## 1. Introduction

[2] A goal of magnetic field reconnection research is to understand the site where magnetic field lines from two different topologies reconnect. One or more physical properties have been used to search for this reconnection site [*Vasyliunas*, 1975; *Mozer*, 2005; *Karimabadi et al.*, 2007; *Phan et al.*, 2007; *Shay et al.*, 2007; *Ji et al.*, 2008]. Possible properties of the reconnection site include: (1) A region of size ∼c/*ω*_{pe} in which (**E** + **U**_{e} × **B**) ≠ 0 that is embedded in a region of size ∼c/*ω*_{pI} where (**E** + **U**_{I} × **B**) ≠ 0, where *ω*_{pe} and *ω*_{pI} are the electron and ion plasma frequencies, **U**_{e} and **U**_{I} are the electron and ion bulk velocities, and **E** and **B** are the electric and magnetic fields; (2) a non-zero parallel electric field; **E**_{∥} (3) a large, spatially confined perpendicular electric field; (4) an electron beta much greater than one; (5) electromagnetic energy conversion, **j** · **E**, where **j** is the current density; (6) super-Alfvenic electron exhaust flow; (7) a density cavity suggestive of a non-zero divergence of the electron pressure; (8) a current channel of enhanced intensity, suggestive of a non-zero inertia term; (9) electron gyroradius ≤c/*ω*_{pe} associated with electron non-gyrotropy; (10) Non-zero heat flux emanating from the region.

[3] The purposes of the present paper are to consider the spatial relationships between some of these regions during asymmetric reconnection, to make the first quantitative comparisons between simulations and space data in such regions, and to emphasize the relative importance of the various terms in the Generalized Ohm's Law. It should be noted that asymmetric reconnection is probably the most prevalent form of reconnection at the sub-solar magnetopause, on the sun, and in all of astrophysics.

[4] It has been shown [*Newcomb*, 1958; *Longmire*, 1963; *Mozer*, 2005] that magnetic field lines moving at the **E** × **B**/B^{2} velocity produce a temporal evolution of the magnetic field identical to that found from Maxwell's equations unless

Because a pair of magnetic field lines moving towards each other at the **E** × **B**/B^{2} velocity cannot reconnect (they would just pass through each other), equation (1) must be satisfied at the reconnection site. This equation is a necessary but not sufficient condition for reconnection because satisfying it means only that the magnetic field geometry must be obtained from Maxwell's equations. Thus, finite regions where equation (1) is satisfied are candidate regions for reconnection and they have **E**_{∥} ≠ 0. Important parallel electric fields have been observed in space [*Mozer et al.*, 2003; *Mozer*, 2005].

[5] The Generalized Ohm's Law is written in a form convenient for analysis with simulation data as

where n is the plasma density, ∇ · **P**_{e} is the divergence of the electron pressure tensor and η is the resistivity. Equivalently, by writing **j** = ne(**U**_{I} − **U**_{e}) in the first term on the right side of equation (2), the Generalized Ohm's Law becomes

[6] Parallel electric field candidate reconnection regions are also regions in which the left sides of equations (2) and (3) are non-zero. However, these left sides can be non-zero in the absence of parallel electric fields because the perpendicular components of the left sides of equations (2) or (3) can be non-zero in the absence of a parallel electric field. Thus, as will be shown, regions where the ideal Ohm's law is non-zero are larger than the parallel electric field candidate reconnection sites and reconnection cannot occur in such larger regions.