Across the Tachocline Divide: Convection and Dynamo Action In M-Dwarf Stars

In this thesis, we explore the convective dynamics taking place in the interiors of M-dwarf stars across the transition from partial to full-sphere convection. Additionally, we search for theoretical explanations for the observed Tachocline Divide, whereby fully convective M-dwarfs seem to maintain their magnetic activity for longer and to flare more aggressively than more massive, partially convective M stars. To this end, we employ the open-source Rayleigh code on massively parallel supercomputers to simulate rotating spherical shells of anelastic magnetohydrodynamic convection in 3D with full nonlinearity. These shells include in their computational domains most of the modeled stars&rsquo; convection zones, as well as the underlying radiative zone and tachocline where applicable. We consider the dynamics of more than 80 simulations falling into three primary classes of models: partially convective spectral type M2 (0.4M) stars without underlying tachoclines, partially convective M2 stars with tachoclines, and fully convective M4 (0.3M) stars.&nbsp; We observe the broad characteristics of dynamo action in the convection zones of both spectral types of M-dwarfs to be shared not only between them, but also with more massive G- and K-stars. We find parity in parameter scalings for the efficiency of their dynamos, the timing of their cycles, and the strength of magnetic feedbacks upon the differential rotation. The effect on the differential rotation is of particular note &ndash; in many of the models we compute, the shear is strongly quenched by the intense magnetic fields, leading to nearly solid body rotation in the convection zone. We find that the most rapidly rotating models we considered tended to produce prograde traveling nests of enhanced convection. We observe these nests to interact strongly with the magnetic fields through their strong helical turbulence, triggering reversals of the magnetic field and accelerating flux emergence. In one particularly novel case, nest-driven field reversal sets the cycle period of the global dynamo. Among the M-dwarfs, we find a striking tendency toward single hemisphere iii dynamo states, which seem to grow more prominent with the increasing depth of the convection zone. We argue that these states result from weakly nonlinear coupling between even and odd eigenmodes of the dynamo solution. We find the generation of buoyant magnetic loops to be a general property of our simulations, despite their diffusivity. We develop a machine-learning pipeline for the autonomous isolation of these intricate magnetic structures, which we dub LoopNet.&nbsp; In consideration of the tachocline divide, we find that the presence of a tachocline in M2 stars significantly increases the prominence of large-scale axisymmetric fields in the global magnetic energy budget. We find that this in turn leads to surface fields which are much more dipolar. Through the spin-down interactions of these magnetic fields with the stellar wind, this trend implies shorter magnetic lifetimes than on M2 stars without tachoclines. However, we find that the surface fields of fully convective M4 stars in our simulations are both stronger and more dipolar than those of even the M2 stars with tachoclines. Even after accounting for stellar sizes, the implication of these findings is that the M4 stars should have shorter magnetic lifetimes than M2 stars. This comes as a surprise, due to the distinct inversion of this prediction in observations. We discuss various avenues by which the tension between our findings and observations might be resolved. Using LoopNet, we find that turbulent stirring by convection in the deep interiors of M4 stars allows them to produce significantly more buoyantly rising magnetic flux ropes than similarly active M2 stars. This result may possibly explain the cause for the enhanced flare rate exhibited by fully convective M-dwarfs when compared to more massive active stars. &nbsp;</p

Cortex-wide, cellular-resolution two-photon microscopy

Functional imaging of the mouse brain in its extreme complexity involves substantial trade-offs. An optical intrinsic spectroscopy system can image the entire cortex but at the expense of spatial and temporal resolution [1]. A two-photon microscope (TPM) can image single neurons with high temporal resolution, but the field of view (FOV) is generally restricted. Advanced techniques like random-access scanning allow for imaging single neurons that are millimeters apart but only by ignoring the neurons and tissue in between [2]. By carefully considering the properties of the optical components as well as the imaging requirements, we present a TPM capable of imaging nearly the entire mouse cortex with 15 Hz frame rates and single neuron resolution. Please click Additional Files below to see the full abstract

Morphometric and physical characteristics distinguishing adult Patagonian lamprey, Geotria macrostoma from the pouched lamprey, Geotria australis

The pouched lamprey, Geotria australis Gray, 1851, has long been considered monotypic in the Geotriidae family with a wide southern temperate distribution across Australasia and South America. Recent studies have provided molecular and morphological evidence for a second Geotria species in South America; Geotria macrostoma (Burmeister, 1868). The aim of this study was to determine morphometric and physical characteristics of adult G. macrostoma that further differentiate this re-instated species of Geotriidae from G. australis. The diagnostic features discriminating immature adult G. macrostoma from G. australis when entering fresh water, are distinct differences in dentition, oral papillae and fimbriae counts and differences in coloration. In addition, G. macrostoma display greater growth of the prebranchial region and oral disc and has a deeper body depth and higher condition factor. All current ecological knowledge of the genus Geotria is based on Australasian populations, which may not be applicable to G. macrostoma. To ensure the conservation and protection of the Patagonian lamprey as a re-identified species, further investigations are needed to understand its life history, biology and ecology throughout its range

Homotopic contralesional excitation suppresses spontaneous circuit repair and global network reconnections following ischemic stroke

Understanding circuit-level manipulations that affect the brain\u27s capacity for plasticity will inform the design of targeted interventions that enhance recovery after stroke. Following stroke, increased contralesional activity (e.g. use of the unaffected limb) can negatively influence recovery, but it is unknown which specific neural connections exert this influence, and to what extent increased contralesional activity affects systems- and molecular-level biomarkers of recovery. Here, we combine optogenetic photostimulation with optical intrinsic signal imaging to examine how contralesional excitatory activity affects cortical remodeling after stroke in mice. Following photothrombosis of left primary somatosensory forepaw (S1FP) cortex, mice either recovered spontaneously or received chronic optogenetic excitation of right S1FP over the course of 4 weeks. Contralesional excitation suppressed perilesional S1FP remapping and was associated with abnormal patterns of stimulus-evoked activity in the unaffected limb. This maneuver also prevented the restoration of resting-state functional connectivity (RSFC) within the S1FP network, RSFC in several networks functionally distinct from somatomotor regions, and resulted in persistent limb-use asymmetry. In stimulated mice, perilesional tissue exhibited transcriptional changes in several genes relevant for recovery. Our results suggest that contralesional excitation impedes local and global circuit reconnection through suppression of cortical activity and several neuroplasticity-related genes after stroke, and highlight the importance of site selection for targeted therapeutic interventions after focal ischemia

Constraints on Earth system functioning at the Paleocene-Eocene Thermal Maximum from the marine silicon cycle

The Paleocene‐Eocene Thermal Maximum (PETM, ca. 56 Ma) is marked by a negative carbon isotope excursion (CIE) and increased global temperatures. The CIE is thought to result from the release of 13C‐depleted carbon, although the source(s) of carbon and triggers for its release, its rate of release, and the mechanisms by which the Earth system recovered are all debated. Many of the proposed mechanisms for the onset and recovery phases of the PETM make testable predictions about the marine silica cycle, making silicon isotope records a promising tool to address open questions about the PETM. We analyzed silicon isotope ratios (δ30Si) in radiolarian tests and sponge spicules from the Western North Atlantic (ODP Site 1051) across the PETM. Radiolarian δ30Si decreases by 0.6‰ from a background of 1‰ coeval with the CIE, while sponge δ30Si remains consistent at 0.2‰. Using a box model to test the Si cycle response to various scenarios, we find the data are best explained by a weak silicate weathering feedback, implying the recovery was mostly driven by nondiatom organic carbon burial, the other major long‐term carbon sink. We find no resolvable evidence for a volcanic trigger for carbon release, or for a change in regional oceanography. Better understanding of radiolarian Si isotope fractionation and more Si isotope records spanning the PETM are needed to confirm the global validity of these conclusions, but they highlight how the coupling between the silica and carbon cycles can be exploited to yield insight into the functioning of the Earth system

Effective connectivity measured using optogenetically evoked hemodynamic signals exhibits topography distinct from resting state functional connectivity in the mouse

Brain connectomics has expanded from histological assessment of axonal projection connectivity (APC) to encompass resting state functional connectivity (RS-FC). RS-FC analyses are efficient for whole-brain mapping, but attempts to explain aspects of RS-FC (e.g., interhemispheric RS-FC) based on APC have been only partially successful. Neuroimaging with hemoglobin alone lacks specificity for determining how activity in a population of cells contributes to RS-FC. Wide-field mapping of optogenetically defined connectivity could provide insights into the brain\u27s structure-function relationship. We combined optogenetics with optical intrinsic signal imaging to create an efficient, optogenetic effective connectivity (Opto-EC) mapping assay. We examined EC patterns of excitatory neurons in awake, Thy1-ChR2 transgenic mice. These Thy1-based EC (Thy1-EC) patterns were evaluated against RS-FC over the cortex. Compared to RS-FC, Thy1-EC exhibited increased spatial specificity, reduced interhemispheric connectivity in regions with strong RS-FC, and appreciable connection strength asymmetry. Comparing the topography of Thy1-EC and RS-FC patterns to maps of APC revealed that Thy1-EC more closely resembled APC than did RS-FC. The more general method of Opto-EC mapping with hemoglobin can be determined for 100 sites in single animals in under an hour, and is amenable to other neuroimaging modalities. Opto-EC mapping represents a powerful strategy for examining evolving connectivity-related circuit plasticity

The Iowa Homemaker vol.20, no.7

Hospitality on a Budget, Mary Ellen Brown, page 2 Plastics Equip the Home, Dorothy Anne Roost, page 4 Designed for Efficiency, Dorothy Gross, page 6 50,000 Words a Day, Betty Bice, page 7 Sally Leads Military Parade, Patricia Hayes, page 8 Self-Investment for Life, Dr. Richard C. Raines, page 10 Home Management Staff, Margaret Kumlien Read, page 11 What’s New in Home Economics, Helen Kubacky, page 12 Defense Challenges the Home Economics, Dr. P. M. Nelson, page 14 Letters from Sumatra and Alaska, page 15 Alums in the News, Bette Simpson, page 16 Vitamins Invade Army Rations, Genevieve Scott, page 17 Flashes from Bacteriology Field, Catherine Raymond, page 18 China on a Budget, Jane Willey, page 19 Behind Bright Jackets, Marjorie Thomas, page 20 Soldiers and Sailors Eat Well, Pat Garberson, page 22 Spindles, Helen Moeckly, page 2