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Magnetic reconnection
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===Anomalous resistivity and Bohm diffusion=== {{see also|Spitzer resistivity#Disagreements with observation}} In the Sweet–Parker model, the common assumption is that the [[magnetic diffusivity]] is constant. This can be estimated using the equation of motion for an electron with mass <math>m</math> and electric charge <math>e</math>: <math display="block">{d{\mathbf{v}} \over dt} = {e \over m}\mathbf{E} - \nu\mathbf{v},</math> where <math>\nu</math> is the collision frequency. Since in the steady state, <math>d{\mathbf{v}}/dt = 0</math>, then the above equation along with the definition of electric current, <math>{\mathbf{J}} = en{\mathbf{v}}</math>, where <math>n </math> is the electron number density, yields <math display="block">\eta = \nu{c^2 \over \omega_{pi}^2}.</math> Nevertheless, if the drift velocity of electrons exceeds the thermal velocity of plasma, a steady state cannot be achieved and magnetic diffusivity should be much larger than what is given in the above. This is called anomalous resistivity, <math>\eta_\text{anom}</math>, which can enhance the reconnection rate in the Sweet–Parker model by a factor of <math>\eta_\text{anom}/\eta</math>. Another proposed mechanism is known as the Bohm diffusion across the magnetic field. This replaces the Ohmic resistivity with <math>v_A^2 (mc/eB)</math>, however, its effect, similar to the anomalous resistivity, is still too small compared with the observations.<ref>{{cite book|last1=Parker|first1=E. G.|title=Cosmical Magnetic Fields|date=1979|publisher=[[Oxford University Press]]|location=Oxford}}</ref>
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