2013 ARTEMIS SCIENCE NUGGETS


Using ARTEMIS to Measure Small Scale Turbulence in the Solar Wind

C. H. K. Chen, Imperial College London


Introduction

Turbulence is one of the most widespread plasma phenomena in the solar system, occurring in the Sun, solar corona, solar wind and planetary magnetospheres. As well as being of fundamental interest, it is also potentially important for determining macroscopic properties of the heliosphere, such as the heating of the corona / solar wind and the propagation of energetic particles. One of the best places to measure plasma turbulence is in the solar wind, where detailed in situ measurements of the rapidly moving and relatively undisturbed plasma are available. As spacecraft instrument resolution has increased, we have been able to probe turbulence at even smaller scales, where the energy is thought to be dissipated causing heating of the plasma. In particular, the ARTEMIS spacecraft have enabled the first measurements of the solar wind density fluctuations at scales well below the typical scale of gyrating ions in the solar wind, allowing us to probe this interesting new range. Here, we describe what these new high resolution density measurements can tell us about the turbulence.

Results

The electric field instrument (EFI) on ARTEMIS measures the potential difference between the spacecraft body and a set of probes that extend far out into the solar wind plasma, sampling at a rate up to 8192 times a second. This potential is directly related to the solar wind density, because at a higher density there is a larger flow of electrons to the spacecraft body. This effect can be seen in Figure 1, where a good correlation is present between the EFI measured spacecraft potential and density measured by the electrostatic analyzer (ESA). Since the ESA is only able to measure the density at the spin rate of the spacecraft (3 seconds), to get higher resolution, the best fit curve from Figure 1 was applied to the high resolution spacecraft potential data.

Figure 1. Density of electrons in the solar wind as a function of the spacecraft potential. The best fit line (red) is applied to high resolution potential data to infer the density at high resolution. Figure from Chen et al. (2012).

This high resolution data can be used to find the energy in the density fluctuations as a function of their frequency. This is shown in Figure 2, along with a similar plot for the magnetic fluctuations. It can be seen that for high frequencies, those greater than 1 Hz, both spectra are steep and the slope in this range has a value close to -2.7. This range corresponds to spatial scales in the solar wind smaller than the gyrating ions but larger than the gyrating electrons. Theories of plasma turbulence predicted that the slopes of density and magnetic fluctuations here should be the same, but the predicted values were -7/3, which is shallower than the measurements.

The relative amplitude of density and magnetic fluctuations can also be used to determine the nature of the turbulence between the ion and electron scales, and in particular distinguish between two popular models: kinetic Alfvén turbulence and whistler turbulence. Kinetic Alfvén turbulence is low frequency, so ions and electrons fluctuate with the magnetic field. Whistler turbulence, however, is high frequency, so the ions can not move fast enough, and density fluctuations are very small compared to the magnetic fluctuations. The fact that the density fluctuations in Figure 2 between the ion and electron scales (1-20 Hz) are comparable to the magnetic fluctuations shows that the turbulence here is kinetic Alfvén, rather than whistler. The high resolution ARTEMIS data set has enabled this distinction to be made for the first time over this range of scales.

Figure 2. Spectrum of normalized density (blue) and magnetic (red) fluctuations in the solar wind. Between the ion and electron scales (vertical dashed lines) the spectra are similar, showing that the turbulence is predominantly kinetic Alfvén in nature. For details of the normalization see Chen et al. (2013).

Conclusion

We have shown that the density fluctuations have a similar behavior to the magnetic fluctuations for scales between the ion and electron scales in the solar wind. The reason for the steep slopes is not fully resolved but may be related to the energy dissipation or intermittency of the turbulence. The fact that the turbulence is predominantly kinetic Alfvén, rather than whistler, is consistent with theoretical expectations and, because of its low frequency, helps to place constraints on the possible turbulence dissipation mechanisms. Knowing that solar wind turbulence transitions to kinetic Alfvén turbulence at small scales may also help us to understand other astrophysical plasmas that are less well diagnosed, such as the interstellar medium and hot accretion flows, where similar turbulence is expected to occur.

Reference

C. H. K. Chen, C. S. Salem, J. W. Bonnell, F. S. Mozer, S. D. Bale, Density Fluctuation Spectrum of Solar Wind Turbulence between Ion and Electron Scales, Phys. Rev. Lett. 109 035001, arXiv:1205.5063 (2012)

C. H. K. Chen, S. Boldyrev, Q. Xia, J. C. Perez, Nature of Subproton Scale Turbulence in the Solar Wind, Phys. Rev. Lett. 110 225002, arXiv:1305.2950 (2013)

Biographical Note

Christopher Chen is a Junior Research Fellow in the Department of Physics, Imperial College London. His current research involves studying the fundamental properties of turbulence in the solar wind (see http://www.imperial.ac.uk/people/christopher.chen for more information).


Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu