Natural radio emissions
Each of the few million lightning flashes that occur on the Earth each day produce bursts of radio energy. At kilohertz frequencies reflection from the ionosphere allows these signals to propagate up to thousands of miles around the Earth, leading to continual intermittent crackling and popping. Particularly at night such signals can also travel within the ionosphere, but different frequencies travel at different rates, leading to so-called tweeks involving ringing or pinging. Signals can sometimes travel through the magnetosphere along magnetic field lines from one hemisphere to the other, yielding so-called whistlers with frequencies that fall off in a highly regular way with time. (Occasionally the signals can also travel back and forth between hemispheres, giving more complex results.) Radio emission can also occur when charged particles from the Sun excite plasma waves in the magnetosphere. And particularly at dawn or when an aurora is present an elaborate chorus of different elements can be produced—and heard directly on a VLF radio receiver.
Sunspots and solar flares make the Sun the most intense radio source in the solar system. Artificial radio signals from the Earth come next. The interaction of the solar wind with the magnetosphere of Jupiter produces radio emissions that exhibit variations reminiscent of gusting.
Outside the solar system, gas clouds show radio emission at discrete gigahertz frequencies from rotational transitions in molecules and spin-flip transitions in hydrogen atoms. (The narrowest lines come from natural masers and have widths around 1 kHz.) The cosmic microwave background, and processes such as thermal emission from dust, radiation from electrons in ionized gases, and synchrotron radiation from relativistic electrons in magnetic fields yield radio emissions with characteristic continuous frequency spectra. A total of over a million radio sources inside and outside our galaxy have now been catalogued, most with frequency spectra apparently consistent with known natural phenomena. Variations of source properties on timescales of months or years are not uncommon; variations of signals on timescales of tens of minutes can be introduced by propagation through turbulence in the interstellar medium.
Most radio emission from outside the solar system shows little apparent regularity. The almost perfectly repetitive signals from pulsars are an exception. Pulsars appear to be rapidly rotating neutron stars—perhaps 10 miles across—whose magnetic fields trap charged particles that produce radio emissions. When they first form after a supernova pulsars have millisecond repetition rates, but over the course of a few million years they slow to repetition rates of seconds through a series of glitches, associated perhaps with cracking in their solid crusts or perhaps with motion of quantized vortices in their superfluid interiors. Individual pulses from pulsars show some variability, presumably largely reflecting details of plasma dynamics in their magnetospheres.