5. A Magnetic Star


Spot on the Sun

Spot on the Sun

. This sunspot consists of a dark umbra inside a light, filamentary penumbra. It is about twice as large as the Earth, or about 24 million meters in diameter. Outside the penumbra the solar granulation is visible. This photograph was taken with exceptional angular resolution of 0.2 arc-seconds or 150 thousand meters at the Sun, using the National Solar Observatory’s Vacuum Tower Telescope at the Sacramento Peak Observatory. [Courtesy of Thomas R. Rimmele, Association of Universities for Research in Astronomy (AURA), National Optical Astronomy Observatories (NOAO), and the National Science Foundation (NSF).]


The Zeeman Effect

The Zeeman Effect

. In a sunspot the spectral lines that are normally at a single wavelength become split into two or three components in the presence of a magnetic field, depending on the orientation of the field with respect to the line of sight. The separation of the outermost components is proportional to the strength of the magnetic field, in this sunspot about 0.4 Tesla, or 4,000 Gauss. The components also have a circular or plane polarization, and the circular polarization, or orientation, indicates the direction, or polarity of the longitudinal magnetic field. [Courtesy of the National Optical Astronomy Observatories (NOAO).]


Magnetogram

Magnetogram

. This magnetogram was taken on 12 February 1989, close to sunspot maximum. Yellow represents positive or north polarity pointing out of the Sun, with red the strongest fields which are around sunspots; blue is negative or south polarity that points into the Sun, with green the strongest. In the northern hemisphere (top half) positive fields lead, in the southern hemisphere (bottom half) the polarities are exactly reversed and the negative fields lead. The Sun rotates east to west so that leading parts of active regions are to the right. [Courtesy of William C. Livingston, National Solar Observatory (NSO), National Optical Astronomy Observatories (NOAO).]


Winding Up The Field

Winding Up The Field

. A model for generating the orientation and polarity of the sunspot magnetic fields. At the beginning of the 11-year cycle of magnetic activity, when the number of sunspots is at a minimum, the magnetic field is the dipolar (poloidal) field seen at the poles of the Sun (a). The internal magnetic fields then runs just below the photosphere from the south to north poles. As time goes on, the highly-conductive, rotating material inside the Sun carries the magnetic field along and winds it up. Because the equatorial regions rotate at a faster rate than the polar ones, the internal magnetic fields become stretched out and wrapped round the Sun’s center, becoming deformed into a partly toroidal field (b) and (c). At the time of activity maximum (c), active regions are formed in two belts, in the northern and southern hemispheres, and bipolar sunspot groups are created when magnetic loops break through the photosphere.


Sunspot Cycle

Sunspot Cycle

. The location of sunspots (upper panel) and their total area (bottom panel) have varied in an 11-year cycle for the past 100 years, but this activity cycle varies both in cycle length and maximum amplitude. As shown in the upper panel, the sunspots form at about 30 degrees latitude at the beginning of the cycle and then migrate to near the Sun’s equator at the end of the cycle. Such an illustration is sometimes called a “butterfly diagram” because of its resemblance to the wings of a butterfly. The total area of the sunspots (bottom panel), given here as a percent of the visible hemisphere, follows a similar 11-year cycle. (Courtesy of David Hathaway, NASA/MSFC.)


The Solar Magnetic Field in Time

The Solar Magnetic Field in Time

. These magnetograms portray the polarity and surface distribution of the magnetism of the solar photosphere. They were made with the Vacuum Tower Telescope of the National Solar Observatory at Kitt Peak from 8 January 1992, during the last maximum in the sunspot cycle (lower left) to 25 July 1999, well into the next maximum (lower left). They show opposite polarities as darker and brighter than the average tint. When the Sun is most active, the number of sunspots is at a maximum, and solar magnetism is dominated by large bipolar sunspots oriented in the east-west (left-right) direction within two parallel bands. At times of low activity (top middle), there are no large sunspots and tiny magnetic fields of different magnetic polarity can be observed all over the photosphere. The haze around the images is the inner solar corona. (Courtesy of Karel J. Schrijver, NSO, NOAO nad NSF.)


Chromosphere During Eclipse

Chromosphere During Eclipse

. A reddened protuberance, called a prominence, provide a brief glimpse of the chromosphere during a total eclipse of the Sun on 11 July 1991. The red light is emitted by the abundant hydrogen atoms in the chromosphere at the hydrogen-alpha electron transition with a wavelength of 656.3 nanometers. (Courtesy of Dennis di Cicco, Sky and Telescope.)


Hydrogen-Alpha Chromosphere

Hydrogen-Alpha Chromosphere

. This photograph of the Sun’s chromosphere was made by tuning into the red line of atomic hydrogen, at the hydrogen-alpha line with a wavelength of 656.3 nanometers. Two round, dark sunspots, each about the size of the Earth, are present in the right half of this image, together with bright plage that marks highly-magnetized regions. (Courtesy of Victor Gaizauskas, obtained at the Ottawa River Solar Observatory, a facility operated by the Herzberg Institute of Astrophysics for the National Research Council of Canada.)


Spicules

Spicules

. Rows of dark spicules, or little spikes, are seen in the red hydrogen-alpha light of the solar chromosphere. A spicule is a short-lived (minutes) narrow jet of gas spurting our of the solar chromosphere at supersonic speeds to heights as great as 15 million meters. Spicules are a thousand times denser than the surrounding gas, so they are seen in absorption against the bright chromospheric background. They appear to cluster in “hedgerows” or “tufts”. [Courtesy of the National Solar Observatory (NSO), the National Optical Astronomy Observatories (NOAO), and the National Science Foundation (NSF).]


The Sun in the Light of Calcium Ions

The Sun in the Light of Calcium Ions

. This global spectroheliogram of the Sun was taken in the light of the singly ionized calcium, or Ca II, at the core of the violet K line with a wavelength of 393.4 nanometers on 02 June 1997. The emission outlines the chromospheric network where magnetic fields are concentrated, like the edges of the tiles in a mosaic. The brightest extended regions are called plage; they are dense places in the chromosphere found above sunspots or other active areas of the photosphere in regions of enhanced magnetic field. [Courtesy of the National Solar Observatory (NSO), the National Optical Astronomy Observatories (NOAO), and the National Science Foundation (NSF).]


Photospheric Magnetism and the Magnetic Network

Photospheric Magnetism and the Magnetic Network

. A high-resolution magnetic map of the quiet part of the photosphere, obtained using the Michelson Doppler Imager (MDI) aboard SOHO, is overlaid with lines of convergence of the horizontal flow, describing the magnetic network. Green circles show the convergence points, red arrows describe the inferred down-flow and blue arrows describe the inferred up-flow. The magnetic field in the photosphere is shown light gray for positive fields directed out of the Sun and black for negative fields pointing in. Magnetic flux disappears in collisions between opposite polarities so fast that all magnetic fields should vanish in a few days. New flux emerging in small ephemeral regions replaces the disappearing flux. (Courtesy of the SOHO MDI consortium. SOHO is a project of international cooperation between ESA and NASA.)


Magnetic Concentration

Magnetic Concentration

. A two-dimensional, vertical cross section of the magnetic-network model of the solar transition region. The motion of supergranular convective cells (bottom) concentrates magnetic fields at their boundaries in the photosphere. The magnetic fields (arrowed lines) are pushed together and amplified up to 0.1 Tesla at the cell edges. Heating in the chromosphere above this magnetic network produces bright calcium emission (Fig. 5.12). The concentrated magnetic fields expand and flare out with height in the overlying corona. Temperature contours between log T = 6..1 (corona) and log T = 5.4 (upper transition region) are marked.


The Sun in the Light of Hydrogen Atoms

The Sun in the Light of Hydrogen Atoms

. This global image of the Sun was taken in the light of hydrogen atoms, at the red hydrogen-alpha transition with a wavelength of 656.3 nanometers, on 11 August 1980. Small, dark regions above sunspots, bright active regions and plage, and dark string-like filaments are seen on the visible disk; bright prominences extend outward over the disk edge or limb. (Courtesy of the Space Environment Center, National Oceanic and Atmospheric Administration (NOAA) under partial support from the United States Air Force.)


Active Prominence

Active Prominence

. When seen at the edge of the Sun, a filament takes the name prominence. This bright loop prominence outlines magnetic fields above sunspots; the loop is big enough to swallow the Earth. The photograph was taken in the green line of ionized iron, designated Fe XIV. [Courtesy of the National Solar Observatory (NSO), the National Optical Astronomy Observatories (NOAO), and the National Science Foundations (NSF).]


Quiescent Prominence

Quiescent Prominence

. Dense, cool gas, seen in the red light of hydrogen alpha at the rim of the Sun, outlines magnetic arches that are silhouetted against the dark background. The prominence material, appearing as a flaming curtain up to 65 million meters above the photosphere, is probably injected into the base of the magnetic loops in the chromosphere. (Courtesy of the Big Bear Solar Observatory.)