BBSO Seminars

Seminars are to be held at 3:00pm, 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)

ABSTRACTS


Title, Name
Abstract
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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.
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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.
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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.
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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]