Latitudinal distribution and magnetic signatures of magnetospheric substorms

Abstract

The Earth’s magnetic field shields the Earth from the solar wind, forming a magnetic cavity inside the solar wind called the magnetosphere. The magnetosphere is a highly dynamic system, constantly interacting with the solar wind. One of its dynamic features is called the magnetospheric substorm, when the magnetosphere unloads energy from the solar wind. Substorm expansions happen in the nightside of the Earth, as the inner magnetic field lines of the magnetotail reconnect and dipolarize closer to the Earth. During this process, magnetospheric currents are redirected along the magnetic field lines, flowing to the Earth’s ionosphere, where they connect to the westward electrojet. The westward electrojet enhances during each substorms, depressing the Earth’s magnetic field.

The magnetic disturbances caused by the westward electrojet are the main subject of this thesis. The magnetic disturbances are studied with ground-based magnetic field measurements. For this purpose, a geomagnetic index called the IL index is formed using the IMAGE magnetometer network to describe the absolute amplitude of these disturbances. The IL index is also used to identify substorm expansion phase onsets. The substorm onsets are identified using an implemented algorithm. A list of substorms is created with these methods for years 1993–2020. This list holds information of the total number of substorms and the duration and amplitude of each substorm. This allows us to study the solar cycle and seasonal variation of these substorm properties. A subset of eleven IMAGE stations is used to study the latitudinal distribution of the substorm properties and the average magnetic signatures using superposed epoch analysis. Also, the solar cycle and seasonal variation of different latitudes is studied.

The magnetic signatures show how the westward electrojet descends to lower latitudes if it is enhanced prior to the substorm onset. The magnetic signatures show positive bays at substorm onset at the three southernmost stations of the subset (62.25◦ N, 60.50◦ N and 58.26◦ N). However, these positive bays become less distinct if the westward electrojet is enhanced prior to the onset. The latitudinal distributions give better understanding of which IMAGE stations find more substorms, and how the solar cycle and seasonal variation of the IL substorm properties are strongly affected due to the majority of substorms found by IMAGE stations at higher latitudes (74.50◦ N, 69.76◦ N and 69.66◦ N).