LOFAR (LOw-Frequency ARray)
LOFAR is a large radio telescope network located mainly in the Netherlands, completed in 2012 by ASTRON, the Netherlands Institute for Radio Astronomy and its international partners, and operated by ASTRON's radio observatory, of the Netherlands Organization for Scientific Research.LOFAR is a new-generation radio interferometer constructed in the north of the Netherlands and across Europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. 1) 2) 3)
LOFAR consists of a vast array of omnidirectional antennas using a new concept in which the signals from the separate antennas are not combined in real time as they are in most array antennas. The electronic signals from the antennas are digitized, transported to a central digital processor, and combined in software to emulate a conventional antenna. The project is based on an interferometric array of radio telescopes using about 20,000 small antennas concentrated in at least 48 stations. Forty of these stations are distributed across the Netherlands and were funded by ASTRON. The five stations in Germany, and one each in Great Britain, France, Sweden and Ireland, were funded by these countries. Further stations may also be built in other European countries. The total effective collecting area is approximately 300,000 m2, depending on frequency and antenna configuration. The data processing is performed by a Blue Gene/P supercomputer situated in the Netherlands at the University of Groningen. LOFAR is also a technology precursor for the SKA (Square Kilometer Array).
LOFAR was conceived as an innovative effort to force a breakthrough in sensitivity for astronomical observations at radio-frequencies below 250 MHz. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g. the One-Mile Telescope or the Very Large Array), arrays of one-dimensional antennas (e.g. the Molonglo Observatory Synthesis Telescope) or two-dimensional arrays of omnidirectional antennas (e.g. Antony Hewish's Interplanetary Scintillation Array).
LOFAR combines aspects of many of these earlier telescopes; in particular, it uses omnidirectional dipole antennas as elements of a phased array at individual stations, and combines those phased arrays using the aperture synthesis technique developed in the 1950s. Like the earlier Cambridge Low Frequency Synthesis Telescope (CLFST) low-frequency radio telescope, the design of LOFAR has concentrated on the use of large numbers of relatively cheap antennas without any moving parts, concentrated in stations, with the mapping performed using aperture synthesis software. The direction of observation ("beam") of the stations is chosen electronically by phase delays between the antennas. LOFAR can observe in several directions simultaneously, as long as their aggregated data rate remains under its cap. This in principle allows a multi-user operation.
LOFAR makes observations in the 10 MHz to 240 MHz frequency range with two types of antennas: Low Band Antenna (LBA) and High Band Antenna (HBA), optimized for 10-80 MHz and 120-240 MHz, respectively. The electric signals from the LOFAR stations are digitized, transported to a central digital processor, and combined in software in order to map the sky. Therefore, LOFAR is a "software telescope". The cost is dominated by the cost of electronics and will therefore mostly follow Moore's law, becoming cheaper with time and allowing increasingly large telescopes to be built. The antennas are simple enough, but there are about 20,000 in the LOFAR array.
The function of the LOFAR WAN (Wide-Area Network) is to transport data between the LOFAR stations and the central processor in Groningen. The main component is the streaming of observational data from the stations. A smaller part of the LOFAR datastream consists of MAC (Monitoring And Control) related data and management information of the active network equipment. Connections of the LOFAR stations in the Netherlands to Groningen run over light-paths (also referred to as managed dark fibers) that are either owned by LOFAR or leased. This ensures the required performance and security of the entire network and the equipment connected to it. Signals from all stations in the core and an area around it are first sent to a concentrator node and subsequently patched through to Groningen.
The LOFAR stations outside the Netherlands are connected via international links that often involve the local NRENs (National Research and Education Networks). In some cases, commercial providers also play a role for part of the way.
For the communication over the light-paths 10 Gbit Ethernet (GbE) technology has been adopted. The high bandwidth connection between the concentrator node in the core and Groningen uses CWDM (Course Wavelength Division Multiplexing) techniques to transfer multiple signals on a single fiber, thereby saving on costs. Since the availability requirement for LOFAR is relatively low (95%), when compared with commercial data communication networks, redundant routing has not been implemented.
Figure 1: Aerial photograph of the Superterp, the heart of the LOFAR core, on which six LOFAR stations are housed. What resembles at first an ancient earthwork in a nature reserve in the northeast of the Netherlands is actually the heart of the most advanced radio telescope in the world, spanning northwestern Europe. The LOFAR, operated by the Astron Netherlands Institute for Radio Astronomy, consists of 51 antenna stations from Sweden to Ireland and Poland tasked with studying some of the lowest frequencies that can be observed from Earth, probing the primordial era before stars and galaxies were formed (image credit: LOFAR/Astron) 6)
Figure 2: LBA antennas of the LOFAR telescope (image credit: Astron)
Figure 3: Black casings in which the HBAs (High Band Antennas) of LOFAR are housed (image credit: ASTRON)
Figure 4: Locations of the International LOFAR Telescope for radio astronomy. The Lofar, operated by the Astron Netherlands Institute for Radio Astronomy, consists of 51 antenna stations from Sweden to Ireland and Poland tasked with studying some of the lowest frequencies that can be observed from Earth, probing the primordial era before stars and galaxies were formed (image credit: LOFAR/Astron)
Some LOFAR application fields:
LOFAR is a multi-purpose sensor array. Its main application is astronomy at low frequencies (10-240 MHz). LOFAR has also applications in Geophysics and Agriculture.
KSP (Key Science Projects) are astronomical applications that helped to drive the design of LOFAR. For each KSP a team of astronomers is involved with ASTRON in realizing the required technical capabilities.
- Cosmic magnetism of the nearby universe
- Ultra high energy cosmic rays
- Epoch of reionization. Astronomers hope that LOFAR will detect the signature the EoR (Epoch of Reionization). If it does, then this will open up an area of study in astronomy that will be even bigger than the CMB (Cosmic Microwave Background). The latter looks at a single epoch in the history of the evolution of the Universe - the EoR refers to a much longer period of time - from the so-called dark ages to the generation of the light from the first stars and galaxies or whatever it was that was around in these times.
- Solar physics and space weather
- Deep extragalactic surveys
- Transients and pulsars.
Geomagnetic Halloween Storm of 29-31 October 2003
The 29-31 October 2003 space weather superstorm, also referred to as the Halloween storm, attracted wide attention both in scientific and industrial communities as well as among the general public. It is probably one of the best publicly reported storms and because of the increasing scientific instrumentation, especially in space, the best recorded storm event ever. A descriptive picture of interest in the storm is given by the number of file transfers from the NOAA Space Environment Center which peaked at 19 million hits per day on 29 October, the average being 0.5 million. The storm period caused a great number of technological impacts varying from enforced alternate airline routes due to the increased particle radiation to the loss of a Japanese US $640 million environment satellite ADEOS-II. An extensive list of effects is given in a recent report by NOAA. Despite the large GIC (Geomagnetically Induced Currents) measured in North America, GIC did not cause any large-scale failures during the storm period. Though a detailed analysis to explain this feature of the storm is lacking, it was most probably due to both the countermeasures taken by the utilities and the fact that the most intense substorms of the period occurred when North America was not in the vicinity of local midnight. 7)
The geomagnetic superstorm knocked down a part of the high-voltage power transmission system in southern Sweden. The blackout lasted for an hour and left about 50,000 customers without electricity. The incident was probably the most severe GIC (Geomagnetically Induced Current) failure observed since the well-known March 1989 Québec blackout. The ‘‘three-phase'' storm produced exceptionally large geomagnetic activity at the Fennoscandian auroral region. Although the diversity of the GIC drivers is addressed in the study, the problems in operating the Swedish system during the storm are attributed geophysically to substorm s, storm sudden commencement, and enhanced ionospheric convection, all of which created large and complex geoelectric fields capable of driving large GIC. On the basis of the basic twofold nature of the failure-related geoelectric field characteristics, a semi-deterministic approach for forecasting GIC-related geomagnetic activity in which average overall activity is supplemented with statistical estimations of the amplitudes of GIC fluctuations is suggested. The study revealed that the primary mode of GIC-related failures in the Swedish high-voltage power transmission system were via harmonic distortions produced by GIC combined with too sensitive operation of the protective relay s. The outage in Malmo¨ on 30 October 2003 was caused by a combination of
Figure5 shows an overview of the interplanetary, magnetospheric and ionospheric conditions during the 29-31 October 2003 storm. The storm period started on 29 October, at about 05:40 UT with sudden southward turning of the IMF (Interplanetary Magnetic Field) associated possibly with the sheath region of the first ICME (Iinterplanetary Coronal Mass Ejection) accompanied by extremely high speed solar wind flow of about 1900 km/s. The magnetosphere responded to this by enhancing the convection, causing the Dst index to decrease to about -180 nT. At the same time, during this ‘‘first'' main phase of the storm, very intense ionospheric disturbances above the Fennoscandian region were observed. Shortly after northward turning of the IMF and entering the recovery phase of the first enhancement of the Dst index, probably the internal field of the ejecta itself caused another southward IMF event starting at about 14:00 UT. This caused the ‘‘second'' main phase of the geomagnetic storm and again the minimum Dst, this time reaching about -360 nT, was accompanied by very strong ionospheric disturbances above Fennoscandia.
Figure 5: Interplanetary, magnetospheric, and ionospheric overview of the 29-30 October 2003 geomagnetic storm period: (top) z component of the interplanetary magnetic field measured by the ACE spacecraft, (middle) Dst index, and (bottom) local variant of the AL index, IL index, computed from the IMAGE magnetometer array measurements. Dashed lines indicate the times of the failures in the Swedish high-voltage power transmission system. Note that interplanetary measurements are not propagated to the magnetopause, causing about an hour lag between the top plot and the other plots (image credit: storm study team)
Figure 6: Model of the well-known 30 October 2003 Halloween solar storm produced by the MIDAS tomographic ionospheric model from the University of Bath (image credit: University of Bath, Ref. 11) 8) 9) 10)
Radio astronomers focus on ionosphere for sharper satellite navigation
• April 2018: Radio astronomers and satellite navigation engineers are focusing their attention on the same point of the sky, looking into methods of improving both satnav accuracy and radio astronomy. 11)
LOFAR, operated by the Astron Netherlands Institute for Radio Astronomy, consists of 51 antenna stations from Sweden to Ireland and Poland tasked with studying some of the lowest frequencies that can be observed from Earth, probing the primordial era before stars and galaxies were formed.
Galileo and other navigation constellations exploit higher microwave frequencies, but radio astronomers and satnav engineers have one major obstacle in common: the ionosphere, the electrically active outer layer of Earth's atmosphere, between 80 km to several thousand kilometers in altitude.
Discovered by early-20th century radio pioneers, the ionosphere creates major satnav errors owing to its tendency to delay and bend navigation signals as they travel the many thousands of kilometers from satellites in orbit down to the ground. - In the worst cases, satnav positioning errors of tens of meters can be introduced by the ionosphere.
A key factor is that the ionosphere is continuously changing, influenced by the Sun and space weather. The astronomy network experiences similar signal interference across its different antennas, but makes up for it by observing a specific strong radio source in the sky as a reference.
For satnav, the favored solution for larger receivers is to use two frequencies – the difference in signal delay reveals the state of the ionosphere. For smaller receivers of the kind found in smartphones and other consumer devices, a simpler adjustment is applied.
Figure 7: Artist's view of a Galileo FOC (Full Operational Capability) satellite. The complete Galileo constellation will consist of 24 satellites along three orbital planes in MEO (Medium Earth Orbit, plus two spares per orbit). The result will be Europe's largest ever fleet of satellites, operating in the new environment of MEO (image credit: ESA, Pierre Carril)
To investigate methods of improving satnav accuracy, ESA worked with Astron and the NLR Netherlands Aerospace Center to measure the ionosphere simultaneously: dual-frequency satnav receivers at Exloo and another Lofar station at Steenwijk, and radio observations were made of celestial objects on locations close to the path of European Galileo, US GPS and Russian Glonass satnav satellites.
The ionospheric error is determined by the TEC (Total Electron Count) of the signal path through space: the higher the count the greater the error. Very high TECs can lead to scintillations that can cause the loss of satnav signal lock. The free electrons making up this count are produced by high-energy particles from the Sun dislodging electrons from atoms at the top of the atmosphere.
The results from the cross-measurements have proved promising. A dual-frequency satnav receiver can give estimates to about a single ‘TEC Unit' (equal to about 10 million billion electrons along a square meter cross section along the signal path). Vertical TEC Unit values can range from a few to several hundred TEC Units. Lofar, however, can shrink this estimate tenfold, down to an accuracy of at least 0.1 TEC Unit relative between a pair of Lofar stations, with much lower noise levels.
Analysis suggests a dedicated ionosphere observation system might be used in the future to improve overall satnav accuracy, while also supporting the calibration of Lofar and comparable radio astronomy systems.
The project was supported through ESA's European GNSS (Global Navigation Satellite System) Evolution Program, undertaking research and development linked to satellite navigation and augmentation systems. It was run for ESA by NLR and Astron.
Some LOFAR Observations/Discoveries
• June 2018: The quiet solar corona emits meter-wavelength thermal bremsstrahlung. Coronal radio emission can only propagate above that radius, Rω, where the local plasma frequency equals the observing frequency. The radio interferometer LOFAR (LOw Frequency ARray) observes in its low band (10–90 MHz) solar radio emission originating from the middle and upper corona. 12)
- Objectives: The first solar aperture synthesis imaging observations is presented in the low band of LOFAR in 12 frequencies each separated by 5 MHz. From each of these radio maps we infer Rω, and a scale height temperature, T. These results can be combined into coronal density and temperature profiles.
- Methods: Radial intensity profiles from the radio images are derived. The focus is on polar directions with simpler, radial magnetic field structure. Intensity profiles were modeled by ray-tracing simulations, following wave paths through the refractive solar corona, and including free-free emission and absorption. Model profiles were fitted to observations with Rω and T as fitting parameters.
- Results: In the low corona, Rω < 1.5 solar radii, high scale height temperatures up to 2.2 x 106 K are found, much more than the brightness temperatures usually found there. But if all Rω values are combined into a density profile, this profile can be fitted by a hydrostatic model with the same temperature, thereby confirming this with two independent methods. The density profile deviates from the hydrostatic model above 1.5 solar radii, indicating the transition into the solar wind.
- Conclusions: These results demonstrate what information can be gleaned from solar low-frequency radio images. The scale height temperatures observed are not only higher than the brightness temperatures, but also higher than temperatures derived from a coronagraph or from EUV (Extreme Ultraviolet) data. Future observations will provide continuous frequency coverage. This continuous coverage eliminates the need for local hydrostatic density models in the data analysis and enables the analysis of more complex coronal structures such as those with closed magnetic fields.
• October 3, 2017: LOFAR (Low Frequency Array) observations at 144 MHz have revealed large-scale radio sources in the unrelaxed galaxy cluster Abell 1132. The cluster hosts diffuse radio emission on scales of ~650 kpc near the cluster center and a head–tail (HT) radio galaxy, extending up to 1 Mpc (Megaparsec), south of the cluster center. The central diffuse radio emission is not seen in NRAO VLA FIRST Survey, Westerbork Northern Sky Survey, nor in C & D array VLA observations at 1.4 GHz, but is detected in our follow-up GMRT (Giant Meterwave Radio Telescope) observations at 325 MHz. Using LOFAR and GMRT data, we determine the spectral index of the central diffuse emission to be α = -1.75 ± 0.19 (S ∞ να). 13)
• September 5, 2017: By following up on mysterious high-energy sources mapped out by NASA's Fermi Gamma-ray Space Telescope, the Netherlands-based LOFAR (Low Frequency Array) radio telescope has identified a pulsar spinning at more than 42,000 revolutions per minute, making it the second-fastest known. 14) 15)
A pulsar is the core of a massive star that exploded as a supernova. In this stellar remnant, also called a neutron star, the equivalent mass of half a million Earths is crushed into a magnetized, spinning ball no larger than Washington, D.C. The rotating magnetic field powers beams of radio waves, visible light, X-rays and gamma rays. If a beam happens to sweep across Earth, astronomers observe regular pulses of emission and classify the object as a pulsar.
"Roughly a third of the gamma-ray sources found by Fermi have not been detected at other wavelengths," said Elizabeth Ferrara, a member of the discovery team at NASA's Goddard Space Center in Greenbelt, Maryland. "Many of these unassociated sources may be pulsars, but we often need follow-up from radio observatories to detect the pulses and prove it. There's a real synergy across the extreme ends of the electromagnetic spectrum in hunting for them."
The new object, named PSR J0952–0607 — or J0952 for short — is classified as a millisecond pulsar and is located between 3,200 and 5,700 light-years away in the constellation Sextans. The pulsar contains about 1.4 times the sun's mass and is orbited every 6.4 hours by a companion star that has been whittled away to less than 20 times the mass of the planet Jupiter. The scientists report their findings in a paper published in the Sept. 10 issue of The Astrophysical Journal Letters and now available online.
At some point in this system's history, matter began streaming from the companion and onto the pulsar, gradually raising its spin to 707 rotations a second, or more than 42,000 rpm, and greatly increasing its emissions. Eventually, the pulsar began evaporating its companion, and this process continues today. Because of their similarity to spiders that consume their mates, systems like J0952 are called black widow or redback pulsars, depending on how much of the companion star remains. Most of the known systems of these types were found by following up Fermi unassociated sources.
The LOFAR discovery also hints at the potential to find a new population of ultra-fast pulsars.
"LOFAR picked up pulses from J0952 at radio frequencies around 135 MHz, which is about 45 percent lower than the lowest frequencies of conventional radio searches," said lead author Cees Bassa at the Netherlands Institute for Radio Astronomy (ASTRON). "We found that J0952 has a steep radio spectrum, which means its radio pulses fade out very quickly at higher frequencies. It would have been a challenge to find it without LOFAR."
Theorists say pulsars could rotate as fast as 72,000 rpm before breaking apart, yet the fastest spin known — by PSR J1748–2446ad, reaching nearly 43,000 rpm — is just 60 percent of the theoretical maximum. Perhaps pulsars with faster periods simply can't form. But the gap between theory and observation may also result from the difficulty in detecting the fastest rotators.
"There is growing evidence that the fastest-spinning pulsars tend to have the steepest spectra," said co-author Ziggy Pleunis, a doctoral student at McGill University in Montreal. The first millisecond pulsar discovered with LOFAR, which was found by Pleunis, is J1552+5437, which spins at 25,000 rpm and also exhibits a steep spectrum. "Since LOFAR searches are more sensitive to these steep-spectrum radio pulsars, we may find that even faster pulsars do, in fact, exist and have been missed by surveys at higher frequencies," he explained.
• February 13, 2017: An international team of astronomers reports the discovery of a new GRG (Giant Radio Galaxy) associated with the galaxy triplet known as UGC 9555. The newly discovered galaxy turns out to be one of the largest GRGs so far detected. 16) 17)
Located some 820 million light years away from the Earth, UGC 9555 is a part of a larger group of galaxies designated MSPM 02158. Recently, a team of researchers led by Alex Clarke of the Jodrell Bank Centre for Astrophysics in Manchester, U.K., has combed through the data provided by the LOFAR (Low Frequency Array) and uncovered new, important information about this distant disturbed galaxy group.
The team has analyzed the data available in the LOFAR MSSS (Multifrequency Snapshot Sky Survey). It is the first northern-sky LOFAR imaging survey that covers the sky north of the celestial equator at frequencies from 119 to 158 MHz in eight separate 2.0 MHz bands. The images obtained as a part of the LOFAR MSSS allowed the scientists to distinguish a new giant radio galaxy.
GRGs are radio galaxies with an overall projected linear length exceeding 6.5 million light years. They are rare objects grown in low-density environments. GRGs are important for astronomers to study the formation and the evolution of radio sources.
The newly detected GRG which has not received any official designation yet has a projected linear size of 8.34 million light years. This makes it one of the largest GRGs known to date. Currently, with a projected size of approximately 16 million light years, the J1420-0545 holds the title of the largest giant radio galaxy discovered so far.
The team noted that the newly detected GRG has integrated flux density at 142 MHz of 1.54 Jy over the whole dual-lobe emission, including underlying background point sources, which gives a total luminosity at 142 MHz of 11.6 septillion W/Hz.
However, the available LOFAR MSSS and archival radio data are still insufficient to confirm the class of this GRG. Radio sources are divided into two classes: Fanaroff and Riley Class I (FRI), and Class II (FRII).
"We cannot clearly classify this GRG as an FR-I or FR-II source based on its morphology in the MSSS and archival radio data. There are no conclusive enhancements of emission from the resolution of the MSSS data (without the contribution from unassociated point sources) from which to use the standard Fanaroff-Riley classification," the paper reads.
Figure 8: Background SDSS (Sloan Digital Sky Survey) image (composite from bands g, r and i) overlaid with white MSSS contours of the GRG at 2, 3, 4, 6 and 8 times the RMS noise (34 mJy/beam). NVSS (NRAO VLA Sky Survey) contours are overlaid in red at 3, 5, 10 and 20 times the RMS noise (0.55 mJy/beam) revealing a bright part of the radio jet towards the north-east. The beam sizes are shown in the lower left (image credit: LOFAR study team)
• March 19, 2013: A team of astronomers led by ASTRON astronomer Dr. George Heald has discovered a previously unknown gigantic radio galaxy, using initial images from a new, ongoing all-sky radio survey. The galaxy was found using the powerful ILT (International LOFAR Telescope), built and designed by ASTRON. The team is currently performing LOFAR's first all-sky imaging survey, the MSSS (Multi-frequency Snapshot Sky Survey). While browsing the first set of MSSS images, Dr. Heald identified a new source the size of the full moon projected on the sky. The radio emission is associated with material ejected from one member of an interacting galaxy triplet system tens to hundreds of millions of years ago. The physical extent of the material is much larger than the galaxy system itself, extending millions of light years across intergalactic space. The MSSS survey is still ongoing, and is poised to discover many new sources like this one. 18)
The new galaxy is a member of a class of objects called Giant Radio Galaxies (GRGs). GRGs are a type of radio galaxy with extremely large physical size, suggesting that they are either very powerful or very old. LOFAR is an effective tool to find new GRGs like this one because of its extreme sensitivity to such large objects, combined with its operation at low frequencies that are well suited to observing old sources.
Figure 9: LOFAR discovers new giant galaxy in all-sky survey. Overlay of the new GRG (blue-white colors) on an optical image from the Digitized Sky survey. The inset shows the central galaxy triplet (image from Sloan Digital Sky Survey). The image is about 2 Mpc (Megaparsec) across (image credit: ASTRON)
1) URL: https://en.wikipedia.org/wiki/LOFAR
2) M. P. van Haarlem, M. W. Wise, A. W. Gunst, G. Heald, J. P. McKean, J. W. T. Hessels, A. G. de Bruyn, R. Nijboer, J. Swinbank, R. Fallows, M. Brentjens, A. Nelles, R. Beck, H. Falcke, R. Fender, J. Hörandel, L. V. E. Koopmans, G. Mann, G. Miley, H. Röttgering, B. W. Stappers, R. A. M. J. Wijers, S. Zaroubi, M. van den Akker, A. Alexov, J. Anderson, K. Anderson, A. van Ardenne, M. Arts, A. Asgekar, I. M. Avruch, F. Batejat, L. Bähren, M. E. Bell, M. R. Bell, I. van Bemmel, P. Bennema, M. J. Bentum, G. Bernardi, P. Best, L. Bîrzan, A. Bonafede, A.-J. Boonstra, R. Braun, J. Bregman, F. Breitling, R. H. van de Brink, J. Broderick, P. C. Broekema, W. N. Brouw, M. Brüggen, H. R. Butcher, W. van Cappellen, B. Ciardi, T. Coenen, J. Conway, A. Coolen, A. Corstanje, S. Damstra, O. Davies, A. T. Deller, R.-J. Dettmar, G. van Diepen, K. Dijkstra, P. Donker, A. Doorduin, J. Dromer, M. Drost, A. van Duin, J. Eislöffel, J. van Enst, C. Ferrari, W. Frieswijk, H. Gankema, M. A. Garrett, F. de Gasperin, M. Gerbers, E. de Geus, J.-M. Grießmeier, T. Grit, P. Gruppen, J. P. Hamaker, T. Hassall, M. Hoeft, H. Holties, A. Horneffer, A. van der Horst, A. van Houwelingen, A. Huijgen, M. Iacobelli, H. Intema, N. Jackson, V. Jelic, A. de Jong, E. Juette, D. Kant, A. Karastergiou, A. Koers, H. Kollen, V. I. Kondratiev, E. Kooistra, Y. Koopman, A. Koster, M. Kuniyoshi, M. Kramer, G. Kuper, P. Lambropoulos, C. Law, J. van Leeuwen, J. Lemaitre, M. Loose, P. Maat, G. Macario, S. Markoff, J. Masters, D. McKay-Bukowski, H. Meijering, H. Meulman, M. Mevius, E. Middelberg, R. Millenaar, J. C. A. Miller-Jones, R. N. Mohan, J. D. Mol, J. Morawietz, R. Morganti, D. D. Mulcahy, E. Mulder, H. Munk, L. Nieuwenhuis, R. van Nieuwpoort, J. E. Noordam, M. Norden, A. Noutsos, A. R. Offringa, H. Olofsson, A. Omar, E. Orrú, R. Overeem, H. Paas, M. Pandey-Pommier, V. N. Pandey, R. Pizzo, A. Polatidis, D. Rafferty, S. Rawlings, W. Reich, J.-P. de Reijer, J. Reitsma, A. Renting, P. Riemers, E. Rol, J. W. Romein, J. Roosjen, M. Ruiter, A. Scaife, K. van der Schaaf, B. Scheers, P. Schellart, A. Schoenmakers, G. Schoonderbeek, M. Serylak, A. Shulevski, J. Sluman, O. Smirnov, C. Sobey, H. Spreeuw, M. Steinmetz, C. G. M. Sterks, H.-J. Stiepe, K. Stuurwold, M. Tagger, Y. Tang, C. Tasse, I. Thomas, S. Thoudam, M. C. Toribio, B. van der Tol, O. Usov, M. van Veelen, A.-J. van der Veen, S. ter Veen, J. P. W. Verbiest, R. Vermeulen, N. Vermaas, C. Vocks, C. Vogt, M. de Vos, E. van der Wal, R. van Weeren, H. Weggemans, P. Weltevrede, S. White, S. J. Wijnholds, T. Wilhelmsson, O. Wucknitz, S. Yatawatta, P. Zarka, A. Zensus, J. van Zwieten, "LOFAR: The LOw-Frequency ARray," Astronomy & Astrophysics, May 21, 2013, DOI:10.1051/0004-6361/201220873, URL: https://arxiv.org/pdf/1305.3550.pdf
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12) C. Vocks, G. Mann, F. Breitling, M. M. Bisi, B. Dabrowski, R. Fallows, P. T. Gallagher, A. Krankowski, J. Magdalenic, C. Marque, D. Morosan, H. Rucker, "LOFAR observations of the quiet solar corona," Astronomy & Astrophysics, Volume 614, 12 June 2018, URL of abstract: https://www.aanda.org/articles/aa/abs/
13) A. Wilber M. Brüggen A. Bonafede F. Savini T. Shimwell R. J. van Weeren D. Rafferty A. P. Mechev H. Intema F. Andrade-Santos A. O. Clarke E. K. Mahony R. Morganti I. Prandoni G. Brunetti H. Röttgering S. Mandal F. de Gasperin M. Hoeft, "LOFAR discovery of an ultra-steep radio halo and giant head–tail radio galaxy in Abell 1132," Monthly Notices of the Royal Astronomical Society, Volume 473, Issue 3, 21 January 2018, pp: 3536–3546, https://doi.org/10.1093/mnras/stx2568
14) "'Extreme' Telescopes Find the Second-fastest-spinning Pulsar," NASA, 5 Sept. 2017, URL: https://www.nasa.gov/feature/goddard/2017
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).