Observational evidence of generation mechanisms for very oblique lower band chorus using THEMIS waveform data

Gao, Xinliang; Mourenas, D.; Li, Wen; Artemyev, А. V.; Lu, Quanming; Tao, X.; Wang, Shui

doi:10.1002/2016ja022915

Cited by 30 publications

(34 citation statements)

References 65 publications

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“…Recently, rising tones of lower band chorus have been thoroughly studied by means of theories [ Omura et al ., ], simulations [ Omura et al ., ; Fu et al ., ], and observations [ Santolik et al ., ; Li et al ., , , ; Gao et al ., , ], but the generation of upper band chorus, or equivalently the power gap between lower and upper bands at about 0.5 f ce , still remains a mystery. Although some potential mechanisms have been proposed to explain the generation of upper band chorus waves and the formation of the power minimum at 0.5 f ce , such as a strong damping at 0.5 f ce [ Omura et al ., ], different source regions [ Bell et al ., ], and different free energy sources [ Fu et al ., ], they are still under debate and lack direct observational evidences.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

Generation of Multiband Chorus in the Earth's Magnetosphere: 1‐D PIC Simulation

Gao

et al. 2017

Geophysical Research Letters

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Multiband chorus waves, where the frequency of upper band chorus is about twice that of lower band chorus, have recently been reported based on THEMIS observations. The generation of multiband chorus waves is attributed to the mechanism of lower band cascade, where upper band chorus is excited via the nonlinear coupling process between lower band chorus and the associated density mode with the frequency equal to that of lower band chorus. In this letter, with a one‐dimensional (1‐D) particle‐in‐cell (PIC) simulation model, we have successfully reproduced multiband chorus waves. During the simulation, the significant density fluctuation is driven by the fluctuating electric field along the wave vector of the pump wave (lower band chorus), which can be directly observed in this self‐consistent plasma system. Then, the second harmonic of the pump whistler‐mode wave (upper band chorus) is generated. After quantitatively analyzing resonant conditions among wave numbers, we can confirm that the generation is caused due to the coupling between the pump wave and the density fluctuation along its wave vector. The third harmonic can also be excited through lower band cascade if the pump whistler‐mode wave has a sufficiently large amplitude. Our simulation results not only provide a theoretical support to the mechanism of lower band cascade to generate multiband chorus but also propose a new pattern of evolution for whistler‐mode waves in the Earth's magnetosphere.

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Section: Conclusion and Discussionmentioning

confidence: 99%

Generation of Multiband Chorus in the Earth's Magnetosphere: 1‐D PIC Simulation

Gao

et al. 2017

Geophysical Research Letters

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“…Currently, the statistical models of the waves distributions are created using wave measurements from various spacecraft and are parameterized by the location of observations and current values for geomagnetic indices (Agapitov et al, ; Aryan et al, ; Gao, Li, Thorne, Bortnik, Angelopoulus, Lu, Tao, et al, ; Gao, Li, Thorne, Bortnik, Angelopoulus, Lu, Tang, et al, ; Gao, Mourenas, et al, ; X. Li, Temerin, et al, ; Meredith et al, ; Pokhotelov et al, ). These current models assume that the preceding state of the magnetosphere plays no role in the current wave distribution in the magnetosphere.…”

Section: Introductionmentioning

confidence: 99%

The Influence of Solar Wind and Geomagnetic Indices on Lower Band Chorus Emissions in the Inner Magnetosphere

et al. 2018

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Statistical wave models, describing the distribution of wave amplitudes as a function of location, geomagnetic activity, and other parameters, are needed as the basis to describe the wave-particle interactions within numerical models of the radiation belts. In this study, we widen the scope of the statistical wave models by investigating which of the solar wind parameters or geomagnetic indices and their time lags have the greatest influence on the amplitudes of lower band chorus (LBC) waves in the inner magnetosphere. The solar wind parameters or geomagnetic indices with the greatest control over the waves were found using the error reduction ratio (ERR) analysis, which plays a key role in system identification modeling techniques. In this application, the LBC magnitudes at different locations are considered as the output data, while the lagged solar wind parameters are the input data. The ERR analysis automatically determines a set of the most influential parameters that explain the variations in the emissions. Both linear and nonlinear applications of the ERR analysis are compared using solar wind inputs and show that the linear ERR analysis can be misleading. The linear results show that the interplanetary magnetic field (IMF) factor has the most influence on at each magnetic local time (MLT) sector. However, the nonlinear ERR analysis shows that the IMF factor coupled with the solar wind velocity has the main contribution to the LBC wave magnitudes. When geomagnetic indices are included as inputs with the solar wind parameters to the nonlinear ERR analysis, the results show that the majority of the variation in emissions may be attributed to the Auroral Electrojet (AE) index. In the dawn sectors between 00 and 12 MLT and 5 < L < 7, the AE index multiplied by the solar wind velocity with zero time lag has the most influence on the amplitudes of LBC. For 5 < L < 7, the parameters with the highest ERR are the AE index multiplied by the solar wind velocity with a 2-hr time lag at 12-16 MLT, the linear AE index with a 2-hr time lag at 16-20 MLT, and AE index multiplied by the IMF factor with zero lag at 20-00 MLT. For 4 < L < 5, the parameters with the highest ERR are the AE index multiplied by the solar wind dynamic pressure with zero time lag at 00-04 MLT, the AE index multiplied by the solar wind velocity with zero time lag between 14 and 12 MLT, the AE index multiplied by the solar wind velocity with a 2-hr time lag at 12-16 MLT, the Dst index with a 6-hr time lag at 12-16 MLT, and the AE index multiplied by the IMF factor with zero lag at 20-00 MLT.Plain Language Summary Lower band chorus (LBC) waves are electromagnetic waves found outside the plasmapause near the geomagnetic equator and are known to modify the local electron distribution through wave-particle interactions leading to the acceleration of the electrons in the radiation belts to relativistic energies. This study aims to identify the solar wind or geomagnetic drivers of the LBC waves using the error reduction ratio analysis. The error reduction ...

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“…Whistler mode waves are intense electromagnetic emissions that occur naturally in the Earth's inner magnetosphere (Burtis & Helliwell, 1969;Gao, Mourenas, et al, 2016;Meredith et al, 2001;Santolík et al, 2003;Tsurutani & Smith, 1974), which are believed to play an important role in regulating electron dynamics in the Van Allen radiation belt (Horne et al, 2003;Horne & Thorne, 1998;Lorentzen et al, 2001). Through resonant wave-particle interactions, whistler mode waves can efficiently accelerate~100 keV electrons tõ MeV energies during geoactive periods, leading to the rapid enhancement of relativistic electrons in the outer radiation belt (Meredith et al, 2001;Summers et al, 1998;Thorne et al, 2013;Xiao et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Nonlinear Evolution of Counter‐Propagating Whistler Mode Waves Excited by Anisotropic Electrons Within the Equatorial Source Region: 1‐D PIC Simulations

Chen

Gao

et al. 2018

JGR Space Physics

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Nonlinear physical processes related to whistler mode waves are attracting more and more attention for their significant role in reshaping whistler mode spectra in the Earth's magnetosphere. Using a 1‐D particle‐in‐cell simulation model, we have investigated the nonlinear evolution of parallel counter‐propagating whistler mode waves excited by anisotropic electrons within the equatorial source region. In our simulations, after the linear phase of whistler mode instability, the strong electrostatic standing structures along the background magnetic field will be formed, resulting from the coupling between excited counter‐propagating whistler mode waves. The wave numbers of electrostatic standing structures are about twice those of whistler mode waves generated by anisotropic hot electrons. Moreover, these electrostatic standing structures can further be coupled with either parallel or antiparallel propagating whistler mode waves to excite high‐k modes in this plasma system. Compared with excited whistler mode waves, these high‐k modes typically have 3 times wave number, same frequency, and about 2 orders of magnitude smaller amplitude. Our study may provide a fresh view on the evolution of whistler mode waves within their equatorial source regions in the Earth's magnetosphere.

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Observational evidence of generation mechanisms for very oblique lower band chorus using THEMIS waveform data

Cited by 30 publications

References 65 publications

Generation of Multiband Chorus in the Earth's Magnetosphere: 1‐D PIC Simulation

Generation of Multiband Chorus in the Earth's Magnetosphere: 1‐D PIC Simulation

The Influence of Solar Wind and Geomagnetic Indices on Lower Band Chorus Emissions in the Inner Magnetosphere

Nonlinear Evolution of Counter‐Propagating Whistler Mode Waves Excited by Anisotropic Electrons Within the Equatorial Source Region: 1‐D PIC Simulations

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