Roussos, Krupp, et al (2019)). Figure 4 shows that these Revs span a single recurrent maximum in GCR flux, with solar wind rarefaction conditions being indicated at the start and end of the interval, and a single major compression event occurring centrally during Rev 280.…”

“…Here we use 6‐hr averaged counts from channel E6, which while designed to detect 1.6–21 MeV electrons, also has a response to >120 MeV protons. By filtering the data, removing the significant enhancements observed during the short intervals when Cassini crosses Saturn's radiation belts and other instrument effects, these high‐energy protons can be isolated to provide a measure of the GCR proton flux (Roussos et al, 2019).…”

Magnetic Structure and Propagation of Two Interacting CMEs From the Sun to Saturn

“…This is because SEPs and GCRs are able to penetrate Saturn's magnetosphere, hence they can be monitored at all times (e.g., Roussos et al, 2008Roussos et al, , 2011. During our period of interest, we find signatures of a Forbush decrease in the GCR data that are indirectly monitored with the LEMMS electron channel E6 (due to penetrating GCR protons; Roussos et al, 2019). This event was included by Roussos et al (2018) in their list of SEP and GCR transients at Saturn between 2004 and 2016.…”

Galactic Cosmic Rays Throughout the Heliosphere and in the Very Local Interstellar Medium

“…This was the case of LEMMS instruments on-board Cassini, originally designed to study low energy particles in the Saturn magnetosphere (see, e.g., Roussos et al. 2011 , 2019 ). The combination of LEMMS with PAMELA and AMS–02 observations provided Roussos et al.…”