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 When we look at the Sun (NEVER EVER look directly at the Sun) we often see that there’s more to it than just a ball of hot gas.  Sunspots  Prominences / Flares  Coronal Mass Ejections (CME)  Solar Storms 2

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 All these phenomena seem related : when there’s more sunspots, there tend to be more prominences, flares, ….  We can get an idea about “solar activity” by counting the number of sunspots ! 10

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 All these phenomena seem related : when there’s more sunspots, there tend to be more prominences, flares, ….  We can get an idea about “solar activity” by counting the number of sunspots !   Average length between two subsequent solar maxima is 11 years.  Location on the Sun depends on where in the cycle we are: butterfly diagrams. 12

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..this is how we know that a new solar cycle started 2 years ago with the occurrence of a first group of sunspots at high latitude! 14

 Let’s have a look at what sunspots really are…. 15

 Sunspots appear dark because they are cooler than the surrounding gas – about 4,000 K.  They continue to exist often for weeks.  If they stay “cool” for so long, something must be preventing hot gas from entering the sunspot area…..  This something turns out to be strong magnetic fields. 16

17 Charged particles have to follow the magnetic field, and thus cannot cross the field lines!

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 Sunspots occur often in pairs, connected by a loop of magnetic field lines.  Gas becomes trapped in these loops, and the result is a prominence.  Prominences can also exist for days or weeks. 19

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 So sunspots and other phenomena related to solar activity are due to magnetic fields.  So why does the Sun have an 11 year cycle then?  Well, we don’t fully understand.  Some combination of magnetic fields, rotation and convection. 21

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 Near the poles, magnetic field lines project out into space.  Along these field lines, charged particles are streaming outward at typically 500 km/s.  This is the Solar wind.  Remember the importance of the Solar Wind in blowing away the hydrogen and helium at some early stage in our Solar System history. 23

 When magnetic field lines become very twisted and knotted, they can sometimes no longer bear the tension and suddenly snap and reorganize themselves.  This generates a tremendous amount of energy, heating the gas (temporarily) to 100 million K and accelerating some of the charged particles to nearly the speed of light : a solar flare. 24

 Large flare will result in large number of particles being ejected – a Coronal Mass Ejection (CME).  If this happens to be aimed in Earth’s direction, the result is a geomagnetic storm.  Forecasting such storm is now dubbed “spaceweather” forecasting. 25

 Beautiful auroras (northern lights)…  … but also disruptions to radio communications, electrical power and electronic components (especially in orbiting satellites).  Example: March 13, 1989: geomagnetic storm disrupted power throughout most of Quebec  Superstorm of 1859: shorted out telegraph wires, with auroras visible even in Hawaii…. 26

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28 Different colors correspond to different transitions of elements in the atmosphere. Oxygen is typically green and red; blue/violet/purple typically nitrogen.