Low Plasma Density Estimation Based on Measurements of Electron Cyclotron Harmonic Waves

The electron plasma frequency to cyclotron frequency ratio is a crucial parameter for accurately evaluating local chorus wave-driven electron acceleration and loss in the inner magnetosphere. In the present work, we propose a method for inferring the plasma frequency from near-equatorial measurements of electron cyclotron harmonic (ECH) waves in low plasma density regions outside the plasmasphere. The basic procedure first consists in a careful selection of ECH waves, such that their frequencies at peak power in two consecutive cyclotron harmonic bands are consistent with their theoretical maximum frequency in these bands. Next, the equatorial plasma frequency can be inferred based on the retained wave measurements, under the assumption of an approximately Maxwellian or Kappa electron distribution (possibly including a small fraction of significantly more energetic electrons). This method has been validated through comparisons with the plasma frequency inferred from the upper hybrid resonance during 16 events that took place during geomagnetic storms in 2014–2018, using data from the Van Allen Probes. The error made with the new method compared to the upper hybrid resonance method usually remains moderate, suggesting that this new method could be a useful complement to the previous methods. However, caution should be exerted when using this method, due to its sensibility to small uncertainties in the electron cyclotron frequency. It is also inapplicable in the presence of a hot electron population of similar or higher density than the cold/cool population.