Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses Second Edition (2015) / Chapter Skim
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2 Scientific Background: Radio Astronomy Service
2 Scientific Background: Radio Astronomy Service
Pages 16-51
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From page 16... ...
Moreover, study by radio astronomers of the same celestial objects that optical astronomers study provides independent insight into the physical processes that are not probed at other wavelengths. Study of the radio emission from celestial sources provides unique insight into the formation, evolution, and physical characteristics of a wide range of astronomical objects and phenomena.
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From page 17... ...
Specific examples of the scientific use of the radio spectrum for astronomical research are highlighted in the following sections. 2.1.1 Types of Radio Emission: Radio Continuum The discovery of radio sources and the bulk of current knowledge about their nature and distribution, and about the processes responsible for the radio emission from them, have come through observations of the continuum radiation.
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From page 18... ...
At the high frequency end, continuum observations at frequencies above 20 GHz are used for the study of the angular distribution, polarization, and fine structure of the 2.7 K cosmic microwave background (see Figure 2.1) , a remnant of the Big Bang.
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From page 19... ...
2.1.2 Types of Radio Emission: Spectral Lines Spectral line radiation is emitted when an atom or molecule undergoes a radiative transition between energy levels. This radiation is emitted at a well-defined frequency and thus results in a line in the radio spectrum (Figure 2.4)
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From page 20... ...
Because planets form as a by-product of star formation, knowledge of interstellar chemistry and the origins of molecular species are vital to an understanding of the early planetary chemistry and the origin of life. Spectral lines from more than 155 different molecular species have now been detected in interstellar
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From page 21... ...
The observations were made with the Green Bank Telescope with a spectral resolution of 390 kHz. At least 727 individual spectral features of dozens of different molecular species are seen.
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FIGURE 2.5 The expansion of the universe results in an apparent Doppler shift of spectral lines for distant sources. The parameter z [(femit − fobs)
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From page 23... ...
and, if they lie in an allocated band, their protection status is listed. In addition to the value of some molecular lines as diagnostic tools, because molecular transitions occur throughout the electromagnetic spectrum, observations of transitions of interstellar molecules at all frequencies improve our understanding of the physical nature and composition of the interstellar medium.
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From page 24... ...
24 HANDBOOK OF FREQUENCY ALLOCATIONS AND SPECTRUM PROTECTION FOR SCIENTIFIC USES TABLE 2.1 Astrophysical Molecules, Grouped by Number of Atoms, Found in Interstellar Clouds of Various Sorts Di-atomic Tri-atomic 4 Atoms 5 Atoms 6 Atoms 7 Atoms H2 C3 c-C3H C5 C 5H C 6H AlF C 2H l-C3H C 4H l-H2C4 CH2CHCN AlCl C 2O C 3N C4Si C 2H 4 CH3C2H C2 C 2S C 3O l-C3H2 CH3CN HC5N CH CH2 C 3S c-C3H2 CH3NC CH3CHO CH+ HCN C 2H 2 H2CCN CH3OH CH3NH2 CN HCO NH3 CH4 CH3SH c-C2H4O CO HCO+ HCCN HC3N HC3NH+ H2CCHOH CO+ HCS+ HCNH+ HC2NC HC2CHO C 6H – CP HOC+ HNCO HCOOH NH2CHO CH3NCO SiC H 2O HNCS H2CNH C 5N HCl H 2S HOCO+ H 2C 2O l-HC4H KCl HNC H2CO H2NCN l-HC4N NH HNO H2CN HNC3 c-H2C3O NO MgCN H2CS SiH4 H2CCNH (?
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From page 25... ...
SCIENTIFIC BACKGROUND: RADIO ASTRONOMY SERVICE 25 8 Atoms 9 Atoms 10 Atoms 11 Atoms 12 Atoms >12 Atoms CH3C3N CH3C4H CH3C5N HC9N c-C6H6 HC11N HC(O)
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From page 26... ...
With modern technology, more bandwidth is essential for high-sensitivity measurements, which depend on averaging for noise reduction.2 The Doppler shift of spectral lines due to the expan 2 Radiometric noise reduction is achieved by increasing the number of effective samples, which means increasing the product of the time spent observing the source and the bandwidth of these observations. Increasing the time spent observing the source is limited by practical considerations, such as amplifier stability and atmospheric variability, which drives the need for wide bandwidths.
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From page 27... ...
. However, as a practical matter, commercial applications that choose to use the opaque bands, between the atmospheric windows, will not only avoid conflict with the radio astronomy service, but also minimize conflicts between other active services.
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From page 28... ...
SOURCE: "The Radio Sun," image, April 11, 1999, http://images.nrao.edu/506; courtesy of NRAO/AUI and Stephen White, University of Maryland. programs with frequency coverage from 1 to 18 GHz provide insight into the nature and evolution of coronal magnetic fields and the temperature and density of nonthermal electrons in active regions.
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From page 29... ...
Many parent molecules are only detectable via radio spectroscopy, so radio observations provide the best way to measure the detailed molecular composition of the cometary ices, which then relate to the volatile composition of the protosolar cloud that formed the Sun and planets. High-resolution radio spectroscopy enables analysis of the dynamics of gas production, the excitation mechanisms affecting coma molecules, and what fraction of the nucleus is actively outgassing.
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From page 30... ...
In addition, radar astrometry places strong constraints on Yarkovsky drift, which results from asymmetrical thermal emissions and can alter the orbits of small objects. Yarkovsky effects are important to the assessment of impact hazards, but also offer another means of estimating masses because the effect is proportional to object size.
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From page 31... ...
In the center are the delay-Doppler images from the S-band Arecibo Planetary Radar observations, where the width of the image is proportional to the asteroid's rotation speed, and the height of the image shows the radar range, related to the object's physical size. At right is how Bennu would appear on the sky viewed from Earth at the time the data were taken (the cross indicates the sub-radar point on the model)
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From page 32... ...
Such observations have the potential to reveal extrasolar planetary magnetic field strengths, the exoplanet's composition, and how bursts may influence habitability for extremely small planetary orbits. In particular, magnetic fields are critical to the establishment of life as they deflect high-energy charged particles and help to confine planetary atmospheres.
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SCIENTIFIC BACKGROUND: RADIO ASTRONOMY SERVICE 33 FIGURE 2.9 A 300 GHz Atacama Large Millimeter Array image of the young star HL Tau and its protoplanetary disk that shows multiple rings and gaps characteristic of emerging planets as they sweep their orbits clear of dust and gas. SOURCE: Courtesy of ALMA (NRAO/ESO/NAOJ)
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From page 34... ...
As with more traditional astronomical studies of weak cosmic radio emission, terrestrial interference poses the greatest challenge to such searches. 2.3 THE MILKY WAY AND OTHER GALAXIES Radio observations of our galaxy and others reveal complex structures from individual stellar systems to extensive stellar nurseries, all of which are situated within a dusty interstellar medium.
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From page 35... ...
2.3.1 The Interstellar Medium: Neutral and Ionized Atomic Gas The material between the stars in the Milky Way and other galaxies includes an inhomogeneous mix of ionized, neutral atomic, and molecular gas. Spectral lines from atomic transitions trace the diffuse component of this interstellar medium.
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From page 36... ...
Allowance for Doppler shifts characteristic of nearby and distant galaxies is essential for adequate protection of radio spectral lines for scientific research. For example, the 100-116 GHz band is used for radio astronomy observations of redshifted CO in distant galaxies and for isotopic transitions of 12CO, 13CO, and C18O in the Milky Way and nearby galaxies.
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From page 37... ...
from dust grains show linear polarization, so grains are elongated and can be aligned by magnetic fields. Dust grains are important catalysts for complex astrochemistry reactions, as they provide surfaces on which molecules may form and then later be ejected into the interstellar medium.
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From page 38... ...
2.3.5 Magnetic Fields Magnetic fields may play a major role in the dynamics of the interstellar gas in galaxies. The strength of the magnetic field along a line of sight can be inferred from its effects on the propagation of radio waves.
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From page 39... ...
The highest mass stars will eject their outer envelopes into the interstellar medium via supernova explosions at the end of their life. While the supernova explosion itself occurs on short timescales, the resulting supernova remnants are relatively long lived and have characteristic non-thermal spectra produced by synchrotron emission from relativistic cosmic ray electrons moving in galactic-scale magnetic fields.
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From page 40... ...
Other structures in this image include supernova remnants (SNRs) and filamentary arcs tracing magnetic field lines.
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From page 41... ...
Furthermore, high spatial resolution observations of atomic and molecular gas trace the circumnuclear accretion disk and provide insight into the feeding and feedback of the supermassive black hole at the center of our galaxy. As mentioned in Section 2.3.1, the 21 cm HI line has been used extensively to trace the atomic gas component of the Milky Way and other galaxies (e.g., see Figure 2.12)
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From page 42... ...
As shown in Figure 6, pulses emitted at lower radio frequencies travel USES HANDBOOK OF FREQUENCY ALLOCATIONS AND SPECTRUM PROTECTION FOR SCIENTIFIC slower through the interstellar medium, arriving later than those emitted at higher frequencies. FIGURE 2.11 Pulse dispersion shown inin this Parkesof the 128 ms of the 128 ms pulsar B1356–60.
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From page 43... ...
Right: The Doppler shifted neutral hydrogen line traces the galaxy kinematics and is used to derive the rotation speed as a function of radius. Discrepancies between the observed rotation speed of galaxies and that predicted based on observed stellar and gaseous distributions led to the hypothesis that galaxies are embedded in extended halos of unobservable material, known as dark matter.
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From page 44... ...
Because of electron energy radiation losses, which are proportional to the square of the electron energy, the radio spectrum often steepens toward high frequencies. The powerful radio emission from quasars and radio galaxies is driven by a "central engine," thought to be the SMBH located at the galactic nucleus, which accretes matter from the dense interstellar medium surrounding the galactic nucleus.
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From page 45... ...
This causes the emerging radiation to be "cut-off" at frequencies below what is called the s ynchrotron self-absorption cutoff frequency, which depends on the magnetic field strength, source size, and flux density. These peaks may be found between 100 MHz and 100 GHz depending on the electron energy population and the magnetic field strength, so measurements of the spectrum surrounding the cutoff frequency as well as at frequencies well above and well below the cutoff frequency provide a powerful technique to study the physics of these small, yet remarkably powerful, relativistic plasmas.
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From page 46... ...
The magnetic fields of clusters of galaxies, individual galaxies, the interstellar medium of our own Milky Way, and even the solar c orona have all been estimated from their observed Faraday rotation. The degree of polarization of radio waves is highest at higher frequencies, but the influence of the ionized interstellar medium is greatest at lower frequencies.
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From page 47... ...
, observations in the 1.2 mm (200-300 GHz) window are important for such fundamental measurement as the FIGURE 2.14 The anisotropies of the cosmic microwave background (CMB)
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From page 48... ...
. In this way, astronomers are able to study the different cosmic populations, including the relation between supermassive black holes and star formation and how they evolve with cosmic time.
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From page 49... ...
The origin of these cluster magnetic fields is an area of active research. 2.5.4 Galaxies in the Early Universe To study the nature of early galaxies as well as other objects, astronomers use radio astronomy data synergistically with optical data from the Hubble Space Telescope, infrared data from the Spitzer Space Telescope, and x-ray data from the Chandra X-ray Observatory (see Figure 2.16)
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From page 50... ...
50 HANDBOOK OF FREQUENCY ALLOCATIONS AND SPECTRUM PROTECTION FOR SCIENTIFIC USES FIGURE 2.15 Very Long Baseline Array 5-GHz maps of the gravitational lens system CLASS B0128+437. This is formed by the deflection of the light from a distant (z = 3.12)
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From page 51... ...
Indeed, HI may be observable in absorption against the cosmic background radiation itself at a redshift of z = 30 (45 MHz)
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