7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Compressive nature of space plasma turbulence: state-of-the-artOwen Wyn Roberts1, Yasuhito Narita2, Rumi Nakamura1, Zoltan Voros1,21Space Research Institute, Austrian academy of sciences, Graz, Austria 2 Institute of Earth Physics
and Space Science, Sopron, Hungary
e-mail (speaker): owen.roberts@oeaw.ac.atThe majority of research on plasma turbulence has usedmagnetic field data. In the solar wind, magnetic
fluctuations’ power is predominantly transverse to themagnetic field at large fluid scales, with very little powerin the compressive (along the mean field direction)
components. At smaller scales where kinetic effects ofions and electrons become important, the relative powerof the compressive component becomes approximately athird of the power.
Measuring the compressive magnetic component is notstraightforward, requiring a mean magnetic field
direction to be defined. The mean field may notnecessarily be well-defined for a turbulent medium. An
alternative to measuring compressive magnetic
fluctuations is to use density fluctuations. The drawback
with measuring the density is that plasma instruments
often do not have the necessary time resolution toresolve ion kinetic scales and below. This is due to thefinite integration time necessary for a sufficient numberof counts in the detector or that the detector scansazimuthally using the spacecraft spin. To overcome theseissues, a different approach is required: calibrating thespacecraft potential.
A spacecraft embedded in a plasma will charge to aparticular value determined by the currents to and fromthe spacecraft. Typically in the solar wind, two currentsdominate, the photoelectron current and the electron
thermal current from the ambient plasma. Sunlit surfacesof the spacecraft emit the photoelectron current and canbe assumed to be constant on short timescales. The
electron thermal current is proportional to the density
and the square root of the electron temperature. Using alower-resolution measurement of the density, we cancalibrate the potential measurement to obtain a density
measurement.
The principal advantages of this method are the higher
time resolution and signal-to-noise ratio. The potential
measurement is made using electric field probes, whichhave much higher time resolutions than particle
measurements. Furthermore, instead of having a verysmall collecting area (as in a plasma instrument), theentire body of the spacecraft is used as a collecting
surface. This method allows us to probe compressive
fluctuations down to sub-ion scales.
Using the spacecraft potential methodology on the
Magnetospheric MultiScale (MMS) mission, we canmeasure the electron density at 8 kHz in burst mode and

32 Hz in fast survey mode. This allows us to probe deep
into the sub-ion range where the power of compressible
fluctuations becomes much more important incomparison to fluid scales. An example of the turbulent
density power spectrum is shown in Figure 1. This isfrom a one hour interval of solar wind plasma.

Figure 1. Power spectral density of density fluctuations
obtained from the spacecraft potential measurement. The
red and blue bars above denote the frequency ranges
accessible from the Fast Plasma Investigation (FPI)
measurement. Adapted from Roberts et al. (2020)
The exceptional time resolution available from MMS
gives a unique way to study compressive fluctuations.
Furthermore, the multi-point capabilities of MMS allow
several multi-point methods to be used, such as thewave-telescope (Roberts et al. (2020a)) andmulti-spacecraft structure functions (Roberts et al.
(2020b)) so that we are also able to make measurementsof compressible plasma turbulence fully in threedimensions. Making this data set truly state-of-the-art.
References[1] Roberts, et al. (2020): Sub-ion scale compressive
turbulence in the solar wind: MMS spacecraft potential
observations, Astrophys. J. Suppl. Ser., 250, 35,
doi:10.3847/1538-4365/abb45d.
[2] Roberts et al. (2020a) Higher-Order Statistics inCompressive Solar Wind Plasma
Turbulence: High-Resolution Density Observations
From the Magnetospheric MultiScale Mission. Front.
Phys. 8:584063.doi:10.3389/fphy.2020.584063[3] Roberts, et al. (2020b): Possible coexistence ofkinetic Alfven and ion Bernstein modes in sub-ion scale
compressive turbulence in the solar wind,
Phys.Rev.Research, 2, 043253,
doi:10.1103/PhysRevResearch.2.043253