We have presented a model to explain the magnetic topology and formation of theta auroras. Data from ISTP/GGS Polar and Wind satellites and the ground-based SuperDARN radars, shown here, support predictions of the model. This polar cap phenomenon is a result of the magnetosphere adjusting its configuration via evolving plasma convection in response to changes of the merging sites and merging rates on the dayside magnetopause, due to the changes of the IMF conditions during geomagnetically quite times. The model is based on Crooker's antiparallel merging hypothesis [Crooker, 1979]. It is not necessary to have active IMF Bz components during the whole process as suggested by Newell and Meng [1995]. Significant changes in By can also be effective. Furthermore, IMF By is an important factor for a strip of closed field lines peeling off from the flank of auroral oval during the onset of the theta aurora. The following conclusions summarize the necessary and sufficient conditions for the formation of theta auroras and main features of the model.
1.The precondition of this auroral pattern is a contracted polar cap as the plasma sheet expands into high latitude regions during prolonged period of northward IMF or |Bz| ~ 0.
2.The theta auroral arcs form approximate 10 mins after southward turnings of IMF Bz or polarity changes in By (or both).
3.The IMF has to remain at the new configuration with |By| 
|Bz| during the initial stage of the arc formation (~ 10 mins).
4.The arcs start on the dawn (dusk) side of the auroral oval and drift duskward (dawnward) across the polar cap for positive (negative) By in the northern hemisphere and conversely in the southern hemisphere.
5.After the theta aurora has formed, changes in IMF By or Bz readjust the merging configuration and continue the auroral pattern.
6.The transpolar arcs are confined on sunward convecting, closed magnetic field lines that map to the boundary plasma sheet in the magnetotail, bifurcating two open sections of the polar cap, achieving a polar cap morphology proposed by Frank et al. [1982].
During the evolution of the transpolar arc, when IMF Bz > 0 and |By| 
Bz, the arc can join the plasma sheet that expands farther into the polar cap, resulting in a configuration similar to the one described by Meng [1981] and Murphree et al. [1982].
7.Enough keV electrons must precipitate into the ionosphere to produce visible UV emissions.
Please send questions, comments, or suggestions about the paper to:
Shen-Wu Chang
Department of Physics and Astronomy, The University of Iowa, Iowa City, IA 52242
Phone:(319)335-3828; Fax:(319)335-1753;
swc@space-theory.physics.uiowa.edu