Ground-level detection of auroral kilometric radiation



[1] The Earth's auroral electrons radiate up to ∼1% of their energy in radio waves, called Auroral Kilometric Radiation (AKR). The emission mechanism, called the electron cyclotron maser (ECM), produces similar emissions at other planets, in the solar atmosphere, and possibly in astrophysical systems such as pulsars and blazars. ECM theory applied to AKR predicts radiation beamed outward that cannot penetrate near the Earth. Nevertheless, over the past several decades there have been occasional reports of AKR-like signals detected at low altitudes. Here we show the first evidence that AKR does indeed penetrate to low altitudes. For three examples of AKR-like emissions detected at South Pole Station, Antarctica, we examined data from the Geotail satellite plasma wave receiver, which had a field of view that included the auroral field lines above the station. The AKR-like emissions detected at ground-level have the same frequency-time structure as simultaneous AKR emissions detected on Geotail 115,000–190,000 km away from the Earth. These first coincident detections of AKR in space and on the ground require the existence of a mechanism to produce the ground-level emissions, suggest that previous AKR-like emissions observed at low altitudes may indeed be AKR, and require revision of the widely-held textbook view that AKR is only detectable from space.