The easy observation of sunspots reveals not only the sun's rotation on itself, to equator, is in 27 days, but that the activity of hot and cold zones of the Sun follow a cycle. The solar cycle is the period during which the Sun's activity varies by up to another. In absolute terms, solar activity is regulated by a cycle of an average period of 11.2 years but the duration can vary between 8 and 15. The 11-year cycle was determined for the first time by the German astronomer Heinrich Schwabe in 1843. In 1849 the Swiss astronomer Johann Rudolf Wolf (1816-1893), establishes a method for calculating the solar activity based on the number of spots. Schwabe cycles are numbered from the maximum of 1761. Cycle 24 will begin in 2012. Variations in solar activity on Earth are reflected by changes in the propagation of radio waves. The frequency range of interest covers the airwaves called HF or short wave, which propagate long distances. During these storms, the strong ionization of upper layers of the atmosphere can disrupt communications satellites, with consequences that may have imagined for telecommunications. Sunspots appear grouped in the hot photosphere (5800 K) as a dark area, cooler (4500 K), surrounded by a lighter region (4500 K to 5800 K) and are due to increased local magnetic field.
These spots can reach dimensions of tens of thousands of km. At the beginning of the solar cycle, the spots usually appear at high latitudes in both hemispheres, north and south. Throughout the cycle, the spots will move closer to the equator until the beginning of the next cycle. The spacecraft Ulysses flew for the first time in succession the parts of the South poles (1994) and North (1995) from the Sun, visible from Earth. To observe the Sun's poles, the probe had to leave the plane of the ecliptic (the plane in which rotate the planets around the sun). It used the enormous gravitational field of Jupiter to change the plane of its orbit. Its orbit is perpendicular to that of the earth, far from the Sun at the distance of about 300 million km. The probe was ideally placed to observe, in July 1994, the collision of Comet Shoemaker-Levy 9 and Jupiter. Ulysses has to know the exact speed of solar winds. This constant stream of particles emitted by the Sun, is twice the size estimated. This speed is about 800 kilometers per second.
Image: The Ulysses probe was launched October 6, 1990 by Space Shuttle Discovery. The mission was terminated June 30, 2009 following the deterioration of the energy source of the probe. This ship of 370 kg, was the first and only fly to the Sun's poles to study the heliosphere, the vast bubble around our star. Designed for a period of 5 years, longevity has been exceptional, a record of 6822 days of operation (18 years 246 days).
Sun seen by Ulysses in the 1990s
Ci-cons, the measures taken by the Ulysses probe, the temperature of the north and south poles of the Sun in thousands of Kelvin. The extension of the mission was decided because of peak solar activity, the poles were again flown in 2000 and 2001, the period of maximum activity. The last flights were made between November 2006 and April 2007 (South Pole) and between November 2007 and March 2008 (South Pole). In 2008, following a decrease in the strength of the probe, the mission seemed about to stop, but scientific evidence has been collected until the end of the mission, which ended on 30 June 2009.
Image: Temperatures of the polar coronal holes of the Sun measured by Ulysses.
Sun seen by SOHO in the 2000s
Our center is powered by thermonuclear fusion reactions that turn in its core at a temperature of 15 million degrees, hydrogen into helium, from 4.57 billion years. Consumption i.e. the loss of solar mass is 4 million tons of hydrogen per second, because it converts 564 million tons of hydrogen into 560 million tons of helium. In early 2010 many groups of sunspots have reappeared, indicating a resumption of activity. The cooler regions are visible as spots on the surface layer, called photosphere. These spots have an average cycle of about 11.2 years. After the minimum activity of 2007, the stains would reappear gradually since announcing the next round of the Sun, but scientists have found nothing for three years. Since December 2009, the Sun appears to wake up, but the maximum solar activity expected in 2012, may be disappointing compared to 2001. The SOHO space probe, is a collaboration between NASA and ESA, was launched December 2, 1995 the base of Cape Canaveral (USA) by an Atlas rocket II.
The SOHO mission was to study the Sun's internal structure, its warm atmosphere, the origins of the solar wind. In operation since February 1996, the mission is still running remarkably in 2010. SOHO operates on a halo orbit around the Lagrangian point L1. At this point, the gravitational forces exerted by the Sun and the Earth on an object are balanced, but balance is unstable and therefore describes a SOHO orbit around this particular point. The period of revolution of SOHO is equal to the period of revolution of the Earth around the Sun, or about 365 days.
Image: These 4 images offered by the Soho space satellite, shows the daily evolution of sunspot groups observed between 8 and 10 February 2010. Credits: Nasa
Sun seen by SDO, in the 2010s
SDO (Solar Dynamics Observatory), is the newest solar observatory, which has since replaced SoHo. SDO was launched by NASA, Thursday, February 11, 2010. SDO will observe the Sun continuously, with a spatial and temporal resolution much higher than Soho. Its orbit is geo synchronized, about 35 000 km altitude. SDO is the first satellite program 'Living with a Star (LWS) of NASA. In geosynchronous orbit, SDO point its instruments at the sun, and sends us wonderful videos on its internal functioning. The most impressive images of the Sun, are those of the grand eruptions. In February 2011, the satellite of NASA SDO filmed a particularly impressive eruption. The explosion of unpredictable boss, followed in the ultraviolet (see animation above-cons), recounts in a few seconds, an event that occurred on 90 minutes. Prominences are filaments of solar material, projected above its surface and characterizing the solar activity. They can reach as March 30, 2010, the impressive size of 230 000 km (17 times the diameter of the Earth), the Sun's diameter is 1,392,000 km.
A solar prominence is a cloud of gas ejected from the solar surface, supported by a loop of the magnetic field of our star. Although very hot, prominences are slightly colder than the surface. An eruptive prominence may continue to levitate about a month and free, through a coronal mass ejection of hot gas throughout the solar system. The exact mechanism for energy leads to the formation of protrusions is still poorly understood, but is the subject of intensive research. The maximum activity is expected in 2012.
Video: The observation of fluctuations in solar activity will allow to know its influence on the Earth's climate. This accelerated video, shows a solar flare that lasted 1:30, which is here, an image every 24 seconds. Credit: NASA / Goddard / SDO AIA Team.