Showing papers on "Heliosphere published in 1993"

Model of the solar wind interaction with the local interstellar medium: Numerical solution of self‐consistent problem

Abstract: 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 experiments.

Detection of interstellar pick-up hydrogen in the solar system.

Abstract: Interstellar hydrogen ionized primarily by the solar wind has been detected by the Solar Wind Ion Composition Spectrometer instrument on the Ulysses spacecraft at a distance of 4.8 AUs from the sun. This 'pick-up' hydrogen is identified by its distinct velocity distribution function, which drops abruptly at twice the local solar wind speed. From the measured fluxes of pick-up protons and singly charged helium, the number densities of neutral hydrogen and helium in the distant regions of the solar system are estimated to be 0.077 +/- 0.015 and 0.013 +/- 0.003 per cu cm, respectively.

Radio Emission from the Heliopause Triggered by an Interplanetary Shock

Abstract: A strong heliospheric radio emission event has been detected by Voyagers 1 and 2 in the frequency range of 2 to 3 kilohertz. This event started in July 1992 and is believed to have been generated at or near the heliopause by an interplanetary shock that originated during a period of intense solar activity in late May and early June 1991. This shock produced large plasma disturbances and decreases in cosmic ray intensity at Earth, Pioneers 10 and 11, and Voyagers 1 and 2. The average propagation speed estimated from these effects is 600 to 800 kilometers per second. After correction for the expected decrease in the shock speed in the outer heliosphere, the distance to the heliopause is estimated to be between 116 and 177 astronomical units.

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

Abstract: 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.

Ulysses observations of a recurrent high speed solar wind stream and the heliomagnetic streamer belt

Abstract: Near-ecliptic solar wind observations by Ulysses on its way to the polar regions of the Sun, compared with those from IMP 8 at 1 AU, showed that high-speed streams decay and broaden with heliocentric distance from IMP 8 to Ulysses, as expected. In July 1992 while traveling south at approximately 13 deg S and 5.3 AU, Ulysses encountered a recurrent high-speed stream, that may also have been observed at IMP 8. The stream has been observed a total of 14 times, once in each solar rotation through June 1993 at approximately 34 deg S. The source of the high-speed stream is an equatorward extension of the south polar coronal hole. From July 1992 through June 1993, averages of solar wind peak speed increased while density decreased with heliographic latitude. Both the stream and a low-speed, high-density flow, presumably associated with the heliomagnetic (coronal) streamer belt encircling the heliomagnetic equator, crossed Ulysses with the solar rotation period until April 1993 when the spacecraft was at approximately 29 deg S heliographic latitude. After this time, as the spacecraft climbed to higher latitudes, the central portion of the streamer belt with lowest speed and highest density disappeared. Therefore, at its maximum inclination, the belt was tilted at approximately 29 deg to the heliographic equator at this point in the solar cycle.

Latitudinal variation of solar wind corotating stream interaction regions: Ulysses

Abstract: Ulysses' initial transit tot high heliographic latitudes at a heliocentric distance of approximately 5 AU has revealed systematic effects in the latitudinal evolution of corotating interaction regions (CIRs). At a latitude corresponding roughly to, but slightly less than, the inferred tilt of the coronal streamer belt and embedded heliospheric current sheet, the strong forward shocks commonly associated with CIRs at lower latitudes disappeared almost entirely; however, the reverse shocks associated with these CIRs persisted to latitudes approximately 10 deg above the streamer belt. Systematic meridional flow deflections observed in association with the forward and reverse waves bounding the CIRs demonstrate that the above effect is the result of the fact that the forward waves propagate to lower latitudes and the reverse waves to higher latitudes with increasing heliocentric distance. These observational results are in excellent agreement with the predictions of a three-dimensional model of corotating solar wind flows that originate in a tilted dipole geometry back at the Sun.

Corotating MeV/amu ion enhancements at ≤1 AU from 1978 to 1986

Abstract: MeV/amu ion enhancements associated with corotating high-speed solar wind streams in 1978–1986 during pre-solar maximum to near solar minimum conditions are studied using ISEE 3/ICE, IMP 8, and Helios 1 data. Around 50% of corotating streams contain energetic ion increases. These increases extend to ∼25 MeV/amu, where they merge into the galactic cosmic ray background, and are most evident approaching solar minimum. Sunward ion streaming in the solar wind frame (first-order anisotropy ∼20%) and positive radial intensity gradients (∼400%/AU) are consistent with acceleration in the outer heliosphere at corotating shocks followed by streaming into the inner heliosphere. The spectra and intensities show little solar cycle variation. The spectra of ions from protons to Fe at ∼2–20 MeV/amu are approximated equally well by exponentials in momentum dJ/dP ≈ exp (−P/P0), P0 = 11–16 MeV c−1 amu−1, or by distribution functions ƒ ≈ exp (−υ/υ0), υ0 = 0.18–0.25 (MeV/amu)1/2, with equivalent power law in energy slopes in the range ∼ −3 to −4. Ion abundances are correlated with the stream peak solar wind speed. In slower corotating streams (maximum solar wind speed <600 km/s), mean abundance ratios are protons/4He = 43 ± 18; 4He/O = 54 ± 23; C/O = 0.62 ± 0.06; Mg/O = 0.19 ± 0.03, and Fe/O = 0.14 ± 0.02. These show some similarity to the corresponding ratios for “solar energetic particles” (SEP) (protons/4He = 70 ± 10; 4He/O = 55 ± 3; C/O = 0.48 ± 0.02; Mg/O = 0.21 ± 0.01 and Fe/O = 0.16 ± 0.02) which are typically accelerated by shocks passing through slow solar wind. In corotating events in higher-speed streams, these ratios become protons/4He = 19 ± 5; 4He/O = 130 ± 35; C/O = 0.89 ± 0.05; Mg/O = 0.14 ± 0.01, and Fe/O = 0.10 ± 0.01 and more closely resemble the corotating event abundance ratios measured in high-speed streams during the mid-1970s solar minimum (protons/4He = 17 ± 7; 4He/O ∼ 160 ± 50; C/O = 0.89 ± 0.1; Mg/O = 0.13 ± 0.03, and Fe/O = 0.096 ± 0.05). Solar wind plasma may also show similar variations in composition with solar wind speed (based on the limited solar wind composition measurements available) so that the energetic ion compositions are consistent with the acceleration of corotating event ions and SEPs from the solar wind. The ordering of corotating event and solar wind abundances by first ionization potential and their variation with solar wind speed suggest that conditions in the ion-neutral fractionation region in the upper chromosphere determine the abundances and are associated in some way with regulation of the solar wind speed.

Disappearance of the heliospheric sector structure at Ulysses

Abstract: In May, 1993, the heliospheric current sheet (HCS) ceased to be seen by the Ulysses spacecraft at a heliocentric latitude of approximately 30 deg S and distance of 4.7 AU. The disappearance of the HCS coincided with the solar wind speed remaining greater than 560 km/s and with the disappearance of one of four interaction regions previously seen on each solar rotation. The heliographic latitude of the disappearance of the HCS at Ulysses was 11 deg equatorward of the latitude of the magnetic neutral sheet computed at the source surface at 2.5 solar radii, and it occurred a half year earlier than predicted on the basis of the persistance of the time profile of the neutral sheet tilt from one solar cycle to the next.

Interplanetary magnetic clouds: Topology and driving mechanism

Abstract: A model is developed to study the origin and propagation of magnetic clouds. Starting with an equilibrium current loop embedded in an ambient plasma consistent with the solar corona, magnetic energy is injected by increasing the loop current. This causes the loop to rise, propelling plasma and magnetic field away from the Sun. Using a simple model of the interplanetary medium, the subsequent dynamics of the loop is calculated to 1 AU and beyond. The macroscopic properties of the resulting structures at 1 AU closely resemble those of observed magnetic clouds. Thermal effects indicate that clouds remain magnetically connected to the Sun in order to yield observed temperatures near 1 AU.

Large-scale solar-wind structure near the Sun detected by Doppler scintillation

Abstract: STUDIES of the solar wind in the inner heliosphere (between the solar-wind source surface at ∼0.01 AU and 0.3 AU) are hampered by the lack of in situ spacecraft measurements. Radio propagation measurements—using both natural1–7 and spacecraft8–14 radio signals—have provided many insights, but information on large-scale solar-wind structure inside 0.3 AU that can be related to coronal features or direct spacecraft measurements at larger distances has nevertheless remained elusive. Here we report the detection of solar-wind structure between 0.08 and 0.53 AU, based on the response of the 13-cm radio signals from the Pioneer Venus Orbiter to electron-density fluctuations and solar-wind speed within this region. These Doppler scintillation measurements were made during the late declining phase of the most recent cycle of solar activity, when the solar wind exhibited recurrent high-speed streams. Near 0.5 AU, we find narrow regions of enhanced scintillation which appear to be associated with compressed plasma at the leading edges of these streams, consistent with previous scintillation measurements15,16. Inside 0.2 AU, however, scintillation enhancements are conspicuously absent from the fast streams, and instead occur in regions where the average solar wind is slow. They exhibit high variability, and appear to be the interplanetary manifestation of coronal mass ejections. The plasma structures giving rise to these enhanced scintillations apparently undergo significant evolution inside 0.3 AU.

Solar wind conditions in the outer heliosphere and the distance to the termination shock

Abstract: The Plasma Science experiment on the Voyager 2 spacecraft has measured 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. Assuming that the interstellar pressure is due to a 5 micro-G magnetic field draped over the upstream face of the heliopause, the 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. There are also large variations in ram pressure on time scales of 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. 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 crossings.

Hybrid simulations of the effects of interstellar pickup hydrogen on the solar wind termination shock

Abstract: Hybrid (kinetic ions/fluid electrons) plasma simulations are used to study the effects of a population of energetic interstellar pickup hydrogen ions on the solar wind termination shock. The pickup hydrogen is treated as a second ion species in the simulations, and thus the effects of the pick-ups on the shock, as well as the effects of the shock on the pickups, are treated in a fully self-consistent manner. For quasi-perpendicular shocks with 10-20 percent pickup hydrogen the pickup ions manifest themselves in a small foot ahead of the shock ramp caused by pickup ion reflection. For oblique shocks with smaller angles between the field and the shock normal, a large fraction of the pickup ions are reflected and move back upstream where they excite large amplitude magnetosonic waves which steepen into shocklets. These backstreaming pickup ions may provide advance warning of a spacecraft encounter with the termination shock.

Deceleration of interstellar hydrogen at the heliospheric interface.

Abstract: High-resolution spectra of nearby stars show absorption lines due to material in the local interstellar cloud. This cloud is deduced to be moving at 26 kilometers per second with respect to the sun, and in the same direction as the "interstellar wind" flowing through the solar system. Measurements by the Ulysses spacecraft show that neutral helium is drifting through the solar system at the same velocity, but neutral hydrogen appears to be moving at only 20 kilometers per second, a result confirmed by new measurements of the hydrogen emission line taken by the High-Resolution Spectrograph on the Hubble Space Telescope. These results indicate that neutral hydrogen atoms from the local interstellar cloud are preferentially decelerated at the heliospheric interface, most likely by charge-exchange with interstellar protons, while neutral helium is unaffected by the plasma. The magnitude of the observed deceleration implies an interstellar plasma density of 0.06 to 0.10 per cubic centimeter, which in turn implies that the heliospheric shock should be less than 100 astronomical units from the sun.

Counterstreaming suprathermal electron events upstream of corotating shocks in the solar wind beyond ∼2 Au: Ulysses

Abstract: Enhanced fluxes of suprathermal electrons are commonly observed upstream of corotating forward and reverse shocks in the solar wind at heliocentric distances beyond approximately 2 AU by the Los Alamos plasma experiment on Ulysses. The average duration of these events, which are most intense immediately upstream from the shocks and which fade with increasing distance from them, is approximately 2.4 days near 5 AU. These events are caused by the leakage of shock-heated electrons into the upstream region. The upstream regions of these shocks face back toward the Sun along the interplanetary magnetic field, so these leaked electrons commonly counterstream relative to the normal solar wind electron heat flux. The observations suggest that conservation of magnetic moment and scattering typically limit the sunward propagation of these electrons as beams to field-aligned distances of approximately 15 AU. Although it seems unlikely that these shock-associated events are an important source of counterstreaming events near 1 AU, remnants of the backstreaming beams may contribute importantly to the diffuse solar wind halo electron population there.

Steady state and dynamical structure of a cosmic-ray-modified termination shock

Abstract: A hydrodynamic model is developed for the structure of a cosmic-ray-modified termination shock. The model is based on the two-fluid equations of diffuse shock acceleration (Drury and Volk, 1981). Both the steady state structure of the shock and its interaction with outer heliospheric disturbances are considered. Under the assumption that the solar wind is decelerated by diffusing interstellar cosmic rates, it is shown that the natural state of the termination shock is a gradual deceleration and compression, followed by a discontinuous jump to a downstream state which is dominated by the pressure contribution of the cosmic rays. A representative model is calculated for the steady state which incorporates both interstellar cosmic ray mediation and diffusively accelerated anomalous ions through a proposed thermal leakage mechanism. The interaction of large-scale disturbances with the equilibrium termination shock model is shown to result in some unusual downstream structure, including transmitted shocks and cosmic-ray-modified contact discontinuities. The structure observed may be connected to the 2-kHz outer heliospheric radio emission (Cairns et al., 1992a, b). The time-dependent simulations also demonstrate that interaction with solar wind compressible turbulence (e.g., traveling interplanetary shocks, etc.) could induce the termination shock to continually fluctuate between cosmic-ray-dominated and gas-dynamic states. This fluctuation may represent a partial explanation of the galactic cosmic ray modulation effect and illustrates that the Pioneer and Voyager satellites will encounter an evolving shock whose structure and dynamic properties are strongly influence by the mediation of interstellar and anomalous cosmic rays.

Cosmic-ray modulation, merged interaction regions, and multifractals

Abstract: The large-scale heliospheric magnetic field fluctuations and their effects on cosmic rays are discussed. Voyager 2 observations from 1983.0 to 1989.6 are used as input to the model of Perko and Burlaga (1992) to show that the few largest merged interaction regions (MIRs) provide essential details in the cosmic-ray intensity (CRI) profile. It is demonstrated that the largest MIRs determine the basic structure of the radial gradients of the CRI. It is shown that the magnetic field strength during the period 1983.0-1989.6 does have a multifractal structure, but that there are different multifractal structures in the three intervals 1983.0-1985.0, 1985.0-1987.5, and 1987.5-1989.6. The multifractal structure can be described by simple functions, the multifractal spectra. These analytical descriptions can be used to generate realizations of the heliospheric magnetic field with the same statistical properties as those observed by Voyager 2. Thus, the starting point is provided for realistic global statistical models of the 11-yr variation of the Galactic CRI.

The gradient of galactic cosmic rays at the solar-wind termination shock

Abstract: We report on a study of the expected spatial variation of the galactic cosmic-ray intensity in the outer heliosphere, in the vicinity of the solar-wind termination shock. Model simulations which contain all of the transport effects, including drifts, predict that the radial gradients change abruptly at the shock, and that the nature of the effect varies significantly with particle energy. At low energies, the radial gradient changes abruptly from a lower value inside the shock to a higher value outside, whereas at high energies, the higher value of the gradient is inside the shock. This effect, which is a consequence of the matching conditions at the shock and is closely related to diffusive shock acceleration, is qualitatively the same for both helisopheric magnetic polarity states and remains much the same in one-dimensional, two-dimensional, and three-dimensional models. Hence drifts do not change the nature of this phenomenon, although they change it quantitatively. The effect is, of course, not present in the absence of a terminal shock and may prove to be an important diagnostic tool for the study of the termination of the solar wind.

Temporal variations in the termination shock distance

Abstract: 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 encountered.

Heliospheric hydrogen beyond 15 AU: Evidence for a termination shock

Abstract: The Voyager and Pioneer 10 spacecraft are moving upstream and downstream into the local interstellar flow, monitoring H Lyman alpha radiation resonantly scattered from heliospheric hydrogen. Voyager Cruise Maneuver observations obtained between 15 and 35 AU reveal that H Lyman alpha intensities in the upstream direction fall as r exp -0.75+/-0.05. Beyond 15 AU downstream, Pioneer 10 intensities fall as r exp -1.07+/-0.1. These trends cannot be simultaneously reproduced using a hot H distribution model that does not include termination shock structure. The Voyager H Lyman alpha intensities also show a distinctive trend to decrease less rapidly with increasing heliocentric distance. Between 15 and 20 AU, Voyager intensities fall as r exp -1, whereas between 30 and 35 AU they fall as r exp -0.35. This flattening trend implies that the upstream H density is increasing rapidly with heliocentric distance beyond about 25 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 alpha observations beyond 15 AU imply the existence of a solar wind termination shock, suggesting that it lies between 70 and 105 AU in the upstream direction.

The role of merged interaction regions and drafts in the heliospheric modulation of cosmic rays beyond 20 AU - A computer simulation

Abstract: Voyager 2 magnetic field measurements are used to simulate merged interaction and rarefaction regions (MIRs and RRs) for 1985-1989 via numerical solutions of the time-dependent, axially symmetric transport equation of cosmic rays in the heliosphere, together with the concurrent use of the wavy neutral sheet as a time-dependent drift parameter. This drift approach was found to be more successful, because it was able to reproduce the intensity levels, the factor modulation, and latitudinal gradients for 1 GeV protons at 23 AU.

Anisotropy and minimum variance of magnetohydrodynamic fluctuations in the inner heliosphere

Abstract: In this paper we examine the anisotropy, minimum variance, and related distinguishing plasma parameters of small-scale fluctuations occurring in the solar wind. We use Helios 2 data taken at solar minimum, at a time when high- and low-speed streams are clearly distinguished and a separation of the characteristics of fluctuations from disparate solar sources is facilitated. We find that while variance directions of fluctuations are generally aligned with the mean magnetic field in regions of high speed and relatively low plasma β, they are more three-dimensional or isotropic in low-speed, high-β intervals. In our analysis period, these latter regions are generally the trailing edges of high-speed streams and slow-flow-containing current sheets. In these low-speed intervals, we find a tendency for large proton density fluctuations to be associated with a preference for fluctuation variance directions to be three-dimensional and also a tendency for field and velocity fluctuations to decouple. In the past it has been emphasized that in high-speed streams, fluctuations are initially outwardly propagating and Alfvenic or two-dimensional in k space and that this Alfvenicity is destroyed by the production of inwardly traveling waves as the flow evolves. It has been suggested that this admixture of waves is produced in a turbulent cascade initiated by stream shears. Here we suggest that this picture is incomplete, that in addition to inwardly propagating plane waves, compressive waves, convected pressure balances, or other density fluctuations can generate or scatter the original spectrum and produce the observed scattering and decoupling of fluctuation directions. Additionally, while we do not dispute the supposition that the long-wavelength free energy in stream-stream interactions can initiate a turbulent cascade in the wind, the role of compressive fluctuations in altering the high-frequency spectrum cannot be ruled out.

Role of drifts and global merged interaction regions in the long-term modulation of cosmic rays

Abstract: The paper shows that the long-term modulation of 0.2- to 3-GV galactic and anomalous cosmic rays over the 22-year heliomagnetic cycle is basically a combination of two solar related processes, the cumulative effect of long-lived global merged interaction regions (GMIRs) and large-scale particle gradient and curvature drifts in the interplanetary magnetic field. This paradigm for cosmic ray modulation is based on the observed changes in cosmic ray intensity from solar minimum to solar maximum over successive solar cycles (21 and 22) using data from 1 AU and from the outer heliosphere. For cycle 21, the 1977-1980 modulation is dominated by GMIRs. In cycle 22, it is shown that drifts are an important but not dominant factor for galactic cosmic ray modulation with the current sheet related drift effects decreasing with increasing rigidity and heliocentric distance.

Termination shock - Solar cycle variations of location and speed

Abstract: Plasma and magnetic field data from Voyagers 1 and 2 over an 11-year period (1978-1988) together with a one-fluid MHD model are used to study the motion of the termination shock. The observed large fluctuations in solar wind parameters cause variations in the speed, in the jump conditions, and in the location of the termination shock. If the interstellar stagnation pressure is equivalent to a magnetic field of 0.5 nT plus draping enhancements, the termination shock moves between 88 AU and 102 AU. The location of the termination shock is anticorrelated with the sunspot number. The termination shock is very strong with pressure ratios of a few thousands, the fast Mach number is about 12, and the plasma beta ratio jumps from about 0.1 to about 14. During the declining phase of the solar cycle, the shock speed is often greater than or comparable to the spacecraft speed. If the crossing of the termination shock by a spacecraft occurs during the declining phase, the spacecraft will cross the termination shock 3 or more times over a period of about 4 years.

Detection of the local interstellar cloud from high-resolution spectroscopy of nearby stars : inferences on the heliospheric interface

Abstract: High-resolution spectroscopic observations of the local interstellar medium between the Sun and nearby stars have been recorded with the Aurelie spectrometer of the Observatoire de Haute Provence (France). They have been used to identify and characterize the local interstellar cloud (LIC) in which the Sun is embedded. The “Doppler triangulation” shows that the local cloud flows toward λ = 74.9°; β = −7.8° at a velocity of 25.7 ± 1.0 kms−1 with respect to the Sun. This is strongly confirmed by Hubble space telescope (HST) UV measurements toward the star Capella. Furthermore, the recent detection of interstellar neutral helium inside the heliosphere, found to flow at 26.0 ± 1.0 kms−1 with the Ulysses Neutral Gas Experiment, brings a definitive proof of this identification. The common interval for the local cloud temperature from these three different types of measurements is 7000 ± 200 K. As a consequence of the LIC velocity determination, the interstellar neutral hydrogen found to flow at 20 kms−1 through the solar system is shown to be decelerated by 6 kms−1 and slightly heated up to 8000 K due to coupling with the plasma at the heliospheric interface. Further high-resolution spectroscopic observations with the HST-Goddard high-resolution spectrometer (provided the short wavelength part can be repaired) should supply the local interstellar electronic density. These informations are of crucial importance with respect to the understanding of the heliospheric cavity.

Motion of the heliospheric termination shock - A gas dynamic model

Abstract: A simple quantitative model is presented for the heliospheric termination shock's anticipated movement in response to upstream solar wind condition variations, under the assumption that the termination shock is initially a strong gasdynamic shock that is at rest relative to the sun, and that there is a discontinuous increase or decrease in the dynamical pressure upstream of the shock. The model suggests that the termination shock is constantly in motion, and that the mean position of the shock lies near the mean equilibrium position which corresponds to the balance between the mean solar wind dynamical pressure and the mean interstellar pressure.

G-Star Astropauses: A Test for Interstellar Pressure

Abstract: Under the assumption of a solar system model, astropause radii are estimated for a sample of 70 G stars near the sun. G-star space velocities and trajectories are calculated. Three stars within 10 pc of the sun have predicted astropause radii larger than 6 arcsec. At least eight stars have traversed low interstellar pressure regions, similar to the path of the sun, over the last 4 Myr. The Galactic influences on these stellar systems thus may be similar to those on the solar system, providing a list of possibly attractive SETI targets. The closest star to the sun 4 Myr ago, in this restricted sample, was HD 147513. The relative dimensions of the astropause radii of G stars with space motions parallel and perpendicular to the surrounding interstellar magnetic field may vary by a factor of 2 and may ultimately yield quantitative estimates of interstellar magnetic field strength near the sun.

The cosmic radiation in the heliosphere at successive solar minima 3. Steady state drift solutions of the transport equation

Abstract: Cosmic ray intensities and density gradients, as observed by the Pioneer 10 and 11, Voyager 1 and 2, and IMP 8 spacecraft during the 1977 and 1987 solar minimum periods, are interpreted in terms of a two-dimensional version of the cosmic ray transport equation that includes drifts. This paper is a follow-up of an earlier no-drift version to elucidate the necessity and limitations of drift effects as demanded by actual observations.

The return of the anomalous cosmic rays to 1 AU in 1992

Abstract: New observations of low energy (approximately 1 to 200 MeV/nuc) cosmic rays measured by three newly launched experiments on Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) during 1992 and 1993 show the strong presence of anomalous cosmic ray (ACR) nitrogen and oxygen, well before the approaching solar minimum. When compared with ACR temporal variations over the past two solar cycles we find that the 1992-1993 fluxes are approximately 5 to 10 times their level at corresponding neutron monitor counting rates in 1969-1970 and 1985

The distribution of interstellar dust in the solar neighborhood

Abstract: We surveyed the IRAS data base at the positions of the 1808 O6-B9.5 stars in The Bright Star Catalog for extended objects with excess emission at 60 microns, indicating the presence of interstellar dust at the location of the star. Within 400 pc the filling factor of the interstellar medium, for dust clouds with a density greater than 0.5/cu cm is 14.6 + or - 2.4%. Above a density of 1.0/cu cm, the density distribution function appears to follow a power law index - 1.25. When the dust clouds are mapped onto the galactic plane, the sun appears to be located in a low-density region of the interstellar medium of width about 60 pc extending at least 500 pc in the direction of longitudes 80 deg - 260 deg, a feature we call the 'local trough'.