Engineering topics

Hathaway, D. H.

Publications and source records attributed to Hathaway, D. H..

At least 37 records · Page 2

NASA NTRS2001

Sun-Earth Day, 2001

To promote awareness of the Sun-Earth connection, NASA's Marshall Space Flight Center, in collaboration with the Stanford SOLAR Center, sponsored a one-day Sun-Earth Day event on April 27, 2001. Although "celebrated" on only one day, teachers and students from across the nation, prepared for over a month in advance. Workshops were held in March to train teachers. Students performed experiments, results of which were shared through video clips and an internet web cast. Our poster includes highlights from student experiments (grades 2 - 12), lessons learned from the teacher workshops and the event itself, and plans for Sun-Earth Day 2002.

Adams, Mitzi L.↗

NASA NTRS2000

Status of Cycle 23 Forecasts

A number of techniques for predicting solar activity on a solar cycle time scale are identified, described, and tested with historical data. Some techniques, e.g,, regression and curve-fitting, work well as solar activity approaches maximum and provide a month- by-month description of future activity, while others, e.g., geomagnetic precursors, work well near solar minimum but provide an estimate only of the amplitude of the cycle. A synthesis of different techniques is shown to provide a more accurate and useful forecast of solar cycle activity levels. A combination of two uncorrelated geomagnetic precursor techniques provides the most accurate prediction for the amplitude of a solar activity cycle at a time well before activity minimum. This precursor method gave a smoothed sunspot number maximum of 154+21 for cycle 23. A mathematical function dependent upon the time of cycle initiation and the cycle amplitude then describes the level of solar activity for the complete cycle. As the time of cycle maximum approaches a better estimate of the cycle activity is obtained by including the fit between recent activity levels and this function. This Combined Solar Cycle Activity Forecast now gives a smoothed sunspot maximum of 140+20 for cycle 23. The success of the geomagnetic precursors in predicting future solar activity suggests that solar magnetic phenomena at latitudes above the sunspot activity belts are linked to solar activity, which occurs many years later in the lower latitudes.

Hathaway, D. H.↗

NASA NTRS2000

Radial Flows in Supergranules

We determine the radial component of the supergranular flow velocity by examining the center-to-limb variation of the Doppler velocity signal. We acquire individual Doppler images obtained with the MDI instrument on the SOHO spacecraft and process them to remove the p-mode oscillation signal, the axisymmetric flows, the convective blueshift signal, and instrumental artifacts. The remaining Doppler signal contains only cellular flow structures. The Doppler signal from the horizontal flows in these cells varies like sin p, where p is the heliocentric angle from disk center. The Doppler signal from radial flows varies like cos p. We fit the center-to-limb variation of the mean squared velocity signal to a straight line in sin(exp 2) rho over the central portion of the disk. The intercept of this line at disk center gives the amplitude of the radial component of the flow. The slope of the line gives the amplitude of the horizontal component. We find that the radial flows for typical supergranules have speeds about 10% that of their associated horizontal flows or about 30 m/s. The ratio of the radial to horizontal flow speed increases from 9% to about 18% as the size of the cells decreases from > 60 Mm to approximately 5 Mm. We use data simulations to check these results and find a ratio that increases from 5% to only about 12% over the same range of cell sizes. These smaller ratios are attributed to an underestimation of the horizontal flow speeds due to the fact that, the transverse component of the horizontal flow is not detected by the Doppler measurements.

Hathaway, D. H.↗

NASA NTRS1999

Multi-Wavelength Analysis of the March 26, 1991: Solar Flare

We have engaged in detailed multi-wavelength analysis of the March 26, 1991, solar flare in order to develop a method of diagnostics of the physical processes responsible for the efficient acceleration of charged particles to high energies and also for diagnostics of the photospheric response to the injection of the accelerated particles. Consideration of this particular flare is of special interest because to date it is the only flare in which the gamma-ray emission with energies of 20-1000 MeV was registered throughout the entire development of the event in the optical, radio and soft x-ray bands. individual registration by the GAMMA-1 telescope of the energy and precise time for each registered gamma-ray photon allowed the maximum detailed investigation of the energy spectrum and flux evolution during the flare.

Kurt, V. G.↗

NASA NTRS1999

Large-Scale Coronal Heating from "Cool" Activity in the Solar Magnetic Network

In Fe XII images from SOHO/EIT, the quiet solar corona shows structure on scales ranging from sub-supergranular (i.e., bright points and coronal network) to multi-supergranular (large-scale corona). In Falconer et al 1998 (Ap.J., 501, 386) we suppressed the large-scale background and found that the network-scale features are predominantly rooted in the magnetic network lanes at the boundaries of the supergranules. Taken together, the coronal network emission and bright point emission are only about 5% of the entire quiet solar coronal Fe XII emission. Here we investigate the relationship between the large-scale corona and the network as seen in three different EIT filters (He II, Fe IX-X, and Fe XII). Using the median-brightness contour, we divide the large-scale Fe XII corona into dim and bright halves, and find that the bright-half/dim half brightness ratio is about 1.5. We also find that the bright half relative to the dim half has 10 times greater total bright point Fe XII emission, 3 times greater Fe XII network emission, 2 times greater Fe IX-X network emission, 1.3 times greater He II network emission, and has 1.5 times more magnetic flux. Also, the cooler network (He II) radiates an order of magnitude more energy than the hotter coronal network (Fe IX-X, and Fe XII). From these results we infer that: 1) The heating of the network and the heating of the large-scale corona each increase roughly linearly with the underlying magnetic flux. 2) The production of network coronal bright points and heating of the coronal network each increase nonlinearly with the magnetic flux. 3) The heating of the large-scale corona is driven by widespread cooler network activity rather than by the exceptional network activity that produces the network coronal bright points and the coronal network. 4) The large-scale corona is heated by a nonthermal process since the driver of its heating is cooler than it is. This work was funded by the Solar Physics Branch of NASA's office of Space Science through the SR&T Program and the SEC Guest Investigator Program.

Falconer, D. A.↗

NASA NTRS1999

Solar Cycle Predictions

We currently lack a comprehensive model for the solar dynamo which might be used for predicting solar cycle activity levels. We can, nonetheless, predict these activity levels with some skill using a variety of techniques based on correlations between activity indicators at different times. A number of the techniques currently in use for predicting solar activity on a solar cycle time-scale have been tested with historical data. Some techniques, e.g., regression and curve fitting, work well as solar activity approaches maximum and provide a month by month description of future activity, while others, e.g., geomagnetic precursors, work well near solar minimum but only provide an estimate of the amplitude of the next cycle. A synthesis of different techniques is shown to provide a more accurate and useful forecast of solar cycle activity levels. The combination of two uncorrelated geomagnetic precursor techniques provides a more accurate prediction for the amplitude of a solar activity cycle of a time well before activity minimum. A mathematical function dependent on the time of cycle initiation and the cycle amplitude can then be used to describe the level of solar activity month by month for the next cycle. As the time of cycle maximum approaches a better estimate of the cycle activity is obtained by including the fit between previous activity levels and this function. The success of the geomagnetic precursors in predicting the amplitude of the next solar cycle supports the idea of extended solar cycle in which the activity cycles overlap. While the current cycle is producing sunspots and active regions in the Sun's equatorial regions the next cycle is already actively influencing the geomagnetic field, The precise nature and source of this influence remains unclear.

Hathaway, D. H.↗

NASA NTRS1999

The Photospheric Convection Spectrum

Spectra of the cellular photospheric flows are determined from observations acquired by the MDI instrument on the SOHO spacecraft. Spherical harmonic spectra are obtained from the full-disk observations. Fourier spectra are obtained from the high-resolution observations. The p-mode oscillation signals and instrumental artifacts are reduced by temporal filtering of the Doppler data. The resulting spectra give power (kinetic energy) per wavenumber for effective spherical harmonic degrees from 1 to over 3000. The spectra show distinct peaks representing granules and supergranules but no distinct features at wavenumbers representative of mesogranu;es or giant cells. The observed cellular patterns and spectra are well represented by a model that only includes granules and supergranules.

Hathaway, D. H.↗

NASA NTRS1998

Network Coronal Bright Points: Coronal Heating Concentrations Found in the Solar Magnetic Network

We examine the magnetic origins of coronal heating in quiet regions by combining SOHO/EIT Fe xii coronal images and Kitt Peak magnetograms. Spatial filtering of the coronal images shows a network of enhanced structures on the scale of the magnetic network in quiet regions. Superposition of the filtered coronal images on maps of the magnetic network extracted from the magnetograms shows that the coronal network does indeed trace and stem from the magnetic network. Network coronal bright points, the brightest features in the network lanes, are found to have a highly significant coincidence with polarity dividing lines (neutral lines) in the network and are often at the feet of enhanced coronal structures that stem from the network and reach out over the cell interiors. These results indicate that, similar to the close linkage of neutral-line core fields with coronal heating in active regions (shown in previous work), low-lying core fields encasing neutral lines in the magnetic network often drive noticeable coronal heating both within themselves (the network coronal bright points) and on more extended field lines rooted around them. This behavior favors the possibility that active core fields in the network are the main drivers of the heating of the bulk of the quiet corona, on scales much larger than the network lanes and cells.

Falconer, D. A.↗

NASA NTRS1998

Large-Scale Coronal Heating, Clustering of Coronal Bright Points, and Concentration of Magnetic Flux

By combining quiet-region Fe XII coronal images from SOHO/EIT with magnetograms from NSO/Kitt Peak and from SOHO/MDI, we show that on scales larger than a supergranule the population of network coronal bright points and the magnetic flux content of the network are both markedly greater under the bright half of the quiet corona than under the dim half. These results (1) support the view that the heating of the entire corona in quiet regions and coronal holes is driven by fine-scale magnetic activity (microflares, explosive events, spicules) seated low in the magnetic network, and (2) suggest that this large-scale modulation of the magnetic flux and coronal heating is a signature of giant convection cells.

Falconer, D. A.↗

NASA NTRS1997

Evaluation of Two Fractal Methods for Magnetogram Image Analysis

Fractal and multifractal techniques have been applied to various types of solar data to study the fractal properties of sunspots as well as the distribution of photospheric magnetic fields and the role of random motions on the solar surface in this distribution. Other research includes the investigation of changes in the fractal dimension as an indicator for solar flares. Here we evaluate the efficacy of two methods for determining the fractal dimension of an image data set: the Differential Box Counting scheme and a new method, the Jaenisch scheme. To determine the sensitivity of the techniques to changes in image complexity, various types of constructed images are analyzed. In addition, we apply this method to solar magnetogram data from Marshall Space Flight Centers vector magnetograph.

Stark, B.↗

NASA NTRS1993

Measurement of p-mode energy propagation in the quiet solar photosphere

We have measured and analyzed the p-mode oscillations in the profile of the Mg I 4571 A line in a quiet region near disk center. The oscillations are found to be mostly standing waves, in agreement with previous work. However, a small propagating component is measured, and we determine the direction, magnitude, and vertical variation of the energy propagation. The work integral indicates an upward energy flow of about 2 x 10 exp 7 ergs/sq cm/s at a height of 50 km above the base of the photosphere for waves with frequencies of 2-16 mHz. This energy flow decreases exponentially with height and drops below 10 exp 5 ergs/sq cm/s in the uppermost photosphere. The energy flow leaving the upper photosphere is at least an order of magnitude too small to constitute a significant source of heating for the chromosphere. However, the p-mode damping in the lower photosphere approaches levels large enough to account for the measured p-mode line widths. The relative amplitudes and phases of the thermodynamic quantities indicate that the p-mode are neither adiabatic nor isothermal in the photosphere.

Fontenla, J. M.↗

NASA NTRS1989

Evaluation of magnetic shear in off-disk center active regions

The changes that projection effects produce in the evaluation of magnetic shear in off-disk center active regions by comparing angular shear calculated in image plane and heliographic coordinates are analyzed, and the procedure for properly evaluating magnetic shear by transforming the observed vector magnetic field into the heliographic system is described. This procedure is then used to evaluate magnetic shear along the magnetic neutral line in an active region that was observed on April 24, 1984 at a longitude offset of -45 deg. In particular, the number of 'critically sheared' pixels along an east-west directed segment of the neutral line in the leader sunspot group changes from 16 in the image plane magnetogram to 14 in the heliographic magnetogram. The critical shear as calculated in the image plane served as a good predictor for the location of flaring activity since the flare ribbons of the great flare of April 24 bracketed the inversion line where the critical shear was located. These results indicate that for this particular region, projection effects did not significantly affect the evaluation of critical shear.

Venkatakrishnan, P.↗

NASA NTRS1988

Elimination of projection effects from vector magnetograms - The pre-flare configuration of active region AR 4474

A simple method of transforming vector magnetograms to heliographic coordinates is demonstrated. The merits of this transformation are illustrated using a vector magnetogram obtained with the MSFC vector magnetograph 80 minutes prior to a white light flare in active region AR 4474 on April 25, 1984. The original magnetogram shows strong magnetic shear along the neutral line at both the flare site and a nonflaring site. The transformation of the magnetogram to heliographic coordinates shows that the elimination of projection effects results in a much shorter length of the sheared region at the nonflaring site than what is inferred from the image plane vector magnetogram. The length of the sheared region at the flare site is relatively less affected by the transformation.

Venkatakrishnan, P.↗

NASA NTRS1987

Asymmetry of the heliosphere

The outflowing solar wind interacts with the local interstellar medium to form the heliospheric cavity within which the solar wind is supersonic. Because the interstellar medium is moving with respect to the sun, and because the solar wind has a latitude dependence, the heliosphere is asymmetric. The flow of the interstellar medium past the heliosphere produces an asymmetry because of the Bernoulli effect, which draws the heliosphere out in a direction orthogonal to the upstream-downstream axis, and because of a viscous interaction, which draws out the heliosphere downstream. A variety of cases are considered and the effects are found to be significant with, typically, the upstream direction having a heliospheric dimension that is 2/3 the downstream dimension. Suggestions have been put forth to the effect that a spacecraft penetration of the heliospheric shock wave may be imminent. Because one of the most distant spacecraft is moving roughly in the upstream direction relative to the interstellar flow, and the other is moving in the downstream direction, the distance to their encounters with the heliospheric shock may differ by as much as 40 AU.

Suess, S. T.↗

NASA NTRS1986

Flow regimes in a shallow rotating cylindrical annulus with temperature gradients imposed on the horizontal boundaries

Experimental flow regime diagrams are determined for a new rotating cylindrical annulus configuration which permits a measure of control over the internal vertical temperature gradient. The new annulus has radial temperature gradients imposed on plane horizontal thermally conducting endwalls (with the cylindrical sidewalls as insulators) and is considered to be more relevant to atmospheric dynamics studies than the classical cylindrical annulus. Observations have revealed that, in addition to the axisymmetric flow and nonaxisymmetric baroclinic wave flow which occur in the classical annulus, two additional nonaxisymmetric flow types occur in the new annulus: boundary-layer thermal convection and deep thermal convection. Flow regime diagrams for three different values of the imposed vertical temperature difference are presented, and explanations for the flow transitions are offered. The new annulus provides scientific backup for the proposed Atmospheric General Circulation Experiment for Spacelab. The apparatus diagram is included.

Hathaway, D. H.↗

NASA NTRS1986

Magnetic reversals of Jupiter and Saturn

The possibility that the gas-giant planets Jupiter and Saturn undergo solar-type magnetic reversals is examined using dynamo theory and radiotelescope data on decametriic emissions from Jupiter. Possible values are found for the effects of the fluctuating velocity field, the magnetic diffusivity, and change in the rotation rate of a dynamo over a characteristic length. The radio emissions from Jupiter decreased in intensity from 1961-72 and rose steadily to the end of 1978, which could have been caused by a change in the Jovian magnetic field. Since Jupiter may have a small rocky core embedded in metallic hydrogen which comprises 75 percent of the radius of the planet, the planetary magnetic field may extend into the cores of its satellites. The dynamo characteristics, like those of Saturn, would be chaotic, although quasi-periodic reversals could occur over intervals on the order of centuries instead of decades such as with the sun and much longer periods such as with the earth.

Hathaway, D. H.↗

NASA NTRS1986

Nonlinear interactions between convection, rotation and flows with vertical shear

The effects of a mean flow with vertical shear on the convective motions in a rotating layer are examined using a three-dimensional and time-dependent numerical model. In the absence of rotation, the convective motions are shown to be dominated by the shear flow when the Richardson number becomes greater than about -1.0. Both heat and momentum are carried down their respective gradients. For rotating cases with vertical rotation vectors, the Coriolis force turns the flow induced by the convection to produce a more complicated shear that changes direction with height. For rotating cases with tilted rotation vectors, the results depend on the direction of the shear. When the imposed flow is in the opposite direction, the convection motions are less energetic and are even suppressed entirely when the shear is strong. When the imposed flow is in the same direction, as that produced by the rotation, the convective motions are enhanced and a countergradient flux of momentum can be produced.

Hathaway, D. H.↗

NASA NTRS1985

Theoretical and experimental design studies for the Atmospheric General Circulation Experiment

The major criterion for the Atmospheric General Circulation Experiment (AGCE) design is that it be possible to realize strong baroclinic instability in the spherical configuration chosen. A configuration was selected in which a hemispherical shell of fluid is subjected to latitudinal temperature gradients on its spherical boundaries and the latitudinal boundaries are insulators. Work in the laboratory with a cylindrical version of this configuration revealed more instabilities than baroclinic instability. Since researchers fully expect these additional instabilities to appear in the spherical configuration also, they decided to continue the laboratory cylindrical annulus studies. Four flow regimes were identified: an axisymmetric Hadley circulation, boundary layer convection, baroclinic waves and deep thermal convection. Regime diagrams were prepared.

Fowlis, W. W.↗