The sun shines every day, even though its light doesn't always come through Earth's atmosphere. Clouds may prevent us on Earth from seeing the sun, but spacecraft orbiting high above us always have the sun in view.
Images are available on the World Wide Web on a daily basis. They come from various ground-based observatories on Earth and from satellites in space. Since it is always daylight somewhere on Earth, we can almost always get a view of the sun from a ground-based observatory. And space images are quickly made available on the World Wide Web.
Many images that you see are made through filters that pass only a very narrow band of spectrum. Typically, an H-alpha filter in the visible will pass only 0.5 Å or less out of the spectrum that extends from 3900 Å in the ultraviolet to 6700 Å in the red.
Daily space observations available
Several telescopes around the world post images on the Web regularly, weather permitting. They include:
Big Bear Solar Observatory: Global H-alpha Network
(Big Bear's own solar images are temporarily unavailable while they considerably upgrade their telescope.)
High Altitude Observatory (H-alpha, K-line, helium, coronagraph image)
San Fernando Observatory
It has at least daily white light and K-line images
It links to
http://www.csun.edu/sfo/astrolinks.html
with other links
Rome Observatory, solar-physics section
The archive of images obtained since 1996 is available at
http://www.mporzio.astro.it/solare/calendarioPSPTa.htm
Culgoora Solar Observatory (Australia), H-alpha low-resolution
Sacramento Peak Observatory/National Solar Observatory:
H-alpha, continuum, helium
Kanzelhoehe Solar Observatory (Austria)
ISOON (Improved Solar Observing Optical Network) H-alpha
ISOON (Improved Solar Observing Optical Network) H-alpha, continuum, helium
Stanford Solar Center links page
Mt. Wilson magnetogram/continuum
Mauna Loa disk and limb H-alpha + coronagraph observations + K-line + continuum + helium
H-alpha prominences, K-line on/off center, radio observations, limb prominences [K3 is the center; K1v (K1-violet) is about 1 Å blueward of K3]
http://bass2000.obspm.fr/home.php?lang=en
SOLIS (Synoptic Optical Long-term Investigations of the Sun) at the National Solar Observatory): photospheric magnetic field; helium (10,830 Å chromosphere); calcium (8,542 Å infrared line)
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The Solar and Heliospheric Observatory ( http://sohowww.nascom.nasa.gov/), launched by the European Space Agency with major participation by NASA, has been aloft since 1995. It is located beyond the Moon in the direction of the sun, where it orbits in a slight halo around the sun-earth line so that its radio signals are not overwhelmed by the solar radio radiation. It contains a dozen instruments, including three that often hourly send back images relevant to this lab.
1) The Extreme-ultraviolet Imaging Spectrograph (EIT), from NASA's Goddard Space Flight Center, sends back four types of image:
a) Helium II (that is, ionized helium, given that He I is neutral helium) at 304 Å. It shows gas at temperatures of about 60,000 K. (Note that K stands for kelvin, a unit using Celsius degrees but starting at absolute zero, –273°C. Since temperatures we are discussing are in the thousands or millions of degrees, we can ignore for our purposes the differences between K and °C.)
b) Iron IX (that is, iron that has lost eight of its electrons) at 171 Å. It is at a temperature of about 1,000,000 K.
c) Iron XII (that is, iron that has lost eleven of its electrons) at 195 Å. It is at a temperature of about 1,500,000 K.
d) Iron XV (that is, iron that has lost fourteen of its electrons) at 284 Å. It is at a temperature of about 2,000,000 to 3,000,000 K.
2) The Large Angle and Spectrographic Coronagraph (LASCO), from the Naval Research Laboratory sends back images using a "coronagraph," a device that blocks out the solar photosphere (the everyday solar surface) to make it possible to see the surrounding corona. Unfortunately, for optical reasons it is not possible for the coronagraph's blocking ("occulting") disk to block out only the photosphere; it has to block out a considerable region surrounding the photosphere as well. The photosphere is drawn in for scale and orientation in the images displayed.
The C1 coronagraph is defunct. The C2 coronagraph blocks the innermost half a sun's radius above the solar disk; its field of view extends from 1.5 solar radii to about 7 solar radii. The C3 coronagraph blocks the innermost 2.5 solar radii above the solar disk; its field of view extends from 3.5 solar radii out to about 33 solar radii. (The sun's radius is about 700,000 km = 440,000 miles at the sun's distance of 1,500,000 km = 93,000,000 miles.)
3) the Michelson Doppler Imager (MDI) and Solar Oscillations Investigation (SOI), from Stanford University, sends back images of the sun that match ordinary white light, though they are in a narrow visible-light filter band. The images can be used to make images that show motions of the sun; for our purposes, we will use them just to show the sunspots; they look as they would appear in white light.
SoHO's Website at http://sohowww.nascom.nasa.gov/ includes links to descriptions of the instruments, to educational matters, and to a gallery of past and present images.
Up-to-date set of 8 images from SoHO:
http://sohowww.nascom.nasa.gov/data/realtime/
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NASA's Transition Region and Coronal Explorer has been aloft since 1998 to make high-resolution observations of the solar corona as well as of the region of transition between the chromosphere and corona. Its field of view covers only part of the sun at one time, but it shows magnetically controlled loops of gas in the corona at exceedingly high detail. It is a project of the Lockheed Martin Solar and Astrophysics Laboratory, with a telescope built by the Smithsonian Astrophysical Observatory.
Its site at http://trace.lmsal.com/POD/NAS2002v2.html shows a wide variety of solar images. The TRACE homepage at http://trace.lmsal.com/ shows a Picture of the Day as well as providing links to other TRACE imagery.
A successor, in many senses, to TRACE and to a defunct Japanese x-ray solar satellite known as Yohkoh was launched in 2006. Called Hinode, the Japanese word for "sunrise," it carries telescopes built by Lockheed Martin Solar and Astrophysics Laboratory and the Smithsonian Astrophysical Observatory and a spectrographic instrument built in England and at the Naval Research Laboratory; it is operated in collaboration with the National Astronomy Observatory of Japan. The x-ray telescope can image the whole sun at a time while maintaining the high-resolution of TRACE.
See
the American Hinode site
and the Japanese Hinode site
The link for daily and archival XRT (x-ray telescope) images is:
http://xrt.cfa.harvard.edu/data/latestimg.php
A pair of spacecraft were sent into orbit in 2006 to image the sun simultaneously from two different angles. Since it gives a stereoscopic or 3-dimensional view, the project is called STEREO, an acronym from Solar Terrestrial Relations Observatories. See:
http://www.nasa.gov/mission_pages/stereo/main/index.html
http://stereo.jhuapl.edu/
Other solar spacecraft
A solar x-ray imager is on a NASA weather satellite, devices to monitor the total amount of solar energy reaching Earth are on at least three satellites, and other spacecraft also play roles in studying the sun.
GOES Solar X-ray Imager: new URL's
http://www.sec.noaa.gov/sxi/
Ground-based imaging
Daily images available
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The U.S. National Solar Observatory's project called Global Oscillation Network Group adopted the name and the acronym GONG because it is used to detect how the solar surface is ringing like a bell. Its network of six telescopes around the world have at least one imaging the sun almost always for years at a time.
http://gong.nso.edu
The latest images showing sunspots and the magnetic field are available at
http://gong.nso.edu/Daily_Images/
Other sets of daily solar images
Lockheed Martin Solar and Astrophysics Laboratory links to
http://www.lmsal.com/solarsites.html
NASA Goddard Space Flight Center
including calcium, coronagraph, and helium, as well as SOHO, and links
http://umbra.nascom.nasa.gov/images/latest.html
Other interesting solar sites
Stanford Solar Center
It posts a video "The Colors of the Sun."
Home page for Ramaty High Energy Solar Spectroscopic Imager
See especially its Public Outreach pages by clicking on the left
and see the latest solar images at:
http://www.solarmonitor.org/rhessidatacenter/
Solar science described at NASA's Marshall Space Flight Center
http://solarscience.msfc.nasa.gov/
The sunspot cycle is graphed at
http://solarscience.msfc.nasa.gov/SunspotCycle.shtml
STEREO 3-D solar images
spaceweather.com
SOHO/MDI of sunspots + map of solar far side + a map of coronal holes
http://www.spaceweather.com
Gary Palmer about high-quality amateur solar observations
http://www.meade4m.com/cgi-bin/article.cgi?ID=90
and
http://www.meade4m.com/cgi-bin/article.cgi?ID=116
Greg Piepol on choosing a camera for solar imaging
http://www.meade4m.com/cgi-bin/article.cgi?ID=144
Astronomy Picture of the Day
http://antwrp.gsfc.nasa.gov/apod/
You can search for sun, corona, eclipse, or other topics.
My eclipse page, with images from past eclipses
http://www.williams.edu/astronomy/eclipse
International Astronomical Union Working Group on Eclipses and the
IAU Program Group on Public Education at the Times of Eclipses
also run by me, with links to maps and other information about observing eclipses
http://www.eclipses.info
http://www.williams.edu/astronomy/eclipses=http://www.totalsolareclipse.net
An older technique can provide even narrower bands of spectrum, which allows purer isolation of different levels of the sun's atmosphere and which allows more accurate study of motions on the sun. This technique uses a device called a spectroheliograph. It was used by the great solar scientist George Ellery Hale at the turn of the 19th century; Hale went on to build the two largest telescopes in the world in succession: first the 100" telescope on Mt. Wilson and then the 200" telescope on Palomar Mountain. (The 200" was the largest in the world through about 1995, when the Keck 400" (10-m) telescopes went into use.)
In a spectroheliograph, sunlight passes through a narrow slit and makes a spectrum. A second slit then isolates some particular wavelength of the spectrum. Next, the two slits scan together from side to side, meaning that the film or, now, electronic detector behind the second slit builds up an image of the sun at the precise wavelength that passes through that slit. An advantage of a spectroheliograph is that the wavelength passed can be very narrow and pure, and any wavelength at all can be selected. But a disadvantage is that the mechanical scanning is imprecise and takes time, allowing the sun to change during a scan.
You can find a brief description of a spectroheliograph at .
A fuller description of a spectroheliograph, by Raymond N. Smartt, formerly of the National Solar Observatory, is available online through the McGraw-Hill Encyclopedia of Science and Technology through their Website
http://www.accessscience.com . (Note that a trial subscription or a full subscription may be necessary.)
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Solar and Heliospheric Observatory
SOHO gallery
Transition Region and Coronal Explorer
Ulysses Probe Homepage
Ramati High Energy Solar Spectroscopic Imager (Goddard)
Ramati High Energy Solar Spectroscopic Imager (Berkeley)
http://hessi.ssl.berkeley.edu
Imager for Magnetopause-to-Aurora Global Exploration (IMAGE)
Thermosphere-Ionosphere-Mesosphere-Energetics and Dynamics (TIMED)
GOES Solar X-ray Imager
http://www.sec.noaa.gov/sxi
NASA Missions, with links
NASA's Living with a Star Program
Solar-B
STEREO (at JHU/APL)
STEREO (at NASA headquarters)
Solar Dynamics Observatory
Solar Probe
Solar Orbiter
Synoptic Optical Long-term Investigations of the Sun (SOLIS)
Yohkoh Public Outreach Project Includes Photos and Movies
Solar Maximum Mission
Solar Physics at Marshall Space Flight Center (with links)
Big Bear Solar Observatory
Swedish Solar Observatory, La Palma
National Solar Observatory
NOAO Image Gallery
Global Oscillation Network Group (GONG)
Stanford SOLAR Center
High Altitude Observatory
Mauna Loa Solar Observatory
Advanced Technology Solar Telescope
Author's Eclipse Expeditions
SEDS (Students for the Exploration and Development of Space) The Sun
From Stargazers to Starships
Exploratorium
Multimedia Tour
Sun-Earth Connection Forum
Sunspot Numbers On-Line
World Radiation Center/Physical Meteorological Observatory Davos, under Projects
spaceweather.com
NOAA Space Environment Center
Lockheed Martin Master List of Solar Web Sites: Space Weather
NOAA data and links
Magnetosphere
Auroras
University Corporation for Atmospheric Research
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