2011 ARTEMIS SCIENCE NUGGETS
ARTEMIS Measurements of Turbulence in the Solar Wind
by C. H. K. Chen, UC Berkeley Space Sciences Laboratory
Introduction
Turbulence is a universal phenomenon seen in many fluids, including the plasma in different space environments throughout the solar system, and is characterized by irregular fluctuations at a broad range of scales. Despite having been widely studied, we are still lacking a comprehensive theory of plasma turbulence. The instrumentation on the ARTEMIS spacecraft gives us an opportunity to measure new properties of turbulence in the solar wind plasma and help us understand its fundamental nature, and the role it plays in the variety of processes that operate throughout the solar system.
Results
One of the main ways to characterize turbulence is through its energy spectrum. The power in the fluctuations can be plotted as a function of their scale (or frequency as measured by the spacecraft). The slope of this spectrum can be measured and compared to theoretical predictions to determine the underlying physics of the turbulence. The power spectrum of electric field fluctuations as measured by ARTEMIS in the solar wind is shown in Figure 1. Two spectra are shown: in blue is the spectrum directly measured by the spacecraft and in green is the spectrum of the electric field that has been transformed into the mean solar wind frame; from electromagnetism theory, we know that the electric field depends on the frame of measurement, even when speeds are non-relativistic (here the frames differ by a few hundred km/s).
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| Figure 1. Spectrum of solar wind electric field fluctuations in the spacecraft (blue) and mean solar wind (green) frames. Reference slopes of -5/3 and -3/2 are also shown. |
The slope of the electric field power spectrum (or "spectral index") is different between the two frames: in the spacecraft frame it is close to -5/3 and in the mean solar wind frame it is close to -3/2. Since this difference is not easy to see by eye, histograms of the spectral index measured in 272 6-hour intervals are shown in Figure 2. The histograms can be seen to clearly peak at different values (although there is significant spread). It can be shown that this difference in spectral index of the electric field between frames is consistent with measured values of the velocity and magnetic field spectral index.
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| Figure 2. Histograms of electric field spectral indices in the spacecraft (upper) and mean solar wind (lower) frames. Reference values of -5/3 and -3/2 are also shown. |
Conclusion
Theoretical attempts are now underway to incorporate these differences in spectral index into models of plasma turbulence. One such theory involves the generation of excess magnetic energy at large scales, which may explain the observations. Due to their unique instrumentation and orbit, the ARTEMIS spacecraft should provide many more new results and enable us to gain further insight into the fundamental nature of plasma turbulence.
Reference
C. H. K. Chen, S. D. Bale, C. Salem, F. S. Mozer (2011), Frame Dependence of the Electric Field Spectrum of Solar Wind Turbulence, Astrophys. J. Lett. 737 L41, arXiv:1105.2390.Biographical Note
Christopher Chen is a postdoctoral scholar at the Space Sciences Laboratory, University of California, Berkeley. His current research involves studying the fundamental properties of turbulence in the solar wind (see http://sprg.ssl.berkeley.edu/~chen for more information).
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu