UCLA Local Group

The results of experiments conducted at the LAPD facility are of interest to both the basic plasma physics community and the space science community. Therefore, research results are published in Journals that reach both communities (Phy. Rev. Lett., J. Geophy. Res., Phy. of Plasmas, Geophy. Res. Lett.). Also research results are reported at the Fall meeting of the American Geophysical Union, the Annual meeting of the American Physical Society, Division of Plasma Physics and various international meetings such as the meeting of the International Association of Geomagnetism and Aeronomy. In addition the LAPD laboratory is engaged in organizing the fourth IPELS (Interrelationship between Experiments in the Laboratory and in Space) conference to be held in 1997.

Areas of Research:

Alfvén waves are low frequency fluctuations propagating in dense, magnetized plasmas. They exhibit both an electric and magnetic field and have large scale size along the magnetic field but microscopic size across the field. They are radiated by fluctuating magnetic field-aligned currents and, in turn, modulate such currents. Localized structures of Alfvén waves lead to acceleration and heating of both electrons and ions. They are of particular importance because they are the principal carrier of energy in macroscale plasmas.
The study of Alfvén waves is recognized to be crucial for understanding the solar corona, generation of the solar wind, the structure of magnetospheres, generation of the aurora, micropulsations of the geomagnetic field, induced currents in the Earth and man-made power and communication grids, and, on an astrophysical scale, the formation of structure in the intergalactic medium. In the interactions among plasma systems, plasma flows, and macroscopic energy exchanges between different plasma regions the Alfvén waves play an important and leading role. They mediate process on a macroscopic scale size as contrasted with high frequency waves such as Langmuir waves which modify the plasma distribution function locally.

As an illustration of the interest in the study of Alfvén wave research, we note that twenty six papers involving spacecraft measurements, computer simulation, and basic and applied theory were presented at the Fall AGU meeting held in San Francisco (Dec 11/15, 1995). It is noteworthy that only three experimental papers were presented, all from the LAPD laboratory. This is not surprising because the experimental study of Alfvén waves requires a special facility that is large in physical size and capable of producing dense magnetized plasmas. It is a tribute to the uniqueness of the LAPD facility that it is the only basic plasma device capable of providing conditions in which Alfvén wave phenomena can be explored with minimal influence of radial boundaries. The only other plasma devices capable of achieving such conditions are tokamaks used for thermonuclear fusion research. However, tokamak devices are intrinsically toroidal resulting in phenomena that are periodic along the magnetic field direction, a situation not present in naturally occurring plasmas.

In addition, Alfvén wave phenomena are intrinsically three dimensional. A complete picture of their spatial structure requires volume measurements. Such measurements are impossible from a single spacecraft and difficult to obtain using multiple spacecraft. However, volume data measurements are straight forward at the LAPD facility because of its computerized data acquisition and extensive three dimensional graphics capability.

Experiments involving Alfvén waves performed to date at the LAPD facility include the study of propagation of low frequency pulses and wave packets, the interference of waves from two sources, the verification of the existence of Alfvén wave cones, Landau damping of Alfvén waves, the spontaneous growth of drift Alfvén waves in a cross field density gradient, the interaction of Alfvén waves with low frequency density fluctuations , Alfvén wave radiation from modulated currents, and electron heating from Alfvén waves. Many more aspects of the Alfvén wave of interest to space plasma physicists remain to be studied, including field line resonances, reflection from boundaries, and generation of Alfvenic turbulence, a topic of special interest to astrophysicists.

Striations:

Another important feature of magnetized plasmas is the formation of magnetic field-aligned plasma structures. Such structures include spicules, granules and coronal arches in the sun, density striations in the auroral and equatorial ionospheres and filamentary magnetic structures in the galactic medium. These types of structures are important in the transport of wave energy through the plasmas and play a role in particle heating. The effect of field-aligned density structures (striations) on various plasma processes can be studied at the LAPD facility because techniques have been developed to produce these structures in a controlled fashion.
Experiments conducted in the LAPD laboratory involving striations include scattering of electromagnetic whistlers and the production of lower hybrid waves through mode conversion at a striation, spontaneous generation of drift Alfvén wave eigenmodes in a striation, and the interaction of Alfven wave cones with density striations.

Other Studies: