Sun

::{{otheruses}} For the astrological significance of the Sun, see Solar system in astrology.

Theoretical problems

Solar neutrino problem

For some time it was thought that the number of neutrinos produced by the nuclear reactions in the Sun was only a third of the number predicted by theory, a result that was termed the solar neutrino problem. Several neutrino observatories were constructed, including the Sudbury Neutrino Observatory and Kamiokande to try to measure the solar neutrino flux. It has recently been found that neutrinos have rest mass, and can therefore transform into harder-to-detect varieties of neutrinos while en route from the Sun to Earth in a process known as neutrino oscillation {{ref|Schlattl}}. Thus, measurement and theory have been reconciled.

Related Topics:
Neutrino - Solar neutrino problem - Sudbury Neutrino Observatory - Kamiokande - Rest mass - Neutrino oscillation

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of about 6,000 K. Above it lies the solar corona with a temperature of one million Kelvin. The high temperature of the corona suggests that it is heated by something other than the photosphere.

Related Topics:
Photosphere - K - Corona

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere. Two main mechanisms have been proposed to explain coronal heating: Wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the

Related Topics:
Corona - Wave

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

form of heat. The other proposed mechanism is flare heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of solar flares and waves. {{ref|Biermann1}}, {{ref|Biermann2}}, {{ref|Alfven}}, {{ref|Parker}}.

Related Topics:
Flare - Magnetic reconnection

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfven waves have been found to dissipate or refract before reaching the corona

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

({{ref|Sturrock}}, {{ref|Priest}}). In addition, Alfven waves do not easily dissipate in the corona {{ref|Parker2}}. Current research focus has therefore shifted towards flare heating

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales {{ref|Parker2}}, but this is still an open topic of investigation.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Faint young sun problem

Theoretical models of the sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75 percent as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

However, the geologic record shows that the Earth has remained at a fairly constant temperature throughout its history. In fact, the young Earth was actually warmer than it is today. Some scientists have suggested that the young Earth's atmosphere contained much larger quantities of greenhouse gases such as carbon dioxide and/or ammonia than are present today {{ref|Kasting}}. Others suggest that cosmic rays might strongly influence the Earth's climate, and that their flux was much higher in the early history of the solar system {{ref|Carslaw}}.

Related Topics:
Greenhouse gas - Carbon dioxide - Ammonia - Cosmic ray

~ ~ ~ ~ ~ ~ ~ ~ ~ ~