Nanodust detection near 1 AU from spectral analysis of Cassini/RPWS radio data

Nanodust grains of a few nanometer in size are produced near the Sun by collisional break-up of larger grains and picked-up by the magnetized solar wind. They have so far been detected at 1 AU by only the two STEREO spacecraft. Here we analyze the spectra measured by the radio and plasma wave instrument onboard Cassini during the cruise phase close to Earth orbit; they exhibit bursty signatures similar to those observed by the same instrument in association to nanodust stream impacts on Cassini near Jupiter. The observed wave level and spectral shape reveal impacts of nanoparticles at about 300 km/s, with an average flux compatible with that observed by the radio and plasma wave instrument onboard STEREO and with the interplanetary flux models

Radio pulses from cosmic ray air showers - Boosted Coulomb and Cherenkov fields

High-energy cosmic rays passing through the Earth's atmosphere produce extensive showers whose charges emit radio frequency pulses. Despite the low density of the Earth's atmosphere, this emission should be affected by the air refractive index because the bulk of the shower particles move roughly at the speed of radio waves, so that the retarded altitude of emission, the relativistic boost and the emission pattern are modified. We consider in this paper the contribution of the boosted Coulomb and the Cherenkov fields and calculate analytically the spectrum using a very simplified model in order to highlight the main properties. We find that typically the lower half of the shower charge energy distribution produces a boosted Coulomb field, of amplitude comparable to the levels measured and to those calculated previously for synchrotron emission. Higher energy particles produce instead a Cherenkov-like field, whose amplitude may be smaller because both the negative charge excess and the separation between charges of opposite signs are small at these energies.Comment: 10 figures - Accepted by Astronomy & Astrophysic

Dust detection by the wave instrument on STEREO: nanoparticles picked up by the solar wind?

The STEREO/WAVES instrument has detected a very large number of intense voltage pulses. We suggest that these events are produced by impact ionisation of nanoparticles striking the spacecraft at a velocity of the order of magnitude of the solar wind speed. Nanoparticles, which are half-way between micron-sized dust and atomic ions, have such a large charge-to-mass ratio that the electric field induced by the solar wind magnetic field accelerates them very efficiently. Since the voltage produced by dust impacts increases very fast with speed, such nanoparticles produce signals as high as do much larger grains of smaller speeds. The flux of 10-nm radius grains inferred in this way is compatible with the interplanetary dust flux model. The present results may represent the first detection of fast nanoparticles in interplanetary space near Earth orbit.Comment: In press in Solar Physics, 13 pages, 5 figure

Nano dust impacts on spacecraft and boom antenna charging

High rate sampling detectors measuring the potential difference between the main body and boom antennas of interplanetary spacecraft have been shown to be efficient means to measure the voltage pulses induced by nano dust impacts on the spacecraft body itself (see Meyer-Vernet et al, Solar Phys. 256, 463 (2009)). However, rough estimates of the free charge liberated in post impact expanding plasma cloud indicate that the cloud's own internal electrostatic field is too weak to account for measured pulses as the ones from the TDS instrument on the STEREO spacecraft frequently exceeding 0.1 V/m. In this paper we argue that the detected pulses are not a direct measure of the potential structure of the plasma cloud, but are rather the consequence of a transitional interruption of the photoelectron return current towards the portion of the antenna located within the expanding cloud

Jet-gas interactions in z~2.5 radio galaxies: evolution of the ultraviolet line and continuum emission with radio morphology

We present an investigation into the nature of the jet-gas interactions in a sample of 10 radio galaxies at 2.3<z<2.9 using deep spectroscopy of the UV line and continuum emission obtained at Keck II and the Very Large Telescope. Kinematically perturbed gas, which we have shown to be within the radio structure in previous publications, is always blueshifted with respect to the kinematically quiescent gas, is usually spatially extended, and is usually detected on both sides of the nucleus. In the three objects from this sample for which we are able to measure line ratios for both the perturbed and quiescent gases, we suggest that the former has a lower ionization state than the latter. We propose that the perturbed gas is part of a jet-induced outflow, with dust obscuring the outflowing gas that lies on the far side of the object. The spatial extent of the blueshifted perturbed gas, typically ~35 kpc, implies that the dust is spatially extended at least on similar spatial scales. We also find interesting interrelationships between UV line, UV continuum and radio continuum properties of this sample.Comment: Accepted for publication in MNRA

Deep spectroscopy of the FUV-optical emission lines from a sample of radio galaxies at z~2.5: metallicity and ionization

We present long-slit NIR spectra, obtained using the ISAAC instrument at the Very Large Telescope, for nine radio galaxies at z~2.5. One-dimensional spectra have been extracted and cross calibrated with optical spectra from the literature to produce line spectra spanning a rest wavelength of ~1200-7000 A. We have also produced a composite of the rest-frame UV-optical line fluxes of powerful, z~2.5 radio galaxies. We have investigated the relative strengths of Ly-alpha, H-beta, H-alpha, HeII 1640 and HeII 4687, and we find that Av can vary significantly from object to object. In addition, we identify new line ratios to calculate electron temperature: [NeV] 1575/[NeV] 3426, [NeIV] 1602/[NeIV] 2423, OIII] 1663/[OIII] 5008 and [OII] 2471/[OII]3728. We model the emission line spectra and conclude they are best explained by AGN-photoionization with the ionization parameter U varying between objects. Single slab photoionization models are unable to reproduce the high- and the low-ionization lines simultaneously: this may be alleviated either by combining two or more single slab photoionization models with different U, or by using mixed-medium models such as those of Binette, Wilson & Storchi-Bergmann (1996). On the basis of NV/NIV] and NIV]/CIV we argue that shocks make a fractional contribution to the ionization of the EELR. We find that in the EELR of z~2 radio galaxies the N/H abundance ratio is close to its solar value. We conclude that N/H and metallicity do not vary by more than a factor of two in our sample. This is consistent with the idea that massive ellipticals are assembled very early in the history of the universe, and then evolve relatively passively up to the present day.Comment: Accepted for publication by MNRA