“Ambipolar Diffusion” and Magnetic Reconnection Tsap Yu. T “Ambipolar Diffusion” and Magnetic Reconnection Tsap Yu.T. , Stepanov A.V. Crimean Astrophysical Observatory Central (Pulkovo) Astronomical Observatory Diffusion and dissipation of the magnetic field in the partially ionized plasma is an important physical process in many cosmic objects Star formation Mestel&Spitzer (1956) - magnetic force prevent star formation if the mass of the magnetic cloud M < 500 Msun since magnetic flux is conserved. 2. Dynamo – alpha-effect due to the magnetic reconnection 3. Flare energy release on the Sun and stars Zaitsev-Stepanov (1991) circuit model of flares Giardini Naxos 2010

What is the “ambipolar diffusion” in the weakly ionized plasma according to Mestel&Spitzer? Three-fluid approximation but “Ambipolar diffusion” (collisional plasma) is the motion of ions through a gas of neutral particles under action of Ampere’s force but classical (real) diffusion is caused by the inhomogenity (two types of diffusion)

Mestel and Spitzer (1956) considered star formation in magnetic dust cloud due to gravity Main problem: since the field is frozen into the contracting cloud the star formation becomes impossible because of the magnetic forces Solution: the distorted magnetic field is able to straighten itself, dragging the ions and electrons with it , while the bulk of the cloud contracts across the magnetic energy This scenario suggests that the magnetic field is frozen into ions, i.e. cloud Very popular expression beginning from Parker (1963)

What is the Joule dissipation in the collisional plasma? The Joule dissipation is the work of the electric field on electric current without the mechanical energy caused by Ampere’s fоrсe, which is equal to the thermal energy release due to ion-neutral, electron-neutral, and ion-electron collisions, i.e. Mestel&Spitzer(1956), Parker(1963) and others.

On the Magnetic Flux Conservation But Parker (1963) Magnetic flux does not conserved due to Joule dissipation

Cowling Conductivity (1957) The degree of ionization is not important as distinguished from Mestel&Spitzer(1956) Main suggestions: 1. Collisional plasma 2. Plasma must be non-stationary over time or space

Generalized Ohm’s Law Cowling conductivity

Sweet-Parker magnetic reconnection Sweet-Parker [Sweet 1958, Parker 1957] Main features: 1. L>>l – the current sheet is long and thin; 2. Plasma is evacuated from the current sheet because of the gas (magnetic) pressure. Plasma evacuation may be caused by Lorentz force due the strength of the magnetic field lines (effect of slingshot) B0 – perpendicular component

Scaling Laws and the Slingshot Effect

Energy Balance of Plasma

Thickness of the Current Sheet under Solar Chromospheric Conditions

Solar Chromospheric Ejections observations with SOT/Hinode in lineСа H Shibata etal.07, De Pontieu et al.07 Spicules Chromospheric jets Thickness < 200 km thickness - 150-300 km,

Hinode Ca H

Models Proposed model Magnetic pressure pushes plasma Shibata et al. 1992 Shock waves push plasma Токовый слой

Conclusions There are two types of ambipolar diffusion in plasma physics – real and formal. The Joule dissipation in partially ionized plasma is determined by collisions between neutrals and ions too. Approach proposed by Cowling (1959) is more adequate than approach proposed by Meste& Spitzer~(1956) in the case of the collisional plasma. Plasma evacuation is an effective cooling mechanism of the current sheet. The Sweet-Parker reconnection in partially ionized plasma can play an important role in the solar chromosphere.

Thank you! RT-22, Crimea