Jovian electrons in the inner heliosphere

Abstract

Context. Jovian electrons serve as an important test-particle distribution in the inner heliosphere and have been used extensively in the past to study the (diffusive) transport of cosmic rays in the inner heliosphere. With new limits on the Jovian source function (i.e. the particle intensity just outside the Jovian magnetosphere), and a new set of in-situ observations at 1 AU for both cases of good and poor magnetic connection between the source and observer, we revisit some of these earlier simulations. Aims. We aim to find the optimal numerical set-up that can be used to simulate the propagation of 6 MeV Jovian electrons in the inner heliosphere. Using such a set-up, we further aim to study the residence (propagation) times of these particles for different levels of magnetic connection between Jupiter and an observer at Earth (1 AU). Methods. Using an advanced Jovian electron propagation model based on the stochastic differential equation (SDE) approach, we calculate the Jovian electron intensity for different model parameters. A comparison with observations leads to an optimal numerical set-up, which is then used to calculate the so-called residence (propagation) times of these particles.Results. Comparing to in-situ observations, we are able to derive transport parameters that are appropriate to study the propagation of 6 MeV Jovian electrons in the inner heliosphere. Moreover, using these values, we show that the method of calculating the residence time applied in former literature is not suited to being interpreted as the propagation time of physical particles. This is due to an incorrect weighting of the probability distribution. We propose and apply a new method, where the results from each pseudo-particle are weighted by its resulting phase-space density (i.e. the number of physical particles that it represents). Thereby we obtain more reliable estimates for the propagation time.

Investigation of Mass‐/Charge‐Dependent Escape of Energetic Ions Across the Magnetopauses of Earth and Jupiter

“…As another possible application, it might be worth considering in a future study whether the escape of relativistic electrons, with their intermediate-sized gyroradii, might also be governed by the processes described in this paper. Such electrons are relatively common in the interplanetary environment as reviewed by Vogt et al (2018). While electrons in the tens to hundreds of kiloelectron volts have gyroradii much smaller than the protons in the same range of energies, electrons at energies between 8 and 13 MeV have gyroradii comparable in size to protons in the 50-to 100-keV range.…”

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

“…Critical for evaluating the presence of JCRPs are observations near and within Jupiter's magnetosphere, which can inform us about the efficiency proton and ion acceleration in that system and what the spectrum of the particles escaping into the heliosphere is. The search for very high energy protons and heavy ions through measurements near Jupiter by Voyager (Krimigis et al 1977;Stilwell et al 1979), Ulysses (Simpson et al 1992), Juno (Mauk et al 2013;Becker et al 2017), and Galileo (Garrard et al 1992;Williams et al 1992) could constrain the spectrum of possible JCRPs escaping from the planet's magnetosphere and allow for a comparison with observations at different heliocentric distances, similar to Vogt et al (2018). Within the Jovian magnetosphere, protons and ions between several MeV n -1 and up to ∼100 MeV n -1 have been observed in the planet's radiation belts, at an acceleration region in Jupiter's middle and outer magnetosphere, or on highlatitude field lines possibly mapping to the auroral region (Zhang et al 1995;Fischer et al 1996;Anglin et al 1997;Selesnick et al (2001).…”

“…Relativistic electrons from Jupiter that have been observed throughout the inner heliosphere are the most characteristic example for this case (Teegarden et al 1974;Krimigis et al 1975;Ferreira et al 2003;Heber et al 2007). When a spacecraft is connected to Jupiter through interplanetary magnetic field (IMF) lines, fluxes of <30 MeV Jovian electrons dominate over those of GCR electrons (Nndanganeni & Potgieter 2018;Vogt et al 2018), even though Jupiter is a point source in the heliosphere. Energetic ions of planetary origin and with subrelativistic energies (<5 MeV) have been observed in the heliosphere upstream of magnetospheric bow shocks (Krimigis et al 2009) or, in the case of Jupiter, within about 1 au of its magnetosphere (Marhavilas et al 2001;Anagnostopoulos et al 2009).…”

The Implications of Simple Estimates of the 2D Outerscale Based on Measurements of Magnetic Islands for the Modulation of Galactic Cosmic-Ray Electrons

“…From previous modulation studies, the Jovian electron contribution at these low energies far outweighs that of galactic electrons (see, e.g., Ferreira et al 2001aFerreira et al , 2001bStrauss et al 2011). The sensitivity of computed low-energy CR electron intensities to the choice of λ out suggests that such a physics-first approach, should a Jovian source be considered, could provide more insight as to possible values for this quantity, as well as providing extra insight into the behavior of other turbulence and large-scale quantities affecting the transport of these particles such as the dissipation range onset wavenumber or the HMF geometry, which may not be readily discernible using currently existing spacecraft observations (see, e.g., Engelbrecht & Burger 2010;Sternal et al 2011;Strauss et al 2011;Engelbrecht & Burger 2013b;Vogt et al 2018) by careful comparisons of model outputs with observations, while also providing a more nuanced understanding of the transport of CRs in the heliosphere than is possible using ad hoc formulations for diffusion coefficients. This will be the subject of future studies.…”

“…Overall, however, the intensities calculated with the model presented in this study remain close to the Adriani et al (2011) observations at high energies, follow the trend of observations at intermediate energies (albeit at different levels of modulation), and are only significantly below observations at the lowest energies. This last point, however, is simply due to the fact that observations below ∼100MeV are dominated by a Jovian electron component (see, e.g., Ferreira et al 2001a;Vogt et al 2018), which is not taken into account in this study. The top panel shows differential intensities calculated for periods of positive (qA<0, blue lines) and negative (qA>0, red lines) magnetic polarity assuming that λ out corresponds to the observed island sizes (dashed lines) and the Adhikari et al (2017a;dotted lines) and Engelbrecht & Burger (2013a; solid lines) assumptions for λ out .…”