Whistler waves are often observed in magnetopause reconnection associated with electron beams. We analyze seven MMS crossings surrounding the electron diffusion region (EDR) to study the role of electron beams in whistler excitation. Waves have two major types: (1) Narrow-band waves with high ellipticities and (2) broad-band waves that are more electrostatic with significant variations in ellipticities and wave normal angles. While both types of waves are associated with electron beams, the key difference is the anisotropy of the background population, with perpendicular and parallel anisotropies, respectively. The linear instability analysis suggests that the first type of wave is mainly due to the background anisotropy, with the beam contributing additional cyclotron resonance to enhance the wave growth. The second type of distribution excites broadband waves via Landau resonance, and as seen in one event, the beam anisotropy induces an additional cyclotron mode. The results are supported by particle-in-cell simulations. We infer that the first type occurs downstream of the central EDR, where background electrons experience Betatron acceleration to form the perpendicular anisotropy; the second type occurs in the central EDR of guide field reconnection. A parametric study is conducted with linear instability analysis. A beam anisotropy alone of above ~3 likely excites the cyclotron mode waves. Large beam drifts cause Doppler shifts and may lead to left-hand polarizations in the ion frame. Future studies are needed to determine whether the observation covers a broader parameter regime and to understand the competition between whistler and other instabilities.

Mantle plumes are thought to recycle material from the Earth’s deep interior. One constraint on the nature and quantity of this recycled material comes from the observation of seismic discontinuities. The detection of the X-discontinuity beneath Hawaii, interpreted as the coesite-stishovite transition, requires the presence of at least 40% basalt. However, previous geodynamic models have predicted that the percentage of high-density basaltic material that mantle plumes can carry to the surface is no higher than 15–20%. We propose this contradiction can be resolved by taking into account the length scale of chemical heterogeneities. While previous modeling studies assumed mechanical mixing on length scales smaller than the model resolution, we here model basaltic heterogeneities with length scales of 30–40~km, allowing for their segregation relative to the pyrolitic background plume material. Our models show that larger basalt fractions than previously thought possible—exceeding 40%—can accumulate within plumes at the depth of the X-discontinuity. Two key mechanisms facilitate this process: (1) The random distribution of basaltic heterogeneities induces large temporal variations in the basalt fraction with cyclical highs and lows. (2) The high density contrast between basalt and pyrolite below the coesite-stishovite transition causes ponding and accumulation of basalt at that depth, an effect that only occurs for intermediate viscosities of pyrolite. These results further constrain the chemical composition of the Hawaiian plume. Beyond that, they provide a geodynamic mechanism that explains the seismologic detection of the X-discontinuity and highlights how recycled material is carried towards the surface.

Distorted crystals carry useful information on processes involved in their formation, deformation and growth. The distortions are accommodated by geometrically necessary dislocations, and therefore characterising those dislocations is an informative task, to assist in, for example, deducing the slip systems that produced the dislocations. Electron Backscatter Diffraction (EBSD) allows detailed quantification of distorted crystal orientations and we summarise here a method for extracting information on dislocations from such data. The Weighted Burgers Vector (WBV) method calculates a vector at each point on an EBSD map, or an average over a region. The vector is a weighted average of the Burgers vectors of dislocation lines intersecting the map surface. It is weighted towards dislocation lines at a high angle to the map but that can be accounted for in interpretation. The method is fast and does not involve specific assumptions about dislocation types. It can be used, with care, to analyse subgrain walls (sharp orientation changes) as well as gradational orientation changes within individual grains. We describe new and published examples of the use of the technique to illustrate its potential; case studies to date mainly use the WBV direction not the magnitude. EBSD orientation data have angular errors, and so does the WBV. We present an analysis of these angular errors, showing there is a trade-off between directional accuracy and area sampled. In summary the technique is fast, free from assumptions, and errors can be taken into account to allow testing of hypotheses about dislocation types.

How the continental crust responds geochemically to progressive extension is one of the interesting questions. The Southern Tibet Rift System (STRS) is one of the active extensional structures. The Yardoi gneiss dome, located within STRS, consists of at least four suites of Miocene granites. As crystallization ages become younger, they are characterized by decrease in Nd(t) and Sr/Y, but increase in 87Sr/86Sr(t) and Rb/Sr. Such temporal trends could be explained by sequential partial melting of first the mafic lower crustal rocks and then progressively shallower metasedimentary rocks. Together with literature data, from north to south along STRS, as the extension proceeds and the heat moves upward, sequential partial melting is common to produce Miocene magmatic rocks. The processes documented in the southern Tibet might be common in other extensional provinces and provides a new insight to unravel the mechanisms for the generation of geochemical variations in contemporaneous granites.

A new database of the Entomological Inoculation Rate (EIR) is used to directly link the risk of infectious mosquito bites to climate in Sub-Saharan Africa. Applying a statistical mixed model framework to high-quality monthly EIR measurements collected from field campaigns in Sub-Saharan Africa, we analyzed the impact of rainfall and temperature seasonality on EIR seasonality and determined important climate drivers of malaria seasonality across varied climate settings in the region. We observed that seasonal malaria transmission requires a temperature window of 15-40 degrees Celsius and is sustained if average temperature is well above the minimum or below the maximum temperature threshold. Our study also observed that monthly maximum rainfall for seasonal malaria transmission should not exceed 600 mm in west Central Africa, and 400 mm in the Sahel, Guinea Savannah and East Africa. Based on a multi-regression model approach, rainfall and temperature seasonality were significantly associated with malaria seasonality in most parts of Sub-Saharan Africa except in west Central Africa. However, areas characterized by significant elevations such as East Africa, topography has a significant influence on which climate variable is an important determinant of malaria seasonality. Malaria seasonality lags behind rainfall seasonality only at markedly seasonal rainfall areas such as Sahel and East Africa; elsewhere, malaria transmission is year-round. The study’s outcome is important for the improvement and validation of weather-driven dynamical malaria models that directly simulate EIR. It can contribute to the development of malaria models fit-for-purpose to support health decision-making towards malaria control or elimination in Sub-Saharan Africa.

Fault zones usually present a granular gouge, coming from the wear material of previous slips. This layer contributes to friction stability and plays a key role in the way elastic energy is released during sliding. Considering a mature fault gouge with a change in the percentage of mineral cementation between particles, we aim to understand the influence of interparticle bonds on slip mechanisms by employing the Discrete Element Method. We consider a direct shear model without fluid in 2D, based on a granular sample with realistic grain sizes and shapes. Focusing on the physics of contacts inside the granular gouge, we explore contact interactions and effective friction coefficient within the fault. Brittleness is enhanced with cementation and even more with dense materials. For the investigated data range, three types of cemented material are highlighted: a mildly cemented material (Couette flow, no cohesion), a cemented material with agglomerates of cemented particles changing the granular flow and acting on slip weakening mechanisms (Riedel shear bands R1), and an ultra-cemented material behaving as a brittle material (with several Riedel bands followed by shear-localization). Effective friction curves present double weakening shapes for dense samples with enough cementation. We find that effective friction of a cemented fault cannot be predicted from Mohr-Coulomb criteria because of the specific stress state and kinematic constraints of the fault zone.

InSight’s seismometer package SEIS was placed on the surface of Mars at about 1.2 m distance from the thermal properties instrument HP3 that includes a self-hammering probe. Recording the hammering noise with SEIS provided a unique opportunity to estimate the seismic wave velocities of the shallow regolith at the landing site. However, the value of studying the seismic signals of the hammering was only realised after critical hardware decisions were already taken. Furthermore, the design and nominal operation of both SEIS and HP3 are non-ideal for such high-resolution seismic measurements. Therefore, a series of adaptations had to be implemented to operate the self-hammering probe as a controlled seismic source and SEIS as a high-frequency seismic receiver including the design of a high-precision timing and an innovative high-frequency sampling workflow. By interpreting the first-arriving seismic waves as a P-wave and identifying first-arriving S-waves by polarisation analysis, we determined effective P- and S-wave velocities of vP = 119+45-21 m/s and vS = 63+11-7 m/s, respectively, from around 2,000 hammer stroke recordings. These velocities likely represent bulk estimates for the uppermost several 10’s of cm of regolith. An analysis of the P-wave incidence angles provided an independent vP/vS ratio estimate of 1.84+0.89-0.35 that compares well with the traveltime based estimate of 1.86+0.42-0.25. The low seismic velocities are consistent with those observed for low-density unconsolidated sands and are in agreement with estimates obtained by other methods.

In the paper a unique phenomenon - the resonant generation of ultra-low frequency (ULF) wave by energetic electrons in the magnetosphere - is presented. On 27 October 2012 in the morning side of the magnetosphere Van Allen Probe A registered an ULF wave with period of 100 s and an amplitude of 0.7 nT. A cloud of energetic electrons was observed simultaneously with the wave. It is established that the electron cloud was injected into the magnetosphere as a result of a substorm. Electron fluxes in several energy channels were modulated with the frequency of the observed wave. It is shown that these flux modulations are caused by the drift resonance of 38 keV electrons and the wave being a fundamental harmonic of an Alfven mode with an azimuthal wavenumber m~ 110-115 propagating to the east, and generated through the gradient instability due to steep earthward density gradient of the resonant electrons.

In recent years, global kilometer-scale convection-permitting models have shown promising results in producing realistic convection and precipitation. Cold pools, which can be represented by km-scale models, are identified as one of the significant mesoscale processes responsible for modulating the life cycle of mesoscale organized convection. However, there is still a lack of understanding about cold pool properties across the spatio-temporal scales, as well as their representation in models. In this study, a 2.5 km global Icosahedral Nonhydrostatic (ICON) model simulation run for 40 days (06 UTC 01 Aug - 23 UTC 10 Sep 2016) from the Dynamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains (DYAMOND) initiative is used to identify thermal cold pools (using Tv) over the tropical oceans. The diurnal cycle of simulated thermal cold pools is compared against NASA’s RapidScat-observed gradient feature (GF) frequency and IMERG precipitation. ICON and IMERG exhibit morning peaks in cold pool activity similar to RapidScat GF frequency but miss the afternoon peak. Advanced Scatterometer (ASCAT) and RapidScat GF spatial climatology is also compared to ICON cold pools, where ICON shows more cold pools over the Indo-Pacific and western Atlantic basins. Random forest regression is applied to identify critical environmental properties with column water vapor found to be most important for controlling cold pool properties. Regional differences between cold pool properties are explored, where easterly waves dominate the eastern Pacific and Atlantic cold pool activity. The western Pacific and the Indian Ocean cold pools are controlled by local mesoscale forcing and intraseasonal oscillations.

Spectrophotometry using a small sample volume has been developed to measure the alkalinity (or bicarbonate). However, the experimental and calculation processes are complicated, and the atmospheric CO2 has to consider in preparing standard solutions. This study aims to quantify the dissolution of the atmospheric CO2 when using spectrophotometry. Also, the effect on the standard solution in titrating that uses to make calibration curve are calculated to understand the effects of the different CO2 concentrations in a laboratory. The difference between the bicarbonate concentrations and the calculated ones based on the updated chemical equilibrium formula was from 0.038 to 5.4×10-6 mg/L. The maximum difference was found at pH 5.0 in the 10 mg/L HCO3- standard solutions. The bicarbonate concentration without the atmospheric CO2 reaction (C1) and with the atmospheric CO2 reaction (C2) was calculated by the PHREEQC. The difference between C1 and C2 ranged from 0.01 to 0.02 mg/L, but the calculated bicarbonate concentrations between the HCl titration and the PHREEQC output were certainly different, which ranged from 3.1 to 11.5 mg/L at the pH 4.3 endpoint. In contrast, at the pH 4.8 endpoint, the difference was significantly decreased from 0.8 to 1.3 mg/L. The effect of the increasing atmospheric CO2 by human breathing in a laboratory is only 0.05 mg/L in the standard solutions when titrating. From the results of this study, the experimental and calculation processes to correct the bicarbonate concentration by the effect of the atmospheric CO2 in a laboratory may be omitted if natural waters are targeted.

The location of the Subtropical Front (STF), the boundary between Subtropical and Subantarctic Water in the Southern Ocean is proposed to be controlled by the strength and location of the Southern Hemisphere westerly winds. We use a hydrodynamic hindcast model and recent observations to test if changes in the westerly winds can cause meridional shifts in the STF over interannual to decadal time scales by modulating local Ekman transport. We find that increased, or northward, shifted westerly winds lead to an enhanced northward Ekman transport over large parts of the Southern Ocean, resulting in a northward shift in the STF. Conversely for weaker or southward shifted westerly winds. Regions with strong eddy variability, such as western boundary current systems of the Agulhas and East Australian Current behave differently, as the Sverdrup balance causes an opposite shift. In these regions an increase in westerly winds lead to a southward shift in the STF. A southward shift of STF has been observed between 2004-2019. However, the shift is smaller than the latitudinal shifts in the location of the zero wind stress curl and maximum westerly winds (-0.4° latitude/decade). This discrepancy is due to positive Ekman trends resulting from the intensification of the westerly winds, which oppose the southward migration. Changes in the Ekman transport and the overall southward shift of the STF have also resulted in an observed positive trend in chlorophyll-a concentrations south of the STF, which could have ramifications for the biological pump and carbon uptake in the Southern Ocean.

True gravity is a three-dimensional vector, g = igλ+jgφ+kgz, with (λ, φ, z) the (longitude, latitude, height) and (i, j, k) the corresponding unit vectors. The vertical direction is along g, not along k, which is normal to the Earth spherical (or ellipsoidal) surface (called deflected-vertical). Correspondingly, the spherical (or ellipsoidal) surfaces are not horizontal surfaces (called deflected-horizontal surfaces). In the (λ, φ, z) coordinates, the true gravity g has longitudinal-latitudinal component, gh = igλ+jgφ, but it is neglected completely in meteorology through using the standard gravity (-g0k, g0 = 9.81 m/s2) instead. Such simplification on the true gravity g has never been challenged. This study uses the atmospheric Ekman layer as an example to illustrate the importance of gh. The standard gravity (-g0k) is replaced by the true gravity g in the classical atmospheric Ekman layer equation with a constant eddy viscosity (K) and a height-dependent-only density ρ(z) represented by an e-folding stratification. New formulas for the Ekman spiral and Ekman pumping are obtained. The second derivative of the gravity disturbance (T) versus z, also causes the Ekman pumping, , in addition to the geostrophic vorticity with DE the Ekman layer thickness and f the Coriolis parameter. With data from the EIGEN-6C4 static gravity model, the global mean strength of the Ekman pumping due to the true gravity is found to be 4.0 cm s-1. Such evidently large value implies the urgency to include the true gravity g into the atmospheric dynamics.

Many processes throughout the heliosphere such as flares, CMEs, storms and substorms have abrupt onsets. The waiting time between these onsets provides key insights as to the underlying dynamical processes. We explore the tail of these waiting time distributions in the context of random processes driven by the solar magnetic activity cycle, which we approximate by a sinusoidal driver. Analytically, we find that the distribution of large waiting times of such a process approaches a power law slope of -2.5 at large enough waiting time, and we find that this power law is primarily controlled by the conditions when the driving is minimum. We find that the asymptotic behavior of the waiting time distributions of solar flares, coronal mass ejections, geomagnetic storms, and substorms exhibit power laws are in reasonable agreement with a sinusoidally driven nonstationary Poisson process.

Internal deformation within the downgoing plate in subduction zones to accommodate the bending of the plate as it starts to subduct is reflected in widespread intraplate seismicity. This seismicity, extending from the outer rise and outer trench slope, down to intermediate depths within the slab, is dominated by the combination of both normal- and thrust-faulting earthquakes reflecting the accumulation and recovery of down-dip curvature. In the idealised case, where all internal deformation is recovered and slabs descend as a straight plate into the deeper mantle, we might expect the seismic moment released in both extension and compression to balance. However, a number of factors may complicate this: the thermal, compositional, and rheological evolution of the slab as it subducts, changes in the proportion of deformation accommodated seismically, and whether the slab undergoes any permanent deformation (e.g., slab necking). Here, we assess earthquake moment release in intraslab settings around the world, focusing on those subduction systems with relatively simple slab geometries. Whilst moment balances for individual regions are often heavily dependent on extreme large-magnitude events, considering the combination of numerous regions around the western Pacific and eastern Indian Ocean indicates that substantially more deformation is accommodated seismically during bending than during unbending, and that in both settings, significantly more moment release reflects down-dip extension than down-dip compression. This suggests that, although the location of seismicity is clearly related to changes in slab curvature, there is a component of permanent, unrecovered down-dip extension in many subducting slabs.

Spectral analyses of the Greenland Ice Sheet Project 2 (GISP2) δ18O record has been interpreted to show a 1/(1470 yr) spectral peak that is highly statistically significant (p<0.01) relative to a null model, H0, consisting of an auto-regressive order one process. In this study H0 is generalized using an auto-regressive moving-average process that is removed from the GISP2 δ18O time series, yielding a spectral estimate that is approximately level. A rule of thumb is proposed for evaluating the adequacy of H0 involving comparing the expected and observed variance of the logarithm of a leveled spectral estimate. After suitably leveling GISP2 δ18O and accounting for multiple hypothesis testing, multitaper spectral estimation indicates that the 1/(1470 yr) peak is insignificant (p>0.05). The proposed techniques for fitting H0 should be generally applicable to evaluating peaks in other geophysical spectral estimates.

This work proposes a robust and non-gaussian version of the shrinkage-based EnKF implementation, the EnKF-KA. The proposed method is based in the robust H filter and in its ensemble time-local version the EnTLHF, using an adaptive inflation factor depending on the shrinkage covariance estimated matrix. This implies a theoretical and solid background to construct robust filters from the well-known covariance inflation technique. The method is tested using the Lorenz-96 model to evaluate the robustness and performance under different scenarios as ensemble size, observation error, errors in the model specifications, and ensemble gaussianity. The results suggest good robustness of the proposed method in all the evaluated cases compared with the standard EnKF, the shrinkage-based EnKF-KA, and the robust EnTLHF.

In this study, We construct the EMD-LSTM model, combined the Empirical Mode Decomposition algorithm (EMD) and the Long Short Term Memory neural network (LSTM), to predict the variation of the >2MeV electron fluxes. The Pc5 power and related geomagnetic indexes as input parameters are used to predict the >2MeV electron fluxes. Compared the prediction results of the model with other classical prediction models, the results shows that the one-day ahead prediction efficiency of the > 2MeV electron fluxes is above 0.80, and the highest prediction efficiency can reach 0.92 in 2011-2013, which is much better than the prediction result of classical prediction models. Selected two high-energy electron flux storm events to verify, the results indicates that the performance of the EMD-LSTM model in the period of the high-energy electron flux storm is also relatively good, especially for the prediction of high-energy electron fluxes at extreme points, and the prediction is closer to actual observation.

Knowledge of the ambient solar wind is important for accurate space-weather forecasting. A simple-but-effective method of forecasting near-Earth solar-wind conditions is “corotation”, wherein solar-wind structure is assumed to be fixed in the reference frame rotating with the Sun. Under this approximation, observations at a source spacecraft can be rotated to a target location, such as Earth. Forecast accuracy depends upon the rate of solar-wind evolution, longitudinal and latitudinal separation between the source and target, and latitudinal structure in the solar wind itself. The time-evolution rate and latitudinal structure of the solar wind are both strongly influenced by the solar cycle, though in opposing ways. A latitudinal offset is typically present, introducing an error to corotation forecasts. In this study, we use observations from the STEREO and near-Earth spacecraft to quantify the latitudinal error. Aliasing between the solar cycle and STEREO orbits means that individual contributions to the forecast error are difficult to isolate. However, by considering an 18-month interval near the end of solar minimum, we find that the latitudinal-offset contribution to corotation-forecast error cannot be directly detected for offsets <6º, but is increasingly important as offsets increase. This result can be used to improve solar-wind data assimilation, allowing representivity errors in solar-wind observations to be correctly specified. Furthermore, as the maximum latitudinal offset between L5 and Earth is ≈5º, corotation forecasts from a future L5 spacecraft should not be greatly affected by latitudinal offset.