Research Article

A Swarm-only model of the magnetosphere to degree and order 3

British Geological Survey, Lyell Centre, Research Avenue South, Edinburgh, UK

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 25 March 2025
Accepted: 30 April 2026

Abstract

In this study, we utilise satellite measurements to investigate variations in the magnetosphere. Using 8- to 12-h time windows, we map the time-varying night-side magnetospheric fields up to degree and order 3, capturing lateral offsets from a simple tilted dipole model. We determine the field over the night-side hemisphere during geomagnetic storms and examine how azimuthal symmetry is disrupted at storm maxima and re-established during the storm recovery phase, as exemplified by the St. Patrick’s Day storm in 2015. For this event, we demonstrate substantial variations in magnetospheric field intensity throughout the storm, followed by a gradual recovery to background levels over several days. Analysing over a decade of data, we investigate long-term seasonal and solar cycle-related variations alongside the impact of intense geomagnetic storms. We observe that the alignment of the Earth’s dipole with the Sun-Earth line plays a key role in shaping the latitudinal geometry of the magnetospheric field. Furthermore, we find that, during quiet conditions, the night-side magnetosphere is characterised by a dipolar field with q₁⁰ ≈ 8 nT. On average, geomagnetic storms induce a tilt of the inducing dipole towards dawn and an enhancement of the equatorial ring current near dusk.