We report here on a novel analysis of the complete set of four Stokes parameters that uniquely determine the linear and/or circular polarization of the radio signal for an extensive air shower. The ...observed dependency of the circular polarization on azimuth angle and distance to the shower axis is a clear signature of the interfering contributions from two different radiation mechanisms, a main contribution due to a geomagnetically-induced transverse current and a secondary component due to the build-up of excess charge at the shower front. The data, as measured at LOFAR, agree very well with a calculation from first principles. This opens the possibility to use circular polarization as an investigative tool in the analysis of air shower structure, such as for the determination of atmospheric electric fields.
Motivated by the recent high-precision measurements of cosmic rays by several new-generation experiments, we have carried out a detailed study to understand the observed energy spectrum and ...composition of cosmic rays with energies up to about 1018 eV. Our study shows that a single Galactic component with subsequent energy cut-offs in the individual spectra of different elements, optimised to explain the observed elemental spectra below ~ 1014 eV and the “knee” in the all-particle spectrum, cannot explain the observed all-particle spectrum above ~ 2 × 1016 eV. We discuss two approaches for a second component of Galactic cosmic rays – re-acceleration at a Galactic wind termination shock, and supernova explosions of Wolf-Rayet stars, and show that the latter scenario can explain almost all observed features in the all-particle spectrum and the composition up to ~ 1018 eV, when combined with a canonical extra-galactic spectrum expected from strong radio galaxies or a source population with similar cosmological evolution. In this two-component Galactic model, the knee at ~ 3 × 1015 eV and the “second knee” at ~ 1017 eV in the all-particle spectrum are due to the cut-offs in the first and second components, respectively. We also discuss several variations of the extra-galactic component, from a minimal contribution to scenarios with a significant component below the “ankle” (at ~ 4 × 1018 eV), and find that extra-galactic contributions in excess of regular source evolution are neither indicated nor in conflict with the existing data. We also provide arguments that an extra-galactic contribution is unlikely to dominate at or below the second knee. Our main result is that the second Galactic component predicts a composition of Galactic cosmic rays at and above the second knee that largely consists of helium or a mixture of helium and CNO nuclei, with a weak or essentially vanishing iron fraction, in contrast to most common assumptions. This prediction is in agreement with new measurements from LOFAR and the Pierre Auger Observatory which indicate a strong light component and a rather low iron fraction between ~ 1017 and 1018 eV.
We use the Low Frequency Array (LOFAR) to probe the dynamics of the stepping process of negatively charged plasma channels (negative leaders) in a lightning discharge. We observe that at each step of ...a leader, multiple pulses of vhf (30-80 MHz) radiation are emitted in short-duration bursts (<10 μs). This is evidence for streamer formation during corona flashes that occur with each leader step, which has not been observed before in natural lightning and it could help explain x-ray emission from lightning leaders, as x rays from laboratory leaders tend to be associated with corona flashes. Surprisingly, we find that the stepping length is very similar to what was observed near the ground, however with a stepping time that is considerably larger, which as yet is not understood. These results will help to improve lightning propagation models, and eventually lightning protection models.
Cosmic rays are routinely measured at LOFAR, both with a dense array of antennas and with the LOFAR Radboud air shower Array (LORA) which is an array of plastic scintillators. In this paper, we ...present two results relating to the cosmic-ray energy scale of LOFAR. First, we present the reconstruction of cosmic-ray energy using radio and particle techniques along with a discussion of the event-by-event and absolute scale uncertainties. The resulting energies reconstructed with each method are shown to be in good agreement, and because the radio-based reconstructed energy has smaller uncertainty on an event-to-event basis, LOFAR analyses will use that technique in the future. Second, we present the radiation energy of air showers measured at LOFAR and demonstrate how radiation energy can be used to compare the energy scales of different experiments. The radiation energy scales quadratically with the electromagnetic energy in an air shower, which can in turn be related to the energy of the primary particle. Once the local magnetic field is accounted for, the radiation energy allows for a direct comparison between the LORA particle-based energy scale and that of the Pierre Auger Observatory. They are shown to agree to within (6±20)% for a radiation energy of 1 MeV, where the uncertainty on the comparison is dominated by the antenna calibrations of each experiment. This study motivates the development of a portable radio array which will be used to cross-calibrate the energy scales of different experiments using radiation energy and the same antennas, thereby significantly reducing the uncertainty on the comparison.
Ultra-high-energy (UHE) cosmic neutrinos interacting with the Moon’s regolith generate particle showers that emit Askaryan radiation. This radiation can be observed from the Earth using ground-based ...radio telescopes like LOFAR. We simulate the effective detection aperture for UHE neutrinos hitting the Moon. Under the same assumptions, results from this work are in good agreement with previous analytic parameterizations and Monte Carlo codes. The dependence of the effective detection aperture on the observing parameters, such as observing frequency and minimum detection threshold, and lunar characteristics like surface topography have been studied. Using a Monte Carlo simulation, we find that the detectable neutrino energy threshold is lowered when we include a realistic treatment of the inelasticity, transmission coefficient, and surface roughness. Lunar surface roughness at large scales enhances the total aperture for higher observation frequencies (
ν
≥
1
GHz
) but has no significant effect on the LOFAR aperture. However, roughness at scales small compared to the wavelength reduces the aperture at all frequencies.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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. Thetype 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 θBn ~ 70°. 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.
Abstract
The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of ...kilometers through the atmosphere forming conducting ionized channels called leaders. Since lightning is an electric discharge phenomenon, there are positively and negatively charged leaders. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily build for radio-astronomy observations. It is observed that a negative leader rather suddenly changes, for a few milliseconds, into a mode where it radiates 100 times more VHF power than typical negative leaders after which it spawns a large number of more typical negative leaders. This mode occurs during the initial stage, soon after initiation, of all lightning flashes we have mapped (about 25). For some flashes this mode occurs also well after initiation and we show one case where it is triggered twice, some 100 ms apart. We postulate that this is indicative of a small (order of 5 km
$$^2$$
2
) high charge pocket. Lightning thus appears to be initiated exclusively in the vicinity of such a small but dense charge pocket.
The TRACER instrument (Transition Radiation Array for Cosmic Energetic Radiation) has been developed for direct measurements of the heavier primary cosmic-ray nuclei at high energies. The instrument ...had a successful long- duration balloon flight in Antarctica in 2003. The detector system and measurement process are described, details of the data analysis are discussed, and the individual energy spectra of the elements O, Ne, Mg, Si, S, Ar, Ca, and Fe (nuclear charge image -26) are presented. The large geometric factor of TRACER and the use of a transition radiation detector make it possible to determine the spectra up to energies in excess of 10 super(14) eV per particle. A power-law fit to the individual energy spectra above 20 GeV amu super(-1) exhibits nearly the same spectral index for all elements, without noticeable dependence on the elemental charge Z.
The LOw-Frequency ARray (LOFAR) is used to make precise measurements of radio emission from extensive air showers, yielding information about the primary cosmic ray. Interpreting the measured data ...requires an absolute and frequency-dependent calibration of the LOFAR system response. This is particularly important for spectral analyses, because the shape of the detected signal holds information about the shower development. We revisit the calibration of the LOFAR antennas in the range of 30–80 MHz. Using the Galactic emission and a detailed model of the LOFAR signal chain, we find an improved calibration that provides an absolute energy scale and allows for the study of frequency dependent features in measured signals. With the new calibration, systematic uncertainties of 13% are reached, and comparisons of the spectral shape of calibrated data with simulations show promising agreement.
The origin of the
knee in the energy spectrum of cosmic rays is an outstanding problem in astroparticle physics. Numerous mechanisms have been proposed to explain the structure in the all-particle ...spectrum. In the article basic ideas of several models are summarized, including diffusive acceleration of cosmic rays in shock fronts, acceleration via cannonballs, leakage from the Galaxy, interactions with background particles in the interstellar medium, as well as new high-energy interactions in the atmosphere. The calculated energy spectra and mean logarithmic masses are compiled and compared to results from direct and indirect measurements.
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