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The sky at night (meteors, Iridium flares, the moon)


Colin
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Why have the Northern Lights moved south?

 

The Northern Lights, or Aurora Borealis, are caused by charged gas particles - that flow away from the Sun as a "solar wind" - interacting with the Earth's magnetic field.

 

This solar wind has its own magnetic field, which can "drag away" the Earth's magnetic field lines, disconnecting them from our planet.

 

Eventually though, the field lines "snap back" into the continuous loop that exits one of the Earth's magnetic poles and re-enters at the other. This snapping back or "reconnection" means that particles that were in the solar wind are pushed into the Earth's atmosphere.

 

The charged particles "excite" gases in our atmosphere and make make them glow - just like gas in a fluorescent tube. The colours depend on the type of gas - a red or green glow is oxygen and the blue and purple colours are produced by nitrogen.

 

The huge ejection of charged particles from the Sun on 3 August disconnected more of the Earth's magnetic field lines and when these snapped back to Earth the resulting auroras were visible much further south than usual.

 

Dr Colin Forsyth from the UCL's Mullard Space Science Laboratory explained that the boundary between "open" (or disconnected) field lines and the rest of the Earth's magnetic field is "where aurora are most likely to occur". And when more field lines are disconnected, that boundary moves south.

 

On Wednesday night stunning light displays were seen over parts of northern Europe, including Germany and Denmark, but so far scientists say there has not been "sufficient activity" over southern parts of the UK to produce a light show.

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