Q&A

# What Factors Could Cause a World to See "Northern" Lights Much Closer to the Equator?

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I'm currently designing a world where the inhabitants see the Aurora Borealis on an almost nightly basis almost all the way to the equator. The lights are so strong they rarely see the stars beyond.

I've been wondering what could cause this, increased solar activity? Stronger magnetic poles? So far I've not found anything conclusive.

What could cause such an enhanced aurora?

Why should this post be closed?

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One way to achieve this would be to simply change the structure of Earth's magnetic field. At the moment, the field is that of a large magnetic dipole, with the field produced by the motion of fluid within the core. Maxwell's equations (see Chapter 4 of these notes) tell us that this motion should produce a magnetic field with radial component $$B(r,\theta)=B(r_{\text{core}})\left(\frac{r}{r_{\text{core}}}\right)^{-3}\cos\theta$$ Here, $B(r_{\text{core}})$ depends on the material properties of the outer core (density $\rho$, electrical conductivity $\sigma$) and the rotation rate of the Earth, $\Omega$: $$B(r_{\text{core}})\sim\sqrt{\frac{\rho\Omega}{\sigma}}$$ Unfortunately, we can't change the angular dependence of this expression, just the amplitude.

Earlier, I mentioned changing the structure of the field. We could do this by adding in some higher-order terms, creating, for instance, a quadrupole field. In the quadrupole case (with vanishing dipole term), there are actually four magnetic poles, not just two. This would presumably lead to auroras in many more places around the globe.

Here's what a quadrupole field would look like:

Image courtesy of Wikipedia user Andre.holzner, CC BY-SA 3.0.

Whether you could create a physical process to generate such a field is another question. We see that $B\propto\sqrt{\Omega}$ because it's the Earth's rotation that drives the motion of the fluids that leads to the dipolar field. It seems unlikely to be that the same mechanism could generate a quadrupole field. Perhaps there are multiple cores in the planet, still in the process of merging? This would involve a complex system of moving material, which would lead to higher-order moments in the field's multipole expansion, and perhaps the effects you're looking for.

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On Earth, solar flares can lead to displays of the aurora that are both more spectacular and visible farther south. Flares in September 2014 led to the aurora being visible in Maine, which is around 45 degrees latitude, so that's halfway between the pole and the equator. (More typically you need to be closer to the arctic circle to see much.)

Solar flares can interfere with electro-magnetic systems, so if your world regularly has auroras visible over most of its surface, you should expect that to affect technological development. I don't know if more and bigger solar flares would, by itself, achieve this effect.

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Actually, and as also pointed out, auroras are sometimes seen in temperate latitudes on Earth, so are not restricted to high latitudes only (although they tend to be more common there).

Taking Wikipedia at face value, there are a few things that can impact the frequency of occurrence of aurora activity:

• They are more common during periods of high solar activity (with our sun, this peaks in an 11-year cycle)
• They are more common during the spring and autumn. The mechanism behind this is not fully known, but at those times of the year, the interplanetary magnetic field and the Earth's magnetic field lines up.
• The solar wind is stronger from the Sun's poles than from its equator.

If we take these together, you'd want:

• a magnetically very active sun
• good alignment between the planet's and its sun's magnetic fields
• a sun rotating at a strong angle relative to the planetary disk's plane; compare Uranus' rotation

I'm not sure if that would be sufficient to produce auroras as far as to the planet's equator, however.

It's quite worthwhile to also note what David Hammen wrote in an answer over on Physics SE (my emphasis):

Regarding Mars, that's fairly simple. Mars is too small. Mars's core froze long ago, and if Mars ever did have plate tectonics, that process stopped long ago. The end of plate tectonics stops any outgassing that would otherwise have replenished the atmosphere. The freezing of Mars's core stopped Mars's magnetic field, if it ever had one. That Mars is small means it has a tenuous hold on its atmosphere. The loss of a magnetic field (if it ever had one) would most likely have exaggerated the atmospheric loss, particularly if this happened when the Sun was young and had a much greater solar wind than it has now. The combination of the above means that even if Mars was habitable long, long ago, that habitability was rather very short lived.

Also, as quite aptly noted by Monica in her answer, allowing for large amounts of aurora will probably cause problems with anything electrically sensitive. My guess is you'd be looking more at something along the lines of vacuum tube style technology or possibly space-hardened technology, and probably less reliance on electricity and electronics, than the highly minituarized electronics technology that we are used to depending so greatly on (because the latter fares very poorly with large induced voltages and currents, which you would see in such a scenario).

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You could eliminate the magnetosphere entirely which would let the solar wind hit the atmosphere directly. Of course this would be on the day side, not the night side. To get it to hit the night side, you need to use the magnetosphere, and that will only pull the particles into rings near the magnetic poles.

You can probably move the magnetic poles around. They tend to align roughly with the rotation of the planet but Uranus is an example of a planet with significantly skewed magnetic poles, although it's obviously not a terrestrial planet. This could produce an aurora elsewhere, but it would still be a smallish ring around each magnetic pole, with some fluctuations. You'll never get an aurora near the magnetic equator.