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This May 4, 2019 SpaceX tweet says

More photos from Falcon 9’s 40th successful launch from Pad 40 in Florida and 39th first stage rocket booster landing to date → http://flickr.com/spacex

and shows two images, including the one below. The image is also shown and discussed in flickr.

Just what is happening here and what does this photo show? What exactly causes those relatively pure red and blue regions of this plume?

They remind me of aurora colors.

Click for full size, it's beautiful and something is going on around the engines...

SpaceX tweeted image "from Falcon 9’s 40th successful launch from Pad 40 in Florida and 39th first stage rocket booster landing"

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    $\begingroup$ Wow! That's a pretty cool looking effect. It looks like the exhaust itself is either glowing or being illuminated by something. $\endgroup$
    – user31448
    May 16, 2019 at 15:28

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Most of the color seen in this picture is not real. In reality, there are parts that are more bluish and some that are more reddish, depending on the temperature of gasses in the different regions.

The striking effect visible in this photograph was definitely produced in post-processing by increasing saturation and contrast by a large amount. There are clear indications visible in the image:

  • there is a huge amount of noise in the red and blue parts of the plume. This happens as the natural noise of the camera is amplified when increasing both contrast and saturation. According to meta data a Nikon D7200 was used - and this camera is far better in this respect than the typical mobile phone or pocket sized camera.

  • there is color banding visible (e.g. in the red parts close to the end of the blue part). This happens if actual colors are very close to each other so that the camera or the image encoding can barely distinguish them and then contrast and saturation are increased

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Falcon 9 uses oxygen and kerosene for its rocket propulsion, which can give off a variety of gases; so it is reasonable to look at some plasma colors produced by different gases in a discaharge lamp (from Wikipedia). In the case of hydrogen we get this:

enter image description here

Source: https://en.wikipedia.org/wiki/File:Hydrogen_discharge_tube.jpg

We may thus be seeing colors from hydrogen plasma, glowing behind the rocket.

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