216 research outputs found
The Effect of Combined Magnetic Geometries on Thermally Driven Winds I: Interaction of Dipolar and Quadrupolar Fields
Cool stars with outer convective envelopes are observed to have magnetic
fields with a variety of geometries, which on large scales are dominated by a
combination of the lowest order fields such as the dipole, quadrupole and
octupole modes. Magnetised stellar wind outflows are primarily responsible for
the loss of angular momentum from these objects during the main sequence.
Previous works have shown the reduced effectiveness of the stellar wind braking
mechanism with increasingly complex, but singular, magnetic field geometries.
In this paper, we quantify the impact of mixed dipolar and quadrupolar fields
on the spin-down torque using 50 MHD simulations with mixed field, along with
10 of each pure geometries. The simulated winds include a wide range of
magnetic field strength and reside in the slow-rotator regime. We find that the
stellar wind braking torque from our combined geometry cases are well described
by a broken power law behaviour, where the torque scaling with field strength
can be predicted by the dipole component alone or the quadrupolar scaling
utilising the total field strength. The simulation results can be scaled and
apply to all main-sequence cool stars. For Solar parameters, the lowest order
component of the field (dipole in this paper) is the most significant in
determining the angular momentum loss.Comment: 15 pages + 9 figures (main), 3 pages + 1 figure (appendix), accepted
for publication to Ap
The Effect of Magnetic Variability on Stellar Angular Momentum Loss II: The Sun, 61 Cygni A, Eridani, Bootis A and Bootis A
The magnetic fields of low-mass stars are observed to be variable on decadal
timescales, ranging in behaviour from cyclic to stochastic. The changing
strength and geometry of the magnetic field should modify the efficiency of
angular momentum loss by stellar winds, but this has not been well quantified.
In Finley et al. (2018) we investigated the variability of the Sun, and
calculated the time-varying angular momentum loss rate in the solar wind. In
this work, we focus on four low-mass stars that have all had their surface
magnetic fields mapped for multiple epochs. Using mass loss rates determined
from astrospheric Lyman- absorption, in conjunction with scaling
relations from the MHD simulations of Finley & Matt (2018), we calculate the
torque applied to each star by their magnetised stellar winds. The variability
of the braking torque can be significant. For example, the largest torque for
Eri is twice its decadal averaged value. This variation is
comparable to that observed in the solar wind, when sparsely sampled. On
average, the torques in our sample range from 0.5-1.5 times their average
value. We compare these results to the torques of Matt et al. (2015), which use
observed stellar rotation rates to infer the long-time averaged torque on
stars. We find that our stellar wind torques are systematically lower than the
long-time average values, by a factor of ~3-30. Stellar wind variability
appears unable to resolve this discrepancy, implying that there remain some
problems with observed wind parameters, stellar wind models, or the long-term
evolution models, which have yet to be understood.Comment: 15 pages + 8 figures, accepted for publication to Ap
Evolution of solar wind sources and coronal rotation driven by the cyclic variation of the Sun's large-scale magnetic field
The strength and morphology of the Sun's magnetic field evolves significantly
during the solar cycle, with the overall polarity of the Sun's magnetic field
reversing during the maximum of solar activity. Long-term changes are also
observed in sunspot and geomagnetic records, however systematic magnetic field
observations are limited to the last four cycles. We investigate the long-term
evolution of the Sun's magnetic field, and the influence this has on the
topology and rotation of the solar corona. The Sun's photospheric magnetic
field was decomposed into spherical harmonics using synoptic Carrington
magnetograms from 1) WSO, 2) MDI onboard the SOHO, and 3) HMI onboard the SDO.
The time-evolution of the spherical harmonic coefficients was used to explore
the variation of the Sun's magnetic field, focusing on the large-scale modes.
PFSS extrapolations of the photospheric field were computed to follow
topological changes in the corona. The footpoints of the Sun's open magnetic
field vary between the polar coronal holes and activity driven features such as
active regions, and equatorial coronal holes. Consequently, the mean rotation
rate of the solar wind is modulated during each cycle by the latitudinal
variation of open field footpoints, with slower rotation during minima and
faster (Carrington-like) rotation during maxima. Thisc variation is sensitive
to cycle to cycle differences in the polar field strengths and hemispherical
flux emergence rates, with the ratio of quadrupole to dipole energy following a
similar variation. Cycle 23 maintained a larger fraction of quadrupolar energy
in the declining phase, which kept the sources of open magnetic flux closer to
the equator, extending the period of faster equator-ward connectivity. The
ratio of quadrupole to dipole energy could be a useful proxy when examining the
impact of differential rotation on the coronae of other Sun-like stars.Comment: Accepted to A&A. 13 Pages + Appendix. 11 Figures + 7 Appendix Figure
Accounting for Differential Rotation in Calculations of the Sun's Angular Momentum-loss Rate
Sun-like stars shed angular momentum due to the presence of magnetised
stellar winds. Magnetohydrodynamic models have been successful in exploring the
dependence of this "wind-braking torque" on various stellar properties, however
the influence of surface differential rotation is largely unexplored. As the
wind-braking torque depends on the rotation rate of the escaping wind, the
inclusion of differential rotation should effectively modulate the angular
momentum-loss rate based on the latitudinal variation of wind source regions.
In order to quantify the influence of surface differential rotation on the
angular momentum-loss rate of the Sun, we exploit the dependence of the
wind-braking torque on the effective rotation rate of the coronal magnetic
field. This quantity is evaluated by tracing field lines through a Potential
Field Source Surface (PFSS) model, driven by ADAPT-GONG magnetograms. The
surface rotation rates of the open magnetic field lines are then used to
construct an open-flux weighted rotation rate, from which the influence on the
wind-braking torque can be estimated. During solar minima, the rotation rate of
the corona decreases with respect to the typical solid-body rate (the
Carrington rotation period is 25.4 days), as the sources of the solar wind
shift towards the slowly-rotating poles. With increasing activity, more solar
wind emerges from the Sun's active latitudes which enforces a Carrington-like
rotation. The effect of differential rotation on the Sun's current wind-braking
torque is found to be small. The wind-braking torque is ~10-15% lower during
solar minimum, than assuming solid body rotation, and a few percent larger
during solar maximum. For more rapidly-rotating Sun-like stars, differential
rotation may play a more significant role, depending on the configuration of
the large-scale magnetic field.Comment: Accepted to A&A. 12 Pages + Appendix. 9 Figures + 4 Appendix Figure
Parker Solar Probe observations of suprathermal electron flux enhancements originating from coronal hole boundaries
Reconnection between pairs of solar magnetic flux elements, one open and the other a closed loop, is theorised to be a crucial process for both maintaining the structure of the corona and producing the solar wind. This 'interchange reconnection' is expected to be particularly active at the open-closed boundaries of coronal holes (CHs). Previous analysis of solar wind data at 1AU indicated that peaks in the flux of suprathermal electrons at slow-fast stream interfaces may arise from magnetic connection to the CH boundary, rather than dynamic effects such as compression. Further, offsets between the peak and stream interface locations are suggested to be the result of interchange reconnection at the source. As a preliminary test of these suggestions, we analyse two solar wind streams observed during the first Parker Solar Probe (PSP) perihelion encounter, each associated with equatorial CH boundaries (one leading and one trailing with respect to rotation). Each stream features a peak in suprathermal electron flux, the locations and associated plasma properties of which are indicative of a solar origin, in agreement with previous suggestions from 1AU observations. Discrepancies between locations of the flux peaks and other features suggest these peaks may too be shifted by source region interchange reconnection. Our interpretation of each event is compatible with a global pattern of open flux transport, although random footpoint motions or other explanations remain feasible. These exploratory results highlight future opportunities for statistical studies regarding interchange reconnection and flux transport at CH boundaries with modern near-Sun missions
Solar angular momentum loss over the past several millennia
The Sun and Sun-like stars lose angular momentum to their magnetized stellar winds. This braking torque is coupled to the stellar magnetic field, such that changes in the strength and/or geometry of the field modifies the efficiency of this process. Since the space age, we have been able to directly measure solar wind properties using in situ spacecraft. Furthermore, indirect proxies such as sunspot number, geomagnetic indices, and cosmogenic radionuclides, constrain the variation of solar wind properties on centennial and millennial timescales. We use near-Earth measurements of the solar wind plasma and magnetic field to calculate the torque on the Sun throughout the space age. Then, reconstructions of the solar open magnetic flux are used to estimate the time-varying braking torque during the last nine millennia. We assume a relationship for the solar mass-loss rate based on observations during the space age which, due to the weak dependence of the torque on mass-loss rate, does not strongly affect our predicted torque. The average torque during the last nine millennia is found to be 2.2 Ă 1030 erg, which is comparable to the average value from the last two decades. Our data set includes grand minima (such as the Maunder Minimum), and maxima in solar activity, where the torque varies from ~1 to 5 Ă 1030 erg (averaged on decadal timescales), respectively. We find no evidence for any secular variation of the torque on timescales of less than 9000 yr
Iterative focused screening with biological fingerprints identifies selective Asc-1 inhibitors distinct from traditional high throughput screening
N-methyl-d-aspartate receptors (NMDARs) mediate glutamatergic signaling that is critical to cognitive processes in the central nervous system, and NMDAR hypofunction is thought to contribute to cognitive impairment observed in both schizophrenia and Alzheimerâs disease. One approach to enhance the function of NMDAR is to increase the concentration of an NMDAR coagonist, such as glycine or d-serine, in the synaptic cleft. Inhibition of alanineâserineâcysteine transporter-1 (Asc-1), the primary transporter of d-serine, is attractive because the transporter is localized to neurons in brain regions critical to cognitive function, including the hippocampus and cortical layers III and IV, and is colocalized with d-serine and NMDARs. To identify novel Asc-1 inhibitors, two different screening approaches were performed with whole-cell amino acid uptake in heterologous cells stably expressing human Asc-1: (1) a high-throughput screen (HTS) of 3 M compounds measuring 35S l-cysteine uptake into cells attached to scintillation proximity assay beads in a 1536 well format and (2) an iterative focused screen (IFS) of a 45âŻ000 compound diversity set using a 3H d-serine uptake assay with a liquid scintillation plate reader in a 384 well format. Critically important for both screening approaches was the implementation of counter screens to remove nonspecific inhibitors of radioactive amino acid uptake. Furthermore, a 15âŻ000 compound expansion step incorporating both on- and off-target data into chemical and biological fingerprint-based models for selection of additional hits enabled the identification of novel Asc-1-selective chemical matter from the IFS that was not identified in the full-collection HTS
Using geographically weighted regression to explore the spatially heterogeneous spread of bovine tuberculosis in England and Wales
An understanding of the factors that affect the spread of endemic bovine tuberculosis (bTB) is critical for the development of measures to stop and reverse this spread. Analyses of spatial data need to account for the inherent spatial heterogeneity within the data, or else spatial autocorrelation can lead to an overestimate of the significance of variables. This study used three methods of analysisâleast-squares linear regression with a spatial autocorrelation term, geographically weighted regression (GWR) and boosted regression tree (BRT) analysisâto identify the factors that influence the spread of endemic bTB at a local level in England and Wales. The linear regression and GWR methods demonstrated the importance of accounting for spatial differences in risk factors for bTB, and showed some consistency in the identification of certain factors related to flooding, disease history and the presence of multiple genotypes of bTB. This is the first attempt to explore the factors associated with the spread of endemic bTB in England and Wales using GWR. This technique improves on least-squares linear regression approaches by identifying regional differences in the factors associated with bTB spread. However, interpretation of these complex regional differences is difficult and the approach does not lend itself to predictive models which are likely to be of more value to policy makers. Methods such as BRT may be more suited to such a task. Here we have demonstrated that GWR and BRT can produce comparable outputs
Kepler main-sequence solar-like stars : surface rotation and magnetic-activity evolution
While the missionâs primary goal was focused on exoplanet detection and characterization, Kepler made and continues to make extraordinary advances in stellar physics. Stellar rotation and magnetic activity are no exceptions. Kepler allowed for these properties to be determined for tens of thousands of stars from the main sequence up to the red giant branch. From photometry, this can be achieved by investigating the brightness fluctuations due to active regions, which cause surface inhomogeneities, or through asteroseismology as oscillation modes are sensitive to rotation and magnetic fields. This review summarizes the rotation and magnetic activity properties of the single main-sequence solar-like stars within the Kepler field. We contextualize the Kepler sample by comparing it to known transitions in the stellar rotation and magnetic-activity evolution, such as the convergence to the rotation sequence (from the saturated to the unsaturated regime of magnetic activity) and the Vaughan-Preston gap. While reviewing the publicly available data, we also uncover one interesting finding related to the intermediate-rotation gap seen in Kepler and other surveys. We find evidence for this rotation gap in previous ground-based data for the X-ray luminosity. Understanding the complex evolution and interplay between rotation and magnetic activity in solar-like stars is crucial, as it sheds light on fundamental processes governing stellar evolution, including the evolution of our own Sun
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