An Earthly haze

Wired Science has a short piece on the Solar Dynamics Observatory‘s eclipse season and features an image of a partially occluded sun, much like the one below.

As the article describes, the shadow of the Earth does not form a clean, smooth arc across the image of the sun as does the Moon because the Earth has an atmosphere, which has variable thickness and does not block the extreme ultraviolet light that SDO detects as efficiently as lunar rock.

It’s worth noting that not all wavelengths are created equal.  This 1700 Å (170 nm; 10 Å = 1nm) image was taken a mere 12 seconds after the 193 Å image above and yet, as you can see there is only a slight dimming at the bottom of the Sun’s disk:

Lest you think that this is the result of the spacecraft moving out of the shadow of Earth’s atmosphere over the course of those 12 seconds, here is an image taken at the slightly shorter 1600 Å wavelength another 10 seconds later.

Clearly a greater portion of the Sun’s disk is blocked, even though the spacecraft was moving out of the Earth’s shadow when it was taken.  All this illustrates an extremely important aspect of our planet’s atmosphere: while it is nearly entirely transparent to light visible to humans (roughly 400-750 nm or 4000-7500 Å), it is opaque to many wavelengths in the infrared and ultraviolet, especially in the far ultraviolet.  This is incredibly important because without this opacity, it would be much more difficult to maintain life on the surface of the Earth.

At this point, an astute reader may have noticed that none of the photographs of the sun displayed in this blog post (four including the header) are the same as the photo used in the Wired Science article.  The SDO website allows a remarkable amount of public access to its data.  To retrieve all these images, I went to the SDO data browser and searched for images starting from 2011-03-29 6:00:00.

This search query wasn’t random.  The Wired Science article mentioned that the picture it featured was taken on March 29, giving me the day.  However, to know what time frame the right image would appear in, I had to know a bit about the orbit of the spacecraft.

SDO is in a geosynchronous orbit around the Earth, an orbital trajectory over 42,000 km above the Earth’s equator that allows the satellite to complete exactly one orbit per day, which means that the satellite will remain over a fixed point on the equator.  This property makes GEO an attractive piece of orbital real estate as is vividly shown in this illustration of satellite locations, courtesy of ESA.

The geosynchronous satellites make up the ring that spans most of the diagram.  As the diagram makes clear, the ring is a fair ways away from the Earth and because the Earth is tilted on its axis, so too is this ring of satellites.  This means that for most of the year, the Earth does not come between the Sun and the geosynchronous satellites.  The exception to this rule comes around the vernal and autumnal equinoxes in March and September, when the orbital nodes of this ring align with the Earth and the Sun, allowing for the Earth to cast a shadow on the satellites while it is midnight on the ground beneath them.

Since the Solar Dynamics Observatory is positioned at 102° W longitude, a station that allows it to transmit data at a high bandwidth via microwave transmission to antennas at White Sands Missle Range, it falls behind the Earth between 6:00 and 7:00 Universal Time.  In the case of the photographs shown here, they were captured between 7:14:37 and 7:15:06 UT.

I’d invite readers that are interested in seeing more to check out the data browser and searching for the photos themselves.  The AIA instrument that captured each of the above photos takes pictures in 10 different wavelengths and though not all of them caught the partial eclipsing of the Sun by the Earth, a good number did, and the effect of the Earth’s atmosphere is different for each wavelength.


About Meng Bomin

Real name Benjamin Main, I am a graduate of Grinnell College with a degree in Biological Chemistry.
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