
The interplanetary magnetic field (IMF), also 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
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
flow to fill the
Solar System
The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...
.
Coronal and solar wind plasma
The
coronal and solar wind
plasmas are highly
electrically conductive
Electrical resistivity (also called volume resistivity or specific electrical resistance) is a fundamental specific property of a material that measures its electrical resistance or how strongly it resists electric current. A low resistivity in ...
, meaning the
magnetic field lines and the plasma flows are
effectively "frozen" together and the magnetic field cannot
diffuse
Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...
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. However, with increasing altitude through the corona, the solar wind accelerates as it extracts energy from the magnetic field through the
Lorentz force
In electromagnetism, the Lorentz force is the force exerted on a charged particle by electric and magnetic fields. It determines how charged particles move in electromagnetic environments and underlies many physical phenomena, from the operation ...
interaction, resulting 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 IMF. This acceleration often leads the IMF to be locally
supersonic
Supersonic speed is the speed of an object that exceeds the speed of sound (Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...
up to 160 AU away from the sun.
The dynamic
pressure
Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...
of the wind dominates over the
magnetic pressure through most of the Solar System (or
heliosphere
The heliosphere is the magnetosphere, astrosphere, and outermost atmospheric layer of the Sun. It takes the shape of a vast, tailed bubble-like region of space. In plasma physics terms, it is the cavity formed by the Sun in the surrounding ...
), so that the magnetic field is pulled into an
Archimedean spiral
The Archimedean spiral (also known as Archimedes' spiral, the arithmetic spiral) is a spiral named after the 3rd-century BC Ancient Greece, Greek mathematician Archimedes. The term ''Archimedean spiral'' is sometimes used to refer to the more gene ...
pattern (the
Parker spiral) 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
current sheet (an
electric current
An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...
that is confined to a curved plane). This
heliospheric current sheet has a shape similar to a twirled
ballerina skirt
A ballerina skirt, also referred to as a Juliet skirt or a romance skirt, is a full skirt that is worn by ballet dancers and is composed of multiple layers of fabric. Ballet dancers wear the longer version of the skirt, while for fashion purposes ...
, and changes in shape through the solar cycle as the Sun's magnetic field reverses about every 11 years.
Magnetic field at Earth orbit

The
plasma in the
interplanetary medium
The interplanetary medium (IPM) or interplanetary space consists of the mass and energy which fills the Solar System, and through which all the larger Solar System bodies, such as planets, dwarf planet
A dwarf planet is a small planetary ...
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 (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 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
Space weather is a branch of space physics and aeronomy, or heliophysics, concerned with the varying conditions within the Solar System and its heliosphere. This includes the effects of the solar wind, especially on the Earth's magnetosphere, ion ...
." 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
In celestial mechanics, the Lagrange points (; also Lagrangian points or libration points) are points of equilibrium for small-mass objects under the gravitational influence of two massive orbiting bodies. Mathematically, this involves t ...
L1; since July 2016, it has been monitored by the
Deep Space Climate Observatory (DSCOVR) satellite, also at the Sun–Earth L1 (with the ACE continuing to serve as a back-up measurement).
The Interplanetary Magnetic Field (IMF)
Space Weather Live. Retrieved 11 February 2020.
See also
* Solar magnetic field
*Solar wind
The solar wind is a stream of charged particles released from the Sun's outermost atmospheric layer, the Stellar corona, corona. This Plasma (physics), plasma mostly consists of electrons, protons and alpha particles with kinetic energy betwee ...
*Magnetosphere
In astronomy and planetary science, a magnetosphere is a region of space surrounding an astronomical object in which charged particles are affected by that object's magnetic field. It is created by a celestial body with an active interior Dynamo ...
References
{{Magnetosphere
Solar System
Outer space
Magnetism in astronomy