Pierre Kaufmann

Metal mesh resonant filters for terahertz frequencies

The interest in terahertz photometric and imaging measurements has motivated the development of bandpass resonant filters to be coupled to multiple-pixel devices such as bolometer arrays. Resonant grids are relatively simple to fabricate, exhibiting high transmission at the central frequency, a narrow bandpass, and good rejection of the side frequencies of the spectrum. We have fabricated filters centered at different frequencies between 0.4 and 10 THz, using photolithography and electroforming techniques. Transmission measurements have shown center frequencies and bandwidths close to the design predictions. The performance of the filters was found not to be critically dependent on small physical deformations in the mesh, becoming more noticeable at higher frequencies (i.e., for smaller physical sizes). Wider bandwidths, needed to attain higher sensitivities in the continuum, were obtained by changing the design parameters for filters at 2 and 3 THz.

New telescopes for ground-based solar observations at submillimeter and mid-infrared

The solar submillimeter-wave telescope (SST) is the only one of its kind dedicated to solar continuous observations. Two radiometers at 0.740 mm (405 GHz), and four at 1.415 mm (212 GHz) are placed in the Cassegrain focal plane of the 1.5-m dish at El Leoncito high altitude site, San Juan, Argentina. The aperture efficiencies are close to design predictions: 20% and 35% for 2 and 4 arcminutes beam sizes at 405 and 212 GHz, respectively. The positioner absolute pointing accuracy is 10 arcseconds. Spectral coverage is complemented by ground-based mid-infrared telescopes developed for high cadence observations in the continuum 10 micron band (30 THz), using small apertures and room-temperature microbolometer cameras. Using the system, a new solar burst spectral component was discovered, exhibiting fluxes increasing for smaller wavelengths, separated from the well known microwave component. Rapid sub-second pulsations are common for all bursts. The pulsations onset times of appear to be connected to the launch times of CMEs. Active regions are brighter for shorter submillimeter-waves. Mid-IR bright regions are found closely associated with calcium plages and magnetic structures near the solar photosphere. Intense and rapid 10 micron brightening was detected on active centers in association with weak flares. These results raise challenging difficulties for interpretation.

Can microbunch instability on solar flare accelerated electron beams account for bright broadband coherent synchrotron microwaves

The physical processes producing bright broadband coherent synchrotron radiation (CSR) bursts in laboratory accelerators are proposed to happen also in solar flares, bringing a plausible explanation to serious interpretation constraints raised by the discovery of a solar flare sub-mm-wave spectral emission component peaking in the terahertz (THz) range simultaneous to the well-known microwaves component. The THz component is due to incoherent synchrotron radiation (ISR) produced by a beam of ultrarelativistic electrons. Beam density perturbations, on a scale of the order of or smaller than the emitting wavelength, sets a microbunch instability producing the intense CSR at lower frequencies. Hard x-ray/γ-ray emissions may include a significant synchrotron emission component from the same ISR spectrum, bringing a new possibility to explain the so called “solar flare electron number paradox.”

Simulating the emission of electromagnetic waves in the terahertz range by relativistic electron beams

Aims. We investigate the dynamics of relativistic electron beams propagating along a uniform magnetic field and the emission process of electromagnetic waves within the terahertz range from the solar photosphere. Our aim is to understand a new solar burst component emitting only in the terahertz range during the solar flare observed by Kaufmann et al. (2004). Methods. We used a 2D3V fully relativistic electromagnetic particle-in-cell (PIC) simulation. Results. We did three different kinds of simulations. The first simulation confirmed that the growth rate of relativistic electron beam instability agrees well with the theoretical estimation. From the second simulation of the electron beam with finite width, we found that the beams are confined along the magnetic field and the electromagnetic waves are generated forward of the electron beams. Some fraction of the electrons are accelerated more than the initial beam velocity. From the third simulation where the electron beams propagate into the high density region, we found that strong electromagnetic waves are generated backward to the electron beams. We also found that the higher frequency emission like 405 GHz, which originate in the strong magnetic field region, becomes stronger than the 212 GHz emission, as shown in the observation by Kaufmann et al. (2004). These simulation results could be applied to the electromagnetic wave emission from the solar photosphere during the solar flares.

Observations of BL Lacertae from the Geodetic VLBI Archive of the Washington Correlator

We present maps of BL Lac obtained from geodetic VLBI data from the archive of the Washington correlator. The observations were made from 1996 March to 1996 November, with periods from one experiment to another as short as 1 month. The dominant structure of the maps is given by a superluminal component (C2) moving predominantly at position angle ~190°. At the later epochs a new superluminal component (C3) emerges from the core and moves along a trajectory at a higher position angle (200°). We also include maps from observations made in 1995 June and August showing the main component (C2) in the early stages of its evolution, as well as an older component (C1). The position angle of the component C1 is nearly the same as that of the new component C3. The component C2 shows indications of nonradial motion, which is discussed in terms of a helically distorted jet. The best fit was obtained for a half-opening angle of the jet of 26 and an angle of the helical axis to the line of sight of 17°.

The effect of a total solar eclipse on long path VLF transmission

Abstract The effects of the total solar eclipse of 12 November 1966 on VLF transmissions at 26·1 kHz transmitted from NPM, Hawaii, and received at Sao Paulo, are presented. The eclipse occurred during the sunrise transition period, making the data interpretation rather difficult. There was a clearly-marked change of phase, corresponding to a maximum delay of 12·3 sec.

Some relationships between solar X-ray bursts and SPA's produced on VLF propagation in the lower ionosphere

This paper discusses SPAs measured at long VLF propagation paths in the lower ionosphere and their association with solar X-ray bursts observed by USNRL satellites in the 0–3 Å, 0–8 Å and 8–20 Å bands. Excellent correlations were found between the SPA importances (in degrees per Mm) and the logarithm of the X-ray burst peak intensities. A hardening of the X-ray burst spectra is evident for increasing importance of SPAs; the threshold energy required for the occurrence of such anomalies was estimated, it is 4.3×10−5 ergs cm−2 sec−1 in the main ionizing band of 0–3 Å. It was also possible to derive the effective recombination coefficient at the normal D-region height of 70 km, this beingαr≈6×10−6 cm3 sec−1; furthermore ion production rates were estimated during SPAs at heights below the reference level.

A high-energy solar flare burst complex and the physical properties of its source region

We discuss a solar flare microwave burst complex, which included a major structure consisting of some 13 spikes of 60 ms FWHM each, observed 21 May, 1984 at 90 GHz (3 mm). It was associated with a simultaneous very hard X-ray burst complex. We suggest that the individual spikes of both bursts were caused by the same electron population: the X-bursts by their bremsstrahlung, and the microwave bursts by their gyrosynchrotron emission. This latter conclusion is based on the evidence that the radio turnover frequency was ≤ 150 GHz. It follows that the emission sources were characterized by an electron density of about 1011 cm−3, a temperature of 5 × 108 K and a magnetic field of about 1400–2000 G. They had a size of about 350 km; if the energy release is caused by reconnection the sources of primary instability could have been smaller and in the form of thin sheets with reconnection speed at a fraction of the Alfvén velocity and burst-like energy injections of ≈ 1027 erg during about 50 ms each. The energized plasma knots lost their injection energy by saturated convective flux (collisionless conduction) in about 30 ms.

Rapid Pulsations in Sub-THz Solar Bursts

A new solar burst emission spectral component has been found showing sub-THz fluxes increasing with frequency, spectrally separated from the well known microwave component. Rapid pulsations are found present in all events observed at the two frequencies of the solar submillimeter-wave telescope: 212 and 405 GHz. They were studied in greater detail for three solar bursts exhibiting the new THz spectral component. The pulse amplitudes are of about 5%-8% of the mean flux throughout the bursts durations, being comparable for both frequencies. Pulsations range from one pulse every few seconds to 8-10 per second. The pulse repetition rates (R) are linearly proportional to the mean burst fluxes (S), following the simple relationship S = kR, suggesting that the pulsations might be the response to discrete flare particle accelerator injections quantized in energy. Although this result is consistent with qualitative trends previously found in the GHz range, the pulse amplitude relative to the mean fluxes at the sub-THz frequencies appear to be nearly ten times smaller than expected from the extrapolation of the trends found in the GHz range. However there are difficulties to reconcile the nearly simultaneous GHz and THz burst emission spectrally separated components, exhibiting rapid pulsations with considerably larger relative intensities in the GHz range.

PROPERTIES OF FAST SUBMILLIMETER TIME STRUCTURES DURING A LARGE SOLAR FLARE

We report properties of fast varying submillimeter emission during one of the strongest solar radio flares of solar cycle 23. Emission was obtained by the Solar Submillimeter-Wave Telescope at 212 and 405 GHz and compared with hard X-ray and γ-ray counts up to few tens of MeV photon energy ranges. We employ different methods to detect and characterize flux density variations and find that during the impulsive phase of the event, the closer in time to the peak flare, the higher the occurrence of the fastest and brightest time structures. The good comparison with hard X-ray and γ-ray count rates indicates that fast submillimeter pulses are the signatures of primary energetic injections. The characteristics of the fast spikes at 212 and 405 GHz, such as their flux density and localization, compared to those of the underlying slower impulsive component, also suggest that their nature is different.