Abstract
Plasma sheet convection is a key element of storm-time plasma dynamics in the magnetosphere. While decades of observations have advanced our understanding of convection in general, specifically storm-time convection remains poorly understood. Using data from ISAS/NASA's Geotail and NASA's MMS, this study characterizes plasma sheet magnetic flux transport across the magnetotail during numerous storms (both recovery and main phases) and contrasts these observations with those from quiet times. Our findings confirm the well-documented enhancement of the convection electric field during geomagnetic storms. Beyond that, our results reveal a significant dawn-dusk asymmetry. At dawn, the elevated convection is realized via relatively faster flows while at dusk, through a stronger northward magnetic field. These findings suggest a complex feedback loop between plasma sheet convection and ring current buildup, whereby the latter asymmetrically inflates the magnetotail on the dusk side, shifting the reconnection site and subsequently enhanced earthward flows toward dawn.
Abstract
While the Cretaceous Normal Polarity Superchron has documented instances of brief reversed polarity intervals, the absence of accurate age determinations for such abrupt shifts poses a challenge in leveraging them as reliable reference tiepoints. This study presents a cyclostratigraphic analysis of gamma-ray data from the DSDP Site 402A. The identification of Milankovitch cycles allowed us to construct a 405-kyr astronomically-tuned age model for two reversed events that occurred in the Aptian. Our results estimate an age of 117.03 ± 0.14 Ma for the Chron M”-1r” or ISEA and 116.17 ± 0.14 Ma for reversal “2,” with timespans of ∼20 and ∼10 kyr, respectively. Sedimentation patterns are influenced by orbital eccentricity cycles with an average sedimentation rate of 5 cm/kyr. The short time intervals related to these reversals exposes the difficulty in their detection in cores through paleomagnetic analysis as well as in deep-tow surveys.
Abstract
The interannual variation of the South China Sea (SCS) summer monsoon onset (SMO) may bring extreme weather and climate disasters in East Asia. However, its skillful forecast still remains challenging. This study investigates the intraseasonal ocean-atmosphere interaction that affects the SCSSMO through diagnostic analysis and numerical experiments. It reveals that the cold sea surface temperature in the Southern SCS during winter (referred as cold tongue, CT) is the key pathway controlling the propagation of the 30–60 days intraseasonal oscillation (ISO) convective system from the Bay of Bengal (BOB) to the SCS. The CT variations affect the interannual variation of the SCSSMO. Specifically, the strong (weak) CT after the peak of La Niña (El Niño) years suppresses (enhances) the propagating ISO from the BOB to the SCS, resulting in a delayed (advanced) SCSSMO. This finding offers the new scientific insights for improving the forecasting of the SCSSMO.
Abstract
The Arase satellite observed the precipitation of monoenergetic electrons accelerated from a very high altitude above 32,000 km altitude on 16 September 2017. The event was selected in the period when the high-angular resolution channel of the electron detector looked at pitch angles within ∼5° from the ambient magnetic field direction, and thereby was the first to examine the detailed distribution of electron flux near the energy-dependent loss cone at such high altitudes. The potential energy below the satellite estimated from the observed energy-dependence of the loss cone was consistent with the energy of the upgoing ion beams, indicating that ionospheric ions were accelerated by a lower-altitude acceleration region. The accelerated electrons inside the loss cone carried a significant net field-aligned current (FAC) density corresponding to ionospheric-altitude FAC of up to ∼3μA/m2. Based on the anisotropy of the accelerated electrons, we estimated the height of the upper boundary of the acceleration region to be >∼2 R
E
above the satellite. The height distribution of the acceleration region below the satellite, estimated from the frequency of auroral kilometric radiation, was ∼4,000–13,000 km altitude, suggesting that the very-high-altitude acceleration region was separated from the lower acceleration region. Additionally, we observed time domain structure (TDS) electric fields on a subsecond time scale with a thin FAC indicated by magnetic deflections. Such a TDS may be generated by the formation of double layers in the magnetotail, and its potential drop could significantly contribute (∼40%–60%) to the parallel energization of precipitating auroral electrons.
Abstract
We use conjugate observations of magnetospheric whistler-mode waves at frequencies up to 16 kHz by the DEMETER spacecraft (at an altitude of approximately 660 km) and the ground-based Kannuslehto station in Finland (L≈5.38) $(L\approx 5.38)$ to investigate the wave propagation to the ground and their characteristic spatial scales. For this purpose, we evaluate correlations between the wave intensities measured by the spacecraft and the ground-based station at various frequencies as a function of their longitudinal and L-shell separations. Two different approaches are used: (a) direct correlation of wave intensities measured at the same times and (b) correlation of wave intensities within corresponding frequency-time windows, focusing on the identification of the same frequency-time wave signatures. We show that the characteristic longitudinal scales of the investigated waves are between about 60° $60{}^{\circ}$ and 90° $90{}^{\circ}$. We further demonstrate that, while the wave intensities measured by DEMETER are generally larger during periods of enhanced geomagnetic activity, wave intensities measured on the ground during increased activity are only slightly larger during the daytime and decrease during the nighttime.
Abstract
The satellite missions GRACE and GRACE Follow-On have undoubtedly been the most important sources to observe mass transport on global scales. Within the Combination Service for Time-Variable Gravity Fields (COST-G), gravity field solutions from various processing centers are being combined to improve the signal-to-noise ratio and further increase the spatial resolution. The time series of monthly gravity field solutions suffer from a data gap of about one year between the two missions GRACE and GRACE Follow-On among several smaller data gaps. We present an intermediate technique bridging the gap between the two missions allowing (1) for a continued and uninterrupted time series of mass observations and (2) to compare, cross-validate and link the two time series. We focus on the combination of high-low satellite-to-satellite tracking (HL-SST) of low-Earth orbiting satellites by GPS in combination with satellite laser ranging (SLR), where SLR contributes to the very low degrees and HL-SST is able to provide the higher spatial resolution at an lower overall precision compared to GRACE-like solutions. We present a complete series covering the period from 2003 to 2022 filling the gaps of GRACE and between the missions. The achieved spatial resolution is approximately 700 km at a monthly temporal resolutions throughout the time period of interest. For the purpose of demonstrating possible applications, we estimate the low degree glacial isostatic adjustment signal in Fennoscandia and North America. In both cases, the location, the signal strength and extend of the signal coincide well with GRACE/GRACE-FO solutions achieving 99.5% and 86.5% correlation, respectively.
Abstract
The fifth GLONASS-K1 satellite with space vehicle number R807 was launched in October 2022. It represents the first spacecraft of the K1+ generation, which offers various technical innovations. Compared to previous K1 satellites, R807 also transmits a code-division multiple access (CDMA) signal in the L2 frequency band in addition to L1 and L2 frequency-division multiple access (FDMA) and L3 CDMA signals. Thus, R807 is the first spacecraft of the K1+ generation. A geometry- and ionosphere-free triple carrier combination is used to analyze the GLONASS R807 clock consistency at different frequencies. Significant inconsistencies were found showing up as variations with a peak-to-peak amplitude of up to 40 cm and periods between 15 min and a few days. Whereas the ultimate explanation for these variations is not known, it is likely that they originate from cross-talk of two oscillators with similar frequency. A short-term clock analysis for integration times up to 100 s based on the one-way carrier phase (OWCP) method shows a superior stability of the R807 clock compared to all other GLONASS satellites including the new K2 generation. The Allan deviation computed from 5 s clock estimates confirms this finding for integration times up to 600 s but shows a significant bump at longer integration times due to the periodic variations mentioned above. Single-frequency OWCP processing confirms consistency of the L1 and L2 FDMA signals whereas the L3 CDMA signal shows a slight phase shift. Although the spurious variations mask the true performance of the K1+ atomic frequency standard, its behavior at short integration times points at a new type of GLONASS satellite clock.
SummaryThe 1934 Mw 8.2 Bihar-Nepal earthquake was one of the devastating earthquakes, which made seismologists realize the importance of proper seismic hazard analysis and design aspects in India. The event occurred way before proper seismic networks were implemented and hence there are no recorded ground motions available for this event. The present study, thus aims to generate possible ground motions for the 1934 Mw 8.2 Bihar-Nepal event. The complex geographical features, ambiguous source information, and lack of ground motion data make the simulation and validation of ground motions very difficult. In this regard, the broadband (BB) ground motions are simulated and validated for the most recent well-documented Himalayan event, i.e., the 2015 Mw 7.9 Nepal earthquake in order to calibrate the model and simulation methodology. For this purpose, the computational model presented by Sreejaya et al. (2023) is extended up to a region of 1000 km × 670 km (longitude 80-89 °E and latitude 23-30 °N) in the Indo-Gangetic Basin to simulate the low-frequency (LF) ground motions using spectral element method (Komatitsch and Tromp 1999). These deterministically simulated LF ground motions are combined with stochastically simulated high-frequency (HF) ground motions based on an improved seismological model following Otarola and Ruiz (2016). The seismic moment and dimensions of the rupture plane presented by Pettanati et al. (2017) are used to generate ten samples for the finite fault source model having different slip distribution along the rupture plane as a random field (Mai and Beroza 2000; 2002). The BB ground motions (0.01–25 Hz) are obtained by merging LF and HF ground motions in the time domain by matching them at a frequency of ∼0.3 Hz. Such BB results are simulated at a grid of stations and at locations where Modified Mercalli Intensity (MMI) intensity values are available. The estimated MMI values and the observed MMI values are compared to emphasize the efficacy of the model. The maximum PGA estimated from the simulated ground motions in horizontal and vertical directions are observed to be 0.48 g and 0.4 g. Further, 5% damped response spectra and spectral amplification are analyzed concerning the sediment depth of the Indo-Gangetic Basin. The results from the study can serve as inputs for dynamic analysis and the design of earthquake-resistant structures across different locations in the Indo-Gangetic Basin.
SummaryFault geometry is a key factor in controling the mechanics of faulting. However, there is currently limited theoretical knowledge regarding the effect of non-planar fault geometry on earthquake mechanics. Here, we address this gap by introducing an expansion of the relation between fault traction and slip, up to second order, relative to the deviation from a planar fault geometry. This expansion enables the separation of the effects of non-planarities from those of planar faults. This expansion is realised in the boundary integral equation, assuming a small fault slope. It provides an interpretation for the effect of complex fault geometry on fault traction, for any fault geometry and any slip distribution. Hence the results are also independent of the friction that applies on the fault. The findings confirm that fault geometry has a strong influence on in-plane faulting (mode II) by altering the normal traction on the fault and making it more resistant to slipping for any fault geometry. On the contrary, for out-of-plane faulting (mode III), fault geometry has a much smaller influence. Additionally, we analyse some singularities that arise for specific fault geometries often used in earthquake simulations and provide guidelines for their elimination. To conclude this study, we discuss the limits of the infinitesimal strain theory when non-planar faults are considered.
SummaryRocks near a fault plane are commonly damaged by multiple earthquake ruptures, forming damage zones. The damage zone is important structures controlling various properties of a fault, yet its fine scale (tens to hundreds of meters) structure is difficult to resolve with surface seismic observations. We propose to use earthquakes that occur at depth within a fault zone as virtual seismometers (VSs) and use surface observations to extract Green's function (GFs) between VS pairs (VSGFs) . This method resembles that of ambient noise tomography and the retrieved VSGFs are related to the structures between event pairs. In this study, we develop the theory about how to extract VSGFs using surface stations deployed across a fault zone. Firstly, we use a half-space model and Fresnel zone analysis to determine the upper and lower limits of the GF frequency band, which is controlled by the station spacing and aperture of a given seismic array. Then, for VS in a fault zone, we demonstrate that the VSGF can be retrieved by linear seismic arrays deployed across the fault, and that the VSGF is equivalent to waves emitted simultaneously from an array of mirror sources of one event and received by the other. Secondly, the half-space result is directly adopted to determine the corresponding frequency band in the damage zone situation. Thirdly, we analyze different combinations of VS pair geometry and conclude that a relatively larger VS distance (much larger than the damage zone width) is more effective to recover damage zone structures for the available frequency bands. In this situation, VSGFs are trapped waves, that is represented by the interference of mirror sources. In such a case, the trapped waves are equivalent to surface waves, which have dispersion features to extract damage zone structures. Finally, we adopt the VSGF method to the Ridgecrest earthquake aftershock monitoring array and use a profile of aftershocks to extract 6 pairs of VSGFs. The spatial variation of VSGFs may reflect the depth-dependent variation of damaged zone. Our analysis shows a promising direction to use VSGFs to extract spatial variations of fault damaged zones.
A nationwide analysis of community-level floodplain development found that over two-million acres of floodplain were developed over the past two decades across the United States, with roughly half of all new floodplain housing built in Florida.
Abstract
Polarization and propagation characteristics of ultra-low frequency (ULF, ≃1−1000 $\simeq 1-1000$ mHz) waves are conventionally studied using arrays of ground-based magnetometers. However, the ground magnetometer observations are subject to distortions due to polarization rotation and spatial integration effects caused by the transition of the magnetohydrodynamic wave into an electromagnetic wave at the lower ionospheric boundary. In contrast, high-frequency (3–30 MHz) radars, like those comprising the Super Dual Auroral Radar Network (SuperDARN), are capable of direct observations of the ULF wave characteristics at ionospheric altitudes via measuring plasma drift velocity variations caused by the wave's electric field. In this work, we use multi-beam data from SuperDARN Hokkaido East, Hokkaido West, and Christmas Valley West radars to identify the dominant polarization modes as well as azimuthal wave numbers of evening-night-side-morning ULF waves in the Pc5 frequency band (1.67–6.67 mHz) propagating over sub-auroral and mid-latitude regions. The observed statistical characteristics of these waves point at the solar wind dynamic pressure variations and Kelvin-Helmholtz instability at the magnetopause as their potential principal sources, although the drift-bounce resonance with trapped energetic ions may contribute to the small-scale part of the observed Pc5 wave population.
The common practice of building dams to prevent flooding can actually contribute to more intense coastal flood events, according to a new study.
Abstract
Standard finite volume or finite difference methods may produce unphysical negative solutions of phase space density when applied to radiation belt diffusion equation with cross diffusion terms. In this work, we apply a recently proposed positivity-preserving finite volume (PPFV) method to a 2D diffusion problem of radiation belt electrons with both structured and unstructured meshes. Our test using a model problem shows that the new method does not produce unphysical negative solutions with both types of meshes even with strong cross-diffusion terms. By applying the method to the 2D pitch angle and energy diffusion problem, we demonstrate that the method achieves positivity of solutions without requiring excessive number of grid points and shows good agreement with previous results obtained using a layer method. The ability of preserving positivity of the solution with unstructured meshes allows the method to handle complex boundary configurations. Our results suggest that the new PPFV method could be useful in modeling radiation belt diffusion processes or in building a physics-based forecast model.
A newly discovered mechanism for the flow and freezing of ice sheet meltwater could improve estimates of sea level rise around the globe.
The movement of carbon dioxide (CO2) from the surface of the ocean, where it is in active contact with the atmosphere, to the deep ocean, where it can be sequestered away for decades, centuries, or longer, depends on a number of seemingly small processes.
A Dartmouth-led study by more than 50 climate scientists worldwide provides the first clear projection of how carbon emissions may drive the loss of Antarctica's ice sheet over the next 300 years.
In September 2023, scientists around the world detected a mysterious seismic signal that lasted for nine straight days. An international team of scientists, including seismologists Alice Gabriel and Carl Ebeling of UC San Diego's Scripps Institution of Oceanography came together to solve the mystery.
Mega ocean warming El Niño events were key in driving the largest extinction of life on planet Earth some 252 million years ago, according to new research.
When a volcano is about to erupt, the surrounding land puffs up like a squeezed balloon. The technical term is "transient deformation," and Virginia Tech researchers have detected and tracked this short-lived movement for the first time using satellite observations of Ol Doinyo Lengai, an active Tanzanian volcano.
