Cycles, our world is full of predictable, regularly repeating events - day/night, seasons, water cycles, carbon cycles, economic cycles, life cycles, climate cycles, Milankovitch cycles (changes in the Earth's precession, obliquity, and orbital eccentricity over thousands to 10's of thousands of years). Our lives are impacted every day by cycles. The sun goes through cycles too that impact the Earth, the solar system, and our daily lives in some surprising ways. Most importantly, the frequency and intensity of solar storms, the biggest explosions of energy in the solar system, ebb and flow with these solar cycles. The periodic variability in the sun's activity includes changes in solar radiation and particle emission that effect Earth's geomagnetic environment, climate, weather, and even the influx of galactic cosmic rays. The sun's electromagnetic and particle radiation also impact other solar system bodies; planetary atmospheres, magnetospheres, satellite surfaces, and comets. To understand what's going on, we need to understand how the sun works.
The sun is a moderate sized (1,400,000 km in diameter), G spectral type, variable star in a vast collection of an estimated 200-400 billion stars in a barred spiral galaxy called the Milky Way. Though it is a smallish star, most stars in our galaxy are smaller, and less massive. Our sun is, like most stars in our galaxy and probably most stars, a main sequence star which means it generates its energy by thermonuclear fusion resulting in the release of gamma ray photons during a process in the core where hydrogen nuclei (protons) are fused together to form helium nuclei. This process, converting hydrogen to helium, driven by the enormous internal pressure of the sun's mass (2x1030 kg or over 300,000 times the mass of the Earth!) is what makes a star a star. Since everything forms from a roughly basic and somewhat consistent cosmic menu of elements (75% H, 24% He,...) the variable that determines whether you get an asteroid, a planet, or a star is mass. Enough mass creates enough pressure to start this fusion process and create a star. Not enough mass and you have a failed star or planet (even less mass and you get an object that is not round at all). It's that simple. The sun formed around the same time that the planets formed, from a molecular cloud - remnant of a super nova explosion. The sun is very old, by human standards but only middle aged by stellar standards, about 4.6 billion years old and it should be around in a more or less similar form for maybe another 5 billion years.
It might be interesting to note that considering only classical (or Newtonian) physics, the physics that you learned in high school, the physics that defined reality for around 400 years, since Isaac Newton gave his three laws of motion, the sun and all the stars cannot shine. Classical physics does not allow two similarly charged protons to bond. In addition, matter - energy equivalency (E=mc2) was not known until the early 1900's. So before Einstein and before quantum physics, there was no plausible explanation for how the sun generated its energy. Want to know more? Read "On the age of mostly everything".
The sun rotates. This was first observed by Galileo Galilei in 1610 when he observed dark "blemishes" (sunspots) on the sun. The observed motion of these sunspots across the face of the sun was the first evidence that the sun rotated on its axis and made it easier to imagine that the Earth rotated too, a concept foreign to the long accepted Ptolemaic and Aristotelian models of the universe which stated that the Earth stood still while the cosmos revolved around it. This was also contrary to church doctrine at the time which accepted the still Earth model and asserted that the heavens were perfect, so no blemishes on the sun. Needless to say, this got Galileo into a lot of trouble. It turns out that almost everything in space rotates, stars, planets, moons, galaxies, clusters of galaxies, clusters of clusters of galaxies, everything rotates. And ironically, the rotation of the sun is responsible for the sun having sunspots! Make a mental note of this for later.
To understand the role of solar rotation (and so, sunspots) in our story, we go back to the 1820's where Michael Faraday, impressed with the work of Hans Christian Oersted who discovered that a magnetic needle (compass) was deflected when placed near a wire with a current flowing through it, developed a theory of electromagnetic induction. Basically, the movement of charge generates a magnetic field and a moving magnetic field generates charge or electrical current. This is the principle behind all electric motors, transformers, and generators today and is the principle that allows the sun to generate a magnetic field. It is also a most central concept in understanding how the solar wind and solar storms impact the rest of the solar system.
Figure 4: The sun's magnetic field. Blue and yellow field lines correspond to north and south polarities
The high pressure in the sun's core heats matter to very high temperatures (around 15 million K) resulting in an abundance of charged/ionized particles (plasma). The sun's rotation combined with convection from the heated material transports this charge around the sun. Now remembering Michael Faraday's discovery that moving charge generates a magnetic field, this is how the sun's magnetic field is generated - electromagnetic induction within a star. When this process operates within a star or planet, we call it a dynamo. Because the sun is composed of plasma and not solid material, its rotation rate is not constant. Therefor, the motion of ionized gas around the sun is not constant. The sun rotates slower at high latitudes (near the poles) than at the equator. This differential motion causes stretching and twisting of the sun's magnetic field creating a complex field that is the sun's magnetosphere. The complexity of the field increases from solar minimum to solar maximum, as lines of magnetic force are wound together and distorted. As field lines, generated deep within the sun emerge above the sun's surface or "photosphere", they create a strong local magnetic field that prevents deeper and hotter, convecting plasma from rising to the surface. The area around this field then appears cooler than the surrounding photosphere and so darker. These dark regions are called sunspots. Since magnets and so magnetic fields always have a north and a south pole, the highly magnetic sunspots often occur in pairs (e.g. north and south poles) and can be seen channeling hot plasma gases between them in structures called prominences.
This occurs because the electrically charged particles comprising the plasma are attracted to magnetic fields or lines of magnetic force and so are pulled along field lines from one pole to another. Other stars have sunspots too! We have both direct visual evidence of this as well as inferred evidence from periodic increases and decreases in a star's brightness. Other evidence includes Zeeman line splitting and Doppler imaging. Since sunspots require a magnetic field, differential rotation, and a convective stellar envelope, we can see visual evidence that other stars are similar to the sun. Massive stars have convective cores and radiative envelopes. This generally produces a weaker magnetic field and fewer sunspots. Astronomers have observed sunspots (well, starspots) ands sunspot cycles on a few other stars which helps determine the star's rotation rate and tell us about the star's internal processes.
Figure 10: Bow shock, about half a light-year across, created from the wind from the star L.L. Orionis colliding with the Orion Nebula flow
The solar wind is the stream of charged (mostly protons and electrons) and neutral particles and associated electric and magnetic fields that moves outward from the sun at high velocities and populates and defines the Interplanetary Magnetic Field or IMF and the heliosphere. A good definition of the heliosphere might be; "that region of space dominated by the IMF". These particles are believed to be accelerated to very high speeds (~500 to 2,000 km/s) by sudden releases of energy associated with a process called magnetic reconnection around highly magnetic sunspot regions. The magnetic field of the solar wind is constantly pushing on the Earth's magnetic field, moving, compressing, and distorting the field and creating electrical currents Solar wind velocities and magnetic field strength vary with the solar cycle, the sun's magnetic latitude, and with radial distance from the sun. At solar maximum, the solar wind velocity is greatest at the sun's equator and less at the poles. Overall, the sun's magnetic field strength is low and polar coronal holes extend to low latitudes. This trend reverses at solar minimum; the solar wind slows down, the sun's magnetic field strength strengthens, and polar coronal holes retreat or disappear altogether.
The realm of the sun's influence extends out beyond the orbits of the planets to the far reaches of the heliosphere, the vast bubble of charged solar wind particles and associated electric and magnetic fields that define the limits of our solar system and interplanetary space. The extent of the heliosphere is not well known but is believed to extend to on the order of 150 A.U. or so from the sun. Expanding out from the sun, energetic particles carrying the sun's magnetic field become dispersed. As their density decreases with increasing distance from the sun, they are not able to push as strongly against particles from interstellar winds. The point where the two pressures balance is called the heliopause. As solar wind particles encounter interstellar particles near this boundary, they slow down transforming their kinetic velocities into thermal energy. The accumulation of these particles near the edge of the heliosphere (think of traffic backups during rush hour) coupled with their deceleration energies creates a sort of shockwave called the termination shock. The two Voyager spacecraft, launched in 1977, are now very near the outer edge of the heliosphere at distances of about 123 AU and 101 AU respectively (Feb 1, 2013) and have already pierced the termination shock at 94 and 84 AU. Beyond the termination shock is the heliosheath, perhaps 10's of AU thick and bounded at its outer point by the heliopause, the final point of influence of the sun's magnetic field. Beyond the heliopause, the Inter Stellar Medium (ISM) populates the vast spaces between the stars. Each of these regions is characterized by the density, strength, and velocities of its fields and particles. The two Voyager apcecraft will sample these regions with a suite of field and particle instruments until about 2025 when their nuclear power sources will run out. Scientists anticipate that the Voyagers will have passed into interstellar space by that time.
The shape of the heliosphere is defined by the opposing pressures of the IMF and ISM. And, the IMF pressure is a function of the speed and direction of the solar wind which is in turn impacted by the solar cycle.
As previously mentioned, our sun goes through cycles of activity. The most prominent of these cycles is the 11 year sunspot cycle though there are other cycles (The Hallstatt solar cycle (2300 years) and the Gleissberg solar cycle (80-90 years), etc.). The number of sunspots in a given year varies predictably over a roughly 11 year period. This year, 2013, is projected to be the peak of the sunspot cycle called SOLAR MAXIMUM (Solar Max). Other changes in the sun can be observed throughout the solar cycle. During Solar Max the frequency of solar storms is at a peak. Solar storms are huge releases of electromagnetic and particle energy driven by magnetic reconnection events in the sun's corona. These storms in the form of solar flares and Coronal Mass Ejections (CMSs) hurl large amounts of high energy electromagnetic and particle radiation into the solar system carrying with it a portion of the sun's magnetic field. As these storms impact solar system bodies, they modify the electrical, magnetic, and chemical environments of planets, moons, and smaller bodies (asteroids and comets). Solar storms as well as the more constant solar wind cause changes in atmospheric chemical composition of planets and moons (e.g. Titan) due to both photo and particle disassociation. Changes to surface properties such as the beautiful reds, and yellows in the surface of Io are a direct result of high energy radiation modifying surface properties. The shapes of the magnetospheres of the gas giants - Jupiter, Saturn, Uranus, and Neptune are modified by changes in solar wind and solar storm pressure interacting with the planet's magnetic dynamo. This also produces auroras observed in the UV in all the outer planets.
There is strong evidence from orbiting spacecraft and surface rover observations that Mars once had a global magnetic field (magnetosphere), warmer temperatures, oceans of liquid water, and much higher atmospheric pressures. Today, Mars is a cold, arid planet with no global magnetic field and a rarified atmosphere by Earthly standards. What happened? It is generally believed that Mars lost its magnetosphere as its internal dynamo cooled and froze out. This gave the solar wind open access to Mars' atmosphere resulting in atmospheric erosion from solar wind particles and the eventual loss of water and most of the rest of the atmosphere. So, the solar wind is in part responsible for the enormous climate change that Mars has experienced over the last few billion years. Other contributors to this process include meteor collisions and absorption of CO2 by the Martian surface.
The shape of the sun's corona changes during this cycle. The shape of the corona is determined by the sun's magnetic field. High energy galactic cosmic rays (GCRs) from violent cosmic events such as super nova explosions are scattered by solar wind particles. Since the solar wind is emphasized during periods of high solar activity, fewer cosmic rays enter the inner solar system and so our atmosphere. Interactions between cosmic rays and Earth's atmosphere produce increased levels of Carbon-14 (14C). This increase in Carbon-14 provides historic records of solar cycles dating back many thousands of years. These records indicate that we are now in a relatively high period of overall solar activity and that many disruptions in the solar cycle have happened in the past. For reasons still not well understood, these disruptions (e.g. Maunder Minimum) have in some cases been correlated with decreases in global temperatures and even mini ice ages.
The amount of solar ultraviolet radiation varies by as much as 400% over the course of the solar cycle, being highest during solar max. However, increased UV also modifies the chemistry in the stratosphere to produce more ozone (O3) which is an effective UV absorber. So, the net result to us hear on the surface is about zero.
Variations in solar activity play important roles in our daily lives. We already know that the interactions between the magnetic field of the solar wind and the Earth's magnetosphere result in beautiful auroras seen in high northern and southern latitudes. These aurora (Aurora Borialis and Aurora Australis) are roughly symmetrical and are driven by similar processes that drive auroras on the outer planets. Auroras have been observed locally in the atmospheres of Mars and Venus as well. Neither planet having a global magnetic field.
Auroras on Earth are seen in the presence of the uniform solar wind. Solar storms, flares and CMEs, intensify these sightings and extend them to much lower latitudes. In fact, newspaper reports of the 1859 super storm record auroral sightings in the northern hemisphere as far south as South America!
Because solar storms pump energy into Earth's magnetosphere, magnetic field lines can come together (reconnect) forming a closed circuit loop. Charged particles in the magnetosphere, attracted to these lines of force, spiral down the lines eventually colliding with atoms in the upper atmosphere. The accelerated spiral motion of these particles produces radio waves which are experienced as radio wave interference with electronics such as TVs and radios (that's why they call them radios!). So disruptions in communications occur much more frequently during times of high solar activity.
Figure 16: A surge of infrared radiation from nitric oxide and carbon dioxide molecules on March 8-10, 2012, signals the biggest upper-atmospheric heating event in seven years. Credit: SABER/TIMED
Increases in solar energy during solar storms also have a pronounced effect on the extent of the atmosphere. We see this most clearly in increased infrared emissions from atmospheric carbon dioxide and nitric oxide. As 10's of billions of kWH of electromagnetic and particle radiation from a flare or CME heats up the atmosphere, these molecules expel most of the added energy as infrared energy which is detectable from Earth orbiting satellites like SABER and TIMED. The added energy expands the thermosphere resulting in increased drag on low orbiting satellites which degrades their orbits and poses risks for untimely reentry. Of course, other orbiting objects such as the plethora of space junk orbiting the Earth are affected in the same way, so solar storms provide a means to sweep debris out of low Earth orbit.
We are protected on Earth from the hazardous space radiation environment by our atmosphere which absorbs almost all high energy radiation (UV, X-Ray, Gamma Ray) and by our magnetosphere which deflects most of the sun's high energy solar wind particles. However, astronauts in space are exposed to much higher levels of both EM and particle radiation. Astronauts on extended missions to the moon, planets, or asteroids would be at risk of much greater exposure due to the length of the mission as well as extensive exposure outside the Earth's protective magnetosphere. Solar storms during these missions pose even greater risks to astronaut safety. This is one of the biggest problems that must be solved if we intend to send humans into the solar system.
Even at altitudes of around 35,000 feet where commercial airlines fly, pilots and frequent flyers are exposed to increased doses of hazardous radiation, as much as 50 to 100 times as much as we experience on the ground. Increased exposure to high energy cosmic rays can result in health problems including cancer. Flights over the poles increase this radiation risk. This is why some airlines prohibit pregnant airline attendants from flying.
Remembering that moving magnetic fields generate current, geomagnetic storms around the Earth from CMEs can jar the magnetosphere enough to produce "Geomagnetically Induced Currents" or GICs in electrical power grids. The added current can cause transformers to overload and create large scale power outages. The economic cost of such an outage can be in the $10's of millions. Such an event blacked out much of Quebec in 1989 for over 9 hours.
The sensitive electronics in satellites and spacecraft are particularly vulnerable to the effects of increased solar activity. Particle radiation can compromise IC circuit boards, flipping bits in memory chips from Single Event Upsets (SEUs) and damaging micro circuit junctions. Solar cells are degraded by long term exposure to energetic solar particles / solar proton events as well as cosmic rays. The defense against this is to radiation harden the electronics, reducing their susceptibility to ionizing EM and particle radiation.
The sun's output is remarkably constant over periods of thousands of years. The "Solar Constant" which is a measure of flux density or the average amount EM radiation at all wavelengths per unit area received by the Earth at its upper atmosphere is measured my Earth orbiting satellites (since 1978) to be 1,361 (kW/m2) and is now known to vary by only 0.1% over a solar cycle which is equal to about 2 watts per square meter, not enough to strongly influence climate. Though small, the solar constant does vary periodically with the 11 year sunspot cycle as well as solar cycles of 88 (Gleisberg Cycle), 208 (DeVries Cycle), and 1,000 years (Eddy Cycle). On much longer time scales, the sun has increased its luminosity by about 30% over its 4.6 billion year life time due to lessoning outward-directed pressure and thus contraction and heating of the core through its thermonuclear fusion process.
Efforts to connect the 11 year sunspot cycle with weather or climate change have been explored in various ways since William Herschel suggested a transient weather connection with solar activity in 1801, noting the Maunder Minimum (1645 - 1715) in sunspot count as one piece of evidence. Many theories predicting Global Climate Change (GCC) over this time frame have been proposed and abandoned. Examples of this are Daansgard's cycles, and a 22 year drought cycle in the western US, but to date no clear relationship with climate has been found. There is some evidence that increased cosmic ray flux creating more aerosol condensation seed nuclei during periods of low solar activity (and so weaker solar magnetic field or IMF) may result in an increase in low altitude cloud cover, particularly at high latitudes though no weather or climate connection has been correlated with these results. It is far more likely that human induced greenhouse gas emission, volcanic activity, and natural changes (internal forcing) are responsible for a rise in global temperatures.
The largest, single, challenge for astronauts traveling to Mars will be to overcome exposure to solar storms and radiation.