Surveys in Geophysics最新文献

Multigrid (MG) methods solve large linear equations on fine grids by projecting them onto progressively coarser grids, on which the problem can be solved more cheaply. They have become among the most effective and prospective solvers for large linear systems. However, due to the abundant null solution space and the inclusion of the air layer, traditional MG methods struggle to converge in three-dimensional (3D) electromagnetic (EM) numerical forward modeling. Served as one major contribution of this review, we provide a complete review on strategies, introduced in recent decades to develop efficient MG algorithms for EM forward modeling. We focus on how these strategies handle the convergence difficulties encountered in EM numerical forward modeling. Another observation is that most state-of-the-art MG solvers have been developed and examined against traditional Krylov subspace iterative solvers, but there is little knowledge on the numerical performance of different strategies. Therefore, another primary contribution of this work is to provide a complete review of the numerical performance of different strategies used in MG solvers for 3D EM forward modeling in geophysical applications. For this purpose, firstly, we briefly introduce on finite difference and finite element numerical discretization of the electrical field partial differential equations to demonstrate why EM forward modeling is challenging to solve. Subsequently, some background information on MG methods is provided to show how they can be implemented in general. Then, different strategies used in different MG methods are introduced in great detail to address the convergence issues encountered in EM forward modeling in geophysical applications, caused by the abundant null solution space and the inclusion of the air layer. Finally, we present four newly developed MG algorithms and compare their overall numerical performance in terms of their parallel ability, stability, efficiency and memory cost by using two increasingly complex models. Since one major motivation for improving the EM forward modeling efficiency is to speed up the inversion process, their perspective of efficiency improvement in EM inversions has been discussed. On this basis, authors and researchers can choose one particular MG solver for their own EM forward modeling problems.

To investigate physical links between the Earth atmosphere and ionosphere, we present data of the medium-scale atmospheric gravity waves (AGWs, periods 25–100 min) observed at auroral latitudes. The AGWs at 80–100 km altitude were inferred from the wind data of the Nordic meteor radar Cluster with spatial/height/time resolution 90 km/5 km/10 min respectively. At the same time, medium-scale traveling ionospheric disturbances (MSTIDs) were detected as variations of the electron density (critical frequency foF2) at the height of F2 maximum (hmF2, 250–350 km) in the data of the ionosonde at Sodankylä Geophysical Observatory (67°N, 27°E, Finland) operating with 1-min time resolution. We found that, except a “fall anomaly” in mid-September–mid-December, the season-local time distributions of AGW at 90 km and MSTID at hmF2 are similar. Namely, larger amplitudes are observed in the dark-sky conditions, such that the separation between smaller and larger amplitudes occurs at solar terminator. However, during the fall anomaly, amplitudes of MSTID at hmF2 are the same as in spring- and wintertime, whereas AGWs at 90 km are practically suppressed. This anomaly starts with the fall transition in the atmospheric circulation and is associated with a sharp change of the phase of semi-diurnal tides. The results are consistent with the idea that the AGWs observed near the mesopause may be generated due to turbulence in the lower atmosphere (below) or due to electrodynamical forces and auroral activity in the ionospheric E-layer. The latter plays a major role in the auroral region and may be more important in dark-sky conditions.

The Active Plasma and E-field Sounders (APES) mission concept aims to resolve orders-of-magnitude errors in modeling transionospheric radio propagation through the midlatitude trough, and to determine which physical mechanism(s) are responsible for generating plasma irregularities there. APES will observe ionospheric electron density profiles and signals from ground transmitters along its orbital track, allowing for a constrained test of propagation models. The mission will also perform small-scale in situ science, differentiating between the long-held temperature gradient instability and the Kelvin–Helmholtz/gradient drift instabilities as potential causes of irregularities in the trough. The centerpiece of the mission is the first-ever oblique topside ionospheric sounder, providing 2D electron density-altitude profiles along the orbital track through cooperative operation between two satellites. The leading satellite will produce swept-frequency HF transmissions that will reflect off the ionosphere before being received by the follower. The following satellite will also receive signals transmitted by the Super Dual Auroral Radar Network (SuperDARN). Both satellites will observe electron density at 1 m along-track resolution, while single-point electron temperature, vector electric field, neutral density and neutral wind will also be provided. The mission will operate in a nominal 350 × 800 km elliptical orbit, with along-track spacing varied from < 1 to 750 km over 12 months of science operations in an inclination between 50–87° and 103–130° (depending on the rideshare). Each bus carries a 250 m/s propulsion system to control eccentricity and for orbit maintenance. The orbital analysis has been used to select orbits with > 500 passes through the trough in each quarter.

Many components of the Earth system feature self-reinforcing feedback processes that can potentially scale up a small initial change to a fundamental state change of the underlying system in a sometimes abrupt or irreversible manner beyond a critical threshold. Such tipping points can be found across a wide range of spatial and temporal scales and are expressed in very different observable variables. For example, early-warning signals of approaching critical transitions may manifest in localised spatial pattern formation of vegetation within years as observed for the Amazon rainforest. In contrast, the susceptibility of ice sheets to tipping dynamics can unfold at basin to sub-continental scales, over centuries to even millennia. Accordingly, to improve the understanding of the underlying processes, to capture present-day system states and to monitor early-warning signals, tipping point science relies on diverse data products. To that end, Earth observation has proven indispensable as it provides a broad range of data products with varying spatio-temporal scales and resolutions. Here we review the observable characteristics of selected potential climate tipping systems associated with the multiple stages of a tipping process: This includes i) gaining system and process understanding, ii) detecting early-warning signals for resilience loss when approaching potential tipping points and iii) monitoring progressing tipping dynamics across scales in space and time. By assessing how well the observational requirements are met by the Essential Climate Variables (ECVs) defined by the Global Climate Observing System (GCOS), we identify gaps in the portfolio and what is needed to better characterise potential candidate tipping elements. Gaps have been identified for the Amazon forest system (vegetation water content), permafrost (ground subsidence), Atlantic Meridional Overturning Circulation, AMOC (section mass, heat and fresh water transports and freshwater input from ice sheet edges) and ice sheets (e.g. surface melt). For many of the ECVs, issues in specifications have been identified. Of main concern are spatial resolution and missing variables, calling for an update of the ECVS or a separate, dedicated catalogue of tipping variables.

A 6-year cycle has long been recognized to influence the Earth’s rotation, the internal magnetic field and motions in the fluid Earth’s core. Recent observations have revealed that a 6-year cycle also affects the angular momentum of the atmosphere and several climatic parameters, including global mean sea level rise, precipitation, land hydrology, Arctic surface temperature, ocean heat content and natural climate modes. In this review, we first present observational evidences supporting the existence of a 6-year cycle in the Earth system, from its deep interior to the climate system. We then explore potential links between the Earth’s core, mantle and atmosphere that might explain the observations, and investigate various mechanisms that could drive the observed 6-year oscillation throughout the whole Earth system.

The risk of earthquakes and their effects on both nature and infrastructure in seismically active regions of India require adaptable and scalable earthquake early warning (EEW) systems. Developing a robust EEW system is crucial to mitigate earthquake risks in the region, but it is a challenging task. Various institutes have attempted to develop EEW systems using different methods. Still, there is no common consensus, and issues remain with response time and reliability of disseminated information to the public. Efforts by institutions like the Indian Institute of Technology, Roorkee, have advanced EEW technologies, focusing on dense seismic sensor networks, real-time data processing algorithms, and effective dissemination mechanisms. Recent initiatives aim to improve sensor sensitivity and accuracy through fast communication systems for quicker earthquake detection. However, challenges persist in making EEW accessible and affordable, particularly in remote areas, due to the lack of a nationwide system. The National Centre for Seismology (NCS), under the Ministry of Earth Sciences (MoES), is piloting an EEW system in the NW Himalayas, which could lead to a nationwide implementation. Developing region-specific algorithms for rapid data analysis and nurturing collaboration between academic institutions, government agencies, and international partners are crucial steps. Public awareness campaigns and educational programs are essential for community resilience and timely response to earthquake alerts. Establishing a robust EEW system in India could significantly enhance earthquake risk mitigation efforts in earthquake-prone zones of the country and should be viewed within the context of a holistic risk reduction framework. EEW systems can enhance mitigation efforts, but they must be complemented by other essential measures, such as improving building resilience and promoting public awareness.