A theory for narrow‐banded radio bursts at Uranus: MHD surface waves as an energy driver

Abstract

We describe a possible scenario for the generation of the narrow‐banded radio bursts (n bursts) detected at Uranus by the Voyager 2 planetary radio astronomy experiment. This particular radio emission is suspected to originate within the Uranian northern polar cusp region. In order to account for the emission burstiness which occurs on time scales of hundreds of milliseconds, we propose that ULF magnetic surface turbulence, generated at the frontside magnetopause, propagates down the open/closed field line boundary and mode converts to kinetic Alfvén waves (KAW) deep within the polar cusp. The oscillating KAW potentials then drive a transient electron stream (beam or loss cone) that creates the bursty radio emission. To substantiate these ideas, we show Voyager 2 magnetometer measurements of enhanced ULF magnetic activity at the frontside magnetopause. We then demonstrate analytically that such magnetic turbulence should mode convert deep in the cusp at a radial distance of 3 RU. A condition for mode conversion from magnetic surface waves to KAW is ƒpe/ƒce < 0.2, which is also a condition favorable for the generation of the R‐X mode radio bursts. The fact that a similar plasma condition is required for both processes lends strong support that the active regions for mode conversion and R‐X mode wave generation are one and the same.