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Reconnection

Magnetic reconnection is one of the processes that govern explosive phenomena in the Plasma Universe. What we mean by explosion in space plasmas is quick transformation in the form of energy, from magnetic to kinetic and thermal energies of plasma gas. Energy can quietly and steadily accumulate in the form of magnetic field. When this is explosively released into plasma gas, the dynamics of the energized gas occasionally lead to photon emission that can be remotely sensed by optical observations. This is how we sense the explosions, solar flares in the corona or substorms in planetary magnetospheres.

Here let us give a brief explanation of what magnetic reconnection is. Let us assume a simple case where anti-parallel magnetic field lines are facing each other across a current sheet. When the field lines are pinched against each other, the thickness of the current sheet in between is made smaller and the current density flowing there is elevated.

In the MHD approximation, which is very good at describing large scale dynamics of plasmas, this is not a problem at all. As the current sheet gets thinner and thinner, the current density is elevated more and more without any problem if the real space plasmas behave as MHD says. In reality, however, current density is sustained by the bulk flow velocity difference between ions and electrons. Intense current density is almost always sustained by fast flowing electrons. When the current density is enhanced, electrons somehow need to flow fast to match up with the required current density by the current sheet pinching.

The current sheet pinching is a part of large-scale dynamics that MHD can describe reasonably well, but what electrons do inside the current sheet is a kind of a picture about which the MHD approximation does not have the resolution to describe: Just because MHD does not know that it can be a problem, it is not in the position to worry about it at all. In real space plasmas, where deviations from MHD description show up sometimes and that often in most exciting situations, in order to accelerate the electrons that carry the intense current density, electric field needs to be, and is, set-up. This electric field at the same time creates a magnetic field component. In term of magnetic field line topology, this newly generated magnetic component changes the two anti-parallel field lines into two field lines forming an X-shaped geometry. This change in the topology is what is directly meant by the word reconnection, but its effects are by far more than that.


coalescence
movie
(11MB)
A movie showing results from one of the most recent huge-scale full particle simulations of magnetic reconnection. Triggering of magnetic reconnection in a cosmic situation would involve multiple X-lines and the generated multiple magnetic islands merge with each other. The merger, called magnetic island coalescence, proceeds in a dynamics manner and very eneregetic electrons are produced at the disappearing X-lines when the size of the magnetic islands subject to the coalescence process is large.

 

When the X-shaped field lines are made, plasma gas is expelled out of the center of the X (diffusion region), say, laterally. Then the evacuated space is filled by gas flowing into the diffusion region vertically. This vertical in-flow brings together with it the field lines to be reconnected. This is how gas and magnetic flux circulation around the X is maintained and how magnetic reconnection develops. Reconnection not only changes the field line topology but also ejects plasma jet laterally out of the diffusion region. There are plasma heating and particle acceleration associated with this ejection of reconnection jets. When the jet collides with the surrounding environment there arises another opportunity for plasma heating and particle acceleration. This collision process is another situation where local particle dynamics at the collision site and large-scale dynamics leading to the collision are tightly coupled.

While reconnection as a whole is a large-scale process, MHD approximation does not necessary capture the most interesting issues related to reconnection. This is very true when one thinks of those that have emerged through advances in reconnection research in recent years. (1) Within the diffusion region, which is the engine that drives the reconnection process, non-MHD effects are known to dominate and elucidating the micro-physics inside the diffusion region is one of the key issues. (2) The micro-physics inside the diffusion region does not stand alone. Instead, there is coupling between the micro-physics and the large-scale dynamics surrounding the diffusion region: The large-scale dynamics determine the boundary conditions for the micro-physics within the diffusion region. The micro-physics is what keeps the reconnection engine running, through which feedback on the spatio-temporal evolution of large-scale dynamics is created. (3) Explosive energy conversion is one of the main characteristics of reconnection. Then it is natural that particle acceleration is one of the heavy duty themes related to reconnection. The theme, however, cannot be addressed by MHD by definition, and requires both particle effects and large-scale dynamics to be taken into account at the same time.

In summary, magnetic reconnection is an important and attractive space plasma process that involves key micro-physics in a key region embedded inside a large area where reconnection as a whole develops. The fundamental understanding of reconnection requires quantifying the coupling between the micro-physics and large-scale dynamics, or the coupling between large-scale MHD dynamics and the particle effects that operate in the key regions to have crucial effects to the whole, such as micro-physics inside the diffusion region.

The SCOPE mission will address the following three fundamental questions on magnetic reconnection:

1 How is magnetic reconnection initiated? :

The triggering theme

2 How is the energy conversion attained in magnetic reconnection? :

From the triggering to large-scale development theme

3 How are non-thermal particles produced in magnetic reconnection? :

The final product theme

Only via simultaneous multi-scale observations to be performed by SCOPE/Cross-Scale, can we address these intriguing questions with the true expectation of giant steps toward the fundamental understanding of one of the most important space plasma processes. We are not only aiming at deeper understanding of magnetospheric phenomena but also determined to contribute to the Plasma Universe theme via hand-on-data basis quantifying scheme.

Can SCOPE sample in-situ data of reconnection? Yes, SCOPE can. Indeed one of the key elements in designing the SCOPE orbit was to have SCOPE immersed in “the tail box” where explosive reconnection in the magnetotail most frequently occurs. SCOPE will also encounter reconnection taking place on the magnetopause surface. Luckily enough, the two reconnections, one in the magnetotail and the other on the magnetopause, seem to have different characteristics in terms of ease to trigger, temporal behavior, and particle acceleration. How different environments/settings lead to these different characteristics is the question that sits at the center of the universal reconnection research. This question can be addressed via in-situ observations in the magnetosphere, and will be addressed by SCOPE.