499 research outputs found
The Twist of the Draped Interstellar Magnetic Field Ahead of the Heliopause: A Magnetic Reconnection Driven Rotational Discontinuity
Based on the difference between the orientation of the interstellar
and the solar magnetic fields, there was an expectation that the magnetic field
direction would rotate dramatically across the heliopause (HP). However, the
Voyager 1 spacecraft measured very little rotation across the HP. Previously we
showed that the twists as it approaches the HP and acquires a strong
T component (East-West). Here we establish that reconnection in the eastern
flank of the heliosphere is responsible for the twist. On the eastern flank the
solar magnetic field has twisted into the positive N direction and reconnects
with the Southward pointing component of the . Reconnection drives a
rotational discontinuity (RD) that twists the into the -T direction
and propagates upstream in the interstellar medium towards the nose. The
consequence is that the N component of is reduced in a finite width
band upstream of the HP. Voyager 1 currently measures angles
() close to solar values. We present MHD simulations
to support this scenario, suppressing reconnection in the nose region while
allowing it in the flanks, consistent with recent ideas about reconnection
suppression from diamagnetic drifts. The jump in plasma (the plasma to
magnetic pressure) across the nose of HP is much greater than in the flanks
because the heliosheath is greater there than in the flanks.
Large-scale reconnection is therefore suppressed in the nose but not at the
flanks. Simulation data suggest that will return to its pristine
value past the HP.Comment: 19 pages, 5 figures, submitte
Can we replicate the findings of EEF trials using school level comparative interrupted time series evaluations? Non-technical report
This report focuses on whether one particular non-experimental method can reproduce the results from experimental evaluations: the comparative interrupted time series (CITS) design. The basic idea is to compare the way in which outcomes in the treatment group deviate from trend after an intervention is introduced, relative to the way in which outcomes in the control group deviate from trend at the same point in time. Under certain assumptions, the difference between these deviations can be interpreted as the effect of the intervention
Reconciling aerosol light extinction measurements from spaceborne lidar observations and in situ measurements in the Arctic
© Author(s) 2014. This work is distributed under the Creative Commons Attribution 3.0 License.In this study we investigate to what degree it is possible to reconcile continuously recorded particle light extinction coefficients derived from dry in situ measurements at Zeppelin station (78.92° N, 11.85° E; 475 m above sea level), Ny-Ålesund, Svalbard, that are recalculated to ambient relative humidity, as well as simultaneous ambient observations with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. To our knowledge, this represents the first study that compares spaceborne lidar measurements to optical aerosol properties from short-term in situ observations (averaged over 5 h) on a case-by-case basis. Finding suitable comparison cases requires an elaborate screening and matching of the CALIOP data with respect to the location of Zeppelin station as well as the selection of temporal and spatial averaging intervals for both the ground-based and spaceborne observations. Reliable reconciliation of these data cannot be achieved with the closest-approach method, which is often used in matching CALIOP observations to those taken at ground sites. This is due to the transport pathways of the air parcels that were sampled. The use of trajectories allowed us to establish a connection between spaceborne and ground-based observations for 57 individual overpasses out of a total of 2018 that occurred in our region of interest around Svalbard (0 to 25° E, 75 to 82° N) in the considered year of 2008. Matches could only be established during winter and spring, since the low aerosol load during summer in connection with the strong solar background and the high occurrence rate of clouds strongly influences the performance and reliability of CALIOP observations. Extinction coefficients in the range of 2 to 130 Mmg-1 at 532 nm were found for successful matches with a difference of a factor of 1.47 (median value for a range from 0.26 to 11.2) between the findings of in situ and spaceborne observations (the latter being generally larger than the former). The remaining difference is likely to be due to the natural variability in aerosol concentration and ambient relative humidity, an insufficient representation of aerosol particle growth, or a misclassification of aerosol type (i.e., choice of lidar ratio) in the CALIPSO retrieval.Peer reviewe
Is Canada really an education superpower? The impact of non-participation on results from PISA 2015
The purpose of large-scale international assessments is to compare educational achievement across countries. For such cross-national comparisons to be meaningful, the participating students must be representative of the target population. In this paper, we consider whether this is the case for Canada, a country widely recognised as high performing in the Programme for International Student Assessment (PISA). Our analysis illustrates how the PISA 2015 sample for Canada only covers around half of the 15-year-old population, compared to over 90% in countries like Finland, Estonia, Japan and South Korea. We discuss how this emerges from differences in how children with special educational needs are defined and rules for their inclusion in the study, variation in school participation rates and the comparatively high rates of pupils’ absence in Canada during the PISA study. The paper concludes by investigating how Canada’s PISA 2015 rank would change under different assumptions about how the non-participating students would have performed were they to have taken the PISA test
Magnetohydrodynamic simulation of an equatorial dipolar paleomagnetosphere
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95191/1/jgra17407.pd
Modeling the Young Sun's Solar Wind and its Interaction with Earth's Paleomagnetosphere
We present a focused parameter study of solar wind - magnetosphere
interaction for the young Sun and Earth, Ga ago, that relies on
magnetohydrodynamic (MHD) simulations for both the solar wind and the
magnetosphere. By simulating the quiescent young Sun and its wind we are able
to propagate the MHD simulations up to Earth's magnetosphere and obtain a
physically realistic solar forcing of it. We assess how sensitive the young
solar wind is to changes in the coronal base density, sunspot placement and
magnetic field strength, dipole magnetic field strength and the Sun's rotation
period. From this analysis we obtain a range of plausible solar wind conditions
the paleomagnetosphere may have been subject to. Scaling relationships from the
literature suggest that a young Sun would have had a mass flux different from
the present Sun. We evaluate how the mass flux changes with the aforementioned
factors and determine the importance of this and several other key solar and
magnetospheric variables with respect to their impact on the
paleomagnetosphere. We vary the solar wind speed, density, interplanetary
magnetic field strength and orientation as well as Earth's dipole magnetic
field strength and tilt in a number of steady-state scenarios that are
representative of young Sun-Earth interaction. This study is done as a first
step of a more comprehensive effort towards understanding the implications of
Sun-Earth interaction for planetary atmospheric evolution.Comment: 16 pages, 7 figure
Magnetic polarizability of hadrons from lattice QCD
We extract the magnetic polarizability from the quadratic response of a
hadron's mass shift in progressively small static magnetic fields. The
calculation is done on a 24x12x12x24 lattice at a = 0.17 fm with an improved
gauge action and the clover quark action. The results are compared to those
from experiments and models where available.Comment: 3 pages, 3 figures, contribution to Lattice 2002 (spectrum
Magnetized jets driven by the sun: the structure of the heliosphere revisited
The classic accepted view of the heliosphere is a quiescent, comet-like shape
aligned in the direction of the Sun's travel through the interstellar medium
(ISM) extending for 1000's of AUs (AU: astronomical unit). Here we show, based
on magnetohydrodynamic (MHD) simulations, that the tension (hoop) force of the
twisted magnetic field of the sun confines the solar wind plasma beyond the
termination shock and drives jets to the North and South very much like
astrophysical jets. These jets are deflected into the tail region by the motion
of the Sun through the ISM similar to bent galactic jets moving through the
intergalactic medium. The interstellar wind blows the two jets into the tail
but is not strong enough to force the lobes into a single comet-like tail, as
happens to some astrophysical jets (Morsony et al. 2013). Instead, the
interstellar wind flows around the heliosphere and into equatorial region
between the two jets. As in some astrophysical jets that are kink unstable
(Porth et al. 2014) we show here that the heliospheric jets are turbulent (due
to large-scale MHD instabilities and reconnection) and strongly mix the solar
wind with the ISM beyond 400 AU. The resulting turbulence has important
implications for particle acceleration in the heliosphere. The two-lobe
structure is consistent with the energetic neutral atoms (ENAs) images of the
heliotail from IBEX (McComas et al. 2013) where two lobes are visible in the
North and South and the suggestion from the CASSINI (Krimigis et al. 2009,
Dialynas et al. 2013) ENAs that the heliosphere is "tailless".Comment: 19 pages, 5 figures; Astrophysical Journal Letters; in pres
Multiple packets of neutral molecules revolving for over a mile
The level of control that one has over neutral molecules in beams dictates
their possible applications. Here we experimentally demonstrate that
state-selected, neutral molecules can be kept together in a few mm long packet
for a distance of over one mile. This is accomplished in a circular arrangement
of 40 straight electrostatic hexapoles through which the molecules propagate
over 1000 times. Up to 19 packets of molecules have simultaneously been stored
in this ring structure. This brings the realization of a molecular low-energy
collider within reach
Electrostatic trapping of metastable NH molecules
We report on the Stark deceleration and electrostatic trapping of NH
() radicals. In the trap, the molecules are excited on the
spin-forbidden transition and detected via
their subsequent fluorescence to the ground state. The 1/e
trapping time is 1.4 0.1 s, from which a lower limit of 2.7 s for the
radiative lifetime of the state is deduced. The spectral
profile of the molecules in the trapping field is measured to probe their
spatial distribution. Electrostatic trapping of metastable NH followed by
optical pumping of the trapped molecules to the electronic ground state is an
important step towards accumulation of these radicals in a magnetic trap.Comment: replaced with final version, added journal referenc
- …