A Multi‐Model Ensemble System for the Outer Heliosphere (MMESH): Solar Wind Conditions Near Jupiter.

How the solar wind influences the magnetospheres of the outer planets is a fundamentally important question, but is difficult to answer in the absence of consistent, simultaneous monitoring of the upstream solar wind and the large‐scale dynamics internal to the magnetosphere. To compensate for the relative lack of in‐situ solar wind data, propagation models are often used to estimate the ambient solar wind conditions at the outer planets for comparison to remote observations or in‐situ measurements. This introduces another complication: the propagation of near‐Earth solar wind measurements introduces difficult‐to‐assess uncertainties. Here, we present the Multi‐Model Ensemble System for the outer Heliosphere (MMESH) to begin to address these issues, along with the resultant multi‐model ensemble (MME) of the solar wind conditions near Jupiter. MMESH accepts as input any number of solar wind models together with contemporaneous in‐situ spacecraft data. From these, the system characterizes typical uncertainties in model timing, quantifies how these uncertainties vary under different conditions, attempts to correct for systematic biases in the input model timing, and composes a MME with uncertainties from the results. For the Juno‐era (04/07/2016–04/07/2023) MME hindcast for Jupiter presented here, three solar wind propagation models were compared to in‐situ measurements from the near‐Jupiter spacecraft Ulysses and Juno spanning diverse geometries and phases of the solar cycle across >14,000 hr of data covering 2.5 decades. The MME gives the most‐probable near‐Jupiter solar wind conditions for times within the tested epoch, outperforming the input models and returning quantified estimates of uncertainty. Plain Language Summary: The sun interacts with all the planets in the solar system through the solar wind, a stream of charged particles which blow outwards from the sun in all directions, carrying the interplanetary magnetic field with them. Both the magnetic field and particles interact with planetary magnetic fields with dramatic effects, including the aurora–which shine not only on the Earth, but on gas giants of the outer solar system, like Jupiter, too. Characterizing the relationship between the solar wind and planetary magnetic fields is easiest with direct spacecraft measurements of both. Spacecraft between the Earth and Sun measure the solar wind, providing valuable context for understanding its interaction with the Earth. Unfortunately, there are no such permanent spacecraft near the other planets. Instead, models can be used to estimate the solar wind at these planets; however, these models can have significant, difficult‐to‐characterize uncertainties. Here we present the Multi‐Model Ensemble System for the outer Heliosphere (MMESH), a framework designed to measure these uncertainties and attempt to correct for them by comparing multiple solar wind models to spacecraft measurements over a long time span. The final result here is an improved solar wind model, with estimated uncertainties, for Jupiter. Key Points: The performance of several existing solar wind propagation models at the orbit of Jupiter is measured for multiple spacecraft epochsA flexible system is developed to generate an ensemble of multiple propagation models so as to best leverage each input model's strengthsOver the epoch tested, the multi‐model ensemble outperforms individual input models by 7%–110% in forecasting the solar wind flow speed [ABSTRACT FROM AUTHOR]