The Case for Combining a Large Low-Band Very High Frequency Transmitter With Multiple Receiving Arrays for Geospace Research: A Geospace Radar

We argue that combining a high‐power, large‐aperture radar transmitter with several large‐aperture receiving arrays to make a geospace radar—a radar capable of probing near‐Earth space from the upper troposphere through to the solar corona—would transform geospace research. We review the emergence of incoherent scatter radar in the 1960s as an agent that unified early, pioneering research in geospace in a common theoretical, experimental, and instrumental framework, and we suggest that a geospace radar would have a similar effect on future developments in space weather research. We then discuss recent developments in radio‐array technology that could be exploited in the development of a geospace radar with new or substantially improved capabilities compared to the radars in use presently. A number of applications for a geospace radar with the new and improved capabilities are reviewed including studies of meteor echoes, mesospheric and stratospheric turbulence, ionospheric flows, plasmaspheric and ionospheric irregularities, and reflection from the solar corona and coronal mass ejections. We conclude with a summary of technical requirements

A deep search for prompt radio emission from the short GRB 150424A with the Murchison Widefield Array

© 2015. The American Astronomical Society. All rights reserved. We present a search for prompt radio emission associated with the short-duration gamma-ray burst (GRB) 150424A using the Murchison Widefield Array (MWA) at frequencies from 80 to 133 MHz. Our observations span delays of 23 s-30 minutes after the GRB, corresponding to dispersion measures of 100-7700 pc cm-3. We see no excess flux in images with timescales of 4 s, 2 minutes, or 30 minutes and set a 3ó flux density limit of 3.0 Jy at 132 MHz on the shortest timescales: some of the most stringent limits to date on prompt radio emission from any type of GRB. We use these limits to constrain a number of proposed models for coherent emission from shortduration GRBs, although we show that our limits are not particularly constraining for fast radio bursts because of reduced sensitivity for this pointing. Finally, we discuss the prospects for using the MWA to search for prompt radio emission from gravitational wave (GW) transients and find that while the flux density and luminosity limits are likely to be very constraining, the latency of the GW alert may limit the robustness of any conclusions

The GMRT 150 MHz all-sky radio survey: First alternative data release TGSS ADR1

We present the first full release of a survey of the 150 MHz radio sky, observed with the Giant Metrewave Radio Telescope (GMRT) between April 2010 and March 2012 as part of the TIFR GMRT Sky Survey (TGSS) project. Aimed at producing a reliable compact source survey, our automated data reduction pipeline efficiently processed more than 2000 h of observations with minimal human interaction. Through application of innovative techniques such as image-based flagging, direction-dependent calibration of ionospheric phase errors, correcting for systematic offsets in antenna pointing, and improving the primary beam model, we created good quality images for over 95 percent of the 5336 pointings. Our data release covers 36 900 deg2 (or 3.6 p steradians) of the sky between-53° and +90° declination (Dec), which is 90 percent of the total sky. The majority of pointing images have a noise level below 5 mJy beam-1 with an approximate resolution of 25''×25'' (or 25''×25''/cos(Dec-19°) for pointings south of 19° declination). We have produced a catalog of 0.62 Million radio sources derived from an initial, high reliability source extraction at the seven sigma level. For the bulk of the survey, the measured overall astrometric accuracy is better than two arcseconds in right ascension and declination, while the flux density accuracy is estimated at approximately ten percent. Within the scope of the TGSS alternative data release (TGSS ADR) project, the source catalog, as well as 5336 mosaic images (5°×5°) and an image cutout service, are made publicly available at the CDS as a service to the astronomical community. Next to enabling a wide range of different scientific investigations, we anticipate that these survey products will provide a solid reference for various new low-frequency radio aperture array telescopes (LOFAR, LWA, MWA, SKA-low), and can play an important role in characterizing the epoch-of-reionisation (EoR) foreground. The TGSS ADR project aims at continuously improving the quality of the survey data products. Near-future improvements include replacement of bright source snapshot images with archival targeted observations, using new observations to fill the holes in sky coverage and replace very poor quality observational data, and an improved flux calibration strategy for less severely affected observational data

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

EFFECTS OF CHANGING OCCUPANCY REQUIREMENTS FOR HIGH-OCCUPANCY VEHICLE LANE: EL MONTE BUSWAY CASE STUDY, JULY 23, 2002

In 1999, the California Legislature passed Senate Bill 63, lowering the vehicle-occupancy requirement on the El Monte Busway on the San Bernardino Interstate 10 Freeway from three persons per vehicle (3+) to two persons per vehicle (2+). The California Department of Transportation (Caltrans) was directed to implement this change on January 1, 2000, and to monitor and evaluate the effects of the 2+ requirement on the operation of the busway and the freeway. Based on the operational effects of the change, new legislation was approved increasing the vehicle-occupancy requirement back to 3+ during the morning and afternoon peak periods effective July 24, 2000, with a 2+ requirement at other times. Information is presented on the effect of the change in the vehicle-occupancy requirement on operation of the busway and freeway, public transit services, violation rates, accidents, and public response. The assessment is based on information from Caltrans, Foothill Transit, the Los Angeles County Metropolitan Transportation Authority, and the California Highway Patrol. Lowering the vehicle-occupancy requirement had a detrimental effect on the busway. At the same time, significant improvements were not realized in the general-purpose freeway lanes. Morning peak-period average travel speeds on the busway were reduced from 65 to 20 mph, while travel speeds in the general-purpose lanes decreased from 25 to 23 mph. Morning peak-period busway vehicle volumes increased from 1,100 to 1,600 with the 2+ designation, but the number of persons carried declined from 5,900 to 5,200. The freeway lane vehicle volumes and passengers per lane per hour remained relatively similar. Peak-period travel times on the busway increased from 20 to 30 minutes. Bus schedule adherence and on-time performance declined significantly and passengers reported delays