Chorus whistler wave source scales as determined from multipoint Van Allen Probe measurements

Abstract

Whistler mode chorus waves are particularly important in outer radiation belt dynamics due to their key role in controlling the acceleration and scattering of electrons over a very wide energy range. The key parameters for both nonlinear and quasi‐linear treatment of wave‐particle interactions are the temporal and spatial scales of the wave source region and coherence of the wave field perturbations. Neither the source scale nor the coherence scale is well established experimentally, mostly because of a lack of multipoint VLF waveform measurements. We present an unprecedentedly long interval of coordinated VLF waveform measurements (sampled at 16384 s−1) aboard the two Van Allen Probes spacecraft—9 h (0800–1200 UT and 1700–2200 UT) during two consecutive apogees on 15 July 2014. The spacecraft separations varied from about 100 to 5000 km (mostly radially); measurements covered an L shell range from 3 to 6; magnetic local time 0430–0900, and magnetic latitudes were ~15 and ~5° during the two orbits. Using time‐domain correlation techniques, the single chorus source spatial extent transverse to the background magnetic field has been determined to be about 550–650 km for upper band chorus waves with amplitudes less than 100 pT and up to 800 km for larger amplitude, lower band chorus waves. The ratio between wave amplitudes measured on the two spacecraft is also examined to reveal that the wave amplitude distribution within a single chorus element generation area can be well approximated by a Gaussian exp(−0.5 · r2/r02), with the characteristic scale r0 around 300 km. Waves detected by the two spacecraft were found to be coherent in phase at distances up to 400 km.

Microscopic Observations of Pulsating Aurora Associated With Chorus Element Structures: Coordinated Arase Satellite‐PWING Observations

“…The estimated transverse (radial and longitudinal directions) size of the wave-particle interaction region (900-km diameter) was comparable to the source size for a coherent chorus element reported by recent Van Allen Probes observations (Agapitov et al, 2017). Intensity modulations of a pulsating aurora are clearly related to successive discrete chorus elements and also possibly associated with chorus subpacket structures at a time scale of tens of milliseconds.…”

“…Using this projection, the source size at the magnetic equator was estimated to be approximately 900 km in diameter in the radial and longitudinal directions. The source size of the wave-particle interaction (900-km diameter) estimated from the auroral patch in this study is almost the same as the chorus source size determined by recent Van Allen Probes satellite observations (Agapitov et al, 2017). Thus, the horizontal patch size is much larger than the Larmor radius for energetic electrons.…”

“…Thus, the horizontal patch size is much larger than the Larmor radius for energetic electrons. A possible dependence of chorus source scale on L shell was also reported (Agapitov et al, 2017), because the chorus source scale can depend on the wavelength, background geomagnetic field, plasma density, and wave normal angle. The source size of the wave-particle interaction (900-km diameter) estimated from the auroral patch in this study is almost the same as the chorus source size determined by recent Van Allen Probes satellite observations (Agapitov et al, 2017).…”

Microburst Scale Size Derived From Multiple Bounces of a Microburst Simultaneously Observed With the FIREBIRD‐II CubeSats

“…The azimuthal scale size (longitudinal scale mapped from LEO) of 555‐keV electrons was greater than 450 ± 10 km, and for the 771‐keV electrons it was greater than 530 ± 10 km. The lower bound microburst scale size is similar to the chorus scale sizes derived by Agapitov et al, (, ) and is discussed bellow.…”

Observations Directly Linking Relativistic Electron Microbursts to Whistler Mode Chorus: Van Allen Probes and FIREBIRD II

“…The absolute value of the transverse (cross‐field) separation at the 500 km altitude of FIREBIRD is plotted in Figure e and is as low as 60 km. This small separation means that both satellites are contained within a larger flux tube defined by a single chorus wave packet, which has a typical transverse size of ∼100–800 km at the equator at L = 4–6 in different case studies (Agapitov et al, , ; Santolík & Gurnett, ; Santolík et al, ). These are the first published observations of simultaneous chorus and microbursts with a separation smaller than a chorus packet.…”