Our sun is an average ordinary star, about half way through its initial hydrogen burning phase.
Only the outer layers are directly observable.
Sunspots [Upper right]
Because the surface layers of the sun rotate at different rates magnetic fields* are wound into complex patterns. Sunspots are localized magnetic storms. They appear a little darker than the surrounding plasma in visible light. Their numbers follow an 11.25 year cycle. The large group shown in the top panel developed in early April 2001, at sunspot maximum.
The cycle period is not exact, varying from 8 to 15 years and the maximum number of large spots can vary by a factor of 2.
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At each sunspot maximum the main magnetic field of the Sun reverses! The magnetic field of the Earth also reverses, but at apparently random million year + intervals. The magnetic reversals probably have something to do with the different rotation rates of different parts of the outer layers (that we can see) and probably of the core (that we cannot see). The core of the Earth rotates little faster than the mantle. |
This image of the sun was made using ultraviolet light emitted by singly ionized Helium atoms in the Solar chromosphere (upper atmosphere).
Helium is not found in the Earth's atmosphere because of its high molecular velocities (above the escape velocity). The element was identified in the spectrum of the Sun before it was identified in the laboratory. Its name is fittingly derived from the Greek word Helios, meaning Sun.
Credit for the discovery of Helium goes to astronomer Joseph Norman Lockyer (born May 17, 1836). Lockyer relied on the then recently developed technique of spectroscopy to disperse sunlight into a spectrum. He knew that each element and ion produces a characteristic spectral pattern of bright lines. He noticed a yellow line in a solar spectrum made during an eclipse which could not be accounted for by elements known on Earth. Almost 27 years later terrestrial Helium was finally identified when the spectrum of a Helium bearing mineral of Uranium provided an exact match to the previously detected element on the Sun. Helium is now known to be the second most abundant element in the Universe (after Hydrogen).
Photo: SOHO - EIT Consortium, ESA, NASA
Question - why exactly is helium found in a uranium bearing mineral?
> Sunquake
Solar flares are explosive events associated with coronal mass ejections [CME's]. They occur at rates of two or three per day at solar maximum. Very large CME's directed toward our planet can have serious effects.
A visible light image can be compared with the Helium UV image above, taken at a different time. Note the different appearance of sunspots in the two images; darker than the surroundings in visible light, and brighter in false color UV. Higher hotter parts of the atmosphere are seen in an extreme UV image and a soft x-ray image shows the 2 000 000 degree outer atmosphere. The bright extended features in the x-ray image are associated with sunspots and the polar coronal holes.
Coronal loops, formed by high energy plasma streaming along magnetic field lines, in far UV and x-ray images, are 30 or more times the diameter of Earth.
Observations of coronal loops with the TRACE satellite show that most of the heating occurs low in the corona, near the base of the loops as they emerge from and return to the solar surface, contrary to past suggestions which involved the possible roles of sound waves and more recently magnetic waves. A current theory, which is gaining ground, is the possibility that the heating is due to small flares, somehow converting the energy stored in magnetic fields into heat energy. Whatever the reason, this 50 year old question continues to fascinate astronomers.
Extending millions of kilometers into space is the tenuous outer corona (lower right).
Coronal mass ejections form what is known as the solar wind which sometimes has dramatic effects here on Earth.
For original images and clips see SOHO
Notice the comet and the effect of cosmic rays hitting the detector in this 1996 sequence.
The first week of November 2003 was marked by a sequence of very large coronal mass ejections which caused some damage to communication satellites. A mass ejection reaches the SOHO camera in the middle of this sequence. The speed of the solar wind varies, depending on the activity of the generating mechanisms on the Sun. In times of high activity the time to reach Earth is reduced from a week by a factor of 2 or 3.