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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Forbush decreases and particle acceleration in the outer heliosphere

Consideration is given to Pioneer 10 and 11 observations of the solar flares that occurred during the period March 6-19, 1989. The observations shown that Forbush decreases propagate with an essentially constant magnitude to 47 AU and with similar magnitude at widely different ecliptic longitudes. The times of recovery from Forbush decreases become progressively greater as the radial distance increases. A scheme is proposed to explain this behavior, giving support to the hypothesis that the solar cycle modulation of the galactic cosmic ray intensity is attributable primarily to overlapping Forbush decreases that are more frequenct and of greater magnitude near times of maximum solar activity.

Van Allen, J. A.↗
NASA NTRS1991

An observation of a heliospheric magnetic cycle dependence for the integral radial gradient of E above 60 MeV cosmic rays

The dependence of the integral radial gradient of the cosmic radiation upon the cosmic ray intensity level and heliocentric radial distance is investigated. The magnitude of the integral radial gradient (IRG) from 1982-1990, during a period of negative polarity, is a strong function of the cosmic ray intensity changing from about 1.5 percent/AU at the time of the peak intensity in 1987 to about 3 percent/AU at times of minimum intensity in 1982 and 1990. In the early period studied from 1977 to 1982, during a period of positive polarity, practically no dependence of the magnitude of the IRG on the cosmic ray intensity was found. The gradient remained at about 3 percent/AU during the entire period. It is argued that these specific changes in the behavior of the gradient in opposite polarity cycles are not predicted by either the drift-wavy current sheet or simple diffusion-convection models of the solar modulation. The results are discussed in the context of the present understanding of the causes of the asymmetry in the 11-year modulation cycles.

Webber, W. R.↗
NASA NTRS1991

Flow downstream of the heliospheric terminal shock - Magnetic field kinematics

A kinematic model of the interplanetary magnetic field in the heliosheath beyond the solar wind terminal shock is presented in order to evaluate the possible importance of MHD effects in that region of space. The need for this evaluation arises because the interplanetary magnetic field is compressed across the terminal shock and further amplified by the decreasing flow speed beyond the shock. Streamlines which approach the stagnation point before turning in the downstream direction lead to the strongest effects due to the extreme slowing of the solar wind and consequent compression of the embedded magnetic field. The magnetic volume force therefore cannot be neglected on streamlines that approach the heliopause in the upstream direction, where the volume containing them is a large fraction of the overall of the heliosheath in the upstream direction. The increase in the magnetic pressure may act to bring the upstream terminal shock significantly closer to the sun, potentially reconciling a conflict between models and observations.

Nerney, S.↗
NASA NTRS1992

Outer heliospheric radio emissions. I - Constraints on emission processes and the source region

Observations by Voyager 1 and 2 of LF radio emissions near 2 and 3 kHz during the interval 1983-1987 while at heliocentric distances from 15 to 27 AU and 11 to 20 AU, respectively, are reported. Constraints on the emission processes and source characteristics are discussed. Minimum Langmuir wave electric fields in the source region, based solely on the kinematics of the radiation processes, lie in the range 1-100 micro-V/m for nominal source and electron beam parameters. Path lengths for fundamental and harmonic emission processes are calculated. The observed levels of radiation can be produced in path lengths and source dimensions smaller than 1 AU provided the participating Langmuir waves have effective temperatures of about 10 exp 17 K. Previous source models for the radiation are discussed.

Cairns, Iver H.↗
NASA NTRS1987

Cosmic-ray variations and magnetic field fluctuations in the outer heliosphere

We have formally confirmed that galactic cosmic ray intensity variations measured by Voyager 2 during recovery from solar maximum are caused by travelling compressions and rarefactions in the mean interplanetary magnetic field. We used Voyager's nearly continuous magnetic field data as input to a time-independent, spherically-symmetric, cosmic ray transport equation in the force-field approximation. The solutions closely followed the count rate of cosmic rays greater than 75 MeV/nucleon over four years. This strongly supports prior theoretical assertions that turbulent interaction regions travelling with the solar wind are the major cause of the solar-cycle variation of galactic cosmic rays in the ecliptic region.

Perko, J. S.↗
NASA NTRS1993

Cosmic ray modulation and the distant heliospheric magnetic field - Voyager 1 and 2 observations from 1986 to 1989

The relation between the cosmic ray (CR) intensity and the magnetic field strength observed by Voyager 1 and 2 during the period from 1986 through most of 1989 beyond 19 AU is examined. It is shown that these observations generally confirm the relation between changes in the CR intensity and the magnetic field intensity observed beyond 10 AU from 1981 to 1985. The CR intensity decreases when merged interaction regions (MIRs) are strong and dominating, and it fluctuates about a plateau when the effects of corotating MIRs are balanced by rarefaction regions. The CR intensity increases when the MIRs are weak and the magnetic field strength is relatively low.

Burlaga, L. F.↗
NASA NTRS1992

Measurements of an intensified CCD detector for the Solar and Heliospheric Observatory

An engineering model intensified CCD detector for the SOHO Coronal Diagnostics Spectrometer has been built and tested at the NASA Goddard Space Flight Center. A windowless MCP intensifier tube converts EUV radiation (30-65 nm) into visible light, which is focused via a lens system onto a Tektronix 1024 x 1024 CCD. Tests have been made of this engineering model to determine the following characteristics: quantum efficiency, resolution, throughput, linearity, statistical variation, readout noise, scattering, and flat-field response. In almost all respects, the detector performed as expected. This detector has been delivered, and work is underway on the flight detector.

Thompson, William T.↗
NASA NTRS1992

The structure of the inner heliosphere from Pioneer Venus and IMP observations

The IMP 8 and Pioneer Venus Orbiter (PVO) spacecraft explore the region of heliographic latitudes between 8 deg N and 8 deg S. Solar wind observations from these spacecraft are used to construct synoptic maps of solar wind parameters in this region. These maps provide an explicit picture of the structure of high speed streams near 1 AU and how that structure varies with time. From 1982 until early 1985, solar wind parameters varied little with latitude. During the last solar minimum, the solar wind developed strong latitudinal structure; high speed streams were excluded from the vicinity of the solar equator. Synoptic maps of solar wind speed are compared with maps of the coronal source surface magnetic field. This comparison reveals the expected correlation between solar wind speed near 1 AU, the strength of the coronal magnetic field, and distance from the coronal neutral line.

Gazis, P. R.↗
NASA NTRS1993

A new way to measure the composition of the interstellar gas surrounding the heliosphere

The composition of neutral gas in the Local Interstellar Medium can be studied by direct, in situ measuring of interstellar neutral atoms penetrating into interplanetary space. A novel experimental approach for in situ atom detection, which has never been used earlier in space, is proposed. The technique is based on the conversion of neutral atoms to negative ions at a specially prepared sensitive surface. Negative ions are subsequently analyzed and detected in an essentially noise-free, multicoincidence mode. It is shown that interstellar hydrogen, deuterium, and oxygen atoms can be measured by the proposed technique. The experiment can be performed from a high-apogee Earth-orbiting satellite or from a deep space probe.

Gruntman, Michael A.↗
NASA NTRS1994

Solar and Heliospheric Observatory (SOHO) Experimenters' Operations Facility (EOF)

This paper describes the SOHO Instrumenters' Operations Facility (EOF) project. The EOF is the element of the SOHO ground system at the Goddard Space Flight Center that provides the interface between the SOHO scientists and the other ground system elements. This paper first describes the development context of the SOHO EOF. It provides an overview of the SOHO mission within the International Solar-Terrestrial Physics (ISTP) project, and discusses the SOHO scientific objectives. The second part of this paper presents the implementation of the SOHO EOF, its innovative features, its possible applications to other missions, and its potential for use as part of a fully integrated ground control system.

Larduinat, Eliane↗
NASA NTRS1994

Potential flow downstream of the heliospheric terminal shock: A non-spherical shock

We have solved for the potential flow downstream of the terminal shock of the solar wind in the limit of small departures from a spherical shock due to a latitudinal ram pressure variation in the supersonic solar wind. The solution connects anisotropic streamlines at the shock to uniform streamlines down the heliotail because we use a non-slip boundary condition on the heliopause at large radii. The rotational velocity about the heliotail in the near-field solution decays as the fourth power of distance from the shock. The polar divergence of the streamlines will have consequences for the previously discussed magnetic pressure ridge that may build-up just inside the heliopause.

Nerney, Steven↗
NASA NTRS1993

Jovian electron propagation in three dimensions of the heliosphere: The Ulysses investigations

We report investigations of Jovian relativistic electrons in the interplanetary medium that provide new insights into both the physical processes by which the Jovian magnetosphere releases its trapped, relativistic electrons into the interplanetary medium, and the modes of their interplanetary propagation. These studies were dependent on the unique postencounter trajectory for Ulysses. The spacecraft remained close to the radial distance of Jupiter (approximately 5.2 AU) and moved southward on the duskside by only approximately 12 deg in heliographic latitude and less than 8 deg in the heliographic azimuth relative to Jupiter for the period of approximately 100 deg days of this study. During this period the nominal Parker spiral interplanetary magnetic field with its alternating polarities sector structure established direct magnetic field line connections frequently between Jupiter and the spacecraft. These unique conditions made it possible to investigate in detail, for approximately four solar rotations, both the Jovian electron burst phenomenon and the continuous, diffusive interplanetary propagation of Jovian electrons.

Simpson, J. A.↗
NASA NTRS1995

On the minimum fluxes of low-energy ions (0.5 MeV/NUC - 8 MeV/NUC) at high heliospheric latitudes

The quiet-time fluxes of ions from 0.5 MeV/nuc to 7 MeV/nuc (for oxygen) have been studied for the time period between mid-1993 through the first 104 days of 1994. During this time period, Ulysses was below the heliographic current sheet. We find that at the quietest times, the ion population is dominated by the anomalous cosmic ray (ACR) component. For more active times, but still avoiding the most active time periods, the population below 3 MeV/nuc contains a CIR component as well as ACR ions.

Blake, J. B.↗
NASA NTRS1995

Forward and reverse shocks in the outer heliosphere: Observations from Voyager 2

Observations from Voyager 2 as it moved from 10 to 14 deg S heliographic latitude in the period from 1992 through 1994 were used to gather statistics on the relative number of forward and reverse shocks. These results can be used to compare with observations from the Ulysses spacecraft which moved from 6 deg S to 70 deg S heliographic latitude during that time period. The Ulysses observations are in agreement with a 3-D, MHD model of the evolution of a steady tilted-dipole solar wind flow configuration prevalent in 1993. The model predicts and the Ulysses observations confirm a preponderance of reverse shocks at Ulysses latitudes poleward of streamer-belt latitudes. A preliminary scan of the Voyager data supports the complementary prediction of the model that forward fronts should dominate at large heliocentric distances near the heliographic equatorial plane during the same time period.

Lazarus, A. J.↗
NASA NTRS1995

The radial component of the heliospheric magnetic field: Ulysses observations

The radial field component, B(sub R), has been monitored continuously since the Ulysses spacecraft left the ecliptic plane in February 1992 travelling toward the southern pole of the Sun. In order to separate spatial from temporal changes, the Ulysses measurements from 0 to 80 heliographic latitude were compared with in-ecliptic measurements of B(sub R) being made simultaneously by IMP-8. The data revealed essentially the same field strengths and time variations at both locations. The conclusion was drawn that there was no significant latitude gradient in B(sub R) and that the stronger polar cap coronal magnetic fields were being transported equatorward to yield a uniform field in the solar wind. The results contrasted with the predictions of the various source surface models which ignore magnetic stresses within 2.5 solar radii. Since the maximum south latitude was attained in September, 1994, Ulysses has traveled northward toward an ecliptic crossing in March 1995 and onward into the north solar hemisphere. The recent results will be presented and compared with those obtained in the southern hemisphere.

Smith, E. J.↗
NASA NTRS1995

Solar wind research with the Large Angle Spectroscopic Coronagraph (LASCO) experiment onboard the Solar and Heliospheric Observatory (SOHO) satellite

The ESA-NASA satellite, to be launched in October 1995, carries three nested coronagraphs, which will image the solar corona from 1.1 R(solar mass) to 30 R(solar mass). Super polished mirrors have been developed for the design of a mirror Lyot coronagraph which has a straylight level comparable with the coronal intensity from 1.1 R, to 30 R(solar mass) Coronal details can be imaged with a spatial resolution of 6 arc seconds. A Fabry Perot interferometer with a spectral resolution of 0.7 A at the wavelength of the green coronal emission line will allow the simultaneous construction of spectra over the entire field of view of 10(exp 6) pixels. The middle coronagraph (1.5 R(solar mass) - 6 R(solar mass)) and the outer coronagraph (3 R(solar mass) - 30 R(solar mass)) are externally occulted lens Lyot coronagraphs. Their straylight level 10(exp -11) B(solar mass) and 10(exp -12) B(solar mass) respectively is an order of magnitude smaller than the intensity of the corona. The sensitivity of LASCO to distinguish between different solar wind acceleration mechanisms will be discussed as well as its ability to discern different CME models.

Brueckner, G. E.↗
NASA NTRS1995

Large-scale variations of solar wind elemental composition and charge states with heliospheric latitude

The Solar Wind Ion Composition Spectrometer (SWICS) onboard Ulysses allows determination of the elemental composition of the solar wind and the charge states of all major solar wind ion species. Ulysses left the ecliptic plane in early 1992, crossed the Sun's south polar region in late 1994 and made a fast approach back towards the ecliptic in the first half of 1995. Data from this period were investigated for long-term variations in the solar wind composition. At midlatitudes Ulysses encountered periodically the fast solar wind stream emerging from the south coronal hole. As a consequence, dramatic variations in the charge-states arise, between high charge-states dominating in the current sheet solar wind and low charge states in the coronal hole stream. However, the initial analysis indicates that from midlatitudes onwards, with Ulysses permanently immersed in the coronal hole stream, the charge state and elemental abundance ratios of the major solar wind ion species stayed essentially constant. This implies that the temperature profile in the coronal hole at solar wind source altitudes exhibit no variation with solar latitude. It confirms that the south coronal hole is essentially unstructured down to scale lengths of several degrees in solar latitude.

Woch, J.↗
NASA NTRS1995

Heliomagnetic latitude dependence of the heliospheric magnetic field

Previous studies have revealed systematic variations of the interplanetary magnetic field with heliographic latitude. Luhmann et al. (1987) modeled Pioneer Venus (PVO) and ISEE-3 observations by assuming an asymmetric dependence on heliolatitude with stronger fields in the northern hemisphere. In a subsequent study, using data from ISEE-3/ICE and IMP-8, Burton et al. (1990) found evidence for a similar asymmetry. However, neither model has been completely successful. The model derived from PVO/ICE observations agrees quite well near solar maximum but shows significant discrepancies during the descending phase of the solar cycle. The model derived from the ICE/IMP-8 comparison suffers from significant phase delays between the difference in field magnitude at the two spacecraft and their latitude difference. In an attempt to account for these phase shifts, the IMP-8 and ICE data have been reexamined in heliomagnetic coordinates which are defined by the orientation of the solar magnetic dipole. The latitude and longitude of the dipole inferred from the data have then been compared with those implicit in source surface calculations. The IMP/ICE correlations have been extended into the recent solar maximum and descending phase. Comparisons have also been carried out between IMP-8 and Ulysses as it traveled to -30 deg south heliographic latitude.

Burton, M. E.↗