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  • 简介:Low-frequencychorusemissionshaverecentlyattractedmuchattentionduetothesuggestionthattheymayplayimportantrolesinthedynamicsoftheVanAllenBelts.However,themechanism(s)generatingtheselow-frequencychorusemissionshavenotbeenwellunderstood.Inthisletter,wereportaninterestingcaseinwhichbackgroundplasmadensityloweredthelowercutofffrequencyofchorusemissionsfromabove0.1fce(typicalordinarychorus)to0.02fce(extremelylow-frequencychorus).Thoseextremelylow-frequencychoruswaveswereobservedinaratherdenseplasma,wherethenumberdensityNewasfoundtobeseveraltimeslargerthanhasbeenassociatedwithobservationsofordinarychoruswaves.Forsuprathermalelectronswhosefreeenergyissuppliedbyanisotropictemperatures,lineargrowthrates(calculatedusingin-situplasmaparametersmeasuredbytheVanAllenProbes)showthatwhistlermodeinstabilitycanoccuratfrequenciesbelow0.1fcewhenthebackgroundplasmadensityNeincreases.EspeciallywhenNereaches90cm^-3ormore,thelowestunstablefrequencycanextendto0.02fceorevenless,whichisconsistentwithsatelliteobservations.Therefore,ourresultsdemonstratethatadensebackgroundplasmacouldplayanessentialroleintheexcitationofextremelylowfrequencychoruswavesbycontrollingthewavegrowthrates.

  • 标签: LOW-FREQUENCY CHORUS EMISSIONS anisotropic temperature instability
  • 简介:TheEmeishanlargeigneousprovince(hereafternamedbyitsacronymELIP)isthefirstacceptedlargeigneousregioninChina.Thecurrentstudytriestoreconstructthedensitystructureofthecrustinthisregion.Forthispurpose,weconductedthegravitysurveyalongan800-km-longprofile,whichstretchedlaterallyalongthelatitude27°NfromLijiang(Yunnanprovince)toGuiyang(Guizhouprovince).Thefieldworkincluded338gravitymeasurementsdistributedfromtheinnerzonetotheouterzoneofthemantleplumehead.Afteraseriesofgravityreductions,wecalculatedtheBouguergravityanomalyandthenconstructedthedensitymodelforELIPbyiterativeforwardmodelingfromaninitialdensitymodeltightlyconstrainedbywide-angleseismicreflectiondata.ThetopographyoftheMoho,herephysicallyinterpretedasadensitydiscontinuityof~0.4g·cm–3,graduallyrisesfromtheinnerzone(~50kmdeep)totheouterzone(~40km),describesathickercrustintheinnerzonethaninanyothersegmentoftheprofileandlargelyreproducestheshapeoftheBouguergravityanomalycurve.BoththeBouguergravityandthedensitystructureshowsignificantdifferenceswithrespecttotheinnerzoneandtheothertwozonesofELIPaccordingtothecommonlyacceptedpartitionoftheEmeishanarea.Athickeranddensermiddle-lowercrustseemstobethemainfeatureofthewesternsectionoftheprofile,whichislikelyrelatedtoitsmaficmagmaticcompositionduetomagmaticunderplatingofthePermianmantleplume.

  • 标签: Bouguer GRAVITY seismic reflection data forward