Micrometeoroids within ten Earth radii

The micrometeoroid population within 10 Earth radii (60,000 km) has been observed by the HEOS 2 dust experiment between 7 February 1972 and 2 August 1974.A total of 431 particles has been observed. Of those 90 particles are classified as random, the rest as particle bursts. The random particles only show a slight increase (factor 3) in flux within 10 Earth radii, compared to the deep space flux at 1 AU and this is interpreted as being due to the gravitational field of the Earth.The bursts are divided into groups and swarms according to their time profiles. The 19 recorded groups are observed both within 10 Earth radii and above, again with a slight increase below 60,000 km. The 15 recorded swarms are exclusively observed within 10 Earth radii. The total micrometeoroid flux in this region is enhanced by 2–3 orders of magnitude. The interpretation is that larger bodies in the 10–10⁶g mass range of the type III fireballs are disintegrating while travelling through the Earth's auroral zones. The fragmentation process proposed is that of electrostatic disruption. This leads to one (or several) swarm(s) of small individual particles, which originally made up the flurry type (cometary) parent body.     

Micrometeoroid impact crater statistics at the boundary of Earth's gravitational sphere of influence

We surveyed craters on a space-exposed surface from the Genesis solar wind sample return mission to find new constraints on the population of micrometeoroids at the edge of the Earth's gravitational sphere of influence. The target was made of 6061-T6 aluminum, identical to the composition of the space-facing end of the Long Duration Exposure Facility satellite, which recorded micrometeoroid impacts in low Earth orbit. We use data from both locations to compare crater frequency as a function of size, with and without gravitational focussing by the Earth. We find that the cratering flux near the Earth-Sun L1 libration point is indistinguishable, within the ∼40% uncertainty of this study, from that in low Earth orbit. The small degree of gravitational focussing between the two locations indicates that particles with geocentric free-space velocities less than a few kilometers per second comprise no more than a few percent of the interplanetary dust complex.     

Impact trajectories of the asteroid Apophis in the 21st century

The asteroid Apophis is one of the most hazardous near-Earth asteroids. As a result of the scattering of Apophis?? potential trajectories after its close approach in 2029, and its possible approach in 2036, there are many dangerous trajectories including impact trajectories after 2036. The purpose of this study is to identify and investigate these trajectories. We use the Everhart integrator; the DE405, DE423, and EPM2008 ephemerides; and two sets of initial data for Apophis (those collected by NASA in 2006 and by the IAA in 2010). More than 50 possible encounters in this century are presented, including 13 encounters between 2036 and 2050. The minimum geocentric distances obtained using a different ephemeris and initial conditions differ little between themselves. Analogous results in (Yeomans et al., 2009) are consistent with our results.     

Lunar microcraters: Implications for the micrometeoroid complex

The contributions of lunar microcrater studies to understand the overall micrometeoroid environment are summarized and compared to satellite data.In comparison with small-scale laboratory studies, most lunar crater morphologies are compatible only with impact velocities > 3·5 km/sec and projectile densities between 1–8 g/cm³; a mean value is most likely 2–4 g/cm³. The particles arenon-porous and fairly equi-dimensional; needles, platelets, rods, whiskers and other highly asymmetric particle shapes can be excluded. Data on projectile chemistry is sparse and non-diagnostic at present.The crater diameters are converted into projectile masses via small scale laboratory impact experiments. Accordingly, the observed span of crater pit diameters (0·1 μm–1 cm) corresponds to a particle mass range of ≈ 10^?15–10^?3 g. This large, dynamic detection range is a unique feature of the lunar rock detector. Absolute crater densities on different rocks vary from “production” to “equilibrium” conditions. After normalization of such densities, relative microcrater size frequencies are obtained to deduce a mass frequency distribution for particles 10^?15–10^?3 g. There is evidence that this distribution is bimodal. A radiation pressure cutoff at 10^?12 g particle mass does not exist. The micrometeoroid flux obtained from lunar rocks is compatible with satellite data. There is indication that the micrometeoroid flux may have been lower in the past. Some speculative astronomical consequences concerning the origin of micrometeoroids are discussed.     

Multi-Spacecraft Study of the 21 January 2005 ICME

We examine the near-Earth Interplanetary Coronal Mass Ejection (ICME) apparently related to the intense Solar Energetic Particle (SEP) event of 20 January 2005. Our purpose is to contribute to the understanding of the macroscopic structure, evolution and dynamics of the solar corona and heliosphere. Using Cluster, ACE and Wind data in the solar wind, and Geotail data in the magnetosheath, we perform a multi-spacecraft analysis of the ICME-driven shock, post-shock magnetic discontinuities and ejecta. Traversals by the well-separated near-Earth spacecraft provide a coherent picture of the ICME geometry. Following the shock, the ICME sequence starts with a hot pileup, i.e.,? a sheath, followed by a fast ejecta characterised by a non-compressive density enhancement (NCDE), which is caused essentially by an enrichment in helium. The plasma and magnetic observations of the ejecta are consistent with the outskirts of a structure in strong expansion, consisting of nested magnetic loops still connected to the Sun. Within the leading edge of the ejecta, we establish the presence of a tilted current sheet substructure. An analysis of the observations suggests that the tilted current sheet is draped within the overlying cloud canopy, ahead of a magnetic cloud-like structure. The flux rope interpretation of this structure near L1, confirmed by observations of the corresponding magnetic cloud, provided by Ulysses at 5.3 AU and away from the Sun?–?Earth line, indicates that the bulk of the cloud is in the northwest sector as seen from the Earth, with its axis nearly perpendicular to the ecliptic. This is consistent with the primary direction of travel of the fast halo CME observed at the Sun. Moreover, the NCDE and helium enrichment are consistent with the position near the streamer belt of the flaring active region NOAA 10720 associated with the CME. However, differences between interplanetary and solar observations indicate a large rotation of the erupting filament and overlying arcade, which can be attributed to the flux rope being subject to the helical kink instability.     

Relativistic effects for near-earth satellite orbit determination

The relativistic formulations for the equations which describe the motion of a near-Earth satellite are compared for two commonly used coordinate reference systems (RS). The discussion describes the transformation between the solar system barycentric RS and both the non-inertial and inertial geocentric RSs. A relativistic correction for the Earth's geopotential expressed in the solar system barycentric RS and the effect of geodesic precession on the satellite orbit in the geocentric RS are derived in detail. The effect of the definition of coordinate time on scale is also examined. A long-arc solution using 3 years of laser range measurements of the motion of the Lageos satellite is used to demonstrate that the effects of relativity formulated in the geocentric RS and in the solar system barycentric RS are equivalent to a high degree of accuracy.     

The Near-Earth Asteroid Size-Frequency Distribution: A Snapshot of the Lunar Impactor Size-Frequency Distribution

It is shown that the size-frequency distribution (SFD) of a time-averaged projectile population derived from the lunar crater SFD of Neukum and Ivanov (in Hazards Due to Comets and Asteroids (T. Gehrels, Ed.), 1994, pp. 359-416, Univ. of Arizona Press, Tucson) provides a convincing fit to the SFD of the current near-Earth asteroid (NEA) population, as deduced from the results of asteroid search programs. Our results suggest that the shape of the SFD of the impactor flux has remained in a steady state since the late heavy bombardment, so that the current NEA population can be viewed as a snapshot of the flux of impactors on the Moon. The number of bodies in the projectile population with diameters of 1 km or more is 700±130, which is in good agreement with recent estimates of the total number of NEAs in this size range. Our results imply that the contribution to the projectile flux from comets is small for diameters below 10 km.     

Temporal fluctuations and anisotropy of the micrometeoroid flux in the Earth-Moon system measured by HEOS 2

The HEOS detector measures the mass and speed of micrometeoroids in the Earth-Moon system. They are detected by the plasma produced by particle impacts on the sensor. During 2 yr of data collection 384 particles have been registered. As shown earlier (COSPAR 1973), they can be divided into 3 categories according to their temporal distribution: particles that are (1) randomly distributed or (2) appear in “groups” or (3) appear in “swarms” In this paper the origin of the groups and swarms is discussed. For this purpose the article orbits with respect to the Earth and the Moon were traced back. The results imply a lunar origin of the groups, whereas the swarms are correlated with the vicinity of the Earth. In addition, the dependence of the cumulative flux upon the detector's viewing direction indicates clearly an anisotropic particle flux.     

脉冲星Geminga MeV 脉冲辐射证据

利用直接解调方法分析COMPTELVPI数据,得到了脉冲星Geminga10-30MeV脉冲辐射存在的证据：全位相及分位相成像均得到了Geminga．分位相成像结果显示,光变曲线的峰值位于pulse1附近,而其它位相区域只能给出上限．由于成像分析不存在选择效应,该成像结果表明,Geminga在MeV能区仍存在脉冲辐射．虽然Geminga的弱MeV辐射使得很难就VP1数据作时间分析,但仍得到了一些周期信号的迹象,并且与成像结果不相矛盾．进一步考察EGRET观测Geminga在30MeV以上的强度比Pulse2／Pulse1,发现在VP1观测的较低能区,Pulse1有渐强于Pulse2的趋势,这和我们在MeV能区的成像及时间分析结果相一致．成像得到的流强表明,Geminga的能谱至少在10MeV以上不应出现偏折．     

Charged dust in the outer planetary magnetospheres

In this paper we consider the dynamics of the electrostatic disruption products of fragile interplanetary dust aggregates which are initially electrically charged on entering the Jovian plasmasphere. On account of their large specific charges, these small dust fragments are strongly effected both by the Lorentz electric force as well as by the polarization electric force resulting from the corotation of the Jovian plasmasphere. The detailed orbits of these charged dust fragments, which are shown to be confined to the equatorial plane, are computed for various launch angles. It is established that the fragments with radii typically around 1 are magneto-gravitationally trapped within the plasma sphere due to the velocity induced oscillation of their surface potentials. The spatial distribution of these fragments are evaluated and the time evolution of the distributions followed. On this basis it is argued that the distribution of micrometeoroid dust within the Jovian magnetospheres, observed by the Pioneer 10 and the recent Voyager spacecraft, is a result of this magneto-gravitational trapping and subsequent orbital evolution of these charged dust fragments. Our discussion includes both the sudden increase, by over an order of magnitude, of the micrometeoroid dust flux at about 30R _J observed by Pioneer 10, and the thin inner dust ring recently observed by the Voyaer spacecraft. The observed brightness asymmetries between the leading and trailing sides of the Galilean satellites appears to be a natural consequence of the impact geometries of these charged dust grains with the satellite surfaces.     

The heliographic latitude dependence and sector structure of the interplanetary magnetic field 1969–1974: Results from the HEOS satellites

Data from the two HEOS satellites obtained during the period December 1968 to August 1974 are used to investigate the large-scale properties of the interplanetary magnetic field.The sector structure has been deduced from the observed times of sector boundary crossings which are tabulated. A two-sector pattern existed throughout most of the period with occasional intervals of 2–3 months duration in which four sectors appeared. The variation of the dominant sector polarity with heliographic latitude showed a reversal in sense during 1971 at the time of the reported reversal in the Sun's polar field. A statistical analysis of the change in polarity distribution with latitude suggests that at Earth's orbit the sector boundaries are inclined to the solar equator on average at an angle of 12 deg.No evidence was found in the HEOS measurements of the north-south field component to confirm the systematic latitude-dependent deviation of the plasma flow away from the solar equatorial plane suggested by several analyses of data from previous spacecraft. The mean field magnitude and the average amplitude of the directional fluctuations appeared to be independent of heliographic latitude within the ±7.3° range explored.     

Trajectory and orbit of the Tunguska meteorite revisited

Abstract— A critical survey is presented of all determinations of the azimuth and inclination of the Tunguska meteorite's trajectory based either on eyewitness testimonies or on the mathematical treatment of the forest-leveling field in the area of the catastrophe. The eyewitness testimonies collected in the neighborhood of the Nizhnyaya Tunguska River indicate the most probable azimuth of the trajectory projection to be 104° from the north to the east, which is close to the most recent azimuth estimate from the forest-leveling field, 99°. For the most part of the trajectory, its inclination could not exceed 15°. However, it is seen from aerodynamic calculations that the combined action of the gravity field and a nonzero aerodynamic lift could increase the inclination to 40° as the end of the trajectory was approached. Meteoroid orbits are calculated for a broad family of trajectories with azimuths ranging from 99° (Fast et al, 1976) to 137° (Krinov, 1949) and geocentric velocities ranging from 25 to 40 km/s. Orbits with large azimuth values (120° and larger) are shown to belong to the asteroidal type. They are succeeded by the orbits of short-period and long-period comets, whereas very small azimuth values and large geocentric velocities correspond to the region of hyperbolic orbits. Certain restrictions on the possible trajectory azimuths and geocentric velocities of the Tunguska body are imposed by this study.     

Ionospheric photoelectrons observed in the magnetosphere at distances up to 7 earth radii

Photoelectrons of ionospheric origin have been observed for the first time at high altitudes (up to 7R_E geocentric distance) using the suprathermal plasma analysers (SPA) on the GEOS satellites. At such high altitudes the photoelectron flux is confined within a few degrees of the magnetic field direction. We show how this flux may be identified and extracted from the background which is a combination of locally produced photoelectrons and ambient plasma. GEOS-2 results are presented to illustrate the “turn-on” of the photoelectron flux at dawn in the ionosphere. Data from GEOS-1 are used to study the behaviour of the photoelectron flux with equatorial geocentric distance from 3 to 7R_E. The results compare favourably with theoretical models and with ionospheric observations at mid latitudes.     

Electrostatic wave variety and the origins of BEN

Spiky pulses in broadband electrostatic “noise” (BEN) in the plasma sheet boundary layer (PSBL) observed by Geotail led to a model for BEN as Bernstein-Greene-Kruskal (BGK) modes, which has been explored by a number of simulations. A recent modified model proposed by Grabbe (Phys. Rev. Lett. 84 (2000a) 3614; Rec. Res. Dev. Plasma 1 (2000b) 89) for BEN well into the magnetotail, in which these trapped-particle modes only occur in the top of the spectrum, whereas the bulk of the spectrum below that which propagates obliquely to the magnetic field consists of standard modes driven by unstable electron/ion beams, predicts that such solitary waves are only present in the source region, whereas they are not in BEN that has propagated well outside the source region. Motivated by that prediction and other features of the model, observations are examined from POLAR of BEN obtained poleward of and within the near-Earth extension of the PSBL. The wave data examined for BEN poleward of the near-Earth PSBL (in the plasma mantle) exhibit frequent turbulent waveforms, but little evidence of solitary waves. However, the wave data near the PSBL source region shows a persistent level of turbulence, within which solitary-like waves are embedded, several of which saturate the receiver. The non-solitary portion of the observed wave data, which appears throughout these regions, fits well the theory of standard electron/ion beam instabilities. However, the higher-frequency portion above the plasma frequency, which is confined to the PSBL vicinity, appears nonlinear in character, with a stronger magnetic field apparently playing a vital role in that nonlinearity.     

Pulsations in the solar wind and on the ground

Measurements of the pulsation activity recorded by the HEOS-1 satellite in the solar wind upstream from the Earth's bow shock are compared with records of Pc 3–4 activity at the Soviet Borok Observatory. Selecting only events in the 0300–1000 U.T. range most suitable for observing at Borok, we obtained eight events with closely similar periods at the satellite and at Borok, while another event showed similar onset times but had rather different dominant periodicities at the two locations.The time delay between the start of the event at HEOS and at Borok depends on the distance between the satellite and the bow shock in a way which suggests that the pulsation activity at the satellite is produced by protons which are counter-streaming along the interplanetary field lines as a result of reflection or energisation at the shock. When the interplanetary field is directed away from the Sun, the period of the disturbance at Borok is most closely similar to the period at HEOS and both are inversely proportional to the magnitude of the interplanetary magnetic field. This coincident dominant periodicity at HEOS and Borok did not seem to exist during periods of Sunward directed interplanetary field.These results are discussed in terms of the possible generating mechanisms for Pc 3–4 activity.     

Radio and X-ray observations of an exceptional radio flare in the extreme z= 4.72 blazar GB B1428+4217

We report on the extreme behaviour of the high-redshift blazar GB B1428+4217 at   z = 4.72  . A continued programme of radio measurements has revealed an exceptional flare in the light curve, with the 15.2-GHz flux density rising by a factor of ∼3 from ∼140 to ∼430  mJy in a rest-frame time-scale of only ∼4 months – much larger than any previous flares observed in this source. In addition to new measurements of the 1.4–43  GHz radio spectrum, we also present the analysis and results of a target-of-opportunity X-ray observation using XMM–Newton , made close to the peak in radio flux. Although the X-ray data do not show a flare in the high-energy light curve, we are able to confirm the X-ray spectral variability hinted at in previous observations. GB B1428+4217 is one of several high-redshift radio-loud quasars that display a low-energy break in the X-ray spectrum, probably due to the presence of excess absorption in the source. X-ray spectral analysis of the latest XMM–Newton data is shown to be consistent with the warm-absorption scenario which we have hypothesized previously. Warm absorption is also consistent with the observed X-ray spectral variability of the source, in which the spectral changes can be successfully accounted-for with a fixed column density of material in which the ionization state is correlated with hardness of the underlying power-law emission.     

Evolution of magnetic flux distribution of solar bipolar active regions

In this paper the magnetic flux distribution of bipolar active regions at the sunspot development stage is analyzed. It is shown that the ratio of the total sunspot area in an active region to the maximum one can be used as a characteristic of the development phase. Such a procedure allows combining the data attributed to different active regions for studying evolutionary changes. The expressions describing the evolution of magnetic flux distribution of bipolar active region were obtained and their interpretation with rise and descent of loop like magnetic flux tube leading to active region formation was justified.     

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.     

On population of hazardous celestial bodies in the near-Earth space

In recent years, following the Chelyabinsk event of February 15, 2013, the lower size limit for presumably dangerous near-Earth objects has been decreased manyfold (essentially, from 140 m to ~10 m). This has drawn an increased attention to the properties of the population of decameter-sized bodies, in particular, the bodies that approach the Earth from the sunward side (daytime sky). The current paper is concerned with various properties of this population. The properties of the ensemble are analyzed using both observational data from other authors and theoretical estimates obtained by cloning virtual bodies. This question is of great practical importance, as the means for detecting such bodies (for example, the SODA project) need to be developed with consideration for the requirements imposed by the population properties. We have shown that the average rate of entering near-Earth space (NES), i.e., at distances less than ~1 million km from the Earth, for decameter-sized and larger bodies from the daytime sky (elongation values of entry points less than 90°) is approximately 620 objects per year for elongation angles of the detection point <90° and approximately 220 objects per year for elongation angles of the detection point <45°.