Something a tad more exciting than the impending change of season has put considerable bounce into my normally staid astronomical gait this past week. The splendid display of the aurora australis I witnessed last Saturday evening has much to do with this. For over five hours, the sky to the south of Hoopers Inlet was daubed by a gorgeous pastel-hued aurora. As this week’s photos show, Saturday’s auroral pageant exhibited prominent beams, which were easily visible to the unaided eye.
This month, I’m hoping, with some confidence, that if the weather gods behave themselves, last week’s aurora won’t be the only good show Otago stargazers will experience.
So why is September a particularly good month for auroras? For reasons astronomers and geophysicists still don’t really understand, statistically more auroras are visible in September and March, which are the months near the spring and autumn equinoxes.
The best explanation proposes that the different tilts of the sun and Earth’s axes in relation to Earth’s orbit are responsible for the preponderance of equinox auroras. As the Earth orbits the sun over the course of a year, it passes through an interplanetary magnetic field (IMF) which originates from different latitudes of the sun (which is tilted at 8 degrees to Earth’s orbit).
As Earth turns, every day its magnetic pole circles around the geographic pole and over the course of a year this changes the daily average angle that our magnetic field makes to the incoming IMF. These factors combined may lead to cyclical changes in the way that the IMF links to Earth’s magnetic field. In turn, this affects the average probability of energy from the solar wind reaching Earth's atmosphere and thereby creating an aurora.
With the moon reaching first quarter on Friday at 23:49, the best time to look for auroras is after moonset when the sky is darkest. Find a place with a good southern horizon, and start looking for the auroral glow. Good luck!