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SpaceX just released this high-speed footage, writing in the comments:

Missions in order of appearance: May JCSAT-14; July CRS-9 launch, stage separation, engine plume interaction, and re-entry burn; December 2015 ORBCOMM landing burn; July CRS-9 landing burn.

Matching the video fragments with that description, the fragment from 0:37-0:50 seems to be the engine plume interaction (the next fragment 0:50-0:55 would then be the re-entry burn):

enter image description here

What is that, actually? What interacts? The two separating dots are obviously the upper and first stages, but what are the filaments/fumes?

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  • $\begingroup$ That is so pretty. Watching the engine plume is quite relaxing :-) $\endgroup$
    – Rory Alsop ♦
    Commented Aug 10, 2016 at 11:23
  • $\begingroup$ @RoryAlsop this one has been in the news recently as well: https://www.youtube.com/watch?v=nPfcwT4Fcy8. $\endgroup$
    – uhoh
    Commented Aug 10, 2016 at 11:37
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    $\begingroup$ @uhoh Stop it! No one will ever answer my question if we all keep watching relaxing videos... $\endgroup$
    – user10509
    Commented Aug 10, 2016 at 11:39
  • $\begingroup$ It's a good question. The mixture is never stoichiometric, and exhaust probably not even completely burned, so maybe some kind of 'afterburner' type of effect? But this question needs a rocket scientist's attention (amateur or otherwise). $\endgroup$
    – uhoh
    Commented Aug 10, 2016 at 11:47

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The two dots are the upper stage boosting to orbit, and the first stage doing its first burn to slow down. (Boostback burn).

It is much clearer to see in this Instagram video.

The interaction is the first stage boosting through the plume of the second stage.

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  • $\begingroup$ Wathc the instagram? The screen shot is a still from it. Much clearer in motion. $\endgroup$
    – geoffc
    Commented Aug 10, 2016 at 12:12
  • $\begingroup$ At first I considered your the first stage boosting through the plume of the second stage very unlikely, but according to the image in this answer second stage ignition is only seconds after separation (not surprisingly, you want to keep that period short). I still find it hard to believe it would give such a (large) effect though, it's almost as if to gases/gas mixtures interact. $\endgroup$
    – user10509
    Commented Aug 10, 2016 at 13:51
  • $\begingroup$ @geoffc oh I understand the dots now - I just misread - my bad. So what is the Nexus-like relaxing thing - unburnt fuel mixture somehow re-ignited? $\endgroup$
    – uhoh
    Commented Aug 10, 2016 at 14:27
  • $\begingroup$ You might be interested in this fantastic video of the Iridium-4 launch: It shows very clearly the plume interactions of first and second stage briefly after stage separation from 2:50. Another video shows the interactions from a different angle, starting at 1:10. $\endgroup$
    – eerie
    Commented Dec 28, 2017 at 13:52