Comments on: Another cosmic-ray puzzle: Are iron nuclei bombarding Earth?

By: David Lambert

David Lambert — Mon, 12 Oct 2009 20:12:15 +0000

Is it possible for the rays detected by this experiment to simply be an unusual reading, such as a “freak” reaction of iron, something that does not normally happen?

By: Hank Glass

Hank Glass — Mon, 31 Aug 2009 14:51:46 +0000

Reply to dave: Detectors have been set up outside Earth’s atmosphere, on satellites such as PAMELA. These detectors are most efficient at studying lower energy cosmic rays. The problem one faces in studying the highest energy cosmic rays is the very low rate: about one particle per square kilometer per year at 1×10^19 eV. For these particles it is only possible to study them by using the Earth’s atmosphere as an amplifier (in a sense). That is, one primary particle is transformed into a very large number of secondary particles, which we then observe on the ground or via the fluorescent trails they produce. It is fairly inexpensive to cover large areas of the Earth with detectors. Covering that area with a detector in space would be prohibitively expensive.

By: dave

dave — Mon, 27 Jul 2009 10:18:32 +0000

So, if you could set up detectors outside Earth’s atmosphere, you’d be able to directly study the primary particles?

By: Michael M. Butler

Michael M. Butler — Fri, 24 Jul 2009 00:17:47 +0000

Hank: Thanks for your response. I look forward to more information as it becomes available.

By: Hank Glass

Hank Glass — Thu, 23 Jul 2009 18:25:04 +0000

Reply to Fred Kratt: The sun is known to be a source of very low energy cosmic rays, but it could not be responsible for the ultra-high energy cosmic rays we measure with Auger. The cosmic ray spectrum spans an enormous range in energy, from less than one GeV (1 GeV = 1 billion electron volts), up to more than 10 billion GeV. Solar cosmic rays are in the range up to maybe a few GeV. Auger studies the highest energy cosmic rays, which are not likely to come from the sun, or anywhere in our own galaxy.

Reply to Michael Butler: You are correct. Cosmic rays have been studied since their discovery in 1912. Direct measurements of low energy cosmic rays show that while more than 90% are protons, the rest are helium and heavier elements, including iron. The composition changes, however, as one looks at higher energy cosmic rays. At the highest energies, the measurement is more difficult because we do not measure the particles directly. Rather, we have to look at the properties of the air showers they produce and infer the composition based on this indirect measurement. The data from several experiments, including Auger, is not yet conclusive: some measurements are consistent with all protons, while Auger’s measurements suggest a mixed composition.

By: Michael M. Butler

Michael M. Butler — Tue, 14 Jul 2009 23:38:49 +0000

I seem to recall the idea that at least some cosmic rays are iron nuclei from long ago–say somewhere in the range 1957-1970. What’s the history of the notion? Am I misremembering that?

By: Fred Kratt

Fred Kratt — Mon, 13 Jul 2009 22:32:01 +0000

Could the data from a recent Lockheed solar probe indicating an iron solar surface below the photosphere shed any light on the source of these iron nuclei?