The Active Spacecraft Potential Control (ASPOC) aboard Magnetospheric Multiscale (MMS) generates beams of indium ions at energies of order 4-12 keV and variable currents of up to 70 μA in order to limit positive spacecraft potentials within several volts and, e.g., thereby improve the measurements obtained by the instruments FPI, HPCA, ADP, and SDP (Torkar et al., 2014). There are two ASPOC per MMS spacecraft, each unit contains four ion emitters and one emitter per instrument is operated at a time.


By determining statistically the photoelectron curve for different science phases, using, plasma current, spacecraft potential, and ASPOC current, plasma density estimation is performed assuming ASPOC current to be a bias current of the spacecraft potential using the method by Andriopoulou et al. (2015).

The code (derivation algorithm) is IDL based and requires spedas to be installed.

The usage is given in (ASPOC Iphoto). The users can run the code to derive the photo electron curve themselves.  For Phase 1 and 2 the derived input parameters for photo electron curves are given in (ASPOC Iphoto curves). The photo electron curves are then used to calculate the density.

For shorter burst mode intervals a local photo curve might improve the density estimation. A code with the photocurve local script can then be used (either ASPOC is on or off).


The solar wind is a weakly compressible turbulent plasma. Although the compressible  fluctuations (i.e. density, magnetic field magnitude) are small with respect to the transverse fluctuations they represent a non-negligible fraction of the fluctuation powers, and become increasingly important at sub ion scales. By using the spacecraft potential to estimate the density in the solar wind much higher time resolutions are possible when compared to the direct measurement. This allows much smaller scales to be investigated than are typically possible with particle detectors. Our solar wind density product uses the plasma density derivation library (see above) to calibrate to density and also removes spin tones in the data. The data archive contains both fast survey mode data sampled at 32Hz and burst mode data sampled at 8kHz. The data can be accessed here. A quick reference guide is also available and the data is described in detail in the paper: Roberts et al., JGR (2020). In the archive, two data formats are available: an IDL .sav format which can be opened with the IDL restore command and an ASCII format.


Strong electric fields have been shown to have an effect on the spacecraft potential causing a larger current from the spacecraft. In effect accelerating more photoelectrons from the surface that ordinarily would not have escaped the potential well of the spacecraft. In these circumstances the electric field effects can have a stronger effect on the spacecraft potential than the density meaning that it can be misleading to use the density estimation. A correction can be performed on the data as discussed in Roberts et al. (2020), so that the density estimation can still be used. An IDL script detailing the method and examples are given here. This routine requires spedas to be installed.