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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⊥