The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard the Aqua (EOS PM-1) satellite generates images of most of the Earth each day, though there are small gaps near the equator where the viewing angle is too extreme.

Below is an example images from an earthobservatory.nasa.gov page, (see the >2 MB full-size image).

What is the repeating bright white streak nearly centered in each pass, and why is it tall and narrow?

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It's the diffuse reflection of sunlight on the oceans. If you look closely, you will see they don't occur over land - although your brain may fill in and make it look like they do until you look closely - the stripe on the east side of South America is a good example of that.

The stripes follow the ground track of the satellite's orbit; as the spacecraft circles the globe it takes many strip images perpendicular to it's motion (horizontal lines) These are assemble to create the vertical stripes shown, and those stripes together are used to make up the view of the globe.

This satellite is in a sun synchronous orbit, which means it always lunch time on the ground underneath the satellite, as a result the band of reflected sunlight always appears in the same place.

There are three effects distorting the diffuse reflection from the circle on might expect.

  1. In order to capture the 2330km broad stripes at the low altitude of 705km required for a sun-synchronous orbit, the capture occurs over a 118 field of view. While the instrument corrects for the fisheye effect, the reflection angle will be a little wider, giving the effect of seeing the reflection in a convex surface (such as a cylindrical mirror, thanks @uhoh) making the reflection narrower.
  2. As the satelite varies in latitude, it turns to face the earth, narrowing the reflection angle, giving the effect of viewing the reflection in a concave surface, making the reflection taller.
  3. The apparent centre of the diffuse reflection will lie proportionally between the point directly under the sun and the point directly under the satellite, so as the satellite moves in latitude, so the diffuse reflection will appear to move. As a result of this motion, the reflection of the sun is smeared out - what "should" be a circle becomes what appears to be a very long streak tapering at the ends. In a very similar way, a vertical slit camera was moved across a model of the Enterprise as it was moved to create the stretching effect for it going into Warp for the Star Trek the Next Generation television series, and can produce other distorted images:

Wikipedia slit scan photography example

  • 3
    $\begingroup$ I'm offended that you believe I chose not to explain it to the best of my ability. $\endgroup$ – JCRM Jun 15 '18 at 12:48
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    $\begingroup$ Uhoh - think about where the sun is. The Earth's axis is tilted at 23.5 degrees relative to our orbital plane. You won't get a diffuse reflection as you get further away from the equator with a camera facing straight down. $\endgroup$ – Rory Alsop Jun 15 '18 at 14:19
  • $\begingroup$ is that any better @uhoh? $\endgroup$ – JCRM Jun 15 '18 at 16:34
  • $\begingroup$ That may exaggerate the effect @uhoh, but it's the slit-scan that makes it appear so long. $\endgroup$ – JCRM Jun 15 '18 at 17:11
  • $\begingroup$ how's that now, @uhoh $\endgroup$ – JCRM Jun 15 '18 at 18:18

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