Sheet Thinning Research Articles

Numerical investigation of magnetic reconnection in Jupiter’s dayside magnetodisc

Context. Dayside magnetodisc reconnection is a unique process that may occur in rapidly rotating magnetospheres with internal plasma sources, and it is not expected at Earth. Several observations suggest that dayside magnetodisc reconnection could be driven in the Kronian magnetosphere. This raises the question as to whether it can occur at Jupiter, the fastest rotating planet in the Solar System. Recent reports have suggested its occurrence, but the limited number of identified events leaves the overall global picture in the Jovian magnetosphere uncertain. Aims. The primary objective of this study is to answer whether dayside magnetodisc reconnection could exist within the Jovian magnetosphere through the use of global magnetohydrodynamics (MHD) simulations. Additionally, we aim to conduct a numerical investigation to reveal its general distribution for possible future exploration. Methods. The numerical simulations for Jovian magnetospheric dynamics are based on three-dimensional MHD calculations, which enable magnetic reconnection through numerical resistivity. In post-processing procedures, the simulated magnetodisc reconnection event is primarily identified by equatorial Bθ reversals, accompanied by an analysis of local plasma flows and global magnetic field lines. Results. Our global MHD simulations reveal the existence of magnetodisc reconnection within the dayside Jovian magnetosphere. The simulation results indicate that dayside magnetodisc reconnection is more likely to occur during solar wind rarefaction, primarily due to the rapid expansion of the dayside magnetospheric volume. This expansion allows a rotation-driven centrifugal force to generate sufficient magnetic field line stretching for current sheet thinning and reconnection. In simulated dayside events, Bθ reversals may occur locally or non-locally through rotational transport from reconnection sites at earlier magnetic local times. Conclusions. The existence and the features derived from simulations are testable as Galileo/Juno has accumulated a considerable amount of dayside magnetospheric data. These results are valuable for future Jovian missions, such as JUICE and Europa Clipper, and provide new insights into interpreting measurements from other rapidly rotating systems.

Picturing Global Substorm Dynamics in the Magnetotail Using Low‐Altitude ELFIN Measurements and Data Mining‐Based Magnetic Field Reconstructions

AbstractA global reconfiguration of the magnetotail characterizes substorms. Current sheet thinning, intensification, and magnetic field stretching are defining features of the substorm growth phase and their spatial distributions control the timing and location of substorm onset. Presently, sparse in‐situ observations cannot resolve these distributions. A promising approach is to use new substorm magnetic field reconstruction methods based on data mining, termed SST19. Here we compare the SST19 reconstructions to low‐altitude electron losses and fields investigation (ELFIN) measurements of energetic particle precipitations to probe the radial profile of the equatorial magnetic field curvature during a 19 August 2022 substorm. ELFIN and SST19 yield a consistent dynamical picture of the magnetotail during the growth phase and capture its key features such as the formation of a thin current sheet and its earthward motion. Furthermore, they resolve a “checkmark” pattern of isotropic electron precipitation boundaries in the time‐energy plane, consistent with earlier observations but now over a broad energy range. It is shown that in the growth phase, the mismatch between SST19 and ELFIN latitudes is much less than one degree, the capability unattainable for any other empirical or first‐principles model.

Dynamical response of the southwestern Laurentide Ice Sheet to rapid Bølling–Allerød warming

Abstract. The shift in climate that occurred between the Last Glacial Maximum (LGM) and the Early Holocene (ca. 18–12 kyr BP) displayed rates of temperature increase similar to present-day warming trends. The most rapid recorded changes in temperature occurred during the abrupt climate oscillations known as the Bølling–Allerød interstadial (14.7–12.9 kyr BP) and the Younger Dryas stadial (12.9–11.7 kyr BP). Reconstructing ice sheet dynamics during these climate oscillations provides the opportunity to assess long-term ice sheet evolution in reaction to a rapidly changing climate. Here, we use glacial geomorphological inversion methods (flowsets) to reconstruct the ice flow dynamics and the marginal retreat pattern of the southwestern sector of the Laurentide Ice Sheet (SWLIS). We combine our reconstruction with a recently compiled regional deglaciation chronology to depict ice flow dynamics that encompass the time period from pre-LGM to the Early Holocene. Our reconstruction portrays three macroscale reorganizations in the orientation and dynamics of ice streaming followed by regional deglaciation associated with rapid warming during the Bølling–Allerød interstadial. Initial westward flow is documented, likely associated with an early set of ice streams that formed during the advance to the LGM. During the LGM ice streaming displays a dominant north to south orientation. Ice sheet thinning at ∼15 ka is associated with a macroscale reorganization in ice stream flow, with a complex of ice streams recording south-eastward flow. A second macroscale reorganization in ice flow is then observed at ∼14 ka, in which southwestern ice flow is restricted to the Hay, Peace, Athabasca, and Churchill river lowlands. Rates of ice sheet retreat then slowed considerably during the Younger Dryas stadial; at this time, the ice margin was situated north of the Canadian Shield boundary and ice flow continued to be sourced from the northeast. Resulting from these changes in ice sheet dynamics, we recognize a three-part pattern of deglacial landform zonation within the SWLIS characterized by active ice margin recession, stagnation and downwasting punctuated by local surging (terrestrial ice sheet collapse): the outer deglacial zone contains large recessional moraines aligned with the direction of active ice margin retreat; the intermediate deglacial zone contains large regions of hummocky and stagnation terrain, in some areas crosscut by the signature of local surges, reflecting punctuated stagnation and downwasting; and the inner deglacial zone contains inset recessional moraines demarcating progressive regional ice margin retreat. We attribute these macroscale changes in ice flow geometry and associated deglacial behaviour to external climatic controls during the Bølling–Allerød and Younger Dryas but also recognize the role of internal (glaciological, lithological and topographic) controls in SWLIS dynamics.

2.5D Magnetohydrodynamic Simulation of the Formation and Evolution of Plasmoids in Coronal Current Sheets

In the present paper, using MPI-AMRVAC, we perform a 2.5D numerical magnetohydrodynamic simulation of the dynamics and associated thermodynamical evolution of an initially force-free Harris current sheet subjected to an external velocity perturbation under the condition of uniform resistivity. The amplitude of the magnetic field is taken to be 10 G, typical of the solar corona. We impose a Gaussian velocity pulse across this current sheet that mimics the interaction of fast magnetoacoustic waves with a current sheet in the corona. This leads to a variety of dynamics and plasma processes in the current sheet, which is initially quasi-static. The initial pulse interacts with the current sheet and splits into a pair of counterpropagating wavefronts, which form a rarefied region that leads to an inflow and a thinning of the current sheet. The thinning results in Petschek-type magnetic reconnection followed by a tearing instability and plasmoid formation. The reconnection outflows containing outward-moving plasmoids have accelerated motions with velocities ranging from 105 to 303 km s−1. The average temperature and density of the plasmoids are found to be 8 MK and twice the background density of the solar corona, respectively. These estimates of the velocity, temperature, and density of the plasmoids are similar to values reported from various solar coronal observations. Therefore, we infer that the external triggering of a quasi-static current sheet by a single-velocity pulse is capable of initiating magnetic reconnection and plasmoid formation in the absence of a localized enhancement of resistivity in the solar corona.

Inherited terrestrial cosmogenic nuclides in landscapes of selective glacial erosion: lessons from Lochnagar, Eastern Grampian Mountains, Scotland

ABSTRACTInheritance from prior exposure often complicates the interpretation of terrestrial cosmogenic nuclide (TCN) inventories in glaciated terrain. Lochnagar, a mountain in eastern Scotland, holds a clear geomorphological record of corrie glaciation and the thinning of the last Scottish ice sheet over the last ~15 ka. Yet attempts to date the main stages in deglaciation after sampling of 21 granite boulders for 10Be, 26Al and 14C from corrie moraines, an ice sheet lateral moraine and boulder spreads revealed widespread, but variable, TCN inheritance. Only the youngest boulder ages fit within the range of expected deglaciation ages. To identify the sources of geological uncertainty, we provide simple models of ice cover duration and erosion histories for plateau, corrie and strath landscape domains, identify the variable nuclide inheritance that derives from different sources for boulders in these domains, and outline the effects of rotation, splitting and erosion of boulders during glacial transport. The combined effects increase clustering around arbitrary mean TCN values that exceed deglaciation ages. A further implication is that boulders have survived beneath overriding ice sheets. Such boulder trapping at Lochnagar may have resulted from topographic controls on katabatic winds and surface ablation acting on a thinning, cold‐based ice sheet.

Coupling MAR (Modèle Atmosphérique Régional) with PISM (Parallel Ice Sheet Model) mitigates the positive melt–elevation feedback

Abstract. The Greenland Ice Sheet is a key contributor to sea level rise. By melting, the ice sheet thins, inducing higher surface melt due to lower surface elevations, accelerating the melt coming from global warming. This process is called the melt–elevation feedback and can be considered by using two types of models: either (1) atmospheric models, which can represent the surface mass balance (SMB), or SMB estimates resulting from simpler models such as positive degree day models or (2) ice sheet models representing the surface elevation evolution. The latter ones do not represent the surface mass balance explicitly as well as polar-oriented climate models. A new coupling between the MAR (Modèle Atmosphérique Régional) regional climate model and the PISM (Parallel Ice Sheet Model) ice sheet model is presented here following the CESM2 (Community Earth System Model; SSP5-8.5, Shared Socioeconomic Pathway) scenario until 2100 at the MAR lateral boundaries. The coupling is extended to 2200 with a stabilised climate (+7 ∘C compared to 1961–1990) by randomly sampling the last 10 years of CESM2 to force MAR and reaches a sea level rise contribution of 64 cm. The fully coupled simulation is compared to a one-way experiment where surface topography remains fixed in MAR. However, the surface mass balance is corrected for the melt–elevation feedback when interpolated on the PISM grid by using surface mass balance vertical gradients as a function of local elevation variations (offline correction). This method is often used to represent the melt–elevation feedback and prevents a coupling which is too expensive in computation time. In the fully coupled MAR simulation, the ice sheet morphology evolution (changing slope and reducing the orographic barrier) induces changes in local atmospheric patterns. More specifically, wind regimes are modified, as well as temperature lapse rates, influencing the melt rate through modification of sensible heat fluxes at the ice sheet margins. We highlight mitigation of the melt lapse rate on the margins by modifying the surface morphology. The lapse rates considered by the offline correction are no longer valid at the ice sheet margins. If used (one-way simulation), this correction implies an overestimation of the sea level rise contribution of 2.5 %. The mitigation of the melt lapse rate on the margins can only be corrected by using a full coupling between an ice sheet model and an atmospheric model.

An investigation of multi-pass incremental sheet forming

This study investigates toolpath strategies of multi-pass incremental sheet forming (MSPIF) including a combination of an IN (I) pass where the tool moves inwards and downwards and an OUT (O) pass where the tool moves outward and upward. MSPIF processes (III, IIO, IOI and IOO) were investigated to form cone and dome shapes using experimental testing and finite element (FE) simulation. It was found that any IN pass after the initial IN pass, produced stepped features due to Rigid Body Translation (RBT) of the formed part, with the magnitude of the step being dependent on the both the quantity and position of the IN passes. The IOO strategy was more effective in producing a flat bottom cup (no stepped features), but due to the succession of OUT passes used, sheet thinning occurred near the centre of the formed part. A 4-stage IOOO strategy successfully increased the final maximum draw angle by 23% relative to the maximum wall angle of a single pass. The results have shown that the MSPIF approach can be used as an effective means to improve the formability in incremental sheet forming.

Double-layer sheets SPR-bonded hybrid joining method of an extendable die

To further improve the strength and quality of the Sheet connection and increase the traditional self-piercing riveted (SPR) interlocking, a self-piercing riveted-bonded (SPR-bonded) hybrid joining method for double-layer sheets based on an extendable die was proposed. Three kinds of heterogeneous sheets expandable die SPR-bonded hybrid forming rules were investigated. The mechanical properties and failure mechanisms of SPR, adhesive, and hybrid joints were compared. Results show that compared with the traditional SPR die, the expandable die can increase the joint interlocking value to a certain extent and thus improve the strength of the connection by delaying the deformation of the lower plate and guiding the rivets to flare twice. The forming load is low when magnesium alloy is used as the upper sheet. Joint stress concentration, strain concentration, and thinning of the lower sheet are light. From the point of view of the shear strength of each joint, the hybrid joints of the three types of sheet materials increased the shear strength of the joints by 11 %, 36 %, and 30 %, respectively, compared with their respective SPR joints. The existence of the adhesive layer in the hybrid joint can improve the overall strength of the hybrid joint on the one hand and delay the destruction of the interlocking structure in the hybrid joint on the other hand. In general, the combination of an extendable die and SPR-bonded hybrid could improve the overall strength and safety of the joint and can be used for joining heterogeneous sheets.

Efficient net shape forming of high-strength sheet metal parts by Transversal Compression Drawing

In this contribution, a new forming method is presented for high-strength steel sheet metal materials called Transversal Compression Drawing (TCD). For TCD, the blankholder of the conventional deep drawing process is replaced by sliders introducing a vertical force along the blank edge. Compared to conventional deep drawing, compressive stresses are thereby superimposed in the flange area of the sheet metal material, reducing the springback of formed components and the forces required during the process. Proof of this is provided here on the basis of numerical investigations in which conventional deep drawing and TCD are compared using the example of an “S-Rail” component made from DP980. Results of these investigations reveal that with TCD a maximum reduction in springback by more than 5 mm is achieved, leading to an improvement in the component´s overall shape accuracy of more than 55%. Furthermore, frictional work is decreased by − 6.92 kJ/kg, resulting in an overall reduction of maximum forming forces by 67.4%. Besides, TCD enables a trim-free net shape forming of the final part contour in a single operation and an increased material utilization of 4.5% from coil to part. Not least, no sheet thinning occurs with TCD, whereas a maximum thinning of 12% is observed with the conventional deep drawing process.

Location and Timing of Magnetic Reconnections in Earth's Magnetotail: Accomplishments of the 29‐Year Geotail Near‐Earth Magnetotail Survey

AbstractThe spacecraft Geotail surveyed the near‐Earth plasma sheet from XGSM = −10 to −31 RE and YGSM = −20 to +20 RE during the period from 1994 to 2022. It observed 243 magnetic reconnection events and 785 tailward flow events under various solar wind conditions during plasma sheet residence time of over 23,000 hr. Magnetic reconnections associated with the onset of magnetospheric substorms occur mostly in the range XGSM = −23 to −31 RE. When the solar wind is intense and high substorm activities continue, magnetic reconnection can occur closer to the Earth. The YGSM locations of magnetic reconnections depend on the solar wind conditions and on previous substorm activity. Under normal solar wind conditions, magnetic reconnection occurs preferentially in the pre‐midnight plasma sheet. Under conditions with intense (weak) solar wind energy input, however, magnetic reconnection can occur in the post‐midnight (duskside) plasma sheet. Continuous substorm activity tends to shift the magnetic reconnection site duskward. The plasma sheet thinning proceeds faster under intense solar wind conditions, and the loading process that provides the preconditions for magnetic reconnection becomes shorter. When magnetic flux piles up during a prolonged period with a strongly northward‐oriented interplanetary magnetic field (IMF) Bz, the time necessary to provide the preconditions for magnetic reconnection becomes longer. Although the solar wind conditions are the primary factors that control the location and timing of magnetic reconnections, the plasma sheet conditions created by preceding substorm activity or the strongly northward IMF Bz can modify the solar wind control.

Contrasting optical properties of dissolved organic matter between oceanic regions near the Getz and Dotson ice shelves in the Amundsen Sea, West Antarctica

The Amundsen Sea, located in West Antarctica, is experiencing rapid melting due to the intrusion of Circumpolar Deep Water, which is causing ice sheet thinning and basal melting. The resulting changes can affect the biogeochemical cycle of dissolved organic matter (DOM) by supplying iron from sea ice and/or glacier, thereby influencing primary production and ocean circulation. Therefore, it is crucial to understand the dynamics of the DOM in this region. In this study, our primary focus was to examine the optical properties of DOM in the oceanic regions adjacent to the West Getz Ice Shelf (WGIS) and the Dotson Ice Shelf (DIS). Significant differences in DOM optical properties, including the chromophoric DOM (CDOM) absorption coefficient at 350 nm (a350), spectral slope coefficient (S275–295), and specific UV absorbance at 254 nm (SUVA254), were observed between the WGIS and DIS regions (t-test, p < 0.05). Notably, the WGIS regions exhibited high a350 values. Additionally, the S275–295 and SUVA254 values, which serve as indices of molecular weight, indicated that the DOM pool in the WGIS regions was dominated by high molecular weight compounds with a substantial proportion of aromatic compounds. In contrast, the low values of a350 and SUVA254 along with the high S275–295 values in the DIS region suggested the dominance of low molecular weight CDOM associated with compounds of lower aromaticity. Furthermore, significant negative correlations were found between biomass of Phaeocystis antarctica (P. antarctica) and phosphate (PO4) in the WGIS regions (r2 = 0.82, p < 0.01 for WGIS 1 and r2 = 0.73, p < 0.01 for WGIS 2). However, no significant relationship was observed in the DIS region. These findings suggest that the high value and molecular weight of a350, extending from the surface layer to the deep layer, in the WGIS regions were associated with autochthonous sources, primarily driven by the colony-forming bloom of P. antarctica. These findings demonstrate that the quantity and quality of DOM in the Amundsen Sea are strongly affected by bloom conditions. The results emphasize that a combination of physical and biological processes interacts in complex ways to determine the characteristics of DOM in the Amundsen Sea.

A new flexible multi-point incremental sheet forming process with multi-layer sheets

In this research, a new flexible multi-point incremental sheet forming process with multi-layer sheets (F-MPIF-MLS) was proposed. The novelty of this new forming process is demonstrated by two specific aspects of contribution. The first is the development of a new flexible multi-point die system to replace the conventional multi-point die, in which the multi-point pins are in contact with and provide support to the blank sheet from the start to the end of the forming operation. The second is the use of multi-layer sheets, which allows the target blank sheet to be deformed without clamping constraints. The new F-MPIF-MLS process clearly shows the benefits of overcoming the limitations of excessive sheet thinning and poor surface finish of the existing conventional multi-point incremental sheet forming (MPIF) process. A detailed comparative investigation into the new F-MPIF-MLS and the conventional MPIF processes is conducted through experimental testing and finite element (FE) simulation. The results show a noticeable reduction of wrinkling by using the new flexible multi-point die over the conventional multi-point die system with multi-layer sheets to form a dome shape. Moreover, in using the new F-MPIF-MLS process, the maximum thickness reduction is significantly reduced from 35% to 5% as compared to the conventional MPIF process to achieve a more uniform thickness distribution of the formed part.

Multiscale Observation of Magnetotail Reconnection Onset: 1. Macroscopic Dynamics

AbstractWe analyze a magnetotail reconnection onset event on 3 July 2017 that was observed under otherwise quiescent magnetospheric conditions by a fortuitous conjunction of six space and ground‐based observatories. The study investigates the large‐scale coupling of the solar wind–magnetosphere system that precipitated the onset of the magnetotail reconnection, focusing on the processes that thinned and stretched the cross‐tail current layer in the absence of significant flux loading during a 2‐hr‐long preconditioning phase. It is demonstrated with data in the (a) upstream solar wind, (b) at the low‐latitude magnetopause, (c) in the high‐latitude polar cap, and (d) in the magnetotail that the typical picture of solar wind‐driven current sheet thinning via flux loading does not appear relevant for this particular event. We find that the current sheet thinning was, instead, initiated by a transient solar wind pressure pulse and that the current sheet thinning continued even as the magnetotail and solar wind pressures decreased. We suggest that field line curvature‐induced scattering (observed by magnetospheric multiscale) and precipitation (observed by Defense Meteorological Satellite Program) of high‐energy thermal protons may have evacuated plasma sheet thermal energy, which may require a thinning of the plasma sheet to preserve pressure equilibrium with the solar wind.

Hot-Air Contactless Single-Point Incremental Forming

Single-point incremental forming (SPIF) has emerged as a time-efficient approach that offers increased material formability compared to conventional sheet-metal forming techniques. However, the physical interaction between the forming tool and the sheet poses challenges, such as tool wear and formability limits. This study introduces a novel sheet-forming technique called contactless single-point incremental forming (CSPIF), which uses hot compressed air as a deformation tool, eliminating the requirement for physical interaction between the sheet and a rigid forming tool. In this study, a polycarbonate sheet was chosen as the case-study material and subjected to the developed CSPIF. The experiments were carried out at an air temperature of 160 °C, air pressure of 1 bar, a nozzle speed of 750 mm/min, and a step-down thickness of 0.75 mm. A Schlieren setup and a thermal camera were used to visualize the motion of the compressed hot air as it traveled from the nozzle to the sheet. The results showed that the CSPIF technique allowed for the precise shaping of the polycarbonate sheet with minimal springback. However, minor deviations from the designed profile were observed, primarily at the starting point of the nozzle, which can be attributed to the bending effects of the sample. In addition, the occurrence of sheet thinning and material buildup on the deformed workpiece was also observed. The average surface roughness (Ra) of the deformed workpiece was measured to be 0.2871 microns.

Interplay of Three‐Dimensional Instabilities and Magnetic Reconnection in the Explosive Onset of Magnetospheric Substorms

AbstractMagnetospheric substorms are preceded by a slow growth phase of magnetic flux loading and current sheet thinning in the tail. Extensive data sets have provided evidence of the triggering of instabilities at substorm onset, including magnetic reconnection and ballooning instabilities. Using an exact kinetic magnetotail equilibrium we present particle‐in‐cell simulations which capture the explosive nature of substorms through a disruption of the dipolarization front by the ballooning instability. We use self‐consistent particle tracking to determine the nonthermal particle acceleration mechanisms.

Formability and Dimensional Accuracy Enhancement with Controlled Tool Rotational Speed in Stretch Forming Process

We present a new stretch forming method for axisymmetric sheet metal parts production. A test bench is designed, fabricated, and mounted on a drilling machine. The blank is fixed rigidly around its periphery in the apparatus, and the forming tool is mounted on the machine spindle. Thus, the control of tool rotational speed, feed rate, and vertical motion from the upper sheet surface becomes possible. Experimental tests are then conducted to form a reference shape. Rotational speed, feed rate, lubrication effect on the final thickness distribution, and geometric profiles are studied. A new multi-pass forming method is also developed and verified. The feed rate decrease, within the range of 0.11–0.18[Formula: see text]mm/rev, reduces the sheet thinning, while its increase improves the dimensional accuracy. The rotational speed increase, within the range of 112–710[Formula: see text]rpm, reduces the thickness, while its decrease enhances the geometrical accuracy. Mineral oil seems to be more effective than greases, so the thickness drop is reduced and the dimensional accuracy is improved. The use of the multi-pass strategy avoids the sheet cracking at great forming depth. The decrease in step size, within the range of 0.3–1[Formula: see text]mm, reduces the thickness minima, while good dimensional accuracy can be obtained in the range of 0.5–1[Formula: see text]mm.

Effect of Wall Angle on Thickness Distribution in Single Point Incremental Forming: Finite Element Analysis and Theoretical Approach

The incremental sheet forming process (ISF) is the emerging forming process in the small size product manufacturing industries where as embossing, stamping, coining operations can be used to produce a product in large quantity. Therefore, ISF is one of the best solution to decrease the cost of production when a small quantity of product is demanded. In present study Efforts are devoted to study the effect of wall angle on sheet thinning, reaction forces, reaction moments, etc. for truncated cone of aluminium alloy (AA1050) using Finite Element simulation. Initially, A MATLAB code was written for spiral tool path. Modeling and simulation was done in ABAQUS software. Thereafter, a theoretical sheet thickness was computed using sine law and it was found out to be in close agreement with simulated sheet thickness.

Contemporary ice sheet thinning drives subglacial groundwater exfiltration with potential feedbacks on glacier flow.

Observations indicate that groundwater-laden sedimentary aquifers are extensive beneath large portions of the Greenland and Antarctic ice sheets. A reduction in the mechanical loading of aquifers is known to lead to groundwater exfiltration, a discharge of groundwater from the aquifer. Here, we provide a simple expression predicting exfiltration rates under a thinning ice sheet. Using contemporary satellite altimetry observations, we predict that exfiltration rates may reach tens to hundreds of millimeters per year under the fastest thinning parts of the Antarctic Ice Sheet. In parts of West Antarctica, predicted rates of exfiltration would cause the total subglacial water discharge rate to be nearly double what is currently predicted from subglacial basal melting alone. Continued Antarctic Ice Sheet thinning into the future guarantees that the rate and potential importance of exfiltration will only continue to grow. Such an increase in warm, nutrient-laden subglacial water discharge would cause changes in ice sliding, melt of basal ice and marine biological communities.

MESSENGER Observations of Multiple Magnetic Energy Releases during Mercury’s Substorm

Substorms are energy storage and dissipation processes that occur in planetary magnetospheres accompanied by numerous phenomena, such as magnetic dipolarizations, plasma sheet thinning and thickening, magnetic flux ropes, and plasmoid ejections. Recent MESSENGER observations have revealed substorm activity in Mercury’s near magnetotail, with average durations of 2–3 minutes for both the growth and expansion phases. Herein, we report MESSENGER spacecraft observations of sporadic and short-term energy dissipation processes during the substorm expansion phase in Mercury’s magnetotail. Three magnetic dipolarizations lasting for ∼4 minutes were detected during the expansion phase and manifested as step-like enhancements in the northward component of the magnetic field. The magnetic field strength decreased during each dipolarization, indicating an intermittent release of magnetic energy. They arise owing to multiple magnetic pileup effects caused by planetward bursty bulk flows ejected from distant magnetotail reconnections. Additionally, in close proximity to each dipolarization, we observed the occurrence of pulse-like fluctuations in the B y component. These new MESSENGER observations suggest that during the expansion phase of Mercury’s substorm, the energy dissipation can be caused by multiple local transient processes, although the duration is much shorter than that of Earth.

Reversible ice sheet thinning in the Amundsen Sea Embayment during the Late Holocene

Abstract. Cosmogenic-nuclide concentrations in subglacial bedrock cores show that the West Antarctic Ice Sheet (WAIS) at a site between Thwaites and Pope glaciers was at least 35 m thinner than present in the past several thousand years and then subsequently thickened. This is important because of concern that present thinning and grounding line retreat at these and nearby glaciers in the Amundsen Sea Embayment may irreversibly lead to deglaciation of significant portions of the WAIS, with decimeter- to meter-scale sea level rise within decades to centuries. A past episode of ice sheet thinning that took place in a similar, although not identical, climate was not irreversible. We propose that the past thinning–thickening cycle was due to a glacioisostatic rebound feedback, similar to that invoked as a possible stabilizing mechanism for current grounding line retreat, in which isostatic uplift caused by Early Holocene thinning led to relative sea level fall favoring grounding line advance.