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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Interplanetary radio storms. I - Extension of solar active regions through the interplanetary medium

About 100 storms of type-III solar radio bursts have been identified in the ISEE-3 radio-experiment data during the 4-yr period around the maximum of the 21st solar cycle. They demonstrate the very frequent presence of streams of suprathermal electrons. Their durations range from 1 to 10 d or more. They are observed up to 100-170 solar radii. Their rate of occurrence is 2 to 3 per solar rotation near solar maximum. It is shown that the time variations of the daily radio-emission intensities correlate with the sunspot-number variations and with the solar activity in general. More specifically, a very good correlation is found with the meter-wave type-III and type-I storms, which demonstrates that the suprathermal electrons responsible for the radio emission have been accelerated below 2 solar radii heliocentric. The different lags observed between the sunspot-number variations, the S-component, and the type-I and type-III storms are discussed.

Bougeret, J.-L.↗
NASA NTRS1980

6 cm observations of a solar active region and bursts with approximately 6 arcsec resolution

The paper examines the solar active region located near N04E49, that was mapped at a 6 cm wavelength over a period of 12 hours, employing the Westerbork synthesis radio telescope. Two impulsive radio bursts were recorded during this time, one of which was associated with a chromospheric flare. One-dimensional fan beam scans of the burst sources were synthesized in order to display their time history with regard to intensity, polarization, position and size.

Erskine, F. T.↗
NASA NTRS1988

Magnetohydrodynamic (MHD) modelling of solar active phenomena via numerical methods

Numerical ideal MHD models for the study of solar active phenomena are summarized. Particular attention is given to the following physical phenomena: (1) local heating of a coronal loop in an isothermal and stratified atmosphere, and (2) the coronal dynamic responses due to magnetic field movement. The results suggest that local heating of a magnetic loop will lead to the enhancement of the density of the neighboring loops through MHD wave compression. It is noted that field lines can be pinched off and may form a self-contained magnetized plasma blob that may move outward into interplanetary space.

Wu, S. T.↗
NASA NTRS1995

Magnetic Observations During the Recent Declining phase of Solar Activity

Changes in the heliospheric magnetic field during the recent declining phase in solar activity are reviewed and compared with observations during past sunspot cycles. The study is based principally on data obtained by IMP-8 and Ulysses. The field magnitude is found to have increased during the declining phase until it reached a maximum value of 11.5 nT in ÷ 19915, approximately two years after sunspot maximum.

Magnetic↗
NASA NTRS1975

Solar activity and the weather

Attempts during the past century to establish a connection between solar activity and the weather are discussed. Some critical remarks about the quality of much of the literature in this field are given, and several recent investigations are summarized. Use of the solar-interplanetary magnetic sector structure in future investigations may add an element of cohesiveness and interaction to these investigations.

Wilcox, J. M.↗
NASA NTRS2005

Decay of Solar Active Regions

We examine the record of sunspot group areas observed over a period of 100 years to determine the rate of decay of solar active regions. We exclude observations of groups when they are more than 60deg in longitude from the central meridian and only include data when at least three days of observations are available following the date of maximum area for a spot group's disk passage. This leaves data for some 24,000 observations of active region decay. We find that the decay rate is a constant 20 microHem/day for spots smaller than about 200 microHem (about the size of a supergranule). This decay rate increases linearly to about 90 microHem/day for spots with areas of 1000 microHem. We find no evidence for significant variations in active region decay from one solar cycle to another. However, we do find that the decay rate is slower at lower latitudes. This gives a slower decay rate during the declining phase of sunspot cycles.

Hathaway, David H.↗
NASA NTRS2016

Patterns of Activity in A Global Model of A Solar Active Region

In this work we investigate the global activity patterns predicted from a model active region heated by distributions of nanoflares that have a range of frequencies. What differs is the average frequency of the distributions. The activity patterns are manifested in time lag maps of narrow-band instrument channel pairs. We combine hydrodynamic and forward modeling codes with a magnetic field extrapolation to create a model active region and apply the time lag method to synthetic observations. Our aim is not to reproduce a particular set of observations in detail, but to recover some typical properties and patterns observed in active regions. Our key findings are the following. (1) Cooling dominates the time lag signature and the time lags between the channel pairs are generally consistent with observed values. (2) Shorter coronal loops in the core cool more quickly than longer loops at the periphery. (3) All channel pairs show zero time lag when the line of sight passes through coronal loop footpoints. (4) There is strong evidence that plasma must be re-energized on a timescale comparable to the cooling timescale to reproduce the observed coronal activity, but it is likely that a relatively broad spectrum of heating frequencies are operating across active regions. (5) Due to their highly dynamic nature, we find nanoflare trains produce zero time lags along entire flux tubes in our model active region that are seen between the same channel pairs in observed active regions.

Bradshaw, S. J.↗
NASA NTRS1977

6 centimeter observations of solar active regions with 6 sec resolution

We have used the Westerbork Synthesis Radio Telescope for observations of solar active regions at 6 cm during 1974 May 8-10. The particular problems of using a sidereal instrument for solar observations are discussed. Maps of total intensity and circular polarization were obtained for four active regions with sunspots and a plage region; the linear polarization was below the noise limit. The brightest components of the 6 cm emission are associated with sunspots, while weaker components are associated with the plage magnetic fields, both longitudinal and transverse. An inversion of the sense of circular polarization was observed in the preceding part of a region with bipolar magnetic-field structure, about two days after the region passed through the central meridian. Nine bursts were also observed, and their one-dimensional positions with respect to the associated active regions are given.

Kundu, M. R.↗
NASA NTRS1993

p-mode frequency variation in relation to global solar activity

We show that p-mode frequency variations correlate remarkably well with the variations of six solar activity indices over a 6 yr period from 1984 October to 1990 November, including both the large variation from solar minimum to solar maximum and smaller variations observed over approximately 1 month intervals during solar maximum. The quality of correlation as seen visually and as measured by two statistical tests differs significantly among the six activity indices, and we briefly speculate on possible reasons for this. Observations used in this study come from the HAO/NSO Fourier tachometer (FTACH) and include the spherical harmonic degree range l greater than 20 and equal to 60 or less and frequency range between 2600 and 3200 micro-Hz. The data are divided into 18 separate epochs with time string duration ranging from a minimum of 18 days to a maximum of 45 days. We have particularly good coverage during the early part of solar maximum of cycle 22.

Bachmann, Kurt T.↗
NASA NTRS1974

Solar activity and the weather

The attempts during the past century to establish a connection between solar activity and the weather are discussed. Some critical remarks about the quality of much of the literature in this field are given. Several recent investigations are summarized. Use of the solar interplanetary magnetic sector structure in future investigations is suggested to perhaps add an element of cohesiveness and interaction to these investigations.

Wilcox, J. M.↗
NASA NTRS1972

Polarization of solar active regions at 9.5 mm wavelength.

A study of the circular polarization structure of solar active regions has been made from data obtained at 9.5 mm. All important active regions observed at 9.5 mm are bipolar in nature, the degree of polarization is about the same for both right and left circular components, and it ranges up to about 4%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates clearly the neutral line between the regions of opposite polarity on magnetograms. Unipolar regions in magnetograms also show up as unipolar regions at 9.5 mm. Magnetic fields as low as 5-10 G on magnetograms manifest as distinctly polarized regions on 9.5 mm maps.

Kundu, M. R.↗
NASA NTRS1973

Polarization of solar active regions at 3.5 millimeter wavelength

A study of the circular polarization structure of solar active regions has been made from data obtained at 3.5 mm wavelength, using the 36-ft-diam radio telescope of the National Radio Astronomy Observatory at Kitt Peak, Arizona. The angular resolution of the telescope at this wavelength is 1.2 min. All important active regions observed at 3.5 mm are bipolar in nature; the degree of polarization ranges from 1 to about 2%. These oppositely polarized components correspond with the Mt. Wilson magnetic regions of opposite polarity; the line of zero polarization delineates the neutral line between the regions of opposite polarity on magnetograms. The longitudinal magnetic fields at the level of 3.5-mm emission computed from the degree of polarization are found to be several hundred gauss.

Kundu, M. R.↗
NASA NTRS2001

Seismic Forecasting of Solar Activity

We have developed and improved helioseismic imaging techniques of the far-side of the Sun as part of a synoptic monitor of solar activity. In collaboration with the MIDI team at Stanford University we are routinely applying our analysis to images within 24 hours of their acquisition by SOHO. For the first time, real-time seismic maps of large active regions on the Sun's far surface are publicly available. The synoptic images show examples of active regions persisting for one or more solar rotations, as well as those initially detected forming on the solar far side. Until recently, imaging the far surface of the Sun has been essentially blind to active regions more than about 50 degrees from the antipode of disk center. In a paper recently accepted for publication, we have demonstrated how acoustic travel-time perturbations may be mapped over the entire hemisphere of the Sun facing away from the Earth, including the polar regions. In addition to offering significant improvements to ongoing space weather forecasting efforts, the procedure offers the possibility of local seismic monitoring of both the temporal and spatial variations in the acoustic properties of the Sun over the entire far surface.

Braun, Douglas↗