|
BBSO Seminars |
Seminars are to be held at 4:00pm, Back Lodge, unless noted otherwise. Speakers, please provide the title, and, if possible, abstract of your talk, when you are ready. For feedback, please direct to Vasyl Yurchyshyn (vayur bbso njit edu)
January 16, 2007: Pilar Montanes-Rodriguez Earthshine applications in the search for distant worlds
December 12, 2006: Enric Palle Bago: Ocean-cloud-albedo interactions at decadal time scales ( abstract)
November 14, 2006: Haisheng Ji: Contracting flaring loops suggests the relaxation of sheared magnetic field (abstract)
February 6, 2006: Jongchul Chae: The Optical Design for the Fast Imaging Solar Spectrograph
January 28, 2006: Jongchul Chae: Study of magnetic structures of chromospheric filaments.
January 23, 2006: Hongqi Zhang: Magnetic properties of flare-CME productive active regions
September 13, 2004: Nancy King: Vegetation Albedoes for Life on Other Planets (abstract)
August 2, 2004: Jongchul Chae: Fast Imaging Solar Spectrograph (FISS) for New Solar Telescope: The First Idea
July 26, 2004: Thomas H. Zurbuchen (): The Structure and Sources of the Solar Wind During the Solar Cycle (videoconference) (abstract)
April 15, 2004: Valentyna Abramenko: Distribution of the magnetic flux in elements of the magnetic field in an active region (abstract)
April 5, 2004: Prof. Cheng Fang: Spectral Diagnostics Of Non-Thermal Particles In The Solar Chromosphere (abstract)
February 10, 2004: Enric Palle Bago: A review on the Earthshine Project. Changes in the earth's reflectance over the past two decades (abstract)
Valentyna Abramenko: Changes in the Spectral Line Profiles During a Solar Flare (abstract)
Reiner Volkmer: Actual Status and Design of the 1.5 m Solar Telescope GREGOR
K.D. Leka: What makes a flare? Determining the magnetic signature of a flaring photosphere. (abstract)
Klaus Hartkorn: Application Of Adaptive Optics To The Spectroscopic Investigation Of Small-Scale Solar Structures. (abstract)
Haisheng Ji: Report On My Work at BBSO
Ayumi Asai: Flare Evolution and Energy Release (abstract)
Valentyna Abramenko: Signature of Avalanche in Solar Flares as Measured by Photospheric Magnetic Fields (abstract)
Vasyl Yurchyshyn: How Directions And Helicity Of The Magnetic Field In Erupted Solar Filaments Define Geoeffectiveness Of Coronal Mass Ejections (abstract)
Yong-Jae Moon: Magnetic Helicity and X-Ray Fluxes of Homologous Flares (abstract)
Martin Woodard: Seismic Imaging of Solar Convection (abstract)
Guo Yang: The introduction to Coronal Mass Ejection and the work I am doing
Abramenko, Magnetic Power Spectra in the Solar Photosphere Derived from Ground and Space Based Observations
Carsten Denker, Center-to-Limb Variation of Small-Scale Magnetic Features
Leonid Didkovsky, Proper Motion of Sunspots Observed with MDI
Michael Steinegger, The H-alpha Network: Overview - Status - Outlook
Louis Strous, THE DYNAMICS OF THE EXCITATION OF SOLAR OSCILLATIONS (abstract)
Jongchul Chae, High Resolution H alpha Observations of Proper Motions in NOAA 8668: Evidence for Filament Mass Injection by Chromospheric Reconnection (abstract, H alpha movie)
Peter Gallagher, Observing the Sun at Radio Wavelengths (abstract)
Jung-Hoon Kim, A Rapid Magnetic Connectivity Change Observed Before a Filament Eruption
Sangwoo Lee, Observational Evidence for Magnetic Dips in Solar Prominences
Title,
Name
Abstract
[Top]
Topological
analyses of eruptive filaments by Olya Panasenco
Erupting
filaments (prominences) that we have analyzed from H" Doppler
data at Helio Research and from SOHO/EIT 304 D show several different
motions along the main axis and legs. Our simple geometrical analyses
of these motions reveal strong coherency in some filaments between
their chirality, and the direction of the vertical motions of the top
of the filaments, and the directions of twisting of their legs.
Viewed from the positive network side, dextral filaments develop
rolling motion toward the observer along with right-hand helicity in
the left leg (clockwise streaming for downward motion) and left-hand
helicity in the right leg. Sinistral filaments, also viewed from the
positive network field side, have the opposite pattern: rolling
motion at the top away from the observer, left-hand helical twist in
the left leg (counterclockwise for downward motion) and right-hand
twist in the right leg. We find consistency between our analyses of
these motions and forms determined from our Ha Doppler observations
and twists or bending deduced from the properties of erupting
filaments observed in EIT images at 304 D. We cannot reconcile these
findings with filaments modeled as magnetic flux ropes that by
definition only have one sign of helicity. In addition, the popular
hypothetical configuration of an eruptive filament as a twisted flux
rope does not account for the complete range of observed shapes in
the erupting filaments. However, we find that a simple flat ribbon or
sheet satisfactorily reproduces nearly all of the observed forms. The
flat ribbon is the most logical beginning topology because filament
spines already have this topology prior to eruption and an initial
long flat sheet with parallel, non twisted threads, as a basic form,
can be bent into many more and different geometrical forms than a
flux rope. To date, we have been able to identify three common
patterns of motion. All are consistent with the ribbon topology and
characterize the helicity of many erupting filaments: (1) Roll of the
top of the filament with horizontal and vertical components, (2)
Twist of the legs consistent in sign with the rolling motion along
the top of the filament, and (3) rotation (writhe) of the whole
filament as it moves and expands outward. Considering that erupting
filament dynamics have large-scale coherency and always occur beneath
CMEs, we suggest that it is reasonable for the top of an erupting
filament to be first in responding to relatively unknown, changing,
coronal forces in its environment. For morphological analyses of
eruptive filaments we used the ratio h/l, where h - is the height of
filament, l - is distance between its legs, and also " - the
angle in degrees of twist or roll during increments of time, t. We
consider three general cases: 1) h/l - 0; 2) h/l > 1, legs close
together; 3) h/l # 1, legs far apart. We also describe the
relationship between the direction of twists in the legs and the roll
direction at the top of the filament using the geometrical terms: l,
h, and ". Starting with our finding that filaments are thin
sheets after their eruption, as known before their eruption, we apply
our geometrical analyses to determinate of the sign of magnetic
helicity and estimate the degree of twisting or bending in different
parts of the top and legs of erupting filaments.
[Top]
The
X-ray Telescope (XRT) aboard the Hinode Observatory by Patricia
Jibben
XRT is a high resolution grazing incidence telescope
capable of observing coronal temperatures from 700,000 to 10,000,000
Kelvin. It has a 34x34 arcmin full field of view and 2" spatial
resolution capable of studying global coronal structures as well as
resolving active region loops. The unique combination of high spatial
resolution and broad temperature range allows observations of energy
buildup, storage and the release process in the corona for any
transient event. XRT was launched on September 22, 2006 and has been
actively observing the sun since November 2006. Recent observations
include transient polar jets within polar coronal hole, C-class
flares, loop evolution, and the so-called coronal 'doughnut'. A
sample of these observations will be presented.
[Top]
Earthshine applications in the search for distant worlds by Pilar Montanes-Rodriguez
Since
the discovery of the first planet outside the solar system, the
number of planet detections is increasing exponentially. Although we
have not been capable of detecting and exploring planets like our own
yet, challenging space missions are already being planned for the
next decades, and the discovery of earth-like planets is only a
matter of time. When the time arrives, one of our main concerns will
be to determine their degree of similarity with our own planet, and
to answer a more intriguing question for the humankind: if there is
life on them. Using real cloud cover observations from satellite, we
have unequivocally detected the vegetation's signature in the Earth's
globally averaged spectrum. The signature is stronger when larger
vegetated regions of the Earth are seen free of clouds. Our results
show that, considering the real cloud cover present in our planet,
previous estimates of the vegetation signal strength were
overoptimistic. Vegetation can be detected on Earth when it is
observed as a distant planet, and although its signal is going to be
much weaker than what we previously though, when detected, it will be
an unquestionable indication of extraterrestrial complex life. Future
applications of earthshine spectroscopy will also be introduced.[Top]
Ocean-cloud-albedo
interactions at decadal time scales by Enric Palle Bago
Over
the past century, our planet has experienced a global warming
generally associated to an increase in anthropogenic greenhouse
gases, superimposed to an unknown intrinsic climate variability. To
understand this warming, it is essential to know the response of the
climate system to external forcing, and the decadal-scale energy
exchange between the oceans and the atmosphere. Presently, two of the
most uncertain elements in the climate are the role of clouds and the
heat storage of the Earth's oceans. Especially intriguing are the
cloud feedback mechanisms, which strongly affect the ability of
global circulation models to predict future climate change. Here, we
compare 20 years of global ocean thermal properties and cloud amount
data to derive their large-scale inter-annual relationships. Despite
some restriction in our conclusions due to the nature of the
datasets, we find significant correlations whose strength and sign
vary with cloud type and latitude. At decadal timescales, we find an
anti-correlation between the global mean cloud amount and the ocean's
heat content, with the clouds leading in the correlation by about 1
year. Our results suggest that cloud changes, whether natural or
anthropogenic in origin, might have a direct influence in the decadal
variability of ocean temperatures and heat content. [Top]
Contracting
flaring loops suggests the relaxation of sheared magnetic field by
Haisheng Ji
In recent years, a new kind of solar flare
phenomenon has been discovered in many flares by different authors.
That is, during the early rising phase of solar flares, hard X-ray
loop top sources or radio/extreme-ultraviolet flaring loops have a
shrinkage motion and, at the same time, flaring ribbons or hard X-ray
footpoints have a converging motion. Only after the rising phase,
there begin to appear an upward expansion for loop top sources and
flaring loops and, also, an outward motion (separation motion) for
flare ribbons or footpoints. So far, the shrinkage motion can not be
fully explained with a 2D flare model. We propose that the new solar
flare phenomenon suggests the relaxation of sheared force-free
magnetic field. [Top]
Vegetation
Albedoes for Life on Other Planets by Nancy King
Scientists
are fast approaching the capability to detect Earth-size planets and
to resolve them spectrally. How can we tell if there is life on
another planet through astronomical observations? Life abounds in
extreme environments and in hidden places of the Earth and could
likewise on other planets. However, on Earth, only photosynthetic
organisms produce unequivocal signs of life that we can detect from
space. These signs are the vegetation "red edge" (the
strong contrast in absorbance by vegetation in the visible light
range and high reflectance in he near-infrared) and the presence of
oxygen in our atmosphere (due to water photolysis by photosynthesis).
In addition, the seasonal cycles of atmospheric carbon dioxide and
methane concentrations are due to the seasonal growth and respiration
of the biosphere. These "biosignatures" are the result of
short-term biophysical processes, which have arisen through long-term
coevolution of the biosphere with the atmosphere, and perhaps due to
evolutionary accidents in the origin of photosynthesis. Could
photosynthesis arise on another planet, and would such organisms
produce the same biosignatures as those on Earth? This talk will
review the state of knowledge on the vegetation red edge and
predicting plant canopy albedoes, the environmental constraints on
photosynthesis, and present potential biosignatures for life around
M-stars, with discussion about their detectability. [Top]
The
Structure and Sources of the Solar Wind During the Solar Cycle
The
solar corona and hence the structure of the heliosphere change
dramatically during the solar cycle. At solar minimum, solar wind
predominantly emerges from polar coronal holes and propagates close
to equatorial regions. Associated with streamers, an intermittent
stream of solar wind is emerging and forming slow solar wind. As
solar activity progresses, this bimodal distribution of the solar
wind becomes much more complicated. Fast and slow solar wind can be
found at all latitudes. We will discuss this transition from low to
high solar activity focusing on solar wind composition data that
provide a new way of observing the evolution of the solar corona over
time. These data show interesting compositional anomalies that
distinguish different sources of solar wind and mark hot ejecta in
the heliosphere. [Top]
Distribution
of the magnetic flux in elements of the magnetic field in an active
region by Valentyna Abramenko
Probability distribution
functions (PDFs) of the unsigned magnetic flux content in flux
concentrations in a mature active region NOAA 9077 were calculated by
using a set of 248 high resolution SOHO/MDI magnetograms. Two
independent routines to outline magnetic flux concentrations were
elaborated. The analysis was performed with 4 different values of the
threshold, p, of the magnetic flux density (p=25, 50, 75, 100 G). We
have found that: i) the best analytical approximation of the observed
PDFs in the range of low flux (F < 100 x 10^18 Mx) is a lognormal
distribution, LN(m, sigma^2), with the expected value m=(0.7 to 5) x
10^18 Mx and the standard deviation sigma = (10 to 45) x 10^18 Mx.
The peak of the lognormal distribution tends to shift toward the
lower flux as the threshold p decreases. This tendency suggests that
the real expected value may be even smaller than 0.7 x 10^18 Mx; ii)
for the flux F > 100 x 10^18 Mx the observed PDFs fall off slower
than the lognormal approximation predicts. In this flux range, the
power law is found to be the best analytical approximation with the
power law index, alpha, approximately equal to 2. The above findings
are consistent with the concept of highly intermittent nature of the
cluster of magnetic flux concentrations in the active region. [Top]
Spectral
Diagnostics Of Non-Thermal Particles In The Solar Chromosphere by
Prof. Cheng Fang
There are at least three effects of the
non-thermal particle bombardment on the solar atmosphere: (1)
non-thermal ionization and excitation; (2) proton-hydrogen charge
exchange; (3) impact line polarization. Due to the non-thermal
ionization and excitation effects of electron bombardments in flares,
H? line is widely broadened and shows a obvious central reversal.
Significant enhancements at the line wings of Ly?and Ly?are also
predicted. In the case of proton bombardment, less strong broadening
and no large central reversal are expected. Based on theoretical
calculations, we proposed a method to estimate the total flux of
electron beam by the measurement of integrated flux in H? line
profiles. Due to the proton-hydrogen charge exchange, the
enhancements at the red wings of Ly? and especially of Ly?lines at
the early impulsive phase of flares are significant. Electron beam
can also in some cases produces visible and UV continuum emission in
white-light flares. However, at the onset phase, a negative "black"
flare may appear in several seconds, due to the increase of the H?
opacity. The impact polarization of atomic lines can provide
complementary information on the energetic particles, the energy
transport and deposit in the solar chromosphere. New results of
spectropolarimetric analysis for the major flare on are also given.
[Top]
A
review on the Earthshine Project. Changes in the earth's
reflectance over the past two decades by Enric Palle Bago
We
correlate an overlapping period of earthshine measurements of the
earth's reflectance (1999 through mid-2001) with satellite
observations of global cloud properties to construct from the latter
a proxy measure of the earth's global shortwave reflectance. This
proxy shows a steady decrease in the earth's reflectance from 1984 to
2000, with a strong drop during the 1990's. During 2001-2003 only
earthshine data are available, and they indicate a complete reversal
of the decline. The radiative forcing implied by either of these
decadal changes in reflectance is climatologically significant.
Understanding how these changes are apportioned between natural
variability, direct forcing, and feedbacks, is fundamental to
confidently assessing and predicting climate change. [Top]
Changes
in the Spectral Line by Profiles During a Solar Flare by Valentyna
Abramenko
The profiles of six photospheric absorption
spectral lines (Fe I 5250A, Fe I 5324A, Fe I 5576A, Ca I 5590A, Ca I
6103A, and Fe I 6165A) measured in the kernel of a 2N solar flare and
in a quiet-sun area, were compared. The observations were carried out
with an echelle spectrograph of Crimean Astrophysical Observatory. It
was shown that compared to the quiet-sun profiles, the flare profiles
are shallower in the line core and are less steep in the wings.
Therefore, measurements of the longitudinal magnetic field made with
a magnetograph system which use the Ca 6103\AA~ spectral line, can be
underestimated by 18-25\% in the areas of bright H$\alpha$ ribbons of
a moderate solar flare of importance 2N. The modeling of the solar
photosphere performed by using a synthesis method, showed that in a
solar flare, the enhanced core emission seems to be related to
heating of the photosphere by the flare, whereas the decrease of the
wings slope was presumably caused by the inhomogeneity of the
photospheric magnetic field. [Top]
What
makes a flare? Determining the magnetic signature of a flaring
photosphere by K.D. Leka.
Big Bear Solar Observatory have
searched for and in many cases, found, changes in photospheric
magnetic fields associated with solar flares; this is demonstrated
with the recent work of Wang et al (2002) which found variations in
magnetic flux temporally associated with six X-Class solar flares. In
this talk I will outline an approach we have developed to distinguish
what may be unique to a flare-imminent solar atmosphere as determined
by the photospheric magnetic field. Using archive data from the
Imaging Vector Magnetograph from U. Hawai`i/Mees Solar Observatory,
we perform statistical evaluations of the state of the photospheric
magnetic field as well as on measures of the inferred coronal
topological complexity (Leka & Barnes 2003; Barnes et al 2003). I
will describe our method which, when applied to the minimal dataset
so far acquired, has indeed been able to distinguish that atmosphere
which is flare-imminent, but only by simultaneously considering six
or more parameters derived from the photospheric magnetic field
vector.[Top]
Application
Of Adaptive Optics To The Spectroscopic Investigation Of Small-Scale
Solar Structures by Klaus Hartkorn.
We study bright
points, umbral dots and the G-band using a two-dimensional
spectrometer and an Adaptive Optics system, which allows us to record
high-resolution dopplergrams and residual intensity images. We find
evidence that bright points are smaller than 120 km in diameter.
Bright points are situated exclusively in regions of enhanced G-band
brightness and do not show a change in their shape or a displacement
in their position of more than 120 km horizontally over a height
range from 0 km to 320 km above photospheric level $\tau =1$. We do
not find velocity differences of more than 100 m/s and a size of 120
km at the locations of bright points compared to the surroundings.
Bright points have a higher contrast in the G-band as well as in the
atomic spectral lines. We suspect the existence of two contrast
enhancement mechanisms for bright points one exclusively for the
G-band, one independent of specific spectral lines. We perform
calculations using the results of a three-dimensional
magneto-hydrodynamical model as input for a radiative transfer
calculation, but find little agreement with our observations. The
core intensity of the G-band CH lines is significantly influenced by
the atmospheric conditions in heights of 160 km and 320 km, but not
heights of 40 km. The velocity investigation of a sunspot shows that
umbral dots seem to consist of two different types. The first type is
the bright part of an intensity pattern of 1000-2000 km size with a
corresponding negatively correlated velocity pattern which is
probably related to umbral oscillations. The second type consists of
localized brightening of a size of not more than 300 km that are
associated with down-flowing plasma. Furthermore, we find penumbral
grains that have penetrated the umbra and appear as brightenings. We
study the velocity signature of penumbral grains and find strong
up-flows of solar plasma associated with the inner, bright parts of
penumbral grains, where as the general correlation between intensity
and velocity within the penumbra is weak.
[Top]
Flare
Evolution and Energy Release by Ayumi Asai.
Studies of
solar flares by using optical (mainly H-alpha) data, combined with
EUV (TRACE), X-rays (Yohkoh & RHESSI), and microwave (Nobeyama)
data. The main results are as follows: 1) We found a good correlation
between motions of H-alpha flare kernels and HXR foot point source
time history. It was shown that the HXR temporal variation can be
explained by a combination between the magnetic field strength and
the separation speed of the H-alpha kernels. 2) Down flow motions
above the EUV arcade correlate with HXR bursts in the impulsive
phase. [Top]
Signature
of Avalanche in Solar Flares as Measured by Photospheric Magnetic
Fields by Valentyna Abramenko.
Turbulent/fractal
parameters of the longitudinal magnetic field, B_z, for four powerful
solar flares were analyzed utilizing the correlation length, \lambda,
of the magnetic energy dissipation field and the scaling exponent,
\beta, which characterizes the measure of intermittency of the B_z
structure. We select a set of four two-ribbon flares, which were
followed by coronal mass ejections, for the study of magnetic
structure. During the course of each flare, we found a peak in \beta
which was followed by a peak in \lambda in all of the cases studied
in this paper. These two peaks were separated by the time interval
\tau_{\lambda} during which a rapid growth of the soft X-ray and
H\alpha flux occurred. The peak in \beta was preceded by a time
period \tau_{\beta} during which \beta increased gradually. For all
of the flares \tau_{\beta} was longer than the time interval
\tau_{\lambda}. The maximum of \lambda occurred nearly
simultaneously, within an accuracy of about 2-5 minutes, with the
maximum of the hard X-ray emission. For the four flares considered in
this paper, we concluded that the more impulsive and/or more powerful
a flare is, the shorter the \beta growth time, \tau_{\beta}, and the
\lambda growth time, \tau_{\lambda}, are. In the framework of the
theory of non-linear dissipative processes, these results may be
interpreted as follows. Before a solar flare occurs there is a
significant increase in the number of magnetic field discontinuities
(\beta increasing), which is followed by an avalanche (increase of
the correlation length) of magnetic energy dissipation events. The
avalanche event occupies the entire active region from the corona to
the photosphere. Our study indicates that the more abrupt is the
avalanche, the stronger and/or more impulsive a flare is. The time
profiles of an avalanche is either Gaussian, which satisfies the
logistic avalanche model, or exponential with an abrupt drop, which
satisfies the exponential avalanche model. The driving time,
\tau_{\beta}, was longer than the avalanching time, \tau_{\lambda},
for all of the events. This qualitatively agrees with the
requirements of the self-organized criticality theory. [Top]
HOW
DIRECTIONS AND HELICITY OF THE MAGNETIC FIELD IN ERUPTED SOLAR
FILAMENTS DEFINE GEOEFFECTIVENESS OF CORONAL MASS EJECTIONS by Vasyl
Yerchyshyn.
Coronal mass ejections (CMEs) are often
associated with erupted magnetic fields or disappeared chromospheric
filaments. The majority of CMEs headed directly toward the earth
(halo CMEs) are observed at 1AU as magnetic clouds (MC). The 3D
structure of a MC can be represented by a force-free flux rope. When
CMEs reach the earth, they may or may not cause magnetic storms. The
geoeffectiveness of CMEs depends on the orientation of the magnetic
field in them. We show that the direction of the axial field in a MC
and its helicity are consistent with the direction of the axial field
and helicity of the erupted filaments. We also suggest that
geoeffectiveness of a CME can be forecasted by using daily Big Bear
Solar Observatory full disk H$\alpha$ and SOHO EIT 195\AA~ images and
SOHO/MDI magnetograms, as well. We continue to study the orientation
of magnetic fields in CMEs and its correlation with the occurrence of
geomagnetic storms. Here we report on the relationship between the
projected speed of CMEs, measured at 20R$_\odot$ from SOHO/LASCO
images, and the hourly averaged magnitude of the southwardly directed
magnetic field, B$_z$, in interplanetary ejecta, as measured by the
ACE magnetometer. CMEs that origin at the central part of the solar
disk ($r < 0.6R_\odot$) are the most geoeffective and the
instensity of the B$_z$ is an exponential function of the CME's
speeds. We propose that the strength of the southward IMF can be
estimated at least one day in advance, immidiatelly after a CME
started. The predicted value of the B$_z$ component can be then used
to estimate the intensity of a geomagnetic storm caused by the
erupteion. The prediction method is based on the correlation between
the speeds of CMEs and magnitudes of the southward IMF as well as the
fact that the orientaion and chirality of the erupted solar filaments
correspond to the orientation and chirality of interplanetary ejecta.
[Top]
Magnetic
Helicity and X-Ray Fluxes of Homologous Flares by Yong-Jae Moon
We present evidence that the occurrence of a series of homologous
flares in an active region is physically related to the accumulation
of magnetic helicity in the corona by shearing motion in the
photosphere. We have analyzed a set of 6.5 hour 1 minute cadence
magnetograms of NOAA 8100 taken by Michelson Doppler Images (MDI) on
board Solar and Heliospheric Observatory (SOHO). During this period,
seven homologous flares took place in the active region, but there
was no remarkable change of magnetic flux. We have determined the
magnetic helicity transport rate via photospheric footpoint shuffling
motions, and found that magnetic helicity was significantly
increasing during the observing period. It was obvious in the case of
a strong M4.1 flare that magnetic helicity injection rate impulsively
increased to a peak during the flaring time as the X-ray flux did. We
also found that the X-ray flux of a flare integrated over the flaring
time interval was strongly correlated with the magnetic helicity
accumulated during the flaring time, with the integrated flux
logarithmically increasing with the accumulated magnetic helicity.
Our results suggest that photospheric shearing motion is an important
driver of coronal activities and, hence, support Choe Cheng's
homologous flare model in which flares can occur in the same plage
repeatedly by continuous photospheric shearing motions. [Top]
Seismic
Imaging of Solar Convection by Martin Woodard
A new
seismic technique is being developed to image inhomogeneous
structure, such as subsurface convective cells, in the Sun. In the
direct imaging approach, a physical model of the solar interior is
inferred directly from correlations in the observed seismic wave
field. A preliminary map of supergranular convection, inferred from
SOHO/MDI helioseismology images, agrees reasonably well with
simultaneous surface Doppler maps of supergranulation. Further
development of the method is expected to improve the technique
considerably. [Top]
THE
DYNAMICS OF THE EXCITATION OF SOLAR OSCILLATIONS by Louis Strous
We investigate seismic events, bursts of seismic waves that
are generated locally just below the solar surface and that we detect
traveling up through the photosphere. We identify a few thousand
seismic events by their traveling wave character, and find that they
are associated with continuum darkening and downflow, and have an
extent of on average about 10 -- 15 minutes and 1 Mm. Their birth
rate is about 8e-16 m^-2 s^-1. The observed upwardly traveling
seismic flux in the average event (as derived from velocities in the
p-mode region of k-omega space) is followed after about 3 minutes by
some reflected downward flux. Only a small fraction of the energy
generated in the hypocenter of the event below the surface travels
straight up for us to see. The bulk of the generated energy is
directed or reflected downward, and is eventually transformed into
p-modes. The seismic events at the surface contain about 1.5e19 J of
seismic energy each, which corresponds to an average flux level of
about 8.5 kW/m^2 over the whole surface. The total energy flow is
likely more than an order of magnitude greater, and is then in the
same ballpark as the estimate of Libbrecht (1988) for the power
required to sustain the p-mode spectrum. We find a roughly linear
relation between the peak seismic flux and the peak downward
convective velocity associated with each seismic event, which does
not fit the highly non-linear relations found theoretically by
Lighthill (1952) and Goldreich & Kumar (1990) for stochastic
excitation by turbulent convection, but does fit the monopole source
deduced by Nigam & Kosovichev (1999) from a study of the p-mode
spectrum.
Reference: Strous, Goode, & Rimmele (2000),
Ap.J. 535, 1000 [Top]
High
Resolution H$\alpha$ Observations of Proper Motion in NOAA 8668:
Evidence for Filament Mass Injection by Chromospheric Reconnection by
Jongchul Chae
There
have been two different kinds of explanations for the source of cool
material in prominences or filaments: coronal condensations from
above and cool plasma injections from below. In this paper, we
present observational results which support filament mass injection
by chromospheric reconnection. The observations of an active filament
in the active region NOAA~8668 were performed on at a wavelength of
H$\alpha-0.6$ \AA\ using the 65~cm vacuum reflector, a Zeiss
H$\alpha$ birefringent filter, and a 12-bit SMD digital camera of Big
Bear Solar Observatory. The best image was selected every 12~s for an
hour based on a frame selection algorithm. All the images were then
co-aligned and corrected for local distortion due to the seeing. The
time-lapse movie of the data shows that the filament was undergoing
ceaseless motion. The H$\alpha$ flow field has been determined as a
function of time using local correlation tracking. Time-averaged flow
patterns usually trace local magnetic field lines, as inferred from
H$\alpha$ fibrils and line-of-sight magnetograms. An interesting
finding is a transient flow field in a system of small H$\alpha$
loops, some of which merge into the filament. The flow is associated
with a cancelling magnetic feature which is located at one end of the
loop system. Initially a diverging flow with speeds below 10 km
s$^{-1}$ is visible at the flux cancellation site. The flow is soon
directed along the loops and accelerated up to 40 km s$^{-1}$ in a
few minutes. Some part of the plasma flow then merges into and moves
along the filament. This kind of transient flow takes place several
times during the observations. Our results clearly demonstrate that
reconnection in the photosphere and chromosphere is a likely way to
supply cool material to a filament, as well as re-organizing the
magnetic field configuration, and, hence, is important in the
formation of filaments. [Top]
Observing
the Sun at Radio Wavelengths by
Peter T. Gallagher
In this talk, I will give an introduction to observing the
Sun using the Owens Valley Solar Array (OVSA). The basic elements of
the radio telescopes together with the mechanisms responsible for
radio emission will first be reviewed. I will then discuss the recent
upgrade of OVSA, data access and analysis, and how to observe using
the array. Finally, some recent scientific results from high
resolution flare observations will be discussed. [Top]