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.