1 Department of Physics and Astronomy, The University of Iowa, Iowa City, Iowa.
2 Mission Research Corporation, Nashua, New Hampshire.
3 Phillips Laboratory, Hanscom Air Force Base, Massachusetts.
4 Lockheed Martin Space Sciences Laboratory, Palo Alto, California.
5 Max-Plank-Institut fur Aeronomie, Katlenburg-Lindau, Germany.
6 The Aerospace Corporation, Los Angeles, California.
7 Johns Hopkins Applied Physics Laboratory, Laurel, Maryland.
8 Goddard Space Flight Center, NASA, Greenbelt, Maryland.
9 Institute of Space and Atmospheric Studies, University of Saskatchewan, Saskatoon, Canada.
10 Laboratoire de Physique et Chimie de l'Environnement Centre National de la Recherche Scientifique, Orléans, France.
11 Department of Physics and Astronomy, University of Leicester, Leicester, United Kingdom.
A model is presented according to which theta auroral arcs form after southward turnings of IMF Bz and/or large variations in By, following prolonged periods of northward IMF or very small Bz, with By 
Bz.
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.
After the theta aurora has formed, changes in IMF By or Bz readjust the merging configuration and continue the auroral pattern.
The transpolar arcs are on closed magnetic field lines that bifurcate two open sections of the polar cap and map to the outer plasma sheet.
Four theta auroral events were studied using data from the ISTP/GGS Polar and Wind spacecraft and the ground-based SuperDARN radars.
Observations that are correctly predicted by our model include:
(1) The formation and evolution of theta auroras observed by VIS are closely related to the IMF patterns measured by MFI.
(2) Both electrons and ions in the transpolar arc and poleward part of the nightside auroral oval exhibit similar spectral characteristics, identified by Hydra and CEPPAD.
The low energy electrons show counter streaming distributions, consistent with them being on closed field lines that magnetically connect to the boundary plasma sheet in the magnetotail.
(3) Ion composition measurements obtained from TIMAS show cold plasma outflows from the ionosphere and hot, isotropic magnetospheric ions in the two regions, also indicating transpolar arcs are on closed field lines.
(4) Large scale polar cap convection inferred by SuperDARN observations is well correlated with IMF patterns.
(5) Plasma convection in the transpolar arcs, inferred from EFI electric field and MFE magnetic field measurements, is sunward.
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