High-Speed Ion Flow Braking and Substorms

Kazuo Shiokawa, Wolfgang Baumjohann, and Gerhard Haerendel

The magnetospheric substorm is an episode of conversion and release of magnetic energy that has piled up in the tail after an interval of magnetic reconnection at the dayside magnetopause. One of the most interesting questions of todays substorm research is the reason for ist sudden onset. The most accepted theory places origin and main conversion process at the formation of an X-type neutral line and associated near-earth tail field reconnection.

However, an argument against the near-earth neutral line (NENL) substorm model is the mismatch between the location of auroral breakup, mapping to 10 RE or less, and the 20-30 RE tailward distance of the NENL. But as Haerendel [1992] pointed out, there is no need for these two locations to map, since they are connected by fast ion flows [Baumjohann, 1993].

Shiokawa et al. [1997a] found that the earthward high-speed flows seen by AMPTE/IRM are not gradually decelerated, but are braked abruptly around 10-15 RE. The associated flow shear creates a current wedge of ~105A and dipolarizes the magnetic field. Shiokawa et al. [1997b] established that the sequence of events is ordered as shown below. A high-speed ion flow, supposedly created by an X-line around 20-30 RE, is braked at the boundary between tail-like and dipolar field, leading to dawnward current and further dipolarization. Only minutes later, substorm effects are seen in ionosphere and inner magnetosphere.

However, the scenario of Shiokawa et al. [1997b] is not a full substorm model. The braking effects last only for about 5 min and the current strength is comparatively weak. High-speed flow braking may often cause only a pseudo-onset or an initial brightening. For a full substorm feedback with the ionosphere or the inner magnetosphere may be needed.

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Haerendel, G.: Disruption, ballooning, or auroral avalanche: On the cause of substorms. In: Proc. 1st Intl. Conf. Substorms (ICS-1), pp. 417-420, ESA, Noordwijk, 1992.

Shiokawa, K., W. Baumjohann and G. Haerendel: Braking of high-speed flows in the near-earth magnetotail. Geophys. Res. Lett. 24, 1179-1182, 1997a.

Shiokawa, K., W. Baumjohann, G. Haerendel, G. Paschmann, J. F. Fennel, E. Friis-Christensen, H. Luehr, G. D. Reeves, C. T. Russell, P. R. Sutcliffe, and K. Takahashi: High-speed ion flow, substorm current wedge, and multiple Pi2 pulsations. J. Geophys. Res., 102, in press, 1997b.