Acceleration of Electrons by Whistler‐Mode Hiss Waves at Saturn

Plasmaspheric hiss waves at the Earth are well known for causing losses of electrons from the radiation belts through wave particle interactions. At Saturn, however, we show that the different plasma density environment leads to acceleration of the electrons rather than loss. The ratio of plasma frequency to electron gyrofrequency frequently falls below one creating conditions for hiss to accelerate electrons. The location of hiss at high latitudes (>25°) coincides very well with this region of very low density. The interaction between electrons and hiss only occurs at these higher latitudes, therefore the acceleration is limited to mid to low pitch angles leading to butterfly pitch angle distributions. The hiss is typically an order of magnitude stronger than chorus at Saturn and the resulting acceleration is rapid, approaching steady state in one day at 0.4 MeV at L = 7 and the effect is stronger with increasing L‐shell. Plain Language Summary The interaction between charged particles and waves that are created within the plasma that sits in the magnetic field of a planet can be very strong. This interaction can move energy from the wave to the particle or vice versa. In the near Earth space environment, known as the magnetosphere, a particular class of waves known as whistler‐mode hiss is well known for changing the direction of motion of charged electrons such that they will collide with the particles of the Earth's atmosphere and be lost from the magnetosphere. This study investigates the effect of these hiss waves at Saturn where the background plasma conditions are different compared to the Earth. The hiss at Saturn exists in a region where both the plasma density is very low and the magnetic field is strong and this causes the electrons to be energized by the waves rather than scattered. Key Points Low ratio of plasma to gyro‐frequency causes whistler‐mode hiss to accelerate rather than scatter electrons Hiss acceleration at high latitudes may lead to butterfly electron pitch angle distributions Rapid acceleration over a timescale of hours which strengthens with increasing L‐shell