Solar X-rays:

Geomagnetic Field:

This link is for a web page with recordings of natural radio from various web sites. This includes recordings of sferics, whistlers, and tweeks.

Here is another explanation of what causes Whistlers - from research done at the  University college of Wales, Aberystwith (courtesy of Charles Wenzel, TECHLIB.COM):


What Causes Whistlers?

Alysson sent a couple of compelling  emails regarding whistlers that I have combined into one below. Although I  have no scientific basis for my thoughts, her explanation seems on target.  It is easy to imagine that a batch of high-velocity ions from the sun  arriving near a pole will spiral in the earth's magnetic field and will  radiate energy in the process. As they slow down it makes perfect sense that  the frequency will drop. Diffuse whistlers  might be due to a spread in arrival times for larger clouds of ions. I  personally don't see the connection to lightning (that's not saying much)  since the ions will travel in spirals simply due to their kinetic energy and  the magnetic field. Also, since all the "whistlers" I've picked up in Texas  have been motors, I wonder if the radiation is more of a near-field  phenomenon. Anyway, enough ranting by an uninformed hack; on to the  excellent emails:

I was just browsing your site, and found your VLF  Whistler Detection article ( 
In that page, you state that: 
Short  whistlers might be due to dispersion, but some whistlers last five  seconds so ordinary dispersion is probably inadequate an explanation. A  radio wave can travel a million miles in five seconds so to accumulate  that much difference in arrival times, the signal would have to travel  hundreds of millions of miles, assuming a pretty steep dispersion curve.  More likely, the whistler is an emission from the magnetosphere  triggered by the lightning pulse.
You may be interested to know that when I was at  University College of Wales, Aberystwith in the late 1970s, the Physics  department research budget was spent almost entirely in the pursuit of  this phenomenon. The research was led by my tutor, Dr A.D. Maude at that  time (sorry, no citation, just my memory of his lectures, but you might  like to try   What was found was that these were generated by the entrapment of  high-energy ions in the Earth’s magnetic field. The ions originate,  reasonably enough, at the solar surface. 
At the relatively low field densities of the  temperate latitudes in the high ionosphere, these particles will  penetrate deep into the field before losing enough energy to become  trapped. They are then deflected along the field, but due to their  momentum, travel in a corkscrew orbit. This acceleration of the charge  results, quite naturally in the emission of radio frequency radiation.
In shedding further energy, the helical orbit  decays becoming more linear (a longer orbital period), and thus a  change in the frequency of the emitted RF. The period of decay for these  high energy particles is of the order of 5 to 12 seconds, but the VLF  signals become difficult to detect without a high-altitude detector. (We  sent up rockets regularly).
I went to look at what remains of my old notes  last night. From what Dr Maude said, the lightning whistlers are  probably caused by ions generated within the storm itself undergoing  the same process, but since their energies are only a few MeV and in  (relatively) low atmosphere, the paths tend to be short (maybe 2-3  seconds only).
The best whistler times  are during periods of intense sunspot activity when large volumes  of solar ions are being pumped out with energies in the GeV and  possibly TeV ranges. My own theory (unsupported) is that the  thunderstorm RF discharges cause a resonant oscillation in the solar  ions which, as you say, will kick them into their helical orbit. The  solar whistlers are, though, observable at any time albeit rather  less frequently.
Dr Maudes rockets were built using (then) the  highest sensitivity receivers available (it was a very generous budget),  the whistlers only being detectable from stratospheric altitudes or  above. I find it amusing that it is now possible to put together a  receiver sensitive enough to make observation of this phenomenon for pocket money prices.
I hope this is of interest.
Alysson Rowan