15 research outputs found
NIRwave: A wave-turbulence-driven solar wind model constrained by PSP observations
We generate a model description of the solar wind based on an explicit
wave-turbulence-driven heating mechanism, and constrain our model with
observational data. We included an explicit coronal heating source term in the
general 3D magnetohydrodynamic code NIRVANA to simulate the properties of the
solar wind. The adapted heating mechanism is based on the interaction and
subsequent dissipation of counter-propagating Alfv\'en waves in the solar
corona, accounting for a turbulent heating rate Q_p. The solar magnetic field
is assumed to be an axisymmetric dipole with a field strength of 1 G. Our model
results are validated against observational data taken by the Parker Solar
Probe (PSP). Our NIRwave solar wind model reconstructs the bimodal structure of
the solar wind with slow and fast wind speeds of 410 km/s and 650 km/s
respectively. The global mass-loss rate of our solar wind model is 2.6e-14
solar masses per year. Despite implementing simplified conditions to represent
the solar magnetic field, the solar wind parameters characterising our
steady-state solution are in reasonable agreement with previously established
results and empirical constraints. The number density from our wind solution is
in good agreement with the derived empirical constraints, with larger
deviations for the radial velocity and temperature. In a comparison to a
polytropic wind model generated with NIRVANA, we find that our NIRwave model is
in better agreement with the observational constraints that we derive.Comment: 14 pages, 12 figures, accepted for publication in A&
Estimating magnetic filling factors from Zeeman–Doppler magnetograms
V.S., S.P.M., and A.J.F.acknowledge funding from the European Research Council (ERC) under the European Unions Horizon 2020 research and innovation programme (grant agreement No. 682393 AWESoMeStars). S.B.S. acknowledges funding via the Austrian Space Application Programme (ASAP) of the Austrian Research Promotion Agency (FFG) within ASAP11, the FWF NFN project S11601-N16 and the sub-project S11604-N16. A. A.V. acknowledges funding received from the Irish Research Council Laureate Awards 2017/2018.Low-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterize them. Zeeman–Doppler imaging (ZDI) can determine the geometry of the large-scale magnetic field while Zeeman broadening can assess the total unsigned flux including that associated with small-scale structures such as spots. In this work, we study a sample of stars that have been previously mapped with ZDI. We show that the average unsigned magnetic flux follows an activity-rotation relation separating into saturated and unsaturated regimes. We also compare the average photospheric magnetic flux recovered by ZDI, BV, with that recovered by Zeeman broadening studies, BI. In line with previous studies, BV ranges from a few % to ~20% of BI. We show that a power-law relationship between BV and BI exists and that ZDI recovers a larger fraction of the magnetic flux in more active stars. Using this relation, we improve on previous attempts to estimate filling factors, i.e., the fraction of the stellar surface covered with magnetic field, for stars mapped only with ZDI. Our estimated filling factors follow the well-known activity-rotation relation, which is in agreement with filling factors obtained directly from Zeeman broadening studies. We discuss the possible implications of these results for flux tube expansion above the stellar surface and stellar wind models.Publisher PDFPeer reviewe
The large scale magnetic field of the G0 dwarf HD 206860 (HN Peg)
International audienc
A change in the relationship between chromospheric activity and the large-scale magnetic field for G stars on the main sequence
International audienceWe present a database of chromospheric activity and magnetic field strengths for 954 F-M dwarf stars Our active sample complements previous chromospheric activity surveys that usually focus on inactive, planet search targets. The internal magnetic fields of cool stars are known to power activity in stellar chromospheres, but there is still much to learn about the nature of this relationship and its dependence on stellar properties. We draw on data from PolarBase, a rich legacy database of observations from the spectropolarimeters ESPaDOnS (CFHT) and NARVAL (TBL), to survey the chromospheric activity and large-scale magnetic field strengths for 954 F-M dwarf stars
Estimating Magnetic Filling Factors from Zeeman–Doppler Magnetograms
International audienceLow-mass stars are known to have magnetic fields that are believed to be of dynamo origin. Two complementary techniques are principally used to characterize them. Zeeman-Doppler imaging (ZDI) can determine the geometry of the large-scale magnetic field while Zeeman broadening can assess the total unsigned flux including that associated with small-scale structures such as spots. In this work, we study a sample of stars that have been previously mapped with ZDI. We show that the average unsigned magnetic flux follows an activity-rotation relation separating into saturated and unsaturated regimes. We also compare the average photospheric magnetic flux recovered by ZDI, , with that recovered by Zeeman broadening studies, . In line with previous studies, ranges from a few % to Ëś20% of . We show that a power-law relationship between and exists and that ZDI recovers a larger fraction of the magnetic flux in more active stars. Using this relation, we improve on previous attempts to estimate filling factors, i.e., the fraction of the stellar surface covered with magnetic field, for stars mapped only with ZDI. Our estimated filling factors follow the well-known activity-rotation relation, which is in agreement with filling factors obtained directly from Zeeman broadening studies. We discuss the possible implications of these results for flux tube expansion above the stellar surface and stellar wind models
Slingshot prominences: coronal structure, mass loss and spin down
International audienc