Cluster observations of whistler mode ducts and banded chorus

Abstract

Bell et al. (2009) proposed that the source region for banded chorus consists of whistler mode ducts of depleted electron density (N-e) for upper band (UB) chorus for all wave normal angles (theta) and ducts of either enhanced or depleted N-e for lower band (LB) chorus for small theta and theta near or greater than the Gendrin angle, respectively. This paper provides support for this model using new high resolution (17 km) N-e observations from the Cluster WHISPER and EFW instruments. Data is examined from January 20, 2004, when strong banded chorus was observed over 3000 km of the Cluster 2 orbit, ending at the magnetic equator. Previous analysis of LB chorus on this day indicated the wave normal angles were larger than the Gendrin angle. Using the N-e data, we show that the LB chorus is generated within depletion ducts in the source region and that the half-width of these ducts (similar to 70 km) is comparable to the transverse scale (similar to 100 km) of the chorus source region. Making use of the fact that the group velocity of the LB chorus waves has a significant cross-L component, we show that the source region extends over 500 km near the magnetic equator.