Devrie S. Intriligator

Preliminary comparison of solar wind plasma observations in the geomagnetospheric wake at 1000 and 500 earth radii

Preliminary comparison of observations in the expected region of the geomagnetic tail with the Ames Research Center solar wind plasma probe at 1000 Earth radii (Pioneer 7, September 1966) and 500 Earth radii (Pioneer 8, January 1968) indicates that the measured plasma characteristics in these regions are similar. In each case intervals of quiescent plasma ion energy spectra are interrupted by abrupt changes in the magnitude and shape of the ion spectra or by complete absence of measurable plasma. Each of these regions is highly disturbed and is most appropriately described by the term ‘geomagnetospheric wake’. A comparison with measurements in the near-Earth region of the geomagnetic tail indicates that the characteristics of the geomagnetic tail apparently undergo significant changes between 80Rz and 500Rz.

Three views of two giant streams: Aligned observations at 1 AU, 4.6 AU, and 5.9 AU

A close radial alignment of the IMP and Pioneers 10 and 11 spacecraft in 1974 allows a nearly unambiguous, empirical study of the radial evolution of the interaction regions of two contrasting weak and strong, giant streams. The study confirms the main aspects of the standard model of corotating interaction regions: an expanding and strengthening pair of forward-reverse shocks sandwich a stream interface. It adds the following concepts: stream group speed — the speed at the stream interface tends to remain constant with distance; corotating stream complexes — interaction regions can include features like noncompressive density enhancements and streamer belts; secondary interfaces — a possible precursor to the reverse shock; and emerging stream interfaces — one emerged between 1 AU and 4.6 AU. The study uses the conservation specific entropy to correlate features between spacecraft.

The solar wind interaction with the geomagnetic field

A review is presented of the interaction of the solar wind with the magnetic field of the earth. The material is developed primarily from an observational point of view. The early observations are covered through late 1963, with primary emphasis on the sunward interaction region. The historical review of the early results is discussed in terms of the significant contributions of each satellite observation and in the light of our present concept of the solar wind-geomagnetic field interaction. Subsequent to 1963 the observations tend to overlap such that a strictly historical treatment is not tractable and the material is presented from a phenomenological approach. The daytime and night-time hemispheres are covered separately in terms of the significant and separable phenomena which dominate the structure and dynamics of these two regions. Satellite and deep space probe data are compared with relevant theory. Further observational eflorts needed to improve our understanding of the details of the solar wind-geomagnetic field interaction are also discussed.

POSITION AND SHAPE OF THE VENUS BOW SHOCK: PIONEER VENUS ORBITER OBSERVATIONS

In this study magnetometer data from the Pioneer Venus Orbiter is used to examine the position and shape of this planets bow shock. Utilizing crossings identified on 86 oc- casions during the first 65 orbits a mean shock surface is defined for sun-Venus-satellite angles of 60-110 o . Both the shock shape and variance in location are found to be very similar to the terrestrial case for the range in SVS angle con- sidered. However, while the spread in shock po- sitions at the earth is due predominantly to the magnetopause location varying in response to so- lar wind dynamic pressure, ionopause altitude variations can have little effect on total obsta- cle radius. Thus, the Cytherean shock is some- times observed much closer to or farther from the planet than previously predicted by gasdynamic theory applied to the deflection of flow about a blunt body which acts neither as source nor sink for any portion of the flow.

Stream interfaces and energetic ions closer than expected: Analyses of Pioneers 10 and 11 observations

An empirical study of corotating interaction regions (CIRs) observed between 3.9 AU and 5.9 AU on Pioneers 10 and 11 shows that the main corotation energetic ion population (CEIP), which is associated with the trailing reverse shock, terminates within the CIR at a definite, structural boundary, which we show here is the stream interface. This new result has significant implications for solar wind and energetic particle modeling. In particular it implies either that the reverse shock forms closer to the stream interface than models suggest or that the theories that treat the generation and transport of these energetic ions, such as preshock Fermi acceleration and cross-field diffusion must be combined or extended. We test these scenarios by comparing the CEIP intensity profiles on the two sides of the stream interface. We find that while each automatically accounts for one or two aspects of the results none of them alone can account for all of our empirical results.

Intermediate transition in the Venus ionosheath

We present the results of an analysis of Pioneer Venus Orbiter (PVO) plasma and electric and magnetic field data with evidence of a plasma transition along the flanks of the Venus ionosheath for 8 PVO passes near the terminator. This transition occurs between the bow shock and the ionopause and represents a stationary change in the properties of the shocked solar wind that streams around the Venus ionosphere. We find that the intermediate transition is characterized by three concurrent features : (1) A noticeable electric field burst measured with the 30 kHz channel of the electric field detector of the PVO ; (2) a severe drop of the magnetic field intensity accompanied by a strong rotation of the magnetic field orientation to a direction closer to the Sun-Venus axis in the inner ionosheath ; and (3) substantially enhanced plasma fluxes detected at the time when these changes in the electric and magnetic fields are measured. The peak particle flux and the peak magnetic field intensity measured at this transition in the data of the 17 PVO orbits are also presented. It is found that large values of the enhanced particle fluxes occur mostly when the peak magnetic field intensity is large.

Interstellar pickup H+ Ions at 8.3 AU: Pioneer 10 plasma and magnetic field analyses

Analysis of Pioneer 10 plasma and magnetic field observations at 8.3 AU in 1975 provides new evidence for the presence of interstellar pickup hydrogen (H + ) ions. Use of plasma sensors that look far from the solar wind direction confirms the spherical shell distribution of the pickup ions in velocity space. Phase space density and flux estimates are closely consistent with those from Ulysses measured under similar conditions. Power spectral analyses of the magnetic field data show a distinct signal a little above the proton gyrofrequency, consistent with the presence of Doppler-shifted ion-cyclotron waves generated by H + pickup ions. These results show that the Pioneer data set has the potential for systematic studies of the global properties of interstellar pickup ions.

Processes associated with particle transport in corotating interaction regions and near stream interfaces

Pioneer and Ulysses observations of energetic particles and solar wind plasma and magnetic fields are analyzed to study particle transport in corotating interaction regions (CIRs) and near stream interfaces (SIs). Energetic particle diffusion with a lowered coefficient near the SIs as compared with the free solar wind may account for the energetic particle profiles in CIRs and for the modulation of Jovian electrons. Our analyses include examination of the magnetic power and variances perpendicular and parallel to the magnetic field for a range of wave numbers and for a range of conditions within two CIRs. Since the magnetic field immediately on either side of a SI, but near it, is not connected to either the forward shock or reverse shock, it is the cross-field motion of the energetic particles that is relevant. Since the cross-field transport is thought to be related to field line mixing, or random walk, we examined magnetic field fluctuations normal to the average field to determine whether any signature of reduced perpendicular particle transport could be found. Evidence consistent with reduced particle transport near the SI was indeed found when we examined time series of the 1-min averages of the magnetic field components. A planar magnetic structure was associated with the trailing unshocked layer following the SIs (i.e., SI to SI + 12 hours) and also with the entire CIRs, but it was not present in the free solar wind. We have quantitatively examined the effects of shear and compression on particle transport and conclude that the effects of shear and compression reduce particle transport in CIRs and in the vicinity of SIs.

Cross-field diffusion in corotating interaction regions

Energetic ions associated with the forward and reverse shocks of corotating interaction regions (CIRs) are commonly observed at places within CIRs that are not magnetically connected to either shock. Examples have been documented with data from Pioneer 10 and 11 and Ulysses. They pose a problem for models that account for these shock-associated, corotating energetic ion populations (CEIPs) in terms of ion energization at the shocks followed by ion propagation along field lines. According to these models, regions magnetically unconnected to a shock should contain no shock-associated ions. Cross-field diffusion has been suggested as a mechanism for populating the shock-unconnected regions of CIRs. Here we quantitatively examine this suggestion. We use the Greens function solution to the convection-diffusion equation applied to idealized CIR geometry, with a source at the reverse shock, to compute the ratio of the ion flux in the heart of a CIR to the flux in the shock-connected part of the CIR and to compare the result with typical ratios observed by the Pioneer spacecraft. The computed ratio for resonant diffusion is many orders of magnitude below the observed ratio. For stochastic field line diffusion, the computed ratio is many orders of magnitude above the observed ratio if a diffusion coefficient appropriate to the free solar wind is used. It agrees with the observed ratio, however, if a reduced diffusion coefficient appropriate to CIRs is used. We conclude that the stochastic field model of cross-field diffusion can account for the presence of energetic ions in regions of CIRs that are magnetically unconnected to shock waves.

Stream interfaces and energetic ions II: Ulysses test of Pioneer results

Ulysses measurements of energetic and solar wind particles taken near 5 AU between 20 and 30 degrees south latitude during a well-developed recurring corotating interaction region (CIR) show that the CIRs corotating energetic ion population (CEIP) associated with the trailing reverse shock starts within the CIR at the stream interface. This is consistent with an earlier result obtained by Pioneers 10 and 11 in the ecliptic plane between 4 and 6 AU. The Ulysses/Pioneer finding is noteworthy since the stream interface is not magnetically connected to the reverse shock, but lies 12–17 corotation hours from it. Thus, the finding seems to be inconsistent with the basic model that generates CEIP particles at the reverse shock and propagates them along field lines. Eliminating the inconsistency probably entails an extension of the standard model such as cross-field diffusion or a non-shock energization process operating near the stream interface closer to the sun.