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Efforts are underway to use high-precision timing of pulsars in order to detect low-frequency gravitational waves. A limit to this technique is the timing noise generated by dispersion in the plasma along the line of sight to the pulsar, including the solar wind. The effects due to the solar wind vary with time, influenced by the change in solar activity on different time-scales, ranging up to ∼11 yr for a solar cycle. The solar wind contribution depends strongly on the angle between the pulsar line of sight and the solar disc, and is a dominant effect at small separations. Although solar wind models to mitigate these effects do exist, they do not account for all the effects of the solar wind and its temporal changes. Since low-frequency pulsar observations are most sensitive to these dispersive delays, they are most suited to test the efficacy of these models and identify alternative approaches. Here, we investigate the efficacy of some solar wind models commonly used in pulsar timing using long-term, high-cadence data on six pulsars taken with the Long Wavelength Array, and compare them with an operational solar wind model. Our results show that stationary models of the solar wind correction are insufficient to achieve the timing noise desired by pulsar timing experiments, and we need to use non-stationary models, which are informed by other solar wind observations, to obtain accurate timing residuals.

 

By direct measurements of the gas temperature, the Atacama Large Millimeter/submillimeter Array (ALMA) has yielded a new diagnostic tool to study the solar chromosphere. Here, we present an overview of the brightness-temperature fluctuations from several high-quality and high-temporal-resolution (i.e. 1 and 2 s cadence) time series of images obtained during the first 2 years of solar observations with ALMA, in Band 3 and Band 6, centred at around 3 mm (100 GHz) and 1.25 mm (239 GHz), respectively. The various datasets represent solar regions with different levels of magnetic flux. We perform fast Fourier and Lomb–Scargle transforms to measure both the spatial structuring of dominant frequencies and the average global frequency distributions of the oscillations (i.e. averaged over the entire field of view). We find that the observed frequencies significantly vary from one dataset to another, which is discussed in terms of the solar regions captured by the observations (i.e. linked to their underlying magnetic topology). While the presence of enhanced power within the frequency range 3–5 mHz is found for the most magnetically quiescent datasets, lower frequencies dominate when there is significant influence from strong underlying magnetic field concentrations (present inside and/or in the immediate vicinity of the observed field of view). We discuss here a number of reasons which could possibly contribute to the power suppression at around 5.5 mHz in the ALMA observations. However, it remains unclear how other chromospheric diagnostics (with an exception of H α line-core intensity) are unaffected by similar effects, i.e. they show very pronounced 3-min oscillations dominating the dynamics of the chromosphere, whereas only a very small fraction of all the pixels in the 10 ALMA datasets analysed here show peak power near 5.5 mHz. This article is part of the Theo Murphy meeting issue ‘High-resolution wave dynamics in the lower solar atmosphere’. 

Abstract We present the full panchromatic afterglow light-curve data of GW170817, including new radio data as well as archival optical and X-ray data, between 0.5 and 940 days post-merger. By compiling all archival data and reprocessing a subset of it, we have evaluated the impact of differences in data processing or flux determination methods used by different groups and attempted to mitigate these differences to provide a more uniform data set. Simple power-law fits to the uniform afterglow light curve indicate a t 0.86±0.04 rise, a t −1.92±0.12 decline, and a peak occurring at 155 ± 4 days. The afterglow is optically thin throughout its evolution, consistent with a single spectral index (−0.584 ± 0.002) across all epochs. This gives a precise and updated estimate of the electron power-law index, p = 2.168 ± 0.004. By studying the diffuse X-ray emission from the host galaxy, we place a conservative upper limit on the hot ionized interstellar medium density, <0.01 cm −3 , consistent with previous afterglow studies. Using the late-time afterglow data we rule out any long-lived neutron star remnant having a magnetic field strength between 10 10.4 and 10 16 G. Our fits to the afterglow data using an analytical model that includes Very Long Baseline Interferometry proper motion from Mooley et al., and a structured jet model that ignores the proper motion, indicates that the proper-motion measurement needs to be considered when seeking an accurate estimate of the viewing angle. 

The West Pacific Warm Pool (WPWP)'s response to increasedpCO₂during the Pliocene is a key model validation target. Different temperature proxies show different trends: The foraminiferal Mg/Ca sea surface temperature (SST) record shows Pliocene WPWP temperatures ~1.2°C cooler than today (Wara et al., 2005,https://doi.org/10.1126/science.1112596), whereas a TEX₈₆study finds a cooling trend and claims the Pliocene WPWP was warmer than today (Zhang et al., 2014,https://doi.org/10.1126/science.1246172). We focus on understanding biases in Mg/Ca data as the best way to constrain the temperature of the Pliocene WPWP. The strongest nonthermal controls on foraminiferal Mg/Ca are Mg/Ca of seawater and dissolution. Dissolution, which imparts a cool bias to Mg/Ca temperatures, depends on Δ[CO₃²⁻], the difference from the carbonate ion concentration needed for calcite saturation. Thus, Pliocene proxy discrepancies might stem from varying Δ[CO₃²⁻] over time. To constrain the effect of changing dissolution on the Mg/Ca data, we collected benthic foraminiferal B/Ca data (a proxy for Δ[CO₃²⁻]) from the WPWP spanning 0–5.5 Ma. We find no long‐term trend in Δ[CO₃²⁻], but variations above and below the threshold of foraminiferal dissolution yield an ~0.4°C cold bias when averaged over the middle to early Pliocene. Changes in seawater Mg/Ca create an ~0.6°C cold bias in the Pliocene Mg/Ca data. After accounting for these biases, we find that the Pliocene WPWP was ~0.1°C cooler than the late Holocene, ranging from −0.5°C to +0.5°C including all uncertainties. Our reconstruction shows a much lower east‐west temperature gradient in the Pliocene tropical Pacific than today, supporting a permanent El Niño‐like “El Padre” state.

 

El Niño‐Southern Oscillation (ENSO) is the strongest mode of interannual climate variability, and its predicted response to anthropogenic climate change remains unclear. Determining ENSO's sensitivity to climatic mean state and the strength of positive and negative feedbacks, notably the thermocline feedback, will help constrain its future behavior. To this end, we collected ENSO proxy data from the early and mid‐Pliocene, a time during which tropical Pacific zonal and vertical temperature gradients were much lower than today. We found that El Niño events had a reduced amplitude throughout the early Pliocene, compared to the late Holocene. By the mid‐Pliocene, El Niño amplitude was variable, sometimes reduced and sometimes similar to the late Holocene. This trend in Pliocene ENSO amplitude mirrors the long‐term strengthening of zonal and vertical temperature gradients and verifies model results showing dampened ENSO under reduced gradients due to a weaker thermocline feedback.

 

The Pliocene Epoch (∼5.3–2.6 million years ago, Ma) was characterized by a warmer than present climate with smaller Northern Hemisphere ice sheets, and offers an example of a climate system in long‐term equilibrium with current or predicted near‐future atmospheric CO₂concentrations (pCO₂). A long‐term trend of ice‐sheet expansion led to more pronounced glacial (cold) stages by the end of the Pliocene (∼2.6 Ma), known as the “intensification of Northern Hemisphere Glaciation” (iNHG). We assessed the spatial and temporal variability of ocean temperatures and ice‐volume indicators through the late Pliocene and early Pleistocene (from 3.3 to 2.4 Ma) to determine the character of this climate transition. We identified asynchronous shifts in long‐term means and the pacing and amplitude of shorter‐term climate variability, between regions and between climate proxies. Early changes in Antarctic glaciation and Southern Hemisphere ocean properties occurred even during the mid‐Piacenzian warm period (∼3.264–3.025 Ma) which has been used as an analog for future warming. Increased climate variability subsequently developed alongside signatures of larger Northern Hemisphere ice sheets (iNHG). Yet, some regions of the ocean felt no impact of iNHG, particularly in lower latitudes. Our analysis has demonstrated the complex, non‐uniform and globally asynchronous nature of climate changes associated with the iNHG. Shifting ocean gateways and ocean circulation changes may have pre‐conditioned the later evolution of ice sheets with falling atmosphericpCO₂. Further development of high‐resolution, multi‐proxy reconstructions of climate is required so that the full potential of the rich and detailed geological records can be realized.

 

Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, dynamically changing the properties of metasurfaces can be a major challenge. Here we demonstrate a reconfigurable hyperbolic metasurface comprised of a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with the phase-change material (PCM) single-crystal vanadium dioxide (VO₂). Metallic and dielectric domains in VO₂provide spatially localized changes in the local dielectric environment, enabling launching, reflection, and transmission of hyperbolic phonon polaritons (HPhPs) at the PCM domain boundaries, and tuning the wavelength of HPhPs propagating in hBN over these domains by a factor of 1.6. We show that this system supports in-plane HPhP refraction, thus providing a prototype for a class of planar refractive optics. This approach offers reconfigurable control of in-plane HPhP propagation and exemplifies a generalizable framework based on combining hyperbolic media and PCMs to design optical functionality.

 

Abstract We present a detailed analysis of the radio galaxy PKS $2250{-}351$ , a giant of 1.2 Mpc projected size, its host galaxy, and its environment. We use radio data from the Murchison Widefield Array, the upgraded Giant Metre-wavelength Radio Telescope, the Australian Square Kilometre Array Pathfinder, and the Australia Telescope Compact Array to model the jet power and age. Optical and IR data come from the Galaxy And Mass Assembly (GAMA) survey and provide information on the host galaxy and environment. GAMA spectroscopy confirms that PKS $2250{-}351$ lies at $z=0.2115$ in the irregular, and likely unrelaxed, cluster Abell 3936. We find its host is a massive, ‘red and dead’ elliptical galaxy with negligible star formation but with a highly obscured active galactic nucleus dominating the mid-IR emission. Assuming it lies on the local M – $\sigma$ relation, it has an Eddington accretion rate of $\lambda_{\rm EDD}\sim 0.014$ . We find that the lobe-derived jet power (a time-averaged measure) is an order of magnitude greater than the hotspot-derived jet power (an instantaneous measure). We propose that over the lifetime of the observed radio emission ( ${\sim} 300\,$ Myr), the accretion has switched from an inefficient advection-dominated mode to a thin disc efficient mode, consistent with the decrease in jet power. We also suggest that the asymmetric radio morphology is due to its environment, with the host of PKS $2250{-}351$ lying to the west of the densest concentration of galaxies in Abell 3936.