Abstract
Turbulent density fluctuations are investigated in the solar wind at sub-ion scales using calibrated spacecraft potential. The measurement technique using the spacecraft potential allows for a much higher time resolution and sensitivity when compared to direct measurements using plasma instruments. Using this novel method, density fluctuations can be measured with unprecedentedly high time resolutions for in situ measurements of solar wind plasma at 1 a.u. By investigating one hour of high-time resolution data, the scale dependant kurtosis is calculated by varying the time lag $\tau$ to calculate increments between observations. The scale-dependent kurtosis is found to increase towards ion scales but then plateaus and remains fairly constant through the sub-ion range in a similar fashion to magnetic field measurements. The sub-ion range is also found to exhibit self-similar monofractal behavior contrasting sharply with the multi-fractal behavior at large scales. The scale-dependent kurtosis is also calculated using increments between two different spacecraft. When the time lags are converted using the ion bulk velocity to a comparable spatial lag, a discrepancy is observed between the two measurement techniques. Several different possibilities are discussed including a breakdown of Taylor's hypothesis, high-frequency plasma waves, or intrinsic differences between sampling directions.
Highlights
The solar wind is an excellent example of turbulent plasma where disordered fluctuations are observed in velocity, temperature, and density as well as in electromagnetic fields [1,2,3,4,5]
There is a flattening in the magnetic field spectra near 5 Hz which is due to instrumental noise
To summarize; we have investigated compressive and incompressive intermittency in the solar wind
Summary
The solar wind is an excellent example of turbulent plasma where disordered fluctuations are observed in velocity, temperature, and density as well as in electromagnetic fields [1,2,3,4,5]. Intermittency in the Solar Wind proton scales, kinetic effects become important, the magnetic spectra steepen [7], and fluctuations become more compressive [12]. In this range, usually called sub-ion range, the Kolmogorovlike phenomenology can be adapted using Hall-MHD and kinetic models. If all other current sources are small it can be assumed that both of these currents are equal and have opposite signs This is typically the case in the solar wind at 1 a.u. Using lower time resolution electron density and temperature data from plasma instruments, the electron thermal current can be calculated as a function of the spacecraft potential. By using the obtained model and the direct measurement of electron temperature the electron density can be derived from the spacecraft potential
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