Multievent study of the correlation between pulsating aurora and whistler mode chorus emissions

Abstract

[1] A multievent study was performed using conjugate measurements of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft and an all-sky imager during periods of intense lower-band chorus waves. The thirteen identified cases support our previous finding, based on two events, that the intensity modulation of lower-band chorus near the magnetic equator is highly correlated with quasiperiodic pulsating auroral emissions near the spacecraft's magnetic footprint, indicating that lower-band chorus is the driver of the pulsating aurora. Furthermore, we identified a fortuitous measurement made simultaneously by two THEMIS spacecraft with small spatial separation. The two spacecraft were found to be located in a single pulsating chorus patch and the spacecraft footprints were in the same pulsating auroral patch when intense chorus bursts were measured simultaneously, whereas only one of the spacecraft's footprints was in a patch when the other spacecraft did not detect intense chorus. On the basis of this event, we can estimate the pulsating chorus patch size by mapping the pulsating auroral patches from the ionosphere toward the magnetic equator, giving a roughly circular region of ∼5000 km diameter for corresponding azimuthally elongated patches with ∼100 km size in the ionosphere. Using a ray-tracing-based calculation of the divergence of chorus raypaths from a point source, together with the corresponding resonant energies, we found that the chorus patch size is most probably not a result of ray divergence but a property of the wave excitation region.Citation: Nishimura, Y., et al. (2011), Multievent study of the correlation between pulsating aurora and whistler mode chorus emissions,

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

“…The 4-Hz modulation, which is related to the spacing of chorus elements, is concentrated at the lower-latitude edge of the auroral patch. However, the observed high correlation region is distributed only around the edge region of the auroral patch with almost the same time difference value of À0.26 s, in contrast to the findings of a previous study (Nishimura et al, 2010(Nishimura et al, , 2011) that used a low temporal resolution (3 s) data set obtained by the THEMIS all-sky imagers (Mende et al, 2008). The observed chorus waves could be in the nonducted mode, because the associated chorus waves were not observed at the ground station at Chistochina (40 km east of Gakona).…”

“…A coordinated geomagnetic conjugate observation could be performed for each of Arase's elliptic orbits. The data set of Arase and PWING have better sensitivities and higher temporal resolutions for waves and aurora measurements than constellation observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites and the THEMIS Ground Based Observatory (GBO) (Nishimura et al, 2010(Nishimura et al, , 2011. The data set of Arase and PWING have better sensitivities and higher temporal resolutions for waves and aurora measurements than constellation observations by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites and the THEMIS Ground Based Observatory (GBO) (Nishimura et al, 2010(Nishimura et al, , 2011.…”

Timescales for electron quasi‐linear diffusion by parallel and oblique lower‐band chorus waves

“…It is worth noting that the resulting minimum timescales of 20 s to 60 s for 15 keV to 100 keV electrons are similar to the observed (16) as a function of the electron density N e at L = 4.5 for E = 1 MeV, with the same other parameters as in Figure 4, and the scaling N e 7/9 from equation (19) (dashed line) and the full numerical results (red diamonds). periods of pulsating auroras, which were recently shown to be highly correlated with lower-band chorus [Nishimura et al, 2011]. These timescales are also similar to, or lower than strong diffusion timescales of isotropization t sd $ 1.8LR E g/ (pca LC 2 ), where R E is the Earth's radius [Lyons, 1973;Schulz, 1974].…”

“…In particular, an important population of plasma sheet low-energy electrons (E $ 10 À 100 keV) are injected near midnight in the outer belt during storms or substorms. Drifting from midnight through dawn to noon, such low-energy electrons are believed to excite lower-band chorus waves near the equator through thermal anisotropy or loss cone instability at small pitch angles, the generated quasi-parallel whistler waves in turn accelerating some electrons to relativistic energies at large pitch angles in the night sector and scattering particles into the loss cone at small pitch angles on the dawnside, finally leading to precipitations in the atmosphere and pulsating auroras Li et al, 2007;Summers et al, 2007b;Ni et al, 2008;Nishimura et al, 2011]. The inferred timescales for precipitation range from hours to days during periods of storm-enhanced convection Li et al, 2007].…”

Chorus intensity modulation driven by time‐varying field‐aligned low‐energy plasma

“…Although it has been rare to observe equatorial and auroral regions simultaneously and to precisely treat magnetic field line modeling, recent studies by Nishimura et al [2010Nishimura et al [ , 2011 determined magnetic field mapping and identified uniquely that lower band chorus is responsible for driving pulsating aurora. Thus, the study of chorus intensity modulation has a direct impact on understanding the driver of auroral pulsation.…”