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.     

Radiation characteristics of quasi-periodic radio bursts in the Jovian high-latitude region

Ulysses had a “distant encounter” with Jupiter in February 2004. The spacecraft passed from north to south, and it observed Jovian radio waves from high to low latitudes (from +80° to +10°) for few months during its encounter. In this study, we present a statistical investigation of the occurrence characteristics of Jovian quasi-periodic bursts, using spectral data from the unified radio and plasma wave experiment (URAP) onboard Ulysses. The latitudinal distribution of quasi-periodic bursts is derived for the first time. The analysis suggested that the bursts can be roughly categorized into two types: one having periods shorter than 30 min and one with periods longer than 30 min, which is consistent with the results of the previous analysis of data from Ulysses’ first Jovian flyby [MacDowall, R.J., Kaiser, M.L., Desch, M.D., Farrell, W.M., Hess, R.A., Stone, R.G., 1993. Quasi-periodic Jovian radio bursts: observations from the Ulysses radio and plasma wave. Experiment. Planet. Space Sci. 41, 1059-1072]. It is also suggested that the groups of quasi-periodic bursts showed a dependence on the Jovian longitude of the sub-solar point, which means that these burst groups are triggered during a particular rotational phase of the planet. Maps of the occurrence probability of these quasi-periodic bursts also showed a unique CML/MLAT dependence. We performed a 3D ray tracing analysis of the quasi-periodic burst emission to learn more about the source distribution. The results suggest that the longitudinal distribution of the occurrence probability depends on the rotational phase. The source region of quasi-periodic bursts seems to be located at an altitude between 0.4 and 1.4 Rj above the polar cap region (L>30).     

Using large radio telescopes at decametre wavelengths

With the aim of evaluating the actual possibilities of doing, from the ground, sensitive radio astronomy at decametre wavelengths (particularly below ), an extensive program of radio observations was carried out, in 1999–2002, by using digital spectral and waveform analysers (DSP) of new generation, connected to several of the largest, decametre radio telescopes in the world (i.e., the UTR-2 and URANs arrays in Ukraine, and the Nançay Decametre Array in France).

We report and briefly discuss some new findings, dealing with decametre radiation from Jupiter and the Solar Corona: namely the discovery of new kinds of hyper fine structures in spectrograms of the active Sun, and a new characterisation of Jupiter's “millisecond” radiation, whose waveform samples, with time resolution down to 40 ns, and correlated measurements, by using far distant antennas (3000 km), have been obtained. In addition, scattering effects, caused by the terrestrial ionosphere and the interplanetary medium, could be disentangled through high time resolution and wide-band analyses of solar, planetary and strong galactic radio sources. Consequences for decametre wavelength imaging at high spatial resolution (VLBI) are outlined. Furthermore, in spite of the very unfavourable electromagnetic environment in this frequency range, a substantial increase in the quality of the observations was shown to be provided by using new generation spectrometers, based on sophisticated digital techniques. Indeed, the available, high dynamic range of such devices greatly decreases the effects of artificial and natural radio interference. We give several examples of successful signal detection in the case of much weaker radio sources than Solar System ones, down to the intensity level.

In summary, we conclude that searching for sensitivity improvement at the decametre wavelength is scientifically quite justified, and is now technically feasible, in particular by building giant, phased antenna arrays of much larger collecting area (as in the LOFAR project). In this task, one must be careful of some specifics of this wavelength range—somewhat unusual in “classical” radio astronomy—i.e., very high level and density of radio interference (telecommunications) and the variable terrestrial ionosphere.     

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.     

Jupiter’s radio spectrum from 74 MHz up to 8 GHz

We carried out a brief campaign in September 1998 to determine Jupiter’s radio spectrum at frequencies spanning a range from 74 MHz up to 8 GHz. Eleven different telescopes were used in this effort, each uniquely suited to observe at a particular frequency. We find that Jupiter’s spectrum is basically flat shortwards of 1-2 GHz, and drops off steeply at frequencies greater than 2 GHz. We compared the 1998 spectrum with a spectrum (330 MHz-8 GHz) obtained in June 1994, and report a large difference in spectral shape, being most pronounced at the lowest frequencies. The difference seems to be linear with log(ν), with the largest deviations at the lowest frequencies (ν).We have compared our spectra with calculations of Jupiter’s synchrotron radiation using several published models. The spectral shape is determined by the energy-dependent spatial distribution of the electrons in Jupiter’s magnetic field, which in turn is determined by the detailed diffusion process across L-shells and in pitch angle, as well as energy-dependent particle losses. The spectral shape observed in September 1998 can be matched well if the electron energy spectrum at L = 6 is modeled by a double power law E^−a (1+(E/E₀))^−b, with a = 0.4, b = 3, E₀ = 100 MeV, and a lifetime against local losses τ₀ = 6 × 10⁷ s. In June 1994 the observations can be matched equally well with two different sets of parameters: (1) a = 0.6, b = 3, E₀ = 100 MeV, τ₀ = 6 × 10⁷ s, or (2) a = 0.4, b = 3, E₀ = 100 MeV, τ₀ = 8.6 × 10⁶ s. We attribute the large variation in spectral shape between 1994 and 1998 to pitch angle scattering, coulomb scattering and/or energy degradation by dust in Jupiter’s inner radiation belts.     

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.     

VLA observations of Jupiter’s synchrotron radiation at 15 and 22 GHz

We observed Jupiter’s synchrotron radiation at frequencies of 15 and 22 GHz using the VLA (Very Large Array) in its most compact configuration (D-array) in March 1991. The spatial brightness distribution of the emission at these high frequencies appears to be very similar to that seen at lower frequencies (5 GHz down to 330 MHz). We measured a total nonthermal flux density at 15 and 22 GHz of 1.5 ± 0.15 Jy and 1.5 ± 0.4 Jy, respectively (both normalized to a geocentric distance of 4.04 AU). These numbers agree well with model spectra of Jupiter’s synchrotron radiation that were obtained by fitting the planet’s nonthermal radio emission between 74 MHz and 8 GHz and suggest a maximum cutoff in electron energies at ∼100 MeV. The degree of linear polarization observed with the VLA is 21.5 ± 1.9% at 15 GHz.     

Shadows in S/NB-events of jovian decametric emission

Narrow-band (NB) events in dynamic spectra and their relation with short (S-) bursts are an unresolved enigma of the jovian decametric emission. This paper is focused on the S/NB-structure with timescales between 0.03 s and 0.3 s. It is shown that the characteristic dash-line appearance of such narrow-band radiation in the dynamic spectrum could be considered as a result of superposition of numerous shadows events. To reproduce such shadows, the concept of the modulator is proposed. The modulator is an activating or amplifying agent, which drifts in the dynamic spectrum toward lower frequencies to stimulate the generation process in the radio source. After the source interaction, the modulator is shielded; one cannot stimulate the emission afterwards. Hence, the S/NB-emission shadows a certain region of the spectrum. This ‘shadow effect’ regularizes the random S-bursts or NB-oscillations into realistic structures in the synthetic spectrum. The resemblance between the real and the synthetic spectra is shown with 2D-correlation analysis.     

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.     

Low-frequency VLA observations of jupiter

We present the first images and measurement of flux density of Jupiter at a frequency of 74 MHz, obtained with the Very Large Array in September 1998. We observed simultaneously at frequencies of 74 and 330 MHz. We compare our data with observations taken during the same time at other frequencies (presented by de Pater and 13 others, 2003, Icarus 163, 434-448) and show that the spectrum of Jupiter appears to flatten, or perhaps turn over, at lower frequencies.     

Search for secular changes in the 3D profile of the synchrotron radiation around Jupiter

We present a summary of Jupiter data taken over an eighteen year span (1981-1998) by the Very Large Array at ∼21.0 cm. At this wavelength the emission is dominated by synchrotron radiation, which is roughly proportional to the product of the electron number density and magnetic field strength (N_eB). At each epoch 8-12 hours of data were taken, which allowed us to examine Jupiter during an entire rotation period. We mapped the longitudinal structure of the synchrotron radiation by using a 3D reconstruction technique developed by Sault et al. [Astron. Astrophys. 324 (1997) 1190] which enabled us to produce plots of the latitude, radial distance, and peak intensity vs. jovian longitude (System III). The results show the shape of the synchrotron radiation has remained stable (except, of course, during the period of comet Shoemaker-Levy 9 impacts). Specifically, the latitudinal structure has remained nearly constant. Furthermore, the general dependence of the radial intensity profile has remained the same throughout the years, though radial distance has slightly, though significantly, changed. This constancy implies that the spatial structure of both the particle distribution and magnetic field have varied little over the eighteen year span. The primary changes in the synchrotron radiation have been seen in the intensity of emission as a function of time. There are certain epochs (e.g., 1987) which show more emissivity than others (e.g., 1981, 1995) at all longitudes. When each epoch is longitudinally averaged, there may be an anti-correlation between the radial distance and corresponding peak intensities of the synchrotron radiation, as one might expect if radial diffusion is important. We examine these trends by comparing the data to plots of the total intensity at 13 cm (by Klein et al., in: Rucker, H.O., et al., Planetary Radio Emissions V. Austrian Acad. Sci. Press, Vienna, p. 221). Overall, variations in our 21-cm data are similar to those measured at 13 cm, but there appears to be a change in spectral index and perhaps in the spatial brightness distribution in 1992. We attribute this to a change in both the spatial and energy distribution of the relativistic electrons.     

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.     

The improved ARTEMIS IV multichannel solar radio spectrograph of the University of Athens

We present the improved solar radio spectrograph of the University of Athens operating at the Thermopylae Satellite Telecommunication Station. Observations now cover the frequency range from 20 to 650 MHz. The spectrograph has a 7-meter moving parabola fed by a log-periodic antenna for 100–650 MHz and a stationary inverted V fat dipole antenna for the 20–100 MHz range. Two receivers are operating in parallel, one swept frequency for the whole range (10 spectrums/sec, 630 channels/spectrum) and one acousto-optical receiver for the range 270 to 450 MHz (100 spectrums/sec, 128 channels/spectrum). The data acquisition system consists of two PCs (equipped with 12 bit, 225 ksamples/sec ADC, one for each receiver). Sensitivity is about 3 SFU and 30 SFU in the 20–100 MHz and 100–650 MHz range respectively. The daily operation is fully automated: receiving universal time from a GPS, pointing the antenna to the sun, system calibration, starting and stopping the observations at preset times, data acquisition, and archiving on DVD. We can also control the whole system through modem or Internet. The instrument can be used either by itself or in conjunction with other instruments to study the onset and evolution of solar radio bursts and associated interplanetary phenomena.     

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.     

Occurrence and source characteristics of the high-latitude components of Jovian broadband kilometric radiation

Ulysses had a “distant encounter” with Jupiter when it was within 0.8 AU of the planet during February, 2004. The passage of the spacecraft was from north to south, and observations of the Jovian radio waves were carried out for a few months from high to low latitudes (+80° to +10°) of Jupiter. The statistical study performed during this “distant encounter” event provided the occurrence characteristics of the Jovian broadband kilometric radiation (bKOM), including the high-latitude component as follows: (1) the emission intensity of bKOM was found to have a sinusoidal dependence with respect to the central meridian longitude (CML), showing a broad peak at ∼180°, (2) bKOM was preferably observed in the magnetic latitudinal range from ∼+30° to +90°, and the emission intensities at the high latitudes were found to be two times larger than that at the equatorial region, and (3) the emission intensity was controlled possibly by the sub solar longitude (SSL) of Jupiter. The intensity had a sharp peak around SSL ∼210°. A 3D ray tracing approach was applied to the bKOM in order to examine the source distribution. It was suggested that: (1) the R-X mode waves generated through the Cyclotron Maser Instability process would be unable to reproduce the intense high-latitude component of the bKOM, (2) the L-O mode, which was assumed to be generated at frequencies near the local plasma frequency, was considered to be the dominant mode for past and present observations at mid- and high-latitudinal regions, and (3) the high-latitude component of bKOM was found to have a source altitude of 0.9-1.5 Rj (Rj: Jovian radii), and to be distributed along magnetic field lines having L>10.     

Jovian emission at 18 cm during the comet S-L 9 impacts observed with RATAN-600 radio telescope

The monitoring of Jupiter synchrotron emission at 18 cm from July 15 till July 31 has shown a large flux density increase up to 29 per cents near July 22 and less but obvious disturbance in the next week.     

Observations of asteroids with ALMA

Thermal observations of large asteroids at millimeter wavelengths have revealed high amplitude rotational lightcurves. Such lightcurves are important constraints on thermophysical models of asteroids, and provide unique insight into the nature of their surface and subsurface composition. A better understanding of asteroid surfaces provides insight into the composition, physical structures, and processing history of these surviving remnants from the formation of our solar system. In addition, detailed observations of the larger asteroids, accompanied by thermophysical models with appropriate temporal and spatial resolution, promise to decrease uncertainties in their flux predictions. Of particular interest are the near-Earth objects, which can be observed at large phase angles, permitting better assessment of the thermal response of their unilluminated surfaces. The high sensitivity of ALMA will enable us to detect many small bodies in all the major groups, to obtain lightcurves for a large sample of main-belt and near-Earth objects, to resolve the surfaces of some large objects, and to separate the emission from primary and secondary objects in binary pairs. In addition to the science goals of asteroid studies, these bodies may also prove useful operationally because those with known shapes and well-characterized lightcurves could be employed for flux calibration by ALMA and other high frequency instruments.     

On the Origin of the Zebra Pattern with Pulsating Superfine Structures on 21 April 2002

Through the data around 3 GHz from the Radio Spectrometer in Huairou, Beijing, zebra-pattern structures from the 21 April 2002 event have been studied. Zebra stripes consist of periodically pulsating superfine structures in this event. An analysis of temporal profiles of intensities at multiple frequency channels shows that the Gaussian temporal profiles of pulse groups on zebra stripes are caused by drifting zebra stripes with Gaussian spectral profiles. The observed quasiperiodic pulsations with about 30 ms period have a peculiar feature of oscillation near a steady state, probably resulting from relaxation oscillations, which modulate the electron cyclotron maser emission that forms the zebra stripes during the process of wave – particle interactions. All the main properties of the zebra stripes with pulsating superfine structures indicate that the double plasma resonance model might be the most suitable one, with the relaxation oscillations, to form the superfine structures. The model of LaBelle et al. (Astrophys. J. 593, 1195, 2003) could not account for the observed properties of zebra-pattern structures in this event nor for most zebra-pattern structures occupying a wide frequency range, mainly because the allowable frequency range of the zebra-pattern structures in their model is too narrow to reproduce the observed zebras.     

Current state and development of the minor planets research in Nikolaev Astronomical Observatory

Regular positional observations of minor planets in Nikolaev Astronomical Observatory have been begun with installation of photographic Zone Astrograph in 1961. The observations of 19 selected minor planets up to 12 magnitude were obtained for 36 years. Accuracy of the photographic positions of minor planets is rather high, 0.15^′′-0.19^′′. These positions were used for improvement of the system of fundamental catalogue and determination of its orientation to the dynamical reference frame. CCD observations of asteroids have been begun at the Zone Astrograph in 2000. There was obtained about the same accuracy, as in photographic observations. During 2004-2006 NAO participated in international collaboration with TUBITAK National Observatory (Turkey) and Kazan State University (Russia) in positional and photometric observations of small Solar system bodies. About four thousands of CCD images for 58 asteroids of 11-18 mag were obtained with internal and external errors of 30-80 mas of a single determination. Some of these observations, as well as the observations of the Minor Planet Center, are being used for the current asteroid mass determinations in Nikolaev observatory. Available results allow us to consider the Russian-Turkish telescope RTT150 as a good candidate for ground-based astrometry support of the future space mission GAIA, moreover in the period before GAIA.