by Caverdale » Jan 24, 2007 10:37 pm
The only article that comes to mind was published about 5 years ago in GEO2, the publication of the Geology and Geography Section, and authored by John Gookin, of NOLS in Wyoming. Much of the article was taken from a foreign publication. My library is still unorganized from a house remodeling last year so I'm unable to look it up.
I am also interested in the subject. For the past decade or so I have been conducting magnetometer surveys over known and unknown lava tubes. Interestingly, at the entrance of breathing lava tubes the magnetometer anomalies indicated many, many lightning strikes, enough so as to totally obliterate the expected anomalies. Away from the entrance lightning strikes are relatively few and randomly distributed. I have presented this result at two papers on magnetic geophysical prospecting for lava tubes at NSS Conventions. Just as you presume, I have mentioned that the lightning strikes are due to ionized air exiting the entrances. Not so, say experts in the audience. More than likely, the lightning channel is created by moisture in the cave air. I had thought that perhaps some radioactivity, such as radon, might create ions, but this was discounted by some people much smarter than I. Since than I have tried find references about ionization of cave air but without any luck.
I have never read about lightning entering and following a cave passage, except in cases where it followed wet ropes or metal cable ladders. However, there have been several mentions in the NSS News and Grotto publications reproduced in the Speleo Digest of cavers being struck by lightning in while inside caves. All of this is due to lightning traveling through the ground, not down the passage. There might even be a case of cavers observing a real lightning discharge inside the cave. This is a vague memory. Hearing thunder booms from lightning while far underground is not uncommon, either.
When measuring magnetic anomalies inside lava tubes, some of them are so intense that they could only be caused by the direct passage of current from lightning strikes on the surface directly to the walls of the tube and there is no evidence that the current came down the passage from the entrance. Unfortunately, I have been unable to find magnetic evidence of lightning strikes around breathing limestone caves because of the lack of magnetic material.
Dale Green
NSS 3669FE