Theoretical analysis on lower band cascade as a mechanism for multiband chorus in the Earth’s magnetosphere

Wang

2018

JGR Space Physics

Self Cite

In this paper, we have studied thousands of multiband chorus events in the Earth's magnetosphere by utilizing seven‐year THEMIS waveform data. There are two types of multiband chorus events in the magnetosphere: electromagnetic multiband chorus (EM‐MBC) event, where two‐band structure is found in both magnetic and electric spectrograms, and quasi‐electrostatic multiband chorus (QE‐MBC) event, where two‐band structure is only detected in the electric spectrogram. EM‐MBC events preferentially occur at low L‐shells on the nightside but at high L‐shells on the dayside. In EM‐MBC events, lower‐band waves typically have relatively larger magnetic amplitudes (>100 pT) and finite wave normal angles (10°–30°). The magnetic amplitudes of upper‐band harmonic waves are found to be typically smaller than 30 pT and peak in the range of <5 pT. QE‐MBC events tend to be detected at low L‐shells on both dayside and nightside. They distribute mainly at relatively higher magnetic latitudes (6° to 15°) in the dawn sector. Lower‐band chorus in QE‐MBC events are typically highly oblique with very large wave normal angles (>60°), and their electric amplitudes peak in the range of 5–15 mV/m. For upper‐band harmonic waves, their electric amplitudes mainly concentrate in the range of 0.15–0.5 mV/m. MBC events occupy a large population in upper‐band chorus observed in the magnetosphere, which even contribute nearly half of upper‐band chorus within L < 10RE. Our study not only provides the first statistical results of MBC events but also reveals the significant role of the lower band cascade in generating upper‐band chorus waves in the magnetosphere.

Section: Observational Resultssupporting

confidence: 62%

Section: Summary and Discussionmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Statistical Results of Multiband Chorus by Using THEMIS Waveform Data

Wang

2018

JGR Space Physics

Self Cite

“…A banded chorus event detected by Van Allen Probes (Fu et al, ), on the other hand, implies that the power gap is a natural consequence of two bands excited by two distinct electron populations. Recently, Gao et al (, ), Gao, Lu, and Wang (), and Gao, Lu, Wang, et al () have done a series of studies about multiband chorus waves in the Earth's magnetosphere, where upper‐band waves are excited through the coupling between electromagnetic and electrostatic components of lower‐band waves (i.e., lower band cascade), and as a result a power gap is formed. Due to the high occurrence of multiband chorus waves [Gao, Lu, Wang, et al, ], they pointed out that the lower‐band cascade could be a significant potential mechanism to generate the banded spectrum.…”

Section: Introductionmentioning

confidence: 99%

Statistical Results of the Power Gap Between Lower‐Band and Upper‐Band Chorus Waves

Geophysical Research Letters

Chen

et al. 2019

Self Cite

The primary power gap of chorus waves is statistically investigated using 7‐year waveform data from THEMIS probes for the first time. Overall, ~2/3 of chorus events have a power gap between the lower and upper bands. Both the gap frequency and the frequency bandwidth of the gap have a broad distribution, which peaks at frequencies of ~0.49fce and ~0.07fce, respectively. The gap frequency tends to increase with increasing |MLAT| (magnetic latitude), while the frequency width gradually increases with L‐shell. In most of banded events, the peak frequency of upper band is roughly twice that of lower band. Two types of events are studied. For Type I events, one population is located around Pup/Plow = 10−1, and the other is around Pup/Plow = 10−3. However, Type II events are roughly concentrated around Pup/Plow = 10−3. The gap frequency positively correlates with the frequency of upper band. Our study provides detailed observational constraints on potential mechanisms of the power gap formation.

“…Moreover, whistler mode waves with a finite wave normal angle exhibit some interesting features, distinct from parallel wave characteristics. Such lower band oblique whistler mode wave has a strong electrostatic component, which can couple with its electromagnetic component to drive the upper band harmonic wave (Gao, Lu, et al, ; Gao et al, ). This lower band cascade mechanism is proposed to explain the generation of multiband chorus waves (Chen et al, ; Gao, Lu, et al, , Gao, Ke, et al, , Gao et al, ).…”

Section: Introductionmentioning

confidence: 99%

The Effects of Thermal Electrons on Whistler Mode Waves Excited by Anisotropic Hot Electrons: Linear Theory and 2‐D PIC Simulations

Fan

et al. 2019

JGR Space Physics

Self Cite

The wave normal angle of excited whistler waves was previously considered to be controlled by the parallel plasma beta (β∥h) of anisotropic hot electrons, while the effects of thermal electrons were usually neglected. By combining both the linear theoretical and 2‐D particle‐in‐cell (PIC) simulation models, we have investigated the effects of thermal electrons on the whistler anisotropy instability. In the high‐beta (β∥h ≥ 0.025) regime, the wave normal angle of the dominant whistler mode with the largest growth rate is always 0°, which is not affected by thermal electrons, while, its wave frequency and linear growth rate decrease with the density and temperature of thermal electrons. These results are also confirmed by PIC simulations. In the low‐beta (β∥h ≤ 0.025) regime, with the increase of the density and temperature of thermal electrons, the wave normal angle of the dominant whistler mode turns to zero from a large value. This change could be due to the stronger damping caused by thermal electrons for oblique whistler mode, since oblique wave usually has a smaller cyclotron resonant velocity than parallel wave. PIC simulations also show a consistent result, but reproduce a broad magnetic spectrum, even in the case including sufficient thermal electrons. Furthermore, thermal electrons with large parallel velocities are resonantly accelerated in the perpendicular direction, while parts of hot electrons are trapped and accelerated in the parallel direction. Our study suggests that the wave normal angle of whistler mode in the Earth's magnetosphere could be determined by both anisotropic and thermal electrons.