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Earth’s Atmosphere & Space AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan.

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Presentation on theme: "Earth’s Atmosphere & Space AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan."— Presentation transcript:

1 Earth’s Atmosphere & Space AS4100 Astrofisika Pengamatan Prodi Astronomi 2007/2008 B. Dermawan

2 Léna et al Physical & Chemical Structure Constituents of the Atmosphere 1. Water vapor Mixing ratio (or fractional content) Léna et al. 1996

3 Physical & Chemical Structure The quantity of precipitable water above altitude z 0 : is the number of molecules per unit volume For normal pressure P 0 and temperature T 0 : Column of precipitable water The scale height of water vapor is considerably less than that of dry air H (  ~3 km)

4 Physical & Chemical Structure 2. Ozone Vertical distribution: depends on the latitude and the season Integrated quantity in the whole atmosphere: 0.24 – 0.38 cm STP (Standard Temperature and Pressure) Maximum concentration occurs at about 16 km (highest ~80 km). It absorbs mainly in the ultraviolet Detection of perturbations due to human activity (industrial products: fluorocarbons)

5 Physical & Chemical Structure 3. Carbondioxide Important source of infrared absorption. It absorbs mainly in the mid-infrared Vertical distribution is similar to those of O 2 and N 2 Mixing ratio is independent of altitude

6 Physical & Chemical Structure 4. Ions Increasingly ionised above 60 km (because of the Sun’s UV radiation)  an excited state of O 2 Recombinations and radiative or collisional de-excitation occur, and hence the electron density is not constant at a given altitude Ionospheric layers: D (height: 60 km; N e : 10 3 cm -3 ), E (100; 10 5 ), F ( ; 2  10 6 ), up to 2000 km N e ~10 4 cm -3

7 Total: transmission window can be defined at a given altitude Partial: the object’s spectra will be modified by telluric absorption bands Absorption of Radiation Atomic and Molecular Transitions Cause absorption at discrete wavelengths Pure rotational (eg. H 2 O, CO 2, O 3 ) Rotational-vibrational (eg. CO 2, NO, CO) Electronic (moleculars: eg. CH 4, CO, H 2 O, O 2, O 3, radicals; atomic: eg. O, N)

8 Absorption of Radiation Optical depth Attenuation of EM radiation by the atm. Bradt 2004

9 Absorption of Radiation Absorptions mm (pure rotational H 2 O & O 2 ) IR & sub-mm (rotational & vibrational H 2 O & O 2 ) Near UV (continuum O 2 ) Far UV (continuum N 2 ) < 10 nm (molecular ionisation is complete & the absorption coefficient is effectively constant) Observation domains Ground-based: visible, near IR ( 0.35  m), cm Space: all the rest including  –ray, X-ray, UV, and IR Balloons (  –ray, X-ray, near UV; alt km), aircraft (IR & sub- mm; alt. 12 km) or on the polar ice caps of the Antarctic plateau

10 Absorption of Radiation Telluric Bands Precise knowledge of the atmospheric absorption band is required to obtain a “true” spectral line Léna et al. 1996

11 Absorption of Radiation Ionospheric plasma The F -layer causes total reflection = 23.5 m for which n = 0 The ionosphere is thus generally transparent to both cm and mm wavelengths

12 Atmospheric Emission Fluorescent Emission (Airglow) Recombination of electrons with ions, which have been produced by daytime reactions of photochemical dissociation, leads to the emission of photons Emission (a continuum & lines) may occur up to several hours after excitation Main sources: O I, Na I, O 2, OH, and H Stable Auroral Red geocorona

13 Atmospheric Emission Thermal emission The atmosphere can be considered as a gas in LTE up to an altitude of km A simple approx. of the intensity Differential measurement techniques To eliminate sky background radiation (fluorescent or thermal origin) Léna et al. 1996

14 Scattering of Radiation Atmospheric scattering Léna et al Causes - The molecules which make up the air: decreases with altitude - Aerosols: depends on winds, climate, type of ground, volcanic activity, industrial pollution, etc.

15 Scattering of Radiation Molecular scattering in the visible and near IR is Rayleigh scattering which has cross-section Rayleigh scattering is not isotropic and actually the cross-section is a function of the angle between the directions of the incident and scattered radiation Aerosol scattering: the particles are bigger than molecules Mie theory: the total effective cross-section If a >>, Q s = Q a = 1   is twice the geometrical cross-section If a >, Q s and Q a have a complicated –dependence. For water droplets or dust grains (silicates) Q s  –1, hence the scattered intensity varies as –1

16 Scattering of Radiation Daylight observation from the ground Léna et al There is a wavelength beyond which thermal emission exceeds daytime scattering emissions, and hence in this range the brightness of the sky is largely independent of the day-night cycle

17 Terrestrial Observing Sites It is essential to choose the best possible site whatever logistic difficulties it may involve Visible, IR, and mm observatories Criteria Absence of cloud: tropical and desert regions, the least cloud regions (10  to 35  N & 0-10  S to 35  -40  S) but fluctuate over different longitudes Léna et al. 1996

18 Terrestrial Observing Sites Photometric quality: stability of atmospheric transparency in the visible (six consecutive hours of clear sky) Infrared and millimeter transparency: minimisation of the height of precipitable water (favors polar and dry tropical sites) Image quality: variation in temperature, and hence in the refraction index on the air, perturb the phase of EM wavefronts. Histogram of its intensity over time must also be taken into consideration

19 Terrestrial Observing Sites Centimeter radio astronomy and beyond Avoid radiofrequency interference, the latitude with a view to covering as much as possible of the two celestial hemispheres, the horizontal surface area available for setting up interferometers Man-made pollution and interference Light pollution in the visible, radiofrequency interference, heat sources (nuclear power stations) modify microclimates, vibrations, industrial aerosols, and the risk of an over- exploitation of space The Antarctic Low temp., dry atmosphere, highest transmission (of IR, sub- mm, mm), weak corresponding emissivity, much reduced turbulence, weak vertical temp. gradient

20 Observation from Space Aspects The launchers: orbit & mass of equipment The energy supply: maneuverability & data transmission capacity The various protection systems: fend off particles, micrometeorites  guaranteeing whatever lifetime is required The quality control & reliability studies: test the system as a whole Observations from atmospheric platforms (aeroplanes at km, stratospheric balloons at km, and rockets up to 300 km) have been included under the denomination of space observation

21 Observation from Space The advantages Overcome three main causes: absorption of radiation, turbulence, and interfering emissions However, some interference remains: Upper atmosphere, solar wind, and zodiacal dust cloud scatter the light from the Sun and emits their own thermal radiation; The flux of particles coming from the Sun or diffusing through the Galaxy can interfere with detectors on board a space observatory  overcome by suitable choice of orbit

22 Observation from Space Sources of perturbation 1. The zodiacal nebula: distribution of dust grains in orbit around the Sun, very close to the ecliptic (inclination ~3  ) Léna et al Jack Newton,

23 Observation from Space 2. High energy particles & photons a. Diffuse cosmic background: mainly of superposition of emissions with different redshifts (in the X- &  -rays regions) Léna et al b. Solar wind: hydrogen plasma ejected from the Sun which travels at high speeds along the field line of the heliosphere. Varies with solar activity

24 Léna et al Observation from Space c. Radiation belts: modified trajectories of charged particles by the lines of force of the Earth’s magnetic field (van Allen belts) lecture09-overhead02.jpg

25 Observation from Space d. Cosmic rays: enter the solar system and interact with the heliosphere which opposes their penetration The flux of cosmic rays in the neighborhood of the Earth is maximum when solar activity is minimum (  solar modulation) Léna et al e. Background from interaction with surrounding matter: highly complex spectrum containing many de-excitation lines superposed upon a continuous emission. Limits the sensitivity of the experiment

26 Observation from Space Choice of orbits Low equatorial orbits (300 – 500 km): communication is easy and repairs are possible. Lifetime is reduced. The Earth blocks 2  sr of the f.o.v, very quick changes between night and day leading to breaks in visibility of the studied source about once per hour High circular orbits (6000 – 100,000 km): pointing is easier, obs. periods are long, reduced the Earth”s blocking of the f.o.v, weak interference (scattering, radiofrequency, thermal emission). Launch energy and for communication are greater (higher cost) Highly elliptical orbits: less power to launch and transmitting data when passes close to the Earth, spends most of its time far from the Earth and its associated interference emissions

27 Observation from Space The best orbits for  astronomy: either very distant (avoiding radiation belts), or else close circular equatorial orbits (avoiding the South Atlantic Anomaly and protected from cosmic rays by the magnetosphere. However, rather inaccessible from the larger launch pads, no interest from the economic and military point of views, and other problems). Distant circular orbits (>60,000 km) or eccentric orbits (apogee ~200,000 km) are the best compromise The Lagrange points: a local minimum of gravitational potential

28 The Moon as an Astronomical Site A long night allows long integration periods on a single source The lunar surface is stable, much lower seismic activity than that of the Earth The absolute instantaneous position of the Moon is known to a very high degree accuracy The ground temp. varies widely between day and night (90 to 400 K) The weak gravity on the Moon makes it is possible to build large structures which are both rigid and light The permanently hidden face of the Moon is entirely free of man- made radiofrequency interference (strongly favor for radio telescope) Disadvantages: higher cost, the continual solar wind and cosmic rays bombardments, the intense solar radiation in the extreme UV and X- ray regions, the incessant impacts of micrometeorites

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