The Sun

Solar Structure

We live in the atmosphere of a medium sized, yellow, variable star.

The interior of the sun contains the core, where the energy producing nuclear fusion takes place, the radiative one and the outer convection zone.

The visible surface of the sun is called the photosphere. Above the photosphere is the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which is the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto.

Solar Wind

The solar wind is part of the heliosphere. It consists of a plasma stream (i.e. charged particles) which are expelled from the upper atmosphere of the sun. The solar wind consists mostly of high-energy electrons and protons that have escaped the sun’s gravity because of the high temperature of the corona and the high kinetic energy particles gain through other processes that we are just beginning to understand.

Coronal Holes

Coronal holes are areas in the Sun’s corona is where the plasma is darker, colder, and has lower than average density. Coronal holes tend to have unipolar concentrations of open magnetic field lines. At the solar minimum, coronal holes are found mainly at the Sun’s polar regions, but can be located anywhere on the sun during solar maximum. The fast-moving component of the solar wind
travels along these open magnetic field lines and is ejected through coronal holes. Favorably positioned coronal holes can cause this high speed component to impact the earth and cause geomagnetic disturbances.

Sunspot Cycle

Sunspots have been observed for thousands of years.Sunspots are visible features on the sun’s surface that appear darker than their surroundings because they are cooler than the surrounding surface area. Sunspots are regions of intense magnetic activity where convection currents are
inhibited by the strong magnetic fields, thus reducing the flow of energy from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. Large sunspots can measure tens of
thousands of kilometers in width.

The number of sunspots that we can observe on the Sun is not constant. The sunspot count varies over an 11-year cycle known as the Solar cycle, with few and sometimes not any, visible at solar minimum. Those that do appear are at high solar latitudes (nearer to the poles). The number of sunspots and their proximity to the equator of the sun increases as the sunspot cycle progresses toward maximum. Sunspots usually exist as pairs with opposite magnetic polarity. The leading sunspot’s magnetic polarity reverses every solar cycle, so that it will be north polarity in one solar cycle and a south in the next.

Space weather is significantly influenced by the solar cycle, as is the Earth’s climate. Solar minimums tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated warmer global temperatures. An unusual thing happened in the 17th century. The solar cycle almost disappeared for several decades — it was a time where very few sunspots were observed. This period is called the Maunder minimum. Europe experienced very cold temperatures or a “little ice-age.” Other similar minima have been discovered through analysis of tree rings and also
appear to have coincided with lower-than-average global temperatures.

Solar Flares & CMEs

A solar flare is a violent explosion in the Sun’s atmosphere releasing a large amount of energy. Solar flares originate in the solar corona and chromosphere, heating plasma to tens of millions of degrees. Flares produce electromagnetic radiation across the electromagnetic spectrum at all wavelengths
from long-wave radio to the shortest wavelength gamma rays. Most flares occur in active regions around sunspots, where intense magnetic fields emerge from the Sun’s surface into the corona. Flares are powered by the sudden (timescales of minutes to tens of minutes) release of magnetic energy stored in the corona.

X-rays and Ultra-Violet radiation emitted by solar flares can affect Earth’s ionosphere and cause black-outs of long-range radio communications. Direct radio emission at microwave frequencies may disturb operation of radars and other devices operating at these frequencies. The radiation from these
flares travel at the speed of light and thus the effects are felt as soon as they are detected.

Solar flares are rated by their intensity and given the designation of A, B, C, M or X according to the peak flux of X-rays near Earth, as measured on the GOES spacecraft. Each class has a peak flux ten times greater than the preceding one, with X class flares being the most intense.

A coronal mass ejection (CME) is an ejection of material from the solar corona and often associated with a solar flare. The ejected material is a plasma consisting primarily of electrons and protons, along with the coronal magnetic field. These CMEs travel slower than the speed of light and may take from 1 to 3 days to reach the earth assuming the earth is in its path.

If the CME reaches the Earth , it may disrupt the Earth’s magnetosphere, causing it to be compressed on the dayside and stretching out the nightside tail. When the magnetosphere’s field lines reconnect on the nightside, it creates a rubber-bandlike reaction and trillions of watts of power are directed back towards the Earth and follow the magnetic field lines down to the earth’s atmosphere. This process creates a substorm which can cause particularly strong auroral displays in North and South polar regions. CME events may cause geomagnetic storms which can disrupt radio transmissions, cause power outages (blackouts), and cause damage to satellites and electrical transmission lines.