Jovian electrons in the inner heliosphere: Opportunities for Multi-spacecraft Observations and Modeling

In this paper we explore the idea of using multi-spacecraft observations of Jovian electrons to measure the 3D distribution of these particles in the inner heliosphere. We present simulations of Jovian electron intensities along selected spacecraft trajectories for 2021 and compare these, admittedly qualitatively, to these measurements. Using the data-model comparison we emphasize how such a study can be used to constrain the transport parameters in the inner heliosphere, and how this can lead to additional insight into energetic particle transport. Model results are also shown along the expected trajectories of selected spacecraft, including the off-ecliptic phase of the Solar Orbiter mission from 2025 onward. Lastly, we revisit the use of historical data and discuss upcoming missions that may contribute to Jovian electron measurements.Comment: Accepted for publication in Ap

Unusually long path length for a nearly scatter-free solar particle event observed by Solar Orbiter at 0.43 au

Context: After their acceleration and release at the Sun, solar energetic particles (SEPs) are injected into the interplanetary medium and are bound to the interplanetary magnetic field (IMF) by the Lorentz force. The expansion of the IMF close to the Sun focuses the particle pitch-angle distribution, and scattering counteracts this focusing. Solar Orbiter observed an unusual solar particle event on 9 April 2022 when it was at 0.43 astronomical units (au) from the Sun. // Aims: We show that the inferred IMF along which the SEPs traveled was about three times longer than the nominal length of the Parker spiral and provide an explanation for this apparently long path. // Methods: We used velocity dispersion analysis (VDA) information to infer the spiral length along which the electrons and ions traveled and infer their solar release times and arrival direction. // Results: The path length inferred from VDA is approximately three times longer than the nominal Parker spiral. Nevertheless, the pitch-angle distribution of the particles of this event is highly anisotropic, and the electrons and ions appear to be streaming along the same IMF structures. The angular width of the streaming population is estimated to be approximately 30 degrees. The highly anisotropic ion beam was observed for more than 12 h. This may be due to the low level of fluctuations in the IMF, which in turn is very probably due to this event being inside an interplanetary coronal mass ejection The slow and small rotation in the IMF suggests a flux-rope structure. Small flux dropouts are associated with very small changes in pitch angle, which may be explained by different flux tubes connecting to different locations in the flare region. // Conclusions: The unusually long path length along which the electrons and ions have propagated virtually scatter-free together with the short-term flux dropouts offer excellent opportunities to study the transport of SEPs within interplanetary structures. The 9 April 2022 solar particle event offers an especially rich number of unique observations that can be used to limit SEP transport models

Anisotropies of solar energetic electrons in the MeV range measured with Solar Orbiter/EPD/HET

Aims. This study analyses relativistic electron measurements obtained by the High Energy Telescope (HET) aboard Solar Orbiter in the energy range from 200 keV to above 10 MeV. Caveats of these measurements are discussed. The purpose of this study is to analyse anisotropies of relativistic solar energetic electrons utilising the different viewing directions of HET. Methods. We identified time periods of interest, that is, those with enhanced electron flux due to a significant solar component, and composed a list of these time periods, including additional observations such as maximum energy and flux as well as the first-order anistropy. Results. This study provides an overview of HET measurements of MeV electrons and a list of time periods of enhanced flux of relativistic solar electrons, 21 in total. For the first time with Solar Orbiter/EPD/HET, the anisotropies of high-energy electrons have been measured. Specifically, we find three time periods with significant anisotropy above 1 MeV within 0.5 au

Multi-spacecraft observations of near-relativistic electron events at different radial distances

Aims. We study the radial evolution of near-relativistic solar energetic electron (SEE) events observed by at least two spacecraft at different heliocentric distances and with small separation angles between their magnetic footpoints at the Sun. Methods. We identified SEE events for which Solar Orbiter and either Wind or STEREO-A had a small longitudinal separation (< 15°) between their nominal magnetic footpoints. For the approximation of the footpoint separation, we followed a ballistic back-mapping approach using in situ solar wind speed measurements. For all the SEE events that satisfied our selection criteria, we determined the onset times, rise times, peak fluxes, and peak values of the first-order anisotropy for electrons in the energy range from ∼50 − 85 keV. We compared the event parameters observed at different spacecraft and derived exponential indices αp for each parameter p, assuming an Rα-dependence on the heliocentric distance R. Results. In our sample of SEE events, we find strong event-to-event variations in the radial dependence of all derived parameters. For the majority of events, the peak flux decreases with increasing radial distance. For the first-order anisotropy and the rise time no clear radial dependence was found. The derived onset delays observed between two spacecraft were found to be too long to be explained by ideal Parker spirals in multiple events. Conclusions. The rudimentary methods presented in this study lead to event parameters with large uncertainties. The absence of a clear radial dependence on the first-order anisotropy and the rise time as well as the ambiguous onset timing of the SEE events found in this study could be the result of general limitations in the methods we used. Further studies, including analyses of the directional fluxes and transport simulations that take the individual instrument responses into account, would allow a better interpretation of the radial evolution of SEE events

Jovian Electrons in the Inner Heliosphere: Opportunities for Multi-spacecraft Observations and Modeling

In this paper we explore the idea of using multi-spacecraft observations of Jovian electrons to measure the 3D distribution of these particles in the inner heliosphere. We present simulations of Jovian electron intensities along selected spacecraft trajectories for 2021 and compare these, admittedly qualitatively, to these measurements. Using the data-model comparison we emphasize how such a study can be used to constrain the transport parameters in the inner heliosphere, and how this can lead to additional insight into energetic particle transport. Model results are also shown along the expected trajectories of selected spacecraft, including the off-ecliptic phase of the Solar Orbiter mission from 2025 onward. Lastly, we revisit the use of historical data and discuss upcoming missions that may contribute to Jovian electron measurements.</p