Measuring Low Plasma Density in the Earth's Equatorial Magnetosphere From Magnetosonic Waves.

The plasma density is one of the most fundamental quantities of any plasma yet measuring it in space is exceptionally difficult when the density is low. Measurements from particle detectors are contaminated by spacecraft photoelectrons and methods using plasma wave emissions are hampered by natural plasma instabilities which dominate the wave spectrum. Here we present a new method which calculates the density from magnetosonic waves near the lower hybrid resonance frequency. The method works most effectively when the ratio of the plasma to cyclotron frequency is fpe/fce < 3.5. The method provides a lower bound on the plasma density. Using the new method we show that wave acceleration of electrons to relativistic energies is increased by orders of magnitude. The method enables years of satellite data to be re‐analyzed for the Earth and the effectiveness of wave acceleration at the Earth, Jupiter and Saturn to be re‐assessed. Plain Language Summary: The electron plasma density is a fundamental quantity of any plasma, but it is very difficult to measure in space using satellites. Satellites charge to different potentials with repel or attract electrons making the true measurement very difficult. The plasma density can be determined from wave oscillations at the plasma frequency, but the waves are difficult to identify as the wave spectrum is often dominated by other much stronger waves. Here we analyze satellite data and show that magnetosonic waves near the lower hybrid resonance frequency can be used to calculate the plasma density. This method provides a lower bound on the density. We show that this lower density leads to much faster electron acceleration by wave‐particle interactions, accelerates electrons to much higher energies and increases the electron flux at one MeV by two orders of magnitude or more. The method enables the importance of electron acceleration at the Earth, Jupiter and Saturn to be re‐evaluated. Key Points: New method to calculate plasma density from magnetosonic waves near the lower hybrid resonance frequencyMethod enables low densities to be measured near the magnetic equatorLow density increases wave acceleration of electrons by orders of magnitude [ABSTRACT FROM AUTHOR]