AW: [ RadSafe ] Solar Flares

Rainer.Facius at Rainer.Facius at
Mon Oct 10 15:55:24 CDT 2005



The observations during the recent events are typical for the vast majority of solar particle events which usually indeed lead to a transient reduction of exposure to atmospheric ionizing radiation. Please note as an aside: the (optical) flare is just another (not always present) signature of the explosive releases of magnetic energy which accelerate the solar particles, hence solar flares as such are never a cause of (ionizing radiation) concern. 


The reason for the reduction is that the particle energies in this vast majority of solar particle events are too low to overcome the joint barriers of the geomagnetic shielding and the mass shielding provided by the 200 to 300 g/cm^2 air mass which the particles have to penetrate to reach aircraft at normal cruising altitudes. The higher the intensities of these low energy particle events, the larger is the magnetic field carried in these particle beams. Once they have reached and passed the earth, this moving magnetic field causes an additional shielding against the regular galactic cosmic ray particles which in turn produces the Forbush decrease and which is the larger the higher the intensities are. 


Yet, about once in a decade a sufficient fraction of solar particles reach energies well above 1 GeV. Then not only astronauts but also aircrew experience increased radiation exposures. Whereas in free space (outside the geomagnetic shielding) some of the known extreme events of this type were associated with a finite risk of early (deterministic) radiation sickness (depending of the space craft of course) none of these events so far would have implied exposures of aircrew violating the annual limits for radiation workers:


Kind regards, Rainer


Von: radsafe-bounces at im Auftrag von Susan Gawarecki
Gesendet: Mo 10.10.2005 21:37
Betreff: [ RadSafe ] Solar Flares

I've not received a digest since Oct. 6 - are things quiet or broken?

Below is a teaser and link to an interesting article. I expect this has
implications for radiation exposure during airline travel as well.

--Susan Gawarecki

Who's Afraid of a Solar Flare? Solar activity can be surprisingly good
for astronauts.

October 7, 2005: Last month, the sun went haywire. Almost every day for
two weeks in early September, solar flares issued from a giant sunspot
named "active region 798/808." X-rays ionized Earth's upper atmosphere.
Solar protons peppered the Moon. It was not a good time to be in space.

Or was it?

During the storms, something strange happened onboard the International
Space Station (ISS): radiation levels dropped.

"The crew of the ISS absorbed about 30% fewer cosmic rays than usual,"
says Frank Cucinotta, NASA's chief radiation health officer at the
Johnson Space Center. "The storms actually improved the radiation
environment inside the station."

Scientists have long known about this phenomenon. It's called a "Forbush
decrease," after American physicist Scott E. Forbush, who studied cosmic
rays in the 1930s and 40s. When cosmic rays hit Earth's upper
atmosphere, they produce a shower of secondary particles that can reach
the ground. By monitoring these showers he noticed, contrary to
intuition, that cosmic ray doses dropped when solar activity was high.


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