LOFAR as an ionospheric probe

At the Low-Frequency Array (LOFAR)(Planet. Space Sci. (2004) these proceedings) frequencies (HF/VHF), extraterrestrial radiation experiences substantial propagation delay as it passes through the ionosphere. The adaptive calibration technique to be employed by LOFAR will use signals from many known bright radio sources in the sky to estimate and remove the effects of this delay. This technique will operate along many simultaneous lines of sight for each of the stations. Measurements will be made on time scales of seconds or shorter, and with accuracies corresponding to path length variations of 1 cm or less. Tomographic techniques can be used to invert the thousands of changing and independent total electron content (TEC) measurements produced by LOFAR into three-dimensional electron density specifications above the array. These specifications will measure spatial and time scales significantly smaller and faster than anything currently available. These specifications will be used to investigate small-scale ionospheric irregularities, equatorial plasma structures, and ionospheric waves. In addition, LOFAR will improve the understanding of the solar drivers of the ionosphere by simultaneously measuring the solar radio bursts and the TEC. Finally, LOFAR, which will be situated to observed the galactic plane, will make continuous, high-resolution observations of the low-latitude ionosphere, an important but under-observed region. This paper will look at LOFAR as an ionospheric probe including comparisons to other ionospheric probes as well as possible methods of operation to optimize ionospheric measurements.     

The radio search for extrasolar planets with LOFAR

The Low Frequency Array (LOFAR) will come on line with unprecedented radio sensitivity and resolution between 10 and 240 MHz. Such a system will provide a factor of 10–30 improvement in sensitivity in the pursuit of the weak radio emission from extrasolar planets. To date, previous examinations of extrasolar planetary systems with the most advanced radio telescopes have yielded a negative result. However, the improvement in sensitivity by LOFAR over current systems will increase the likelihood of extrasolar planet detection in the radio. We apply radiometric models derived previously from the study of planets in our solar system to the known extrasolar planets, and demonstrate that approximately 3–5 of them should emit in the proper frequency range and with enough power to possibly become detectable at Earth with LOFAR.     

Low-frequency solar radiophysics with LOFAR and FASR

Low-frequency radio observations offer unique diagnostics of the solar corona and solar wind. After a prolongued hiatus, there is renewed interest in this important frequency regime. Two new ground-based instruments will provide critical new low-frequency observations: the low-frequency array (LOFAR) and the frequency agile solar radiotelescope (FASR). This brief topical review summarizes low-frequency radio phenomena that will be accessible to detailed study by LOFAR and FASR in the coming decade. Energy release, drivers of space weather, and studies of the solar wind are emphasized. Both instruments are expected to play important roles in both basic research problems and national and international space weather capabilities. While FASR is a solar-dedicated instrument, LOFAR is not. Solar observing requirements for LOFAR are briefly discussed.     

LOFAR: The potential for solar and space weather studies

The Low Frequency array (LOFAR) will be a next generation digital aperture synthesis radio telescope covering the frequency range from 10 to 240 MHz. The instrument will feature full polarisation and multi-beaming capability, and is currently in its design phase. This work highlights the solar, heliospheric and space weather applications where LOFAR, with its unique and unprecedented capabilities, can provide useful information inaccessible by any other means. The relevant aspects of the LOFAR baseline design are described, and the most promising techniques of interest are enumerated. These include tracking coronal mass ejections (CMEs) out to large distances using interplanetary scintillation (IPS) methods, tomographic reconstruction of the solar wind in the inner heliosphere using IPS, direct imaging of the radio emission from CMEs and finally possible Faraday rotation studies of the magnetic field structure of the heliosphere and the CMEs. This work is a part of an effort directed towards ensuring the compatibility of LOFAR design with solar and space weather applications, in collaboration with the wider community.     

Scaling Astro-WISE to LOFAR long term archive

The Astro-WISE information system was developed to handle data processing for the KIDS survey. In this paper we describe the adaptation of the WISE concept to allow scaling to support archives containing tens of petabytes of stored data and the changes we introduced to accommodate the system for the LOFAR Long Term Archive. With this we provide an example of how Astro-WISE technology can be adapted to support a wider range and scale of data.     

Study of solar system planetary lightning with LOFAR

Radio signatures of lightning discharges have been detected by the Voyager spacecraft near Saturn and Uranus up to 40 MHz. Corresponding flux densities at the distance of the Earth are up to 1000 Jansky (Jy) for Saturn (1 event per minute above 50 Jy, with 30–300 ms duration) and up to a few tens of Jansky for Uranus. Low Frequency ARray LOFAR will allow us to detect and monitor the lightning activity at these two planets. Imaging will allow us to locate lightning sources on Saturn's disk (even if with moderate accuracy), which could then be correlated to optical imaging of clouds. Such observations could provide new information on electrification processes, atmospheric dynamics, composition, and geographical and seasonal variations, compared to the Earth. In addition, lightning may play a role in the atmospheric chemistry, through the production of non-equilibrium trace organic constituents potentially important for biological processes. LOFAR observations can also help us to assess the existence of lightning at Neptune (marginally detected by Voyager), at Venus (where their existence is very controversial), and at Mars (possibly resulting from dust cloud charging). At Jupiter, low-altitude ionospheric layers of meteoritic origin and/or intrinsically long discharge duration seem to prevent the emission and escape of high-frequency radio waves associated with lightning. LOFAR thus presents good possibilities for the detection and study of solar system planetary lightning; we also discuss its relevance to bring new information on Terrestrial lightning-related upper atmosphere transient phenomena (sprites, TIPPs…). Instrumental constraints are outlined.     

LOFAR and Jupiter''s radio (synchrotron) emissions

Jupiter's radio emissions at frequencies below 300 MHz have never been imaged at high spatial resolution. In this paper we discuss the role of LOFAR to image Jupiter's synchrotron radiation at low frequencies to study the low-energy, barely relativistic, electron population in the planet's radiation belts. Radio spectra of Jupiter's synchrotron radiation have revealed significant modifications over time at frequencies between 100 and 1000 MHz, suggestive of processes such as pitch angle scattering by plasma waves, Coulomb scattering and perhaps energy degradation by dust. With LOFAR we may begin investigating the cause of such variability through its imaging capabilities at frequencies 200 MHz at high angular resolution. In particular, quasi-simultaneous observations with LOFAR and higher frequency arrays, such as the Very Large Array (VLA), may provide the necessary data to identify the cause of such variability, which is tightly coupled to the origin and mode of transport (including source/loss terms) of the high-energy electrons in Jupiter's inner radiation belts.     

Solar, interplanetary, planetary, and related extra-solar system science for LOFAR

The low frequency array (LOFAR) radiotelescope will be a powerful instrument for answering fundamental, unresolved scientific questions concerning solar system radio phenomena and related emissions from nearby stellar systems. This paper reviews the phenomena, emission mechanisms, open scientific questions, and LOFAR's capabilities. LOFAR will detect metric solar radio bursts in the corona and interplanetary medium, out to distances of order 10 solar radii, as well as Jovian radio emissions. Arguments are given that LOFAR may be sufficiently sensitive to detect stellar analoges of solar type II and III bursts, and may detect cyclotron-maser emissions from extra-solar planets. LOFAR may also aid space weather research, by passively detecting coronal mass ejections (CMEs) via scintillation and Faraday rotation effects, or by detecting radar signals bounced off CMEs and coronal density structures if a suitable solar radar is developed.     

Fast radio imaging of Jupiter''s magnetosphere at low-frequencies with LOFAR

Jupiter emits intense decameter (DAM) radio waves, detectable from the ground in the range 10–40 MHz. They are produced by energetic electron precipitations in its auroral regions (auroral-DAM), as well as near the magnetic footprints of the Galilean satellite Io (Io-DAM). Radio imaging of these decameter emissions with arcsecond angular resolution and millisecond time resolution should provide:

(1) an improved mapping of the surface planetary magnetic field, via imaging of instantaneous cyclotron sources of highest frequency;

(2) measurements of the beaming angle of the radiation relative to the local magnetic field, as a function of frequency;

(3) detailed information on the Io–Jupiter electrodynamic interaction, in particular the lead angle between the Io flux tube and the radio emitting field line;

(4) direct information on the origin of the sporadic drifting decameter S-bursts, thought to be electron bunches propagating along magnetic field lines, and possibly revealing electric potential drops along these field lines;

(5) direct observation of DAM emission possibly related to the Ganymede–Jupiter, Europa–Jupiter and/or Callisto–Jupiter interactions, and their energetics;

(6) information on the magnetospheric dynamics, via correlation of radio images with ultraviolet and infrared images of the aurora as well as of the Galilean satellite footprints, and study of their temporal variations;

(7) an improved mapping of the Jovian plasma environment (especially the Io torus) via the propagation effects that it induces on the radio waves propagating through it (Faraday rotation, diffraction fringes, etc.);

(8) possibly on the long-term a better accuracy on the determination of Jupiter's rotation period.

Fast imaging should be permitted by the very high intensity of Jovian decameter bursts. LOFAR's capability to measure the full polarization of the incoming waves will be exploited. The main limitation will come from the maximum angular resolution reachable. We discuss several approaches for bringing it close to the value of 1^″ at 30–40 MHz, as required for the above studies.

LOFAR antenna development and initial observations of solar bursts

We are developing and testing active baluns and electrically short dipoles for possible use as the primary wide band receiving elements in the low-frequency array (LOFAR) for long wavelength radio astronomy. Several dipoles of various designs and dimensions have been built and tested. Their useful range occurs when the dipole arms are approximately to one wavelength long and the feedpoint is less than wavelength above ground. An eight-element NRL LOFAR test array (NLTA) interferometer has been built and fringes have been observed from the brightest celestial sources in the frequency range from 10 to 50 MHz. The antenna temperatures vary from about 10% to 100% of the average brightness temperature of the galactic background. With these parameters it is easy to make the amplifier noise levels low enough that final system temperature is dominated by the galactic background.     

LOFAR Observations of Fine Spectral Structure Dynamics in Type IIIb Radio Bursts

Solar radio emission features a large number of fine structures demonstrating great variability in frequency and time. We present spatially resolved spectral radio observations of type IIIb bursts in the 30?–?80 MHz range made by the Low Frequency Array (LOFAR). The bursts show well-defined fine frequency structuring called “stria” bursts. The spatial characteristics of the stria sources are determined by the propagation effects of radio waves; their movement and expansion speeds are in the range of \((0.1\,\mbox{--}\,0.6)c\). Analysis of the dynamic spectra reveals that both the spectral bandwidth and the frequency drift rate of the striae increase with an increase of their central frequency. The striae bandwidths are in the range of \({\approx}\,(20\,\mbox{--}\,100)\) kHz and the striae drift rates vary from zero to \({\approx}\,0.3~\mbox{MHz}\,\mbox{s}^{-1}\). The observed spectral characteristics of the stria bursts are consistent with the model involving modulation of the type III burst emission mechanism by small-amplitude fluctuations of the plasma density along the electron beam path. We estimate that the relative amplitude of the density fluctuations is of \(\Delta n/n\sim10^{-3}\), their characteristic length scale is less than 1000 km, and the characteristic propagation speed is in the range of \(400\,\mbox{--}\,800~\mbox{km}\,\mbox{s}^{-1}\). These parameters indicate that the observed fine spectral structures could be produced by propagating magnetohydrodynamic waves.     

The Dynamic Spectrum of Interplanetary Scintillation: First Solar Wind Observations on LOFAR

The LOw Frequency ARray (LOFAR) is a next-generation radio telescope which uses thousands of stationary dipoles to observe celestial phenomena. These dipoles are grouped in various ‘stations’ which are centred on the Netherlands with additional ‘stations’ across Europe. The telescope is designed to operate at frequencies from 10 to 240 MHz with very large fractional bandwidths (25?–?100 %). Several ‘beam-formed’ observing modes are now operational and the system is designed to output data with high time and frequency resolution, which are highly configurable. This makes LOFAR eminently suited for dynamic spectrum measurements with applications in solar and planetary physics. In this paper we describe progress in developing automated data analysis routines to compute dynamic spectra from LOFAR time–frequency data, including correction for the antenna response across the radio frequency pass-band and mitigation of terrestrial radio-frequency interference (RFI). We apply these data routines to observations of interplanetary scintillation (IPS), commonly used to infer solar wind velocity and density information, and present initial science results.     

The shape of the radio wavefront of extensive air showers as measured with LOFAR

Extensive air showers, induced by high energy cosmic rays impinging on the Earth’s atmosphere, produce radio emission that is measured with the LOFAR radio telescope. As the emission comes from a finite distance of a few kilometers, the incident wavefront is non-planar. A spherical, conical or hyperbolic shape of the wavefront has been proposed, but measurements of individual air showers have been inconclusive so far. For a selected high-quality sample of 161 measured extensive air showers, we have reconstructed the wavefront by measuring pulse arrival times to sub-nanosecond precision in 200 to 350 individual antennas. For each measured air shower, we have fitted a conical, spherical, and hyperboloid shape to the arrival times. The fit quality and a likelihood analysis show that a hyperboloid is the best parameterization. Using a non-planar wavefront shape gives an improved angular resolution, when reconstructing the shower arrival direction. Furthermore, a dependence of the wavefront shape on the shower geometry can be seen. This suggests that it will be possible to use a wavefront shape analysis to get an additional handle on the atmospheric depth of the shower maximum, which is sensitive to the mass of the primary particle.     

Probing the growth of supermassive black holes at z > 6 with LOFAR

H  ii regions surrounding supermassive black holes (SMBHs) in an otherwise still neutral intergalactic medium (IGM) are likely to be the most easily detectable sources by future 21-cm experiments like LOFAR. We have made predictions for the size distribution of such H  ii regions for several physically motivated models for BH growth at high redshift and compared this to the expected LOFAR sensitivity to these sources. The number of potentially detectable H  ii regions does not only depend on the ionization state of the IGM and the decoupling of the spin temperature of the neutral hydrogen from the cosmic microwave background temperature, but is also strongly sensitive to the rate of growth of BHs at high redshift. If the SMBHs at redshift 6 were built up via continuous Eddington-limited accretion from low mass seed BHs at high redshift, then LOFAR is not expected to detect isolated QSO H  ii regions at redshifts much larger than 6, and only if the IGM is still significantly neutral. If the high-redshift growth of BHs starts with massive seed BHs and is driven by short-lived accretion events following the merging of BH hosting galaxies then the detection of H  ii regions surrounding SMBHs may extend to redshifts as large as 8–9 but is still very sensitive to the redshift to which the IGM remains significantly neutral. The most optimistic predictions are for a model where the SMBHs at z > 6 have grown slowly. H  ii regions around SMBHs may then be detected to significantly larger redshifts.     

Telescope calibration using polarization calibration data

This article describes the use of the telescope output Stokes vector measured during a polarization calibration to infer the properties of mirrors in the telescope itself. Polarization calibrations performed at the National Solar Observatory Dunn Solar Telescope are used to demonstrate this technique (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A parameterization for the radio emission of air showers as predicted by CoREAS simulations and applied to LOFAR measurements

Measuring radio emission from air showers provides excellent opportunities to directly measure all air shower properties, including the shower development. To exploit this in large-scale experiments, a simple and analytic parameterization of the distribution of the radio signal at ground level is needed. Data taken with the Low-Frequency Array (LOFAR) show a complex two-dimensional pattern of pulse powers, which is sensitive to the shower geometry. Earlier parameterizations of the lateral signal distribution have proven insufficient to describe these data. In this article, we present a parameterization derived from air-shower simulations. We are able to fit the two-dimensional distribution with a double Gaussian, requiring five fit parameters. All parameters show strong correlations with air shower properties, such as the energy of the shower, the arrival direction, and the shower maximum. We successfully apply the parameterization to data taken with LOFAR and discuss implications for air shower experiments.     

The low-frequency array (LOFAR): opening a new window on the universe

We present an overview of the low-frequency array (LOFAR) that will open a window on one of the last and most poorly explored regions of the electromagnetic spectrum. LOFAR will be a large (baselines up to 400 km), low-frequency aperture synthesis array with large collecting area ( at ) and high resolution (1.5^″ at 100 MHz), and will provide sub-mJy sensitivity across much of its operating range. LOFAR will be a powerful instrument for solar system and planetary science applications as reviewed by papers in this monogram. Key astrophysical science drivers include acceleration, turbulence, and propagation in the galactic interstellar medium, exploring the high red-shift universe and transient phenomena, as well as searching for the red-shifted signature of neutral hydrogen from the cosmologically important epoch of re-ionization.     

Absolute magnitude calibration for red clump stars

We combined the (K _s , J?K _s ) data in Laney et al. (Mon. Not. R. Astron. Soc. 419:1637, 2012) with the V apparent magnitudes and trigonometric parallaxes taken from the Hipparcos catalogue and used them to fit the $M_{K_{s}}$ absolute magnitude to a linear polynomial in terms of V?K _s colour. The mean and standard deviation of the absolute magnitude residuals, ?0.001 and 0.195 mag, respectively, estimated for 224 red clump stars in Laney et al. (2012) are (absolutely) smaller than the corresponding ones estimated by the procedure which adopts a mean $M_{K_{s}}=-1.613~\mbox{mag}$ absolute magnitude for all red clump stars, ?0.053 and 0.218 mag, respectively. The statistics estimated by applying the linear equation to the data of 282 red clump stars in Alves (Astrophys. J. 539:732, 2000) are larger, $\Delta M_{K_{s}}=0.209$ and σ=0.524 mag, which can be explained by a different absolute magnitude trend, i.e. condensation along a horizontal distribution.     

Position calibration methodology for scanning sky monitor for ASTROSAT

Scanning Sky Monitor (SSM) on ASTROSAT is an X-ray sky monitor which has a large Field of View (FOV) and scans the sky to detect and locate X-ray transient sources in the energy range 2 to 10 keV. Experiments are carried out to calibrate SSM detectors for position response and to verify the calibration constants derived. In this paper we discuss the methodology of position calibration of proportional counters for SSM and results from various experiments.