Excitation of extremely low-frequency chorus emissions: The role of background plasma density
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摘要
Low-frequency chorus emissions have recently attracted much attention due to the suggestion that they may play important roles in the dynamics of the Van Allen Belts.However, the mechanism(s) generating these low-frequency chorus emissions have not been well understood..In this letter, we report an interesting case in which background plasma density lowered the lower cutoff frequency of chorus emissions from above 0.1 f_(ce)(typical ordinary chorus) to 0.02 f_(ce)(extremely low-frequency chorus).Those extremely low-frequency chorus waves were observed in a rather dense plasma, where the number density N_e was found to be several times larger than has been associated with observations of ordinary chorus waves.For suprathermal electrons whose free energy is supplied by anisotropic temperatures, linear growth rates(calculated using in-situ plasma parameters measured by the Van Allen Probes) show that whistler mode instability can occur at frequencies below 0.1 f_(ce) when the background plasma density N_e increases.Especially when N_e reaches 90 cm–3 or more, the lowest unstable frequency can extend to 0.02 f_(ce) or even less, which is consistent with satellite observations.Therefore, our results demonstrate that a dense background plasma could play an essential role in the excitation of extremely lowfrequency chorus waves by controlling the wave growth rates.
Low-frequency chorus emissions have recently attracted much attention due to the suggestion that they may play important roles in the dynamics of the Van Allen Belts.However, the mechanism(s) generating these low-frequency chorus emissions have not been well understood..In this letter, we report an interesting case in which background plasma density lowered the lower cutoff frequency of chorus emissions from above 0.1 f_(ce)(typical ordinary chorus) to 0.02 f_(ce)(extremely low-frequency chorus).Those extremely low-frequency chorus waves were observed in a rather dense plasma, where the number density N_e was found to be several times larger than has been associated with observations of ordinary chorus waves.For suprathermal electrons whose free energy is supplied by anisotropic temperatures, linear growth rates(calculated using in-situ plasma parameters measured by the Van Allen Probes) show that whistler mode instability can occur at frequencies below 0.1 f_(ce) when the background plasma density N_e increases.Especially when N_e reaches 90 cm–3 or more, the lowest unstable frequency can extend to 0.02 f_(ce) or even less, which is consistent with satellite observations.Therefore, our results demonstrate that a dense background plasma could play an essential role in the excitation of extremely lowfrequency chorus waves by controlling the wave growth rates.
Low-frequency chorus emissions have recently attracted much attention due to the suggestion that they may play important roles in the dynamics of the Van Allen Belts.However, the mechanism(s) generating these low-frequency chorus emissions have not been well understood..In this letter, we report an interesting case in which background plasma density lowered the lower cutoff frequency of chorus emissions from above 0.1 f_(ce)(typical ordinary chorus) to 0.02 f_(ce)(extremely low-frequency chorus).Those extremely low-frequency chorus waves were observed in a rather dense plasma, where the number density N_e was found to be several times larger than has been associated with observations of ordinary chorus waves.For suprathermal electrons whose free energy is supplied by anisotropic temperatures, linear growth rates(calculated using in-situ plasma parameters measured by the Van Allen Probes) show that whistler mode instability can occur at frequencies below 0.1 f_(ce) when the background plasma density N_e increases.Especially when N_e reaches 90 cm–3 or more, the lowest unstable frequency can extend to 0.02 f_(ce) or even less, which is consistent with satellite observations.Therefore, our results demonstrate that a dense background plasma could play an essential role in the excitation of extremely lowfrequency chorus waves by controlling the wave growth rates.
引文
Bell,T.F.,Inan,U.S.,Haque,N.,and Pickett,J.S.(2009).Source regions of banded chorus.Geophys.Res.Lett.,36(11),L11101.https://doi.org/10.1029/2009GL037629
Cattell,C.A.,Breneman,A.W.,Thaller,S.A.,Wygant,J.R.,Kletzing,C.A.,and Kurth,W.S.(2015).Van Allen Probes observations of unusually low frequency whistler mode waves observed in association with moderate magnetic storms:Statistical study.Geophys.Res.Lett.,42(18),7273-7281.https://doi.org/10.1002/2015GL065565
Chen,L.,R.M.Thorne,V.K.Jordanova,C.-P.Wang,M.Gkioulidou,L.Lyons,and R.B.Horne(2010).Global simulation of EMIC wave excitation during the 21April 2001 storm from coupled RCM-RAM-HOTRAY modeling.J.Geophys.Res.,115,A07209.https://doi.org/10.1029/2009JA015075
Chen,L.J.,Thorne,R.M.,Li,W.,and Bortnik,J.(2013).Modeling the wave normal distribution of chorus waves.J.Geophys.Res.Space Phys.,118(3),1074-1088.https://doi.org/10.1029/2012JA018343
Chen,Y.,Reeves,G.D.,and Friedel,R.H.W.(2007).The energization of relativistic electrons in the outer Van Allen radiation belt.Nat.Phys.,3(9),614-617.https://doi.org/10.1038/nphys655
Funsten,H.O.,Skoug,R.M.,Guthrie,A.A.,MacDonald,E.A.,Baldonado,J.R.,Harper,R.W.,Henderson,K.C.,Kihara,K.H.,Lake,J.E.,…Chen,J.(2013).Helium,oxygen,proton,and electron(HOPE)mass spectrometer for the radiation belt storm probes mission.Space Sci.Rev.,179(1-4),423-484.https://doi.org/10.1007/s11214-013-9968-7
Gao,Z.L.,Su,Z.P.,Zhu,H.,Xiao,F.L.,Zheng,H.N.,Wang,Y.M.,Shen,C.,and Wang,S.(2016).Intense low-frequency chorus waves observed by Van Allen Probes:Fine structures and potential effect on radiation belt electrons.Geophys.Res.Lett.,43(3),967-977.https://doi.org/10.1002/2016GL067687
Goldstein,J.,De Pascuale,S.,Kletzing,C.,Kurth,W.,Genestreti,K.J.,Skoug,R.M.,Larsen,B.A.,Kistler,L.M.,Mouikis,C.,and Spence,H.(2014).Simulation of Van Allen Probes plasmapause encounters.J.Geophys.Res.Space Phys.,119(9),7464-7484.https://doi.org/10.1002/2014JA020252
Gurnett,D.A.,and O'Brien,B.J.(1964).High-latitude geophysical studies with satellite Injun 3:5.Very-low-frequency electromagnetic radiation.J.Geophys.Res.,69(1),65-89.https://doi.org/10.1029/JZ069i001p00065
Horne,R.B.,and Thorne,R.M.(1993).On the preferred source location for the convective amplification of ion cyclotron waves.J.Geophys.Res.Space Phys.,98(A6),9233-9247.https://doi.org/10.1029/92JA02972
Huang J.,Gu X.D.,Ni B.B.,Luo Q.,Fu S.,Xiang Z.,and Zhang W.X.(2018).Importance of electron distribution profiles to chorus wave driven evolution of Jovian radiation belt electrons.Earth Planet.Phys.,2(5),371-383.https://doi.org/10.26464/epp2018035
Jordanova,V.K.,Thorne,R.M.,Li,W.,and Miyoshi,Y.(2010).Excitation of whistler mode chorus from global ring current simulations.J.Geophys.Res.Space Phys.,115(A5),A00F10.https://doi.org/10.1029/2009JA014810
Katoh,Y.,and Omura,Y.(2011).Amplitude dependence of frequency sweep rates of whistler mode chorus emissions.J.Geophys.Res.Space Phys.,116(A7),A07201.https://doi.org/10.1029/2011JA016496
Kennel,C.(1966).Low-frequency whistler mode.Phys.Fluids,9(11),2190-2202.https://doi.org/10.1063/1.1761588
Kim,J.-H.,Lee,D.Y.,Cho,J.H.,Shin,D.K.,Kim,K.C.,Li,W.,and Kim,T.K.(2015).A prediction model for the global distribution of whistler chorus wave amplitude developed separately for two latitudinal zones.J.Geophys.Res.Space Phys.,120(4),2819-2837.https://doi.org/10.1002/2014JA020900
Kletzing,C.A.,Kurth,W.S.,Acuna,M.,MacDowall,R.J.,Torbert,R.B.,Averkamp,T.,Bodet,D.,Bounds,S.R.,Chutter,M.,…Tyler,J.(2013).The electric and magnetic field instrument suite and integrated science(EMFISIS)on RBSP.Space Sci.Rev.,179(1-4),127-181.https://doi.org/10.1007/s11214-013-9993-6
LeDocq,M.J.,Gurnett,D.A.,and Hospodarsky,G.B.(1998).Chorus source locations from VLF Poynting flux measurements with the Polar spacecraft.Geophys.Res.Lett.,25(21),4063-4066.https://doi.org/10.1029/1998GL900071
Li,W.,Bortnik,J.,Thorne,R.M.,and Angelopoulos,V.(2011).Global distribution of wave amplitudes and wave normal angles of chorus waves using THEMISwave observations.J.Geophys.Res.Space Phys.,116(A12),A12205.https://doi.org/10.1029/2011JA017035
Li,W.,Thorne,R.M.,Bortnik,J.,Tao,X.,and Angelopoulos,V.(2012).Characteristics of hiss-like and discrete whistler-mode emissions.Geophys.Res.Lett.,39(18),L18106.https://doi.org/10.1029/2012GL053206
Li,W.,Bortnik,J.,Thorne,R.M.,Cully,C.M.,Chen,L.,Angelopoulos,V.,Nishimura,Y.,Tao,J.B.,Bonnell,J.W.,and LeContel,O.(2013).Characteristics of the Poynting flux and wave normal vectors of whistlermode waves observed on THEMIS.J.Geophys.Res.Space Phys.,118(4),1461-1471.https://doi.org/10.1002/jgra.50176
Li,W.,Chen,L.,Bortnik,J.,Thorne,R.M.,Angelopoulos,V.,Kletzing,C.A.,Kurth,W.S.,and Hospodarsky,G.B.(2015).First evidence for chorus at a large geocentric distance as a source of plasmaspheric hiss:Coordinated THEMISand Van Allen Probes observation.Geophys.Res.Lett.,42(2),241-248.https://doi.org/10.1002/2014GL062832
Meredith,N.P.,Horne,R.B.,Thorne,R.M.,and Anderson,R.R.(2003).Favored regions for chorus-driven electron acceleration to relativistic energies in the Earth’s outer radiation belt.Geophys.Res.Lett.,30(16),1871.https://doi.org/10.1029/2003GL017698
Meredith,N.P.,Horne,R.B.,Li,W.,Thorne,R.M.,and Sicard-Piet,A.(2014).Global model of low-frequency chorus(f LHR Moullard,O.,Masson,A.,Laakso,H.,Parrot,M.,Décréau,P.,Santolik,O.,and Andre,M.(2002).Density modulated whistler mode emissions observed near the plasmapause.Geophys.Res.Lett.,29(20),36-1-36-4.https://doi.org/10.1029/2002GL015101
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Omura,Y.,Furuya,N.,and Summers,D.(2007).Relativistic turning acceleration of resonant electrons by coherent whistler mode waves in a dipole magnetic field.J.Geophys.Res.Space Phys.,112(A6),A06236.https://doi.org/10.1029/2006JA012243
Omura,Y.,Katoh,Y.,and Summers,D.(2008).Theory and simulation of the generation of whistler-mode chorus.J.Geophys.Res.Space Phys.,113(A4),A04223.https://doi.org/10.1029/2007JA012622
Omura,Y.,Nakamura,S.,Kletzing,C.A.,Summers,D.,and Hikishima,M.(2015).Nonlinear wave growth theory of coherent hiss emissions in the plasmasphere.J.Geophys.Res.Space Phys.,120(9),7642-7657.https://doi.org/10.1002/2015JA021520
Santolík,O.,Gurnett,D.A.,Pickett,J.S.,Parrot,M.,and Cornilleau-Wehrlin,N.(2003a).Spatio-temporal structure of storm-time chorus.J.Geophys.Res.Space Phys.,108(A7),1278.https://doi.org/10.1029/2002JA009791
Santolík,O.,Parrot,M.,and Lefeuvre,F.(2013b).Singular value decomposition methods for wave propagation analysis.Radio Sci.,38(1),1010.https://doi.org/10.1029/2000RS002523
Shprits,Y.Y.,Li,W.,and Thorne,R.M.(2006).Controlling effect of the pitch angle scattering rates near the edge of the loss cone on electron lifetimes.J.Geophys.Res.Space Phys.,111(A12),A12206.https://doi.org/10.1029/2006JA011758
Shprits,Y.Y.,Subbotin,D.A.,Meredith,N.P.,and Elkington,S.R.(2008).Review of modeling of losses and sources of relativistic electrons in the outer radiation beltⅡ:Local acceleration and loss.J.Atmos.Sol.Terr.Phys.,70(14),1694-1713.https://doi.org/10.1016/j.jastp.2008.06.014
Shue,J.H.,Chao,J.K.,Fu,H.C.,Russell,C.T.,Song,P.,Khurana,K.K.,and Singer,H.J.(1997).A new functional form to study the solar wind control of the magnetopause size and shape.J.Geophys.Res.Space Phys.,102(A5),9497-9511.https://doi.org/10.1029/97JA00196
Su,Z.P.,Xiao,F.L.,Zheng,H.N.,and Wang,S.(2011).CRRES observation and STEERB simulation of the 9 October 1990 electron radiation belt dropout event.Geophys.Res.Lett.,38(6),L06106.https://doi.org/10.1029/2011GL046873
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Cattell,C.A.,Breneman,A.W.,Thaller,S.A.,Wygant,J.R.,Kletzing,C.A.,and Kurth,W.S.(2015).Van Allen Probes observations of unusually low frequency whistler mode waves observed in association with moderate magnetic storms:Statistical study.Geophys.Res.Lett.,42(18),7273-7281.https://doi.org/10.1002/2015GL065565
Chen,L.,R.M.Thorne,V.K.Jordanova,C.-P.Wang,M.Gkioulidou,L.Lyons,and R.B.Horne(2010).Global simulation of EMIC wave excitation during the 21April 2001 storm from coupled RCM-RAM-HOTRAY modeling.J.Geophys.Res.,115,A07209.https://doi.org/10.1029/2009JA015075
Chen,L.J.,Thorne,R.M.,Li,W.,and Bortnik,J.(2013).Modeling the wave normal distribution of chorus waves.J.Geophys.Res.Space Phys.,118(3),1074-1088.https://doi.org/10.1029/2012JA018343
Chen,Y.,Reeves,G.D.,and Friedel,R.H.W.(2007).The energization of relativistic electrons in the outer Van Allen radiation belt.Nat.Phys.,3(9),614-617.https://doi.org/10.1038/nphys655
Funsten,H.O.,Skoug,R.M.,Guthrie,A.A.,MacDonald,E.A.,Baldonado,J.R.,Harper,R.W.,Henderson,K.C.,Kihara,K.H.,Lake,J.E.,…Chen,J.(2013).Helium,oxygen,proton,and electron(HOPE)mass spectrometer for the radiation belt storm probes mission.Space Sci.Rev.,179(1-4),423-484.https://doi.org/10.1007/s11214-013-9968-7
Gao,Z.L.,Su,Z.P.,Zhu,H.,Xiao,F.L.,Zheng,H.N.,Wang,Y.M.,Shen,C.,and Wang,S.(2016).Intense low-frequency chorus waves observed by Van Allen Probes:Fine structures and potential effect on radiation belt electrons.Geophys.Res.Lett.,43(3),967-977.https://doi.org/10.1002/2016GL067687
Goldstein,J.,De Pascuale,S.,Kletzing,C.,Kurth,W.,Genestreti,K.J.,Skoug,R.M.,Larsen,B.A.,Kistler,L.M.,Mouikis,C.,and Spence,H.(2014).Simulation of Van Allen Probes plasmapause encounters.J.Geophys.Res.Space Phys.,119(9),7464-7484.https://doi.org/10.1002/2014JA020252
Gurnett,D.A.,and O'Brien,B.J.(1964).High-latitude geophysical studies with satellite Injun 3:5.Very-low-frequency electromagnetic radiation.J.Geophys.Res.,69(1),65-89.https://doi.org/10.1029/JZ069i001p00065
Horne,R.B.,and Thorne,R.M.(1993).On the preferred source location for the convective amplification of ion cyclotron waves.J.Geophys.Res.Space Phys.,98(A6),9233-9247.https://doi.org/10.1029/92JA02972
Huang J.,Gu X.D.,Ni B.B.,Luo Q.,Fu S.,Xiang Z.,and Zhang W.X.(2018).Importance of electron distribution profiles to chorus wave driven evolution of Jovian radiation belt electrons.Earth Planet.Phys.,2(5),371-383.https://doi.org/10.26464/epp2018035
Jordanova,V.K.,Thorne,R.M.,Li,W.,and Miyoshi,Y.(2010).Excitation of whistler mode chorus from global ring current simulations.J.Geophys.Res.Space Phys.,115(A5),A00F10.https://doi.org/10.1029/2009JA014810
Katoh,Y.,and Omura,Y.(2011).Amplitude dependence of frequency sweep rates of whistler mode chorus emissions.J.Geophys.Res.Space Phys.,116(A7),A07201.https://doi.org/10.1029/2011JA016496
Kennel,C.(1966).Low-frequency whistler mode.Phys.Fluids,9(11),2190-2202.https://doi.org/10.1063/1.1761588
Kim,J.-H.,Lee,D.Y.,Cho,J.H.,Shin,D.K.,Kim,K.C.,Li,W.,and Kim,T.K.(2015).A prediction model for the global distribution of whistler chorus wave amplitude developed separately for two latitudinal zones.J.Geophys.Res.Space Phys.,120(4),2819-2837.https://doi.org/10.1002/2014JA020900
Kletzing,C.A.,Kurth,W.S.,Acuna,M.,MacDowall,R.J.,Torbert,R.B.,Averkamp,T.,Bodet,D.,Bounds,S.R.,Chutter,M.,…Tyler,J.(2013).The electric and magnetic field instrument suite and integrated science(EMFISIS)on RBSP.Space Sci.Rev.,179(1-4),127-181.https://doi.org/10.1007/s11214-013-9993-6
LeDocq,M.J.,Gurnett,D.A.,and Hospodarsky,G.B.(1998).Chorus source locations from VLF Poynting flux measurements with the Polar spacecraft.Geophys.Res.Lett.,25(21),4063-4066.https://doi.org/10.1029/1998GL900071
Li,W.,Bortnik,J.,Thorne,R.M.,and Angelopoulos,V.(2011).Global distribution of wave amplitudes and wave normal angles of chorus waves using THEMISwave observations.J.Geophys.Res.Space Phys.,116(A12),A12205.https://doi.org/10.1029/2011JA017035
Li,W.,Thorne,R.M.,Bortnik,J.,Tao,X.,and Angelopoulos,V.(2012).Characteristics of hiss-like and discrete whistler-mode emissions.Geophys.Res.Lett.,39(18),L18106.https://doi.org/10.1029/2012GL053206
Li,W.,Bortnik,J.,Thorne,R.M.,Cully,C.M.,Chen,L.,Angelopoulos,V.,Nishimura,Y.,Tao,J.B.,Bonnell,J.W.,and LeContel,O.(2013).Characteristics of the Poynting flux and wave normal vectors of whistlermode waves observed on THEMIS.J.Geophys.Res.Space Phys.,118(4),1461-1471.https://doi.org/10.1002/jgra.50176
Li,W.,Chen,L.,Bortnik,J.,Thorne,R.M.,Angelopoulos,V.,Kletzing,C.A.,Kurth,W.S.,and Hospodarsky,G.B.(2015).First evidence for chorus at a large geocentric distance as a source of plasmaspheric hiss:Coordinated THEMISand Van Allen Probes observation.Geophys.Res.Lett.,42(2),241-248.https://doi.org/10.1002/2014GL062832
Meredith,N.P.,Horne,R.B.,Thorne,R.M.,and Anderson,R.R.(2003).Favored regions for chorus-driven electron acceleration to relativistic energies in the Earth’s outer radiation belt.Geophys.Res.Lett.,30(16),1871.https://doi.org/10.1029/2003GL017698
Meredith,N.P.,Horne,R.B.,Li,W.,Thorne,R.M.,and Sicard-Piet,A.(2014).Global model of low-frequency chorus(f LHR
Olsen R.C.,Shawhan S.D.,Gallagher D.L.,Green J.L.,Chappell C.R.,and Anderson R.R.(1987).Plasma observations at the Earth’s magnetic equator.J.Geophys.Res.Space Phys.,92(A3),2385-2407.https://doi.org/10.1029/JA092iA03p02385
Omura,Y.,Furuya,N.,and Summers,D.(2007).Relativistic turning acceleration of resonant electrons by coherent whistler mode waves in a dipole magnetic field.J.Geophys.Res.Space Phys.,112(A6),A06236.https://doi.org/10.1029/2006JA012243
Omura,Y.,Katoh,Y.,and Summers,D.(2008).Theory and simulation of the generation of whistler-mode chorus.J.Geophys.Res.Space Phys.,113(A4),A04223.https://doi.org/10.1029/2007JA012622
Omura,Y.,Nakamura,S.,Kletzing,C.A.,Summers,D.,and Hikishima,M.(2015).Nonlinear wave growth theory of coherent hiss emissions in the plasmasphere.J.Geophys.Res.Space Phys.,120(9),7642-7657.https://doi.org/10.1002/2015JA021520
Santolík,O.,Gurnett,D.A.,Pickett,J.S.,Parrot,M.,and Cornilleau-Wehrlin,N.(2003a).Spatio-temporal structure of storm-time chorus.J.Geophys.Res.Space Phys.,108(A7),1278.https://doi.org/10.1029/2002JA009791
Santolík,O.,Parrot,M.,and Lefeuvre,F.(2013b).Singular value decomposition methods for wave propagation analysis.Radio Sci.,38(1),1010.https://doi.org/10.1029/2000RS002523
Shprits,Y.Y.,Li,W.,and Thorne,R.M.(2006).Controlling effect of the pitch angle scattering rates near the edge of the loss cone on electron lifetimes.J.Geophys.Res.Space Phys.,111(A12),A12206.https://doi.org/10.1029/2006JA011758
Shprits,Y.Y.,Subbotin,D.A.,Meredith,N.P.,and Elkington,S.R.(2008).Review of modeling of losses and sources of relativistic electrons in the outer radiation beltⅡ:Local acceleration and loss.J.Atmos.Sol.Terr.Phys.,70(14),1694-1713.https://doi.org/10.1016/j.jastp.2008.06.014
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