Spatio‐temporal structure of storm‐time chorus

Abstract

[1] We discuss chorus emissions measured by the four Cluster spacecraft at close separations during a geomagnetically disturbed period on 18 April 2002. We analyze the lower band of chorus below one half of the electron cyclotron frequency, measured at a radial distance of 4.4 Earth's radii, within a 2000 km long source region located close to the equator. The characteristic wave vector directions in this region are nearly parallel to the field lines and the multipoint measurement demonstrates the dynamic character of the chorus source region, changing the Poynting flux direction at time scales shorter than a few seconds. The electric field waveforms of the chorus wave packets (forming separate chorus elements on power spectrograms) show a fine structure consisting of subpackets with a maximum amplitude above 30 mV/m. To study this fine structure we have used a sine-wave parametric model with a variable amplitude. The subpackets typically start with an exponential growth phase, and after reaching the saturation amplitude they often show an exponential decay phase. The duration of subpackets is variable from a few milliseconds to a few tens of milliseconds, and they appear in the waveform randomly, with no clear periodicity. The obtained growth rate (ratio of the imaginary part to the real part of the wave frequency) is highly variable from case to case with values obtained between a few thousandths and a few hundredths. The same chorus wave packets simultaneously observed on the different closely separated spacecraft appear to have a different internal subpacket structure. The characteristic scale of the subpackets can thus be lower than tens of kilometers in the plane perpendicular to the field line, or hundreds of kilometers parallel to the field line (corresponding to a characteristic time scale of few milliseconds during the propagation of the entire wave packet). Using delays of time-frequency curves obtained on different spacecraft, we have found the same propagation direction as obtained from the simultaneous Poynting flux calculations. The delays roughly correspond to the whistler-mode group velocity estimated from the cold plasma theory. We have also observed delays corresponding to antiparallel propagation directions for two neighboring chorus wave packets, less than 0.1 s apart.

Electron acceleration at Jupiter: input from cyclotron-resonant interaction with whistler-mode chorus waves

“…This type of frequency time structure is observed at Jupiter (Gurnett and Scarf, 1983) as well as the Earth (e.g., Burtis and Helliwell, 1969;Tsurutani and Smith, 1974;Santolík et al, 2003) and Saturn . Individual chorus bursts may only last a few milliseconds, but they often overlap in time and occur repeatedly for periods of hours and longer.…”

Large-amplitude transmitter-associated and lightning-associated whistler waves in the Earth's inner plasmasphere atL< 2

“…The subsecond time scale is many orders of magnitude quicker than time scales for quasi-linear diffusion appropriate to previously known whistler populations which had amplitudes of up to ∼30 mV/m [Santolík et al, 2003] but are more typically observed with amplitudes of <1 mV/m.…”

Characteristics of magnetospherically reflected chorus waves observed by CLUSTER

“…Complex fine structure of chorus wave packets was recently found in their source region . The sources of chorus are located within a few thousands of km of the equatorial plane (Dunkel and Helliwell, 1969;Burtis and Helliwell, 1969;Burton and Holzer, 1974;Tsurutani and Smith, 1977;Nagano et al, 1996;LeDocq et al, 1998;Santolík et al, 2003b), with scales of the order of a few wavelengths perpendicular to the field lines (Santolík and Gurnett, 2003). Chorus thus propagates from its well localized source in the vicinity of the equatorial plane, but no evidence of reflected chorus waves returning to the equator was found until recently.…”