Solar transition region
Encyclopedia
The solar transition region is a region of the Sun
's atmosphere, between the chromosphere
and corona
. It is visible from space
using telescopes that can sense ultraviolet
. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:
Helium ionization
is important because it is a critical part of the formation of the corona
: when solar material is cool enough that the helium within it is only partially ionized (i.e. retains one of its two electron
s), the material cools by radiation very effectively via both black body radiation and direct coupling to the helium Lyman continuum
. This condition holds at the top of the chromosphere
, where the equilibrium temperature is a few tens of thousands of kelvin
s.
Applying slightly more heat causes the helium to ionize fully, at which point it ceases to couple well to the Lyman continuum and does not radiate nearly as effectively. The temperature jumps up rapidly to nearly one million kelvin, the temperature of the solar corona. This phenomenon is called the temperature catastrophe and is a phase transition
analogous to boiling
water
to make steam
; in fact, solar physicists refer to the process as evaporation by analogy to the more familiar process with water. Likewise, if the amount of heat being applied to coronal material is slightly reduced, the material very rapidly cools down past the temperature catastrope to around one hundred thousand kelvin, and is said to have condensed. The transition region consists of material at or around this temperature catastrophe.
The transition region is visible in far-ultraviolet (FUV) images from the TRACE spacecraft, as a faint nimbus above the dark (in FUV) surface of the Sun and the corona. The nimbus also surrounds FUV-dark features such as solar prominence
s, which consist of condensed material that is suspended at coronal altitudes by the magnetic field.
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...
's atmosphere, between the chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
and 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...
. It is visible from space
Outer space
Outer space is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos....
using telescopes that can sense ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
. It is important because it is the site of several unrelated but important transitions in the physics of the solar atmosphere:
- Below, gravity dominates the shape of most features, so that the Sun may be described in terms of layers and horizontal features (like sunspots); above, dynamic forces dominate the shape of most features, so that the transition region itself is not a well-defined layer at a particular altitude.
- Below, most of the helium is not fully ionized, so that it radiates energy very effectively; above, it is fully ionized. This has a profound effect on the equilibrium temperature (see below).
- Below, the material is opaque to the particular colors associated with spectral lineSpectral lineA spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s, so that most spectral lines formed below the transition region are absorption lines in infraredInfraredInfrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
, visible light, and near ultraviolet, while most lines formed at or above the transition region are emission lines in the far ultraviolet (FUV)UltravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
and X-rayX-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s. This makes radiative transferRadiative transferRadiative transfer is the physical phenomenon of energy transfer in the form of electromagnetic radiation. The propagation of radiation through a medium is affected by absorption, emission and scattering processes. The equation of radiative transfer describes these interactions mathematically...
of energy within the transition region very complicated. - Below, gas pressurePressurePressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
and fluid dynamicsFluid dynamicsIn physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow—the natural science of fluids in motion. It has several subdisciplines itself, including aerodynamics and hydrodynamics...
dominate the motion and shape of structures; above, magnetic forcesMagnetismMagnetism is a property of materials that respond at an atomic or subatomic level to an applied magnetic field. Ferromagnetism is the strongest and most familiar type of magnetism. It is responsible for the behavior of permanent magnets, which produce their own persistent magnetic fields, as well...
dominate the motion and shape of structures, giving rise to different simplifications of magnetohydrodynamicsMagnetohydrodynamicsMagnetohydrodynamics is an academic discipline which studies the dynamics of electrically conducting fluids. Examples of such fluids include plasmas, liquid metals, and salt water or electrolytes...
. The transition region itself is not well studied in part because of the computational cost, uniqueness, and complexity of Navier-Stokes combined with electrodynamics.
Helium ionization
Ionization
Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation. A substance may dissociate without necessarily producing ions. As an example, the molecules of table sugar...
is important because it is a critical part of the formation of the 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...
: when solar material is cool enough that the helium within it is only partially ionized (i.e. retains one of its two electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s), the material cools by radiation very effectively via both black body radiation and direct coupling to the helium Lyman continuum
Lyman series
In physics and chemistry, the Lyman series is the series of transitions and resulting ultraviolet emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1...
. This condition holds at the top of the chromosphere
Chromosphere
The chromosphere is a thin layer of the Sun's atmosphere just above the photosphere, roughly 2,000 kilometers deep....
, where the equilibrium temperature is a few tens of thousands of kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s.
Applying slightly more heat causes the helium to ionize fully, at which point it ceases to couple well to the Lyman continuum and does not radiate nearly as effectively. The temperature jumps up rapidly to nearly one million kelvin, the temperature of the solar corona. This phenomenon is called the temperature catastrophe and is a phase transition
Phase transition
A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another.A phase of a thermodynamic system and the states of matter have uniform physical properties....
analogous to boiling
Boiling
Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...
water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
to make steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
; in fact, solar physicists refer to the process as evaporation by analogy to the more familiar process with water. Likewise, if the amount of heat being applied to coronal material is slightly reduced, the material very rapidly cools down past the temperature catastrope to around one hundred thousand kelvin, and is said to have condensed. The transition region consists of material at or around this temperature catastrophe.
The transition region is visible in far-ultraviolet (FUV) images from the TRACE spacecraft, as a faint nimbus above the dark (in FUV) surface of the Sun and the corona. The nimbus also surrounds FUV-dark features such as solar prominence
Solar prominence
A prominence is a large, bright feature extending outward from the Sun's surface, often in a loop shape. Prominences are anchored to the Sun's surface in the photosphere, and extend outwards into the Sun's corona...
s, which consist of condensed material that is suspended at coronal altitudes by the magnetic field.
External links
- Animated explanation of the Transition Region (and Chromosphere) (University of Glamorgan).
- Animated explanation of the temperature of the Transition Region (and Chromosphere) (University of Glamorgan).