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1. Investigating Spectral Distortion of Local Volcano Infrasound by Nonlinear Propagation at Sakurajima Volcano, Japan

   https://doi.org/10.1029/2019JB018284  

   Maher, Sean P.; Matoza, Robin S.; de Groot‐Hedlin, Catherine D.; Gee, Kent L.; Fee, David; Yokoo, Akihiko (March 2020, Journal of Geophysical Research: Solid Earth)

   Abstract Sound waves generated by erupting volcanoes can be used to infer important source dynamics, yet acoustic source‐time functions may be distorted during propagation, even at local recording distances (15 km). The resulting uncertainty in source estimates can be reduced by improving constraints on propagation effects. We aim to quantify potential distortions caused by wave steepening during nonlinear propagation, with the aim of improving the accuracy of volcano‐acoustic source predictions. We hypothesize that wave steepening causes spectral energy transfer away from the dominant source frequency. To test this, we apply a previously developed single‐point, frequency domain, quadspectral density‐based nonlinearity indicator to 30 acoustic signals from Vulcanian explosion events at Sakurajima Volcano, Japan, in an 8‐day data set collected by five infrasound stations in 2013 with 2.3‐ to 6.2‐km range. We model these results with a 2‐D axisymmetric finite‐difference method that includes rigid topography, wind, and nonlinear propagation. Simulation results with flat ground indicate that wave steepening causes up to2 dB (1% of source level) of cumulative upward spectral energy transfer for Sakurajima amplitudes. Correction for nonlinear propagation may therefore provide a valuable second‐order improvement in accuracy for source parameter estimates. However, simulations with wind and topography introduce variations in the indicator spectra on order of a few decibels. Nonrandom phase relationships generated during propagation or at the source may be misinterpreted as nonlinear spectral energy transfer. The nonlinearity indicator is therefore best suited to small source‐receiver distances (e.g.,2 km) and volcanoes with simple sources (e.g., gas‐rich strombolian explosions) and topography. 

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2. $1/f$ Noise in the Heliosphere: A Target for PUNCH Science

   https://doi.org/10.1007/s11207-024-02401-z  

   Wang, Jiaming; Matthaeus, William H; Chhiber, Rohit; Roy, Sohom; Pradata, Rayta A; Pecora, Francesco; Yang, Yan (December 2024, Solar Physics)

   Abstract We present a broad review of$$1/f$$ $1/f$noise observations in the heliosphere, and discuss and complement the theoretical background of generic$$1/f$$ $1/f$models as relevant to NASA’s Polarimeter to UNify the Corona and Heliosphere (PUNCH) mission. First observed in the voltage fluctuations of vacuum tubes, the scale-invariant$$1/f$$ $1/f$spectrum has since been identified across a wide array of natural and artificial systems, including heart rate fluctuations and loudness patterns in musical compositions. In the solar wind the interplanetary magnetic field trace spectrum exhibits$$1/f$$ $1/f$scaling within the frequency range from around$$\unit[2 \times 10^{-6}]{Hz}$$to around$$\unit[10^{-3}]{{Hz}}$$at 1 au. One compelling mechanism for the generation of$$1/f$$ $1/f$noise is the superposition principle, where a composite$$1/f$$ $1/f$spectrum arises from the superposition of a collection of individual power-law spectra characterized by a scale-invariant distribution of correlation times. In the context of the solar wind, such a superposition could originate from scale-invariant reconnection processes in the corona. Further observations have detected$$1/f$$ $1/f$signatures in the photosphere and corona at frequency ranges compatible with those observed at 1 au, suggesting an even lower altitude origin of$$1/f$$ $1/f$spectrum in the solar dynamo itself. This hypothesis is bolstered by dynamo experiments and simulations that indicate inverse cascade activities, which can be linked to successive flux tube reconnections beneath the corona, and are known to generate$$1/f$$ $1/f$noise possibly through nonlocal interactions at the largest scales. Conversely, models positing in situ generation of$$1/f$$ $1/f$signals face causality issues in explaining the low-frequency portion of the$$1/f$$ $1/f$spectrum. Understanding$$1/f$$ $1/f$noise in the solar wind may inform central problems in heliospheric physics, such as the solar dynamo, coronal heating, the origin of the solar wind, and the nature of interplanetary turbulence. 

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3. Evolution of the Interplanetary Turbulence and the Associated Turbulence Anisotropy in the Outer Heliosphere: VOYAGER 2 Observations

   https://doi.org/10.3847/1538-4357/ad2eae  

   Zhu, Xingyu; He, Jiansen; Zank, Gary P; Verscharen, Daniel; Zhao, Ling-Ling; Duan, Die; Lin, Rong (April 2024, The Astrophysical Journal)

   Abstract We study the radial evolution of the inertial-range solar wind plasma turbulence and its anisotropy in the outer heliosphere. We use magnetic field (B) measurements from the Voyager 2 spacecraft for heliocentric distancesRfrom 1 to 33 au. We find that the perpendicular and trace power spectral densities (PSDs) of the magnetic field ( $E_{{\mathrm{B}}_{\perp }}$and $E_{{\mathrm{B}}_{\mathrm{Tr}}}$) still follow a Kolmogorov-like spectrum until 33 au. The parallel magnetic field PSD, $E_{B_{\parallel }}$, transits from a power-law index of −2 to −5/3 as the distance crossesR∼ 10 au. The PSD at frequencies 0.01 Hz <f< 0.2 Hz flattens atR> 20 au, gradually approaching anf⁻¹spectrum, probably due to instrument noise. At 0.002 Hz <f< 0.1 Hz, quasi-parallel propagation dominates at 1 au <R< 7 au, with quasi-perpendicular propagation gradually emerging atR> 5 au. ForR> 7 au, oblique propagation becomes the primary mode of propagation. At smaller frequencies off< 0.01 Hz, $E_{{\mathrm{B}}_{\perp }}$increases with propagation angle at 1 au <R< 5 au, and in contrast decreases with propagation angle atR> 5 au due to the enhanced power level at propagation angles smaller than 20°. Such enhancement may derive from the injection of wave energy from the pickup ion source into the background turbulent cascade, and the injected wave energy is transferred across scales without leaving local enhancements in $E_{{\mathrm{B}}_{\perp }}$or $E_{{\mathrm{B}}_{\mathrm{Tr}}}$. 

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4. Deep Einstein@Home Search for Continuous Gravitational Waves from the Central Compact Objects in the Supernova Remnants Vela Jr. and G347.3-0.5 Using LIGO Public Data

   https://doi.org/10.3847/1538-4357/ad8b9e  

   Ming, J; Papa, M A; Eggenstein, H-B; Beheshtipour, B; Machenschalk, B; Prix, R; Allen, B; Bensch, M (December 2024, The Astrophysical Journal)

   Abstract We perform a search for continuous nearly monochromatic gravitational waves from the central compact objects associated with the supernova remnants Vela Jr. and G347.3. Over 10¹⁸different waveforms are considered, covering signal frequencies between 20 and 1300 Hz (20 and 400 Hz) for G347.3-0.5 (Vela Jr.) and a very broad range of frequency derivatives. The data set used for this first search is from the second observing run of LIGO (O2). Thousands of volunteers donating compute cycles through the computing project Einstein@Home have made this endeavor possible. Following the Einstein@Home search, we perform multistage follow-ups of over 5 million waveforms. The threshold for selecting candidates from the Einstein@Home search is such that, after the multistage follow-up, we do not expect any surviving candidate due to noise. The very last stage uses a different data set, namely, the LIGO O3 data. We find no significant signal candidate for either targets. Based on this null result, for G347.3-0.5, we set the most constraining upper limits to date on the amplitude of gravitational-wave signals, corresponding to deformations below 10⁻⁶in a large part of the search band. At the frequency of best strain sensitivity, near 161 Hz, we set 90% confidence upper limits on the gravitational-wave intrinsic amplitude of $h_0^{90\%}\approx 6.2\times {10}^{-26}$. Over most of the frequency range, our upper limits are a factor of 10 smaller than the indirect age-based upper limit. For Vela Jr., near 163 Hz, we set $h_0^{90\%}\approx 6.4\times {10}^{-26}$. Over most of the frequency range, our upper limits are a factor of 15 smaller than the indirect age-based upper limit. The Vela Jr. upper limits presented here are slightly less constraining than the most recent upper limits of R. Abbott et al., but they apply to a broader set of signals. 

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5. Steady Radiating Gravity waves: An Exponential Asymptotics Approach

   https://doi.org/10.1007/s42286-023-00081-z  

   Kataoka, Takeshi; Akylas, T. R. (January 2024, Water Waves)

   Abstract The radiation of steady surface gravity waves by a uniform stream$$U_{0}$$ $U_0$over locally confined (width$$L$$ $L$) smooth topography is analyzed based on potential flow theory. The linear solution to this classical problem is readily found by Fourier transforms, and the nonlinear response has been studied extensively by numerical methods. Here, an asymptotic analysis is made for subcritical flow$$D/\lambda > 1$$ $D/\lambda >1$in the low-Froude-number ($$F^{2} \equiv \lambda /L \ll 1$$ $F^2\equiv \lambda /L\ll 1$) limit, where$$\lambda = U_{0}^{2} /g$$ $\lambda =U_0^2/g$is the lengthscale of radiating gravity waves and$$D$$ $D$is the uniform water depth. In this regime, the downstream wave amplitude, although formally exponentially small with respect to$$F$$ $F$, is determined by a fully nonlinear mechanism even for small topography amplitude. It is argued that this mechanism controls the wave response for a broad range of flow conditions, in contrast to linear theory which has very limited validity. 

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