Owen Roberts
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.
This poster explores the nature of compressive density fluctuations in the solar wind using high time resolution spacecraft potential measurements. There are several observations of the radial variation of the temperature in the solar wind, which has observed that the solar wind cools much slower than what is expected for an adiabatically expanding gas (first figure). This result implies there is a local source of heating.
Heating also seems to be intermittently distributed in the plasma and concentrated near regions that have large gradients (second figure). The second figure shows an increase in the temperature where the gradients in the magnetic fields are higher.
Therefore understanding the statistical properties of the large amplitude gradients will help us in understanding the heating.
Slide 2
To be able to have a density measurement with high time resolution we use the spacecraft potential which can be calibrated to give a measurement of the electron density. The spacecraft potential is governed by the currents going to and from the spacecraft. The two dominant currents are photoelectrons emitted from the sunlit surfaces and the electron thermal current collected by the spacecraft (first figure). The MMS consist of four spacecraft which means that differences can be measured from a time lag or from a spatial lag (second figure).
Slide 3
Here we plot the scale-dependent kurtosis of the density fluctuations (first figure) this shows a power-law increase at large scales and then a plateau after the ion scales. The points where a spatial lag has been calculated show a difference with the time lags. This could be due to a number of reasons i.e. breakdown of Taylor's hypothesis, wave activity, differences in the directions, larger structures than spacecraft separation sizes a sampling effect. The magnetic field fluctuations show a similar trend although they are affected by noise at a smaller time lag.
Slide 4
Here, as an additional indicator of the intermittency properties of the density we calculate the structure functions (first figure) and the scaling exponents (second figure). An intermittent process also normally show multifractality. The structure functions show two distinct ranges that can be fitted. There is a hint of a third transition range but this is too short to be fit convincingly. From the gradients in the inertial and sub-ion range, the variation of the scaling exponents versus the moment is shown and the inertial range is shown to be much more intermittent than the sub ion range as was shown with the scale-dependent kurtosis.
Conclusions
The spacecraft potential gives an excellent high time resolution method to study sun ion scale turbulence
The scale-dependent kurtosis of density and magnetic field fluctuations suggests a plateau in kurtosis at ion scales. The scaling exponents also suggest this region is not intermittent.
Time lags give good scale coverage but poor directional coverage. Spatial lags give good directional coverage but poor scale coverage. Future mission concepts should strive to have good directional and scale coverage
There is a difference between time lags and spatial lags which we have several hypotheses to explain (breakdown of Taylor, structure sizes, sampling effect, waves, directional differences)