explanation blue bibcodes open ADS page with paths to full text
Author name code: komm
ADS astronomy entries on 2022-09-14
author:"Komm, Rudolf W." OR author:"Komm, Rudi"
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Title: Is the Subsurface Meridional Flow Zero at the Equator?
Authors: Komm, R.
2022SoPh..297...99K Altcode:
We study the subsurface meridional flow and its divergence from the
surface to a depth of about 16 Mm at the equator and its variation
with the solar cycle derived with ring-diagram analysis applied to
Michelson Doppler Imager (MDI) Dynamics Program, Global Oscillation
Network Group (GONG), and Helioseismic and Magnetic Imager (HMI)
Dopplergrams. The meridional flow at the equator is small but nonzero
and is mainly negative (southward) during Solar Cycle 23 with an
average of −1.1 ±0.2 ms−<SUP>1</SUP> at depths shallower than 7
Mm and positive (northward) during Solar Cycle 24 with an average of
+1.3 ±0.1 ms−<SUP>1</SUP> over the same depth range derived from
supersynoptic maps of combined HMI and GONG data (scaled to match HMI
flow amplitudes). The divergence in supersynoptic maps is positive at
all times and clearly varies with the solar cycle with large values
during cycle maxima and small values during minima. On time scales
of synoptic maps, we found that at depths shallower than 10 Mm the
cross-equatorial flow is, on average, toward the hemisphere with the
larger amount of flux. The meridional flow at the equator has broad
distributions with widths that are at least five times larger than the
mean values. The distributions of Solar Cycles 23 and 24 overlap but
are distinguishable. For a high-activity subset, the cross-equatorial
flow is predominantly toward locations with high activity and the
divergence is greater than average. The nonzero cross-equatorial
flow is in this case a consequence of the inflows present near active
regions and the imbalance of activity between the hemispheres. For a
quiet-region subset, the cross-equatorial flow is, on average, in the
same direction as the average flow over a solar cycle with a similar
broad distribution, while the quiet-region divergence is smaller than
the grand average.
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Title: Subsurface Horizontal Flows During Solar Cycles 24 and 25
with Large-Tile Ring-Diagram Analysis
Authors: Komm, R.
2021SoPh..296..174K Altcode:
We study the large-scale subsurface flows to a depth of about 32 Mm
covering the near-surface shear layer (NSSL). The flows were derived
with a ring-diagram analysis applied to Helioseismic and Magnetic Imager
(HMI) Dopplergrams using tiles with 30<SUP>∘</SUP> diameter instead
of the commonly used 15<SUP>∘</SUP> ones. This allows us to determine
flows at greater depths in exchange for coarser spatial resolution. We
confirm that the average zonal flow increases with increasing depth
and reaches a plateau in the NSSL. There is a hint of a local maximum
or saddle point much closer to the surface at about 8 Mm. The average
meridional flow is poleward at all depths in both hemispheres. The
average amplitude is 14.3 ±0.2 m s<SUP>−1</SUP> at 30<SUP>∘</SUP>
and 12.4 ±0.2 m s<SUP>−1</SUP> at 15<SUP>∘</SUP> latitude at depths
of 20 Mm and shallower, while amplitudes at these latitudes decrease at
greater depths. The solar-cycle variation of the zonal and meridional
flow are clearly noticeable from the surface throughout the NSSL. The
dominant features of the zonal flow are bands of faster-than-average
flow associated with Solar Cycles 24 and 25. The onset of the fast bands
happens almost simultaneously at all depths. For Cycle 25, the fast
bands appear in the southern hemisphere about one year before those
in the northern one and both fast bands appear several years before
magnetic activity appeared at the surface in either hemisphere. The
meridional flow shows a similar pattern after subtracting the temporal
mean at each latitude. The bands of converging residual meridional flow
move from mid- to low latitudes during a solar cycle. These bands appear
at low latitudes almost at the same time at all depths, similar to the
fast bands of the zonal flow. However, at 45<SUP>∘</SUP> latitude
they appear first in layers near 32 Mm and about two years later at the
solar surface, as if the pattern were rising through the outer layers.
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Title: Subsurface Plasma Flows and the Flare Productivity of Solar
Active Regions
Authors: Biji, Lekshmi; Jain, Kiran; Komm, Rudolf; Nandy, Dibyendu
2021AGUFMSH54A..07B Altcode:
Highly energetic solar events such as solar flares and Coronal
Mass Ejections (CMEs) can lead to extreme space weather. Hence, it
is essential to understand their physical drivers and explore what
governs their occurrence and intensity. By using the near-surface
velocities derived by the ring-diagram analysis of active region
patches using Global Oscillation Network Group (GONG) Doppler
velocity measurements, we seek to explore the connection between
subsurface flow properties and solar flares. The temporal evolution
of vorticity and kinetic helicity of flaring and non-flaring active
regions is investigated. The integrated vorticity, kinetic and current
helicities, and magnetic flux one day prior to the flare are observed
to be correlated with the integrated flare intensity. We show that
active regions with strong subsurface vorticity and kinetic helicity
tend to generate high intensity flares. We hypothesize that this is
achieved via energy injection into subsurface magnetic flux systems
by helical plasmas flows.
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Title: Subsurface Flow Measurements in the Near Surface Shear Layer
over Two Solar Cycles
Authors: Tripathy, Sushanta; Jain, Kiran; Komm, Rudolf; Kholikov,
Shukirjon
2021AGUFMSH53C..02T Altcode:
Helioseismic studies have illustrated that the most significant
changes with the solar cycle occur in the near-surface shear layer
(NSSL) where the density changes by several orders of magnitude. This
layer approximately occupies the outer 5% of the solar interior. It is
also believed that a nonlinear alpha-omega dynamo could be operating
in the NSSL where the velocity shear converts a part of the poloidal
magnetic field into the toroidal field in addition to the global
dynamo operating in the tachocline region. With the advent of local
helioseismic technique of ring diagram, subsurface flows in the outer 2%
have been well studied. Here we extend the measurements of large-scale
flows to deeper layers to provide observational constraints on the
temporal as well as latitudinal variations of the zonal and meridional
flows during the last two solar cycles. The study is based on the GONG
data and uses the technique of ring diagram and 30 degree tiles to
probe deeper layers. We will also compare GONG with HMI results for
solar cycle 24 and beyond.
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Title: Kinetic and Magnetic Helicity in Solar Active Regions
Authors: Liu, Yang; Komm, Rudolf; Brummell, Nicholas; Hoeksema, J.
2021AGUFMSH54A..08L Altcode:
Flows and magnetic fields in the Sun interact with each other,
especially in the solar interior where flow dominates. Kinetic and
magnetic helicity are proxies for links between flows and the magnetic
field. Studying magnetic helicity, its evolution, physical scale, and
the relationship with flows (kinetic helicity) provides insight for
understanding the solar dynamo and magnetic flux emergence. This study
explores the relationship between kinetic helicity in the interior and
magnetic helicity in the atmosphere in active regions. We investigate
16 active regions to search for a possible sign relationship between
the magnetic and kinetic helicity. This sample is representative
because it includes active regions with various magnetic properties --
8 emerging and 8 well developed active regions having simple and complex
magnetic configurations, and that exhibit a range of solar activity,
including ones with multiple major flares and no flare activity. We
determine the relationship between kinetic and magnetic helicity for
active regions with various properties and discuss implications for
solar interior activity.
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Title: A Comparative Study of Measurements of the Suns Axisymmetric
Flows: A COFFIES Effort
Authors: Upton, Lisa; Jain, Kiran; Komm, Rudolf; Mahajan, Sushant;
Pevtsov, Alexei; Roudier, Thierry; Tripathy, Sushanta; Ulrich, Roger;
Zhao, Junwei; Basu, Sarbani; Chen, Ruizhu; DeRosa, Marc; Hess Webber,
Shea; Hoeksema, J.
2021AGUFMSH55D1871U Altcode:
Consequence Of Fields and Flows in the Interior and Exterior of
the Sun (COFFIES) is a Phase-1 NASA DRIVE Science Center (DSC),
with the primary objective of developing a data driven model of
solar activity. One of COFFIES five primary science questions is
What drives varying large-scale motions in the Sun? To address this
question, we are developing a comprehensive catalog of the variable
differential rotation and meridional circulation flow patterns. This
catalog includes measurements of these flows as obtained by several
measurement techniques: Doppler imaging, granule tracking, magnetic
pattern tracking, magnetic feature tracking, as well as both time
distance and ring diagram helioseismology. We show a comparison of
these flows across these varied techniques, with a particular focus
on the MDI/HMI/GONG/Mount Wilson overlap period (May-July 2010). We
investigate the uncertainties and attempt to reconcile any discrepancies
(e.g., due to flow depth or systematics associated with the different
measurement techniques). This analysis will pave the way toward
accurately determining the global patterns of axisymmetric flows and
their regular and irregular variations during the cycle.
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Title: Global solar flows and their impact on magnetic activity
Authors: Dikpati, Mausumi; Braun, Douglas; Featherstone, Nicholas;
Hindman, Bradley; Komm, Rudolf; Liu, Yang; Scherrer, Philip; Upton,
Lisa; Wang, Haimin
2021AGUFMSH55D1872D Altcode:
This poster presents the second year progress report of the LWS
focused-science team-4 of 2019. The main science objective is to
jointly develop the most comprehensive, dynamically consistent picture
of solar flows at the surface, in the convection zone and tachocline,
and determine the MHD effects induced by these motions. Our major
team-achievements in the second year include: (i) consensus about active
regions' flow and their contributions in modifying the global flow;
(ii) long-term global flow map from various magnetograms, and their
specific properties as function of cycle phase, (iii) impacts of the
flows in polar field evolutions, (iv) simulations of global flows with
various solar-like interior conditions, (v) roles of simulated flows
in driving the nonlinear dynamics of spot-producing magnetic fields
and producing their spatio-temporal patterns, which are compared with
that manifested as active regions patterns in surface magnetograms. We
will describe in detail how these observationally constrained local and
global flows are leading us to improved simulations of model-outputs of
magnetic activity and flows. In turn, these outputs can reliably be used
as inputs to heliospheric models, for example, for simulating properties
of reconnection of active regions' magnetic fields, high-speed streams,
sector passages, all of which have profound influence on various
aspects of space weather and impact on terrestrial atmosphere.
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Title: Understanding the Consequences Of Fields and Flows in the
Interior and Exterior of the Sun (COFFIES)
Authors: Hoeksema, J. T.; Brummell, N.; Bush, R.; Hess Webber, S.;
Kitiashvili, I.; Komm, R.; Kosovichev, A.; Mendez, B.; Scherrer, P.;
Upton, L.; Wray, A.; Zevin, D.; The Coffies Team
2021AAS...23811322H Altcode:
The solar activity cycle is the Consequence Of Fields and Flows in the
Interior and Exterior of the Sun (COFFIES). As a Phase-1 NASA DRIVE
Science Center (DSC), COFFIES ultimately aims to develop a data-driven
model of solar activity. To attain this goal COFFIES members are
learning to work together effectively to perform the investigations
needed to answer five primary science questions: <P />1) What drives
varying large-scale motions in the Sun? <P />2) How do flows interact
with the magnetic field to cause varying activity cycles? <P />3) Why
do active regions emerge when and where they do? <P />4) What do the
manifestations of activity and convection reveal about the internal
processes? <P />5) How does our understanding of the Sun as a star
inform us more generally about activity dynamics and structure? <P />The
virtual COFFIES center brings together a broad spectrum of observers,
data analysts, theorists, computational scientists, and educators
who collaborate through interacting working groups of four principal
science teams. The principal objectives of the four primary science
teams are to 1) understand the generation of quasi-periodic stellar
magnetic cycles, 2) further develop 3D physical models of interior
dynamics and convection, 3) establish clear physical links between solar
flow fields and near-surface observations, and 4) develop more robust
helioseismic techniques to resolve solar interior flows. Additional
cross-team activities are facilitated by teams for numerical modeling,
center effectiveness, outreach and eduction, and diversity, equity,
inclusion and access (DEIA).
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Title: Divergence and Vorticity of Subsurface Flows During Solar
Cycles 23 and 24
Authors: Komm, R.; Howe, R.; Hill, F.
2021SoPh..296...73K Altcode:
We study the solar-cycle variation of the divergence and vorticity
of subsurface horizontal flows from the surface to a depth of 16
Mm. The flows were derived with ring-diagram analysis applied to
Michelson Doppler Imager (MDI) Dynamics Program, Global Oscillation
Network Group (GONG), and Helioseismic and Magnetic Imager (HMI)
Dopplergrams. We study their variation for the complete data set and
for two subsets representing active and quiet regions. All three data
sets show alternating bands of diverging and converging flows and bands
of cyclonic and anticyclonic flows moving from mid-latitudes toward
the equator during a solar cycle. For Solar Cycle 24, these bands are
precursors of the magnetic activity appearing several years before
magnetic activity is present at a given latitude even leading the fast
bands of the flows. The amplitude differences between the cyclonic and
anticyclonic and the converging and diverging bands during a solar
cycle agree within the error bars between the complete data set and
the two subsets. For Solar Cycle 24, the amplitude differences are 6.0
±0.7 10<SUP>−8</SUP>s−<SUP>1</SUP> for the bands of vorticity and
−4.9 ±0.6 10<SUP>−8</SUP>s−<SUP>1</SUP> for those of divergence
averaged over 2.0 - 11.6 Mm using the complete data set. The amplitude
differences of Solar Cycle 23 are 26 ±3 % smaller than those of Solar
Cycle 24. The flows of the active-region subset are more converging
and cyclonic than those of the quiet-region subset with an extra
vorticity of 1.3 ±0.1 10<SUP>−8</SUP>s−<SUP>1</SUP> and an extra
divergence of −6.7 ±0.3 10<SUP>−8</SUP>s−<SUP>1</SUP> averaged
over 7.5<SUP>∘</SUP> - 30<SUP>∘</SUP> and all depths and epochs. The
amplitude of the extra divergence of active regions is about a factor of
1.3 larger at depths shallower than 6 Mm and decreases with increasing
depth, while the extra vorticity is nearly constant with depth.
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Title: COFFIES - Developing a Reliable Physical Model of the Solar
Activity Cycle
Authors: Hoeksema, J. T.; Brummell, N.; Bush, R. I.; Komm, R.;
Kosovichev, A. G.; Mendez, B.; Scherrer, P. H.; Upton, L.; Wray,
A. A.; Zevin, D.
2020AGUFMSH0020007H Altcode:
The solar activity cycle is the Consequence Of Fields and Flows in
the Interior and Exterior of the Sun (COFFIES). The COFFIES Drive
Science Center ultimately aims to develop a data driven model of solar
activity. The challenging goals are 1) to understand the generation
of the quasi-periodic stellar magnetic cycles, 2) further develop 3D
physical models of interior dynamics and convection, 3) establish the
physical links between solar flow fields and near-surface observations,
and 4) develop more robust helioseismic techniques to resolve solar
interior flows. To reach these goals the COFFIES team is focusing on
what is needed to answer five primary science questions: 1) What drives
varying large-scale motions in the Sun? 2) How do flows interact with
the magnetic field to cause varying activity cycles? 3) Why do active
regions emerge when and where they do? 4) What do the manifestations
of activity and convection reveal about the internal processes? And
5) How does our understanding of the Sun as a star inform us more
generally about activity dynamics and structure? The virtual COFFIES
center is bringing together a broad spectrum of observers, analysts,
theorists, computational scientists, and educators who collaborate
through interacting teams focused on helioseismology, dynamos, solar
convection, surface links, numerical modeling, center effectiveness,
outreach, education, diversity and inclusion.
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Title: Global Solar Flows and Magnetic Fields: Observing, Simulating
and Predicting Their Impact on the Heliosphere and Terrestrial
Atmosphere
Authors: Dikpati, M.; Braun, D. C.; Featherstone, N. A.; Komm, R.;
Liu, Y.; Scherrer, P. H.; Upton, L.; Wang, H.
2020AGUFMSH0020008D Altcode:
Understanding origins and evolution of solar magnetic activity occurring
on a wide range of time-scales, and the space weather effects caused
by the particles and electromagnetic outputs that reach the Earth,
requires knowledge of the physical origins of this activity below
photosphere. Despite much progress, our knowledge of processes
responsible for driving the magnetohydrodynamics of flows and fields
below photosphere and their relation to observed flows and magnetic
activity is far from complete. For example, there is no consensus as to
the number of meridional circulation-cells that exist in the Sun and the
depth at which the poleward-flow switches direction to equatorward. Main
objective of our LWS focused-science team is to jointly develop the
most comprehensive, dynamically consistent picture of solar flows
at the surface, in the convection zone and tachocline, and determine
the MHD effects induced by these motions. We will present how we are
developing consensus sets of observational constraints and simulating
model-outputs of magnetic activity and flows, which can reliably be
used as inputs to heliospheric and terrestrial- atmospheric models. The
ultimate success will be in our ability to predict the features of
solar cycle 25, including the active-latitudes and -longitudes, global-
and localized-flows several months to years ahead.
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Title: Helioseismic Constraints on the Solar Interior Dynamics
and Dynamo
Authors: Kosovichev, A. G.; Brummell, N.; Dikpati, M.; Guerrero,
G.; Kitiashvili, I.; Komm, R.; Korzennik, S.; Pipin, V.; Reiter, J.;
Stejko, A.; Ulrich, R. K.; Warnecke, J.
2020AGUFMSH007..04K Altcode:
Uninterrupted helioseismic observations from the SoHO/MDI, SDO/HMI and
GONG instruments for more than two decades provide unique observational
data for studying the solar-cycle variations of the differential
rotation, large-scale and meridional flows. The data also allows
us to investigate changes in the thermodynamic structure associated
with dynamo-generated magnetic fields. The wealth of global and local
helioseismic data provides theoretical constraints on the solar dynamics
and dynamo models. The synergy of helioseismic inferences with advanced
MHD modeling sheds light on the origin of the solar activity cycles. It
helps to understand better the physical processes that control the
strength and duration of the cyclic magnetic activity and leads to
new physics-based approaches for prediction of the solar cycles. We
briefly overview the current status, discuss the solar dynamical
structure and evolution revealed by helioseismic inversions and the
forward-modeling method, and focus on the most critical points of the
problem. In particular, we discuss recent advances in measurements and
modeling of the solar-cycle variations of the meridional circulation
and migrating zonal flows (torsional oscillations) on the solar surface
and in the subsurface layers, the deep convection zone, and the solar
tachocline. The relationships between the internal dynamics and the
evolution of global magnetic fields lead to new ideas of how magnetic
fields are generated and affect the solar flows and structure.
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Title: Solar-Cycle Variation of the Subsurface Flows of Active and
Quiet Regions
Authors: Komm, R.
2020AGUFMSH007..03K Altcode:
We study the solar-cycle variation of subsurface flows for both
active and quiet solar regions. We derive the temporal variation of
the zonal and meridional flows from the surface to a depth of 16 Mm
using ring-diagram analysis applied to Dopplergrams obtained with
the Helioseismic and Magnetic Imager (HMI) instrument onboard the
Solar Dynamics Observatory (SDO) spacecraft. We use thresholds in
magnetic activity to separate the flows into subsets of active and
quiet regions. In addition, we derive the subset of intermediate
regions that are excluded from either the quiet- or active-region
subset. The subsurface flows associated with active and quiet regions
show the same variation with the solar cycle with alternating bands
of faster- and slower-than-average zonal and meridional flows moving
from mid-latitudes toward the equator during the course of a cycle
but with different amplitudes. We will derive the flow patterns of the
intermediate-region subset as well and compare their amplitudes with
those of the other subsets. We will study the north-south variation
of the resulting flow patterns and their implications on the activity
of solar cycle 25.
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Title: Solar-Cycle Variation of the Subsurface Flows of Active-
and Quiet-Region Subsets
Authors: Komm, R.; Howe, R.; Hill, F.
2020SoPh..295...47K Altcode:
We study the solar-cycle variation of subsurface flows for both active
and quiet solar regions. We derive flows from the surface to a depth
of 16 Mm using ring-diagram analysis applied to Dopplergrams obtained
with the Michelson Doppler Imager (MDI) Dynamics Program, the Global
Oscillation Network Group (GONG), and the Helioseismic and Magnetic
Imager (HMI) instrument. We derive the temporal variation of the zonal
and meridional flows in a consistent manner for Solar Cycles 23 and
24 combining the flows from the three data sources scaled to match
HMI-derived flows. The subsurface flows associated with active and quiet
regions show the same variation with the solar cycle with alternating
bands of faster- and slower-than-average zonal and meridional flows
moving from mid-latitudes toward the equator during the course of
a cycle. We derive the differences between the amplitudes of the
extrema of the fast and the slow flows. For Cycle 24, the average
difference between the fast- and slow-flow amplitude is 9.5 ±0.5
ms−<SUP>1</SUP> for the zonal flows and 7.0 ±0.4 ms−<SUP>1</SUP>
for the meridional flows of the quiet-region subset averaged over 2
to 12 Mm within ±30<SUP>∘</SUP> latitude. For the active-region
subset, the average difference is 10.4 ±0.9 ms−<SUP>1</SUP> for
the zonal flows and 9.3 ±0.7 ms−<SUP>1</SUP> for the meridional
flows. We subtract the flows of the quiet-region subset from those
of the active-region one to determine the contribution of active
regions to the long-term flow pattern. The resulting meridional flow
associated with active regions has a maximum amplitude near 3.1 Mm and
its amplitude decreases with depth. This implies that the converging
flows attributed to active regions are a shallow-layer phenomenon.
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Title: Kinetic Helicity and Lifetime of Activity Complexes During
Solar Cycle 24
Authors: Komm, R.; Gosain, S.
2019ApJ...887..192K Altcode:
We study magnetic features on the solar surface that exist for
several rotations during solar cycle 24. To identify them, we average
synoptic maps over a range in latitude and stack the resulting
longitudinal strips in time. We use synoptic maps of magnetograms
obtained with the NSO/Synoptic Optical Long-term Investigations of
the Sun instrument and create synoptic maps of the kinetic helicity
of subsurface flows integrated over 2.0-7.1 Mm based on Solar
Dynamics Observatory/Helioseismic and Magnetic Imager Dopplergrams. To
distinguish between active and quiet regions, we sort the grid points of
the synoptic maps by their activity level and divide the data into four
subsets with 25% of activity each and into two subsets with the highest
or lowest 12.5% of activity values. The kinetic helicity of these
six subsets follows the hemispheric helicity rule with, on average,
positive values in the southern and negative values in the northern
hemisphere. However, the helicity of the subset with the highest
activity is about four times higher than that of the other subsets,
and the mid-quartile subsets show the weakest hemispheric helicity
rule. We define the lifetime of complexes in each subset and find
that for the high-activity subset, the amplitude of magnetic activity
and kinetic helicity increases almost linearly with the lifetime of
complexes. The distribution of flares closely resembles that of the
high-activity subset. The flare-productive locations in long-lived
complexes produce, on average, the same number of flares as those of
short-lived complexes. However, long-lived complexes have a higher
fractional number of these locations than the short-lived complexes
and thus produce more flares not just because they live longer.
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Title: Long-Lived Activity Complexes, their Kinetic Helicity,
Lifetime, and Flare Activity
Authors: Komm, Rudolf W.; Gosain, Sanjay
2019shin.confE..53K Altcode:
We study long-lived activity complexes using stackplots of magnetic
activity derived from NSO/SOLIS synoptic magnetograms. We focus on
the kinetic helicity below the surface determined with ring-diagram
analysis applied to full-disk Dopplergrams from SDO/HMI during
Solar Cycle 24. The kinetic helicity of activity complexes follows
the hemispheric helicity rule with mainly positive values in the
southern hemisphere and negative ones in the northern hemisphere. To
distinguish between active and quiet regions, we divide the data into
subsets with high and low levels of activity and create stackplots of
surface magnetic activity and subsurface kinetic helicity for each
subset. The distribution of flares in a stackplot resembles closely
that of the high-activity subset. The flare-productive locations in
long-lived complexes produce, on average, the same number of flares
as those of short-lived ones. However, long-lived complexes have a
larger number of these locations and thus a higher flare-production
rate than short-lived ones. We will present the latest results.
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Title: Kinetic and Current Helicity of Long-Lived Activity Complexes
During Solar Cycle 24
Authors: Komm, Rudolf; Gosain, Sanjay
2018csc..confE...6K Altcode:
We study long-lived activity complexes during Solar Cycle 24. We focus
on the kinetic helicity below the surface determined with ring-diagram
analysis applied to full-disk Dopplergrams from SDO/HMI. In addition,
we study the current helicity at the solar surface of these activity
complexes determined from synoptic vector magnetograms. Current and
kinetic helicity of activity complexes follow the hemispheric helicity
rule with mainly positive values in the southern hemisphere and negative
ones in the northern hemisphere. The locations with the dominant sign of
kinetic helicity are more organized than those of secondary sign even
if they are not part of an activity complex, while locations with the
secondary sign are more fragmented. We will present the latest results.
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Title: GONG p-Mode Parameters Through Two Solar Cycles
Authors: Kiefer, René; Komm, Rudi; Hill, Frank; Broomhall, Anne-Marie;
Roth, Markus
2018SoPh..293..151K Altcode: 2018arXiv181009324K
We investigate the parameters of global solar p-mode oscillations,
namely damping width Γ , amplitude A , mean squared velocity
«v<SUP>2</SUP>», energy E , and energy supply rate d E /d t , derived
from two solar cycles' worth (1996 - 2018) of Global Oscillation Network
Group (GONG) time series for harmonic degrees l =0 -150 . We correct for
the effect of fill factor, apparent solar radius, and spurious jumps in
the mode amplitudes. We find that the amplitude of the activity-related
changes of Γ and A depends on both frequency and harmonic degree of the
modes, with the largest variations of Γ for modes with 2400 μ Hz≤ν
≤3300 μ Hz and 31 ≤l ≤60 with a minimum-to-maximum variation of
26.6 ±0.3 % and of A for modes with 2400 μ Hz≤ν ≤3300 μ Hz and
61 ≤l ≤100 with a minimum-to-maximum variation of 27.4 ±0.4 % . The
level of correlation between the solar radio flux F<SUB>10.7</SUB> and
mode parameters also depends on mode frequency and harmonic degree. As
a function of mode frequency, the mode amplitudes are found to follow
an asymmetric Voigt profile with ν<SUB>max</SUB>=3073.59 ±0.18 μ
Hz. From the mode parameters, we calculate physical mode quantities
and average them over specific mode frequency ranges. In this way, we
find that the mean squared velocities «v<SUP>2</SUP>» and energies
E of p modes are anticorrelated with the level of activity, varying by
14.7 ±0.3 % and 18.4 ±0.3 % , respectively, and that the mode energy
supply rates show no significant correlation with activity. With this
study we expand previously published results on the temporal variation
of solar p-mode parameters. Our results will be helpful to future
studies of the excitation and damping of p modes, i.e., the interplay
between convection, magnetic field, and resonant acoustic oscillations.
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Title: Subsurface Flows During Cycle 23 and 24
Authors: Komm, Rudolf; Howe, Rachel; Hill, Frank
2018csc..confE..54K Altcode:
We study the solar-cycle variation of subsurface flows from the
surface to a depth of 16 Mm. We have used ring-diagram analysis to
analyze Dopplergrams obtained with the MDI Dynamics Program, the GONG,
and the SDO/HMI instrument. We combine the zonal and meridional flows
from the three data sources and we derive their temporal variation
in a consistent manner for Solar Cycle 23 and 24. For Cycle 24, the
flow patterns are precursors of the magnetic activity. The timing
difference between the occurrence of the flow pattern and the magnetic
one increases almost linearly with increasing latitude. For example,
the fast zonal and meridional flow appear about 2.1 years and 2.5 years
respectively before the magnetic pattern at 30 degree latitude in the
northern hemisphere, while in the southern one the differences are
3.2 years and 2.6 years. The flow patterns of Cycle 25 are present
and have reached 30 degree latitude. The amplitude differences of
Cycle 25 are about 22% smaller than those of Cycle 24 but comparable
to those of Cycle 23. In addition, we divide the data into subsets of
low and high magnetic activity and study the variation of the quiet-
and active-region flows during Solar Cycle 23 and 24.
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Title: Subsurface Zonal and Meridional Flow During Cycles 23 and 24
Authors: Komm, R.; Howe, R.; Hill, F.
2018SoPh..293..145K Altcode:
We study the solar-cycle variation of subsurface flows from the
surface to a depth of 16 Mm. We have used ring-diagram analysis to
analyze Dopplergrams obtained with the Michelson Doppler Imager (MDI)
Dynamics Program, the Global Oscillation Network Group (GONG), and the
Helioseismic and Magnetic Imager (HMI) instrument. We combined the zonal
and meridional flows from the three data sources and scaled the flows
derived from MDI and GONG to match those from HMI observations. In
this way, we derived their temporal variation in a consistent manner
for Solar Cycles 23 and 24. We have corrected the measured flows for
systematic effects that vary with disk positions. Using time-depth
slices of the corrected subsurface flows, we derived the amplitudes
and times of the extrema of the fast and slow zonal and meridional
flows during Cycles 23 and 24 at every depth and latitude. We find an
average difference between maximum and minimum amplitudes of 8.6 ±0.4
ms−<SUP>1</SUP> for the zonal flows and 7.9 ±0.3 ms−<SUP>1</SUP>
for the meridional flows associated with Cycle 24 averaged over a
depth range from 2 to 12 Mm. The corresponding values derived from
GONG data alone are 10.5 ±0.3 ms−<SUP>1</SUP> for the zonal and
10.8 ±0.3 ms−<SUP>1</SUP> for the meridional flow. For Cycle 24,
the flow patterns are precursors of the magnetic activity. The timing
difference between the occurrence of the flow pattern and the magnetic
one increases almost linearly with increasing latitude. For example,
the fast zonal and meridional flow appear 2.1 ±0.6 years and 2.5 ±0.6
years, respectively, before the magnetic pattern at 30<SUP>∘</SUP>
latitude in the northern hemisphere, while in the southern hemisphere,
the differences are 3.2 ±1.2 years and 2.6 ±0.6 years. The flow
patterns of Cycle 25 are present and have reached 30<SUP>∘</SUP>
latitude. The amplitude differences of Cycle 25 are about 22%
smaller than those of Cycle 24, but are comparable to those of
Cycle 23. Moreover, polynomial fits of meridional flows suggest that
equatorward meridional flows (counter-cells) might exist at about
80<SUP>∘</SUP> latitude except during the declining phase of the
solar cycle.
---------------------------------------------------------
Title: Signatures of Solar Cycle 25 in Subsurface Zonal Flows
Authors: Howe, R.; Hill, F.; Komm, R.; Chaplin, W. J.; Elsworth, Y.;
Davies, G. R.; Schou, J.; Thompson, M. J.
2018ApJ...862L...5H Altcode: 2018arXiv180702398H
The pattern of migrating zonal flow bands associated with the solar
cycle, known as the torsional oscillation, has been monitored with
continuous global helioseismic observations by the Global Oscillations
Network Group (GONG), together with those made by the Michelson
Doppler Imager (MDI) on board the Solar and Heliospheric Observatory
(SOHO) and its successor, the Helioseismic and Magnetic Imager (HMI)
on board the Solar Dynamics Observatory (SDO), since 1995, giving us
nearly two full solar cycles of observations. We report that the flows
now show traces of the mid-latitude acceleration that is expected to
become the main equatorward-moving branch of the zonal flow pattern for
Cycle 25. Based on the current position of this branch, we speculate
that the onset of widespread activity for Cycle 25 is unlikely to be
earlier than the middle of 2019.
---------------------------------------------------------
Title: 22 Year Solar Magnetic Cycle and its relation to Convection
Zone Dynamics
Authors: Jain, Kiran; Tripathy, Sushanta; Komm, Rudolf; Hill, Frank;
Simoniello, Rosaria
2018IAUS..340....9J Altcode: 2018arXiv180505371J
Using continuous observations for 22 years from ground-based network
GONG and space-borne instruments MDI onboard SoHO and HMI onboard SDO,
we report both global and local properties of the convection zone and
their variations with time.
---------------------------------------------------------
Title: The Sun in transition? Persistence of near-surface structural
changes through Cycle 24
Authors: Howe, R.; Davies, G. R.; Chaplin, W. J.; Elsworth, Y.; Basu,
S.; Hale, S. J.; Ball, W. H.; Komm, R. W.
2017MNRAS.470.1935H Altcode: 2017arXiv170509099H
We examine the frequency shifts in low-degree helioseismic modes
from the Birmingham Solar-Oscillations Network covering the period
from 1985 to 2016, and compare them with a number of global activity
proxies well as a latitudinally resolved magnetic index. As well as
looking at frequency shifts in different frequency bands, we look at a
parametrization of the shift as a cubic function of frequency. While the
shifts in the medium- and high-frequency bands are very well correlated
with all of the activity indices (with the best correlation being with
the 10.7-cm radio flux), we confirm earlier findings that there appears
to have been a change in the frequency response to activity during solar
Cycle 23, and the low-frequency shifts are less correlated with activity
in the last two cycles than they were in Cycle 22. At the same time,
the more recent cycles show a slight increase in their sensitivity to
activity levels at medium and higher frequencies, perhaps because a
greater proportion of activity is composed of weaker or more ephemeral
regions. This lends weight to the speculation that a fundamental change
in the nature of the solar dynamo may be in progress.
---------------------------------------------------------
Title: Solar-Cycle Variation of Subsurface-Flow Divergence: A Proxy
of Magnetic Activity?
Authors: Komm, R.; Howe, R.; Hill, F.
2017SoPh..292..122K Altcode:
We study the solar-cycle variation of subsurface flows from the surface
to a depth of 16 Mm. We have analyzed Global Oscillation Network Group
(GONG) Dopplergrams with a ring-diagram analysis covering about 15
years and Helioseismic and Magnetic Imager (HMI) Dopplergrams covering
more than 6 years. After subtracting the average rotation rate and
meridional flow, we have calculated the divergence of the horizontal
residual flows from the maximum of Solar Cycle 23 through the declining
phase of Cycle 24. The subsurface flows are mainly divergent at quiet
regions and convergent at locations of high magnetic activity. The
relationship is essentially linear between divergence and magnetic
activity at all activity levels at depths shallower than about 10
Mm. At greater depths, the relationship changes sign at locations of
high activity; the flows are increasingly divergent at locations with
a magnetic activity index (MAI) greater than about 24 G. The flows are
more convergent by about a factor of two during the rising phase of
Cycle 24 than during the declining phase of Cycle 23 at locations of
medium and high activity (about 10 to 40 G MAI) from the surface to at
least 10 Mm. The subsurface divergence pattern of Solar Cycle 24 first
appears during the declining phase of Cycle 23 and is present during
the extended minimum. It appears several years before the magnetic
pattern of the new cycle is noticeable in synoptic maps. Using linear
regression, we estimate the amount of magnetic activity that would be
required to generate the precursor pattern and find that it should be
almost twice the amount of activity that is observed.
---------------------------------------------------------
Title: Parametrizing the time variation of the `surface term' of
stellar p-mode frequencies: application to helioseismic data
Authors: Howe, R.; Basu, S.; Davies, G. R.; Ball, W. H.; Chaplin,
W. J.; Elsworth, Y.; Komm, R.
2017MNRAS.464.4777H Altcode: 2016arXiv161004113H
The solar-cycle variation of acoustic mode frequencies has a frequency
dependence related to the inverse mode inertia. The discrepancy
between model predictions and measured oscillation frequencies for
solar and solar-type stellar acoustic modes includes a significant
frequency-dependent term known as the surface term, which is also
related to the inverse mode inertia. We parametrize both the surface
term and the frequency variations for low-degree solar data from
Birmingham Solar-Oscillations Network (BiSON) and medium-degree data
from the Global Oscillations Network Group (GONG) using the mode inertia
together with cubic and inverse frequency terms. We find that for the
central frequency of rotationally split multiplets, the cubic term
dominates both the average surface term and the temporal variation,
but for the medium-degree case, the inverse term improves the fit to
the temporal variation. We also examine the variation of the even-order
splitting coefficients for the medium-degree data and find that, as
for the central frequency, the latitude-dependent frequency variation,
which reflects the changing latitudinal distribution of magnetic
activity over the solar cycle, can be described by the combination of
a cubic and an inverse function of frequency scaled by inverse mode
inertia. The results suggest that this simple parametrization could be
used to assess the activity-related frequency variation in solar-like
asteroseismic targets.
---------------------------------------------------------
Title: Sub-photosphere to Solar Atmosphere Connection
Authors: Komm, Rudolf; De Moortel, Ineke; Fan, Yuhong; Ilonidis,
Stathis; Steiner, Oskar
2017hdsi.book..173K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Subsurface Zonal and Meridional Flows from SDO/HMI
Authors: Komm, Rudolf; Howe, Rachel; Hill, Frank
2016usc..confE..55K Altcode:
We study the solar-cycle variation of the zonal and meridional
flows in the near-surface layers of the solar convection zone from
the surface to a depth of about 16 Mm. The flows are determined from
SDO/HMI Dopplergrams using the HMI ring-diagram pipeline. The zonal and
meridional flows vary with the solar cycle. Bands of faster-than-average
zonal flows together with more-poleward-than-average meridional flows
move from mid-latitudes toward the equator during the solar cycle and
are mainly located on the equatorward side of the mean latitude of solar
magnetic activity. Similarly, bands of slower-than-average zonal flows
together with less-poleward-than-average meridional flows are located
on the poleward side of the mean latitude of activity. Here, we will
focus on the variation of these flows at high latitudes (poleward of
50 degree) that are now accessible using HMI data. We will present
the latest results.
---------------------------------------------------------
Title: Solar origins of space weather
Authors: Jain, Kiran; Komm, Rudolf W.
2016AsJPh..25..363J Altcode:
Space weather refers to the varying conditions in the space environment
near Earth that are driven by the Sun and its changing magnetic
field. The magnetic field originates in the interior of the Sun and
extends throughout the solar atmosphere. We discuss the solar sources
of space weather and focus on coronal mass ejections (CMEs), flares,
and solar energetic particles (SEP) and on the on-going efforts to
predict these eruptive events and their effect on space weather.
---------------------------------------------------------
Title: Are subsurface flows evidence of hidden magnetic flux during
cycle minimum?
Authors: Komm, Rudolf; Howe, Rachel; Hill, Frank
2016SPD....47.0708K Altcode:
Subsurface flows vary during the course of a solar cycle showing bands
of faster- and slower-than-average rotation and bands of converging
meridional flow. These flow patterns migrate with latitude; they first
appear during the declining phase of a solar cycle and are present
during cycle minimum. They appear several years before the magnetic
pattern of a new cycle is apparent in synoptic maps and the values
of magnetic flux at these locations are comparable to other quiet-Sun
locations without such flow patterns. Do the precursory flow patterns
thus indicate the presence of magnetic flux that is too small-scale
or short-lived to be noticed in synoptic maps? How much flux would be
required to generate these flow patterns?We quantify the relationship
between subsurface flow patterns and magnetic activity during Cycles 23
and 24 and address these questions. We have analyzed GONG and SDO/HMI
Dopplergrams using a dense-pack ring-diagram analysis and determined
flows in the near-surface layers of the solar convection zone to a
depth of about 16 Mm.
---------------------------------------------------------
Title: Horizontal Flows in Active Regions from Ring-diagram and
Local Correlation Tracking Methods
Authors: Jain, Kiran; Tripathy, S. C.; Ravindra, B.; Komm, R.; Hill, F.
2016ApJ...816....5J Altcode: 2015arXiv151103208J
Continuous high-cadence and high spatial resolution Dopplergrams allow
us to study subsurface dynamics that may be further extended to explore
precursors of visible solar activity on the surface. Since the p-mode
power is absorbed in the regions of high magnetic field, the inferences
in these regions are often presumed to have large uncertainties. In
this paper, using the Dopplergrams from space-borne Helioseismic
Magnetic Imager, we compare horizontal flows in a shear layer below the
surface and the photospheric layer in and around active regions. The
photospheric flows are calculated using the local correlation tracking
(LCT) method, while the ring-diagram technique of helioseismology
is used to infer flows in the subphotospheric shear layer. We find
a strong positive correlation between flows from both methods near
the surface. This implies that despite the absorption of acoustic
power in the regions of strong magnetic field, the flows inferred
from the helioseismology are comparable to those from the surface
measurements. However, the magnitudes are significantly different;
the flows from the LCT method are smaller by a factor of 2 than the
helioseismic measurements. Also, the median difference between the
direction of corresponding vectors is 49°.
---------------------------------------------------------
Title: Sub-photosphere to Solar Atmosphere Connection
Authors: Komm, Rudolf; De Moortel, Ineke; Fan, Yuhong; Ilonidis,
Stathis; Steiner, Oskar
2015SSRv..196..167K Altcode: 2013SSRv..tmp...93K
Magnetic fields extend from the solar interior through the
atmosphere. The formation and evolution of active regions can be studied
by measuring subsurface flows with local helioseismology. The emergence
of magnetic flux from the solar convection zone is associated with
acoustic perturbation signatures. In near-surface layers, the average
dynamics can be determined for emerging regions. MHD simulations
of the emergence of a twisted flux tube show how magnetic twist
and free energy are transported from the interior into the corona
and the dynamic signatures associated with such transport in the
photospheric and sub-photospheric layers. The subsurface twisted flux
tube does not emerge into the corona as a whole in emerging active
regions. Shear flows at the polarity inversion line and coherent
vortical motions in the subsurface flux tubes are the major means by
which twist is transported into the corona, leading to the formation
of sigmoid-shaped coronal magnetic fields capable of driving solar
eruptions. The transport of twist can be followed from the interior
by using the kinetic helicity of subsurface flows as a proxy of
magnetic helicity; this quantity holds great promise for improving
the understanding of eruptive phenomena. Waves are not only vital for
studying the link between the solar interior and the surface but for
linking the photosphere with the corona as well. Acoustic waves that
propagate from the surface into the magnetically structured, dynamic
atmosphere undergo mode conversion and refraction. These effects
enable atmospheric seismology to determine the topography of magnetic
canopies in the solar atmosphere. Inclined magnetic fields lower
the cut-off frequency so that low frequency waves can leak into the
outer atmosphere. Recent high resolution, high cadence observations of
waves and oscillations in the solar atmosphere, have lead to a renewed
interest in the potential role of waves as a heating mechanism. In light
of their potential contribution to the heating of the solar atmosphere,
some of the recent observations of waves and oscillations and ongoing
modelling efforts are reviewed.
---------------------------------------------------------
Title: Solar Subsurface Flows During Solar Cycle 24
Authors: Komm, R.; Howe, R.; Hill, F.
2015AGUFMSH23A2420K Altcode:
We study the variation of subsurface flows in the presence of magnetic
activity during the current solar cycle. To determine flows in the
near-surface layers of the solar convection zone from the surface to
a depth of about 16 Mm, we have analyzed Dopplergrams obtained with
the Global Oscillation Network Group (GONG) and the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO)
using a dense-pack ring-diagram analysis. We will compare the zonal
and meridional flows during Cycle 24 with those during Cycle 23. The
zonal and meridional flows at high latitudes are now accessible thanks
to HMI data. The zonal- and meridional-flow patterns track the mean
latitude of activity and are precursors of magnetic activity appearing
about three years before activity is visible in synoptic maps of
the solar surface. The poleward branch of the zonal-flow pattern is
noticeable during Cycle 24 but is considerably weaker than during the
previous cycle. We will also present the latest results from global
helioseismology for comparison.
---------------------------------------------------------
Title: Persistent Near-Surface Flow Structures from Local
Helioseismology
Authors: Howe, Rachel; Komm, R. W.; Baker, D.; Harra, L.; van
Driel-Gesztelyi, L.; Bogart, R. S.
2015SoPh..290.3137H Altcode: 2015arXiv150706525H; 2015SoPh..tmp..115H
Near-surface flows measured by the ring-diagram technique of local
helioseismology show structures that persist over multiple rotations. We
examine these phenomena using data from the Global Oscillation Network
Group (GONG) and the Helioseismic and Magnetic Imager (HMI) and show
that a correlation analysis of the structures can be used to estimate
the rotation rate as a function of latitude, giving a result consistent
with the near-surface rate from global helioseismology and slightly
slower than that obtained from a similar analysis of the surface
magnetic field strength. At latitudes of 60<SUP>∘</SUP> and above,
the HMI flow data reveal a strong signature of a two-sided zonal flow
structure. This signature may be related to recent reports of "giant
cells" in solar convection.
---------------------------------------------------------
Title: Solar-Cycle Variation of Subsurface Meridional Flow Derived
with Ring-Diagram Analysis
Authors: Komm, R.; González Hernández, I.; Howe, R.; Hill, F.
2015SoPh..290.3113K Altcode: 2015SoPh..tmp...83K
We study the solar-cycle variation of the meridional flow in the
near-surface layers of the solar convection zone from the surface to
a depth of 16 Mm. We have analyzed Global Oscillation Network Group
(GONG) Dopplergrams with a ring-diagram analysis covering about 13
years (July 2001 - October 2014), from the maximum of Cycle 23 through
the rising phase of Cycle 24, and Helioseismic and Magnetic Imager
(HMI) Dopplergrams covering more than four years (May 2010 - January
2015). GONG and HMI lead to similar meridional flows during common
epochs and latitudes. The meridional flow averaged over a Carrington
rotation is poleward up to about 70<SUP>∘</SUP> in both hemispheres at
all depths after correcting for systematic effects. The flow amplitude
peaks at about 40<SUP>∘</SUP> latitude with an amplitude of about 16
to 20 ms−<SUP>1</SUP> depending on depth. The meridional flow varies
with the solar cycle; the flow amplitudes are larger during cycle
minimum than during maximum at low- and mid-latitudes. The flows are
mainly faster or more-poleward-than-average on the equatorward side of
the mean latitude of activity and slower or less-poleward-than-average
on its poleward side. The residual meridional flow converges near
the mean latitude of activity. A comparison with the corresponding
zonal flow derived from GONG and HMI data shows that the bands
of more-poleward-than-average meridional flow coincide with the
bands of faster-than-average zonal flow and that the bands of
less-poleward-than-average meridional flow coincide with the bands of
slower-than-average zonal flow. This implies that the residual flows
are cyclonic. The bands of fast meridional flow appear at mid-latitudes
about three years before magnetic activity of Cycle 24 is present in
synoptic maps.
---------------------------------------------------------
Title: Temporal evolution of the solar torsional oscillation and
implications for cycle 25
Authors: Hill, Frank; Howe, Rachel; Komm, Rudolf; Schou, Jesper;
Thompson, Michael; Larson, Timothy
2015TESS....110502H Altcode:
The zonal flow known as the torsional oscillation has been observed
on the Sun’s surface since 1980 and in its interior since 1995. It
has two branches that migrate during the solar cycle, with one moving
towards the equator and the other towards the poles. The rate at which
these branches migrate in latitude is tightly correlated with the
timing of the solar cycle, as seen during the long minimum between
cycles 23 and 24. The poleward branch generally becomes visible 10
to 12 years before the appearance of the magnetic activity associated
with the corresponding sunspot cycle as it did for the current cycle
24. However, the poleward flow for cycle 25, which was expected to
appear in 2008-2010, was not observed. Subsequent analysis showed
that it is a very weak flow, and is masked by an apparent change
in the background solar differential rotation rate. We will present
the latest observations of the zonal flow as determined from global
helioseismology, and will discuss the implications for the strength
and timing of cycle 25.
---------------------------------------------------------
Title: Subsurface Zonal and Meridional Flow Derived from GONG and
SDO/HMI: A Comparison of Systematics
Authors: Komm, R.; González Hernández, I.; Howe, R.; Hill, F.
2015SoPh..290.1081K Altcode: 2015SoPh..tmp...22K
We study the subsurface flows in the near-surface layers of the
solar convection zone from the surface to a depth of 16 Mm derived
from Global Oscillation Network Group (GONG) and Helioseismic and
Magnetic Imager (HMI) Dopplergrams using a ring-diagram analysis. We
characterize the systematic east-west and north-south variations
present in the zonal and meridional flows and compare flows derived
from GONG and HMI data before and after the correction. The average
east-west variation with depth of one flow component resembles the
average north-south variation with depth of the other component. The
east-west variation of the zonal flow together with the north-south
variation of the meridional flow can be modeled as a systematic
radial velocity. This indicates a solar center-to-limb variation as
the underlying cause. The north-south variation of the zonal flow and
the east-west variation of the meridional flow require two separate
functions. The east-west variation of the meridional flow consists
mainly of an annual variation with the B<SUB>0</SUB> angle, while the
north-south trend of the zonal flow consists of a constant non-zero
component in addition to an annual variation. This indicates a geometric
projection artifact. After compensating for these systematic effects,
the meridional and zonal flows derived from HMI data agree well with
those derived from GONG data. An offset remains between the zonal flow
derived from GONG and HMI data. The equatorward meridional flows at
high latitude that appear episodically depending on the B<SUB>0</SUB>
angle are absent from the corrected flows.
---------------------------------------------------------
Title: Solar-cycle variation of subsurface flows during 20 years
Authors: Komm, Rudolf; Howe, Rachel; Hill, Frank
2015TESS....121406K Altcode:
We study the solar-cycle variation of the zonal and meridional flow
in the near-surface layers of the solar convection zone from the
surface to a depth of about 16 Mm. We have analyzed Dopplergrams
obtained with the Michelson Doppler Imager (MDI) onboard the Solar and
Heliospheric Observatory (SOHO), the Global Oscillation Network Group
(GONG),and the Helioseismic and Magnetic Imager (HMI) onboard the Solar
Dynamics Observatory (SDO) with a dense-pack ring-diagram analysis. The
three data sets combined cover almost two solar cycles. The zonal and
meridional flows vary with the solar cycle. Their amplitude variation
tracks the mean latitude of activity and appears about three years
before magnetic activity is visible in synoptic maps of the solar
surface. We focus on the variation of the zonal and meridional flows,
including their long-term variation at mid- and low-latitudes using
GONG and MDI data and their variation at the high latitudes that are
now accessible using HMI data. We will present the latest results.
---------------------------------------------------------
Title: Subsurface helicity of active regions 12192 and 10486
Authors: Komm, Rudolf; Tripathy, Sushant; Howe, Rachel; Hill, Frank
2015TESS....110506K Altcode:
The active region 10486 that produced the Halloween flares in 2003
initiated our interest in the kinetic helicity of subsurface flows
associated with active regions. This lead to the realization that the
helicity of subsurface flows is related to the flare activity of active
regions. Eleven years later, a similarly enormous active region (12192)
appeared on the solar surface. We plan to study the kinetic helicity of
the subsurface flows associated with region 12192 and compare it to that
of region 10486. For 10486, we have analyzed Dopplergrams obtained with
the Michelson Doppler Imager (MDI) onboard the Solar and Heliospheric
Observatory (SOHO) and the Global Oscillation Network Group (GONG)
with a dense-pack ring-diagram analysis. For 12192, we have analyzed
Dopplergrams from GONG and the Helioseismic and Magnetic Imager (HMI)
onboard the Solar Dynamics Observatory (SDO). We will present the
latest results.
---------------------------------------------------------
Title: Current and Kinetic Helicity of Long-lived Activity Complexes
Authors: Komm, Rudolf; Gosain, Sanjay
2015ApJ...798...20K Altcode:
We study long-lived activity complexes and their current helicity at
the solar surface and their kinetic helicity below the surface. The
current helicity has been determined from synoptic vector magnetograms
from the NSO/SOLIS facility, and the kinetic helicity of subsurface
flows has been determined with ring-diagram analysis applied to
full-disk Dopplergrams from NSO/GONG and SDO/HMI. Current and kinetic
helicity of activity complexes follow the hemispheric helicity rule
with mainly positive values (78%; 78%, respectively, with a 95%
confidence level of 31%) in the southern hemisphere and negative ones
(80%; 93%, respectively, with a 95% confidence level of 22% and 14%,
respectively) in the northern hemisphere. The locations with the
dominant sign of kinetic helicity derived from Global Oscillation
Network Group (GONG) and SDO/HMI data are more organized than those of
the secondary sign even if they are not part of an activity complex,
while locations with the secondary sign are more fragmented. This is
the case for both hemispheres even for the northern one where it is
not as obvious visually due to the large amount of magnetic activity
present as compared to the southern hemisphere. The current helicity
shows a similar behavior. The dominant sign of current helicity is
the same as that of kinetic helicity for the majority of the activity
complexes (83% with a 95% confidence level of 15%). During the 24
Carrington rotations analyzed here, there is at least one longitude in
each hemisphere where activity complexes occur repeatedly throughout
the epoch. These "active" longitudes are identifiable as locations of
strong current and kinetic helicity of the same sign.
---------------------------------------------------------
Title: Solar Meridional Flow from Helioseismic Observations
Authors: Komm, R.
2014AGUFMSH44A..03K Altcode:
The meridional flow in the solar interior has been studied with
helioseismic techniques applied to observations from the ground-based
Global Oscillation Network Group (GONG), the Michelson Doppler Imager
(MDI) onboard the Solar and Heliospheric Observatory (SOHO), and most
recently from the Helioseismic and Magnetic Imager (HMI) instrument
onboard the Solar Dynamics Observatory (SDO). The observed meridional
flow is on average poleward in the near-surface layers of the Sun and
its amplitude varies with the solar cycle. The meridional circulation
plays an important role in dynamo theories. For example, the structure
and the strength of the meridional flow might determine the duration of
the solar cycle and set the timing of the reversals of the Sun's polar
fields in flux-transport dynamo models. To improve the understanding of
the relationship between dynamo and meridional flow, major topics that
need to be addressed are the variation of the meridional flow with the
solar cycle, its variation with depth, and the meridional flow at high
latitudes. I will discuss recent results from helioseismic observations.
---------------------------------------------------------
Title: Current and Kinetic Helicity of Long-Lived Activity Complexes
Authors: Komm, R.; Gosain, S.
2014AGUFMSH41B4137K Altcode:
We focus on long-lived activity complexes and their helicity below and
above the solar surface. These locations of recurrent flux emergence
in or close to a pre-existing active region, last for typically five
to seven solar rotations. It is known that emergence of new magnetic
flux in pre-existing magnetic region causes an increase in topological
complexity of the magnetic field which leads to flares and Coronal Mass
Ejections (CMEs). A quantitative measure of topological complexity
of magnetic fields is given by the magnetic helicity which measures
twisting and linking of the magnetic field. The current helicity
determined from vector magnetograms is the equivalent of the kinetic
helicity determined from subsurface flows. The helicity is thus an ideal
quantity to investigate the linkage of magnetic fields in the solar
atmosphere with flows in the upper solar convection zone. The subsurface
flows from the surface to a depth of 16 Mm are determined with a
ring-diagram analysis of GONG and SDO/HMI Dopplergrams and the current
helicity density is determined from SOLIS vector magnetograms. We
will study the kinetic and current helicity as a function of time and
Carrington longitude, averaged over a suitable range of latitudes in
either hemisphere. We will present the latest results.
---------------------------------------------------------
Title: A Combined Study of Photospheric Magnetic and Current
Helicities and Subsurface Kinetic Helicities of Solar Active Regions
during 2006-2013
Authors: Seligman, D.; Petrie, G. J. D.; Komm, R.
2014ApJ...795..113S Altcode: 2014arXiv1409.0764S
We compare the average photospheric current helicity H<SUB>c</SUB>
, photospheric twist parameter α (a well-known proxy for the
full relative magnetic helicity), and subsurface kinetic helicity
H<SUB>k</SUB> for 194 active regions observed between 2006-2013. We use
2440 Hinode photospheric vector magnetograms, and the corresponding
subsurface fluid velocity data derived from GONG (2006-2012) and
Helioseismic and Magnetic Imager (2010-2013) dopplergrams. We find a
significant hemispheric bias in all three parameters. The subsurface
kinetic helicity is preferentially positive in the southern hemisphere
and negative in the northern hemisphere. The photospheric current
helicity and the α parameter have the same bias for strong fields (|B|
> 1000 G) and no significant bias for weak fields (100 G <|B|
< 500 G). We find no significant region-by-region correlation
between the subsurface kinetic helicity and either the strong-field
current helicity or α. Subsurface fluid motions of a given handedness
correspond to photospheric helicities of both signs in approximately
equal numbers. However, common variations appear in annual averages
of these quantities over all regions. Furthermore, in a subset of 77
regions, we find significant correlations between the temporal profiles
of the subsurface and photospheric helicities. In these cases, the sign
of the linear correlation coefficient matches the sign relationship
between the helicities, indicating that the photospheric magnetic
field twist is sensitive to the twisting motions below the surface.
---------------------------------------------------------
Title: Solar-Cycle Variation of Subsurface Zonal Flow
Authors: Komm, R.; Howe, R.; González Hernández, I.; Hill, F.
2014SoPh..289.3435K Altcode: 2014SoPh..tmp...29K
We study the solar-cycle variation of the zonal flow in the near-surface
layers of the solar convection zone from the surface to a depth of 16
Mm covering the period from mid-2001 to mid-2013 or from the maximum
of Cycle 23 through the rising phase of Cycle 24. We have analyzed
Global Oscillation Network Group (GONG) and Helioseismic and Magnetic
Imager (HMI) Dopplergrams with a ring-diagram analysis. The zonal flow
varies with the solar cycle showing bands of faster-than-average flows
equatorward of the mean latitude of activity and slower-than-average
flows on the poleward side. The fast band of the zonal flow and the
magnetic activity appear first in the northern hemisphere during
the beginning of Cycle 24. The bands of fast zonal flow appear
at mid-latitudes about three years in the southern and four years
in the northern hemisphere before magnetic activity of Cycle 24 is
present. This implies that the flow pattern is a direct precursor of
magnetic activity. The solar-cycle variation of the zonal flow also
has a poleward branch, which is visible as bands of faster-than-average
zonal flow near 50° latitude. This band appears first in the southern
hemisphere during the rising phase of the Cycle 24 and migrates slowly
poleward. These results are in good agreement with corresponding
results from global helioseismology.
---------------------------------------------------------
Title: Hemispheric Distribution of Subsurface Kinetic Helicity and
Its Variation with Magnetic Activity
Authors: Komm, R.; Gosain, S.; Pevtsov, A. A.
2014SoPh..289.2399K Altcode: 2014SoPh..tmp...25K
We study the hemispheric distribution of the kinetic helicity of
subsurface flows in the near-surface layers of the solar convection
zone and its variation with magnetic activity. We determine subsurface
flows with a ring-diagram analysis applied to Global Oscillation Network
Group (GONG) Dopplergrams and Dynamics Program data from the Michelson
Doppler Imager (MDI) instrument onboard the Solar and Heliospheric
Observatory (SOHO). We determine the average kinetic helicity density
as a function of Carrington rotation and latitude. The average kinetic
helicity density at all depths and the kinetic helicity, integrated
over 2 - 7 Mm, follow the same hemispheric rule as the current/magnetic
helicity proxies with predominantly positive values in the southern
and negative ones in the northern hemisphere. This holds true for all
levels of magnetic activity from quiet to active regions. However,
this is a statistical result; only about 55 % of all locations follow
the hemispheric rule. But these locations have larger helicity values
than those that do not follow the rule. The average values of helicity
density increase with depth for all levels of activity, which might
reflect an increase of the characteristic size of convective motions
with greater depth. The average helicity of subsets of high magnetic
activity is about five times larger than that of subsets of low
activity. The solar-cycle variation of helicity is thus mainly due to
the presence or absence of active regions. During the rising phase of
cycle 24, locations of high magnetic activity at low latitudes show
a weaker hemispheric behavior compared to the rising phase of cycle 23.
---------------------------------------------------------
Title: Daily Normalized Helicity of Subsurface Flows
Authors: Komm, Rudolf; Reinard, Alysha; Hill, Frank
2014AAS...22421801K Altcode:
Flare-productive active regions are associated with subsurface flows
with large values of kinetic helicity density. Kinetic helicity is
related to mixing and turbulence of fluids. Reinard et al. 2010 have
developed a parameter that captures the variation of kinetic helicity
with depth and time, the so-called Normalized Helicity Gradient Variance
(NHGV). This parameter increases 2-3 days before a flare occurs and the
NHGV values for flaring and non-flaring active regions represent clearly
separate populations. We derive subsurface flows from the surface to
a depth of 16 Mm using GONG and SDO/HMI Dopplergrams analyzed with
the ring-diagram technique and calculate kinetic helicity density as
a function of position on the solar disk. We will then calculate the
NHGV parameter exploring different normalization schemes and depth
ranges. We will present cases studies of active regions observed with
GONG and SDO/HMI.
---------------------------------------------------------
Title: Current and Kinetic Helicity of Long-Lived Activity Complexes
Authors: Komm, Rudolf; Gosain, S.
2014shin.confE..60K Altcode:
We focus on long-lived activity complexes and their helicity below and
above the solar surface. These locations of recurrent flux emergence
in or close to a pre-existing active region, last for typically five
to seven solar rotations. It is known that emergence of new magnetic
flux in pre-existing magnetic region causes an increase in topological
complexity of the magnetic field which leads to flares and Coronal Mass
Ejections (CMEs). A quantitative measure of topological complexity
of magnetic fields is given by the magnetic helicity which measures
twisting and linking of the magnetic field. The current helicity
determined from vector magnetograms is the equivalent of the kinetic
helicity determined from subsurface flows. The helicity is thus an ideal
quantity to investigate the linkage of magnetic fields in the solar
atmosphere with flows in the upper solar convection zone. The subsurface
flows from the surface to a depth of 16 Mm are determined with a
ring-diagram analysis of GONG and SDO/HMI Dopplergrams and the current
helicity density is determined from SOLIS vector magnetograms. We
will study the kinetic and current helicity as a function of time and
Carrington longitude, averaged over a suitable range of latitudes in
either hemisphere. We will present the latest results.
---------------------------------------------------------
Title: Daily Normalized Kinetic Helicity of Subsurface Flows
Authors: Komm, Rudolf; Reinard, A.; Hill, F.
2014shin.confE.163K Altcode:
Flare-productive active regions are associated with subsurface flows
with large values of kinetic helicity density. Kinetic helicity is
related to mixing and turbulence of fluids. Reinard et al. 2010 have
developed a parameter that captures the variation of kinetic helicity
with depth and time, the so-called Normalized Helicity Gradient Variance
(NHGV). This parameter increases 2-3 days before a flare occurs and the
NHGV values for flaring and non-flaring active regions represent clearly
separate populations. We derive subsurface flows from the surface to
a depth of 16 Mm using GONG and SDO/HMI Dopplergrams analyzed with
the ring-diagram technique and calculate kinetic helicity density as
a function of position on the solar disk. We will then calculate the
NHGV parameter exploring different normalization schemes and depth
ranges. We will present cases studies of active regions observed with
GONG and SDO/HMI.
---------------------------------------------------------
Title: Photospheric and sub-photospheric Flows in Active Regions
Authors: Jain, Kiran; Komm, Rudolf W; Tripathy, Sushanta; Ravindra,
B.; Hill, Frank
2014AAS...22421821J Altcode:
The availability of continuous high-cadence and high-spatial resolution
Dopplergrams allows us to study sub-surface dynamics that may be further
extended to explore precursors of the solar activity. Since p-mode power
is absorbed in high magnetic field regions, the helioseismic inferences
in these regions are associated with large errors. In order to validate
results, we use Dopplergrams from both space-borne (Helioseismic
Magnetic Imager-HMI) and ground-based (Global Oscillation Network
Group-GONG) observations to infer horizontal flows in photospheric
and sub-photospheric layers in and around several active regions with
different characteristics. The photospheric flows are calculated using
local correlation tracking (LCT) method while ring-diagram analysis
technique is used to infer flows in the sub-photospheric regions. A
detailed comparison between flows in shear layer and photospheric
layer will be made in order to study similarities and discrepancies
in these results.
---------------------------------------------------------
Title: Active Regions with Superpenumbral Whirls and Their Subsurface
Kinetic Helicity
Authors: Komm, R.; Gosain, S.; Pevtsov, A.
2014SoPh..289..475K Altcode:
We search for a signature of helicity flow from the solar interior
to the photosphere and chromosphere. For this purpose, we study two
active regions, NOAA 11084 and 11092, that show a regular pattern of
superpenumbral whirls in chromospheric and coronal images. These two
regions are good candidates for comparing magnetic/current helicity with
subsurface kinetic helicity because the patterns persist throughout the
disk passage of both regions. We use photospheric vector magnetograms
from SOLIS/VSM and SDO/HMI to determine a magnetic helicity proxy, the
spatially averaged signed shear angle (SASSA). The SASSA parameter
produces consistent results leading to positive values for NOAA
11084 and negative ones for NOAA 11092 consistent with the clockwise
and counter-clockwise orientation of the whirls. We then derive
the properties of the subsurface flows associated with these active
regions. We measure subsurface flows using a ring-diagram analysis of
GONG high-resolution Doppler data and derive their kinetic helicity,
h<SUB>z</SUB>. Since the patterns persist throughout the disk passage,
we analyze synoptic maps of the subsurface kinetic helicity density. The
sign of the subsurface kinetic helicity is negative for NOAA 11084
and positive for NOAA 11092; the sign of the kinetic helicity is
thus anticorrelated with that of the SASSA parameter. As a control
experiment, we study the subsurface flows of six active regions without
a persistent whirl pattern. Four of the six regions show a mixture
of positive and negative kinetic helicity resulting in small average
values, while two regions are clearly dominated by kinetic helicity
of one sign or the other, as in the case of regions with whirls. The
regions without whirls follow overall the same hemispheric rule in
their kinetic helicity as in their current helicity with positive
values in the southern and negative values in the northern hemisphere.
---------------------------------------------------------
Title: Solar Origins of Space Weather and Space Climate: Preface
Authors: González Hernández, I.; Komm, R.; Pevtsov, A.; Leibacher,
J. W.
2014SoPh..289..437G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: A Combined Study of Photospheric Magnetic and Current
Helicities and Subsurface Kinetic Helicities of Solar Active Regions
during 2006-2012
Authors: Seligman, Darryl; Petrie, G.; Komm, R.
2014AAS...22315801S Altcode:
We compare the average photospheric current helicity H_c, photospheric
twist parameter α (a well-known proxy for the full relative magnetic
helicity), and subsurface kinetic helicity K_h for 128 active
regions observed between 2006-2012. We use 1436 Hinode photospheric
vector magnetograms and subsurface fluid velocity data from GONG
Dopplergrams. We find a significant hemispheric bias in all three
parameters. The K_h parameter is preferentially positive/negative in
the southern/northern hemisphere. The H_c and α parameters have the
same bias for strong fields |{B}|>1000 G). We examine the temporal
variability of each parameter for each active region and identify
a significant subset of regions whose three helicity parameters all
exhibit clear increasing or decreasing trends. The temporal profiles
of these regions have the same bias: positive/negative helicity in the
northern/southern hemisphere. The results are consistent with Longcope
et al.'s Σ-effect. This work is carried out through the National Solar
Observatory Research Experiences for Undergraduate (REU) site program,
which is co-funded by the Department of Defense in partnership with
the NSF REU Program. The National Solar Observatory is operated by
the Association of Universities for Research in Astronomy, Inc. (AURA)
under cooperative agreement with the National Science Foundation.
---------------------------------------------------------
Title: Subsurface Flows in Active Region 11158
Authors: Jain, K.; Tripathy, S. C.; Komm, R.; González Hernández,
I.; Hill, F.
2013ASPC..478..225J Altcode:
We apply the ring-diagram technique to study the temporal evolution of
horizontal velocity in sub-photospheric layers beneath active regions
as they move across the solar disk. Here we present results for the
AR 11158 for six days and investigate how flows get organized within
the active region by the morphology of individual sunspots or vice
versa. We find abrupt changes in depth profiles for smaller regions
in going from one day to another, however the average flows for the
active region do not show significant temporal variation.
---------------------------------------------------------
Title: Medium-Degree Global-Mode Frequency Shifts in Solar Cycles
23 and 24: Is There Any Difference?
Authors: Howe, R.; Komm, R.; Hill, F.
2013ASPC..478..155H Altcode:
It is well established that the frequencies of acoustic modes vary
with the solar cycle, being strongly correlated with the temporal
and spatial distribution of magnetic activity as measured by the
magnetic field strength or by intensity proxies. With nearly eighteen
years of data from GONG and MDI, we check for differences between the
sensitivity to the Kitt Peak magnetic index between the rising phases
of Solar Cycles 23 and 24. We find no significant difference.
---------------------------------------------------------
Title: Solar Cycle Variation of High-Degree Acoustic Mode Frequencies
Authors: Tripathy, S. C.; Jain, K.; Komm, R. W.; Hill, F.
2013ASPC..478..221T Altcode:
We investigate the temporal variations of the high-degree mode
frequencies measured over localized regions of the Sun though
the technique of ring-diagrams. We observe that the high-degree
mode frequencies have a solar cycle variation similar to those of
intermediate-degree modes but ten times greater. We also find that
the averaged frequency shifts are linearly correlated with routinely
measured solar activity indices e.g. 10.7 cm radio flux. We do not,
however, find any evidence of a quadratic relation between the
frequencies of individual multiplets and solar activity indices as
reported earlier from the study of global high-degree modes.
---------------------------------------------------------
Title: Solar-Cycle Variation of Subsurface Zonal Flow Derived from
Ring-Diagram Analysis
Authors: Komm, R.; Howe, R.; González Hernández, I.; Hill, F.;
Bogart, R. S.; Haber, D.
2013ASPC..478..217K Altcode:
We study the solar-cycle variation of the zonal flow in the near-surface
layers of the solar convection zone from the surface to a depth of
16 Mm. We have analyzed Global Oscillation Network Group (GONG)
and Helioseismic and Magnetic Imager (HMI) Dopplergrams with the
ring-diagram analysis covering about 12 years combined. The zonal
flow varies with the solar cycle showing faster-than-average flows
equatorward of the mean latitude of activity and slower-than-average
flows on the poleward side. The bands of fast zonal flow appear at
mid-latitudes about two years before magnetic activity of cycle 24
is seen. The poleward branch of this variation is visible as bands of
fast zonal flow near 50° latitude in both HMI and GONG data.
---------------------------------------------------------
Title: The Torsional Oscillation and the Timing of the Solar Cycle:
Is it Maximum Yet?
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Larson, T. P.; Schou, J.; Thompson, M. J.
2013ASPC..478..303H Altcode:
After the late start to Cycle 24 there are some indications that
activity may have peaked as early as late 2011 and that the polar-field
reversal has already occurred in the North. We use helioseismic
measurements of the migrating zonal flow pattern known as the torsional
oscillation to estimate the length of the solar cycle, and find that
it has held steady at about 12.3 years since late 2009, which would
point to solar maximum in 2013 as expected.
---------------------------------------------------------
Title: Magnetic Polarity Streams and Subsurface Flows
Authors: Howe, R.; Baker, D.; Harra, L.; van Driel-Gesztelyi, L.;
Komm, R.; Hill, F.; González Hernández, I.
2013ASPC..478..291H Altcode:
An important feature of the solar cycle is the transport of unbalanced
magnetic flux from active regions towards the poles, which eventually
results in polarity reversal. This transport takes the form of distinct
“polarity streams” that are visible in the magnetic butterfly
diagram. We compare the poleward migration rate estimated from such
streams to that derived from the subsurface meridional flows measured
in helioseismic data from the GONG network since 2001, and find that
the results are in reasonable agreement.
---------------------------------------------------------
Title: Subsurface Meridional Flow from HMI Using the Ring-Diagram
Pipeline
Authors: Komm, R.; González Hernández, I.; Hill, F.; Bogart, R.;
Rabello-Soares, M. C.; Haber, D.
2013SoPh..287...85K Altcode: 2012SoPh..tmp..177K
We have determined the meridional flows in subsurface layers for 18
Carrington rotations (CR 2097 to 2114) analyzing high-resolution
Dopplergrams obtained with the Helioseismic and Magnetic Imager
(HMI) instrument onboard the Solar Dynamics Observatory (SDO). We are
especially interested in flows at high latitudes up to 75<SUP>∘</SUP>
in order to address the question whether the meridional flow remains
poleward or reverses direction (so-called counter cells). The flows
have been determined in depth from near-surface layers to about 16 Mm
using the HMI ring-diagram pipeline. The measured meridional flows show
systematic effects, such as a variation with the B<SUB>0</SUB>-angle
and a variation with central meridian distance (CMD). These variations
have been taken into account to lead to more reliable flow estimates
at high latitudes. The corrected average meridional flow is poleward
at most depths and latitudes with a maximum amplitude of about 20~m
s^{-1} near 37.5<SUP>∘</SUP> latitude. The flows are more poleward
on the equatorward side of the mean latitude of magnetic activity at
22<SUP>∘</SUP> and less poleward on the poleward side, which can be
interpreted as convergent flows near the mean latitude of activity. The
corrected meridional flow is poleward at all depths within ±
67.5<SUP>∘</SUP> latitude. The corrected flow is equatorward only at
75<SUP>∘</SUP> latitude in the southern hemisphere at depths between
about 4 and 8 Mm and at 75<SUP>∘</SUP> latitude in the northern
hemisphere only when the B<SUB>0</SUB> angle is barely large enough to
measure flows at this latitude. These counter cells are most likely the
remains of an insufficiently corrected B<SUB>0</SUB>-angle variation
and not of solar origin. Flow measurements and B<SUB>0</SUB>-angle
corrections are difficult at the highest latitude because these flows
are only determined during limited periods when the B<SUB>0</SUB>
angle is sufficiently large.
---------------------------------------------------------
Title: Latest Results Found with Ring-Diagram Analysis
Authors: Baldner, C. S.; Basu, S.; Bogart, R. S.; Burtseva, O.;
González Hernández, I.; Haber, D.; Hill, F.; Howe, R.; Jain, K.;
Komm, R. W.; Rabello-Soares, M. C.; Tripathy, S.
2013SoPh..287...57B Altcode: 2012SoPh..tmp..294B
Ring-diagram analysis is a helioseismic tool useful for studying
the near-surface layers of the Sun. It has been employed to study
near-surface shear, meridional circulation, flows around sunspots,
and thermal structure beneath active regions. We review recent results
obtained using ring-diagram analysis, state some of the more important
outstanding difficulties in the technique, and point out several
extensions to the technique that are just now beginning to bear fruit.
---------------------------------------------------------
Title: Preface
Authors: Mansour, Nagi N.; Kosovichev, Alexander G.; Komm, Rudolf;
Longcope, Dana; Leibacher, John W.
2013SoPh..287....1M Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Are subsurface flows and coronal holes related?
Authors: Komm, R.; Howe, R.; González Hernández, I.; Harra, L.;
Baker, D.; van Driel-Gesztelyi, L.
2013JPhCS.440a2022K Altcode:
We study synoptic maps of solar subsurface flows covering six Carrington
rotations (2050 to 2055). The subsurface flows are determined with
a ring-diagram analysis of GONG high-resolution Doppler data. We
identify the locations of coronal holes in synoptic maps of EUV images
at 195Å from the EIT instrument and determine the characteristics
of associated subsurface flows. We study two long-lived coronal holes
that are present during this epoch. We find that large-scale patterns
are present in the subsurface flows but appear to be unrelated to
these coronal holes. The horizontal subsurface flows associated with
the two long-lived coronal holes are weakly divergent (upflows) with
small cyclonic vorticity. These flows are thus similar to subsurface
flows of quiet regions with regard to the vertical flows and similar
to flows of active regions with regard to vorticity.
---------------------------------------------------------
Title: How do the active region subsurface flow properties differ
based on hemisphere and CME association?
Authors: Reinard, Alysha; Komm, R.; Hill, F.
2013shin.confE..60R Altcode:
The investigation of subsurface flows beneath active regions offers
insight into the processes that occur prior to and during flare/CME
eruptions. We present new research on this topic that considers both
the flare location (specifically northern vs southern hemisphere) and
the presence or absence of an associated CME to determine whether there
is any difference in the subsurface flow pattern. We find essentially
no difference in subsurface flows below eruptive (i.e. CME associated)
and non-eruptive flares, indicating that the underlying processes
are similar in each case and the magnetic configuration of the active
region determines the eruptive potential. We do find a difference in
events originating in the northern and southern hemisphere with the
kinetic helicity density at the deepest layers being of opposite signs
in each hemisphere. This effect is stronger for CME-associated flares,
perhaps because such flares tend to be larger.
---------------------------------------------------------
Title: Helicity of Subsurface Flows and Magnetic Activity in the
Photosphere
Authors: Komm, Rudolf; Gosain, S.; Pevtsov, A. A.
2013shin.confE..43K Altcode:
Subsurface flows associated with active regions show generally
large values of kinetic helicity density. The vertical component
of kinetic helicity is defined as the product of the curl of the
horizontal velocities and the vertical velocity component. It is
thus the equivalent of current helicity determined from vector
magnetograms. The vertical component of kinetic helicity follows
on average the hemispheric rule established for current helicity
with negative values in the northern hemisphere and positive values
in the southern one. We analyze 11 years of GONG Dopplergrams and
derive subsurface flows from the surface to a depth of 16 Mm with
the ring-diagram technique. From these velocities, we calculate
the kinetic helicity density and integrate it over selected depth
ranges. We will study the kinetic helicity as a function of time and
latitude for different levels of magnetic activity, such as active
and quiet regions. We will present the latest results.
---------------------------------------------------------
Title: Subsurface flows associated with non-Joy oriented active
regions: a case study
Authors: González Hernández, Irene; Komm, Rudolf; van
Driel-Gesztelyi, Lidia; Baker, Deborah; Harra, Louise; Howe, Rachel
2013JPhCS.440a2050G Altcode:
Non-Joy oriented active regions (ARs) are a challenge for solar magnetic
field modelers. Although significant deviations from Joy's law are
relatively rare for simple bipolar ARs, understanding the causes of
their particularity could be critical for the big picture of the solar
dynamo. We explore the possibility of the sub-surface local dynamics
being responsible for the significant rotation of these ARs. We apply
the ring-diagram technique, a local helioseismology method, to infer
the flows under and surrounding a non-Joy oriented AR and present the
results of a case study in this paper.
---------------------------------------------------------
Title: Daily Normalized Helicity of Subsurface Flows
Authors: Komm, Rudolf; Reinard, A.; Hill, F.
2013shin.confE..44K Altcode:
Flare-productive active regions are associated with subsurface flows
with large values of kinetic helicity density. Kinetic helicity is
related to mixing and turbulence of fluids. Reinard et al. 2010 have
developed a parameter that captures the variation of kinetic helicity
with depth and time, the so-called Normalized Helicity Gradient Variance
(NHGV). This parameter increases 2-3 days before a flare occurs and
the NHGV values for flaring and non-flaring active regions represent
clearly separate populations. We derive subsurface flows from the
surface to a depth of 16 Mm using GONG Dopplergrams analyzed with the
ring-diagram technique. From the measured velocities, we calculate
kinetic helicity density as a function of position on the solar
disk. We will then calculate the NHGV parameter exploring different
normalization schemes and depth ranges. We will calculate daily NHGV
maps of the solar disk for different levels of magnetic activity. We
will present the latest results.
---------------------------------------------------------
Title: The High-latitude Branch of the Solar Torsional Oscillation
in the Rising Phase of Cycle 24
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Larson, T. P.; Rempel, M.; Schou, J.; Thompson, M. J.
2013ApJ...767L..20H Altcode:
We use global heliseismic data from the Global Oscillation Network
Group, the Michelson Doppler Imager on board the Solar and Heliospheric
Observatory, and the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory, to examine the behavior, during the rising phase
of Solar Cycle 24, of the migrating zonal flow pattern known as the
torsional oscillation. Although the high-latitude part of the pattern
appears to be absent in the new cycle when the flows are derived by
subtracting a mean across a full solar cycle, it can be seen if we
subtract the mean over a shorter period in the rising phase of each
cycle, and these two mean rotation profiles differ significantly
at high latitudes. This indicates that the underlying high-latitude
rotation has changed; we speculate that this is in response to weaker
polar fields, as suggested by a recent model.
---------------------------------------------------------
Title: Subsurface Flows in and Around Active Regions with Rotating
and Non-rotating Sunspots
Authors: Jain, K.; Komm, R. W.; González Hernández, I.; Tripathy,
S. C.; Hill, F.
2012SoPh..279..349J Altcode: 2012arXiv1205.2356J
The temporal variation of the horizontal velocity in sub-surface layers
beneath three different types of active region is studied using the
technique of ring diagrams. In this study, we select active regions
(ARs) 10923, 10930, 10935 from three consecutive Carrington rotations:
AR 10930 contains a fast-rotating sunspot in a strong emerging active
region while other two have non-rotating sunspots with emerging flux
in AR 10923 and decaying flux in AR 10935. The depth range covered is
from the surface to about 12 Mm. In order to minimize the influence
of systematic effects, the selection of active and quiet regions is
made so that these were observed at the same heliographic locations
on the solar disk. We find a significant variation in both components
of the horizontal velocity in active regions as compared to quiet
regions. The magnitude is higher in emerging-flux regions than in the
decaying-flux region, in agreement with earlier findings. Further,
we clearly see a significant temporal variation in depth profiles
of both zonal and meridional flow components in AR 10930, with the
variation in the zonal component being more pronounced. We also notice
a significant influence of the plasma motion in areas closest to the
rotating sunspot in AR 10930, while areas surrounding the non-rotating
sunspots in all three cases are least affected by the presence of the
active region in their neighborhood.
---------------------------------------------------------
Title: Subsurface flows associated with eruptive and non-eruptive
flares
Authors: Reinard, Alysha; Krista, Larisza; Komm, Rudi; Hill, Frank
2012shin.confE.144R Altcode:
Subsurface flows beneath active regions offer insight into the processes
that occur prior to and during flare/CME eruptions. We have developed
a technique to forecast solar flares based on subsurface flows. We
present new research on this topic that involves comparing eruptive
and non-eruptive flares to determine whether there is any difference
in the flow pattern. We also look at active region characteristics
associated with these events.
---------------------------------------------------------
Title: Active regions with superpenumbral whirls and their subsurface
flow vorticity
Authors: Komm, Rudolf W.; Gosain, S.; Pevtsov, A.
2012shin.confE.119K Altcode:
We search for a signature of helicity flow from the solar interior to
the photosphere and chromosphere. We study two active regions NOAA
11084 and NOAA 11092 that show a regular pattern of superpenumbral
whirls in H-alpha. The pattern persists throughout the disk passage
of both regions. We use photospheric vector magnetograms from
SOLIS/VSM to determine two helicity proxies: vertical component of
the current helicity density (Hc_z=Jz.Bz) and the mean twist parameter
(alpha_z=<Jz/Bz>), and to study their evolution. We compare the
two proxies of magnetic helicity with the properties of the subsurface
flows below the active regions. For this purpose, we analyze subsurface
flows measured with a ring-diagram analysis of GONG high-resolution
Doppler data and derive their vorticity. As a control experiment,
we study the subsurface flows of six active regions that do not show
a regular whirl pattern in the chromosphere.
---------------------------------------------------------
Title: Are subsurface flows and coronal holes related?
Authors: Komm, Rudolf W.; Howe, R.; González Hernández, I.; Harra,
L.; Baker, D.; van Driel-Gesztelyi, L.
2012shin.confE.120K Altcode:
We study subsurface flows measured with a ring-diagram analysis of GONG
high-resolution Doppler data. In previous studies, we have focused on
the relationship between active regions and subsurface flows associated
with them. Synoptic subsurface flow maps show also large-scale patterns
that are not obviously associated with active regions. It is unknown
whether these flow patterns correlate with any large-scale magnetic
features. Here, we explore whether there is a relationship between
subsurface flows and coronal features. We analyze synoptic maps of
subsurface flows covering 18 Carrington rotations during the years
2006 and 2007 (CR 2038-2055). Long-lived coronal holes are present
during this epoch at low latitudes, which are accessible by ring-diagram
analysis of GONG data. We compare subsurface flow maps with EIT synoptic
maps of EUV images at 195A (http://sun.stanford.edu/synop/EIT/) and
will present the latest results.
---------------------------------------------------------
Title: Temporal Variation of Subsurface Flows in Active Regions
Authors: Komm, Rudolf W.; Jain, K.; Tripathy, S. C.; Gonzalez
Hernandez, I.; Hill, F.
2012shin.confE.121K Altcode:
We apply the ring-diagram technique to study the temporal variation of
horizontal velocity in sub-photospheric layers beneath active regions
as they move across the solar disk. We categorize these active regions
on the basis of their dynamical characteristics and investigate
how velocity components beneath rotating sunspots differ from that
in non-rotating sunspots. Our study clearly shows that there is a
singnificant temporal variation in depth profiles of both zonal and
meridional components in active regions with rotating sunspots while
these variations remain small for non-rotating sunspots.
---------------------------------------------------------
Title: Coronal relative magnetic helicities and subsurface kinetic
helicities of active regions
Authors: Petrie, Gordon; Komm, Rudolf; Amari, Tahar
2012shin.confE.117P Altcode:
Excess of helicity in coronal magnetic structures has often been
linked to their instability and eruption. Moreover, active regions
associated with subphotospheric patterns of strong subsurface kinetic
helicity have been found to be more flare productive. How are these
atmospheric magnetic and subsurface fluid helicities related? Using
SOLIS vector magnetic magnetograms, the XTRAPOL nonlinear force-free
field extrapolation code and GONG helioseismic data we study the coronal
relative magnetic helicities of a set of active regions in combination
with their subsurface kinetic helicities in order to better understand
the transport processes of helicity in solar activity.
---------------------------------------------------------
Title: The Evolution of Large-Scale Subsurface Flow Patterns in
the Sun
Authors: Bogart, Richard S.; Baldner, C. S.; Basu, S.; Burtseva, O.;
Gonzalez-Hernandez, I.; Haber, D. A.; Hill, F.; Howe, R.; Jain, K.;
Komm, R. W.; Rabello-Soares, M. C.; Tripathy, S.
2012AAS...22020509B Altcode:
Ring-diagram analysis permits us to infer large-scale flow fields at the
photosphere and down to depths of about 0.95 R. We present comparisons
of the mean zonal and meridional velocity profiles determined from
uniform analysis techniques applied to three observational data sets,
those from the SDO/HMI and SOHO/MDI missions and the GONG project,
over the last 18 years. We pay special attention to measurements
obtained during the summer of 2010, when observations from all three
observatories were available. We discuss systematic effects affecting
the individual datasets in order to analyse evolution of global flows
over the time scale of the solar cycle.
---------------------------------------------------------
Title: Data From The HMI Ring-Diagram Pipelines
Authors: Bogart, Richard S.; Baldner, C. S.; Basu, S.;
Gonzalez-Hernandez, I.; Haber, D. A.; Hill, F.; Howe, R.; Jain, K.;
Komm, R. W.; Rabello-Soares, M. C.; Tripathy, S.
2012AAS...22020512B Altcode:
The HMI data pipeline for measurement of sub-surface flows with
ring-diagram analysis has been running for nearly two years, and
virtually all HMI Doppler data have been analyzed. Nearly 5 million
local-area power spectra have been produced and fitted for regions of
various sizes, and inversions for the depth structure of flows have
been performed for over 150,000 of the larger regions. The pipeline
for determination of the sub-surface thermal structure is still under
active development, with test results for a number of strong active
regions currently available for analysis. We describe the ring-diagram
pipelines, report on their performance, describe the data products
available, and discuss outstanding problems and issues for further
development.
---------------------------------------------------------
Title: Latest Results on the Torsional Oscillation and Solar Cycle 25
Authors: Hill, Frank; Howe, R.; Schou, J.; Thompson, M.; Larson, T.;
Komm, R.
2012AAS...22012302H Altcode:
The Torsional Oscillation in the Sun is a zonal (East-West) flow
that is slightly faster than the background differential rotation
profile. The location of this flow slowly migrates in latitude over
a period of several years. There are two branches of the flow: an
equatorward branch that underlies the active regions, and a poleward
branch. The timing of the equatorward migration is correlated with the
timing of the solar cycle such that the activity for a cycle appears
when the center of the flow reaches latitude 25 degrees. In addition,
the poleward branch appears about 12 years prior to the activity for
a cycle. Thus we should have observed the onset of Cycle 25 in 2008,
but did not. This poster will update the observations to 2012, and
present a new analysis that shows that the Cycle 25 flow appeared
in 2010, but was hidden by a change in the background differential
rotation profile. These results suggest that the next minimum will be
two years longer than average, and that Cycle 25 will begin in 2022.
---------------------------------------------------------
Title: Vorticity of Subsurface Flows of Emerging and Decaying
Active Regions
Authors: Komm, R.; Howe, R.; Hill, F.
2012SoPh..277..205K Altcode:
We study the temporal variation of the vorticity of subsurface flows
of 828 active regions and 977 quiet regions. The vorticity of these
flows is derived from measured subsurface velocities. The horizontal
flows are determined by analyzing high-resolution Global Oscillation
Network Group Doppler data with ring-diagram analysis covering
a range of depths from the surface to about 16 Mm. The vertical
velocity component is derived from the divergence of the measured
horizontal flows using mass conservation. We determine the change in
unsigned magnetic flux density during the disk passage of each active
region using Michelson Doppler Imager (MDI) magnetograms binned to
the ring-diagram grid with centers spaced by 7.5° ranging ± 52.5°
in latitude and central meridian distance with an effective diameter
of 15° after apodization. We then sort the data by their flux change
from decaying to emerging flux and divide the data into five subsets
of equal size. We find that the vorticity of subsurface flows increases
during flux emergence and decreases when active regions decay. For flux
emergence, the absolute values of the zonal and meridional vorticity
components show the most coherent variation with activity, while for
flux decrease the strongest signature is in the absolute values of the
meridional and vertical vorticity components. The temporal variation of
the enstrophy (residual vorticity squared) is thus a good indicator for
either flux increase or decrease. There are some indications that the
increase in vorticity during flux emergence happens about a day later
at depths below about 8 Mm compared to layers shallower than about
4 Mm. This timing difference might imply that the vorticity signal
analyzed here is caused by the interaction between magnetic flux and
turbulent flows near the solar surface. There are also hints that the
vorticity decrease during flux decay begins about a day earlier at
layers deeper than about 8 Mm compared to shallower ones. However, the
timing difference between the change at different depths is comparable
to the time step of the analysis.
---------------------------------------------------------
Title: The Global Context of Solar Activity During the Whole
Heliosphere Interval Campaign
Authors: Webb, D. F.; Cremades, H.; Sterling, A. C.; Mandrini, C. H.;
Dasso, S.; Gibson, S. E.; Haber, D. A.; Komm, R. W.; Petrie, G. J. D.;
McIntosh, P. S.; Welsch, B. T.; Plunkett, S. P.
2011SoPh..274...57W Altcode:
The Whole Heliosphere Interval (WHI) was an international observing and
modeling effort to characterize the 3-D interconnected "heliophysical"
system during this solar minimum, centered on Carrington Rotation
2068, March 20 - April 16, 2008. During the latter half of the WHI
period, the Sun presented a sunspot-free, deep solar minimum type
face. But during the first half of CR 2068 three solar active regions
flanked by two opposite-polarity, low-latitude coronal holes were
present. These departures from the quiet Sun led to both eruptive
activity and solar wind structure. Most of the eruptive activity,
i.e., flares, filament eruptions and coronal mass ejections (CMEs),
occurred during this first, active half of the interval. We determined
the source locations of the CMEs and the type of associated region,
such as active region, or quiet sun or active region prominence. To
analyze the evolution of the events in the context of the global solar
magnetic field and its evolution during the three rotations centered
on CR 2068, we plotted the CME source locations onto synoptic maps of
the photospheric magnetic field, of the magnetic and chromospheric
structure, of the white light corona, and of helioseismological
subsurface flows. Most of the CME sources were associated with the
three dominant active regions on CR 2068, particularly AR 10989. Most
of the other sources on all three CRs appear to have been associated
with either isolated filaments or filaments in the north polar crown
filament channel. Although calculations of the flux balance and
helicity of the surface magnetic features did not clearly identify a
dominance of one region over the others, helioseismological subsurface
flows beneath these active regions did reveal a pronounced difference
among them. These preliminary results suggest that the "twistedness"
(i.e., vorticity and helicity) of subsurface flows and its temporal
variation might be related to the CME productivity of active regions,
similar to the relationship between flares and subsurface flows.
---------------------------------------------------------
Title: Solar dynamics at high latitudes and deep in the convection
zone
Authors: Gonzalez Hernandez, I.; Komm, R.; Howe, R.; Kholikov, S.;
Hill, F.; Bogart, R. S.; Rabello-Soares, M.
2011AGUFMSH33A2041G Altcode:
For a long time, helioseismic inferences have provided a window into
the solar interior. In the last two decades, the development of local
helioseismology tools has extended the capability of helioseismology
by allowing the study of localized structure changes and dynamics. In
particular, it has revealed the subsurface flows and its variation
throughout the solar cycle. Both the torsional oscillation and the
meridional circulation present interesting patterns leading to solar
cycle 24, which continue during the onset of this cycle. Yet, many
questions related to the behavior of such flows at high latitudes as
well as deep down in the convection zone that are key to solar dynamo
models remain unanswered. Long-term helioseismic studies using both
Global Oscillation Network Group (GONG) and Michelson Doppler Imager
(MDI)data have uncovered the difficulties of properly interpreting data
far from disk center due to systematics and solar effects, limiting
the helioseismic inferences to only specific areas. The Helioseismic
and Magnetic Imager (HMI) instrument on board the Solar Dynamics
Observatory (SDO) provides an excellent opportunity to explore the
until now unreachable territories thanks to its higher resolution. In
addition, newly available artificial data sets offer an unprecedented
opportunity for disentangling and modeling the different effects. We
present here a review of the main features observed in the subsurface
flows in the recent years and discuss future plans to extend the
inferences at higher latitudes and deep down in the convection zone.
---------------------------------------------------------
Title: Multi-spectral Analysis of Heliseismic Acoustic Mode Parameters
Authors: Jain, Kiran; Tripathy, S.; Basu, S.; Bogart, R.; Gonzalez
Hernandez, I.; Hill, F.; Howe, R.; Kholikov, S.; Komm, R.
2011sdmi.confE..33J Altcode:
Simultaneous measurements at different wavelengths from SDO offer
the prospect of studying the sensitivity of helioseismic inferences
to the choice of observing height both in quiet-Sun and magnetically
active regions. In this poster, we present comparison of mode parameters
obtained with different observables, quantify differences, and interpret
results in the context of the formation height and the anticipated phase
relationships between the oscillations at those heights. This work is
expected to enhance our understanding of the excitation and damping
of the oscillations and the uncertainties in helioseismic inferences.
---------------------------------------------------------
Title: Subsurface kinetic helicity of flows near active regions
Authors: Komm, R.; Jain, K.; Petrie, G.; Pevtsov, A.; González
Hernández I.; Hill, F.
2011sdmi.confE..68K Altcode:
We study the temporal variation of subsurface flows associated with
emerging and decaying active regions on the Sun. We measure the
subsurface flows analyzing GONG high-resolution Doppler data with
ring-diagram analysis. We can detect the emergence of magnetic flux
in these flows when averaging over a sufficiently large sample. In a
previous study, we have found that emerging flux has a faster rotation
than the ambient fluid and pushes it up, as indicated by enhanced
vertical velocity and faster-than-average zonal flow. Here, we show
that the kinetic helicity density of subsurface flows increases when
new flux emerges and decreases when flux decays.
---------------------------------------------------------
Title: Latest Results Found With Ring-Diagram Analysis
Authors: Haber, D. A.; Baldner, C.; Basu, S.; Bogart, R. S.;
González-Hernández, I.; Hill, F.; Howe, R.; Jain, K.; Komm, R. W.;
. Rabello-Soares, C.; Pinkerton, S.; Tripathy, S.
2011sdmi.confE..51H Altcode:
This talk will mainly be a preview of the posters generated by
the HMI Rings Team on large-scale (meridional and zonal) flows;
characterizations of active regions at various stages of evolution using
data from AIA as well as from HMI; systematic changes in frequencies,
flows, and other fitted parameters as a function of disk placement,
underlying magnetism, B angle, etc.; and the status of the Rings
pipeline. It will also include any new ring-diagram results from GONG
and MDI.
---------------------------------------------------------
Title: Large-scale flows from HMI using the ring-diagram pipeline
Authors: Komm, R.; González Hernández, I.; Hill, F.; Bogart, R. S.;
Rabello-Soares, M. C.
2011sdmi.confE..72K Altcode:
We determine the zonal and meridional flows in subsurface layers derived
from HMI Doppler data processed with the HMI ring-diagram pipeline. We
analyze subsurface flow measurements obtained during Carrington rotation
2097 to 2113. We are especially interested in flows at latitudes of
60 degree and higher, since previous observations have been limited
to lower latitudes (using local helioseismic techniques). Systematic
effects, such as B0-angle variations, have to be taken into account
to lead to reliable flow estimates at high latitudes. We will present
the latest results.
---------------------------------------------------------
Title: Subsurface flows associated with rotating sunspots
Authors: Jain, Kiran; Komm, Rudolf; Hernández, Irene González;
Tripathy, Sushant C.; Hill, Frank
2011IAUS..273..356J Altcode: 2011arXiv1107.5032J
In this paper, we compare components of the horizontal flow below
the solar surface in and around regions consisting of rotating and
non-rotating sunspots. Our analysis suggests that there is a significant
variation in both components of the horizontal flow at the beginning
of sunspot rotation as compared to the non-rotating sunspot. The flows
in surrounding areas are in most cases relatively small. However,
there is a significant influence of the motion on flows in an area
closest to the sunspot rotation.
---------------------------------------------------------
Title: Solar subsurface flows of active regions: flux emergence and
flare activity
Authors: Komm, Rudolf; Howe, Rachel; Hill, Frank; Jain, Kiran
2011IAUS..273..148K Altcode:
We study the temporal variation of subsurface flows associated
with active regions within 16 Mm of the solar surface. We have
analyzed the subsurface flows of nearly 1000 active and quiet regions
applying ring-diagram analysis to Global Oscillation Network Group
(GONG) Dopplergram data. We find that newly emerging active regions
are characterized by enhanced upflows and fast zonal flows in the
near-surface layers, as expected for a flux tube rising from deeper
layers of the convection zone. The subsurface flows associated with
strong active regions are highly twisted, as indicated by their large
vorticity and helicity values. The dipolar pattern exhibited by the
zonal and meridional vorticity component leads to the interpretation
that these subsurface flows resemble vortex rings, when measured on
the spatial scales of the standard ring-diagram analysis.
---------------------------------------------------------
Title: Helioseismic Observations of Solar Convection Zone Dynamics
Authors: Hill, Frank; Howe, Rachel; Komm, Rudi; Hernández, Irene
González; Kholikov, Shukur; Leibacher, John
2011IAUS..271...15H Altcode:
The large-scale dynamics of the solar convection zone have been inferred
using both global and local helioseismology applied to data from the
Global Oscillation Network Group (GONG) and the Michelson Doppler
Imager (MDI) on board SOHO. The global analysis has revealed temporal
variations of the “torsional oscillation” zonal flow as a function of
depth, which may be related to the properties of the solar cycle. The
horizontal flow field as a function of heliographic position and depth
can be derived from ring diagrams, and shows near-surface meridional
flows that change over the activity cycle. Time-distance techniques
can be used to infer the deep meridional flow, which is important for
flux-transport dynamo models. Temporal variations of the vorticity can
be used to investigate the production of flare activity. This paper
summarizes the state of our knowledge in these areas.
---------------------------------------------------------
Title: Subsurface kinetic helicity of flows near active regions
Authors: Komm, Rudolf; Jain, K.; Petrie, G.; Pevtsov, A.; González
Hernández, I.; Hill, F.
2011shin.confE.142K Altcode:
We study the flows in the upper solar convection zone determined from
GONG data using the standard dense-pack ring-diagram analysis and derive
daily and synoptic maps of the velocity components. We also calculate
the vorticity and the kinetic helicity density of the flows. Previous
studies have shown that the vorticity is enhanced near locations
of active regions and that the kinetic helicity density associated
with active regions correlates well with the X-ray flare intensity of
active regions. These fluid dynamics descriptors are thus promising
indicators for investigating the relation between active regions and
associated subsurface flows. Here, we focus on the temporal evolution of
subsurface kinetic helicity density during flux emergence and decay. We
will present the latest results.
---------------------------------------------------------
Title: Large-scale Zonal Flows During the Solar Minimum -- Where Is
Cycle 25?
Authors: Hill, Frank; Howe, R.; Komm, R.; Christensen-Dalsgaard, J.;
Larson, T. P.; Schou, J.; Thompson, M. J.
2011SPD....42.1610H Altcode: 2011BAAS..43S.1610H
The so-called torsional oscillation is a pattern of migrating zonal flow
bands that move from mid-latitudes towards the equator and poles as the
magnetic cycle progresses. Helioseismology allows us to probe these
flows below the solar surface. The prolonged solar minimum following
Cycle 23 was accompanied by a delay of 1.5 to 2 years in the migration
of bands of faster rotation towards the equator. During the rising phase
of Cycle 24, while the lower-level bands match those seen in the rising
phase of Cycle 23, the rotation rate at middle and higher latitudes
remains slower than it was at the corresponding phase in earlier cycles,
perhaps reflecting the weakness of the polar fields. In addition,
there is no evidence of the poleward flow associated with Cycle 25. We
will present the latest results based on nearly sixteen years of global
helioseismic observations from GONG and MDI, with recent results from
HMI, and discuss the implications for the development of Cycle 25.
---------------------------------------------------------
Title: Subsurface Vorticity of Flaring versus Flare-Quiet Active
Regions
Authors: Komm, R.; Ferguson, R.; Hill, F.; Barnes, G.; Leka, K. D.
2011SoPh..268..389K Altcode: 2010SoPh..tmp...78K
We apply discriminant analysis to 1023 active regions and their
subsurface-flow parameters, such as vorticity and kinetic helicity
density, with the goal of distinguishing between flaring and non-flaring
active regions. We derive synoptic subsurface flows by analyzing GONG
high-resolution Doppler data with ring-diagram analysis. We include
magnetic-flux values in the discriminant analysis derived from NSO
Kitt Peak and SOLIS synoptic maps binned to the same spatial scale
as the helioseismic analysis. For each active region, we determine
the flare information from GOES and include all flares within 60°
central meridian distance to match the coverage of the ring-diagram
analysis. The subsurface-flow characteristics improve the ability to
distinguish between flaring and non-flaring active regions. For the C-
and M-class flare category, the most important subsurface parameter
is the so-called structure vorticity, which estimates the horizontal
gradient of the horizontal-vorticity components. The no-event skill
score, which measures the improvement over predicting that no events
occur, reaches 0.48 for C-class flares and 0.32 for M-class flares, when
the structure vorticity at three depths combined with total magnetic
flux are used. The contributions come mainly from shallow layers within
about 2 Mm of the surface and layers deeper than about 7 Mm.
---------------------------------------------------------
Title: Subsurface Velocity of Emerging and Decaying Active Regions
Authors: Komm, R.; Howe, R.; Hill, F.
2011SoPh..268..407K Altcode: 2011SoPh..tmp....6K
We study the temporal variation of subsurface flows of 828 active
regions and 977 quiet regions. The horizontal flows cover a range of
depths from the surface to about 16 Mm and are determined by analyzing
Global Oscillation Network Group high-resolution Doppler data with
ring-diagram analyses. The vertical velocity component is derived
from the divergence of the measured horizontal flows using mass
conservation. For comparison, we analyze Michelson Doppler Imager
(MDI) Dynamics Run data covering 68 active regions common to both
data sets. We determine the change in unsigned magnetic flux during
the disk passage of each active region using MDI magnetograms binned
to the ring-diagram grid. We then sort the data by their flux change
from decaying to emerging flux and divide the data into five subsets
of equal size. We find that emerging flux has a faster rotation than
the ambient fluid and pushes it up, as indicated by enhanced vertical
velocity and faster-than-average zonal flow. After active regions
are formed, downflows are established within two days of emergence in
shallow layers between about 4 and 10 Mm. Emerging flux in existing
active regions shows a similar scenario, where the upflows at depths
greater than about 10 Mm are enhanced and the already established
downflows at shallower depths are weakened. When active regions decay,
the corresponding flow pattern disappears as well; the zonal flow slows
down to values comparable to that of quiet regions and the upflows
become weaker at deeper layers. The residual meridional velocity is
mainly poleward and shows no obvious variation. The magnitude of the
residual velocity, defined as the sum of the squares of the residual
velocity components, increases with increasing magnetic flux and
decreases with decreasing flux.
---------------------------------------------------------
Title: Solar-cycle variation of zonal and meridional flow
Authors: Komm, R.; Howe, R.; Hill, F.; González Hernández, I.;
Haber, D.
2011JPhCS.271a2077K Altcode:
We study the variation with the solar cycle of the zonal and meridional
flows in the near-surface layers of the solar convection zone. We have
analyzed MDI Dynamics-Program data with ring-diagram analysis covering
the rising phase of cycle 23, while the analyzed GONG high-resolution
data cover the maximum and declining phase of cycle 23. For the zonal
flow, the migration with latitude of the flow pattern is apparent in
the deeper layers, while for the meridional flow, a migration with
latitude is apparent only in the layers close to the surface. The
faster-than-average bands of the zonal flow associated with the new
cycle are clearly visible. Similarly, a pattern related to the new
cycle appears in the residual meridional flow. We also study the flow
differences between the hemispheres during the course of the solar
cycle. The difference pattern of the meridional flow is slanted in
latitude straddling the faster-than-average band of the torsional
oscillation pattern in the zonal flow. The difference pattern of the
zonal flow, on the other hand, resembles the cycle variation of the
meridional flow. In addition, the meridional flow during the minimum
of cycle 23/24 appears to be slightly stronger than during the previous
minimum of cycle 22/23.
---------------------------------------------------------
Title: Solar flares and temporal changes in subsurface vorticity
measurements
Authors: Komm, R.; Jain, K.; Reinard, A.; Howe, R.; Hill, F.
2011JPhCS.271a2019K Altcode:
We derive the kinetic helicity density of subsurface flows applying
ring-diagram analysis to Global Oscillation Network Group (GONG)
data. Here, we focus on flows derived from times series of 8 hours
and compare them to daily values for a high- and a low-activity
sample. Compared with daily values, the horizontal flows derived from
8-hour time series are reasonable near disk center and less reliable
near the limb. Also, the errors are larger for shorter time series. A
dipolar helicity pattern is present in the flows derived from 8-hour
and 24-hour time series of flare-productive active region 10808. For
the quiet-Sun sample, the subsurface kinetic helicity is considerably
smaller without any pattern.
---------------------------------------------------------
Title: First Global Rotation Inversions of HMI Data
Authors: Howe, R.; Larson, T. P.; Schou, J.; Hill, F.; Komm, R.;
Christensen-Dalsgaard, J.; Thompson, M. J.
2011JPhCS.271a2061H Altcode:
We present the first 2-dimensional global rotational inversions of
medium-degree p-mode data from the Helioseismic and Magnetic Imager,
and compare the results with inversions of Michelson Doppler Imager
data for the same time period. The inferred rotation profiles show
good agreement between the two instruments.
---------------------------------------------------------
Title: Comparison of HMI Dopplergrams with GONG and MDI data
Authors: Howe, R.; Jain, K.; Hill, F.; Komm, R.; González Hernández,
I.; Bogart, R.
2011JPhCS.271a2060H Altcode:
We compare sample Dopplergrams from the Helioseismic and Magnetic
Imager, the Michelson Doppler Imager and the Global Oscillation Network
Group. Each instrument has a distinct static velocity patterm across
the disk; once this has been subtracted and the images interpolated
to a common grid, the agreement is satisfactory.
---------------------------------------------------------
Title: The torsional oscillation and the new solar cycle
Authors: Howe, R.; Hill, F.; Komm, R.; Christensen-Dalsgaard, J.;
Larson, T. P.; Schou, J.; Thompson, M. J.; Ulrich, R.
2011JPhCS.271a2074H Altcode:
We present updated observations of the pattern of migrating solar
zonal flows known as the torsional oscillation, covering 15 years of
helioseismic measurements with GONG and MDI and 30 years of surface
Doppler observations from Mount Wilson. We compare the behavior of the
flows during the extended solar minimum following Cycle 23 with that in
earlier minima. We demonstrate that the timing of the migration of the
zonal flow belts may be of some use in predicting the start of the new
cycle. We also note that the behavior of the high-latitude part of the
pattern currently differs from that seen early in the previous cycle,
with the high-latitude poleward-migrating branch still not established.
---------------------------------------------------------
Title: Local helioseismology of sunspot regions: Comparison of
ring-diagram and time-distance results
Authors: Kosovichev, A. G.; Basu, S.; Bogart, R.; Duvall, T. L., Jr.;
Gonzalez-Hernandez, I.; Haber, D.; Hartlep, T.; Howe, R.; Komm, R.;
Kholikov, S.; Parchevsky, K. V.; Tripathy, S.; Zhao, J.
2011JPhCS.271a2005K Altcode: 2010arXiv1011.0799K
Local helioseismology provides unique information about the subsurface
structure and dynamics of sunspots and active regions. However,
because of complexity of sunspot regions local helioseismology
diagnostics require careful analysis of systematic uncertainties
and physical interpretation of the inversion results. We present new
results of comparison of the ring-diagram analysis and time-distance
helioseismology for active region NOAA 9787, for which a previous
comparison showed significant differences in the subsurface sound-speed
structure, and discuss systematic uncertainties of the measurements
and inversions. Our results show that both the ring-diagram and
time-distance techniques give qualitatively similar results, revealing
a characteristic two-layer seismic sound-speed structure consistent
with the results for other active regions. However, a quantitative
comparison of the inversion results is not straightforward. It must
take into account differences in the sensitivity, spatial resolution
and the averaging kernels. In particular, because of the acoustic
power suppression, the contribution of the sunspot seismic structure
to the ring-diagram signal can be substantially reduced. We show that
taking into account this effect reduces the difference in the depth
of transition between the negative and positive sound-speed variations
inferred by these methods. Further detailed analysis of the sensitivity,
resolution and averaging properties of the local helioseismology methods
is necessary for consolidation of the inversion results. It seems to
be important that both methods indicate that the seismic structure of
sunspots is rather deep and extends to at least 20 Mm below the surface,
putting constraints on theoretical models of sunspots.
---------------------------------------------------------
Title: Rotation-rate variations at the tachocline: An update
Authors: Howe, R.; Komm, R.; Hill, F.; Christensen-Dalsgaard, J.;
Larson, T. P.; Schou, J.; Thompson, M. J.; Toomre, J.
2011JPhCS.271a2075H Altcode:
After 15 years of GONG and MDI observations of the solar interior
rotation, we revisit the issue of variations in the rotation rate near
the base of the convection zone. The 1.3-year period seen in the first
few years of the observations disappeared after 2000 and has still
not returned. On the other hand, the agreement between GONG and MDI
observations suggests that variations seen in this region have some
solar origin, whether a true rotation-rate change or possibly mere
stochastic variation; we present a numerical experiment supporting
this contention.
---------------------------------------------------------
Title: A search for coherent structures in subsurface flows
Authors: Komm, R.; Howe, R.; Hill, F.
2011JPhCS.271a2065K Altcode:
We search for coherent patterns in horizontal subsurface flows obtained
from Global Oscillation Network Group (GONG) Dopplergram data using
ring-diagram analysis. The existence of north-south aligned downflow
patterns near the equator has been predicted by numerical models of the
solar convection zone. We analyze time series of daily flow measurements
near the solar equator focusing on the vertical velocity component and
the derivative of the zonal flow in the east-west direction. To reduce
the influence of surface magnetic activity, we analyze observations
during the minimum phase of the solar cycle. We find coherent
equatorial structures that persist for several days in the zonal
velocity derivative and the vertical velocity component and are not
associated with surface magnetic activity. We use a cross-correlation
analysis to measure the strength and rotation rate of these coherent
patterns. Our results are consistent with other studies that have
observed north-south aligned patterns in supergranulation.
---------------------------------------------------------
Title: Ring-diagram parameter comparisons for GONG, MDI and HMI
Authors: Howe, R.; Tripathy, S.; González Hernández, I.; Komm, R.;
Hill, F.; Bogart, R.; Haber, D.
2011JPhCS.271a2015H Altcode:
We examine the differences between ring-diagram mode frequency estimates
from samples of Global Oscillation Network Group [GONG], Michelson
Doppler Imager [MDI] and Helioseismic and Magnetic Imager [HMI] data,
and find that different instruments and analysis pipelines do result
in small systematic differences which may not be uniform across the
solar disk.
---------------------------------------------------------
Title: Low-degree helioseismology with AIA
Authors: Howe, R.; Hill, F.; Komm, R.; Broomhall, A. -M.; Chaplin,
W. J.; Elsworth, Y.
2011JPhCS.271a2058H Altcode:
We form unresolved-sun time series from the 1600 and 1700 Angstrom
images produced by the Atmospheric Imaging Assembly, and find a clean
low-degree p-mode spectrum at each wavelength. The time series and
spectra are compared with Doppler velocity and continuum intensity
time series from the Helioseismic and Magnetic Imager and velocity
series from the Birmingham Solar Oscillation Network. The UV data
have a slight phase shift with respect to the velocity, and show more
sensitivity to high-frequency and less to low-frequency modes. Unlike
the HMI (visible) continuum observations, the UV spectra show little
or no granulation noise at low frequencies and thus potentially allow
more low-frequency modes to be recovered. These results suggest that
asteroseismology at near-UV wavelengths should be very feasible and
even an improvement on visible-wavelength intensity measurements,
at least in low-activity stars.
---------------------------------------------------------
Title: Meridional Flow Observations: Implications for the current
Flux Transport Models
Authors: González Hernández, Irene; Komm, Rudolf; Kholikov, Shukur;
Howe, Rachel; Hill, Frank
2011JPhCS.271a2073G Altcode:
Meridional circulation has become a key element in the solar dynamo
flux transport models. Available helioseismic observations from several
instruments, Taiwan Oscillation Network (TON), Global Oscillation
Network Group (GONG) and Michelson Doppler Imager (MDI), have made
possible a continuous monitoring of the solar meridional flow in the
subphotospheric layers for the last solar cycle, including the recent
extended minimum. Here we review some of the meridional circulation
observations using local helioseismology techniques and relate them
to magnetic flux transport models.
---------------------------------------------------------
Title: Testing the GONG ring-diagram pipeline with HMI Dopplergrams
Authors: Jain, Kiran; Tripathy, S. C.; González Hernández, I.;
Kholikov, S.; Hill, F.; Komm, R.; Bogart, R.; Haber, D.
2011JPhCS.271a2017J Altcode:
The GONG ring-diagram pipeline was developed to analyze GONG+
Dopplergrams in order to extract information about solar subsurface
flows and has been extensively tested for this purpose. Here we present
preliminary results obtained by analyzing the HMI Dopplergrams with
the GONG pipeline and compare them with those obtained from the HMI
ring-diagram pipeline.
---------------------------------------------------------
Title: Helioseismic Studies of a Sunspot using HMI Data
Authors: Tripathy, S. C.; Jain, K.; Gonzalez Hernandez, I.; Komm,
R.; Hill, F.; McManus, S.; Bogart, R.; Rabello-Soares, M. C.; Basu,
S.; Baldner, C.; Haber, D. A.
2010AGUFMSH11A1603T Altcode:
We study the mode parameters and sub-surface properties of the sunspot
in NOAA active region 10093 during its disk passage between August
6-14, 2010. This is one of the major active regions recorded so far
during the cycle 24 and continuous observations are available from the
Helioseismic and Magnetic Imager (HMI). We will present the results
using the HMI data processed through the HMI ring-diagram pipeline and
compare those obtained with the GONG pipeline. We will also present
results by analyzing the GONG observations through GONG pipeline.
---------------------------------------------------------
Title: The acoustic and magnetic solar cycle
Authors: Komm, R.
2010AN....331..873K Altcode:
The frequency variation of global modes provides information about
the solar structure and the rotation rate of the solar interior. The
solar-cycle variation of the rotation rate, the so-called torsional
oscillation, extends throughout the convection zone. Alternating bands
of faster- and slower-than-average rotation move from mid-latitudes
toward the equator during the solar cycle. The variation of the
meridional flow, measured with local helioseismic techniques, is
of similar amplitude. The zonal- and the meridional-flow variation
related to a new cycle are noticeable years before magnetic activity
of the new cycle is present at the surface. The sound-speed variations
show a small, but significant variation with the solar cycle near the
base of the convection zone, which might be interpreted as a change
in magnetic field strength. The mode characteristics, such as acoustic
radius and frequency shifts, provide information about changes in the
near-surface layers, where acoustic modes are excited.
---------------------------------------------------------
Title: Predictions of active region flaring probability using
subsurface helicity measurements
Authors: Reinard, A. A.; Komm, R.; Hill, F.
2010AGUFMSH43B1818R Altcode:
Solar flares are responsible for a number of hazardous effects on the
earth such as disabling high-frequency radio communications, interfering
with GPS measurements, and disrupting satellites. However, forecasting
flare occurrence is currently very difficult. One possible means for
predicting flare occurrence lies in helioseismology, i.e. analysis of
the region below the active region for signs of an impending flare. Time
series helioseismic data collected by the Global Oscillation Network
Group (GONG) has been analyzed for a subset of active regions that
produce large flares and a subset with very high magnetic field strength
that produce no flares. A predictive parameter has been developed and
analyzed using discriminant analysis as well as traditional forecasting
tools such as the Heidke skill score. Preliminary results show that
this parameter predicts the flaring probability of an active region
2-3 days in advance with a relatively high degree of success.
---------------------------------------------------------
Title: Solar Subsurface Flows derived with Ring-Diagram Analysis
Authors: Komm, R.; Howe, R.; Gonzalez Hernandez, I.; Hill, F.; Haber,
D. A.
2010AGUFM.S32A..06K Altcode:
Local helioseismology makes it possible to map the horizontal flows in
the outer convection zone of the Sun. For the ring-diagram analysis,
we start from full-disk Doppler velocity images of the Sun and track a
region at about the surface rotation rate for a period of a day. Each
tracked data cube of velocity is then Fourier transformed. The resulting
3-D power spectrum shows structures that correspond to the acoustic
waves. These structures appear as rings in a 2-D plane at a given
temporal frequency. Since acoustic waves are advected by subsurface
flows, the velocity of these horizontal flows can be determined from
the offset of the ring centers. Using ring-diagram analysis of Doppler
images of the Sun obtained with the ground-based Global Oscillation
Network Group (GONG) and the Michelson Doppler Imager (MDI) instrument
on board the Solar and Heliospheric Observatory spacecraft (SOHO),
we are studying, for example, the large-scale subsurface flows (E-W
rotation and N-S meridional flow) and their variation with the solar
cycle of magnetic activity. We are also studying subsurface flows
associated with active regions on the Sun focusing on their evolution
(emergence and decay). In addition, we have started to analyze data from
the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics
Observatory (SDO) spacecraft. We will present some recent results.
---------------------------------------------------------
Title: Subsurface Flows from SDO, SOHO, and GONG (Invited)
Authors: Komm, R.
2010AGUFMSH21C..01K Altcode:
Active regions (magnetic fields, in general) provide a point of
contact between helioseismic studies of the upper convection zone
and studies of the solar atmosphere as well as the whole heliophysics
system. Helioseismic observations allow us to derive subsurface flows
and sound-speed structures associated with active regions. For example,
recent helioseismic studies have shown that the emergence and decay of
active regions can be observed as changes in subsurface flow patterns
and sound-speed variations on time scales of days. SDO will allow us
to study these phenomena in unprecedented detail and the comparison
with GONG and SOHO data sets will provide cross-calibration. On time
scales of years, the intercomparison of the SDO, SOHO, and GONG data
sets will allow us to study large-scale subsurface flows and their
variation during two (or more) solar cycles. The cycle-related patterns
of the zonal and the meridional flow appeared before magnetic activity
of cycle 24 was present at the solar surface. With SDO and GONG, we
will be able to observe these patterns during the current cycle and
compare them with the patterns of the previous one observed by GONG
and SOHO. Subsurface flows of active regions are also highly twisted
and this appears to be related to the flare production of active
regions. Helicity-loaded fields are very probably responsible for
the most geoeffective solar phenomena such as coronal mass ejections
and flares. It is thus of interest, for example, to find out how
the kinetic helicity of subsurface flows derived from SDO and GONG
Dopplergrams relates to the magnetic helicity of flux tubes derived
from SDO vector magnetograms. I will discuss some of the plans for
intercomparison of SDO data sets with SOHO and GONG data.
---------------------------------------------------------
Title: Global and Local Helioseismology from HMI and AIA
Authors: Howe, R.; Komm, R.; Gonzalez Hernandez, I.; Jain, K.; Hill,
F.; Haber, D. A.; Bogart, R.
2010AGUFMSH11A1601H Altcode:
Data from the HMI [Helioseismic and Magnetic Imager] and AIA
[Atmospheric Imaging Assembly] instruments aboard the Solar Dynamics
observatory have been available for some months. We present some
preliminary results from these data, including subsurface flow maps
and activity-related local mode parameter shifts from helioseismic
ring-diagram analysis of HMI data, HMI helioseismic sensing of the far
side of the Sun, and low-degree p-mode spectra from the high-photosphere
bands on AIA as well as from HMI velocity and continuum intensity. The
results will be compared with those from the Michelson Doppler Imager
and the Global Oscillation Network Group.
---------------------------------------------------------
Title: Modeling the Subsurface Structure of Sunspots
Authors: Moradi, H.; Baldner, C.; Birch, A. C.; Braun, D. C.; Cameron,
R. H.; Duvall, T. L.; Gizon, L.; Haber, D.; Hanasoge, S. M.; Hindman,
B. W.; Jackiewicz, J.; Khomenko, E.; Komm, R.; Rajaguru, P.; Rempel,
M.; Roth, M.; Schlichenmaier, R.; Schunker, H.; Spruit, H. C.;
Strassmeier, K. G.; Thompson, M. J.; Zharkov, S.
2010SoPh..267....1M Altcode: 2009arXiv0912.4982M; 2010SoPh..tmp..171M
While sunspots are easily observed at the solar surface, determining
their subsurface structure is not trivial. There are two main
hypotheses for the subsurface structure of sunspots: the monolithic
model and the cluster model. Local helioseismology is the only means
by which we can investigate subphotospheric structure. However, as
current linear inversion techniques do not yet allow helioseismology to
probe the internal structure with sufficient confidence to distinguish
between the monolith and cluster models, the development of physically
realistic sunspot models are a priority for helioseismologists. This
is because they are not only important indicators of the variety of
physical effects that may influence helioseismic inferences in active
regions, but they also enable detailed assessments of the validity of
helioseismic interpretations through numerical forward modeling. In
this article, we provide a critical review of the existing sunspot
models and an overview of numerical methods employed to model wave
propagation through model sunspots. We then carry out a helioseismic
analysis of the sunspot in Active Region 9787 and address the serious
inconsistencies uncovered by Gizon et al. (2009a, 2009b). We find that
this sunspot is most probably associated with a shallow, positive
wave-speed perturbation (unlike the traditional two-layer model)
and that travel-time measurements are consistent with a horizontal
outflow in the surrounding moat.
---------------------------------------------------------
Title: Using helioseismology to understand and predict the solar cycle
Authors: Hill, Frank; Komm, Rudi; Howe, Rachel; Gonzalez Hernandez,
Irene; Kholikov, Shukur; Leibacher, John
2010shin.confE.156H Altcode:
Helioseismology is now being used to investigate the subsurface
flows that are related to the solar cycle. The relevant flows are the
east-west zonal flows (torsional oscillation), and the north-south
meridional flows. This poster will summarize the relationship of the
timing of the solar cycle with the characteristics of the zonal and
meridional flows; as well as what we know about the nature of the deep
meridional flows that play a role in the dynamo.
---------------------------------------------------------
Title: What Solar Oscillation Tell Us About the Solar Minimum
Authors: Jain, K.; Tripathy, S. C.; Burtseva, O.; H´Ndez, I. G.;
Hill, F.; Howe, R.; Kholikov, S.; Komm, R.; Leibacher, J.
2010ASPC..428...57J Altcode: 2010arXiv1002.2411J
The availability of continuous helioseismic data for two consecutive
solar minima has provided a unique opportunity to study the changes
in the solar interior that might have led to this unusual minimum. We
present preliminary analysis of intermediate-degree mode frequencies in
the 3 mHz band during the current period of minimal solar activity and
show that the mode frequencies are significantly lower than those during
the previous activity minimum. Our analysis does not show any signature
of the beginning of cycle 24 until the end of 2008. In addition, the
zonal and meridional flow patterns inferred from inverting frequencies
also hint at a delayed onset of a new cycle. The estimates of travel
time are higher than the previous minimum confirming a relatively weak
solar activity during the current minimum.
---------------------------------------------------------
Title: Meridional Circulation During the Extended Solar Minimum:
Another Component of the Torsional Oscillation?
Authors: González Hernández, I.; Howe, R.; Komm, R.; Hill, F.
2010ApJ...713L..16G Altcode: 2010arXiv1003.1685G
We show here a component of the meridional circulation developing
at medium-high latitudes (40°-50°) before the new solar cycle
starts. Like the torsional oscillation of the zonal flows, this extra
circulation seems to precede the onset of magnetic activity at the solar
surface and moves slowly toward lower latitudes. However, the behavior
of this component differs from that of the torsional oscillation
regarding location and convergence toward the equator at the end
of the cycle. The observation of this component before the magnetic
regions appear at the solar surface has only been possible due to the
prolonged solar minimum. The results could settle the discussion as
to whether the extra component of the meridional circulation around
the activity belts, which has been known for some time, is or is not
an effect of material motions around the active regions.
---------------------------------------------------------
Title: Evidence That Temporal Changes in Solar Subsurface Helicity
Precede Active Region Flaring
Authors: Reinard, A. A.; Henthorn, J.; Komm, R.; Hill, F.
2010ApJ...710L.121R Altcode:
We report on the analysis of subsurface vorticity/helicity measurements
for flare producing and quiet active regions. We have developed a
parameter to investigate whether large, decreasing kinetic helicity
density commonly occurs prior to active region flaring. This new
parameter is effective at separating flaring and non-flaring active
regions and even separates among C-, M-, and X-class flare producing
regions. In addition, this parameter provides advance notice of flare
occurrence, as it increases 2-3 days before the flare occurs. These
results are striking on an average basis, though on an individual
basis there is still considerable overlap between flare associated
and non-flare associated values. We propose the following qualitative
scenario for flare production: subsurface rotational kinetic energy
twists the magnetic field lines into an unstable configuration,
resulting in explosive reconnection and a flare.
---------------------------------------------------------
Title: Subsurface Flow Properties of Flaring versus Flare-Quiet
Active Regions
Authors: Ferguson, R.; Komm, R.; Hill, F.; Barnes, G.; Leka, K. D.
2009ASPC..416..127F Altcode:
We apply discriminant analysis to 1009 active regions and their
subsurface flow parameters, such as vorticity and kinetic helicity
density, with the goal of distinguishing between flaring and non-flaring
active regions. Flow and flux variables lead to better classification
rates than a no-event prediction. The Heidke skill score, which measures
the improvement over predicting that no events occur, increases by
about 25% and 50% for C- and M-class flares when several subsurface
characteristics are included compared to using a single magnetic
flux measure.
---------------------------------------------------------
Title: Subsurface Zonal Flows of Active and Quiet Regions
Authors: Komm, R.; Howe, R.; Hill, F.; González Hernández, I.
2009ASPC..416..123K Altcode:
We study the zonal flow in solar subsurface layers, analyzing about
six years of GONG+ high-resolution Doppler data with a ring-diagram
analysis. We focus on the variation of the zonal flow with magnetic
activity over a range of depths from the surface to about 16 Mm. We
calculate the average zonal flow for a quiet- and an active-region
subset defined as dense-pack locations with an unsigned magnetic flux
less than 3.4 G and locations with greater than 65.0 G respectively. The
average zonal flow of active regions is about 4 ms<SUP>-1</SUP> faster
than the average flow of quiet regions and this difference increases
slightly with increasing depth at depths greater than about 5 Mm. The
difference shows no apparent pattern in time and latitude; it shows
no variation with the solar cycle.
---------------------------------------------------------
Title: Temporal Variation of Subsurface Flows of Active Regions
Authors: Komm, R.; Howe, R.; Hill, F.
2009ASPC..416..115K Altcode:
We study the temporal variation of subsurface flows associated with
955 active regions. The subsurface kinetic helicity density varies
with the magnetic flux and its values at deeper layers are correlated
with the total flare intensity. The average vertical velocity shows
a downflow at depths shallower than about 12 Mm and upflows at
greater depth. Daily ring-diagram measurements thus confirm previous
synoptic measurements. In addition we find, that at some depths, the
crosscorrelation between the vertical velocity and the unsigned magnetic
flux is positive at negative lag time. This implies that the temporal
variation of the vertical velocity might be a precursor of flux changes.
---------------------------------------------------------
Title: The internal dynamics and magnetism of the sun -- the
perspective from global helioseismology (Invited)
Authors: Howe, R.; Hill, F.; Komm, R.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.
2009AGUFMSH11B..04H Altcode:
Helioseismology allows us to probe the interior dynamics of the
Sun. Observations over the past three decades reveal the interior
rotation profile, with a near-surface shear layer, differential
rotation throughout the convection zone, a strong shear layer -- the
tachocline -- at the base of the convection zone, and approximately
uniform rotation in the radiative interior. Since the mid 1990's,
continuous observations from the Global Oscillations Network Group and
the Michelson Doppler Imager have allowed the study of subtle temporal
variations in the rotation within the convection zone. The so-called
"torsional oscillation" pattern of migrating zonal flows accompanying
the surface activity migration during the solar cycle has been shown to
penetrate deep within the convection zone. During the current extended
solar minimum, the flow bands can be seen to migrate more slowly towards
the equator than was seen in the previous minimum. There have also been
(still unconfirmed) findings of shorter-term variations in the rotation
rate close to the tachocline during the early years of the previous
solar cycle. This review will describe the important results and give
an update on the most recent observations of the interior dynamics as
we await the rise of solar cycle 24.
---------------------------------------------------------
Title: Ring Diagram Analysis of an Artificial 96 × 96 × 20 Mm
Data Set
Authors: Howe, R.; González Hernández, I.; Komm, R.; Hill, F.
2009ASPC..416..151H Altcode:
A 16-hour time series of data from a 96 × 96 × 20 Mm hydrodynamic
convection simulation has recently been made available. We will present
the preliminary results of applying the ring-diagram technique of local
helioseismology to this dataset, in comparison with similar quiet-Sun
observations from GONG and MDI, and show that it is possible to recover
the underlying horizontal flow profile, at least in the upper half of
the region.
---------------------------------------------------------
Title: Using subsurface helicity measurements to predict flare
occurrence
Authors: Reinard, A. A.; Henthorn, J.; Komm, R.; Hill, F.
2009AGUFMSH21C..06R Altcode:
Solar flares are responsible for a number of hazardous effects including
disabling high-frequency radio communications, interfering with GPS
measurements, and disrupting satellites. Forecasting flare occurrence
is very difficult, giving little advanced notice of these events. One
possible means for predicting flare occurrence lies in helioseismology,
i.e. analysis of the region below the active region for signs of
an impending flare. Time series helioseismic data collected by the
Global Oscillation Network Group (GONG) have been analyzed for a
subset of active regions that produce large flares and a subset with
very high magnetic field strength that produce no flares. A predictive
parameter has been developed and analyzed using discriminant analysis
as well as traditional forecasting tools such as the Heidke skill
score. Preliminary results indicate this parameter predicts flare
occurrence with a high success rate.
---------------------------------------------------------
Title: Large-Scale Solar Subsurface Flows During Solar Cycle Minimum
Authors: Komm, R.; Howe, R.; Hill, F.
2009AGUFMSH11A1482K Altcode:
The long solar minimum between cycles 23/24 allows us to study
large-scale flows, such as rotation and meridional flow, as a dynamical
process without bias due to magnetic fields. We study the subsurface
flows in the near-surface layers of the convection zone with a local
helioseismic technique, called ring-diagram analysis, using Global
Oscillation Network Group (GONG) data obtained during the years 2008
and early 2009. We focus on the meridional flow and the divergence and
vorticity of subsurface flows during this exceptional solar minimum. We
also search for large-scale coherent structures that might be present
in the convection zone but might be easily obscured by the effect of
surface magnetic fields during other epochs of the solar cycle. We
will present the most recent results.
---------------------------------------------------------
Title: The Torsional Oscillation and the Solar Minimum
Authors: Howe, R.; Hill, F.; Komm, R.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.
2009AGUFM.U34A..03H Altcode:
The so-called torsional oscillation is a pattern of zonal flow bands,
detected at the solar surface by direct Doppler measurements and within
the convection zone by helioseismic measurements such as those carried
out by the Global Oscillations Network Group and the Michelson Doppler
Imager, that migrates from mid-latitudes towards the equator and poles
with each solar cycle. In the current minimum the low-latitude branch
of the pattern can be seen to have taken at least a year longer to
migrate towards the equator than was the case in the previous minimum. A
flow configuration matching that of the previous minimum was reached
during 2008, and by early 2009 the fast-rotating belt associated with
the new cycle had reached the latitude at which the onset of activity
was seen in Cycle 23, but magnetic activity has remained low. We will
present the most recent results and consider the implications for the
new solar cycle.
---------------------------------------------------------
Title: The Torsional Oscillation and the Solar Cycle: Is it Minimum
Yet?
Authors: Howe, R.; Komm, R.; Hill, F.; Larson, T.; Schou, J.; Thompson,
M. J.; Ulrich, R. K.
2009ASPC..416..269H Altcode:
The torsional oscillation pattern of migrating zonal flows is related to
the solar activity cycle. In the approach to solar minimum, we compare
the current flow profile with that seen at the previous minimum, using
Global Oscillation Network Group (GONG) and Michelson Doppler Imager
(MDI) data as well as Mount Wilson Doppler observations that reach
further back in time. Will the flow pattern at the upcoming minimum
match that for the previous one?
---------------------------------------------------------
Title: A Note on the Torsional Oscillation at Solar Minimum
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Schou, J.; Thompson, M. J.
2009ApJ...701L..87H Altcode: 2009arXiv0907.2965H
We examine the evolution of the zonal flow pattern in the upper solar
convection zone during the current extended solar minimum, and compare
it with that during the previous minimum. The results suggest that
a configuration matching that at the previous minimum was reached
during 2008, but that the flow band corresponding to the new cycle has
been moving more slowly toward the equator than was observed in the
previous cycle, resulting in a gradual increase in the apparent length
of the cycle during the 2007-2008 period. The current position of the
lower-latitude fast-rotating belt corresponds to that seen around the
onset of activity in the previous cycle.
---------------------------------------------------------
Title: Emerging and Decaying Magnetic Flux and Subsurface Flows
Authors: Komm, R.; Howe, R.; Hill, F.
2009SoPh..258...13K Altcode:
We study the temporal variation of subsurface flows of 788 active
regions and 978 quiet regions. The vertical-velocity component used in
this study is derived from the divergence of the measured horizontal
flows using mass conservation. The horizontal flows cover a range of
depths from the surface to about 16 Mm and are determined by analyzing
about five years of GONG high-resolution Doppler data with ring-diagram
analysis. We determine the change in unsigned magnetic flux during
the disk passage of each active region using MDI magnetograms binned
to the ring-diagram grid. We then sort the data by their flux change
from decaying to emerging flux and divide the data into five subsets
of equal size. The average vertical flows of the emerging-flux
subset are systematically shifted toward upflows compared to the
grand average values of the complete data set, whereas the average
flows of the decaying-flux subset show comparably more pronounced
downflows especially near 8 Mm. For flux emergence, upflows become
stronger with time with increasing flux at depths greater than about
10 Mm. At layers shallower than about 4 Mm, the flows might start to
change from downflows to upflows, when flux emerges, and then back to
downflows after the active regions are established. The flows in the
layers between these two depth ranges show no response to the emerging
flux. In the case of decaying flux, the flows change from strong upflows
to downflows at depths greater than about 10 Mm, whereas the flows do
not change systematically at other depths. A cross-correlation analysis
shows that the flows in the near-surface and the deeper layers might
change about one day before flux emerges. The flows associated with
the quiet regions fluctuate with time but do not show any systematic
variation.
---------------------------------------------------------
Title: Solar flares and solar subphotospheric vorticity
Authors: Komm, R.; Hill, F.
2009JGRA..114.6105K Altcode: 2009JGRA..11406105K
We explore the relation between surface magnetic flux of the sun and
subsurface flow vorticity for flaring and nonflaring solar active
regions. For this purpose, we use a data set consisting of 1009 active
regions, including the vorticity measurements of their subsurface
flows derived from high-resolution global oscillation network group
(GONG) helioseismology data and the corresponding X-ray flare data
from the geostationary operation environmental satellite (GOES). Using
quantities averaged over the disk passage of active regions, we find
that, while there is a considerable spread of the flux and vorticity
values, they are more or less linearly related. We distinguish the
level of flare activity by X-ray flare class and find that large
flux or large vorticity values are sufficient for an active region
to produce low-intensity C-class flares. Active regions that produce
high-intensity X-class flares are characterized by large values of
both flux and vorticity. Active regions that produce M-class flares of
intermediate intensity are characterized by large vorticity values. The
inclusion of solar subsurface vorticity thus helps to distinguish
between flaring and nonflaring active regions.
---------------------------------------------------------
Title: Subsurface Flow Properties of Flaring Versus Flare-quiet
Active Regions
Authors: Ferguson, Ryan M.; Komm, R.; Hill, F.; Barnes, G.; Leka, K. D.
2009SPD....40.1908F Altcode:
Previous studies have shown that the flare activity of active regions is
intrinsically linked with the vorticity of subsurface flows on temporal
and spatial scales comparable to the size and lifetime of active
regions. We begin to address the question whether the measured vorticity
of subsurface flows associated with active regions can help to improve
flare forecasting. For this purpose, we apply statistical tests based
on discriminant analysis to several subsurface flow parameters with the
goal to differentiate between flaring and non-flaring active regions. <P
/>We will present the latest results. This work is carried out through
the National Solar Observatory Research Experiences for Undergraduate
(REU) site program, which is co-funded by the Department of Defense
in partnership with the National Science Foundation REU Program.
---------------------------------------------------------
Title: A Helioseismic Comparison of the Solar Minima Preceding Cycles
23 and 24
Authors: Hill, Frank; Howe, R.; Komm, R.; Gonzallez Hernandez, I.;
Tripathy, S.; Jain, K.
2009SPD....40.2401H Altcode:
The current solar minimum is clearly unusual in a variety of
ways, including length, solar wind pressure, cosmic ray flux, and
marked absence of sunspots. This talk will compare the current
minimum with the previous one in terms of its helioseismic and
subsurface flow characteristics. The helioseismic characteristics are
primarily activity-related changes in the frequencies, amplitudes
and lifetimes. The relevant flows are the torsional oscillation,
meridional flow, subsurface vorticity, and the subsurface rotation rate.
---------------------------------------------------------
Title: Subsurface Zonal Flows
Authors: Komm, R.; Howe, R.; Hill, F.; González Hernández, I.
2009SoPh..254....1K Altcode: 2008SoPh..tmp..189K
We study the zonal flow in solar subsurface layers, analyzing about
six years of GONG++ high-resolution Doppler data with ring-diagram
analysis. We focus on the variation of zonal flow with magnetic activity
over a range of depths from the surface to about 16 Mm. There is a
positive correlation between unsigned magnetic flux and zonal flow at
most depths. We calculate the average zonal flow for a quiet- and an
active-region subset defined as dense-pack locations with an unsigned
magnetic flux less than 3.4 G and locations with greater than 65.0 G,
respectively. The average zonal flow of active regions is about 4 m
s<SUP>−1</SUP> larger than the average flow of quiet regions. This
difference increases slightly with increasing depth, which might be
explained by a nonradial inclination of the flux tubes or a different
extent in depth of different magnetic features. The difference shows
no apparent pattern in time and latitude, which makes it unlikely that
it is simply a manifestation of the torsional-oscillation pattern. As
a byproduct, we find that the size of the North - South asymmetry of
the rotation rate decreases during the same epoch.
---------------------------------------------------------
Title: Subsurface Meridional Circulation in the Active Belts
Authors: González Hernández, I.; Kholikov, S.; Hill, F.; Howe, R.;
Komm, R.
2008SoPh..252..235G Altcode: 2008SoPh..tmp..167G; 2008arXiv0808.3606G
Temporal variations of the subsurface meridional flow with the solar
cycle have been reported by several authors. The measurements are
typically averaged over periods of time during which surface magnetic
activity existed in the regions where the velocities are calculated. The
present work examines the possible contamination of these measurements
due to the extra velocity fields associated with active regions plus
the uncertainties in the data obtained where strong magnetic fields
are present. We perform a systematic analysis of more than five years
of GONG data and compare meridional flows obtained by ring-diagram
analysis before and after removing the areas of strong magnetic
field. The overall trend of increased amplitude of the meridional flow
towards solar minimum remains after removal of large areas associated
with surface activity. We also find residual circulation toward the
active belts that persists even after the removal of the surface
magnetic activity, suggesting the existence of a global pattern or
longitudinally-located organized flows.
---------------------------------------------------------
Title: Kinetic helicity of subsurface flows and magnetic flux
Authors: Komm, Rudolf; Hill, Frank; Howe, Rachel
2008JPhCS.118a2035K Altcode:
We study the relation between the vorticty of solar subsurface flows
and surface magnetic activity, analyzing more than five years of GONG+
data with ring-diagram analysis. We focus on the enstrophy, defined
as the square of vorticity, and the kinetic helicity density, defined
as the scalar product of velocity and vorticity, and derive them from
the surface to a depth of about 16 Mm. We find that enstrophy and
helicity density of subsurface flows are rather constant at low flux
values (less than about 10 G), while at higher flux values there is a
linear relation between flux and the logarithm of enstrophy or unsigned
helicity. In addition, we analyze the temporal variation of thirteen
emerging active regions. At the locations of these active regions,
there is little enstrophy or helicity before the regions emerge, while
after flux emergence the vorticity and helicity values are large. The
crosscorrelation in time between flux and enstrophy shows that they
are correlated and that shallow layers lag behind deeper layers. This
signal might be a hint of the emergence of active regions.
---------------------------------------------------------
Title: Rotation Rate of Sunspots and Subsurface Zonal Flows
Authors: Komm, R.; Howe, R.; Hill, F.; González Hernández, I.
2008AGUSMSP41A..08K Altcode:
From surface observations, it is well known that sunspots rotate faster
than the surrounding plasma. Helioseismic observations have confirmed
this behavior for near-surface layers. Here, we study the zonal flow
of active regions in solar subsurface layers over a range of depths
from the surface to about 16 Mm. We have analyzed about six years of
GONG+ high-resolution Doppler data with the dense-pack ring-diagram
analysis. We calculate the average zonal flow for a quiet- and an
active-region subset defined as dense-pack patches (of 15 degree
diameter) with an unsigned magnetic flux less than 3.4 G and greater
than 65.0 G respectively. The average zonal flow of active regions is
about 4m/s larger than the average flow of quiet regions on dense-pack
length scales. This difference increases slightly with increasing depth
and shows no apparent pattern in time and latitude. As a byproduct,
we study the north-south asymmetry of the rotation rate in these
subsurface layers and find that the asymmetry decreases during the
declining phase of solar cycle 23.
---------------------------------------------------------
Title: Views of the Solar Torsional Oscillation
Authors: Howe, R.; Komm, R. W.; Hill, F.; Schou, J.; Thompson, M. J.
2008AGUSMSP41A..05H Altcode:
The pattern of zonal flows migrating towards the equator over the
solar cycle, known as the torsional oscillation, is well established
from both helioseismology and surface Doppler measurements. However,
the exact appearance of the pattern will vary depending on the form of
the overall rotation profile that has been subtracted, even when the
data cover a full eleven-year cycle. Here we compare the appearance
of the flow pattern when applying several different methods to the
MDI and GONG data for Solar Cycle 23.
---------------------------------------------------------
Title: Space Weather with GONG+ Data
Authors: Hill, F.; Komm, R.; Gonzalez-Hernandez, I.; Petrie, G.;
Harvey, J. W.
2008AGUSMSP54A..08H Altcode:
The Global Oscillation Network Group (GONG) is now routinely
producing several data products that are useful for space weather
predictions. These products are one-minute cadence full-disk
magnetograms obtained continually; ten-miniute averages of these
magnetograms; one-hour cadence synoptic magnetic field maps and
potential field source-surface extrapolations; and twelve-hour far-side
maps that show the presence of large active regions. Most of these these
products are made available over the Internet in near-real time. In
addition, we are developing flare predictors based on subsurface
vorticity obtained from helioseismic ring diagrams in conjunction
with surface magnetic field observations. We find that, when both the
subsurface vorticity and the surface magnetic field are above certain
thresholds for a specific active region, then that active region has
a very high probability of producing vigorous flare activity. We will
present the quantitative results for this predictor and also report
on progress developing a predictor based on the temporal evolution of
the vorticity.
---------------------------------------------------------
Title: Emerging Active Regions Studied with Ring-Diagram Analysis
Authors: Komm, R.; Morita, S.; Howe, R.; Hill, F.
2008ApJ...672.1254K Altcode:
We study the temporal variation of subsurface flows associated with
emerging active regions, focusing on four regions in detail. Two of
them, AR 10314 and AR 10488, emerge near disk center and the other two,
AR 10365 and AR 10375, are older regions where new flux emerges during
their disk passage. We measure the horizontal subsurface flows from
high-resolution Global Oscillation Network Group (GONG) data using
ring-diagram analysis and derive the vertical flow component. Before
flux emergence, we find upflows in AR 10314, while the other emerging
region, AR 10488, shows mainly weak vertical flows. Both aging regions,
AR 10365 and AR 10375, initially show downflows, as expected from
already established regions. When new flux emerges, the weaker one of
the two, AR 10365, shows upflows, while AR 10375 shows an even stronger
downflow. In strong active regions, such as AR 10375 and AR 10488,
strong downflows are present after the region has been established. In
all four regions, the transition occurs on timescales of about one
to two days. As a control experiment, we repeat the analysis for the
same locations as those of the four active regions in 53 Carrington
rotations and find that it is unlikely that the temporal variations of
the vertical velocity are caused by systematics such as a projection
effect. We then search our data set for emerging regions with similar
characteristics to AR 10314 and AR 10488, i.e., emergence near disk
center and large flux increase. From an analysis of 13 emerging regions,
we conclude that there is a small preference for upflows before the
emergence of new flux and for a transition toward downflows after
flux emergence.
---------------------------------------------------------
Title: Helioseismic Frequency Shifts in Active Regions
Authors: Howe, R.; Haber, D. A.; Hindman, B. W.; Komm, R.; Hill, F.;
Gonzalez Hernandez, I.
2008ASPC..383..305H Altcode:
The variation in the frequencies of solar acoustic modes over the
activity cycle is well established. We discuss some of the historical
findings, and present some recent results obtained using both global and
local helioseismic analysis of data from the Global Oscillation Network
Group and the Michelson Doppler Imager. The results are consistent with
earlier work; the frequencies of modes in the five-minute band generally
show a positive correlation with the local surface magnetic field
strength, while those above the acoustic cutoff show an anticorrelation.
---------------------------------------------------------
Title: Subsurface and Atmospheric Influences on Solar Activity
Authors: Howe, R.; Komm, R. W.; Balasubramaniam, K. S.; Petrie,
G. J. D.
2008ASPC..383.....H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Subsurface Flows near Four Emerging Active Regions Studied
with Ring-Diagram Analysis
Authors: Komm, R.; Howe, R.; Hill, F.; Morita, S.
2008ASPC..383...83K Altcode:
We study the temporal variation of subsurface flows associated with
four emerging active regions. Two of them, AR~10314 and AR~10488,
emerge near disk center and the other two, AR~10365 and AR~10375, are
older regions where new flux emerges during their disk passage. We
measure the horizontal subsurface flows from high-resolution Global
Oscillation Network Group (GONG+) data using ring-diagram analysis
and derive the vertical flow component. Before flux emergence, we
find upflows in AR~10314, while the other emerging region, AR~10488,
shows only weak vertical flows hinting at upflows. Both aging regions,
AR~10365 and AR~10375, show initially downflows, as expected from
already established regions. When new flux emerges, the weaker one
of the two, AR~10365, shows upflows, while AR~10375 shows stronger
downflows. In strong active regions, such as AR~10375 and AR~10488,
strong downflows are present after the region has been established.
---------------------------------------------------------
Title: Divergence and Vorticity of Solar Subsurface Flows Derived
from Ring-Diagram Analysis of MDI and GONG Data
Authors: Komm, R.; Howe, R.; Hill, F.; Miesch, M.; Haber, D.;
Hindman, B.
2007ApJ...667..571K Altcode:
We measure the relation between divergence and vorticity of
subsurface horizontal flows as a function of unsigned surface magnetic
flux. Observations from the Michelson Doppler Imager (MDI) Dynamics
Program and Global Oscillation Network Group (GONG) have been analyzed
with a standard ring-diagram technique to measure subsurface horizontal
flows from the surface to a depth of about 16 Mm. We study residual
horizontal flows after subtracting large-scale trends (low-order
polynomial fits in latitude) from the measured velocities. On average,
quiet regions are characterized by weakly divergent horizontal flows and
small anticyclonic vorticity (clockwise in the northern hemisphere),
while locations of high activity show convergent horizontal flows
combined with cyclonic vorticity (counterclockwise in the northern
hemisphere). Divergence and vorticity of horizontal flows are
anticorrelated (correlated) in the northern (southern) hemisphere. This
is especially noticeable at greater depth, where the relation between
divergence and vorticity of horizontal flows is nearly linear. These
trends show a slight reversal at the highest levels of magnetic flux;
the vorticity amplitude decreases at the highest flux levels, while
the divergence changes sign at depths greater than about 10 Mm. The
product of divergence and vorticity of the horizontal flows, a proxy
of the vertical contribution to the kinetic helicity density, is on
average negative (positive) in the northern (southern) hemisphere. The
helicity proxy values are greater at locations of high magnetic activity
than at quiet locations.
---------------------------------------------------------
Title: Solar Cycle Changes Over 11 Years of Medium-Degree Helioseismic
Observations
Authors: Howe, Rachel; Komm, R. W.; Hill, F.; Christensen-Dalsgaard,
J.; Schou, J.; Thompson, M. J.
2007AAS...210.2218H Altcode: 2007BAAS...39..127H
The Global Oscillations Network Group (GONG) has now completed, and
the Michelson Doppler Imager (MDI) aboard SOHO will soon complete,
a full eleven years of continuous observations of the medium-degree
solar oscillations. This enables us to follow changes in the acoustic
mode parameters and interior dynamics over a full solar cycle. We
present results from observations of convection-zone dynamics, in
which the torsional oscillation pattern seen at the surface can be
followed throughout most of the bulk of the convection zone, and also
changes in the frequency, lifetime and amplitude of the modes which
can be shown to be closely related in space and time to the migrating
pattern of surface activity. <P />This work utilizes data obtained by
the Global Oscillation Network Group (GONG) program, managed by the
National Solar Observatory, which is operated by AURA, Inc. under a
cooperative agreement with the National Science Foundation. SOHO is
a mission of international cooperation between ESA and NASA.
---------------------------------------------------------
Title: Value-added maps: fluid-dynamics descriptors from ring diagrams
Authors: Komm, R. W.
2007AN....328..269K Altcode:
We describe fluid-dynamics descriptors derived from maps of the
horizontal flow components measured with ring-diagram analysis. Here,
we focus on quantities, such as vorticity and kinetic helicity density,
and discuss three examples of results derived from them: subsurface
flows associated with active regions, subsurface flows and flare
activity, and large-scale behavior of horizontal flows.
---------------------------------------------------------
Title: Temporal variations in solar rotation at the bottom of the
convection zone: The current status
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Schou, J.; Thompson, M. J.; Toomre, J.
2007AdSpR..40..915H Altcode:
We present the most recent results on the short-period variations
in the solar rotation rate near the base of the convection zone. The
1.3-year period which was reported in the early years of solar cycle 23
appears not to persist after 2001, but there are hints of fluctuations
at a different period during the declining phase of the cycle.
---------------------------------------------------------
Title: Peak parameter shifts from large-aperture ring diagram analysis
Authors: Howe, R.; González Hernández, I.; Hill, F.; Komm, R.
2006ESASP.624E..68H Altcode: 2006soho...18E..68H
No abstract at ADS
---------------------------------------------------------
Title: B0-angle effect on zonal and meridional flow determinations
from 3 years ring diagram analysis of GONG++ data
Authors: Zaatri, A.; Komm, R.; González Hernández, I.; Howe, R.;
Corbard, T.
2006ESASP.624E..55Z Altcode: 2006soho...18E..55Z
No abstract at ADS
---------------------------------------------------------
Title: Solar Convection Zone Dynamics: How Sensitive Are Inversions
to Subtle Dynamo Features?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Hill, F.;
Komm, R.; Larsen, R. M.; Schou, J.; Thompson, M. J.
2006ApJ...649.1155H Altcode:
The nearly 10 year span of medium-degree helioseismic data from the
Global Oscillation Network Group and the Michelson Doppler Imager has
allowed us to study the evolving flows in the solar convection zone
over most of solar cycle 23. Using two independent two-dimensional
rotation inversion techniques and extensive studies of the resolution
using artificial data from different assumed flow profiles, including
those generated from sample mean field dynamo models, we attempt to
assess the reality of certain features seen in the inferred rotation
profiles. Our results suggest that the findings from observations of
a substantial depth dependence of the phase of the zonal flow pattern
in the low latitudes, and the penetration of the flows deep into the
convection zone, are likely to be real rather than artifacts of the
inversion process.
---------------------------------------------------------
Title: Subsurface flows from numerical simulations compared with
flows from ring analysis
Authors: Ustyugov, S.; Komm, R.; Burtseva, O.; Howe, R.; Kholikov, S.
2006ESASP.624E..54U Altcode: 2006soho...18E..54U
No abstract at ADS
---------------------------------------------------------
Title: Subsurface flows measured with big rings
Authors: Komm, R.; González Hernández, I.; Howe, R.; Hill, F.
2006ESASP.624E..53K Altcode: 2006soho...18E..53K
No abstract at ADS
---------------------------------------------------------
Title: Divergence and Vorticity of Subsurface Flows Derived from
Ring-Diagram Analysis of MDI and GONG Data
Authors: Komm, R.; Howe, R.; Hill, F.; Miesch, M.; Haber, D.;
Hundman, B.
2006ESASP.617E..42K Altcode: 2006soho...17E..42K
No abstract at ADS
---------------------------------------------------------
Title: Flares, Magnetic Fields, and Subsurface Vorticity: A Survey
of GONG and MDI Data
Authors: Mason, D.; Komm, R.; Hill, F.; Howe, R.; Haber, D.; Hindman,
B. W.
2006ApJ...645.1543M Altcode:
We search for a relation between flows below active regions and flare
events occurring in those active regions. For this purpose, we determine
the subsurface flows from high-resolution Global Oscillation Network
Group (GONG) and Michelson Doppler Imager (MDI) Dynamics Program data
using the ring-diagram technique. We then calculate the vorticity
of the flows associated with active regions and compare it with a
proxy of the total X-ray flare intensity of these regions using data
from the Geostationary Operation Environmental Satellite (GOES). We
have analyzed 408 active regions with X-ray flare activity from GONG
and 159 active regions from MDI data. Both data sets lead to similar
results. The maximum unsigned zonal and meridional vorticity components
of active regions are correlated with the total flare intensity; this
behavior is most apparent at values greater than 3.2×10<SUP>-5</SUP>
W m<SUP>-2</SUP>. These vorticity components show a linear relation
with the logarithm of the flare intensity that is dependent on the
maximum unsigned magnetic flux; vorticity values are proportional to
the product of total flare intensity and maximum unsigned magnetic flux
for flux values greater than about 36 G. Active regions with strong
flare intensity show a dipolar pattern in the zonal and meridional
vorticity component that reverses at depths between ~2 and 5 Mm. A
measure of this pattern shows the same kind of relation with total
flare intensity as the vorticity components. The vertical vorticity
component shows no clear relation to flare activity.
---------------------------------------------------------
Title: North South Asymmetry of Zonal and Meridional Flows
Determined From Ring Diagram Analysis of Gong ++ Data
Authors: Zaatri, A.; Komm, R.; González Hernández, I.; Howe, R.;
Corbard, T.
2006SoPh..236..227Z Altcode:
We study the North-South asymmetry of zonal and meridional components
of horizontal, solar subsurface flows during the years 2001-2004,
which cover the declining phase of solar cycle 23. We measure the
horizontal flows from the near-surface layers to 16 Mm depth by
analyzing 44 consecutive Carrington rotations of Global Oscillation
Network Group (GONG) Doppler images with a ring-diagram analysis
technique. The meridional flow and the errors of both flow components
show an annual variation related to the B<SUB>0</SUB>-angle variation,
while the zonal flow is less affected by the B<SUB>0</SUB>-angle
variation. After correcting for this effect, the meridional flow is
mainly poleward but it shows a counter cell close to the surface at
high latitudes in both hemispheres. During the declining phase of the
solar cycle, the meridional flow mainly increases with time at latitudes
poleward of about 20<SUP>˚</SUP>, while it mainly decreases at more
equatorward latitudes. The temporal variation of the zonal flow in both
hemispheres is significantly correlated at latitudes less than about
20<SUP>˚</SUP>. The zonal flow is larger in the southern hemisphere
than the northern one, and this North-South asymmetry increases with
depth. Details of the North-South asymmetry of zonal and meridional flow
reflect the North-South asymmetry of the magnetic flux. The North-South
asymmetries of the flows show hints of a variation with the solar cycle.
---------------------------------------------------------
Title: MDI and GONG Inferences of the Changing Sun
Authors: Burtseva, O.; González Hernández, I.; Hill, F.; Howe, R.;
Jain, K.; Kholikov, S.; Komm, R.; Leibacher, J.; Toner, C.; Tripathy,
S.; Haber, D.; Hindman, B.; Ladenkov, O.; Chou, D. -Y.
2006ESASP.617E..41B Altcode: 2006soho...17E..41B
No abstract at ADS
---------------------------------------------------------
Title: Anomalous variations in low-degree helioseismic mode
frequencies
Authors: Howe, R.; Chaplin, W. J.; Elsworth, Y.; Hill, F.; Komm,
R. W.; Isaak, G. R.; New, R.
2006MNRAS.369..933H Altcode: 2006MNRAS.tmp..504H
We compare changes in the frequencies of solar acoustic modes with
degree between 0 and 2, as derived from Global Oscillation Network Group
(GONG), Birmingham Solar Oscillations Network (BiSON) and Michelson
Doppler Imager (MDI) spectra obtained between 1995 and 2003. We find
that, after the solar-activity dependence has been removed from the
frequencies, there remain variations that appear to be significant,
and are often well correlated between the different data sets. We
consider possible explanations for these fluctuations, and conclude
that they are likely to be related to the stochastic excitation of the
modes. The existence of such fluctuations has possible relevance to
the analysis of other low-degree acoustic mode spectra such as those
from solar-type stars.
---------------------------------------------------------
Title: Solar Flares, Magnetic Fields, And Subsurface Vorticity. A
Survey Of Gong And Mdi Data
Authors: Mason, Douglas J.; Komm, R.; Howe, R.; Hill, F.; Haber, D.;
Hindman, B.
2006SPD....37.0506M Altcode: 2006BAAS...38..224M
We search for a relation between flare events and magnetic fields
in active regions and the subsurface flows associated with these
regions. For this purpose, we determine the solar subsurface flows
from high-resolution Global Oscillation Network Group (GONG) and and
Michelson Doppler Imager (MDI) Dynamics Program data using ring-diagram
analysis (a local helioseismology technique). For the first time,
we have been able to search out these connections with a statistical
analysis of consecutive data that encompass many years. We have
found that the vorticity of the flow field below the solar surface,
specifically the maximum flow vorticity within each active region,
correlates well with the total X-ray flare intensity data for the
region (provided by GOES, the Geostationary Operation Environmental
Satellite). Above a certain threshold of flare activity, vorticity
values exhibit a linear relationship with total flare activity
that is dependent on the magnetic flux of the active region.This
work is carried out through the National Solar Observatory Research
Experiences for Undergraduate (REU) site program, which is co-funded
by the Department of Defense in partnership with the National Science
Foundation REU Program.
---------------------------------------------------------
Title: Large-Scale Zonal Flows Near the Solar Surface
Authors: Howe, R.; Komm, R.; Hill, F.; Ulrich, R.; Haber, D. A.;
Hindman, B. W.; Schou, J.; Thompson, M. J.
2006SoPh..235....1H Altcode:
Migrating bands of weak, zonal flow, associated with the activity
bands in the solar cycle, have been observed at the solar surface for
some time. More recently, these flows have been probed deep within the
convection zone using global helioseismology and examined in more detail
close to the surface with the techniques of local helioseismology. We
compare the near-surface results from global and local helioseismology
using data from the Michelson Doppler Imager and the Global Oscillation
Network Group with surface Doppler velocity measurements from the Mount
Wilson 150-foot tower and find that the results are in reasonable
agreement, with some explicable differences in detail. All of the
data sets show zones of faster rotation approaching the equator from
mid-latitudes during the solar cycle, with a variation at any given
location that can be approximately, but not completely, described by a
single sinusoid and an amplitude that does not drop off steeply below
the surface.
---------------------------------------------------------
Title: Meridional Circulation Variability from Large-Aperture
Ring-Diagram Analysis of Global Oscillation Network Group and
Michelson Doppler Imager Data
Authors: González Hernández, I.; Komm, R.; Hill, F.; Howe, R.;
Corbard, T.; Haber, D. A.
2006ApJ...638..576G Altcode:
Ring-diagram analysis, a local helioseismology technique, has proven to
be very useful for studying solar subsurface velocity flows down to a
depth of about 0.97 R<SUB>solar</SUB>. The depth range is determined by
the modes used in this type of analysis, and thus depends on the size
of the area analyzed. Extending the area allows us to detect lower
spherical harmonic degree (l) modes which, at a constant frequency,
penetrate deeper in the Sun. However, there is a compromise between
the size of the area and the validity of the plane-wave approximation
used by the technique. We present the results of applying the ring
diagrams to 30° diameter areas over the solar surface in an attempt
to reach deeper into the solar interior. Meridional flows for 25
consecutive Carrington rotations (1985-2009) are derived by applying
this technique to Global Oscillation Network Group (GONG) and Michelson
Doppler Imager (MDI) data. This covers a time span of almost 2 yr,
starting at the beginning of 2002. The amplitude of the meridional
flow shows a variation of the order of 5 m s<SUP>-1</SUP> during this
period. Our results indicate that the flows increase toward the interior
of the Sun for the depth range studied. We find a 1 yr periodicity in
the appearance of an equatorward meridional cell at high latitudes that
coincides with maximum values of the solar inclination toward the Earth
(B<SUB>0</SUB> angle).
---------------------------------------------------------
Title: Helioseismic sensing of the solar cycle
Authors: Komm, R.; Howe, R.; Hill, F.
2006AdSpR..38..845K Altcode:
All quantities observed with helioseismic methods, such as
frequencies, width, and amplitudes of acoustic waves, vary with the
solar cycle. However, they relate to the dynamics and structure in
different parts of the solar convection zone. The rotation rate varies
with the solar cycle, showing the so-called torsional oscillation
pattern, from the surface throughout most of the convection zone. Near
the tachocline, the current observations do not show this solar-cycle
variation, but there is some evidence that there is a 1.3-year variation
of the rotation rate. The meridional flow, observed in the outer 2% of
the solar radius, varies with the solar cycle showing flows converging
toward the mean latitude of magnetic activity at depths less than about
10 Mm and flows diverging at greater depth. There is some evidence
for a counter-cell in the northern hemisphere during epochs of high
activity. Structure inversions show variations in asphericity near
the surface where the sound speed varies with the distribution of
surface activity. There are hints but no conclusive evidence that such
variations exist in the convection zone. The damping of acoustic modes
increases and the mode energy decreases with increasing activity. Their
variation with time and latitude shows that even global modes sense
the local distribution of the surface magnetic activity.
---------------------------------------------------------
Title: Large-scale Flows in Subsurface Layers
Authors: Komm, R.; Howe, R.; González Hernández, I.; Hill, F.;
Toner, C.
2005ASPC..346...33K Altcode:
We analyze Global Oscillation Network Group (GONG) observations obtained
during 14 consecutive Carrington rotations CR 1979--1992. We use a
ring-diagram technique in order to measure the zonal and meridional flow
components in the upper solar convection zone from the near-surface
layers to 16 Mm in depth and derive the vertical velocity component
assuming mass conservation. The average flows show the patterns that
are expected as solar-cycle related variations. For example, the
vertical flow shows downflows near the mean latitude of activity and
upflows near the equator. This long-term pattern seems to be the net
effect of flows at locations that do not coincide with strong active
regions. Locations of strong active regions show downflows at depths
less than about 12 Mm on average and strong upflows at greater depths
independent of latitude. At these locations, the zonal flow is faster on
average than the average flow over regions with less magnetic activity.
---------------------------------------------------------
Title: Solar Convection-Zone Dynamics, 1995-2004
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R.;
Schou, J.; Thompson, M. J.
2005ApJ...634.1405H Altcode:
The nine-year span of medium-degree helioseismic data from the Global
Oscillation Network Group (GONG) and the Michelson Doppler Imager (MDI)
allows us to study the evolving zonal flows in the solar convection
zone over the rising phase, maximum, and early declining phase
of solar cycle 23. Using two independent two-dimensional rotation
inversion techniques, we investigate the depth profile of the flow
pattern known as the torsional oscillation. The observations suggest
that the flows penetrate deep within the convection zone-perhaps to
its base-even at low latitudes, and that the phase of the pattern is
approximately constant along lines of constant rotation rather than
lines of constant latitude.
---------------------------------------------------------
Title: How Sensitive are Rotation Inversions to Subtle Features of
the Dynamo?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Schou, J.;
Thompson, M. J.; Komm, R.; Hill, F.
2005ASPC..346...99H Altcode:
Global rotation inversions can probe the pattern of zonal flows
well into the convection zone. In this paper, we test the ability
of the inversions to constrain the predictions of dynamo models. A
flux-transport dynamo model, including a mean-field theory of
differential rotation and allowing for feedback of the Lorentz force
on differential rotation and meridional flow, was used to produce a
22-year cycle of simulated rotation profiles. These were then subjected
to simulated inversions with realistic mode sets and errors, in order
to test how well the subtle subsurface features of the input profile
could be recovered. The preliminary results are quite encouraging.
---------------------------------------------------------
Title: Solar Flares, Magnetic Fields, and Subsurface Vorticity. A
survey of GONG data
Authors: Mason, D.; Komm, R.; Hill, F.; Howe, R.
2005AAS...20711103M Altcode: 2005BAAS...37.1341M
We search for a relation between flows below active regions on the Sun
and flare events in those active regions. For this purpose, we determine
the solar subsurface flows from high-resolution Global Oscillation
Network Group (GONG) data using the ring-diagram technique. We then
calculate the vorticity of the flows associated with active regions
and compare it with the X-ray flare intensity of these regions from
the Geostationary Operation Environmental Satellite (GOES). The maximum
unsigned vorticity of an active region is correlated with its maximum
magnetic flux and the maximum unsigned zonal and meridional vorticity
component are also correlated with flare intensity greater than 3.2 ×
10<SUP>-5</SUP> W/m<SUP>2</SUP>. Above this threshold, large vorticity
values will accompany large magnetic flux for a given flare intensity
and larger flare activity will accompany lower magnetic field for a
given vorticity value. Active regions with strong flare intensity
additionally show a dipolar pattern in the zonal and meridional
vorticity component. We define a structure component as a measure of
this dipolar pattern and find that it can be represented as a linear
function of the logarithm of flare intensity where the slope is linearly
dependent on the unsigned flux. <P />This work is carried out through
the National Solar Observatory Research Experiences for Undergraduate
(REU) site program, which is co-funded by the Department of Defense in
partnership with the National Science Foundation REU Program. Travel
is funded by the University of Southern California.
---------------------------------------------------------
Title: Ring Analysis of Solar Subsurface Flows and Their Relation
to Surface Magnetic Activity
Authors: Komm, R.; Howe, R.; Hill, F.; González-Hernández, I.;
Toner, C.; Corbard, T.
2005ApJ...631..636K Altcode:
We measure the horizontal flows in the outer 2% of the Sun by analyzing
14 consecutive Carrington rotations of Global Oscillation Network Group
(GONG) Doppler images and two of Michelson Doppler Imager (MDI) Dynamics
Program data with the ring-diagram technique. The zonal and meridional
flows show no variation with activity at low to medium activity levels
(below 71 G). At active region locations, the zonal flow increases
with increasing activity; active regions rotate faster than their
quieter surroundings. The meridional flow at active region locations
is more equatorward than on average at depths less than about 10 Mm;
the flow converges toward the mean latitude of activity. At depths
greater than about 10 Mm, some active region locations show poleward
and others equatorward motions indicating strong outflows from active
regions. The estimated vertical flow decreases with increasing activity
levels except at active region locations at depths greater than about
10 Mm; active regions show downflows near the surface and upflows at
depths greater than about 10 Mm. The velocity errors increase somewhat
with increasing activity at flux levels below 71 G, but they increase
rapidly up to about 2 times the median error at higher flux values. This
increase occurs at all depths. The flows averaged over all longitudes
show the patterns expected from solar cycle variations. The quiet and
the intermediate activity subsets show the same flow pattern, while
the active region subset shows a mixture of solar cycle flow pattern
and local flow behavior.
---------------------------------------------------------
Title: Kinetic Helicity Density in Solar Subsurface Layers and Flare
Activity of Active Regions
Authors: Komm, R.; Howe, R.; Hill, F.; González Hernández, I.;
Toner, C.
2005ApJ...630.1184K Altcode:
We search for a relation between subsurface flows below active regions
and flare events occurring in those regions. For this purpose,
we use a ring-diagram analysis to determine the subsurface flows
from high-resolution Global Oscillation Network Group (GONG) and
Michelson Doppler Imager (MDI) data and derive the kinetic helicity
as a measure of the topology of the subsurface flows. We compare it
with X-ray flare data from the Geostationary Operational Environmental
Satellite (GOES). We study active regions in three Carrington rotations
(CR 1982, 1988, and 2009), which represent different levels of flare
activity. The maximum value of the unsigned kinetic helicity density
associated with each active region correlates remarkably well with the
total flare X-ray intensity of the active regions; active regions with
strong flare activity show large values of kinetic helicity density
in subsurface flows.
---------------------------------------------------------
Title: Kinetic Helicity in Solar Subsurface Layers and Flare Activity
of Active Regions
Authors: Komm, R.; Howe, R.; Hill, F.; González Hernández, I.;
Toner, C. G.
2005AGUSMSP43B..03K Altcode:
We search for a relation between subsurface flows below active
regions and flare events occuring in those regions. For this purpose,
we use a ring-diagram analysis to determine the subsurface flows from
high-resolution Global Oscillation Network Group (GONG) and Michelson
Doppler Imager (MDI) data and derive the kinetic helicity as a measure
of the topology of the subsurface flows. We compare it with X-ray flare
data from Geostationary Operational Environmental Satellite (GOES). We
study active regions in three Carrington rotations (CR~1982, 1988, and
2009), which represent different levels of flare activity. The maximum
value of the unsigned kinetic helicity density associated with each
active region correlates remarkably well with the total flare X-ray
intensity of the active regions; active regions with strong flare
activity show large values of kinetic helicity density in subsurface
flows. This work has ben supported by NASA grant NAG 5-11703.
---------------------------------------------------------
Title: Global, Local and Surface Measurements of Large-Scale Zonal
Flows Near the Solar Surface
Authors: Howe, R.; Komm, R. W.; Haber, D. A.; Hindman, B. W.; Ulrich,
R. K.; Schou, J.; Thompson, M. J.; Hill, F.
2005AGUSMSP32A..03H Altcode:
Migrating bands of weak zonal flow, associated with the activity
bands in the solar cycle, have been observed at the solar surface
for some time. More recently, these flows have been probed deep
within the convection zone using global helioseismology, and
examined in more detail close to the surface with the techniques of
local helioseismology. We compare the results from global and local
helioseismology using data from the Michelson Doppler Imager and the
GONG network and also Doppler measurements from Mount Wilson, and find
that the results are in reasonable agreement, with some explicable
differences in detail. This was work was supported by the National
Science Foundation and NASA.
---------------------------------------------------------
Title: 2 Years of Meridional Circulation from GONG Ring Diagrams
Authors: Gonzalez Hernandez, I.; Komm, R.; Corbard, T.; Hill, F.;
Howe, R.; Haber, D. A.
2005AGUSMSP32A..01G Altcode:
Large Aperture Ring Diagram analysis has been used to search
for meridional circulation variability using a 2-year GONG data
series. This technique uses patches that are four times the size
of the typically studied sections of 15 degrees in diameter, so we
are able to recover information about lower l modes that penetrate
deeper into the Sun. Although extending the working area allow us
to reach further into the solar interior, there is a compromise
between the size of the patch and the validity of the plane wave
approximation used by the technique. In this particular study, we
search for variability of the meridional flows as a function of depth
for 25 consecutive Carrington rotations. We have studied patches of
30-degree diameter over the solar surface as they crossed the solar
central meridian. The range of modes recovered with these larger
regions goes down to l~100 and reach a maximum depth of approximately
0.96Rsun. A set of 15 overlapping sections, centered at latitudes
0,+/-7.5,+/-15,+/-22.5,+/-30.0,+/-37.5,+/-45.0 and +/-52.5, has been
analyzed for 24 intervals of 1664 minutes covering each Carrington
rotation from CR1985 to CR2009 (Jan-2002 to Dec-2003). Meridional
circulation results from standard ring diagram analysis and this
large-aperture technique are compared, as well as results obtained
from two different instruments GONG and MDI. This work was supported
in part by NASA grant NAG5-11703. SOHO is a project of international
cooperation between ESA and NASA. This work utilizes data obtained by
the Global Oscillation Network Group (GONG) Program, managed by the
National Solar Observatory, which is operated by AURA, Inc. under a
cooperative agreement with the National Science Foundation. The data
were acquired by instruments operated by the Big Bear Solar Observatory,
High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar
Observatory, Instituto de Astrofisico de Canarias, and Cerro Tololo
Interamerican Observatory.
---------------------------------------------------------
Title: Local Helioseismic Mode Frequency Shifts With Magnetic
Activity, From GONG and MDI
Authors: Howe, R.; Komm, R. W.; Gonzalez Hernandez, I.; Hill, F.;
Haber, D. A.; Hindman, B. W.
2005AGUSMSP11B..06H Altcode:
We use the ring-diagram technique of local helioseismology to study the
frequency shifts of high-degree solar acoustic modes from over 600 days
of data from the Global Oscillations Network Group (GONG), covering
the period 2001-2004. The data are compared with contemporaneous
data from the Michelson Doppler Imager (MDI) dynamics program, where
available. We examine both synoptic charts and the day-to-day variations
in selected active regions. The results, once instrumental effects have
been removed, show strong dependence of the mode frequency on the local
magnetic flux, with the frequencies generally increasing with magnetic
index. We relate these findings to results from global modes. This
work was supported by the National Science Foundation and NASA
---------------------------------------------------------
Title: Comparison of Mode Parameters Between Velocity and Intensity
Acoustic Spectra via Ring Diagrams
Authors: Tripathy, S. C.; Hill, F.; González Hernández, I.; Howe,
R.; Komm, R. W.; Toner, C. G.
2005AGUSMSP24A..03T Altcode:
We analyse the local acoustic spectra at different locations over the
solar disk using both velocity and intensity images from MDI. These
spectra were fitted to obtain different mode parameters: e.g., acoustic
frequencies, mode amplitudes and life time using symmetric fits. We
find differences between frequencies derived from velocity and intensity
filtergrams, and it appears that the mode frequencies vary as a function
of location on the disk. Since the apparent frequency shift between an
oscillation observed in velocity and intensity can not be a property
of the mode, the analysis is expected to provide important information
about the driving and damping of local acoustic oscillations.
---------------------------------------------------------
Title: Five-Minute Power Maps From GONG and MDI.
Authors: Howe, R.; Komm, R. W.; Hill, F.; Haber, D. A.; Hindman, B. W.
2005AGUSMSP13A..01H Altcode:
The presence of magnetic active regions on the solar surface is well
known to influence the detected power of the oscillation signal. We
consider maps of the five-minute power in the velocity signal from
Global Oscillations Network Group (GONG) observations covering much
of the disk over multiple Carrington Rotations, and compare these in
detail with magnetic and continuum intensity images, with estimates of
the velocity power from ring diagram helioseismic analysis, and also
with a small sample of contemporaneous MDI (Michelson Doppler Imager)
data. The comparison of power maps with magnetograms is carried out at
a pixel-by-pixel level, for averages over patches of 16× 16 degrees
in heliographic latitude and longitude, and at some intermediate
scales. This research was supported by the National Science Foundation
and NASA.
---------------------------------------------------------
Title: Vorticity and Kinetic Helicity in Solar Subsurface Layers
from GONG and MDI data
Authors: Komm, R.; Howe, R.; Hill, F.; Haber, D. A.; González
Hernández, I.
2005AGUSMSP43B..04K Altcode:
We use a ring-diagram analysis to determine the subsurface flows from
high-resolution Global Oscillation Network Group (GONG) and Michelson
Doppler Imager (MDI) data and derive the vorticity and the kinetic
helicity of the subsurface flows in the upper 16~Mm of the convection
zone. We separate the contributions of large-scale horizontal flows,
such as differential rotation, from those of small-scale variations,
such as the ones due to active regions, and analyze the large-scale and
the residual component independently. We study the relation between
magnetic activity and subsurface flows by comparing synoptic maps of
the derived residual quantities with maps of photospheric magnetic
activity. By comparing synoptic maps derived from GONG and MDI data,
we are able to cross-validate the results. We will present the latest
findings. This work has been supported by NASA and NSF.
---------------------------------------------------------
Title: Solar Subsurface Flows of Active Region AR~0696
Authors: Komm, R.; Howe, R.; Donaldson Hanna, K.; Hill, F.; Sheeley, N.
2005AGUSMSP24A..02K Altcode:
We use a ring-diagram analysis to determine the subsurface flows in
the upper 16~Mm of the convection zone from high-resolution Global
Oscillation Network Group (GONG) data obtained during the first two
weeks of November 2004. The active region AR~0696 emerges near the
eastern limb and moves across the disk during this time period. The
region produced several terrestrially effective flares and halo
CMEs during its transit across the disk. During its disk passage,
AR~0696 is the only large active region in the northern hemisphere
and almost the only flare producing region on the sun. This makes it a
good candidate for investigating the relation between active regions,
their flare activity, and associated subsurface flows. We will present
the latest results. This work was supported by NASA grant NAG 5-11703.
---------------------------------------------------------
Title: The Effects of Magnetically-Induced Spectral Line Profile
Changes on Helioseismic and Flare Observations
Authors: Edelman, F.; Hill, F.; Howe, R.; Komm, R.
2005AGUSMSP13A..03E Altcode:
We have modeled the effect of changes in the shape of the spectral
line used for the GONG and MDI observations, and we investigate
the consequences for measurements of properties of oscillations and
flares. We find that magnetic field measurements are not very sensitive
to line shape changes, but velocity estimates do strongly depend on
line variations. Using simulated observations of a flare we find that
recently observed associated magnetic field changes are not due to
line shape changes. On the other hand, a simulation of an oscillation
indicates that at least part of the observed amplitude suppression in
an active region is due to variations in the line shape. We also report
preliminary results of the effect of vertical phase variations across
the line profile on the helioseismic observations. This work is carried
out through the National Solar Observatory Research Experiences for
Undergraduate (REU) site program, which is co-funded by the Department
of Defense in partnership with the National Science Foundation REU
Program. This work utilizes data obtained by the Global Oscillation
Network Group (GONG) project, managed by the National Solar Observatory,
which is operated by AURA, Inc. under a cooperative agreement with
the National Science Foundation. The data were acquired by instruments
operated by the Big Bear Solar Observatory, High Altitude Observatory,
Learmonth Solar Observatory, Udaipur Solar Observatory, Instituto de
Astrofísica de Canarias, and Cerro Tololo Interamerican Observatory.
---------------------------------------------------------
Title: How Sensitive are Rotation Inversions to Subtle Features of
the Dynamo?
Authors: Howe, R.; Rempel, M.; Christensen-Dalsgaard, J.; Hill, F.;
Komm, R. W.; Schou, J.; Thompson, M. J.
2004ESASP.559..468H Altcode: 2004soho...14..468H
No abstract at ADS
---------------------------------------------------------
Title: Searching for Subsurface Signatures of X-Class Flares
Authors: Komm, R.; Howe, R.; González Hernández, I.; Hill, F.;
Sudol, J.; Toner, C.
2004ESASP.559..158K Altcode: 2004soho...14..158K
No abstract at ADS
---------------------------------------------------------
Title: Solar Subsurface Flows and Vorticity
Authors: Komm, R.; Howe, R.; González Hernández, I.; Hill, F.;
Haber, D.; Hindman, B.; Corbard, T.
2004ESASP.559..520K Altcode: 2004soho...14..520K
No abstract at ADS
---------------------------------------------------------
Title: Local Frequency Shifts from GONG and MDI
Authors: Howe, R.; Komm, R. W.; González Hernández, I.; Hill, F.;
Haber, D. A.; Hindman, B. W.
2004ESASP.559..484H Altcode: 2004soho...14..484H
No abstract at ADS
---------------------------------------------------------
Title: The Phase of the Torsional Oscillation Pattern
Authors: Howe, R.; Komm, R. W.; Hill, F.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.
2004ESASP.559..476H Altcode: 2004soho...14..476H
No abstract at ADS
---------------------------------------------------------
Title: Meridional Variability from Large-Aperture Ring Diagrams
Authors: González Hernández, I.; Komm, R.; Hill, F.; Howe, R.;
Corbard, T.
2004ESASP.559..444G Altcode: 2004soho...14..444G
No abstract at ADS
---------------------------------------------------------
Title: A tale of Two Regions: Acoustic Power Maps and Magnetic
Activity in AR 10486 and AR 10488
Authors: Howe, R.; Komm, R. W.; González Hernández, I.; Hill, F.;
Haber, D. A.; Hindman, B. W.
2004ESASP.559..480H Altcode: 2004soho...14..480H
No abstract at ADS
---------------------------------------------------------
Title: Flowmaps Covering Six Consecutive Carrington Rotations
Authors: Komm, R.; Howe, R.; Bolding, J.; Donaldson Hanna, K.;
González Hernández, I.; Hill, F.; Toner, C.
2004ESASP.559..516K Altcode: 2004soho...14..516K
No abstract at ADS
---------------------------------------------------------
Title: The Effect of Spectral Line Shape Changes on GONG Observations
of Oscillations and Flares
Authors: Edelman, F.; Hill, F.; Howe, R.; Komm, R.
2004ESASP.559..416E Altcode: 2004soho...14..416E
No abstract at ADS
---------------------------------------------------------
Title: An Automated Image Rejection System for GONG
Authors: Clark, R.; Toner, C.; Hill, F.; Hanna, K.; Ladd, G.; Komm,
R.; Howe, R.; Gonzalez-Hernandez, I.; Kholikov, S.
2004ESASP.559..381C Altcode: 2004soho...14..381C
No abstract at ADS
---------------------------------------------------------
Title: Convection-Zone Dynamics from GONG and MDI, 1995-2004
Authors: Howe, R.; Komm, R. W.; Hill, F.; Christensen-Dalsgaard, J.;
Haber, D. A.; Schou, J.; Thompson, M. J.
2004ESASP.559..472H Altcode: 2004soho...14..472H
No abstract at ADS
---------------------------------------------------------
Title: Activity-related Changes in Local Solar Acoustic Mode
Parameters from Michelson Doppler Imager and Global Oscillations
Network Group
Authors: Howe, R.; Komm, R. W.; Hill, F.; Haber, D. A.; Hindman, B. W.
2004ApJ...608..562H Altcode:
We use the ring-diagram technique of local helioseismology to study
the amplitude and line width of high-degree solar acoustic modes
from 474 days of data from the Michelson Doppler Imager Dynamics
program, covering the period 1996-2002. The 2002 data are compared
with contemporaneous data from the Global Oscillations Network Group
network. The results, once instrumental effects have been removed,
show a strong dependence of the amplitude and lifetime of the modes on
the local magnetic flux, with the amplitude and lifetime decreasing in
the 5 minute band and a reversed trend at high frequencies. We relate
these findings to results from global modes and from other approaches
for analyzing high-degree local oscillations.
---------------------------------------------------------
Title: Vorticity of Solar Subsurface Flows and Torsional Oscillations
Authors: Komm, R.; Howe, R.; Gonzalez-Hernandez, I.; Hill, F.; Haber,
D.; Hindman, B.; Corbard, T.
2004AAS...204.5308K Altcode: 2004BAAS...36Q.757K
We study horizontal flows in the upper solar convection zone
derived from GONG and MDI Dynamics Program data using ring-diagram
analysis. We start exploring the dynamics of the near surface layers
and the interaction between flows and magnetic flux by deriving the
vorticity of the horizontal flow components. A preliminary analysis of
synoptic flow maps shows that the vorticity is enhanced near locations
of active regions. In this study, we focus on the question of how
these flows with vorticity near active regions are related to the
so-called torsional oscillation pattern shown by zonal flows. During
a solar cycle, alternating bands of faster- and slower-than-average
rotation move from high latitudes toward the solar equator with the
faster-than-average band being equatorward of active regions. The
solar-cycle variation of the zonal flows thus contributes to the
vorticity measured in daily or synoptic flow maps. We plan to determine
the size of this contribution and will present our latest results. <P
/>This work was supported by grants from NASA and NSF.
---------------------------------------------------------
Title: Meridional Circulation Variability from Large-Aperture Ring
Diagrams
Authors: Gonzalez-Hernandez, I.; Komm, R.; Corbard, T.; Hill, F.;
Howe, R.
2004AAS...204.5307G Altcode: 2004BAAS...36..756G
Ring Diagram analysis, a local helioseismology technique, has proven
very useful in the study of solar subsurface velocity flows to a
depth of about 0.97Rsun. The depth range is determined by the modes
recovered with this method and thus depends on the size of the area
analyzed. Extending the working area allows us to detect lower l modes
that penetrate deeper into the Sun. However, there is a compromise
between the size of the patch and the validity of the plane wave
approximation used by the technique. <P />Here we search for variability
of the meridional flows as a function of depth for three consecutive
Carrington rotations. We have studied patches of 30-degree diameter over
the solar surface as they crossed the solar central meridian. These
patches are twice the size of the typically studied sections of 15
degrees in diameter. The range of modes recovered with these larger
regions goes down to l 100. A set of 15 overlapping sections, centered
at latitudes 0 +/-7.5,+/-15,+/-22.5,+/-30.0,+/-37.5,+/-45.0 and +/-52.5,
has been analyzed for 25 intervals of 1664 minutes in each Carrington
rotation: CR1987, CR1988 and CR1989. Both GONG and MDI full disk
Dopplergrams have been used for the work. <P />This work was supported
in part by NASA grant NAG5-11703. SOHO is a project of international
cooperation between ESA and NASA. This work utilizes data obtained by
the Global Oscillation Network Group (GONG) Program, managed by the
National Solar Observatory, which is operated by AURA, Inc. under a
cooperative agreement with the National Science Foundation. The data
were acquired by instruments operated by the Big Bear Solar Observatory,
High Altitude Observatory, Learmonth Solar Observatory, Udaipur Solar
Observatory, Instituto de Astrofisica de Canarias, and Cerro Tololo
Interamerican Observatory.
---------------------------------------------------------
Title: Acoustic Power Maps of High Spatial Resolution Sunspot Data
Authors: Astley, V.; Komm, R.; Howe, R.
2004AAS...204.5311A Altcode: 2004BAAS...36..757A
We analyzed a time series of high spatial resolution (.04" per pixel)
intensity images of a sunspot region observed on July 15, 2002 with
the Swedish Solar Telescope to study the interaction of acoustic modes
with small-scale features, such as granules near a sunspot, and their
evolution over short time-scales (approximately one hour). The images
were taken in G-band intensity with a cadence of 22 seconds. The time
series were filtered with Empirical Mode Decomposition to eliminate
low frequency power generated by the evolution of solar surface
features and a simple 1-D FFT was used to obtain the power spectra in
temporal frequency at each spatial location. Preliminary results show
a correlation between small-scale structure and frequency-dependent
acoustic power emission. Granules show increased power output in the
3 mHz range while the same area shows acoustic suppression at higher
frequencies confirming results by other authors. We find that the
distribution of acoustic power is spatially very localized especially
in penumbral filaments and umbral sub-structures of the sunspot. <P
/>This work is carried out through the National Solar Observatory
Research Experiences for Undergraduate (REU) site program, which is
co-funded by the Department of Defense in partnership with the National
Science Foundation REU Program.
---------------------------------------------------------
Title: Solar Subsurface Fluid Dynamics Descriptors Derived from
Global Oscillation Network Group and Michelson Doppler Imager Data
Authors: Komm, R.; Corbard, T.; Durney, B. R.; González Hernández,
I.; Hill, F.; Howe, R.; Toner, C.
2004ApJ...605..554K Altcode:
We analyze Global Oscillation Network Group (GONG) and Michelson Doppler
Imager (MDI) observations obtained during Carrington rotation 1988
(2002 March 30-April 26) with a ring-diagram technique in order to
measure the zonal and meridional flow components in the upper solar
convection zone. We derive daily flow maps over a range of depths up
to 16 Mm on a spatial grid of 7.5d in latitude and longitude covering
+/-60° in latitude and central meridian distance and combine them
to make synoptic flow maps. We begin exploring the dynamics of the
near-surface layers and the interaction between flows and magnetic flux
by deriving fluid dynamics descriptors such as divergence and vorticity
from these flow maps. Using these descriptors, we derive the vertical
velocity component and the kinetic helicity density. For this particular
Carrington rotation, we find that the vertical velocity component is
anticorrelated with the unsigned magnetic flux. Strong downflows are
more likely associated with locations of strong magnetic activity. The
vertical vorticity is positive in the northern hemisphere and negative
in the southern hemisphere. At locations of magnetic activity,
we find an excess vorticity of the same sign as that introduced by
differential rotation. The vertical gradient of the zonal flow is
mainly negative except within 2 Mm of the surface at latitudes poleward
of about 20°. The zonal-flow gradient appears to be related to the
unsigned magnetic flux in the sense that locations of strong activity
are also locations of large negative gradients. The vertical gradient
of the meridional flow changes sign near about 7 Mm, marking a clear
distinction between near-surface and deeper layers. GONG and MDI data
show very similar results. Differences occur mainly at high latitudes,
especially in the northern hemisphere, where MDI data show a counter
cell in the meridional flow that is not present in the corresponding
GONG data.
---------------------------------------------------------
Title: Helioseismic sensing of the solar cycle
Authors: Komm, R.; Howe, R.; Hill, F.
2004cosp...35.1397K Altcode: 2004cosp.meet.1397K
In the last decade, great progress has been made in understanding
the activity-cycle variation of the dynamics and structure of
the solar interior. Observations from SoHO/MDI, GONG, BiSON,
and Mt. Wilson have now provided detailed helioseismic information
over two decades. Parameters such as frequency, width, and amplitude
describing global acoustic modes sense the varying local distribution
of the surface magnetic activity with the solar cycle. Mode width
and amplitude contain information about the damping and excitation of
acoustic modes and hence have implications for the understanding of
the near-surface layers where the acoustic modes are generated. The
frequency variation of the global modes provides information about
the solar structure and interior rotation rate. The rotation rate
in the upper convection zone varies with the solar cycle: the zonal
flows are detectable in at least the upper third of the convection
zone. At the base of the convection zone, the rotation rate varies
with a period of about 1.3 yr which might indicate an exchange of
angular momentum between the radiative interior and the convection
zone. With local helioseismology techniques such as ring-diagram or
time-distance analysis, it is possible to measure the flow component
in the meridional direction. Its variation with depth and solar cycle
can provide insights into the operation of the solar dynamo.
---------------------------------------------------------
Title: A Comparison of Solar p-Mode Parameters from MDI and GONG:
Mode Frequencies and Structure Inversions
Authors: Basu, S.; Christensen-Dalsgaard, J.; Howe, R.; Schou, J.;
Thompson, M. J.; Hill, F.; Komm, R.
2003ApJ...591..432B Altcode:
Helioseismic analysis of solar global oscillations allows investigation
of the internal structure of the Sun. One important test of the
reliability of the inferences from helioseismology is that the
results from independent sets of contemporaneous data are consistent
with one another. Here we compare mode frequencies from the Global
Oscillation Network Group and Michelson Doppler Imager on board SOHO
and resulting inversion results on the Sun's internal structure. The
average relative differences between the data sets are typically less
than 1×10<SUP>-5</SUP>, substantially smaller than the formal errors in
the differences; however, in some cases the frequency differences show
a systematic behavior that might nonetheless influence the inversion
results. We find that the differences in frequencies are not a result
of instrumental effects but are almost entirely related to the data
pipeline software. Inversion of the frequencies shows that their
differences do not result in any significant effects on the resulting
inferences on solar structure. We have also experimented with fitting
asymmetric profiles to the oscillation power spectra and find that,
compared with the symmetric fits, this causes no significant change
in the inversion results.
---------------------------------------------------------
Title: Flow maps from GONG+ ring diagrams
Authors: Komm, R.; Bolding, J.; Corbard, T.; Hill, F.; Howe, R.;
Toner, C.
2003SPD....34.0811K Altcode: 2003BAAS...35..823K
We show first results derived from one month or more of GONG++
data analyzed with a ring-diagram technique as part of the GONG++
local helioseimology analysis pipeline. We focus on observations
obtained during spring 2002 and especially on Carrington rotation 1988
(2002/3/30 - 2002/4/26) and measure horizontal flow components over a
range of depths up to 16 Mm on a spatial grid of 7.5 degree in latitude
and longitude. We calculate zonal and meridional flow components and
compare the average zonal flows with corresponding results of a global
rotation inversion. We create and analyze synoptic maps of large-scale
flows and compare them with corresponding synoptic maps of magnetic
activity. We will present the latest results. <P />RH and RK are
partially supported by NASA Grant S-92698-F. NSO is operated by AURA,
Inc under a co-operative agreement with the National Science Foundation.
---------------------------------------------------------
Title: A Comparison of Low-Degree Solar p-Mode Parameters from BiSON
and GONG: Underlying Values and Temporal Variations
Authors: Howe, R.; Chaplin, W. J.; Elsworth, Y. P.; Hill, F.; Komm,
R.; Isaak, G. R.; New, R.
2003ApJ...588.1204H Altcode:
Approximately 5 years of the l=0 time series from the GONG project
have been analyzed using the algorithm developed for the BiSON
zero-dimensional data. The data cover the period 1995-2000. The results
are compared with those from a parallel analysis of contemporaneous
BiSON data and also with the results of the traditional GONG analysis
of the low-degree time series. The spectra analyzed were prepared
using the multitaper spectral analysis technique used in the recent
reanalysis of the GONG data. We consider both solar cycle trends
and temporally averaged values for mode frequencies, line widths,
amplitudes, and asymmetry parameters.
---------------------------------------------------------
Title: The LoHCo Project. 1 -- Comparison of Ring-Diagram Local
Helioseismology on GONG++, MDI and Mt. Wilson Data Sets
Authors: Bogart, R. S.; Schou, J.; Basu, S.; Bolding, J.; Hill, F.;
Howe, R.; Komm, R. W.; Leibacher, J. W.; Toner, C. G.; Corbard, T.;
Haber, D. A.; Hindman, B. W.; Toomre, J.; Rhodes, E. J.; Rose, P. J.;
LoHCo Project Team
2003SPD....34.0804B Altcode: 2003BAAS...35..822B
Full deployment of the GONG+ enhanced observing network in October
2001 and implementation of ring-diagram helioseismology in the
GONG++ analysis pipeline this year has enabled us to make a detailed
intercomparison of results obtained through multiple paths, from
observation through each of the analysis steps. Such comparisons
can provide a certain degree of validation of the implementations
of the analysis procedures, hints of systematic errors, and better
characterization of the observations, possibly leading to improved
calibrations. The Local Helioseismology Comparison (LoHCo) Project
has been established to provide standards for intercomparison of
results obtained with different local helioseismic analysis techniques
applied to the available observational data sources. We present here
a detailed comparison of ring-diagram determinations of localized
sub-surface flows and frequency shifts obtained from both MDI and
GONG in common observing intervals during Carrington Rotation 1988
(2002/3/30 -- 2002/4/26), using both the MDI and the GONG analysis
pipelines. We also present preliminary results of similar analyses of
data obtained by the Mt. Wilson MOF during the same times. <P />This
work is partially supported by grants from NASA and NSF.
---------------------------------------------------------
Title: Localized Frequency Shifts from GONG+
Authors: Howe, R.; Komm, R. W.; Hill, F.; Bolding, John; Toner, Cliff;
Corbard, Thierry
2003SPD....34.0802H Altcode: 2003BAAS...35Q.822H
Ring Diagram analysis uses 3-dimensional power spectra from small
areas of the solar disk to measure the local frequency of high-degree
modes and follow local flows below the surface. The pipeline for
processing ring diagrams from the 1024x 1024 pixel data generated
by the GONG+ network has now been implemented and the first data
has been analyzed. We will present our latest initial results on the
local variations in the mode frequency and their correlation with the
local magnetic index over a month or more of observations. <P />RK,
CT, and RH in part, are supported by NASA contract S-92698-F. NSO is
operated by AURA, Inc under a co-operative agreement with the National
Science Foundation.
---------------------------------------------------------
Title: Temporal Variation of Angular Momentum in the Solar Convection
Zone
Authors: Komm, R.; Howe, R.; Durney, B. R.; Hill, F.
2003ApJ...586..650K Altcode:
We derive the angular momentum as a function of radius and time with the
help of the rotation rates resulting from inversions of helioseismic
data obtained from the Global Oscillation Network Group (GONG) and
the Michelson Doppler Imager (MDI) and the density distribution from
a model of the Sun. The base of the convection zone can be identified
as a local maximum in the relative angular momentum after subtracting
the contribution of the solid-body rotation. The angular momentum as
a function of radius shows the strongest temporal variation near the
tachocline. This variation extends into the lower convection zone and
into the radiative interior and is related to the 1.3 yr periodicity
found in the equatorial rotation rate of the tachocline. In the upper
convection zone, we find a small systematic variation of the angular
momentum that is related to torsional oscillations. The angular momentum
integrated from the surface to a lower limit in the upper convection
zone provides a hint that the torsional oscillation pattern extends
deep into the convection zone. This is supported by other quantities
such as the coefficients of a fit of Legendre polynomials to the
rotation rates as a function of latitude. The temporal variation of the
coefficient of P<SUB>4</SUB>, indicative of torsional oscillations,
suggests that the signature of these flows in the inversion results
extend to about r~0.83R<SUB>solar</SUB>. With the lower limit of
integration placed in the middle or lower convection zone, the angular
momentum fluctuates about the mean without apparent trend, i.e., the
angular momentum is conserved within the measurement errors. However,
when integrated over the layers slightly below the convection zone
(0.60-0.71R<SUB>solar</SUB>), the angular momentum shows the 1.3 yr
period and hints at a long-term trend that might be related to the
solar activity cycle.
---------------------------------------------------------
Title: Temporal variation of angular momentum in the convection zone
Authors: Komm, R.; Howe, R.; Durney, B. R.; Hill, F.
2003ESASP.517...97K Altcode: 2003soho...12...97K
We derive the angular momentum as a function of radius and time with the
help of the rotation rates resulting from inversions of helioseismic
data obtained from the Global Oscillation Network Group (GONG)
and the Michelson Doppler Imager (MDI) and the density distribution
from a model of the Sun. The angular momentum as a function of radius
shows the strongest temporal variation near the base of the convection
zone. This variation extends into the lower convection zone and into
the radiative interior and is related to the 1.3-yr periodicity found in
the equatorial rotation rate of the tachocline. In the upper convection
zone, we find a small systematic variation of the angular momentum that
is related to torsional oscillations. The angular momentum integrated
from the surface to a lower limit in the upper convection zone provides
a hint that the torsional oscillation pattern extends deep into the
convection zone. With the lower limit of integration placed in the
lower half of the convection zone, the angular momentum fluctuates
about the mean without apparent trend, i.e. the angular momentum is
conserved within the measurement errors. However, when integrated over
the layers slightly below the convection zone, the angular momentum
shows the 1.3-yr period and hints at a long-term trend which might be
related to the solar activity cycle.
---------------------------------------------------------
Title: Activity related variation of width and energy of global
p-modes
Authors: Komm, R.; Howe, R.; Hill, F.
2003ESASP.517..325K Altcode: 2003soho...12..325K
We derived mode width, energy, and energy supply rate from 66 108-day
GONG time series currently processed with multitapers. We show the
temporal variation of these mode parameters from the previous minimum
to the maximum of the current solar cycle localized in latitude. Mode
width and energy of global modes clearly sense the local distribution
of surface magnetic activity. The relation between magnetic activity
and localized mode energy and width is linear within the measurement
uncertainties. The energy supply rate however does not show such
a relation with the latitudinal distribution of surface magnetic
activity. The results presented here are consistent with previously
published results, where we analyzed periodograms instead of
multitapered spectra.
---------------------------------------------------------
Title: Comparing results from the GONG l = 0 and BiSON time series
Authors: Howe, R.; Chaplin, W. J.; Elsworth, Y.; Isaak, G. R.; Komm,
R. W.; New, R.
2003ESASP.517..303H Altcode: 2003soho...12..303H
Approximately 5 years of the l = 0 time series from the GONG project
have been analysed using the algorithm developed for the BiSON
0-dimensional data. The data cover the period 1995-2000. The results
are compared with those from a parallel analysis of contemporaneous
BiSON data, and also with the results of the traditional GONG analysis
of the low-degree time series. The spectra analysed were prepared
using the multitaper spectral analysis technique used in the recent
re-analysis of the GONG data. We consider both solar-cycle trends
and temporally averaged values for mode frequencies, linewidths,
amplitudes and asymmetry parameters.
---------------------------------------------------------
Title: Transient oscillations near the solar tachocline
Authors: Toomre, Juri; Christensen-Dalsgaard, Jorgen; Hill, Frank;
Howe, Rachel; Komm, Rudolf W.; Schou, Jesper; Thompson, Michael J.
2003ESASP.517..409T Altcode: 2003soho...12..409T
We report on further developments in the 1.3-yr quasi-periodic
oscillations reported by Howe et al. (2000). These are small (6 to 8
nHz peak-to-peak) oscillations in the inferred rotation rate near the
bottom of the convection zone and in the outer part of the radiative
interior. The oscillations are strongest and most coherent at about a
fractional radius of 0.72 in the equatorial region. Further monitoring
of the oscillations near the equator shows that they continued for a
period after the end of the data analyzed by Howe et al., but appear to
have now diminished in amplitude. This is reminiscent of the transient
behavior of similar (1.3 to 1.4 yr) periodicities in solar-wind and
geomagnetic datasets previously reported. We speculate that the near
tachocline oscillation is associated with the rising phase of the
solar cycle. We discuss tests performed to eliminate various possible
explanations of the oscillations due to systematic errors in the data
and in their analyses.
---------------------------------------------------------
Title: Localizing the Solar Cycle Frequency Shifts in Global p-Modes
Authors: Howe, R.; Komm, R. W.; Hill, F.
2002ApJ...580.1172H Altcode:
The 6.5 yr span of observations from the Global Oscillation
Network Group and the Michelson Doppler Imager aboard the Solar
and Heliospheric Observatory allows a detailed study of the solar
cycle-related frequency shifts at the level of central frequencies
and a-coefficients from individual multiplets and even of individual
modes within a multiplet. We analyze such data and show that the
shifts at all levels of averaging are consistent with the hypothesis
that the global p-mode frequency shifts are closely related to the
surface magnetic field distribution. Furthermore, the evolution of the
surface magnetic flux distribution can be reconstructed by an inversion
technique operating on the shifts within individual (n, l) multiplets.
---------------------------------------------------------
Title: Detectability of large-scale flows in global helioseismic
data - A numerical experiment
Authors: Roth, M.; Howe, R.; Komm, R.
2002A&A...396..243R Altcode:
Convective motions affect the solar p-modes by shifting their
frequencies. In comparison to the frequency splitting caused by the
differential rotation, this is only a small additional effect. As the
spatial resolution of the inversions for the differential rotation
becomes better, it is important to know how these additional frequency
shifts modify the splitting coefficients and how these two effects might
be disentangled. Therefore we carry out a numerical experiment. We use
quasi-degenerate perturbation theory to create frequencies of p-modes
that are affected by differential rotation and by large-scale flows. The
simulated frequency sets are analyzed and inverted for differential
rotation. We use changes in the (l, nu ) coverage, the multiplets,
and the inversion results as diagnostics to draw conclusions about the
detectability of large-scale flows in global helioseismic data. The
result is a detectability limit of the order of 10 m s<SUP>-1</SUP>
for large-scale flows in the convection zone. A sectoral poloidal
flow with greater amplitude will lead to a noticeable distortion of
the rotation rate, while a zonal poloidal flow with greater amplitude
will lead to distorted even-a coefficients and disrupted multiplets.
---------------------------------------------------------
Title: MDI and GONG inferences of the changing solar interior
Authors: Barban, C.; Howe, R.; Hill, F.; Komm, R. W.; Leibacher, J.;
Toner, C.; Bogart, R.; Braun, D.; Haber, D.; Hindman, B.; Lindsey, C.
2002ESASP.508...55B Altcode: 2002soho...11...55B
The Global Oscillation Network Group (GONG) and the Solar Oscillations
Investigation (SOI) using the Michelson Doppler Imager (MDI) instrument
aboard the SOHO spacecraft provide combined data sets that now cover
more than six years and allow us to probe the changing dynamics of the
convection zone in unprecedented detail. Here we present the latest
combined results from both projects, showing the evolution of the
migrating zonal flows close to the surface and also changes close to
and below the base of the convection zone, as well as changes in the
mode parameters related to surface magnetic activity variation in time
and latitude.
---------------------------------------------------------
Title: Localizing Width and Energy of Solar Global p-Modes
Authors: Komm, R.; Howe, R.; Hill, F.
2002ApJ...572..663K Altcode:
We present the first attempt at localizing in latitude the temporal
variation of mode energy, energy supply rate, and lifetime of global
acoustic modes. We use Global Oscillation Network Group (GONG) and
Michelson Doppler Imager data analyzed with the GONG peak-fitting
algorithm to measure mode width and amplitude of individual (l, n, m)
modes. While measured amplitude and width values are inherently noisier
than frequency measurements, it is possible to use the (m/l) dependence
of these mode parameters to extract their variation in latitude. With
the currently analyzed data sets, we construct maps in time and latitude
of acoustic mode energy, lifetime (inverse of mode width), and energy
supply rate covering the rising phase of the current solar cycle from
the previous minimum to the current maximum. We find that the energy
and width of global modes vary in latitude as well as in time and
that the variation is clearly related to the distribution of magnetic
flux. After removing the average quantity, the residual mode width
shows a linear correlation with magnetic activity with a correlation
coefficient of 0.88, while the corresponding residual mode energy is
anticorrelated with magnetic activity with a correlation coefficient
of -0.90. These mode parameters derived from global p-modes respond to
the local distribution of surface magnetic activity. The energy supply
rate shows no correlation with the latitudinal distribution of magnetic
activity within the limits of the current measurements. We estimate the
variation of global mode energy in response to an individual magnetic
feature, such as a plage, and find that the global mode energy and the
mode lifetime are reduced by about 40% by an active region compared
to the quiet Sun.
---------------------------------------------------------
Title: New Results from GONG `classic' data
Authors: Kras, S.; Howe, R.; Komm, R.; Hill, F.
2002AAS...200.0408K Altcode: 2002BAAS...34R.644K
We reprocessed all 59 108-day time series obtained during the operation
of GONG in `classic' mode (mid-1995--mid-2001) using multitapers. The
multitaper method helps to improve the fitting of mode parameters by
producing smoothed power spectra. The main benefit is an increase
in the number of modes that are fitted well, which leads to an
increase of the order of 10%\ in the number of multiplets for each
time sample. We will compare multitapered with previous untapered
results to show the improvement gained, for example, in the rotation
rates of the solar interior by this reprocessing step. In addition,
we analyzed 1-year and 3-year GONG classic time series in order to
improve the number of well-fitted modes at low frequencies (below 1.5
mHz). Preliminary results show an improvement in the number of modes at
low frequencies is gained by increasing the length of the time series
from 108 days to three years. We will present our latest results. The
Global Oscillation Network Group (GONG) project is managed by the
National Solar Observatory, which is operated by AURA, Inc. under a
cooperative agreement with the National Science Foundation.
---------------------------------------------------------
Title: Temporal Variation of Angular Momentum in the Solar Convection
Zone
Authors: Komm, R.; Howe, R.; Durney, B.; Hill, F.
2002AAS...200.0404K Altcode: 2002BAAS...34Q.644K
We present the temporal variation of the solar angular momentum
derived from helioseismic observations. In the absence of `true'
angular momentum inversions, we use the rotation rates resulting from
rotation inversions of GONG data and the density distribution from a
model of the Sun. We focus especially on the layers near the base of
the convection zone and the layers near the solar surface. We derive
the angular momentum as a function of depth and the corresponding
solid-body rotation. The angular momentum decreases with increasing
radius following essentially the product of density times the fourth
power of radius. The tachocline can be identified as a local maximum
in the radial gradient of the angular momentum and as a local maximum
in the relative angular momentum after subtracting the contribution
of the solid-body rotation. The angular momentum shows the strongest
temporal variation near the tachocline. This variation is reminiscent
of the 1.3-yr periodicity found in the equatorial rotation rate of the
tachocline, which is not too surprising since the angular momentum of
a spherical shell is heavily weighted toward the equator. We discuss
the extension of this variation into the convection zone and into
the radiative interior. In addition, we fit the rotation rates as
functions of latitude with Legendre polynomials to cross-validate
the numerical results and to draw conclusions about the zonal flows
(`torsional oscillations') in the upper convection zone. This work
was supported by NASA Grant S-92698-F.
---------------------------------------------------------
Title: A Comparison of Solar p-Mode Parameters from the Michelson
Doppler Imager and the Global Oscillation Network Group: Splitting
Coefficients and Rotation Inversions
Authors: Schou, J.; Howe, R.; Basu, S.; Christensen-Dalsgaard, J.;
Corbard, T.; Hill, F.; Komm, R.; Larsen, R. M.; Rabello-Soares, M. C.;
Thompson, M. J.
2002ApJ...567.1234S Altcode:
Using contemporaneous helioseismic data from the Global Oscillation
Network Group (GONG) and Michelson Doppler Imager (MDI) onboard SOHO,
we compare frequency-splitting data and resulting inversions about the
Sun's internal rotation. Helioseismology has been very successful in
making detailed and subtle inferences about the solar interior. But
there are some significant differences between inversion results
obtained from the MDI and GONG projects. It is important for making
robust inferences about the solar interior that these differences are
located and their causes eliminated. By applying the different analysis
pipelines developed by the projects not only to their own data but
also to the data from the other project, we conclude that the most
significant differences arise not from the observations themselves
but from the different frequency estimation analyses used by the
projects. We find that the GONG pipeline results in substantially fewer
fitted modes in certain regions. The most serious systematic differences
in the results, with regard to rotation, appear to be an anomaly in
the MDI odd-order splitting coefficients around a frequency of 3.5 mHz
and an underestimation of the low-degree rotational splittings in the
GONG algorithm.
---------------------------------------------------------
Title: Solar-cycle variation of the sound-speed asphericity from
GONG and MDI data 1995-2000
Authors: Antia, H. M.; Basu, S.; Hill, F.; Howe, R.; Komm, R. W.;
Schou, J.
2001MNRAS.327.1029A Altcode: 2001astro.ph..9326A
We study the variation of the frequency splitting coefficients
describing the solar asphericity in both GONG and MDI data, and
use these data to investigate temporal sound-speed variations as a
function of both depth and latitude during the period 1995-2000 and a
little beyond. The temporal variations in even splitting coefficients
are found to be correlated to the corresponding component of magnetic
flux at the solar surface. We confirm that the sound-speed variations
associated with the surface magnetic field are superficial. Temporally
averaged results show a significant excess in sound speed around
r=0.92R<SUB>solar</SUB> and latitude of 60°.
---------------------------------------------------------
Title: Empirical Mode Decomposition and Hilbert Analysis Applied to
Rotation Residuals of the Solar Convection Zone
Authors: Komm, R. W.; Hill, F.; Howe, R.
2001ApJ...558..428K Altcode:
We apply empirical mode decomposition (EMD) and Hilbert analysis
to time series of rotation residuals at all latitudes and at all
depths in the convection zone derived from 49 Global Oscillation
Network Group data sets covering the period 1995 May 7 to 2000
May 15. Hilbert analysis combined with EMD is a tool to analyze
nonlinear and nonstationary signals and is used to localize events
in time-frequency space. We calculate Hilbert power spectra, power
as a function of time and frequency, for each time series in order to
determine whether the rotation rate in the convection zone shows any
other systematic temporal variation besides the so-called torsional
oscillation pattern in the upper convection zone and the periodicity
of 1.3 yr near the base of the convection zone. In other regions of the
convection zone, the temporal variations of the rotation residuals are
compatible with a noise signal except near about 0.86 R<SUB>solar</SUB>
in radius, where we find indications of a long-term period of about 6
yr. However, it is uncertain whether this signal is of solar origin,
since the available data set is too short to rule out the possibility
of an artifact. In addition, we calculate the amount of power contained
in the torsional oscillation signal as a function of time, latitude, and
radius to study the variation of the torsional oscillation pattern. The
depth to which the pattern extends apparently changes with time. For
example, at midlatitudes the pattern extends to deeper layers with
increasing time. The degree of stationarity doubles from the surface
to about 0.92 R<SUB>solar</SUB> in radius, which indicates that the
torsional oscillation pattern disappears with increasing depth in
agreement with previous results.
---------------------------------------------------------
Title: Asymmetric Line Profiles Applied to Gong Helioseismic Data
Authors: Landy, D.; Howe, R.; Komm, R.; Hill, F.
2001AGUSM..SP21C04L Altcode:
P-mode frequencies have traditionally been fit using a symmetric
Lorentzian profile. Due to clear asymmetries in the peaks, researchers
have begun moving toward asymmetric near-Lorentzian profiles; amongst
the most popular is the Nigam profile. In this paper, results are
presented from a fit of the Nigam profile to two datasets, both
taken from GONG data, using a new peak fitting engine (PEAKFIND
Mark II). Details of PEAKFIND Mark II are presented. Results of the
Nigam model fits are compared with those achieved using a symmetric
Lorentzian line profile (those reported by the GONG network). Results
are also shown to be largely consistent with other published fits in
the low frequency region.
---------------------------------------------------------
Title: Detectability of Large-Scale Convection in Global Helioseismic
Data
Authors: Roth, M.; Howe, R.; Komm, R. W.
2001AGUSM..SP31A08R Altcode:
Convection affects the solar p-modes by additonally shifting the
frequencies. This effect is small in comparison to the frequency
splitting caused by the differential rotation. But, as the spatial
resolution of the inversions for the differential rotation becomes
better, it is important to know how these additional frequency shifts
are blended into the splitting coefficients and how both might be
disentangled. Therefore we carry out a numerical experiment. We
calculate with quasi-degenerate perturbation theory the frequencies
of p-modes that are affected by differential rotation and large-scale
convection cells. This simulated data are inverted for differential
rotation, and on that basis upper limits for the detectability of
large-scale convection in global helioseismic data are derived.
---------------------------------------------------------
Title: Hilbert Spectral Analysis Applied to Helioseismic Time Series
Authors: Komm, R.; Hill, F.; Howe, R.
2001AGUSM..SP31A07K Altcode:
We apply Empirical Mode Decomposition and Hilbert spectral analysis
to helioseismic time series to study excitation and damping of solar
p-modes. We use time series from the Solar Oscillations Investigation
(SOI) project using the Michelson Doppler Imager (MDI) aboard the Solar
and Heliospheric Observatory (SOHO) spacecraft. The Hilbert spectral
analysis is a tool to analyze nonlinear and nonstationary signals and
is used to localize events in time-frequency space. The solar acoustic
oscillations are thought to be stochastically excited by the release of
acoustic energy from sources near the top of the turbulent convection
zone of the Sun. Individual modes are present during some time periods
and absent during others. We analyze time series of different l and m
values in order to detect individual excitation or damping events. In
addition, we compare data sets obtained during different levels of
solar magnetic activity to study the influence of magnetic activity on
solar p-modes. We will present the latest results of this investigation.
---------------------------------------------------------
Title: Comparing Global Solar Rotation Results from MDI and GONG
Authors: Howe, R.; Komm, R. W.; Hill, F.; Christensen-Dalsgaard, J.;
Schou, J.; Thompson, M. J.; Corbard, T.
2001AGUSM..SP31A14H Altcode:
The GONG (Global Oscillations Network Group) project and the Solar
Oscillations Investigation (SOI) using the Michelson Doppler Imager
(MDI) instrument aboard the SOHO spacecraft have jointly accumulated
more than five years of data on medium-degree solar p-modes, including
nearly four years of contemporaneous observations. The inferences of
interior solar rotation from the two projects are broadly consistent
and show similar temporal variations, but there are also significant
systematic differences. We report here on the results of an ongoing
attempt to cross-compare the results and analysis techniques of the
two projects. Three 108-day periods, at low, medium and high solar
activity epochs, have been analysed, with both MDI and GONG analysis
being applied to each data set, and the results are compared.
---------------------------------------------------------
Title: Solar Cycle Changes in p-Mode Frequencies and Asphericity
1995-2000
Authors: Howe, R.; Hill, F.; Komm, R. W.
2001AGUSM..SP21C03H Altcode:
With 5 years of analysed data from the GONG (Global Oscillation Network
Group) project and the Solar Oscillations Investigation (SOI) using
the Michelson Doppler Imager (MDI) aboard the SOHO spacecraft, we can
investigate the solar-cycle changes in the medium-degree solar p-mode
frequencies in much more detail than has previously been possible. The
quality of the data allows us to study the variations in the central
frequencies of individual (l,n) multiplets, and also to demonstrate
that the latitudinal variation of the frequency changes within a
multiplet correlates closely with the latitudinal distribution of
surface magnetic activity. We report here on the latest results of
such an investigation, and the implications for our understanding of
the relationship between p-mode frequencies and solar activity.
---------------------------------------------------------
Title: Variations in Rotation Rate Within the Solar Convection Zone
From GONG and MDI 1995-2000
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Schou, J.; Thompson, M. J.; Toomre, J.
2001AGUSM..SP31A15H Altcode:
Helioseismic measurements with the Michelson Doppler Imager (MDI)
instrument aboard SOHO, and complementary measurements from the Global
Oscillation Network Group (GONG) project, are revealing changes deep
within the Sun as the solar cycle progresses. We will present the
latest results based on recent data from both experiments, including
flows in the upper part of the convection zone and variations in the
rotation rate near its base.
---------------------------------------------------------
Title: Hilbert Spectral Analysis Applied to Rotation Residuals of
the Solar Convection Zone
Authors: Komm, R.; Hill, F.; Howe, R.
2001AGUSM..SP31A06K Altcode:
We apply Empirical Mode Decomposition and Hilbert spectral analysis to
time series of rotation residuals at all latitudes and at all depths
in the solar convection zone derived from 49 data sets obtained by
the Global Oscillation Network Group (GONG) project covering the
period 1995 May 7 to 2000 May 15. The Hilbert spectral analysis is
a tool to analyze nonlinear and nonstationary signals and is used
to localize events in time-frequency space. We calculate Hilbert
power spectra, power as a function of time and frequency, for each
time series in order to determine whether the rotation rate in the
solar convection zone shows any other systematic temporal variation
besides the torsional oscillation pattern in the upper convection zone
and the 1.3-yr periodicity near the base of the convection zone. In
addition, we calculate the amount of power contained in the torsional
oscillation signal as a function of time, latitude, and radius to study
the variation of the torsional oscillation pattern. For example, the
degree of stationarity of the torsional oscillation doubles between
surface layers and a depth of about 8%\ of the solar radius. This
indicates that the torsional oscillation pattern disappears with
increasing depth in agreement with previous studies. We will present
the latest results of this investigation.
---------------------------------------------------------
Title: Studying asphericity in the solar sound speed from MDI and
GONG data
Authors: Antia, H. M.; Basu, S.; Hill, F.; Howe, R.; Komm, R. W.;
Schou, J.
2001ESASP.464...45A Altcode: 2001soho...10...45A
We study the variation of the frequency splitting coefficients
describing the solar asphericity in both GONG and MDI data, and
use these data to investigate temporal sound-speed variations as a
function of both depth and latitude during the period 1995-2000. The
temporal variations in even splitting coefficients are found to
be correlated with the corresponding component of magnetic flux at
the solar surface. The sound-speed variations associated with the
surface magnetic field appear to be superficial. Temporally averaged
results show a significant excess in sound speed around r = 0.92
R<SUB>solar</SUB> and latitude of 60°.
---------------------------------------------------------
Title: Comparing mode frequencies from MDI and GONG
Authors: Howe, R.; Hill, F.; Basu, S.; Christensen-Dalsgaard, J.;
Komm, R. W.; Munk Larsen, R.; Roth, M.; Schou, J.; Thompson, M. J.;
Toomre, J.
2001ESASP.464..137H Altcode: 2001soho...10..137H
We present results of analyses of MDI and GONG time series covering
the same time intervals, and using both the MDI and GONG peakbagging
algorithms. We discuss some of the likely causes of differences between
the inferred frequencies and frequency splittings. In addition, we
consider the effect of these differences on the results of inversions
for the solar internal rotation and sound speed.
---------------------------------------------------------
Title: Solar cycle changes in convection zone dynamics from MDI and
GONG 1995 - 2000
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Munk Larsen, R.; Schou, J.; Thompson, M. J.; Toomre, J.
2001ESASP.464...19H Altcode: 2001soho...10...19H
The combined GONG and MDI medium-degree helioseismic data sets now cover
just over 5 years and allow us to probe the changing dynamics of the
convection zone in unprecedented detail. Here we present the latest
results from both projects, showing the evolution of the migrating
zonal flows close to the surface and also changes close to and below
the base of the convection zone.
---------------------------------------------------------
Title: Background amplitudes of solar p-modes observed by GONG
Authors: Komm, R.; Howe, R.; Hill, F.
2001ESASP.464..645K Altcode: 2001soho...10..645K
We analyzed the mode background amplitudes, derived from the 45
currently processed 108-day GONG time series. We found that the
background amplitudes and their solar-cycle variation qualitatively
resembles more the mode amplitude, A, than the quantity mode
amplitude times width squared, AΓ<SUP>2</SUP>. If the measured
background consists only of the tails of p-modes and leaks, then
the background amplitude should qualitatively follow the behavior of
AΓ<SUP>2</SUP>. This unexpected behavior might be a subtle artifact
of the temporal window correction or it might indicate that the tails
of leaks and modes are not the only contributors to the measured
background.
---------------------------------------------------------
Title: Interior Solar-Cycle Changes Detected by Helioseismology
Authors: Howe, R.; Hill, F.; Komm, R. W.; Christensen-Dalsgaard, J.;
Munk Larsen, R.; Schou, J.; Thompson, M. J.; Toomre, J.
2001IAUS..203...40H Altcode:
Helioseismic measurements with the MDI instrument aboard SOHO,
and complementary measurements from the GONG network, are revealing
changes deep within the Sun as the solar cycle progresses. We will
present results based on recent data from both experiments, including
variations in the rotation rate deep inside the convection zone.
---------------------------------------------------------
Title: The effect of magnetic flux distribution on individual-m
frequencies
Authors: Howe, R.; Landy, D. H.; Komm, R. W.; Hill, F.
2001ESASP.464...91H Altcode: 2001soho...10...91H
The GONG PEAKFIND algorithm generates a frequency for each m in an l,n
multiplet. For most purposes, we then fit orthogonal polynomials to the
frequencies to derive α-coefficients. The even-order coefficients are
strongly correlated with the distribution of the magnetic flux. With
over 4 years of GONG data, we can now demonstrate that the frequencies
of individual n, l, m components experience shifts correlated with
the surface flux distribution in the region sampled by that mode. At
high activity levels, this can give rise to visible distortion of
the "S-curve" shape within a multiplet, which in turn means that
higher-order α coefficients are needed to correctly represent the
shape and estimate the central frequency of the multiplet.
---------------------------------------------------------
Title: Width and energy of solar p-modes observed by GONG 1995 - 1999
Authors: Komm, R.; Howe, R.; Hill, F.
2001ESASP.464...33K Altcode: 2001soho...10...33K
We present measurements of mode width, mean-square velocity power, the
energy per mode, and the energy supply rate, derived from all currently
processed 108-day GONG time series and discuss their implications
for p-mode excitation and damping. The mode width shows the familiar
"plateau" between 2.5 and 3.1 mHz with a "dip" near 2.9 mHz. This
dip is most prominent during solar-cycle minimum and disappears with
increasing magnetic activity. The mode energy, which reaches a maximum
value of about 2.2×10<SUP>28</SUP> erg near 3.15 mHz, decreases with
increasing activity. The decrease is frequency dependent and shows
a maximum near 3 mHz with a change of about -13% from the previous
activity minimum to the currently highest level of activity. The energy
supply rate, reaching a maximum value of about 2.5×10<SUP>23</SUP>
erg s<SUP>-1</SUP> near 3.6 mHz, decreases on average by about 3%
and its solar-cycle variation shows no frequency dependenc which
is in marked contrast to the other mode parameters. Therefore, the
variation in the energy supply rate might be compatible with a zero
change. We speculate that the excess of the supplied energy might be
transferred to the increasing number of flux tubes and might, in this
way, contribute to the irradiance variation.
---------------------------------------------------------
Title: Exploring time series analysis techniques
Authors: Komm, R.; Hill, F.; Howe, R.; Toner, C.
2001ESASP.464..351K Altcode: 2001soho...10..351K
Multitaper power spectra show greatly reduced noise compared
to single taper spectra, such as periodograms. We performed a
random-restart test to show that multitaper spectra do not bias the
fitted mode parameters. Then, we show a different way to increase the
signal-to-noise ratio of spectra by calculating interleaved shifted
cross-spectra. Finally, we start exploring the Hilbert spectral analysis
which is a tool to localize events in time-frequency space.
---------------------------------------------------------
Title: Width and Energy of Solar p-Modes Observed by Global
Oscillation Network Group
Authors: Komm, R. W.; Howe, R.; Hill, F.
2000ApJ...543..472K Altcode:
We present measurements of mode width, Γ, and mean square
velocity power, 2>, derived from all currently processed 108 day
Global Oscillation Network Group (GONG) time series and discuss
their implications for p-mode excitation and damping. Assuming
stochastic excitation, we estimate the energy per mode, E, and the
energy supply rate, dE/dt. For modes with l=9-150, the mean square
velocity power and the mode energy peak at about 3.15 mHz reaching
values of 2>~1.4×10<SUP>3</SUP> cm<SUP>2</SUP> s<SUP>-2</SUP>
and E~2.2×10<SUP>28</SUP> ergs. The energy supply rate reaches
a maximum value of dE/dt~2.5×10<SUP>23</SUP> ergs s<SUP>-1</SUP>
near 3.6 mHz. The mode width shows the familiar “plateau” between
2.5 and 3.1 mHz with a “dip” near 2.9 mHz, which is strongest
for l~40. This dip is most prominent during solar cycle minimum and
disappears with increasing magnetic activity. The energy supply rate
decreases on average by about 2.7% from the previous activity minimum
to the currently highest level of activity. The solar cycle variation
of dE/dt shows no frequency dependence, which is in marked contrast
to the other mode parameters. The mode parameters are adequately
represented by power laws in several frequency ranges, for example,
dE/dt~ν<SUP>6.89+/-0.07</SUP> for 2.4 mHz<=ν<3.0 mHz and
dE/dt~ν<SUP>-5.62+/-0.27</SUP> for 3.75 mHz<=ν<4.5 mHz. The
solar cycle variation of these parameters can then be expressed as
changes of a few percent in the power-law exponents and multipliers. Our
results agree reasonably well with previous studies of Birmingham
Solar-Oscillations Network and Big Bear Solar Observatory data.
---------------------------------------------------------
Title: Studying Asphericity in the Solar Sound Speed from MDI and
GONG Data 1995-1999
Authors: Schou, J.; Antia, H. M.; Basu, S.; Howe, R.; Hill, F.; Komm,
R. W.
2000SPD....31.0111S Altcode: 2000BAAS...32..803S
We study the variation of the frequency splitting coefficients
describing the solar asphericity in both GONG and MDI data, and use
these data to investigate temporal sound-speed variations as a function
of both depth and latitude during the period 1995--99. We confirm that
the sound-speed variations associated with the surface magnetic field
are superficial.
---------------------------------------------------------
Title: Solar-Cycle Changes in Convection-Zone Dynamics from SOI and
GONG Data
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
2000SPD....31.0113H Altcode: 2000BAAS...32..803H
The combined GONG and MDI medium-degree helioseismic data sets now cover
more than 4.5 years and allow us to probe the changing dynamics of the
convection zone in unprecedented detail. Here we present the latest
results from both projects, showing the evolution of the migrating zonal
flows close to the surface and also changes close to and below the base
of the convection zone. This work utilizes data obtained by the Global
Oscillation Network Group (GONG) project, managed by the National Solar
Observatory, a Division of the National Optical Astronomy Observatories,
which is operated by AURA, Inc. under a cooperative agreement with the
National Science Foundation. SOHO is a joint project of ESA and NASA.
---------------------------------------------------------
Title: Width and Energy of Solar P-Modes Observed by GONG
Authors: Komm, R. W.; Howe, R.; Hill, F.
2000SPD....31.0114K Altcode: 2000BAAS...32..803K
We present mode width, Γ , and mean-square velocity power, <
v<SUP>2</SUP> >, derived from all currently processed 108-day GONG
time series and discuss their implications for p-mode excitation and
damping. Assuming stochastic excitation, we estimate the energy per
mode, E, and the energy supply rate, dE / dt. For modes with l =
9 - 150, the mean-square velocity power and the mode energy peak
at about 3.15 mHz reaching values of < v<SUP>2</SUP> > ≈
1.4 ; 10<SUP>3</SUP> cm<SUP>2</SUP> s<SUP>-2</SUP> and E ≈ 2.2 ;
10<SUP>28</SUP> ergs. The energy supply rate reaches a maximum value of
dE / dt ≈ 2.5 ; 10<SUP>23</SUP> ergs s<SUP>-1</SUP> near 3.6 mHz. The
mode width shows the familiar `plateau' between 2.5 and 3.1 mHz with a
`dip' near 2.9 mHz, which is strongest for l ≈ 40. This dip is most
prominent during solar-cycle minimum and disappears with increasing
magnetic activity. The energy supply rate decreases on average
by about 2.7%\ from the previous activity minimum to the currently
highest level of activity. The solar-cycle variation of dE / dt shows
no frequency dependence, which is in marked contrast to the other mode
parameters. The mode parameters are adequately represented by power laws
in several frequency ranges, for example, dE / dt ~ ν <SUP>6.89</SUP>
+/- 0.07 for 2.4 <= ν < 3.0 mHz and dE / dt ~ ν <SUP>-5.62</SUP>
+/- 0.27 for 3.75 <= ν < 4.5 mHz. The solar-cycle variation of
these parameters can then be expressed as changes of a few percent in
the power-law exponents and multipliers. Our results agree reasonably
well with previous studies of BiSON and BBSO data.
---------------------------------------------------------
Title: Deeply Penetrating Banded Zonal Flows in the Solar Convection
Zone
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
2000ApJ...533L.163H Altcode: 2000astro.ph..3121H
Helioseismic observations have detected small temporal variations
of the rotation rate below the solar surface that correspond to the
so-called “torsional oscillations” known from Doppler measurements of
the surface. These appear as bands of slower- and faster-than-average
rotation moving equatorward. Here we establish, using complementary
helioseismic observations over 4 yr from the GONG network and from
the MDI instrument on board SOHO, that the banded flows are not
merely a near-surface phenomenon: rather, they extend downward at
least 60 Mm (some 8% of the total solar radius) and thus are evident
over a significant fraction of the nearly 200 Mm depth of the solar
convection zone.
---------------------------------------------------------
Title: Variations in solar sub-surface rotation from GONG data
1995-1998
Authors: Howe, R.; Komm, R.; Hill, F.
2000SoPh..192..427H Altcode:
We have completed an analysis of the first 35 GONG Months (1 GM = 36
days) covering the last solar minimum and the rising phase of cycle
23. The mode parameters have been estimated from 33 time series,
each of 3-GM duration, with centers spaced by 1 GM. We report on the
temporal evolution of the rotational splitting coefficients up to 15th
order. The coefficients do not correlate well with any surface magnetic
flux measure yet considered, but we find small but significant trends in
their temporal evolution. Inverting the coefficients for two-dimensional
rotation information and looking at deviations from the mean produces
a picture of a systematic zonal flow migrating towards lower latitudes
during the rising phase of the cycle. This flow is probably associated
with the torsional oscillation. Similar trends are seen in the 1986
-1990 BBSO data.
---------------------------------------------------------
Title: Dynamic Variations at the Base of the Solar Convection Zone
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
2000Sci...287.2456H Altcode:
We have detected changes in the rotation of the sun near the base of
its convective envelope, including a prominent variation with a period
of 1.3 years at low latitudes. Such helioseismic probing of the deep
solar interior has been enabled by nearly continuous observation of
its oscillation modes with two complementary experiments. Inversion of
the global-mode frequency splittings reveals that the largest temporal
changes in the angular velocity Ω are of the order of 6 nanohertz
and occur above and below the tachocline that separates the sun's
differentially rotating convection zone (outer 30% by radius) from
the nearly uniformly rotating deeper radiative interior beneath. Such
changes are most pronounced near the equator and at high latitudes and
are a substantial fraction of the average 30-nanohertz difference in Ω
with radius across the tachocline at the equator. The results indicate
variations of rotation close to the presumed site of the solar dynamo,
which may generate the 22-year cycles of magnetic activity.
---------------------------------------------------------
Title: Solar-Cycle Changes in Gong P-Mode Widths and Amplitudes
1995-1998
Authors: Komm, R. W.; Howe, R.; Hill, F.
2000ApJ...531.1094K Altcode:
We search for a solar-cycle variation in mode widths and amplitudes
derived from 3 month GONG time series. The variation of mode width
and amplitude observed in GONG data are the combined effects of
fill factor, temporal variation, and measurement uncertainties. The
largest variation is caused by the fill factor resulting in modes with
increased width and reduced amplitude when fill is lower. We assume
that the solar-cycle variation is the only other systematic variation
beside the temporal window function effect. We correct all currently
available data sets for the fill factor and simultaneously derive the
solar-cycle variation. We find an increase of about 3% on average in
mode width from the previous minimum to October 1998 and a decrease of
about 7% and 6% in mode amplitude and mode area (widthxamplitude). We
find no l dependence of the solar-cycle changes. As a function of
frequency, these changes show a maximum between 2.7 and 3.3 mHz with
about 47% higher than average values for mode width and about 29%
and 36% higher ones for mode amplitude and area. We estimate the
significance of these rather small changes by a prewhitening method
and find that the results are significant at or above the 99.9% level,
with mode area showing the highest level of significance and mode width
the lowest. The variation in background amplitude is most likely not
significant and is consistent with a zero change.
---------------------------------------------------------
Title: The Solar Cycle is More than Skin Deep!
Authors: Komm, R.; Howe, R.; Hill, F.
2000ESASP.463...15K Altcode: 2000sctc.proc...15K
No abstract at ADS
---------------------------------------------------------
Title: Helioseismic detection of temporal variations of solar rotation
rate near the base of the convection zone
Authors: Howe, R.; Christensen-Dalsgaard, J.; Hill, F.; Komm, R. W.;
Larsen, R. M.; Schou, J.; Thompson, M. J.; Toomre, J.
1999AAS...19510702H Altcode: 1999BAAS...31R1530H
The differential rotation of the Sun and its ability to generate
large-scale magnetic fields through cyclic dynamo action appear to be
intimately linked. It is now commonly thought that the global dynamo
behavior responsible for the emergence of active regions is derived
from strong organized toroidal magnetic fields generated by rotational
shear in a thin region (the tachocline) at the base of the convection
zone. The magnetic field could well have a feedback effect on the fluid
motions in the region. We are thus motivated to use helioseismology
to look for changes in rotation profiles near the tachocline as the
Sun's magnetic cycle progresses. This approach has become possible
using frequency-splitting data for p- and f-mode oscillations derived
over four years (from May 1995 to Sept 1999) of full-disk Doppler
observations from the ground-based Global Oscillation Network Group
(GONG) project and from the Michelson Doppler Imager (MDI) experiment
aboard the SOHO spacecraft. Inversions using two different methods of
the splittings from these two independent data sets reveal systematic
variations of the rotation rate close to the base of the convection
zone, with different behavior at low and high latitudes. Notable
are variations of order 6 nHz in rotation rates near the equator,
to be compared with the radial angular velocity contrast across the
tachocline of about 30 nHz. These exhibit several nearly repetitive
changes with a period of about 1.2-1.4 years and appear to be real
changes in the deep convection zone and tachocline rotation rates that
need to be followed as the solar cycle progresses. The GONG project is
managed by the National Solar Observatory, a Division of the National
Optical Astronomy Observatories, which is operated by AURA, Inc. under
a cooperative agreement with the National Science Foundation. SOHO is
a joint project of ESA and NASA.
---------------------------------------------------------
Title: Solar Cycle Changes in GONG P-Mode Frequencies, 1995-1998
Authors: Howe, R.; Komm, R.; Hill, F.
1999ApJ...524.1084H Altcode:
We have analyzed 27 3 month sets of Global Oscillaiton Network Group
(GONG) data from the end of cycle 22 and the beginning of cycle
23 and here present evidence of significant shifts in the central
frequencies and the even a-coefficients of the frequency splittings of
the modes. The temporal behavior of the even a-coefficients is better
reproduced by the corresponding coefficients of a Legendre polynomial
decomposition of the surface magnetic field than by the total flux;
i.e., the temporal variation is strongly correlated with the latitudinal
distribution of the surface magnetic activity. These changes are
consistent with available data from previous solar cycles. The even
a-coefficients, which sense the asphericity of the solar structure,
appear to show similar temporal evolution at all depths. The odd
a-coefficients, which sense the internal differential rotation, show
no significant variation with time or depth. In particular they show
no significant correlation with either the magnetic flux or with the
corresponding odd Legendre components of the flux. This suggests that
the solar cycle related variation of the oscillation frequencies is
not due to contamination of observed Doppler shifts by the surface
magnetic fields.
---------------------------------------------------------
Title: Multitaper Spectral Analysis and Wavelet Denoising Applied
to Helioseismic Data
Authors: Komm, R. W.; Gu, Y.; Hill, F.; Stark, P. B.; Fodor, I. K.
1999ApJ...519..407K Altcode:
Estimates of solar normal mode frequencies from helioseismic
observations can be improved by using multitaper spectral analysis
(MTSA) to estimate spectra from the time series, then using wavelet
denoising of the log spectra. MTSA leads to a power spectrum
estimate with reduced variance and better leakage properties than
the conventional periodogram. Under the assumption of stationarity
and mild regularity conditions, the log multitaper spectrum has a
statistical distribution that is approximately Gaussian, so wavelet
denoising is asymptotically an optimal method to reduce the noise in
the estimated spectra. We find that a single m-ν spectrum benefits
greatly from MTSA followed by wavelet denoising and that wavelet
denoising by itself can be used to improve m-averaged spectra. We
compare estimates using two different five-taper estimates (Slepian
and sine tapers) and the periodogram estimate for Global Oscillation
Network Group (GONG) time series at selected angular degrees l. We
compare those three spectra with and without wavelet denoising,
both visually and in terms of the mode parameters estimated from the
preprocessed spectra using the GONG peak-fitting algorithm. The two
multitaper estimates give equivalent results. The number of modes
fitted well by the GONG algorithm is 20%-60% larger (depending on l
and the temporal frequency) when applied to the multitaper estimates
than when applied to the periodogram. The estimated mode parameters
(frequency, amplitude, and width) are comparable for the three power
spectrum estimates, except for modes with very small mode widths (a
few frequency bins), where the multitaper spectra broaden the modes
compared with the periodogram. At frequencies below 3 mHz, wavelet
denoising of the log multitaper power spectra tends to increase the
number of modes for which the GONG peak-fitting algorithm converges
well. Close to 3 mHz, where all modes are resolved, wavelet denoising
makes little difference. At higher frequencies close to the acoustic
cutoff frequency, where modes are blended into ridges, wavelet denoising
the multitaper spectra reduces the number of good fits. We tested the
influence of the number of tapers used and found that narrow modes
at low n-values are broadened to the extent that they can no longer
be fitted if the number of tapers is too large. For helioseismic time
series of this length and temporal resolution, the optimal number of
tapers is less than 10.
---------------------------------------------------------
Title: Solar Cycle Changes in GONG Data 1995-1998
Authors: Komm, R.; Howe, R.; Hill, F.
1999AAS...194.5601K Altcode: 1999BAAS...31..911K
The GONG project has now analysed 3-month velocity time series
covering the period late 1995 to mid-1998, covering the end of solar
cycle 22 and the beginning of cycle 23. We here present an analysis
of the highly significant shifts in the central frequencies and the
a_2 and a_4 coefficients of the modes, and relate them to magnetic
activity indices and to the corresponding coefficients of a Legendre
polynomial decomposition of the surface magnetic field. These changes
are confirmed by an analysis of the MDI-SOI time series for some of
the equivalent time periods using the GONG peakfinding algorithm,
and are consistent with available data from previous solar cycles. In
addition, we study mode widths and amplitudes derived from the GONG
data and search for a variation of these mode parameters with the solar
cycle. With increased activity from cycle minimum to mid-1998, we find
a small increase in mode width of about 2% on average and about 4% in
the frequency range from 2.9 to 3.3 mHz. We find a decrease of similar
size in mode amplitude and mode area (width times amplitude). The
change in background amplitude is not significant and is consistent
with a zero change.
---------------------------------------------------------
Title: Helioseismology and the Solar Cycle
Authors: Komm, R.
1999AAS...194.4204K Altcode: 1999BAAS...31R.882K
Helioseismic data show temporal variations that are clearly related to
the solar cycle. It is well-established that the central frequencies
of p-modes vary with solar activity, while it is a recent result
that the even-a coefficients vary with the corresponding components
of the surface magnetic fields. The frequency variations seem to
be mainly due to near surface effects, but the physical relation
between these variations and the surface magnetic fields has not been
established yet. The solar-cycle variation of other mode parameters,
such as mode width and amplitude, which relate to the excitation and
damping of p-modes, are less well known. With GONG operating for over
3 years by now and SOHO for 2 years, we have data sets available which
continually cover the end of solar cycle 22 and the beginning of solar
cycle 23. This allows us to study the solar cycle variation of p-mode
parameters with unprecedented detail. I will discuss recent results
and some of their implications.
---------------------------------------------------------
Title: Short-term periodicities of the sun's `mean' and differential
rotation
Authors: Javaraiah, J.; Komm, R. W.
1999SoPh..184...41J Altcode:
We have looked for periodicities in solar differential rotation on
time scales shorter than the 11-year solar cycle through the power-
spectrum analysis of the differential rotation parameters determined
from Mt. Wilson velocity data (1969-1994) and Greenwich sunspot
group data (1879-1976). We represent the differential rotation by a
set of Gegenbauer polynomials (ω(φ)= + (5sin<SUB>2</SUB>φ−1)+
(21sin<SUB>4</SUB>φ−14sin<SUB>2</SUB>φ+1)). For the Mt. Wilson
data, we focus on observations obtained after 1981 due to the reduced
instrumental noise and have binned the data into intervals of 19
days. We calculated annual averages for the sunspot data to reduce
the uncertainty and corrected for outliers occuring during solar
cycle minima. The power spectrum of the photospheric `mean rotation' ,
determined from the velocity data during 1982-1994, shows peaks at the
periods of 6.7-4.4 yr, 2.2 ± 0.4 yr, 1.2 ± 0.2 yr, and 243 ± 10 day
with ≥99.9% confidence level, which are similar to periods found in
other indicators of solar activity suggesting that they are of solar
origin. However, this result has to be confirmed with other techniques
and longer data sets. The 11-yr periodicity is insignificant or absent
in . The power spectra of the differential rotation parameters and ,
determined from the same subset, show only the solar cycle period with
a ≥99.9% confidence level.
---------------------------------------------------------
Title: Solar Cycle Changes in Oscillation Parameters From the First
35 Months of GONG
Authors: Hill, F.; Howe, R.; Komm, R.
1999soho....9E..63H Altcode:
We have completed an analysis of the first 35 GONG Months (1 GM = 36
days) covering the last solar minimum and the rising phase of Cycle
23. The mode parameters have been estimated from 33 time series,
each of 3-GM duration, with centers spaced by 1 GM. We report on
the temporal evolution of the frequency, splitting coefficients up
to 15th order, widths, and amplitudes. We clearly observe the bulk
frequency shift that is well-correlated with the surface magnetic
flux, and are able to discern fine details in this evolution. We find
that the even frequency splitting coefficients up to 14th order are
highly correlated not with the total magnetic surface flux, but instead
with the corresponding Legendre components of the average latitudinal
surface flux. On the other hand, the odd coefficients do not correlate
well with any surface magnetic flux measure yet considered. We find
small yet significant trends in the temporal evolution of the odd
coefficients up to 15th order. The widths and amplitudes also show
evidence of temporal evolution, with widths slightly increasing (up
to 10%) and amplitudes similarly decreasing as the activity level rises.
---------------------------------------------------------
Title: Solar-Cycle Changes in SOI and GONG Data for 1996-7
Authors: Anderson, E. R.; Howe, R.; Komm, R.
1998ESASP.418..901A Altcode: 1998soho....6..901A
We have analysed two 3-month sets of GONG data from the end of cycle 22
(GONG months 12-14, June-September 1996) and the beginning of cycle 23
(GONG months 21-23, May-August 1997), and here present evidence of small
but significant shifts in the central frequencies and the a<SUB>4</SUB>
coefficients of the modes, when averaged over all l and over 70-mu
Hz frequency bins. These changes are confirmed by an analysis of the
MDI-SOI time series for the equivalent time periods using the GONG
peakfinding algorithm. Although small, these changes are consistent with
what we would expect based on measurements of the activity-dependence
of the frequencies in previous solar cycles. (Elsworth et al 1994,
Woodard and Libbrecht 1993) We look forward to following the rise of
solar cycle 23 in detail.
---------------------------------------------------------
Title: Comparison of SOHO-SOI/MDI and GONG Spectra
Authors: Komm, R. W.; Anderson, E.; Hill, F.; Howe, R.; Kosovichev,
A. G.; Scherrer, P. H.; Schou, J.; Fodor, I.; Stark, P.
1998ESASP.418..253K Altcode: 1998soho....6..253K
We compare solar p-mode parameters, such as central frequency, width,
and amplitude, derived from GONG and SOHO-SOI/MDI Medium-l Program
time series obtained during the same time period. With the excellent
data available now from GONG and SOHO-SOI/MDI, there exist data
sets long enough to make such a comparison useful. For this study,
we have chosen time series of three ell values (ell = 30, 65, and 100)
corresponding to GONG month 16 (Oct 28 -- Dec 2, 1996). For each time
series, we calculated multitaper power spectra using generalized
sine tapers to reduce the influence of the gap structure, which is
different for the two data sets. Then, we applied the GONG peakfitting
algorithm to the spectra to derive mode parameters and selected `good'
fits common to both MDI and GONG spectra, according to three selection
criteria. Preliminary results show that mode frequencies determined
from MDI spectra are essentially the same as the frequencies from
GONG spectra and that the difference is, in general, well within one
formal error bar. The background slope at frequencies above 5mHz is
different between MDI and GONG spectra depending on ell. At present,
we are analyzing 3-month time series of ell = 0 to ell = 150. We intend
to present the results of the on-going comparison.
---------------------------------------------------------
Title: Multitaper Spectral Analysis and Wavelet Denoising Applied
to Helioseismic Data
Authors: Komm, Rudolf; Gu, Yeming; Hill, Frank; Stark, Phil; Fodor,
Imola
1998ASPC..154..783K Altcode: 1998csss...10..783K
Our goal is to improve the estimates of mode frequencies, amplitudes,
and widths derived from helioseismic observations. To this end, we
apply Multitaper Spectral Analysis (MTSA) to the observed time series
to derive power spectrum estimates, and then we apply wavelet denoising
to the log spectra to further improve the signal-to-noise ratio of the
modes. The rationale behind this approach is that MTSA leads to a power
spectrum estimate with reduced variance and better leakage properties
than the conventional periodogram and that since the log multitaper
spectrum is close to Gaussian, distributed wavelet denoising is the
optimum method to reduce the noise level in the calculated spectra. We
applied MTSA and wavelet denoising to GONG and SOHO-SOI/MDI time series
and found that a single m-nu spectrum benefits greatly from MTSA plus
wavelet denoising and that wavelet denoising by itself can be used to
improve m-averaged spectra.
---------------------------------------------------------
Title: Multitaper Analysis Applied to a 3-month Time Series
Authors: Komm, R. W.; Anderson, E.; Hill, F.; Howe, R.; Fodor, I.;
Stark, P.
1998ESASP.418..257K Altcode: 1998soho....6..257K
We show the benefit of multitapering by applying this technique to
a 3-month helioseismic time series, then deriving p-mode parameters
using the GONG peakfitting algorithm. A multitaper spectrum is an
average over uncorrelated spectra derived from the same time series
by applying a set of orthogonal tapers. Thus, a multitaper spectrum
has less variance or noise than a single taper spectrum and has better
leakage properties than a periodogram. We use generalized sine tapers,
which are orthogonal tapers taking the gap structure of the time
series into account. We applied this technique with great success to
a variety of time series from SOHO-SOI/MDI and GONG. The benefit of
multitapering is that more modes can be fitted than in a periodogram
due to the reduced noise. The improvement depends on ell and other
details of the time series and is typically between 20% and 60% for
low to medium ell values for GONG as well as MDI data. For example,
for the 3-month GONG time series covering months 12--14, the number
of good fits increases by 10% on average for all modes from ell =
0--150, using 5 generalized sine tapers. The largest improvement is
at ell <= 70 where at low frequencies one extra ridge can be fitted
in the multitaper spectrum.
---------------------------------------------------------
Title: Estimated Mode Parameters from the Fitting of GONG Spectra
Authors: Hill, F.; Anderson, E.; Howe, R.; Jefferies, S. M.; Komm,
R.; Toner, C.
1998ESASP.418..231H Altcode: 1998soho....6..231H
The estimation of mode parameters is a critical step in the helioseismic
data reduction process. Several estimation methods are currently in
use, and a comparison of the resulting frequencies from a common data
set shows small, yet significant, differences. This suggests that the
fitting procedures can introduce systematic errors. These errors will
affect subsequent inversions of the data. For example, the presence of
a high-latitude jet in the solar rotation rate appears to depend on the
type of spectral fitting used to estimate the splitting coefficients. In
addition, as the available helioseismic observations have improved,
it has become apparent that several effects have been neglected in the
peak fitting techniques. These effects include line profile asymmetry,
coupling between the background and the mode signal, fine details in
the leakage matrix, and the differences in the oscillation spectrum
when observed in Doppler velocity or total intensity. Here we report
on the latest GONG fitting methods and present the resulting mode
parameter estimates. The GONG fitting technique now includes improved
mode quality assurance checks and asymmetrical line profiles. Currently
under development are multi-dimensional fitting, multi-taper time
series analysis, background/mode coupling, simultaneous fitting
of velocity and intensity spectra, and the inclusion of a leakage
matrix. The improvements have resulted in higher-quality frequency
estimates that are now being computed for 108-day long time series
spaced by 36 days. After completion, each frequency table is made
freely available to the helioseismic community.
---------------------------------------------------------
Title: GONG Spectra in three observables: What is a p-mode frequency?
Authors: Harvey, J.; Hill, F.; Komm, R.; Leibacher, J.; Pohl, B.;
GONG Team
1998IAUS..185...49H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Multitaper Spectral Analysis and Wavelet Analysis of Daily
and Monthly Sunspot Numbers
Authors: Komm, R. W.
1998ASPC..140..431K Altcode: 1998ssp..conf..431K
No abstract at ADS
---------------------------------------------------------
Title: Multitaper Spectral Analysis and Wavelet Denoising Applied
to Helioseismic Data
Authors: Komm, R.; Gu, Y.; Stark, P.; Hill, F.
1997SPD....28.0215K Altcode: 1997BAAS...29..895K
Our goal is to improve the estimates of mode frequencies, amplitudes,
and widths derived from helioseismic observations. To this end, we
apply Multitaper Spectral Analysis (MTSA) to the observed time series
to derive power spectrum estimates, and then we apply wavelet denoising
to the spectra to improve the signal-to-noise ratio of the modes. The
rationale behind this approach is that MTSA leads to a more accurate
and robust power spectrum estimate than the conventional periodogram and
that since the log multitaper spectrum is close to Gaussian distributed
wavelet denoising is the optimum method to reduce the noise level
in the calculated spectra. We have put together a `pipeline' to
calculate a multitaper spectral estimate from a given time series,
to apply wavelet denoising to the log spectra and then to derive
mode parameters using the GONG peak-fitting algorithm. This pipeline
was applied to a set of simple artificial data in order to check for
systematic errors and consistency. The wavelet denoising method was
already applied to m-averaged South Pole spectra and to some GONG
spectra of different L values reducing the noise level considerably
and improving the fit. At the moment, we apply the pipeline to GONG
and SOHO-SOI/MDI time series. We intend to present a comparison of
two multitaper estimates using Slepian and Sinusoidal tapers with a
conventional periodogram and a comparison of each of the three spectra
with the corresponding wavelet denoised spectrum. This comparison will
allow us to discuss the benefits of adding these methods to existing
helioseismic data analysis packages.
---------------------------------------------------------
Title: A spatial and spectral maximum entropy method as applied to
OVRO solar data
Authors: Komm, R. W.; Hurford, G. J.; Gary, D. E.
1997A&AS..122..181K Altcode:
We present first results of applying a Maximum Entropy Method (MEM)
algorithm that acts in both the spatial and spectral domains to
data obtained with the frequency-agile solar interferometer at Owens
Valley Radio Observatory (OVRO) taken at 45 frequencies in the range
1-18 GHz. The traditional MEM algorithm does not exploit the spatial
information available at adjacent frequencies in the OVRO data, but
rather applies separately to each frequency. We seek an algorithm that
obtains a global solution to the visibilities in both the spatial and
spectral domains. To simplify the development process, the algorithm
is at present limited to the one-dimensional spatial case. We apply
our 1-d algorithm to observations taken with the OVRO frequency-agile
interferometer of active region AR 5417 near the solar limb on March
20, 1989 (vernal equinox). The interferometer's two 27 m antennas
and 40 m antenna were arranged in a linear east-west array, which
at the vernal equinox gives a good match to the 1-d algorithm. Our
results show that including the spectral MEM term greatly improves the
dynamic range of the reconstructed image compared with a reconstruction
without using this information. The derived brightness temperature
spectra show that for AR 5417 the dominant radio emission mechanism
is thermal gyroresonance and we use this information to deduce the
spatial variation of electron temperature and magnetic field strength
in the corona above the active region.
---------------------------------------------------------
Title: Spatial/Spectral MEM Applied to OVRO Flare Data
Authors: Komm, R. W.; Gary, D. E.; Hurford, G. J.
1996AAS...188.8502K Altcode: 1996BAAS...28..961K
We present observations of a flare that occured on 1995 Oct 11, in AR
7912 obtained with the frequency-agile solar interferometer at Owens
Valley Radio Observatory (OVRO) made with both high spatial and spectral
resolution. We analyzed data at 32 frequencies in the range 1.2-12.4 GHz
for both left-handed and right-handed circular polarizations applying
the recently completed Spatial/Spectral Maximum Entropy Method (MEM). In
contrast to the traditional MEM algorithm, which does not exploit the
spatial information available at adjacent frequencies in the OVRO data,
the new algorithm obtains a global solution to the visibilities in
both the spatial and spectral domains and leads to spectra which are
greatly improved in smoothness and dynamic range. A comparison with
BBSO data shows that the optical flare is associated with the trailing
sunspot of the active region, while the leading sunspot shows no flare
related brightning. The reconstructed radio images show two sources;
one of them, the primary source, is associated with the optical flare
and has its peak emission at about 7.0 GHz, while the other one, the
secondary source, is associated with the leading sunspot with a peak
emission at about 2.8 GHz. The slopes of the brightness temperature
spectra imply that in both cases non-thermal gyrosynchrotron emission
is the process responsible for the microwave radiation. A preliminary
model fit of the spectra shows that the emission of both sources comes
from low-order harmonics and that the estimated magnetic field strength
and the estimated density of nonthermal electrons of the primary
source are much larger than the corresponding values of the secondary
source. The results suggest that the two sources are connected by a
large secondary loop and that the emission of the secondary source is
caused by nonthermal electrons escaping from the primary source. The
time evolution of the two sources within about +/- 3 min of the flux
peak time can be described by an increase in the number of nonthermal
electrons up to the peak time and by a subsequent decrease.
---------------------------------------------------------
Title: Temperature and velocity fluctuations in the deeper layers
of the solar atmosphere.
Authors: Vollmoeller, P.; Komm, R.; Mattig, W.
1996A&A...306..294V Altcode:
To study temperature and velocity fluctuations in the lower photosphere
of the Sun, we analyzed spectrograms obtained with the Vacuum Tower
Telescope of the Observatorio del Teide in Tenerife in a spectral
range at about 410nm, the secondary minimum of the continuum absorption
coefficient. We find much larger temperature fluctuations than deduced
by previous investigations at almost all photospheric heights. The
velocity fluctuations show the same height dependence as deduced by
earlier studies at mid- and upper photospheric layers (at heights
larger than about 100km above τ_0_=1), but they do not increase as
rapidly with decreasing height in the lower photosphere. The results
indicate the existence of a velocity maximum located at the bottom
of the photosphere or slightly below in subphotospheric layers. The
location of this maximum agrees reasonably well with predictions of
convection zone models.
---------------------------------------------------------
Title: Characteristic Size and Diffusion of Quiet Sun Magnetic
Patterns
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1995SoPh..158..213K Altcode:
We have previously studied large-scale motions using high-resolution
magnetograms taken from 1978 to 1990 with the NSO Vacuum Telescope on
Kitt Peak. Latitudinal and longitudinal motions were determined by
a two-dimensional crosscorrelation analysis of pairs of consecutive
daily observations using small magnetic features as tracers. Here we
examine the shape and amplitude of the crosscorrelation functions. We
find a characteristic length scale as indicated by the FWHM of the
crosscorrelation functions of 16.6 ± 0.2 Mm. The length scale
is constant within ±45° latitude and decreases by about 5% at
52.5° latitude; i.e., the characteristic size is almost latitude
independent. The characteristic scale is within 3% of the average value
during most times of the solar cycle, but it increases during cycle
maximum at latitudes where active regions are present. For the time
period 1978-1981 (solar cycle maximum), the length scale increases
up to 1.7 Mm or 10% at 30° latitude. In addition, we derive the
average amplitude of the crosscorrelation functions, which reflects
the diffusion of magnetic elements and their evolutionary changes
(including formation and decay). We find an average value of 0.091 ±
0.003 for the crosscorrelation amplitude at a time lag of one day, which
we interpret as being caused by the combined effect of the lifetime
of magnetic features and a diffusion process. Assuming a lifetime
of one day, we find a value of 120 km<SUP>2</SUP> s<SUP>−1</SUP>
for the diffusion constant, while a lifetime of two days leads to 230
km<SUP>2</SUP> s<SUP>−1</SUP>.
---------------------------------------------------------
Title: Wavelet Analysis of a Magnetogram
Authors: Komm, R. W.
1995SoPh..157...45K Altcode:
I analyze a quiet-Sun magnetogram with an orthogonal wavelet transform,
which allows me to define an entropy measure. The entropy measure of
the magnetogram as a function of spatial scale obeys a scaling law,
which leads to a fractal dimension ofD<SUB>f</SUB> = 1.7. Furthermore,
the entropy scaling law is directly related to the intermittency of
magnetic features, which increases for decreasing spatial scales,
as expected for a turbulent signal. In this context, the scaling law
parameter can be interpreted as a fractional reduction in volume from
one step of the turbulent cascade to the next.
---------------------------------------------------------
Title: The Application of Spatial and Spectral MEM to OVRO Solar Data
Authors: Komm, R. W.; Hurford, G. J.; Gary, D. E.
1995SPD....26.1301K Altcode: 1995BAAS...27..986K
No abstract at ADS
---------------------------------------------------------
Title: Hurst Analysis of Mt. Wilson Rotation Measurements
Authors: Komm, R. W.
1995SoPh..156...17K Altcode:
I study the temporal variation of the solar rotation on time scales
shorter than the 11-year cycle by analyzing the daily Mt. Wilson
Doppler measurements from 1967 to 1992. The differential rotation is
represented by the three coefficients,A, B, andC, of the following
expansion:ω =A +B sin<SUP>2</SUP>(θ) +C sin<SUP>4</SUP>(θ). TheA,
B, andC time series show clearly the 11-year solar cycle and they also
show high-frequency fluctuations. The Hurst analysis of these time
series shows that a Gaussian random process such as observational
noise can only account for fluctuations on time scales shorter than
20 days. For time scales from 20 days to 11 years, the variations of A
give rise to a Hurst exponent ofH = 0.83, i.e., the variations ofA are
`persistent'. The temporal variations ofB show the same behavior asC,
which is different fromA. From one to 11 years, theB andC variations
are dominated by the 11-year cycle, while for time lags shorter than
about 250 days, theB andC fluctuations give rise to a Hurst exponent
ofH = 0.66, which lies betweenH = 1/2, of a Gaussian random process,
and the exponent of the persistent process shown byA. An analysis of the
equivalent coefficients of the first three even Legendre polynomials,
computed usingA, B, andC, provides additional information. For time
scales between 100 and 1000 days, the ranges,R/S, of Legendre polynomial
coefficients decrease with increasing order of the polynomials which
suggests that the persistent process operates mainly on large spatial
scales. The Hurst exponent ofH = 0.83 for variations inA is the same asH
for monthly sunspot numbers with time scales between 6 months and 200
years and for<SUP>14</SUP>C radiocarbon data with time scales between
120 years and 3000 years, previously analyzed by other authors. The
combined results imply that the underlying solar process shows the
same persistent behavior for time scales as short as about 20 days up
to time scales of a few thousand years.
---------------------------------------------------------
Title: The Covariance of Latitudinal and Longitudinal Motions of
Small Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1994SoPh..151...15K Altcode:
We study the covariance of longitudinal and latitudinal motions of small
magnetic features after subtracting long-term averages of differential
rotation and meridional flow. The covariance is generally interpreted
as Reynolds stress and linked to the equatorward transport of angular
momentum. Using high-resolution magnetograms taken daily with the NSO
Vacuum Telescope on Kitt Peak, we determine large-scale motions by
a two-dimensional crosscorrelation analysis of pairs of consecutive
daily observations from which active regions are excluded, i.e., we
analyze the motions of small magnetic features. In the present work,
we focus on 107 day pairs obtained during the year 1988 and on 472
day pairs taken in selected intervals from 1978 to 1990. We find
that all covariance values are very small (below 250 m<SUP>2</SUP>
s<SUP>−2</SUP>), which is about one to two orders of magnitude smaller
than the values from sunspot measurements derived by other authors. At
active region latitudes, the masking process increases the noise,
which increases the chance that the covariances at these latitudes
are not significantly different from zero. We find that the results
depend strongly on the temporal averaging involved. Daily unaveraged
crosscorrelations lead to no apparent correlation between the residual
velocities, while in the monthly averages of the 1988 data, we find a
covariance of −37 ± 15 m<SUP>2</SUP> s<SUP>−2</SUP> at 45° with
a linear correlation of −0.59, which is significantly different from
zero and has the right sign for an equatorial transport of angular
momentum. When we average over longer time periods, the covariance
values decrease again. The annual averages of the 1978-1990 data
show both no significant covariances and the smallest errors. These
small covariances imply that the motions of small magnetic features
do not reflect the transport of angular momentum via the mechanism of
Reynolds stress.
---------------------------------------------------------
Title: Meridional Flow and Rotation of Active Regions
Authors: Komm, R. W.
1994SoPh..149..417K Altcode:
Meridional flow and rotation are studied using high-resolution
magnetograms taken during the year 1988 with the NSO Vacuum Telescope on
Kitt Peak. Motions are determined by a two-dimensional crosscorrelation
analysis of 107 pairs of consecutive daily observations. The analysis
was done first with active regions included and repeated with active
regions excluded. The difference between the two analyses provides an
estimate of the motions of active regions. I find that active regions
rotate slower than small magnetic features and that active regions
show a meridional flow toward the mean latitude of activity.
---------------------------------------------------------
Title: Wavelet Analysis of Active Regions
Authors: Komm, Rudolf W.
1994ASPC...68...24K Altcode: 1994sare.conf...24K
No abstract at ADS
---------------------------------------------------------
Title: Wavelet Analysis of Solar Magnetic Structures
Authors: Komm, Rudolf W.
1994ASPC...64..432K Altcode: 1994csss....8..432K
No abstract at ADS
---------------------------------------------------------
Title: Solar non-rotational motions
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1994smf..conf...68K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of the solar granulation.
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig,
W.; Staiger, J.
1993A&A...279..599N Altcode:
This investigation is based on a series of spectrograms of extraordinary
spatial resolution taken with the vacuum tower telescope (VTT) at Izana
(Tenerife) in 1990. The quantitative analysis of these spectrograms
reveals an asymmetrical character of the granular flow (non-Benard like
convection). We suggest that a typical granule consists of a region
of high intensity and low turbulence in its interior and a region of
high turbulence and moderate intensity at its border. In other words,
we surmise that reigons of enhanced turbulence outline the borders
of granules. By means of power and coherence analyses we found two
different scaling laws for the small scale range: both the velocity
and intensity power as well as various cross-correlation functions
change their behavior near log k approximately = 0.8.
---------------------------------------------------------
Title: Meridional Flow of Small Photospheric Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1993SoPh..147..207K Altcode:
We study the meridional flow of small magnetic features, using
high-resolution magnetograms taken from 1978 to 1990 with the NSO
Vacuum Telescope on Kitt Peak. Latitudinal motions are determined by a
two-dimensional crosscorrelation analysis of 514 pairs of consecutive
daily observations from which active regions are excluded. We find a
meridional flow of the order of 10 m s<SUP>−1</SUP>, which is poleward
in each hemisphere, increases in amplitude from 0 at the equator,
reaches a maximum at mid-latitude, and slowly decreases poleward. The
average observed meridional flow is fit adequately by an expansion
of the formM (θ) = 12.9(±0.6) sin(2θ) + 1.4(±0.6) sin(4θ), in m
s<SUP>−1</SUP> whereθ is the latitude and which reaches a maximum of
13.2 m s<SUP>−1</SUP> at 39°. We also find a solar-cycle dependence
of the meridional flow. The flow remains poleward during the cycle, but
the amplitude changes from smaller-than-average during cycle maximum to
larger-than-average during cycle minimum for latitudes between about
15° and 45°. The difference in amplitude between the flows at cycle
minimum and maximum depends on latitude and is about 25% of the grand
average value. The change of the flow amplitude from cycle maximum to
minimum occurs rapidly, in about one year, for the 15-45° latitude
range. At the highest latitude range analyzed, centered at 52.5°,
the flow is more poleward-than-average during minimumand maximum,
and less at other times. These data show no equatorward migration of
the meridional flow pattern during the solar cycle and no significant
hemispheric asymmetry. Our results agree with the meridional flow and
its temporal variation derived from Doppler data. They also agree on
average with the meridional flow derived from the poleward migration
of the weak large-scale magnetic field patterns but differ in the
solar-cycle dependence. Our results, however, disagree with the
meridional flow derived from sunspots or plages.
---------------------------------------------------------
Title: The Covariance of Latitudinal and Longitudinal Motions of
Small Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1993BAAS...25.1220K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Rotation Rates of Small Magnetic Features from Two-Dimensional
and One-Dimensional Cross-Correlation Analyses
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1993SoPh..145....1K Altcode:
We present results of an analysis of 628 high-resolution magnetograms
taken daily with the NSO Vacuum Telescope on Kitt Peak from 1975 to
1991. Motions in longitude on the solar surface are determined by a
two-dimensional cross-correlation analysis of consecutive day pairs. We
find that the measured rotation rate of small magnetic features, i.e.,
excluding active regions, is in excellent agreement with the results
of the previous one-dimensional analysis of the same data (Komm,
Howard, and Harvey, 1993). The polynomial fits show magnetic torsional
oscillations, i.e., a more rigid rotation during cycle maximum and
a more differential rotation during cycle minimum, but with smaller
amplitudes than the one-dimensional analysis. The full width at half
maximum of the cross-correlations is almost constant over latitude
which shows that the active regions are effectively excluded. The
agreement between the one- and two-dimensional cross-correlation
analyses shows that the two different techniques are consistent and that
the large-scale motions can be divided into rotational and meridional
components that are not affected by each other.
---------------------------------------------------------
Title: Rapid Variations in the Intergranular Space
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig,
W.; Staiger, J.
1993ASPC...46..222N Altcode: 1993mvfs.conf..222N; 1993IAUCo.141..222N
No abstract at ADS
---------------------------------------------------------
Title: Torsional Oscillations and Internal Rotation
Authors: Komm, R. W.; Harvey, J. W.; Howard, R. F.
1993ASPC...42..269K Altcode: 1993gong.conf..269K
No abstract at ADS
---------------------------------------------------------
Title: Torsional Oscillation Patterns in Photospheric Magnetic
Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1993SoPh..143...19K Altcode:
We analyzed 689 high-resolution magnetograms taken daily with the NSO
Vacuum Telescope on Kitt Peak from 1975 to 1991. Motions in longitude on
the solar surface are determined by a one-dimensional crosscorrelation
analysis of consecutive day pairs. The main sidereal rotation rate
of small magnetic features is best fit byω = 2.913(±0.004) −
0.405(±0.027) sin<SUP>2</SUP>φ − 0.422(±0.030) sin<SUP>4</SUP>φ,
in µrad s<SUP>−1</SUP>, whereφ is the latitude. Small features and
the large-scale field pattern show the same general cycle dependence;
both show a torsional oscillation pattern. Alternating bands of
faster and slower rotation travel from higher latitudes toward the
equator during the solar cycle in such a way that the faster bands
reach the equator at cycle minimum. For the magnetic field pattern,
the slower bands coincide with larger widths of the crosscorrelations
(corresponding to larger features) and also with zones of enhanced
magnetic flux. Active regions thus rotate slower than small magnetic
features. This magnetic torsional oscillation resembles the pattern
derived from Doppler measurements, but its velocities are larger by a
factor of more than 1.5, it lies closer to the equator, and it leads
the Doppler pattern by about two years. These differences could be
due to different depths at which the different torsional oscillation
indicators are rooted.
---------------------------------------------------------
Title: Solar Meridional Flow Detected in Small Magnetic Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.
1992AAS...181.8102K Altcode: 1992BAAS...24.1252K
We present results of an analysis of 514 high-resolution magnetograms
taken daily with the NSO Vacuum Telescope on Kitt Peak from 1978 to
1990. Motions in latitude on the solar surface are determined by a
two-dimensional crosscorrelation analysis of consecutive day pairs
after excluding large active regions. We find a meridional flow of the
order of 10 ms(-1) , which is poleward in each hemisphere, increases
in amplitude from 0 at the equator, reaches a maximum at mid-latitude,
and slowly decreases poleward. The average meridional flow is fit by an
expansion of derivatives of even Legendre polynomials $M(theta ) = 8.88
(+/- 0.45) {{partial P_2}/{partial theta }} - 0.66 (+/-0.26) {{partial
P_4}/{partial theta }} in ms^{-1}\ where \theta is the latitude, which
reaches a maximum of 13.2 ms^{-1}\ at 39 deg. We also find a solar
cycle dependence of the meridional flow. The flow remains poleward
during the cycle, but the amplitude (at latitudes poleward of 20 deg)
changes from smaller-than-average during maximum to larger-than-average
during minimum. The meridional flow fit of the maximum activity years
1980--1982 peaks at 10.9 ms^{-1}, while the fit of the minimum years
1984--1986 reaches a maximum velocity of 14.5 ms^{-1}$; the difference
is about 27% of the average value.
---------------------------------------------------------
Title: Crosscorrelation Analysis of Small Photospheric Magnetic
Features
Authors: Komm, Rudolf W.; Howard, Robert F.; Harvey, John W.
1992AAS...180.5110K Altcode: 1992BAAS...24..815K
We present results of an analysis of high-resolution magnetograms
taken daily with the NSO Vacuum Telescope on Kitt Peak from 1975 to
1991. Motions in longitude on the solar surface are determined by a
one-dimensional crosscorrelation analysis of consecutive day pairs. The
mean sidereal rotation rate of small magnetic features is best fit by
$omega = 2.913 (+/- 0.004) -0.405 (+/- 0.027) sin(2phi ) -0.422 (+/-
0.030) sin(4phi ) in \mu rad s^{-1} where \phi$ is the latitude. The
small features show a torsional oscillation pattern; alternating bands
of faster and slower rotation travel from higher latitudes toward the
equator during the solar cycle in such a way that the faster bands
reach the equator at cycle minimum. The magnetic torsional oscillation
resembles the pattern derived from Doppler measurements, but is
different in three respects. Its velocities are larger by a factor of
more than 1.5, it lies closer to the equator, and leads the Doppler
pattern by about two years. Motions in longitude and also in latitude
are determined by a two-dimensional crosscorrelation analysis. The mean
sidereal rotation rate of the two-dimensional analysis is in excellent
agreement with the one-dimensional rate which assures the robustness of
the two-dimensional analysis. In latitude, we find meridional motions
of the order of 10 m/s which are poleward in each hemisphere.
---------------------------------------------------------
Title: What are the Boundaries of Solar Granules?
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig,
W.; Staiger, J.
1992AAS...180.5109N Altcode: 1992BAAS...24..814N
This investigation is based on a series of spectrograms of extraordinary
spatial resolution taken with the vacuum tower telescope(VTT) at Iza\
na (Tenerife) in 1990. The quantitative analysis of these spectrograms
revealed an asymmetrical character of the granular flow (non-Benard
like convection). The intensity maximum and the maximum of the upward
line-of-sight velocity do not coincide. In most cases the maximum of
the velocity lies near the border of the granule and falls rapidly to
the adjacent intergranular lane(from 1.5 to 0.2kmsec(-1) over 200km),
but moderately towards the other intergranular lane. In some granules
the position with zero velocity coincides with the position of highest
intensity, whereas maxima of velocities with different signs lie at
their border, thus reflecting a typical velocity profile of a rotating
eddy. The low correlation(of less than 0.5) between intensity and
Doppler velocity fluctuations along the spectrograph slit reflects
the asymmetric character of the solar granular flow. Concerning
the border of granules we find that bright regions often exhibit
downward, instead of the expected upward velocity. Moreover, by
investigating the broadening of a non-magnetically sensitive line,
we were able to localize regions with enhanced turbulence within the
intergranular space. We find that these regions do not always cover
the whole intergranular lane, but are concentrated at the border of
the granules, especially where the steep decrease of the velocity
takes place. On the basis of these findings we suggest that a typical
granule consists of a region of high intensity and low turbulence in
its interior and a region of high turbulence and moderate intensity
at its border. In other words, we surmise that regions of enhanced
turbulence outline the borders of granules. Using our time series
of spectrograms, which were taken every 15sec over a total of 5min,
we followed the dynamics of these properties and the evolution of the
steep intensity and velocity changes along the slit. These changes
are connected with shear instabilities and turbulence production. The
findings from non-active regions will be compared with those from
active regions based on magnetically sensitive lines.
---------------------------------------------------------
Title: Velocity Fluctuations; Energy Dissipation in the Solar
Photosphere
Authors: Komm, R.; Mattig, W.; Nesis, A.
1992ASPC...26..175K Altcode: 1992csss....7..175K
No abstract at ADS
---------------------------------------------------------
Title: Rotation Rate Determined from Small Photospheric Magnetic
Features
Authors: Komm, R. W.; Howard, R. F.; Harvey, J. W.; Forgach, S.
1992ASPC...27..325K Altcode: 1992socy.work..325K
No abstract at ADS
---------------------------------------------------------
Title: Dynamics of the solar granulation. I - A phenomenological
approach
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig,
W.; Staiger, J.
1992A&A...253..561N Altcode:
High-spatial-resolution spectrograms taken with the vacuum tower
telescope in Tenerife were used to investigate the dynamics of the deep
photospheric layers by tracing the motions of small-scale structures
such as granulation. Based on a time series of these spectrograms,
traces of line Doppler shifts were detected which show strong
asymmetries within solar granules. The results are discussed within
the framework of different granulation flow models.
---------------------------------------------------------
Title: The decay of granular motions and the generation of gravity
waves in the solar photosphere
Authors: Komm, R.; Mattig, W.; Nesis, A.
1991A&A...252..827K Altcode:
The solar photosphere was investigated using a coherence analysis of
rms-velocities. Results confirm that there is a distiction between
the granular structures of the lower photosphere and the secondary
structures of the higher photosphere. It is shown that the conversion
of motions occurs well below a height of 200 km. In the layers of
the higher photosphere (above 170 km) structures are found in the
wavenumber range from 2.5 M/m to 7.0 M/m with a dominant scale of
about 4.0 M/m. It is concluded that secondary motions are generated
by decaying granular motions.
---------------------------------------------------------
Title: The height dependence of velocity-intensity fluctuations and
several non-dimensional parameters in the solar photosphere
Authors: Komm, R.; Mattig, W.; Nesis, A.
1991A&A...252..812K Altcode:
The quiet photosphere was studied using autocorrelation functions
(ACFs) of intensity-velocity fluctuations. It is found that all ACFs
get broader with height for the whole center-to-limb variation. For
the length scale L, there is no significant center-to-limb variation
but a general height dependence. In the lower photospheric layers
(less than 150 km), L is of the order of 500 km, while in the higher
layers L increases to 700 km. The vorticity is found to be 0.025/s
in the lower photosphere and 0.001/s in the higher layers. The height
dependence of several independent nondimensional parameters has been
deduced to study the effect of dissipative processes on the granular
motions. The Reynolds number is found to decrease from 5 x 10 exp 9
at the continuum layers to 10 exp 8 at the temperature minimum; the
Peclet number decreases from 70 to 1; and the magnetic Reynolds number
is of the order of 500,000. It is concluded that granules are buoyantly
rising turbulent structures which disintegrate due to turbulent mixing
with the environment.
---------------------------------------------------------
Title: Granulation Spectroscopy: First Results from VTT-Tenerife
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig,
W.; Staiger, J.
1991BAAS...23R1048N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The small-scale velocity field in the solar photosphere
Authors: Komm, R.; Mattig, W.; Nesis, A.
1991A&A...243..251K Altcode:
The center-to-limb variation of velocity fluctuations derived from
several spectral lines is presented and, from these data, the height
dependence of the vertical and horizontal components of the small-scale
velocity field is deduced. A strong decrease in the lower photosphere
and a flat gradient in the upper photosphere are observed for both the
horizontal and vertical velocity. It is concluded that the convective
motions decay in the middle photosphere up to a height of about 170
km and so-called secondary motions dominate the upper layers of the
photosphere. A stability criterion is used to interpret this conversion
of motions and, by utilizing the Richardson number and several length
scales, it is shown that the stable stratification of the photosphere
causes the decay of the granular convective motions.
---------------------------------------------------------
Title: On the Dynamics of Granulation in Active Regions and the
Heating Problem (With 2 Figures)
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.
1991mcch.conf...36N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The height dependence of intensity structures in the solar
photosphere
Authors: Komm, R.; Mattig, W.; Nesis, A.
1990A&A...239..340K Altcode:
The results are presented of a power and coherence analysis of intensity
variations derived from the wings of the solar Mg b2 line. It is found
that the power spectra can be represented by a power law function in
the range of wavenumbers between 2.8/Mm. The deep photosphere shows
the Kolmogorov (-5/3)-scaling law. The values of the exponent and also
of the rms intensity itself decrease with height, attain a minimum,
and increase again. While small structures are coherent up to higher
photospheric layers, the coherence of the larger structures breaks
down in the same layer where the rms intensity shows its minimum. It
is concluded that the large intensity structures reflect the effect
of convective overshoot, and the breakdown of the coherence reflects
the disappearance of convective structures up to a certain height in
the photosphere, while the small structures are of turbulent origin.
---------------------------------------------------------
Title: The decay process of the granulation and its influence on
the absorption lines.
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.
1990AGAb....5...33N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Influence of the Granulation on the Absorption Lines
I. Nonactive Regions
Authors: Nesis, A.; Hanslmeier, A.; Hammer, R.; Komm, R.; Mattig, W.
1990PDHO....7..108N Altcode: 1990ESPM....6..108N; 1990dysu.conf..108N
No abstract at ADS
---------------------------------------------------------
Title: Results on the height dependence of granular velocity
fluctuations
Authors: Komm, R.; Mattig, W.
1989hsrs.conf..330K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Granulare Overshoot-Schichten als Randbedingungen
Authors: Nesis, A.; Komm, R.; Mattig, W.
1986MitAG..67..289N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Einiges zur Streulichtkorrektur
Authors: Komm, R.; Hessenbruch, A.; Mattig, W.
1986MitAG..67..286K Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Balmerlinien in Sonnenflecken
Authors: Komm, R.
1983MitAG..60..279K Altcode:
No abstract at ADS
