Abstract
Our nearest shining star, the Sun, source of radiations and energy, sometimes generates severe events and phenomena in space which can affect our technology and biosphere. On the other hand, space weather, as defined by National Aeronautics and Space Administration (NASA), is conditions on the Sun and in the solar wind, magnetosphere, ionosphere, and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. A brief description of the Sun-Earth connection is firstly presented. Secondly, a particular attention is given to highlight the Sun's variability and the link between the space weather and climate change by means of some recent studies.
TopThe Sun-Earth Connection
In fact, our star is a giant sphere of electrically-charged hot gas. This charged gas moves, generating a powerful magnetic field, which goes through a cycle, called the solar magnetic cycle or simply, solar cycle. Its periodicity is 11 years or so, and during this magnetic field of the Sun completely flips. To flip back again it takes about another 11 years. The solar cycle affects activity on the surface of the Sun, such as sunspots which are caused by the Sun's magnetic fields.
Sunspots are sites of very strong magnetic fields that are cooler than the rest of the photosphere (see Figure 1). This is why they appear dark against the photosphere. A medium size sunspot is bigger than the Earth’s diameter. Small sunspots may only last days, larger sunspots and sunspot groups may last several months. Sunspots usually come in groups with two sets of spots. One set will have positive or north magnetic field while the other set will have negative or south magnetic field. The field is strongest in the darker parts of the sunspots (the umbra), and is weaker and more horizontal in the lighter part (the penumbra). Sunspots have been, historically, important manifestation of variable solar activity.
It’s known that solar activity is a very complex process, involving the whole Sun and its atmosphere, but sunspot numbers retain their value as a simple measure of solar activity. As the magnetic fields change, so does the amount of activity on the Sun's surface.
In addition, the Sun impacts not only our planet but all the solar system with electromagnetic radiations, from gamma rays to radio waves which takes about eight minutes to reach our land (only Infra-Red (IR), Visible and near Ultra-Violets (UV) rays) after all the dangerous ones have been absorbed by the different layers of our atmosphere (see Figure 2), and with particles intercepted by the magnetosphere.
These particles and radiations can be generated by different processes (Eddy, 2009):
Key Terms in this Chapter
Magnetosphere: The region around a planet dominated by the planet's magnetic field. Other planets in our solar system have magnetospheres, but Earth has the strongest one of all the rocky planets: Earth's magnetosphere is a vast, comet-shaped bubble. Life on Earth initially developed and continues to be sustained under the protection of this magnetic environment. The magnetosphere shields our planet from solar and cosmic particle radiation, as well as erosion of the atmosphere by the solar wind. It is generated by the convective motion of charged, molten iron, far below the surface in Earth's outer core. Constant bombardment by the solar wind compresses the sun-facing side of our magnetic field. The sun-facing side, or dayside, extends a distance of about six to 10 times the radius of the Earth. The side of the magnetosphere facing away from the sun - the night side - stretches out into an immense magnetotail, which fluctuates in length and can measure hundreds of Earth radii, far past the moon's orbit at 60 Earth radii.
Solar Wind: A flow of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma consists of mostly electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV . Embedded within the solar-wind plasma is the interplanetary magnetic field. The solar wind varies in density, temperature and speed over time and over solar latitude and longitude. Its particles can escape the Sun's gravity because of their high energy resulting from the high temperature of the corona, which in turn is a result of the coronal magnetic field.
SunSPOTS: Areas where the magnetic field is about 2500 times stronger than Earth's one, much higher than anywhere else on the Sun. Because of the strong magnetic field, the magnetic pressure increases while the surrounding atmospheric pressure decreases. This in turn lowers the temperature relative to its surroundings because the concentrated magnetic field inhibits the flow of hot, new gas from the Sun's interior to the surface. One interesting aspect of the Sun is its sunspots.
Cloud Cover: Known also as cloudiness or cloud amount, it refers to the fraction of the sky obscured by clouds when observed from a particular place.
Albedo: In Latin meaning “whiteness,” it is the measure of the diffuse reflection of solar radiation out of the total solar radiation received by an astronomical body (e.g. a planet like Earth). It is dimensionless and measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects all incident radiation).
Cosmic Rays: High energy particles, generally originating outside the Solar System. They are mainly 89% of protons (nuclei of hydrogen), they also include nuclei of helium (10%) and heavier nuclei (1%). When they arrive at Earth, they collide with the nuclei of atoms in the upper atmosphere, creating more particles.
Solar Coronal Mass Ejections: Large solar expulsions of plasma and magnetic field from the Sun’s corona. CMEs travel outward from the Sun at speeds ranging from 250 km/s to 3000 km/s. The fastest Earth-directed CMEs can reach our planet in as little as 15-18 hours. Slower CMEs can take several days to arrive. They expand in size as they propagate away from the Sun and larger CMEs can reach a size comprising nearly a quarter of the space between Earth and the Sun by the time it reaches our planet.
Hurricane: Tropical cyclone with winds of 119 kilometers per hour or greater that occurs particularly in the western Atlantic, that is usually accompanied by rain, thunder, and lightning, and that sometimes moves into temperate latitudes.
Solar Cycle: Known also as solar magnetic activity cycle which have an average duration of about 11 years characterizing changes in the Sun's activity (including changes in the levels of solar radiation and ejection of solar material) and appearance (changes in the number and size of sunspots, flares, and other manifestations). Solar maximum and solar minimum refer respectively to periods of maximum and minimum sunspot counts. Cycles cover from one minimum to the next.
Kp-Index: Derived by calculating a weighted average of K -indices from a network of geomagnetic observatories.
Geomagnetic Storms: A momentary disturbance of the Earth's magnetosphere, which is associated with solar coronal mass ejections, coronal holes, or solar flares. It’s caused by a solar wind shock wave when it strikes the geomagnetic field 24 to 36 hours after the event.
Solar Flare: An abrupt flash of increased brightness on the Sun, usually observed near its surface. The flare ejects clouds of electrons, ions, and atoms along through the Sun's corona into outer space, and also emits radio waves. According to the peak flux in watts per square metre (W/m 2 ) of X-rays with wavelengths 0.1 to 0.8 nanometer, as measured at the Earth by the Geostationary Operational Environmental Satellite ( GOES) spacecraft, the classification system for solar flares uses the letters A (< 10 -7 W/m 2 ), B (10 -7 – 10 -6 W/m 2 ), C (10 -6 – 10 -5 W/m 2 ), M (10 -5 – 10 -4 W/m 2 ) or X (> 10 -4 W/m 2 ).
Total Solar Irradiance: A measure of the solar power over all wavelengths per unit area incident on the Earth's upper atmosphere. It is measured perpendicular to the incoming sunlight. The solar constant is a conventional measure of mean TSI at a distance of one astronomical unit (AU).
K-Index: Quantifies disturbances in the horizontal component of the geomagnetic field with an integer from 0 to 9, with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed during a three-hour interval.
Thermosphere: The layer of the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions in the ionosphere.