E5 Astrophysical Plasma Physics

Topic

The central element of this model is astrophysical plasma—a fully ionized gas in which electrons and ions move freely under the influence of electromagnetic fields, governed at all times by the Lorentz force.

The plasma frequency is the natural frequency at which this astrophysical plasma oscillates collectively as its electrons shift away from the ions and then return toward them. If the frequency of a given radiation falls below this plasma frequency, the radiation cannot propagate through the plasma at all; the plasma itself acts as a shield, blocking any radiation with a frequency that is too low.

The Larmor radius determines the degree to which the trajectory of a charged particle curves as it moves through a magnetic field within this astrophysical plasma; cosmic rays, for instance, trace paths with radii of curvature on the order of a parsec within the galaxy's magnetic field. Because the Larmor radius governs the curvature of each trajectory, the galaxy's magnetic field effectively dictates—particle by particle—the precise path each charged particle follows as it moves through the plasma.

The Debye length defines the scale at which the plasma screens an external charge: at scales larger than the Debye length, the plasma behaves as an electrically neutral whole, whereas at smaller scales, the individual charges of its electrons and ions remain distinguishable. This distinction means that a single astrophysical plasma can behave either as a neutral medium or as a collection of perfectly distinguishable individual charges, depending on the scale of observation. The cyclotron frequency is the frequency at which a charged particle gyrates around the magnetic field lines threading the plasma; this causes cyclotron emission—or synchrotron emission, in the relativistic limit—to occur precisely at multiples of that same gyration frequency. Thus, from the astrophysical plasma with which this document begins to the cyclotron and synchrotron emission with which it concludes—and encompassing the plasma frequency, Larmor radius, and Debye length—the physics of astrophysical plasma dictates that the same ionized gas responds differently depending on the scale, frequency, or magnetic field used to measure it.

ID:3926