The residence-time of Jovian electrons in the inner heliosphere

Abstract

Context. Jovian electrons serve an important role in test-particle distribution in the inner heliosphere. They 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, that is, the particle intensity just outside the Jovian magnetosphere, and a new set of in-situ observations at 1 AU for cases of both 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 setup, 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 approach, we calculated the Jovian electron intensity for different model parameters. A comparison with observations leads to an optimal numerical setup, which was then used to calculate the so-called residence (propagation) times of these particles. Results. Through a comparison with in-situ observations, we were able to derive transport parameters that are appropriate for the study of 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 the existing 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 applied 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). We thereby obtained more reliable estimates for the propagation time.

Numerical and experimental evidence for a new interpretation of residence times in space

“…Fig. 1: The simulation setup according to Vogt et al (2020). On the right side a visualisation is shown of how the Jovian source spectrum determines the resulting differential intensity.…”

“…In an extension of the work presented in Vogt et al (2020), the same SDE code as discussed by Dunzlaff et al (2015) is utilized in order to perform the simulations. Since it is written in CUDA in order to optimize the performance time, several simplifications haven been made due to the limited internal memory on Graphic Processing Units (GPUs).…”

“…The solar wind velocity is chosen to be directed radially outward and constant throughout the simulation setup. Following the deeper discussion on parameter setting by Vogt et al (2020), the value for this quantity is set to u S W = 400 km/s, in agreement with solar minimum conditions. The corresponding HMF is assumed to be a Parker field.…”

“…In order to estimate the residence times of charged particles in the context of Stochastic Differential Equation (SDE) modelling, Vogt et al (2020) proposed a new approach with results more realistic than the ones by previous attempts. This study will investigate further the energy dependence of residence times regarding how they are related to adiabatic energy changes as modelled by the Parker TPE in particular.…”

“…As Vogt et al (2020) focused on 6 MeV Jovian electrons, this work subsequently aims to expand the focus to energies covering the whole observed range of E Jov = [0.3, 100.0] MeV as indicated by the Jovian source spectrum according to Vogt et al (2018). The goal is to gain further insights as to the role that adiabatic energy changes play relative to diffusion to mathematically model charged particle propagation.…”