On the applicability of single-spacecraft interferometry methods using electric field probes

TL;DR

This paper evaluates and compares three interferometry methods for determining plasma wave phase velocity using electric field probes on a spacecraft, highlighting their sensitivities and applicability conditions.

Contribution

It provides a detailed analysis and comparison of three interferometry techniques applied to spacecraft electric field measurements for plasma wave velocity estimation.

Findings

Potential method sensitive to wave planarity and spacecraft potential changes.

E-field methods less affected by spacecraft potential, with the second method generally preferable.

The second E-field method reliably estimates spin-plane velocity when propagation direction is known.

Abstract

When analyzing plasma waves, a key parameter to determine is the phase velocity. It enables us to, for example, compute wavelengths, wave potentials, and determine the energy of resonant particles. The phase velocity of a wave, observed by a single spacecraft equipped with electric field probes, can be determined using interferometry techniques. While several methods have been developed to do this, they have not been documented in detail. In this study, we use an analytical model to analyze and compare three interferometry methods applied on the probe geometry of the Magnetospheric Multiscale spacecraft. One method relies on measured probe potentials, whereas the other two use different E-field measurements: one by reconstructing the E-field between two probes and the spacecraft, the other by constructing four pairwise parallel E-field components in the spacecraft spin-plane. We find that the potential method is sensitive both to how planar the wave is, and to spacecraft potential changes due to the wave. The E-field methods are less affected by the spacecraft potential, and while the reconstructed E-field method is applicable in some cases, the second E-field method is almost always preferable. We conclude that the potential based interferometry method is useful when spacecraft potential effects are negligible and the signals of the different probes are very well correlated. The method using two pairs of parallel E-fields is practically always preferable to the reconstructed E-field method and produces the correct velocity in the spin-plane, but it requires knowledge of the propagation direction to provide the full velocity.