A Guide for Solar Watchers pt.1 - Sun basics
For all those who don't quit understand what is happening with our Sun and what is space weather we decided to make A Guide for Solar Watchers. We will start with solar basics - structure, characteristics, solar activity etc. In addition we will present spacecrafts and observatories which task is to observe solar activity. And at the end we will show you how to read solar charts, diagrams and other solar data to make sure you understand what you are reading in our Solar activty posts. So, let's start from the beginning.
Our solar system is composed of the Sun and all things which orbit around it: the planets, asteroids, and comets.
Radius: 696 000 km (109 times the radius of the Earth).
Mass: 1.99 x 10^30 kg (333 000 times the mass of the Earth).
Luminosity (rate of energy radiation): 3.86 x 10^26 W.
Mean density: 1400 kg/m3 (1.4 times that of water).
The Sun is 150 million kilometers (93 million miles) away from the Earth (this distance varies slightly throughout the year, because the Earth's orbit is an ellipse and not a perfect circle) and this distance we measure in AU (Astronomical units. It would take 8 minutes and 19 seconds from Sun to Earth by speed of light.
Crude diagram of Sun shining light on Earth with distance marked. Sunlight takes about 8 minutes, 19 seconds to reach the Earth (based on the average distance).
1 AU = about 149,597,870.7 kilometres (92,955,807.3 mi)= 8.317 light minutes
1 light-year ≈ 63,241 AU
The Sun is a star - with a diameter of about 1.4 million kilometers (860,000 miles) it would take 110 Earths strung together to be as long as the diameter of the Sun. The Sun is mostly made up of hydrogen (about 92.1% of the number of atoms, 75% of the mass) and helium can also be found in the Sun (7.8% of the number of atoms and 25% of the mass). The other 0.1% is made up of heavier elements, mainly carbon, nitrogen, oxygen, neon, magnesium, silicon and iron. Its mass (about 2×1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System.
Stars like the Sun shine for nine to ten billion years. The Sun is about 4.5 billion years old, judging by the age of moon rocks. Based on this information, current astrophysical theory predicts that the Sun will become a red giant in about five billion (5,000,000,000) years.
Earth compared to Sun and solar flare
The Sun is neither a solid nor a gas but is actually plasma. This plasma is tenuous and gaseous near the surface, but gets denser down towards the Sun's fusion core. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields. As the Sun consists of a plasma and is not solid, it rotates faster at its equator than at its poles. This behavior is known as differential rotation, and is caused by convection in the Sun and the movement of mass, due to steep temperature gradients from the core outwards.
4 minute overview video of the Sun
STRUCTURE OF THE SUN
The Sun can be divided into six layers. From the center out, the layers of the Sun are as follows: the solar interior composed of the core (which occupies the innermost quarter or so of the Sun's radius), the radiative zone, and the the convective zone, then there is the visible surface known as the photosphere, the chromosphere, and finally the outermost layer, the corona.
The energy produced through fusion in the Sun's core powers the Sun and produces all of the heat and light that we receive here on Earth. The process by which energy escapes from the Sun is very complex. Since we can't see inside the Sun, most of what astronomers know about this subject comes from combining theoretical models of the Sun's interior with observational facts such as the Sun's mass, surface temperature, and luminosity (total amount of energy output from the surface).
All of the energy that we detect as light and heat originates from nuclear reactions deep inside the Sun's high-temperature "core." This core extends about one quarter of the way from the center of Sun (where the temperature is around 15.7 million kelvin (K), or 28 million degrees Fahrenheit) to its surface, which is only 5778 K or 5505 °C.
This diagram shows a cross-section of a solar-type star. (Source: High Energy Astrophysics Science Archive Research Center, NASA Goddard Space Flight Center)
In its core, the Sun fuses 620 million metric tons of hydrogen each second. Once regarded by astronomers as a small and relatively insignificant star, the Sun is now thought to be brighter than about 85% of the stars in the Milky Way galaxy, most of which are red dwarfs.
SOLAR CYCLES, SUN's MAGNETIC FIELD
The Sun has a complicated and changing magnetic field, which forms things like sunspots and active regions. The magnetic field sometimes changes explosively, spitting out clouds of plasma and energetic particles into space and sometimes even towards Earth. The solar magnetic field changes on an 11 year cycle. Every solar cycle, the number of sunspots, flares, and solar storms increases to a peak, which is known as the solar maximum. Then, after a few years of high activity, the Sun will ramp down to a few years of low activity, known as the solar minimum. This pattern is called the "sunspot cycle", the "solar cycle", or the "activity cycle".
Solar Cycle 24
The Sun's magnetic field leads to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System. Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth's outer atmosphere.
The solar magnetic field extends well beyond the Sun itself. The magnetized solar wind plasma carries Sun's magnetic field into the space forming what is called the interplanetary magnetic field.
Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field causes strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. Sunspots expand and contract as they move across the surface of the Sun and can be as large as 80,000 kilometers (50,000 mi) in diameter, making the larger ones visible from Earth without the aid of a telescope. They may also travel at relative speeds ("proper motions") of a few hundred m/s when they first emerge onto the solar photosphere.
Seen in a close-up, sunspots have a distinctive appearance as seen in this solar telescope photo; the surrounding surface contains small, irregular bright areas called granules which are the upwelled part of numerous local convection currents that carry hot Hydrogen gas to the photosphere (Earth shown for scale). (Source: RST Cosmology)
When a magnetic field line arcs away from the Sun's surface as a coronal loop, plasma is drawn along by the magnetic field, forming a chromospheric prominence/filament. When the loop reconnects to another part of the Sun's magnetic field, field lines from different regions are broken and joined together with field lines from other regions. (This is thought to occur between field lines in the loop and those that surround and extend into space). This reconnection generates an immense electric currents that flow to try and oppose the magnetic change. This releases colossal amounts of energy
(the electrical resistance of the plasma heats it up by dissipating the electric current) resulting in flares, coronal loops, prominences/filaments and coronal mass ejections.
The Sun's hot corona continuously expands in space creating the solar wind, a stream of charged particles that extends to the heliopause at roughly 100 astronomical units. The bubble in the interstellar medium formed by the solar wind, the heliosphere, is the largest continuous structure in the Solar System. Solar wind mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time. These particles can escape the Sun's gravity because of their high kinetic energy and the high temperature of the corona.
The solar wind creates the heliosphere, a vast bubble in the interstellar medium that surrounds the Solar System. Other phenomena include geomagnetic storms that can knock out power grids on Earth, the aurora (northern and southern lights), and the plasma tails of comets that always point away from the Sun.
Solar flares are massive explosions in the Sun's atmosphere that brighten in a few minutes and then fade over the course of about an hour. They involve all layers of the atmosphere (photosphere, chromosphere and corona). A typical flare ejects plasma at speeds close to the speed of light. Flares emit a burst of electromagnetic radiation, in the forms of visible light, radio waves and gamma waves. At the site of reconnection, it is though that a helix of unconnected magnetic field lines radiates from the Sun, carrying plasma with it in a coronal mass ejection.
Example of solar flare
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 operation of radars and other devices operating at these frequencies.
Active region 10486, already under close scrutiny by several instruments on SOHO and other satellites, as well as numerous ground observatories, started up a spectacular two-part show in the morning on Tuesday 28 October 2003. An X 17.2 flare, the second largest flare observed by SOHO, was setting off a strong high energy proton event and a fast-moving Coronal Mass Ejection, hitting Earth early on Wednesday 29 October. (Source: SOHO)
Solar flares are classified as A, B, C, M or X according to the peak flux (in watts per square meter, W/m2) of 100 to 800 picometer 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 having a peak flux of order 10−4 W/m2. Within a class there is a linear scale from 1 to 9, so an X2 flare is twice as powerful as an X1 flare, and is four times more powerful than an M5 flare. The more powerful M and X class flares are often associated with a variety of effects on the near-Earth space environment.
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, known as a solar proton event, or "coronal mass ejection" (CME). These particles can impact the Earth's magnetosphere and present radiation hazards to spacecraft, astronauts and cosmonauts.
CORONAL MASS EJECTION (CME)
A coronal mass ejection (CME) is a massive burst of solar wind, other light isotope plasma, and magnetic fields rising above the solar corona or being released into space.
Coronal mass ejections are often associated with other forms of solar activity, most notably solar flares. Most ejections originate from active regions on Sun's surface, such as groupings of sunspots associated with frequent flares. Near solar maximum the Sun produces about 3 CMEs every day, whereas near solar minimum there is about 1 CME every 5 days.
Normal Condition: Earth’s magnetic field deflects the charged particles streaming out from the Sun
The ejected material is a plasma consisting primarily of electrons and protons, but may contain small quantities of heavier elements such as helium, oxygen, and even iron. It is associated with enormous changes and disturbances in the coronal magnetic field.
When the ejection is directed towards the Earth and reaches it as an interplanetary CME (ICME), the shock wave of the traveling mass of Solar Energetic Particles causes a geomagnetic storm that may disrupt the Earth's magnetosphere, compressing it on the day side and extending the night-side magnetic tail. When the magnetosphere reconnects on the nightside, it releases power on the order of terawatt scale, which is directed back toward the Earth's upper atmosphere.
Earth directed coronal mass ejection
This process can cause particularly strong auroras in large regions around Earth's magnetic poles. These are also known as the Northern Lights (aurora borealis) in the northern hemisphere, and the Southern Lights (aurora australis) in the southern hemisphere. Coronal mass ejections, along with solar flares of other origin, can disrupt radio transmissions and cause damage to satellites and electrical transmission line facilities, resulting in potentially massive and long-lasting power outages.
Solar particles interact with Earth's magnetosphere.
Humans in space or at high altitudes, for example, in airplanes, risk exposure to intense radiation. Short-term damage may include skin irritation with potential increased risk of developing skin cancer, but it's likely that any affected individuals would recover from any such exposure.
A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in the interplanetary medium. A geomagnetic storm is caused by a solar wind shock wave and/or cloud of magnetic field which interacts with the Earth's magnetic field. The increase in the solar wind pressure initially compresses the magnetosphere and the solar wind magnetic field will interact with the Earth’s magnetic field and transfer an increased amount of energy into the magnetosphere. Both interactions cause an increase in movement of plasma through the magnetosphere (driven by increased electric fields inside the magnetosphere) and an increase in electric current in the magnetosphere and ionosphere.
During the main phase of a geomagnetic storm, electric current in the magnetosphere create magnetic force which pushes out the boundary between the magnetosphere and the solar wind. The disturbance in the interplanetary medium which drives the geomagnetic storm may be due to a solar coronal mass ejection (CME) or a high speed stream (CIR) of the solar wind originating from a region of weak magnetic field on the Sun’s surface. The frequency of geomagnetic storms increases and decreases with the sunspot cycle. CME driven storms are more common during the maximum of the solar cycle and CIR driven storms are more common during the minimum of the solar cycle.
Aurora Borealis or Northern Light (Credit: KaseyJoan)
EFFECTS OF GEOMAGNETIC STORMS
Intense solar flares release very-high-energy particles that can cause radiation poisoning to humans (and mammals in general) in the same way as low-energy radiation from nuclear blasts.
Earth's atmosphere and magnetosphere allow adequate protection at ground level, but astronauts in space are subject to potentially lethal doses of radiation. The penetration of high-energy particles into living cells can cause chromosome damage, cancer, and a host of other health problems. Large doses can be fatal immediately.
Solar protons with energies greater than 30 MeV are particularly hazardous. In October 1989, the Sun produced enough energetic particles that, if an astronaut were to have been standing on the Moon at the time, wearing only a space suit and caught out in the brunt of the storm, he would probably have died; the expected dose would be about 7000 rem. Note that astronauts who had time to gain safety in a shelter beneath moon soil would have absorbed only slight amounts of radiation. The cosmonauts on the Mir station were subjected to daily doses of about twice the yearly dose on the ground, and during the solar storm at the end of 1989 they absorbed their full-year radiation dose limit in just a few hours.
Geomagnetic Storms - the effects of Space Weather on Modern Technology (Source: SpaceWeather.gc.ca)
Solar proton events can also produce elevated radiation aboard aircraft flying at high altitudes. Although these risks are small, monitoring of solar proton events by satellite instrumentation allows the occasional exposure to be monitored and evaluated, and eventually the flight paths and altitudes adjusted in order to lower the absorbed dose of the flight crews.
The K-index quantifies disturbances in the horizontal component of earth's magnetic field with an integer in the range 0-9 with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations of horizontal components observed on a magnetometer during a three-hour interval.
So, this image bellow is the summary of terms we defined above:
A composite diagram of Solar structure (made with photos courtesy of SOHO/NASA and a Pov-Ray representation of the interior structure). The labels are as follows: C: core; CME: coronal mass ejection; Co: corona; CoSt: coronal streamers; CS: chromosphere; CZ: convective zone; F: filament; Fl: flare; G: granules; P: plages (bright spots); Pr: prominence; RZ: radiative zone; SG: supergranules; Sp: sprites; SS: sunspots; SW: solar wind.
(Compiled material from SOHO, Wikipedia, Cronodon)