Solar Distance Research Articles

Influence of the Solar Activity Cycle on the Gasdynamic Structureof the Heliospheric Interface

The effects of the 11 year solar activity cycle on the heliospheric plasma interface in the presence of neutral H-atoms have been investigated. Our calculations show that nonstationary processes of such kind lead to1) a decrease of the mean interstellar plasma density in the interface;2) a sequence of shocks and rarefaction waves moving from the heliopause (HP)to the bow shock (BS); 3) an expansion of the region between the BS and HP;4) the TS excursion along the upwind direction is within 30%of the mean solar distance.

The spectrum of magnetic field irregularities in the solar corona and in interplanetary space

Results are presented of a spectral analysis of Faraday rotation fluctuations observed during the solar occultations of the Helios spacecraft. The temporal spectra of Faraday rotation fluctuations in the frequency range between 10 −3 Hz and 10 −2 Hz usually assume the form of a power law. The power-law exponent is ⋍1.6 at heliocentric distances 3 R ⊙ < r < 6 R ⊙ (R ⊙ = solar radius) and ⋍1.1 at distances 6 R ⊙ < r < 12 R ⊙. The Faraday rotation data are compared with in situ measurements of magnetic field fluctuations in the range of solar distances 0.3 < r < 1 AU.

Long-Term Radio Scintillation Variations in the Circumsolar Plasma

Long-term scintillation measurements of the solar wind formation zone at solar elongations ranging from 1°–8° (Sun impact parameters: 4–30 R⊙) were recorded using the water maser source IRC-20431 at the wavelength λ=1.35 cm during its annual solar occultations in December 1981–1998. Dramatic changes in the spatial dependence of the scintillation index were recorded over the course of the 11-year solar cycle. Markedly diminished scattering, attributed to a pronounced heliolatitude effect, was observed at the closest solar approach distances in the years around solar activity minimum. From parallel investigations of the solar magnetic field structure it was determined that the field strength at the source of the solar wind streamlines is the governing factor for the solar wind acceleration process. Particularly apparent in the scintillation data during solar activity minimum is the increasing role of the polar coronal holes with their associated open magnetic field structure. The dependence of the solar scattering intensity on heliolatitude fades in the years of high solar activity as the level of scintillations increases at polar latitudes.

Stellar Content of Local Group Galaxies - An Introduction

In The Realm of the Nebulae, Hubble (1936) first drew attention to the fact that the Milky Way system and the Andromeda galaxy belong to a small cluster that also contains M32, M33, the Magellanic Clouds, NGC 205, NGC 6822 and IC 1613. Hubble also listed IC 10 as a possible member of what he referred to as “the Local Group”. Inspection of the prints of the Palomar Sky Survey shows (van den Bergh 1962) that a large fraction of all field galaxies are located in such small groups or clusters. Our Milky Way system therefore appears to be situated in a rather typical region of space. All of the well-established Local Group members that are listed above are at distances D ≤ 1.0 Mpc. A conservative limit D &lt; 1.5 Mpc may therefore be used to search for new Local Group members. An additional criterion for physical membership in the Local Group is that a candidate member with solar apex distance θ and radial velocity Vr should lie close to the Vr versus cos θ relation for well-established Local Group members (Courteau &amp; van den Bergh 1999). Finally candidates may be disqualified from membership if they appear projected on nearby groups of galaxies that are centered at distances greater than 1.5 Mpc. In particular the Local Group candidates NGC 1560, NGC 1569, UGC-A86 and Cassiopeia 1 were excluded because they appear projected on (or near) the IC 342/Maffei group. Furthermore NGC 55 and UKS 2323-326 were excluded because they appear projected on (or near) the Sculptor (= South Polar) group. Observational data on 35 probable Local Group members are given in Table 1.

Applying international space station (ISS) and solar-sail technology to the exploration and diversion of small, dark near earth objects (NEO's)

Space-Station technology, micron-thick 100-m solar sails, and a near-Earth propulsion system allow exploration of small, dark Near-Earth-Objects (NEO's) of cometary origin. Early crewed missions to NEO's could analyze the objects' properties and evaluate resource-mining possibilities. Later missions to Earth-threatening 100-m radius NEO's could deploy NEO-centered, high-area, low-mass reflective structures. The solar gravitational parameter (GM sun) on a NEO is slightly reduced by increased radiation pressure. The central-force-field equations reveal that NEO eccentricity and average solar distance are thereby slightly increased. Given decades of warning and long-lived reflective canopies, such structures can convert Earth-impacts into near-misses. Although not suitable for NEO mining, such structures are superior to nuclear detonations because (as revealed by the 1994 Jupiter-comet interaction) NEO calving may be a consequence of explosions. NEO despinning is not required.

Thermal radiation from dust grains in Edgeworth-Kuiper Belt

We calculate the temperature of dust grains produced in Edgeworth-Kuiper Belt (EKB) based on the grain model for water-ice and silicate mixtures. The dust grains with radii ranging from 0.1 μm to 1 mm have low temperatures of about 20 K to 50 K in EKB, depending on their size, solar distance, and a volume mixing ratio of silicate to water-ice. We also estimate the thermal radiation from dust cloud in EKB. The result of thermal emission shows the spectral feature of water-ice at the wavelength of about 60 μm. Although it is difficult to estimate the possibility to detect the thermal emission spectrum of EKB dust cloud, due to large uncertainties in its spatial density, we found that the thermal emission of dust cloud in EKB lies below the IRAS data of foreground zodiacal emission. The maximum value of the thermal emission derived from the acceptable dust cloud model in EKB, however, becomes to be comparable to that of foreground zodiacal emission in far-infrared and submillimeter wavelength domains. Since the EKB dust cloud seems to concentrate near the ecliptic plane, a scanning of infrared observation along a line perpendicular to the ecliptic plane may reveal the presence of such dust cloud in the future.

Collisional evolution and the resulting mass distribution of interplanetary dust

On the basis of numerical approaches for the collisional and orbital evolution of dust particles, the number density distribution of interplanetary dust with the mass range of m ≥ 10−12g is investigated. The slope of the mass distribution of dust particles strongly depends on the radial dependence of dust production by their parent bodies, and the collisional interaction between particles. Specifically, the m −7/3 dependence of the number density distribution at 1 AU for m ≥ 10−6g can be explained through the balance between the collisional loss of particles and the dust supply, whereas the m −4/3 dependence for 10−12g ≤ m ≥ 10−6g particles is derived from simple Poynting-Robertson orbital decay. A possible model of the dust populations of asteroidal, cometary, and Edgeworth-Kuiper belt origin that is consistent with the observed dust flux at a solar distance of 1 AU is presented.

On non-baryonic dark objects in galactic haloes

Taking into account that a hierarchical pattern of clustering appears in cold dark matter scenarios, the existence of cold dark matter substructure in galactic haloes is explored. For this purpose, well-known dynamical constraints are imposed on non-baryonic objects, at solar Galactocentric distance. In this paper, aggregates of axions, aggregates of fermions and clusters of these aggregates have been considered, and ranges for their masses and radii have been estimated. Non-baryonic objects with radii and masses different from those allowed must have been disrupted by dynamical effects, so that their components must be orbiting freely within the Galactic halo. Upper limits for the ratio between the mass (per volume unit) that remains bound to dark objects and the cold dark matter total mass (per volume unit) in the Galactic halo have also been estimated.

The Car–Sgr arm as outlined by superclouds and the grand design of the Galaxy

Abstract It is found that H I superclouds are regularly spaced along the very long Car–Sgr arm, the mutual distances being mostly 0.1 and 0.2 in units of the Solar distance to the centre. Such regularity is instrinsic to the grand design galaxies with density-wave spiral arms. A higher density of old stars and star clusters within the Car-Sgr arm is observed, implying a strong density wave. A symmetric second arm should exist in a grand design galaxy yet this arm is well seen only in the II quadrant, being behind the Per arm. The positions of most superclouds, the giant H II regions and GMCs over the Galaxy are compatible with a four-arm spiral structure with two less pronounced additional arms midway between the two principal arms; the nearby Per arm is one of the secondary arms. The pitch angle of such a grand design spiral pattern is 10°–12°. If this pattern does exist, the long segments of the arms have none the above-mentioned spiral-arm tracers. At any rate the Car-Sgr arm is certainly much brighter and more regular than all the others. The most probable arm class of the Galaxy is 9–12; it is multiarm or grand design galaxy.

HST-FGS parallaxes of two high-velocity stars.

view Abstract Citations (6) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS HST-FGS parallaxes of two high-velocity stars. MacConnell, D. Jack ; Osborn, Wayne H. ; Miller, Richard J. Abstract Astrometric position measurements with the Hubble Space Telescope's Fine Guidance System have been used to measure the trigonometric parallaxes of two high-velocity stars, R50 = G166-37 and W624 = G16-25. In both cases, the parallaxes are small and confirm large velocities in the galactocentric rest frame. We derive pi_ {abs} = 5.2 +/- 0.7 mas and vrf = 467 kms for R50. The results for W624 are pi_ {abs} = 3.8 +/- 1.0 and vrf = 326 kms with an uncertainty of about 30%. Our results support a value for the galactic escape velocity at the solar distance of about 475 kms. Publication: The Astronomical Journal Pub Date: September 1997 DOI: 10.1086/118560 Bibcode: 1997AJ....114.1268M Keywords: ASTROMETRY full text sources ADS | data products SIMBAD (11) MAST (1) ESA (1)

Galactic kinematics of Cepheids from Hipparcos proper motions

The Hipparcos proper motions of 220 galactic Cepheids, together with relevant ground-based photometry, have been analysed. The effects of galactic rotation are very clearly seen. Mean values of the Oort constants, A = 14.82 +/- 0.84 km/s/kpc, and B = -12.37 +/- 0.64 km/s/kpc, and of the angular velocity of circular rotation at the Sun, Omega_0 = 27.19 +/- 0.87 km/s/kpc, are derived. Comparison of the value of A with values derived from recent radial velocity solutions confirm, within the errors, the zero-points of the PL and PLC relations derived directly from the Hipparcos trigonometrical parallaxes of the same stars. The proper motion results suggest that the galactic rotation curve is declining slowly at the solar distance from the galactic centre (dTheta/dR = -2.4 +/- 1.2 km/s/kpc). The component of the solar motion towards the North Galactic Pole is found to be +7.61 +/- 0.64 km/s. Based on the increased distance scale deduced in the present paper the distance to the galactic centre derived in a previous radial velocity study is increased to R_0 = 8.5 +/- 0.5 kpc.

Antiproton Catalyzed Fusion Propulsion for Interplanetary Missions

Interplanetary trips using chemical propellants require years to complete. A recently completed study on an antiproton catalyzed fusion reaction propulsion system has shown that the specie c impulses that can be obtained are between 1500 s for a contained system to over 100,000 s for a system that directly uses the fusion reaction products. Thrust-to-weight ratios exceeding 1 can be sustained. This allows considerably shorter solar system travel times than conventional chemical propellants. Missions considered range from inner to outer solar system distances. A tradeoff can be made between reducing travel time and reducing initial mass in low Earth orbit. Missions to the inner planets can be shortened considerably for a given mass ratio, whereas missions to the outermost planets will be several weeks in duration.

Measurement of the propagation speed of plasma inhomogeneities in the solar corona using an uplink/downlink cross‐correlation method

Radio sounding investigations were performed over a period of about 30 days during the 1991 solar occultation of the Ulysses spacecraft (August and September 1991). Dual‐frequency Doppler and ranging measurements were obtained using the NASA Deep Space Network. For this experiment the radio ray path from Ulysses to Earth moved essentially parallel to the solar equator, sounding the circumsolar plasma on the east (ingress phase) and west (egress phase) solar limbs at solar offset distances from about 4 to 40 RS (solar radii). Dual‐frequency, two‐way Doppler data were used to determine the propagation speed of coronal plasma inhomogeneities by cross‐correlation analysis between the uplink and downlink ray paths. The motion of Earth and spacecraft with respect to Sun results in a spatial separation (typically 20,000 km) between the uplink and downlink ray path during the round‐trip travel time of the signal. The specific configuration of the Ulysses radio system (dual‐frequency S/X band downlinks coherently locked to a S band uplink) provides the means for separating the uplink and downlink plasma contributions. The propagation speed of plasma inhomogeneities intersecting both ray paths can be determined by computing the cross‐correlation of the Doppler data from both ray paths as a function of time lag. The time lag of maximum cross‐correlation was used to calculate the propagation speed of plasma inhomogeneities. By evaluating the Ulysses data using this uplink and downlink cross‐correlation method, significant cross‐correlation peaks were obtained in the range of 7 to 34 RS. A high symmetry of the propagation speeds for both sides of the solar limb was found. An increase in propagation speed with increasing solar distance can be seen, starting from about 50 km/s at a distance of 7 RS up to 390 km/s at a distance of 29.7 RS (east) and from about 120 km/s at 9.8 RS up to 415 km/s at 33.8 RS (west). A significant enhancement (550–660 km/s) was observed at a distance of about 18 RS on both sides of the solar limb due to a solar event still being investigated.

Anomalous He Acceleration, the Particle Source and the Transport Coefficient

Anomalous He+component acceleration at the heliospheric termination shock is modelled numerically via a steady-state solution to the combined cosmic-ray transport equation and shock boundary condition via a matrix inversion technique. This numerical solution automatically provides a no-drift modulation solution for the He+. Consistency with experimental data on the anomalous component is obtained for injection at 10 keV nucl-1, at 120 AU, with a distribution function f(v)=2.75 × 10-24 m-6 s3 and a radial diffusion coefficient k=2.24× 1022 cm2 s-1 at 20 AU and for 100 MeV nucl-1 particles but which varied proportional to vγrδ where v and r are, respectively, particle velocity and solar distance,γ=1.3 and δ=0.5. However, a range of values of (γ,δ) between (1,0) and (2.4,1.4) were found to yield acceptable fits to the data. Pre-acceleration of ionised He at CIRs is possible as a source, although there is sufficient quiet-solar-wind-associated He+ for the required injection flux and the constraints on the injection efficiency are less at the terminal shock. These conclusions are insensitive to the terminal shock position and to the value of the injection energy.

TWO-FLUID MODEL FOR HEATING OF THE SOLAR CORONA AND ACCELERATION OF THE SOLAR WIND BY HIGH-FREQUENCY ALFVÉN WAVES

A model of the solar corona and wind is developed which includes for the first time the heating and acceleration effects of high-frequency Alfven waves in the frequency range between 1 Hz and 1 kHz. The waves are assumed to be generated by the small-scale magnetic activity in the chromospheric network. The wave dissipation near the gyro-frequency, which decreases with increasing solar distance, leads to strong coronal heating. The resulting heating function is different from other artificial heating functions used in previous model calculations. The associated thermal pressure-gradient force and wave pressure-gradient force together can accelerate the wind to high velocities, such as those observed by Helios and Ulysses. Classical Coulomb heat conduction is also considered and turns out to play a role in shaping the temperature profiles of the heated protons. The time-dependent two-fluid (electrons and protons) model equations and the time-dependent wave-spectrum equation are numerically integrated versus solar distance out to about 0.3 AU. The solutions finally converge and settle on time-stationary profiles which are discussed in detail. The model computations can be made to fit the observed density profiles of a polar coronal hole and polar plume with the sonic point occurring at 2.4 R⊙ and 3.2 R⊙, respectively. The solar wind speeds obtained at 63 R⊙ are 740 km s-1 and 540 km s-1; the mass flux is 2.1 and 2.2 × 108 cm-2 s-1 (normalized to 1 AU), respectively. The proton temperature increases from a value of 4 × 105 K at the lower boundary to 2 × 106 K in the corona near 2 R⊙.

Signal degradation of the AVHRR shortwave channels of NOAA 11 and NOAA 14 by daily monitoring of desert targets

The only indication of sensor degradation affecting the shortwave channels of the AVHRR is the changing response of targets which are assumed to be invariant in time. Using daily data of the whole lifetime of NOAA 11 three different long term trends in the signal response could be identified and quantitatively separated: real sensor degradation, atmospheric influence caused by the changing local time of observation, and surface anisotropy by increasing solar zenith distance. Together they cause the decrease of the measured signal and should be called effective signal degradation. If atmospheric corrections are applied to get surface reflectances it is important to avoid duplicate corrections that would lead to long term trends caused by changing atmospheric attenuation. The degradation coefficients were calculated by postulating a constant NDVI of the desert area. Corrections for surface ansisotropy to the time series of the reflectances were applied. Thus a precise quantitative inter-calibration of the NOAA 14 to the NOAA 11 time series could be performed. The resulting degradation coefficients can be updated and applied in real time.

Origin of C+ ions in the heliosphere

C+ pickup ions were investigated with the SWICS instrument along the trajectory of Ulysses, covering a broad range of solar latitude and distance. Whereas nearly all the observed H+, He+, N+, O+ and Ne+ pickup ions are created from the interstellar gas penetrating deep into the heliosphere, C+ comes primarily from an “inner source” which is located at a solar distance below a few AU and extends over all heliospheric latitudes investigated up to now. We present evidence that the C+ originates from carbon compounds evaporating from interstellar grains. This inner source also produces some O+ and N+ with estimated relative abundances ofC+/O+ ∼ 1 and N+/O+ ∼ 0.2. However, the total amount of O+ and N+ produced by this inner source is only of the order of 10−3 as compared to the total production of O+ and N+ from the interstellar gas in the heliosphere, respectively. Thus the inner source does not significantly contribute to oxygen or nitrogen in the anomalous cosmic rays (ACR) but its contribution to ACR-carbon may not be negligible.

Ulysses observations of differential alpha‐proton streaming in the solar wind

Data from the solar wind spectrometer on the Ulysses spacecraft are used to study the differential streaming between the alpha particles and protons in the solar wind over the heliographic distance range of 1.3 to 5.4 AU and latitudes from 0° to ±80° during the period December 1990 through September 1995. The study is based on 6‐hour averages of the parameter Vαp = |Vα ‐ Vp| where Vα and Vp are the vector velocities of the alpha particles and protons, respectively. It is found that Vαp decreases with increasing distance from the Sun and with decreasing solar wind speed. The distance and velocity dependencies can be combined into a single dependence on travel time Tfrom the Sun to the point of observation, with Vαp declining, on the average, as T−0.70±0.07. After normalization by this travel time factor, there is no residual dependence of Vαp, on heliographic latitude thus ruling out any rotational effects on either the acceleration or deceleration of the alphas relative to the protons. There is also no significant difference in the normalized values of Vαp between quasi‐stationary and transient (coronal mass ejection) flows. The ratios Vαp/VA, where VA is the Alfvén speed, and Vαp/Vwave, where Vwave is the observed propagation speed of Alfvénic fluctuations, both decline with increasing distance from the Sun, but Vαp/Vwave remains in the range of 1.0 to 1.5 out to a travel time of 5 or 10 days. There are weak correlations between the normalized value of Vαp and the amplitudes of fluctuations in both the magnitude and the direction of the interplanetary magnetic field. Although Vαp anticorrelates strongly with the ratio of the Coulomb collision time to the solar wind expansion time, it is believed that the correlation is not evidence of a cause and effect relation between those two parameters over much of the solar wind regime observed by Ulysses. Where comparisons are possible, the Ulysses data closely agree with extrapolations of the Helios data to greater solar distances.

Differential flow between solar wind protons and alpha particles: First WIND observations

Alpha particle and proton measurements in the solar wind made using the SWE Faraday cup detectors on the WIND spacecraft are reported. Some overall trends observed confirm past observations: the ratios of alpha particle to proton density Nα/NP, thermal speed Wα/WP, as well as the differential velocity Vα‐VP (hereafter VαP) are generally correlated with bulk solar wind flow speed. The detailed WIND measurements enable us to investigate instances when the alpha‐proton differences deviate from these overall general trends. Occasionally, difference velocities as large as 80 km/s were seen, with the ratio of |VαP| to the Alfvén speed VA near unity, characteristics more typical of observations at solar distances less than 1 AU. An example is presented where |Vα|‐|VP| reverses sign while |VαP| stays nearly constant. Comparison of the vector velocities and the magnetic field suggests that the speed reversal is associated with a localized kink in the magnetic field. Finally we show an instance where |VαP| exceeds the observed wave speed for Alfvénic fluctuations (Vwave=B0ΔV/ΔB) resulting in alpha particle velocity fluctuations that anti‐correlate with the wave. Though this phenomenon has been previously reported in high‐latitude measurements beyond 1 AU, it is shown here to also occur at 1 AU in the ecliptic.

Anisotropic Angular Broadening in the Solar Wind

AbstractWe present Very Large Array observations at wavelengths of 2, 3.5, 6, and 20 cm, of angular broadening of radio sources due to the solar wind in the region 2-16 solar radii. Angular broadening is anisotropic with axial ratios in the range 2-16. Larger axial ratios are observed preferentially at smaller solar distances. Assuming that anisotropy is due to scattering blobs elongated along magnetic field lines, the distribution of position angles of the elliptically broadened images indicates that the field lines are non-radial even at the largest heliocentric distances observed here. At 5R⊙, the major axis scattering angle is ~ 0.7" at λ = 6 cm and it varies with heliocentric distance as R−1.6. The level of turbulence, characterized by the wave structure function at a scale of 10 km along the major axis, normalized to λ = 20 cm, has a value 20±7 at 5R⊙ and varies with heliocentric distance as R−3. Comprison with earlier results suggest that the level of turbulence is higher during solar maximum. Assuming a power-law spectrum of electron density fluctuations, the fitted spectral exponents have values in the range 2.8–3.4 for scales sizes between 2–35 km. The data suggests temporal fluctuations (of up to 10%) in the spectral exponent on a time scale of a few tens of minutes. The observed structure functions at different solar distances do not show any evidence for an inner scale; the upper limits are 1 km at 2R⊙ and 4 km at 13R⊙. These upper limits are in conflict with earlier determinations and may suggest a reduced inner scale during solar maximum.