29 research outputs found
Final Report for DoE Grant DE-FG02-06ER54878, Laboratory Studies of Reconnection in Magnetically Confined Plasmas
The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF). The first configuration is characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be specifically relevant to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In this Report we review the general motivation of this work, the experimental set-up, and the main physics results
On the temperature of the solar wind
Solar wind provides an example of a weakly collisional plasma expanding from
a thermal source in the presence of spatially diverging magnetic field lines.
Observations show that in the inner heliosphere, the electron temperature
declines with the distance approximately as , which is significantly slower than the adiabatic expansion law . Motivated by such observations, we propose a kinetic theory
that addresses the non-adiabatic evolution of a nearly collisionless plasma
expanding from a central thermal source. We concentrate on the dynamics of
energetic electrons propagating along a radially diverging magnetic flux tube.
Due to the conservation of their magnetic moments, the electrons form a beam
collimated along the magnetic field lines. Due to weak energy exchange with the
background plasma, the beam population slowly loses its energy and heats the
background plasma. We propose that no matter how weak the collisions are, at
large enough distances from the source a universal regime of expansion is
established where the electron temperature declines as . This is close to the observed scaling of the solar wind temperature
in the inner heliosphere. Our first-principle kinetic derivation may thus
provide an explanation for the slower-than-adiabatic temperature decline in the
solar wind. More broadly, it may be useful for describing magnetized winds from
G-type stars.Comment: 9 pages, 3 figure
Final Report: Laboratory Studies of Spontaneous Reconnection and Intermittent Plasma Objects
The study of the collisionless magnetic reconnection constituted the primary work carried out under this grant. The investigations utilized two magnetic configurations with distinct boundary conditions. Both configurations were based upon the Versatile Toroidal Facility (VTF) at the MIT Plasma Science and Fusion Center and the MIT Physics Department. The NSF/DOE award No. 0613734, supported two graduate students (now Drs. W. Fox and N. Katz) and material expenses. The grant enabled these students to operate the VTF basic plasma physics experiment on magnetic reconnection. The first configuration was characterized by open boundary conditions where the magnetic field lines interface directly with the vacuum vessel walls. The reconnection dynamics for this configuration has been methodically characterized and it has been shown that kinetic effects related to trapped electron trajectories are responsible for the high rates of reconnection observed. This type of reconnection has not been investigated before. Nevertheless, the results are directly relevant to observations by the Wind spacecraft of fast reconnection deep in the Earth magnetotail. The second configuration was developed to be relevant to specifically to numerical simulations of magnetic reconnection, allowing the magnetic field-lines to be contained inside the device. The configuration is compatible with the presence of large current sheets in the reconnection region and reconnection is observed in fast powerful bursts. These reconnection events facilitate the first experimental investigations of the physics governing the spontaneous onset of fast reconnection. In the Report we review the general motivation of this work and provide an overview of our experimental and theoretical results enabled by the support through the awards
Particle acceleration by magnetic reconnection in geospace
Particles are accelerated to very high, non-thermal energies during explosive
energy-release phenomena in space, solar, and astrophysical plasma
environments. While it has been established that magnetic reconnection plays an
important role in the dynamics of Earth's magnetosphere, it remains unclear how
magnetic reconnection can further explain particle acceleration to non-thermal
energies. Here we review recent progress in our understanding of particle
acceleration by magnetic reconnection in Earth's magnetosphere. With improved
resolutions, recent spacecraft missions have enabled detailed studies of
particle acceleration at various structures such as the diffusion region,
separatrix, jets, magnetic islands (flux ropes), and dipolarization front. With
the guiding-center approximation of particle motion, many studies have
discussed the relative importance of the parallel electric field as well as the
Fermi and betatron effects. However, in order to fully understand the particle
acceleration mechanism and further compare with particle acceleration in solar
and astrophysical plasma environments, there is a need for further
investigation of, for example, energy partition and the precise role of
turbulence.Comment: Submitted to Space Science Review
Exploiting Laboratory and Heliophysics Plasma Synergies
Recent advances in space-based heliospheric observations, laboratory experimentation, and plasma simulation codes are creating an exciting new cross-disciplinary opportunity for understanding fast energy release and transport mechanisms in heliophysics and laboratory plasma dynamics, which had not been previously accessible. This article provides an overview of some new observational, experimental, and computational assets, and discusses current and near-term activities towards exploitation of synergies involving those assets. This overview does not claim to be comprehensive, but instead covers mainly activities closely associated with the authors’ interests and reearch. Heliospheric observations reviewed include the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) on the National Aeronautics and Space Administration (NASA) Solar Terrestrial Relations Observatory (STEREO) mission, the first instrument to provide remote sensing imagery observations with spatial continuity extending from the Sun to the Earth, and the Extreme-ultraviolet Imaging Spectrometer (EIS) on the Japanese Hinode spacecraft that is measuring spectroscopically physical parameters of the solar atmosphere towards obtaining plasma temperatures, densities, and mass motions. The Solar Dynamics Observatory (SDO) and the upcoming Solar Orbiter with the Heliospheric Imager (SoloHI) on-board will also be discussed. Laboratory plasma experiments surveyed include the line-tied magnetic reconnection experiments at University of Wisconsin (relevant to coronal heating magnetic flux tube observations and simulations), and a dynamo facility under construction there; the Space Plasma Simulation Chamber at the Naval Research Laboratory that currently produces plasmas scalable to ionospheric and magnetospheric conditions and in the future also will be suited to study the physics of the solar corona; the Versatile Toroidal Facility at the Massachusetts Institute of Technology that provides direct experimental observation of reconnection dynamics; and the Swarthmore Spheromak Experiment, which provides well-diagnosed data on three-dimensional (3D) null-point magnetic reconnection that is also applicable to solar active regions embedded in pre-existing coronal fields. New computer capabilities highlighted include: HYPERION, a fully compressible 3D magnetohydrodynamics (MHD) code with radiation transport and thermal conduction; ORBIT-RF, a 4D Monte-Carlo code for the study of wave interactions with fast ions embedded in background MHD plasmas; the 3D implicit multi-fluid MHD spectral element code, HiFi; and, the 3D Hall MHD code VooDoo. Research synergies for these new tools are primarily in the areas of magnetic reconnection, plasma charged particle acceleration, plasma wave propagation and turbulence in a diverging magnetic field, plasma atomic processes, and magnetic dynamo behavior.United States. Office of Naval ResearchNaval Research Laboratory (U.S.