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Many, many popular science articles claim that if the Earth didn't have a magnetic field, then the much higher concentration of cosmic rays that reached the surface would cause health damage to humans. (It's easy to find a zillion such articles with a simple Google search, so I won't bother linking to them.) Some of these article just say that the higher concentration of radiation would be "dangerous", while others use stronger language like "catastrophic". But I couldn't find any articles that give any hard numbers, or even qualitative language more specific than broad terms.

If the Earth's magnetic field vanished overnight, then how much more radiation (in grays or sievert or whatever quantity this can be most easily answered) would Earth receive at sea level?

My guess is that the health issues wouldn't be that serious, since the Earth's magnetic field has changed orientation many times and left the Earth with a negligible magnetic field for thousands of years - and as far as I know, those events didn't cause mass extinctions or anything. (Although I believe that the last flip occurred before the evolution of anatomically modern humans, so we don't have any direct evidence about human life in the long-term absence of a geomagnetic field.)

A related question is about the claim that without a geomagnetic field, the stronger solar wind "would" or "could" or "might" (depending on the pop-sci article) strip away the Earth's atmosphere. If this is true, then I'd be curious what the time scale would be. It must be geologically long, given the many periods of thousands of years without a geomagnetic field during which the atmosphere was apparently negligibly affected.

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    $\begingroup$ I’m voting to close this question because it's about biological impacts due to a hypothetical scenario, rather than about physics. $\endgroup$
    – Kyle Kanos
    Commented Jan 30, 2023 at 17:46
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    $\begingroup$ @KyleKanos Well, it seems to me that to answer it, you need to figure out what the radiation flux would be without a geomagnetic field, which is a physics question. So it seems to me that the physics and the biology sides of the question are equally important. $\endgroup$
    – tparker
    Commented Jan 30, 2023 at 18:36
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    $\begingroup$ the question could trivially be rephrased as "what is the radiation does in the absence of the geomagnetic field" which clearly puts that in the realm of physics $\endgroup$
    – rfl
    Commented Jan 30, 2023 at 19:42
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    $\begingroup$ @KyleKanos I respectfully disagree. In order to answer this question, you need to answer both the first half (about the physics) and the second half (about the biology), so I wouldn't say that either half is uniquely "the crux" of the question. (And to be honest, I suspect that there are more physicists than biologists who are qualified to give a decent answer to both halves of the question, so I don't think it's very likely that I'd get a good answer on Biology SE.) Also, it seems to me that the impact of radiation on the human body is just as much a biophysics question as a bio question. $\endgroup$
    – tparker
    Commented Jan 31, 2023 at 1:23
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    $\begingroup$ I don't believe there is any clearly better Stack Exchange site for this interdisciplinary question, so I do not think it should be closed. Biology SE is a poor fit because there is too much physics/astrophysics/geophysics involved. Earth Science SE or Astronomy SE might be as good as Physics SE, but not obviously better, so I would leave it to the Question Poster to pick among the 3. $\endgroup$
    – David Bailey
    Commented Feb 4, 2023 at 2:26

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There is no expectation of any catastrophic biospheric effects from the loss of the Earth's magnetic field. The increase in human ionizing radiation dose would not be significant.

The current world average radiation dose from all natural sources (radon/thoron, internal, terrestrial, cosmic) is about 2.4 mSv/year. According to one study, even in the worst case, losing the Earth's magnetic protection would only increase the typical cosmic ray radiation Earth surface dose from 0.3 to 1.2 mSv/year, i.e. the total average dose could increase from 2.4 to 3.3 mSv/year. This is far smaller than natural geographic variations in background radiation. For example, the average natural background radiation dose in Florida is only 1.3 mSv/year, while in South Dakota it is 9.6 mSv/year.

If you live in northern latitudes, the biggest radiation risk from losing the magnetic field might actually be from increases in UV-B radiation due to enlarged ozone holes caused by increased high-energy proton fluxes in the upper atmosphere.

It is not clear if the net effect of the Earth's magnetosphere is to protect the Earth's atmosphere from erosion by the solar wind. Losing the Earth's magnetic field would increase some atmospheric losses but decrease others. The magnetic field protects the atmosphere from direct interactions with the solar wind, but a large magnetosphere also collects energy from the solar wind that can heat up atmospheric ions. Venus and Mars do not have geodynamos, and their atmospheric masses are respectively two orders-of-magnitude larger and smaller than Earth, but all three planets have very similar atmospheric escape rates. Losing the Earth's geodynamo should not have any large effect on the Earth's atmospheric erosion rate.

Note that unless you stop the solar wind, it is not possible for all protective magnetic fields around the Earth to disappear. Turning off the Earth's geodynamo is not sufficient. This paper on "Solar wind induced magnetic field around the unmagnetized Earth" suggests if the geodynamo stopped, the solar wind would quickly induce a magnetic field that could provide similar protection from cosmic rays. This is similar to the induced magnetosphere of Venus. The only obvious way to stop the solar wind is turn off the Sun, in which case worries about changes in cosmic ray radiation doses would not be our biggest concern.

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    $\begingroup$ Terrific answer, thanks so much. $\endgroup$
    – tparker
    Commented Feb 4, 2023 at 21:30
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The geomagnetic field is most effective at shielding the equator, and not very effective at shielding the poles. So we see aurorae frequently at the poles, rarely at the equator. The aurorae represent radiation blocked by the atmosphere, not the geomagnetic field. They don't extend to the surface, so the atmosphere is obviously blocking most the the radiation. There is not much latitude variation in cosmic radiation at the surface. Thus, we may conclude that the atmosphere is highly effective at blocking cosmic radiation, and we should not expect much variation with changes in the geomagnetic field.

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  • $\begingroup$ Do you have any thoughts on the proposition that without the geomagnetic field, the solar wind would eventually strip away the Earth's atmosphere (as some claim happened to Mars)? $\endgroup$
    – tparker
    Commented Feb 4, 2023 at 20:28
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    $\begingroup$ @tparker Modeling atmosphere loss is difficult. For Venus, with no (current) intrinsic magnetic field, it seems to have stripped the hydrogen but left the other elements. Mars has considerably less gravity, so it's a different case. $\endgroup$
    – John Doty
    Commented Feb 4, 2023 at 20:43
  • $\begingroup$ > "They don't extend to the surface, so the atmosphere is obviously blocking most the the radiation." Aurora does not extend to surface maybe for other reasons, such as high density of gas. Electric current in gas (plasma) radiates only when the gas is diluted enough. $\endgroup$
    – Ján Lalinský
    Commented Feb 5, 2023 at 14:03
  • $\begingroup$ The aurora are caused by accelerated electrons and protons from the geomagnetic tail of Earth's magnetosphere exciting atoms and molecules in the upper atmosphere. These precipitating particles don't hit the surface because they are at lower energies (i.e., only 10s of keV). Solar energetic particle (SEP) events can and do penetrate to Earth's surface, but only in the most extreme cases because their gyroradii are large enough and energies high enough. With a magnetic field, these would more often damage our ozone, atmosphere, and biosphere. $\endgroup$
    – honeste_vivere
    Commented Mar 13, 2023 at 13:08
  • $\begingroup$ ^without not with a magnetic field [facepalm] $\endgroup$
    – honeste_vivere
    Commented Apr 14, 2023 at 13:07

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