I was feeling bold, a little too bold on this trip. I had no fresh film with me, and only loaded a roll of decades expired Fujichrome Velvia 50 at box speed to capture the fall colors. So, the universe decided it was time to teach me a lesson.
How Does Aurora Form?
Never thought being a physics TA would ever come in handy. In the simplest terms, aurora forms when charged particles ejected from the Sun collide with Earth's magnetic field.
Earth's Magnetic Dipole:
It is crazy to imagine that Earth has a magnetic field, supposing it's just a giant rock. But things are not so simple. Earth has layers - in the outer core, a metallic, conductive mixture of molten nickel and iron forms convection currents (convection: think heat - heat escapes from the inner core in a natural process called geodynamo), the motion of which generates powerful electrical currents, which in turn generate magnetic fields. One can easily deduce then, that the magnetic poles are not necessarily fixed. They are constantly shifting and moving on the geological timescale. At the end of the day, however, the movements owe to the rotation of the Earth, and the magnetic axes average out to the center of rotation.
It is equally important to distinguish between our theoretical estimation of the magnetic poles (north/south geomagnetic poles) and the actual magnetic poles. Here are some illustrations from Wikipedia:
The geomagnetic and the actual magnetic pole are going to be in slightly different positions around the geographic north pole at this moment in history. Also, as part of the geomagnetic dipole model, the "magnet" inside our Earth points the opposite direction (south pole of the magnet is oriented towards the north magnetic pole, and vice versa because opposites attract).
Solar Storms & Cycles:
Our Sun is a volatile star, and its activities wax and wane according to a roughly 11-year solar cycle. Sunspots are indicators of high concentrations of magnetic flux, reducing convection and hence temperature/brightness. These regions are associated with heightened solar activity, and solar flares and coronal mass ejections (CMEs) typically originate around these regions.
Solar flares are individual, localized bursts of electromagnetic radiation, and usually associated with CMEs, which are plasma ejections from the Sun's corona into the heliosphere. If the CMEs reach far enough they are named ICMEs, which stands for interplanetary CMEs. Both solar flares and ICMEs are responsible for the formation of aurorae (worse things can happen depending on their strength, such as the Carrington Event) as they release a large amount of charged, excited particles which slam into Earth's magnetosphere, collide with atoms/molecules in our atmosphere to bring about the magnificent natural light show.
In fact, solar flares and CMEs occur fairly frequently. During solar maxima, the Sun produces about 3 CMEs daily, whereas during solar minima, there is still a CME about once every 5 days. Solar minima/maxima are defining characteristics of the solar cycle, which lasts around 11 years. We are in the midst of the 25th solar cycle, where the minimum took place around 2019, and coming up on the solar maximum in the next year or so. The number of sunspots have gradually increased, and as a result the world witnessed two wide-scale aurorae this year so far. Both were caused by CMEs, some of the strongest in the past 20 years. We can look forward to seeing more of them in 2025. So keep your eyes peeled!
Fun fact: during the total solar eclipse as I peaked through my camera, I could see some very visible sunspots, which I later used to align my partial solar eclipse photos.
Where Do the Colors Come From?
As explained above, the charged particles from the Sun interact with Earth's atmosphere to create the aurora. But where exactly are the colors coming from?
We need to look down to the molecular level. Our atmosphere consists of compounds that are essential to life, including oxygen, nitrogen, hydrogen. They each play a major part in the colors we see:
Oxygen green: roughly at altitudes around 100 - 150km up;
Oxygen red: oxygen has another excited state which releases red light, but only at altitudes around 200 - 250km up where the atmosphere is much less dense;
Nitrogen blue/violet/pink: released by nitrogen molecules;
Nitrogen turquoise/green: emitted by nitrogen atoms but often overshadowed by oxygen green;
Hydrogen pink/crimson: hydrogen atoms are generally lower in the atmosphere, so in more energetic displays these colors can be seen at the bottom of a green band of aurora.
Photographing the Aurora:
There have been two major aurora-inducing ICMEs this year - one in May, and one in October. When the first one came around, it hit most of the northern parts of the country, but visible in northern California as well. Had my partner stayed in Yosemite that night, he and his buddy would have seen an awesome display. Unfortunately we were all in the city by night, and could only catch a tiny glimpse of the hue with long exposures on our phones. It was fairly disappointing, because light pollution is not forgiving in SF.
Second time around, I was up in New England in October to see fall colors. That's right, just a roll of expired Velvia 50 slide film with no suspicions whatsoever. Then one night, when we frantically checked the news and looked up in the sky, we could see weak hues of pink and green in the night sky. Some quick iPhone shots confirmed the aurora is in sight:
I had put my faith in the expired slide film, but unfortunately, the magenta cast destroyed any visible signs of the aurora. It was still beautiful to see though, and at least I made out with a few digital photos. Obviously, it was not as visible to the naked eye, but you can certainly make out the green patches, and the sky was brighter than normal.
It was my fault, and I learned my lesson - ALWAYS test expired film before a trip, and ALWAYS have some fresh film available. Do not trust that eBay seller that said the film was cold stored and will come out fine. There are opportunities in this life that one simply cannot miss.
However, I did make it out with a gorgeous photo of long exposure star trails:
So not all was lost :').
Sources:
NASA, NOAA: Sun Reaches Maximum Phase in 11-Year Solar Cycle: https://science.nasa.gov/science-research/heliophysics/nasa-noaa-sun-reaches-maximum-phase-in-11-year-solar-cycle/
NOAA: Hello Solar Cycle 25: https://www.weather.gov/news/201509-solar-cycle
BBC: Bands, rays and arcs of green and purple. This is what causes the different shapes and colours of the aurora: https://www.skyatnightmagazine.com/space-science/what-causes-different-shapes-colours-aurora
Wikipedia