2,115 research outputs found
A Statistical Survey of Low‐Frequency Magnetic Fluctuations at Saturn
Low‐frequency waves are closely related to magnetospheric energy dissipation processes. The Cassini spacecraft explored Saturn's magnetosphere for over 13 years, until September 2017, covering a period of more than a complete solar cycle. Using this rich heritage dataset, we systematically investigated key physical parameters of low‐frequency waves in Saturn's magnetosphere, including their local time distribution and the dependence on solar activity. We found that the wave activity peaked in the near noon sector. For the nightside, the wave intensity also appeared to peak pre and post‐midnight. Due to the limited local time coverage for each solar phase, we were not able to draw a firm conclusion on the wave's dependence on solar activity. In general, the wave power showed a monotonically decreasing trend towards larger distances in nightside sectors especially during the declining phase, which implied that low‐frequency waves mainly originate from the relatively inner regions of the magnetosphere. On the dayside, stronger waves were mostly located at/within ∼25 Rs, near the magnetopause. The study shows a global picture of low‐frequency waves in Saturn's magnetosphere, providing important implications for how magnetospheric energy dissipates into Saturn's polar ionosphere and atmosphere
Plasma Sheet Pressure Variations in the Near‐Earth Magnetotail During Substorm Growth Phase: THEMIS Observations
We investigate the plasma sheet pressure variations in the near‐Earth magnetotail (radius distance, R, from 7.5 RE to 12 RE and magnetic local time, MLT, from 18:00 to 06:00) during substorm growth phase with Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations. It is found that, during the substorm growth phase, about 39.4% (76/193) of the selected events display a phenomenon of equatorial plasma pressure (Peq) decrease. The occurrence rates of Peq decrease cases are higher in the dawn (04:00 to 06:00) and dusk (18:00 to 20:00) flanks (> 50%) than in the midnight region (20:00 to 04:00, −16%). The mean value of Peq increase percentages at the end of substorm growth phase is the highest (~ 40%) in the premidnight MLT bin (22:00 to 00:00) and is almost unchanged in the dawn and dusk flanks. Further investigations show that 13.0% of the events have more than 10% of Peq decrease at the end of substorm growth phase comparing to the value before the growth phase, and ~ 28.0% of the events have small changes (< 10%), and ~ 59.0% events have a more than 10% increase. This study also reveals the importance of electron pressure (Pe) in the variation of Peq in the substorm growth phase. The Pe variations often account for more than 50% of the Peq changes, and the ratios of Pe to ion pressure often display large variations (~ 50%). Among the investigated events, during the growth phase, an enhanced equatorial plasma convection flow is observed, which diverges in the midnight tail region and propagates azimuthally toward the dayside magnetosphere with velocity of ~ 20 km/s. It is proposed that the Peq decreases in the near‐Earth plasma sheet during the substorm growth phase may be due to the transport of closed magnetic flux toward the dayside magnetosphere driven by dayside magnetopause reconnection. Both solar wind and ionospheric conductivity effects may influence the distributions of occurrence rates for Peq decrease events and the Peq increase percentages in the investigated region.Key PointsAbout 40% of the selected events in the near‐tail region display a phenomenon of equatorial plasma pressure decreaseAn enhanced equatorial convection with speed of ~ 20 km/s is observed in our cases during the substorm growth phaseStatistical studies for the distributions of Peq properties and electron pressure variations are performedPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/141851/1/jgra53963.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/141851/2/jgra53963_am.pd
Plasmapause surface wave oscillates the magnetosphere and diffuse aurora
Energy circulation in geospace lies at the heart of space weather research. In the inner
magnetosphere, the steep plasmapause boundary separates the cold dense plasmasphere,
which corotates with the planet, from the hot ring current/plasma sheet outside. Theoretical
studies suggested that plasmapause surface waves related to the sharp inhomogeneity exist
and act as a source of geomagnetic pulsations, but direct evidence of the waves and their role
in magnetospheric dynamics have not yet been detected. Here, we show direct observations
of a plasmapause surface wave and its impacts during a geomagnetic storm using multisatellite and ground-based measurements. The wave oscillates the plasmapause in the
afternoon-dusk sector, triggers sawtooth auroral displays, and drives outward-propagating
ultra-low frequency waves. We also show that the surface-wave-driven sawtooth auroras
occurred in more than 90% of geomagnetic storms during 2014–2018, indicating that they
are a systematic and crucial process in driving space energy dissipation
Theory for superconductivity in (Tl,K)FeSe as a doped Mott insulator
Possible superconductivity in recently discovered (Tl,K)FeSe
compounds is studied from the viewpoint of doped Mott insulator. The Mott
insulating phase is examined to be preferred in the parent compound at
due to the presence of Fe vacancies. Partial filling of vacancies at the
Fe-sites introduces electron carriers and leads to electron doped
superconductivity. By using a two-orbital Hubbard model in the strong coupling
limit, we find that the s-wave pairing is more favorable at small Hund's
coupling, and d wave pairing is more favorable at large Hund's
coupling.Comment: 4+ pages, 3 figures, to appear in EP
Competitions of magnetism and superconductivity in FeAs-based materials
Using the numerical unrestricted Hartree-Fock approach, we study the ground
state of a two-orbital model describing newly discovered FeAs-based
superconductors. We observe the competition of a mode spin-density
wave and the superconductivity as the doping concentration changes. There might
be a small region in the electron-doping side where the magnetism and
superconductivity coexist. The superconducting pairing is found to be spin
singlet, orbital even, and mixed s + d wave (even
parity).Comment: 5 pages, 3 figure
Probing the superconducting pairing symmetry from spin excitations in BiS based superconductors
Starting from a two-orbital model and based on the random phase
approximation, spin excitations in the superconducting state of the newly
discovered BiS superconductors with three possible pairing symmetries are
studied theoretically. We show that spin response is uniquely determined by the
pairing symmetry. Possible spin resonance excitations might occur for the
d-wave symmetry at an incommensurate momentum about . For the
p-wave symmetry the transverse spin excitation near is enhanced. For
the s-wave pairing symmetry there is no spin resonance signature. These
distinct features may be used for probing or determining the pairing symmetry
in this newly discovered compound.Comment: 4 pages, 5 figure
Experimental observation of topological Fermi arcs in type-II Weyl semimetal MoTe2
Weyl semimetal is a new quantum state of matter [1-12] hosting the condensed
matter physics counterpart of relativisticWeyl fermion [13] originally
introduced in high energy physics. The Weyl semimetal realized in the TaAs
class features multiple Fermi arcs arising from topological surface states [10,
11, 14-16] and exhibits novel quantum phenomena, e.g., chiral anomaly induced
negative mag-netoresistance [17-19] and possibly emergent supersymmetry [20].
Recently it was proposed theoretically that a new type (type-II) of Weyl
fermion [21], which does not have counterpart in high energy physics due to the
breaking of Lorentz invariance, can emerge as topologically-protected touching
between electron and hole pockets. Here, we report direct spectroscopic
evidence of topological Fermi arcs in the predicted type-II Weyl semimetal
MoTe2 [22-24]. The topological surface states are confirmed by directly
observing the surface states using bulk-and surface-sensitive angle-resolved
photoemission spectroscopy (ARPES), and the quasi-particle interference (QPI)
pattern between the two putative Fermi arcs in scanning tunneling microscopy
(STM). Our work establishes MoTe2 as the first experimental realization of
type-II Weyl semimetal, and opens up new opportunities for probing novel
phenomena such as exotic magneto-transport [21] in type-II Weyl semimetals.Comment: submitted on 01/29/2016. Nature Physics, in press. Spectroscopic
evidence of the Fermi arcs from two complementary surface sensitive probes -
ARPES and STS. A comparison of the calculated band structure for T_d and 1T'
phase to identify the topological Fermi arcs in the T_d phase is also
included in the supplementary informatio
Rapamycin Pharmacokinetic and Pharmacodynamic Relationships in Osteosarcoma: A Comparative Oncology Study in Dogs
Signaling through the mTOR pathway contributes to growth, progression and chemoresistance of several cancers. Accordingly, inhibitors have been developed as potentially valuable therapeutics. Their optimal development requires consideration of dose, regimen, biomarkers and a rationale for their use in combination with other agents. Using the infrastructure of the Comparative Oncology Trials Consortium many of these complex questions were asked within a relevant population of dogs with osteosarcoma to inform the development of mTOR inhibitors for future use in pediatric osteosarcoma patients.This prospective dose escalation study of a parenteral formulation of rapamycin sought to define a safe, pharmacokinetically relevant, and pharmacodynamically active dose of rapamycin in dogs with appendicular osteosarcoma. Dogs entered into dose cohorts consisting of 3 dogs/cohort. Dogs underwent a pre-treatment tumor biopsy and collection of baseline PBMC. Dogs received a single intramuscular dose of rapamycin and underwent 48-hour whole blood pharmacokinetic sampling. Additionally, daily intramuscular doses of rapamycin were administered for 7 days with blood rapamycin trough levels collected on Day 8, 9 and 15. At Day 8 post-treatment collection of tumor and PBMC were obtained. No maximally tolerated dose of rapamycin was attained through escalation to the maximal planned dose of 0.08 mg/kg (2.5 mg/30 kg dog). Pharmacokinetic analysis revealed a dose-dependent exposure. In all cohorts modulation of the mTOR pathway in tumor and PBMC (pS6RP/S6RP) was demonstrated. No change in pAKT/AKT was seen in tumor samples following rapamycin therapy.Rapamycin may be safely administered to dogs and can yield therapeutic exposures. Modulation pS6RP/S6RP in tumor tissue and PBMCs was not dependent on dose. Results from this study confirm that the dog may be included in the translational development of rapamycin and potentially other mTOR inhibitors. Ongoing studies of rapamycin in dogs will define optimal schedules for their use in cancer and evaluate the role of rapamycin use in the setting of minimal residual disease
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