113 research outputs found
Turbulent 3D MHD dynamo model in spherical shells: regular oscillations of the dipolar field
We report the results of three-dimensional numerical simulations of convection-driven dynamos in relatively thin rotating spherical shells that show a transition from an strong non-oscillatory dipolar magnetic field to a weaker regularly oscillating dipolar field. The transition is induced primarily by the effects a stress-free boundary condition. The variation of the inner to outer radius ratio is found to have a less important effect
Properties of Flares-Generated Seismic Waves on the Sun
The solar seismic waves excited by solar flares (``sunquakes'') are observed
as circular expanding waves on the Sun's surface. The first sunquake was
observed for a flare of July 9, 1996, from the Solar and Heliospheric
Observatory (SOHO) space mission. However, when the new solar cycle started in
1997, the observations of solar flares from SOHO did not show the seismic
waves, similar to the 1996 event, even for large X-class flares during the
solar maximum in 2000-2002. The first evidence of the seismic flare signal in
this solar cycle was obtained for the 2003 ``Halloween'' events, through
acoustic ``egression power'' by Donea and Lindsey. After these several other
strong sunquakes have been observed. Here, I present a detailed analysis of the
basic properties of the helioseismic waves generated by three solar flares in
2003-2005. For two of these flares, X17 flare of October 28, 2003, and X1.2
flare of January 15, 2005, the helioseismology observations are compared with
simultaneous observations of flare X-ray fluxes measured from the RHESSI
satellite. These observations show a close association between the flare
seismic waves and the hard X-ray source, indicating that high-energy electrons
accelerated during the flare impulsive phase produced strong compression waves
in the photosphere, causing the sunquake. The results also reveal new physical
properties such as strong anisotropy of the seismic waves, the amplitude of
which varies significantly with the direction of propagation. The waves travel
through surrounding sunspot regions to large distances, up to 120 Mm, without
significant decay. These observations open new perspectives for helioseismic
diagnostics of flaring active regions on the Sun and for understanding the
mechanisms of the energy release and transport in solar flares.Comment: 12 pages, 4 figures, submitted to Ap
No Evidence Supporting Flare Driven High-Frequency Global Oscillations
The underlying physics that generates the excitations in the global
low-frequency, < 5.3 mHz, solar acoustic power spectrum is a well known process
that is attributed to solar convection; However, a definitive explanation as to
what causes excitations in the high-frequency regime, > 5.3 mHz, has yet to be
found. Karoff and Kjeldsen (Astrophys. J. 678, 73-76, 2008) concluded that
there is a correlation between solar flares and the global high-frequency solar
acoustic waves. We have used the Global Oscillations Network Group (GONG)
helioseismic data in an attempt to verify Karoff and Kjeldsen (2008) results as
well as compare the post-flare acoustic power spectrum to the pre-flare
acoustic power spectrum for 31 solar flares. Among the 31 flares analyzed, we
observe that a decrease in acoustic power after the solar flare is just as
likely as an increase. Furthermore, while we do observe variations in acoustic
power that are most likely associated with the usual p-modes associated with
solar convection, these variations do not show any significant temporal
association with flares. We find no evidence that consistently supports flare
driven high-frequency waves.Comment: 20 pages, 9 figures, Accepted for publication in Solar Physic
Interpreting Helioseismic Structure Inversion Results of Solar Active Regions
Helioseismic techniques such as ring-diagram analysis have often been used to
determine the subsurface structural differences between solar active and quiet
regions. Results obtained by inverting the frequency differences between the
regions are usually interpreted as the sound-speed differences between them.
These in turn are used as a measure of temperature and magnetic-field strength
differences between the two regions. In this paper we first show that the
"sound-speed" difference obtained from inversions is actually a combination of
sound-speed difference and a magnetic component. Hence, the inversion result is
not directly related to the thermal structure. Next, using solar models that
include magnetic fields, we develop a formulation to use the inversion results
to infer the differences in the magnetic and thermal structures between active
and quiet regions. We then apply our technique to existing structure inversion
results for different pairs of active and quiet regions. We find that the
effect of magnetic fields is strongest in a shallow region above 0.985R_sun and
that the strengths of magnetic-field effects at the surface and in the deeper
(r < 0.98R_sun) layers are inversely related, i.e., the stronger the surface
magnetic field the smaller the magnetic effects in the deeper layers, and vice
versa. We also find that the magnetic effects in the deeper layers are the
strongest in the quiet regions, consistent with the fact that these are
basically regions with weakest magnetic fields at the surface. Because the
quiet regions were selected to precede or follow their companion active
regions, the results could have implications about the evolution of magnetic
fields under active regions.Comment: Accepted for publication in Solar Physic
Imaging Spectroscopy of a White-Light Solar Flare
We report observations of a white-light solar flare (SOL2010-06-12T00:57,
M2.0) observed by the Helioseismic Magnetic Imager (HMI) on the Solar Dynamics
Observatory (SDO) and the Reuven Ramaty High-Energy Solar Spectroscopic Imager
(RHESSI). The HMI data give us the first space-based high-resolution imaging
spectroscopy of a white-light flare, including continuum, Doppler, and magnetic
signatures for the photospheric FeI line at 6173.34{\AA} and its neighboring
continuum. In the impulsive phase of the flare, a bright white-light kernel
appears in each of the two magnetic footpoints. When the flare occurred, the
spectral coverage of the HMI filtergrams (six equidistant samples spanning
\pm172m{\AA} around nominal line center) encompassed the line core and the blue
continuum sufficiently far from the core to eliminate significant Doppler
crosstalk in the latter, which is otherwise a possibility for the extreme
conditions in a white-light flare. RHESSI obtained complete hard X-ray and
\Upsilon-ray spectra (this was the first \Upsilon-ray flare of Cycle 24). The
FeI line appears to be shifted to the blue during the flare but does not go
into emission; the contrast is nearly constant across the line profile. We did
not detect a seismic wave from this event. The HMI data suggest stepwise
changes of the line-of-sight magnetic field in the white-light footpoints.Comment: 14 pages, 7 figures, Accepted by Solar Physic
Photospheric and Subphotospheric Dynamics of Emerging Magnetic Flux
Magnetic fields emerging from the Sun's interior carry information about
physical processes of magnetic field generation and transport in the convection
zone. Soon after appearance on the solar surface the magnetic flux gets
concentrated in sunspot regions and causes numerous active phenomena on the
Sun. This paper discusses some properties of the emerging magnetic flux
observed on the solar surface and in the interior. A statistical analysis of
variations of the tilt angle of bipolar magnetic regions during the emergence
shows that the systematic tilt with respect to the equator (the Joy's law) is
most likely established below the surface. However, no evidence of the
dependence of the tilt angle on the amount of emerging magnetic flux, predicted
by the rising magnetic flux rope theories, is found. Analysis of surface plasma
flows in a large emerging active region reveals strong localized upflows and
downflows at the initial phase of emergence but finds no evidence for
large-scale flows indicating future appearance a large-scale magnetic
structure. Local helioseismology provides important tools for mapping
perturbations of the wave speed and mass flows below the surface. Initial
results from SOHO/MDI and GONG reveal strong diverging flows during the flux
emergence, and also localized converging flows around stable sunspots. The wave
speed images obtained during the process of formation of a large active region,
NOAA 10488, indicate that the magnetic flux gets concentrated in strong field
structures just below the surface. Further studies of magnetic flux emergence
require systematic helioseismic observations from the ground and space, and
realistic MHD simulations of the subsurface dynamics.Comment: 21 pages, 15 figures, to appear in Space Science Review
Transient Magnetic and Doppler Features Related to the White-light Flares in NOAA 10486
Rapidly moving transient features have been detected in magnetic and Doppler
images of super-active region NOAA 10486 during the X17/4B flare of 28 October
2003 and the X10/2B flare of 29 October 2003. Both these flares were extremely
energetic white-light events. The transient features appeared during impulsive
phases of the flares and moved with speeds ranging from 30 to 50 km s.
These features were located near the previously reported compact acoustic
\cite{Donea05} and seismic sources \cite{Zharkova07}. We examine the origin of
these features and their relationship with various aspects of the flares, {\it
viz.}, hard X-ray emission sources and flare kernels observed at different
layers - (i) photosphere (white-light continuum), (ii) chromosphere (H
6563\AA), (iii) temperature minimum region (UV 1600\AA), and (iv) transition
region (UV 284\AA).Comment: 26 pages, 13 figures, 2 tables, accepted for publication in Solar
Physic
Time-distance analysis of the emerging active region NOAA 10790
We investigate the emergence of Active Region NOAA 10790 by means of time – distance helioseismology. Shallow regions of increased sound speed at the location of increased magnetic activity are observed, with regions becoming deeper at the locations of sunspot pores. We also see a long-lasting region of decreased sound speed located underneath the region of the flux emergence, possibly relating to a temperature perturbation due to magnetic quenching of eddy diffusivity, or to a dense flux tube. We detect and track an object in the subsurface layers of the Sun characterised by increased sound speed which could be related to emerging magnetic-flux and thus obtain a provisional estimate of the speed of emergence of around 1 km s−1
Time--Distance Helioseismology Data Analysis Pipeline for Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results
The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory
(SDO/HMI) provides continuous full-disk observations of solar oscillations. We
develop a data-analysis pipeline based on the time-distance helioseismology
method to measure acoustic travel times using HMI Doppler-shift observations,
and infer solar interior properties by inverting these measurements. The
pipeline is used for routine production of near-real-time full-disk maps of
subsurface wave-speed perturbations and horizontal flow velocities for depths
ranging from 0 to 20 Mm, every eight hours. In addition, Carrington synoptic
maps for the subsurface properties are made from these full-disk maps. The
pipeline can also be used for selected target areas and time periods. We
explain details of the pipeline organization and procedures, including
processing of the HMI Doppler observations, measurements of the travel times,
inversions, and constructions of the full-disk and synoptic maps. Some initial
results from the pipeline, including full-disk flow maps, sunspot subsurface
flow fields, and the interior rotation and meridional flow speeds, are
presented.Comment: Accepted by Solar Physics topical issue 'Solar Dynamics Observatory
Signatures of Emerging Subsurface Structures in Acoustic Power Maps
We show that under certain conditions, subsurface structures in the solar
interior can alter the average acoustic power observed at the photosphere above
them. By using numerical simulations of wave propagation, we show that this
effect is large enough for it to be potentially used for detecting emerging
active regions before they appear on the surface. In our simulations,
simplified subsurface structures are modeled as regions with enhanced or
reduced acoustic wave speed. We investigate the dependence of the acoustic
power above a subsurface region on the sign, depth, and strength of the wave
speed perturbation. Observations from the Solar and Heliospheric
Observatory/Michelson Doppler Imager (SOHO/MDI) prior and during the emergence
of NOAA active region 10488 are used to test the use of acoustic power as a
potential precursor of magnetic flux emergence.Comment: 7 pages, 5 figures, accepted for publication in Solar Physics on 21
March 201
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