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Current sheet and open field lines with footpoints near the edge of the polar cap. The magnetic axis is inclined relative to the rotation axis by 60 degrees. Red
field lines originate on the north polar cap and green field lines in the right panel originate on the south polar cap. Purple and grey colors indicate positive and negative net
local charge density in the current sheet, which is shown between 1.2-2 light cylinder radii.
Current sheet and open field lines with footpoints near the edge of the polar cap. The magnetic axis is inclined relative to the rotation axis by 90 degrees. Red field lines originate on the north polar cap and green field lines in the right panel originate on the south polar cap. Purple and grey colors indicate positive and negative net local charge density in the current sheet, which is shown between 1.2-2 light cylinder radii.
Magnetic field lines and current sheets for an orbiting neutron star binary with the magnetic moments of both
stars aligned with the rotation axis. The stars are not spinning, i.e., R_{LC,∗} = ∞.
Fields are by and large confined
to the half of the magnetosphere closer to their source star.
This movie shows the corotating field pattern as the orbit progresses.
Magnetic field lines and current sheets for an orbiting neutron star binary with the magnetic moments of both
stars aligned with the rotation axis. The stars are spinning
rapidly at ∼ ms periods, with R_{LC,∗}/R_∗ = 2.7. Stellar spin
winds fields backwards toroidally, and they can propagate to
the far side of the magnetosphere closer to the opposing star.
This movie shows the corotating field pattern as the orbit progresses.
Magnetic field lines and current sheets for an orbiting neutron star binary with the magnetic moment of one
star aligned with the rotation axis, and the magnetic moment of the other star tilted and antialigned with the rotation axis.
The stars are not spinning, i.e., R_{LC,∗} =
∞. Fields from each star encircle the other star and force
fields coming off the second star backwards toroidally.
This movie shows the corotating field pattern as the orbit progresses.
Magnetic field lines and current sheets for an orbiting neutron star binary with the magnetic moment of one star
aligned with the rotation axis, and the magnetic moment of the
other star tilted and antialigned with the rotation axis. The
stars are spinning rapidly at ∼ ms periods, with R_{LC,∗} /R_∗ =
2.7.
Stellar spin winds fields backwards toroidally.
This movie shows the corotating field pattern as the orbit progresses.
This movie shows the dynamical behavior of field lines seeded on one of the stars. We find
a clear cyclical process operating in the magnetosphere. First, field lines from one star can attach to the second star. Second, as the orbit progresses these field lines
develop twist and are expelled outward past the second
star as closed loops. Third, these loops open up to infinity and then reconnect on the far side of the first star
opposite to the second. Fourth, the orbital motion will
bring the second star back into contact with the closed
loops, and they reattach to the second star.
The Magnetospheric Multiscale (MMS) mission has given us unprecedented access to high cadence particle and field data of magnetic reconnection at Earth's magnetopause. MMS first passed very near an X-line on 16 October 2015, the Burch event, and has since observed multiple X-line crossings. Subsequent 3D particle-in-cell (PIC) modeling efforts of and comparison with the Burch event have revealed a host of novel physical insights concerning magnetic reconnection, turbulence induced particle mixing, and secondary instabilities. In this study, we employ the Gkeyll simulation framework to study the Burch event with different classes of extended, multi-fluid magnetohydrodynamics (MHD), including models that incorporate important kinetic effects, such as the electron pressure tensor, with physics-based closure relations designed to capture linear Landau damping. Such fluid modeling approaches are able to capture different levels of kinetic physics in global simulations and are generally less costly than fully kinetic PIC. We focus on the additional physics one can capture with increasing levels of fluid closure refinement via comparison with MMS data and existing PIC simulations. In particular, we find that the ten-moment model well captures the agyrotropic structure of the pressure tensor in the vicinity of the X-line and the magnitude of anisotropic electron heating observed in MMS and PIC simulations. However, the ten-moment model has difficulty resolving the lower hybrid drift instability, which has been observed to plays a fundamental role in heating and mixing electrons in the current layer.
Protein sequence space is vast; nature uses only an infinitesimal fraction of possible sequences to sustain life. Are there solutions to biological problems other than those provided by nature? Can we create artificial proteins that sustain life? To investigate this question, the Hecht lab has created combinatorial collections, or libraries, of novel sequences with no homology to those found in living organisms. These libraries were subjected to screens and selections, leading to the identification of sequences with roles in catalysis, modulating gene regulation, and metal homeostasis. However, the resulting functional proteins formed dynamic rather than well-ordered structures. This impeded structural characterization and made it difficult to ascertain a mechanism of action.
To address this, Christina Karas's thesis work focuses on developing a new model of libraries based on the de novo protein S-824, a four-helix bundle with a very stable three-dimensional structure. The first part of this research focused on mutagenesis of S-824 and characterization of the resulting proteins, revealing that this scaffold tolerates amino acid substitutions, including buried polar residues and the removal of hydrophobic side chains to create a putative cavity.
Distinct from previous libraries, Karas targeted variability to a specific region of the protein, seeking to create a cavity and potential active site. The second part of this work details the design and creation of a library encoding 1.7 x 10^6 unique proteins, assembled from degenerate oligonucleotides. The third and fourth parts of this work cover the screening effort for a range of activities, both in vitro and in vivo. I found that this collection binds heme readily, leading to abundant peroxidase activity. Hits for lipase and phosphatase activity were also detected.
This work details the development of a new strategy for creating de novo sequences geared toward function rather than structure.
