The Marscher & Gear model: Initially the magnetic field is compressed and electrons heated. With time the volume at the enhanced magnetic field (shock) filled with electrons grows larger. When the shock moves out, it expands adiabatically, and the flux decreases.
The Hughes, Aller & Aller model: The shock is formed inside the tau=1 surface of the jet, and while larger parts of the shock become visible, the flux starts to increase. The flux decrease is caused by adiabatic expansion, like in the M&G model (and initially already in the van der Laan model).
Last update: 2001-07-30 / mtt