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What are those white lines connecting the ground to the sky on the left side of this photo?

I've see these before in the nuclear bomb test films too. They're apparently already in place upon detonation, as the bright backlight from the mushroom cloud suddenly makes these lines appear in the foreground.

I imagine test devices are being shot into the sky before detonation but I'd like to know more about this. Google searching has been fruitless as I don't even know what words I'm supposed to be searching for.

Looking for a brief explanation along with a source citation.

enter image description here

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Sounding rockets or smoke flares may be launched just before a device explodes so that their vapor trails may be used to record the passage of the otherwise invisible shock wave.

To learn about every detail of these tests, contact your nearest FBI agent.

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Thanks! I don't need intricate detail... just wanted to know what they were. – Sparky Nov 14 '11 at 19:21

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