22E01
A New Measure of the Dissipation Region in Collisionless Magnetic
Reconnection
無衝突磁気リコネクションの磁気拡散領域の新定義	 
Seiji Zenitani, Michael Hesse, Alex Klimas and Masha Kuznetsova
銭谷	  誠司, Michael Hesse, Alex Klimas, Masha Kuznetsova
National Astronomical Observatory of Japan, Division of Theoretical Astronomy
2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
国立天文台 理論研究部	  〒181-8588 東京都三鷹市大沢2-21-1
We propose a new measure to identify a small-scale dissipation region in collisionless magnetic
reconnection. Formulating the energy transfer in the electron's rest frame, we introduce an electron-frame
dissipation measure. The measure is an invariant scalar and extensively tested by particle-in-cellsimulations: the innermost region surrounding the reconnection site is accurately located in various cases.
We further discuss implications for nonideal MHD dissipation and reconnection structures.

1. Background
Collisionless magnetic reconnection is an
important process in many plasma systems,
ranging from solar-terrestrial environments toextreme astrophysical settings. It is known thatthe reconnection process is critically controlled
by the compact dissipation region (DR) near theX-point and therefore the structure of the DR has
been of great interest. Typically, the innermost
DR has been identified by the out-of-planeelectron nonidealness, E + v e x B ≠ 0. However,
it is not clear whether this is applicable to generalcases. In fact, recent numerical simulations
exhibited puzzling results and reconnection
scientists were at a loss how to find the DR [1-6].
2. A new measure of the DR
In this work, we present our solution to theseimportant problems. Considering the energy
transfer from the electromagnetic field to plasmas
in the electron’s fluid frame, we introduce theelectron-frame dissipation measure D e [7]:     De = j '⋅ E ' = γ e $% j ⋅ E + ve × B − ρc ve ⋅ E &'()

(1)

This measure is a Lorentz-invariant, i.e.
frame-independent scalar quantity. We test ourmeasure by two-dimensional particle-in-cellsimulations, and show that the measure accurately
identifies the innermost region surrounding thereconnection site (see Figure). From the MHD
viewpoint, it is related to the nonideal energy
conversion.
We further extend our theory to structures ofreconnection sites: the central dissipation region,
which is controlled by electron physics, ananti-dissipative electron jet, and an electron shock
are identified. Our measure provides a newperspective on understanding the inner structuresin collisionless magnetic reconnection.

Fig.1. 2D particle-in-cell simulation from Ref. [7]Top:A conventional measure [E + ve x B] yBottom: Our new dissipation measure De (Eq. 1)
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