Why is aurora borealis circular in shape when viewed from space?

Question

These are some false color images of the northern lights captured by NASA's satellite Dynamics Explorer-1.

Image credits: Dynamics Explorer - 1 (DE-1) Spin-Scan Auroral Imaging (SAI) Photo Gallery

It's interesting that what we see as longitudinal bands from Earth are part of larger rings when viewed from space. I thought that the aurora is formed when charged particles from the sun ionize the earth's atmosphere. I cannot understand why that can happen only in this circular ring. Are there any simple explanations for this phenomenon?

Answer 1

Only some of the aurora patterns are circular; most of them are more irregular and blobby in shape.

It is known that the aurora itself is created when electrons in the earth's geomagnetic tail and high-speed electrons and protons (called the solar wind) shot into space by the sun interact with the earth's magnetic field in space (called the magnetosphere).

Those particles have their trajectories bent by the magnetic field as they penetrate it. The amount of bend depends on a bunch of stuff but mostly by how fast they are moving and what the shape and extent of the magnetosphere happens to be (which itself changes from day to day).

If most of the particles in the wind have approximately the same energy, then they will get bent in the same amount which tends to focus and accelerate a cloud of incoming particles into a ring as they approach the earth.

Answer 2

Yes, the field is stronger at the north pole, but it also has very little surface area perpendicular to the sun. The majority of the north pole field lines perpendicular to the sun are looped around in the other direction far away from the earth where the field density is much lower.

Consider electrons traveling toward the earth with only one velocity. As they get closer to the earth the magnetic field gets stronger. Eventually the electrons will spin around the field lines of sufficient strength and travel along just that field line. Since that field line intersects the surface of the earth at just one geomagnetic latitude, the electrons will only interact at that latitude forming a band.

In the below picture, the field density is eventually strong enough to deflect the particles along the field lines. Before that point, it is interacting with the field lines from the magnetic north pole.

Now the electrons do not immediately interact with the atmosphere, but they instead get reflected back and forth between the poles and drift eastward (west for positive ions) while doing so, causing the night sky to be illuminated just as well as the day side when the electrons eventually interact with the atmosphere.

That should explain why the aurora is not as prevalent at the north pole and why it is visible as a ring around the day and night side.

Answer 3

Why is aurora borealis circular in shape when viewed from space?

The aurora emissions you see are not actually circular, but rather oval shaped. They can look somewhat circular but they usually distend toward the nightside (i.e., anti-sunward side) because the Earth's magnetosphere is stretched on the nightside (forming the geomagnetic tail or magnetotail). The tail forms due to the asymmetry in the solar wind dynamic pressure. In fact, most of the electrons that generate the aurora come directly from the magnetotail, not the solar wind.

I thought that the aurora is formed when charged particles from the sun ionize the earth's atmosphere.

I wrote two other detailed answers at https://physics.stackexchange.com/a/382414/59023 and https://physics.stackexchange.com/a/335325/59023. The basic idea here is that electrons with kinetic energies in the ~1-10 keV range strike neutral oxygen or nitrogen and excite their orbital electrons which then emit photons during the de-excitation of the excited electron. The characteristic bright green you see is from oxygen.

Unfortunately, many of the popular science articles get the source wrong. They often say that charged particles from the Sun hit the upper atmosphere. Most of the particles come from the geomagnetic tail, not directly from the solar wind. They are accelerated toward Earth by processes like magnetic reconnection and wave-particle interactions with electromagnetic fluctuations like Alfven waves and whistler waves.

Why the aurora focus around the geomagnetic poles is also confused in these popsci articles. The has to due to mirror forces acting on charged particles orbiting a magnetic field and moving along it. The more detailed explanation involves so called adiabatic invariants of charged particle motion (e.g., see answer at https://physics.stackexchange.com/a/670591/59023). The basic premise is that the dominant magnetic field geometry of Earth, close to the surface, is that of a giant dipole (it's tiltled off from the geographic poles by ~11 degrees). To move toward the Earth near the equator would require particles to move across the magnetic field. It is possible to do this, but the Lorentz force tends to make this very difficult. The only places where the magnetic field points towards the Earth's surface are near the geomagnetic poles (Note that magnetic north is near the south geographic pole). So the one place where particles can move roughly along the magnetic field toward the surface are near the geomagnetic poles.

I cannot understand why that can happen only in this circular ring.

It's not actually circular. It's more oval-shaped or oblate than circular, most of the time. It distends into the nightside more than the dayside due to the stretching/compression of the geomagnetic field.

Are there any simple explanations for this phenomenon?

The distortion of the nominal dipole magnetic topology due to the solar wind and other magnetic anomalies tends to make the magnetic poles distorted toward the nightside (i.e., away from the solar wind) more than not. This results in the oval shape. It can become more circular during "quiet times."

Answer 4

Geomagnetic flux lines originating inside the auroral ovals are dragged by the solar wind to form the outer lobes of the magnetotail and ultimately connect to the interplanetary magnetic field (the IMF) up to many Earth radii downstream. These lines completely bypass the plasma sheet nearer Earth at the core of the magnetotail and thus bypass any electrons the sheet may potentially leak into the field. These lobes contain a much lower concentration of electrons - 20 to 50 times less than the region immediately surrounding the central plasma sheet - and so contribute very little to the aurorae (typically just a faint, diffuse glow in the dark center visible only to instruments).

Fig. 1 - Simplified schematic of Earth's magnetosphere.

Flux lines originating (that is, exiting Earth's surface) at the auroral oval on the tailward side are stretched to form the inner 'surface' (as it were) of the lobes and thus border the central plasma sheet directly. Electrons from the sheet diffuse into these flux lines. As the lines are progressively stretched tailward, they eventually come together and reconnect near the far end of the plasma sheet, releasing considerable energy.

Fig. 2 - Magnetic reconnection.

Upon reconnection, the flux lines rapidly contract Earthward, like a rubber band snapping back from being stretched, launching Alfvén waves that move Earthward along the sheet (their counterpart flux lines likewise snap in the opposite direction, often entraining a bit of plasma sheet and accelerating it - called a 'plasmoid' - tailward). Electrons in the vicinity that already have velocities in the Earthward direction similar to the Alfvén waves' group velocity are snagged and carried along like surfers on a wave, and accelerated by the electric field of the Alfvén waves to energies in the ten keV range (a process called Landau damping). These electrons spiral along the flux lines to the poles, forming a field-aligned current, where they impact Earth's upper atmosphere at around 20,000 km/s, creating auroras. As the flux lines further inside the oval lead elsewhere and so do not participate in this process, there are no auroras there as those lines don't lead to a ready supply of electrons nor the means to accelerate them significantly. This completes the magnetotail side of the auroral oval.

Fig. 3 - Auroral electrons surfing Alfvén waves generated by a magnetic reconnection event. (note the yellow vertical lines on the left represent the IMF, not the solar wind as labelled in yellow. This label applies to the faint blue arrows beneath pointing to the right. The unfortunate mix of colors makes it a bit confusing.)

The weaker, sunward side of the auroral oval is supplied by electrons from the dayside magnetopause which are funneled down through the cusps to the poles along flux lines that intersect the upper atmosphere at the auroral oval. The aurorae on the sunward side of the oval are generally more diffuse than those facing the magnetotail. These electrons are also accelerated by reconnection events, but the picture here is a bit more complicated and beyond the scope of this discussion. Suffice it to say that these events are also a source of accelerated, aurora-bound electrons.

Flux lines originating outside the auroral ovals (that is, lines originating more equatorward) stay relatively close to Earth, within a few Earth radii, and so are largely closed off from sources of auroral electrons. These lines do not participate in auroral processes except during especially strong magnetic storms which can greatly distort the field near Earth.

This is a highly simplified explanation of course, but it does describe in general terms why aurorae tend to form an oval pattern with a dark center. It is these particular sets of flux lines that connect to ready supplies of electrons along with the means to accelerate them poleward.