Solar forcing of the terrestrial...

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CRAS2A-2847; No of Pages 16

Please cite this article in press as: T. Dudok de Wit, J. Watermann, Solar forcing of the terrestrial atmosphere, C. R. Geoscience(2009), doi:10.1016/j.crte.2009.06.001

External geophysics, climate and environment (Climate)

Solar forcing of the terrestrial atmosphereThierry Dudok de Wit a,*, Jürgen Watermann a,b

aUMR 6115 CNRS, laboratoire de physique et chimie de l’environnement et de l’espace, université d’Orléans,3A, avenue de la Recherche-Scientifique, 45071 Orléans, France

b Le Studium, Orléans, France

Received 31 January 2009; accepted after revision 12 May 2009

Written on invitation of the Editorial Board

Abstract

The Sun provides the main energy input to the terrestrial atmosphere, and yet the impact of solar variability on long-termchanges remains a controversial issue. Direct radiative forcing is the most studied mechanism. Other much weaker mechanisms,however, can have a significant leverage, but the underlying physics is often poorly known. We review the main mechanisms bywhich solar variability may impact the terrestrial atmosphere on time scales ranging from days to millennia. This includes radiativeforcing, but also the effect of interplanetary perturbations and energetic particle fluxes, all of which are eventually driven by thesolar magnetic field. To cite this article: T. Dudok de Wit, J. Watermann, C. R. Geoscience xxx (2009).# 2009 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Résumé

Le forçage solaire sur l’atmosphère terrestre. Le Soleil est la principale source d’énergie de l’atmosphère terrestre, maisl’impact de sa variabilité reste un sujet à controverse. Le mécanisme le plus étudié est le forçage radiatif direct. Or d’autresmécanismes bien moins intenses peuvent avoir un effet de levier non négligeable. La plupart est mal comprise. Nous passons enrevue les divers mécanismes par lesquels le Soleil peut affecter l’atmosphère terrestre sur des échelles de temps allant du jour auxmillénaires. La liste inclut le forçage radiatif, mais aussi l’effet des perturbations interplanétaires et des particules de haute énergie.Tous ces mécanismes sont in fine entraînés par le magnétisme solaire. Pour citer cet article : T. Dudok de Wit, J. Watermann, C. R.Geoscience xxx (2009).# 2009 Académie des sciences. Publié par Elsevier Masson SAS. Tous droits réservés.

Keywords: Solar variability; Solar forcing; Solar irradiance; Atmosphere; Climate change

Mots clés : Variabilité solaire ; Forçage solaire ; Irradiance solaire ; Atmosphère ; Changements climatiques terrestres

1. Introduction

In two decades, the connection between solaractivity and the Earth’s atmosphere has moved from

a mere curiosity to a hotly debated topic. Many reviewshave been written, emphasising either the radiativeforcing from a solar viewpoint [21,25,46,48,49] or froma terrestrial viewpoint [29,30], solar variability ingeneral [1,8,10,22,53,64,67], historical aspects andlong-term effects [3,6,15,34,87,94], and other, indirectmechanisms [56,84]. Here, we review the solar inputs to

C. R. Geoscience xxx (2009) xxx–xxx

* Corresponding author.E-mail address: [email protected] (T. Dudok de Wit).

1631-0713/$ – see front matter # 2009 Académie des sciences. Published by Elsevier Masson SAS. All rights reserved.doi:10.1016/j.crte.2009.06.001

http://dx.doi.org/10.1016/j.crte.2009.06.001


mailto:[email protected]

the terrestrial atmosphere and focus on their origin, theunderlying physics and their observation.

The Sun–Earth connection is a world of paradoxes.Until recently, this seamless system was widelyconsidered as a stack of independent layers, and onlyin recent times did the interactions between these layersreally attract attention. The role of the Sun in our solarsystem goes undisputed, and yet the effect of solarvariability on the atmosphere remains quite controver-sial. As we shall see later, the main mechanisms bywhich the Sun affects the Earth are not the mostimmediate ones in terms of energetic criteria.

The Sun – like any living star – continuously radiatesenergy outward into the heliosphere. The radiatedenergy is carried by:

! electromagnetic waves over a frequency band rangingfrom radio waves to hard X-rays;

! a stream of hot plasma (the solar wind) consistingprimarily of electrons and protons with a smallfraction of heavier ions;

! an interplanetary magnetic field (IMF) which iscarried along with the solar wind (often referred to asa frozen-in magnetic field);

! violent solar outbreaks such as solar flares andcoronal mass ejections (CME) [37].

The solar radiative output is nearly constant in timeand accounts for about 1365 W/m2 at a solar distance of1 Astronomical Unit (AU), with a solar cycle dependentvariation of the order of 0.1%. Under quiet solarconditions, the flow rates of the kinetic energy of thesolar wind bulk motion and the solar wind thermalenergy amount to about 5 " 10#4 W/m2 each at 1 AU,i.e., a million times less than the radiative input. Theenergy flow rate of the IMF is another two orders ofmagnitude smaller, about 5 " 10#6 W/m2. Yet, thesedifferent mechanisms all have a distinct impact on theterrestrial atmosphere and none of them can be ruledout, a priori.

Nearly 70% of the solar radiation that arrives at thetop of the Earth’s atmosphere is absorbed in theatmosphere or at the Earth’s surface; the rest isimmediately reflected. In contrast, the efficiency ofenergy transfer from the solar wind into the magneto-sphere is only 1–10%, depending on the orientation ofthe IMF.

Wave and particle emissions are not the only meansby which the Sun can influence the Earth’s atmosphere.The solar wind plasma, more precisely, the IMFassociated with it, modifies the rate of penetration ofinterstellar energetic particles into the heliosphere and

eventually into the atmosphere. This has led to one ofthe more controversial aspects of Sun–climate studies.

In this review, we first start with an illustration of solarvariability on time scales from days to decades (Section2). Section 3 then addresses the solar radiative output andits effects, and Section 4 the role of orbital changes.Thereafter, we focus on indirect effects, the electriccircuit (Section5, includinggalactic cosmic rays [GCR]),atmospheric convection under quiet (Section 6) andactive (Section 7) solar conditions, and the role of thecoupling with upper atmospheric layers (Section 8).Conclusions follow in Section 9. External forcings thatare not related to the Sun (such as volcanic activity) andinternal forcings are not addressed.

2. Solar variability

Solar activity affects the Earth’s environment on timescales of minutes to millions of years. The shorter timescales are of particular interest in the frame of spaceweather1 [72], but will not as much be considered here.Long-term changes of solar and heliospheric conditionsand their manifestation in the Earth’s space andatmospheric environment are typically considered tobe in the realm of space climate [61]. It is often believedthat only slow variations (i.e. time scales of years andabove) can affect climate. This is not fully correct in thesense that the rate of occurrence of fast transients suchas solar flares is modulated in time, so that all timescales eventually matter.

To give a glimpse on the complexity of solarvariability, we illustrate in Fig. 1 the variation of somekey solar-terrestrial parameters; several of them will bediscussed in later sections. The long time interval (leftpanel) covers 3 decades only because very few accuratesolar observations were available before the advent ofthe space age. One of the main tasks in solar-terrestrialphysics today is to extrapolate these tracers backward intime.

The tracers (or proxies, as they are usually called) ofsolar activity that are shown in Fig. 1 are respectively:

! X-ray: the soft X-ray flux between 0.1 and 0.8 nm,which is indicative of the energy released during solareruptive phenomena such as flares. Most of thisradiation is absorbed in the upper atmosphere (above60 km) and above;

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1 Space weather mostly deals with short-term impacts and forecast-ing of solar activity, with a particular focus on its societal effects:impacts on space systems, navigation, communications, ground tech-nology, etc.


! Lya: the intensity of the bright H Lyman-a line at121.57 nm, which is mainly emitted in the solartransition region and is absorbed in the ionosphere(above 90 km);

! MgII: the core-to-wing ratio of the Mg II line at279.9 nm, which is a good proxy for the solarirradiance in the ultraviolet (UV). This radiation isprimarily absorbed in the stratosphere, where itaffects ozone concentration;

! TSI: the Total Solar Irradiance (TSI), which repre-sents the total radiated power measured at 1 AU,above the atmosphere. This quantity summarises thetotal radiative energy input to the Earth;

! 10.7 cm: the radio flux emitted at 10.7 cm, ordecimetric index. This radiation has no direct impacton climate, but it is widely used in Global CirculationModels (GCM) as a proxy for solar activity. It ismeasured daily since 1947;

! ISN: the International Sunspot Number (ISN), one ofthe most ancient gauges of solar activity, with almostdaily measurements since 1749;

! jBj: the intensity of the IMF at the L1 Lagrange point,just upstream of the Earth;

! np: the proton density, also measured in the solarwind. This quantity, combined with the solar windbulk speed, gives the solar wind dynamic pressure,

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Fig. 1. Relative variation (in percentage) of some of the key solar-terrestrial parameters. The left plots shows 3 decades of observations, withmonthly averaged data and the right plot a 1-year excerpt with hourly or daily observations. From top to bottom: soft X-ray flux (from GOES/SEM),irradiance in the EUV (Lyman-a line composite from LASP, Boulder), irradiance in the UV (MgII index from NOAA), Total Solar Irradiance (TSIcomposite from PMOD-WRC, Davos), radio flux at 10.7 cm (from Penticton Observatory), sunspot number (ISN, from SIDC, Brussels), intensity ofthe magnetic field in the solar wind (jBj, fromOMNIWeb), proton density in the solar wind (np, fromOMNIWeb), aa geomagnetic index (from ISGI,Paris) and neutron flux at mid-latitude (fn, from SPIDR). All quantities are normalised with respect to their time-average. Some of the vertical scalesdiffer between the two plots.Fig. 1. Variation relative (en pourcentage) de quelques paramètres-clé des relations Soleil–Terre. La figure de gauche montre trois décenniesd’observations, avec des moyennes hebdomadaires ; celle de droite couvre une année d’observations, avec des moyennes journalières ou horaires. Dehaut en bas : le flux de rayons X mous (de GOES/SEM), l’irradiance dans l’EUV (composite de la raie Lyman-a ; LASP, Boulder), l’irradiance dansl’UV (indice MgII ; NOAA), l’irradiance solaire totale (composite de la TSI ; PMOD-WRC, Davos), flux radio à 10,7 cm (Observatoire dePenticton), nombre de taches solaires (ISN ; SIDC, Bruxelles), intensité du champ magnétique dans le vent solaire (jBj ; OMNIWeb), densité deprotons dans le vent solaire (np ; OMNIWeb), indice géomagnétique aa (ISGI, Paris) et flux de neutrons à moyenne latitude (fn ; SPIDR). Toutes lesquantités ont été normalisées par rapport à leur moyenne temporelle. Certaines échelles verticales peuvent différer entre les deux graphes.


which is the main solar parameter to define the shapeof the magnetosphere;

! aa: the aa-index, which is a 3-hourly range measureof the level of geomagnetic field fluctuations at mid-latitudes. Its amplitude reflects the amount ofmagnetic energy that is released in the terrestrialenvironment;

! fn: the atmospheric neutron flux, measured on Earth,at mid-latitude. This flux is indicative of the highlyenergetic galactic cosmic ray flux, which is not ofsolar origin, but is modulated by solar activity. Part ofthis ionising radiation is absorbed in the middleatmosphere, where it might affect cloud condensa-tion.

The left panel reveals a conspicuous modulation ofabout 11 years, which is known as the solar cycle andwhose origin is rooted in the solar magnetic dynamo[11]. Solar magnetism is indeed the ultimate driverbehind all the quantities we shall encounter here [15].Its great complexity, and the wide range of spatial andtemporal scales covered by its dynamics allow for a richvariety of manifestations.

The solar cycle, which is best evidenced by thenumber of dark sunspots occurring on the solar surface,is probably the best documented manifestation of solaractivity on our terrestrial environment. Statisticallyrobust signatures of the solar cycle have been reportedin a large variety of atmospheric records, includingstratospheric temperatures [43], ozone concentration[28,73], changes in circulation in the middle [42] andlower [97] atmosphere, tropospheric temperatures [13],ocean surface temperature [65,96], and many more[30,33,34,94].

The important point in Fig. 1 is the occurrence of thesame 11-year cycle in all solar-terrestrial parameters.As a consequence, disentangling their individualimpacts on the atmosphere is almost impossible withoutthe contribution of physical models. All quantities arecorrelated, but not all are necessarily causally related toatmospheric changes.

A look at shorter time scales (right panel in Fig. 1)reveals a different and, in some sense, much morecomplex picture. Some quantities exhibit an occasional27-day modulation associated with solar rotation, butcorrelations are not systematic anymore. For the samereason, the properties of the 11-year cycle may not bereadily extrapolated to longer time scales either.

Another distinctive feature of Fig. 1 is the highlyintermittent nature of some quantities, such as the softX-ray flux and geomagnetic indices. The presence ofrare but extreme events suggests that the rate of

occurrence of such events may affect climate, eventhough the lifetime of each individual event is orders ofmagnitude below the characteristic response time of theatmosphere.

3. The solar radiative output

The largest solar energy input to the terrestrialenvironment comes through electromagnetic waves.The Sun radiates over the entire spectrum, with a peakin the visible part (400–750 nm). The actual shape of thespectrum is dictated by the composition of the solaratmosphere and its temperature, which increases fromnear 6000 K in the photosphere to millions of degrees inthe corona.

The bulk of the solar spectrum is relatively welldescribed by the emission of a black body at 5770 K. Ontop of this smooth spectrum come numerous discretefeatures associated with absorption and emissionprocesses [46]. The UV part of the solar spectrum(12–400 nm) is partly depleted by such absorptionprocesses, whereas the Extreme UV (EUV, 10–120 nm)is strongly enhanced by contributions from the hotterpart of the solar atmosphere. The visible and near-infrared contributions both represent about 45% of thetotal radiated power, whereas the UV represents about8% and the EUV less than 10#3%. Although thedifferent layers of the solar atmosphere are stronglycoupled by the solar magnetic field, the variability of thesolar spectrum is remarkably complex and cannotproperly be described by a single parameter.

3.1. The total solar irradiance

When studying the Earth’s global energy budget[38,39], the solar radiative forcing is often representedby a single convenient parameter, called TSI. The TSI isthe power integrated over the entire solar spectrum. Fora long time, it was believed to be constant, hence itsancient name solar constant.

The TSI can only be measured from space since theterrestrial atmosphere absorbs part of the radiation. Thefirst measurements started in 1978 and revealed a smallbut significant variation. Several missions have mea-sured the TSI since, giving an average value of 1365 W/m2 [24]. The relative modulation amplitude2 over asolar cycle is 0.1% but short-term variations of up to0.25% may occur during periods of intense solaractivity [99].


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2 Defined here as (maximum # minimum)/time average.


Different TSI observations agree on the short-termrelative variability, but significant differences existbetween their long-term trends. There exist today threecomposites of the TSI, based on how the data fromdifferent instruments are stitched together (Fig. 2). Thedisagreement between these three versions regardingthe existence of a secular trend has fuelled a fiercedebate. Indeed, the composite of the PMOD group [24]suggests the existence of a recent downward trend in theTSI, whereas the ACRIM group [98] claims theopposite.

Two key issues with the TSI are the origin of itsvariability and the reconstruction of past values. TheSun is photometrically quiet and the short-termvariability mainly results from a competition betweenan irradiance deficit due to sunspots and an enhance-ment due to bright photospheric features called faculae[25]. The two effects are connected, but the variabilityaffects different spectral bands. The secular trend in theTSI is more directly related to weak changes inbrightness during spotless periods (called quiet Sun),which means that trends are best observed bycomparing minima in the solar cycle. The origin ofthese slow brightness changes is still unclear, althoughit is certainly related to the solar magnetic field [21].

A reconstruction of pre-1978 values of the TSI is ofcourse a major issue for climate studies. There is strongobservational evidence for solar surface magnetism tobe the major driver of TSI changes on time scales ofdays to years [41]. Based on this, Fligge et al. [19]

developed a semi-empirical model for reconstructingTSI changes from the surface distribution of the solarmagnetic field, using solar magnetograms inferred fromsolar images of the Ca K line emission. Unfortunately,few images exist before 1915, which limits theapplicability of the method.

The only direct solar proxy that is sufficientlyhomogeneous for reconstructing the TSI back to theMaunder minimum is the sunspot number. The Maunderminimum (1645–1715) is of particular interest since theSunwas very inactive at that time and temperatures in theNorthern hemisphere were unusually low [18,75]. Byusing reconstructions of the sunspot number going backto 1610 as inputs to open magnetic flux transportsimulations, several authors [40,47,95] have demon-strated that the TSI was lower during the Maunderminimum than today. The uncertainty on the actualchange in TSI, however, is high. Present estimates give achange in radiative forcing (the net downward radiativeflux) from+0.06 to +0.3 W m#2 [20], which is equivalentto aDT = +0.04 to+0.18 K increase in global temperaturesince the Maunder minimum. The IntergovernmentalPanel onClimate Change (IPCC) concludes that this barechange is insufficient to explain the observed globaltemperature increase [20]. The same conclusions hold forreconstructions made since 1978.

For TSI reconstructions on time scales of centuries tomillennia, a different approach must be used. The mostreliable proxies are cosmogenic isotopes such as 14Cand 10B, whose production rate is modulated by solaractivity [3]. Bard et al. [4] have shown that relativevariations in the abundance of such cosmogenicisotopes are in excellent agreement with sunspot-basedTSI reconstructions. There have been attempts toreconstruct solar activity up to hundreds of thousandyears into the past [87]. For such long periods, however,the slowly but erratically varying geomagnetic fieldbecomes a major source of uncertainty. Discrepanciesbetween paleomagnetic reconstructions based ondifferent deep-sea cores are still too important toproperly quantify the solar contribution 20 kiloyearsand more backward [3].

The relatively small impact of solar radiative forcingon climate has been questioned by several. Scafetta andWest, for example, used a phenomenological model toconclude that at least 50% of the global warmingobserved since 1900 had a solar origin [70,71]. Threerecurrent arguments are:

! recent solar activity is better reflected by the TSIcomposite from the ACRIM group than from thePMOD group;


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Fig. 2. Comparison of three composites of the total solar irradiance,averaged over 81 days. The composites are: PMOD version d41-61-0807 [24], ACRIM version 11/08 [98], and SARR version 3/08 [58].For better visibility, all curves have been shifted vertically to share thesame average value for 1986–1987.Fig. 2. Comparaison entre trois composites de l’irradiance solairetotale, moyennées sur 81 jours. Ces composites sont : PMOD, versiond41-61-0807 [24], ACRIM, version 11/08 [98], et SARR, version 3/08[58]. Par souci de lisibilité, les courbes ont été décalées verticalementet ont la même valeur moyenne entre 1986 et 1987.


! short-term statistical fluctuations and longer-termcycles have distinct effects [70], which may explainwhy such clear signatures of solar cycles (11-year, butalso the weaker 90-year Gleissberg cycle) have beenfound in atmospheric records;

! feedback mechanisms are not sufficiently wellunderstood and positive feedback may be muchstronger than expected [64,77].

Lockwood and Fröhlich [54] argue that the PMODcomposite is the most reliable, and so solar activity hasnot increased at the end of the 20th century. Objectionsagainst the second and third arguments have been madeby climate modellers who do not see evidence for sucheffects in GCM, for example the comment by Lean [50].

Most of the TSI consists of visible and near-infraredradiation, which are primarily absorbed by oceans andland surfaces, and in the lower troposphere by watervapour and by CO2. For that reason, a direct connectionbetween TSI change and tropospheric temperaturechange can be established. This direct forcing isinsufficient to explain the observed temperatureincrease. However, several effects such as the hydro-logical cycle [74] and stratospheric water vapourfeedback [78] could have an impact on the forcing-response relationship. The debate continues unabated.

3.2. The solar spectral irradiance

A significant portion of the solar radiative outputdoes not account for a direct radiative forcing because itis absorbed in the middle and upper atmosphere where itaffects photochemistry. Spectrally resolved observa-tions are required to study these effects.

The principal features of the solar spectrum and itsvariability are illustrated in Fig. 3. The main result isthe large relative variability in the UV band and below,which exceeds that of the TSI by orders of magnitude.In absolute terms, this spectral variability peaks in theUV between 200 and 400 nm. Below 310 nm, thisradiation is strongly absorbed in the mesosphere (from50 km to about 80–90 km), and in the stratosphere bythe ozone Hartley band [26]. During periods of intensesolar activity, the ozone concentration thus increases,heating the stratosphere and higher layers, whichaffects the downward radiative flux. This also impactsthe meridional temperature gradient, altering planetaryand gravity waves, and finally affecting globalcirculation [28]. Haigh first introduced this generalpicture, which is now widely accepted [30,44,75]. Themain effects are a warming of the upper and lowerstratosphere at low and middle latitudes, and a

strengthening of the winter stratospheric polar nightjet. Direct heating by absorption of the UV can explainmost of the temperature response in the upperstratosphere but not in the troposphere and lower


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Fig. 3. From top to bottom: the solar irradiance spectrum with(dashed) a black-body model at 5770 K, the altitude at which theUV and EUV components are predominantly absorbed (unit opticaldepth), the relative and absolute variability of the irradiance from solarmaximum to solar minimum. All these results refer to the (Oct. 2003–Jan. 2009) time span. This plot is based on observations from SORCE/XPS, TIMED/EGS, SORCE/SOLSTICE and SORCE/SIM.Fig. 3. De haut en bas : le spectre solaire avec en tireté l’irradianced’un corps noir émettant à 5770 K, l’altitude à laquelle le rayonnementUVet EUVest préférentiellement absorbé (profondeur optique unité),et les variabilités relative et absolue du spectre solaire, entre lemaximum et le minimum solaire. Ces données concernent la périoded’octobre 2003 à janvier 2009 et proviennent des instruments SORCE/XPS, TIMED/EGS, SORCE/SOLSTICE and SORCE/SIM.


stratosphere. The final temperature response dependscritically on the ozone concentration profiles and ondetails of the couplingmechanisms. Thesemechanismsare non-linear, and so a meaningful radiation budgetcannot be established without resorting to GCM. Thesemodels show important discrepancies and yet, recentcomparisons seem to converge toward a mean modelresponse of up to about 2.5% in ozone and 0.8 K intemperature during a typical solar cycle [2].

Less than 0.01% of the total irradiance comes fromwavelengths below 200 nm. This small contribution ismostly absorbed in the lowermost ionosphere, wherephotodissociation affects the local composition andgenerates heat. Because this part of the solar spectrumis highly variable, it has a noticeable effect. On timescales of hours to days, solar flares, for example, canincrease the electron density by orders of magnitude[51]. Long-term signatures of solar activity are alsoevident in many ionospheric parameters; the mostconspicuous one is the 11-year solar cycle [36,45]. Thesolar-cycle dependence of the height of constantplasma density in the lower ionosphere is attributedto the competing effects of a higher ionisation rate(resulting in higher plasma density at a given fixedheight) and increased atmospheric heating and upwel-ling (resulting in lower plasma density at the sameheight) at solar maximum as compared to solarminimum. A slow global cooling has also beenobserved [7], similar to that found in the meso- andstratosphere. This global cooling is most likely relatedto a contraction of the atmosphere due to an increasingconcentration in greenhouse gases.

We conclude at this stage that the photochemical anddynamical impacts of the solar UV component have asignificant leverage on the stratosphere and on climate.According to the IPCC [20], this mechanism cannotexplain the temperature increase observed during the20th century; it would require an amplification that isnot reproduced by present GCM. Three important issuesare:

! to better understand the physical coupling mechan-isms within the middle atmosphere and with the loweratmosphere;

! to include in GCM originally designed for the loweratmosphere a proper description of the upper atmo-sphere and, reciprocally, to include in modelsoriginally designed for the upper atmosphere a betterdescription of the lower atmosphere;

! to improve the solar inputs to these models in order toobtain a better response of ozone concentration versustime and position.

Concerning the last issue, we note that solar spectralirradiance observations are highly fragmented andinaccurate. Indeed, such measurements must be carriedout from space, where detectors suffer from degrada-tion. An ‘‘overlap strategy’’ is frequently used, wheresuccessive satellite experiments are directly comparedto improve their long-term accuracy. For the TSI,uncertainties of one part in 105 per annum can beobtained, whereas for the EUV–UV range, errors ofmore than 50% unfortunately are not exceptional.

The first continuous observations of the EUV–UVspectrum started in 2002 with the TIMEDmission [100],later complemented by SORCE. Because of this severelack of radiometrically accurate observations, most usersofUVdata, including climatemodellers, have resigned tousing proxies. The radio flux at 10.7 cm (or f10.7 index,Fig. 1) is often used in atmospheric studies, for it can beconveniently measured from ground. The MgII index[82] has been advocated as a better proxy for the UV, butnone of these quantities can properly reproduce thespectral variability [17].

4. Orbital changes and solar diameter variations

Orbital changes and variations in the solar diameterhave very little in common. Both, however, lead to a slowmodulationof the solar irradiance that can be described ingeometrical terms. In this sense, they fall under thepreceding section. Orbital changes are well understood[14] and are discussed in an article by D. Paillard [62].

The evidence for a variability of the apparent solardiameter has on the contrary remained elusive. Groundand space observations yield relative amplitudes of lessthan 0.06% over one cycle but do not agree [83]. Theeffect on climate is likely to be small, but cannot beruled out. The upcoming Picard mission, which will belaunched in 2010, precisely aims at measuring the solardiameter during the rising phase of the solar cycle withunprecedented accuracy.

5. Solar impact on atmospheric electricity

Atmospheric electricity is an old field of research butits role in the Sun–Earth coupling has recently attractedconsiderable interest and controversy. The effect of ionson the atmosphere is discussed inmore detail by E. Blanc[9]; here, we concentrate on the role of the Sun only.

5.1. Effect of the atmospheric current

A minute current of $ 2 pA/m2 permanently flowsdown from the ionosphere through the troposphere to


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the terrestrial surface, generating charges that arecapable of affecting the nucleation of water droplets toform clouds. This current responds to internal but alsoto solar forcings, providing a mechanism by which solaractivity affects various atmospheric parameters such ascloud cover, temperature and precipitation [68,85].Tinsley [84] has shown that there are at least fourindirect solar inputs which modulate the process:

! variations in the galactic cosmic ray flux, mediated bysolar activity (Section 5.2);

! solar energetic particle fluxes that are occasionallygenerated by intense solar flares or CME associatedshocks;

! relativistic electrons coming from the Earth’s radia-tion belts;

! polar cap ionospheric electric potential changes(Section 6).

The latter two are mainly induced by geomagneticactivity driven by interplanetary perturbations.

Most of the mechanisms listed above occurerratically and on time scales of days and so theirlong-term impact is difficult to assess. Recent advanceshave been made in the study of transient luminousevents [9], which provide an unexpected energy linkbetween the lower ionosphere and the upper tropo-sphere.

5.2. Effect of galactic cosmic rays

During the active part of the 11-year solar cycle thesolar magnetic field and its heliospheric extension, theIMF, are generally stronger and more turbulent thanaround solar minimum. A stronger IMF will moresuccessfully guide and deflect interstellar protons than aweaker IMF, with the result that the solar cycle imposesan 11-year modulation on the flux of GCR reaching theEarth’s atmosphere. The contribution of cosmic rays toion production in the atmosphere on short and long timescales is well established [5]. At present, at least threemodels in use describe this process: one developed inOulu [89], another in Bern [16] and a third one in Sofia[93,92]. A comparison of model simulations withballoon-borne ion density measurements has shown thatmodels and measurements are in good agreement [88].

Svensmark and coworkers [80,81] promoted amechanism in which an increased intensity of theGCR flux is, at least in part, responsible for an enhanceddensity of free ions and electrons in the troposphere.The free electrons, liberated by cosmic rays, assist inproducing ionised aerosols which in turn should act as

water vapour condensation nuclei in the troposphere.Tinsley and coworkers [85] suggested that a GCR fluxmodulation changes the aerosol ionisation which in turnchanges the ice nucleation efficiency of the aerosol. Inboth cases, the net effect is an enhancement of theglobal low-altitude cloud coverage, a modification ofthe Earth albedo and eventually a modulation of theglobal tropospheric temperature in correlation with the11-year solar activity cycle. In short, it is suggested [80]that the cloud coverage is modulated by the solar cycle,at least at heights below some 3 km.

This view is cautioned byothers. SunandBradley [79]cast doubt on the usefulness of the selection of data usedby Svensmark and Friis-Christensen [81] and demon-strate that results become different if different analysisintervals are considered. They conclude that no solidobservational evidence exists for the suggested GCR–cloud coverage relation. Harrison and Carslaw [32] andUsoskin [87] conclude that neither the GCR–cloudcoverage link proposed by Tinsley and coworkers nor theone proposed by Svensmark and coworkers can beexcluded but find that someelements in the chains of bothmechanisms remain contentious, and they doubt whetherthe processes are efficient enough to contribute sign-ificantly to a modulation of low cloud formation. Sloanand Wolfendale [76] estimate that on a solar cycle scale,less than 23% of the 11-year cycle change in the globallyaveraged cloud cover is due to the change in the rate ofionisation from the solar modulation of cosmic rays.

The controversy is still going on, and the lack ofaccurate long-term observations of cosmic ray intensityand especially global cloud coverage presently does notallow to accept or discard a potential influence of theGCR-cloud connection on long-term changes of thetropospheric mean temperature. The CLOUD experi-ment that is planned at CERN should help betterquantify the cloud formation rate [80]. Experimentalevidence gathered so far appears to suggest that on shorttime scales (a few days) and on interannual time scales alink between cosmic ray flux and low cloud coverageexists. The correlation between low cloud area coverageand cosmic ray induced ionisation has been found to bedependent on latitude and geographic region. It issignificantly positive at mid-latitudes but poor (andpossibly negative) in the tropics [63,91]. Depending onthe time interval considered, better correlations existover the Atlantic (1983–2000) or over the Pacific(1983–1993) [63]. Europe and the North and SouthAtlantic exhibit the best correlation over the period1984–2004 [90]. The pronounced regional variation ofthe correlation eventually results in a poor globalcorrelation [91].


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Let us stress again that all solar variability iseventually driven by the solar magnetic field, and so it isdifficult to quantify the real contribution of eachmechanism. As an illustration, Lockwood et al. [55]found the open solar magnetic flux to increase duringthe 20th century. This results in an increased shieldingagainst GCR and possibly a reduced cloud coverage.The same open magnetic flux, however, is also stronglycorrelated with the TSI [52] and with the level ofgeomagnetic activity, both of which lead to atemperature change.

6. Atmospheric convection under quiet solarconditions

Under quiet solar conditions, the transfer of energyfrom the Sun into the Earth’s atmosphere leads to thedevelopment of an electric current system (the solarquiet or Sq system) which consists of two components,one driven by solar electromagnetic radiation (Sq0) andthe other by the interaction between the solar wind andthe geomagnetic field. Note that the influence of thegeomagnetic field on the motion of charged particles israther strong such that the electrons (which above some70–80 km altitude are little affected by collisions andare the most important carriers of ionospheric electriccurrents) move preferentially perpendicular to both theelectric and magnetic fields (known as Hall effect).

Solar UV/EUV heating increases the scale height ofthe neutral constituents and causes their daytimeupwelling, which is accompanied by a systematicneutral gas redistribution via tidal winds. The ionisedpart of the upper atmosphere between about 90 and140 km altitude, dynamically strongly coupled to theneutral gas via collisions between ions and neutralatoms and molecules, expands and contracts with theneutral gas. As this motion takes place in the presence ofthe geomagnetic field, the charged particles experiencea dynamo force and move along closed streamlines.They form the Sq0 current system, which is significantbetween northern and southern auroral latitudes butpractically negligible at polar cap latitudes. Thecorotation electric field (due to the frictional couplingof the neutral atmosphere to the Earth rotation)exercises a strong influence at low, middle andsubauroral latitudes and imposes a systematic eastwardshift on the Sq0 pattern.

Seen from an observer at a fixed point in a Sun–Earthcoordinate system, i.e., not rotating with the Earth (forinstance, at rest in a geocentric solar magnetospheric[GSM] system), the solar wind together with the IMFcreate a~ySW "~BIMF electric field, usually termed ‘‘solar

wind merging electric field’’ along the high-latitudemagnetospheric boundary (with~ySW and~BIMF denotingthe solar wind bulk speed and IMF vectors, respec-tively). The electric field maps down to the Earth’satmosphere along geomagnetic field lines (which can beconsidered equipotential lines in the magnetosphere)and is observed as an electric field from dawn to duskacross the polar cap. This electric field, combined withthe geomagnetic field (downward in the northern andupward in the southern polar cap) supports a Hallcurrent from the nightside to the dayside across thepolar cap, closed by return currents (known as auroralelectrojets) at slightly lower but still auroral latitudes.Such return currents must flow in the ionospherebecause the ionospheric Hall currents are divergencefree. This is the second contribution to the Sq currents.The coupling of the atmosphere to the rotating Earthand the magnitude of the east–west component of theIMF modify the preferential orientation of the convec-tion pattern in the sense that it may become more or lessshifted, mostly in westward but sometimes in eastwarddirection.

Although the rate of solar radiation on the topsideatmosphere depends solely on geographic latitude andlongitude, the Sq current system also depends ongeomagnetic latitude and longitude, as a result of theionospheric plasma density distribution. The latter is notonly governed by charge production via UV and EUVradiation but also by the electric conductivity tensor,which depends on the orientation of the geomagneticfield vector. For instance, close to the geomagneticequator, the magnetic field is nearly horizontal. Theonly way to move electric charges across thegeomagnetic field is along the equator as any verticalelectric current would immediately be quenched byspace charges accumulating at the lower and upperboundaries of the ionosphere. This effect facilitatesconsiderably the establishment of a narrow electriccurrent strip in the dayside upper atmosphere along thegeomagnetic equator (known as equatorial electrojet).

The Sq current system is strongly dependent onseason, with a remarkable increase in the summer and adecrease in the winter hemisphere. The Sq systemfurther depends on the solar cycle; the somewhat higheraverage solar wind speed and the enhanced atmosphericionisation due to more intense UV/EUV radiation andenergetic particle precipitation increase the electricalconductivity and contribute to more intense ionosphericelectric currents during the maximum and earlydeclining phases of the solar cycle.

Fig. 4 [57] shows the Sq current system generated bysolar electromagnetic radiation alone (Sq0, right hand


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side) and the combined electromagnetic and solar windgenerated Sq system (left hand side).

7. The impact of solar activity on the Earth’satmosphere

The steady-state conditions representing the quietSun are not typical for the maximum and early decliningphases of the solar cycle. The impact of short-term(transient) events on the Earth’s atmosphere can beprofound [37]. Several types of eruptions are known tooccur, with solar flares and CME being the most violentones (as far as the effects on the Earth’s environment areconcerned). Just as under quiet solar conditions, bothelectromagnetic radiation and solar energetic particlefluxes play important roles for the state of the upperatmosphere under the various types of active solarconditions.

Solar flares, a bursty type of energy release, radiatebroadband electromagnetic waves whose intensities aremuch higher than steady-state solar radiation. Therather strong X-ray component associated with flares

penetrates deep into the atmosphere and enhances theionisation level between 60 and 90 km altitude. This hasdeleterious effects on HF radiowave propagation.

Some solar flares and CME are accompanied bystreams of very energetic protons (up to hundreds ofMeV) ejected from the Sun and accelerated in the solarcorona and beyond. Unlike the typical solar windprotons (%1 keV), these high-energy protons canpenetrate into the outer magnetosphere nearly unhin-dered by the geomagnetic field (which normally shieldsthe Earth environment from the direct entry of solarwind particles) and propagate along the field linestoward the Earth. Protons with energies up to 10 MeVionise the polar atmosphere at altitudes significantlybelow 100 km, which facilitates considerably theabsorption of HF radiowaves propagating at polarlatitudes (referred to as polar cap absorption [PCA]).The flare-associated proton flux may last for severaldays which is the time it takes to bring the plasmadensity back to a normal level.

A different category of solar activity, with lessprofound effects on the average, follows a recurrent


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Fig. 4. Ionospheric Sq current system as inferred from ground-based magnetometer observations at 40 sites over the period May–June 1965. Righthand side: tidal currents only, left-hand side: combined tidal and polar cap currents. Current intensity between adjacent lines is 10 kAcounterclockwise (solid) and clockwise (dotted) [57].Fig. 4. Le système de courant ionosphérique Sq, déterminé à partir des mesures faites au sol avec un réseau de 40 magnétomètres, pour la périodeallant de mai à juin 1965. À droite, figurent uniquement les courants de marée, et à gauche la combinaison des courants de marée et de ceux de lacalotte polaire. L’écart entre les courbes de niveau est de 10 kA. Le courant s’écoule dans le sens horaire (tiretés) et anti-horaire (trait continu) [57].


pattern. At the boundary between low-speed(% 400 km/s) and high-speed (% 700 km/s) solar windflow regimes, one often observes a shock front that isproduced by the high-speed plasma pushing the low-speed plasma. The flow regime boundary is fixed to thesolar surface, rotates with the Sun and is likely to persistfor longer than one solar rotation such that theassociated solar wind structures show a tendency to

hit the Earth’s space environment again after one solarrotation (approximately 27 days).

Fig. 5 (from NOAA-NGDC) shows, among otherparameters, solar X-ray and energetic particle fluxesobserved at geostationary orbit during the geomagneticstorm on 14 July 2000 which became famous as the‘‘Bastille day storm’’. On 14 July, the X-ray fluxes inboth channels reach X-class intensity which is


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Fig. 5. Observations by the SEM instrument onboard GOES-10 during the Bastille day storm. Top panel: soft X-ray intensity in the 0.05–0.3 nm(XL) and 0.1–0.8 nm (XS) bands. Second panel: energetic particle flux intensity. Protons > 1 MeV (I1), > 5 MeV (I2),> 10 MeV (I3),> 30 MeV(I4), > 50 MeV (I5), > 60 MeV (I6), > 100 MeV (I7); electrons > 2 MeV (E1 – mostly no data); a particles 150–250 MeV (A5), 300–500 MeV(A6). Third panel: magnetic field perpendicular to GOES orbital plane (i.e., practically in geographic northward direction). Fourth panel: neutronflux from the McMurdo neutron monitor in the Antarctic. Note the ground level enhancement at 11 UTon 14 July and the Forbush decrease (deepestin early morning of 16 July). Open and closed triangles mark the position of GOES-10 at local noon and midnight, respectively (figure generated byNOAA’s National Geophysical Data Center).Fig. 5. L’orage géomagnétique du jour de la Bastille (14 juillet 2000), observé par l’instrument SEM sur GOES-10. Le graphe du haut montre le fluxde rayons X mous dans les bandes de 0,05–0,3 nm (XL) et de 0,1–0,8 nm (XS). Le second graphe montre le flux de particules de haute énergie, avecdes protons > 1 MeV (I1), > 5 MeV (I2), > 10 MeV (I3), > 30 MeV (I4), > 50 MeV (I5), > 60 MeV (I6), > 100 MeV (I7) ; des élec-trons > 2 MeV (E1 – avec des données manquantes) ; des particules a entre 150–250 MeV (A5), 300–500 MeV (A6). Dans le troisièmegraphe : le champ magnétique perpendiculaire au plan orbital de GOES-10 (correspondant quasiment au Nord géographique). Dans le derniergraphe : flux de neutrons dumoniteur à neutrons deMcMurdo, en Antarctique. Notez l’augmentation du flux (ground level enhancement) le 14 juilletà 11 heures TU, suivie d’une décroissance de Forbush qui atteint son minimum tôt le matin du 16 juillet. Les triangles vides et pleins indiquentrespectivement le midi local et le minuit local à l’emplacement de GOES-10 (figure générée par le National Geophysical Data Center de la NOAA).


considered severe by space scientists. While the X-rayflux returns to near pre-flare intensities after severalhours, the particle flux remains highly elevated for morethan a day and moderately elevated for several days.

Solar energetic particles can penetrate the Earth’satmosphere down to stratospheric and even troposphericheights. For instance, chemically-induced changes inthe abundance of nitric oxide constituents in thestratosphere resulting from such fluxes were observedwith the UARS satellite [35]. In another case, extremelyenergetic solar cosmic rays associated with the intensesolar X-ray flare and CME of 20 January 2005 led to asubstantial ground level enhancement and an increase ofthe aerosol density over Antarctica as inferred from theTOMS Aerosol Index [60].

Auroral activity, triggered by the impact of solaractivity on the Earth’s magnetosphere, is one of thevarious sources of atmospheric gravity waves. Gravitywaves play a significant role in the momentum andenergy budget of the mesosphere and lower thermo-sphere [23].

Both electromagnetic radiation and charged particleprecipitation into the atmosphere can lead to amodification of the neutral air density in the upperatmosphere. Excessive UV and EUV radiation asso-ciated with solar activity, and to a smaller extent keVparticle precipitation and Joule heating (caused by themotion of the ionospheric plasma forced by strongelectric fields) can heat the atmosphere at the altitudesof Low Earth Orbiting (LEO) satellites – between about300 km and more than 1000 km above the ground –thereby increasing the neutral air density at a givenheight and eventually leading to increased satellite drag.At the lowest satellite altitudes (300–400 km), the airdensity can reach several times the value typical forquiet conditions.

A connection between solar activity and the atmo-sphere that is specific to the Antarctic continent wasproposed by Troshichev [86]. The solar wind mergingelectric field maps, via field-aligned currents, down tothe atmosphere to establish a transpolar cap electricpotential whose changes can, via electric connection tothe troposphere, influence the large-scale verticalcirculation system that forms above the Antarcticcontinent in the winter season. In this circulationsystem, air masses descend above the central Antarcticridge and ascend near the coast. If the vertical windsbecome very strong (for instance, as a result of field linemerging at the magnetopause), they disturb the thermalequilibrium which results in an increased cloudcoverage over Antarctica and they disturb the large-scale horizontal wind system, thereby quenching the

circumpolar wind vortex. Indirect evidence for thiseffect was inferred from regular meteorologicalobservations made at Antarctic stations.

8. Coupling of atmospheric layers

The coupling between the ionised and neutral gascomponents of the upper atmosphere up to about140 km is a two-way process. If the electric field and theneutral wind measured in an Earth-fixed referenceframe are denoted by ~E and~u, respectively, the electriccurrent density in the presence of the geomagneticfield, ~B0, is expressed as ~J ¼

Pð~E þ~u"~B0Þ with

P

denoting the electric conductivity tensor. An electricfield (of external origin, for instance) influences the ionvelocity and, via collisional coupling, the neutral gaswhile the neutral wind (due to pressure, gravity and theCoriolis force, for instance) is equivalent to a ~u"~B0

electric field (in an Earth-fixed frame) and influences inreturn the ion and electron velocities. In other words,solar energy may be transferred from the electricallycharged to the neutral component of the upperatmosphere via frictional heating while kinetic energymay be transferred from the neutral to the chargedcomponent via a neutral wind associated electric field.

In addition to dynamic coupling between the neutraland electrically charged components of the ionosphere,it has become evident that different atmospheric heightregions are also coupled. Planetary waves are primecandidates for linking different altitudes [69]. They arelarge-scale oscillations of the lower, middle and upperatmosphere with periods preferentially (but not exclu-sively) near 5, 10 and 16 days. In some cases, planetarywaves are generated in the lower atmosphere (tropo-sphere and stratosphere) and propagate upward into themiddle and upper atmosphere. In other cases, theyappear to have been generated in the middle atmosphereand propagate latitudinally.

Goncharenko and Zhang [27] conclude that seasonaltrend, solar flux and geomagnetic activity cannotaccount for temperature variations in the thermospherewhich they had observed during an incoherent scatterradar campaign in January–February 2008. Theysuggest that the variations are associated with strato-spheric warming and hence demonstrate a link betweenthe lower and the upper atmosphere. Yigit et al. [101]demonstrate the penetration of gravity waves andsubsequent momentum deposition from the lowertroposphere and stratosphere to the middle thermo-sphere.

Supported by the observational evidence acquiredover the years, it became clear that kinetic and


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electromagnetic coupling between atmospheric layersexists and the need for developing coupled atmosphere–thermosphere–ionosphere–plasmasphere modelsemerged. As a consequence, GCM of the terrestrialupper atmosphere have evolved. About a decade ago, thetime-dependent three-dimensional Coupled Thermo-sphere Ionosphere Plasmasphere (CTIP) model wasdeveloped [59]. TheCTIPmodel consists of three distinctcomponents, a global thermosphere model, a high-latitude ionosphere model and a mid- and low-latitudeionosphere/plasmasphere model.

The CoupledMiddle Atmosphere and Thermospheremodel (CMAT) is one of the advanced modelsultimately derived from the CTIP model. Its range ofvalidity was originally extended down to 30 km altitude[31] and a further improved version (CMAT2 [12])extends from exospheric heights (from 104 km altitudefor the ionospheric flux tubes) down to 15 km altitude.The extensions to CTIP mean that lower atmospheredynamic effects such as gravity waves can be included,and conversely the effects of ionospheric inputs such asauroral precipitation on middle and lower atmospherecan be examined.

AThermosphere General CirculationModel (TGCM)family, developed at theNationalCenter forAtmosphericResearch by Richmond et al. [66], comprises three-dimensional, time-dependent modules representing theEarth’s neutral upper atmosphere. Recent models in theseries include a self-consistent aeronomic scheme for thecoupled Thermosphere/Ionosphere system, the Thermo-sphere Ionosphere Electrodynamic General CirculationModel (TIEGCM) and the TIME-GCM, which extendsthe lower boundary to 30 km and includes the effects ofthe prevailing physical and chemical processes.

Optical phenomena such as lightning-inducedsprites, jets and elves and the electromagnetic fieldsassociated with them have become a topic of intensestudy over the last decade. They are of too small a scaleto be handled properly by global circulation andcoupling models. This kind of electromagnetic activityis discussed in a companion article by E. Blanc.

9. Conclusions

Solar radiation is by far the most intense source ofenergy supplied to the terrestrial atmosphere, and thereis a wealth of evidence in favour of the response ofatmospheric parameters to solar variations. Most of theattention has focused so far on the sole variability of theTSI, which gives a simplistic view of the complexity ofthe solar driver. Indeed, solar variability manifests itselfin a variety of different (but coupled) mechanisms; most

of the underlying feedback mechanisms remain poorlyknown, which hampers the quantification of individualprocesses. For that reason, there has been and is stillmuch debate about the real impact of solar variability onclimate. According to the IPCC [20], over the lastcentury, this impact has most likely been small ascompared to anthropogenic effects.

There are several important working fronts as far asthe Sun–Earth connection is concerned. Most GCMwhose development started in the lower atmosphere stilllargely ignore the upper part of the atmosphere onwhich solar variability has the largest impact. Oneobvious issue is therefore the upward extension of thesemodels, and a better description of the mechanisms bywhich the upper layers may couple to the stratosphereand eventually to the troposphere. This also involves abetter understanding on how solar variability affectsregional climate data. On the other hand, GCM models,like the CITP which started from the thermosphere, facethe challenge of an appropriate downward extension tothe stratosphere (and eventually the troposphere).

A second issue is the definition of reference spectralirradiance in the EUVand UV bands for different levelsof solar activity. These bands have an importantleverage of the middle atmosphere and the reconstruc-tion of past levels is still lacking today. In all thesereconstruction attempts, however, one should be carefulagainst inbreeding of models.

A third issue is the understanding of the micro-physics associated with atmospheric electricity and inparticular the quantitative role of ions and electrons forstimulating the production of water vapour condensa-tion nuclei. All three issues involve a much closerinteraction between the space and atmospheric com-munities, which is definitely the highest priority of all.

Acknowledgements

During the preparation of this paper, JW benefitedfrom a grant provided by Le Studium, Agence régionalede recherche et d’accueil international de chercheursassociés en région centre, France. We also gratefullyacknowledge the numerous institutes and teams thatprovided the experimental data we used in the plots.

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