Andy Smith
Magnetospheric substorms are a cycle of energy storage and release in the Earth’s magnetosphere. The growth phase describes the storage of energy, generated through the interaction of the Earth’s magnetic field with the field carried by the solar wind. The growth phase proceeds until the system reaches a state of instability, known as substorm onset. The explosive energy release that follows converts the stored magnetic and thermal plasma energy into plasma kinetic energy. The clearest indicator of this process, and the accompanying large-scale reconfiguration of the nightside magnetosphere, is a bright auroral display that is triggered. Recent work has shown that the initial structure and brightening of the most equatorward auroral arc is related to an instability in the near-Earth magnetotail. In this work we analyse in situ observations of instability-driven wave activity observed by the THEMIS spacecraft, and compare them with observations made on the ground.
The substorm is a process of energy storage and release in the Earth’s magnetosphere. Energy is stored through the interaction between the Earth’s magnetic field and the interplanetary magnetic field, carried by the solar wind. The energy accumulates on the night side of the Earth until the system becomes unstable. This is known as substorm onset. The clearest indication that a substorm is occurring is the rapid brightening of the aurora during substorm onset. Recent work has shown that azimuthal structure, known as auroral beads, is almost always present within the brightening aurora [Kalmoni et al., 2017]. An example of the development of this auroral structure is shown by panels a - d. Quantitative analysis of this structure has linked its presence and development with magnetic waves driven by a process in the near-Earth magnetotail [e.g. Kalmoni et al., 2018]. The increases in ground and conjugate space based ULF wave power are shown in panels e and f, and are remarkably similar. This suggests that they are related to the same phenomena, and that the spacecraft are near the source region.
Slide 2
We now look at the period (or frequency) dependence of the magnetic fluctuations in space for a similar event. As in the first example event, in panel a all three spacecraft see an increase in wave power of several orders of magnitude prior to substorm onset. Panels b – d show wavelet spectrograms of the same interval, for waves with periods between 10 and 100s. The wavelet spectrograms suggest that the wave power is broadband, and over the full range of periods presented. Observations made by the ‘the’ spacecraft show strong wave power first, and therefore that this spacecraft might be the closest to the source.
Slide 3
We can perform the same analysis on an array of ground stations across the North American continent. These spectrograms look different to those from the spacecraft, and vary significantly from station to station. Broadly, the longer period wave power is seen to start first, while the shorter period waves are seen later. The length of this delay varies between stations. However, some stations do not see shorter period waves with significant wave power at all.
Slide 4
If we take the times at which each station first sees significant wave power (in each band) we can plot maps of the relative timing. The colour on these maps indicates the timing relative to the very first station to see significant wave power in any band. Each panel shows a different period, and there is a clear difference between the panels. The shortest periods (e.g. panel a) are only seen at a few stations around substorm onset, while the longer periods (e.g. panel f, g) are seen over a very large region. Meanwhile, the shortest delays are seen close to the onset location, and at the longest periods. The contours show that the spread of the signal is fastest westwards, and it only propagates slowly eastwards or north/south. The physical reason for the difference between the shorter and longer wave periods on the ground (but not in space) could be that the waves are travelling at different velocities, or that the shorter periods are more damped and therefore appear to take longer to rise above background.