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Hare, B. M., Scholten, O., Bonardi, A., Buitink, S., Corstanje, A., Ebert, U., . . . Winchen, T. (2018). LOFAR Lightning Imaging: Mapping Lightning With Nanosecond Precision. Journal of Geophysical Research - Atmospheres, 123(5), 2861-2876
Open this publication in new window or tab >>LOFAR Lightning Imaging: Mapping Lightning With Nanosecond Precision
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2018 (English)In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 123, no 5, p. 2861-2876Article in journal (Refereed) Published
Abstract [en]

Lightning mapping technology has proven instrumental in understanding lightning. In this work we present a pipeline that can use lightning observed by the LOw-Frequency ARray (LOFAR) radio telescope to construct a 3-D map of the flash. We show that LOFAR has unparalleled precision, on the order of meters, even for lightning flashes that are over 20km outside the area enclosed by LOFAR antennas (approximate to 3,200km(2)), and can potentially locate over 10,000 sources per lightning flash. We also show that LOFAR is the first lightning mapping system that is sensitive to the spatial structure of the electrical current during individual lightning leader steps.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2018
Keywords
lightning, lightning mapping, LOFAR, time of arrival, leader propagation
National Category
Physical Sciences
Research subject
Natural Science, Physics
Identifiers
urn:nbn:se:lnu:diva-72691 (URN)10.1002/2017JD028132 (DOI)000428437100028 ()
Available from: 2018-04-13 Created: 2018-04-13 Last updated: 2019-02-27Bibliographically approved
Arias, M., Vink, J., de Gasperin, F., Salas, P., Oonk, J. B., van Weeren, R. J., . . . Zucca, P. (2018). Low-frequency radio absorption in Cassiopeia A. Astronomy and Astrophysics, 612, Article ID A110.
Open this publication in new window or tab >>Low-frequency radio absorption in Cassiopeia A
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 612, article id A110Article in journal (Refereed) Published
Abstract [en]

Context. Cassiopeia A is one of the best-studied supernova remnants. Its bright radio and X-ray emission is due to shocked ejecta. Cas A is rather unique in that the unshocked ejecta can also be studied: through emission in the infrared, the radio-active decay of Ti-44, and the low-frequency free-free absorption caused by cold ionised gas, which is the topic of this paper. Aims. Free-free absorption processes are affected by the mass, geometry, temperature, and ionisation conditions in the absorbing gas. Observations at the lowest radio frequencies can constrain a combination of these properties. Methods. We used Low Frequency Array (LOFAR) Low Band Antenna observations at 30-77 MHz and Very Large Array (VLA) L-band observations at 1-2 GHz to fit for internal absorption as parametrised by the emission measure. We simultaneously fit multiple UV-matched images with a common resolution of 17 '' (this corresponds to 0.25 pc for a source at the distance of Cas A). The ample frequency coverage allows us separate the relative contributions from the absorbing gas, the unabsorbed front of the shell, and the absorbed back of the shell to the emission spectrum. We explored the effects that a temperature lower than the similar to 100-500 K proposed from infrared observations and a high degree of clumping can have on the derived physical properties of the unshocked material, such as its mass and density. We also compiled integrated radio flux density measurements, fit for the absorption processes that occur in the radio band, and considered their effect on the secular decline of the source. Results. We find a mass in the unshocked ejecta of M = 2.95 +/- 0.48 M-circle dot for an assumed gas temperature of T = 100 K. This estimate is reduced for colder gas temperatures and, most significantly, if the ejecta are clumped. We measure the reverse shock to have a radius of 114 '' +/- 6 '' and be centred at 23:23:26, +58:48:54 (J2000). We also find that a decrease in the amount of mass in the unshocked ejecta (as more and more material meets the reverse shock and heats up) cannot account for the observed low-frequency behaviour of the secular decline rate. Conclusions. To reconcile our low-frequency absorption measurements with models that reproduce much of the observed behaviour in Cas A and predict little mass in the unshocked ejecta, the ejecta need to be very clumped or the temperature in the cold gas needs to be low (similar to 10 K). Both of these options are plausible and can together contribute to the high absorption value that we find.

Keywords
supernovae: individual: Cas A, ISM: supernova remnants, radiation mechanisms: general, radio continuum: general
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-76769 (URN)10.1051/0004-6361/201732411 (DOI)000431715700003 ()
Available from: 2018-07-11 Created: 2018-07-11 Last updated: 2019-02-27Bibliographically approved
Zucca, P., Morosan, D. E., Rouillard, A. P., Fallows, R., Gallagher, P. T., Magdalenic, J., . . . Zarka, P. (2018). Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR. Astronomy and Astrophysics, 615, Article ID A89.
Open this publication in new window or tab >>Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 615, article id A89Article in journal (Refereed) Published
Abstract [en]

Context. Type II radio bursts are evidence of shocks in the solar atmosphere and inner heliosphere that emit radio waves ranging from sub-meter to kilometer lengths. These shocks may be associated with coronal mass ejections (CMEs) and reach speeds higher than the local magnetosonic speed. Radio imaging of decameter wavelengths (20-90 MHz) is now possible with the Low Frequency Array (LOFAR), opening a new radio window in which to study coronal shocks that leave the inner solar corona and enter the interplanetary medium and to understand their association with CMEs. Aims. To this end, we study a coronal shock associated with a CME and type II radio burst to determine the locations at which the radio emission is generated, and we investigate the origin of the band-splitting phenomenon. Methods. The type II shock source-positions and spectra were obtained using 91 simultaneous tied-array beams of LOFAR, and the CME was observed by the Large Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric Observatory (SOHO) and by the COR2A coronagraph of the SECCHI instruments on board the Solar Terrestrial Relation Observatory (STEREO). The 3D structure was inferred using triangulation of the coronographic observations. Coronal magnetic fields were obtained from a 3D magnetohydrodynamics (MHD) polytropic model using the photospheric fields measured by the Heliospheric Imager (HMI) on board the Solar Dynamic Observatory (SDO) as lower boundary. Results. The type II radio source of the coronal shock observed between 50 and 70 MHz was found to be located at the expanding flank of the CME, where the shock geometry is quasi-perpendicular with theta(Bn)similar to 70 degrees. The type II radio burst showed first and second harmonic emission; the second harmonic source was cospatial with the first harmonic source to within the observational uncertainty. This suggests that radio wave propagation does not alter the apparent location of the harmonic source. The sources of the two split bands were also found to be cospatial within the observational uncertainty, in agreement with the interpretation that split bands are simultaneous radio emission from upstream and downstream of the shock front. The fast magnetosonic Mach number derived from this interpretation was found to lie in the range 1.3-1.5. The fast magnetosonic Mach numbers derived from modelling the CME and the coronal magnetic field around the type II source were found to lie in the range 1.4-1.6.

Keywords
Sun: corona, Sun: coronal mass ejections (CMEs), Sun: radio radiation
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-77396 (URN)10.1051/0004-6361/201732308 (DOI)000439525400003 ()
Available from: 2018-08-29 Created: 2018-08-29 Last updated: 2019-02-27Bibliographically approved
Trinh, T. N., Scholten, O., Bonardi, A., Buitink, S., Corstanje, A., Ebert, U., . . . Winchen, T. (2017). Circular polarization of radio emission from air showers in thunderstorm conditions. In: Buitink, S Horandel, JR DeJong, S Lahmann, R Nahnhauer, R Scholten, O (Ed.), 7th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2016): . Paper presented at 7th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA), JUN 07-10, 2016, Groningen, NETHERLANDS. E D P SCIENCES, Article ID UNSP 03002.
Open this publication in new window or tab >>Circular polarization of radio emission from air showers in thunderstorm conditions
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2017 (English)In: 7th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA 2016) / [ed] Buitink, S Horandel, JR DeJong, S Lahmann, R Nahnhauer, R Scholten, O, E D P SCIENCES , 2017, article id UNSP 03002Conference paper, Published paper (Refereed)
Abstract [en]

We present measured radio emission from cosmic-ray-induced air showers under thunderstorm conditions. We observe for these events large differences in intensity, linear polarization and circular polarization from the events measured under fair-weather conditions. This can be explained by the effects of atmospheric electric fields in thunderclouds. Therefore, measuring the intensity and polarization of radio emission from cosmic ray extensive air showers during thunderstorm conditions provides a new tool to probe the atmospheric electric fields present in thunderclouds.

Place, publisher, year, edition, pages
E D P SCIENCES, 2017
Series
EPJ Web of Conferences, ISSN 2100-014X ; 135
National Category
Physical Sciences
Research subject
Natural Science, Physics
Identifiers
urn:nbn:se:lnu:diva-79221 (URN)10.1051/epjconf/201713503002 (DOI)000407847700024 ()978-2-7598-9015-6 (ISBN)
Conference
7th International Conference on Acoustic and Radio EeV Neutrino Detection Activities (ARENA), JUN 07-10, 2016, Groningen, NETHERLANDS
Available from: 2018-12-14 Created: 2018-12-14 Last updated: 2019-02-27Bibliographically approved
Scholten, O., Bonardi, A., Buitink, S., Corstanje, A., Ebert, U., Falcke, H., . . . Winchen, T. (2017). Precision study of radio emission from air showers at LOFAR. In: Morselli, A Capone, A Fernandez, GR (Ed.), RICAP16, 6TH ROMA INTERNATIONAL CONFERENCE ON ASTROPARTICLE PHYSICS: . Paper presented at 6th Roma International Conference on Astroparticle Physics (RICAP), JUN 21-24, 2016, Rome, ITALY. , Article ID UNSP 02012.
Open this publication in new window or tab >>Precision study of radio emission from air showers at LOFAR
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2017 (English)In: RICAP16, 6TH ROMA INTERNATIONAL CONFERENCE ON ASTROPARTICLE PHYSICS / [ed] Morselli, A Capone, A Fernandez, GR, 2017, article id UNSP 02012Conference paper, Published paper (Refereed)
Abstract [en]

Radio detection as well as modeling of cosmic rays has made enormous progress in the past years. We show this by using the subtle circular polarization of the radio pulse from air showers measured in fair weather conditions and the intensity of radio emission from an air shower under thunderstorm conditions.

Series
EPJ Web of Conferences, ISSN 2100-014X ; 136
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-67510 (URN)10.1051/epjconf/201713602012 (DOI)000405958300023 ()
Conference
6th Roma International Conference on Astroparticle Physics (RICAP), JUN 21-24, 2016, Rome, ITALY
Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2019-02-27Bibliographically approved
Acero, F., Aloisio, R., Amans, J., Amato, E., Antonelli, L. A., Aramo, C., . . . Wagner, R. (2017). Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7-3946. Astrophysical Journal, 840(2), Article ID 74.
Open this publication in new window or tab >>Prospects for Cherenkov Telescope Array Observations of the Young Supernova Remnant RX J1713.7-3946
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2017 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 840, no 2, article id 74Article in journal (Refereed) Published
Abstract [en]

We perform simulations for future Cherenkov Telescope Array (CTA) observations of RX J1713.7-3946, a young supernova remnant (SNR) and one of the brightest sources ever discovered in very high energy (VHE) gamma rays. Special attention is paid to exploring possible spatial (anti) correlations of gamma rays with emission at other wavelengths, in particular X-rays and CO/H I emission. We present a series of simulated images of RX J1713.7-3946 for CTA based on a set of observationally motivated models for the gamma-ray emission. In these models, VHE gamma rays produced by high-energy electrons are assumed to trace the nonthermal X-ray emission observed by XMM-Newton, whereas those originating from relativistic protons delineate the local gas distributions. The local atomic and molecular gas distributions are deduced by the NANTEN team from CO and H I observations. Our primary goal is to show how one can distinguish the emission mechanism(s) of the gamma rays (i.e., hadronic versus leptonic, or a mixture of the two) through information provided by their spatial distribution, spectra, and time variation. This work is the first attempt to quantitatively evaluate the capabilities of CTA to achieve various proposed scientific goals by observing this important cosmic particle accelerator.

Keywords
cosmic rays, gamma rays: ISM, ISM: individual objects (RX J1713.7-3946, G347.3-0.5)
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-67049 (URN)10.3847/1538-4357/aa6d67 (DOI)000403228100003 ()
Available from: 2017-07-20 Created: 2017-07-20 Last updated: 2019-02-27Bibliographically approved
Corstanje, A., Bonardi, A., Buitink, S., Falcke, H., Horandel, J. R., Mitra, R., . . . Winchen, T. (2017). The effect of the atmospheric refractive index on the radio signal of extensive air showers. Astroparticle physics, 89, 23-29
Open this publication in new window or tab >>The effect of the atmospheric refractive index on the radio signal of extensive air showers
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2017 (English)In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 89, p. 23-29Article in journal (Refereed) Published
Abstract [en]

For the interpretation of measurements of radio emission from extensive air showers, an important systematic uncertainty arises from natural variations of the atmospheric refractive index n. At a given altitude, the refractivity N = 10(6) (n - 1) can have relative variations on the order of 10% depending on temperature, humidity, and air pressure. Typical corrections to be applied to N are about 4%. Using CoREAS simulations of radio emission from air showers, we have evaluated the effect of varying N on measurements of the depth of shower maximum X-max. For an observation band of 30-80 MHz, a difference of 4% in refractivity gives rise to a systematic error in the inferred X-max between 3.5 and 11 g/cm(2), for proton showers with zenith angles ranging from 15 to 50 degrees. At higher frequencies, from 120 to 250 MHz, the offset ranges from 10 to 22 g/cm(2). These offsets were found to be proportional to the geometric distance to X-max. We have compared the results to a simple model based on the Cherenkov angle. For the 120-250 MHz band, the model is in qualitative agreement with the simulations. In typical circumstances, we find a slight decrease in X-max compared to the default refractivity treatment in CoREAS. While this is within commonly treated systematic uncertainties, accounting for it explicitly improves the accuracy of X-max measurements. (C) 2017 Elsevier B.V. All rights reserved.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Cosmic rays, Extensive air showers, Radio emission, Atmospheric effects
National Category
Physical Sciences
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-64215 (URN)10.1016/j.astropartphys.2017.01.009 (DOI)000395614500004 ()
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2019-02-27Bibliographically approved
Horandel, J. R., Bonardi, A., Buitink, S., Corstanje, A., Falcke, H., Mitra, P., . . . Winchen, T. (2017). The mass composition of cosmic rays measured with LOFAR. In: Morselli, A Capone, A Fernandez, GR (Ed.), RICAP16, 6TH ROMA INTERNATIONAL CONFERENCE ON ASTROPARTICLE PHYSICS: . Paper presented at 6th Roma International Conference on Astroparticle Physics (RICAP), JUN 21-24, 2016, Rome, ITALY. , Article ID UNSP 02001.
Open this publication in new window or tab >>The mass composition of cosmic rays measured with LOFAR
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2017 (English)In: RICAP16, 6TH ROMA INTERNATIONAL CONFERENCE ON ASTROPARTICLE PHYSICS / [ed] Morselli, A Capone, A Fernandez, GR, 2017, article id UNSP 02001Conference paper, Published paper (Refereed)
Abstract [en]

High-energy cosmic rays, impinging on the atmosphere of the Earth initiate cascades of secondary particles, the extensive air showers. The electrons and positrons in the air shower emit electromagnetic radiation. This emission is detected with the LOFAR radio telescope in the frequency range from 30 to 240 MHz. The data are used to determine the properties of the incoming cosmic rays. The radio technique is now routinely used to measure the arrival direction, the energy, and the particle type (atomic mass) of cosmic rays in the energy range from 10(17) to 10(18) eV. This energy region is of particular astrophysical interest, since in this regime a transition from a Galactic to an extra-galactic origin of cosmic rays is expected. For illustration, the LOFAR results are used to set constraints on models to describe the origin of high-energy cosmic rays.

Series
EPJ Web of Conferences, ISSN 2100-014X ; 136
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-67509 (URN)10.1051/epjconf/201713602001 (DOI)000405958300012 ()
Conference
6th Roma International Conference on Astroparticle Physics (RICAP), JUN 21-24, 2016, Rome, ITALY
Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2019-02-27Bibliographically approved
Trinh, T. N., Scholten, O., Bonardi, A., Buitink, S., Corstanje, A., Ebert, U., . . . Winchen, T. (2017). Thunderstorm electric fields probed by extensive air showers through their polarized radio emission. Physical Review D: covering particles, fields, gravitation, and cosmology, 95(8), Article ID 083004.
Open this publication in new window or tab >>Thunderstorm electric fields probed by extensive air showers through their polarized radio emission
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2017 (English)In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 95, no 8, article id 083004Article in journal (Refereed) Published
Abstract [en]

We observe a large fraction of circular polarization in radio emission from extensive air showers recorded during thunderstorms, much higher than in the emission from air showers measured during fair-weather circumstances. We show that the circular polarization of the air showers measured during thunderstorms can be explained by the change in the direction of the transverse current as a function of altitude induced by atmospheric electric fields. Thus by using the full set of Stokes parameters for these events, we obtain a good characterization of the electric fields in thunderclouds. We also measure a large horizontal component of the electric fields in the two events that we have analyzed.

Place, publisher, year, edition, pages
American Physical Society, 2017
National Category
Physical Sciences
Research subject
Natural Science, Physics
Identifiers
urn:nbn:se:lnu:diva-67505 (URN)10.1103/PhysRevD.95.083004 (DOI)000406878500003 ()
Available from: 2017-08-30 Created: 2017-08-30 Last updated: 2019-02-27Bibliographically approved
Buitink, S., Corstanje, A., Falcke, H., Hörandel, J. R., Huege, T., Nelles, A., . . . Zensus, J. A. (2016). A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations. Nature, 531(7592), 70-73
Open this publication in new window or tab >>A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations
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2016 (English)In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 531, no 7592, p. 70-73Article in journal (Refereed) Published
Abstract [en]

Cosmic rays are the highest-energy particles found in nature. Measurements of the mass composition of cosmic rays with energies of 1017–1018 electronvolts are essential to understanding whether they have galactic or extragalactic sources. It has also been proposed that the astrophysical neutrino signal1 comes from accelerators capable of producing cosmic rays of these energies2. Cosmic rays initiate air showers—cascades of secondary particles in the atmosphere—and their masses can be inferred from measurements of the atmospheric depth of the shower maximum3 (Xmax; the depth of the air shower when it contains the most particles) or of the composition of shower particles reaching the ground4. Current measurements5 have either high uncertainty, or a low duty cycle and a high energy threshold. Radio detection of cosmic rays6, 7, 8 is a rapidly developing technique9 for determining Xmax (refs 10, 11) with a duty cycle of, in principle, nearly 100 per cent. The radiation is generated by the separation of relativistic electrons and positrons in the geomagnetic field and a negative charge excess in the shower front6, 12. Here we report radio measurements of Xmax with a mean uncertainty of 16 grams per square centimetre for air showers initiated by cosmic rays with energies of 1017–1017.5 electronvolts. This high resolution in Xmax enables us to determine the mass spectrum of the cosmic rays: we find a mixed composition, with a light-mass fraction (protons and helium nuclei) of about 80 per cent. Unless, contrary to current expectations, the extragalactic component of cosmic rays contributes substantially to the total flux below 1017.5 electronvolts, our measurements indicate the existence of an additional galactic component, to account for the light composition that we measured in the 1017–1017.5 electronvolt range.

Keywords
High-energy astrophysics, Particle astrophysics
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Astroparticle Physics
Identifiers
urn:nbn:se:lnu:diva-51518 (URN)10.1038/nature16976 (DOI)
Note

Corrigendum: Buitink et al. (2016) A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations. Nature, 537: 572.DOI: 10.1038/nature18936

Available from: 2016-03-29 Created: 2016-03-29 Last updated: 2019-02-27Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-7066-3614

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