It is now approaching the maximum of the Sun’s eleven year cycle when the Sun’s poles flip and the solar wind is at its strongest. The collision of the solar wind and the Earth’s magnetic field causes the aurora. On Thursday evening (Oct 10, 2024) it was grand. It was seen by many people, and many artistic images were taken and posted.
In the absence of the Earth’s magnetic field, the solar wind would continuously bombard and gradually strip away our atmosphere. And no atmosphere would have meant, no life. This is what has happened on Mars which lost its magnetic field, then much of its atmosphere, and is now desolate. But here, the Earth’s magnetic field repulses most of the solar wind and allows the presence of an atmosphere and life.
Although the vast majority of the Sun’s solar wind is repulsed by the Earth’s magnetic field, a small portion does enter near the poles arriving from about 650 km to 100 km above the Earth’s surface where it collides with our upper atmosphere.
The colours that result, depend on the altitude and the gas involved. If ions of the solar wind strike oxygen atoms above about 240 km in the atmosphere, the interaction produces a red glow. The most familiar display, a green-yellow hue, occurs as ions strike oxygen at below this level. The purplish and bluish light, that often appears in the lower fringes of auroras, is produced by ions striking atoms of nitrogen above about 100 km.
Now, the aurora is the only time that humans directly see a display of the Earth’s magnetic field. If there had been no magnetic field, there would have been no aurora and the solar wind would strip away our atmosphere and life would be gone. So, our magnetic field is one of the major reasons that we have life on Earth. The sight of a grand auroral display is the sight of why we exist.
By all means enjoy the artistic beauty of aurora, but pause for a moment to reflect that in watching an aurora we also watching a major reason for our existence.
Early in the evening, there were reds green and yellow. This was taken with a fish-eye lens.
Still looking west the activity has increased.
Now, looking north, the streaks are impressive.
Looking toward the west and the zenith the red streaks are major. That is the moon on the left edge. Picture by Cynthia Fraser.
A wonderful explanation. I had no idea, and now am certainly reflecting on the Aurora and our precious life. Thank you, Alistair.
Wonderful images and explanation, Alastair – such a gift.
If air is composed of about 75% nitrogen and 21% oxygen, why do we see so much more green and red than blue-violet?
Tom, that may have resulted from my selection bias.
Thank you Alistair and Cynthia!
Thanks so much for sharing these lovely photos, and especially for framing the story of the northern lights in this way. I’ve come across bits and pieces of this story over the years, but not until tonight has it come together so powerfully for me. This is a tale I won’t forget!
I saw..J narvelled..I took photos..did not know any of your explanation! I must have been ‘away many days..magnetic field..solar winds..major existence factor…fascinating to think on..thanks for this!!