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The Long Wavelength Array is a radio telescope array located at the Sevilleta National Wildlife Refuge in La Joya, New Mexico, well suited and situated for the observation of lightning. The array consists of 256 high-sensitivity dual polarization antennas arranged in a 100 m diameter. This paper demonstrates some of the capabilities that the array brings to the study of lightning. Once 32 or more antennas are used to image lightning radio sources, virtually every integration period longer than the impulse response of the array includes at least one identifiable lightning emitter, independent of the integration period used. The use of many antennas also allows multiple simultaneous lightning radio sources to be imaged at sub-microsecond timescales; for the flash examined, 51% of the images contained more than one lightning source. Finally, by using many antennas to image lightning sources, the array is capable of locating sources fainter than the galactic background radio noise level, yielding possibly the most sensitive radio maps of lightning to date. This incredible sensitivity enables, for the first time, the emissions originating from the positive leader tips of natural in-cloud lightning to be detected and located. The tip emission is distinctly different from needle emission and is most likely due to positive breakdown.

 

We present recent 2-port vector network analyzer (VNA) measurements of the complete set of scattering parameters for the antenna used within the Long Wavelength Array (LWA) and the associated front end electronics (FEEs). Full scattering parameter measurements of the antenna yield not only the reflection coefficient for each polarization, S11 and S22, but also the coupling between polarizations, S12 and S21. These had been previously modeled using simulations, but direct measurements had not been obtained until now. The measurements are used to derive a frequency dependent impedance mismatch factor (IMF) which represents the fraction of power that is passed through the antenna–FEE interface and not reflected due to a mismatch between the impedance of the antenna and the impedance of the FEE. We also present results from a two-antenna experiment where each antenna is hooked up to a separate port on the VNA. This allows for cross–antenna coupling to be measured for all four possible polarization combinations. Finally, we apply the newly measured IMF and FEE forward gain corrections to LWA data to investigate how well they remove instrumental effects.

 

Next-generation aperture arrays are expected to consist of hundreds to thousands of antenna elements with substantial digital signal processing to handle large operating bandwidths of a few tens to hundreds of MHz. Conventionally, FX correlators are used as the primary signal processing unit of the interferometer. These correlators have computational costs that scale as $\mathcal {O}(N^2)$ for large arrays. An alternative imaging approach is implemented in the E-field Parallel Imaging Correlator (EPIC) that was recently deployed on the Long Wavelength Array station at the Sevilleta National Wildlife Refuge (LWA-SV) in New Mexico. EPIC uses a novel architecture that produces electric field or intensity images of the sky at the angular resolution of the array with full or partial polarization and the full spectral resolution of the channelizer. By eliminating the intermediate cross-correlation data products, the computational costs can be significantly lowered in comparison to a conventional FX or XF correlator from $\mathcal {O}(N^2)$ to $\mathcal {O}(N \log N)$ for dense (but otherwise arbitrary) array layouts. EPIC can also lower the output data rates by directly yielding polarimetric image products for science analysis. We have optimized EPIC and have now commissioned it at LWA-SV as a commensal all-sky imaging back-end that can potentially detect and localize sources of impulsive radio emission on millisecond timescales. In this article, we review the architecture of EPIC, describe code optimizations that improve performance, and present initial validations from commissioning observations. Comparisons between EPIC measurements and simultaneous beam-formed observations of bright sources show spectral-temporal structures in good agreement.

 

We present recent improvements to the search for the global Cosmic Dawn signature using the Long Wavelength Array station located on the Sevilleta National Wildlife Refuge in New Mexico, USA (LWA–SV). These improvements are both in the methodology of the experiment and the hardware of the station. An improved observing strategy along with more sophisticated temperature calibration and foreground modeling schemes have led to improved residual RMS limits. A large improvement over previous work using LWA–SV is the use of a novel achromatic beamforming technique which has been developed for LWA–SV. We present results from an observing campaign which contains 29 days of observations between March 10, 2021 and April 10, 2021. The reported residual RMS limits are six times above the amplitude of the potential signal reported by the Experiment to Detect the Global EoR Signature (EDGES) collaboration. 

ABSTRACT We observed the flare stars AD Leonis, Wolf 424, EQ Pegasi, EV Lacertae, and UV Ceti for nearly 135 h. These stars were observed between 63 and 83 MHz using the interferometry mode of the Long Wavelength Array. Given that emission from flare stars is typically circularly polarized, we used the condition that any significant detection present in Stokes I must also be present in Stokes V at the same time in order for us to consider it a possible flare. Following this, we made one marginal flare detection for the star EQ Pegasi. This flare had a flux density of 5.91 Jy in Stokes I and 5.13 Jy in Stokes V, corresponding to a brightness temperature 1.75 × 1016(r/r*)−2 K. 

The search for the spectral signature of hydrogen from the formation of the first stars, known as Cosmic Dawn or First Light, is an ongoing effort around the world. The signature should present itself as a decrease in the temperature of the 21[Formula: see text]cm transition relative to that of the Cosmic Microwave Background and is believed to reside somewhere below 100[Formula: see text]MHz. A potential detection was published by the Experiment to Detect the Global EoR Signal (EDGES) collaboration with a profile centered around 78[Formula: see text]MHz of both unexpected depth and width (Bowman et al. [2018] Nature 555, 67). If validated, this detection will have profound impacts on the current paradigm of structure formation within [Formula: see text]CDM cosmology. We present an attempt to detect the spectral signature reported by the EDGES collaboration with the Long Wavelength Array station located on the Sevilleta National Wildlife Refuge in New Mexico, USA (LWA-SV). LWA-SV differs from other instruments in that it is a 256 element antenna array and offers beamforming capabilisties that should help with calibration and detection. We report first limits from LWA-SV and look toward future plans to improve these limits.