2023 THEMIS SCIENCE NUGGETS
Statistical characterization of the dynamic near-Earth plasma sheet relative to Ultra-Low Frequency (ULF) wave growth at substorm onset
Andrew Smith
Assistant Researcher,
Senior Research Fellow, Department: Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle, UK
Introduction
Substorms are an explosive phenomenon in near-Earth space. Energy is stored in the Earth’s magnetic field through interactions with the incoming flow of the solar wind, energy which is later released explosively on the night side of the Earth, resulting in consequences including the generation of the northern lights, or aurora. A number of physical processes occur in near-Earth space during the rapid, impressive brightening of the aurora, but their timing and the chain of causality that links them is unclear to this day. Nonetheless, it is commonly observed that bright patches or “beads” form within the brightening aurora, and therefore it is theorized that the process responsible for the beads may play an important role in the development and initiation of a substorm. In this work we investigate the conditions in near-Earth space that correspond to the source region of these bead-like structures.
Specifically, we examine 68 intervals during which the THEMIS spacecraft are measuring the near-Earth plasma sheet region that maps – through the Earth’s magnetic field - to the exponentially brightening auroral arc. This inference is made by comparing the ULF (Ultra- Low Frequency) magnetic waves observed by the spacecraft with those on the ground. We hypothesize that the growing ULF waves provide an excellent “clock” by which to evaluate changes in the conditions measured by the spacecraft.
Results
We first examined changes in the magnetic field and plasma flows measured by the THEMIS spacecraft. Figure 1 shows how the local plasma flows (left) and magnetic field (right) change during the time interval in which the ULF waves grow. The start and end of the rapid growth in ULF wave power is indicated with black vertical lines, and represents the epochs to which the data are normalized. Before the ULF waves grow (T = -30 to 0) the plasma is fairly quiescent, with small azimuthal (~Vy, Figure 1c) flows present. During the interval in which the ULF waves begin to grow (i.e. time epoch 0 to 5 minutes) we see large increases in plasma velocity in all three directions, with the strongest being directed towards the planet (Vx) and azimuthally (|Vy|). Comparing the observations closer to the Earth (blue), with those made further from the planet (orange), we see that the flows are stronger farther from the Earth.
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| Figure 1. The local plasma velocity (left) and magnetic field (right) observations relative to the growth of ULF waves. Top row: X component (towards/away from the Sun); middle row: Y component (approximately in the dawn-dusk direction); bottom row: Z component (north/south relative to the Earth’s magnetic poles). The observations are split into those closer to the Earth (blue) or further from the Earth (orange). Time has been normalized to align the observations to the start and end of the exponentially growing ULF waves (black vertical dashed lines). |
Meanwhile, in the magnetic field we see evidence of the magnetotail stretching, with Bx increasing and a reduction in Bz before the ULF waves start to grow. Closer to the Earth (blue), we see that the stretching of the tail occurs closer to the ULF wave onset (i.e. T0), while farther from the Earth (orange) the stretching occurs over a longer period of time (~30 minutes).
We also examined the energy density of the local environment around the spacecraft. If the process responsible for the bead-like structures was local, then we might expect to see a decrease in energy of the source, as energy is transferred to the growing ULF waves. Figure 2 shows how the local measured energy densities change over time, relative to the time when exponentially growing ULF waves are seen. Before the ULF waves begin to grow (i.e. between -30 and 0 minutes) we see that most parameters are relatively steady (Figure 2), with the ion and electron thermal energies showing decreases of between 5 and 20%. During the increasing ULF wave power (between 0 and 5 minutes), we see the electromagnetic ULF wave power increase dramatically by three orders of magnitude. We resolve these ULF plasma waves in the wave-related ion kinetic velocity (Figure 2c in red, which also increases sharply), and also see that the bulk ion kinetic energy (Figure 2c, orange) increases indicating that the bulk plasma flow velocity becomes enhanced during this time. However, we do not see a corresponding decrease in another parameter that could explain the source of the ULF waves. Suggesting that there is not a clear local source of energy. We observe that the total local energy density (in red, Figures 2a and b) increases during this time, suggesting the injection of energy into the local region.
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| Figure 2. The local energy density relative to the growth of ULF waves in the magnetotail, in a similar format to Figure 1. Panels (a) and (b) show the median energy density (and interquartile range in a) of the thermal plasma and magnetic field. Panel (c) shows the energy density of the electromagnetic waves, along with wave and bulk ion kinetic energy density. |
Conclusion
In this work we have evaluated the conditions in near-Earth space that are linked to the brightening aurora during the start of substorms, using timing derived from the exponential growth of ULF waves. Before the exponential growth in ULF waves we generally see quiescent conditions dominate, with the magnetotail showing signs that it is stretching, likely as a result of the addition of energy into the magnetospheric system. This stretching occurs more slowly farther from the planet, while close to the planet it only occurs closer to the onset of ULF waves. During the period in which the waves grow we see large increases in plasma flow towards the Earth and azimuthally. A third to half of the energy of these increasing flows can be linked to the growing waves. We find no clear local source of energy for these ULF waves, in fact we observe an increase in local energy density, suggesting the injection of energy from an external source.
References
Smith, A. W., Rae, I. J., Forsyth, C., Watt, C. E. J., & Murphy, K. R. (2023). Statistical characterization of the dynamic near-Earth plasma sheet relative to Ultra-Low Frequency (ULF) wave growth at substorm onset. Journal of Geophysical Research: Space Physics, 128, e2022JA030491. https://doi.org/10.1029/2022JA030491Biographical Note
Andy Smith completed his PhD in space environment physics at the University of Southampton in 2018. He then held a post-doctoral position at University College London until 2022 when he was awarded a NERC Independent Research Fellowship, which he currently holds at Northumbria University.doi.org/10.3847/1538-4357/abaf03
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu