Spatial Extent and Temporal Correlation of Chorus and Hiss: Statistical Results From Multipoint THEMIS Observations

Abstract

The spatial localization of whistler mode waves, determined by their generation process, propagation, and damping, is important for assessing the efficiency of wave-particle interactions and the dynamics of the radiation belts. We use 11 years of multipoint wave measurements in 2007-2017 from five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft covering L = 2-10 at low magnetic latitudes over all magnetic local times (MLTs) to characterize both the instantaneous spatial extent of chorus and hiss active regions and their wave amplitude distribution in the vicinity of their respective generation regions, making use of the time domain correlation technique. The correlation of lower band chorus wave power dynamics from two spacecraft measurements remains significant up to 250-800 km transverse to the background magnetic field, in agreement with the typical scale of a chorus element, with a shorter correlation length for higher peak wave power. Additionally, a coordinated analysis of average chorus amplitudes at separate locations shows that the radial extent of the chorus active region varies from 0.2 R E on the nightside to 1 R E on the dayside, with an azimuthal width of 1 MLT. The wave power correlation scale of hiss has no fine structure and is generally larger (1,500-3,000 km); its correlation remains nonnegligible up to 1.5 R E . Such observations are consistent with chorus generation by localized electron injections and subsequent hiss amplification at lower L shells from seed chorus waves having propagated and spread over a wider region inside the plasmasphere. The connection between hiss and chorus is further explored statistically through the correlation between their respective wave power variations, suggesting a significant link between them.

Van Allen Probes Observations of Chorus Wave Vector Orientations: Implications for the Chorus‐to‐Hiss Mechanism

“…It is worth noting that the requirement for strong azimuthal density gradients in the form of plumes provides an explanation for the spatial structure observed by Agapitov et al (2018), who found that correlation between chorus and hiss waves was observed most frequently between 12 and 16 MLT. This spatial limitation is also consistent with the direct observation of concurrent, or slightly delayed, enhancements between chorus and hiss measured between 13 and 17 MLT by Bortnik et al (2009) and between 12 and 13 MLT by Li et al (2015).…”

“…Further investigation revealed chorus at larger radial distances, ∼10 R E , could be a source of plasmaspheric hiss with observed time delays of 5–8 s, consistent with propagation times predicted by ray tracing (Li et al, ). Agapitov et al () showed substantial correlation between chorus and hiss wave power observed by the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, occurring most frequently in the post‐noon sector, with ∼20% of chorus observed between 12 and 16 in magnetic local time (MLT) being well correlated with hiss waves observed inside of the plasmasphere immediately afterward. Through extensive modeling efforts it has also been demonstrated that this chorus‐to‐hiss mechanism could reproduce the frequency range, wave power, and the spatial structure of plamsaspheric hiss (e.g., Chen, Bortnik, et al, , ; Chen, Li, et al, ; Meredith et al, ).…”

The Storm Time Development of Source Electrons and Chorus Wave Activity During CME‐ and CIR‐Driven Storms

“…Chorus waves are generated via the instability created by the temperature anisotropy of these hot electrons, and we present evidence that the spatial location of chorus wave power follows the changing access and drift history of these source electrons. Agapitov et al, 2018), furthering showing the importance in understanding the generation of chorus waves in the afternoon sector. To our knowledge, our results are the first showing the storm phase progression over MLT and Lshell with direct chorus observations.…”

Statistical Analysis of Transverse Size of Lower Band Chorus Waves Using Simultaneous Multisatellite Observations

“…Interestingly, from THEMIS FBK statistics, Agapitov et al (2018) found that cluster scale chorus waves with larger B w have smaller scale sizes, which may be consistent with our results, since larger B w chorus waves are more likely observed near the equator region, especially in the nightside magnetosphere (W. Li et al, 2009). Moreover, the scale size of cluster scale chorus waves is larger on the dayside than on the nightside, which is suggested to be caused by the fact that injected energetic electrons spread wider as they drift from the nightside to the dayside (Agapitov et al, 2018). Thus, it is not surprising that a relatively small scale size of chorus waves,~100 km, was reported by Santolík and Gurnett (2003) near 21 MLT.…”