143 research outputs found
Surface magnetic field effects in local helioseismology
Using helioseismic holography strong evidence is presented that the phase (or
equivalent travel-time) of helioseismic signatures in Dopplergrams within
sunspots depend upon the line-of-sight angle in the plane containing the
magnetic field and vertical directions. This is shown for the velocity signal
in the penumbrae of two sunspots at 3, 4 and 5 mHz. Phase-sensitive holography
demonstrates that they are significantly affected in a strong, moderately
inclined magnetic field. This research indicates that the effects of the
surface magnetic field are potentially very significant for local helioseismic
analysis of active regions.Comment: 6 pages, 8 figure
Convectively stabilised background solar models for local helioseismology
In local helioseismology numerical simulations of wave propagation are useful
to model the interaction of solar waves with perturbations to a background
solar model. However, the solution to the equations of motions include
convective modes that can swamp the waves we are interested in. For this
reason, we choose to first stabilise the background solar model against
convection by altering the vertical pressure gradient. Here we compare the
eigenmodes of our convectively stabilised model with a standard solar model
(Model S) and find a good agreement.Comment: 3 pages, 3 figures, HELAS NA3, The Acoustic Solar Cycle, Birmingham,
6-8 January 200
Fragile detection of solar g modes by Fossat et al
The internal gravity modes of the Sun are notoriously difficult to detect,
and the claimed detection of gravity modes presented in Fossat et al. 2017 is
thus very exciting. Given the importance of these modes for understanding solar
structure and dynamics, the results must be robust. While Fossat et al. 2017
described their method and parameter choices in detail, the sensitivity of
their results to several parameters were not presented. Therefore, we test the
sensitivity to a selection of them. The most concerning result is that the
detection vanishes when we adjust the start time of the 16.5 year velocity time
series by a few hours. We conclude that this reported detection of gravity
modes is extremely fragile and should be treated with utmost caution.Comment: 15 pages, 11 Figure
Constraining differential rotation of Sun-like stars from asteroseismic and starspot rotation periods
In previous work we identified six Sun-like stars observed by Kepler with
exceptionally clear asteroseismic signatures of rotation. Here, we show that
five of these stars exhibit surface variability suitable for measuring
rotation. In order to further constrain differential rotation, we compare the
rotation periods obtained from light-curve variability with those from
asteroseismology. The two rotation measurement methods are found to agree
within uncertainties, suggesting that radial differential rotation is weak, as
is the case for the Sun. Furthermore, we find significant discrepancies between
ages from asteroseismology and from three different gyrochronology relations,
implying that stellar age estimation is problematic even for Sun-like stars.Comment: Accepted for publication in A&A. 5 pages, 4 figure
Magneto-acoustic wave propagation and mode conversion in a magnetic solar atmosphere: comparing results from the CO5BOLD code with ray theory
We present simulations of magneto-acoustic wave propagation in a magnetic,
plane-parallel stratified solar model atmosphere, employing the CO5BOLD-code.
The tests are carried out for two models of the solar atmosphere, which are
similar to the ones used by Cally (2007) and Schunker & Cally (2006). The two
models differ only in the orientation of the magnetic field. A qualitative
comparison shows good agreement between the numerical results and the results
from ray theory. The tests are done in view of the application of the present
numerical code for the computation of energy fluxes of propagating acoustic
waves into a dynamically evolving magnetic solar atmosphere. For this, we
consider waves with frequencies above the acoustic cut-off frequency.Comment: 4 pages, 4 figures, will appear in HELAS-IV Conference Proceedin
Rotational splitting as a function of mode frequency for six Sun-like stars
Asteroseismology offers the prospect of constraining differential rotation in
Sun-like stars. Here we have identified six high signal-to-noise main-sequence
Sun-like stars in the Kepler field, which all have visible signs of rotational
splitting of their p-mode frequencies. For each star, we extract the rotational
frequency splitting and inclination angle from separate mode sets (adjacent
modes with l=2, 0, and 1) spanning the p-mode envelope. We use a Markov chain
Monte Carlo method to obtain the best fit and errors associated with each
parameter. We are able to make independent measurements of rotational
splittings of ~8 radial orders for each star. For all six stars, the measured
splittings are consistent with uniform rotation, allowing us to exclude large
radial differential rotation. This work opens the possibility of constraining
internal rotation of Sun-like stars.Comment: Published in Astronomy and Astrophysics. 4 pages, 3 figure
Observing and modeling the poloidal and toroidal fields of the solar dynamo
Context. The solar dynamo consists of a process that converts poloidal field
to toroidal field followed by a process which creates new poloidal field from
the toroidal field.
Aims. Our aim is to observe the poloidal and toroidal fields relevant to the
global solar dynamo and see if their evolution is captured by a
Babcock-Leighton dynamo.
Methods. We use synoptic maps of the surface radial field from the KPNSO/VT
and SOLIS observatories to construct the poloidal field as a function of time
and latitude, and Wilcox Solar Observatory and SOHO/MDI full disk images to
infer the longitudinally averaged surface azimuthal field. We show that the
latter is consistent with an estimate of that due to flux emergence and
therefore closely related to the subsurface toroidal field.
Results. We present maps of the poloidal and toroidal magnetic field of the
global solar dynamo. The longitude-averaged azimuthal field observed at the
surface results from flux emergence. At high latitudes this component follows
the radial component of the polar fields with a short time lag (1-3 years). The
lag increases at lower latitudes. The observed evolution of the poloidal and
toroidal magnetic fields is described by the (updated) Babcock-Leighton dynamo
model.Comment: A&
SDO/HMI survey of emerging active regions for helioseismology
Observations from the Solar Dynamics Observatory (SDO) have the potential for
allowing the helioseismic study of the formation of hundreds of active regions,
which would enable us to perform statistical analyses. Our goal is to collate a
uniform data set of emerging active regions observed by the SDO/HMI instrument
suitable for helioseismic analysis up to seven days before emergence. We
restricted the sample to active regions that were visible in the continuum and
emerged into quiet Sun largely avoiding pre-existing magnetic regions. As a
reference data set we paired a control region (CR), with the same latitude and
distance from central meridian, with each emerging active region (EAR). We call
this data set, which is currently comprised of 105 emerging active regions
observed between May 2010 and November 2012, the SDO Helioseismic Emerging
Active Region (SDO/HEAR) survey. To demonstrate the utility of a data set of a
large number of emerging active regions, we measure the relative east-west
velocity of the leading and trailing polarities from the line-of-sight
magnetogram maps during the first day after emergence. The latitudinally
averaged line-of-sight magnetic field of all the EARs shows that, on average,
the leading (trailing) polarity moves in a prograde (retrograde) direction with
a speed of 121 +/- 22 m/s (-70 +/- 13 m/s) relative to the Carrington rotation
rate in the first day. However, relative to the differential rotation of the
surface plasma, the east-west velocity is symmetric, with a mean of 95 +/- 13
m/s. The SDO/HEAR data set will not only be useful for helioseismic studies,
but will also be useful to study other features such as the surface magnetic
field evolution of a large sample of EARs.Comment: Accepted by Astronomy and Astrophysics, 11 figures, one longtable;
update corrects units in Figure
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