7. The Violent Sun


Hydrogen-Alpha Flare Ribbons

Hydrogen-Alpha Flare Ribbons

. A large solar flare observed in the red light of hydrogen alpha (Ha), showing two extended, parallel flare ribbons in the chromosphere. Each image is 200 million meters in width, subtending an angle of 300 arc seconds or about one sixth of the angular extent of the Sun. These photographs were taken at the Big Bear Solar Observatory on 29 April 1998. (Courtesy of Haimin Wang).


Lyman Alpha Explosion

Lyman Alpha Explosion

. A massive surge moves to a height of 150 million meters at the edge of the Sun, shining in the light of the Lyman alpha emission from hydrogen atoms at 121.6 nanometers The emitting gas is at a temperature of 10,000 to 20,000 degrees Kelvin, characteristic of the chromosphere. This image shows that the radiation is emitted from numerous long, thin magnetic filaments. It was taken from the Transition Region and Coronal Explorer, or TRACE, a mission of the Stanford-Lockheed Institute for Space Research and part of the NASA Small Explorer Program. (Courtesy of the TRACE consortium and NASA.)


An X-ray Flare

An X-ray Flare

. As shown in the lower left of this X-ray image, a solar flare can result in soft X-ray radiation that outshines the entire Sun at these wavelengths. Less luminous coronal loops are found in quiescent, or non-flaring, active regions on other parts of the Sun, and dark coronal holes are also present at both poles (top and bottom). This image was taken with the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite on 25 October 1991. (Courtesy of Keith Strong, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Impulsive and Gradual Phases of a Flare

Impulsive and Gradual Phases of a Flare

. The time profile of a solar flare at hard X-ray energies, above 30 keV, is characterized by an impulsive feature that lasts for about one minute. This impulsive phase coincides with the acceleration of high-speed electrons that emit non-thermal bremsstrahlung at hard X-ray wavelengths and non-thermal synchrotron radiation at centimeter radio wavelengths. The less-energetic emission shown here, below 30 keV, can be composed of two components, an impulsive component followed by a gradual one. The latter component builds up slowly and becomes most intense during the gradual decay phase of solar flares when thermal radiation dominates. At even lower soft X-ray energies (about 10 keV), the gradual phase dominates the flare emission. This data was taken on 15 November 1991 with the Hard X-ray Telescope (HXT) aboard Yohkoh. (Courtesy of NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Solar Flares in Varying Perspectives

Solar Flares in Varying Perspectives

. During the early impulsive phase of a solar flare, electrons accelerated to high energies and very rapid speeds emit radio bursts and hard X-rays. The radio emission is at frequencies from 100 to 3000 MHz, or at wavelengths between 3 and 0.1 meters; the hard X-rays have photon energies greater than 30 keV. The subsequent gradual phase is detected with soft X-rays, at energies of about 10 keV or less, as an aftereffect of the impulsive radiation. The soft X-rays are the thermal radiation, or bremsstrahlung, of a gas heated to temperatures of tens of millions of degrees.


Bremsstahlung

Bremsstahlung

. When a hot electron moves rapidly and freely outside an atom, it inevitably moves near a proton in the ambient gas. There is an electrical attraction between the electron and proton because they have equal and opposite charge, and this pulls the electron toward the proton, bending the electron’s trajectory and altering its speed. The electron emits electromagnetic radiation in the process. This radiation is known as bremsstrahlung from the German word for “braking radiation”.


Double Flaring Sources

Double Flaring Sources

. Hard X-ray flares often appear as double sources that are aligned with the photosphere footpoints of a flaring magnetic loop detected at soft X-ray wavelengths. These footpoints can also be the sites of white-light flare emission. The time profiles of this flare, detected on 15 November 1991, show that the increase of white-light emission matches almost exactly that of the hard X-ray flux. This and the simultaneity of hard X-ray emission from the two footpoints establish that non-thermal electrons transport the impulsive-phase energy along the flare loops. These soft X-ray, hard X-ray and white-light images of the solar flare were taken with telescopes aboard the Yohkoh mission. (Courtesy of NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Gamma Rays During Solar Flares

Gamma Rays During Solar Flares

. Energetic protons, accelerated during solar flares, can encounter the nuclei of carbon and other elements found in the solar atmosphere. The nucleus is excited to a higher energy level during each collision. It then emits gamma radiation (called a gamma-ray photon) with a specific energy characteristic of the nucleus involved.


Plasma Frequency and Coronal Electron Density

Plasma Frequency and Coronal Electron Density

. The plasma frequency (right axis) that corresponds with the electron density (left axis) in the corona, as inferred from measurements during total eclipses of the Sun. The dashed line is for a coronal hole. As the corona expands out to larger distances from the Sun, and into an ever-larger volume of space, it becomes thinner and less dense, and the plasma frequency is smaller.


Solar Radio Bursts

Solar Radio Bursts

. A large solar flare can be associated with several different kinds of intense radio emission, depending of the frequency (left vertical axis) and time after the explosion (bottom axis). The impulsive, or flash, phase of the solar flare, starting at 0 hours, normally lasts about 10 minutes and is associated with a powerful microwave burst. Dynamic spectra at frequencies of about 108 Hz, show type II and type III bursts that drift from high to low frequencies as time goes on, but at different rates depending on the type of burst. The height scale (right vertical axis) is in units of the Sun’s radius of 696 million meters; it is the height at which the coronal electron density yields a plasma frequency corresponding to the frequency on the left-hand side.


Synchrotron Radiation

Synchrotron Radiation

. High-speed electrons moving at velocities near that of light emit a narrow beam of synchrotron radiation as they spiral around a magnetic field. This emission is sometimes called non-thermal radiation because the electron speeds are much greater than those of thermal motion at any plausible temperature. The name “synchrotron” refers to the man-made, ring-shaped synchrotron particle accelerator where this type of radiation was first observed; a synchronous mechanism keeps the particles in step with the acceleration as they circulate in the ring.


Site of Radio Burst

Site of Radio Burst

. Electrons are rapidly accelerated just above the tops of coronal loops during the early stages of the flare, emitting the powerful loop-like radio signals mapped here with white contours. The underlying hydrogen-alpha (Ha) emission is shown in the accompanying photographs. These 10-second snapshot radio maps were obtained with the Very Large Array (VLA) at a wavelength of 20 centimeters, or a frequency of 1,420 MHz. The angular extent of each 20-cm flaring loop is about one minute of arc, or one thirtieth of the angular width of the visible solar disk. (Courtesy of Ken Lang.)


Coronal Mass Ejection

Coronal Mass Ejection

. A huge coronal mass ejection is seen in this coronagraph image, taken on 27 February 2000 with the Large Angle Spectrometric COronagraph (LASCO) on the SOlar and Heliospheric Observatory (SOHO). The white circle denotes the edge of the photosphere, so this mass ejection is about twice as large as the visible Sun. The black area corresponds to the occulting disk of the coronagraph that blocks intense sunlight and permits the corona to be seen. About one hour before this image was taken, another SOHO instrument, the Extreme ultraviolet Imaging Telescope or EIT, detected a filament eruption lower down near the solar chromosphere. (Courtesy of the SOHO LASCO consortium. SOHO is a project of international cooperation between ESA and NASA.)


Eruptive Prominence

Eruptive Prominence

. This large erupting prominence was observed in the extreme ultraviolet light of ionized helium (He II at 30.4 nanometers) on 24 July 1999. The comparison image of the Earth shows the enormous extent of the prominence; the inset full-disk solar image indicates that the eruption looped out for a distance almost equal to the Sun’s radius. (Courtesy of the SOHO EIT consortium. SOHO is a project of international collaboration between ESA and NASA.)


Three-part coronal mass ejection

Three-part coronal mass ejection

. A coronal mass ejection (top bright loop) rises above a dark cavity, followed by a rising prominence (central bright oval). An occulting disk of a coronagraph has blocked the intense sunlight from the photosphere, revealing the surrounding faint corona, and the white circle denotes the edge of the photosphere.  This image was taken with the Large Angle Spectrometric COronagraph, abbreviated LASCO, on the SOlar and Heliospheric Observatory, or SOHO for short.  (Courtesy of the SOHO LASCO consortium and NASA.  SOHO is a project of international collaboration between ESA and NASA.)


Stitching Up The Wound

Stitching Up The Wound

. A time sequence of three radio images show an eruptive prominence above the solar disk at the north-east (top left) during a 1.5 hour period. A negative soft X-ray image shows an arcade of loops formed by magnetic reconnection at a slightly later time, as if the Sun was stitching itself back together after the eruption. The radio images are from the Nobeyama Radioheliograph, while the X-ray image was obtained with the Soft X-ray Telescope (SXT) on the Yohkoh satellite. (Courtesy of Shinzo Enome, Nobeyama Radio Observatory.)


Magnetic Interaction

Magnetic Interaction

. A pair of oppositely-directed, twisted coronal loops come in contact, releasing magnetic energy to power a solar flare. Arrows indicate the direction of the magnetic field lines and the sense of twist. Such magnetic encounters can occur when newly emerging magnetic fields rise through the photosphere to merge with pre-existing ones in the corona, or when the twisted coronal loops are forced together by underlying motions.


Cusp Geometry

Cusp Geometry

. A large, soft X-ray cusp structure (lower right) is detected after a coronal mass ejection on 25 January 1992. The cusp, seen edge-on at the top of the arch, is the place where the oppositely directed magnetic fields, threading the two legs of the arch, are stretched out and brought together. Several similar images have been taken with the Soft X-ray Telescope (SXT) aboard Yohkoh, showing that magnetic reconnection is a common method of energizing solar explosions. (Courtesy of Loren W. Acton, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Hard X-ray Flare in the Corona

Hard X-ray Flare in the Corona

. White contour maps show three impulsive hard X-ray sources from high-energy electrons accelerated during a solar flare, superposed on the loop-like configuration of soft X-rays emitted during the flare gradual or decay phase. In addition to the double-footpoint sources, a hard X-ray source exists in the low corona above the corresponding soft X-ray magnetic loop structure, with an intensity variation similar to those of the other two hard X-ray sources. This indicates that the flare is energized from a site near the loop top. These simultaneous images were taken on 13 January 1992 with the Hard X-ray Telescope (HXT) and the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite. (Courtesy of Satoshi Masuda, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Particle Acceleration Site

Particle Acceleration Site

. Time-of-flight localization of the acceleration site (labeled with a cross) indicates that the energetic flaring electrons originate above the coronal loops detected at soft X-rays and the hard X-ray bursts (contours) located at the loop footpoints. These images were obtained with the hard X-ray (HXT) and soft X-ray (SXT) telescopes aboard the Yohkoh spacecraft at the dates and times indicated on each image. The precise timing of the hard X-rays was obtained with the large-area hard X-ray detectors aboard the Compton Gamma Ray Observatory (CGRO), (Courtesy of Markus J. Aschwanden, NASA and ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


Solar Flare Model

Solar Flare Model

. A solar flare is powered by magnetic energy released from a magnetic interaction site above the top of the loop shown here. Electrons are accelerated to high speed, generating a burst of radio energy as well as impulsive loop-top hard X-ray emission. Some of these non-thermal electrons are channeled down the loop and strike the chromosphere at nearly the speed of light, emitting hard X-rays by electron-ion bremsstrahlung at the loop footpoints. When beams of accelerated protons enter the dense, lower atmosphere, they cause nuclear reactions that result in gamma-ray spectral lines and energetic neutrons. Material in the chromosphere is heated very quickly and rises into the loop, accompanied by a slow, gradual increase in soft X-ray radiation. This upwelling of heated material is called chromospheric evaporation.


Model of three-part coronal mass

Model of three-part coronal mass

. In this model of a three-part coronal mass ejection, portrayed by Terry Forbes (2000), swept-up, compressed mass and a bow shock have been added to the eruptive-flare portrayal of Tadashi Hirayama (1974). The combined representation includes compressed material at the leading edge of a low-density, magnetic bubble or cavity, and dense prominence gas. The prominence and its surrounding cavity rise through the lower corona, followed by sequential magnetic reconnection and the formation of flare ribbons at the footpoints of a loop arcade. [Adapted from Hugh S. Hudson, Jean-Louis Bougeret and Joan Burkepile (2006).]


Model of solar eruption

Model of solar eruption

. A magnetic reconnection takes place at a current sheet (dark vertical line) beneath a prominence and above closed magnetic field lines. The coronal mass ejection, abbreviated CME, traps hot plasma below it (hatched region). The solid curve at the top is the bow shock driven by the CME. The closed field region above the prominence (center) is supposed to become a flux rope in the interplanetary medium. [Adapted from Petrus C. Martens and N. Paul Kuin (1989).]


Magnetic Shapes

Magnetic Shapes

. This X-ray image shows loop-like emission, with contorted, twisted geometry, as well as bright, compact, active-region coronal loops and relatively faint and long magnetic loops. It was taken on 11 June 1992 with the Soft X-ray Telescope (SXT) aboard the Yohkoh mission of the Japanese Institute of Space and Astronautical Science (ISAS). (Courtesy of NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)


The Sun Getting Ready To Explode

The Sun Getting Ready To Explode

. When the coronal magnetic fields get twisted into an S, or sigmoid, shape, they become dangerous, like a coiled rattlesnake waiting to strike. Statistical studies indicate that the appearance of such a large S or inverted S shape in soft X-rays is likely to be followed by an explosion in just a few days. This image was taken on 8 June 1998 with the Soft X-ray Telescope (SXT) aboard the Yohkoh satellite (Courtesy of Richard C. Canfield, NASA, ISAS, the Lockheed-Martin Solar and Astrophysics Laboratory, the National Astronomical Observatory of Japan, and the University of Tokyo.)