Search for: All records

Abstract The quantum dimer magnet, with antiferromagnetic intradimer and interdimer Heisenberg exchange between spin-1/2 moments, is known to host an$$(\left|\uparrow \downarrow \right\rangle -\left|\downarrow \uparrow \right\rangle )/\sqrt{2}$$ $\left(\left(\uparrow \downarrow \right)-\left(\downarrow \uparrow \right)\right)/\sqrt{2}$ singlet ground state when the intradimer exchange is dominant. Rare-earth-based quantum dimer systems with strong spin-orbit coupling offer the opportunity for tuning their magnetic properties by using magnetic anisotropy as a control knob. Here, we present bulk characterization and neutron scattering measurements of the quantum dimer magnet Yb₂Be₂SiO₇. We find that the Yb³⁺ions can be described by an effective spin-1/2 model at low temperatures and the system does not show signs of magnetic order down to 50 mK. The magnetization, heat capacity, and neutron spectroscopy data can be well-described by an isolated dimer model with highly anisotropic exchange that stabilizes a singlet ground state with a wavefunction$$(\left|\uparrow \uparrow \right\rangle -\left|\downarrow \downarrow \right\rangle )/\sqrt{2}$$ $\left(\left(\uparrow \uparrow \right)-\left(\downarrow \downarrow \right)\right)/\sqrt{2}$ or$$(\left|\uparrow \uparrow \right\rangle+\left|\downarrow \downarrow \right\rangle )/\sqrt{2}$$ $\left(\left(\uparrow \uparrow \right)+\left(\downarrow \downarrow \right)\right)/\sqrt{2}$ . Our results show that strong spin-orbit coupling can induce unusual entangled states of matter in quantum dimer magnets. 

Abstract While whistler‐mode waves are generated by injected anisotropic electrons on the nightside, the observed day‐night asymmetry of wave distributions raises an intriguing question about their generation on the dayside. In this study, we evaluate the distributions of whistler‐mode wave amplitudes and electrons as a function of distance from the magnetopause (MP) on the dayside from 6 to 18 hr in magnetic local time (MLT) within ±18° of magnetic latitude using the Time History of Events and Macroscale Interaction During Substorms measurements from June 2010 to August 2018. Specifically, under different levels of solar wind dynamic pressure and geomagnetic index, we conduct a statistical analysis to examine whistler‐mode wave amplitude, as well as anisotropy and phase space density (PSD) of source electrons across 1–20 keV energies, which potentially provide a source of free energy for wave generation. In coordinates relative to the MP, we find that lower‐band (0.05–0.5f_ce) waves occur much closer to the MP than upper‐band (0.5–0.8f_ce) waves, wheref_ceis electron cyclotron frequency. Our statistical results reveal that strong waves are associated with high anisotropy and high PSD of source electrons near the equator, indicating a preferred region for local wave generation on the dayside. Over 10–14 hr in MLT, as latitude increases, electron anisotropy decreases, while whistler‐mode wave amplitudes increase, suggesting that wave propagation from the equator to higher latitudes, along with amplification along the propagation path, is necessary to explain the observed waves on the dayside. 

Abstract Unlike Earth, where whistler‐mode chorus is generally observed outside the plasmasphere and hiss is predominantly confined within it, Jupiter's magnetosphere frequently hosts both wave modes simultaneously. Using Juno observations, we present a global survey of whistler‐mode waves at Jupiter, demonstrating that both chorus and hiss are prevalent over 6 < M < 13, with chorus confined to lower latitudes and hiss extending to higher latitudes. Based on the observed wave and plasma characteristics, we apply physics‐based global diffusion modeling to assess the separate and combined effects of chorus and hiss waves on energetic electron dynamics. The simulations show that, near regions of peak wave activity, chorus waves lead to net electron losses below ∼1 MeV while producing acceleration at higher energies. Hiss waves primarily drive electron losses, but in combination with chorus waves, they can also enable electron acceleration above ∼1 MeV at specific pitch angles. 

Self-supervised learning(SSL) is essential to obtain foundation models in NLP and CV domains via effectively leveraging knowledge in large-scale unlabeled data. The reason for its success is that a suitable SSL design can help the model to follow the neural scaling law, i.e., the performance consistently improves with increasing model and dataset sizes. However, it remains a mystery whether existing SSL in the graph domain can follow the scaling behavior toward building Graph Foundation Models~(GFMs) with large-scale pre-training. In this study, we examine whether existing graph SSL techniques can follow the neural scaling behavior with the potential to serve as the essential component for GFMs. Our benchmark includes comprehensive SSL technique implementations with analysis conducted on both the conventional SSL setting and many new settings adopted in other domains. Surprisingly, despite the SSL loss continuously decreasing, no existing graph SSL techniques follow the neural scaling behavior on the downstream performance. The model performance only merely fluctuates on different data scales and model scales. Instead of the scales, the key factors influencing the performance are the choices of model architecture and pretext task design. This paper examines existing SSL techniques for the feasibility of Graph SSL techniques in developing GFMs and opens a new direction for graph SSL design with the new evaluation prototype. Our code implementation is available online to ease reproducibility https://github.com/HaitaoMao/GraphSSLScaling. 

Abstract We investigate the association between wave‐driven relativistic (>700 keV) electron precipitation (REP) and the two main geomagnetically effective solar wind (SW) drivers: coronal mass ejections (CMEs) and high‐speed streams (HSSs). We combine SW observations from OMNI and a data set of clear wave‐driven REP events (covering 2012–2023, from low Earth orbit electron measurements from the Polar Operational Environmental Satellites and Meteorological Operational satellites) to determine when and under which conditions CMEs and HSSs lead to REP. Out of 91 CMEs and 309 HSSs, 76%, and 70% are associated with REP, respectively, indicating that both drivers are effective at triggering relativistic electron loss into the atmosphere. However, only ∼25% of all REP events over 2012–2023 are associated with these major SW structures, suggesting that large‐scale drivers are not required in most cases. We also found that the strongest controlling factor for REP occurrence is not the driver type or storm intensity alone, but the preconditioning of the outer belt: drivers associated with REP typically impact a magnetosphere with elevated fluxes of trapped >700 keV electrons. REP occurs primarily during the main phase, although HSS‐driven REP is more often sustained into the recovery phase. In terms of properties, HSS‐driven REP appears broader in radial extent and is associated with higher electron flux, whereas CME‐driven REP shows larger precipitation‐to‐trapped flux ratios, indicating more efficient wave scattering. These results support a scenario in which SW forcing, storm timing, and radiation belt preconditioning jointly control the occurrence and properties of wave‐driven REP. 

Abstract Whistler‐mode chorus waves have been widely examined in the Earth's magnetosphere since they play an important role in the dynamics of the radiation belts around the Earth and energetic electron precipitation into the upper atmosphere. In this study, we examine the relationship between the low‐energy (<800 eV) electrons and the linear instability of chorus waves through a statistical analysis of 12 years of in situ observations from the THEMIS mission. Our results show that 31% of all lower‐band chorus wave events observed near the magnetic equator (|MLAT| < 10°) are linearly unstable if the low‐energy electron density is considered, whereas only 4% are unstable when the low‐energy electrons are ignored. Additionally, we identify a critical transition in chorus wave properties, where emission types shift from predominantly electromagnetic ( < 0.03) to a mixture of electromagnetic and quasi‐electrostatic components ( ∼ 0.1) depending on the low‐energy electron density ratio. Moreover, an analysis of correlation coefficients () associated with temporal variations of wave amplitude reveals that the low‐energy electron density is the dominant factor (∼43% with  > 0.2) of chorus wave modulation, while the hot electron temperature (>800 eV) ratio and electron parallel beta contribute only 15% and 21%, respectively. These results emphasize the important contribution of the low‐energy electrons to the instability and modulations of chorus waves in the Earth's magnetosphere. 

An advantage of Large Language Models (LLMs) is their contextualization capability – providing different responses based on student inputs like solution strategy or prior discussion, to potentially better engage students than standard feedback. We present a design and evaluation of a proof-of-concept LLM application to offer students dynamic and contextualized feedback. Specifically, we augment an Online Programming Exercise bot for a college-level Cloud Computing course with ChatGPT, which offers students contextualized reflection triggers during a collaborative query optimization task in database design. We demonstrate that LLMs can be used to generate highly situated reflection triggers that incorporate details of the collaborative discussion happening in context. We discuss in depth the exploration of the design space of the triggers and their correspondence with the learning objectives as well as the impact on student learning in a pilot study with 34 students. 

Abstract Electromagnetic ion cyclotron waves in the Earth's outer radiation belt drive rapid electron losses through wave‐particle interactions. The precipitating electron flux can be high in the hundreds of keV energy range, well below the typical minimum resonance energy. One of the proposed explanations relies on nonresonant scattering, which causes pitch‐angle diffusion away from the fundamental cyclotron resonance. Here we propose the fractional sub‐cyclotron resonance, a second‐order nonlinear effect that scatters particles at resonance ordern = 1/2, as an alternate explanation. Using test‐particle simulations, we evaluate the precipitation ratios of sub‐MeV electrons for wave packets with various shapes, amplitudes, and wave normal angles. We show that the nonlinear sub‐cyclotron scattering produces larger ratios than the nonresonant scattering when the wave amplitude reaches sufficiently large values. The ELFIN CubeSats detected several events with precipitation ratio patterns matching our simulation, demonstrating the importance of sub‐cyclotron resonances during intense precipitation events. 

Abstract Interchange instability is known to drive fast radial transport of electrons and ions in Jupiter's inner and middle magnetosphere. In this study, we conduct a statistical survey to evaluate the properties of energetic particles and plasma waves during interchange events using Juno data from 2016 to 2023. We present representative examples of interchange events followed by a statistical analysis of the spatial distribution, duration and spatial extent. Our survey indicates that interchange instability is predominant atM‐shells from 6 to 26, peaking near 17 with an average duration of minutes and a correspondingM‐shell width of <∼0.05. During interchange events, the associated plasma waves, such as whistler‐mode, Z‐mode, and electron cyclotron harmonic waves exhibit a distinct preferential location. These findings provide valuable insights into particle transport and the source region of plasma waves in the Jovian magnetosphere, as well as in other magnetized planets within and beyond our solar system.