How the Earth's Magnetic field deflects the solar wind

[kimbyd]

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.

 

[davenn]

I finally found this description:
https://www.nap.edu/read/11188/chapter/5

Sadly, that needs a user account to access

Are you able to copy and paste the part of the text that is relevant please

 

[davenn]

As a result, the orientation of the field and the charge of the particles are actually irrelevant.

I would strongly suggest that that isn't correct. Tho the orientation of the Earth's magnetic isn't changing greatly.
The orientation of the solar winds' magnetic field does change and it's during the changes of the solar winds' field
and it's interaction with the Earths' field that geomagnetic storms occur.
Those that forecast geo-storms and us others that want to observe/photo aurora look out for that change in the
field direction of the solar wind.

SpaceWeather.com -- News and information about meteor showers, solar flares, auroras, and near-Earth asteroids

down the page a bit and on the left, shows the monitoring of the interplanetary magnetic field orientation

It also links to data sites

ace-mag-24-hour.gif (640×512) (noaa.gov)

Real Time Solar Wind | NOAA / NWS Space Weather Prediction Center

 

[kimbyd]

Sadly, that needs a user account to access

Are you able to copy and paste the part of the text that is relevant please

Sorry for the delay. That is odd. Perhaps it's region-linked?

Anyway, in responding to this, I realized that I did get it wrong again. Re-reading that link, here's what I think happens:

1) When the solar wind interacts with the Earth's magnetic field, the solar wind particles are actually captured by the magnetic field much of the time. There probably is a bow shock around the captured plasma which deflects more of it, but the magnetic field itself does a lot of the deflecting.
2) How this works is that the captured plasma is directed towards the poles, but then often reconnects with the magnetic field lines which trail behind the Earth rather than traveling down towards the poles. I imagine this is because of the linear momentum the incoming plasma has. Once the plasma is on the trailing magnetic field lines, it just flows past, as those lines stretch behind the Earth without reconnecting.

So I think a better explanation for this is that the interaction between the solar wind, interplanetary magnetic field, and the Earth's magnetic field drastically alters the magnetic field around the Earth, making it look sort of like a teardrop with the blunt end facing the Sun and "pinched" at the north and south poles where some particles travel towards said poles. Here's the relevant diagram and quote describing this process in the link above:

Drawing (not to scale) showing the structure of Earth’s magnetosphere and illustrating the process of reconnection between the interplanetary and geomagnetic fields. Magnetic fields reconnect or annihilate where the fields point in opposite directions, at the sunward boundary of the magnetosphere and downstream of Earth in the magnetotail. At “X-Line 1,” the interplanetary magnetic field (IMF) and the closed geomagnetic field cancel or annihilate, producing open field lines that have one end at Earth and the other in the solar wind. The open field lines are carried by the solar wind downstream of Earth, toward a second reconnection site (X-Line 2) in the magnetotail. The field lines above and below this site have opposite direction and reconnect, producing plasma outflows from the reconnection site both toward and away from Earth.

 

[kimbyd]

I would strongly suggest that that isn't correct. Tho the orientation of the Earth's magnetic isn't changing greatly.
The orientation of the solar winds' magnetic field does change and it's during the changes of the solar winds' field
and it's interaction with the Earths' field that geomagnetic storms occur.
Those that forecast geo-storms and us others that want to observe/photo aurora look out for that change in the
field direction of the solar wind.

What I meant by that is that the orientation of the Earth's magnetic field doesn't matter. If the Earth's magnetic field is disrupted (e.g. during a "pole flip"), then the structure of the magnetic field and its strength are changed drastically. But as long as the field is stable, the orientation of said field is irrelevant.

 

[jasonRF]

As you found when you started looking, this can be a complex but fascinating topic. I studied space plasma physics in graduate school, so took a class on solar-terrestrial physics that spent a lot of time on this topic. I think it would be difficult for a non-expert to sum it up and give it justice (while not leaving the reader with misconceptions) in a single post, and I am not an expert!

Assuming you know electrodynamics at least at the level of Griffiths, if you want to learn more then I recommend looking at "Introduction to Space Physics" by Kivelson and Russell. I wrote a little review of it here
Introduction to Space Physics by M.G.Kivelson and C.T.Russell | Physics Forums
It is well written and includes a lot of schematics to help the reader understand what is going on. There is also a second edition that I am not familiar with since I have been out of the space physics field for 20+ years
https://www.amazon.com/dp/1107098823/?tag=pfamazon01-20

jason