Auroras are created when ionized particles coming from the sun hit Earth's magnetic field and travel along it to the north and south poles where they react with the atmosphere. These charged particles are ejected from the sun by the solar wind or coronal mass ejections(CMEs).

solar wind

The solar wind is the constant ejection of charged particles by the sun. Mostly electrons and protons but also alpha particles(ionized helium) and a few heavier elements. Thanks to the solar wind, faint auroras are very frequent. The end of our solar system is where the solar wind is too faint to be measured.

The solar wind is created when charged particles in the corona(sun's atmosphere) get accelerated by open magnetic field lines. These open magnetic field lines extend out from the poles of our star into space away from the sun creating the interplanetary magnetic field(IMF). Since the sun is made of plasma the particles are charged either positively or negatively and so are either attracted or repelled from these magnetic field lines. If the particles are of opposite charge as the magnetic field lines they follow the field lines at height speeds. If the particles are of same charge as the field lines they get repelled and either fall back on the sun or get thrown off into space.
Small amounts of the solar wind are also caused by the other type of magnetic field lines, closed magnetic field lines. These field lines stay on the sun creating loops on the surface. The particles that are of opposite charge as the field lines follow the field lines and make them visible if they create loops. If the particles are of same charge as the field lines they get flung into space and become part of the solar wind.

CMEs (coronal mass ejections)

A coronal mass ejection is an ejection of a big amount of light and plasma(ionized gas) and contrarily to the solar wind which is ejected in all directions CMEs are directional because they are emitted at one spot and not everywhere on the surface. Even if the CME is not directed directly towards earth part of the plasma can still get to us because since the sun rotates on its own axe some particles are ejected at one spot and a little later other particles are ejected a little to the side creating a spiral shape.

The sun is made of plasma meaning that our star is not a solid ball and so does not rotate evenly. The material at the equator rotates faster than the material at the poles(this phenomenon is still an area of research). This causes the magnetic field lines to get entangled and eventually to snap at one or more spots creating an ejection of material. If the ejection is big enough it creates a CME otherwise the ejected material falls back on to the sun creating a loop(solar prominence). When the ejected material touches the surface of the sun it makes a bright flash of light(solar flare) and some of the material gets energized by the energy released during the reconnection of the magnetic field lines. The energized particles have enough energy to defy gravity and fly away into space. If the particles are directed towards earth these ejected particles create bright auroras and can disrupt long range radio communication because radio waves are low frequency electromagnetic waves and auroras do not only emit visible light but also low frequency radio waves. CMEs on the other hand (especially the big ones) cause a lot more damage because there are more particles ejected. The charged particles can attract or repel electrons in atoms of high orbit satellites making the electrons move. This can create high voltage and brake the electronics in the satellites. But CMEs can also cause power outages on earth because power lines are flowing electrons meaning the power lines each have a magnetic field(MF). When a lot of charged particles with each their own magnetic field fly through the MF of the power lines the MFs amplify or cancel out. If it would only be a few ionized particles it would not be a problem but because there are so many charged particles amplified parts can get even more amplified. This can cause high voltage.

frequency of auroras

The variation in the intensity of the solar wind and the frequency of CMEs depends on the variation of the magnetic field of the sun. All magnetic fields are created by electric current. Since the sun is made of plasma(charged particles) and that this plasma is moving due to convection(difference in temperature(inside is hotter than outside because fusion in core creats heat)) an electric current is formed and so also a magnetic field. Changes in the movement of the plasma causes changes in the magnetic field. The magnetic field of the sun switches poles every 11 years. Meaning that if the magnetic north is at the geographic north pole of the sun then 11 years later it will be at the geographic south of the sun. During this 11 year cycle the solar activity either increases or decreases. One easy way of observing this cycle other than through the variation of the solar wind and the amount of coronal mass ejections occurring is by observing the amount of sunspots.


There are 11 years between sunspot maximum and minimum. Sunspots are places where magnetism is stronger because the MF lines are very close together due to the magnetic field lines getting entangled because the suns plasma does not rotate evenly. It rotates faster at the equator and that is why most sunspots are close to the equator. The strong magnetic field slows the movement of convection. The plasma with the same charge as the MF gets repelled and there for can't flow up. Convection is the down movement of denser and usually colder material and the up movement of lighter and usually hotter material. Since the hot plasma can't rise up the colder plasma stays on the surface making sunspots, regions which are a little less hot and so darker.

There are several types of auroras:

  1. night-time auroras

    These are the famous dancing curtains of light. This type of aurora is produced by a magnetic field sending charged particles to the poles where they react with the atmosphere. Since there are more particles at one spot there is also more energy. This is why we say that the magnetic field amplifies the solar wind particles energy.

  2. cusp auroras

    The solar wind particles react with the atmosphere without the intervention of a magnetic field. Unlike the night-time auroras which only occur at and around the magnetic poles of a planet, cusp auroras can occur everywhere in the atmosphere.

  3. proton auroras

    They are very similar to the cusp auroras.
    Protons coming from the sun interact with the upper atmosphere by stealing electrons and making themselves neutral in charge. Since magnetic fields only deviate charged particles the now neutral protons continue undisturbed into the rest of the atmosphere where, due to their great speed,they bump into atoms several times. These atoms then get energy from the neutral protons and through luminescence(electrons jumping up and down energy levels) emit a photon, producing the auroras.

auroras on Earth

Ones the ionized particles hit Earth's magnetic field they either bounce off and change direction because they are of same charge as the magnetic field lines or the charged particles follow the field lines to the poles because they are of opposite charge.(opposites attract)

At the poles the particles react with the atmosphere and create high voltage. The particles give energy to the atoms in the atmosphere making there electrons move (voltage) and jump up to a higher energy level. After a while the electrons fall back down and emit a photon. Different atoms emit different colors because the distance between the energy levels varys from element to element.
The altitude also influences the color because in low altitudes atoms are close together and if an atom that got exited by charged particles from the sun gets hit by another atom before it emitted a photon the atom that hit the energized atom steals the energy and transforms it into vibration(heat). So in other words, only the energized atoms with the highest energy can emit a photon because other vise it takes to long and the energy gets stolen by other atoms. In the high atmosphere on the other hand the atoms are farther apart and so have more time to emit a photon so energized atoms with less energy still have enough time to emit a photon. For example oxygen at high altitudes emits red light(long wave lengths(less energy)) while at low altitudes this molecule emits green light(smaller wave lengths(more energy)).
Auroras can occur at daytime and at night but during the day we do not see them because the sun is very bright and the auroras become too faint to see. That's why they are best visible in winter because in winter it's almost always night time at the poles and in summer its almost always daytime.

The movement of the auroras is caused by the particles interacting with Earth's magnetic field. Each of the particles has their own magnetic field because they are charged particles which are moving. The ionized particles magnetic field interacts with Earth's MF moving Earth's magnetic field lines. Since the flow of particles from the sun is not constant there are times where there are no particles on parts of the MF lines so these parts move back to their normal position.
The change in brightness of the auroras is caused by the Earth's MF and the interplanetary magnetic field(IMF) either amplifying or canceling each other out because the two magnetic fields do not align perfectly. And as explained why above, the field lines of the two MFs move, changing the place where they amplify and cancel each other out.

On our planet mostly night time auroras occur. But occasionally proton auroras can occur close to the poles.

extraterrestrial auroras

Auroras occur on all bodies (planets, moons, etc...) with a proper atmosphere that orbit a star.


Jupiter's auroras are very bright in UV light because its atmosphere is mostly made of hydrogen. They are also visible in other wave lengths like x-rays. But not visible light.
Other than the solar wind, Io, a moon of Jupiter, contributes to the gas giants auroras because Io's volcanoes throw charged particles into outer space and Jupiter's magnetic field picks them up.
Another interesting feature about the auroras of this gas giant is that they never seem to cease. Why is still an unsolved mystery.


Mars does not have a global magnetic field anymore because over time the mantel has slowly cooled and hardened and since Mars is a small planet this process happens faster than on other planets. So there are only few magma flows beneath the surface and so only remnants of a global magnetic field. These remnants called local magnetic fields have the shape of umbrellas and are mostly on the southern hemisphere of the planet because early in Mars's history a big object containing a lot of iron smashed into Mars's southern half creating a magma ocean. Difference in temperature in the magma(which like Earth's outer core was made of a big amount of liquid iron) caused convection which created a magnetic field. Since Mars already had a global magnetic field before the impact because the core is made of iron and nickel, the southern MF got stronger do to the impact and so lasted longer.
Since Mars no longer has a global magnetic field the night-time auroras do not occur at the poles like on Earth but at the local magnetic fields.
Because a big part of the atmosphere is not shielded by a magnetic field, cusp auroras are common there. Since there is hydrogen in the height atmosphere of the red planet, some auroras can only be seen in UV light.
Proton auroras are also common on the red planet because its magnetic field is not very strong and so does not extend as far out as Earth's.


Venus does not have a magnetic field even though it has an iron core because the planet rotates too slowly. To generate a MF you need a liquid metal outer core with the fluid moving a lot due to convection and the planets rotation. Venus has a liquid metal outer core but the fluid does not move fast enough to creat a global MF.
Due to a lack of a global magnetic field only cusp auroras occur on this planet. The particles emitted by the sun hit the planet pretty much equally because there is no magnetic field to guide the particles to the poles. Since the ionized particles are not concentrated at the poles but are spread out everywhere in the atmosphere the auroras are very faint.

If no magnetic field why such a dense atmosphere?

Even though Venus does not have an intrinsic(interior) magnetic field the solar wind interacts with its atmosphere to create an induced magnetic field. Over time the ionized particles coming from the sun "stole" electrons from the atoms in the high atmosphere and UV light separated electrons from their atoms creating the ionosphere. This layer of the atmosphere is made of charged particles which attract or repel the charged particles coming from the sun.
Other planets like Earth and Mars also have an induced MF but Venus, unlike Mars, still has a sick atmosphere because since it has approximately the same size as Earth(so bigger then Mars) it probably still has active volcanoes who continuously regenerate the atmosphere which is getting eroded by the solar wind because there is no magnetic field to protect it. Also Venus is bigger and heavier then Mars so it has a stronger gravitational attraction which allows it to better hold on to its atmosphere.
Venus has a denser atmosphere then Earth because our planet has most of its carbon stored in limestone. At high temperatures the carbon is released into the atmosphere creating CO2. So if Venus would be colder it would also store most of its carbon(from the CO2) in its crust and have a lighter atmosphere.

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