POLAR Provides First Experimental Documentation of Magnetic Reconnection

1. POLAR Provides First ExperimentalDocumentation of Magnetic Reconnection Reconnection, the fundamental process for transferring/exchanging energy in the universe, occurs near earth, at the sun, and at other star systems Magnetic reconnection is no longer just a theoretical construct with schematic illustrations The Hydra, EFI and MFE instrument teams, using POLAR particle and field data are producing maps of the reconnection site Discovery: The first experimental documentation that the process of collisionless magnetic connection works as theoretically foreseen and can be a quasi-stationary process

2. POLAR Provides First ExperimentalDocumentation of Magnetic Reconnection • MEASUREMENTS: • 3.5 hours of full resolution magnetic field, electric field, ion and electron particle data • ~2 million pieces of information Reconnection region will wash over POLAR as compression occurs • OBSERVATIONS: • POLAR orbit was tangential to the magnetopause current layer • Although stationary within its own moving frame of reference, the motion of the reconnection site relative to the moving spacecraft meant that the spacecraft, over time, wandered in and about the reconnection region, sampling and resampling the layers. • Superposed epoch processing of the ~2 million pieces of information yielded experimental confirmation of 12 theoretical predictions regarding collisionless magnetic reconnection. Position of POLAR (red) with respect to reconnection region (yellow) is dynamic, both are moving.

3. Fingerprints of Collisionless Reconnection:Magnetic field geometry Simulation based on theory Theory predicts that the magnetic field strength, shown in color, should approach zero at the reconnection site. The POLAR analysis resolves this expected magnetic field structure to a resolution approaching 1 km At one point during the interaction, the magnetic field strength is comparable to uncertainty levels for the magnetometer toward magnetosphere toward solar wind Superposed epoch image of reconnection site

4. Fingerprints of Collisionless Reconnection:The Electron Pressure “Ridge” Simulation of electron pressure ridge As material flows inward, particle density builds and the particles are heated. If the electrodynamics is to be steady, without resistivity, an electron pressure “ridge” will form at right angles to the inflowing material. POLAR observes this high pressure of electrons along the direction separating the solar wind from the magnetosphere side toward magnetosphere toward solar wind Superposed epoch image of electron pressure at reconnection site

5. Fingerprints of Collisionless Reconnection:Unmagnetized Electrons • A byproduct of magnetic reconnection is that particles initially located on a magnetic field line do not remain there once the reconnection has taken place. • Near a neutral point, at the magnetic null, charged particles will no longer be constrained to gyromotion about the field. • Low energy electrons, with their small gyroradius, are ideal tracers for this condition. • Shown is the ratio of electron gyroradius to the scale length of B. When this number is >1, individual electrons are no longer controlled by the magnetic field and are highly likely to move from one field line to another toward magnetosphere toward solar wind Superposed epoch image of degree to which electrons are controlled by the magnetic field

6. POLAR Provides First ExperimentalDocumentation of Magnetic Reconnection • INTERPRETATION: • Short spatial scales are present within this event • These short scales are compatible with the MHD description for thin current layers near a reconnection site expected when the ambipolar and Hall electric fields control the electrodynamics • IMPLICATIONS: • Unlike the traditional view of anomalous resistivity leading to magnetic reconnection, the present work demonstrates that collisionless effects can provide the necessary departure from ideal MHD to permit reconnection to occur.