The Solar System in the Galactic environment

Tuorla Observatory, University of Turku     

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.     

Galactic Cosmic Ray Modulation and the Last Solar Minimum

In this work the galactic cosmic ray modulation in relation to solar activity indices and heliospheric parameters during the years 1996??C?2010 covering solar cycle 23 and the solar minimum between cycles 23 and 24 is studied. A new perspective of this contribution is that cosmic ray data with a rigidity of 10 GV at the top of the atmosphere obtained from many ground-based neutron monitors were used. The proposed empirical relation gave much better results than those in previous works concerning the hysteresis effect. The proposed models obtained from a combination of solar activity indices and heliospheric parameters give a standard deviation <?10?% for all the cases. The correlation coefficient between the cosmic ray variations of 10?GV and the sunspot number reached a value of r=?0.89 with a time lag of 13.6±0.4 months. The best reproduction of the cosmic ray intensity is obtained by taking into account solar and interplanetary indices such as sunspot number, interplanetary magnetic field, CME index, and heliospheric current sheet tilt. The standard deviation between the observed and calculated values is about 7.15?% for all of solar cycle 23; it also works very well during the different phases of the cycle. Moreover, the use of the cosmic ray intensity of 10?GV during the long minimum period between cycles 23 and 24 is of special interest and is discussed in terms of cosmic ray intensity modulation.     

Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution

1 INTRODUCTIONRadial abundance gradient along the Galactic disk constitutes one of the most importantobservational constraillts fOr models of the evolution of the Galactic disk. The existence of sucha gradient is now well established, through radio and optical observations of HII regions, diskstars, pIanetary nebulae (see Henry and Worthey 1999 for a detailed review) and open clusters(Friel 1995, 1999). An average gradiellt of dlog(X/H)/dR ~ --0.06 dex kpc--' is observed inthe Milky …     

Probing the Fundamental Physics of the Solar Corona with Lunar Solar Occultation Observations

Imaging and spectroscopy of the solar corona, coupled with polarimetry, are the only tools available at present to capture signatures of physical processes responsible for coronal heating and solar wind acceleration within the first few solar radii above the solar limb. With the recent advent of improved detector technology and image processing techniques, broad-band white light and narrow-band multi-wavelength observations of coronal forbidden lines, made during total solar eclipses, have started to yield new views about the thermodynamic and magnetic properties of coronal structures. This paper outlines these unique capabilities, which until present, have been feasible primarily with observations during natural total solar eclipses. This work also draws attention to the exciting possibility of greatly increasing the frequency and duration of solar eclipse observations with Moon orbiting observatories utilizing lunar limb occultation of the solar disk for coronal measurements.     

太阳和月球射电辐射对40m天线数据接收影响分析

利用标准射电源和标准噪声源,对宁静太阳和月球射电辐射的噪声温度进行了测量,估计它们对40m天线接收绕月卫星信号的影响,结论是:(1)40m天线指向偏离太阳超过2°时,能够满足正常数据接收要求;(2)40m天线指向月球时,系统接收的噪声温度增加值为87.1K, 此时系统的总噪声温度能够满足正常数据接收要求.     

太阳邻域银河系薄盘标高研究

利用Hipparcos卫星精确天体测量观测数据,尝试研究由不同样本恒星所反映的太阳邻域银河系薄盘的标高.根据Tycho测光系统的色指数,并考虑观测样本的完备性,分别在主序段和水平支上选取了几类恒星样本,以考察银河系薄盘标高的演化特征.分析结果发现,在比较完备的样本空间上,由O-B型主序星定义的银盘标高为103.1±3.0 pc,太阳位于其平均平面以上的15.2±7.3 pc处;水平支恒星定义的银盘标高为144.0±10.0 pc,太阳位于其平均平面以上3.5±5.4pc处.A、F、G、K、M型的主序星由于观测样本完备性的限制而无法进行可信的标高研究.     

Observable Parameters of Solar Microwave Pulsating Structure and Their Implications for Solar Flare

From the observations with the Chinese Solar Broadband Radiospectrometer (SBRS/Huairou) in the frequency range of 1.10?–?2.06 GHz and 2.60?–?3.80 GHz during 2004?–?2006, we select 14 flare events which were associated with numerous fast microwave subsecond pulsating structures (period: P<0.5 s). In order to describe these subsecond pulsating structures comprehensively, we defined a set of observable parameters including emission frequency (f ₀), bandwidth (b _w), polarization degree (r), period (P), duration (D), modulation depth (M), quality factor (Q), single pulse frequency drifting rate (R _spfd), global frequency drifting rate (R _gfd), and symmetrical factor of the pulse profile (S). Then based on a detailed analysis of the spectrograms of the fast pulsations which occurred in one of these flares (an X3.4 flare/CME event occurred on 13 Dec. 2006), we discuss the possible relations among these observable parameters and their physical implications for the dynamical processes of solar eruptive events, and applied them to interpret the nature of the pulsations in the flare/CME event. Such study of microwave periodic pulsations provides us with a useful tool to probe the details of the flare kernels, and understand the physical mechanism of solar eruptive processes.     

Determination of the Thermophysical Characteristics of the Lunar Soil Using Solar Heat

Solar System Research - The possibility of determining the thermophysical characteristics of the lunar soil by a penetrating probe when using solar heat as a heater is considered. The effectiveness...     

Gas as a Tracer of the Galactic Potential

We consider a disk-like dark matter model for the Milky Way andcompare a few predictions with observations. The observed gaseousflaring for HI and molecular gas fits the model predictions indetail. The global HI distribution in the Milky Way needs to beexplained by a multiphase medium. The dark matter distribution in theGalactic halo is traced by a low density component of halogas. High-velocity clouds with distances up to ～ 50 kpc may beexplained as condensations which originate from instabilities withinthe gaseous halo. Our model explains also ‘beards’ and ‘forbiddenvelocities’ as observed in the rotation curves of externalgalaxies. A disk-like dark matter model is self-consistent and inexcellent agreement with observations.     

"嫦娥一号"探月卫星发射期间太阳活动现象监测

为发射"嫦娥一号"探月卫星,保障其在空间环境运行的安全和国家载人航天计划的正常实施,云南天文台全日面Hα色球望远镜为中国科学院空间环境研究预报中心实时提供太阳活动数据资料,协作完成特殊时期空间环境监测任务.     

The Implications of 3D for Solar MHD Modelling

The effects of three-dimensionality on the modelling of solar magnetic fields are described. We focus on two processes that are believed to play an important role in coronal heating – the braiding of field lines by photospheric motions and the reconnection of colliding flux tubes. First, it is shown that a proper treatment of boundary conditions at the photosphere in 3D entails qualitatively new physical processes that are not present in 2D. The numerical resolution of even simple boundary velocity patterns in 3D leads to obstacles which have no counterpart in the 2D case. We conclude that adaptive mesh refinement is necessary for capturing the essential 3D physics of a braiding motion at the photosphere. Next, the effects of 3D on magnetic reconnection are discussed. Reconnection in 3D can lead to an evolution of interacting flux tubes, magnetic tunneling, that is not only impossible in lower dimensionality, but is strikingly counterintuitive. The implications of these results for the structure of the solar magnetic field and for coronal heating are described.     

A Study of the Scale Height of the Thin Galactic Disk in the Solar Neighborhood

An attempt is made to determine the scale height of the thin Galactic disk in the solar neighborhood using various stellar samples from the accurate astrometric data of the Hipparcos satellite. Using the Tycho color indices and considering the completeness of the samples, several samples were selected from the main sequence and the horizontal branch. Based on two rather complete samples, it is found that the scale height of the Galactic disk defined by the O-B type main sequence stars is 103.1 ± 3.0 pc and the Sun is located at height of 15.2 ± 7.3 pc above its mean plane; while the scale height of the Galactic disk defined by the horizontal branch stars is 144.0 the ±10.0 pc and the Sun is located at height 3.5 ± 5.4 pc above the mean plane. Owing to the incompleteness of observational samples, it is not possible to use the main sequence stars of spectral types A, F, G, K and M to make a reliable determination of the scale height.     

Cosmogenic Radionuclides In Meteorites and Solar Modulation of Galactic Cosmic Rays In the Internal Heliosphere

Solar System Research - Cosmogenic radionuclides with half-life periods T1/2 ranging from several days to a million years, produced in the nuclear reactions of galactic cosmic rays (GCR) with...     

On the Relationship of the 27-day Variations of the Solar Wind Velocity and Galactic Cosmic Ray Intensity in Minimum Epoch of Solar Activity

We study the relationship of the 27-day variations of the galactic cosmic ray intensity with similar variations of the solar wind velocity and the interplanetary magnetic field based on observational data for the Bartels rotation period # 2379 of 23 November 2007 – 19 December 2007. We develop a three-dimensional (3-D) model of the 27-day variation of galactic cosmic ray intensity based on the heliolongitudinally dependent solar wind velocity. A consistent, divergence-free interplanetary magnetic field is derived by solving Maxwell’s equations with a heliolongitudinally dependent 27-day variation of the solar wind velocity reproducing in situ observations. We consider two types of 3-D models of the 27-day variation of galactic cosmic ray intensity, i) with a plane heliospheric neutral sheet, and ii) with the sector structure of the interplanetary magnetic field. The theoretical calculations show that the sector structure does not significantly influence the 27-day variation of galactic cosmic ray intensity, as had been shown before, based on observational data. Furthermore, good agreement is found between the time profiles of the theoretically expected and experimentally obtained first harmonic waves of the 27-day variation of the galactic cosmic ray intensity (with a correlation coefficient of 0.98±0.02). The expected 27-day variation of the galactic cosmic ray intensity is inversely correlated with the modulation parameter ζ (with a correlation coefficient of −0.91±0.05), which is proportional to the product of the solar wind velocity V and the strength of the interplanetary magnetic field B (ζ∼VB). The high anticorrelation between these quantities indicates that the predicted 27-day variation of the galactic cosmic ray intensity mainly is caused by this basic modulation effect.     

Study of the Geoeffectiveness and Galactic Cosmic-Ray Response of VarSITI-ISEST Campaign Events in Solar Cycle 24

We analyze and compare the geomagnetic and galactic cosmic-ray (GCR) response of selected solar events, particularly the campaign events of the group International Study of Earth-affecting Solar Transients (ISEST) of the program Variability of the Sun and Its Terrestrial Impact (VarSITI). These selected events correspond to Solar Cycle 24, and we identified various of their features during their near-Earth passage. We evaluated the hourly data of geomagnetic indices and ground-based neutron monitors and the concurrent data of interplanetary plasma and field parameters. We recognized distinct features of these events and solar wind parameters when the geomagnetic disturbance was at its peak and when the cosmic-ray intensity was most affected. We also discuss the similarities and differences in the geoeffectiveness and GCR response of the solar and interplanetary structures in the light of plasma and field variations and physical mechanism(s), which play a crucial role in influencing the geomagnetic activity and GCR intensity.     

Chemical Evolution of the System of Galactic Planetary Nebulae

The problem of the chemical evolution of the system of Galactic planetary nebulae, starting with the early stage of development of the Galaxy, is investigated. The radial and vertical gradients of C, N, O, Ne, Ar, Cl, and S abundances are determined for different ages of the precursor stars of the nebulae. A statistically significant age dependence of the gradients is derived.     

Modulation of Galactic Protons in the Heliosphere During the Unusual Solar Minimum of 2006 to 2009

The last solar minimum activity period, and the consequent minimum modulation conditions for cosmic rays, was unusual. The highest levels of galactic protons were recorded at Earth in late 2009 in contrast to expectations. Proton spectra observed for 2006 to 2009 from the PAMELA cosmic ray detector on-board the Resurs-DK1 satellite are presented together with the solutions of a comprehensive numerical model for the solar modulation of cosmic rays. The model is used to determine what mechanisms were mainly responsible for the modulation of protons during this period, and why the observed spectrum for 2009 was the highest ever recorded. From mid-2006 until December 2009 we find that the spectra became significantly softer because increasingly more low energy protons had reached Earth. To simulate this effect, the rigidity dependence of the diffusion coefficients had to decrease significantly below ～?3 GeV. The modulation minimum period of 2009 can thus be described as relatively more ‘diffusion dominated’ than previous solar minima. However, we illustrate that drifts still had played a significant role but that the observable modulation effects were not as well correlated with the waviness of the heliospheric current sheet as before. Protons still experienced global gradient and curvature drifts as the heliospheric magnetic field had decreased significantly until the end of 2009, in contrast to the moderate decreases observed during previous minimum periods. We conclude that all modulation processes contributed to the observed increases in the proton spectra for this period, exhibiting an intriguing interplay of these major mechanisms.