# How does the Earth's magnetic field protect it from the solar wind?

Several sources (see linked questions at the end) say that earth's magnetic field shields us from the solar wind. The general consensus is that without earth's magnetic field, the atmosphere would be stripped away and life would not be possible.

My understanding is that the solar wind is fairly balanced between positively and negatively electrically charged particles.

Wouldn't this mean that all of the positive particles would be attracted to and sucked into one of the earth's poles, and the negative particles sucked into the other pole? Giving the effect that the earth's magnetic field doesn't protect us from the solar wind at all, it just concentrates it at the poles? If it's merely concentrated at the poles then we still receive the same amount of particles overall than if the earth had no magnetic field, so why isn't the atmosphere stripped? Is it about the direction of travel of the particles?

I know the above paragraph probably contains misunderstandings since evidence that we're still here doesn't line up with the assumption. I only have a basic understanding of magnetism and electric charge, so a good answer would explain how and why I'm wrong in layman's terms. I think my misunderstanding may have something to do with the difference between magnetic and electric fields.

I think the main question I would like answered, more specifically is: What exactly does the magnetic field do to the particles such that they are stopped from stripping the atmosphere?

Would a magnetic rod through Mars shield it from solar wind?

• Do you know about en.wikipedia.org/wiki/Lorentz_force ? – infinitezero Jul 12 at 12:37
• @infinitezero I did study that in A level physics but that was around 9 years ago. My understanding is that a charged particle moving in a magnetic field will experience a force in the opposite direction to it's motion. If that's true then shouldn't the particles in the solar wind slow down but then still fall to earth at the poles? If they are slowing down then where does their energy go? – Karl Jul 12 at 12:49
• The mangetic fiedl of the earth crystalinks.com/earthsmagneticfield.html . There are no magnetic charges. Charged particles spiral down towards the poles, on the way they lose their energy hitting atmsphere molecules, and most ending as part of the atmosphere in the north and south poles. A lot with high enough energy, particularly muons, still reach sea level because the earth's magnetic field is not strong enough. cosmic.lbl.gov/SKliewer/Cosmic_Rays/Muons.htm – anna v Jul 12 at 13:36
• Look at an electron spiraling down in the magnetic field of a bubble chamber (mf perpendicular to the picture) hst-archive.web.cern.ch/archiv/HST2005/bubble_chambers/… The hypothesis is that the atmosphere high up is not directly hit for it to diminish, although this is a model. There are questions , see space.com/11187-earth-magnetic-field-solar-wind.html – anna v Jul 12 at 13:38
• @Karl The energy goes to the atmospheric molecules the particles hit and slow down their spiral. By the time they reach the poles it is also goin into light , as the other comment says, the auroras. light is also energy. In the model, the particles hitting away from the poles are trapped around the magnetic field lines, that is how the magnetic field protects us. As I said before , it is the dominant model . With more data coming in from planets there might be a change in the model, as the link I gave seems to say. – anna v Jul 12 at 14:46

It has nothing to do with pressure in the thermodynamic sense nor with virtual particles. There is an intrinsic magnetic field generated somehow in Earth's core (dynamo discussion could fill volumes) and that field interacts with the magnetic field and charged particles of the solar wind. Since the solar wind is supersonic, there is a bow shock generated. This decelerates and deflects the solar wind around the magnetosphere, which stands off from the Earth. Without this, the solar wind's convective electric field (i.e., basically a $$\mathbf{E}_{sw}$$ = $$- \mathbf{V}_{sw} \times \mathbf{B}_{sw}$$ field due to the motion of charged particles carrying a magnetic field past the Earth) would drag the ionized upper atmosphere off Earth very quickly.

Giving the effect that the earth's magnetic field doesn't protect us from the solar wind at all, it just concentrates it at the poles?

This is wrong, it does protect Earth's atmosphere from the solar wind, as I stated above. The drift velocity induced by the solar wind's convective electric field on newly ionized particles (called pick up ions) is called the ExB-drift, and it ranges in speed from 10s of km/s to 100s of km/s. The escape speed from Earth at the surface is only ~11.2 km/s. Thus, if the ionized upper atmosphere were suddenly exposed to $$\mathbf{E}_{sw}$$, the ions and electrons would immediately be accelerated up to 10s to 100s of km/s, easily escaping Earth's gravitational field.

• The Earth's core is to hot to generate a magnetic field. It's the dynamo effect of the Earth's electric field which creates the Earth's magnetic field. The Earth's geomagnetic north pole is a magnetic south pole. – Cinaed Simson Jul 30 at 21:59
• I am not sure from where you are getting this information, but it is wrong. High temperature liquids and gases are perfectly capable of carrying currents, which generate magnetic fields. The sun has magnetic fields all the way down to the core and the core of the sun is significantly higher than Earth's core. Yes, I am aware that the magnetic north is near the geographic south pole, though I fail to see why that's relevant. – honeste_vivere Jul 30 at 23:10

It is the ozone layer, that needs to be protected, so it can protect us against UV radiation (photons).

It is very important to understand that solar wind is made up by:

1. electrons

2. protons

3. alpha particles

The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma consists of mostly electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV.

https://en.wikipedia.org/wiki/Solar_wind

Now the magnetic field of the Earth, which is produced by the liquid outer iron core (electric currents) of the Earth, extends to space beyond the ionosphere.

https://en.wikipedia.org/wiki/Earth%27s_magnetic_field

The magnetosphere is the region above the ionosphere that is defined by the extent of the Earth's magnetic field in space. It extends several tens of thousands of kilometers into space, protecting the Earth from the charged particles of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that protects the Earth from harmful ultraviolet radiation.

Now the reason we need the magnetosphere is because it protects the ionosphere. Why? Because the ionosphere includes the mezosphere.

The ionosphere (/aɪˈɒnəˌsfɪər/1[2]) is the ionized part of Earth's upper atmosphere, from about 60 km (37 mi) to 1,000 km (620 mi) altitude, a region that includes the thermosphere and parts of the mesosphere and exosphere.

https://en.wikipedia.org/wiki/Ionosphere

Now why do we need the mezosphere? Because it protects the stratosphere, that includes the ozone layer.

The lowest part of the Earth's atmosphere, the troposphere extends from the surface to about 10 km (6.2 mi). Above that is the stratosphere, followed by the mesosphere. In the stratosphere incoming solar radiation creates the ozone layer.

Now if all the radiation (charged particles) would be deflected, then we would not have an ozone layer, and we would not be protected from UV radiation.

Ultraviolet (UV), X-ray and shorter wavelengths of solar radiation are ionizing, since photons at these frequencies contain sufficient energy to dislodge an electron from a neutral gas atom or molecule upon absorption. In this process the light electron obtains a high velocity so that the temperature of the created electronic gas is much higher (of the order of thousand K) than the one of ions and neutrals. The reverse process to ionization is recombination, in which a free electron is "captured" by a positive ion. Recombination occurs spontaneously, and causes the emission of a photon carrying away the energy produced upon recombination. As gas density increases at lower altitudes, the recombination process prevails, since the gas molecules and ions are closer together. The balance between these two processes determines the quantity of ionization present.

It is very important to understand that the solar wind could strip away the ozone layer.

But your question is about the stripping away of the atmosphere, and why the solar wind does not do that. Now the solar wind exerts a pressure. If this pressure reached the atmosphere, it would strip it away.

Now the magnetosphere has a pressure too, and it counterbalances the pressure of the solar wind.

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).[24] 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.[25] Sunward of the magnetopause is the bow shock, the area where the solar wind slows abruptly.

1. So basically the charged particles are making up the solar wind, but the pressure of the solar wind is what is important, and that the magnetosphere counterbalances it so the atmosphere is not stripped away.

2. It is the ozone layer that needs to be protected, to protect us from UV radiation.

Some of the charged particles do get into the magnetosphere. These spiral around field lines, bouncing back and forth between the poles several times per second. In addition, positive ions slowly drift westward and negative ions drift eastward, giving rise to a ring current.

The answer to your question is that most of the charged particles do get deflected.

After the comment, the question is more about the QM level explanation of how the pressure of the magnetosphere deflects the electrons, protons and alpha particles of the solar wind.

Now the magnetosphere has energy, and as the solar wind's particles reach the magnetosphere, the magnetic field of the Earth starts to interact with the solar wind's particles. This interaction is mediated by virtual photons.

Virtual photons are not real photons, they are off mass shell, though, they are a mathematical way of describing the interaction between the field and the solar wind's particles.

Now as the solar wind's particles interact with the magnetic field of the Earth, the magnetic field's energy is transferred to the solar wind's particles as kinetic energy and momentum, via virtual photons, thus the solar wind's particles trajectory, momentum changes so that they move away from the Earth.

• This answer speaks a lot about why the magnetic field is important to protect us, but my question was more "how" it protects us. There is some discussion that the magnetic field slows the solar wind and the particles get deflected but I think it's clear from the question I already know that, there isn't much detail about specifically how the particles get deflected. – Karl Jul 13 at 16:01
• @Karl it is the pressure, my answer states that the magnetosphere's pressure is what deflects the charged particles. Since the radiation is made of particles of EM charge, the magnetosphere's pressure deflects them before they could reach the ionosphere. Are you asking how the charged particles of radiation at the QM level are deflected by the magnetosphere's EM field? It happens via virtual particles. The radiation of the solar wind includes electrons, protons, and alpha particles. – Árpád Szendrei Jul 13 at 16:19
• @Karl As these charged particles reach the magnetosphere, the magnetic field of the Earth interacts with them via virtual particles (virtual photons). The interaction alters the trajectory of the charged particles of the radiation, because the magnetic field's EM energy is transferred to these charged particles of the solar wind as kinetic energy in a direction away from the Earth. As the electrons, protons, and alpha particles start interacting with the magnetic field of the Earth, they gain kinetic energy and momentum from the magnetic field via virtual photons. – Árpád Szendrei Jul 13 at 16:22
• @Karl Thus, the trajectory of these particles gets altered, away from the Earth. – Árpád Szendrei Jul 13 at 16:22
• @Karl I edited my answer. – Árpád Szendrei Jul 13 at 16:28