Quantifying the importance of an L4 or L5 magnetograph for modelling the global solar magnetic field

Abstract

Currently, space weather observations and predictions are primarily made using observations from the L1 Lagrange point (on the Sun-Earth line). A mission to place a satellite at the L5 viewpoint (60° behind Earth) has been proposed to enhance our space weather predictive capabilities. This thesis explores the potential impact of an additional magnetograph at L5, as well as at the L4 viewpoint (60° in front of the Earth). We first conduct a study utilising a spherical harmonic magnetic flux transport (MFT) and potential field source surface (PFSS) combined model to simulate the Sun’s global magnetic field. We also perform a study employing an MFT and 3D magnetofrictional combined model. In both studies, the first simulation run is called a “Reference Sun” simulation, which assumes a full 360° longitude view of the Sun to produce synthetic bipole data. This simulation, along with every simulation in this thesis, is conducted over 4 solar cycles.
In the first study, we conduct 12 “limited data” simulations, where we vary the limited field of view (FOV; L1, L1+L5 or L1+L4+L5), the inclusion of rotational updates (RUs) and the initial polar field strength (B₀). For the most accurate scenario, with the addition of an L4 or L5 magnetograph, we find accuracy increases of 38–40% for different flux quantities, 5–9% for the B_r polarities, 23–37% for the B_r error, 4–13% for the B_r correlation, 24–28% for the heliospheric current sheet (HCS) location and 2–3% for the connectivity of the photospheric field.
The second study conducts three limited data simulations using bipole data from L1, L1+L5, and L1+L4+L5 FOVs, including RUs and reducing B₀ to ensure correct polar field oscillations. In addition to the quantities investigated in the first study, we can examine the non-potential and free magnetic energy, the electric current and the global non-potential magnetic field with this simulating method. Including L5 data with L1, we find accuracy increases of 32–38% for globally integrated quantities, 15–25% for the 3D magnetic field, 10–36% for the 2D spatial distributions of non-potential quantities and 26% for the HCS. Similarly, including L4 with L1+L5 improves the accuracy by 11–37% for globally integrated quantities, 9–17% for the magnetic field, 16– 39% for the 2D non-potential quantities and 13% for the HCS. These potential improvements in modelling accuracy indicate more reliable space weather predictions and the importance for future space programs to incorporate L5 and L4 observations.

Date of Award	3 Jul 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Duncan Hendry Mackay (Supervisor) & Thomas Neukirch (Supervisor)