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Flare Ribbon Signatures of Plasmoids

Solar flares are explosive space weather events that rapidly convert stored magnetic energy into bulk motion, plasma heating, and particle acceleration. Understanding the structure and dynamics of the magnetic reconnection that powers flares is critical for predicting the energy release. In particular, the amount of energy transferred to energetic particles is thought to be highly dependent on whether the reconnection is primarily turbulent (e.g., plasmoid dominated) or instead laminar. We present new high-resolution MHD simulations of three-dimensional reconnection in an eruptive flare and compare the results to recent data. Although flare reconnection is challenging to observe directly in the corona, highly detailed constraints on its dynamics can be obtained from observations of flare ribbons that track the chromospheric footpoints of newly reconnected field lines. The analogues of flare ribbons in our simulations are identified by tracking discontinuous changes in field-line magnetic connectivity due to the reconnection. In our highest-resolution calculations, we find that these ribbon analogues are highly structured and exhibit many ‘whorl’ patterns that are linked to turbulent plasmoids in the reconnecting current sheet. Such flare ribbon fine structure therefore reveals crucial information about the fundamental turbulent vs. laminar nature of the reconnection. Plasmoid-driven particle acceleration models would therefore predict co-location of energetic particle signatures with plasmoid-associated ribbon fine structure.

J T Dahlin↗

NASA NTRS1992

Interstellar absorption along the line of sight to Theta Carinae using Copernicus observations

A profile fitting technique is employed to identify the velocities and Doppler b values for H I and H II clouds along the line of sight to Theta Car. Total abundances and depletions for 12 elements, plus column densities for the J = 0 to J = 5 rotational levels of H2 are obtained. Electron densities for both clouds are calculated from the ratios of the fine-structure levels of C II and N II, obtaining 0.08/cu cm and 1.2/cu cm. The fine-structure levels of C I, which led to 120/cu cm, are used to calculate the neutral hydrogen density for the H I region. D I is also present in the data from the Theta Car line of sight, yielding a D/H ratio of 5 x 10 exp -6. Elemental depletions are calculated for the H I region as well. Comparison of the results for Theta Car and those for Zeta Oph and Alpha Vir shows that the absolute depletions are different; however, the relative depletions are remarkably stable for different physical conditions.

Allen, M. M.↗

NASA NTRS2003

Spectral behavior of hydrated sulfate salts: implications for Europa mission spectrometer design

Remote sensing of the surface of Europa with near-infrared instruments has suggested the presence of hydrated materials, including sulfate salts. Attention has been focused on these salts for the information they might yield regarding the evolution of a putative interior ocean, and the evaluation of its astrobiological potential. These materials exhibit distinct infrared absorption features due to bound water. The interactions of this water with the host molecules lead to fine structure that can be used to discriminate among these materials on the basis of their spectral behavior. This fine structure is even more pronounced at the low temperatures prevalent on icy satellites. Examination of hydrated sulfate salt spectra measured under cryogenic temperature conditions provides realistic constraints for future remote-sensing missions to Europa. In particular, it suggests that a spectrometer system capable of 2-5 nm spectral resolution or better, with a spatial resolution approaching 100 m, would be able to differentiate among proposed hydrated surface materials, if present, and constrain their distributions across the surface. Such information would provide valuable insights into the evolutionary history of Europa.

Astronomy↗

NASA NTRS1987

Airborne spectrophotometry of Eta Carinae from 4.5 to 7.5 microns and a model for source morphology

Spectrophotometric observations of Eta Car between 4.5 and 7.5 microns show a featureless thermal-like spectrum with no fine-structure lines or broad emission or absorption features. The color temperature of the spectrum is approximately 375 K. High spatial resolution maps at 3.5, 4.8, and 10 microns obtained from the ground are used to discuss the dust distribution and temperature structure, and to present a model for general source morphology. The upper limit to the brightness of the forbidden Ar II fine-structure emission line at 6.98 microns is less than 7 x 10 to the -16th W/sq cm, which still allows for a significant overabundance of argon and is consistent with the evolved nature of the source.

Russell, Ray W.↗

NASA NTRS1986

The microwave spectrum of solar millisecond spikes

The microwave radiation from solar flares sometimes shows short and intensive spikes which are superimposed on the burst continuum. In order to determine the upper frequency limit of their occurrence and the circular polarization, a statistical analysis was performed on digital microwave observations from 3.2 to 92.5 GHz. Additionally, fine structures were investigated with a fast 32-channel spectrometer at 3.47 GHz. It was found that about 10 percent of the bursts show fine structures at 3.2 and 5.2 GHz, whereas none occurred above 8.4 GHz. Most of the observed spikes were very short and their bandwidth varied from below 0.5 MHz to more than 200 MHz. Simultaneous observations at two further frequencies showed no coincident spikes at the second and third harmonic. The observations can be explained by the theory of electron cyclotron masering if the observed bandwidths are determined by magnetic field inhomogeneities or if the rise times are independent of the source diameters. The latter would imply source sizes between 50 and 100 km.

Staehli, M.↗

NASA NTRS1972

Magnetic coupling of the active chromosphere to the solar interior.

Evidence is summarized to show that the configuration of e lines which governs the appearance of H-alpha fine structure in active regions is set mainly by motions in the subphotosphere where these lines are anchored. It is shown that H-alpha fine structure is directly coupled to a layer probably more than 5000 km below the photosphere, and little distortion of the strong fields is expected in the ines. The shorter rotation period of active regions observed by Howard and others (compared to the photospheric gas) reted as a result ofthis direct coupling of the strong field to a more rapidly rotating solar interior. The effects of dragging such a field through a photosphere of finite resistivity are briefly considered for features of various observed cross-sections.

Foukal, P.↗

NASA NTRS1985

Rocket measurements of mesospheric ionization irregularities

The Langmuir probe technique for measurement of electron concentration in the mesosphere is capable of excellent altitude resolution, of order 1 m. Measurements from nine daytime rocket flights carrying an electron density fine structure experiment frequently show small scale ionization structures in the altitude region 70 to 90 km. The irregularities are believed to be the result of turbulent advection of ions and electrons. The fine structure experiment flown by the University of Illinois is described and methods of analyzing the collected data is presented. Theories of homogeneous, isotropic turbulence are reviewed. Power spectra of the measured irregularities are calculated and compared to spectra predicted by turbulence theories.

Stoltzfus, R. B.↗

NASA NTRS1974

Ion and electron temperatures in the SUMMA mirror device by emission spectroscopy

Ion temperatures were obtained in the SUMMA mirror device by observing the Doppler-broadened charge-exchange component of the 667.8and 587.6-nm He lines in He plasma and the H alpha and H beta lines in H2 plasma. The second moment of the line profiles was used as the parameter to determine ion temperature. Corrections for magnetic splitting, fine structure, monochromator slit function, and variation in charge-exchange cross section with energy are derived and included. Even for constant cross section, no magnetic splitting or fine structure, and infinitely narrow slit function, the line profile is not Gaussian, because the excitation results from a change-exchange process. Comparison is made with temperatures from a neutral particle analyzer.-

Patch, R. W.↗

NASA NTRS2001

Resonance Scattering of Fe XVII X-ray and EUV Lines

Over the years a number of calculations have been carried out to derive intensities of various X-ray and EUV lines in Fe XVII to compare with observed spectra. The predicted intensities have not agreed with solar observations, particularly for the line at 1.5.02 Angstroms; resonance scattering has been suggested as the source for much of the disagreement. The atomic data calculated earlier used seven configurations having n=3 orbitals and the scattering calculations were carried out only for incident energies above the threshold of the highest fine-structure level. These calculations have now been extended to thirteen configurations having n=4 orbitals and the scattering calculations are carried out below as well as above the threshold of the highest fine structure level. These improved calculations of Fe XVII change the intensity ratios compared to those obtained earlier, bringing the optically thin F(15.02)/F(16.78) ratio and several other ratios closer to the observed values. However, some disagreement with the solar observations still persists, even thought the agreement of the presently calculated optically thin F(15.02)/F(15.26) ratio with the experimental results of Brown et al. (1998) and Laming et al. (2000) has improved. Some of the remaining discrepancy is still thought to be the effect of opacity, which is consistent with expected physical conditions for solar sources. EUV intensity ratios are also calculated and compared with observations. Level populations and intensity ratios are calculated, as a function of column density of Fe XVII, in the slab and cylindrical geometries. As found previously, the predicted intensities for the resonance lines at 15.02 and 15.26 Angstroms exhibit initial increases in flux relative to the forbidden line at 17.10 Angstroms and the resonance line at 16.78 Angstroms as optical thickness increases. The same behavior is predicted for the lines at 12.262 and 12.122 Angstroms. Predicted intensities for some of the allowed EUV lines are also affected by opacity.

Bhatia, A. K.↗

NASA NTRS2014

Dynamics of the Transition Corona

Magnetic reconnection between the open and closed magnetic fields in the corona is believed to play a crucial role in the corona/heliosphere coupling. At large scale, the exchange of open/closed connectivity is expected to occur in pseudo-streamer (PS) structures. However, there is neither clear observational evidence of how such coupling occurs in PSs, nor evidence for how the magnetic reconnection evolves. Using a newly developed technique, we enhance the off-limb magnetic fine structures observed with the Atmospheric Imaging Assembly and identify a PS-like feature located close to the northern coronal hole. We first identify that the magnetic topology associated with the observation is a PS, null-point (NP) related topology bounded by the open field. By comparing the magnetic field configuration with the extreme ultraviolet (EUV) emission regions, we determined that most of the magnetic flux associated with plasma emission are small loops below the PS basic NP and open field bounding the PS topology. In order to interpret the evolution of the PS, we referred to a three-dimensional MHD interchange reconnection modeling the exchange of connectivity between small closed loops and the open field. The observed PS fine structures follow the dynamics of the magnetic field before and after reconnecting at the NP obtained by the interchange model. Moreover, the pattern of the EUV plasma emission is the same as the shape of the expected plasma emission location derived from the simulation. These morphological and dynamical similarities between the PS observations and the results from the simulation strongly suggest that the evolution of the PS, and in particular the opening/closing of the field, occurs via interchange/slipping reconnection at the basic NP of the PS. Besides identifying the mechanism at work in the large-scale coupling between the open and closed fields, our results highlight that interchange reconnection in PSs is a gradual physical process that differs from the impulsive reconnection of the solar-jet model.

magnetic fields↗

NASA NTRS1981

Color and temperature signatures of ocean fronts observed with the Nimbus-7 CZCS

The Nimbus-7 Coastal Zone Color Scanner has a unique capability for imaging simultaneously the optical (ocean color) and surface temperature (thermal infrared) signatures of ocean fronts. Examples of such color and temperature visualizations of some ocean fronts delineate virtually identical morphology. In other cases, however, the two signatures are displaced horizontally by tens of km, and display markedly different morphology and fine structure. For example, an image of the Gulf Stream's north wall over the Grand Banks shows the sharpest color front to occur approximately 50 km away from the sharpest temperature front. Moreover, the fine structure of the color front is qualitatively suggestive of disorganized turbulent diffusion, whereas that of the thermal front shows distinctive shear instabilities. These differences can occur because the infrared temperature image represents the top 0.05 millimeter of the sea surface, whereas the ocean color image represents the upper meters to decameters of the sea.

Mueller, J. L.↗

NASA NTRS2015

Spectral Confusion for Cosmological Surveys of Redshifted C II Emission

Far-infrared cooling lines are ubiquitous features in the spectra of star-forming galaxies. Surveys of redshifted fine-structure lines provide a promising new tool to study structure formation and galactic evolution at redshifts including the epoch of reionization as well as the peak of star formation. Unlike neutral hydrogen surveys, where the 21 cm line is the only bright line, surveys of redshifted fine-structure lines suffer from confusion generated by line broadening, spectral overlap of different lines, and the crowding of sources with redshift. We use simulations to investigate the resulting spectral confusion and derive observing parameters to minimize these effects in pencilbeam surveys of redshifted far-IR line emission. We generate simulated spectra of the 17 brightest far-IR lines in galaxies, covering the 150-1300 μm wavelength region corresponding to redshifts 0 < z < 7, and develop a simple iterative algorithm that successfully identifies the 158 μm [C II] line and other lines. Although the [C II] line is a principal coolant for the interstellar medium, the assumption that the brightest observed lines in a given line of sight are always [C II] lines is a poor approximation to the simulated spectra once other lines are included. Blind line identification requires detection of fainter companion lines from the same host galaxies, driving survey sensitivity requirements. The observations require moderate spectral resolution 700 < R < 4000 with angular resolution between 20″ and 10′, sufficiently narrow to minimize confusion yet sufficiently large to include a statistically meaningful number of sources.

galactic evolution at redshifts↗

NASA NTRS1991

The temperature of the circumstellar envelope of Alpha Orionis

The processes that heat and cool the circumstellar envelope of Alpha Ori are analyzed and the temperature calculated as a function of distance from the star. Cooling by fine structure lines is included for the first time in a general way in the theory of the thermal balance of a circumstellar envelope. Adiabatic cooling, dust-drag heating, and various line coolants contribute in the outer envelope, whereas the fine structure lines of O I and Si II and adiabatic cooling are dominant in the inner envelope. The rotational levels of CO cool the gas only at very small and at very large distances; elsewhere they provide a small amount of heating. The temperature declines very rapidly at first and then varies roughly as r exp -1/2 in the outer envelope. At very large distances, the temperature distribution flattens out due to photoelectron heating and reaches a minimum of about 4 K at about 250 arcsec.

Rodgers, Bernadette↗

NASA NTRS1990

High-resolution reflectance spectra of Mars in the 2.3-micron region - Evidence for the mineral scapolite

Spatially resolved reflectance spectra of Mars obtained from the Cooled Grating Array Spectrometer at the NASA Infrared Telescope Facility during August 26-30, 1988 were used to investigate the mineralogic origin of the 2.36-micron band that was revealed previously in Martian spectra. The new high-resolution data demonstrate that the absorption envelope centered near 2.36 microns has considerable spectral fine structure. It is shown that, while some of the observed absorption is due to CO in the Martian atmosphere, a significant amount of the observed spectral behavior is due to a solid mineral phase on the Martian surface. The solid-phase absorption features observed were found to match the spectral fine structure of minerals in the scapolite family. Scapolites of differing compositions were found which closely reproduce the regional spectral variations observed on Mars.

Clark, Roger N.↗

NASA NTRS1974

Interstellar carbon I lines in zeta Ophiuchi

With two f-values of C I lines obtained from the spectra of zeta Pup and gamma(2) Vel, it is shown that the interstellar C I spectrum in zeta Oph obeys an optical-depth relation similar to that for Na I, and the relative strengths of 19 multiplets are determined from this relation. The dominant processes which excite neutral carbon are used to calculate populations of the fine-structure levels as functions of the density of H nuclei and the temperature, and evidence is found that the C I lines are formed in the dense cloud in front of zeta Oph at a heliocentric velocity of -14.4 km/s. It is noted that the observed column-density ratios of the neutral carbon fine-structure levels lead to a total H nuclei density of about 220 to 660 per cu cm, while the electron density and observed C(+) abundance indicate a value of 10,000 per cu cm, with a 0.05 pc thick gas layer.

De Boer, K. S.↗

NASA NTRS1987

Ray propagation path analysis of acousto-ultrasonic signals in composites

The most important result was the demonstration that acousto-ultrasonic (AU) energy introduced into a laminated graphite/resin propagates by two modes through the structure. The first mode, along the graphite fibers, is the faster. The second mode, through the resin matrix, besides being slower is also more strongly attenuated at the higher frequencies. This demonstration was accomplished by analyzing the time and frequency domain of the composite AU signal and comparing them to the same for a neat resin specimen of the same chemistry and geometry as the composite matrix. Analysis of the fine structure of AU spectra was accomplished by various geometrical strategies. It was shown that the multitude of narrow peaks associated with AU spectra are the effect of the many pulse arrivals in the signal. The shape and distribution of the peaks is mainly determined by the condition of nonnormal reflections of ray paths. A cepstrum analysis was employed which can be useful in detecting characteristic times. Analysis of propagation modes can be accomplished while ignoring the fine structure.

Kautz, Harold E.↗

NASA NTRS1990

Iridium profile for 10 million years across the Cretaceous-Tertiary boundary at Gubbio (Italy)

The iridium anomaly at the Cretaceous-Tertiary (KT) boundary was discovered in the pelagic limestone sequence at Gubbio on the basis of 12 samples analyzed by neutron activation analysis (NAA) and was interpreted as indicating impact of a large extraterrestrial object at exactly the time of the KT mass extinction. Continuing controversy over the shape of the Ir profile at the Gubbio KT boundary and its interpretation called for a more detailed follow-up study. Analysis of a 57-meter-thick, 10-million-year-old part of the Gubbio sequence using improved NAA techniques revealed that there is only one Ir anomaly at the KT boundary, but this anomaly shows an intricate fine structure, the origin of which cannot yet be entirely explained. The KT Ir anomaly peaks in a 1-centimeter-thick clay layer, where the average Ir concentration is 3000 parts per trillion (ppt); this peak is flanked by tails with Ir concentrations of 20 to 80 ppt that rise above a background of 12 to 13 ppt. The fine structure of the tails is probably due in part to lateral reworking, diffusion, burrowing, and perhaps Milankovitch cyclicity.

Alvarez, Walter↗

NASA NTRS1981

High-resolution ultraviolet solar observations from sounding rockets and Spacelab

High spatial (about 1 arcsec) and temporal (20 sec) resolution UV solar spectroscopy has been carried out by the Naval Research Laboratory from sounding rockets since 1975. The High Resolution Telescope and Spectrograph payload, consisting of a 300 mm diameter x 4500 mm focal length Cassegrain telescope and the tandem Woodsworth spectrograph, was used to make the observations. Using this instrument, a multitude of highly energetic phenomena can be observed in the chromosphere, transition zone, and corona. High-resolution photographs of the UV continuum at 1600 A, used to study the fine structure and evolution of the temperature minimum layer, show that because of its strong spatial and temporal intensity fluctuations, the 1600 A continuum emission comes from not only the temperature minimum, but a variety of temperature regimes. Similar spatial and temporal behavior is seen in lower chromospheric lines of Si I. Typical line profiles of the C IV 1550 A line illustrate the fine structure and dynamics of the transition zone; the mass and energy flux balance in this zone is also determined.

Brueckner, G. E.↗