[Kan] 

Harang discontinuity originated in the ionosphere is proposed as a possible direct cause of dipolarization in the near-Earth plasma sheet at the substorm expansion onset [posted on 3-5-01; modified on 3-6-01]:

• Harang discontinuity appears to be a characteristic feature of the substorm growth phase as indicated by the SUPERDARN data summarized in a personal statement by Bristow posted on this Website under item #5.
  
  
• Harang discontinuity is the zonal distortion of enhanced convection in the midnight sector originated from the nonuniformity of the ionospheric Hall and Pedersen conductances.
  
  
• Hanrang discontinuity intensifies in the pre-dipolarization phase between the growth phase and the expansion onset.
  
  "A pre-dipolarization phase is unavoidable." A personal statement of Vasyliunas posted on this Website under item #5.
  
  
• The enhanced zonal convection originated by the Harang discontinuity in the ionosphere leads to a whiplash of rapid azimuthal convection in the near-Earth plasma sheet (~6-10 Re).
  
  
• The resulting rapid azimuthal convection in the near-Earth plasma sheet leads to a local reduction of plasma pressure to allow the magnetic tension to snap earthward, resulting in rapid piling up of magnetic flux on the earthward side of the near-Earth plasma sheet. This is proposed to be the direct cause of dipolarization at the substorm expansion onset.
  
  The proposed direct cause of dipolarization is consistent with a consensus statement of the Substorm-Onset Workshop (posted on this Website under item 5): "Any explanation for substorm expansion must account for plasma pressure reduction in near-Earth (~6-10 Re) plasma sheet."  
  
  
• The Harang discontinuity originated in one hemisphere can cause dipolarization in the near-Earth plasma sheet to result in substorm expansion onset in both hemispheres with different intensities.
  
  
• In summary, development of the Harang discontinuity in the ionosphere produces a whiplash of rapid azimuthal convection in the near-Earth plasma sheet, which in turn produces a backlash of rapid pile up of magnetic flux on the earthward side (~6-8 Re) of the near-Earth plasma sheet. This is proposed here as the direct cause of the dipolarization at the substorm expansion onset.