2014 THEMIS SCIENCE NUGGETS
Discrete Auroral Arcs Generated by Ionospheric Feedback Instability
by Nan Jia, Dartmouth College
Introduction
Aurora are one of the earliest geophysical phenomenon discovered by mankind. However, the mechanisms that generate discrete auroral arcs of different structures are still not fully understood. Numerical simulation is a powerful tool to investigate the spatial structures and temporal dynamics of aurora. The main motivation of this research is to use a newly developed three-dimensional numerical model to reproduce the discrete auroral arcs observed by THEMIS during a substorm event in Alaska in 2008.
Our simulation results have successfully reproduced both the linear, quiet and the nonlinear, dynamic auroral arcs in the observation. Figure 1 shows the comparison between simulated and observed aurora. The formation of auroral with a variety of structures can be explained by the feedback mechanism between the auroral ionosphere and magnetic field-aligned currents carried by Alfven waves. Varying parameters such as electric field and plasma density in the ionosphere resulted in the formation of different of auroral structures.
To further demonstrate this theory, we can also compare the temporal dynamics between simulated and observed aurora (Figure 2). Figure 2 shows the comparison of temporal dynamics between the intensity of the observed aurora and the magnetic field in the simulation. In order to demonstrate the similarity between them, Fast Fourier Transform (FFT) is applied to the results to get the power spectra. From Figure 2, we can see that the frequency of the simulated magnetic field is in the same range as the observed data (near 3.4mHz).
Results
Our simulation results have successfully reproduced both the linear, quiet and the nonlinear, dynamic auroral arcs in the observation. Figure 1 shows the comparison between simulated and observed aurora. The formation of auroral with a variety of structures can be explained by the feedback mechanism between the auroral ionosphere and magnetic field-aligned currents carried by Alfven waves. Varying parameters such as electric field and plasma density in the ionosphere resulted in the formation of different of auroral structures.
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| Figure 1. Comparison between the simulated parallel current density in the northern ionosphere and high-resolution images of discrete auroral forms taken on 10/29/2008 at Fort Yukon, Alaska (from THEMIS imager, courtesy of E. Donovan). Top row shows comparison between simulations and “homogeneous” discrete auroral arc observed at 05:39 UT. Bottom row shows comparison between simulations and “non-homogeneous”, active discrete auroral arc observed at 06:16 UT. The blue regions in the simulated results correspond to the bright regions of the observed aurora. |
To further demonstrate this theory, we can also compare the temporal dynamics between simulated and observed aurora (Figure 2). Figure 2 shows the comparison of temporal dynamics between the intensity of the observed aurora and the magnetic field in the simulation. In order to demonstrate the similarity between them, Fast Fourier Transform (FFT) is applied to the results to get the power spectra. From Figure 2, we can see that the frequency of the simulated magnetic field is in the same range as the observed data (near 3.4mHz).
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| Figure 2. Comparison of the power spectra between the signal intensity of the observed aurora (left) and the signal of the magnetic field in the simulation (right). |
Conclusion
The results from our 3D simulations are applied to explain the structure of discrete auroral arcs observed during the October 29, 2008 substorm at Fort Yukon, Alaska. The main conclusion is that the ionosphere is responsible for the formation of a variety of structures in the discrete auroral arcs.
Reference
Jia, N., and A. V. Streltsov (2014), Ionospheric feedback instability and active discrete auroral forms, J. Geophys. Res. Space Physics, 119, 2243–2254, doi:10.1002/2013JA019217.Biographical Note
Nan Jia received his PhD degree at Dartmouth College, Hanover, USA. His research interests include waves in plasma and ionosphere-magnetosphere coupling.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu