Lesson 4: Magnetospheric substormskho.unis.no/misc/AGF351/Lectures/Aikio_lecture4.pdfMagnetospheric...
Transcript of Lesson 4: Magnetospheric substormskho.unis.no/misc/AGF351/Lectures/Aikio_lecture4.pdfMagnetospheric...
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Lesson4:Magnetosphericsubstorms
AGF‐351Op=calmethodsinauroralphysicsresearch
UNIS,24.‐25.11.2011
AnitaAikioUniversityofOulu
Finland
Photo:J.Jussila
IMFBz<0(southward)=>idealizedsteadyreconnec=on+plasmaconvec=on
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Energytodrivemagnetosphericplasmaconvec=on,field‐alignedcurrentsandaurora
comesfromthesolarwindApartoftheenergycarriedwiththesolarwindcanpenetrateintothemagnetosphere,especiallywhenIMFhasasouthwardcomponent.Onewidelyusedes?matefortheenergyinputistheepsilonparameterbyAkasofu:
ε =107vB2sin4(θ/2)lo2
wheretheunitofεisW,loisalengthof7RE,vandBarethesolarwindspeedandmagne?cfield,respec?vely,andθistheclockangleoftheIMFdefinedas
θ =arctan(By/Bz).
Severalothercouplingfunc?onsexistandthesehavebeencomparedtodifferentindicesofgeomagne?cac?vity,especiallytoAE,ALandDst(e.g.Newelletal,2007).
MagnetosphericsubstormsWhenIMFBz<0,someenergyisstoredasmagne?cenergyinthenightsidemagnetotail.Theexplosivereleaseofthisenergyiscalledmagnetosphericsubstorm.Thisanima?onshowsforma?onofthenear‐Earthneutralline(NENL)inthetail.
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Auroraduringsubstorm• Growthphase(A):arcsare
dri[ingequatorward.• Substormonset(B):themost
equatorwardarcbrightens.• Expansionphase(C):auroral
bulgeexpandspoleward,towardseastandwest=>westwardtravelingsurge,WTS.
• Lateexpansionphase(D):Omegabandstraveleastward.
• Recoveryphase(E):attheequatorwardedgeoftheeasternregion,dimpulsa?ngpatchesofauroraappear,dri[ingeastward.TheWTSdegenerates.Aurorarecoverstolowerla?tudes.
• Laterecoveryphase(F):inthemorningsectorpulsa?ngauroraproceedsforsome?me.
Currentsduringsubstorm
The beginning of the 1980’s: Is the substorm a directly driven or a loading-unloading process (or both)?
Directly driven electrojets or the substorm current wedge –or both?
ClauerandMcPherron(1974)
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Grouptask4:Whathappensduringsubstormphasesinthemagnetosphereandtheionosphere?
Fillintheform.
Extraques?on:Whichmagne?cindexisagoodmeasureofsubstormac?vity?
Configura=onalchangesinthemagnetosphereThe original NENL model of Hones
Hones(1979)
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Modelforsubstormonset
THEMIS Ground-Based Coordination – SuperDARN Workshop - 2006
THEMISMission Science Objectives
Primary – What macroscale instability causes substorm onset?
Secondary – How are radiation belt electrons energized?Tertiary – Dayside solar wind-magnetosphere coupling processes.
MHDscenarioforcross‐tailcurrentdisrup=on
!"#$% &'()*$$#"%
CHAPTER 5. PLASMA CONVECTION AND MAGNETOSPHERIC CURRENTS 66
Figure 5.15: Example of contours of constant plasma pressure (red) and flux tubevolume (blue) in the equatorial plane, which give rise to a field-aligned current to theionosphere (Auroral Plasma Physics, 2002).
Current conservation ! · j! = "! · j" gives
! · j! = "! · j" = "! ·!B#!p
B2
"
. (5.64)
It can be shown (Vasyliunas, 1970; Heinemann and Pontius, 1990) that this equationyields
#ion
eq
j!B
= "Beq
B2eq
·!peq #!V , (5.65)
the so called Vasyliunas equation. Here V is the di!erential flux tube volume (i.e. thevolume of a magnetic flux tube of unit magnetic flux). This volume is given by
V =$ ion
eq
ds
B, (5.66)
where the integral is extended along a magnetic field line from the equatorial plane tothe ionosphere. If, for simplicity, we assume that j! vanishes in the equatorial plane,eq. (5.65) gives the parallel current density in the ionosphere. This approach doesn’timply any generation mechanism, it just addresses diversion from the perpendicular tothe parallel current.
For the current to be diverted accordig to eq. (5.65), it is necessary that contours ofconstant pressure p and constant flux tube volume V in the equatorial plane are notaligned with each other. Thus e.g. reduction of plasma pressure in the equatorial plane(or change in flux tube volume) may lead to a field-aligned current (Fig. 5.15).
If the pressure gradient term in eq. (5.62) is small, the first term, the inertial term maydominate. In this case, the perpendicular current reduces to
j" = "!m
B2
dv
dt#B . (5.67)
TheNENLproducesfastflowsEarthward.ThevolumesoffluxtubesdecreasewhentheyapproachtheEarthandanoutwardpressuregradientdevelops.Thepressuregradientproducesbrakingoftheflows,dv/dt.Thisproducestheiner?alcurrentintheoppositedirec?onasthecross‐tailcurrent:
dv/dt B
j⊥