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Solar Activity: Sunspots, Magnetism & Flares

Solar activity refers to a series of complex phenomena in the solar atmosphere, including sunspots, flares, prominences, coronal transient events, etc. These activities are usually caused by electromagnetic processes in the solar atmosphere. They are sometimes strong and sometimes weak, and have a certain periodicity, with an average period of about 11 years. During periods of intense solar activity, the sun will radiate large amounts of ultraviolet rays, Phenomena such as ionospheric disturbances, and even the increase or decrease in the number of sunspots can affect the Earth's climate. Therefore, research on solar activity is of extremely important significance for humans. From a micro perspective, this is a need for further development of solar cells. From a macro perspective, understand the universe, monitor the space environment, develop science and technology, and prevent natural disasters.

Sunspot Cycles
Sunspot "cycles" were first observed in 1843 by Samuel Schwabe, who after 17 years of observations, noticed the cyclical pattern. The idea of standardizing the method of counting sunspots was initiated by Rudolf Wolf in 1848 and his counting methodology has been continued to this day. Sunspots have been recorded since Galileo in the early 1600's. However, it was about 150 years later that Schwabe observed the cycles in the sunspots to be approximately 11 years.
Sunspots 250 years

Sunspot cycles have been accurately measured for the past 250 years. The chart shows cycles 1 through 23. In 1919 George Hale showed that the sunspot cycles coincided with the sun's magnetic cycles. However the magnetic cycle was a 22 year cycle, double the 11 year sunspot cycle. Sunspots are a very good measure of the sun's internal magnetic activity and of its surface activity, i.e. flares, prominences, CMEs, etc. It is now believed that the twisting magnetic action in the plasma Convection Zone below the sun's surface causes sunspots, flares, etc. to form, and the sun's magnetic field to reverse itself every 22 years. (The earth's magnetic field also reverses itself, but only about once every million years. These reversals are recorded in rocks and their signatures can be seen in striped magnetic formations on the ocean floor.)

The Sun's Magnetic Properties
The sun is a very active "magnetic star". Its internal regions are 100% plasma. Plasma is a gas whose temperature has risen to such a high level that it becomes sensitive to magnetism. The sun's rotating magnetic fields affect the gases of the Solar Wind creating the Magnetic Current Sheet, which is a humongous continuous magnetic wave of ion particles in the Heliosphere. The spiral wavy shape can be compared to a rotating lawn sprinkler, except that the waves keep growing until they encounter the Termination Shock. Extending throughout the Heliosphere, the Magnetic Current Sheet is considered the largest structure in our solar system. The sun's strong changing magnetic field varies from year to year and amazingly reverses itself on average every 10.7 years. The "differential rotation" of the Convection Zone (explained above in the Convection Zone section) causes the magnetic field lines to become twisted over time which then causes magnetic field loops to erupt on the sun's surface triggering the formation of dramatic Solar Prominences and Coronal Mass Ejections (CMEs).
Cme 8-1-10 2
The ultraviolet photo from NASA's Solar Dynamic Laboratory (SDO) shows the sun's whole northern hemisphere exploding as examples of huge Solar Flares and a Coronal Mass Ejection. Different colors in the image represent different gas temperatures. Here is a quote from NASA, "On August 1, 2010, almost the entire earth facing side of the sun erupted in a tumult of activity. This image shows the large solar flare, a solar tsunami (wave-like structure upper right) multiple filaments of magnetism lifting off the stellar surface, large-scale shaking of the solar Corona, radio bursts, a Coronal Mass Ejection, and more."

The events on the sun's surface were not isolated events, they were all magnetically collated into one massive instantaneous explosion. NASA announced "Explosions on the sun are not localized or isolated events. Instead, solar activity is interconnected by magnetism over breathtaking distances. Solar Flares, Tsunamis, Coronal Mass Ejections - they can go off all at once, hundreds of thousands of miles apart, in a dizzyingly complex concert of mayhem." The magnetic forces were traced by the SDO spacecraft. Previously these types of events were thought to be isolated from one another. While scientists know a lot about what happened that day, they are still trying to piece together the causes. It may be that a new theory has to evolve to explain massive explosions like this one. Not all explosions are massive, so there may be more than one type of activity going on.

The CME from August 1st, 2010 headed directly towards the earth and three days later the earth's magnetic field reverberated from the CME's impact which sparked auroras as far south as Wisconsin and Iowa. However, this particular intense solar storm was an exception. Weeks and sometimes a month had gone by without a single sunspot. The sun in August 2010 was coming out of an exceptionally long low period of sun activity, the longest low in more than a century. Sun activity cycles are measured by the number of sunspots recorded in a given year. Sunspots are dark regions caused by strong magnetic fields that only appear dark because the local magnetic field is so strong that it blocks the upward flow of heat from the sun's interior. 

Solar Flares and CMEs
A solar Flare is a very large explosion on the surface of the sun with its plasma suddenly roaring to millions of degrees. Flares occur in the active regions around Sunspots. The Flare shown on the left was an X-class Flare (most powerful) which occurred on August 9, 2011. The image is an extreme ultraviolet picture taken by NASA's SDO satellite. While this Flare produced a Coronal Mass Ejection (CME), the CME was not headed towards
the earth and no local effects were observed. The energy emitted from a Flare is about one sixth of the sun's total energy output each second. Strong Flares eject streams of electrons, ions, and atoms (Solar Storms) into outer space.
Xclass flare 8-9-11

Flares are formed when intense magnetic fields from below the sun's surface link up with magnetic fields in the outer Corona in a process called "Magnetic Reconnection". Flares are powered by the sudden release of magnetic energy stored in the sun's Corona. The same energy release may also produce a CME, but not always. And, sometimes CMEs form without Flares. The connection between Flares and CMEs is not well understood. Magnetic Reconnection is a physical process in highly conductive plasmas where magnetic fields clash, re-configure themselves into a lower energy level, and the excess magnetic energy is then converted into kinetic and thermal energy. Big Flares are equivalent to billions of megatons of TNT exploding within a few seconds. Electrons, protons, and other particles that are accelerated by Magnetic Reconnection in a Flare approach the speed of light. It is still not possible to predict when a CME or Flare will erupt because the trigger mechanism isn't known. (The Northern Lights are an example of magnetic reconnection in the earth's atmosphere.)

Flares produce radiation across the electromagnetic spectrum, although with different intensities. Most of their energy goes to frequencies outside of our visual range so the majority of Flares are not visible to the naked eye and must be observed with special instruments. While not very intense at white light, they can be very bright at particular frequencies. Solar Flares are classified as A, B, C, M or X according to the peak flux of X-rays at a specified frequency range. Each class has a peak flux ten times greater than the preceding one. Within a class there is a linear scale from 1 to 9, so an X2 Flare is twice as powerful as an X1 Flare. Solar Flares strongly influence space weather in the vicinity of the earth. They can produce streams of highly energetic particles (alpha particles) in the Solar Wind.

The biggest solar storm ever recorded was the Carrington Flare of 1859, named after Richard Carrington, a prominent English astronomer who observed it. It was the first Solar Flare ever recorded. Skies erupted in red, green, and purple auroras so brilliant that newspapers could be read in the dark as easily as in daylight. Stunning auroras pulsated as far south as Cuba, El Salvador, and Hawaii. Telegraph systems worldwide went haywire. Spark discharges shocked telegraph operators and set the telegraph paper on fire. The Carrington Flare was the largest Flare in the past 500 years as measured by radiation particles locked in the polar ice. A similar Solar Flare that generates a "massive" CME could knock out current day electrical grid power for months.

Most radiation storms take two hours from the time of visual detection to reach earth's orbit. CMEs are clouds of plasma and particles that travel much slower than radiation storms (about 300 miles a second vs. 30,000). We do have satellites far out in space that monitor the sun day and night. NASA is working on programs to detect CME storms and give utilities an early warning of up to 30 minutes. NASA announced that about 1 in 7 Flares experience an aftershock. About ninety minutes after the Flare initially dies down, it springs to life again producing an extra surge of extreme ultraviolet radiation. The energy in the second phase can exceed the energy of the primary phase by as much as a factor of four. The late phase is thought to result from some of the Sunspot's magnetic loops re-forming. The extra energy from the late phase can have a big effect on earth. Extreme ultraviolet wavelengths are particularly good at heating and ionizing earth's upper atmosphere. When our planet's atmosphere is heated by extreme UV radiation it puffs up, accelerating the decay of low-orbiting satellites. Furthermore, the ionizing action of extreme UV can bend radio signals and disrupt the normal operation of GPS satellites.

Solar Nanoflares
When you see the word "nano", one naturally expects the object to be something small. And sure enough, solar nanoflares are a "billion" times less energetic than ordinary solar flares. But, compared to an explosion here on earth, each nanoflare has the energy equivalent of 10,000 atomic bombs. The sun can go months without producing an ordinary solar flare. Nanoflares, on the other hand, are crackling on the sun non-stop and many go off at the same time.

For more than a half-century, scientists have trying to figure out what causes the sun's corona to be so hot. It is one of the most vexing problems in astrophysics. One theory is that nanoflares might be involved. They appear to be active throughout the 11 year solar cycle, which would explain why the corona remains hot during the Solar Minimum. And while each individual nanoflare falls very short of the energy required to heat the sun's corona, collectively they might have no trouble doing to job. 

Spicules and Alfvénic Waves
Spicules (spik'-cules) are the dark red images in the picture. They are dynamic jet spouts about 300 miles in diameter shooting up into the cromosphere from the photosphere (surface of the sun). An individual spicule typically reaches up to 30,000 miles above the photosphere at any one time there are aboout 60,000 to 70,000 active spicules on the sun's surface. They are found in regions of strong magnetic fields. Spicules live for about 5 to 10 minutes. NASA compares the spicules to "seaweed" in the ocean swaying back and forth in the ocean's waves. Only in the sun's corona, magnetic field ripples called "Alfvénic Waves", cause the the spicules to sway. A close up view from the Solar Dynamics Observatory (SDO) spacecraft using ultraviolet techology shows the Spicules in the picture in the left below. The different colors represent various gas temperatures.
Spicule jets

Alfvénic Waves, named after Hannes Alfvén, are magnetically induced waves in electrically conducting fluids. Conducting fluid examples are salt water, electrolytes, liquid metals, and of course plasmas. Alfvén discovered this phenomenon in 1942 and won the Nobel Prize in Physics for it in 1970. Alfvén suggested that large plasmas could carry huge electric currents capable of generating "galactic magnetic fields" (i.e. the sun's Magnetic Current Sheet described above). Alfvénic Waves travel up and down a magnetic field line much the way a wave travels up and down a plucked guitar string. NASA's SDO is now able to measure how much energy is carried by the Alfvénic Waves spewing out from the jets of Spicules. The research shows that the waves carry 100 times more energy than previously thought. While the Alfvénic Waves carry enough energy to drive the intense heating of the Corona to 200 times hotter than the sun's surface and the Solar Winds up to 1.5 million miles per hour, how much of this energy is actually transferred is unknown.

The Alfvénic Waves as we know them today could account for the energy of the Corona and Solar Wind, but there is not enough energy to account for the huge mass of plasma materials ejected during CMEs. Says Scott McIntosh at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, “We still don't perfectly understand the process going on, but we're getting better and better observations. The next step is for people to improve the theories and models to really capture the essence of the physics that's happening. Now that the real power of the waves has been revealed in the Corona, the next step in unraveling the mystery of its extreme heat is to study how the waves transfer their energy to the plasma.”   

The Heliosphere
Helios in Greek means the “sun”. Hence Heliosphere means the “sphere of the sun”. The Heliosphere is a comet-like shaped bubble with a trailing tail filled with hydrogen and helium gases from the Solar Wind. The Solar Wind is a constant stream of charged particles emanating from the sun's extremely hot outer Corona atmosphere, which is 4,000 Kelvin at the surface, but 20,000,000 Kelvin at the outermost hottest points! We do not have a theory at this time to explain these extreme temperatures.

The concept of a "flow of charged particles from the sun" surfaced in the scientific literature during the 1940s and 50s, but it was controversial. It was not until 1962 when the Mariner II spacecraft detected a continuous flowing particle Wind that the issue was put to bed. The mariner also discovered that the Solar Wind fluctuated in intensity in a 27 day cycle which was in concert with the rotation of the sun. The solar wind particles (ions of hydrogen and helium) can escape the sun's gravity because of their high energy.

For the first six billion miles from the sun, the Solar Wind travels about a million miles per hour. Even at a million miles per hour, it takes the solar Wind about a year to reach the outer limits of the Heliosphere. As it begins to come in contact with the Interstellar Wind, it slows down and finally comes to a stop. The point where the Wind slows down to subsonic speed (below the speed of sound) is called the Termination Shock, which is about 90 AU (Astronomical Units) from the sun. The edge of our solar system is called the Heliopause which is where the solar Wind and the Interstellar sund pressures balance. The area between the Termination Shock and the Heliopause is called the Heliosheath and is extremely large, about 100 AU in the front and includes the long tail in the rear. The area where the Interstellar Wind coming from outer space meets the solar Wind is called the Bow Shock. The Bow Shock is so named because it is similar to the water wave in front of a ship's Bow, except in this case it is a gaseous wave. 
Solar map

In 1977 Voyagers 1 and 2 were launched and headed out into space at 39,000 and 35,000 miles per hour respectively. After completing their planetary missions, they continued towards outer space in somewhat opposite directions but still had enough power to continue to communicate back to Earth. In December of 2004, Voyager I passed the Termination Shock (TS) at 94 AU and Voyager 2 passed the TS in August of 2007 at 84 AU. The voyagers are now (July 2011, 34 years later) proceeding through the Heliosheath and are getting close to the Heliopause, the edge of the Heliosphere the outer boundary of our solar system.

In June of 2011, NASA surprised everyone when they announced that the Voyagers were encountering huge frothy magnetic bubbles at the Heliosphere boundary. See the diagram at the left. (The red and blue wavy lines represent the sun's magnetic waves.) This was totally unexpected. Some of the bubbles are 100 million miles wide. NASA explains: "Because the sun spins, its magnetic field becomes twisted and wrinkled, a bit like a ballerina's skirt. Far, far away from the sun, where the Voyagers are now, the folds of the skirt bunch up. The crowded folds of the skirt reorganize themselves, sometimes explosively, into foamy magnetic bubbles. The actual bubbles appear to be self-contained and substantially disconnected from the broader solar magnetic field." This is all recent news and scientists are still studying this bubble phenomenon.
Old new view

The speed and magnetic strength of the Wolar Wind varies in step with the sunspot cycles. During the sun's quiet periods like the one ending in 2008, the speed of the Solar Wind slowed considerably. The Ulysses spacecraft measured the decrease in speed to be as much as 20% during the last solar minimum compared to the previous solar minimum. These variations do affect our weather here on Earth. The last "little ice age" corresponds in time to the "maunder minimum" period in the sunspot cycles. Most scientists believe that these variations also affect the size of the Heliosphere - i.e. the Heliosphere expands and contracts in synch with the sunspot cycles. However, at the moment this is just theory. Perhaps the Voyagers will shed some light on this phenomenon.

The Big Questions
Corona Heating
Sun coronaThe sun's outer atmosphere, the Corona, is extremely hot (millions of degrees K) at its outer edges while the Photosphere (the visible surface of the sun) has a temperature of only 5,800 K. The processes that superheat the Corona, maintain it at extreme temperatures, and accelerate the Solar Wind are still somewhat of a mystery. Usually, temperatures decrease as you move away from a heat source. This is true in the sun's Core right up to the surface. Then over a relatively small distance, the temperatures suddenly rise to extreme highs - 20 million degrees K! While recent data show that Spicules expel enough energy to do the heating (see Spicules section above) how they transfer their heat to the Corona and how much heat is transferred is still unknown.

Nature of Solar Flares and CMEs
Areas on the sun near Sunspots often flare up heating the plasma to millions of degrees in just seconds and blasting billions of tons of plasma material into space. The precise causes of Solar Flares and Coronal Mass Ejections (CMEs) are solar mysteries. We do know that Spicules do not have enough energy to expel CMEs. We think we understand the basic magnetic mechanisms. However, many pieces of the puzzle are missing. We can not reliably predict when and where a flare will occur or how big it will be.

Origin of Sunspot Cycles
Sunspot cyclesOver an approximate 11 year period, the monthly daily average number of Sunspots observed on the sun's surface increases from nearly zero to over 100 and then decreases to near zero again as the next cycle starts. The nature and underpinnings of the Sunspot Cycle constitute some of the great mysteries of solar astronomy. While we know many details about Sunspot Cycles and also about some of the processes that play a role in producing them, we are still unable to produce a scientific model that explains Sunspot occurrences using basic physics principles. 

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