Measuring plasma parameters in the upper solar corona and inner heliosphere is challenging because of the region’s weakly emissive nature and inaccessibility for most in situ observations. Radio imaging of broadened and distorted background astronomical radio sources during solar conjunction can provide unique constraints for the coronal material along the line of sight. In this study, we present radio spectral imaging observations of the Crab Nebula (Tau A) from 2024 June 9 to June 22 when it was near the Sun with a projected heliocentric distance of 5–27 solar radii, using the Owens Valley Radio Observatory’s Long Wavelength Array at multiple frequencies in the 30–80 MHz range. The imaging data reveal frequency-dependent broadening and distortion effects caused by anisotropic wave propagation through the turbulent solar corona at different distances. We analyze the brightness, size, and anisotropy of the broadened images. Our results provide detailed observations showing that the eccentricity of the unresolved source increases as the line of sight approaches the Sun, suggesting a higher anisotropic ratio of the plasma turbulence closer to the Sun. In addition, the major axis of the elongated source is consistently oriented in the direction perpendicular to the radial direction, suggesting that the turbulence-induced scattering effect is more pronounced in the direction transverse to the coronal magnetic field. Lastly, when the source undergoes large-scale refraction as the line of sight passes through a streamer, the apparent source exhibits substructures at lower frequencies. This study demonstrates that observations of celestial radio sources with lines of sight near the Sun provide a promising method for measuring turbulence parameters in the inner heliosphere.

A number of theoretical studies have proposed a prompt or precursor low-frequency radio counterpart to gravitational wave events detected by LIGO and Virgo. Detection of such events would offer a new window on the immediate environment of the merger and provide an avenue to rapid localization. However, identifying fast transients in real-time in localization regions spanning hundreds to thousands of square degrees presents severe technical challenges. To address these challenges, we present a novel technique embodied in the Time Machine, a system featuring a two-stage voltage buffer and subsequent processing pipeline designed for the Long Wavelength Array at the Owens Valley Radio Observatory. This array is developed to instantaneously image the entire viewable sky. We detail the system’s buffer structure that allows data collection from several minutes before a trigger event, up to 30 minutes after an event. The processing of this voltage data involves beamforming and searching the full 90th-percentile localization region above the horizon with ms-time resolution and the ability to detect events with ∼100 Jy ms (7 σ ) fluence within the 55–85 MHz band. Furthermore, we incorporate an offline cross-correlation pipeline to improve positional accuracy of identified transients to within subarcminute levels. We present a full overview of the system design and initial testing results.

Here we present near vertical angle of arrival and Doppler observations using a Digisonde Portable Sounder 4D (DPS4D) as a transmitter and the Long Wavelength Array at Sevilleta (LWA‐SV) radio telescope as a receiver, over an 80‐km baseline. With 256 dual‐polarization dipole antennas, LWA‐SV provides unparalleled sensitivity and angular resolution to high frequency sounding measurements of traveling ionospheric disturbances (TIDs). We conducted a study over 3 days in March 2018 and detected several TIDs ranging from 10‐ to 45‐min periods. These medium scale TIDs were detected using an implementation of frequency‐and‐angular sounding (FAS), which was first described in Beley et al. (1995, https://doi.org/10.1029/95RS01992). FAS assumes an ionospheric mirror model and provides estimates of the wavelength, direction, and speed of TIDs using the measured angle of arrival and Doppler of a high frequency ionospheric reflection. We experiment with this method and show that LWA‐SV is capable of achieving observations that are unmatched in sensitivity and resolution, allowing for measurements limited primarily by the errors accumulated by the assumptions within different manifestations of the FAS algorithm.

Modern techniques for specifying the ionosphere include the use of ionosondes, which both transmit and receive sweep soundings in the high‐frequency (HF) band. We replicate this process by observing the broad band emission from lightning using the Long Wavelength Array, Sevilleta, radio telescope. We use this station to observe the powerful broadband radio bursts from the breakdown of air that occurs during lightning flashes. For nearby lightning we observe both the direct line of sight and the delayed ionospheric reflection. By correlating the amplitude time series from the direct line of sight to that of the ionospheric reflection, we can accurately measure the group delay as a function of frequency. By separating into right‐hand circular and left‐hand circular, we can derive both the O mode and X mode ionograms. This novel technique allows for accurate ionograms to be made even in restricted frequency bands and provides a means to probe density profiles at multiple locations of the ionosphere on short time scales with a single receiver station.

Abstract Utilizing the all‐sky imaging capabilities of the first station of the Long Wavelength Array along with a host of all‐sky optical cameras, we have now observed 44 optical meteor counterparts to radio afterglows. Combining these observations, we have determined the geographic positions of all 44 afterglows. Comparing the number of radio detections as a function of altitude above sea level to the number of expected bright meteors, we find a strong altitudinal dependence characterized by a cutoff below ∼90 km, below which no radio emission occurs, despite the fact that many of the observed optical meteors penetrated well below this altitude. This cutoff suggests that wave damping from electron collisions is an important factor for the evolution of radio afterglows. This finding agrees with the hypothesis that the emission is the result of electron plasma wave emission.

Using the narrowband all-sky imager mode of the LWA1 we have now detected 30 transients at 25.6 MHz, 1 at 34 MHz, and 93 at 38.0 MHz. While we have only optically confirmed that 37 of these events are radio afterglows from meteors, evidence suggests that most, if not all, are. Using the beam-forming mode of the LWA1 we have also captured the broadband spectra between 22.0 and 55.0 MHz of four events. We compare the smooth, spectral components of these four events and fit the frequency dependent flux density to a power law, and find that the spectral index is time variable, with the spectrum steepening over time for each meteor afterglow. Using these spectral indices along with the narrow band flux density measurements of the 123 events at 25.6 and 38 MHz, we predict the expected flux densities and rates for meteor afterglows potentially observable by other low frequency radio telescopes.

We present a new passive, bistatic high‐frequency (HF) radar system consisting of the transmitters for the radio station WWV and the dipole antenna array that comprises the first station of the Long Wavelength Array (LWA) or “LWA1.” We demonstrate that these two existing facilities, which are operated for separate purposes, can be used together as a unique HF radar imager, capable of monitoring the entire visible sky. In this paper, we describe in detail the techniques used to develop all‐sky radar capability at 10, 15, and 20 MHz. We show that this radar system can be a useful tool for probing ionospheric structure and its effect on over‐the‐horizon (OTH) geolocation. The LWA1+WWV radar system appears to be especially adept at detecting and characterizing structures associated with sporadic‐E. In addition, we also demonstrate how this system may be used for long‐distance, OTH mapping of terrain/ocean HF reflectivity. Finally, we discuss the potential improvements in the utility of these applications as more LWA stations are added.

After the newly constructed LEDA outriggers at LWA1 became operational Hank Tillman and Steve Ellingson performed TBN observations on September 5, 2013. They only looked at the 5 outriggers and performed dipole-dipole correlations for those baselines. They noted discrepancies in the magnitude and phase of the correlation products going from baseline to baseline. More importantly they reported a strong DC offset/component in the fringe rate plots for most of the baselines that desensitizes them to the point that they are not useable as-is. This initial report lead to the detailed analysis of the situation documented in this memo.

The first station of the Long Wavelength Array, LWA1, is a new low frequency radio telescope currently operating in New Mexico, USA. The primary data products delivered by the telescope consist of raw voltage time series data. As such, the data have large volumes with a high degree of flexibility. The LWA Software Library (LSL) is designed to process these data and provides a variety of signal processing functions that are not generally available. LSL also serves as the software available on the LWA1 User's Computing Facility, a six node cluster available to LWA1 observers for data reduction.

The initial cluster mass function (ICMF) is a fundamental property of star formation in galaxies. To gauge its universality, we measure and compare the ICMFs in irregular and spiral galaxies. Our sample of irregular galaxies is based on 13 nearby galaxies selected from a volume-limited sample from the fifth data release of the Sloan Digital Sky Survey (SDSS). The extinctions, ages, and masses were determined by comparing their u'g'i'z' magnitudes to those generated from starburst models. Completeness corrections were performed using Monte Carlo simulations in which artificial clusters were inserted into each galaxy. We analyzed three nearby spiral galaxies with SDSS data in exactly the same way to derive their ICMF based on a similar number of young, massive clusters as the irregular galaxy ICMF. We find that the ICMFs of irregular and spiral galaxies for masses >3x10^4 M_sun are statistically indistinguishable. For clusters more massive than 3x10^4 M_sun, the ICMF of the irregular galaxies is reasonably well fit by a power law dN(M)/dM M^-a_M with a_M = 1.88 +/- 0.09. Similar results were obtained for the ICMF of the spiral galaxy sample but with a_M = 1.75 +/- 0.06. We discuss the implications of our result for theories of star cluster formation: the shape of the ICMF appears to be independent of metallicity and galactic shear rate.