On nights when the magnetic field of the Earth is particularly active, a familiar boundary on an aurora map moves with a kind of mechanical precision: the viewing line shifts south, and people living nowhere near the Arctic find themselves with a reason to look up.
The viewing geometry is simple. NOAA’s aurora forecast products display a forecast oval centered on the magnetic pole of the Earth, and a ‘viewline’ showing where aurorae can be viewed low down on the southern horizon. The model of choice has significance because it relies upon the OVATION model and correlates to maximum expected geomagnetic activity over night. As the oval increases in brightness and size, so too does the viewline, and this makes aurora viewing possible when it would not have been otherwise.
With increased geomagnetic activity, aurora is possible across a broad swath of northern states. NOAA viewlines tend to include the area around the border between the U.S. and Canada and sometimes large areas of the Upper Midwest, New England, or the Plains states, with the chances improving for darker, more northerly sites. This is a viewing problem rather than a promise the aurora might be out while the ground-level sky is not suitable for viewing because of weather and lighting.
But timing still focuses opportunity.
The activity may become most spectacular just before or at local midnight, so a time range that works for sky gazing is to head out from 10 p.m. to 2 a.m. This range is dependent on local conditions for how much illumination is actually seen. Clouds may negate a good geomagnetic display, or a bright moon may obscure contrast even when the aurora is active. The best path to take when looking for low-friction improvements is almost always geographical: go north if you can, and make sure you have a good view north from a spot that isn’t illuminated.
However, the phenomenon of the “southward reach” can only be comprehended by analyzing it as a chain of energy transfer rather than a phenomenon that is visible. Geomagnetic storms occur due to a change in the solar wind or a coronal mass ejection containing charged particles that move towards the earth. The charged particles collide with the magnetosphere, out of which some are directed through the magnetic fields towards the upper atmosphere. This collision generates light. The intensity as well as the footprint changes during intense storms. The footprint or the oval shape expands during intense storms. The area where it can be viewed from earth also increases. One does not need to stand directly under it. The aurora can often be viewed from a distance of 1000 km if it is intense.
For people photographing the display, there are no restrictions other than physical ones: low light, motion, and maintaining steady framing. A very popular starting point is a wide field of view, a tripod, and an exposure that captures detail and auroral motion simultaneously. A useful starting point, based on field guides, is to start with a lens opened to maximum aperture, ISO 1600, and exposure times of 10 to 15 seconds, and adjust depending on the speed of motion and detail needed in the foreground. A shorter exposure is needed if the aurora is about to become active and filamentous, or if it is, and a longer exposure may result in smearing the motion into a bright band across the sky.
Nothing about this challenges the underlying observing principle, which is the aurora is as much about atmosphere and infrastructure as it is about astronomy. This is the same geomagnetic forcing function which causes the auroral oval to expand, and it is the reason why serious storms are of great interest regarding the potential impact upon satellites, radio, navigation, and the power grid. The observing principle for aurora watchers is simple and straightforward.
