How and why does the Sun’s energy change, and how does the Earth respond? We care about these changes, and seek improved understanding of their causes and consequences. We do this because society urgently seeks to quantify anthropogenic and natural causes of climate change, because we are increasingly reliant on the technological benefits of space assets, and because we utilize and explore extensive environmental domains well beyond the surface where we live.
Introduction

Figure 1. The coupled Sun-Earth system – a schematic. Primary regions of the Sun-Earth system include the Sun, which is the system’s source of energy (shown in yellow), the heliosphere and space environment near Earth (shown in green) and the Earth’s atmosphere and surface (shown in blue). Arrows indicate the flow of energy among regions. Galactic cosmic rays originate beyond the Sun-Earth system and are modulated by it.

Our planet inhabits the neighborhood of the Sun, a somewhat capricious, middle-aged star. The Sun’s output – in the form of energetic particles (mainly electrons and protons) and magnetic fields – is sufficiently stable to maintain a terrestrial environment that supports human life and enables space-based technologies. But the energy output is not constant. A dynamo located beneath the Sun’s visible surface causes solar activity to wax and wane with a cycle of roughly 11 years. Solar activity – a measure of the amount of magnetic flux that erupts onto the Sun’s surface and into its atmosphere, driven by the dynamo below – changes the Sun’s energy outputs. Even the total electromagnetic output, called the total solar irradiance (TSI, in units of Watt per meter² at a distance of one Astronomical Unit), is higher during peaks of the solar activity cycle, warming Earth’s surface slightly. In atmospheric layers at increasingly higher altitudes above the Earth’s surface, the terrestrial impact of the solar cycle grows dramatically, dominating the variations in temperature and density beyond 100 km. In addition, erratic solar eruptions expel particles and magnetic fields into the solar wind, further roiling the Earth’s outer atmosphere and the space environment near earth.

With the advent of the space era – which commenced barely fifty years ago – fledgling, intermittent observations of the Sun and Earth have evolved to databases of multiple solar and terrestrial parameters with unprecedented spatial and temporal coverage. These databases provide a phenomenal opportunity to characterize the changing Sun and the changing Earth, and to explore, understand and specify the complex coupled system (Figure 1) in which we live.

Faces of the Sun

Figure 2. Solar phenomena and time scales of solar variability.Images of the Sun made in different wavelengths of radiation reveal myriad phenomena including bright, coronal loops (upper left), flares and active regions (upper middle, movie), coronal mass ejections (lower left) and dark sunspots (lower middle, movie). Flares and coronal mass ejections produce energy output variations on time scales of minutes to hours. Rotation of sunspots, faculae and plage across the Sun’s face generate energy output variations on times scales of days and weeks. A dynamo inside the Sun (upper right), located at the bottom of the convention zone, generates the 11-year activity cycle, shown in sunspot numbers (right) and in coronal evolution seen in soft X-ray images made from the Yohkoh spacecraft (lower right). Images and Movies: NASA SOHO & Trace, ISAS Yohkoh

Were the Sun the pristine, blemish-free orb that Galileo‘s seventeenth century Catholic Church decreed, solar energy output would vary little on time scales of days to decades. However, the Sun’s face exhibits myriad features (Figure 2) that are distinguished from the background “quiet” Sun by their altered levels of magnetic field strength. Magnetic features and associated phenomena emerge, evolve and decay with increased occurrence and size when the Sun is active. Observations at different wavelengths reveal aspects of the different features. For example, compact black spots, sunspots, are apparent in visible light on the nominal “surface” of the Sun (defined as the gaseous shell of the Sun’s atmosphere which emits visible photons). Large sunspots, many times the size of the Earth, may cover a few percent of the Sun’s disk. Multiple clumps of bright regions, called faculae (Latin for “torch”), often surround sunspots. When viewed in ultraviolet light that is emitted from higher in the Sun’s atmosphere, above the visible surface, larger and brighter plage are seen to overlie the faculae. In the outer layers of the Sun’s atmosphere – the corona – larger still, and even brighter, regions of strong magnetic flux sometimes cover a significant fraction of the Sun’s face. Also populating the corona are extended dark patches, called coronal holes, where mass from the solar atmosphere flows into the surrounding heliosphere (the sphere of “helio”, Greek for “Sun”), producing the solar wind that envelops the Earth and planets.

Sunspots have been recorded since their telescopic discovery in the early 1600s, and variations in sunspot numbers (Figure 2) characterize the archetypal solar activity cycle. Sunspots, faculae, plages and coronal holes are all generated by protrusions of magnetic field into the Sun’s atmosphere, driven by the subsurface dynamo. Because electromagnetic radiation from these magnetic features is altered relative to the quiet Sun, their occurrence modulates, in distinct ways, the Sun’s net outward flowing energy. Thus, superimposed on the dominant 11-year activity cycle (Figure 2) are semi-regular fluctuations on time scales of days to weeks, produced by the Sun’s rotation on its axis and the evolution of magnetic regions. On shorter time scales still, active region magnetic fields can reconnect, producing large fluxes of electromagnetic radiation at very high energies, called flares. The rapid and unpredictable eruptions that produce flares also spew massive amounts of matter into the solar wind, called coronal mass ejections, which accelerate particles and tangle fields throughout the heliospshere.

Figure 3. Variations in total solar irradiance.Variations in the total solar irradiance (upper left) are currently made by the Total Irradiance Monitor (TIM) on the Solar Radiation and Climate (SORCE) spacecraft (lower right), to be followed by the Glory mission in solar cycle 24. The primary sources of the observed total solar irradiance variations are bright faculae and dark sunspots whose contributions are shown over the solar cycle (lower left) and during a short-term period when large sunspots on the disk produced a significant irradiance reduction (upper right). Images: NASA SOHO, BBSO, LASP

Since the space era coincides approximately with the Modern Maximum of high solar activity (characterized by high sunspot numbers after ∼1950 in Figure 2), most of our observational knowledge of the Sun and its variations pertains to overall high activity levels. The dearth of sunspots during the Maunder Minimum (from 1645 to 1715 in Figure 2) indicates an absence of magnetic fields presumably accompanied by reduced energy outputs.

Bathed in Light

Photons carry the bulk of the Sun’s energy directly to Earth, depositing most of it – at near-UV, visible and IR wavelengths – near the surface. Although historically referred to as the solar “constant”, the total solar irradiance varies because dark sunspots and bright faculae on the Sun’s disk reduce and enhance, respectively, the net photon emission (Figure 3). There is an overall increase in total solar irradiance during the solar cycle because enhanced emission in bright faculae more than offsets (by a factor of ∼2) the decreased emission in sunspots. However, when solar rotation carries large sunspots onto the face of the Sun visible at the Earth, short-term sunspot dimming can exceed facular brightening by as much as a factor of 5 (Figure 3). This produces significant depletions in radiant energy that are seen in Figure 3 superimposed on the 11-year solar irradiance cycle.
Solar photons at wavelengths less than 300 nm deposit their energies at increasingly higher altitudes in the Earth’s atmosphere (Figure 4); UV radiation is absorbed in the stratosphere (~25 to 50 km above the surface) and the extreme UV (EUV) radiation in the thermosphere and ionosphere (100 to 500 km above the surface). Electromagnetic radiation at the shorter (EUV) wavelengths of the solar spectrum are formed higher in the solar atmosphere, absorbed higher in the Earth’s atmosphere, and vary much more than does radiation in the visible spectrum. For example, solar cycle changes in EUV radiation exceed 100%, compared with the 0.1% cycle of the visible spectrum. This is because plage emission at EUV wavelengths is an order of magnitude (or more) brighter than the background Sun, whereas the contrast of the underlying visible light faculae that alter the total irradiance is only a few percent.

Figure 4. Deposition of solar radiation in the Earth’s atmosphere.Shown on the left, by the red curve, is the approximate altitude at which solar electromagnetic radiation at different wavelengths is absorbed in the Earth’s atmosphere. Primary atmospheric “spheres” – the troposphere, stratosphere and thermosphere – are identified. On the right are the average temperature (T) and density (ρ) profiles of the atmosphere, with the arrows indicating approximately where the energy in specific bands is deposited. Radiation at wavelengths longer than ∼300 nm reaches the surface and troposphere. Solar energy heating warms the atmosphere, reversing the cooling trend with altitude in the troposphere and producing the hot “thermo” sphere. Images: NRL, NASA, Google Image

There is much debate about how the Sun’s changing energy outputs influence Earth’s climate, including speculation that solar variations, rather than anthropogenic gases, have caused significant recent global warming. The Sun’s activity cycle does influence the Earth’s surface temperature, but the changes are modest and difficult to detect. The solar signal must be isolated from other climatic influences that operate simultaneously. The El Niño Southern Oscillation (ENSO, an atmospheric-ocean coupling in the tropical Pacific), volcanic aerosols, increasing concentrations of greenhouse gases (GHGs) and industrial aerosols, and land use changes all affect the surface and troposphere, in different ways, in different amounts, in different geographical regions. When these changes are accounted for in the surface temperature record, a solar-driven global temperature cycle of about 0.1 K (from activity min to max) is identified (Figure 5). For comparison, the globe warmed 0.2 K during the 1997 “super” ENSO event, cooled 0.3 K as a result of the Pinatubo volcano, and has warmed 0.4 K since 1980 in response to changes in anthropogenic gas concentrations. Recent global warming is therefore primarily anthropogenic, rather than solar, in origin.

Temperatures in the Earth’s atmosphere respond to the Sun’s changing energy outputs with increasing amplitude at increasing height above the surface (Figure 5). Anthropogenic influences significantly exceed the solar cycle signal near the surface, but they are comparable in the stratosphere. In the thermosphere-ionosphere, for example near 450 km, the solar cycle global temperature increase of 400 K overwhelms the 3 K long-term anthropogenic cooling in recent decades. Here, the deposition of solar EUV radiation (Figure 4) causes the atmosphere to be much hotter (∼1,000 K) than at the surface (288 K), and solar energy output changes are the undisputed orchestrator of temperature and density variations. For example, solar activity is the dominant cause of total electron content (TEC) variability, which alters the ionosphere transmission and refraction of radio waves, affecting systems such as the Global Positioning System (GPS) which utilize these frequencies for communication and navigation around the globe.

Figure 5. Global temperature responses to solar, anthropogenic, volcanic and ENSO influences. Compared are the natural and anthropogenic sources of global temperature change at the Earth’s surface (left)) and in the lower stratosphere (right), obtained by multiple regression. The observed global surface and atmosphere temperature changes since 1980 (shown as symbols in the two bottom panels) are modeled by linear combinations of four different signals; solar irradiance and anthropogenic gases (greenhouse gases, GHG, and chlorofluorocarbons, CFC, upper panels) the El Niño Southern Oscillation (ENSO) and volcanic aerosols (middle panels). Images: NASA, Google Image.

The magnitude and terrestrial consequences of long-term solar activity – especially on Earth’s climate – is uncertain because of the lack of comprehensive observations prior to the space era. Four of the largest amplitude sunspot cycles have occurred in the space era but during the Maunder Minimum sunspots were absent from the Sun’s disk for extended periods, the cycle perhaps interrupted by a lethargic dynamo. Presumably solar energy outputs were reduced, but the actual levels of brightness and solar wind speed is speculative. One approach for estimating past changes in the Sun’s energy output is to model the dynamo-driven transport of surface magnetic flux (by diffusion, meridional flow and differential rotation as the Sun rotates on its axis) using sunspots as indicators of emerging magnetic fields. Long-term evolution of the total magnetic flux is then assumed to produce irradiance changes (Figure 6). When simulated in this way, solar irradiance variations account for less than 10% of the total global warming in the past century.

Buffeted by Wind

A steady stream of mass – mainly protons – flows from coronal holes, where magnetic fields in the Sun’s outer atmosphere extend into the heliosphere, rather than being anchored, loop-like, at the Sun’s surface. With a speed of about 450 km per sec, this steady solar wind transports particles and magnetic fields to Earth (150 million km downstream) in a few days. At Earth, semi-regular, recurring wind streams occur as dark coronal holes rotate across the Sun’s face.

Impinging upon the Earth, the flowing solar wind contorts and reshapes the geomagnetic field into an elongated teardrop, with an upwind bow shock at about 10 R_E and a much longer, elongated tail that extends some 100 R_E downwind. The magnetosphere envelops the plasmasphere (a region of low density ions where magnetic field lines are anchored at Earth), which in turn surrounds – and protects – the Earth’s outer atmosphere and ionosphere.

Figure 6. Estimating long-term changes in solar activity and irradiance. A sub-surface dynamo (upper image, far left) causes magnetic fields to penetrate the solar surface. The pattern of magnetic flux (upper image, second left) evolves continually as surface magnetic flux is transported by differential rotation, meridional flow and diffusion (three images, upper right). These transport processes alter the amount of closed and open flux, which in turn alter solar irradiance and the solar wind, respectively. Flux transport model calculations simulate long-term solar energy output changes, and suggest that solar irradiance has increased 0.05% since the seventeenth century Maunder Minimum (lower graph). Images: NASA, Y.-M. Wang.

Magnetic fields on the Sun can reconnect abruptly, propelling explosions of photons and mass towards the Earth (Figure 7). Flare photons arrive almost immediately (in 8 minutes), followed by highly energetic solar particles (within hours) then, over a few days, by the ejected coronal mass. As coronal mass ejections propagate through the heliosphere, solar wind speeds can increase to over 700 km per sec.

The bow shock is first to feel the brunt of the agitated solar wind impact as heliospheric and terrestrial fields intermix and reconnect, producing at times dramatic reverberations that reconfigure the entire magnetosphere (Figure 8, upper left image). Particles that were trapped in belts of magnetic field are funneled into the Earth’s polar regions where the fields converge, depositing their energy by collisions with oxygen and nitrogen gases in the Earth’s lower thermosphere (near 100 km altitude). Auroras are visible evidence of the resultant resistive heating. The heated gases expand upward and outward, fountain-like, transporting energy and mass over the poles to the night side of the Earth, and eventually to mid latitudes around the globe, further carried by the Earth’s rotation (Figure 8, middle). Dramatic temperature and composition changes ensue, including rapid alterations of ionospheric electron densities that impact communication and navigation in sudden (and unpredictable) ways, and neutral densities (Figure 8, lower left) that impose severe drag (acceleration) on spacecraft in low-earth orbit.

Geomagnetic storm impacts are muted in the stratosphere and at lower altitudes because the increasingly dense atmosphere (at decreasing altitudes, Figure 4, right panel) strongly attenuates all but the most energetic particles. Nevertheless, surges of solar energetic particles – high energy protons – can penetrate deep into the Earth’s atmosphere and alter ozone chemistry. Galactic cosmic rays also penetrate to the lower atmosphere; their flux attenuated by the intervening heliosphere such that high solar activity diminishes the flux and reduces the content of ¹⁴C in tree rings and ¹⁰Be in ice cores. Thus these cosmogenic isotope archives preserve in terrestrial format a history of solar activity over past millennia, specifically the changes in the open magnetic flux that modulate the helisopshere.

Integrating the Sun-Earth System

Figure 7. Anatomy of the erupting Sun-Earth system. During the “Halloween” storm on 28 Oct 2003, a large active region, observed as a sunspot in a white light image made at the Big Bear Solar Observatory (upper left image) and plage brightening in EUV emission by the Extreme ultraviolet Imaging Telescope(EIT) on the Solar and Heliospheric Observatory (SOHO) (upper right image), erupted when it was near the center of the solar disk. Detectors on the Geostationary Operational Environmental Satellites (GOES) recorded an X-class flare (upper right plot). The Large Angle and Spectrometric Coronagraph Experiment (LASCO) on SOHO observed an associated coronal mass ejection (lower left image). Many hours later, the energetic particles associated with the eruption reached the LASCO detector at L1, upstream from Earth, and saturated its detectors (lower right image). GOES detectors recorded the proton fluxes (lower right plot), which were reported by the National Weather Service Space Weather Prediction Center. Images: BBSO, NASA/SOHO, NOA/SWPC.

The Sun-Earth system is huge, vastly unobserved, and not yet modeled as a fully-integrated regime. Of the entire volume from the Earth’s surface to 20,000 km (where GPS spacecraft orbit) more than 99% resides above 50 km. Solar-driven impacts are qualitatively recognized throughout the domain but quantitative representation is poor, in part because of the lack of regular, global observations in the upper atmosphere and space environment, and the daunting challenge of observing with adequate spatial and temporal coverage.

Physical models are therefore crucial to capture the entire system. Emerging is a next generation of terrestrial system models that encapsulates new understanding and assimilates observations. General circulation climate models are extending upwards from the surface. Rather than using a few token “buffer” layers in the stratosphere to represent the upper boundary of the climate system, the models are reaching to 100 km, with many stratosphere and mesospheric layers, interactive ozone chemistry and coupled chemical, dynamical and radiative schemes. For the space environment, geospace system models are being constructed by coupling models of individual regimes, integrating the thermosphere and ionosphere electrodynamically with the magnetosphere and heliosphere (Figure 9) and ingesting state-of-the-art solar drivers, including irradiance at all wavelengths, solar wind speed, density (and temperature) and magnetic field. Above about 50 km, however, current knowledge of dynamical motions, vertical interactions and system coupling mechanisms is fledgling compared with knowledge of the atmosphere and surface below. Whereas general circulation climate models have been under development for almost 50 years (since the 1960s) the first coupled thermosphere-ionosphere–magnetosphere model was attempted only in 2001.

Figure 8. Terrestrial impact of a solar-geomagnetic storm. The upper left image depicts a coronal mass ejection from the sun impinging on the magnetosphere, allowing trapped particles to spill into the thermosphere and ionosphere, producing a geomagnetic storm and a bright aurora. The image in the middle illustrates the consequences: Heating by precipitating particles at high latitudes produces upward mass flow over polar regions, and downwelling equatorial winds, which the Earth’s rotation transports to the day (sunlit) side, producing large scale changes in temperature and composition. Thermopsheric densities can change dramatically in response to such solar-terrestrial eruptions. In the bottom panel, the solar-terrestrial changes during the Halloween geomagnetic storm are seen to be superimposed on the more regular changes associated with the EUV photon variations during solar rotation. Image: R. Meier, NASA

Ultimately, assimilation of comprehensive observations will advance models throughout the entire Sun-Earth system, not just in the troposphere – as is done today. A future challenge is to secure the needed observations to constrain the models by developing the capability to image the geospace environment on large spatial scales, not just in selected domains, with adequate cadence to properly capture the space-time variations so as to extract geophysical parameters relevant to the models. Seamless specification and forecasting of the entire, expanded terrestrial system and its solar-driven fluctuations will then ensue.

Judith Lean
Space Science Division http://spacescience.nrl.navy.mil/
Naval Research Laboratory www.nrl.navy.mil
Washington DC 20375
Acknowledgements: Funded by NASA and ONR. Much appreciated and valued collaborations with David Rind, Mike Picone, Yi-Ming Wang, Joe Huba, Robert Meier and others at NRL, LASP and elsewhere.

Figure 9. Components of an Integrated Sun-Earth System Model.Components of the Sun-Earth system are integrated using fully-coupled physical models, enabling simulations of environmental change from the Sun to the Earth. The models are available at Community Coordinated Modeling Center (CCMC).