Earth’s Atmosphere & Space

AS4100 Astrofisika PengamatanProdi Astronomi 2007/2008

B. Dermawan

Léna et al. 1996

Physical & Chemical Structure Constituents of the Atmosphere

kg

gr3

32

mper air of mass

mper OH of mass

1. Water vaporMixing ratio (or fractional content)

Léna et al. 1996

Physical & Chemical StructureThe quantity of precipitable water above altitude z0:

zNz d )(0

OH0 2

is the number of molecules per unit volumeOH2N

For normal pressure P0 and temperature T0:

0

2

2

d)(] [][

)(103.4][

30OH

00

253OH

z

Hz

zezrcmgcmh

zrT

T

P

PmN

Column of precipitable water

The scale height of water vapor is considerably less than that of dry air H ( ~3 km)

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)

Physical & Chemical Structure

3. Carbondioxide

Important source of infrared absorption. It absorbs mainly in the mid-infrared

Vertical distribution is similar to those of O2 and N2

Mixing ratio is independent of altitude

Physical & Chemical Structure

4. Ions

eheh OOO OO 2*

22

Increasingly ionised above 60 km (because of the Sun’s UV radiation)

*2O an excited state of O2

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; Ne: 103 cm-3), E (100; 105), F (150-300; 2106), up to 2000 km Ne ~104 cm-3

Total: transmission window can be defined at a given altitudePartial: 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. H2O, CO2, O3)Rotational-vibrational (eg. CO2, NO, CO)Electronic (moleculars: eg. CH4, CO, H2O, O2, O3, radicals; at

omic: eg. O, N)

Absorption of RadiationOptical depth

0

d )()()(),( 00

z

iii zzzrz

Attenuation of EM radiation by the atm.

i

i zI

zI),(

cos

1exp

)(

)(0

0

0

Bradt 2004

Absorption of RadiationAbsorptions

• mm (pure rotational H2O & O2)• IR & sub-mm (rotational & vibrational H2O & O2)• Near UV (continuum O2)• Far UV (continuum N2) < 10 nm (molecular ionisation is complete & the ab

sorption coefficient is effectively constant)

Observation domains

Ground-based: visible, near IR ( < 25 m), mm ( > 0.35 m), cm

Space: all the rest including –ray, X-ray, UV, and IR Balloons (–ray, X-ray, near UV; alt. 30-40 km), aircraft (IR & sub-mm; alt. 12 km) or on the polar ice caps of the Antarctic plateau

Absorption of Radiation

• Telluric Bands

Precise knowledge of the atmospheric absorption band is required to obtain a “true” spectral line

Léna et al. 1996

Absorption of Radiation

• Ionospheric plasma

Nm

eNn e

pp

p 3

02

222

2 1097.84

Hz ; 11

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

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, O2, OH, and H

www.albany.edu

Stable Auroral Red

geocorona

www.albany.edu

Atmospheric Emission• Thermal emissionThe atmosphere can be considered as a gas in LTE up to an altitude of 40-60 km

A simple approx. of the intensity cos

1)()( TBzI

• Differential measurement techniquesTo eliminate sky background radiation (fluorescent or thermal origin)

Léna et al. 1996

Scattering of Radiation

• Atmospheric scattering

Léna et al. 1996

Causes- The molecules which make up

the air: decreases with altitude- Aerosols: depends on winds,

climate, type of ground, volcanic activity, industrial pollution, etc.

Scattering of RadiationMolecular scattering in the visible and near IR is Rayleigh scattering which has cross-section

42

223 )1(

3

8)(

N

nR

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 moleculesMie theory: the total effective cross-section )(2

absscat QQa

If a >> , Qs = Qa = 1 is twice the geometrical cross-sectionIf a > , Qs and Qa have a complicated –dependence. For water droplets or dust grains (silicates) Qs –1, hence the scattered intensity varies as –1

Scattering of Radiation

Daylight observation from the groundLéna et al. 1996

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

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

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

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 interferenceLight 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 AntarcticLow temp., dry atmosphere, highest transmission (of IR, sub-mm, mm), weak corresponding emissivity, much reduced turbulence, weak vertical temp. gradient

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 10-20 km, stratospheric balloons at 20-40 km, and rockets up to 300 km) have been included under the denomination of space observation

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

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. 1996

Jack Newton, http://www.arizonaskyvillage.com

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. 1996b. Solar wind: hydrogen plasma ejected from the Sun which travels at high speeds along the field line of the heliosphere. Varies with solar activity

Léna et al. 1996

Observation from Spacec. Radiation belts: modified trajectories of charged particles by

the lines of force of the Earth’s magnetic field (van Allen belts)

http://www.astro.psu.edu/users/niel/astro485/lectures/lecture09-overhead02.jpg srag-nt.jsc.nasa.gov/AboutSRAG/What/What.htm

Observation from Spaced. 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. 1996

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

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

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

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