Astrophysics (Index)About

magnetic field

(magnetic force as distributed over a space)

Astronomical bodies, including stars, planets and galaxies can have magnetic fields, generally generated by a dynamo (rotating material that is electrically conductive). They also be a remnant of earlier magnetism, preserved by ferromagnetism, the "permanent magnet" effect of iron and some other materials.

An object's magnetic field can be dipole, basically arranged with the two magnetic polarities in opposite directions, or multipole, arranged so that more than one region of its surface has each polarity. The dynamo includes moving material conducting electricity, e.g., something conductive convecting. A body (planet or star) showing a dipole field suggests the magnetic field's origin is largely the product of a single large dynamo or aligned dynamos. A multipole field may be the result of ferromagnetism generated at an earlier age, or may be multiple misaligned dynamos. The latter is more likely in larger bodies and in slower rotation. A magnetic field can be "basically" dipole, i.e., include only small, weak regions other than what is expected in a dipole. Jupiter has a dipole field, but also magnetic spots, which might share characteristics of sunspots. (The largest is informally termed the Great Blue Spot though it is not actually visible.)

An angular power spectrum of the magnetic field strength (a magnetic power spectrum, using power in the sense of "the square of the multipole expansion coefficients") around a spherical magnetized object (or sphere-shaped surface concentric with the center of an object) yields a characteristic of the field, e.g., to what degree it is organized into multiple poles at various scales.

There is a tendency to align a body's magnetic field with its rotation, but it can be off, often by several degrees, a dipole tilt. Some cited solar system magnetic fields (sources I've found are not always consistent):

Body Topology Dipole Tilt Field at equator
Mercury dipole 14° 0.01×Earth's
Venus N/A N/A basically none
Earth dipole 11° about 0.3 gauss
Moon N/A N/A basically none
Mars N/A N/A basically none
Jupiter dipole 10° 14×Earth's
Ganymede dipole 0.024×Earth's
Saturn dipole basically none 0.71×Earth's
Uranus multipole -59° 0.74×Earth's
Neptune multipole -47° 0.42×Earth's
Pluto N/A N/A basically none

Io, Europa, Callisto, and Titan have basically none. The Sun's varies over its 22-year cycle during which it flips polarity twice. Its topology varies, more dipole-like during solar minimum (fewest sunspots), the tilt for most of the cycle is 10° or less, and its magnetic flux density is on the order of 100 times Earth's. A magnetic field throughout much of the solar system, the interplanetary magnetic field (IMF or heliospheric magnetic field, HMF) is effectively carried out from the Sun by the solar wind, electrically-conductive plasma. The Earth's magnetic field extends through a region within the Moon's orbit, generating the Van Allen belts out of solar wind particles.

Current models and simulations produces some of the features seen in the various solar system magnetic fields, but have not been made to consistently reproduce all the observed features.

Stellar magnetic fields (beyond that of the Sun) can be detected and studied through Zeeman-Doppler imaging. Compact objects have strong fields. The entire Milky Way has a magnetic field (galactic magnetic field or GMF, terms that may also sometimes be used to refer to other galaxies), generally a few μg, e.g., 6 μg in the general region around the Sun (solar neighborhood). Among the methods of detecting ISM magnetic fields:

Dust emission polarization has been found to correspond across the sky with neutral atomic hydrogen, which gives hints to the 3-D structure of the galactic magnetic field.

Further reading:

Referenced by pages:
AGN corona
Alfvén wave
adaptive mesh refinement (AMR)
Ap star
atmospheric escape
anomalous X-ray pulsar (AXP)
beta (β)
black hole shadow
binary star
Blandford-Znajek mechanism (BZ process)
conformal field theory (CFT)
Chandrasekhar limit
CMB polarization
coronal mass ejection (CME)
cyclotron emission radiation spectroscopy (CRES)
crustal magnetism
current sheet
curvature radiation
cyclotron radiation
dark matter
electric dipole radiation
equilibrium condensation model
extra-solar planet
Faraday rotation
field lines
flux freezing
flux rope
Forbush decrease
gauss (G)
geomagnetic storm
giant planet
gravitomagentic field
magnetic field strength (H)
Hall effect
Hanle effect
Hayashi limit
high-B radio pulsar (HBRP)
hypermassive neutron star (HMNS)
inflated radii
interplanetary medium (IPM)
interstellar magnetic field (ISMF)
Lorentz force
lunar swirl
magnetic anomaly
magnetic dipole braking
magnetic dipole radiation
magnetic energy spectrum
magnetic flux (Φ)
magnetic flux density (B)
magnetic induction
magnetosonic wave
magnetospheric truncation radius
mass spectrometer
mathematical field
Maxwell's equations
Magnetospheric Multiscale Mission (MMS)
molecular cloud
magnetorotational instability (MRI)
multi-messenger astronomy
permeability (κ)
physical field
plasma wave
polarization modes
Poynting vector (S)
Project 8
pulsar (PSR)
pulsar characteristic age (τ)
pulsar wind nebula (PWN)
quantum field theory (QFT)
quadratic field strength
quantum mechanics (QM)
radiation belt
magnetic reconnection
Reynolds number (Re)
SGR J1745-2900
chromospheric activity index
solar flare
spinning dust emission
standard model of a flare
stellar flare
Sun surface features
synchrotron radiation
tesla (T)
Tianwen-1 (TW-1)
torus coordinates
Tolman-Oppenheimer-Volkoff limit (TOV)
Van Allen belts
variable star
Vlasov-Poisson equation
white dwarf (WD)
X-ray source
young stellar object (YSO)
Zeeman-Doppler imaging (ZDI)
Zeeman effect
zonal flow