Westward Drift Research Articles

Midlatitude Plasma Blob‐like Structures Along With Super Equatorial Plasma Bubbles During the May 2024 Great Geomagnetic Storm

AbstractPlasma blob is generally a low‐latitude phenomenon occurring at the poleward edge of equatorial plasma bubble (EPB) during post‐sunset periods. Here we report a case of midlatitude ionospheric plasma blob‐like structures occurring along with super EPBs over East Asia around sunrise during the May 2024 great geomagnetic storm. Interestingly, the blob‐like structures appeared at both the poleward and westward edges of EPBs, reached up to 40°N magnetic latitudes, and migrated westward several thousand kilometers together with the bubble. The total electron content (TEC) inside the blob‐like structures was enhanced by ∼50 TEC units relative to the ambient ionosphere. The blob‐like structure at the EPB poleward edge could be partly linked with field‐aligned plasma accumulation due to poleward development of bubble. For the blob‐like structure at the EPB west side, one possible mechanism is that it was formed and enhanced accompanying the bubble evolution and westward drift.

Characterization of hydromagnetic waves propagating over a steady, non-axisymmetric background magnetic field

Motivated by recent observations of rapid (interannual) signals in the geomagnetic data, and by advances in numerical simulations approaching the Earth’s outer core conditions, we present a study on the dynamics of hydromagnetic waves evolving over a static base state. Under the assumption of timescales separation between the rapid waves and the slow convection, we linearize the classical magneto-hydrodynamics equations over a steady non-axisymmetric background magnetic field and a zero velocity field. The initial perturbation is a super-rotating pulse of the inner core, which sets the amplitude and length scales of the waves in the system. The initial pulse triggers axisymmetric, outward propagating torsional Alfvén waves, with characteristic thickness scaling with the magnetic Ekman number as E k M 1 / 4 . Because the background state is non-axisymmetric, the pulse also triggers non-axisymmetric, quasi-geostrophic (QG) Alfvén waves. As these latter waves propagate outwards, they turn into QG, magneto-Coriolis (QG-MC) waves as the Coriolis force supersedes inertia in the force balance. The period of the initial wave packet is preserved across the shell but the QG-MC wavefront disperses and a westward drift is observed after this transformation. Upon reaching the core surface, the westward drift of the QG-MC waves presents an estimated phase speed of about 1100 k m y − 1 . This analysis confirms the QG-MC nature of the rapid magnetic signals observed in geomagnetic field models near the Equator.

A Detailed Study of Jupiter’s Great Red Spot over a 90-day Oscillation Cycle

Jupiter’s Great Red Spot (GRS) is known to exhibit oscillations in its westward drift with a 90-day period. The GRS was observed with the Hubble Space Telescope on eight dates over a single oscillation cycle in 2023 December to 2024 March to search for correlations in its physical characteristics over that time. Measured longitudinal positions are consistent with a 90-day oscillation in drift, but no corresponding oscillation is found in latitude. We find that the GRS size and shape also oscillate with a 90-day period, having a larger width and aspect ratio when it is at its slowest absolute drift (minimum date-to-date longitude change). The GRS’s UV and methane gas absorption-band brightness variations over this cycle were small, but the core exhibited a small increase in UV brightness in phase with the width oscillation; it is brightest when the GRS is largest. The high-velocity red collar also exhibited color changes, but out of phase with the other oscillations. Maximum interior velocities over the cycle were about 20 m s−1 larger than minimum velocities, slightly larger than the mean uncertainty of 13 m s−1, but velocity variability did not follow a simple sinusoidal pattern as did other parameters such as longitude width or drift. Relative vorticity values were compared with aspect ratios and show that the GRS does not currently follow the Kida relation.

Analysis of the drift of the South Atlantic Anomaly using particle monitors onboard Insight-HXMT

Using the particle monitors aboard Insight-HXMT satellite, we analyzed over 6.5 years of continuous data to study the South Atlantic Anomaly (SAA) position, average count rate, and surface size, as well as its evolution from June 2017 to March 2024. We confirm the anti-correlation between the particle count rate obtained by particle monitors and the solar activity, as well as the anti-correlation between the evolution of the SAA surface size and the solar activity. Furthermore, this study confirms earlier measurements of a westward drift of the SAA with a rate of 0.343±0.002o/yr in longitude. However, it reveals a smaller northward drift with a rate of 0.028±0.002o/yr in latitude. Two irregular drifts both in longitude and latitude were observed. We suggest that the irregular drifts may be not driven by solar cycle variation of the atmosphere but may be associated with geomagnetic jerks.

Dynamic Characterization of Equatorial Plasma Bubble Based on Triangle Network‐Joint Slope Approach

AbstractThis paper introduces a Triangle Network‐Joint Slope (TN‐JS) approach to characterize the spatial and temporal dynamics of Equatorial Plasma Bubbles (EPBs) during geomagnetic storms. To collaboratively determine the EPB drift directions from multiple stations, a Delaunay triangle network is constructed, utilizing the distribution of Ionospheric Piercing Points (IPPs). The Time Difference of Arrival (TDOA) is extracted through cross‐correlating the Rate of Total Electron Content (ROT). The EPB drift direction can be approximately calculated by considering TDOA and IPP distances within each individual triangle of the network. This calculation is then refined through a joint statistical analysis. Using a reference station as the origin, the remaining stations within the network are projected along the estimated EPB drift direction. A spatial‐temporal color map illustrating regional ionospheric anomaly ROT observations is constructed. The EPB drift velocity among multiple stations can be collectively estimated by fitting the slope of this map, facilitating outlier exclusion. Accounting for satellite dynamic effects and the diverse orbit characteristics of GPS and BDS, corresponding IPP scan velocity compensation is performed and analyzed for EPB dynamic estimation. Using the geomagnetic storm event that occurred on September 8 as a case study, the spatial‐temporal kinetic properties of EPBs is characterized by analyzing Global Navigation Satellite System (GNSS) observations from 17 Hong Kong monitoring stations with the proposed TN‐JS approach. The results indicate during this magnetic event, that EPBs exhibit a westward drift trend with velocities ranging from a few tens to hundreds of meters per second in GPS and BDS observations.

Evolution and disappearance of the paleo-West Pacific Anomaly: Implications to the future of South Atlantic Anomaly

Palaeomagnetic and modern geomagnetic measurements indicate that the South Atlantic Anomaly (SAA) has undergone rapid changes over the past few hundred years. Its minimum intensity decreased at an average rate of 26 not yr−1, accompanied by a continuous westward drift and spatial expansion. Recently, a secondary minimum of SAA emerged near southern Africa, leading to speculation that expansion of the SAA could indicate an impending geomagnetic reversal. Here, we focus on the evolution and disappearance of the paleo-West Pacific Anomaly (WPA), as another SAA-like structure, which may have implications for the future of SAA evolution. We regard the WPA as SAA-like due to its feature and its association with a reversal flux patch on the core-mantle boundary. Consequently, we suggest that the observed evolutionary pattern in the WPA can serve as a reference for other negative anomalies, such as the SAA. By analysing models that combine datasets of archaeomagnetic and historical records, such as gufm1 and HistKalmag, it is found that the WPA occurred between 1600 and 1820 CE. Over its duration, the WPA experienced phases of rapid expansion, drift, and division. Eventually, its primary component faded away, giving rise to a new segment that continued to expand. The initial two evolutionary phases of the WPA are similar to the evolution of the SAA over the past century. According to the WPA's evolution, it suggests that the current state of the SAA may correspond to an early stage of splitting. Forecasts based on the evolution of the WPA indicate a rapid expansion of the anomalous region in the short term, followed by a gradual reduction in its primary component and continued expansion of a new local minimum. This study provides valuable insight into the evolution of the SAA and highlights the potential utility of the WPA as an evolutionary reference for such geomagnetic phenomena.

A Statistical Analysis of the Morphology of Storm‐Enhanced Density Plumes Over the North American Sector

AbstractThe storm‐enhanced density (SED) is a large‐scale midlatitude ionospheric electron density enhancement in the local afternoon sector, which exhibits substantial spatial gradients and thus can impose detrimental effects on modern navigation and communication systems, causing potential space weather hazards. This study has identified a comprehensive list of 49 SED events over the continental US and adjacent regions, by examining strong geomagnetic storms occurring between 2000 and 2023. The ground‐based Global Navigation Satellite System (GNSS) total electron content and data from a new TEC‐based ionospheric data assimilation system were used to analyze the characteristics of SED. For each derived SED events, we have quantified its morphology by employing a Gaussian function to parameterize key characteristics of the SED, such as the plume intensity, central longitude, and half‐width. A statistical analysis of SEDs was conducted for the first time to characterize their climatological features. We found that the SED distribution exhibits a higher peak intensity and a narrower width as geomagnetic activity strengthens. The peak intensity of SED has maximum values around the equinoxes in their seasonal distribution. Additionally, we observed a solar cycle dependence in the SED distribution, with more events occurring during the solar maximum and declining phases compared to the solar minimum. SED plumes exhibit a sub‐corotation feature with respect to the Earth, characterized by a westward drift speed between 50 and 400 m/s and a duration of 3–10 hr. These information advanced the current understanding of the spatial‐temporal variation of SED characteristics.

The impact of the Hunga Tonga–Hunga Ha’apai volcanic eruption on the Peruvian atmosphere: from the sea surface to the ionosphere

The eruption of the Hunga Tonga Hunga Ha’apai volcano on 15 January 2022 significantly impacted the lower and upper atmosphere globally. Using multi-instrument observations, we described disturbances from the sea surface to the ionosphere associated with atmospheric waves generated by the volcanic eruption. Perturbations were detected in atmospheric pressure, horizontal magnetic field, equatorial electrojet (EEJ), ionospheric plasma drifts, total electron content (TEC), mesospheric and lower thermospheric (MLT) neutral winds, and ionospheric virtual height measured at low magnetic latitudes in the western South American sector (mainly in Peru). The eastward Lamb wave propagation was observed at the Jicamarca Radio Observatory on the day of the eruption at 13:50 UT and on its way back from the antipodal point (westward) on the next day at 07:05 UT. Perturbations in the horizontal component of the magnetic field (indicative of EEJ variations) were detected between 12:00 and 22:00 UT. During the same period, GNSS-TEC measurements of traveling ionospheric disturbances (TIDs) coincided approximately with the arrival time of Lamb and tsunami waves. On the other hand, a large westward variation of MLT winds occurred near 18:00 UT over Peru. However, MLT perturbations due to possible westward waves from the antipode have not been identified. In addition, daytime vertical plasma drifts showed an unusual downward behavior between 12:00 and 16:00 UT, followed by an upward enhancement between 16:00 and 19:00 UT. Untypical daytime eastward zonal plasma drifts were observed when westward drifts were expected. Variations in the EEJ are highly correlated with perturbations in the vertical plasma drift exhibiting a counter-equatorial electrojet (CEEJ) between 12:00 and 16:00 UT. These observations of plasma drifts and EEJ are, so far, the only ground-based radar measurements of these parameters in the western South American region after the eruption. We attributed the ion drift and EEJ perturbations to large-scale thermospheric wind variations produced by the eruption, which altered the dynamo electric field in the Hall and Pedersen regions. These types of multiple and simultaneous observations can contribute to advancing our understanding of the ionospheric processes associated with natural hazard events and the interaction with lower atmospheric layers.Graphical

Geochronology and geochemistry of the El Salvador plutonic complex (Sierra de Tamaulipas, NE Mexico): cenozoic tectonic implications of the eastern Mexican alkaline province

ABSTRACT The El Salvador plutonic complex is a 9.2 km2 circular body within the Sierra de Tamaulipas, part of the Eastern Mexican Alkaline Province (EMAP). Three alkaline magmatic suites were identified from El Salvador from the geochemical analysis of 20 samples. Suite A (SiO2 = 58.3–65.3 wt.%) is ferroan (Fe* = 0.761–0.873) and alkalic (MALI = 4.83–9.89). In contrast, Suites B (SiO2 = 52.5–60.8 wt.%; Fe* = 0.680–0.756) and C (SiO2 = 50.5–67.1 wt.%; Fe* = 0.616–0.749) are magnesian. Suite B is alkali-calcic (MALI = -2.18–4.64), while Suite C is alkalic (MALI = 4.48–8.59). All suites display arc-related signatures. U-Pb and fission-track geochronology data reveal two uplift episodes during the cooling history of Suite A. One in the Late Eocene was based on U-Pb zircon (38.42 ± 0.21 Ma) and titanite ages (35.54 ± 3.77 Ma). The other was during the Oligocene from U-Pb apatite (29.9 ± 6.54 Ma) and fission-track titanite (30.2 ± 5.53 Ma) and apatite ages (32.7 ± 7.06 Ma). Integrating the arc-related signatures of El Salvador rocks with the well-documented Palaeogene arc magmatism of the Sierra Madre del Sur, we propose that the mantle beneath the EMAP experienced metasomatism during the Early Permian (and possibly the Early Jurassic) but not after the Late Cretaceous, ruling out Cenozoic slab subduction in eastern Mexico. In the absence of a slab to explain the El Salvador magmatism, we suggest a long-lasting, widespread mantle upwelling beneath Mexico’s northern half in response to the Farallon slab’s break-up. Under this context, the westward drift of the North American plate led to the Mexican Foreland Basin lithosphere reaching this massive mantle upwelling in late Eocene times to produce the EMAP magmatism.

Reversal of drift direction during the Laschamp geomagnetic excursion

Earth's magnetic field changes in both space and time: the temporal changes are called geomagnetic and paleomagnetic secular variations. Westward drift has been noted as a feature of secular variation for several centuries, but eastward drift has received less attention. We use three global geomagnetic field models covering the past 100 kyr to extend temporal coverage for tracking the zonal (azimuthal) motion of the radial magnetic field. The models we use are GGF100k (100–0 ka), GGFSS70 (70–15 ka), LSMOD.2 (50–30 ka); the extent of the models enables the inclusion of the extreme secular variations found during excursions, particularly the Laschamp excursion (42–40 ka). GGFSS70 and LSMOD.2 have higher temporal resolution than GGF100k, but their underlying data have poorer spatial coverage. Spatial structure is greatly diminished in all models for spherical harmonic degrees l>4.We use two types of time-longitude plots, one of the full radial field to expose reverse and intense flux patches at the core-mantle boundary. The second time-longitude plot is processed to enhance zonal motion signatures and allows us to use Radon drift analyses to uncover characteristic time scales of both westward and eastward drift at mid to high latitudes in both the northern and southern hemispheres. Our results differ across the three models, which we attribute to varying degrees of resolution, accuracy, and data distribution. Nevertheless, recurrent episodes of both eastward and westward drift ranging from ±0.05o/yr to ±0.18o/yr occur in both the northern and southern hemispheres. Westward drift dominates. We also observe 8–20 kyr intervals between occurrences of high-latitude reverse flux patches correlated with strong drift signals. Focusing on the period 50–30 ka, we observe dominant eastward drift preceding the Laschamp excursion and westward drift subsequently. In a period not associated with an excursion, 90–80 ka, we see strong mid to high latitude drift signatures in the northern hemisphere.

Theory of continental drift – causes of the motion. Outline of the theory

The theory of mantle convection currents as the cause of lithospheric plate movements has several major problems, including the absence of an adequate energy source. As shown in our previous contribution, an unbiased interpretation of geochemical data does not support the assumptions of a significant amount of radionuclides in the lower mantle or even in the core. It is our assertion that solar radiation is the primary energy source in the lithosphere. This energy is converted into mechanical energy via thermoelastic waves, even in depths with minimal temperature fluctuations. This has been confirmed by various methods of continuous stress measurement. The periodic and quasiperiodic thermoelastic reversible deformations, such as the circadian and annual cycles (including tidal periods), can also cause irreversible deformations due to the ratcheting mechanism. The 2D model showed that the strength limit is exceeded in 0.3 % of all diurnal cycles during the year. As a consequence, continents tend to extend while the oceanic lithosphere is pushed and overthrusted between continents. The middle-ocean ridges, similar to continental rifts, are filled by ascending magma which is one example of the ratcheting mechanism. The final plate movements are determined by the distribution of major continents and the overall westward drift of the lithosphere, which is slower for deep-rooted plates like the Indian one. Large asteroid impacts are important triggers (and possibly significant energy sources) of discrete events, like the formation of hotspots and large igneous provinces.

The Westward Drift of Jupiter's Polar Cyclones Explained by a Center‐of‐Mass Approach

AbstractThe first orbits around Jupiter of the Juno spacecraft in 2016 revealed a symmetric structure of multiple cyclones that remained stable over the next 5 years. Trajectories of individual cyclones indicated a consistent westward circumpolar motion around both poles. In this paper, we propose an explanation for this tendency using the concept of beta‐drift and a “center‐of‐mass” approach. We suggest that the motion of these cyclones as a group can be represented by an equivalent sole cyclone, which is continuously pushed by beta‐drift poleward and westward, embodying the westward motion of the individual cyclones. We support our hypothesis with 2D model simulations and observational evidence, demonstrating this mechanism for the westward drift. This study joins consistently with previous studies that revealed how aspects of these cyclones result from vorticity‐gradient forces, shedding light on the physical nature of Jupiter's polar cyclones.

Simultaneous Ground and Satellite Measurements of the Polarization Jet at the Yakutsk Station Meridian

The data of simultaneous measurements of the polarization jet from the Yakutsk ground-based vertical radio sounding station and satellite observations of narrow electron density dips or fast westward drifts of the ionospheric plasma from satellites of the Defense Meteorological Satellite Program (DMSP) series. The events are based on ground-based ionospheric measurements and cover the time interval from March 1989 to December 2015, i.e. about 26 years. The simultaneity of observations is ensured by a time period of approximately ±1.5 h from the time of registration of signs of a polarization jet according to the data of a ground-based ionospheric sounding station or by the an orbital period of DMSP satellites around the Earth. Based on data of long-term simultaneous satellite and ground-based measurements (126 events), it was shown and confirmed that the presence of characteristic additional traces of reflections on ionograms indicates the presence of narrow and fast drifts of ionospheric plasma or a polarization jet near the zenith of the observation station. It is also shown that the quasi-instantaneous longitude extent of a polarization jet at subauroral latitudes can in some cases reach 8 h or 120° by longitude.

Tidal drag and westward drift of the lithosphere

Tidal forces are generally neglected in the discussion about the mechanisms driving plate tectonics despite a worldwide geodynamic asymmetry also observed at subduction and rift zones. The tidal drag could theoretically explain the westerly shift of the lithosphere relative to the underlying mantle. Notwithstanding, viscosity in the asthenosphere is apparently too high to allow mechanical decoupling produced by tidal forces. Here, we propose a model for global scale geodynamics accompanied by numerical simulations of the tidal interaction of the Earth with the Moon and the Sun. We provide for the first time a theoretical proof that the tidal drag can produce a westerly motion of the lithosphere, also compatible with the slowing of the Earth’s rotational spin. Our results suggest a westerly rotation of the lithosphere with a lower bound of ω≈(0.1-0.2)°/Myr in the presence of a basal effective shear viscosity η≈1016 Pa·s, but it may rise to ω>1°/Myr with a viscosity of η≲3×1014 Pa·s within the Low-Velocity Zone (LVZ) atop the asthenosphere. This faster velocity would be more compatible with the mainstream of plate motion and the global asymmetry at plate boundaries. Based on these computations, we suggest that the super-adiabatic asthenosphere, being vigorously convecting, may further reduce the viscous coupling within the LVZ. Therefore, the combination of solid Earth tides, ultra-low viscosity LVZ and asthenospheric polarized small-scale convection may mechanically satisfy the large-scale decoupling of the lithosphere relative to the underlying mantle. Relative plate motions are explained because of lateral viscosity heterogeneities at the base of the lithosphere, which determine variable lithosphere-asthenosphere decoupling and plate interactions, hence plate tectonics.

Quasi-geostrophic monopoles in a sheared zonal flow: Influence of the beta-effect and variable shear

This work builds on and continues a suite of earlier studies of the interaction between a monopole and a sheared zonal flow in the framework of a 1.5-layer quasi-geostrophic model. In Reznik and Kravtsov [Phys. Fluids 33, 116606 (2021); hereafter RK21], this problem was considered under an f-plane approximation for the case in which the dependence of the zonal velocity U¯(y) on latitude y was linear. Here, the conclusions stemming from that work are generalized for the case of a beta-plane and a variable shear of the background flow. Namely, numerical experiments with singular vortices using the algorithm of Kravtsov and Reznik [“Numerical solutions of the singular vortex problem,” Phys. Fluids 31, 066602 (2019); hereafter KR19] confirm the existence of the trapping latitude ytr, which attracts (repels) prograde (retrograde) vortices and clarifies the underlying mechanisms. Unlike in the case of a linear shear on an f-plane, the latitude ytr here does not necessarily coincide with the latitude at which the effective beta-parameter β¯=β−∂yyU¯+Rd−2U¯ vanishes (here, β denotes the derivative of the Coriolis parameter with respect to latitude and Rd is the Rossby radius of deformation). Another important difference is that in the presence of nonzero β≠0, a trapped prograde vortex exhibits a near-zonal westward drift with the zonal velocity close to the phase speed of long Rossby waves −βRd2 and the meridional velocity at least two orders of magnitude smaller than that. On the other hand, the meridional velocity of a retrograde vortex appears to be unrestricted; such a vortex can rapidly move in any direction, including the direction across the zonal current.

First Observations of Equatorial Ionospheric Electric Fields Driven by Storm‐Time Rapidly Recurrent Magnetospheric Substorms

AbstractGeomagnetic storm ionospheric electrodynamics at the magnetic equator are examined in detail using upgraded, higher time resolution, Jicamarca incoherent scatter radar drift observations, SuperMAG geomagnetic field, and solar wind data to study equatorial ionospheric electric fields during the 8 September 2017 storm main and early recovery phases. We show that during a period of mostly large and southward Interplanetary Magnetic Field (IMF) Bz there were numerous daytime prompt penetration vertical and zonal plasma drifts. These prompt penetration events were closely associated with rapidly recurring (quasi‐periods ∼30 − 60 min) magnetospheric substorms with expansion phases corresponding to upward and westward drift (eastward and downward electric field) perturbations, and recovery phases to opposite polarity changes. The magnitudes of the zonal prompt penetration drifts were about twice larger than those of the vertical drifts. Our study suggests that magnetospheric substorms are the main drivers of prompt penetration electric fields during extended periods (over ∼2 hr) of nearly steady southward IMF Bz. We also find that in general substorms are major driving factors for disturbance electric fields during geomagnetic active times. Our data highlight the powerful capabilities of the Jicamarca incoherent scatter radar as a tool for detailed studies of short‐lived solar wind‐magnetosphere‐ionosphere coupling processes.

Continental tapering of South America caused asymmetric non-uniform opening of the South Atlantic Ocean

South Atlantic opening has been typically modelled as being related to symmetric and static thermal upwelling and seafloor spreading that drive divergent continental drift of South America and Africa. Comparative analyses, however, show that South Atlantic opening is asymmetric and non-uniform. For neither asymmetric nor non-uniform opening are the underlying mechanisms clear. Here I use geological and geophysical data to inform analytical modelling, revealing that westward drifting and southward tapering of the South American continent have controlled the asymmetry and the non-uniformity in South Atlantic opening. I interpret that the asymmetric non-uniform seafloor spreading caused the ridge and hotspots to migrate, leaving behind non-linear seamount trails that are indicative of the speed of hotspot migration rather than direction of plate movement. The findings point towards a chain reaction from continental drifting, through seafloor spreading to ridge-hotspot interaction, which is instrumental in understanding the geodynamics for global plate tectonics.

Results of long-term radiation environment monitoring by the Russian RMS system on board Zvezda module of the ISS

The Radiation monitoring system (RMS) continuously operated in various configurations since the launch of the Zvezda module of the International Space Station (ISS). The RMS consisted of 7 units, namely: the R-16 dosimeter, 4 DB-8 dosimeters, utility and data collection units. The obtained data covers a time of 22 years. This paper analyses the radiation environment variations on board the “Zvezda” module. Variations of the onboard daily dose rate associated with changes of ISS altitude and 11-year cycle galactic cosmic rays’ variations are analyzed and discussed. It is shown that the observed increase in the daily dose from 0.20 - 0.25 to 0.35 - 0.50 mGy/day is mostly due to the increase of ISS orbit altitude, resulting in a substantial increase of the dose contribution from the South Atlantic Anomaly (SAA) Region. Dose rate variations in the SAA as well as latitude and longitude dose rate distributions are discussed in detail. Analysis confirms that the well-known westward drift effect of the SAA is clearly visible from radiation dose measurements on the ISS.

Ocean expansion: The role of detachment faults

AbstractDetachment faulting at slow spreading ocean ridge axes is recognized as a major surface creation mechanism, yet the structural relationships of these faults with feeder‐dykes of on‐axis volcanoes remained unresolved. This summary study shows that surface creation leading to ocean widening is exclusively controlled by detachment faults, shallow tracers of tectonic stresses induced by the westward drift of plates. Volcanoes are fed by feeder‐dykes following on‐axis rotational detachment faults. Once formed volcanoes are dragged along the detachment whose footwall is made of mantle material sometimes hosting gabbro sills. Due to the faster drift of the uppermost lithospheric layer, the feeder‐dykes are then intersected by active deeper low‐angle detachments, become inactive and are replaced by new ones on‐axis. Rooted vertically on either side of the plate boundary, the detachment flexing is all the earlier and more progressive as the faults are far from the axis, positioning gabbro as sills at shallow level within a deformed mantle interspersed with cataclasite horizons. Correlations between shallow and deep lithospheric processes are then clarified.

Global coarse-grained mesoscale eddy statistics based on integrated kinetic energy and enstrophy correlations

Abstract. Recently, Jánosi et al. (2019) introduced the concept of a “vortex proxy” based on an observation of strong correlations between integrated kinetic energy and integrated enstrophy over a large enough surface area. When mesoscale vortices are assumed to exhibit a Gaussian shape, the two spatial integrals have particularly simple functional forms, and a ratio of them defines an effective radius of a “proxy vortex”. In the original work, the idea was tested over a restricted area in the Californian Current System. Here we extend the analysis to global scale by means of 25 years of AVISO altimetry data covering the (ice-free) global ocean. The results are compared with a global vortex database containing over 64 million mesoscale eddies. We demonstrate that the proxy vortex representation of surface flow fields also works globally and provides a quick and reliable way to obtain coarse-grained vortex statistics. Estimated mean eddy sizes (effective radii) are extracted in very good agreement with the data from the vortex census. Recorded eddy amplitudes are directly used to infer the kinetic energy transported by the mesoscale vortices. The ratio of total and eddy kinetic energies is somewhat higher than found in previous studies. The characteristic westward drift velocities are evaluated by a time-lagged cross-correlation analysis of the kinetic energy fields. While zonal mean drift speeds are in good agreement with vortex trajectory evaluation in the latitude bands 30–5∘ S and 5–30∘ N, discrepancies are exhibited mostly at higher latitudes on both hemispheres. A plausible reason for somewhat different drift velocities obtained by eddy tracking and cross-correlation analysis is the fact that the drift of mesoscale eddies is only one component of the surface flow fields. Rossby wave activities, coherent currents, and other propagating features on the ocean surface apparently contribute to the zonal transport of kinetic energy.