Editing Magnetopause (section)

==Characteristics==
[[File:dipole-and-image-dipole.png|thumb|300px|Schematic representation of a planetary dipole magnetic field in a vacuum (right side) deformed by a region of plasma with infinite conductivity. The Sun is to the left. The configuration is equivalent to an image dipole (green arrow) being placed at twice the distance from the planetary dipole to the interaction boundary.<ref name="CB1940">{{cite book|title=Geomagnetism, Vol. II|date=1940|publisher=[[Oxford Univ. Press]] | author=Sydney Chapman|author2=J. Bartels}}</ref>]]

Prior to the age of space exploration, interplanetary space was considered to be a vacuum.  The coincidence of the first observation of a [[solar flare]] and the [[geomagnetic storm of 1859]] was evidence that plasma was ejected from the Sun during the flare event.  Chapman and Ferraro<ref>{{cite journal|last=Chapman|first=Sidney|author2=V. C. A. Ferraro|title=A new theory of magnetic storms|journal=Terrestrial Magnetism| date=1931| volume=36| issue=2| pages=77–97| doi=10.1029/TE036i002p00077| bibcode=1931TeMAE..36...77C}}</ref><ref>{{cite journal|last=Chapman|first=Sidney| author2=V. C. A. Ferraro|title=A new theory of magnetic storms|journal=Terrestrial Magnetism| date=1931| volume=36| issue=3| pages=171–186| doi=10.1029/TE036i003p00171|bibcode=1931TeMAE..36..171C}}</ref><ref>{{cite journal| last=Chapman|first=Sidney| author2=V. C. A. Ferraro|title=A new theory of magnetic storms, II. The main phase| journal=Terrestrial Magnetism|date=1933|volume=38|pages=79|doi=10.1029/TE038i002p00079}}</ref><ref>{{cite journal| last=Chapman|first=Sidney|author2=V. C. A. Ferraro|title= The theory of the first phase of the geomagnetic storm |journal=Terrestrial Magnetism|date=1940|volume=45|issue=3| pages=245|doi=10.1029/te045i003p00245| bibcode=1940TeMAE..45..245C}}</ref> proposed that a plasma was emitted by the Sun in a burst as part of a flare event which disturbed the planet's magnetic field in a manner known as a geomagnetic storm.  The collision frequency of particles in the plasma in the interplanetary medium is very low and the electrical conductivity is so high that it could be approximated to an infinite conductor.  

A magnetic field in a vacuum cannot penetrate a volume with infinite conductivity.  Chapman and Bartels (1940)<ref name="CB1940" /> illustrated this concept by postulating a plate with infinite conductivity placed on the dayside of a planet's dipole as shown in the schematic.  The field lines on the dayside are bent.  At low latitudes, the magnetic field lines are pushed inward.  At high latitudes, the magnetic field lines are pushed backwards and over the polar regions.  The boundary between the region dominated by the planet's magnetic field (i.e., the [[magnetosphere]]) and the plasma in the interplanetary medium is the magnetopause.  The configuration equivalent to a flat, infinitely conductive plate is achieved by placing an image dipole (green arrow at left of schematic) at twice the distance from the planet's dipole to the magnetopause along the planet-Sun line.  Since the solar wind is continuously flowing outward, the magnetopause above, below and to the sides of the planet are swept backward into the geomagnetic tail as shown in the artist's concept.  The region (shown in pink in the schematic) which separates field lines from the planet which are pushed inward from those which are pushed backward over the poles is an area of weak magnetic field or day-side cusp.  Solar wind particles can enter the planet's magnetosphere through the cusp region.  Because the solar wind exists at all times and not just times of solar flares, the magnetopause is a permanent feature of the space near any planet with a magnetic field.

The magnetic field lines of the planet's magnetic field are not stationary.  They are continuously joining or merging with magnetic field lines of the interplanetary magnetic field in a process called [[magnetic reconnection]].  The joined field lines are swept back over the poles into the planetary magnetic tail.  In the tail, the field lines from the planet's magnetic field are re-joined and start moving toward night-side of the planet. The physics of this process was first explained by Dungey (1961).<ref>{{cite journal| last=Dungey|first=J. W.|title=Interplanetary Magnetic Field and the Auroral Zones|journal=Phys. Rev. Lett.| date=Jan 1961|volume=6|issue=2|pages=47–48|url=http://prl.aps.org/|access-date=12 July 2011|bibcode = 1961PhRvL...6...47D |doi = 10.1103/PhysRevLett.6.47 }}</ref> As such, the process is now referred to as the [[Dungey Cycle]].

If one assumed that magnetopause was just a boundary between a magnetic field in a vacuum and a plasma with a weak magnetic field embedded in it, then the magnetopause would be defined by electrons and ions penetrating one gyroradius into the magnetic field domain.  Since the gyro-motion of electrons and ions is in opposite directions, an electric current flows along the boundary.  The actual magnetopause is much more complex.<ref>Physics of the Magnetopause, Edited by P. Song, B. U. Ö. Sonnerup, [[Michelle Thomsen|M. F. Thomsen]], American Geophys. Union, Washington, D.C., Geophysical Monograph Series, Volume 90, 1995. 447 pages, {{ISBN|0-87590-047-X}}</ref>