Response of the Earth’s environment to solar radiative forcing Ingrid Cnossen British Antarctic Survey Image credit: NASA

Outline This lecture Intro to atmosphere structure and processes Absorption of solar radiation Atmosphere composition Energy balance Ionization Conductivity Low latitude currents Next lecture Effects of variations in solar radiative forcing (examples) High-latitude currents Coupling to magnetosphere

Vertical structure of the atmosphere D layer F2 layer F1 layer E layer Thermosphere Mesosphere Stratosphere Troposphere 2 3 4 5 6 7 400 350 300 250 200 150 100 50 600 800 1000 Electron density (log10 cm-3) Temperature (K) Height (km) Low High Air density     scale height 3

The yellow indicates absorption, scattering/reflection by the atmosphere. Main absorber at larger wavelengths (>1000 nm) is H2O (water). CO2 (carbon-dioxide) absorbs around 2000 nm. The sharp dip near 750-800 nm is due to O2 (oxygen). Ozone absorbs in the UV range around 250 nm in the stratosphere. Shorter wavelengths get absorbed at higher altitudes in the middle and upper atmosphere.

Absorption of solar radiation Balance between Intensity of incoming solar radiation Atmosphere density

Atmospheric absorption cross-sections X-ray EUV FUV middle UV near UV From Cnossen et al. (2007)

What happens with absorbed solar energy? EUV photon Photo-ionization Photo-dissociation Excitation Fast electron (photo-electron) Slow ion (He+) He atom EUV photon Heating Chemical reactions Radiative loss (cooling) 8

Transformation of radiative energy From Fomichev (JASTP, 2009) LTE applies only in a dense atmosphere, where collisions are frequent, so energy can be considered to be instantly converted to heat (very quick recombination; quick release of energy from excited states). In a less dense atmosphere, collisions are not as frequent, and energy stored in an excited state or in chemical potential energy is much longer lived, and can be transported over considerable distance. LTE radiative processes non-LTE radiative processes non-LTE chemical processes non-LTE kinetic processes LTE = local thermodynamic equilibrium

Energy Thresholds for Processes Species Dissociation (Å) (eV) Ionization H He O O2 N2 NO 2423.7 1270.4 1910 5.11 9.76 6.49 911.75 504.27 910.44 1027.8 796 1340 13.6 24.58 13.62 12.06 15.57 9.25 From Heubner et al., Astrophys. Space Sci., 195, 1-294, 1992

Neutral atmosphere composition Ar O2 N2 Important minor species: NO (nitric oxide) CO2 (carbon dioxide) O3 (ozone) heterosphere Density per cubic meter. At 100 km we have about 10^20, which is about 10^14 cm^-3 turbopause (balance between molecular and eddy diffusion) homosphere

Absorption of solar radiation within the atmosphere Image credit: John Emmert/NRL

Energy balance in the atmosphere planetary balance zonal mean absorbed solar radiation zonal mean outgoing infrared radiation From Mlynczak (2000)

Winds in the thermosphere

Radiative energy terms in the middle atmosphere IR = infrared cooling CP = chemical heating Sol = solar heating LTE non-LTE From Fomichev (JASTP, 2009)

Radiative energy terms in the middle atmosphere What latitude? Tropical latitudes, because heating exceeds cooling! IR = infrared cooling CP = chemical heating Sol = solar heating LTE non-LTE From Fomichev (JASTP, 2009)

Middle atmosphere radiative heating and cooling solar heating infrared cooling H2O (>18.5 μm) CO2 CO2 O3 H2O (<18.5 μm) O2 (<205 nm) O3 O2 (>205 nm) H2O CH4 LTE non-LTE From Fomichev (JASTP, 2009)

Thermosphere heating and cooling rates (solar min) Roble et al. (1987) O2 absorption exothermic chemical reactions total neutral heating total neutral cooling NO 5.3 μm emission O3 absorption CO2 15 μm emission collisions with ions/electrons quenching of O(1D) atomic oxygen cooling molecular thermal conduction auroral particle precipitation eddy thermal conduction O recombination Joule heating

Photochemistry in the upper atmosphere From Richards (JGR, 2011)

Ionosphere composition Major species: O2+, NO+, and O+ Molecular ions dominate < ~150 km; approximate photo-chemical equilibrium O+ dominant > ~200 km

Conductivity of the ionosphere   𝑁_𝑒

Solar quiet (Sq) current system Winds drive currents Observations from l’Aquila Currents perturb the magnetic field

Summary Solar activity affects… Heating and cooling processes throughout the atmosphere Pressure gradients and winds Composition Ionization Conductivity Currents in the ionosphere These processes also interact with each other!