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@aspieRommel said:
How was the rocket able to “glowworm” like that? What conditions were just right for it to happen?

Axioms
1. The Earth is round.
2. The Earth rotates about its north-south axis once per day.
3. Other laws of physics (particularly relating to chemistry) are valid.

At local sunset, the light from the sun seems to be traveling exactly parallel to local observers on the ground. However, in actuality it is traveling tangent to the round surface of the Earth as shown in the figure below (Wikipedia).

The time of sunset as a function of altitude can be determined trigonometrically knowing the radius of Earth (R ≈ 6370 km) and its rotation rate (15°/hr). The conditions of sunset suitably match the small angle approximation for sine though such that sunset is 1 minute later for each 1.5 km of altitude gained for relatively small altitudes (h ≪ R).

Local sunset at Lompoc, CA near the launch site was 4:56 PM on the day of the launch and liftoff occurred at 5:27:34 PM or roughly 30 minutes after sunset. As such, Falcon would fly out of twilight and into direct sunlight at an altitude of roughly 45 km. Lets take a look at the webcast:

It takes a couple more minutes for the rocket to reach 45 km altitude so by then the rocket must reach an altitude of ~50 km before hitting daylight. It's pretty evident in the webcast when this happens at ~00:02:15 into flight. When we tune into Stage 2 following stage separation we can see it's in daylight. It's still dark on the ground though so anything in the sky lit by daylight is going to appear brilliant (i.e. like the moon).

Now that we've established the lighting conditions, let's turn our attention to the thing being lit. According to publicly-available information, Falcon 9 Stage 1 converts 2.5 metric tons of liquid kerosene and liquid oxygen per second into mostly steam and some carbon dioxide, soot, and other random organic molecules. Stage 2 does the same thing at roughly an order of magnitude lower rate (9 engines vs 1 engine with all else being equal). Steam is nothing more than hot water vapor and is being exhausted from Merlin at a fairly high temperature and pressure into a fairly cold vacuum. Chemistry tells us that these sorts of conditions cause the expelled vapor to condense and form clouds. I'll leave the exercise of working through steam phase transitions as an exercise for the reader. The generic name for clouds formed this way is "condensation trails" or "contrails".

So, now we have high altitude clouds lit up by daylight seen by observers in the Earth's shadow.

Awesome!