![\"Caption:](\"https:\/\/blog.aurorasaurus.org\/wp-content\/uploads\/2017\/09\/usatoday.png\")
<\/a>The graphic focuses primarily on the solar wind, or the continuous flow of plasma \u2014 a high-temperature mix of charged particles \u2014 from the sun. \u00a0But while the solar wind is essential to understanding auroras, there are other factors involved in creating these celestial light displays. By only considering the solar wind, we leave some key questions unanswered. \u00a0For example,<\/span> why do we see the aurora at night <\/b>(when we\u2019re facing away from the sun)?<\/span><\/p>\n Before we can tackle that, we need to recognize that charged particles aren\u2019t the only things the sun sends us. \u00a0The sun is actually a giant magnet \u2014 and just like an ordinary bar magnet, it has a north and a south pole, with invisible magnetic field lines stretching from one to the other. \u00a0As scientists first discovered in the 1800s, electricity and magnetism have a very special relationship with each other. \u00a0A defining feature of this relationship is that electric charges can\u2019t cross magnetic field lines. \u00a0Instead, they spiral around them, like this:<\/span><\/p>\n You can think of the particles as being \u201ctrapped\u201d by the field lines. \u00a0And that\u2019s precisely what\u2019s happening in the solar wind! \u00a0Bound up in the plasma are bits of the sun\u2019s magnetic field. \u00a0Earth is a giant magnet, too, and ultimately it\u2019s the interaction between the sun\u2019s magnetic field and Earth\u2019s that leads to the aurora. \u00a0The underlying mechanism of that interaction, which transfers energy between the two fields, is known as <\/span>magnetic reconnection<\/span><\/i><\/a>.<\/span><\/i><\/p>\n The idea behind reconnection is fairly simple: \u00a0magnetic field lines meet, break off, and form connections with other field lines. \u00a0But what does that mean, exactly? \u00a0Have a look:<\/span><\/p>\n Each magnetic field line has a direction (as indicated by the arrows). \u00a0By convention, we say that a magnet\u2019s field lines extend from the north pole to the south pole. \u00a0(Fun fact: \u00a0<\/i>the Earth\u2019s geographic north pole is actually its magnetic south pole<\/i><\/a>.)\u00a0<\/i>Reconnection occurs when opposing field lines come into contact.<\/span><\/p>\n Now let\u2019s jump back to the <\/span>USA Today<\/span><\/i> graphic. \u00a0See how Earth\u2019s magnetic field is depicted there? \u00a0It\u2019s a gray circle, with Earth in the middle. \u00a0In reality, it looks more like this:<\/span><\/p>\n When the solar wind hits Earth\u2019s magnetic field, the wind slows down dramatically, creating a shock wave called the <\/span>bow shock.<\/span><\/i> \u00a0The bow shock diverts the stream of charged particles (and the magnetic field it carries) into paths that run alongside Earth\u2019s magnetic field. \u00a0In certain spots, the solar and planetary magnetic fields point in opposite directions, and reconnection takes place. \u00a0The resulting new field lines \u2014 a combination of the sun\u2019s and Earth\u2019s \u2014 are swept back toward the magnetotail. \u00a0<\/span><\/p>\n The process is a bit like <\/span>swimming breaststroke in an endless pool<\/span><\/a> within the ecliptic plane. \u00a0We\u2019ll explore that analogy further in an upcoming post. \u00a0Stay tuned!<\/span><\/p>\n So why does aurora happen when we\u2019re facing away from the sun? Reconnection also happens on Earth\u2019s night side. \u00a0As the energy from the solar wind is transferred into the magnetotail, it interacts with the charged particles already trapped there. \u00a0When opposing field lines approach one another in the magnetotail, a portion of the magnetic field pinches off into space. \u00a0The rest, meanwhile, <\/span>snaps back toward us<\/span><\/a>. <\/span><\/p>\n As the charged particles accelerate toward Earth\u2019s poles, they pick up even more energy. \u00a0Once they slam into the atmosphere, that energy is transferred to the atoms they encounter. \u00a0But the atoms can\u2019t hold on to that extra energy for long \u2014 so they re-emit it as light. \u00a0The color of the light corresponds to the amount of energy absorbed by a given atom, as well as the type of atom it is. \u00a0Oxygen, for example, can produce either green or red emissions, whereas nitrogen\u2019s are generally violet or blue. <\/span><\/p>\n These emissions are what we perceive as the aurora.<\/span><\/p>\n Now you know the real story: \u00a0<\/span>magnetic reconnection in the magnetotail<\/i><\/strong> is why you can see the aurora even when you\u2019re facing away from the sun. \u00a0Nighttime auroras cannot be explained by the solar wind alone! \u00a0The magnetosphere is a complex jungle of plasma, and it\u2019s the greatest physics lab we can actually sample so let\u2019s make sure we give credit where credit is due!<\/span><\/p>\n <\/p>\n <\/p>\n![\"A](\"https:\/\/blog.aurorasaurus.org\/wp-content\/uploads\/2017\/09\/bfield.png\")
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By Maschen (Own work) [CC0], via Wikimedia Commons<\/figcaption><\/figure>\n![\"By](\"https:\/\/blog.aurorasaurus.org\/wp-content\/uploads\/2017\/09\/Reconnection.gif\")
<\/a>![\"By](\"https:\/\/blog.aurorasaurus.org\/wp-content\/uploads\/2017\/09\/Magnetosphere_rendition.jpg\")
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