An Etymological Dictionary of Astronomy and Astrophysics
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An instrument especially designed for solar observations.

→ solar; → instrument.

The radiative power per unit area in all wavelengths from the Sun received by the Earth at its average distance from the Sun. Its mean value is called the → solar constant. The solar irradiance changes over a year by about 6.6% due to the variation in the Earth/Sun distance. Moreover, solar activity variations cause irradiance changes of up to 1%.

→ solar; → irradiance.

The edge of the → disk of the → Sun.

→ solar; → limb.

The ecliptic longitude of the Sun. It varies from 0° (at the vernal equinox) to 360° during the year. By Kepler's Second Law, the rate of change of the solar longitude is such that the Earth sweeps out equal areas on the ecliptic plane in equal times.

→ solar; → longitude.

The total → radiant energy, in all wavelengths, emitted by the Sun in all directions. It is 3.828 × 10²⁶ W or 3.828 × 10³³ erg sec^-1 (International Astronomical Union, Resolution B3, 14 August 2015, Honolulu, USA). This is the luminosity unit conventionally used to give the luminosities of stars. See also: → solar constant. When the Earth first formed, 4.56 billion years ago, the Sun radiated 30% less energy than it does today, thus giving rise to the so-called → faint early Sun paradox. Ever since then, its power has increased by 7% every billion years (I. Ribas, 2009, arXiv:0911.4872).

→ solar; → luminosity.

The period of time, about 22 years, after which the magnetic → polarity of the Sun returns to its earlier state. It consists of two consecutive → solar cycles.

→ solar; → magnetic; → cycle.

The Sun's magnetic field which is probably created by the → differential rotation of the Sun together with the movement of charged particles in the → convective zone. Understanding how the solar magnetic field comes about is the fundamental problem of Solar Physics. The solar magnetic field is responsible for all solar magnetic phenomena, such as → sunspots, → solar flares, → coronal mass ejections, and the → solar wind. The solar magnetic fields are observed from the → Zeeman broadening of spectral lines, → polarization effects on radio emission, and from the channeling of charged particles into visible → coronal streamers. The strength of Sun's average magnetic field is 1 → gauss (twice the average field on the surface of Earth, around 0.5 gauss), and can be as strong as 4,000 Gauss in the neighborhood of a large sunspot.

→ solar; → magnetic; → field.

The amount of mass in our Sun, 1.99 x 10³³ g, about 330,000 times the Earth's mass. The solar mass is also the unit in which the masses of other stars, galaxies, and other large celestial bodies are expressed.

→ solar; → mass.

The month(s) during the 11 year → solar cycle when the number of → sunspots reaches a maximum.

→ solar; → maximum.

The proportion of the solar matter made up of → chemical elements heavier than → helium. It is denoted by Z, which represents the sum of all elements heavier than → helium, in mass fraction. The most recent determination of the solar Z gives a value of 0.0134 (Asplund et al. 2009, ARAA 47, 481), corresponding to the present-day photospheric composition.

→ solar; → metallicity.

The month(s) during the 11 year → solar cycle when the number of → sunspots is lowest.

→ solar; → minimum.

The cloud of interstellar gas and dust from which the Sun and the rest of the solar system initially formed.

→ solar; → nebula.

That part of the Milky Way galaxy lying near the Sun. In fact there is no definition of the exact radius of this region. It is referred to the immediate solar neighborhood (within about 5 pc), the solar neighborhood (within about 25 pc), and the extended solar neighborhood (within a few hundred pc).

→ solar; → neighborhood.

A neutrino generated in the → Sun. The main source of solar neutrinos is the → proton-proton chain of reactions: 4 × p→ He + 2e⁺ + 2ν_e, in which an energy of +28 MeV is shared between the reaction products. These are called → low-energy neutrinos. There are less important reactions in the Sun yielding a smaller flux of higher energy neutrinos. The solar neutrino flux can be estimated from the → solar luminosity (L), as follows Since there are two neutrinos for each 28 MeV of energy, the neutrino flux at the Earth distance (d) is given by: ν flux = 2Lsun/(28 MeV) × (1/4πd²) = 6 × 10¹⁰ cm^-2 s^-1. See also the → solar neutrino problem.

→ solar; → neutrino; → flux.

A major discrepancy between the flux of neutrinos detected at Earth from the solar core and that predicted by current models of solar nuclear fusion and our understanding of neutrinos themselves. The problem, lasting from the mid-1960s to about 2002, was a considerably lesser detected number of neutrons compared with theoretical predictions. The discrepancy has since been resolved by new understanding of neutrino physics, requiring a modification of the → standard model of particle physics, in particular → neutrino oscillation.

→ solar; → neutrino; → problem.

A measure of the flux of neutrinos from the Sun reaching the Earth. 1 SNU is equal to 10^-36 solar neutrinos captured per target atom per second.

→ solar; → neutrino; → unit.

A → European Space Agency (ESA) mission with strong → National Aeronautics and Space Administration (NASA) participation aimed at studying the Sun up close and from high latitudes, launched on 10 February 2020. Solar Orbiter is equipped with 10 instruments and will provide the first images of the Sun's poles. It will make a close approach of the Sun every six months. Its distance from the Sun varies from within the orbit of → Mercury to close to the orbit of Earth. At closest approach, Solar Orbiter will be about approximately 42 million km from the Sun. Solar Orbiter will combine in situ measurements of the → solar wind around the spacecraft with remote sensing, looking at the Sun's features from afar, to connect the two together. The spacecraft has been tested to withstand temperatures up to 500 °C -- enduring thirteen times the amount of solar heating that satellites in Earth's orbit experience. Solar Orbiter will help us understand how our star creates and controls the → heliosphere, i.e. the giant bubble of → plasma that surrounds the whole → Solar System and influences the planets within it.

→ solar; → orbiter.

The angle subtended (8''.79) by the → equatorial radius of the Earth at a distance of 1 → astronomical unit.

→ solar; → parallax.

The abundance of a → chemical element as determined from the observation of solar → spectral lines. The solar chemical composition is an important ingredient in our understanding of the formation, structure and evolution of both the Sun and our solar system. Furthermore, it is an essential reference standard against which the elemental contents of other astronomical objects are compared (Asplund et al. 2009, arXiv:0909.0948). The photospheric abundances relative to hydrogen are not representative of the → protosun, or global → solar system abundances. This is because heavy-element fractionation in the Sun has altered photospheric abundances (Lodders 2003, ApJ 591, 1220).

→ solar; → photospheric; → abundance.

The branch of astrophysics concerned with the study of the physical properties of the Sun based on the most detailed observations which can be obtained for a star.

→ solar; → physics.