1. 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).

2. ■ Outline I will review observations of changes associated with substorm dipolarization in the near-Earth magnetotail which 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 of fast flows, but I will focus on the near-Earth dipolarization.) from Lui [2004]

3. ■ 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 min 42 s original difference

4. ■ Azimuthal Auroral Forms A bead-like structure appears during the early stage of the development of an auroral bulge. Wavelength: ~130-600 km (m=30-135) Possibly correspond to ballooning instability in the near-Earth magnetotail Elphinstone et al. [1995]

5. ■ Westward Traveling Surge Possibly corresponds to the ballooning instability in the near-Earth magnetotail. Roux et al. [1991]

6. ■ Timing & Location VxΔBzΔPt A statistical study of magnetotail evolution around substorm onsets [Miyashita et al., 2009] Magnetic reconnection begins at X ~ -16 to -20 Re 2 min before auroral onset. Dipolarization begins at X ~ -7 to -10 Re almost simultaneously (within 2 min). Then the dipolarization region expands in all directions. Auroral Onset Y X -15 15 -5-32 increase decreaseearthward tailward

7. ■ 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

8. ■ 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]

9. ■ 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

10. ■ 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

11. ■ 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 BHBH BVBV El Ion

12. ■ 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 the ballooning instability El Ion flux grad E FAC radial δBδB

13. ■ 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|

14. ■ 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]

15. ■ 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]

16. 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 EDED ΔBHΔBH S//

17. ■ 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┴ φ azimuthal δB┴ ψ

18. ■ 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 E D 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 δEDδED δB// Ion Flux

19. ■ 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

20. ■ 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]

21. ■ 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] BzVx

22. ■ 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.

23. ■ 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.

24. ■ 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)

26. ■ Statistical Study Vx ΔBz ΔPt Magnetotail evolution at substorm onsets Reconnection at X ~ -18 Re Dipolarization 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] Auroral Onset Y X -15 15 -5-32 increase decreaseearthward tailward

27. ■ Ion Pressure In the initial dipolarization region (X > -10 Re and 2 < Y < 6 Re), the ion pressure increases, not decreases, in association with dipolarization. In the surrounding regions, the ion pressure first decreases and then tends to increase after dipolarization begins. Miyashita et al. [2010] ΔBzPpΔPp Y X -15 15 -5-32 increase decrease

28. ■ Ion Pressure In the initial dipolarization region, the pressure increase is largely contributed by high-energy particles. (Pp (high) increases.) Pp (low) increases or decreases. Pb decreases. In the surrounding regions, Pp (low) decreases. Pp high does not change. Miyashita et al. [2010] increase decrease Y X -15 15 -5-32 ΔPp highΔPp lowΔPb

29. ■ Ion β At the magnetic equator, the ion β enhances at the time of the dipolarization in the region of the initial dipolarization at X ~ -8 Re. This high-β condition is favorable for the ballooning instability. Miyashita et al. [2010]