Orbital and physical characteristics of micrometeoroids in the inner solar system as observed by Helios 1

The Helios 1 spacecraft was launched in December 1974 into a heliocentric orbit of 0.3 AU perihelion distance. Helios 2 followed one year later on a similar orbit. Both spaceprobes carry on board micrometeoroid experiments each of which contains two sensors with a total sensitive area of 121 cm². To date, only preliminary data are available from Helios 2. Therefore the results presented here mainly apply to data from Helios 1. The ecliptic sensor of Helios 1 measures dust particles which have trajectories with elevations from ?45° to + 55° with respect to the ecliptic plane. The south sensor detects dust particles with trajectory elevations from ?90° (ecliptic south-pole) to ?4°. The ecliptic sensor is covered by a thin film (3000 Å parylene coated with 750 Å aluminium) as protection against solar radiation. The other sensor is shielded by the spacecraft rim from direct sunlight and has an open aperture. Micrometeoroids are detected by the electric charge produced upon impact. During the first 6 orbits of Helios 1 around the sun the experiment registered a total of 168 meteoroids, 52 particles were detected by the ecliptic sensor and 116 particles by the south sensor. This excess of impacts on the south sensor with regard to the impacts on the ecliptic sensor is due predominantly to small impacts which are characterized by small pulse heights of the charge signals. But also large impacts were statistically significantly more abundant on the south sensor than on the ecliptic sensor. Most impacts on the ecliptic sensor were observed when it was pointing in the direction of motion of Helios (apex direction). In contrast to that the south sensor detected most impacts when it was facing in between the solar and antapex direction. Orbit analysis showed that the “apex” particles which are predominantly detected by the ecliptic sensor have eccentricities e < 0.4 or semi-major axes a ? 0.5 AU. From a comparison with corresponding data from the south sensor it is concluded that the average inclination f of “apex” particles is -i < 30°. The excess of impacts on the south sensor, called “eccentric” particles, have orbit eccentricities e > 0.4 and semimajor axes a > 0.5AU. β-meteoroids leaving the solar system on hyperbolic orbits are directly identified by the observed imbalance of outgoing (away from the sun) and ingoing particles. It is shown that “eccentric” particles, due to their orbital characteristics, should be observable also by the ecliptic sensor. Since they have not been detected by this sensor it is concluded that the only instrumental difference between both sensors, i.e. the entrance film in front of the ecliptic sensor, prevented them from entering it. A comparison with penetration studies proved that particles which do not penetrate the entrance film must have bulk densities ρ(g/cm³) below an upper density limit ρ_max. It is shown that approximately 30% of the “eccentric” particles have densities below ρ_max = 1 g/cm³.     

The definition of the ecliptic

The ecliptic as a mean orbital plane of the Sun in Le Verrier's theory is a mean orbital plane determined from the secular parts of the longitude of the ascending node and the inclination of the Sun with respect to a reference plane. On the other hand, the ecliptic in Newcomb's theory is so chosen that the latitude with respect to his ecliptic does not have cosg nor sing whereg is the mean anomaly of the Sun. The two definitions are really different in spite of their apparent similarity. Standish (1981) defined the ecliptic from a kinematical point of view, and it is shown that the ecliptic defined by Standish (in the rotating sense) does coincide with the ecliptic defined by Newcomb.     

Photometric lightcurves and pole determination of 433 Eros

Fourteen photometric lightcurves of 433 Eros were made at the Astronomical Observatory of Torino during the 1974–75 close passage. The absolute magnitude of the primary maximum (10^m78), the phase coefficient (0.023 mag/degree), the synodic and sidereal period of rotation (0^d.21956 and 0^d.21959, respectively) and the ecliptic coordinates of the pole (λ = 17°, β = 10°) were deduced.     

Counterglow from the Earth-Moon libration points

Results from the OSO-6 Rutgers Zodiacal Light Analyzer experiment show photometric perturbations above the background in the anti-Sun line of sight. Sixteen successive lunations were examined, and the accumulated perturbations show a maximum value in the direction of the L₄ and L₅ Earth-Moon libration points. This is interpreted as a counterglow from a cloud of particles at the libration points. The average brightness of these libration clouds is 20 S₁₀ Vis. The average angular size of the libration clouds is approximately 6 degrees. Their position varies from one lunation to the next, within an ellipsoidal zone centered on the libration point direction, with its semi-major axis, of approximately 6 degrees, nominally in the ecliptic and its semi-minor axis, of approximately 2 degrees perpendicular to the ecliptic. The position of these clouds with respect to the Lagrangian L₄ and L₅ points, is towards the Moon in the northern summer and away from the Moon in the northern winter.     

The three-dimensional shape of the magnetopause

Nearly 1000 magnetopause crossings from HEOS-2, HEOS-1, OGO-5 and 5 IMP space-craft covering most of the northern and part of the southern dayside and near-Earth tail magnetopause (X >?15 R_E) have been used to perform a detailed study of the three-dimensional shape and location of the magnetopause. The long-term influence of the solar wind conditions on the average magnetopause geometry has been reduced by normalising the radial distances of the observed magnetopause crossings to an average dynamical solar wind pressure. Best-fit ellipsoids have been obtained to represent the average magnetopause surface in geocentric solar ecliptic (GSE) and (as a function of tilt angle) in solar magnetic (SM) coordinates. Average geocentric distances to the magnetopause for the 1972–1973 solar wind conditions (density 9.4 cm^?3, velocity 450 km s^?1) are 8.8 R_E in the sunward direction, 14.7 R_E in the dusk direction, 13.4 R_E in the dawn direction and 13.7 R_E in the direction normal to the ecliptic plane. The magnetopause surface is tilted by 6.6° ± 2° in a direction consistent with that expected from the aberration effect of the radial solar wind. Our data suggest that the solar wind plasma density and the interplanetary magnetic field (IMF) orientation affect the distance to the polar magnetopause, larger distances corresponding to higher plasma density and southward fields. Our best-fit magnetopause surface shows larger geocentric distances than predicted by the model of Choe et al. [Planet Space Sci. 21, 485 (1973).] normalised to the same solar wind pressure.     

Three years of Ulysses dust data: 1993–1995

The Ulysses spacecraft is orbiting the Sun on a highly inclined ellipse (i = 79°). After its Jupiter flyby in 1992 at a heliocentric distance of 5.4 AU, the spacecraftreapproached the inner solar system, flew over the Suns south polar region in September 1994,crossed the ecliptic plane at a distance of 1.3 AU in March 1995, and flew over the Suns northpolar region in July 1995. We report on dust impact data obtained with the dust detector onboardUlysses between January 1993 and December 1995. We publish and analyse the complete dataset of 509 recorded impacts of dust particles with masses between 10⁻¹⁶ g–10⁻⁷ g. Together with 968 dust impacts from launch until the end of 1992 published earlier (Grün et al., 1995c), information about 1477 particles detected with theUlysses sensor between October 1990 and December 1995 is now available. The impact ratemeasured between 1993 and 1995 stayed relatively constant at about 0.4 impacts per day andvaried by less than a factor of ten. Most of the impacts recorded outside about 3.5 AU arecompatible with particles of interstellar origin. Two populations of interplanetary particles havebeen recognized: big micrometer-sized particles close to the ecliptic plane and smallsub-micrometer-sized particles at high ecliptic latitudes. The observed impact rate is comparedwith a model for the flux of interstellar dust particles which gives relatively good agreement withthe observed impact rate. No change in the instruments noise characteristics or degradation of thechanneltron could be revealed during the three-year period.     

Radial velocity discriminated coronal photometric measurements at the July 11, 1991 total eclipse

The results from a set of 12 solar corona radial velocity measurements in the 400-440 nm spectral band during the total solar eclipse of July 11, 1991 are reported. The measurements show that the orbital motion of the F-corona material near the sun in the ecliptic plane is consistent with Keplerian motion and predominantly, but not exclusively, prograde, as is usually assumed. This work demonstrates a method of using the measured radial velocities to sort out the relative amounts of K-corona, near-earth F-corona, near-solar F-corona, and scattered light in each measurement for each observation point W and E of the sun between 2.5R_{o(solar radii)} and 5R_o along the celestial equator and at three points north of the sun. The near-solar F-corona component is quite weak, contributing only 7-14% of the total signal in each case. The stronger diffraction component from near-earth F-corona is estimated to have been produced by particles with radii of about 11μ. In contrast, the scattered light component appears as strong zero-velocity features dominating all the measurements. The measurements W and E of the sun and near the ecliptic plane also show evidence of a red-shift velocity of at least 330 km s⁻¹, suggestive of a high-speed dust outflow from the sun.     

On the analytic lunar and solar perturbations of a near Earth satellite

The disturbing function of the Moon (Sun) is expanded as a sum of products of two harmonic functions, one depending on the position of the satellite and the other on the position of the Moon (Sun). The harmonic functions depending on the position of the perturbing body are developed into trigonometric series with the ecliptic elementsl, l′, F, D and Γ of the lunar theory which are nearly linear with respect to time. Perturbation of elements are in the form of trigonometric series with the ecliptic lunar elements and the equatorial elements ω and Ω of the satellite so that analytic integration is simple and the results accurate over a long period of time.     

Low-Resolution STELab IPS 3D Reconstructions of the Whole Heliosphere Interval and Comparison with in-Ecliptic Solar Wind Measurements from STEREO and Wind Instrumentation

We present initial 3D tomographic reconstructions of the inner heliosphere during the Whole Heliosphere Interval (WHI) – Carrington Rotation 2068 (CR2068) – using Solar-Terrestrial Environment Laboratory (STELab) Interplanetary Scintillation (IPS) observations. Such observations have been used for over a decade to visualise and investigate the structure of the solar wind and to study in detail its various features. These features include co-rotating structures as well as transient structures moving out from the Sun. We present global reconstructions of the structure of the inner heliosphere during this time, and compare density and radial velocity with multi-point in situ spacecraft measurements in the ecliptic; namely STEREO and Wind data, as the interplanetary medium passes over the spacecraft locations.     

Photometric properties of zodiacal light particles

From published ground-base, spacecraft, and rocket photometry and polarimetry of the zodiacal light, a number of optical and physical parameters have been derived. It was assumed that the number density, mean particle size, and albedo vary with heliocentric distance, and shown that average individual interplanetary particles have a small but definite opposition effect, a mean single-scattering albedo in the V band at 1-AU heliocentric distance of 0.09 ± 0.01, and a zero-phase geometric albedo of 0.04. Modeled by a power law, both albedos decrease with increasing heliocentric distance as r^?0.54. The corresponding exponents for changes in mean particle size and number density are related in a simple way. The median orbital inclination of zodiacal light particles with respect to the ecliptic is 12°, close to the observed median value for faint asteroids and short-period comets. Furthermore, the color of dust particles and its variation with solar phase angle closely resemble those of C asteroids. These findings are, at least, consistent with the zodiacal cloud originating primarily from collisions among asteroids. Finally, a value of ?10¹⁸?_ErmE g was derived for the mass of the zodiacal cloud, where ?_E is the mean particle radius (in micrometers) at 1-AU-heliocentric distance. For extinction in the ecliptic, $\text{Δm = 10}{}^{\mathrm{?5}}\text{?}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{mag}$ was obtained, where ? is the solar elongation in degrees.     

Constraining the Kinematics of Coronal Mass Ejections in the Inner Heliosphere with In-Situ Signatures

We present a new approach to combine remote observations and in-situ data by STEREO/HI and Wind, respectively, to derive the kinematics and propagation directions of interplanetary coronal mass ejections (ICMEs). We use two methods, Fixed-? (F?) and Harmonic Mean (HM), to convert ICME elongations into distance, and constrain the ICME direction such that the ICME distance–time and velocity–time profiles are most consistent with in-situ measurements of the arrival time and velocity. The derived velocity–time functions from the Sun to 1?AU for the three events under study (1?–?6 June 2008, 13?–?18 February 2009, 3?–?5 April 2010) do not show strong differences for the two extreme geometrical assumptions of a wide ICME with a circular front (HM) or an ICME of small spatial extent in the ecliptic (F?). Due to the geometrical assumptions, HM delivers the propagation direction further away from the observing spacecraft with a mean difference of ≈?25°.     

Statistical Survey of Type III Radio Bursts at Long Wavelengths Observed by the Solar TErrestrial RElations Observatory (STEREO)/Waves Instruments: Goniopolarimetric Properties and Radio Source Locations

We have performed a statistical analysis of a large number of Type III radio bursts observed by STEREO between May 2007 and February 2013. Only intense, simple, and isolated cases have been included in our data set. We focused on the goniopolarimetric (GP, also referred to as direction-finding) properties at frequencies between 125 kHz and 2 MHz. The apparent source size γ is very extended (≈?60^°) for the lowest analyzed frequencies. Observed apparent source sizes γ expand linearly with a radial distance from the Sun at frequencies below 1 MHz. We show that Type III radio bursts statistically propagate in the ecliptic plane. The calculated positions of radio sources indicate that scattering of the primary beam pattern plays an important role in the propagation of Type III radio bursts in the interplanetary medium.     

The Spin-Orbit Resonant Rotation of Mercury: A Two Degree of Freedom Hamiltonian Model

The paper develops a hamiltonian formulation describing the coupled orbital and spin motions of a rigid Mercury rotation about its axis of maximum moment of inertia in the frame of a 3:2 spin orbit resonance; the (ecliptic) obliquity is not constant, the gravitational potential of mercury is developed up to the second degree terms (the only ones for which an approximate numerical value can be given) and is reduced to a two degree of freedom model in the absence of planetary perturbations. Four equilibria can be calculated, corresponding to four different values of the (ecliptic) obliquity. The present situation of Mercury corresponds to one of them, which is proved to be stable. We introduce action-angle variables in the neighborhood of this stable equilibrium, by several successive canonical transformations, so to get two constant frequencies, the first one for the free spin-orbit libration, the other one for the 1:1 resonant precession of both nodes (orbital and rotational) on the ecliptic plane. The numerical values obtained by this simplified model are in perfect agreement with those obtained by Rambaux and Bois [Astron. Astrophys. 413, 381–393]. This revised version was published online in July 2006 with corrections to the Cover Date.     

Helium abundance variations in the solar wind

Helium abundance variations in the solar wind have been studied using data obtained with Los Alamos plasma instrumentation on IMP 6, 7, and 8 from 1971 through 1978. For the first time, average flow characteristics have been determined as a function of helium abundance, A(He). Low and average values of A(He) are each preferentially identified with a different characteristic plasma ‘state’ these correspond to what have previously been recognized as the signatures of interplanetary magnetic field polarity reversals and high speed streams, respectively. Helium enhancements at 1 AU also can be identified with a characteristic plasma state, which includes high magnetic field intensity and low proton temperature. This is further evidence that such enhancements are a signal of coronal transient mass ejections. Long-term averages of A(He) at least partially reflect the relative frequency with which coronal streamers, holes, and transients extend their influence into the ecliptic plane at 1 AU. As a result, there is a real and pronounced solar cycle variation of solar wind H(He).     

Photoelectric photometry of 21 asteroids

Photoelectric lightcurves of 21 asteroids are presented. The observations were carried out from 1978 to 1982 at the Astronomical Observatory of Torino (at the Astrophysical Observatory of Catania for 137 Meliboea). For 10 objects a reliable rotation period has been obtained, while for two others a rough estimate is given. In several cases the analysis of the observed amplitudes versus the ecliptic longitudes indicates the most favorable future oppositions for period and/or pole determination. For some asteroids transformations to UBV Standard System were performed.     

Phase function and polarization curve of interplanetary scatterers from zodiacal light photopolarimetry

Wide-angle ecliptic measurements of zodiacal light brightness (Z) and polarization (P) lead to fundamental results about optical properties of interplanetary scatterers, under a few reasonable assumptions (that they depend upon heliocentric distance by a r^?n law, and suffer no significant distortion of their scattering indicatrix between 0.5 and 2 a.u.): 1. The phase function σ(θ) is expressed (Equation 6) as a function of n and of (Z) data. 2. At the elongation ? = 90°, the derivative $\text{dZ}\text{d?}$ yields an absolute determination of the intensity $\text{T}$ scattered at right angles from the Sun by a single unit-volume of interplanetary medium (Equation 7). 3. The polarization degree $\text{P}$(θ) of the sunlight scattered by a single volume is derived (Equation 12) from n and from (Z + P) data. For two special values of the scattering angle θ, n vanishes in Equation (12), so that a fair knowledge of the polarization curve (Fig. 2) is reached prior to any assumption, or any forthcoming Jupiter-probe measure, about the value of n.Should n be provided by the Pioneers, then a thorough treatment of the whole problem of phase function and polarization curve can be performed by means of Equations (6) and (12) supplied with available zodiacal light photopolarimetric observations.     

The effect of the Earth’s oblateness on the Moon’s physical libration in latitude

The Moon’s physical libration in latitude generated by gravitational forces caused by the Earth’s oblateness has been examined by a vector analytical method. Libration oscillations are described by a close set of five linear inhomogeneous differential equations, the dispersion equation has five roots, one of which is zero. A complete solution is obtained. It is revealed that the Earth’s oblateness: a) has little effect on the instantaneous axis of Moon’s rotation, but causes an oscillatory rotation of the body of the Moon with an amplitude of 0.072″ and pulsation period of 16.88 Julian years; b) causes small nutations of poles of the orbit and of the ecliptic along tight spirals, which occupy a disk with a cut in a center and with radius of 0.072″. Perturbations caused by the spherical Earth generate: a) physical librations in latitude with an amplitude of 34.275″; b) nutational motion for centers of small spiral nutations of orbit (ecliptic) pole over ellipses with semi-major axes of 113.850″ (85.158″) and the first pole rotates round the second one along a circle with radius of 28.691″; c) nutation of the Moon’s celestial pole over an ellipse with a semi-major axis of 45.04″ and with an axes ratio of about 0.004 with a period of T = 27.212 days. The principal ellipse’s axis is directed tangentially with respect to the precession circumference, along which the celestial pole moves nonuniformly nearly in one dimension. In contrast to the accepted concept, the latitude does not change while the Moon’s poles of rotation move. The dynamical reason for the inclination of the Moon’s mean equator with respect to the ecliptic is oblateness of the body of the Moon.     

Physical properties of asteroid 433 Eros

Newly available photometric, polarimetric, spectroscopic, thermal-radiometric, radar, and occultation results are synthesized in order to derive a coherent model for Eros. The geometric albedo is 0.19±0.01 at the visual wavelength, and the overall dimensions are approximately 13 × 15 × 36km. The rotation is about the short axis, in the direct sense, with a sidereal period of 5^h16^m13^s.4. The pole of rotation lies within a few degrees of ecliptic coordinates λ = 16° and β = + 11°.Eros is uniformly coated with a particulate surface layer several millimeters thick. It has an iron-bearing silicate composition, similar to that of a minority of main-belt asteroids, and probably identifiable with H-type ordinary chondrites.     

Speckle interferometry of asteroids II. 532 Herculina

Speckle interferometry of 532 Herculina performed on January 17 and 18, 1982, yields triaxial ellipsoid dimensions of (263 ± 14) × (218 ± 12) × (215 ± 12) km, and a north pole for the asteroid within 7° of RA = 7^b47^m and DEC = ?39° (ecliptic coordinates γ = 132° β = ?59°). In addition, a “spot” some 75% brighter than the rest of the asteroid is inferred from both speckle observations and Herculina's lightcurve history. This bright complex, centered at asterocentric latitude ?35°, longitude 145–165°, extends over a diameter of 55° (115 km) of the asteroid's surface. No evidence for a satellite is found from the speckle observations, which leads to an upper limit of 50 km for the diameter of any satellite with an albedo the same as or higher than Herculina.     

Pickup interstellar helium ions in the region of the solar gravitational cone

Our numerical analyses of the velocity and spatial distributions of pickup interstellar helium ions in the region of the solar gravitational cone in the ecliptic plane at a distance of 1 AU show that the ion density maximum must be displaced relative to the neutral helium cone axis in the direction of the Earth’s revolution around the Sun. The solar wind parameters in the numerical model correspond to their observed values during the crossing of the helium cone by the ACE spacecraft in 1998. At these parameters, the calculated angular displacement is 5°. The absence of a similar displacement in the ACE measurements is shown to stem from the fact that the spectrometer onboard ACE records and identifies only a fraction of the pickup helium ions with fairly high magnitudes and certain directions of the velocities.