Sistemi radar per la rilevazione di corpi cosmici vicini

Abstract

Ground-based radio observations to probe the ozone content in the meteor region

Radar studies of the plasma irregularities produced by meteoroid ablation provide a powerful diagnostic probe of the Earth's atmosphere. This role is especially important as an inexpensive route for studying several atmospheric processes in comparison with other remote sensing techniques or satellite measurements. Ozone concentration has been indirectly measured in the upper mesosphere/lower thermosphere region by the BLM (Bologna-Lecce-Modra) Forward Scatter (FS) meteor radar by the detection of meteoroids interacting with the Earth's atmosphere. Results of variations of mesospheric ozone concentration at different height levels and time scales were deduced in 1992-2000 from the cumulative duration distributions of overdense echoes. Data of the BLM radar obtained in the last millennium decade confi rm the existence of a secondary ozone layer at atmospheric heights of 85-90 km and show a gradual yearly depletion of the ozone content, similarly to the decrease measured in the eighties by the Solar Mesosphere Explorer (SME) satellite throughout consecutive years (Rusch et al., 1990). Radio observations show in addition large seasonal variations at middle latitudes where the abundance at a secondary ozone maximum at 85-90 km is found to be as much as a factor of two higher in spring months than that in summer months

Progetto 2004/2.2: Misure di concentrazione di gas in traccia e delle caratteristiche ottiche delle particelle di aerosol a Baia Terra Nova e Dôme C (DO3meCO2)

La comprensione dei meccanismi che determinano la concentrazione dei gas in traccia che maggiormente influenzano il sistema climatico, in particolare CO2, O3, CH4 e CO, è diventato un argomento di primaria importanza per la comunità scientifica che si occupa di questioni climatiche. Analoga importanza rivestono inoltre le ricerche miranti a valutare gli effetti che a scala regionale possono essere prodotti da altri componenti dell’atmosfera come gli aerosol e le nubi. Nel caso della CO2, la conoscenza della distribuzione della sua concentrazione alle alte latitudini è molto importante perché, trovandosi a grandissima distanza da regioni vegetate ed industriali, consente di valutare con accuratezza non solo il rateo medio annuo di crescita in condizioni background, ma anche le piccole fluttuazioni annuali determinate da fenomeni a grande scala. Questi studi e misure permettono inoltre la validazione dei modelli che descrivono il ciclo del carbonio su scala globale. Nel caso dell’ozono, il ruolo che tale gas riveste nella fotochimica della troposfera e in particolare nel determinarne la capacità ossidativa, il suo ruolo di tracciante dei processi di scambio stratosfera-troposfera e dei processi di trasporti di masse d’aria inquinate, rende fondamentale la conoscenza del rapporto di rimescolamento al suolo e dei suoi andamenti stagionali ed interannuali. A dispetto della loro bassa concentrazione nell’area antartica, gli aerosol possono rivestire un ruolo molto importante nel sistema climatico antartico, in particolare a causa dei bassi valori di elevazione solare e agli alti valori di albedo della superficie, condizioni che amplificano notevolmente gli effetti radiativi diretti degli aerosol. La scarsità di siti di misura in Antartide e nella zona del Mare di Ross, rendono le misure effettuate presso la stazione Mario Zucchelli (MZS), nell’abito del Programma Nazionale Ricerche in Antartide (PNRA), oltremodo preziose per offrire una copertura della zona costiera del continente. Nel presente report si descrivono le attività sperimentali condotte presso la stazione di Campo Icaro, lungo la costa del Mare di Ross, nel periodo 31 ottobre 2004 -17 febbraio 2005. Inoltre sono riportati i risultati preliminari dell’analisi dei dati raccolti durante tale campagna di misura

UAV-aided calibration for commissioning of phased array radio telescopes

Calibration of antenna positions and instrumental response is a crucial step in the commissioning of a phased array radio telescope. The Low Frequency Aperture Array system of the Square Kilometre Array (SKA) is envisaged to consist of about 131,000 antennas. In this paper, we propose a strategy to efficiently conduct commissioning calibration of such a large phased array radio telescope using a near-field probe mounted on an nmanned Aerial Vehicle (UAV). We demonstrate the effectiveness of the proposed method using simulations. This indicates that potentially cost-saving relaxation of requirements on placement accuracy is possible. We also propose to validate this method in practice using the Low Frequency Array (LOFAR)

Comparison between Measured and Simulated Antenna Patterns for a LOFAR LBA array

A UAV-based system has been employed for a measurement campaign on a station of the radio telescope LOFAR to characterize the individual Low Band Antenna patterns. The experimental set-up has been then simulated with a full-wave software and numerical embedded element patterns have been compared to the measured results. A statistical analysis of the differences between the two data sets has been finally carried out to estimate the accuracy of the electromagnetic model

The Northern Cross fast radio burst project–I: overview and pilot observations at 408 MHz

Fast radio bursts (FRBs) remain one of the most enigmatic astrophysical sources. Observations have significantly progressed over the last few years, due to the capabilities of new radio telescopes and the refurbishment of existing ones. Here, we describe the upgrade of the Northern Cross radio telescope, operating in the 400–416 MHz frequency band, with the ultimate goal of turning the array into a dedicated instrument to survey the sky for FRBs

The Northern Cross Fast Radio Burst project -- III. The FRB-magnetar connection in a sample of nearby galaxies

Fast radio bursts (FRBs) are millisecond radio transients observed at cosmological distances. The nature of their progenitors is still a matter of debate, although magnetars are invoked by most models. The proposed FRB-magnetar connection was strengthened by the discovery of an FRB-like event from the Galactic magnetar SGR J1935+2154. In this work, we aim to investigate how prevalent magnetars such as SGR J1935+2154 are within FRB progenitors. We carried out an FRB search in a sample of seven nearby (< 12 Mpc) galaxies with the Northern Cross radio telescope for a total of 692 h. We detected one 1.8 ms burst in the direction of M101 with a fluence of $58 \pm 5$ Jy ms. Its dispersion measure of 303 pc cm$^{-3}$ places it most-likely beyond M101. Considering that no significant detection comes indisputably from the selected galaxies, we place a 38 yr$^{-1}$ upper limit on the total burst rate (i.e. including the whole sample) at the 95\% confidence level. This upper limit constrains the event rate per magnetar $\lambda_{\rm mag} < 0.42$ magnetar$^{-1}$ yr$^{-1}$ or, if combined with literature observations of a similar sample of nearby galaxies, it yields a joint constraint of $\lambda_{\rm mag} < 0.25$ magnetar$^{-1}$ yr$^{-1}$. We also provide the first constraints on the expected rate of FRBs hypothetically originating from ultraluminous X-ray (ULX) sources, since some of the galaxies observed during our observational campaign host confirmed ULXs. We obtain $< 13$ yr$^{-1}$ per ULX for the total sample of galaxies observed. Our results indicate that bursts with energies $E>10^{34}$ erg from magnetars like SGR J1935+2154 appear more rarely compared to previous observations and further disfavour them as unique progenitors for the cosmological FRB population, leaving more space open to the contribution from a population of more exotic magnetars, not born via core-collapsed supernovae.Comment: 9 pages, 4 figures, published in A&