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.
Partial lunar eclipse
I have only rarely photographed a lunar eclipse. The last time was a total eclipse, a decade ago (blog.kootenay-lake.ca/?p=11796), and I spotted a bluish light that is rarely mentioned in the somewhat superficial stories offered by most websites.
Last night I photographed a partial lunar eclipse and expected to see nothing that required an explanation. Yet, there was a mystery that gave me a temporary pause. I will show the picture and attempt an explanation.
This partial lunar eclipse is about at its maximum. Yet, the portion of the moon that is in the Earth’s umbra is much smaller than the shaded region at the top.
When the moon is eclipsed by the Earth, there are two regions of shadow: the umbra and the penumbra. The umbra is the region of total shade: the Earth blocks all of the Sun’s light from falling on the moon. The penumbra is the region of partial shade: the Earth only blocks some of the light from the Sun.
The thing about the penumbra is that it is not uniformly dark. Some regions are much darker than other regions, depending upon how much of the sunlight gets past the Earth. In this view of the partial lunar eclipse, only a tiny portion at the top is in the umbra, and the rest is all penumbra with its variable darkness.
It is probably easier to understand by imagining that one is walking on the moon, but starting at the top and so deep in the umbra. As you look back towards the Earth, the Sun is completely blocked. As you walk south, you quickly come to a place where now a tiny bit of the Sun peeks around the edge of the Earth. It is still markedly dark where you are, although you have just entered the penumbra.
Further southward walking reveals more and more of the Sun from behind the Earth and so the moon’s surface becomes brighter and brighter.