The Northern Lights came to California — but will it happen again?

If you were in California this past week, you probably spent more time than usual gazing at the stars.

Why? Because the Northern Lights, or Aurora Borealis, made a surprise trip down to the continental United States, lighting up skies as far south as Joshua Tree and giving Californians a rare look at a celestial show that usually requires getting on a plane.

Those that missed it (or those who caught it and loved it) may wonder if this was this a once-in-a-lifetime opportunity, or if it could happen here again. To find out, we asked three experts at the UC Berkeley Space Sciences Laboratory, Chris Chaston, Harald Frey and Yan Li. Chaston and Frey are research physicists and Li is an associate research physicist at the lab.

Q: First things first — what are the Northern Lights? Why do we call them that? Where are they usually seen?

Chris Chaston (CC): The Northern Lights, or Aurora Borealis, are emissions of light from atoms and molecules in the upper atmosphere stimulated by collisions with energetic electrons and ions streaming downward from near-Earth space. They are usually seen at high latitudes — above 60 degrees and during the night when the light becomes visible. They occur in an oval roughly centered around the poles. In the Southern Hemisphere they are referred to as the Aurora Australis.

Harald Frey (HF): There is a widespread misconception that aurora on Earth is generated by energetic particles coming directly from the sun. The real processes are more complicated. The sun continuously emits the solar wind, as a stream of energetic electrons and protons that carries electric and magnetic fields with it. The interaction of these fields with the Earth’s magnetic field distorts it and transfers energy into it. Our magnetic field cannot accumulate endless amounts of energy and has to release this excess energy. This energy release generates energetic electrons in the so-called auroral acceleration region about 6000 kilometers above ground. When these energetic electrons collide with the atoms and molecules of the upper atmosphere at about 100-200 km altitude, these atoms and molecules emit light that we consider as aurora. Very often we refer to is as just aurora or polar lights that occur in the north and south.

Q: Why do you think people are fascinated by the Northern Lights? 

HF: You have to consider the aspect of surprise. You likely look up into the sky hundreds of times and nothing is happening, except for the occasional meteor. But when suddenly the whole sky lights up, changes color, and forms change and dance around, you are admiring what nature is capable of doing.

CC: Because it’s one of those things that ‘blows you away’ when you see it in full swing. It’s not just the light itself, but more how it moves or ‘dances’ — like a kaleidoscope in the sky, truly wonderful.

Q: Did you get a chance to see the lights over California this past weekend?

CC: I missed it on Friday night but I did try after sunset on Saturday from a high point in the Bay area. It was perhaps just a faint glow above the horizon to the North. I read of people driving up to Shasta on Friday who saw a good show late in the evening.

Q: Why were they visible in California over the past weekend, and as far south and east as Florida? Is that normal — or worrisome?

CC: There was a major geomagnetic storm over the weekend due to the shedding of matter and magnetic field from the Sun in the form of a series of coronal mass ejections (CMEs) that collided with Earth, or more specifically Earth’s magnetic field. Under these circumstances Earth’s magnetic field gets compressed and the low latitude edge where those energetic particles stream downward to collide with the atmosphere moves southward in the Northern Hemisphere. This southward shift is related to the strength of the storm. The weekend’s storm was a particularly strong event such that the aurora could be seen from Northern California and the southern states. It’s not something to be especially worried about, but it can impact the power grid due to currents induced in electrical transmissions lines and radio communications may be disrupted due to variations in the upper atmosphere or more correctly the ionosphere.

HF: The impact of such geomagnetic storms on the ground is generally rather limited, but similar events in the past have caused damage to electric power lines and transformers. More worrisome is the potential damage to satellites in orbit around the Earth. The energetic electrons can damage the electronic components of satellites and make them unusable. One way to limit damage to satellites is to turn them off temporarily, which was actually done with the three THEMIS satellites around Earth that are managed by our UC Berkeley Space Sciences Laboratory. Many other satellite operators must have taken similar precautions.

Q: Some reports suggest that this may happen in the continental U.S. more frequently in the next few years. Do you agree with that? Why is that?

Yan Li (YL): Solar flares and CMEs as energetic solar activity are more frequent during the maximum of the solar cycle. These violent solar activity produce intense solar energetic electrons and ions. The Sun is on the rising phase of the solar cycle and approaching its activity maximum
soon. Therefore, it is reasonable to expect more frequent aurora sightings in the next few years.

CC: Solar cycle prediction is an evolving art and sometimes things don’t work out as expected, but around 11 years is the average and we are close to the peak now, I think. Here’s a link to the sunspot number charts that define where we are in the solar cycle: Solar Cycle Progression | NOAA / NWS Space Weather Prediction Center.

On a short-term basis ‘hotspots’ on the Sun can stick around for a while, so with an average solar rotation period of 4 weeks there could be some increased auroral action in a little under a month from now.

Q: Why do the Northern Lights appear in different colors here than the usual green we see in photos from Alaska?

CC: The color of light that is seen is dependent on the atmospheric composition and the energy and type of colliding particle that stimulates the light emitted. The classic green aurora is due to energized electrons colliding with atomic oxygen and is the most common auroral emission reported.  At lower latitudes, such as here in California, the downward streaming electrons which stimulate the light seen from the ground have a different distribution in energy than those observed at high latitudes and the aurora is only observed from the side looking northward, rather than from below. These factors lead to us observing different colors and mixtures of colors.

Q: Can they be seen by the naked eye? Why are iPhones so much better at capturing them?

CC: Certainly, the aurora is observed by the naked eye. Cameras with long exposure times and sensitive light sensors (CCD and CMOS) can capture faint aurora but the best way to view the Northern Lights for the amateur observer is with the naked eye.

HF: The human eye is most sensitive to green light, less to red, and even less to blue. The optical detectors of iPhones try to mimic the human eye sensitivity as best as possible but they are still more sensitive to red. Therefore a mix of blue/green/red will appear slightly different to the naked eye and on a picture taken by the phone.

Q: Is there any way to know when they will be visible near us? (Friends of mine now send me solar forecasts, but I have no idea how to interpret them!) 

CC: Yes. I can suggest NOAA’s Space Weather Prediction Center’ at Homepage | NOAA / NWS Space Weather Prediction Center that provides forecasts every day.

YL: The accuracy of Solar forecasts for flares and CMEs are improving, but it’s still very difficult. Even when a forecast of flare or CME is successful, the direction of the energetic electrons produced needs to be right to reach the earth. So, a direct forecast of auroral sightings is not simple. But when there are super strong flares (like X-class flares) and super fast (>1000km/s) CMEs, energetic electrons produced can span a large angle, sometimes almost in all directions around the Sun, in which cases we might be in luck for auroral activity while we might also be in 'luck' for blackouts and loss of satellite communications, using cell phones etc.

Q: If you really want to see the Northern Lights, should you wait for them to come to California, or where might you go?

CC: You might be waiting a long time to see it here again in California and even then, you will likely not see it at its best. If you want to see the aurora in all its glory, I would suggest booking into a hotel near Fairbanks, Alaska for a couple of weeks during mid-winter.

HF: Other good places are northern Norway, Sweden, and Finland and central Canada. You need a clear sky and as little moonlight as possible. There is aurora over Iceland, but this region is known for many cloudy days and coastal fog. The weather is more stable in inland regions, like Fairbanks, Alaska.

Q: What if you want to go somewhere warmer?

CC: You can also see aurora in the Southern Hemisphere. This past weekend I heard it was good from Hobart, Tasmania, and was supposed to be visible all the way up to Coffs Harbour in New South Wales, Australia, although I have not searched for images.

Q: When should I plan this kind of trip, in terms of timing around solar cycles?

CC: Aurora is observed throughout the solar cycle at high latitudes. I spent several weeks in Fairbanks in the winters of 1996 and 1997, i.e., around solar minimum, and saw some fantastic displays. So I would not be too concerned with the timing  — just make sure there is plenty of darkness and as Harald pointed out it’s better if the moon is out of view.

HF: Last week you would not have seen the bright aurora from Fairbanks because the aurora was much too far south. May through August are bad times for aurora viewing from Alaska because the sun does not completely set in the far northern regions and the background light would have made it difficult (impossible?) to see aurora, even if it were there. If you want to have it warm while watching the Northern Lights, you have to stay in California and wait for the rare ~10-year event. 

Q: When you look for the Northern Lights, what direction should you be looking in?

CC: This depends on where you are, but in the continental U.S. just looking north is a good start. 

Chris Chaston has been studying the physics of auroral particle acceleration since arriving at UC Berkeley in 1996. This work has involved NASA’s FAST, POLAR, THEMIS and Van Allen Probes missions, the European Space Agency’s Cluster mission and Japan’s Reimei satellite as well as sounding rockets. His work on this topic has focused on the physics of Alfvén waves and how these waves can drive auroral arc formation through the acceleration of ions and electrons.

Harald Frey received his education in Germany and joined the Space Sciences Laboratory at UC Berkeley in 1997. He has been working on the detection and analysis of optical phenomena in near-Earth space using cameras on satellites and on the ground. He was the instrument scientist for the Far Ultraviolet Imager on the NASA IMAGE mission that observed aurora, the U.S. project scientist for the Imager for Sprites and Upper Atmospheric Lightning (ISUAL), and instrument scientist for the Far Ultraviolet Imager on the NASA Ionospheric Connection Explorer (ICON). He is now retired.

Yan Li has been a research physicist at the UC Berkeley Space Sciences Laboratory since 1999 and is an expert on solar coronal mass ejections that trigger aurora events. Before arriving at UC Berkeley more than 20 years ago, her work carried her from the Institute of Space Physics at the Space Academy in Beijing, China, the University of Sydney in Australia and Kyushu University in Japan.