A solar flare is a sudden brightening observed over the Sun’s surface or the solar limb, which is interpreted as a large energy release of up to 6 x 1025 joules of energy. They are mainly followed by a colossal coronal mass ejection also known as a CME. The flare ejects clouds of electrons, ions, and atoms through the corona of the sun into space. These clouds typically reach Earth a day or two after the event. The term is also used to refer to similar phenomena in other stars, where the term stellar flare applies.
Solar flares affect all layers of the solar atmosphere, when the plasma medium is heated to tens of millions of kelvins the electrons, protons, and heavier ions are accelerated to near the speed of light. They produce radiation across the electromagnetic spectrum at all wavelengths, from radio waves to gamma rays, although most of the energy is spread over frequencies outside the visual range and for this reason the majority of the flares aren’t visible to the naked eye and must be observed with special instruments. Flares occur in active regions around sunspots, where intense magnetic fields penetrate the photosphere to link the corona to the solar interior. Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona. The same energy releases may produce coronal mass ejections (CME), although the relation between CMEs and flares is still not well established.
X-rays and UV radiation emitted by solar flares can affect Earth’s ionosphere and disrupt long-range radio communications. Direct radio emission at decimetric wavelengths may disturb operation of radars and other devices operating at these frequencies.
Solar flares were 1st observed on the Sun by Richard Christopher Carrington and independently by Richard Hodgson in 1859 as localized visible brightenings of small areas within a sunspot group. Stellar flares have also been observed on a variety of other stars.
The frequency of occurrence of solar flares varies, from several per day when the Sun is particularly “active” to less than one every week when the Sun is “quiet”, following the 11-year cycle. Large flares are less frequent than smaller ones.
Although there is a general agreement on the flares’ causes, the details are still not well known. It isn’t clear how the magnetic energy is transformed into the particle kinetic energy, nor is it known how the particles are accelerated to energies as high as 10 MeV and beyond. There are also some inconsistencies regarding the total number of accelerated particles, which sometimes seems to be greater than the total number in the coronal loop. Scientists are unable to forecast flares, even to this day.
Solar flares are classified as A, B, C, M or X according to the peak flux of 100 to 800 picometre X-rays near Earth, as measured on the GOES spacecraft.
A flare then is classified taking S or a number that represents its size and a letter that represents its peak intensity, v.g.: Sn is a normal subflare.
Solar flares strongly influence the local space weather in the vicinity of the Earth. They can produce streams of highly energetic particles in the solar wind, known as a solar proton event, or “coronal mass ejection”. These particles can impact the Earth’s magnetosphere (see main article at geomagnetic storm), and present radiation hazards to spacecraft and astronauts.
Massive solar flares are sometimes associated with CMEs which can trigger geomagnetic storms that have been known to knock out electric power for extended periods of time.
The soft X-ray flux of X class flares increases the ionization of the upper atmosphere, which can interfere with short-wave radio communication and can heat the outer atmosphere and thus increase the drag on low orbiting satellites, leading to orbital decay. Energetic particles in the magnetosphere contribute to the aurora borealis and aurora australis. Energy in the form of hard x-rays can be damaging to spacecraft electronics and are generally the result of large plasma ejection in the upper chromosphere.
The radiation risks posed by coronal mass ejections are a major concern in discussions of a manned mission to Mars, the moon, or other planets. Energetic protons can pass through the human body, causing biochemical damage, presenting a hazard to astronauts during interplanetary travel. Some kind of physical or magnetic shielding would be required to protect the astronauts. Most proton storms take at least two hours from the time of visual detection to reach Earth’s orbit. A solar flare on January 20, 2005 released the highest concentration of protons ever directly measured, giving astronauts as little as 15 minutes to reach shelter.
Flares produce radiation across the electromagnetic spectrum, although with different intensity. They aren’t very intense at white light, but they can be very bright at particular atomic lines. They normally produce bremsstrahlung in X-rays and synchrotron radiation in radio.
Optical Observations. Richard Carrington observed a flare for the 1st time on 1 September 1859 projecting the image produced by an optical telescope, without filters. It was an extraordinarily intense white light flare. Since flares produce copious amounts of radiation at Hα, adding a narrow passband filter centered at this wavelength to the optical telescope, allows the observation of not very bright flares with small telescopes. For years Hα was the main, if not the only, source of information about solar flares. Other passband filters are also used.
Space Telescopes. Since the beginning of Space exploration, telescopes have been sent to space, where they work at wavelengths shorter than UV, which are completely absorbed by the atmosphere, and where flares may be very bright. Since the 1970s, the GOES series of satellites observe the Sun in soft X-rays, and their observations became the standard measure of flares, diminishing the importance of the Hα classification. Hard X-rays were observed by many different instruments, the most important today being the Reuven Ramaty High Energy Solar Spectroscopic Imager. Nonetheless, UV observations are today the stars of solar imaging with their incredible fine details that reveal the complexity of the solar corona. Spacecraft may also bring radio detectors at very very long wavelengths (as long as a few kilometers) that cannot propagate through the ionosphere.
The following spacecraft missions have flares as their main observation target.
The most powerful flare ever observed was the 1st one to be observed, on September 1, 1859, and was reported by British astronomer Richard Carrington and independently by an observer named Richard Hodgson. The event is named the Solar storm of 1859, or the “Carrington event”. The flare was visible to a naked-eye, and produced stunning auroras down to tropical latitudes such as Cuba or Hawaii, and set telegraph systems on fire. The flare left a trace in Greenland ice in the form of nitrates and beryllium-10, which allow its strength to be measured today. Cliver and Svalgaard reconstructed the effects of this flare and compared with other events of the last 150 years. In their words: While the 1859 event has close rivals or superiors in each of the above categories of space weather activity, it is the only documented event of the last ∼150 years that appears at or near the top of all of the lists.
Other large solar flares also occurred on April 2, 2001, October 28, 2003 (X17.2 and 10), September 7, 2005 (X17), February 17, 2011 (X2), August 9, 2011 (X6.9), March 7, 2012 (X5.4), July 6, 2012 (X1.1). July 6, 2012- The solar storm hit just after 12 midnight UK time, when an X1.1 solar flare fired out of the AR1515 sunspot. Another X1.4 solar flare from AR 1520 region of the Sun, 2nd in the week, reached the earth on July 15, 2012 with a geomagnetic storm of G1–G2 level. A X1.8-class flare was recorded on October 24, 2012. There has been major solar flare activity in early 2013, notably within a 48 hour period starting on May 12, 2013, a total of four X-class solar flares were emitted ranging from an X1.2 and upwards of an X3.2., the latter of which was one of the largest flares of the year so far.
Flare sprays are a type of eruption associated with solar flares. They involve faster ejections of material than eruptive prominences, and reach velocities of 500 to 1200 kilometers per second.
Current methods of flare prediction are problematic, and there is no certain indication that an active region on the Sun will produce a flare. However, many properties of sunspots and active regions correlate with flaring. For example, magnetically complex regions called delta spots produce largest flares. A simple scheme of sunspot classification due to McIntosh is commonly used as a starting point for flare prediction. Predictions are usually stated in terms of probabilities for occurrence of flares above M or X GOES class within 24 or 48 hours. The U.S. National Oceanic and Atmospheric Administration (NOAA) issues forecasts of this kind.
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