The Solar Cycle and its Relation to Solar Weather

 by Dave Lengyel

When we plan our vacations, outings or observing sessions, we make it a point to check the weather forecast. We’ve learned over the years that the jet stream, Canadian air masses or high pressure systems over the Atlantic Ocean, though far away, have a definite effect on our weather, and so we pay attention to what they are doing. But, should we also direct our attention to something even further away, like 93 million miles? No doubt, most everyone is aware of how important the Sun is to our very existence here on Earth, but mostly we consider it to be a stalwart of steady energy output with little variation. But what goes on in the not-so-empty regions of space between our star and us can change rapidly and dramatically and can have both short-term and long-term effects on us. In other words, maybe we should pay closer attention to our solar weather forecasts.

In order to understand how the changeable Sun can affect us on Earth, we need to take a closer look at the Sun’s “anatomy”. The Sun produces energy through nuclear fusion which occurs only when temperatures are high enough to allow protons to get close enough together so that they fuse and don’t just bounce away from each other due to their like positive charges. The temperature in the Sun’s core, about 10,000,000 K, allow nuclear fusion to occur. This process produces huge amounts of energy in the form of photons. These high-energy photons cannot just move directly out of the core due to its high density. In fact, it may take upwards of 200,000 years for the energy originally produced by fusion in the core to finally leave the Sun and travel outwards into space. The core and the region just outside it compose about three-fourths of the solar radius. Here, energy is transferred outward by radiation in which the photons produced by fusion are absorbed in the high density region by other particles and then reemitted. As energy continues to make it’s way outward, the density in those regions begin to decrease, and particles are freer to move about in bulk motion. The churning around of material in this convective layer is where things get interesting, at least in terms of the connection between the Sun and the Earth. Since the material in the Sun is not quite like our everyday matter here on Earth, but rather exists in the plasma state where protons and electrons are separated, some special things take place in this convective zone. The method of heat transfer here is dependent upon these huge blobs of plasma rising, losing energy and cooling, then falling back down to be heated again much like water in a kettle.

Above the convection zone is the rather thin photosphere, the only part of the Sun we normally see from Earth. It is from this layer of the Sun that photons of energy which formed long ago and far away in the core can finally break free and leave, and these are the photons that our eyes detect. Since the source of the Sun’s energy, nuclear fusion, occurs deep in the core, it would stand to reason that the Sun’s outer layers would be cooler the further from the core that they are. In fact, the temperature of the top of the photosphere is about 4400 K. But in terms of temperature, this cooling pattern from the center out changes.

The chromosphere is the thin lower-density area above the photosphere, and in this layer the temperature begins to increase as you go upwards until a temperature of about 25000 K is reached. Above this region is the Sun’s corona, the region that amateur astronomers find so interesting that they travel thousands of miles to see it, when it briefly appears during those frantic moments of totality during a solar eclipse. This milky-white solar “atmosphere” of the Sun is much dimmer than the photosphere and can only be seen when the Sun is up and the sky is dark, conditions that occur during totality. Professional astronomers are also very interested in this part of the Sun, one reason being that it has a temperature of about 2 million degrees K in its lower regions. The mechanism for the heating of the corona is not understood at this time, but was studied intensely during the 1991 total solar eclipse from Mauna Kea and is continuing to be investigated by various solar observing spacecraft such as TRACE, SOHO and ACE.

The Sun does not send out its energy in ways that are constant and easily predictable and much of this is due to the Sun’s intense magnetic field. This magnetic field, thousands of times more intense than Earth’s, originates in the convective layer of the Sun. Since the Sun is composed of matter in the plasma state, when the free electrons and atomic nuclei move around in convection currents, this means that what we have are charged particles in motion, or an electrical current and a magnetic field results. Since the Sun is not a solid body, as it rotates parts of it move at different speeds. The polar regions rotate in about 35 days and the equatorial regions rotate in about 25 days. This differential rotation plays havoc with the lines of magnetic force and lead to the Sun’s erratic output of energy. As the different latitudes of the Sun rotate differently, the magnetic flux tubes around the Sun get twisted and kinked as lines that were originally going north and south get spread out and wound around the Sun. Since the magnetic lines of force do not go too deeply into the Sun, these flux tubes are often brought upwards through the photosphere by convection. The magnetic fields tend to confine the normal convection that brings energy to the photosphere and so that part of the photosphere is somewhat cooler than the surrounding regions, so less light is produced there that eventually reaches Earth, and a sunspot occurs. When viewed through a telescope fitted with a solar filter, sunspots appear dark, but this is really due to the photosphere being so bright. A typical sunspot is about as bright as the full Moon. Despite the fact that the twisted magnetic fields disrupt convection through the photosphere, the overall Sun is actually brighter when there are more sunspots. In part this is due to the formation of faculae, brighter areas associated with weaker magnetic fields, and the formation of solar flares in the active regions around sunspots.

It has been known since about the mid 19th century that sunspot numbers are cyclic, generally peaking every eleven years, but there is a bit more to the story. The Sun reverses its magnetic polarity every eleven years also, and so the sunspot cycle might be better thought of as a twenty-two year cycle, with sunspot numbers reaching a maximum about every eleven years. Since the regions around sunspot are often the locations of much energetic activity, we see a correlation between the sunspot cycle and the amount of material blasted away from the Sun, forming the solar weather.

The Sun is constantly sending material outward, some of which interacts with Earth. This stream of charged particles, mostly protons and electrons, is called the solar wind. During times of high numbers of sunspots, the solar wind can become quite changeable. The magnetic fields which form sunspots can become disconnected as they twist and rise and reconnect part way up. When this happens, large amounts of energy and particles are blasted away from the Sun as a solar flare. Also associated with the active regions around sunspots are filaments and prominences, which are filaments seen on the Sun’s limb during a total solar eclipse. These structures are vast areas of hot magnetic gas that extend hundreds of thousands of miles out from the photosphere into the corona. They are held to the Sun where their lines of magnetic force go into the Sun. When these magnetic lines are cut, there is a tremendous release of energy into space called a coronal mass ejection. This adds a whole lot of energy to the solar wind and unleashes a solar storm, perhaps heading towards Earth.

Fortunately, we here on Earth are not defenseless against the onslaught of high-energy particles from the Sun. Our own magnetic field saves the day, at least most of the time. As the particles of the solar wind encounter our magnetic field, a bow-shock forms which deflects the majority of the charged particles away from the Earth. When a coronal mass ejection occurs, it may send its blast toward us. If so, our magnetic field is put to the test. If the coronal mass ejection’s magnetic field is aligned with Earth’s, the two fields repel and our magnetosphere is able to absorb the shock and rebound slowly. If the fields are oriented oppositely, then we lose our protective magnetic field for a while, and high-energy particles can enter our region of space and this can be damaging.

The interaction of the solar wind with our magnetosphere and our atmosphere can have a variety of effects on Earth. It causes the auroras to occur as the particles pass out of the Van Allen radiation belts and interact with particles in our upper atmosphere. The resulting exchange of energy boosts the electrons in our atmospheric gases to higher levels and when they return, a photon is emitted. This effect of the solar wind is usually relatively benign, but a really strong solar wind caused by an active Sun (lots of sunspots) can cause very strong auroras. The currents that flow down Earth’s magnetic lines of force are direct currents and during a powerful aurora, this can disrupt electrical power grids, which use alternating current, as it did in March of 1989 in parts of North America. Another nasty effect of an active Sun and coronal mass ejections are possible silencing of orbiting satellites due to the destruction of their solar panels and sensitive electronics by the increased solar wind. Cellular phone service could be disrupted (is this always a bad thing?), military communications and intelligence gathering could be affected and modern navigational systems which depend on global positioning systems could fail. Astronauts in Earth orbit, especially if they happen to be outside their spacecraft, would be vulnerable to the adverse effects of coronal mass ejections. If we return to the Moon, astronauts there would have it even worse, since the Moon has no appreciable magnetic field to offer protection.