Editing Aurora (section)

== Interaction of the solar wind with Earth ==
Earth is constantly immersed in the [[solar wind]], a flow of magnetized hot plasma (a gas of free electrons and positive ions) emitted by the Sun in all directions, a result of the two-million-degree temperature of the Sun's outermost layer, the [[solar corona|corona]]. The solar wind reaches Earth with a velocity typically around 400&nbsp;km/s, a density of around 5 ions/cm<sup>3</sup>, and a magnetic field intensity of around 2–5 nT (for comparison, Earth's surface field is typically 30,000–50,000 nT). During [[magnetic storm]]s, in particular, flows can be several times faster; the [[interplanetary magnetic field]] (IMF) may also be much stronger. [[Joan Feynman]] deduced in the 1970s that the long-term averages of solar wind speed correlated with geomagnetic activity.<ref>{{cite journal|url=https://ntrs.nasa.gov/search.jsp?R=19770051690|title=On the high correlation between long-term averages of solar wind speed and geomagnetic activity|journal=Journal of Geophysical Research|author1=Crooker, N. U.|author2=Feynman, J.|author3=Gosling, J. T.|date=1 May 1977|volume=82|issue=13|page=1933|doi=10.1029/JA082i013p01933|bibcode=1977JGR....82.1933C|access-date=10 November 2017|archive-date=4 November 2016|archive-url=https://web.archive.org/web/20161104205820/https://ntrs.nasa.gov/search.jsp?R=19770051690|url-status=live}}</ref> Her work resulted from data collected by the [[Explorer 33]] spacecraft.

The solar wind and magnetosphere consist of [[Plasma (physics)|plasma]] (ionized gas), which conducts electricity. It is well known (since [[Michael Faraday]]'s work around 1830) that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cuts ''across'' (or is cut ''by''), rather than ''along'', the lines of the magnetic field, an electric current is induced within the conductor. The strength of the current depends on a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together, and d) the distance between the conductor and the magnetic field, while the ''direction'' of flow is dependent upon the direction of relative motion. [[Dynamo]]s make use of this basic process ("the [[dynamo theory|dynamo effect]]"), any and all conductors, solid or otherwise are so affected, including plasmas and other fluids.

The IMF originates on the Sun, linked to the [[sunspot]]s, and its [[magnetism|field lines (lines of force)]] are dragged out by the solar wind. That alone would tend to line them up in the Sun-Earth direction, but the rotation of the Sun angles them at Earth by about 45 degrees forming a spiral in the ecliptic plane, known as the [[Eugene Parker|Parker spiral]]. The field lines passing Earth are therefore usually linked to those near the western edge ("limb") of the visible Sun at any time.<ref>[http://gse.gi.alaska.edu/recent/javascript_movie.html Alaska.edu] {{webarchive|url=https://web.archive.org/web/20061220050940/http://gse.gi.alaska.edu/recent/javascript_movie.html|date=20 December 2006 }}, Solar wind forecast from a [[University of Alaska]] website</ref>

The solar wind and the magnetosphere, being two electrically conducting fluids in relative motion, should be able in principle to generate electric currents by dynamo action and impart energy from the flow of the solar wind. However, this process is hampered by the fact that plasmas conduct readily along magnetic field lines, but less readily perpendicular to them. Energy is more effectively transferred by the temporary magnetic connection between the field lines of the solar wind and those of the magnetosphere. Unsurprisingly this process is known as [[magnetic reconnection]]. As already mentioned, it happens most readily when the interplanetary field is directed southward, in a similar direction to the geomagnetic field in the inner regions of both the [[north magnetic pole]] and [[south magnetic pole]].

Auroras are more frequent and brighter during the intense phase of the solar cycle when [[coronal mass ejections]] increase the intensity of the solar wind.<ref>{{cite web|url=http://www.nasa.gov/worldbook/aurora_worldbook.html|archive-url=https://web.archive.org/web/20050905165404/http://www.nasa.gov/worldbook/aurora_worldbook.html|url-status=dead|archive-date=5 September 2005|title=NASA – NASA and World Book|publisher=Nasa.gov|date=7 February 2011|access-date=26 July 2011}}</ref>

=== Magnetosphere ===
[[File:Structure of the magnetosphere LanguageSwitch.svg|lang=en|thumb|upright=1.4|Schematic of Earth's [[magnetosphere]]]]

Earth's [[magnetosphere]] is shaped by the impact of the solar wind on Earth's magnetic field. This forms an obstacle to the flow, diverting it, at an average distance of about 70,000&nbsp;km (11 Earth radii or R<sub>e</sub>),<ref>{{cite journal|last1=Shue|first1=J.-H|first2=J. K.|last2=Chao|first3=H. C.|last3=Fu|first4=C. T.|last4=Russell|first5=P.|last5=Song|first6=K. K.|last6=Khurana|first7=H. J.|last7=Singer|title=A new functional form to study the solar wind control of the magnetopause size and shape|journal=J. Geophys. Res.|date=May 1997|volume=102|issue=A5|pages=9497–9511|doi=10.1029/97JA00196|bibcode=1997JGR...102.9497S }}</ref> producing a [[bow shock]] 12,000&nbsp;km to 15,000&nbsp;km (1.9 to 2.4 R<sub>e</sub>) further upstream. The width of the magnetosphere abreast of Earth is typically 190,000&nbsp;km (30 R<sub>e</sub>), and on the night side, a long "magnetotail" of stretched field lines extends to great distances (> 200 R<sub>e</sub>).

The high-latitude magnetosphere is filled with plasma as the solar wind passes Earth. The flow of plasma into the magnetosphere increases with additional turbulence, density, and speed in the solar wind. This flow is favoured by a southward component of the IMF, which can then directly connect to the high latitude geomagnetic field lines.<ref>{{cite journal|last1=Lyons|first1=L. R.|first2=H.-J.|last2=Kim|first3=X.|last3=Xing|first4=S.|last4=Zou|first5=D.-Y.|last5=Lee|first6=C.|last6=Heinselman|first7=M. J.|last7=Nicolls|first8=V.|last8=Angelopoulos|first9=D.|last9=Larson|first10=J.|last10=McFadden|first11=A.|last11=Runov|first12=K.-H.|last12=Fornacon|title=Evidence that solar wind fluctuations substantially affect global convection and substorm occurrence|journal=J. Geophys. Res.|year=2009|volume=114|issue=A11306|pages=1–14|doi=10.1029/2009JA014281|bibcode=2009JGRA..11411306L|doi-access=free }}</ref> The flow pattern of magnetospheric plasma is mainly from the magnetotail toward Earth, around Earth and back into the solar wind through the [[magnetopause]] on the day-side. In addition to moving perpendicular to Earth's magnetic field, some magnetospheric plasma travels down along Earth's magnetic field lines, gains additional energy and loses it to the atmosphere in the auroral zones. The cusps of the magnetosphere, separating geomagnetic field lines that close through Earth from those that close remotely allow a small amount of solar wind to directly reach the top of the atmosphere, producing an auroral glow.

On 26 February 2008, [[THEMIS]] probes were able to determine, for the first time, the triggering event for the onset of [[magnetospheric substorm]]s.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/themis/auroras/themis_power.html|title=NASA – THEMIS Satellites Discover What Triggers Eruptions of the Northern Lights|publisher=Nasa.gov|access-date=26 July 2011| archive-url= https://web.archive.org/web/20110629043044/http://www.nasa.gov/mission_pages/themis/auroras/themis_power.html| archive-date= 29 June 2011| url-status=live}}</ref> Two of the five probes, positioned approximately one-third the distance to the Moon, measured events suggesting a [[magnetic reconnection]] event 96 seconds prior to auroral intensification.<ref>{{cite journal|doi=10.1126/science.1160495|title=Tail Reconnection Triggering Substorm Onset|year=2008|last1=Angelopoulos|first1=V.|last2=McFadden|first2=J. P.|last3=Larson|first3=D.|last4=Carlson|first4=C. W.|last5=Mende|first5=S. B.|last6=Frey|first6=H.|last7=Phan|first7=T.|last8=Sibeck|first8=D. G.|last9=Glassmeier|first9=K.-H.|journal=Science|volume=321|issue=5891|pages=931–5|pmid=18653845|bibcode=2008Sci...321..931A|last10=Auster|first10=U.|last11=Donovan|first11=E.|last12=Mann|first12=I. R.|last13=Rae|first13=I. J.|last14=Russell|first14=C. T.|last15=Runov|first15=A.|last16=Zhou|first16=X.-Z.|last17=Kepko|first17=L.|s2cid=206514133 |doi-access=free}}</ref>

[[Geomagnetic storm]]s that ignite auroras may occur more often during the months around the [[equinox]]es. It is not well understood, but geomagnetic storms may vary with Earth's seasons. Two factors to consider are the tilt of both the solar and Earth's axis to the ecliptic plane. As Earth orbits throughout the year, it experiences an interplanetary magnetic field (IMF) from different latitudes of the Sun, which is tilted at 8 degrees. Similarly, the 23-degree tilt of Earth's axis about which the geomagnetic pole rotates with a diurnal variation changes the daily average angle that the geomagnetic field presents to the incident IMF throughout the year. These factors combined can lead to minor cyclical changes in the detailed way that the IMF links to the magnetosphere. In turn, this affects which energy from the solar wind can reach Earth's inner magnetosphere and thereby enhance auroras. Recent evidence in 2021 has shown that individual separate substorms may in fact be correlated networked communities.<ref>{{cite journal|title=Network community structure of substorms using SuperMAG magnetometers, L. Orr, S. C. Chapman, J. W. Gjerloev & W. Guo|issue=1|page=1842|journal=Nature Communications|date=23 March 2021|volume=12|doi=10.1038/s41467-021-22112-4|last1=Orr|first1=L.|last2=Chapman|first2=S. C.|last3=Gjerloev|first3=J. W.|last4=Guo|first4=W.|pmid=33758181|pmc=7988152 }}</ref>