A Model of Substorm Onset Initiated From the Ionosphere

L. H. Lyu and M. Q. Chen

Institute of Space Science, National Central University, Chung-Li, Taiwan, R.O.C.

In this study we try to show that:

A particular electric field distribution in the night-side ionosphere can trigger onset of substorm.

Preliminary report of this study has been presented in ICS-5 [Lyu and Chen, 2000].

1. Reflection of Alfven Wave at a Boundary with Anisotropic Conductivities

• M-I coupling is mainly accomplished by Alfven wave [e.g., Kan and Sun, 1985; Kan et al., 1988].
• Alfven wave can carry information such as electric field, electric current, and field-aligned current, along the magnetic field line. (Figure 1)
• Electric field in an Alfven wave will rotate left-handed with respect to the background magnetic field when the Alfven wave is reflected at a boundary with anisotropic conductivities (Hall conductivity & Pedersen conductivity). (Figure 2)
• Diffuse aurora can enhance anisotropic conductivities in the night-side ionosphere.
• Effective conductivities in a thin plasma sheet:
  • Enhanced convection during growth phase can lead to thinning of plasma sheet.
  • Electric currents in the thin plasma sheet consist of (1) diamagnetic current due to plasma pressure gradient at PSBL and (2) effective Hall current and Pedersen current due to non-adiabatic motion of ions (particular hot heavy ions) in the thin plasma sheet.
  • Effective Hall conductivity and Pedersen conductivity increase with increasing ion temperature or with decreasing current sheet thickness.
  • Effective anisotropic conductivities can also lead to left-hand rotation of electric field.

2. Our Model of Substorm Onset

• Non-uniform distribution of anisotropic conductivities at poleward boundary of diffuse aurora can lead to faster rotation of electric field in lower latitude (around 65 MLT) and slower rotation of electric field in higher latitude (around 75 MLT). (Figure 3)
• At the end of growth phase, we expect there will be a convergent movement of electrons (due to E cross B drift) near the midnight region, when the distribution of perpendicular electric field is similar to the one shown in Figure 3.
• The cluster of electrons (Figure 3) can result in a field-aligned potential difference with lower electric potential at the ionosphere boundary.
• Electrons are accelerated toward the plasma sheet by this potential drop and result in a downward field-aligned current in the lower-latitude pre-midnight region. (Figure 3)
• Another way to view this process:
  • The convergent movement of electrons can result in a convergent E_per distribution, where E_per is the component of electric field perpendicular to the local magnetic field.
  • Alfven waves lunched from such an electric field distribution will carry an intense downward field-aligned current at center part of this region and weak upward field-aligned current around this region (i.e., a region with non-uniform anisotropic conductivities).
• This downward field-aligned current in the pre-midnight region can lead to explosive thinning of near-earth plasma sheet in the midnight region [Ohtani et al., 1992].
• Explosive thinning of near-earth plasma sheet can enhance the effective anisotropic conductivities and lead to faster rotation of electric field in the near-Earth plasma sheet.
• Again, we expect that there will be a convergent movement of electrons (E cross B drift) at the tailward boundary of the explosive thinning region due to non-uniform electric field distribution as shown in Figure 4.
• The cluster of electrons (Figure 4) can result in a field-aligned potential difference with lower electric potential at the plasma-sheet boundary.
• Electrons are accelerated toward the ionosphere by this potential drop and result in an upward field-aligned current in the lower-latitude pre-midnight region. (Figure 4)
• This upward field-aligned current in the pre-midnight region can (1) cause the initial breakup of the most equatorward aurora arc; and (2) trigger dipolarization of near-Earth plasma sheet in the midnight region.
• Dipolarization of near-Earth plasma sheet will lead to formation of downward field-aligned current in the post-midnight region.

3. Discussion

3.1 Expansion Phase/Pseudo Break-up

• If there are enough hot plasmas being trapped in the thin current sheet prior to onset of substorm, then change of magnetic field configuration will change the pitch-angle distribution of these hot plasmas [e.g., Kaufmann, 1987]. These hot plasmas will then move toward ionosphere. A field-aligned potential drop will be setup by these hot plasmas at top of ionosphere due to convergent of magnetic field line [e.g., Wagner et al., 1980]. Electrons will be accelerated by this potential drop and result in aurora expansion onset.
• If there are not enough hot plasmas in the thin current sheet prior to onset of substorm, there will be no expansion phase but only a pseudo break-up of aurora arc in the lower latitude ionosphere.
• Positive feedback effects during expansion phase:
  • We recall that electric currents in a thin plasma sheet consist of (1) diamagnetic current due to enhanced pressure gradient at PSBL and (2) effective Hall current and Pedersen current due to non-adiabatic motion of ions (particular hot heavy ions) in the thin plasma sheet.
  • The effective Hall current and Pedersen current shown in Figure 5 become important when there are enough hot ions in the thin current sheet prior to onset of substorm.
  • The effective Hall current and Pedersen current will be reduced by the initial dipolarization, because these currents are very sensitive to current sheet thickness.
  • These processes provide an initial positive feedback effect for the dipolarization process.
  • Both pressure and pressure gradient in the near Earth plasma sheet will be reduced when the hot plasmas begin to move toward the ionosphere.
  • Reducing of plasma pressure gradient will reduce the diamagnetic current and further enhance the dipolarization process.
  • Reducing of plasma pressure in the near Earth plasma sheet can launch a rarefaction wave propagating tailward.
  • Plasma sheet thinning at the wave front of the rarefaction wave can enhance the effective Hall and Pedersen conductivities and result in upward field-aligned current due to convergent movement of electrons in the plasma sheet.
  • Recovery phase begins when the rarefaction wave front propagates into a region (at distant tail) without enough energetic ions to continue these positive feedback processes for dipolarization.
  • Based on Faraday’s law, current sheet thinning will take place outside of the field-aligned current wedges. Enhancement of effective Hall and Pedersen conductivities in these regions can lead to formation of upward field-aligned current due to convergent movement of electrons in the plasma sheet.
  • As a result, during expansion phase, our model expects that the dipolarization region should always expand from the midnight region to the morning and evening sectors with alternate distribution of upward and downward field-aligned currents in the morning sector.

3.2 Northward Turning of IMF

• Enhanced convection flow can slow down the left-hand rotation of electric field.
• Northward turning of IMF can slow down flow convection, and thus speed up the left-hand rotation of electric field in the ionosphere. (This process can be analogous to our daily experience that we have to decelerate our car in order to make a sharp turn on the road.)
• As a result, northward turning of IMF may trigger onset of substorm [e.g., Kamide, 1991] or lead to a pseudo break-up.

3.3 Magnetic Field Signatures during Explosive Thinning

Effective Hall current and Pedersen current in the thin plasma sheet can explain the observed magnetic field signatures during explosive thinning of near-Earth plasma sheet [Ohtani et al., 1992].

• Figure 6(a) sketches the expected electric current distribution in the near-Earth plasma sheet.
• Based on this electric current distribution, we can obtain the change of magnetic field configuration in the north lobe and south lobe of the explosive thinning region (i.e., the yellow region in Figure 6(a)).
• Figure 6(b) sketches the expected changes of magnetic field components in north lobe and south lobe near the explosive thinning region.
• The expected magnetic field signatures shown in Figure 6(b) are consistent with the observations given by Ohtani et al. [1992].

3.4 Comparison with Other Observations:

4. Summary

In summary, we have shown a possible nonlinear process in which substorm onset is initiated from the ionosphere. Our model can also provide a reasonable explanation for:

(1) The cause of explosive thinning;

(2) Onset of substorm after northward turning of IMF;

(3) A delay of expansion onset after initial breakup.

• Electrons, that are responsible for initial breakup, are accelerated by field-aligned potential drop all the way from plasma sheet to ionosphere.
• Electrons, that are responsible for expansion onset, move from plasma sheet to ionosphere at thermal speed. They are not accelerated until they reach to the top of ionosphere.

However, we still suspect that different substorm onset may be triggered by different nonlinear process. Therefore, we shall not exclude other substorm onset models at this moment.

5. References

• Kan, J. R., and W. Sun, Simulation of westward traveling surge and Pi2 pulsations during substorms, J. Geophys. Res., 90, 10,911, 1985.
• Kan, J. R., L. Zhu, and S.-I. Akasofu, A theory of substorms: Onset and subsidence, J. Geophys. Res., 93, 5624, 1988.
• Kamide, Y., The auroral electrojets: Relative importance of ionospheric conductivities and electric field, in Auroral Physics, p. 385, edited by C.-I. Meng, M. J. Rycroft, and L. A. Frank, Cambridge University Press, Cambridge, 1991.
• Kaufmann, R. L., Substorm currents: Growth phase and onset, J. Geophys. Res., 92, 7471, 1987.
• Lyu, L. H., and M. Q. Chen, A kinetic M-I coupling model with unloading instability at onset of substorm, in Proc. 5th International Conference on Substorms, St. Petersburg, Russia, 16-20 May 2000 (ESA SP-443, July 2000), p. 315, 2000.
• Ohtani, S., K. Takahashi, L. J. Zanetti, T. A. Potemra, R. W. McEntire, and T. Iijima, Initial signatures of magnetic field and energetic particle fluxes at tail reconfiguration: Explosive growth phase, J. Geophys. Res., 97, 19,311, 1992
• Wagner, J. S., T. Tajima, J. R. Kan, J. N. Leboeuf, S.-I. Akasofu, and J. M. Dawson, V-potential double layers and the formation of auroral arcs, Phys. Rev. Lett., 45, 803, 1980.