ISSN:0148-0227    

DOI:10.1029/93JA01169

年代:1993

数据来源: WILEY

摘要:

The interaction of the solar wind with the interstellar medium is expected to result in a complex, probably dynamic outer heliospheric boundary region. In increasing distance from the Sun the boundary region includes the solar wind termination shock, the heliopause, and perhaps a heliobow‐shock. The continuing missions of Voyager 1 and 2 and Pioneer 10 and 11 provide a unique opportunity to make in situ particle and field observations of the boundaries of the heliosphere and associated phenomena. Observations already made by these spacecraft suggest that the termination shock may be located 60‐100 AU from the Sun. Voyager 1 will reach 60 AU in 1995 and 100 AU in 2006. NASA's Space Physics Division is planning with the Voyager and Pioneer Projects to ensure timely and maximum scientific return from spacecraft encounters with the solar wind termination region. Initial emphasis is on being able to reliably determine when an encounter is imminent, so that spacecraft tracking and spacecraft and instrument configurations can be changed to encounter modes. It is anticipated that multiple encounters with heliospheric boundaries will occur due to motion of the boundaries in response to large variations in the solar wind press

ISSN:0148-0227    

DOI:10.1029/93JA01174

年代:1993

数据来源: WILEY

摘要:

The plasma wave receivers on the Voyager spacecraft will likely provide indicators of both the actual crossing of the termination shock as well as precursors of the shock crossing. Langmuir waves are commonly observed in the electron foreshock regions associated with planetary bow shocks; hence we expect to observe similar emissions ahead of the termination shock. Since the electron foreshock can extend considerable distances upstream of the termination shock, the detection of these waves can provide as many as several weeks warning that a crossing of the termination shock is imminent. Electrostatic turbulence associated with planetary bow shocks themselves is also an expected feature of the solar wind termination shock and will provide an important signature with which to identify the shock and to provide information on its thickness and fundamental processes. Since this turbulence is collocated with the shock, it cannot provide any advanced warning of the shock. Both upstream Langmuir waves and electrostatic wave turbulence can often be found in conjunction with interplanetary shocks, although the generally weaker nature of these shocks often leads to weaker plasma wave signatures than observed at planetary bow shocks. We demonstrate with Voyager observations that the amplitudes expected for each of these phenomena are well within the range of detectability by the Voyager plasma wave receiver even for termination shock distances exceeding 100 AU.

ISSN:0148-0227    

DOI:10.1029/93JA01176

年代:1993

数据来源: WILEY

摘要:

The heliospheric termination shock is expected to move in response to variation in upstream solar wind conditions; we present a simple quantitative model of this motion. In the model it is assumed that the termination shock is initially a strong gas dynamic shock at rest with respect to the Sun and that upstream of the shock there is a discontinuous increase or decrease in dynamical pressure. This jump is taken to be a contact discontinuity, i.e., an increase (decrease) in density without change in speed at the discontinuity. We then analyze what happens after the discontinuity encounters the shock. The postinteraction configuration is a moving termination shock, a postshock contact discontinuity, and a compression or rarefaction signal propagating into the downstream medium. The analysis is also extended to consider the successive passage of contact discontinuities through the termination shock. On the basis of this model we suggest that the termination shock is constantly in motion and that the following picture emerges: (1) the mean position of the shock is near the mean equilibrium position corresponding to balance between mean solar wind dynamical pressure and mean interstellar pressure; but (2) the shock makes inward and outward excursions over several (or even several tens?) astronomical units and at any given moment its position is determined by the recent (past several month) history of variations of solar wind dynamical pressure. The inward or outward speed of the shock depends on the magnitude of the change in upstream dynamical pressure but is typically of the order of 100 km/s. Therefore the first detection of this shock would be due to the shock moving inward through the spacecraft location rather than the spacecraft reaching a fixed shock location. A kinematic analysis due to Suess (this issue) leads to generally similar conclusions, although his conjecture that the speed of the termination shock may be much larger for outward motion than for inward motion is not supported by our dynamical analysis.

ISSN:0148-0227    

DOI:10.1029/92JA02895

年代:1993

数据来源: WILEY

摘要:

A kinematic analysis is made of solar wind driven temporal variations in the heliospheric termination shock distance. This has become possible because the large‐scale dynamics of, and temporal variations in the distant solar wind are now well enough known from a combination of in situ and remote measurements. Conversely, nothing is known of the corresponding properties in the local interstellar medium, and hence these will be ignored. Given specific assumptions for how the termination shock responds to solar wind fluctuations, it is shown that the termination shock is very agile, moving in and out by up to a few AU in a month. This conclusion holds for a broad range of shock response assumptions. Because a spacecraft moves slowly in comparison to the termination shock, the shock will sweep back and forth over the spacecraft once the shock is first encountere

ISSN:0148-0227    

DOI:10.1029/93JA01170

年代:1993

数据来源: WILEY

摘要:

A self‐consistent gasdynamic model of the solar wind interaction with the local interstellar medium (LISM), which took into account the mutual influence of the plasma component (electrons and protons) of the LISM and the LISM H atoms that penetrate into the heliosphere was constructed by Baranov et al. (1981) in the approximation of axial symmetry. This model, however, had a number of defects. In particular, the motion of the H atoms was described by hydrodynamical equations, although the mean free path of the H atoms and the characteristic length of the problem were comparable. An iterative method, that used a Monte Carlo simulation of H atom motion in the field of the plasma component hydrodynamic parameters, was suggested by Baranov et al. (1991) and only the first step of the iteration was realized (non‐self‐consistent problem solution). In this paper the results of the self‐consistent problem solution for a single set of the undisturbed solar wind and LISM parameters are presented. The structure of the upwind as well as wake regions of the flow is calculated. The geometrical pattern of the flow (bow shock, heliopause, termination shock, Mach disc, etc), the bulk velocity and the number densities of H atoms and plasma component are obtained and analyzed as a function of the distance from the Sun for different values of the polar angle. The effects of resonance charge exchange of the LISM H atoms as well as energetic H atoms “born” in the solar wind are taken into account. It is interesting to note that the effect of H atoms penetrating the solar wind results in the disappearance of the complicated flow structure as well as the supersonic regions between the heliopause and termination shock in the downwind region. In future we are going to compare our theoretical results with the results of Voyager 1/2, Pioneer 10/11, Ulysses spacecraft, and other

ISSN:0148-0227    

DOI:10.1029/93JA01171

年代:1993

数据来源: WILEY

摘要:

The radial gradient of anomalous cosmic ray oxygen measured instantaneously between Voyager 2 and Pioneer 10 during 1985‐1988 is correlated with the inferred tilt of the heliospheric neutral sheet. This is consistent with a simple model in which the radial gradient is related to the length of the neutral sheet between the two spacecraft. With this model we show that the radial gradient and the tilt of the neutral sheet near the solar wind termination shock can be inferred from the Voyager and Pioneer observations. By comparing the time history of the inferred tilt with that derived from solar observations, we estimate that the termination shock was at AU at solar minimum in 1987. At solar maximum the shock should be located at ∼90 AU due to the increased pressure of the solar w

ISSN:0148-0227    

DOI:10.1029/93JA01173

年代:1993

数据来源: WILEY

摘要:

Modeling the kinematic magnetic field in the solar wind beyond the terminal shock shows that a ridge of magnetic pressure is produced just inside the heliopause. This ridge has its maximum amplitude in the plane defined by the solar rotation axis and the heliotail and decreases to zero amplitude in the solar equatorial plane. The ridge is sufficiently large that it will cause the layer immediately inside the heliopause to thicken, pushing the heliopause outward and slightly affecting its position relative to the terminal shock. However, the ridge is far too thin to cause an important change in the distance of the terminal shock from the Sun. The kinematic assumption prevents us from estimating the actual magnitude of the ridge, but we show that these conclusions are a simple consequence of geometrical arguments for incompressible, steady, laminar flows. Moreover, the heliopause magnetic field originates on the terminal shock near the substagnation point. Consequently, the heliospheric current sheet field reversals are painted onto the inside surface of the heliopause. Alternate magnetic polarity strips will be oppositely directed relative to the interstellar magnetic field, implying that reconnection inevitably occurs on a fine scale near the nose of the heliosphere. This suggests that the heliopause is a leaky, diffuse surface.

ISSN:0148-0227    

DOI:10.1029/93JA01177

年代:1993

数据来源: WILEY

摘要:

The Plasma Science experiment on the Voyager 2 spacecraft has measured to date the properties of solar wind protons from 1 to 40.4 AU. We use these observations to discuss the probable location and motion of the termination shock of the solar wind. A least squares fit of proton ram pressure to heliocentric distanceRover this distance yields a ram pressure equal to (1.67 × 10−8dynes cm−2)R−2.00 ± 0.02, whereRis measured in astronomical units. Assuming that the interstellar pressure is due to a 5 µG magnetic field draped over the upstream face of the heliopause, this radial variation of ram pressure implies that the termination shock will be located at an average distance near 89 AU. This distance scales inversely as the assumed field strength, i.e., for a 7 µG field, the termination shock will be located on average at 64 AU. In addition to the global falloff with distance, there are large variations in ram pressure on relatively short time scales (tens of days), due primarily to large variations in solar wind density at a given radius. Such rapid changes in the solar wind ram pressure can cause large perturbations in the location of the termination shock. Using a simple kinematic model, we study the nonequilibrium location of the termination shock as it responds to these ram pressure changes. The results of this study suggest that the position of the termination shock can vary by as much as 10 AU in a single year, depending on the nature of variations in the ram pressure, and that multiple crossings of the termination shock by a given outer heliosphere spacecraft are likely. After the first crossing, such models of shock motion will be useful for predicting the timing of subsequent

ISSN:0148-0227    

DOI:10.1029/93JA01178

年代:1993

数据来源: WILEY

摘要:

The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman α radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman α intensities in the upstream direction fall asr−0.75±0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall asr−1.07±0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. Radiative transfer calculations using the hot H model predict that upstream intensities should fall more rapidly as a function of heliocentric distance than downstream intensities, precisely opposite to the observed trends. The Voyager H Lyman α intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall asr−1, whereas between 30 and 35 AU they fall asr−0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond ≈25 AU. A simple analysis suggests that the density distribution changes from nearly uniform between 15 and 20 AU, tor0.65dependence between 30 and 35 AU. This steepening trend is significant because similar H density gradients are predicted in models which include the effects of the termination shock. Taken together, the Voyager and Pioneer 10 H Lyman α observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the ups

ISSN:0148-0227    

DOI:10.1029/93JA01175

年代:1993

数据来源: WILEY