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AuroraThe Solar Spectrum

Atmospheres Interact with LightX raysionize atoms & moleculesdissociate moleculesabsorbed by almost all gasesUltraviolet (UV)dissociate some moleculesabsorbed well by O3 & H2OVisible (V)passes right through gasessome photons are scatteredInfrared (IR)absorbed by greenhouse gases

3Canonical Plot of = 1

Recall Lambert-Beer exponential absorption law:

SpeciesIonizationDissociation(eV)()(eV)()N2O2ONNOHHe 15.5812.0813.6114.549.2513.5924.58796102691185313409125049.765.12

6.5112702422

1905Ionization and dissociation threshold energies andwavelengths Thresholds wavelengths are for the formation of ions in the ground state

A substantial fraction of ions and neutrals is produced in electronically excitedstates and in enhanced vibrational population distributionsNeutral and ion/electron densities in the upper atmosphere

Aurora

Final outcome of complex Magnetospheric, Ionospheric and Thermospheric interactions with the Solar wind and Interplanetary magnetic filed

What is Airglow

Airglow is faint luminescence found in the upper atmosphere due to excitations and deexcitations of atoms and molecules by interacting with solar radiation.Airglow emission is generally observed in the visible spectrum which is mainly due the following processes.Fluorescence1. A + hv A* A* A + hv Chemiluminescence2. A + B A* + B A* A + hvDue to the large availability of EUV photons in the upper atmosphere along with a smaller concentration of atoms and molecules every atom and molecules when traverses back to the ground state gives out its characteristic wavelength.Salient features of airglow: Airglow was discovered in 1868 by Anders Angstrom in the form of green light in the upper atmosphere when aurora was absent. Solar radiation is solely responsible directly and indirectly for the formation of airglow. Half of the luminosity of the night sky is contributed from Airglow. The silhouette of an object held against the dark sky is due to the presence of airglow. Brightest occurrence in the range of 1020 km at an altitude of 100km Airglow intensities are measured in the units of Rayleigh ( 106 photons/cm3). Typical intensities are of the order of few hundred to thousand Rayleigh.Airglow bears closest resemblance to one of the very important optical phenomenon in the upper atmosphere i.e., Aurora

Most of the airglow emanates from the region about 70 to 300 km.

Aurora is usually observed in Polar regions.

Aurora and Airglow

Although airglow bears resemblance to aurora it is very distinct in comparison to AuroraAirglow is global which is seen at every point of the earth. Aurora is local to the polar regions of earth.

Airglow is due to the interaction of solar radiation with the atmospheric constituents. Aurora is due to the interaction of high energetic solar wind particles with the constituents of atmosphere in the presence of magnetic field of earth.

Airglow intensities are very low of the order of few hundred Rayleigh. Auroral intensities are very high and they are easily observed by naked eye. The typical intensities are of the order of few kilo Rayleigh.

Luminosity of Airglow is relatively uniform. Luminosity of Aurora is highly structured.

Picture of Airglow taken from an all sky PhotometerAurora as seen from a site in North Pole

Types of Airglow:

Depending on the time of observation airglow is mainly classified into three types they are

At any given time 42 45 % of the Earth is Day 33 35 % of the Earth is Night 20 25 % of the Earth is Twilight

AirglowDayglow Twilightglow NightglowPhotoabsorption / Resonant/fluorescent ExothermalEnergetic particles reactions chemical reactions14Sources and Mechanisms of Airglow

The important sources of production of airglow are O2 and N

These species participate in variety of processes that are initiated directly or indirectly by solar radiation.

Solar EUV photons are mainly responsible in initiating the reactions leading to formation of airglow.15Radiating SpeciesMTransitionWavelength ( nm )Altitudes ( km )AirglowIntensity (kR )Aurora IntensityH1Electronic656.3100-20000.0151 He4Electronic1083250-1500110N14Electronic246.6, 520.0 1044, 820130-270130-3000.7, 0.2, 10, 3N+14Electronic575.5, 658.4130-4000.5-0.8O16Electronic557.7, 630.0, 844.690-240160-320130-2800.2700.1000.01310010005O+16Electronic732.0130-4000.015OH17Electronic Vibrational766.070-11010001000Few Important Airglow emitting species:557.7 nm Greenline Dayglow Emission:

Brightest and the most readily observed airglow emission in the atmosphere.

O(1S) O(1D) + 557.7 nm

Atomic oxygen Energy level diagram

Vertical Profile of greenline0.91 sec110 secSpectral states 17Sources of Production of O(1S) in mesosphere

Photodissociation of molecular oxygen by solar EUV photons

O2 + hv (