Evolution of galactic magnetic fields Rodrigues, L F S; Chamandy, L; Shukurov, A ...
Monthly notices of the Royal Astronomical Society,
02/2019, Volume:
483, Issue:
2
Journal Article
Peer reviewed
Open access
ABSTRACT
We study the cosmic evolution of the magnetic fields of a large sample of spiral galaxies in a cosmologically representative volume by employing a semi-analytic galaxy formation model and ...numerical dynamo solver in tandem. We start by deriving time- and radius-dependent galaxy properties using the galform galaxy formation model, which are then fed into the non-linear mean-field dynamo equations. These are solved to give the large-scale (mean) field as a function of time and galactocentric radius for a thin disc, assuming axial symmetry. A simple prescription for the evolution of the small-scale (random) magnetic field component is also adopted. We find that, while most massive galaxies are predicted to have large-scale magnetic fields at redshift $z$ = 0, a significant fraction of them is expected to contain negligible large-scale field. Our model indicates that, for most of the galaxies containing large-scale magnetic fields today, the mean-field dynamo becomes active at $z$ < 3. Moreover, the typical magnetic field strength at any given galactic stellar mass is predicted to decline with time up until the present epoch, in agreement with our earlier results. We compute the radial profiles of pitch angle, and find broad agreement with observational data for nearby galaxies.
ABSTRACT
Volume complete sky surveys provide evidence for a binary origin for the formation of isolated white dwarfs with magnetic fields in excess of a MegaGauss. Interestingly, not a single ...high-field magnetic white dwarf has been found in a detached system, suggesting that if the progenitors are indeed binaries, the companion must be removed or merge during formation. An origin scenario consistent with observations involves the engulfment, inspiral, and subsequent tidal disruption of a low-mass companion in the interior of a giant star during a common envelope phase. Material from the shredded companion forms a cold accretion disc embedded in the hot ambient around the proto-white dwarf. Entrainment of hot material may evaporate the disc before it can sufficiently amplify the magnetic field, which typically requires at least a few orbits of the disc. Using three-dimensional hydrodynamic simulations of accretion discs with masses between 1 and 10 times the mass of Jupiter inside the core of an Asymptotic Giant Branch star, we find that the discs survive for at least 10 orbits (and likely for 100 orbits), sufficient for strong magnetic fields to develop.
ABSTRACT Volume complete sky surveys provide evidence for a binary origin for the formation of isolated white dwarfs with magnetic fields in excess of a MegaGauss. Interestingly, not a single ...high-field magnetic white dwarf has been found in a detached system, suggesting that if the progenitors are indeed binaries, the companion must be removed or merge during formation. An origin scenario consistent with observations involves the engulfment, inspiral, and subsequent tidal disruption of a low-mass companion in the interior of a giant star during a common envelope phase. Material from the shredded companion forms a cold accretion disc embedded in the hot ambient around the proto-white dwarf. Entrainment of hot material may evaporate the disc before it can sufficiently amplify the magnetic field, which typically requires at least a few orbits of the disc. Using three-dimensional hydrodynamic simulations of accretion discs with masses between 1 and 10 times the mass of Jupiter inside the core of an Asymptotic Giant Branch star, we find that the discs survive for at least 10 orbits (and likely for 100 orbits), sufficient for strong magnetic fields to develop.
The first in situ nightside electron density and temperature profiles at Mars are presented as functions of altitude and local time (LT) from the Langmuir Probe and Waves (LPW) instrument on board ...the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission spacecraft. LPW is able to measure densities as low as ∼100
cm−3, a factor of up to 10 or greater improvement over previous measurements. Above 200 km, near‐vertical density profiles of a few hundred cubic centimeters were observed for almost all nightside LT, with the lowest densities and highest temperatures observed postmidnight. Density peaks of a few thousand cubic centimeters were observed below 200 km at all nightside LT. The lowest temperatures were observed below 180 km and approach the neutral atmospheric temperature. One‐dimensional modeling demonstrates that precipitating electrons were able to sustain the observed nightside ionospheric densities below 200 km.
Key Points
First in situ nightside electron density and temperature profiles on the nightside of Mars
Electron temperatures approach neutral atmospheric temperatures below 200 km
Electron densities below 200 km require additional ionization source
Le mentorat est un moyen efficace d'aider les employés à améliorer leurs compétences, à resserrer les liens entre eux et à favoriser l'apprentissage dans le milieu de travail. L'approche des mentors ...peut notamment mener à ces répercussions positives. Par contre, des études indiquent que le mentorat peut aussi être nuisible ou inefficace pour certains mentorés, et ce, en raison des comportements interpersonnels des mentors. Deux études avec des modèles complémentaires ont été menées : une étude avec une approche transversale (N = 226) et une seconde avec une approche prospective (N = 278). Elles visaient à répondre à l'objectif d'examiner l'effet des comportements interpersonnels des mentors (selon la perspective des mentorés) sur différentes conséquences en milieu de travail des mentorés (intention de quitter le travail, engagement au travail, bien-être, qualité de la relation de mentorat) par l'entremise de deux types de motivation contextuelle (travail et mentorat). Les résultats suggèrent, en général, que les comportements interpersonnels de soutien émis par les mentors contribuent à une motivation autonome au travail et dans la relation de mentorat, ce qui est lié à un meilleur fonctionnement des mentorés. À l'inverse, quand les comportements interpersonnels des mentors sont contraignants, ils favorisent une motivation contrôlée et peuvent avoir des effets négatifs sur l'engagement au travail, le bien-être au travail et la qualité de la relation de mentorat, et des effets positifs sur l'intention de quitter le travail.
Mentoring is an effective way to help employees improve their skills, strengthen their relationships, and promote learning in the workplace. However, studies indicate that mentoring can also be harmful or ineffective for some mentees, particularly due to mentors' interpersonal behaviors. Two studies with complementary designs were conducted: one with a cross-sectional approach (N = 226) and a second with a prospective approach (N = 278). Both studies aimed to examine the effect of mentors' interpersonal behaviors (from the mentees' perspective) on mentees' workplace outcomes (turnover intentions, work engagement, well-being at work, quality of the mentoring relationship) through two types of contextual motivation (work and mentoring). Overall, the results suggest that the need-supportive interpersonal behaviors exhibited by mentors contribute to greater autonomous motivation at work and in the mentoring relationship, which is associated with more positive functioning among mentees. In contrast, restrictive interpersonal behaviors are related to greater controlled forms of motivation, which decreases mentees' work engagement, their well-being at work and the quality of the mentoring relationship and increases their turnover intentions.
Intérêt public
Pris dans leur ensemble, les résultats suggèrent que les mentors qui soutiennent les besoins psychologiques des mentorés peuvent favoriser une motivation plus optimale à l'égard de l'emploi et de la relation de mentorat, et l'épanouissement professionnel des mentorés. Les comportements des mentors jouent ainsi un rôle important dans la qualité de la motivation des mentorés et leur expérience dans le milieu de travail.
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