University of Cambridge

Sunspots and earth's atmosphere

The atmosphere of the sun has a profound impact on the earth. Our everyday life is directly influenced by the effect of solar radiation. Earth’s climate is ruled by a very simple and fundamental principle: “energy coming in must go out”.  Energy comes in by way of sunlight mostly in the form of visible and ultraviolet (UV) light. Energy travels back out to space via infrared light shining up from the earth’s surface and atmosphere.

The earth’s energy flux in and out can be wildly out of balance at any given instant or location, but on average for the entire planet and over a long time, the energy flux must balance. If the global energy fluxes are out of balance, the temperature of the earth’s surface rise or falls, seeking a new equilibrium (1).

A change in earth’s temperature affects the outgoing energy flux according to a fundamental law of physics called the Stephan- Boltzmann relation, which says that the outgoing energy flux from an object increases as the object gets wormer. According to this relation, an object radiates energy at a rate equal to εσT4, where this energy flux is in units of W/m2, epsilon (ε) is the emissivity, reflecting the efficiency with which the material radiates energy, sigma (σ) is the Stefan-Boltzmann constant which has the value 5.67X10-8W/ (m2k4), and T is the temperature in Kelvin. The Stefan-Boltzmann reaction says that an ordinary object emits light all the time as long as its temperature is greater than absolute zero (-4590F or -2730C).  For example, an electric burner on a stove glows visibly with red light when it is hot to the touch. But at room temperature, the burner still emits radiation, but in infrared frequencies that our eyes cannot see (1).

Archer and Rahmstorf (1) stated that the brightness of the sun varies naturally over time. The clearest variation in solar intensity, and the easiest to measure, seems to be a part of the roughly 11 year sunspot cycle. In the recent decades, the maximum in the sunspot cycle brings may be 100 dark spots on the surface of the sun, in the low part of the cycle, there may be no sunspot at all. Although each individual sunspot is a cool region, overall the sun shines more brightly when there are lots of sunspots. Accurate measurements of the solar intensity go back about three decades, and they reveal the sunspot cycle alters the intensity of the sun by 0.08%.  When this intensity is averaged over the entire surface of the earth and corrected for the earth’s albedo, it results in radiative forcing variations of about 0.2 W/m2.

In 1610, shortly after viewing the sun with his new telescope, Galileo Galilei (or was it Thomas Harriot?) made the first European observations of sunspots. Continuous daily observations were

Started at the Zurich Observatory in 1849 and earlier observations have been used to extend the records back to 1610. The sunspot number is calculated by first counting the number of sunspot groups and then the number of individual sunspots (2).

Sunspots

Sunspots

Sunspots are dark, planet-sized regions that appear on the “surface” of the sun. Sunspots are “dark” because they are colder than the areas around them. A large sunspot might have a temperature of about 4,000 K (about 3,700° C or 6,700° F). This is much lower than the 5,800 K (about 5,500° C or 10,000° F) temperature of the bright photosphere that surrounds the sunspots.

Sunspots are only dark in contrast to the bright face of the sun. If you could cut an average sunspot out of the sun and place it in the night sky, it would be about as bright as a full moon. Sunspots have a lighter outer section called the penumbra, and a darker middle region named the umbra. Sunspots are caused by the sun’s magnetic field welling up to the photosphere, the sun’s visible “surface”. The powerful magnetic fields around sunspots produce active regions on the sun, which often lead to solar flares and Coronal Mass Ejections (CMEs). The solar activity of flares and CMEs are called “solar storms”. Sunspots form over periods lasting from days to weeks, and can last for weeks or even months. The average number of spots that can be seen on the face of the sun is not always the same, but goes up and down in a cycle (3).

The “sunspot number” is then given by the sum of the number of individual sunspots and ten times the number of groups. Since most sunspot groups have, on average, about ten spots, this formula for counting sunspots gives reliable numbers even when the observing conditions are less than ideal and small spots are hard to see. Monthly averages (updated monthly) of the sunspot numbers show that the number of sunspots visible on the sun waxes and wanes with an approximate 11-year cycle (2).

Maunder Minimum

Early records of sunspots indicate that the sun went through a period of inactivity in the late 17th century. Very few sunspots were seen on the sun from about 1645 to 1715, this time period is called Maunder Minimum. Although the observations were not as extensive as in later years, the sun was in fact well observed during this time and this lack of sunspots is well documented. This period of solar inactivity also corresponds to a climatic period called the “Little Ice Age” when rivers that are normally ice-free froze and snow fields remained year-round at lower altitudes. There is evidence that the sun has had similar periods of inactivity in the more distant past. The connection between solar activity and terrestrial climate is an area of on-going research (2). The lack of sunspots implies a cooler sun at that time, driven by a radiative forcing decrease of about 0.12 W/m2, much smaller than the increased radiative forcing from greenhouse gases of about 3.6 W/m2(1).

The Butterfly Diagram

Butterfly DiagramDetailed observations of sunspots have been obtained by the Royal Greenwich Observatory since 1874. These observations include information on the sizes and positions of sunspots as well as their numbers. These data show that sunspots do not appear at random over the surface of the sun but are concentrated in two latitude bands on either side of the equator. A butterfly diagram (updated monthly) showing the positions of the spots for each rotation of the sun since May 1874 shows that these bands first form at mid-latitudes, widen, and then move toward the equator as each cycle progresses (2).

The Greenwich Sunspot Data

The Royal Greenwich Observatory data has been appended with data obtained by the US Air Force Solar Optical Observing Network since 1977. This newer data has been reformatted to conform to the older Greenwich data and both are available in a local directory of ASCII files. Each file contains records for a given year with individual records providing information on the daily observations of active regions (2).

Direct observations over the past four centuries show that the number of sunspots observed on the sun’s surface varies periodically, going through successive maxima and minima. Following sunspot cycle 23, the sun went into a prolonged minimum characterized by a very weak polar magnetic field and an unusually large number of days without sunspots. Sunspots are strongly magnetized regions generated by a dynamo mechanism that recreates the solar polar field mediated through plasma flows. Here Nandy et al (4) report results from kinematic dynamo simulations which demonstrate that a fast meridional flow in the first half of a cycle, followed by a slower flow in the second half, reproduces both characteristics of the minimum of sunspot cycle 23. Their model (4) predicts that, in general, very deep minima are associated with weak polar fields. Sunspots govern the solar radiative energy and radio flux, and, in conjunction with the polar field, modulate the solar wind, the heliospheric open flux and, consequently, the cosmic ray flux at earth.

An analysis of satellite data challenges the intuitive idea that decreasing solar activity cools earth, and vice versa. In fact, solar forcing of earth’s surface climate seems to work the opposite way around — at least during the current sun cycle. Joanna Haigh, an atmospheric physicist at Imperial College London, and her colleagues (5) analyzed daily measurements of the spectral composition of sunlight made between 2004 and 2007 by NASA’s Solar Radiation and Climate Experiment (SORCE) satellite. They found that the amount of visible light reaching earth increased as the sun’s activity declined — warming the earth’s surface. The study period covers the declining phase of the current solar cycle. Solar activity, which in the current cycle peaked around 2001, reached a pronounced minimum in late 2009 during which no sunspots were observed for an unusually long period. Sunspots, dark areas of reduced surface temperature on the sun caused by intense magnetic activity, are the best-known visible manifestation of the 11-year solar cycle. They have been regularly observed and recorded since the dawn of modern astronomy in the seventeenth century. But measurements of the wavelengths of solar radiation have until now been scant. Radiation leak Haigh’s team compared SORCE’s solar spectrum data with wavelengths predicted by a standard empirical model based mainly on sunspot numbers and area, and noticed unexpected differences. The amount of ultraviolet radiation in the spectrum was four to six times smaller than that predicted by the empirical model, but an increase in radiation in the visible wavelength, which warms the earth’s surface, compensated for the decrease (5).

Contrary to expectations, the net amount of solar energy reaching earth’s troposphere — the lowest part of the atmosphere — seems to have been larger in 2007 than in 2004, despite the decline in solar activity over that period. The spectral changes seem to have altered the distribution of ozone molecules above the troposphere. In a model simulation, ozone abundance declined below an altitude of 45 kilometres altitude in the period 2004–07, and increased further up in the atmosphere. The modelled changes are consistent with space-based measurements of ozone during the same period (5). The full implications of the discovery are unclear. Haigh (5) said that the current solar cycle could be different from previous cycles, for unknown reasons. But it is also possible that the effects of solar variability on atmospheric temperatures and ozone are substantially different from what has previously been assumed.

Michael Lockwood, a space physicist at the University Of Reading, UK, says that the data seem incredibly important and if solar activity is out of phase with solar radiative forcing, it could change his understanding of how processes in the troposphere and stratosphere act to modulate earth’s climate. Some meteorologists believe that during phases of low solar activity, ‘blocking events’ — unusual patterns in westerly air currents that can cause cold snaps and freak weather in Europe — occur more frequently. A blocking event is thought to have caused the southward transport of ash clouds following the eruption in March of the Icelandic volcano Eyjafjallajökull, which disrupted air traffic throughout Europe. But any links between recent weather anomalies and possible peculiarities in the current solar cycle are speculative for now, says Lockwood.

Over the three-year study period, the observed variations in the solar spectrum have caused roughly as much warming of earth’s surface as have increases in carbon dioxide emissions. But because solar activity is cyclic it should have no long-term impact on climate, regardless of whether similar spectral changes have occurred during previous solar cycles (5).

The sun’s magnetic activity swings from a minimum to maximum over an average 11 years. The variations (which influence the quantum of radiation that reaches the earth) are gauged by the number and placement of sunspots visible. Increased solar activity, which entails huge eruptions of charged particles and emission of intense radiation, can adversely affect satellites, communication and power systems, as well as pose serious health risk to astronauts. The next solar maximum is expected around 2013. The geomagnetic storms may be caused by the solar flares and associated mass ejection from the solar corona. It happens when the charged solar particles interact with the earth’s magnetic field. Sunspots influence the energy output of the sun- the greater the number of sunspots, the higher is the energy output. This, in turn, increases the heat received by the earth, which determines the planet’s weather and climate.

References:

  1. Archer. D and Rahmstorf. S. In: The Climate Crisis, An Introductory Guide to Climate Change, Cambridge University Press, Cambridge, New York, (2010).
  2. http://solarscience.msfc.nasa.gov/SunspotCycle.shtml
  3. http://www.windows2universe.org/sun/atmosphere/sunspots.html
  4. Nandy, D. et al .Nature 471: 80–82 ( 2011)
  5. http://www.nature.com/news/2010/101006/full/news.2010.519.html

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