Interfacing MHD Single Fluid and Kinetic Exospheric Solar Wind Models and Comparing Their Energetics

An exospheric kinetic solar wind model is interfaced with an observation-driven single-fluid magnetohydrodynamic (MHD) model. Initially, a photospheric magnetogram serves as observational input in the fluid approach to extrapolate the heliospheric magnetic field. Then semi-empirical coronal models are used for estimating the plasma characteristics up to a heliocentric distance of 0.1 AU. From there on, a full MHD model that computes the three-dimensional time-dependent evolution of the solar wind macroscopic variables up to the orbit of Earth is used. After interfacing the density and velocity at the inner MHD boundary, we compare our results with those of a kinetic exospheric solar wind model based on the assumption of Maxwell and Kappa velocity distribution functions for protons and electrons, respectively, as well as with in situ observations at 1 AU. This provides insight into more physically detailed processes, such as coronal heating and solar wind acceleration, which naturally arise from including suprathermal electrons in the model. We are interested in the profile of the solar wind speed and density at 1 AU, in characterizing the slow and fast source regions of the wind, and in comparing MHD with exospheric models in similar conditions. We calculate the energetics of both models from low to high heliocentric distances.     

Coronal Temperature Profiles Obtained from Kinetic Models and from Coronal Brightness Measurements Obtained During Solar Eclipses

Coronal density, temperature, and heat-flux distributions for the equatorial and polar corona have been deduced from Saito’s model of averaged coronal white-light (WL) brightness and polarization observations. These distributions are compared with those determined from a kinetic collisionless/exospheric model of the solar corona. This comparison indicates similar distributions at large radial distances (>?7 R_⊙) in the collisionless region. However, rather important differences are found close to the Sun in the acceleration region of the solar wind. The exospheric heat flux is directed away from the Sun, while that inferred from all WL coronal observations is in the opposite direction, i.e. conducting heat from the inner corona toward the chromosphere. This could indicate that the source of coronal heating extends up into the inner corona, where it maximizes at r>1.5 R_⊙, well above the transition region.     

Using Polar Coronal Hole Area Measurements to Determine the Solar Polar Magnetic Field Reversal in Solar Cycle 24

An analysis of solar polar coronal hole (PCH) areas since the launch of the Solar Dynamics Observatory (SDO) shows how the polar regions have evolved during Solar Cycle 24. We present PCH areas from mid-2010 through 2013 using data from the Atmospheric Imager Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) instruments onboard SDO. Our analysis shows that both the northern and southern PCH areas have decreased significantly in size since 2010. Linear fits to the areas derived from the magnetic-field properties indicate that, although the northern hemisphere went through polar-field reversal and reached solar-maximum conditions in mid-2012, the southern hemisphere had not reached solar-maximum conditions in the polar regions by the end of 2013. Our results show that solar-maximum conditions in each hemisphere, as measured by the area of the polar coronal holes and polar magnetic field, will be offset in time.     

Standard and Non-Standard Solar Models

We summarize the physical input and assumptions commonly adopted in modern standard solar models that also produce good agreement with solar oscillation frequencies. We discuss two motivations for considering non-standard models: the solar neutrino problem and surface lithium abundance problem. We begin to explore the potential for mixed core models to solve the neutrino problem, and compare the structure, neutrino flux, and oscillation frequency predictions for several models in which the inner 25% of the radius is homogenized, taking into account the effects of non-local equilibrium abundances of ³He. The results for the neutrino flux and helioseismic predictions are far from satisfactory, but such models have the potential to reduce the predicted ⁷Be/⁸B neutrino flux ratio, and further studies are warranted. Finally, we discuss how much the neutrino problem can be alleviated in the framework of the standard solar model by using reaction rates, abundances and neutrino capture cross-sections at the limits of their uncertainties, while still satisfying the constraints of helioseismology.     

High-Resolution Studies of the Solar Polar Magnetic Fields

We present high-resolution studies of the solar polar magnetic fields near sunspot maximum in 1989 and towards sunspot minimum in 1995. We show that, in 1989, the polar latitudes were covered by several unipolar regions of both polarities. In 1995, however, after the polar field reversal was complete, each pole exhibited only one dominant polarity region.Each unipolar region contains magnetic knots of both polarities but the number count of the knots of the dominant polarity exceeds that of the opposite polarity by a ratio of order 4:1, and it is rare to find opposite polarity pairs, i.e., magnetic bipoles.These knots have lifetimes greater than 7 hours but less than 24 hours. We interpret the longitudinal displacement of the knots over a 7-hour period as a measure of the local rotation rate. This rotation rate is found to be generally consistent with Snodgrass' (1983) magnetic rotation law.In an attempt to obtain some insight into the operation of the solar dynamo, sketches of postulated subsurface field configurations corresponding to the observed surface fields at these two epochs of the solar cycle are presented.     

Polar Coronal Holes During Solar Cycles 22 and 23

Data from the Solar Wind Ion Composition Spectrometer (SWICS) on Ulysses and synoptic maps from Kitt Peak are used to analyze the polar coronal holes of solar activity cycles 22 and 23 (from 1990 to end of 2003). In the beginning of the declining phase of solar cycles 22 and 23, the north polar coronal holes (PCHs) appear about one year earlier than the ones in the south polar region. The solar wind velocity and the solar wind ionic charge composition exhibit a characteristic dependence on the solar wind source position within a PCH. From the center toward the boundary of a young PCH, the solar wind velocity decreases, coinciding with a shift of the ionic charge composition toward higher charge states. However, for an old PCH, the ionic charge composition does not show any obvious change, although the latitude evolution of the velocity is similar to that of a young PCH.     

Fast Solar Wind Velocity in a Polar Coronal Hole during Solar Minimum

We present a study of the outflow velocity of the fast wind in the northern polar coronal hole observed on 21 May 1996, during the minimum of solar activity, in the frame of a joint observing program of the SOHO (Solar Heliospheric Observatory) mission. The outflow velocity is inferred from an analysis of the Doppler dimming of the intensities of the Ovi 1032, 1037 and Hi L 1216 lines observed between 1.5 R and 3.5 R with the Ultraviolet Coronagraph Spectrometer (UVCS), operating onboard SOHO. The analysis shows that for a coronal plasma characterized by low density, as derived for a polar hole at solar minimum by Guhathakurta et al. (1999), and low temperature, as directly measured at the base of this coronal hole by David et al. (1998), the oxygen outflow speed derived spectroscopically is consistent with that implied by the proton flux conservation. The hydrogen outflow is also consistent with flux conservation if the deviation from isotropy of the velocity distribution of the hydrogen atoms is negligible. Hence, for this cool and tenuous corona, the oxygen ions and neutral hydrogen atoms flow outward roughly at the same speed, which increases from 40 km s^–1 at 1.5 R to 360 km s^–1 at 3.1 R , with an average acceleration of the order of 4.5×10³ cm s^–2. The highly anisotropic velocity distributions of the Ovi ions found in the analysis confirm that the process which is heating the oxygen ions acts preferentially across the magnetic field.     

Colliding Winds in Binary Star Systems: Theory, Models

The properties of the interaction region of colliding winds in hot star binaries and evolved low mass binaries are discussed in the frame of 2D and 3D hydrodynamical numerical simulations. The emission of such systems and their nebulae, in particular in X-rays, is reviewed and new results regarding orbital line profile variations are presented. The connection between colliding winds, thin shell instabilities and structure formation is outlined. This revised version was published online in July 2006 with corrections to the Cover Date.     

基于深空探测器下行信号的太阳风观测及通信链路的影响综述

在上合期间,日冕和太阳风严重影响深空通信链路。论述了非均匀太阳风对深空通信的影响,综述了基于深空探测器下行信号的太阳风观测,以及通过反演技术进行太阳风和日冕特性研究的国内外进展。以我国的深空探测为背景,提出一套基于探测器下行信号的太阳风观测方案,并可利用现有探测器进行预先观测研究。在我国深空探测任务开始后,在数据通信和测轨的同时,进行全过程的观测实验。     

Cloud Models for Solar Faculae

Assuming the clouds as plane parallel structures above the photosphere, center-to-limb contrast variations of various cloud models for solar faculae with approximations such as optically thin or thick, hot or cold, and with or without surface reflections, have been investigated. It has been found that the observed facular contrast data from Frazier (1971) and Taylor et al. (1998) at the 525 nm continuum is best represented by a cloud which is 230 K hotter than the undisturbed photosphere, with an optical depth =0.4283, and with isotropic surface reflections causing 11% of the background photons to be lost before penetrating into the cloud. This model and some other cloud models are shown to provide a fit better than the other physical and non-physical facular models presented previously.     

Energy Gain of Positive Ions in Solar Polar Coronal Holes

We have investigated heating of solar polar coronal holes and acceleration of fast solar wind by means of lower hybrid (LH) waves. A three-fluid Maxwell model comprising electrons, protons, and α-particles is employed at around two solar radii heliocentric distance, where wave dissipation starts to be dominated by collisionless processes. We suggest specific wavenumber ranges corresponding to LH as well as stochastic instabilities and find that these instabilities may bring about a significant energy gain in positive ions.     

Models for Polar Haze Formation in Jupiter's Stratosphere

We present coupled chemical-microphysical models for the formation, growth, and physical properties of the jovian polar haze based on a gas-phase photochemical model for the auroral regions developed by A. S. Wong et al. (2000, Astrophys. J.534, L215-217). In this model, auroral particle precipitation provides an important energy source for enhanced decomposition of methane and production of benzene and polycyclic aromatic hydrocarbons (PAHs). We find that at high altitude, A₄ (pyrene, a hydrocarbon consisting of four fused aromatic rings) should homogeneously nucleate to form tiny primary particles. At lower altitudes, A₃ (phenanthrene) and A₂ (naphthalene) heterogeneously nucleate on the A₄ nuclei. These particles subsequently grow by additional condensation of A₂ on the nucleated particles and by coagulation and eventually sediment out to the troposphere. We run different cases of the aerosol microphysical model for different assumptions regarding the fractal dimension of aggregate particles formed by the coagulation process. If coagulation is assumed to produce spherical particles (of dimensionality 3), then their mean radius at altitudes below the 20-mbar pressure level is computed to be approximately 0.1 μm. If coagulation produces fractal aggregates of dimension 2.1, then their equivalent mean radius below the 20-mbar level is much larger, of order 0.7 μm. Aggregates with fractal dimensions between 2.1 and 3 form with equivalent mean radii between 0.1 and 0.7 μm. In every case, mean particle radius is found to decrease with increasing altitude, as expected for a system approximately in sedimentation-coagulation equilibrium. The predicted range of altitudes where aerosol formation occurs and the mean size to which particles grow are found to be generally consistent with observations. However, our calculations cannot presently account for the large amount of total aerosol loading inferred by M. G. Tomasko et al. (1986, Icarus65, 218-243). We suggest that the primarily neutral chemical pathway to heavy hydrocarbon and PAH formation proposed by Wong et al. (2000) may proceed too slowly to produce a sufficient amount of condensible material. Inclusion of ion and ion-neutral reactions in the chemical scheme could potentially lead to the prediction of higher PAH production rates, higher nucleation rates, and greater aerosol loading, producing better agreement with the observations.     

太阳风电子动力学不稳定性

电子是太阳风粒子中最为重要的组分之一,它可以通过多种机制对太阳风产生影响.太阳风中的电子通常具有温度各向异性和束流两种非热平衡分布特征,这些偏离热平衡分布的特征可以通过波粒相互作用激发电子不稳定性和等离子体波动,激发的等离子体波动又可以通过波粒相互作用调制太阳风粒子的分布,从而加热太阳风中的背景粒子.因此电子动力学不稳定性在太阳风的演化过程中扮演了极为重要的角色.详细介绍了太阳风中常见的电子动力学不稳定性,并基于等离子体动力论,详细介绍太阳风传播过程中所出现的各种不稳定性,尤其是在近日球层和太阳大气区域所出现的电子声热流不稳定性以及低混杂热流不稳定性,并分析其波粒相互作用机制,以便更加深入地研究太阳风传播过程中的电子分布函数演化.     

Nitrate in Polar Ice: A New Tracer of Solar Variability

Knowledge of the long-term variability of solar activity is of both astrophysical and geoscientific interest. Reconstructions of solar activity over multiple millennia are traditionally based on cosmogenic isotopes ¹⁴C or ¹⁰Be measured in natural terrestrial archives, but the two isotopes exhibit significant differences on millennial time scales, so that our knowledge of solar activity at this time scale remains somewhat uncertain. Here we present a new potential proxy of solar activity on the centennial-millennial time scale, based on a chemical tracer, viz. nitrate content in an ice core drilled at Talos Dome (Antarctica). We argue that this location is optimal for preserving the solar signal in the nitrate content during the Holocene. By using the firn core from the same location we show that the 11-year and Gleissberg cycles are present with the variability of 10??C?25?% in nitrate content in the pre-industrial epoch. This is consistent with the results of independent efforts of modeling HNO₃ and NO _y in Antarctic near surface air. However, meteorological noise on the interannual scale makes it impossible to resolve individual solar cycles. Based on different processes of formation and transport compared to cosmogenic isotopes, it provides new, independent insight into long-term solar activity and helps resolve the uncertainties related to cosmogenic isotopes as diagnostics of solar activity.     

Deterministically Driven Avalanche Models of Solar Flares

We develop and discuss the properties of a new class of lattice-based avalanche models of solar flares. These models are readily amenable to a relatively unambiguous physical interpretation in terms of slow twisting of a coronal loop. They share similarities with other avalanche models, such as the classical stick–slip self-organized critical model of earthquakes, in that they are driven globally by a fully deterministic energy-loading process. The model design leads to a systematic deficit of small-scale avalanches. In some portions of model space, mid-size and large avalanching behavior is scale-free, being characterized by event size distributions that have the form of power-laws with index values, which, in some parameter regimes, compare favorably to those inferred from solar EUV and X-ray flare data. For models using conservative or near-conservative redistribution rules, a population of large, quasiperiodic avalanches can also appear. Although without direct counterparts in the observational global statistics of flare energy release, this latter behavior may be relevant to recurrent flaring in individual coronal loops. This class of models could provide a basis for the prediction of large solar flares.     

Ultra-High Resolution Nitrate in Polar Ice as Indicator of Past Solar Activity

Ionization in the polar atmosphere causes the formation of nitrate compounds, which are frozen out and incorporated into the layers of the polar ice sheets. From a 122-m ice core collected in 1992 on the central Greenland ice sheet, it has been possible to examine a solar signal in a ultra-high resolution record of nitrate concentrations. The sequence extends over a period of 415 years at a temporal resolution of no less than one analysis per month (total number of samples 7776 resulting from 1.5 cm sampling along the entire core). This type of measurement reveals major nitrate anomalies which are thought to result from the injection of individual solar proton events into the winter polar stratosphere. For this reason, the large nitrate anomalies provide the possibility to delineate a signal of solar activity well beyond the known geophysical records.     

Constructing and Characterising Solar Structure Models for Computational Helioseismology

In local helioseismology, numerical simulations of wave propagation are useful to model the interaction of solar waves with perturbations to a background solar model. However, the solution to the linearised equations of motion include convective modes that can swamp the helioseismic waves that we are interested in. In this article, we construct background solar models that are stable against convection, by modifying the vertical pressure gradient of Model S (Christensen-Dalsgaard et al., 1996, Science 272, 1286) relinquishing hydrostatic equilibrium. However, the stabilisation affects the eigenmodes that we wish to remain as close to Model S as possible. In a bid to recover the Model S eigenmodes, we choose to make additional corrections to the sound speed of Model S before stabilisation. No stabilised model can be perfectly solar-like, so we present three stabilised models with slightly different eigenmodes. The models are appropriate to study the f and p ₁ to p ₄ modes with spherical harmonic degrees in the range from 400 to 900. Background model CSM has a modified pressure gradient for stabilisation and has eigenfrequencies within 2% of Model S. Model CSM_A has an additional 10% increase in sound speed in the top 1 Mm resulting in eigenfrequencies within 2% of Model S and eigenfunctions that are, in comparison with CSM, closest to those of Model S. Model CSM_B has a 3% decrease in sound speed in the top 5 Mm resulting in eigenfrequencies within 1% of Model S and eigenfunctions that are only marginally adversely affected. These models are useful to study the interaction of solar waves with embedded three-dimensional heterogeneities, such as convective flows and model sunspots. We have also calculated the response of the stabilised models to excitation by random near-surface sources, using simulations of the propagation of linear waves. We find that the simulated power spectra of wave motion are in good agreement with an observed SOHO/MDI power spectrum. Overall, our convectively stabilised background models provide a good basis for quantitative numerical local helioseismology. The models are available for download from http://www.mps.mpg.de/projects/seismo/NA4/ .     

Predictive Capabilities of Avalanche Models for Solar Flares

We assess the predictive capabilities of various classes of avalanche models for solar flares. We demonstrate that avalanche models cannot generally be used to predict specific events because of their high sensitivity to the embedded stochastic process. We show that deterministically driven models can nevertheless alleviate this caveat and be efficiently used for predictions of large events. Our results suggest a new approach for predictions of large (typically X-class) solar flares based on simple and computationally inexpensive avalanche models.     

The Shape of the Solar Limb: Models and Observations

In this paper we compare observed, empirical, and modelled solar limb profiles and discuss their potential use to derive physical properties of the solar atmosphere. The PHOENIX, SolMod3D, and COSI radiative transfer codes as well as VAL-C models are used to calculate the solar limb shape under different assumptions. The main properties of each model are shown. The predicted limb shape as a function of wavelength for different features on the solar disk, such as quiet Sun, sunspots, and faculae, is investigated. These models provide overall consistent limb shapes with some discrepancies that are discussed here in terms of differences in solar atmosphere models, opacities, and the algorithms used to derive the solar limb profile. Our analysis confirms that the most common property of all models is limb shapes that are much steeper than what is observed, or predicted by the available empirical models. Furthermore, we have investigated the role of the Fraunhofer lines within the spectral domain used for the solar limb measurements. Our results show that the presence of the Fraunhofer lines significantly displaces the limb inflection point from its position estimated assuming only the photospheric continuum. The PICARD satellite, launched on 15 June 2010, will provide measurements of the limb shape at several wavelengths. This work shows that the precision of these measurements allows for discrimination among the available models.