A. Corstanje

Vrije Universiteit Brussel

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H. Falcke

Radboud University Nijmegen

S. ter Veen

Netherlands Institute for Radio Astronomy

A. Nelles

Friedrich-Alexander-Universität Erlangen-Nürnberg

J.R. Hörandel

Radboud University Nijmegen

S. Thoudam

Khalifa University of Science and Technology

O. Schölten

Vrije Universiteit Brussel

S. Buitink

Vrije Universiteit Brussel

J. P. Rachen

Vrije Universiteit Brussel

T. N. G. Trinh

Can Tho University

T. Huege

Karlsruhe Institute of Technology

Cooperative Institutions

Netherlands Institute for Radio Astronomy

121

University of Groningen

Max Planck Institute for Radio Astronomy

Radboud University Nijmegen

University of Amsterdam

Max Planck Society

University of Manchester

Leiden University

Astronomical Institute of the Slovak Academy of Sciences

Astronomy and Space

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Measuring a Cherenkov ring in the radio emission from air showers at 110–190MHz with LOFAR

Astroparticle Physics (2014)

A. Nelles P. Schellart S. Buitink A. Corstanje K. D. de Vries

LOFAR

Air shower

10.1016/j.astropartphys.2014.11.006

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Citations (64)

LOFAR Lightning Imaging: Mapping Lightning With Nanosecond Precision

Journal of Geophysical Research Atmospheres (2018)

B. M. Hare O. Schölten A. Bonardi S. Buitink A. Corstanje

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 20 km outside the area enclosed by LOFAR antennas (∼3,200 km

LOFAR

Lightning

Upper-atmospheric lightning

10.1002/2017jd028132

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Citations (31)

LOFAR discovery of a quiet emission mode in PSR B0823+26

Monthly Notices of the Royal Astronomical Society (2015)

C. Sobey Neil J. Young J. W. T. Hessels P. Weltevrede A. Noutsos

PSR B0823+26, a 0.53-s radio pulsar, displays a host of emission phenomena over time-scales of seconds to (at least) hours, including nulling, subpulse drifting, and mode-changing. Studying pulsars like PSR B0823+26 provides further insight into the relationship between these various emission phenomena and what they might teach us about pulsar magnetospheres. Here we report on the LOFAR (Low-Frequency Array) discovery that PSR B0823+26 has a weak and sporadically emitting ‘quiet’ (Q) emission mode that is over 100 times weaker (on average) and has a nulling fraction forty-times greater than that of the more regularly-emitting ‘bright’ (B) mode. Previously, the pulsar has been undetected in the Q mode, and was assumed to be nulling continuously. PSR B0823+26 shows a further decrease in average flux just before the transition into the B mode, and perhaps truly turns off completely at these times. Furthermore, simultaneous observations taken with the LOFAR, Westerbork, Lovell, and Effelsberg telescopes between 110 MHz and 2.7 GHz demonstrate that the transition between the Q mode and B mode occurs within one single rotation of the neutron star, and that it is concurrent across the range of frequencies observed.

LOFAR

Mode (computer interface)

QUIET

10.1093/mnras/stv1066

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Citations (38)

A large light-mass component of cosmic rays at 1017–1017.5 electronvolts from radio observations

Nature (2016)

S. Buitink A. Corstanje H. Falcke J.R. Hörandel T. Huege

Component (thermodynamics)

10.1038/nature16976

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Citations (138)

Measurement of the cosmic-ray energy spectrum above 1016 eV with the LOFAR Radboud Air Shower Array

Astroparticle Physics (2015)

S. Thoudam S. Buitink A. Corstanje J. Emilio Enriquez H. Falcke

LOFAR

Air shower

Shower

10.1016/j.astropartphys.2015.06.005

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Citations (19)

Discovery of carbon radio recombination lines in absorption towards Cygnus A

Monthly Notices of the Royal Astronomical Society (2013)

J. B. R. Oonk R. J. van Weeren F. Salgado L. K. Morabito A. G. G. M. Tielens

We present the first detection of carbon radio recombination line absorption along the line of sight to Cygnus A. The observations were carried out with the LOw Frequency ARray in the 33 to 57 MHz range. These low frequency radio observations provide us with a new line of sight to study the diffuse, neutral gas in our Galaxy. To our knowledge this is the first time that foreground Milky Way recombination line absorption has been observed against a bright extragalactic background source. By stacking 48 carbon $\alpha$ lines in the observed frequency range we detect carbon absorption with a signal-to-noise ratio of about 5. The average carbon absorption has a peak optical depth of 2$\times$10$^{-4}$, a line width of 10 km s$^{-1}$ and a velocity of +4 km s$^{-1}$ with respect to the local standard of rest. The associated gas is found to have an electron temperature $T_{e}\sim$ 110 K and density $n_{e}\sim$ 0.06 cm$^{-3}$. These properties imply that the observed carbon $\alpha$ absorption likely arises in the cold neutral medium of the Orion arm of the Milky Way. Hydrogen and helium lines were not detected to a 3$\sigma$ peak optical depth limit of 1.5$\times$10$^{-4}$ for a 4 km s$^{-1}$ channel width. Radio recombination lines associated with Cygnus A itself were also searched for, but are not detected. We set a 3$\sigma$ upper limit of 1.5$\times$10$^{-4}$ for the peak optical depth of these lines for a 4 km s$^{-1}$ channel width.

Line-of-sight

Line (geometry)

10.1093/mnras/stt2158

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Citations (17)

Distinguishing features of high altitude negative leaders as observed with LOFAR

Atmospheric Research (2021)

O. Schölten B. M. Hare J. R. Dwyer Ningyu Liu C. Sterpka

We present high resolution observations of negative leaders at high altitude using the LOFAR radio telescope. We show that the structure of negative leaders at high altitude (altitudes larger than 7 km) differs in several respects from that of negative leaders at lower altitudes. In particular, the High Altitude Negative Leaders (HANLs) show very distinct steps of a few hundred meters, stepping times of the order of a few milliseconds and a filamentary structure that extends outward over several hundreds of meters; as opposed to lower altitude (≲ 5 km) leaders, which have stepping times and distances around 0.01 ms and 10 m. Similar to lower altitude leaders, high altitude leaders emit copious VHF radiation from their propagating tip and have propagation velocities of the order of 105 m/s. Corona-flash like bursts can be distinguished when zooming in to meter and nanosecond scales.

LOFAR

10.1016/j.atmosres.2021.105688

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Citations (27)

Needle Propagation and Twinkling Characteristics

Journal of Geophysical Research Atmospheres (2021)

B. M. Hare O. Schölten J. R. Dwyer C. Strepka S. Buitink

Abstract Recently, a new lightning phenomena, termed needles, has been observed in both VHF and in optical along positive lightning leaders. They appear as small (<100 m) leader branches that undergo dielectric breakdown at regular intervals (called twinkles). Providing a coherent and consistent explanation for this phenomenon is challenging as each twinkle is a form of negative breakdown that propagates away from the positive leader. In this study, we provide detailed observations of needles in VHF, observed during two lightning flashes. We show distributions of different needle properties, including twinkle propagation speeds, time between twinkles, and needle lengths, among others. We show a return stroke and multiple recoil leaders that quench needle activity. We also show that nearby needle activity does not seem to correlate together, and that needle twinkling can slow down by 10%–30% per twinkle. We conclude by presenting possibilities for how the positive leader could induce negative propagation away from the positive channel, and we argue that twinkles can propagate like a stepped leader or like a recoil leader depending on the temperature of the needle, which implies that needle twinkles can probably propagate without emitting VHF.

Recoil

Lightning

10.1029/2020jd034252

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Citations (18)

Lunar occultation of the diffuse radio sky: LOFAR measurements between 35 and 80 MHz

Monthly Notices of the Royal Astronomical Society (2015)

H. K. Vedantham L. V. E. Koopmans A. G. de Bruyn S. J. Wijnholds M. A. Brentjens

We present radio observations of the Moon between $35$ and $80$ MHz to demonstrate a novel technique of interferometrically measuring large-scale diffuse emission extending far beyond the primary beam (global signal) for the first time. In particular, we show that (i) the Moon appears as a negative-flux source at frequencies $35<\nu<80$ MHz since it is `colder' than the diffuse Galactic background it occults, (ii) using the (negative) flux of the lunar disc, we can reconstruct the spectrum of the diffuse Galactic emission with the lunar thermal emission as a reference, and (iii) that reflected RFI (radio-frequency interference) is concentrated at the center of the lunar disc due to specular nature of reflection, and can be independently measured. Our RFI measurements show that (i) Moon-based Cosmic Dawn experiments must design for an Earth-isolation of better than $80$ dB to achieve an RFI temperature $<1$ mK, (ii) Moon-reflected RFI contributes to a dipole temperature less than $20$ mK for Earth-based Cosmic Dawn experiments, (iii) man-made satellite-reflected RFI temperature exceeds $20$ mK if the aggregate cross section of visible satellites exceeds $80$ m$^2$ at $800$ km height, or $5$ m$^2$ at $400$ km height. Currently, our diffuse background spectrum is limited by sidelobe confusion on short baselines (10-15% level). Further refinement of our technique may yield constraints on the redshifted global $21$-cm signal from Cosmic Dawn ($40>z>12$) and the Epoch of Reionization ($12>z>5$).

Radio occultation

LOFAR

Zodiacal light

10.1093/mnras/stv746

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Citations (22)

The effect of the atmospheric refractive index on the radio signal of extensive air showers

Astroparticle Physics (2017)

A. Corstanje A. Bonardi S. Buitink H. Falcke J.R. Hörandel

Zenith

10.1016/j.astropartphys.2017.01.009

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Citations (20)