A microscopic and nanoscopic view of storm‐time chorus on 31 March 2001

We investigate intense whistler‐mode chorus emissions which occurred during the geomagnetic storm on 31 March 2001. We use multipoint measurements obtained by the Cluster spacecraft in the premidnight equatorial region outside the plasmasphere at a radial distance of 4 Earth radii ( L = 4.0 − 4.2). Observed spatio‐temporal variations of the direction of the Poynting flux manifest a consistent pattern: the central position of the chorus source fluctuates at time scales of minutes within 1000–2000 km of the geomagnetic equator. We demonstrate that estimates of the electromagnetic planarity can be used to characterize the extent of the source, obtaining a range of 3000–5000 km. Discrete wave packets of chorus are observed to rise in frequency between 0.13 and 0.5 of the local electron cyclotron frequency, at a rate up to 20 kHz/s, having the maximum peak amplitudes of ∼20 mV/m. We observe a fine structure of subpackets with large amplitudes embedded in the interior of the wave packets. This fine structure has a typical delay of a few milliseconds between the two neighboring maxima of the wave amplitude. Longer delays occur with a decreasing probability density.