86 research outputs found
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
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
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
Average motion of emerging solar active region polarities I: Two phases of emergence
Our goal is to constrain models of active region formation by tracking the
average motion of active region polarity pairs as they emerge onto the surface.
We measured the motion of the two main opposite polarities in 153 emerging
active regions (EARs) using line-of-sight magnetic field observations from the
Solar Dynamics Observatory Helioseismic Emerging Active Region (SDO/HEAR)
survey (Schunker et al. 2016). We first measured the position of each of the
polarities eight hours after emergence and tracked their location forwards and
backwards in time. We find that, on average, the polarities emerge with an
east-west orientation and the separation speed between the polarities
increases. At about 0.1 days after emergence, the average separation speed
reaches a peak value of 229 +/- 11 m/s, and then starts to decrease, and about
2.5 days after emergence the polarities stop separating. We also find that the
separation and the separation speed in the east-west direction are
systematically larger for active regions with higher flux. Our results reveal
two phases of the emergence process defined by the rate of change of the
separation speed as the polarities move apart. Phase 1 begins when the opposite
polarity pairs first appear at the surface, with an east-west alignment and an
increasing separation speed. We define Phase 2 to begin when the separation
speed starts to decrease, and ends when the polarities have stopped separating.
This is consistent with the picture of Chen, Rempel, & Fan (2017): the peak of
a flux tube breaks through the surface during Phase 1. During Phase 2 the
magnetic field lines are straightened by magnetic tension, so that the
polarities continue to move apart, until they eventually lie directly above
their anchored subsurface footpoints.Comment: accepted A&
Acoustic power absorption and enhancement generated by slow and fast MHD waves
We used long duration, high quality, unresolved (Sun-as-a star) observations
collected by the ground based network BiSON and by the instruments GOLF and
VIRGO on board the ESA/NASA SOHO satellite to search for solar-cycle-related
changes in mode characteristics in velocity and continuum intensity for the
frequency range between 2.5mHz < nu < 6.8mHz. Over the ascending phase of solar
cycle 23 we found a suppression in the p-mode amplitudes both in the velocity
and intensity data between 2.5mHz <nu< 4.5mHz with a maximum suppression for
frequencies in the range between 2.5mHz <nu< 3.5mHz. The size of the amplitude
suppression is 13+-2 per cent for the velocity and 9+-2 per cent for the
intensity observations. Over the range 4.5mHz <nu< 5.5mHz the findings hint
within the errors to a null change both in the velocity and intensity
amplitudes. At still higher frequencies, in the so called High-frequency
Interference Peaks (HIPs) between 5.8mHz <nu < 6.8mHz, we found an enhancement
in the velocity amplitudes with the maximum 36+-7 per cent occurring for 6.3mHz
<nu< 6.8mHz. However, in intensity observations we found a rather smaller
enhancement of about 5+-2 per cent in the same interval. There is evidence that
the frequency dependence of solar-cycle velocity amplitude changes is
consistent with the theory behind the mode conversion of acoustic waves in a
non-vertical magnetic field, but there are some problems with the intensity
data, which may be due to the height in the solar atmosphere at which the VIRGO
data are taken.Comment: Accepted for publication in A&A. 10 pages, 9 figures
Helioseismology of sunspots: how sensitive are travel times to the Wilson depression and to the subsurface magnetic field?
In order to assess the ability of helioseismology to probe the subsurface
structure and magnetic field of sunspots, we need to determine how helioseismic
travel times depend on perturbations to sunspot models. Here we numerically
simulate the propagation of f, p1, and p2 wave packets through magnetic sunspot
models. Among the models we considered, a ~50 km change in the height of the
Wilson depression and a change in the subsurface magnetic field geometry can
both be detected above the observational noise level. We also find that the
travel-time shifts due to changes in a sunspot model must be modeled by
computing the effects of changing the reference sunspot model, and not by
computing the effects of changing the subsurface structure in the quiet-Sun
model. For p1 modes the latter is wrong by a factor of four. In conclusion,
numerical modeling of MHD wave propagation is an essential tool for the
interpretation of the effects of sunspots on seismic waveforms.Comment: 5 pages, 3 figures: submitted to A&
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