Search for: All records

Abstract Drift periodic echoes of electrons in the inner belt appear as structured bands in energy spectrograms, also known as “zebra stripes”. Such phenomenon is normally observed at energies from 10s of keV to ∼250 keV. We report multiple series of zebra stripes of relativistic electrons observed by the recent Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. The high energy resolution measurements taken by the REPTile‐2 (Relativistic Electron and Proton Telescope integrated little experiment‐2) instrument onboard CIRBE show that zebra stripes of radiation belt electrons can be observed from 300 keV to >1 MeV, crossing theLrange from 1.18 to >3, from quiet times to storm times. Through test particle simulations, we show that a prompt electric field with a peak amplitude ∼5 mV/m in near‐Earth space can trigger zebra stripes of relativistic electrons. Azimuthal inhomogeneity of electron distribution caused by the prompt electric field modulates the electron energy spectrum by energy‐dependent drift phases to form zebra stripes. Though zebra stripes are observed in both belts, they tend to last longer and appear more frequently in the inner belt. Zebra stripes in the outer belt will have a shorter lifetime due to more perturbations there, including energy and pitch‐angle diffusion, which diminish the structure. This study demonstrates the important role of electric fields in the dynamics of relativistic electrons and contributes to the understanding of the mechanisms creating and diminishing zebra stripes. 

Abstract Ultra‐low frequency (ULF) waves radially diffuse hundreds‐keV to few‐MeV electrons in the magnetosphere, as the range of drift frequencies of such electrons overlaps with the wave frequencies, leading to resonant interactions. Theoretically this process is described by analytic expressions of the resonant interactions between electrons and ULF wave modes in a background magnetic field. However, most expressions of the radial diffusion rates are derived for equatorially mirroring electrons and are based on estimates of the power of ULF waves that are obtained either from spacecraft close to the equatorial plane or from the ground but mapped to the equatorial plane. Based on recent statistical in situ observations, it was found that the wave power of magnetic fluctuations is significantly enhanced away from the magnetic equator. In this study, the distribution of the wave amplitudes as a function of magnetic latitude is compared against models simulating the natural modes of oscillation of magnetospheric field lines, with which they are found to be consistent. Energetic electrons are subsequently traced in 3D model fields that include a latitudinal dependence that is similar to measurements and to the natural modes of oscillation. Particle tracing simulations show a significant dependence of the radial transport of relativistic electrons on pitch angle, with off‐equatorial electrons experiencing considerably higher radial transport, as they interact with ULF wave fluctuations of higher amplitude than equatorial electrons. These findings point to the need for incorporating pitch‐angle‐dependent radial diffusion coefficients in global radiation belt models. 

Abstract Following the largest magnetic storm in 20 years (10 May 2024), REPTile‐2 on NASA's CIRBE satellite identified two new radiation belts containing 1.3–5 MeV electrons aroundL = 2.5–3.5 and 6.8–20 MeV protons aroundL = 2. The region aroundL = 2.5–3.5 is usually devoid of relativistic electrons due to wave‐particle interactions that scatter them into the atmosphere. However, these 1.3–5 MeV electrons in this new belt seemed unaffected until a magnetic storm on 28 June 2024, perturbed the region. The long‐lasting nature of this new electron belt has physical implications for the dependence of electron wave‐particle interactions on energy, plasma density, and magnetic field strength. The enhancement of protons aroundL = 2 exceeded an order of magnitude between 6.8 and 15 MeV forming a distinct new proton belt that appears even more stable. CIRBE, after a year of successful operation, malfunctioned 25 days before the super storm but returned to functionality 1 month after the storm, enabling these discoveries. 

This paper reports the molecular organization and mechanical properties of electrospun, post-drawn polyacrylonitrile (PAN) nanofibers. Without post-drawing, the polymer chain was kinked and oriented in hexagonal crystalline structures. Immediate post-drawing in the semi-solid state disrupted the crystal structures and chain kink at maximum draw ratio. Structural re-orientation at maximum draw resulted in a 500% increase in Young's modulus and a 100% increase in ultimate tensile strength. By applying post-drawing to electrospinning it may be possible to obtain PAN fibers and PAN-derived carbon fibers with enhanced mechanical properties compared to available fabrication technologies. 

Abstract Collagen is the major structural protein in myocardium and contributes to tissue strength and integrity, cellular orientation, and cell–cell and cell‐matrix interactions. Significant post‐myocardial infarction related loss of cardiomyocytes and cardiac tissue, and their subsequent replacement with fibrous scar tissue, negatively impacts endogenous tissue repair and regeneration capabilities. To overcome such limitations, tissue engineers are working toward developing a 3D cardiac patch which not only mimics the structural, functional, and biological hierarchy of the native cardiac tissue, but also could deliver autologous stem cells and encourage their homing and differentiation. In this study, we examined the utility of electrospun, randomly‐oriented, type‐I collagen nanofiber (dia= 789 ± 162 nm) mats on the cardiomyogenic differentiation of human bone marrow‐derived mesenchymal stem cells (BM‐MSC) spheroids, in the presence or absence of 10 μM 5‐azacytidine (aza). Results showed that these scaffolds are biocompatible and enable time‐dependent evolution of early (GATA binding protein 4: GATA4), late (cardiac troponin I: cTnI), and mature (myosin heavy chain: MHC) cardiomyogenic markers, with a simultaneous reduction in CD90 (stemness) expression, independent of aza‐treatment. Aza‐exposure improved connexin‐4 expression and sustained sarcomeric α‐actin expression, but provided only transient improvement in cardiac troponin T (cTnT) expression. Cell orientation and alignment significantly improved in these nanofiber scaffolds over time and with aza‐exposure. Although further quantitativein vitroandin vivostudies are needed to establish the clinical applicability of such stem‐cell laden collagen nanofiber mats as cardiac patches for cardiac tissue regeneration, our results underscore the benefits of 3D milieu provided by electrospun collagen nanofiber mats, aza, and spheroids on the survival, cardiac differentiation and maturation of human BM‐MSCs. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 3303–3312, 2018. 

Abstract Flare frequency distributions represent a key approach to addressing one of the largest problems in solar and stellar physics: determining the mechanism that counterintuitively heats coronae to temperatures that are orders of magnitude hotter than the corresponding photospheres. It is widely accepted that the magnetic field is responsible for the heating, but there are two competing mechanisms that could explain it: nanoflares or Alfvén waves. To date, neither can be directly observed. Nanoflares are, by definition, extremely small, but their aggregate energy release could represent a substantial heating mechanism, presuming they are sufficiently abundant. One way to test this presumption is via the flare frequency distribution, which describes how often flares of various energies occur. If the slope of the power law fitting the flare frequency distribution is above a critical threshold,α= 2 as established in prior literature, then there should be a sufficient abundance of nanoflares to explain coronal heating. We performed >600 case studies of solar flares, made possible by an unprecedented number of data analysts via three semesters of an undergraduate physics laboratory course. This allowed us to include two crucial, but nontrivial, analysis methods: preflare baseline subtraction and computation of the flare energy, which requires determining flare start and stop times. We aggregated the results of these analyses into a statistical study to determine thatα= 1.63 ± 0.03. This is below the critical threshold, suggesting that Alfvén waves are an important driver of coronal heating.