| |
Radial Propagation of the Substorm Injection Region
Key Data Sets
LANL Geosynchronous Energetic Particles,
CRRES MEB Energetic Particles
Key Results
1) Definitive results showing that substorm
injections propagate radially inward
2) Speed of propagation is approximately 24 km/s which may represent braking
of faster bursty bulk flows when they hit the inner magnetosphere.
Contact
Geoffrey D. Reeves, Los Alamos National
Lab., 505/665-3877, reeves@lanl.gov
More Information
Reeves, G. D., R. W. H. Friedel, M. G.
Henderson, A. Korth, P. S. McLachlan, and R. D. Belian, Radial propagation
of substorm injections, Proc. of ICS-3, Versailles France, 12-17 May, 579-584,
1996.
http://leadbelly.lanl.gov/reeves/papers/96_ics-3_lanl_crres.pdf
Abstract
A central controversy in substorm physics
is the location in the magnetosphere where substorm onsets actually begin.
An answer to this question would take us a long way toward understanding
the onset mechanism itself. The near-earth neutral line model of substorms
originally placed onset in the near-earth tail at a distance of perhaps
20 Earth Radii (Re) and the onset was attributed to magnetic reconnection.
In the near-earth neutral line model substorm effects propagated earthward
and into the ionosphere. Substorm injections were attributed to an earthward-directed
"convection surge". 
Two later observations called this picture into question. Studies of flows in the magnetotail showed that reconnection seldom occurred earthward of 25 Re and development of field new magnetic models that showed the auroral regions are magnetically connected to the tail at distances of 6-10 Re. This led to the opposing "current disruption" theory which suggested that the substorm onset started in the 6-10 Re region and that substorm effects propagated tailward where they might eventually cause reconnection through a tailward-propagating rarefaction wave.
The direction of propagation of substorm effects was a central debate in many case studies. To investigate this question thoroughly we analyzed substorm injections observed by two satellites separated in radius. Injections in which fluxes at all energies are enhanced simultaneously are referred to as dispersionless injections and they indicate that the spacecraft was within the injection region itself. We surveyed data from the entire CRRES mission to find dispersionless substorm onsets that were observed by both CRRES and any one of the three operating LANL geosynchronous satellites.
Twenty nine events were found that had unambiguous onset signatures in both satellites. We calculated the time delay between the injection at geosynchronous orbit and the injection at CRRES, inside geosynchronous orbit. In all cases CRRES observed the injection later indicating that the injection was propagating earthward. For nine of the 29 events CRRES and the LANL satellites were within one hour of local time - lined up in radius. For those events the delay time was essentially linearly related to radial separation and we could determine that the propagation velocity was approximately 24 km/s.
The implications were unambiguous. Inside geosynchronous orbit (R less than 6.6Re) substorm injections propagate earthward. Although this does not rule out the current disruption model as a possible onset mechanism it appears to better support the near-earth neutral line model. In particular this observation has played a key role in discussions of the relationship between reconnection and bursty bulk flows. Many investigators now suspect that bursty bulk flows (BBFs) may be the link between processes in the more distant magnetotail and the near-earth and auroral effects. In one scenario reconnection launches a fast flow of plasma (a BBF) that travels earthward at several hundred or even thousands of kilometers per second. When the BBF enters the near-earth region where the magnetic field is rigid it slows and is diverted. It is this slowing and diversion that causes the earthward propagating injection, formation of the "substorm current wedge", and the well-known auroral displays.
|
