What are Solar Flares?

solar flare is a sudden flash of increased Sun‘s brightness, usually observed near its surface. Flares are often, but not always, accompanied by a coronal mass ejection.[1]

Solar flares fall on a very broad spectrum of emissions, an energy release of typically 1020 joules of energy is considered to be the median for a well-observed event, while a major event can emit up to 1025 joules[2]

The flare ejects clouds of electrons, ions, and atoms along with the electromagnetic waves through the Sun‘s corona into outer space. The phenomenon therefore provides an early example of multi-messenger astronomy. If ejection is in the direction of the Earth the particles hitting the upper atmosphere can cause bright auroras, and may even disrupt long range radio communication. It usually takes a day or two for these clouds to reach Earth.[3] The term is also used to refer to similar phenomena in other stars, where the term stellar flare applies.

Physics

Solar flares affect all layers of the solar atmosphere (photospherechromosphere, and corona), when the plasma medium is heated to tens of millions of degrees kelvin, while the cosmic-ray-like electronsprotons, 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 are not 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 the operation of radars and other devices that use those frequencies.

Solar flares were first observed on the Sun by Richard Christopher Carrington and independently by Richard Hodgson in 1859[4] as localized visible brightenings of small areas within a sunspot group. Stellar flares can be inferred by looking at the lightcurves produced from the telescope or satellite data of 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 (the solar cycle). Large flares are less frequent than smaller ones.

The strength of an event within a class is noted by a numerical suffix ranging from 1 to 9, which is also the factor for that event within the class. Hence, an X2 flare is twice the strength of an X1 flare, an X3 flare is three times as powerful as an X1, and only 50% more powerful than an X2 [10]. An X2 is four times more powerful than an M5 flare.[11]

Hazards

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 or stellar wind, known as a solar proton event. These particles can impact the Earth’s magnetosphere (see main article at geomagnetic storm), and present radiation hazards to spacecraft and astronauts. Additionally, massive solar flares are sometimes accompanied by coronal mass ejections (CMEs) which can trigger geomagnetic storms that have been known to disable satellites and knock out terrestrial electric power grids 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 solar flares 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,[14] 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,[15] giving astronauts as little as 15 minutes to reach shelter.

 

 

PHOTO: By NASA Goddard Space Flight Center – Flickr: Magnificent CME Erupts on the Sun – August 31, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=21422679

wikipedia.org/wiki/Solar_flare