Meteors appear to be raining down on New Zealand, but why are some of them bright green?
New Zealand may appear to be under a meteor strike right now. After a large meteor exploded over the ocean near Wellington on July 7, creating a sonic boom that could be heard at the bottom of the South Island, a smaller fireball was captured two weeks later over Canterbury.
Fireballs Aotearoa, a collaboration between astronomers and citizen scientists aiming to recover a recently fallen meteorite, has received a lot of questions about this event. One of the most frequent is about the color bright green, and is it the same green that the aurora produces.
Wikimedia CommonsCC BY-ND
Green fireballs have been reported and filmed in New Zealand regularly. Bright meteors often signal the arrival of an asteroid boulder, which can be between a few centimeters and a meter in diameter when it hits the atmosphere.
Some of these asteroids contain nickel and iron and hit the atmosphere at speeds of up to 60 km per second. It releases a large amount of heat very quickly, and the evaporated iron and nickel emit a green glow.
But is this the same as the bright green of the aurora? For recent meteors, the answer is primarily no, but it’s not really that simple.
Read more: The equivalent of 1,800 tonnes of TNT: what we now know about the meteor that lit up the daytime sky over New Zealand
Meteor trail color
The green glow of the aurora is caused by oxygen ions in the upper atmosphere, which are created by collisions between atmospheric oxygen molecules and particles emitted by the sun.
These oxygen ions recombine with electrons to produce oxygen atoms, but the electrons can remain in an excited state for several seconds. In an energy transition known as “forbidden” because it doesn’t obey the usual quantum rules, they then emit auroral green light at a wavelength of 557nm.
Meteors can also shine through this route, but only if very quickly. Very fast meteors heat up in the thin atmosphere above 100 km where aurorae form.
Read more: Are the northern lights caused by ‘particles from the Sun’? Not exactly
If you want to see green aurorae from meteors, watch out for the Perseid meteor shower, which has now begun and will peak on August 13 in the Southern Hemisphere.
Also arriving at a speed of about 60 km per second, the Perseids are a very fast part of the comet Swift-Tuttle. Some of the Perseids follow a beautiful, luminous, clear green trail behind them, especially at the start of their journey.
After the Canterbury meteor hit on July 22, fluctuating winds from the upper atmosphere twisted a soft glowing trail, producing a pale yellow glow towards the end (as seen in the GIF below, also noted by Greg Price for the previous meteor). This is due to the sodium atoms being constantly excited in catalytic reactions involving ozone.
Are we being bombarded by meteors?
Yes and no. The arrival of a large exploding green meteor and the fall of a meteorite is not uncommon in New Zealand, but it is rare to recover the rock. Fireballs Aotearoa works to increase the recovery rate.
In an average year, perhaps four meteorites hit New Zealand. We encourage citizen scientists to build their own meteor camera systems so they can capture these events.
By comparing meteors against a starry background and triangular images captured by multiple cameras, we can determine the meteor’s position in the atmosphere within tens of meters.
Campbell Duncan/NASA/CAMS NZ, Author provided
That not only helps us find the rock, but also tells us what the meteoroid’s orbit was before the impact, which in turn tells us which part of the solar system it came from. This is a fairly efficient way to sample the solar system without having to launch a space mission.
Aotearoa Fireball and International Meteor Association, Author provided
The Aotearoa Fireball quickly filled Otago with meteor cameras and there were half a dozen others in other parts of the South Island. The North Island isn’t well covered yet, and we want more people (on both islands) to build or buy meteor cameras and point them at the sky.
Then when a bright meteor explodes with an explosion over New Zealand, we might be able to take that meteorite and do some good science with it.
Many thanks for the input from Jim Rowe of the British Fireball Alliance, and Greg Price who photographed the July 22 meteor and the persistent train.
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