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On a previous thread (now locked) I was wondering about how, precisely, the Earth's magnetic field protects us from the solar wind. Posting this here because what I wrote in that thread is very wrong, and I think it's an interesting topic.
I had a hell of a time finding good information. I figured that either the particles would be captured by the magnetic field, which, if we ignore the magnetic field of the captured particles, would tend to deflect particles towards the poles and result in an increase in the overall flux. If, on the other hand, the particle trajectories are merely bent by the magnetic field and not captured, then it would act as a lens that would either focus or deflect the particles depending upon the charge and orientation of the magnetic field.
Neither of these made sense, so I hunted around for a description that actually did make sense. And it looks like the answer is that I was wrong to think that the magnetic field of the captured particles could be ignored. I finally found this description:
https://www.nap.edu/read/11188/chapter/5
It looks like what happens is the magnetic field captures a number of charged particles, and those captured particles form a diffuse plasma that surrounds the Earth. The densest part of this plasma is the Van Allen Radiation Belt, but it goes much further outward than this. This diffuse plasma, known as the magnetosphere, is held in place by the magnetic field and appears to be what actually deflects the incoming particles. This plasma creates an outward pressure which pushes against any incoming charged particles that would otherwise it. The edge of the magnetosphere is where the pressure of the solar wind is equal to the pressure of the magnetosphere.
As a result, the orientation of the field and the charge of the particles are actually irrelevant. The only things that matter are the strength of the magnetic field (which determines how far from the planet this plasma extends) compared against the strength of the solar wind and the interplanetary magnetic field. There are two "openings" of this plasma that are close to the Earth's magnetic poles which direct a number of particles towards the poles, but most of the incoming solar wind particles are deflected. These holes aren't right at the poles because of the complex interactions between the solar wind, this plasma, and the Earth's magnetic field. And apparently other holes frequently open up in the magnetosphere which allow solar wind to come through. I imagine this can happen as a result of solar flares, for instance.
I've never looked closely at the astrophysics of the Earth's magnetic field, so I'd be interested to hear any corrections.
I had a hell of a time finding good information. I figured that either the particles would be captured by the magnetic field, which, if we ignore the magnetic field of the captured particles, would tend to deflect particles towards the poles and result in an increase in the overall flux. If, on the other hand, the particle trajectories are merely bent by the magnetic field and not captured, then it would act as a lens that would either focus or deflect the particles depending upon the charge and orientation of the magnetic field.
Neither of these made sense, so I hunted around for a description that actually did make sense. And it looks like the answer is that I was wrong to think that the magnetic field of the captured particles could be ignored. I finally found this description:
https://www.nap.edu/read/11188/chapter/5
It looks like what happens is the magnetic field captures a number of charged particles, and those captured particles form a diffuse plasma that surrounds the Earth. The densest part of this plasma is the Van Allen Radiation Belt, but it goes much further outward than this. This diffuse plasma, known as the magnetosphere, is held in place by the magnetic field and appears to be what actually deflects the incoming particles. This plasma creates an outward pressure which pushes against any incoming charged particles that would otherwise it. The edge of the magnetosphere is where the pressure of the solar wind is equal to the pressure of the magnetosphere.
As a result, the orientation of the field and the charge of the particles are actually irrelevant. The only things that matter are the strength of the magnetic field (which determines how far from the planet this plasma extends) compared against the strength of the solar wind and the interplanetary magnetic field. There are two "openings" of this plasma that are close to the Earth's magnetic poles which direct a number of particles towards the poles, but most of the incoming solar wind particles are deflected. These holes aren't right at the poles because of the complex interactions between the solar wind, this plasma, and the Earth's magnetic field. And apparently other holes frequently open up in the magnetosphere which allow solar wind to come through. I imagine this can happen as a result of solar flares, for instance.
I've never looked closely at the astrophysics of the Earth's magnetic field, so I'd be interested to hear any corrections.