Explaining Cosmic Rays

Well, before disappearing into a long session of thinking about some funny behaviour my strings are up to (more later) I’d like to do a quick report on the departmental colloquium that I went to just now. We had Gary Zank, the Director of the Institute for Geophysics and Planetary Physics of the UC system (he’s based out of UCR) give us a talk entitled: “Particle Acceleration in Cosmic Plasmas”, and it was quite fascinating (and very well presented).

It is all about the physics of cosmic rays (here’s a NASA link called Cosmicopia for a little background on them). Here is a section of the abstract he sent to us prior to the talk:

An outstanding problem in astrophysics is to explain the origin of the almost featureless cosmic ray spectrum extending up to energies of some 10²⁰ eV. A very small feature is apparent at between about 10¹³ – 10¹⁵ eV, the “knee.” In the late 1970’s, a suite of papers was published establishing the idea of diffusive shock acceleration for cosmic rays, essentially a first-order Fermi mechanism, which appeared to provide an explanation for the observed cosmic ray spectrum up to the knee. Diffusive shock acceleration is probably the most widely used particle acceleration mechanism in astrophysics and space physics, yet the theory is based on some stringent simplifications. The detailed [plasma] physics of the acceleration mechanism requires elucidation. We are fortunate in that very detailed observations of particle acceleration at shock waves, particularly in the guise of Space Weather, are providing considerable experimental insight into the basic physics of particle acceleration at a shock wave.

He gave us an overview of the remarkably detailed series of studies that his group has been carrying out (with the aid of an impressive multitude of computer simulations of the magnetohydrodynamics involved) in converting the various suggested acceleration mechanisms into detailed output that can be compared to experimental observations. Here’s a bit from their website:

The dynamical acceleration of particles at shocks waves propagating in the heliosphere is very poorly understood, yet shock waves are ubiquitous and almost all shocks are observed to energize ions and electrons. An understanding of particle acceleration at solar wind shocks has far reaching astrophysical implications. Furthermore, since energetic particles accelerated in either solar flares or in CME-driven shocks arrive at the Earth well before solar ejecta driven disturbances, an understanding of particle acceleration at interplanetary shocks is an integral part of the NSF and NASA Space Weather program.

He spoke quite a bit about shock wave mechanisms and how they work in supernovae as well. The physics of then how those “Galactic” cosmic rays wander around the galaxy and get to us (about a million years later) is quite intricate indeed, but the simulations give a lot of insight into how they end up here, and what we observe. All of this helps cover the physics of cosmic rays right up to the aforementioned “knee” in the spectrum. There’s still a big mystery about the kink in the spectrum that happens at the knee, and what happens for energies after that (and also the part of the spectrum called, yes, “the ankle”, which suggests an underlying completely separate part of the spectrum that spreads to ultra-high energies that is often a cause for speculations involving all sorts of exotic things lying in people’s notebooks).

Simply put, nobody has any idea where those super-high energy cosmic rays come from. That’s why this is on lots of people’s top ten list of outstanding problems in physics.

[Update: our Librarian, Sara Tompson, has constructed a partial bibliography for this (and other) talk’s material. It is here.]

-cvj