The outermost atmosphere of the Sun immediately above the chromosphere, which can be seen during a total solar eclipse. It consists of hot, extremely tenuous gas extending for millions of kilometer fr
om the Sun's surface.
An area in the solar corona which appears dark in X-rays and ultraviolet light. The gas density in these areas are very low, about 100 times less than that of coronal active regions. The magnetic fiel
d lines in a coronal hole extend out into interplanetary space rather than returning to the Sun's surface, as they do in other parts of the Sun (open magnetic field line). Ionized hot gas can escape easily along such a path, and this brings about high speed solar wind streams.
An emission line in the spectrum of the solar corona caused by highly ionized metal ions; especially those of iron, such as the red and green lines at 6375 Å and 5303 Å [Fe X] and [Fe XIV], respective
ly. From their discovery in 1870 until 1939, it was believed that these forbidden lines would be due to an unknown element, called coronium. Ultraviolet and X-ray coronal lines are also detectable in stars.
An arc-like structure in the Sun's corona that is found around sunspots and in active regions. These structures are associated with the closed magnetic field lines that connect magnetic regions on the
solar surface. The loops are sometimes as high as 10,000 km with their two ends situated in photosphere regions of opposite magnetic polarity. This implies that the coronal loops are tubes of magnetic flux filled with hot plasma. They last for days or weeks but most change quite rapidly.
A huge eruption of material from regions of the solar corona in which the magnetic field is closed, but which suffer an extremely energetic disruption. Over the course of several hours up to 10,000 bi
llion kg of this material is ejected into interplanetary space with a speed of as high as 3000 km/s. CMEs are most spectacularly observed by a white light coronagraph located outside Earth's atmosphere. Such observations from Skylab in the early 1970's were the first to reveal this phenomenon. CME's disrupt the flow of the solar wind and can produce intense electromagnetic disturbances that can severely damage satellites and disrupt power grids on Earth. When these ejections reach the Earth, they give rise to geomagnetic storms. The frequency varies with the solar cycle; during solar minimum they come at a rate of about one per week, and during maximum there is an average of about two or three per day.
A bright, narrow stream of particles traveling through the Sun's corona, visible in images taken with a coronagraph or during a total solar eclipse. Coronal streamers represent the most outwardly exte
nded structures in the solar corona and result from the interaction between the solar slow wind and the large-scale magnetic field.
The periodic variation in frequency or number of solar active events (sunspots, prominences, flares, and other solar activity) occurring with an interval of about 11 years. The solar cycle was discove
red in 1843 by Samuel Heinrich Schwabe (1789-1875), a German apothecary and amateur astronomer, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. Solar cycle numbering goes back to the 18th century, when the Cycle 1 peak occurred in 1760. Cycle 23 peaked in 2000, and the following Cycle 24 will reach its maximum in 2013.
The sun goes through 11-year variations or cycles of high and low activity - solar flares and coronal mass ejections, for example, based on the regular increase and decrease of sunspots. Cycles as sho
rt as 9 years and as long as 14 years have also been observed. The cycle is caused by the fact that the sun's magnetic north and south poles flip every 11 years.