 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Interplanetary magnetic field" – news ·newspapers ·books ·scholar ·JSTOR(December 2016) (Learn how and when to remove this message) |
Theinterplanetary magnetic field (IMF), also commonly referred to as theheliospheric magnetic field (HMF),[2] is the component of thesolar magnetic field that is dragged out from the solarcorona by thesolar wind flow to fill theSolar System.
Thecoronal and solar windplasmas are highlyelectrically conductive, meaning themagnetic field lines and the plasma flows areeffectively "frozen" together[3][4] and the magnetic field cannotdiffuse through the plasma on time scales of interest. In the solar corona, the magnetic pressure greatly exceeds the plasma pressure and thusthe plasma is primarily structured and confined by the magnetic field. However, with increasing altitude through the corona, the solar wind accelerates as it extracts energy from the magnetic field through theLorentz force interaction, resulting in the flow momentum exceeding the restrainingmagnetic tension force and the coronal magnetic field is dragged out by the solar wind to form the IMF. This acceleration often leads the IMF to be locallysupersonic up to 160 AU away from the sun.[5]
The dynamicpressure of the wind dominates over themagnetic pressure through most of the Solar System (orheliosphere), so that the magnetic field is pulled into anArchimedean spiral pattern (theParker spiral[6]) by the combination of the outward motion and theSun's rotation. In near-Earth space, the IMF nominally makes an angle of approximately 45° to the Earth–Sun line, though this angle varies with solar wind speed. The angle of the IMF to the radial direction reduces with helio-latitude, as the speed of the photospheric footpoint is reduced.
Depending on the polarity of the photospheric footpoint, the heliospheric magnetic field spirals inward or outward; the magnetic field follows the same shape of spiral in the northern and southern parts of the heliosphere, but with opposite field direction. These two magnetic domains are separated by acurrent sheet (anelectric current that is confined to a curved plane). Thisheliospheric current sheet has a shape similar to a twirledballerina skirt, and changes in shape through the solar cycle as the Sun's magnetic field reverses about every 11 years.
Theplasma in theinterplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth being over 100 times greater than originally anticipated. If space were a vacuum, then the Sun's magnetic dipole field — about 10−4teslas at the surface of the Sun[citation needed] — would reduce with the inverse cube of the distance to about 10−11 teslas. But satellite observations show that it is about 100 times greater at around 10−9 teslas.[citation needed]Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field induces electric currents, which in turn generates magnetic fields — and, in this respect, it behaves like anMHD dynamo.[citation needed]
The interplanetary magnetic field at the Earth's orbit varies with waves and other disturbances in the solar wind, known as "space weather." The field is a vector, with components in the radial and azimuthal directions as well as a component perpendicular to the ecliptic. The field varies in strength near the Earth from 1 to 37 nT, averaging about 6 nT.[7] Since 1997, the solar magnetic field has been monitored in real time by theAdvanced Composition Explorer (ACE) satellite located in a halo orbit at the Sun–EarthLagrange Point L1; since July 2016, it has been monitored by theDeep Space Climate Observatory (DSCOVR) satellite, also at the Sun–Earth L1 (with the ACE continuing to serve as a back-up measurement).[8]
