I think the answer has more than adequately been given, but I just had to add at least one pic I took when on holiday in Sweden last year.
Driving back from Kiruna in late November on the E10 I heard a lovely sound, and looked up to see the odd contrail from a few JAS 39 Gripens from F21 (assumed) at Luleå. It was hard to follow what was happening +30,000ft above me, but from the engine noises, contrails, and odd flash of reflected winter light and colour from the aircraft I believe there were at least four Gripens up on an Air-Air training exercise.
I assumed when I took this photo that after being "Killed" the dead aircraft had to exit the engagement to the east, fly a racetrack orbit, before being able to leave the holding point and re-engage to the west (flying right over head in the process)
It was interesting as I figured you could tell the change in throttle setting by the density of the contrail, then several moments later you could hear the change. Unfortunately I didn't witness much of the actual engagements due to light cirrus clouds, occasionally masking the contrail, and I feel the actual engagements were taking place below the freezing layer.
Eventually they made their way further west or onto another ex, so I hurriedly drove the remaining 100km back to the E4
. I couldn't believe it, but +1hr later as I approached the bridge crossing the river prior to the Luleå Flygstation exit I saw two Gripens do a low overfly of the airfield break into a tight circuit from upwind and then disappear below the treeline while turning onto final. By the time I got to the military side of the airfield they would've been shut down and the pilots most likely returning their kit.
Things I learnt - 3x optical zoom isn't enough, and when aircraft are about and driving, always have your camera handy!
Sorry for the long and irrelevant post - but hey, the RM12 is just variant of the F404 that powers the Hornet - and yes they contrail!