Engineering topics
Publications and source records attributed to Mihalov, J. D..
In order to understand better source and loss processes for energetic trapped protons near Jupiter, a modification of de Pater and Goertz' finite difference diffusion calculations for Jovian equatorial energetic electrons is made to apply to the case of protons inside the orbit of Metis. Explicit account is taken of energy loss in the Jovian ring. Comparison of the results is made with Galileo Probe measurements.
Energetic trapped electrons were measured with the Galileo Jupiter Probe, with samples from inside Io's orbit, down to just above the atmosphere. The energetic electron fluxes and spectra agree well with the earlier results from the Pioneer spacecraft, where comparison may be made under the assumption of simple power law spectra. New features from the Galileo measurements include direct observations of the electron pitch angle distributions and spectral softening, both as the atmosphere is approached and at smaller pitch angles at each measurement location.
Possible signatures of interstellar pickup hydrogen have been identified in data from the Pioneer 10 and 11 Ames plasma analyzers at locations out to beyond the orbit of Saturn, and at a wide range of longitudes in the heliosphere, both upstream and downstream in the interstellar wind. These signatures have their highest amplitudes near shocks and CIRs in the solar wind stream structure, and lower amplitudes elsewhere. Also the signatures have lower amplitudes in the downstream direction with respect to the interstellar flow.
A population of low-energy (0-250 V E/q) ions with tailward directed velocity vectors and energies above that for escape from Venus is evident in nightside data from the Ames plasma analyzer on the Pioneer Venus Orbiter spacecraft. Good correlations with solar wind parameters were not obtained for the magnitudes of these ion fluxes, but tendencies for occurrence at times of tailward oriented magnetic fields and for alignment of the ion flows with the magnetic field were found. These tendencies seemed to be enhanced for higher-energy ions. In a few cases where comparisons were made, the ion fluxes were consistent with simultaneous O(+) measurements by the neutral mass spectrometer experiment on the spacecraft. The mean flux observed of the escaping nightside ions, averaged over an approximately 10-week-long spacecraft nightside season, was less than 2 x 10(exp 6)/sq cm/s.
A population of low-energy (0-250 V E/q) ions with tailward directed velocity vectors and energies above that for escape from Venus is evident in nightside data from the Ames plasma analyzer on the Pioneer Venus Orbiter spacecraft. Good correlations with solar wind parameters were not obtained for the magnitudes of these ion fluxes, but tendencies for occurrence at times of tailward oriented magnetic fields and for alignment of the ion flows with the magnetic field were found. These tendencies seemed to be enhanced for higher-energy ions. In a few cases where comparisons were made, the ion fluxes were consistent with simultaneous O(+) measurements by the neutral mass spectrometer experiment on the spacecraft. The mean flux observed of the escaping nightside ions, averaged over an approximately 10-week-long spacecraft nightside season, was less than 2 x 10(exp 6) cm(exp -2) s(exp -1).
The Pioneer 10 and 11 Ames plasma analyzers included off-angle integrating counters used for the first exploration of magnetospheric plasmas at Jupiter and Saturn. When summed over multi-day intervals during interplanetary cruise, at times of relatively constant solar wind speed during 1972 to 1977, the resulting count rates show a dependence on acceptance energies of the plasma analyzer deflection plates. The count rates could be produced by energetic charged particles, solar wind protons moving at a large angle to the bulk flow, and interstellar pickup ions. We interpret the more energetic of two peaks that are sometimes observed as the signature of interstellar pickup hydrogen. This peak is located at just below twice the solar wind speed (V(sub SW)), when near 3 AU heliocentric distance, decreasing to just above V(sub SW) as the heliocentric distance increases. Also, in the 8 to 12 AU range of heliocentric distances, we identify the pickup hydrogen signature as a shelf that ends at an edge located below 2V(sub SW). During these observations, the spacecraft longitude relative to the upstream interstellar flow changes roughly from 25 deg, to 155 deg at the larger heliocentric distance. The peak at the smaller heliocentric distances is most consistent with a velocity distribution that is a shell in phase space, with limited thickening as the pickup ions are assimilated into the solar wind flow.
At the end of 1992, the Pioneer 10, Pioneer 11, and Voyager 2 spacecraft were at heliocentric distances of 56.0, 37.3, and 39.0 AU and heliographic latitudes of 3.3 deg N, 17.4 deg N, and 8.6 deg S, respectively. Pioneer 11 and Voyager 2 are at similar celestial longitudes, while Pioneer 10 is on the opposite side of the Sun. All three spacecraft have working plasma analyzers, so intercomparison of data from these spacecraft provides important information about the global character of the solar wind in the outer heliosphere. The averaged solar wind speed continued to exhibit its well-known variation with solar cycle: Even at heliocentric distances greater than 50 AU, the average speed is highest during the declining phase of the solar cycle and lowest near solar minimum. There was a strong latitudinal gradient in solar wind speed between 3 deg and 17 deg N during the last solar minimum, but this gradient has since disappeared. The solar wind temperature declined with increasing heliocentric distance out to a heliocentric distance of at least 20 AU; this decline appeared to continue at larger heliocentric distances, but temperatures in the outer heliosphere were suprisingly high. While Pioneer 10 and Voyager 2 observed comparable solar wind temperatures, the temperature at Pioneer 11 was significantly higher, which suggests the existence of a large-scale variation of temperature with heliographic longitude. There was also some suggestion that solar wind temperatures were higher near solar minimum.
Solar wind data from the Ames experiment aboard the Pioneer Venus Orbiter, associated with a period of high solar activity that began at the end of May, 1991, are summarized and discussed. Some particularly large, strong shocks and solar ejecta were observed. The solar longitude of Venus, relative to associated flares. varies over a wide range, for a series of flares that produced X-rays that saturated the GOES X-ray counters.
The EPI instrument operates during the pre-entry phase of the Galileo Probe. The main objective is the study of the energetic particle population in the inner Jovian magnetosphere and in the upper atmosphere. This will be achieved through omnidirectional measurements of electrons, protons, alpha-particles and heavy ions (Z greater than 2) and recording intensity profiles with a spatial resolution of about 0.02 Jupiter radii. Sectored data will also be obtained for electrons, protons, and alpha-particles to determine directional anisotropies and particle pitch angle distributions. The detector assembly is a two-element telescope using totally depleted circular silicon surface-barrier detectors surrounded by cylindrical tungsten shielding. The lower energy threshold of the particle species investigated during the Probe's pre-entry phase is determined by the material thickness of the Probe's rear heat shield which is required for heat protection of the scientific payload during entry into the Jovian atmosphere. The EPI instrument is combined with the Lightning and Radio Emission Detector and both instruments share one interface of the Probe's power, command, and data unit.
Pioneers 10 and 11, and Voyager 2, have active plasma analyzers as they proceed through heliocentric distances of the order of 30-50 AU, facilitating comparative studies of the global character of the outer solar wind and its variation over the solar cycle. Careful study of these data show that wind ion temperature remains constant beyond 15 AU, and that there may be large-scale variations of temperature with celestial longitude and heliographic latitude. There has thus far been no indication of a heliospheric terminal shock.
The Pioneer 10, Pioneer 11, and Voyager 2 spacecraft are now at heliocentric distances of 50, 32 and 33 AU, and heliographic latitudes of 3.5 deg N, 17 deg N, and 0 deg N, respectively. Pioneer 11 and Voyager 2 are at similar celestial longitudes, while Pioneer l0 is on the opposite side of the sun. The baselines defined by these spacecraft make it possible to resolve radial, longitudinal, and latitudinal variations of solar wind parameters. The solar wind temperature decreases with increasing heliocentric distance out to a distance of 10-15 AU. At larger heliocentric distances, this gradient disappears. These high solar wind temperatures in the outer heliosphere have persisted for at least 10 years, which suggests that they are not a solar cycle effect. The solar wind temperature varied with heliographic latitude during the most recent solar minimum. The solar wind temperature at Pioneer 11 and Voyager 2 was higher than that seen at Pioneer 10 for an extended period of time, which suggests the existence of a large-scale variation of temperature with celestial longitude, but the contribution of transient phenomena is yet to be clarified.
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.
Beginning in 1989, the active phase of the present solar cycle became manifest in the outer heliosphere as large disturbances in solar wind velocity as observed by the Ames plasma analyzers aboard Pioneer 10 (46-50 AU heliocentric distance) and Pioneer 11 (about 28 AU). Inner heliospheric baseline plasma observations from the Pioneer Venus Orbiter (0.7 AU) and IMP 8 (1 AU) are useful for attempts to correlate solar events with the outer heliospheric disturbances. With regard to the onset of activity at Pioneer 11, Pioneer Venus observations are pertinent, and some of these in turn correspond with CMEs (coronal mass ejections) observed in SMM coronagraph data. In particular, enhanced solar wind speeds observed at Pioneer Venus during December 1988 to February 1989 are associated with seven large solar wind shocks (or shock candidates); corresponding CMEs may be identified. Two of these seven shocks were identified as candidates for a precursor to the onset of the disturbances at Pioneer 11. At Pioneer 10 the disturbed period includes two large disturbances, associated with the passage of shocks. There are several candidate CMEs in the SMM observations, one of which may be associated with the second Pioneer 10 shock.
A statistical characterization is made of the combined ion and magnetic field properties of the Venus magnetosheath and magnetotail, on the basis of plasma and magnetic field data from 223 Pioneer Venus orbits; no assumptions are made as to existing regions or their plasma and field characteristics. Plasma is found to flow tailward in all locations, and the magnetotail is highly draped. Weak magnetic field asymmetries are associated with the plasma dropouts. A high-E/q plasma population, previously interpreted as planetary-pickup ions, is found asymmetrically both within the tail and in the adjacent sheath. The Venus tail is filled with plasma that is primarily shocked solar wind, at fluxes that are sometimes undetectable; the tail coexists with a photoion population which generates asymmetries in the bulk plasma and magnetic field properties.
Changes in solar wind speed and magnetic polarity observed at the Pioneer spacecraft are discussed here in terms of the changing magnetic geometry implied by SMM coronagraph observations over the period 1985-1987. The pattern of recurrent solar wind streams, the long-term average speed, and the sector polarity of the interplanetary magnetic field all changed in a manner suggesting both a temporal variation, and a changing dependence on heliographic latitude. Coronal observations during this epoch show a systematic variation in coronal structure and the magnetic structure imposed on the expanding solar wind. These observations suggest interpretation of the solar wind speed variations in terms of the familiar model where the speed increases with distance from a nearly flat interplanetary current sheet, and where this current sheet becomes aligned with the solar equatorial plane as sunspot minimum approaches, but deviates rapidly from that orientation after minimum.
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.
Pioneer Venus in situ measurements of N(e), total solar EUV flux, and solar wind dynamic pressure P(sw) are used to investigate the solar cycle and orbit-to-orbit variations of the Venus nightside ionosphere. It is found that the density of the main body of the nightside ionosphere below 600 km responds to the solar wind dynamic pressure variations in much the same way that the density of the ionotail does: both regions are depleted at times of high P(sw). Solar cycle variations in the EUV flux near solar minimum produce very large changes in the electron density in ionotail, changes that are consistent with the ionotail being supplied by nightward ion flow. However, short-term variations in the EUV flux produce a weak negative response in the density at altitudes between 150 and 600 km, at least at solar maximum. The nightside variations are consistent with enhanced solar wind scavenging of plasma from the nightside ionosphere during periods of higher P(sw).
Suprathermal ions with arrival directions quite distinct from those of the solar wind have been detected upstream of the Venus bow shock. The possibility that these events could be caused by instrumental or spacecraft effects or that they could be either solar wind disturbances, planetary pickup ions, or suprethermal ions upstream of the Venus bow shock is examined. It is concluded that they are consistent with upstream suprathermal ions associated with the bow shock that are observed downstream from the points of intersection of their extrapolated trajectories with the shock.