Magnetospheric Multiscale (MMS), a NASA four-spacecraft constellation mission launched on March 12, 2015, will investigate magnetic reconnection in the boundary regions of the Earth's magnetosphere, particularly along its dayside boundary with the solar wind and the neutral sheet in the magnetic tail. The most important goal of MMS is to conduct a definitive experiment to determine what causes magnetic field lines to reconnect in a collisionless plasma. The significance of the MMS results will extend far beyond the Earth's magnetosphere because reconnection is known to occur in interplanetary space and in the solar corona where it is responsible for solar flares and the disconnection events known as coronal mass ejections. Active research is also being conducted on reconnection in the laboratory and specifically in magnetic-confinement fusion devices in which it is a limiting factor in achieving and maintaining electron temperatures high enough to initiate fusion. Finally, reconnection is proposed as the cause of numerous phenomena throughout the universe such as comet-tail disconnection events, magnetar flares, supernova ejections, and dynamics of neutron-star accretion disks. The MMS mission design is focused on answering specific questions about reconnection at the Earth's magnetosphere. The prime focus of the mission is on determining the kinetic processes occurring in the electron diffusion region that are responsible for reconnection and that determine how it is initiated; but the mission will also place that physics into the context of the broad spectrum of physical processes associated with reconnection. Connections to other disciplines such as solar physics, astrophysics, and laboratory plasma physics are expected to be made through theory and modeling as informed by the MMS results.

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Magnetospheric Multiscale Overview and Science Objectives

It is shown how satellite magnetometer data at a magnetopause penetration can be used to determine the vector normal to the magnetopause current layer and the magnetic-field component along this normal. According to theory such a component is a measure of the amount of field reconnection at magnetopause. Results from 22 Explorer 12 boundary penetrations are presented indicating normal-field components of less than 5 γ in two-thirds of the cases. Measured field variations within the current layer are presented to demonstrate the existence of two fundamentally different types of boundary structure, the rotational and the tangential discontinuity. The former of these permits a nonzero normal field component, whereas the latter does not. The rotational discontinuity seems to occur predominantly during magnetic storms and two of these cases, involving substantial normal-field components, provide compelling evidence that field reconnection takes place during the storm main phase. Finally, the calculated normal vector is compared with the normal to the surface of the Mead-Beard magnetosphere model.

Magnetopause structure and attitude from Explorer 12 observations.

https://ntrs.nasa.gov/api/citations/19660009062/downloads/19660009062.pdf

Hydromagnetic flow around the magnetosphere

Eleven passes of the ISEE satellites through the frontside terrestrial magnetopause were identified, where the plasma velocity in the magnetopause and boundary layer was substantially larger than in the magnetosheath. The nature of the plasma flow, magnetic field, and energetic particle fluxes in these regions were examined, with a view to determining whether the velocity enhancements can be explained by magnetic field reconnection.

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Evidence for magnetic field reconnection at the Earth's magnetopause

Geometrical considerations are presented to show that dayside magnetic merging when constrained to act only where the fields are antiparallel results in lines of merging that converge at the polar cusps. An important consequence of this geometry is that no accelerated flows are predicted across the dayside magnetopause. Acceleration owing to merging acts in opposition to the magnetosheath flow at the merging point and produces the variably directed, slower-than-magnetosheath flows observed in the entry layer. Another consequence of the merging geometry is that much of the time closed field lines constitute the subsolar region of the magnetopause. The manner in which the polar cap convection patterns predicted by the proposed geometry change as the interplanetary field is rotated through 360 deg provides a unifying description of how the observed single circular vortex and the crescent-shaped double vortex patterns mutually evolve under the influence of a single operating principle.

Dayside merging and cusp geometry

: Numerous phenomena observed at the surface of theearth, in the magnetospheric cavity, and in the neighborhood of its boundary are related to the flow of the solar wind. The extent to which this flow can be analyzed by the techniques of continuum fluid mechanics is discussed in relation to the observational and theoretical evidence. Numerous calculations have been made to determine the shape of the magnetosphere cavity, the location of the bow shock, and the variations in flow properties within the shock layer. Observations of auroral motions and alignments have indicated that flow patterns with appreciable velocities also exist inside the magnetosphere, thus suggesting appreciable coupling across the boundary. Analysis of such a boundary leads to a rate of reconnection of the field lines which decreases only logarithmically with the magnetic Reynolds number. The logarithmic dependence implies a rapid rate of reconnection which is insensitive to uncertainties in the effective conductivity of the plasma. Applying this result to the flow over the magnetosphere, the conclusion is that more than 10% of the magnetic field lines brought to the surface of the magnetosphere by the solar wind get broken and reconnected to the earth's polar field lines. (Author)

Aerodynamic aspects of the magnetospheric flow

The extent to which solar wind flow can be analyzed by the techniques of continuum fluid mechanics is discussed in relation to the observational and theoretical evidence. Numerous calculations have been made to determine the shape of the magnetosphere cavity, the location of the bow shock, and the variations in flow properties within the shock layer. Observations of auroral motions and alignments have indicated that flow patterns with appreciable velocities also exist inside the magnetosphere, thus suggesting appreciable coupling across the boundary. In general, an appreciable change in magnetic field direction across this boundary is expected. Analysis of such a boundary leads to a rate of reconnection of the field lines which decreases only logarithmically with the magnetic Reynolds number. The logarithmic dependence implies a rapid rate of reconnection which is very insensitive to the uncertainties in the effective conductivity of the plasma. Applying this result to the flow over the magnetosphere, it is concluded that more than 10% of the magnetic field lines brought to the surface of the magnetosphere by the solar wind get broken and reconnected to the earth's polar field lines. This rapid rate of reconnection is in rough quantitative agreement with the suggestion that themore » internal flow results from the field lines which extend into the interplanetary plasma being dragged by the wind. 48 references. (auth)« less

H. E. Petschek

Papers

Aerodynamic aspects of the magnetospheric flow

Aerodynamic aspects of the magnetospheric flow

AERODYNAMIC ASPECTS OF THE MAGNETOSPHERIC FLOW. Research Report No. 170