Sun Key Terms

Correlation

A measure of the extent to which two factors vary together, and thus of how well either factor predicts the other.

Solar Dynamics Observatory

A NASA mission which has been observing the Sun since 2010. Launched on February 11, 2010, the observatory is part of the Living With a Star program.

Solar Flare

A brilliant outbreak in the Sun's outer atmosphere, usually associated with active groups of sunspots.

Plasma

A high-temperature gas consisting entirely of ions, instead of neutral atoms or molecules. Because of the high temperature, the atoms strike each other hard enough to keep at least the outer electrons knocked off.

Sunspot

A magnetic disturbance on the Sun's surface that is cooler than the surrounding area.

Gamma Rays

A penetrating electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves and so imparts the highest photon energy.

Magnetic Field

A region around a magnetic material or a moving electric charge within which the force of magnetism acts.

SOHO

A spacecraft built by a European industrial consortium led by Matra Marconi Space that was launched on a Lockheed Martin Atlas II AS launch vehicle on December 2, 1995 to study the Sun. SOHO has also discovered over 3,000 comets.

Photosphere

A star's outer shell from which light is radiated.

Geomagnetic Storm

A temporary disturbance of the Earth's magnetosphere caused by a solar wind shock wave and/or cloud of magnetic field that interacts with the Earth's magnetic field.

X-Rays

An electromagnetic wave of high energy and very short wavelength, which is able to pass through many materials opaque to light.

Magnetic Pressure

An energy density associated with a magnetic field. Any magnetic field has an associated magnetic pressure contained by the boundary conditions on the field.

Prominence

An eruption of a flamelike tongue of relatively cool, high-density gas from the solar chromosphere into the corona where it can be seen during a solar eclipse or by observing strong spectral lines in its emission spectrum.

Solar Wind

An outrush of gas past the Earth and beyond the outer planets. Near the Earth, the solar wind travels at velocities near 600 km/s, sometimes reaching 1000 km/s.

Plage

An unusually bright region on the sun.

Auroras

Glowing, often moving colored light forms seen near the north and south magnetic poles of the Earth; caused by radiation from high-altitude air molecules excited by particles from the Sun and Van Allen belts.

Flux

Density measuring mean solar electromagnetic radiation (solar irradiance) per unit area. It is measured on a surface perpendicular to the rays, one astronomical unit (AU) from the Sun (roughly the distance from the Sun to the Earth).

Chromosphere

One of the Sun's outer layers, visible for a few minutes as a spectacular halo during during a total eclipse of the Sun.

Convection

One of three modes of transmission of heat (energy) from hot regions to cold regions; involves motions of masses of material.

Solar Eclipse

Partial or total blocking of the Sun's light by an astronomical body (in most usages, by the Moon).

Infrared

Radiation of wavelength too long to see, usually about 1 to 100 µm.

Nuclear Chain Reaction

Refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats.

Ultraviolet

Region of the electromagnetic spectrum just beyond the visible range, corrosponding to wavelengths slightly shorter than blue light.

Kelvin

The SI base unit of thermodynamic temperature, equal in magnitude to the degree Celsius.

Pixel

The angular distance between two objects on the sky that fall one pixel apart on the detector

Umbra

The dark central part of a sunspot.

Equinox

The date when the Sun passes through the Earth's equatorial plane (occurs twice annually).

Solstice

The date when the Sun reaches maximum distance from the celestial equator (occurs twice annually).

Penumbra

The less dark outer part of a sunspot, surrounding the dark core.

Solar Fusion

The merging of light atomic nuclei into heavier atomic nuclei, with the release of particles and radiation.

Maunder Minimum

The name used for the period around 1645 to 1715 during which sunspots became exceedingly rare, as was then noted by solar observers.

Sunspot Cycle

The number of sunspots seen on the "surface" of the Sun changes from year to year. This rise and fall in sunspot counts is a cycle. The length of the cycle is about eleven years on average.

Solar Convection Zone

The outer-most layer of the interior. It extends from a depth of 200,000 km up to the visible surface of the Sun. Energy is transported by convection in this region. The surface of the convection zone is where light (photons) is created.

Corona

The outermost atmosphere of the Sun, having a temperature of about 1 to 2 million Kelvin.

Solar Maximum

The period in the sunspot cycle when solar activity is highest and sunspots are most abundant.

Solar Minimum

The period in the sunspot cycle when solar activity is lowest and sunspots are least abundant.

Magnetosphere

The region surrounding the earth or another astronomical body in which its magnetic field is the predominant effective magnetic field.

Solar Radioactive Zone

The section of the solar interior between the innermost core and the outer convective zone.

Nuclear Fission

The splitting of a massive atom into two or more lighter atoms to release energy.

Sun

The star around which the earth orbits.

Archaeastronomy

The study of the astronomical and cultural practices of ancient cultures.

Dynamo

The theoretical explanation of the basic solar activity cycle which is mainly (but not exclusively) manifested in the periodic appearance of sunspots. Its mean period is 11 yrs (i.e. 22 yrs for the cycle time of the full magnetic oscillation) but this value varies between 8 and 14 yrs.

Active Region

an area on the Sun where magnetic fields are concentrated; sunspots, prominences, flares, and CMEs all tend to occur in active regions

Coronal Mass Ejections

bursts of charged particles from the Sun's corona that travel outward into space