MMS Observations of Whistler-Mode Waves: Comparison Between the Observed and Theoretically Predicted Electric Field

Journal article, 2026

Electric field instruments function by electrically coupling to the surrounding plasma, resulting in a response function that varies depending on local conditions. This variable coupling can complicate quantitative interpretation of wave measurements, yet is rarely considered. While this effect has recently been quantified for the Van Allen Probes, we present the first quantitative analysis for the Magnetospheric MultiScale (MMS) mission. First, MMS observations are evaluated against Faraday's Law, revealing that the angle between the measured whistler-mode electric and magnetic fields directly depends on the wave propagation direction, a feature consistent with sheath impedance effects. A novel technique for determining electric field observations along each measurement direction is introduced, addressing limitations of previous works. This reveals that, for MMS in low-density plasma, spin-plane amplitudes are ∼60% of expected values, with small phase shifts, while spin-axis measurements are accurate within 5%–10%, with phase shifts up to −20°. At intermediate densities, spin-plane amplitudes match, or slightly exceed, expected values whereas spin-axis observations can be overestimated by 70% and experience frequency-dependent phase shifts. At high-density, spin-plane measurements generally agree with expected values, but spin-axis observations are overestimated by ∼30% and experience ∼30°phase shifts. Accurate measurements are critical, with electric field fluctuations increasingly being used to infer wave-particle energy exchange rates. If electric field observations can be under or over-measured depending on the local plasma environment, this directly impacts these computations of energy exchange rates, potentially leading to a misinterpretation of the fundamental physical mechanisms that drive particle dynamics.