I like that picture a lot. You have probably described it several times by now but....
Please :)
It’s a glow-plug.
Diesel engines don’t use spark plugs.
The fuel is ignited by compression induced heat.
During the start-up and early running of the engine there isn’t enough heat to run the engine so these little beauties glow white hot to start with then red hot after the engine starts. They turn off after 20 mins, as the cylinder head is hot enough to sustain the ignition process using compression alone, at 21:1 ratio.
Neither of those two have enough energy to spark, and could only glow in an emergency. Possably. :)