This chapter summarizes the observational lessons learned after two decades of observations of the Cosmic Dawn (CD) and Epoch of Reionization (EoR) with SKAO pathfinders and precursors. We will describe the effort towards building accurate simulation pipelines for actual observations and summarize the approaches that different groups have taken to calibrate and mitigate systematic effects such as sky model incompleteness, limited instrument models and antenna mutual coupling. We conclude by discussing the impact that these lessons may have on the design and analysis of upcoming SKAO observations of the Cosmic Dawn and Epoch of Reionization.

Instruments targeting 21 cm emission at high redshifts need a spectral dynamic range of better than ten thousand to distinguish the 21 cm background against bright foregrounds. Systematics arising from the antenna pattern are a leading limitation for current instruments and must be addressed in future experiments. Antenna pattern measurements could help reach this precision. Pattern measurements are complicated by the large scale of the instruments and interaction with the local environment. In-situ beam mapping methods have been investigated but the required accuracy remains ill defined. One consideration is whether the calibration source is in the far field. Near field measurements require more elaborate measurement and such an expense must be well motivated. The far field distance is set by the effective size of the antenna. Reflections and interactions with surroundings extend the effective size of the antenna to scales well beyond the physical aperture. Here we give a new, instrument-agnostic method for calculating beam calibration requirements. Using 21cm models and instrument noise we prescribe bounds on the geometric reflection size scales. These scales must be shown via measurement to be below noise. This prescription depends weakly on instrument-specific noise and for interferometers, on the characteristic baseline length, but is otherwise independent of any detailed simulation of antenna or analysis pipeline. Example calculations for HERA-like and EDGES-like instruments find cosmological structures map to reflection scales of 100 m. This far field distance puts ground-based transmitters close to the horizon and drone sources well above typical or legal operating heights. A near-field measurement approach is necessary. Phase-locked systems have been demonstrated with promising results but more work is necessary to validate an antenna pattern at the necessary dynamic range.

The Space Weather Around Young Suns (SWAYS) program was introduced in I. Davis et al. as a multiwavelength monitoring program for studying the activity and particle environments of nearby, young solar-type stars. The SWAYS program currently includes the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA) operating between 13 and 87 MHz to search for stellar equivalents of solar type II and III bursts, which are associated with bulk plasma motion in the corona and interplanetary medium. These observations are accompanied by simultaneous photometric data from the high-precision optical instrument Flarescope to identify associated flare events. These two instruments have collectively acquired nearly 900 hr of data with approximate to 70% overlap between 2023 November through 2024 June, dedicated to six stars. Here, we present the results of this first season of the SWAYS observing campaign, which include a superflare from the star EK Draconis with no accompanying low-frequency particle flux signal. The novelty of the coordination at these specific parts of the spectrum allow us to uniquely evaluate the conditions that may have inhibited a radio detection. We find that the exceptionally hot, dense coronae of incredibly active stars may not be conducive to the development of the instabilities required for type II and III bursts, or else inspire new expectations for when we should expect to observe a signal relative to the time of the flare. This may represent the plasma-density complement to the magnetospheric limitations to observing space-weather signatures at low frequencies.

We report systematic issues commonly observed in the Hydrogen Epoch of Reionization Array observations that cause abrupt changes in power over time, resulting in temporal discontinuities. To identify these effects, we applied the Temporal Discontinuity Index and Spectral Discontinuity Index metrics, which complement existing diagnostic tools by detecting discontinuities and revealing their potential relationships to bandpass and power-related issues. Analysis of 30 data sets, each corresponding to a single observing night, shows that such systematics appear consistently in many antennas and across multiple nights. Based on these metrics, we identify three main types of discontinuity-related issues: periodic broad-band discontinuities linked to Inter-Integrated Circuit (I2C) polling, broad-band discontinuities associated with degraded post-amplifier modules (PAMs), and sudden power changes related to the digital backend of the antenna system. The second type, associated with degraded PAMs, occurs less frequently within a single night compared to the periodic polling effect, though both are broadband. The third type affects the largest number of antennas and is most prevalent in the H6C data. The width of the discontinuities related to the digital backend corresponds to the frequency range handled by a single correlator box, scaled by the number of boxes involved. Although 30 data sets were examined, this paper presents a focused analysis of four representative nights to highlight the main discontinuity issues. We also present correlations between flagging due to discontinuities and flagging caused by other failure modes to investigate how these issues are interrelated.

We identify a systematic distortion of the gain-versus-frequency function of radio telescopes caused by digital flattening ("whitening") of the signal's spectrum followed by re-quantization, a common pair of processes in the signal processing of modern telescopes. Wide-bandwidth telescopes often have a large variation of signal power over frequency. Flattening of the spectrum allows samples of the channelized signal to be represented in a small number of bits, allowing efficient downstream processing. However, we show that this produces subtle systematic error in the measured spectra. We explore this effect in data from the Owens Valley Radio Observatory's Long Wavelength Array and through detailed semi-analytic simulations. Although the effect can be small so that it has heretofore been unrecognized, we demonstrate that it produces distortion of the spectrum at a level that is problematic for some science, in particular 21 cm cosmology. Finally, we explore mitigation strategies, showing that the effect can be substantially reduced by careful choice of the gain distribution along the signal path or by incorporating dithering in the re-quantization step.

The electron density of the solar corona is a fundamental parameter in many areas of solar physics. Traditionally, routine estimates of coronal density have relied exclusively on white-light observations. However, these density estimates, obtained by inverting the white-light data, require simplifying assumptions, which may affect the robustness of the measurements. Hence, to improve the reliability of coronal density measurements, it is highly desirable to explore other complementary methods. In this study, we estimate the coronal electron densities in the middle corona, between approximately 1.7 and 3.5 R circle dot, using low-frequency radio observations from the recently commissioned Long Wavelength Array at the Owens Valley Radio Observatory (OVRO-LWA). The results demonstrate consistency with those derived from white-light coronagraph data and predictions from theoretical models. We also derive a density model valid between 1.7 and 3.5 r circle dot, given by rho(r ')=1.27r '-2+29.02r '-4+71.18r '-6 , where r '=r/R circle dot , with r the heliocentric distance. OVRO-LWA is a solar-dedicated radio interferometer that provides science-ready images with low latency, making it well suited for generating regular and independent estimates of coronal densities to complement existing white-light techniques.

We report the first upper limits on the power spectrum of 21 cm fluctuations during the Epoch of Reionization and Cosmic Dawn from Phase II of the Hydrogen Epoch of Reionization Array (HERA) experiment. HERA Phase II constitutes several significant improvements in the signal chain compared to Phase I, most notably resulting in expanded frequency bandwidth, from 50–250 MHz. In these first upper limits, we investigate a small two-week subset of the available Phase II observations, with a focus on identifying new systematic characteristics of the instrument, and establishing an analysis pipeline to account for them. We report 2 σ upper limits in eight spectral bands, spanning 5.6 ≤ z ≤ 24.4 that are consistent with thermal noise at the 2 σ level for k ≳ 0.6–0.9 h Mpc ^−1 (band dependent). Our tightest limit during Cosmic Dawn ( z > 12) is 1.13 × 10 ^6 mK ^2 at ( k = 0.55 h Mpc ^−1 , z = 16.78), and during the EoR (5.5 < z < 12), it is 1.78 × 10 ^3 mK ^2 at ( k = 0.70 h Mpc ^−1 , z = 7.05). We find that mutual coupling has become our dominant systematic, leaking foreground power that strongly contaminates the low- k modes, resulting in the loss of modes from k = 0.35 to 0.55 compared to Phase I data.

The brief (10 nanoseconds) transient radio emission from cosmic ray air showers carries key information about the energy and mass composition of high energy cosmic rays, but anthropogenic radio frequency interference has historically prevented radio-based cosmic ray studies from being carried out independently from other types of detectors. We describe a cosmic ray detection system for the Owens Valley Radio Observatory Long Wavelength Array that searches for radio emission from cosmic ray air showers without relying on an external trigger, and runs alongside the other observing modes of the array. The OVRO-LWA, located in Eastern California, recently completed an expansion to 352 dual-polarization antennas and new signal processing infrastructure. In order to detect cosmic rays in the presence of radio frequency interference (RFI), initial event classification and RFI rejection is performed on Field Programmable Gate Array boards, which each process a sampled voltage timeseries from both polarizations of a subarray of 32 antennas. Each board uses dedicated RFI veto antennas outside the air shower radio footprint to reject RFI events. We present the trigger design, RFI flagging strategy, and candidate cosmic rays.

We report on a search for prompt, low-frequency radio emission from the gravitational-wave (GW) merger S250206dm using the Owens Valley Radio Observatory Long Wavelength Array (OVRO-LWA). Early alerts favored a neutron-star-containing merger, making this a compelling target. Motivated by theoretical predictions of coherent radio bursts from mergers involving a neutron star, we utilized the OVRO-LWA Time Machine system to analyze voltage data recorded around the time of the event. Time Machine is a two-stage voltage buffer and processing pipeline that continuously buffers raw data from all antennas across the array’s nearly full-hemisphere instantaneous field of view, enabling retrospective beamforming, dedispersion, and fast-transient candidate identification. For this event, we analyzed a 30 minute interval beginning 3.5 minutes after the merger, which included 2 minutes of pre-alert data recovered by the ring buffer. We searched the 50% localization probability region with millisecond time resolution in the 69–86 MHz frequency band. No radio counterpart was detected above a 7 σ fluence detection threshold of ∼150 Jy ms. Using Bayesian analysis, we place a 95% confidence upper limit on the source luminosity of L _95 = 4 × 10 ^41 erg s ^−1 . These constraints start to probe the bright end of the coherent-emission parameter space predicted by jet–interstellar medium shock processes, magnetar and blitzar-like mechanisms, and recent simulation-based scenarios for neutron-star-containing mergers. This study presents the first sensitive, large-area, millisecond-timescale search for prompt low-frequency radio emission from a GW merger with the OVRO-LWA, establishing a framework in which about 10 additional events will yield stringent population-level constraints.

Strong solar activities are often accompanied by a variety of radio bursts. These radio bursts not only serve as valuable diagnostics of coronal and heliospheric processes but also as potential tools in space weather monitoring and forecasting. However, space weather applications call for the capability for low-latency and high-sensitivity radio burst recording and reporting, which has remained lacking. In this work, we present the development of a near-real-time radio burst recording and reporting system with the Owens Valley Radio Observatory's Long Wavelength Array. The system directly clips data from the real-time buffer and streams it as a live real-time radio dynamic spectrogram. The spectrograms are then fed to a deep learning-based burst identification module for type III radio bursts. The identifier is built on a You Only Look Once architecture, trained by synthetic type III radio bursts generated by using a physics-based model to achieve accurate and robust detection. This system enables continuous real-time radio spectrum streaming and the automatic reporting of type III radio bursts within similar to 10 s of their occurrence.

Strong solar activity is often accompanied by a variety of radio bursts. These bursts are valuable diagnostics of coronal and heliospheric processes and also have potential applications in space weather monitoring and forecasting. However, space weather applications require low-latency, high-sensitivity radio burst recording and reporting capabilities, which have remained limited. In this work, we present the development of a near-realtime radio burst recording and reporting system using the Owens Valley Radio Observatory Long Wavelength Array. The system directly clips data from a realtime buffer and streams them as a live radio dynamic spectrogram. These spectrograms are then processed by a deep-learning-based burst identification module for type III radio bursts. The identifier is based on a YOLO (You Only Look Once) architecture and is trained on synthetic type III radio bursts generated using a physics-based model to achieve accurate and robust detection. This system enables continuous realtime radio spectrum streaming and automatic reporting of type III radio bursts within approximately 10 seconds of their occurrence.

We present an imaging-domain search for technosignatures at decametric wavelengths with the Owens Valley Radio Observatory Long Wavelength Array, targeting ultra-narrowband continuous-wave signals between 50 and 86 MHz. We implement an offline graphics processing unit pipeline that processes raw voltage data with upchannelization to approximately 10 Hz frequency resolution, producing all-sky images for each fine channel and totaling more than 3 & times; 106 images for a single 30 s epoch. Candidate selection is performed using multikernel matched filtering across frequencies, empirical noise standardization, and false-discovery-rate control. After applying quality cuts that remove extended sources, corrupted images, and obvious radio frequency interference, three narrowband candidates with signal-to-noise ratios above 10 sigma were selected for detailed analysis. By reimaging these candidates with finer temporal and spectral resolution, we resolved their structure and found them to be inconsistent with compact celestial narrowband emitters. Consequently, we report no detection of extraterrestrial technosignatures. The representative sensitivity of the search is similar to 100 Jy per channel across the entire visible hemisphere. For an unresolved emitter, this corresponds to 10 sigma equivalent isotropic radiated power limits of about 1014 W at a distance of 10 pc and 1018 W at 1 kpc. The wide field of view and ultrafine spectral resolution of this approach enable simultaneous probing of technosignature signals from millions of stellar systems. This method further establishes a scalable framework for deeper integrations and stacked searches toward neutron star targets relevant to axion-like particle line conversion.

The precise characterization and mitigation of systematic effects is one of the biggest roadblocks impeding the detection of the fluctuations of cosmological 21 cm signals. Missing data in radio cosmological experiments, often due to radio frequency interference (RFI), pose a particular challenge to power spectrum analysis as this could lead to the ringing of bright foreground modes in the Fourier space, heavily contaminating the cosmological signals. Here we show that the problem of missing data becomes even more arduous in the presence of systematic effects. Using a realistic numerical simulation, we demonstrate that partially flagged data combined with systematic effects can introduce significant foreground ringing. We show that such an effect can be mitigated through inpainting the missing data. We present a rigorous statistical framework that incorporates the process of inpainting missing data into a quadratic estimator of the 21 cm power spectrum. Under this framework, the uncertainties associated with our inpainting method and its impact on power spectrum statistics can be understood. These results are applied to the latest Phase II observations taken by the Hydrogen Epoch of Reionization Array, forming a crucial component in power spectrum analyses as we move toward detecting 21 cm signals in the ever more noisy RFI environment.

Decades of solar coronal observations have provided substantial evidence for accelerated particles in the corona. In most cases, the location of particle acceleration can be roughly identified by combining high spatial and temporal resolution data from multiple instruments across a broad frequency range. In almost all cases, these nonthermal particles are associated with quiescent active regions, flares, and coronal mass ejections (CMEs). Only recently, some evidence of the existence of nonthermal electrons at locations outside these well-accepted regions has been found. Here, we report for the first time multiple cases of transient nonthermal emissions, in the heliocentric range of ∼3–7 R _⊙ , which do not have any obvious counterparts in other wave bands, like white-light and extreme ultraviolet. These detections were made possible by the regular availability of high dynamic-range low-frequency radio images from the Owens Valley Radio Observatory’s Long Wavelength Array. While earlier detections of nonthermal emissions at these high heliocentric distances often had comparable extensions in the plane of sky, they were primarily associated with radio CMEs, unlike the cases reported here. Thus, these results add on to the evidence that the middle corona is extremely dynamic and contains a population of nonthermal electrons, which is only becoming visible with high dynamic-range low-frequency radio images.

Routine measurements of the magnetic field of coronal mass ejections (CMEs) have been a key challenge in solar physics. Making such measurements is important both from a space weather perspective and for understanding the detailed evolution of the CME. In spite of significant efforts and multiple proposed methods, achieving this goal has not been possible to date. Here we report the first possible detection of gyroresonance emission from a CME. Assuming that the emission is happening at the third harmonic, we estimate that the magnetic field strength ranges from 7.9 to 5.6 G between 4.9 and 7.5 R _⊙ . We also demonstrate that this high magnetic field is not the average magnetic field inside the CME, but most probably is related to small magnetic islands, which are also being observed more frequently with the availability of high-resolution and high-quality white-light images.

A major challenge in understanding the initiation and evolution of coronal mass ejections (CMEs) is measuring the magnetic field of the magnetic flux ropes (MFRs) that drive CMEs. Recent developments in radio imaging spectroscopy have paved the way for diagnosing the CMEs’ magnetic field using gyrosynchrotron radiation. We present magnetic field measurements of a CME associated with an X5-class flare by combining radio imaging spectroscopy data in microwaves (1–18 GHz) and meter waves (20–88 MHz), obtained by the Owens Valley Radio Observatory’s Expanded Owens Valley Solar Array (EOVSA) and Long Wavelength Array (OVRO-LWA), respectively. EOVSA observations reveal that the microwave source, observed in the low corona during the initiation phase of the eruption, outlines the bottom of the rising MFR-hosting CME bubble seen in extreme ultraviolet and expands as the bubble evolves. As the MFR erupts into the middle corona and appears as a white-light CME, its meter-wave counterpart, observed by OVRO-LWA, displays a similar morphology. For the first time, using gyrosynchrotron spectral diagnostics, we obtain magnetic field measurements of the erupting MFR in both the low and middle corona, corresponding to coronal heights of 0.02 and 1.83 R _⊙ . The magnetic field strength is found to be around 300 G at 0.02 R _⊙ during the CME initiation and about 0.6 G near the leading edge of the CME when it propagates to 1.83 R _⊙ . These results provide critical new insights into the magnetic structure of the CME and its evolution during the early stages of its eruption.

The Deployable Optical Receiver Aperture (DORA) experiment is a pathfinder for large-scale space-based radio interferometry with experiments addressing scalable laser communications within swarms and precision low frequency radio astronomy instruments. The collection of prototype instru-ments will characterize background noise which might limit future optical links and radio astronomy measurements. The DORA laser terminal concept uses arrays of solid state optical/ infrared sensors to form a wide-field high sensitivity detector theoretically capable of speeds up to 1 Gbps at a range of up to 1000 km between cubesats without requiring precision pointing. The freedom to rotate as needed for science or power purposes, without interrupting data transfer, will enable future swarm radio arrays of free-flying antennas operating as a single correlated array. The first DORA mission aims to test fly a silicon photomultiplier sensor and measure background light from reflected sunlight, moonlight and city lights. DORA's radio pathfinder aims to flight-test compact solid-state radio technol-ogy needed for precision low-frequency receivers targeting the cosmic dawn and epoch of reionization. Solid-state RF (radio frequency) switches and related elements which have not yet been demonstrated in space at the precision necessary have been formed into a dual-mode radio spectrometer attached to a deployable monopole antenna. A filter bank VHF (very high frequency) spectrometer provides continuous monitoring from 20 to 180 MHz spectrum in 20 MHz channels while a miniature software defined radio makes high spectral resolution scans of the same band. The satellite was built and tested by students in Arizona State University's (ASU) Low frequency Cosmology Lab and ASU's Interplanetary Initiative Lab (IPL). Vacuum testing, vibration testing, and day-in-the-life testing were all performed at ASU. The design and build began during the Covid pandemic which had many effects on the availability of people and components. As a result, late changes to payload, structure, and power systems were necessary. During testing, there were several component failures, all of which were resolved before delivery. These can ultimately be traced to late changes and provide instructive examples for future projects. DORA was aboard Northrop Grumman's 21st Cygnus flight on August 4th and was deployed on October 8th, 2024. Connection to DORA was maintained until November 27th, 2024, and on December 1st, it is estimated to have re-entered Earth's orbit. [1]

Observation of 21cm radio emission by neutral hydrogen from before reionization is a promising window as a probe of fundamental physics and origin of the first stars or other objects. The line temperature of intergalactic hydrogen is uniquely sensitive to the large scale flow of energy from objects forming at a time difficult to probe by other means. The redshifted line from the birth of the first stars at redshift 25 down to the end of reionization at redshift 5 is observed at wavelengths between one and five meters. The observing challenges include sensitivity for which significant collecting area is required, interference in the heavily used band, and instrumental precision which must reject unwanted chromaticity at one part in 100,000. The Hydrogen Epoch of Reionization Array (HERA) is a second generation 21cm experimental interferometer targeting 21cm emission at redshifts 5 to 23. HERA has reported limits on the 21cm power spectrum which constrain galaxy formation models and confirmed that x-rays were likely present to heat the early universe. After an upgrade several seasons have been recorded which are forecast to have much deeper sensitivity. The year six (2022) campaign has been partially reduced and a description has been publicly released. This reduction includes flagging, calibration, and averaging of two weeks of nightly observing. At this sensitivity small effects begin to appear. Small levels of interference become confused with otherwise unobjectionable systematics. Mutual coupling between antennas becomes visible above the noise level and transient astronomical objects are observed. These results suggest lessons which are generally applicable to other present and future instruments.

Low-frequency radio emission in the form of type II and III bursts is a direct indicator of plasma motion in the solar corona and interplanetary medium. However, detecting equivalent events on solar analogs requires thousands of observing hours and complementary multiwavelength observations to constrain the origin of the radio emission. To address this, we have begun the Study of Space Weather Around Young Suns (SWAYS), a multiwavelength program for monitoring space weather around young, solar-type stars. This program currently focuses on five solar-type stars spanning 100–800 Myr in age. It includes a dedicated observing scheme from the recently upgraded Owens Valley Radio Observatory (OVRO) Long Wavelength Array (LWA) operating at 13–86 MHz to search for stellar analogs of solar type II and III bursts. We have built the optical photometry instrument Flarescope to operate simultaneously with OVRO-LWA observations to investigate whether radio bursts are accompanied by magnetic reconnection events. We analyze the performance based on a 1 hr observation of π ^1 UMa, which shows that Flarescope can reach submillimagnitude precision through nondifferential photometry on π ^1 UMa in 60 s integration times when diffusing the light with engineered diffusers. A small field of OVRO-LWA cross-correlated data centered on π ^1 UMa reaches a noise level of 740 mJy at 10 s integration time, consistent with confusion noise. With this precision, we should be able to detect large optical flares and related radio bursts that may indicate accompanying coronal mass ejections and energetic particle events. In this paper, we present the design, framework, and performance of the SWAYS program.

Measurement of the 21 cm emission of neutral hydrogen in the intergalactic medium probes the era of the first luminous objects and the era of the intergalactic medium becoming fully ionized by the first stars, the Epoch of Reionization (EoR). However, the 21 cm signal is orders of magnitude fainter than astrophysical foregrounds, as well as other sources of radio frequency interference, making it challenging to measure. It is therefore imperative that any instrumental systematic effects which could further distort spectral structure are properly understood and mitigated. Kolopanis et al. (2023) identified a systematic in Phase II data from the MWA which manifests as excess correlation in the 21 cm power spectrum. This introduces a bias that can make foregrounds couple into 21 cm background modes. The source of the systematic is not known but one possible cause is mutual coupling between antennas. There are multiple physical mechanisms which cause mutual coupling, but we have focused on re-radiation for this work. Re-radiation is caused by impedance mismatch at the antenna receiver which causes part of the sky signal to be reflected and re-radiate to the surrounding antennas. This can cause excess correlation in the resulting observation.