Interplanetary magnetic field

The interplanetary magnetic field (IMF), commonly referred to as the heliospheric magnetic field (HMF), is the component of the solar magnetic field that is dragged out from the solar corona by the solar wind flow to fill the Solar System.

The coronal and solar wind plasmas are highly electrically conductive, meaning the magnetic field lines and the plasma flows are effectively “frozen” together and the magnetic field cannot diffuse through the plasma on time scales of interest. In the solar corona, the magnetic pressure greatly exceeds the plasma pressure and thus the plasma is primarily structured and confined by the magnetic field.

With increasing altitude through the corona, solar wind acceleration results in the flow momentum exceeding the restraining magnetic tension force and the coronal magnetic field is dragged out by the solar wind to form the HMF.

The dynamic pressure of the wind dominates over the magnetic pressure through most of the Solar System (or heliosphere), so that the magnetic field is pulled into an Archimedean spiral pattern by the combination of the outward motion and the Sun’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 a two current sheet (an electric current that is confined to a curved plane). This heliospheric current sheet has a shape similar to a twirled ballerina skirt, and changes in shape through the solar cycle as the Sun’s magnetic field reverses about every 11 years.

The plasma in the interplanetary 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 10−4 teslas at the surface of the Sun — 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. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid in a magnetic field induces electric currents, which in turn generates magnetic fields — and, in this respect, it behaves like an MHD dynamo.

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.

Since 1997, the solar magnetic field has been monitored in real time by the Advanced Composition Explorer (ACE) satellite located in a halo orbit at the Sun–Earth Lagrange Point L1; since July 2016, it has been monitored by the Deep Space Climate Observatory (DSCOVR) satellite, and also at Sun–Earth L1 (with the ACE continuing to serve as a back-up measurement).

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