Simulating Outer Space

You can't simulate space in the laboratory!

The problem is that plasmas in outer space are too different from the plasmas we can create in the laboratory. In space, the sizes are much bigger (10s of kilometers versus 10s of centimeters), the densities are much lower (1 particle per cubic centimeter vs. 10¹³) and the magnetic fields are weaker (nanogauss versus kilogauss). Yet, over the years, a research community has evolved that explores basic plasma phenomena in the laboratory with emphasis towards understanding space plasma physics. What good are these experiments?

There are several problems with making measurements in space. Foremost, most of space is inaccessible to localized measurement; only measurement of optical properties is possible from which all other relevant plasma parameters must be interpreted. For those regions near the earth accessible to satellites there are other limitations. There is the time-space conundrum, where because the satellite is moving, temporal changes cannot be distinguished from spatial variations. In both situations, measurements are generally able to characterize the plasma on only one scale. Imaging reveals only macroscopic structure, while satellites measure microscopic phenomena, often on scales smaller than the Debye length. Simultaneous measurement of both scales is rare, making it difficult to interpret the interrelationship of the two. Moreover, large-scale space events of interest are infrequent and unique, and hence do not lend themselves to general models of space plasma dynamics.

It is in this context that laboratory experiments can fill a niche in trying to understand space plasmas, complementing and enhancing observations. One can make detailed measurements of both micro- and macroscale phenomena simultaneously. Moreover, lab experiments can be carefully controlled to give reproducible results. Despite the proscription of the introduction, several investigations have in fact been performed with the intention of "simulating" astrophysical phenomena to a limited degree. The essential limitations of satellite measurements mean that large-scale global structure must be pieced together from myriad highly localized measurements. Here, laboratory simulations can offer insight into both qualitative and sometimes quantitative global structure and dynamics. These experiments typically try to reproduce the large-scale features: currents, plasma flow, particle transport, macroscopic evolution, etc. They can also model interaction between regions with disparate plasma parameters. Although some small scale structure many be reproduced, more generally the stringent criteria on scaling mean that the dynamics at this level are likely very different from those in space.

More typically, however, these laboratory experiments generally do not directly simulate astrophysical phenomena because of the disparate scales. Instead, they address specific physics issues, studying individual processes relevant to the macroscopic space plasma phenomenon under investigation. Such studies might confirm heating processes or wave dispersion relations. These might confirm the postulated dynamics. More generally, however, these experiments can test theories, which can then be applied to problems relevant to space plasma physics. Again, the parameters of space cannot be scaled to the laboratory. Thus these studies confirm the dynamics in the laboratory regime, allowing testing of theoretical predictions on one scale, which lends credence to their interpretation of space observations. Importantly, such studies promote refinement of these models. Moreover, these studies can provide direction for future satellite measurements in characterizing the dynamics of the astrophysical regime.

At the University of New Mexico, this approach is used to explore:

Alfvén waves, waves that travel along magnetic field lines like waves on a string, are important in heating the solar atmosphere, Earth's magnetosphere and the formation of Earth's aurora.
Magnetic reconnection is the tearing and reconnection of the magnetic field. This is causes solar flares and major changes in Earth's magnetosphere.
Expanding plasmas, where a dense magnetized plasma expands into a less dense region. Here we are trying to understand the formation of astrophysical jets.

The two experiments used to study these phenomena at UNM are the HelCat device, and previously the Auburn Linear Experiment for Space Plasma Investigations (ALESPI), at Auburn University.

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