Many topics discussed here in Earth Science SE, tend to be about facts that are of consensus in the scientific community but not widely accepted by the general public. Instead, this one is widely accepted by the general public but it seems to generate doubts and debates between the leading scientists in the field.
We have always heard that Earth's magnetic field protects us from harmful cosmic radiation and solar wind, that would otherwise strip the atmosphere away and kill us all due to high doses ionizing radiation.
Well, all that seem to actually be under debate. Because (if I understand the issue correctly), the magnetic field deflects most of the incoming charged particles, but on the other hand it increases dramatically the cross sectional area through which Earth interacts with the solar wind.
This figure from Wikipedia's Magnetosphere page will illustrate the problem:
As you can see, most of the charged particles are deflected away from Earth (orange lines), but a fraction of them gets funneled towards the poles through the so called "polar cusps" (yellow lines), that's why auroras (a.k.a. northern lights) happen around the poles. As you can see, the size of the polar cusps "funnels" is comparable to the size of the Earth, therefore giving rise to the discussion if the magnetic field increases or decreases the amount of charged particles that hit the atmosphere: If the funnels are smaller than the Earth the magnetic field would decrease the amount of particles hitting the Earth, but if the funnels are larger the particles hitting us would increase.
These charged particles are usually cited as relevant due to two reasons:
- They are harmful to life by increasing the dose of ionizing radiation that reach the surface. Such radiation in turn generates mutations in the DNA.
- They can hit molecules in the atmosphere fast enough to send them to outer space, leading to atmospheric escape and the thinning of our beloved atmosphere.
The first point is only partially true, because the atmosphere itself is an extraordinary shield against cosmic and solar radiation. Therefore, even in the eventual shutdown of Earth's magnetic field, not much radiation would reach the surface anyway. That's perhaps one of the reasons why nobody has been able to link magnetic reversals to mass extinctions. More on that is discussed in this question.
The second point is also questionable, because although the magnetic field does redirect the charged particles to the poles, it is unclear if the amount of particles currently hitting polar areas is more or less than the amount that would hit the entire Earth in the absence of a magnetic field. Notably, atmospheric escape would be equally bad if it happen distributed over the globe or if it is concentrated at the poles.
It is important to note that in this question I refer to the intrinsic magnetic field. Because, all bodies with atmosphere that are exposed to the solar wind develop an induced magnetic field. This induced magnetic field is observed in Venus, Mars and even comets. However, it is much weaker that Earth's intrinsic magnetic field.
The lack of intrinsic magnetic field is often blamed as one of the reasons that Mars lost its atmosphere. However, Venus have always challenged that idea: Venus have an atmosphere much thicker than Earth's and it doesn't have an intrinsic magnetic field. This article from Space.com describes how this discussion was restarted due to:
recent observations that show Mars and Venus are losing oxygen ions from their atmospheres into space at about the same rate as Earth
And they cite Robert Strangeway (a leading scientist in the field) saying "My opinion is that the magnetic shield hypothesis is unproven". Strangeway also presented a poster on last year's AGU fall meeting describing the same skepticism: "Does an Intrinsic Magnetic Field Inhibit or Enhance Planetary Ionosphere Outflow and Loss?"
In that work he backs the above cite of Space.com with actual numbers:
The oxygen ion outflow is frequently assumed to be a proxy for the loss of water from the planetary atmosphere. In terms global outflow rates for the Earth the rate varies from $10^{25}$ to $10^{26}$ s$^{−1}$, depending on geomagnetic activity. For both Venus and Mars global rates of the order $5 \times 10^{24}$ s$^{−1}$ have been reported.
Giving some credit to his skepticism, his work was cited in a 2018 paper written by most if not all the leading scientist behind the MAVEN probe that have been orbiting Mars since 2014. There they say:
our expectation is that stripping of atmospheric gas by the solar wind is minimized by the presence of a magnetic field (e.g., Hutchins et al., 1997), although observations of escaping ions at Earth suggest that this may not be strictly correct (Strangeway et al.,2017).
The second author of that paper: Dave Brain, also gave a very interesting TED talk where he mentions the topic: What a planet needs to sustain life (starting at minute 7:31).
I'm fascinated by the topic, but my understanding of the physics of Earth's magnetic field is minimal. So I wonder: What evidence is there supporting that the intrinsic magnetic field really protects Earth's atmosphere and its living organisms?
In other words: In the absence of models and speculations of how the intrinsic magnetic field interacts with charged particles: Is there any reason to believe that a stronger magnetic field offers more protection than a weak or negligible one?
