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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
NASA NTRS2006

Solar Wind Helium Abundance as a Function of Speed and Heliographic Latitude: Variation through a Solar Cycle

We present a study of the variation of the relative abundance of helium to hydrogen in the solar wind as a function of solar wind speed and heliographic latitude over the previous solar cycle. The average values of A(sub He), the ratio of helium to hydrogen number densities, are calculated in 25 speed intervals over 27-day Carrington rotations using Faraday Cup observations from the Wind spacecraft between 1995 and 2005. The higher speed and time resolution of this study compared to an earlier work with the Wind observations has led to the discovery of three new aspects of A(sub He), modulation during solar minimum from mid-1995 to mid-1997. First, we find that for solar wind speeds between 350 and 415 km/s, A(sub He), varies with a clear six-month periodicity, with a minimum value at the heliographic equatorial plane and a typical gradient of 0.01 per degree in latitude. For the slow wind this is a 30% effect. We suggest that the latitudinal gradient may be due to an additional dependence of coronal proton flux on coronal field strength or the stability of coronal loops. Second, once the gradient is subtracted, we find that A(sub He), is a remarkably linear function of solar wind speed. Finally, we identify a vanishing speed, at which A(sub He), is zero, is 259 km/s and note that this speed corresponds to the minimum solar wind speed observed at one AU. The vanishing speed may be related to previous theoretical work in which enhancements of coronal helium lead to stagnation of the escaping proton flux. During solar maximum the A(sub He), dependences on speed and latitude disappear, and we interpret this as evidence of two source regions for slow solar wind in the ecliptic plane, one being the solar minimum streamer belt and the other likely being active regions.

Kasper, J. C.↗
NASA NTRS1980

Microwave, EUV, and X-ray observations of active region loops - Evidence for gyroresonance absorption in the corona

The combined data from 3.7 and 11 cm radio maps, EUV spectroheliograms, and X-ray photographs of an active region (McMath 12379) are analyzed. The near equality of the radio brightness temperatures and the electron temperature determined from the EUV and X-ray data implies that the radio emission is thermal. Since the free-free absorption is inadequate to account for this, the large optical depth must be produced by gyroresonance absorption. It is inferred that the magnetic scale height must be of the same order as the observed width of the coronal loops (one-billion cm). The scale of emission in the radio, and hence the scale of the absorbing region transverse to the line of sight, is also of this order.

Kundu, M. R.↗
NASA NTRS1994

Transient microwave brightenings in solar active regions: Comparison between VLA and Yohkoh observations

We report observations of transient microwave (2 cm) brightenings and their relationship with brightenings in soft X-rays. The peak flux of the microwave brightenings observed by the Very Large Array (VLA) is smaller than the previously reported fluxes by two orders of magnitude. The microwave sources were highly polarized (up to 100%) and were situated on the periphery of a sunspot umbra. Among the many transients observed in X-rays by Yohkoh, two were observed simultaneously in microwaves. The microwave sources were found to be closer to the umbra of the sunspot than were the X-ray loops. It seems that the microwave sources are located at the footpoints of the looplike X-ray transients. Using the combined VLA, Yohkoh, and Mees data set, we determine the physical parameters of the loop in which the brightenings occur. We find that an increase in emission measure accompanied by small-scale heating can account for the X-ray brightening. The microwave emission can be interpreted as thermal gyroresonance or nonthermal gyrosynchrotron processes during the X-ray brightening. The magnetic field in the microwave-source region is found to be 1200-1800 G. The observations also provide evidence for temperature gradient in the coronal loops.

Gopalswamy, N.↗
NASA NTRS1991

The response of the chromosphere during a stellar flare

A set of chromospheric models was developed, having a coronal loop geometry, energy balance through the entire loop from photosphere to corona, and a rigorous treatment of the radiative transfer in the important, optically thick, chromospheric emission lines. The models show that the soft X-ray emission and thermal conduction from a long lived hot corona are effective heating agents in the lower atmosphere during the gradual phase of stellar flares. The model predictions show the correct order of magnitude for the emission lines produced during the gradual phase of the flare with a reasonable coronal temperature evolution.

Hawley, Suzanne L.↗
NASA NTRS1986

Evidence of large-scale structures in the atmosphere of the active K-dwarf component of V471 Tauri

Contemporaneous IUE ultraviolet spectra and visible photoelectric data of the eclipsing binary V471 Tauri (K2V + DA) between 1979 and 1985 was analyzed. The combined data detail the three-dimensional structure of atmospheric loops and their associated starspots on the K dwarf. The distribution of starspot regions on the surface of the K star was inferred from the visible photometry. When spots are located near the limb of the K dwarf prior to and shortly after the total eclipse of the white dwarf, absorption lines such as C II, C III, c IV, and Si IV appear superimposed on the continuum of the white dwarf. These absorption lines are likely caused by cool coronal loops overlying the spots in the atmosphere of the K dwarf. The loops can extend nearly one stellar radius above the surface of the K2V star.

Guinan, E. F.↗
NASA NTRS1992

Transient periodicities in X-ray-active red dwarfs - First results from Mount Cuba and interpretation with an oscillating loop model

Results from a program at the Mount Cuba Astronomical Observatory to optically monitor transient periodicities in flare stars are reported. The data are analyzed for periodicities by means of a modified autocorrelation approach. A randomization technique is used to assess the statistical significance of periods. In AD Leonis, variability with amplitude 0.4 percent and a period of 4.1 min was detected during one run on March 6, 1991: the probability that this periodicity is due to chance is 10 exp -5. Further observations obtained on AD Leo within 1-2 hr on the same night showed no significant variability (0.01). It is shown that optical periodicities of the sort detected may be interpreted as arising from oscillations in coronal loops.

Mullan, D. J.↗
NASA NTRS2003

Parametric Transformation Analysis

Because twisted coronal features are important proxies for predicting solar eruptive events, and, yet not clearly understood, we present new results to resolve the complex, non-potential magnetic field configurations of active regions. This research uses free-form deformation mathematics to generate the associated coronal magnetic field. We use a parametric representation of the magnetic field lines such that the field lines can be manipulated to match the structure of EUV and SXR coronal loops. The objective is to derive sigmoidal magnetic field solutions which allows the beta greater than 1 regions to be included, aligned and non-aligned electric currents to be calculated, and the Lorentz force to be determined. The advantage of our technique is that the solution is independent of the unknown upper and side boundary conditions, allows non-vanishing magnetic forces, and provides a global magnetic field solution, which contains high- and low-beta regimes and is consistent with all the coronal images of the region. We show that the mathematical description is unique and physical.

Gary, G. Allan↗
NASA NTRS1998

Constructing the Coronal Magnetic Field: by Correlating Parameterized Magnetic Field Lines with Observed Coronal Plasma Structures

The reconstruction of the coronal magnetic field is carried out using a perturbation procedure. A set of magnetic field lines generated from magnetogram data is parameterized and then deformed by varying the parameterized values. The coronal fluxtubes associated with this field are adjusted until the correlation between the field lines and the observed coronal loops is maximized. A mathematical formulation is described which ensures (1) that the normal component of the photospheric field remains unchanged, (2) that the field is given in the entire corona, (3) that the field remains divergence free, and (4) that electrical currents are introduced into the field. It is demonstrated that a simple radial parameterization of a potential field, comprising a radial stretching of the field, can provide a match for a simple bipolar active region, AR 7999, which crossed the central meridian on 1996 Nov 26. At a coronal height of 30 km, the resulting magnetic field is a non-force free magnetic field with the maximum Lorentz force being on the order of 2.6 x 10(exp -9) dyn resulting from an electric current density of $0.13 mu A/ sq m. This scheme is an important tool in generating a magnetic field solution consistent with the coronal flux tube observations and the observed photospheric magnetic field.

Gary, G. A.↗
NASA NTRSDate not supplied

Heating of Coronal X-Ray Bright Points

The X-ray bright points (XBPs) are consistently present on the solar disk throughout time and space. Studying their contribution to overall coronal emissions and heating is intriguing. XBPs are prominently visible during the solar minimum, especially in the absence of bright active regions (ARs). In this discussion, we'll talk about the contribution of XBPs to coronal X-ray emissions during the cycle minimum. This analysis is based on spectroscopic observations by Solar X-ray Monitor (XSM) onboard Chandrayaan 2, alongside concurrent observations from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and the X-Ray Telescope (XRT) on the Hinode observatory. Similar to ARs, XBPs consist of coronal loops visible in EUV/X-ray images. We investigated the possibility of nanoflares sustaining the heating of these XBP loops by comparing observed emissions with field-aligned hydrodynamic simulations. Our results suggest that nanoflares could indeed maintain the heating of the XBPs. Furthermore, we conducted a detailed study of a single XBP observed during the successful flight of the Marshall Grazing Incidence Spectrometer (MaGIXS). The unique observations made by MaGIXS, combined with 1D hydrodynamic simulations, enabled us to determine the nanoflare heating frequency for this particular XBP.

coronal heating↗
NASA NTRS2017

Formation of Heliospheric Arcs of Slow Solar Wind

A major challenge in solar and heliospheric physics is understanding the origin and nature of the so-called slow solar wind. The Sun's atmosphere is divided into magnetically open regions, known as coronal holes, where the plasma streams out freely and fills the solar system, and closed regions, where the plasma is confined to coronal loops. The boundary between these regions extends outward as the heliospheric current sheet (HCS). Measurements of plasma composition strongly imply that much of the slow wind consists of plasma from the closed corona that escapes onto open field lines, presumably by field-line opening or by interchange reconnection. Both of these processes are expected to release closed-field plasma into the solar wind within and immediately adjacent to the HCS. Mysteriously, however, slow wind with closed-field plasma composition is often observed in situ far from the HCS. We use high-resolution, three-dimensional, magnetohydrodynamic simulations to calculate the dynamics of a coronal hole with a geometry that includes a narrow corridor flanked by closed field and is driven by supergranule-like flows at the coronal-hole boundary. These dynamics produce giant arcs of closed-field plasma that originate at the open-closed boundary in the corona, but extend far from the HCS and span tens of degrees in latitude and longitude at Earth. We conclude that such structures can account for the long-puzzling slow-wind observations.

Higginson, A. K.↗
NASA NTRS1986

Beam-induced pressure gradients in the early phase of proton-heated solar flares

The pressure gradient induced in a coronal loop by proton beam momentum deposition is calculated and compared with the thermal pressure gradient arising from nonuniform deposition of beam energy; it is assumed that the transfer of momentum and energy from beam to target occurs via the Coulomb interaciton. Results are presented for both a low mean energy and a high mean energy proton beam injected at the loop apex and characterized by a power-law energy spectrum. The present treatment takes account of the breakdown of the cold target approximation for the low-energy proton beam in the corona, where the thermal speed of target electrons exceeds the beam speed. It is found that proton beam momentum deposition plays a potentially significant role in flare dynamics only in the low mean energy case and only in the corona, where it may dominate the acceleration of target material for as long as several tens of seconds. This conclusion suggest that the presence of low-energy nonthermal protons may be inferred from velocity-sensitive coronal observations in the early impulsive phase.

Tamres, David H.↗
NASA NTRS1985

Alfvenic pulses in the solar atmosphere

Some nonlinear aspects of Alfvenic pulses propagating in coronal loops and the underlying chromosphere are numerically investigated. Heat conduction and radiation are included. The Alfvenic pulses are modeled as axisymmetric twists on a vertical cylindrical flux tube. They nonlinearly couple into acoustic-gravity waves propagating along the flux tube. A single Alfvenic pulse is found to leave two acoustic-gravity pulses in its wake. These pulses can result in significant motions of the transition region and underlying chromosphere. These motions do not resemble spicules, but they may correspond to a variety of observations indicating that the solar atmosphere is in a continual dynamic state. It is suggested that a dynamic chromosphere and transition region may be the inevitable consequence of the coronal heating process itself.

Mariska, J. T.↗
NASA NTRSDate not supplied

Heating of Coronal X-Ray Bright Points

The X-ray bright points (XBPs) are consistently present on the solar disk throughout time and space. Studying their contribution to overall coronal emissions and heating is intriguing. XBPs are prominently visible during the solar minimum, especially in the absence of bright active regions (ARs). In this discussion, we'll talk about the contribution of XBPs to coronal X-ray emissions during the cycle minimum. This analysis is based on spectroscopic observations by Solar X-ray Monitor (XSM) onboard Chandrayaan 2, alongside concurrent observations from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory (SDO) and the X-Ray Telescope (XRT) on the Hinode observatory. Similar to ARs, XBPs consist of coronal loops visible in EUV/X-ray images. We investigated the possibility of nanoflares sustaining the heating of these XBP loops by comparing observed emissions with field-aligned hydrodynamic simulations. Our results suggest that nanoflares could indeed maintain the heating of the XBPs. Furthermore, we conducted a detailed study of a single XBP observed during the successful flight of the Marshall Grazing Incidence Spectrometer (MaGIXS). The unique observations made by MaGIXS, combined with 1D hydrodynamic simulations, enabled us to determine the nanoflare heating frequency for this particular XBP.

coronal heating↗
NASA NTRSDate not supplied

3-D Mapping of the Magnetic Field in the Active Region by the CLASP2.1 Rocket Experiment

Comprehensive magnetic field measurements of the solar atmosphere are crucial for understanding energy transport from the photosphere to the corona and its dissipation. However, observations of the magnetic field in the chromosphere and the upper atmospheric layers above the chromosphere, where the gas pressure dominance changes from (β>1) to magnetic pressure dominance (β<1), are overwhelmingly lacking. Therefore, we have focused on the polarization of ultraviolet radiation emitted from the upper chromosphere and transition layer and have conducted the Japan-U.S.-EU observation rocket experiment CLASP in order to demonstrate its usefulness. In CLASP2.1 conducted on October 8, 2021, scan observations were made at 16 locations in the active region, and Stokes (intensity $I$, linearly polarized $Q$, $U$, circularly polarized $V$) spectra in the 280~nm wavelength range were obtained. Looking at the observed region with AIA 171~{¥AA} on board the SDO satellite, we see that it consists of a region where a structure corresponding to the foot of a high-temperature loop called moss (moss) is seen, and a region where a low-temperature loop spreading from a sunspot is seen. We focused on the ionized magnesium $h$ & $k$ lines (emitted from the middle and uppermost of the chromosphere) and the manganese lines (emitted from the low part of the chromosphere), which show particularly prominent circular polarization, and derived the line-of-sight magnetic fields in the low, middle and uppermost parts of the chromosphere by applying weak field approximation to them. Furthermore, by combining the results with observations by the Solar Optical Telescope onboard the solar observing satellite HINODE, we obtained three-dimensional information on the magnetic field in the active region from the photosphere to the uppermost part of the chromosphere. In general, the magnetic field in the active region becomes weaker and smoother as one goes up in the sky, as reported by the CLASP2 observation (Ishikawa et al. 2021). However, in some regions, polarity reversal was observed only in the uppermost part of the chromosphere, and comparison with high spatial resolution transition layer and coronal images recorded by SDO/AIA revealed the connection between the magnetic field structure in the chromosphere and coronal loops.

R. Ishikawa↗
NASA NTRS1975

The large coronal transient of 10 June 1973. I - Observational description

Ground-based and Skylab coronagraph observations are used to trace a coronal loop transient from its inception near the surface to a distance of 5 solar radii. Events in the lower solar atmosphere associated with this feature are outlined chronologically, and its physical appearance is described. The densities, mass, and energy of the transient are estimated, its shape is determined, and its typical and atypical characteristics are noted. It is inferred that this transient was caused by the emergence of an active region. It is shown that it was not immediately preceded by a flare and that the material at its leading edge did not decelerate appreciably, even at approximately 5 solar radii.

Hildner, E.↗
NASA NTRS1976

The quantitative properties of three soft X-ray flare kernels observed with the AS&E X-ray telescope on Skylab

The physical parameters for the kernels of three solar X-ray flare events have been deduced using photographic data from the S-054 X-ray telescope on Skylab as the primary data source and 1-8 and 8-20 A fluxes from Solrad 9 as the secondary data source. The kernels had diameters of about 5-7 seconds of arc and in two cases electron densities at least as high as 0.3 trillion per cu cm. The lifetimes of the kernels were 5-10 min. The presence of thermal conduction during the decay phases is used to argue: (1) that kernels are entire, not small portions of, coronal loop structures, and (2) that flare heating must continue during the decay phase. We suggest a simple geometric model to explain the role of kernels in flares in which kernels are identified with emerging flux regions.

Kahler, S. W.↗
NASA NTRS1979

A query into the source of proton emission from solar flares, report 2

Seven solar flares that were followed by major proton events were examined to determine the diverse and common properties of major flares. The most probable site of primary proton acceleration is cospatial with the site and instant of formation of coronal loops. Because loop formation occurs through the entire duration of major solar flares over significantly large areas of active centers, it is proposed that proton injection occurs from a relatively large volume of space in the corona of active centers and is continuous throughout, and possibly even after, the visible duration of the related chromospheric flare. The flare veil is hypothesized to occur as a result of proton charge exchange taking place in the white-light transient. The Kopp and Pneuman model of loop formation by magnetic reconnection is suggested as an adequate and satisfactory model for all major flares with the provision that the beginning of rapid magnetic field reconnection is coincident with flare start.

Martin, S. F.↗
NASA NTRS1979

The energetics of resistive magnetic tearing

A new treatment of the energetics of resistive magnetic tearing is presented. This instability is the only known field-reconnection mechanism which develops in time, and is thus a primary candidate for a number of astrophysical 'flare' processes occurring in stressed magnetic fields. The tearing process is elucidated by a subdivision into magnetic regions of different characteristics: first, as dominated by resistive or highly conductive dynamics, and then by function as a source or sink of the energy flux. Matching conditions between these zones provide a determination of the growth rate and its physical-parameter dependence. The observable plasma energy outputs, due mainly to two distinct magnetically driven acceleration mechanisms, are estimated and their directive spatial properties shown. Finally, the generality of resistive tearing is demonstrated by an application of the previous considerations to realistic sheared-field configurations, including the case of a solar coronal loop, for which flare-instability thresholds and energy-deposition sites are described.

Van Hoven, G.↗