Magnetogram
Encyclopedia
The term magnetogram has two meanings, used separately in the contexts of magnetic fields of the Sun
and the Earth
.
In the context of the magnetic field
of the Sun
, the term magnetogram refers to a pictorial representation of the spatial variations in strength of the solar magnetic field. Magnetograms are often produced by exploiting the Zeeman effect
(or, in some cases, the Hanle effect
), which George Ellery Hale
employed in the first demonstration that sunspots
were magnetic in origin, in 1908. Solar magnetograms are produced by suitably instrumented telescopes
referred to as magnetographs
. Some magnetographs can only measure the component of the magnetic field
along the line of sight from the observer to the source (the field's "longitudinal" component). One example of such a "line-of-sight" or "longitudinal" magnetograph is the Michelson Doppler Imager (MDI), a scientific instrument that takes magnetograms of the Sun
in order to measure velocity and magnetic fields in the Sun's photosphere
to learn about the convection zone
and about the magnetic fields which control the structure of the solar corona
. A vector magnetograph
also measures the component of the magnetic field perpendicular to the line of sight (the field's "transverse" component), from which all three components of the magnetic field
vector can be deduced. Two examples include the the National Solar Observatory's
SOLIS instrument and the Helioseismic and Magnetic Imager aboard NASA's Solar Dynamics Observatory
satellite.
In the context of geophysics
, a magnetogram is a measurement of temporal variation the local strength and direction of the geomagnetic field. Such magnetograms have existed since Victorian times, and the British Geological Survey
has preserved records from the 1850s, showing the effects of the 1859 Carrington Event
, the worst magnetic storm known to have hit Earth.
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
and the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
.
In the context of the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, the term magnetogram refers to a pictorial representation of the spatial variations in strength of the solar magnetic field. Magnetograms are often produced by exploiting the Zeeman effect
Zeeman effect
The Zeeman effect is the splitting of a spectral line into several components in the presence of a static magnetic field. It is analogous to the Stark effect, the splitting of a spectral line into several components in the presence of an electric field...
(or, in some cases, the Hanle effect
Hanle effect
The Hanle effect is a reduction in the polarisaton of light when the atoms emitting the light are subject to a magnetic field in a particular direction, and when they have themselves been excited by polarised light....
), which George Ellery Hale
George Ellery Hale
George Ellery Hale was an American solar astronomer.-Biography:Hale was born in Chicago, Illinois. He was educated at MIT, at the Observatory of Harvard College, , and at Berlin . As an undergraduate at MIT, he is known for inventing the spectroheliograph, with which he made his discovery of...
employed in the first demonstration that sunspots
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
were magnetic in origin, in 1908. Solar magnetograms are produced by suitably instrumented telescopes
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
referred to as magnetographs
Magnetograph
A magnetograph is one of two types of scientific instrument:* A special type of magnetometer that records a time plot of the local magnetic field near the instrument; or...
. Some magnetographs can only measure the component of the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
along the line of sight from the observer to the source (the field's "longitudinal" component). One example of such a "line-of-sight" or "longitudinal" magnetograph is the Michelson Doppler Imager (MDI), a scientific instrument that takes magnetograms of the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
in order to measure velocity and magnetic fields in the Sun's photosphere
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
to learn about the convection zone
Convection zone
The convection zone of a star is the range of radii in which energy is transported primarily by convection. In the radiation zone, energy is transported by radiation...
and about the magnetic fields which control the structure of the solar corona
Corona
A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph...
. A vector magnetograph
Vector magnetograph
A vector magnetograph is a type of imaging telescope that can estimate the 3-D vector of the magnetic field on a distant body with a resolved line spectrum...
also measures the component of the magnetic field perpendicular to the line of sight (the field's "transverse" component), from which all three components of the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
vector can be deduced. Two examples include the the National Solar Observatory's
National Solar Observatory
The mission of the National Solar Observatory is to advance knowledge of the Sun, both as an astronomical object and as the dominant external influence on Earth, by providing forefront observational opportunities to the research community...
SOLIS instrument and the Helioseismic and Magnetic Imager aboard NASA's Solar Dynamics Observatory
Solar Dynamics Observatory
The Solar Dynamics Observatory is a NASA mission which will observe the Sun for over five years. Launched on February 11, 2010, the observatory is part of the Living With a Star program...
satellite.
In the context of geophysics
Geophysics
Geophysics is the physics of the Earth and its environment in space; also the study of the Earth using quantitative physical methods. The term geophysics sometimes refers only to the geological applications: Earth's shape; its gravitational and magnetic fields; its internal structure and...
, a magnetogram is a measurement of temporal variation the local strength and direction of the geomagnetic field. Such magnetograms have existed since Victorian times, and the British Geological Survey
British Geological Survey
The British Geological Survey is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS headquarters are in Keyworth, Nottinghamshire, but other centres...
has preserved records from the 1850s, showing the effects of the 1859 Carrington Event
Solar storm of 1859
The solar storm of 1859, also known as the Solar Superstorm, or the Carrington Event, which occurred during solar cycle 10, was the most powerful solar storm in recorded history, and the largest flare, observed by Richard Christopher Carrington, became known as the Carrington Super...
, the worst magnetic storm known to have hit Earth.
See also
- MagnetographMagnetographA magnetograph is one of two types of scientific instrument:* A special type of magnetometer that records a time plot of the local magnetic field near the instrument; or...
- Vector MagnetographVector magnetographA vector magnetograph is a type of imaging telescope that can estimate the 3-D vector of the magnetic field on a distant body with a resolved line spectrum...
- Solar and Heliospheric ObservatorySolar and Heliospheric ObservatoryThe Solar and Heliospheric Observatory is a spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas IIAS launch vehicle on December 2, 1995 to study the Sun, and has discovered over 2100 comets. It began normal operations in May...
- Solar Dynamics ObservatorySolar Dynamics ObservatoryThe Solar Dynamics Observatory is a NASA mission which will observe the Sun for over five years. Launched on February 11, 2010, the observatory is part of the Living With a Star program...
External links
- For a discussion of solar magnetograms, see the article "Solar Magnetism: Magnetograms"
- For a repository of geophysical magnetograms, see