SEARCH · Engineering Papers
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Theories and reported results from investigations of cosmic ray modulation and acceleration are summarized. Aspects considered include microscopic or fundamental theory, gradient and curvature drifts in modulation, and interplanetary acceleration of shocks and particles.
Recommendations are made for programs to meet high-priority objectives for solar and heliospheric physics. These priority areas include: the physics of the ISM and the heliospheric boundary; solar particle acceleration, transport, and composition; the ultraheavy composition of solar energetic particles; the large-scale structure of the heliosphere and solar modulation; the microphysics of interplanetary acceleration and shock acceleration. Measures that should be taken to continue and enhance ongoing programs are summarized.
Solar cycle 23 witnessed an unprecedented array of space- and ground-based instruments observing the violent eruptions from the Sun that had huge impact on the heliosphere. It was possible to characterize corona) mass ejections (CMEs) that cause extreme solar energetic particle events and geomagnetic storms, the two aspects that concern the space weather community. In this paper I discuss the special populations of CMEs that have significant interplanetary consequences: shock-driving CMEs identified based on their association with type 11 radio bursts and in-situ shocks, SEP-producing CMEs, and geoeffective CMEs (those that produce geomagnetic storms). I discuss the kinematic and solar-source properties of these populations and how they vary with the solar activity cycle. I also compare their properties with the general population of CMEs, so one can recognize when and where these events occur on the Sun.
The earth's collisionless plasma bow shock has, overall, a nonuniform structure whose magnetic profile is simultaneously that of a monotonic or laminar perpendicular shock and of a multigradient oblique shock, depending on the local orientation of the interplanetary field to the nominal shock surface. A 'pulsation index' Ip has been devised from empirical results to provide a simple convenient means of assessing the probable local character of the shock's structure; Ip = 0 or 1, according to whether local field geometry favors perpendicular or oblique structure, respectively, at a chosen point of observation on the nominal shock surface.
Recently, several events described as hot expanding diamagnetic cavities have been observed upstream from the earth's bow shock using the ISEE 1 and 2 spacecraft. It has been suggested that fast shocks may form at the edges of some of these events because of the rapid expansion of the cavities. Here, plasma density, temperature, velocity, and total field changes across the edges of several events were examined, and these changes were found to be consistent with the presence of shocks there. The presence of flat-topped electron distributions and occasional electron beams at and down-stream from the edges provides additional evidence for shocks. Plasma wave observations also show shocklike electrostatic noise at the edges of several events. It is concluded that the edges of diamagnetic cavity events are often shocks, with a range of shock strengths similar to that observed in the interplanetary medium. The range of shock strengths may be the result of different convection and/or expansion speeds of the cavities.
Solar-wind observations associated with the enhanced levels of solar activity in August 1972 are reviewed with emphasis on recent analyses which more unambiguously characterize the changes in the interplanetary medium. Observations from Pioneer 9 at about 0.8 AU, Pioneer 10 at 2.2 AU, and in the vicinity of earth are reviewed and reinterpreted in the light of new data. Recent calculations of local shock velocities are reviewed, and their implications are discussed. These results indicate that, contrary to previously published observations, there is no significant deceleration of the interplanetary shocks between 0.8 AU and 2.2 AU.
Observations of plasma and magnetic field variations in the near-Earth solar wind are discussed. Both a corotating stream and a driven shock are present. The driver gas seems to be enveloped in the rising speed phase of this stream; this appearance is attributed to a convoluted surface separating the two plasma domains. The magnetic field in the post shock flow (0030-1230 UT of July 29) has a large and geoeffective southward component at times; the energy coupling coefficient reaches approximately 5.4 x 10 to the 19th power ergs/s. In the driver gas (1230 UT of July 29 to 0110 of July 30) the magnetic field is dominantly northward. The density and dynamic pressure decrease by almost two orders of magnitude (100 to 2 cm/3) from just behind the interplanetary shock to approximately 3 hours into the driver gas flow. The dominant magnetic field variation in the driver gas is modeled by a cloud-like structure. Significant plasma parameter variations within the driver gas are attributed to structure in the parent solar mass ejection event and to interplanetary kinematics.
Definite evidence for particle acceleration in the solar wind came around a decade ago. Two likely sources are known to exist: particles may be accelerated by the turbulence resulting from the superposition of Alfven and Magnetosonic waves (Statistical Acceleration) or they may be accelerated directly at shock fronts formed by the interaction of fast and slow solar wind (CIR's) or by traveling shocks due to sporadic coronal mass ejections. Naurally both mechanisms may be operative. In this work the acceleration problem was tackled numerically using Helios 1 and 2 data to create a realistic representation of the Heliospheric plasma. Two 24 hour samples were used: one where there are only wave like fluctuations of the field (Day 90 Helios 1) and another with a shock present in it (Day 92 of Helios 2) both in 1976 during the STIP 2 interval. Transport coefficients in energy space have been calculated for particles injected in each sample and the effect of the shock studied in detail.
The complex interplanetary structures during 7 to 8 Nov 2004 are analyzed to identify their properties as well as resultant geomagnetic effects and the solar origins. Three fast forward shocks, three directional discontinuities and two reverse waves were detected and analyzed in detail. The three fast forward shocks 'pump' up the interplanetary magnetic field from a value of approx.4 nT to ~44 nT. However, the fields after the shocks were northward, and magnetic storms did not result. The three ram pressure increases were associated with major sudden impulses (SI + s) at Earth. A magnetic cloud followed the third forward shock and the southward Bz associated with the latter was responsible for the superstorm. Two reverse waves were detected, one at the edge and one near the center of the magnetic cloud (MC). It is suspected that these 'waves' were once reverse shocks which were becoming evanescent when they propagated into the low plasma beta MC. The second reverse wave caused a decrease in the southward component of the IMF and initiated the storm recovery phase. It is determined that flares located at large longitudinal distances from the subsolar point were the most likely causes of the first two shocks without associated magnetic clouds. It is thus unlikely that the shocks were 'blast waves' or that magnetic reconnection eroded away the two associated MCs. This interplanetary/solar event is an example of the extremely complex magnetic storms which can occur in the post-solar maximum phase.
The observation of one of the largest magnetic clouds ever observed at a distance of 1 AU, with a diameter of greater than about 0.4 AU, is reported. The cloud is shown to be almost unchanged structurally by interaction with the earth bow shock. The first observations are reported of an auroral activity response to the passage of a magnetic cloud, with a nearly immediate increase in auroral activity when the IMF theta(B) angle reversed polarity to negative near the cloud center. The results provide strong evidence that turbulent magnetic fields behind interplanetary shocks are a possible cause of Forbush decreases, but contest the idea that relatively smooth, strong fields in clouds are a cause of such decreases. The cloud field modeling supports the existence of magnetic force-free fields in describing cloud structure.
The pulsation model of the earth's bow shock proposed a nonuniform shock having both perpendicular (abrupt, monotonic) and oblique (oscillatory, multigradient) properties simultaneously, depending on local orientation of the shock surface to the interplanetary field B sub sw in parallel planes defined by B sub sw and solar wind velocity. Multiple, concurrent, satellite observations of the shock and solar wind conditions were used. Twenty-six potentially useful intervals of concurrent Explorer 33 and 35 data acquisition were examined, of which six were selected for closer study. In addition, two years of OGO-5 and HEOS 1 magnetometer data were examined for possible conjunctions to these spacecraft having applicable data. One case of clear nonuniformity and several of field-dependent structure were documented. A computational aid, called pulsation index, was developed.
A study of data from the Apollo lunar surface suprathermal ion detector experiment (Side) package shows that plasma flow and energy parameters in the dusk magnetosheath are much better correlated with geomagnetic activity than those in the dawn magnetosheath. This result is in agreement with a dawn-dusk asymmetry in the magnetosheath magnetic field and in the bow shock configuration. The different orientations between the mean interplanetary magnetic field direction and the shock normal for the magnetosheaths suggest an explanation of the difference in the plasma parameters on the two sides.
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.
Results are presented of detailed comparisons between Doppler scintillation and in situ plasma measurements to improve the understanding of Doppler scintillation transients. During a combined observing period of nearly 3 mo in 1981-1982 near solar maximum, 22 transients were observed by the Pioneer Venus Orbiter spacecraft and 23 shocks were observed by Helios 1. It is found that at least 84 percent of the transients are shocks, while at least 90 percent of the shocks are transients. Although the temporal profiles of Doppler scintillation and mass flux density are similar, the magnitudes of the Doppler scintillation transients may not simply reflect those of mass flux density. Only one pronounced solar wind event that was observed in the mass flux density measurements showed no signature in the scintillation data; field and particle measurements by Helios 1 suggest that it is a noncompressive density enhancement and/or a magnetic cloud. It is shown that Doppler scintillation measurements can now be used by themselves to detect and locate interplanetary shocks near the sun with a relatively high degree of certainty.
Specific fields of current investigation by satellite observation and ground-based radio-astronomical and optical techniques are discussed. Topics include: aspects of energetic particles trapped in the earth's magnetic field and transiently present in the outer magnetosphere and the solar, interplanetary, and terrestrial phenomena associated with them; plasma flows in the magnetosphere and the ionospheric effects of particle precipitation, with corresponding studies of the magnetosphere of Jupiter, Saturn, and possibly Uranus; the origin and propagation of very low frequency radio waves in the earth's magnetosphere and ionosphere; solar particle emissions and their interplanetary propagation and acceleration; solar modulation and the heliocentric radial dependence of the intensity of galactic cosmic rays; radio frequency emissions from the quintescent and flaring sun; shock waves in the interplanetary medium; radio emissions from Jupiter; and radio astronomy of pulsars, flare stars, and other stellar sources.
We present for the first time the Voyager 1 and 2 plasma instrument observations within the interplanetary space of upstream suprathermal E < 6 keV electrons during times when close < 100 RJ from Jupiter’s bow shock. These measurements are a biproduct of our recovery of the Voyager Jupiter flyby measurements of the plasma electrons made by the Plasma Science Experiment (PLS) (Bridge et al., 1977). The initial ion and electron plasma observations were first reported by Bridge et al. (1979a, b). The periods were chosen to follow those reported by Zwickl et al. (1981) using energetic particle data with E ≥ 30 keV from the Low Energy Charged Particle (LECP) instrument (Krimigis et al., 1977). For this study we also use Voyager magnetic field data (Ness et al., 1979a, b) which is critical for this study. Basically, whenever the interplanetary magnetic field is nearly radially aligned relative to the spacecraft sun line, connection to the Jovian bow shock is believed to occur and one would expect those times when the keV suprathermal electron would be observed, but we also see them when the field is not radial but close to Jupiter’s bow shock. Whenever the magnetic field is radial the plasma instrument’s D cup or side sensor, which makes the electron observations (10 eV ≤ E ≤ 6 keV), is viewing ∼ 90 degree pitch angle electrons but evidently its wide field-of-view (FOV) allows one to see the keV electrons assumed to be field aligned. So, we will be looking into FOV alignment issues relative to the local magnetic field vector and any evidence for pitch angle scattering by plasma waves (see Scarf et al., 1981) that might broaden their width in pitch angle and allow their detection.
The interaction between collisionless shocks is examined from a kinetic viewpoint using computer simulations of shock collisions spanning the range of ambient plasma and shock parameters typical of the interplanetary medium. It is found that the interaction between two subcritical, perpendicular shocks is largely fluidlike with no evidence for accelerated particles. The electromagnetic fields associated with each shock, but not the plasmas, pass through each other during the collision. This interaction is due to self-consistently generated electric fields which adjust to the value needed to bring to rest the two incoming streams. The interaction between two supercritical perpendicular collisionless shocks is similar to the subcritical case, in that the electromagnetic fields associated with each shock are transmitted whereas particles are not. However, the interaction is also highly dynamic with substantial acceleration of ions. Increasing the Mach number gives rise to an increasing number of energetic particles.
A fundamental feature of the heliosphere is the three-dimensional structure of the interplanetary magnetic field. The magnetic field investigation on Ulysses, the first space probe to explore the out-of-ecliptic and polar heliosphere, aims at determining the large-scale features and gradients of the field, as well as the heliolatitude dependence of interplanetary phenomena so far only observed near the ecliptic plane. The Ulysses magnetometer uses two sensors, one a Vector Helium Magnetometer, the other a Fluxgate Magnetometer. Onboard data processing yields measurements of the magnetic field vector with a time resolution up to 2 vectors/second and a sensitivity of about 10 pT. Since the switch-on of the instrument in flight on 25 October 1990, a steady stream of observations has been made, indicating that at this phase of the solar cycle the field is generally disturbed: several shock waves and a large number of discontinuities have been observed, as well as several periods with apparently intense wave activity. The paper gives a brief summary of the scientific objectives of the investigation, followed by a detailed description of the instrument and its characteristics. Examples of wave bursts, interplanetary shocks and crossings of the heliospheric current sheet are given to illustrate the observations made with the instrument.