Toward a Defensible Catalogue of Properties That Typify the Electron Diffusion Region of Collisionless Magnetic Reconnection

Details Toward a Defensible Catalogue of Properties That Typify the Electron Diffusion Region of Collisionless Magnetic Reconnection Toward a Defensible Catalogue of Properties That Typify the Electron Diffusion Region of Collisionless Magnetic Reconnection

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmsm41a1446s&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #SM41A-1446

Statistics

7815 Electrostatic Structures, 7835 Magnetic Reconnection (2723, 7526), 7894 Instruments And Techniques

Scientific paper

Many researchers are "looking" for "the" electron diffusion region layers with in situ data. Some publish papers about the statistics of such regions operationally "defined" to be electron diffusion regions, pending contradiction. Theorists also use a variety of recipes to identify diffusion regions in their numerical simulations. For example, some would say that the electron diffusion region is wherever E ≠ -U_e×B where U_e is the electron fluid velocity. Others in the "diffusion region hunt" identify the (ion) "diffusion region" as a proxy for the detection of the "electron diffusion region", in spite of the fact that any current carrying layer in space is an "ion diffusion region", whether it is attached to the reconnection site or not! Some observationalists insist that E∥ \ne 0 in the presence of enhanced perpendicular electric fields are de facto signals of entry into the electron diffusion regions. Unfortunately, such loose recipes also conclude that collisionless shock layers are electron diffusion regions, as are most discontinuities common in space plasmas. Soon with MMS an entire flotilla of instrumentation will be flown, dedicated to elucidating the observed properties of "the" electron diffusion region. Using reconnecting layers of PIC simulations with open boundary conditions for magnetic geometries with guide field, multiple islands, and anti-parallel current sheets, we have collated the properties of electron diffusion regions, known to be electron diffusion regions by tests that can be performed on simulation variables (like the vector potential, ∇ × E∥ and multiple time scales) that are not accessible to spacecraft bound observers. Significant agyrotropy in the electron pressure tensor is a common property of these certified electron diffusion regions. Agyrotropy reflects, in an integrated and observable way, that the thermal electrons have been recently demagnetized, so that a guiding center drift picture for the typical electron is untenable in that locale. A second property of such layers is the strong acceleration of the electron fluid velocity (in the presence of this agyrotropy) that is quasi-perpendicular to the magnetic field and approaches the local electron Alfven speed (1000's of km/s in the magnetopause regions) and should easily be detectable. While neither signature is exclusively the property of the electron diffusion region, their joint detection eliminates other locales of short scales where agyrotropy and field aligned flow are enhanced such as along the separatrices, or the anecdotal possibility of such strong electron bulk flows in the absence of non-gyrotropic pressure tensors of the thermal electrons. Both of these signatures are essential to the reconnection process as it is currently understood: agyrotropy for the demagnetization of the electrons and the strong electron flow as the inertially limited electron acceleration across a region where the magnetic field is not impeding the acceleration from the stagnation point. Strong perpendicular electric fields in the presence of parallel electric fields are found in the diffusion region but are also commonplace in other locales in space that are not electron diffusion regions.

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