المؤلفون: Jiang Yu, Xiaoman Liu, Aojun Ren, Jing Wang, Zuzheng Chen, Zhaoguo He, Nigang Liu, Liuyuan Li, Jun Cui, Jinbin Cao

المصدر: Geophysical Research Letters, Vol 51, Iss 10, Pp n/a-n/a (2024)

مصطلحات موضوعية: magnetosonic waves, wave propagation, ray tracing, plasmaspheric plume, Geophysics. Cosmic physics, QC801-809

الوصف: Abstract Plasmaspheric density structures are considered to control the propagation trajectories of fast magnetosonic (MS) waves in the inner magnetosphere. However, whether the plasmaspheric plume can effectively alter the propagation of MS waves remains unknown. Based on the analytical model of plasma density, ray tracing simulations are performed to investigate the propagation of exactly perpendicular MS waves in the equatorial plane in the magnetosphere containing a plasmaspheric plume. We find that plasmatrough and plume MS waves propagating toward the plasmaspheric plume can be reflected into the plasmaspheric core by the plume, then potentially migrating globally and thus quasi‐trapped inside the plasmaspheric core. The simulations also indicate that lower‐frequency MS waves approaching the plasmaspheric plume are more easily reflected and quasi‐trapped inside the plasmaspheric core. Our findings illustrate a previously unexplored way that plasmatrough MS waves could access and be trapped inside the plasmaspheric core via azimuthal plasmaspheric density structures.

وصف الملف: electronic resource

العلاقة: https://doaj.org/toc/0094-8276Test; https://doaj.org/toc/1944-8007Test

الوصول الحر: https://doaj.org/article/592b18b0f526418483230fd2853d0d42Test

المؤلفون: Bruce A. Campbell, Gareth A. Morgan, Beatriz Sánchez‐Cano

المصدر: Geophysical Research Letters, Vol 51, Iss 4, Pp n/a-n/a (2024)

مصطلحات موضوعية: Mars, ionospheres, wave propagation, Geophysics. Cosmic physics, QC801-809

الوصف: Abstract The electron density of the Martian ionosphere is modulated by solar wind forcing and crustal magnetic fields. Sounding observations from the orbital Shallow Radar (SHARAD) map the ionospheric total electron content (TEC) at a spatial resolution of ∼35 km. Averaging over a 250‐km diameter window from data collected weeks to years apart yields the first map of long‐term stable dayside martian TEC features. An extensive region of suppressed TEC in the southern highlands correlates with strong radial magnetic fields, but in other areas no simple correlation is observed. The TEC maps do follow the outlines of exposed Noachian crust and patches of magnetization in Tharsis not reset by volcanic activity. SHARAD TEC mapping may capture magnetic field strength at an intermediate height between the surface and the altitudes of orbital measurements underlying spherical harmonic models. Existing and future data will allow SHARAD TEC mapping to ∼100 km spatial resolution.

وصف الملف: electronic resource

العلاقة: https://doaj.org/toc/0094-8276Test; https://doaj.org/toc/1944-8007Test

الوصول الحر: https://doaj.org/article/e4e4beac19c24b61aa086494ac1cacd1Test

المؤلفون: Sinevich, A. A.^1,2 (AUTHOR) sinevich.aa@gmail.com, Chernyshov, A. A.^1,2 (AUTHOR), Chugunin, D. V.¹ (AUTHOR), Oinats, A. V.³ (AUTHOR), Clausen, L. B. N.⁴ (AUTHOR), Miloch, W. J.⁴ (AUTHOR), Nishitani, N.⁵ (AUTHOR), Mogilevsky, M. M.¹ (AUTHOR)

المصدر: Geophysical Research Letters. 4/28/2022, Vol. 49 Issue 8, p1-11. 11p.

مصطلحات موضوعية: *RADIO wave propagation, *IONOSPHERIC plasma, *HUMAN geography, *MAGNETIC storms, *ELECTRON density, *GEOMAGNETISM, *LATITUDE

مستخلص: We study the spatial structure of a polarization jet/Sub‐Auroral Ion Drift (PJ/SAID) based on data from the NorSat‐1 and Swarm satellites during a geomagnetic storm. Observations of plasma parameters inside the PJ/SAID are obtained with NorSat‐1 using a system of Langmuir probes with a nominal sampling rate of up to 1 kHz, which allowed measurements with such a high temporal resolution for the first time. A comparative analysis of plasma parameters and electron density spectra inside PJ according to the data from both satellites is presented. Our results show that fluctuations of plasma parameters inside the PJ increase at all scales with increasing geomagnetic activity. Small‐scale irregularities in the PJ are measured in situ down to hundreds of meters. The role of large‐scale effects in the PJ increases in comparison with the small‐scale ones during high geomagnetic activity. The PJ consists of structures ∼0.2° latitude in size within which small‐scale irregularities are present. Plain Language Summary: Polarization jet (PJ), also known as Sub‐Auroral Ion Drift (SAID), events are fast westward plasma drifts with a narrow latitudinal extent, occurring at subauroral latitudes in the Earth's ionosphere. The decrease in the density of the ionospheric plasma inside PJ/SAID significantly affects the conditions for the propagation of shortwave radio waves, which indicates the practical importance of studying this phenomenon. Despite the importance of using a variety of ground‐based observation facilities for studying and analyzing PJ/SAID properties, as well as developing analytical models and numerical modeling, in situ observations are the most valuable. Such in situ observations can be obtained only with satellites flying through a developing PJ/SAID. Large‐scale features of PJ/SAID are currently well understood, but small‐scale processes within PJ/SAID are practically not studied, and many open questions remain. In this work, we study the small‐scale structures in PJ/SAID during a geomagnetic storm of 20 April 2018, using multi‐instrumental approach involving low‐Earth orbit. Key Points: Fluctuations of plasma parameters inside the polarization jet (PJ) increase at all scales during higher geomagnetic activitySmall‐scale irregularities inside the PJ are measured in situ down to hundreds of metersThe role of large‐scale effects in the PJ increases in comparison with small‐scale ones with geomagnetic activity [ABSTRACT FROM AUTHOR]

المؤلفون: Andoh, Satoshi¹ (AUTHOR) andoh@kugi.kyoto-u.ac.jp, Saito, Akinori¹ (AUTHOR), Shinagawa, Hiroyuki² (AUTHOR)

المصدر: Geophysical Research Letters. 4/16/2022, Vol. 49 Issue 7, p1-11. 11p.

مصطلحات موضوعية: *RADIO wave propagation, *LATITUDE, *ATMOSPHERIC models, *PHASE velocity, *COMPUTER simulation, *DENSE plasmas, *ZONAL winds

مستخلص: To reveal mechanisms of day‐to‐day variations of the low‐latitude sporadic E (Es) layers, Es layer simulations were performed and compared to plasma layers observed by the Arecibo radar. Many studies have been conducted about the Es layers till now. However, few studies investigated the day‐to‐day variations of the Es layers especially at the low‐latitudes. Herein, for the first time, our numerical model generally succeeded in reproducing features of the day‐to‐day variations of the low‐latitude Es layers. We found that the day‐to‐day variations of the Es layers are created by a combination of day‐to‐day variations of zonal/meridional‐wind shears driven by the tides and the downward phase velocity of the tides. The wind‐driven mechanism generally explains the day‐to‐day variations of the low‐latitude Es layers. Plain Language Summary: The sporadic E (Es) layers are highly dense plasma layers appearing irregularly at altitudes 90–130 km. They cause unexpected reflections and abnormal propagations of radio waves used for broadcast communications and navigations. The Es layers exhibit day‐to‐day variations in their behaviors and structures, making it difficult to accurately predict their occurrence. In this study, our numerical ionospheric model, coupled with the neutral winds of a whole atmospheric model, reproduced day‐to‐day variations of the Es layers observed by the radar at Arecibo. This is the first time such a modeling has been performed at Arecibo after decades of research on Es layers. Our numerical model, which is based on the wind‐driven mechanism, can reproduce their daily behaviors. Day‐to‐day variations of the winds are crucial for behaviors of the low‐latitude Es layers. Key Points: Our numerical model generally reproduced day‐to‐day variations of the sporadic E (Es) layers observed at Arecibo for the first timeDay‐to‐day variations of the low‐latitude Es layers are generally created by those of the atmospheric diurnal/semi‐diurnal tidesBelow ∼100 km at the low‐latitudes, the zonal‐wind shears of the diurnal tides form the Es layers, and the vertical winds do not [ABSTRACT FROM AUTHOR]

المؤلفون: J. D. Huba, H.‐L. Liu

المصدر: Geophysical Research Letters, Vol 50, Iss 20, Pp n/a-n/a (2023)

مصطلحات موضوعية: plasmasphere ducts, whistler wave propagation, gravity waves, plasmasphere irregularities, Geophysics. Cosmic physics, QC801-809

الوصف: Abstract We show that atmospheric gravity waves can generate plasma ducts and irregularities in the plasmasphere using the coupled SAMI3/WACCM‐X model. We find the equatorial electron density is irregular as a function of longitude which is consistent with CRRES measurements (Clilverd et al., 2007, https://doi.org/10.1029/2007ja012416Test). We also find that plasma ducts can be generated for L‐shells in the range 1.5–3.0 with lifetimes of ∼ 0.5 hr; this is in line with observations of ducted VLF wave propagation with lifetimes of 0.5–2.0 hr (Clilverd et al., 2008, https://doi.org/10.1029/2007ja012602Test; Singh et al., 1998, https://doi.org/10.1016/s1364-6826Test(98)00001-7).

وصف الملف: electronic resource

العلاقة: https://doaj.org/toc/0094-8276Test; https://doaj.org/toc/1944-8007Test

الوصول الحر: https://doaj.org/article/82b39560b80a461b95c51986dfade352Test

المؤلفون: Sato, H.¹ (AUTHOR) hiroatsu.sato@dlr.de, Rietveld, M. T.^2,3 (AUTHOR), Jakowski, N.¹ (AUTHOR)

المصدر: Geophysical Research Letters. 2/28/2021, Vol. 48 Issue 4, p1-8. 8p.

مصطلحات موضوعية: *RADIO wave propagation, *PLASMA physics, *GLOBAL Positioning System, *INCOHERENT scattering, *IONOSPHERIC plasma

مستخلص: We report on simultaneous observation of artificial plasma density irregularities near the magnetic zenith (MZ) by incoherent scatter radar and GNSS satellite in the high latitude. During an EISCAT (European Incoherent Scatter Scientific Association) HF heating experiment, a GLONASS satellite signal intersected the disturbed ionospheric volume along the local magnetic field lines. The satellite signal amplitude and phase were simultaneously perturbed when the electron temperature increased in the F region through O‐mode HF waves. The field‐aligned irregularities (FAIs) and associated density perturbations are most significantly found in the MZ direction. The growth of FAI reached the saturation level in 30 s while large‐scale electron density perturbation on the order of 0.1 TECU developed in a few minutes. The observed density perturbations agree well with recent numerical studies of FAI generation due to the thermal self‐focusing process. Plain Language Summary: Powerful high‐frequency radio waves are used to study electron heating process in the high latitude ionosphere. We observed the development of plasma density structures by analyzing small changes of amplitude and phase of ground‐based satellite signals and incoherent scatter radar measurements. It is concluded that the electron heating causes irregular electron density structures along local magnetic field lines. This experiment shows that coordinated ground‐based satellite and incoherent scatter radar measurements essentially help to better understand the physics of ionospheric plasma dynamics and radio wave propagation. Key Points: HF‐induced field‐aligned plasma irregularities are simultaneously observed by GLONASS satellite and incoherent scatter radarThe satellite signal amplitude fluctuation and TEC deviation are generally correlatedObserved positive density deviations suggest that the density perturbations are likely caused by the thermal self‐focusing process [ABSTRACT FROM AUTHOR]

المؤلفون: Toh, A., Capdeville, Y., Chi, W.‐C., Ide, S.

المصدر: Geophysical Research Letters; 4/28/2023, Vol. 50 Issue 8, p1-9, 9p

مصطلحات موضوعية: FAULT zones, EARTHQUAKE zones, ELASTIC wave propagation, TREMOR, SEISMOMETERS, OCEAN bottom, EARTHQUAKES, SUBDUCTION zones, PALEOSEISMOLOGY

مستخلص: Tremors are a type of slow earthquake with long‐duration signals compared to ordinary earthquakes. The long signals have been considered to solely reflect their long source process. However, here, we provide evidence suggesting that the source processes of tremors are not always long. We refer to these observations as short‐duration tremors. They were recorded by ocean‐bottom seismometers placed very close to the source. Although these tremors exhibit a short‐duration signal when recorded near the source, they exhibit a typical long‐duration signal elsewhere. Our numerical simulations demonstrate that the features can be captured by considering a strongly scattering medium around their source. One such structure could be small low‐velocity inclusions distributed around the seismic source. The inclusions may represent the seismic expression of geologically detected aquifers in tremor source regions. Furthermore, this medium could be embedded along the slow earthquake fault zone and play a critical role in their source process. Plain Language Summary: At least two types of earthquakes occur in shallow subduction zones: ordinary earthquakes and tremors. Tremors are known to exhibit long signal duration compared to ordinary earthquakes. To date, tremors' long‐duration signal has been solely interpreted by their source process. Here, we discovered tremors that exhibit short duration signals when recorded close from the source which we referred to as "short‐duration tremors". They suggest that tremors' source process is not always long and structural effects may partially form the typical long‐duration signals. We performed numerical simulations on elastic wave propagation and demonstrated that the observations can be qualitatively reproduced by assuming a strongly scattering material surrounding the seismic source. On the other hand, it has been reported based on ocean bottom drilling project that tremor source region may consist of patchily distributed aquifers. The inclusions in our model may be the seismic expressions of the geologically detected aquifers. Further, such a structure could be embedded along the slow‐earthquake fault zone and play a key role in their source process. Key Points: Short‐duration tremors in ocean‐bottom seismometer records suggest that the source process of tremors is not always longShort‐duration tremors can be interpreted by placing a strongly scattering medium around their sourceSuch a medium embedded along the slow earthquake fault zone could play a key role in the tremor source process [ABSTRACT FROM AUTHOR]

: Copyright of Geophysical Research Letters is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

المؤلفون: Di Martino, Maria Del Pilar, De Siena, Luca, Tisato, Nicola

المصدر: Geophysical Research Letters; 9/28/2022, Vol. 49 Issue 18, p1-9, 9p

مصطلحات موضوعية: VOLCANIC ash, tuff, etc., ULTRASONIC propagation, ELASTIC wave propagation, ROCK texture, ROCK properties, SPECTRAL element method

مستخلص: Pore space controls the mechanical and transport properties of rocks. At the laboratory scale, seismic modeling is usually performed in relatively homogeneous settings, and the influence of the pore space on the recorded wavefields is determined by rock‐fluid interactions. Understanding this influence in dry rocks is instrumental for assessing the impact of pore topology on waves propagating in heterogeneous environments, such as volcanoes. Here, we simulated the propagation of shear waves as a function of pore space parameters in computational models built as proxies for volcanic rocks. The spectral‐element simulations provide results comparable with ultrasonic experiments, and the outcome shows that the size, shape, volume, and location of pores impact amplitudes and phases. These variations intensify in waveform coda after multiple scattering. Our results confirm that pore topology is one of the primary regulators of the propagation of elastic waves in dry rocks regardless of porosity. Plain Language Summary: Pores control the non‐elastic behavior and, in general, the petrophysical and mechanical properties of rocks. Such properties are essential to assess potential resources such as aquifers and reservoirs or hazards posed by earthquakes, volcanoes, and constructions. The factors controlling the elasticity of rocks are texture, pore space and the fluids filling the pores. While volcanoes represent a key target for rock characterization, measuring and modeling these factors in volcanic rocks remains challenging due to their intrinsic heterogeneities. In this study, we analyzed how pore space parameters influence the overall elastic properties of rocks by changing one parameter at a time. We created synthetic samples and performed computational simulations that show the individual contribution of the amount, size, location, and shape on waveform phases and amplitudes. The findings demonstrate that we can constrain the pore space in heterogeneous rocks in simple but realistic scenarios. Our results are the first step to provide computationaly‐driven forward models of seismic signals in heterogeneous volcanic media, necessary to predict the responses of volcanic rocks to stress. Key Points: Computational modeling quantifies the influence of pore space topology on S‐wave propagation in volcanic rocksAmount, size and location of pores impact ultrasonic wave propagation in dry rocks independently of porosityPath effects dominate the waveforms and depend on the location of the pores [ABSTRACT FROM AUTHOR]

: Copyright of Geophysical Research Letters is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

المؤلفون: Kong, Qingkai, Wang, Ruijia, Walter, William R., Pyle, Moira, Koper, Keith, Schmandt, Brandon

المصدر: Geophysical Research Letters; 7/16/2022, Vol. 49 Issue 13, p1-10, 10p

مصطلحات موضوعية: DEEP learning, EXPLOSIONS, ELASTIC wave propagation, MACHINE learning, SHEAR waves, PHYSICS

مستخلص: This paper combines the power of deep‐learning with the generalizability of physics‐based features, to present an advanced method for seismic discrimination between earthquakes and explosions. The proposed method contains two branches: a deep learning branch operating directly on seismic waveforms or spectrograms, and a second branch operating on physics‐based parametric features. These features are high‐frequency P/S amplitude ratios and the difference between local magnitude (ML) and coda duration magnitude (MC). The combination achieves better generalization performance when applied to new regions than models that are developed solely with deep learning. We also examined which parts of the waveform data dominate deep learning decisions (i.e., via Grad‐CAM). Such visualization provides a window into the black‐box nature of the machine‐learning models and offers new insight into how the deep learning derived models use data to make decisions. Plain Language Summary: This paper presents a new method to distinguish earthquakes from explosions using seismic data. The method combines features implicitly defined by a deep learning algorithm with features explicitly defined from physical models of seismic sources and elastic wave propagation. The combination of these two types of features makes our method perform better on new data sets. By visualizing the performance of our combined model, we gain insight into what the deep learning model relies on to make its decisions. Key Points: Discrimination of earthquakes and explosions can be enhanced by combining physics‐based features with those derived from machine learningVisualizing which parts of the input the deep learning model relies on can provide more insight into the processes underlying decisionsThe deep learning model focuses on different frequency bands for P and S waves to make the decision [ABSTRACT FROM AUTHOR]

: Copyright of Geophysical Research Letters is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

المؤلفون: Vasko, I. Y., Mozer, F. S., Bale, S. D., Artemyev, A. V.

المصدر: Geophysical Research Letters; 6/16/2022, Vol. 49 Issue 11, p1-10, 10p

مصطلحات موضوعية: ELECTRIC field strength, ION acoustic waves, THERMAL electrons, ELECTRIC potential, DEBYE length, ELECTRIC waves, CORONAL mass ejections, ACOUSTIC wave propagation

مستخلص: We present analysis of electrostatic waves around the ramp of a quasi‐perpendicular Earth's bow shock observed by the Magnetospheric Multiscale spacecraft. The electrostatic waves have amplitudes up to 800 mV/m, which is the largest value ever reported in the Earth's bow shock. In contrast to previous studies, the electrostatic waves have large amplitudes of the electrostatic potential, up to 20 V or 20% of local electron temperature. The wavelengths are from 150 m to 3 km, that is from 15 to 300 Debye lengths and typically from 0.4 to 1.5 thermal electron gyroradii. Importantly, these waves can propagate not only quasi‐parallel or oblique, but also almost perpendicular to local magnetic field. The electrostatic waves are interpreted in terms of ion‐acoustic waves, although the presence of electron cyclotron harmonic waves cannot be entirely ruled out. These results suggest that electrostatic waves may strongly affect the dynamics of electrons in collisionless shocks. Plain Language Summary: Electrostatic fluctuations are always present around collisionless shocks observed in the heliosphere. However, the effects of these fluctuations on the shock dynamics have not been quantified yet, mostly because experimental studies of their properties are scarce and, moreover, limited to electric field measurements in a spacecraft spin plane. We present analysis of electrostatic waves in a quasi‐perpendicular Earth's bow shock using three‐component electric field measurements aboard the Magnetospheric Multiscale spacecraft. We show that electrostatic waves have rather high amplitudes of the electrostatic potential, wavelengths comparable with thermal electron gyroradius, and can propagate at various angles to local magnetic field, including almost perpendicular propagation. The revealed properties indicate that electrostatic waves may be efficient in pitch‐angle scattering/demagnetization of electrons in collisionless shocks, thereby potentially facilitating electron heating and thermalization. Key Points: Ion‐acoustic waves have electric field amplitudes up to 800 mV/m and electrostatic potential amplitudes up to 20 VThe typical wavelength of ion‐acoustic waves is comparable with thermal electron gyroradiusIon‐acoustic waves can propagate at arbitrary angles to local magnetic field, including almost perpendicular propagation [ABSTRACT FROM AUTHOR]

: Copyright of Geophysical Research Letters is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)