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At least 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2010

WHERE THE GRANULAR FLOWS BEND

Based on IMaX/SUNRISE data, we report on a previously undetected phenomenon in solar granulation. We show that in a very narrow region separating granules and intergranular lanes, the spectral line width of the Fe I 5250.2 A line becomes extremely small. We offer an explanation of this observation with the help of magneto-convection simulations. These regions with extremely small line widths correspond to the places where the granular flows bend from upflow in granules to downflow in intergranular lanes. We show that the resolution and image stability achieved by IMaX/SUNRISE are important requisites to detect this interesting phenomenon.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

DETECTION OF VORTEX TUBES IN SOLAR GRANULATION FROM OBSERVATIONS WITH SUNRISE

We have investigated a time series of continuum intensity maps and corresponding Dopplergrams of granulation in a very quiet solar region at the disk center, recorded with the Imaging Magnetograph eXperiment (IMaX) on board the balloon-borne solar observatory SUNRISE. We find that granules frequently show substructure in the form of lanes composed of a leading bright rim and a trailing dark edge, which move together from the boundary of a granule into the granule itself. We find strikingly similar events in synthesized intensity maps from an ab initio numerical simulation of solar surface convection. From cross sections through the computational domain of the simulation, we conclude that these granular lanes are the visible signature of (horizontally oriented) vortex tubes. The characteristic optical appearance of vortex tubes at the solar surface is explained. We propose that the observed vortex tubes may represent only the large-scale end of a hierarchy of vortex tubes existing near the solar surface.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

STATISTICAL DISTRIBUTION OF SIZE AND LIFETIME OF BRIGHT POINTS OBSERVED WITH THE NEW SOLAR TELESCOPE

We present results of 2 hr non-interrupted observations of solar granulation obtained under excellent seeing conditions with the largest aperture ground-based solar telescope-the New Solar Telescope (NST)-of Big Bear Solar Observatory. Observations were performed with adaptive optics correction using a broadband TiO filter in the 705.7 nm spectral line with a time cadence of 10 s and a pixel size of 0.''0375. Photospheric bright points (BPs) were detected and tracked. We find that the BPs detected in NST images are cospatial with those visible in Hinode/SOT G-band images. In cases where Hinode/SOT detects one large BP, NST detects several separated BPs. Extended filigree features are clearly fragmented into separate BPs in NST images. The distribution function of BP sizes extends to the diffraction limit of NST (77 km) without saturation and corresponds to a log-normal distribution. The lifetime distribution function follows a log-normal approximation for all BPs with lifetime exceeding 100 s. A majority of BPs are transient events reflecting the strong dynamics of the quiet Sun: 98.6% of BPs live less than 120 s. The longest registered lifetime was 44 minutes. The size and maximum intensity of BPs were found to be proportional to their lifetimes.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

IBEX RIBBON: WHAT COULD IT TELL ABOUT THE LOCAL INTERSTELLAR MAGNETIC FIELD?

We show that the shape of the IBEX Ribbon can be reproduced assuming energetic neutral atoms originate in regions beyond the heliopause where the interstellar magnetic field is the strongest and perpendicular to radial directions from the Sun. The best fit to the observed ribbon was obtained for the local interstellar magnetic field B{sub {infinity}} = 3.0 {+-} 1.0 {mu}G pointing from ecliptic/galactic coordinates ({lambda}, {beta})/(l, b) = (225{sup 0} {+-} 5{sup 0}, 35{sup 0} {+-} 5{sup 0})/(27{sup 0} {+-} 5{sup 0}, 51{sup 0} {+-} 5{sup 0}) close to the apparent ribbon center ({lambda}, {beta})/(l, b) = (221{sup 0}, 39{sup 0})/(33{sup 0}, 55{sup 0}). The geometrical considerations presented below should prove useful in identifying the mechanism of ribbon formation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

RAPID ENHANCEMENT OF SHEARED EVERSHED FLOW ALONG THE NEUTRAL LINE ASSOCIATED WITH AN X6.5 FLARE OBSERVED BY HINODE

We present G-band and Ca II H observations of NOAA AR 10930 obtained by Hinode/SOT on 2006 December 6 covering an X6.5 flare. The Local Correlation Tracking technique was applied to the foreshortening-corrected G-band image series to acquire horizontal proper motions in this complex {beta}{gamma}{delta} active region. With the continuous high-quality, spatial and temporal resolution G-band data, we not only confirm the rapid decay of outer penumbrae and darkening of the central structure near the flaring neutral line, but also unambiguously detect for the first time the enhancement of the sheared Evershed flow (average horizontal flow speed increased from 330 {+-} 3.1 to 403 {+-} 4.6 m s{sup -1}) along the neutral line right after the eruptive white-light flare. Post-flare Ca II H images indicate that the originally fanning out field lines at the two sides of the neutral line get connected. Since penumbral structure and Evershed flow are closely related to photospheric magnetic inclination or horizontal field strength, we interpret the rapid changes of sunspot structure and surface flow as the result of flare-induced magnetic restructuring down to the photosphere. The magnetic fields turn from fanning out to inward connection causing outer penumbrae decay, meanwhile those near the flaring neutral line become more horizontal leading to stronger Evershed flow there. The inferred enhancement of horizontal magnetic field near the neutral line is consistent with recent magnetic observations and theoretical predictions of flare-invoked photospheric magnetic field change.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

SOLAR CYCLE VARIATIONS OF THE OCCURRENCE OF CORONAL TYPE III RADIO BURSTS AND A NEW SOLAR ACTIVITY INDEX

This Letter presents the results of studies of solar cycle variations of the occurrence rate of coronal type III radio bursts. The radio spectra are provided by the Learmonth Solar Radio Observatory (Western Australia), part of the USAF Radio Solar Telescope Network (RSTN). It is found that the occurrence rate of type III bursts strongly correlates with solar activity. However, the profiles for the smoothed type III burst occurrence rate differ considerably from those for the sunspot number, 10.7 cm solar radio flux, and solar flare index. The type III burst occurrence rate (T3BOR) is proposed as a new index of solar activity. T3BOR provides complementary information about solar activity and should be useful in different studies including solar cycle predictions and searches for different periodicities in solar activity. This index can be estimated from daily results of the Automated Radio Burst Identification System. Access to data from other RSTN sites will allow processing 24 hr radio spectra in near-real time and estimating true daily values of this index. It is also shown that coronal type III bursts can even occur when there are no visible sunspots on the Sun. However, no evidence is found that the bursts are not associated with active regions. It is also concluded that the type III burst productivity of active regions exhibits solar cycle variations.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2001

The Stellar Imager (SI) Mission Concept

The Stellar Imager (SI) is envisioned as a space-based, UV-optical interferometer composed of ten or more one-meter class elements distributed with a maximum baseline of 0.5 km. It will image stars and binaries with 100 to 1000 resolution elements on their surface and enable long-term studies of stellar magnetic activity patterns and their evolution with time for comparison with those on the sun. It will also sound their interiors through asteroseismology to image internal structure, differential rotation, and large-scale circulations. SI will enable us to understand the various effects of magnetic fields of stars, the dynamos that generate them, and the internal structure and dynamics of the stars in which they exist. The ultimate goal is to achieve the best-possible forecasting of solar activity on times scales ranging up to decades, and an understanding of the impact of stellar magnetic activity on astrobiology and life in the Universe. The road to that goal will revolutionize our understanding of stars and stellar systems, the building blocks of the Universe. Fitting naturally within the NASA long-term time line, SI complements defined missions, and with them will show us entire other solar systems, from the central star to their orbiting planets.

Carpenter, Kenneth G.↗
NASA NTRS2001

The Stellar Imager (SI) Mission Concept

The Stellar Imager (SI) is envisioned as a space-based, UV-optical interferometer composed of 10 or more one-meter class elements distributed with a maximum baseline of 0.5 km. It will image stars and binaries with one hundred to one thousand resolution elements on their surface and enable long-term studies of stellar magnetic activity patterns and their evolution with time, for comparison with those on the sun. It will also sound their interiors through asteroseismology to image internal structure, differential rotation, and large-scale circulations. SI will enable us to understand the various effects of magnetic fields of stars, the dynamos that generate them, and the internal structure and dynamics of the stars in which they exist. The ultimate goal is to achieve the best-possible forecasting of solar activity on times scales ranging up to decades, and an understanding of the impact of stellar magnetic activity on astrobiology and life in the Universe. The road to that goal will revolutionize our understanding of stars and stellar systems, the building blocks of the Universe. Fitting naturally within the NASA and ESA long-term time lines, SI complements defined missions, and with them will show us entire other solar systems, from the central star to their orbiting planets. In this paper we will describe the scientific goals of the mission, the performance requirements needed to address those goals, and the design concepts now under study.

Carpenter, Kenneth G.↗
NASA NTRS2001

The Stellar Imager (SI) Mission Concept

The Stellar Imager (SI) is envisioned as a space-based, UV optical interferometer composed of 10 or more one-meter class elements distributed with a maximum baseline of 0.5 km. It will image stars and binaries with one hundred to one thousand resolution elements on their surface and enable long-term studies of stellar magnetic activity patterns and their evolution with time, for comparison with those on the sun. It will also sound their interiors through asteroseismology to image internal structure, differential rotation, and large-scale circulations. SI will enable us to understand the various effects of magnetic fields of stars, the dynamos that generate them, and the internal structure and dynamics of the stars in which they exist. The ultimate goal is to achieve the best-possible forecasting of solar activity on times scales ranging up to decades, and an understanding of the impact of stellar magnetic activity on astrobiology and life in the Universe. The road to that goal will revolutionize our understanding of stars and stellar systems, the building blocks of the Universe. Fitting naturally within the NASA and ESA long-term time lines, SI complements defined missions, and with them will show us entire other solar systems, from the central star to their orbiting planets. In this paper we will describe the scientific goals of the mission, the performance requirements needed to address those goals, and the design concepts now under study.

Carpenter, Kenneth G.↗
NASA NTRS2001

Stellar Imager

The Stellar Imager (SI) is envisioned as a space-based, UV-optical interferometer composed of 10 or more one-meter class elements distributed with a maximum baseline of 0.5 approx. km. It will image stars and binaries with one hundred to one thousand resolution elements on their surface and enable long-term studies of stellar magnetic activity patterns and their evolution with time, for comparison with those on the sun. It will also sound their interiors through asteroseismology to image internal structure, differential rotation, and large-scale circulations. SI will enable us to understand the various effects of magnetic fields of stars, the dynamos that generate them, and the internal structure and dynamics of the stars in which they exist. The ultimate goal is to achieve the best-possible forecasting of solar activity on times scales ranging up to decades, and an understanding of the impact of stellar magnetic activity on astrobiology and life in the Universe. The road to that goal will revolutionize our understanding of stars and stellar systems, the building blocks of the Universe. Fitting naturally within the NASA and ESA long-term time lines, SI complements defined missions, and with them will show us entire other solar systems, from the central star to their orbiting planets. In this paper we will describe the scientific goals of the mission, the performance requirements needed to address those goals, and the design concepts now under study.

Carpenter, Kenneth G.↗
NASA NTRS2001

Stellar Imager

The Stellar Imager (SI) is envisioned as a space-based, UV-optical interferometer composed of 10 or more one-meter class elements distributed with a maximum baseline of approx. 0.5 km. It will image stars and binaries with one hundred to one thousand resolution elements on their surface and enable long-term studies of stellar magnetic activity patterns and their evolution with time, for comparison with those on the sun. It will also sound their interiors through asteroseismology to image internal structure, differential rotation, and large-scale circulations. SI will enable us to understand the various effects of magnetic fields of stars, the dynamos that generate them, and the internal structure and dynamics of the stars in which they exist. The ultimate goal is to achieve the best-possible forecasting of solar activity on time scales ranging up to decades, and an understanding of the impact of stellar magnetic activity on astrobiology and life in the Universe. The road to that goal will revolutionize our understanding of stars and stellar systems, the building blocks of the Universe. Fitting naturally within the NASA and ESA long-term time lines, SI complements defined missions, and with them will show us entire other solar systems, from the central star to their orbiting planets. In this paper we will describe the scientific goals of the mission, the performance requirements needed to address those goals, and the design concepts now under study.

Carpenter, Kenneth G.↗
DOE OSTI2010

Long-term and transient forcing of the low ionosphere monitored by SAVNET

In this paper we present the main findings obtained by the South America VLF NETwork since its installation in the South America territory. In particular we show the capability of the VLF technique to monitor the long-term solar activity and transient solar and geomagnetic disturbances. On long timescales they are indices on the possibility of monitoring the Lyman-{alpha} solar radiation. On shorter timescales we show that the VLF technique is a very sensitive mean of detecting solar X-ray flares. Those small events with a peak power {>=}5x10{sup -7} W/m{sup 2} are detected with a 100% probability. A lower limit for the X-ray power of {approx}2.7 10{sup -7} W/m{sup 2} has been found in order to produce a ionospheric disturbance, and we confirm the important role of the Lyman-{alpha} radiation to form and maintain the low ionospheric D-region. SAVNET has also observed for the first time the ionospheric disturbances produced by outbursts from the magnetar SGR 1550-5408. This genuine detection suggests the possibility of monitoring on a routine basis these objects of fundamental importance in high-energy astrophysics. Finally, we show that SAVNET is well suited for participating to the search for seismo-ionospheric disturbances in order to study the possibility of earthquake events prediction.

58 GEOSCIENCES↗
DOE OSTI2010

Modified Burgers' equation resulting from a hydrodynamic model of shock waves with reflected particles

Ion reflection at perpendicular, collisionless shock waves is treated in a simple gas dynamical context. The effect of pickup ions reflected by the cross-shock potential and their subsequent energization in the motional electric field is included through the introduction of source terms in the basic inviscid fluid equations. From a weak source description, a Burgers' equation is derived modified by the addition of a quadratically nonlinear term. The construction of Burgers' equation demonstrates explicitly that particle reflection is a dissipation mechanism for collisionless shocks. Numerical solutions for the modified Burgers' equation (MBE) are presented. From these solutions it is implied that the dissipative term of the MBE is associated with the length scale of the shock foot, while the additional term is associated with the shock ramp. The general physical model is discussed in terms of the reflection and energization of pickup ions at outer heliospheric perpendicular shocks.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Evidence for Coexistence of SOC, Intermittent Turbulence and Low-Dimensional Chaos Processes in Solar Flare Dynamics

In this study, we present evidence for the co-existence of a SOC-like mechanism, of intermittent turbulence and a hidden low dimensional chaotic process underlying the solar activity. In particular, the original signal reveals a high dimensional stochastic character and a critical state according to a SOC process, since there in no clear discrimination between the original signal and its surrogate data concerning geometrical and dynamical characteristics of the solar time series. Furthermore, using the flat coefficient F we found evidence for intermittent turbulence behaviour. This result was based on the non-Gaussian character and the presence of long-range correlated plasma vorticity. This is in agreement with the low dimensional chaotic self-organization that was revealed after the application of a high pass filter to the original solar flares signal and provides further evidence for self organization in solar activity dynamics. Finally, the above results indicate the existence of a phase transition-like process in the solar corona dynamics, from high dimensional stochastic states to strong intermittent turbulence and low dimensional chaotic states, according to a general theory of critical dynamics applied to the driven and distributed solar corona system.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

A nonlocal wave-wave interaction among Alfven waves in an intermediate-{beta} plasma

A nonlocal coupling mechanism to directly transfer the energy from large-scale magnetohydrodynamic (MHD) Alfven waves to small-scale kinetic Alfven waves is presented. It is shown that the interaction between a MHD Alfven wave and a reversely propagating kinetic Alfven wave can generate another kinetic Alfven wave, and this interaction exists in the plasmas where the thermal to magnetic pressure ratio is larger than the electron to ion mass ratio. The proposed nonlocal interaction may have a potential application to account for the observed electron scale kinetic Alfven waves in the solar wind and solar corona plasmas.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Ion-Neutral Collisions in the Interstellar Medium: Wave Damping and Elimination of Collisionless Processes

Most phases of the interstellar medium contain neutral atoms in addition to ions and electrons. This introduces differences in plasma physics processes in those media relative to the solar corona and the solar wind at a heliocentric distance of 1 astronomical unit. In this paper, we consider two well-diagnosed, partially-ionized interstellar plasmas. The first is the Warm Ionized Medium (WIM) which is probably the most extensive phase in terms of volume. The second is the gas of the Local Clouds of the Very Local Interstellar Medium (VLISM). Ion-neutral interactions seem to be important in both media. In the WIM, ion-neutral collisions are relatively rare, but sufficiently frequent to damp magnetohydrodynamic (MHD) waves (as well as propagating MHD eddies) within less than a parsec of the site of generation. This result raises interesting questions about the sources of turbulence in the WIM. In the case of the VLISM, the ion-neutral collision frequency is higher than that in the WIM, because the hydrogen is partially neutral rather than fully ionized. We present results showing that prominent features of coronal and solar wind turbulence seem to be absent in VLISM turbulence. For example, ion temperature does not depend on ion mass. This difference may be due to ion-neutral collisions, which distribute power from more effectively heated massive ions such as iron to other ion species and neutral atoms.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2010

ON THE SOLAR CHROMOSPHERE OBSERVED AT THE LIMB WITH HINODE

Broadband images in the Ca II H line, from the Broadband Filter Imager (BFI) instrument on the Hinode spacecraft, show emission from spicules emerging from and visible right down to the observed limb. Surprisingly, little absorption of spicule light is seen along their lengths. We present formal solutions to the transfer equation for given (ad hoc) source functions, including a stratified chromosphere from which spicules emanate. The model parameters are broadly compatible with earlier studies of spicules. The visibility of Ca II spicules down to the limb in Hinode data seems to require that spicule emission be Doppler shifted relative to the stratified atmosphere, either by supersonic turbulent or organized spicular motion. The non-spicule component of the chromosphere is almost invisible in the broadband BFI data, but we predict that it will be clearly visible in high spectral resolution data. Broadband Ca II H limb images give the false impression that the chromosphere is dominated by spicules. Our analysis serves as a reminder that the absence of a signature can be as significant as its presence.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2010

DEPENDENCE OF THE ANOMALOUS RESISTIVITY ON THE INDUCED ELECTRIC FIELD IN SOLAR FLARES

Anomalous resistivity is a critical parameter for triggering the fast magnetic reconnection and interpreting the eruption of solar flares in the nearly collisionless coronal plasma. However, the mechanism for the production of anomalous resistivity and its evolution are weakly understood. In this paper, the one-dimensional Vlasov equation was numerically solved with the typical solar coronal parameters and realistic mass ratio in the presence of strong inductive electric field, and the relationship between the anomalous resistivity and the reconnecting electric field was inferred for the area near the center of reconnecting current sheets. Our principal findings are summarized as follows. (1) The relationship between the anomalous resistivity and the reconnecting electric field E {sub 0} may be represented by {eta}{sub eff} = [10.82-10.99 exp (-0.36 E {sub 0})]{Omega} m. (2) If E{sub 0} is small enough, it may be described by {eta}{sub eff} = [4.02 E {sub 0} - 0.18]{Omega} m, which is basically consistent with the early experimental results on the plasma response to the applied electric field. (3) In comparison with theoretical formulas for the current-driven ion-acoustic and Buneman anomalous resistivities, if E{sub 0} is small, the anomalous resistivity may be due to the ion-acoustic instability; if E{sub 0} is large, the anomalous resistivity may be due to the Buneman instability. These results are also basically consistent with early experiments.

79 ASTRONOMY AND ASTROPHYSICS↗