Observations of the ballooning

and interchange instabilities

in the near-Earth magnetotail

at substorm expansion onsets

Yukinaga Miyashita(STEL, Nagoya University)

If you have any comments or questions,please feel free to contact me(miyasita@stelab.nagoya-u.ac.jp).

■ Outline

I will review observations of changes associated withsubstorm dipolarization in the near-Earth magnetotailwhich is possibly related to (or consistent with)the ballooning instability.

•Introduction• Azimuthal auroral forms (bead-like structure)• Location and timing of dipolarization

•Observations of dipolarization• low-frequency fluctuations, pressure, flow, etc.

•Comparison between observations and theories•Summary

(The interchange instability may be seen at the front offast flows, but I will focus on the near-Earth dipolarization.)

from Lui [2004]

■ Azimuthal Auroral Forms

• A bead-like structure appears during the early stage of the development of an onset arc and extends in the azimuthal direction. [Elphinstone et al.,1995; Donovan et al., 2007]

• Spatial scale: <~10 km x ~1-2 h MLT

• Wavelength: ~100 km (m=100-300)

• The forms may correspond to the ballooning instability in the near-Earth magnetotail.

(The westward traveling surge may correspond to the ballooning instability [Roux et al., 1991].) Donovan et al. [2007]

1 min42 s

original difference

■ Timing & LocationVx ΔBz ΔPt

A statistical study ofmagnetotail evolutionaround substorm onsets[Miyashita et al., 2009]

Magnetic reconnectionbegins at X ~ -16 to -20 Re2 min before auroral

onset.

Dipolarizationbegins at X ~ -7 to -10 Realmost simultaneously(within 2 min).Then the dipolarization

regionexpands in all directions.

AuroralOnset

Y

X

-15

15-5 -32

increase decreaseearthward tailward

■ In Situ Observations of Dipolarization

• Background conditions (Where are low-frequency fluctuations seen?)• pressure gradient• thin current sheet• Low-frequency fluctuations are localized near the magnetic equator?

• Changes at the beginning of dipolarization• low-frequency fluctuations with a period of ~60 s• pressure gradient and anisotropy• β• flow

■ Background Pressure Gradient

• The pressure gradient is important for ballooning.• Large earthward pressure gradient at X > ~-10 Re.• The gradient is small at X < ~-10 Re.

from Wang et al. [2001]

■ Thin Current Sheet

• Current sheet thickness at X > ~-10 Re during the growth phase: ~100-1000 km [Sergeev et al., 1990]• The curvature radius becomes small. (~2000-6000 km, less than for dipole) [Korth et al., 1991; Pu et al., 1992]• These imply intense cross-tail current and considerable

taillike magnetic field configuration.

• A few min before onset the current sheet further thins [Liang et al., 2009].

• The considerable taillike region extends from X~-5 to -20 Re. [Miyashita et al., 2009] Liang et al. [2009]

Bx

Ni

Bz

Ti

■ Where are Magnetic Fluctuations Seen?

• Higher-frequency B fluctuations associated with dipolarization are larger near the equator. (< ~60 s period) [Shiokawa et al., 2005]

• Lower-frequency fluctuations are seen away from the equator, but the amplitude seems to be smaller.

ballooning, not interchange?

Shiokawa et al. [2005]

Eq

■ Roux et al. [1991] (1)

• The first paper suggesting that ballooning occurs associated with dipolarization and corresponds to the westward traveling surge.

• dispersionless flux increases local process

BH

BV

El

Ion

■ Roux et al. [1991] (2)

• low-frequency fluctuations of radial B• high electron fluxes when dipole-like• B and flux fluctuations are in anti-phase• earthward gradient of ion flux• E fluctuations• westward propagating waves• alternate localized FACs

consistent with theballooning instability

El

Ion

flux grad

E

FAC

radialδB

■ Identification of Ballooning Mode Waves (1)

• M. Saito et al. [2008]• Identification Criteria:• magnetic equator near midnight at X ~ -8 to -12 Re• perturbations: |δBx| > |δBz| > |δBy|• discrete low-frequency |δBx|

■ Identification of Ballooning Mode Waves (2)

• Ballooning mode waves were identified for the events with β > ~20 (consistent with theories incorporating kinetic effects and/or compression effects)

• δBx with 0.01-0.02 Hz 1-3 min before dipolarization and auroral onset

• no δBy

• ω ~ 0 in the plasma rest frame

M. Saito et al. [2008]

■ Identification of Ballooning Mode Waves (3)

• wavelength λy ~ 1000-6000 km ~ ion Larmor radius ~ 100-600 km in the ionosphere

• The wavelength was larger near auroral onset MLT.

M. Saito et al. [2008]

Erickson et al. [2000]1.growth phase: oscillations with ~60-90 s period (drift wave?)2.dawnward E and energy flow toward the ionosphere (S//) (trigger waves)3.dipolarization onset (cross-tail current reduction and SCW)4-5. explosive increase in S//, westward E B compression large FAC “explosive growth phase” ( wave?)

■ Trigger Waves

ED

ΔBH

S//

■ Low-Frequency Waves

• Waves appear at discrete frequencies.• ~0.01 Hz: a few min before dipolarization onset• higher frequencies:

just before or at onset [Liang et al., 2009; Park et al., 2010]

• Compressional δB// is dominant.

• δB┴ perpendicular to the azimuthal direction is also large (linearly polarized) for some events.

Park et al. [2010]

δB//

δB┴ φazimuthal

δB┴ ψ

■ Coupling of Alfvén and Slow Mode Waves

Phase differences of low-frequency waves (~45-65 s period)within the current sheet at auroral onset

•δBv and δ ED are 90 deg out of phase. (standing Alfvén)

•δB// and ion flux are 180 deg out of phase (slow mode)

Holter et al. [1995]

δB//

δBv

δED

δB//

Ion Flux

■ Plasma Pressure and β

• The plasma pressure increases, not decrease, after dipolarization onset. [Miyashita et al., 2010]

• nearly isotropic before onset [Lui et al., 1992; Pu et al., 1992]

• The ion β increases around onset. [Lui et al., 1992, Miyashita et al., 2010]

Miyashita et al. [2010]

Lui et al. [1992]

Bz

Pi

β

■ Pressure Gradient around Onset

• The density (pressure) gradient is large and earthward before onset relaxes after onset [Korth et al., 1991; Pu et al., 1992; Chen et al., 2003]

• Waves propagate westward at ~100-400 km/s [Chen et al., 2003]

time delay of E/T fluxes

dusk/dawn anisotropy (earthward grad P)

Chen et al. [2003]

■ Plasma Flow

• Earthward flows are dominant at the beginning of dipolarization, but tailward flows are also seen. The direction changes alternately. • Some dipolarizations begin with tailward flows.

M. Saito et al. [2010]

Bz Vx

■ Comparison with Theory (1)

• Several previous studies tested theoretical destabilization conditions with in situ observations.

• Satisfied• Roux et al. [1991], Korth et al. [1991], Pu et al. [1992] (incompressibility, u//=0)• Pu et al. [1997] (one of two modes)

• Unsatisfied• Ohtani and Tamao [1993] (compressibility)

• Whether the destabilization conditions are satisfied or unsatisfied depends on the assumptions and the neglected terms in formulation.

■ Comparison with Theory (2)

• Different theoretical studies made different assumptions.• equilibrium field shape• coupling of Alfvén and slow mode waves• compressibility• parallel velocity perturbation• wavelength (finite Larmor radius effect)• kinetic effects• pressure anisotropy• ionospheric boundary condition

different destabilization conditions• MHD theories: low β (< ~1) or high β• a kinetic theory: high β (> ~20)

Each assumption should be validated from observations (if possible) to understand the substorm triggering mechanism in the real magnetotail.

■ Summary

• Low-frequency waves appear 1-2 min before dipolarization and auroral onset. Their characteristics are consistent with the ballooning instability under the coupling of Alfvén and slow mode waves .

• However, further studies are needed to clarify whether or not these waves really trigger the dipolarization and auroral breakup.

• What causes the low-frequency waves just before dipolarization?• spontaneously generated there?• caused by fast flow or wave generated by reconnection in the midtail?

(causal relationship between reconnection and current disruption)

■ Statistical StudyVx ΔBz ΔPt

Magnetotail evolutionat substorm onsets

Reconnection at X ~ -18 ReDipolarization at X ~ -8 Re 2 min before onset.

Total Pressure (Pi + Pb)- largely decreases (energy is largely released) at -10> X > -18 Re seen more widely than fast earthward flows- increases at X > -10 Re (dipolarization)

Miyashita et al. [2009]

AuroralOnset

Y

X

-15

15-5 -32

increase decreaseearthward tailward

■ Ion Pressure• In the initialdipolarization region(X > -10 Re and2 < Y < 6 Re),the ion pressureincreases,not decreases,in association withdipolarization.

• In the surroundingregions, the ionpressure firstdecreases and thentends to increaseafter dipolarizationbegins.

Miyashita et al. [2010]

ΔBz Pp ΔPp

Y

X

-15

15-5 -32

increase decrease

■ Ion PressureIn the initialdipolarization region,the pressure increaseis largely contributedby high-energyparticles. (Pp (high) increases.)

Pp (low) increasesor decreases.Pb decreases.

In the surroundingregions, Pp (low)decreases.

Pp high does notchange.

Miyashita et al. [2010]

increase decrease

Y

X

-15

15-5 -32

ΔPp high ΔPp low ΔPb

■ Ion β

At the magnetic equator,the ion β enhances at thetime of the dipolarizationin the region of the initial dipolarization at X ~ -8

Re.

This high-β conditionis favorable for theballooning instability.

Miyashita et al. [2010]