Protection from radiation by earths mag field

[NickSpain96]

I've heard that the Earth's magnetic field protects us from radiation but am curious as to how this works.

I assume it would have something to do with charged particle motion in a mag field because radiation is ionizing but I can't seem to nut out the mechanics behind it.
Thanks in advance.

 

[Evo]

I don't know if these answers are enough.

http://www.Newton.dep.anl.gov/askasci/phy00/phy00976.htm

http://www.windows2universe.org/glossary/plasmaspheric_gain.html

Wikipedia has a simple description of how the Earth's magnetic field deflects solar radiation.

Earth's magnetic field, predominantly dipolar at its surface, is distorted further out by the solar wind. This is a stream of charged particles leaving the Sun's corona and accelerating to a speed of 200 to 1000 kilometres per second. They carry with them a magnetic field, the interplanetary magnetic field (IMF).[20]

The solar wind exerts a pressure, and if it could reach Earth's atmosphere it would erode it. However, it is kept away by the pressure of the Earth's magnetic field. The magnetopause, the area where the pressures balance, is the boundary of the magnetosphere. Despite its name, the magnetosphere is asymmetric, with the sunward side being about 10 Earth radii out but the other side stretching out in a magnetotail that extends beyond 200 Earth radii.[21] Sunward of the magnetopause is the bow shock, the area where the solar wind slows abruptly.[20]

continued...

http://en.wikipedia.org/wiki/Earth's_magnetic_field#Magnetosphere

 

[Astronuc]

Adding to Evo's response, charged particles revolve in magnetic fields, which is the basis of cyclotrons (besides the accelerating potential) and mass spectrometers.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/maspec.html

The solar radiation and galactic cosmic radiation (mostly high energy protons and electrons) interact with the Earth's magnetic field, high above the atmosphere. Also, some particles collide with the nitrogen and oxygen in the atmosphere and produce spallation and transmutation reactions.

The solar wind actually helps shield the Earth from some of the cosmic radiation.

 

[Jon Richfield]

I encounter a good deal of assurance to the effect that without the protective effect of our magnetic field:
1: We would lose our atmosphere very quickly (spallation?)
2: We would be irradiated to frazzles in jig time.
3: Horrible things would happen to our industries.

Now, those surely all must be true, but I would love a coherent, authoritative, simple explanation of why we believe all this. Consider a few silly thoughts.

A: Sure, Mars has a weak magnetic field and this explains its practically non-existent atmosphere, right? So we can deduce that at present Venus is undergoing a transient magnetic field flip for a few fleeting centuries, because its atmosphere contains about as much nitrogen as ours, and roughly a hundred times as many oxygen atoms, right? And its magnetic field currently is well, a bit on the modest side for banishing spallation, no?

B: When our field traps charged particles, they either get stored in belts or get passed on. The particles in the belts go where? I could imagine a few ending up in outer space, but would expect most of them either to crash into the atmosphere near the poles, or elsewhere and fall to earth. If not, then we should see a steady accumulation of atoms accumulating up there, mostly hydrogen and a bit of He, probably outweighing our atmosphere in a few million years. But we don't see that, do we? So the reduction of our planetary accumulation of mildly energetic particles should not in the long term be much affected by loss of our mag field?

C: High-energy particles pay little attention to our magnetic field, and it doesn't affect photons very much either, nor do spallation neutrons, so the only relevant protection if any, is against charged particles of moderate or low energy. But our atmosphere absorbs those extremely efficiently within short stopping distances, so how important is that defence at all? There is an argument about defence for our ozone shield, but how well has it been established that the effect on our ozone is substantial, let alone that any plausible reduction in ozone at high levels would be important? How much harm has been traced to our ozone holes at their deepest?

D: Our atmosphere stops the most dramatically harmful radiation (UVC, X-rays and gamma) nearly stone cold, UVA is not much problem, and only a fraction of UVB is stopped by the ozone layer, without causing much obvious harm in the high latitudes. Most of it gets stopped by the bulk of the atmosphere. And yet a fair number of people live above something like 2/3 of the atmosphere without suffering obviously unacceptable radiation harm. With or without the ozone layer, why would the magnetic field seem so important? We have had thousands of mag field reversals, a dozen or so in the last million years alone, but there does not seem to be much evidence of correlation between polar inversions and extinction events.

E. OK, so solar storms could cause some nasty events to our power lines etc, though that is a mid-20th century emergent problem. If we learned that within 20 years the mag field was about to vanish (not just flip!) I bet that we could develop means of containing that problem affordably. If we learned tomorrow that the field had suddenly gone for twenty years, that would be worse, and a lot more expensive, but I bet we could handle it bearably and expensively within just a few years. I am unsure what would happen to our ionosphere, and we had better install a planet-wide set of magnetic beacons unless someone invents a gyroscopic compass or something equally exotic for campers or boy-scouts, but how big a problem would it be in the grand scheme of things?

Bottom line: Just why such a big worry about the magnetic field anyway? And why the emphasis on the sort of variable that I have described? I have asked this for years, and the answers have never varied in their lack of cogency.

 

[davenn]

A: Sure, Mars has a weak magnetic field and this explains its practically non-existent atmosphere, right? So we can deduce that at present Venus is undergoing a transient magnetic field flip for a few fleeting centuries, because its atmosphere contains about as much nitrogen as ours, and roughly a hundred times as many oxygen atoms, right? And its magnetic field currently is well, a bit on the modest side for banishing spallation, no?

No ... The major reason for a lesser atmosphere on Mars, is because of its much smaller size and lower gravitational field strength that is unable to hold an atmosphere. Spallation accounts for less than 1/3 of the loss of Mars' atmosphere.

Another reason Spallation is insignificant ... Take Venus, it has no magnetosphere, yet it is substantially closer to the sun than Mars and therefor subjected to a much stronger solar wind, yet losses due to solar wind are minimal.
The dominant loss process for Venus' atmosphere is through electric force field acceleration. As electrons are less massive than other particles, they are more likely to escape from the top of Venus's ionosphere.



cheers
Dave

some info from Wiki