Sunday, June 28, 2015
The Case For Particle Accelerator Experiments in Near Earth Space
What do we know about the aurora? Well, we know that high above Earth's surface, high-energy electrons and ions rain down on the upper atmosphere - spiraling along the planet's magnetic field lines. When the particles strike the upper atmosphere they excite (ionize) nitrogen or oxygen molecules producing the glowing display known as the aurora.
While computer simulations and numerical analysis have dominated auroral studies, such as at the Geophysical Institute of the University of Alaska, there is a less well known way to study this phenomenon. This entails injecting electrons artificially using a space -borne accelerator.
Few are aware that NASA has previously flown devices that fired beams with energies of a few to tens of kiloelectron volts (keV). In a new study, Marshall et al (Journal of Geophysical Research, Space Physics, 2014) have used computer simulations to explore the capabilities of a small but powerful particle accelerator positioned at an altitude of 300 km and aimed at the atmosphere.
This accelerator would feature energies three orders of magnitude greater than the actual ones already used by NASA. Thus, it would be capable of generating a beam with particles in the 0.5 - 10 MeV range via pulses lasting 0.1 microsecond and carrying 0.1 amp of current. The simulated specs for the device are based on the Compact Accelerator for Space Science (COMPASS) currently being designed by the SLAC National Accelerator Laboratory and the nonprofit research institute SRI international.
The authors note that deploying such a device would offer many scientific opportunities to study the behavior of particles in the upper atmosphere. We'd see, for example, the paths of injected electrons trace out the arcing field lines of Earth's magnetic field - illustrating how they bend and snap during intense geomagnetic activity caused by magnetic storms.
We'd also see how visible, aurora-like emissions would be generated as the particles descend into the Earth's atmosphere and strike atmospheric molecules. Make no mistake this is exciting stuff especially for those involved in space physics. If such a particle accelerator could shed more light on magnetic substorms it would be more than welcome.
In most current substorm models it is required that there exist some dynamo action which sends currents to specific regions to provide a Lorentz force: (J ⊥ X B). At a particular altitude then, these J ⊥ currents can trigger a substorm. In the proper space physics (magnetospheric) context, a “neutral wind” arises from a force associated with the neutral air of the Earth’s atmosphere (e.g. Hargreaves, The Solar-Terrestrial Environment, Cambridge Univ. Press, 1992, p. 24). This force can be expressed (ibid.):
F = mU f
where f is the collision frequency. It is also noted that this wind blows perpendicular to the geomagnetic field (ibid.)
If one solves for f above, and uses the magnitude of magnetic force (F = qvB) where B is the magnetic induction, and v the velocity one arrives at two horizontal flows for electrons and ions moving in opposite directions. mU f = qvB = (-e) vB = (e) vB
v1 = mU f / (-e) B and v2 = mU f / (e) B
A terrific articulation of the process has been done in a paper by Syun-Ichi Akasofu ( Auroral Substorms: Paradigm Shifts in Research', Eos Transactions, Vol. 91, No. 31, August 3, 2010, p. 269) wherein the author points out:
"The transfer mechanism of solar wind energy to the magnetosphere...is now known to be a dynamo process that converts the kinetic energy of the solar wind to electrical energy on the magnetopause - because most auroral and geomagnetic phenomena are various manifestations of energy dissipation processes."
This amounted to a brilliant recognition of the energy aspects but more work needs to be done, especially in developing self-consistent models for "loading" and "unloading" processes. These refer to two distinct energy paradigms, the B-v (driven) and E-J . In the former observations of the magnetic field and motions govern the models, in the latter it is the magnitude of currents or current densities (J ).
Ideally, use of the type of particle accelerator proposed may actually one day be able to replicate actual auroras such as the one we observed and photographed in Chena Hot Springs, Alaska in March, 2005 e.g.
Meanwhile, Marshall et al's computer modeling shows the injected beam would leave a visible, glowing trail that could be measured from the ground with a peak emission occurring at an altitude of about 44 km. How to explain? As the incoming electrons are jerked around by the molecules of the atmosphere they'd also emit x-rays that could be seen from high altitude balloons operating at around 44 km - thereby providing a further prediction for the model.
One drawback, we need to be sure to inform the public via popular science articles or blog posts so as not to incur a hysterical reaction - such as been the case with the HAARP (High Frequency Active Auroral Research Program). See more on this program here:
As noted in the above link:
"HAARP was a target of conspiracy theorists, who claimed that it was capable of modifying weather, disabling satellites and exerting mind control over people, and that it was being used as a weapon against terrorists. Such theorists blamed the program for causing earthquakes, droughts, storms and floods, diseases such as Gulf War syndrome and chronic fatigue syndrome, the 1996 crash of TWA Flight 800, and the 2003 destruction of the space shuttle Columbia. Commentators and scientists say that proponents of these theories are "uninformed", because most theories put forward fall well outside the abilities of the facility "
Once again, then, we see the emergence of really nutty conspiracy theories which must always be judiciously and relentlessly separated from those which have validity. Simple conflation of all conspiracies is, of course, the easiest path to take - but also the intellectual coward's and non-critical thinker's. We cannot be that blasé and lazy.
The way to do this for space physics developments - like the proposed particle accelerator- is to keep the public informed, and also hope that current science education can keep up with current scientific advances! Briefly put, we need much more attention to high school physics - as well as college physics!