Editing Earth's magnetic field (section)

== Magnetosphere ==
{{See also|Magnetosphere}}
[[File:Magnetosphere Levels.svg|thumb|An artist's rendering of the structure of a magnetosphere. 1)&nbsp;Bow shock. 2)&nbsp;Magnetosheath. 3)&nbsp;Magnetopause. 4)&nbsp;Magnetosphere. 5)&nbsp;Northern tail lobe. 6)&nbsp;Southern tail lobe. 7)&nbsp;Plasmasphere.]]

Earth's magnetic field, predominantly dipolar at its surface, is distorted further out by the solar wind. This is a stream of charged particles leaving the [[solar corona|Sun's corona]] and accelerating to a speed of 200 to 1000&nbsp;kilometres per second. They carry with them a magnetic field, the [[interplanetary magnetic field]] (IMF).<ref name="MerrillMagnetosphere">{{harvnb|Merrill|2010}}, pages 126–141</ref>

The solar wind exerts a pressure, and if it could reach Earth's atmosphere it would erode it. However, it is kept away by the pressure of the Earth's magnetic field. The [[magnetopause]], the area where the pressures balance, is the boundary of the magnetosphere. Despite its name, the magnetosphere is asymmetric, with the sunward side being about 10&nbsp;[[Earth radius|Earth radii]] out but with the other side stretching out in a [[magnetotail]] that extends beyond 200&nbsp;Earth radii.<ref name="ParksIntro" />
Sunward of the magnetopause is the [[bow shock]], the area where the solar wind slows abruptly.<ref name="MerrillMagnetosphere" />

Inside the magnetosphere is the [[plasmasphere]], a donut-shaped region containing low-energy charged particles, or [[Plasma (physics)|plasma]]. This region begins at a height of 60&nbsp;km, extends up to 3 or 4 Earth radii, and includes the ionosphere. This region rotates with the Earth.<ref name="ParksIntro" /> There are also two concentric tire-shaped regions, called the [[Van Allen radiation belt]]s, with high-energy ions (energies from 0.1 to 10&nbsp;[[Electronvolt|MeV]]). The inner belt is 1–2&nbsp;Earth radii out while the outer belt is at 4–7&nbsp;Earth radii. The plasmasphere and Van Allen belts have partial overlap, with the extent of overlap varying greatly with solar activity.<ref>{{cite press release |last1=Darrouzet |first1=Fabien |last2=De Keyser |first2=Johan |last3=Escoubet |first3=C. Philippe |date=10 September 2013 |title=Cluster shows plasmasphere interacting with Van Allen belts |url=http://sci.esa.int/cluster/52802-cluster-shows-plasmasphere-interacting-with-van-allen-belts/   |publisher=European Space Agency |access-date=22 October 2013}}</ref>

As well as deflecting the solar wind, the Earth's magnetic field deflects [[cosmic ray]]s, high-energy charged particles that are mostly from outside the [[Solar System]]. Many cosmic rays are kept out of the Solar System by the Sun's magnetosphere, or [[heliosphere]].<ref>{{cite news |title=Shields Up! A breeze of interstellar helium atoms is blowing through the solar system |author=<!-- Author not provided --> |date=27 September 2004 |newspaper=Science@NASA |url=https://science.nasa.gov/science-news/science-at-nasa/2004/27sep_shieldsup/ |access-date=23 October 2013 |archive-date=21 June 2013 |archive-url=https://web.archive.org/web/20130621115829/http://science.nasa.gov/science-news/science-at-nasa/2004/27sep_shieldsup/ |url-status=dead }}</ref> By contrast, astronauts on the Moon risk exposure to radiation. Anyone who had been on the Moon's surface during a particularly violent solar eruption in 2005 would have received a lethal dose.<ref name="MerrillMagnetosphere" />

Some of the charged particles do get into the magnetosphere. These spiral around field lines, bouncing back and forth between the poles several times per second. In addition, positive ions slowly drift westward and negative ions drift eastward, giving rise to a [[ring current]]. This current reduces the magnetic field at the Earth's surface.<ref name="MerrillMagnetosphere" /> Particles that penetrate the ionosphere and collide with the atoms there give rise to the lights of the [[aurora (astronomy)|aurorae]] while also emitting [[X-rays]].<ref name="ParksIntro" />

The varying conditions in the magnetosphere, known as [[space weather]], are largely driven by solar activity. If the solar wind is weak, the magnetosphere expands; while if it is strong, it compresses the magnetosphere and more of it gets in. Periods of particularly intense activity, called [[geomagnetic storm]]s, can occur when a [[coronal mass ejection]] erupts above the Sun and sends a shock wave through the Solar System. Such a wave can take just two days to reach the Earth. Geomagnetic storms can cause a lot of disruption; [[Halloween solar storms, 2003|the "Halloween" storm of 2003]] damaged more than a third of NASA's satellites. The largest documented storm, the [[Carrington Event]], occurred in 1859. It induced currents strong enough to disrupt telegraph lines, and aurorae were reported as far south as Hawaii.<ref name="MerrillMagnetosphere" /><ref>{{cite journal |first=Sten |last=Odenwald |title=The great solar superstorm of 1859 |journal=Technology Through Time |volume=70 |year=2010 |url=http://sunearthday.gsfc.nasa.gov/2010/TTT/70.php |archive-url=https://web.archive.org/web/20091012045135/http://sunearthday.gsfc.nasa.gov/2010/TTT/70.php |url-status=dead |archive-date=12 October 2009 |access-date=24 October 2013}}</ref>