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Magnetic reconnection is one of the most universal processes in space plasma that is responsible for charged particle acceleration, mixing and heating of plasma populations. In this paper we consider a triggering process of reconnection that is driven by interaction of two discontinuities: solar wind rotational discontinuity and tangential discontinuity at the Earth's magnetospheric boundary, magnetopause. Combining the multispacecraft measurements and global hybrid simulations, we show that solar wind discontinuities may drive the magnetopause reconnection and cause the mixing of the solar wind and magnetosphere plasmas around the magnetopause, well downstream of the solar wind flow. Since large-amplitude discontinuities are frequently observed in the solar wind and predicted for various stellar winds, our results of reconnection driven by the discontinuity-discontinuity interaction may have a broad application beyond the magnetosphere.

Two unresolved questions at galaxy centers, namely the formation of the nuclear star cluster (NSC) and the origin of the gamma-ray excess in the Milky Way (MW) and Andromeda (M31), are both related to the formation and evolution of globular clusters (GCs). They migrate towards the galaxy center due to dynamical friction, and get tidally disrupted to release the stellar mass content including millisecond pulsars (MSPs), which contribute to the NSC and gamma-ray excess. In this study, we propose a semi-analytical model of GC formation and evolution that utilizes the Illustris cosmological simulation to accurately capture the formation epochs of GCs and simulate their subsequent evolution. Our analysis confirms that our GC properties at z=0 are consistent with observations, and our model naturally explains the formation of a massive NSC in a galaxy similar to the MW and M31. We also find a remarkable similarity in our model prediction with the gamma-ray excess signal in the MW. However, our predictions fall short by approximately an order of magnitude in M31, indicating distinct origins for the two gamma-ray excesses. Meanwhile, we utilize the catalog of Illustris halos to investigate the influence of galaxy assembly history. We find that the earlier a galaxy is assembled, the heavier and spatially more concentrated its GC system behaves at z=0. This results in a larger NSC mass and brighter gamma-ray emission from deposited MSPs, Comment: 11 pages, 18 figures. Accepted for publication in MNRAS. Comments welcome!

We present VisionFM, a foundation model pre-trained with 3.4 million ophthalmic images from 560,457 individuals, covering a broad range of ophthalmic diseases, modalities, imaging devices, and demography. After pre-training, VisionFM provides a foundation to foster multiple ophthalmic artificial intelligence (AI) applications, such as disease screening and diagnosis, disease prognosis, subclassification of disease phenotype, and systemic biomarker and disease prediction, with each application enhanced with expert-level intelligence and accuracy. The generalist intelligence of VisionFM outperformed ophthalmologists with basic and intermediate levels in jointly diagnosing 12 common ophthalmic diseases. Evaluated on a new large-scale ophthalmic disease diagnosis benchmark database, as well as a new large-scale segmentation and detection benchmark database, VisionFM outperformed strong baseline deep neural networks. The ophthalmic image representations learned by VisionFM exhibited noteworthy explainability, and demonstrated strong generalizability to new ophthalmic modalities, disease spectrum, and imaging devices. As a foundation model, VisionFM has a large capacity to learn from diverse ophthalmic imaging data and disparate datasets. To be commensurate with this capacity, in addition to the real data used for pre-training, we also generated and leveraged synthetic ophthalmic imaging data. Experimental results revealed that synthetic data that passed visual Turing tests, can also enhance the representation learning capability of VisionFM, leading to substantial performance gains on downstream ophthalmic AI tasks. Beyond the ophthalmic AI applications developed, validated, and demonstrated in this work, substantial further applications can be achieved in an efficient and cost-effective manner using VisionFM as the foundation.

Electromagnetic ion cyclotron waves are expected to pitch-angle scatter and cause atmospheric precipitation of relativistic ($> 1$ MeV) electrons under typical conditions in Earth's radiation belts. However, it has been a longstanding mystery how relativistic electrons in the hundreds of keV range (but $<1$ MeV), which are not resonant with these waves, precipitate simultaneously with those $>1$ MeV. We demonstrate that, when the wave packets are short, nonresonant interactions enable such scattering of $100$s of keV electrons by introducing a spread in wavenumber space. We generalize the quasi-linear diffusion model to include nonresonant effects. The resultant model exhibits an exponential decay of the scattering rates extending below the minimum resonant energy depending on the shortness of the wave packets. This generalized model naturally explains observed nonresonant electron precipitation in the hundreds of keV concurrent with $>1$ MeV precipitation.

Phase space holes, double layers and other solitary electric field structures, referred to as time domain structures (TDSs), often occur around dipolarization fronts in the Earth's inner magnetosphere. They are considered to be important because of their role in the dissipation of the injection energy and their potential for significant particle scattering and acceleration. Kinetic Alfv\'en waves are observed to be excited during energetic particle injections, and are typically present in conjunction with TDS observations. Despite the availability of a large number of spacecraft observations, the origin of TDSs and their relation to kinetic Alfv\'en waves remains poorly understood to date. Part of the difficulty arises from the vast scale separations between kinetic Alfv\'en waves and TDSs. Here, we demonstrate that TDSs can be excited by electrons in nonlinear Landau resonance with kinetic Alfv\'en waves. These electrons get trapped by the parallel electric field of kinetic Alfv\'en waves, form localized beam distributions, and subsequently generate TDSs through beam instabilities. A big picture emerges as follows: macroscale dipolarization fronts first transfer the ion flow (kinetic) energy to kinetic Alfv\'en waves at intermediate scale, which further channel the energy to TDSs at the microscale and eventually deposit the energy to the thermal electrons in the form of heating. In this way, the ion flow energy associated with dipolarization fronts is effectively dissipated in a cascade from large to small scales in the inner magnetosphere., Comment: Accepted for publication in Journal of Geophysical Research

After protoplanets have acquired sufficient mass to open partial gaps in their natal protostellar disks, residual gas continues to diffuse onto horseshoe streamlines under effect of viscous dissipation, and meander in and out of the planets' Hill sphere. Within the Hill sphere, the horseshoe streamlines intercept gas flow in circumplanetary disks. The host stars' tidal perturbation induces a barrier across the converging streamlines' interface. Viscous transfer of angular momentum across this tidal barrier determines the rate of mass diffusion from the horseshoe streamlines onto the circumplanetary disks, and eventually the accretion rate onto the protoplanets. We carry out a series of numerical simulations to test the influence of this tidal barrier on super thermal planets. In weakly viscous disks, protoplanets' accretion rate steeply decreases with their masses above the thermal limit. As their growth timescale exceeds the gas depletion time scale, their masses reach asymptotic values comparable to that of Jupiter. In relatively thick and strongly viscous disks, protoplanets' asymptotic masses exceed several times that of Jupiter. Two dimensional numerical simulations show that such massive protoplanets strongly excite the eccentricity of nearby horseshoe streamlines, destabilize orderly flow, substantially enhance the diffusion rate across the tidal barrier, and elevate their growth rate until their natal disk is severely depleted. In contrast, eccentric streamlines remain stable in three dimensional simulations. Based on the upper falloff in the observe mass distribution of known exoplanets, we suggest their natal disks had relatively low viscosity alpha sim 0.001, modest thickness H/R sim 0.03 to 0.05, and limited masses comparable to that of minimum mass solar nebula model., Comment: Accepted by ApJ

Robust topology optimization (RTO) improves the robustness of designs with respect to random sources in real-world structures, yet an accurate sensitivity analysis requires the solution of many systems of equations at each optimization step, leading to a high computational cost. To open up the full potential of RTO under a variety of random sources, this paper presents a momentum-based accelerated mirror descent stochastic approximation (AC-MDSA) approach to efficiently solve RTO problems involving various types of load uncertainties. The proposed framework can perform high-quality design updates with highly noisy stochastic gradients. We reduce the sample size to two (minimum for unbiased variance estimation) and show only two samples are sufficient for evaluating stochastic gradients to obtain robust designs, thus drastically reducing the computational cost. We derive the AC-MDSA update formula based on $\ell_1$-norm with entropy function, which is tailored to the geometry of the feasible domain. To accelerate and stabilize the algorithm, we integrate a momentum-based acceleration scheme, which also alleviates the step size sensitivity. Several 2D and 3D examples with various sizes are presented to demonstrate the effectiveness and efficiency of the proposed AC-MDSA framework to handle RTO involving various types of loading uncertainties., Comment: 38 pages (including reference)

During their formation, emerging protoplanets tidally interact with their natal disks. Proto-gas-giant planets, with Hills radius larger than the disk thickness, open gaps and quench gas flow in the vicinity of their orbits. It is usually assumed that their type II migration is coupled to the viscous evolution of the disk. Although this hypothesis provides an explanation for the origin of close-in planets, it also encounter predicament on the retention of long-period orbits for most gas giant planets. Moreover, numerical simulations indicate that planets migrations are not solely determined by the viscous diffusion of their natal disk. Here we carry out a series of hydrodynamic simulations combined with analytic studies to examine the transition between different paradigms of type II migration. We find a range of planetary mass for which gas continues to flow through a severely depleted gap so that the surface density distribution in the disk region beyond the gap is maintained in a quasi-steady state. The associated gap profile modifies the location of corotation \& Lindblad resonances. In the proximity of the planet's orbit, high-order Lindblad \& corotation torque are weakened by the gas depletion in the gap while low-order Lindblad torques near the gap walls preserves their magnitude. Consequently, the intrinsic surface density distribution of the disk determines delicately both pace and direction of planets' type II migration. We show that this effect might stall the inward migration of giant planets and preserve them in disk regions where the surface density is steep., Comment: 21 pages, 12 figures, accepted by ApJ

The frequent detection of binary mergers of $\sim 30 M_{\odot}$ black holes (BHs) by the Laser Interferometer Gravitational-Wave Observatory (LIGO) rekindled researchers' interest in primordial BHs (PBHs) being dark matter (DM). In this work, we looked at PBHs distributed as DM with a monochromatic mass of $30 M_{\odot}$ and examined the encounter-capture scenario of binary formation, where the densest central region of DM halo dominates. Thus, we paid special attention to the tidal effect by the supermassive black hole (SMBH) present. In doing so, we discovered a necessary tool called loss zone that complements the usage of loss cone. We found that the tidal effect is not prominent in affecting binary formation, which also turned out insufficient in explaining the totality of LIGO's event rate estimation, especially due to a microlensing event constraining the DM fraction in PBH at the mass of interest from near unity to an order smaller. Meanwhile, early-universe binary formation scenario proves so prevailing that the LIGO signal in turn constrains the PBH fraction below one percent. Thus, people should put more faith in alternative PBH windows and other DM candidates.

In the $\Lambda$CDM scenario, small galaxies merge to produce larger entities. Since supermassive black holes (SMBHs) are found in galaxies of all sizes, SMBH binaries (SMBHBs) are generally expected to form during the amalgamation of galaxies. It is unclear what fraction of these binaries could eventually merge, but a general consensus is that initially the orbital decay is mediated by the surrounding gas and stars. In this Letter, we show that in active galactic nulcei (AGNs) the radiation field also causes the orbits of the accreting SMBHs to shrink. The corresponding mechanism, known as the "Poynting-Robertson drag" (PR drag), takes effect on a well-defined timescale $CT_{\rm Sal}$, where $T_{\rm Sal}$ is the Salpeter timescale of the AGN, presumably coincide with the primary SMBH, and $C=4\xi^{-1}\epsilon^{-1}q^{1/3}(1+q)^{2/3}(1-\epsilon)$ is a constant determined by the radiative efficiency $\epsilon$, the mass ratio $q$ of the two black holes, and a parameter $\xi$ characterizing the size of the circum-secondary accretion disk. We find that when $q\lesssim$a few$\times10^{-5}$, the PR drag is more efficient in shrinking the binary than many other mechanisms, such as dynamical friction and type-I migration. Our finding points to a possible new channel for the coalescence of unequal SMBHBs and the clearing of intermediate-massive black holes in AGNs., Comment: 6 pages, 2 figures, accepted for publication in ApJ Letters

We report the photometry of six transits of the hot Jupiter HAT-P-29b obtained from 2013 October to 2015 January. We analyze the new light curves, in combination with the published photometric, and Doppler velocimetric, and spectroscopic measurements, finding an updated orbital ephemeris for the HAT-P-29 system, $T_{\rm C}[0]= 2456170.5494(15)\,[\rm{BJD_{TDB}}]$ and $P=5.723390(13) \,{\rm days}$. It is $17.63\,{\rm s}$ ($4.0\,\sigma$) longer than the previously published value, amounting to errors exceeding $2.5\,\rm{hrs}$ at the time of writing (on UTC 2018 June 1). The measured transit mid-times for HAT-P-29b show no compelling evidence of timing anomalies from a linear model, which rules out the presence of a perturbers with masses greater than $0.6$, $0.7$, $0.5$, and $0.4\,{\rm M_\oplus}$ near the $1:2$, $2:3$, $3:2$, and $2:1$ resonances with HAT-P-29b, respectively., Comment: Accepted for publication in AJ

Extended gaps in the debris disks of both Vega and Fomalhaut have been observed. These structures have been attributed to tidal perturbations by multiple super-Jupiter gas giant planets. Within the current observational limits, however, no such massive planets have been detected. Here we propose a less stringent `lone-planet' scenario to account for the observed structure with a single eccentric gas giant and suggest that clearing of these wide gaps is induced by its sweeping secular resonance. During the depletion of the disk gas, the planet's secular resonance propagates inward and clears a wide gap over an extended region of the disk. Although some residual intermediate-size planetesimals may remain in the gap, their surface density is too low to either produce super-Earths or lead to sufficiently frequent disruptive collisions to generate any observable dusty signatures. The main advantage of this lone-planet sweeping-secular-resonance model over the previous multiple gas giant tidal truncation scenario is the relaxed requirement on the number of gas giants. The observationally inferred upper mass limit can also be satisfied provided the hypothetical planet has a significant eccentricity. A significant fraction of solar or more massive stars bear gas giant planets with significant eccentricities. If these planets acquired their present-day kinematic properties prior to the depletion of their natal disks, their sweeping secular resonance would effectively impede the retention of neighboring planets and planetesimals over a wide range of orbital semi-major axes., Comment: 20 pages, 12 figures. Accepted for publication in ApJ