Probing solar and stellar interior dynamics and dynamo

Solar and stellar activity is a result of complex interaction between magnetic field, turbulent convection and differential rotation in a star’s interior. Magnetic field is believed to be generated by a dynamo process in the convection zone. It emerges on the surface forming sunspots and starspots. Localization of the magnetic spots and their evolution with the activity cycle is determined by large-scale interior flows. Thus, the internal dynamics of the Sun and other stars hold the key to understanding the dynamo mechanism and activity cycles. Recently, significant progress has been made for modeling magnetohydrodynamics of the stellar interiors and probing the internal rotation and large-scale dynamics of the Sun by helioseismology. Also, asteroseismology is beginning to probe interiors of distant stars. I review key achievements and challenges in our quest to understand the basic mechanisms of solar and stellar activity.     

Current status of asteroseismology

Asteroseismology, the extension of helioseismology to stars – solar-alike or not – has been an exciting and active field of research for about two decades. While over that period helioseismology has had great success in revealing the solar structure and its dynamics, progress in asteroseismology has been hampered by the observational challenge to carry out, primarily from the ground, high precision measurements on unresolved objects with a substantially lower flux.     

Recent progress in time–distance helioseismology

Time–distance helioseismology is one of the local helioseismology techniques that are used to derive the interior properties of the Sun. It has been used to study the structures and flow fields beneath sunspots on local scales, as well as used to derive interior rotational rates and meridional flow velocities on global scales. In addition to the efforts in improving time–distance measurements and inversions, theoretical modeling is also carried out to enhance the accuracy of sensitivity kernels. Recently, by use of realistic numerical simulation on solar convection, we have also started to investigate the validity of time–distance studies.     

Seismological studies of the sun and other stars

Observations of a large number of different oscillation frequencies in the Sun provide an opportunity for detailed testing of the theory of stellar structure and evolution. At present highly significant discrepancies remain between observed and computed frequencies, and so our models of the solar interior have to be modified. With further improvements in the observations it might become possible to make a direct empirical determination of the density structure throughout the Sun.Similar oscillations have so far not been detected in other stars, but attempts to do so are under way. Theoretical estimates indicate that amplitudes somewhat greater than for the Sun might be expected for early F stars on the main sequence, and that the amplitude increases rapidly with decreasing gravity. Observation of such oscillations would enable investigations of the structure of these stars, and would in addition provide valuable information about the excitation mechanism of the oscillations.     

The dynamics of the solar radiative zone

The picture of the solar radiative zone is evolving quickly. This review is separated in two parts. We first recall how the two powerful probes of the solar interior, namely the neutrinos and helioseismology have scrutinized the microscopic properties of the solar radiative plasma. Recent observations stimulate today complementary activities beyond the standard stellar model through theoretical modeling of angular momentum transport by rotation, internal waves or (and) by magnetic fields to get access to the dynamical motions of this important region of the Sun. So in the second part, we summarize the first impact of such processes on the radiative zone.     

Accurate determination of the successive moments of the sun: a new window open on the sun''s interior

Despite its great importance for solar physics, mainly in the fields of solar fundamental astrometry, helioseismology, planetary motions and relativistic effects, the successive zonal harmonics of the Sun still remain elusive and subject to some controversy. Direct observations from the ground suffer from atmospheric effects and are not of enough accuracy. Up to now, space flights (SOHO) or balloon missions give consistent data but lead to spurious results due to the noise. As far as indirect observations are concerned, the more precise determination of the successive moments (mainly J₂, J₄ and even J₆), will be provided unambiguously by the study of the orbit of a spacecraft flying close to the Sun or around Mercury. This has been scheduled, but not yet achieved. In this paper we will first emphasize why it is important to know the successive zonal harmonics of the Sun with a high accuracy. We will show how their precise knowledge permits to obtain informations on the Sun's interior, mainly the shear's regions (tacholine or leptocline). Then we will give an up-to-date review of both theories (including the heliosismology approach) and data. We will explain some of the difficulties, mainly due to the differential rotation and we will give an insight of what the PICARD's mission will bring in that field. Then we will propose a novel concept for a Sun's mission, which would lead to the most accurate determination of the successive solar moments (that could be part of another project), and thus opening a new window on the Sun's interior.     

Cool luminous stars

A broad theme emerging from $\text{IUE}$ and $\text{Einstein}$ observations of cool stars is that magnetic fields control the structure and energy balance of the outer atmospheres of these stars. I summarize the phenomena associated with magnetic fields in the Sun and show that similar phenomena occur in cool luminous stars. High dispersion spectra are providing unique information concerning densities, atmospheric extension, and emission line widths. A recent unanticipated discovery is that the transition lines are redshifted (an antiwind) in β Dra (G2 Ib) and perhaps other stars, which I interpret as indicating downflows in closed magnetic flux tubes as are seen in the solar flux tubes above sunspots. Finally, I classify the G and K giants and supergiants into three groups — active stars, quiet stars, and hybrid stars — depending on whether their atmospheres are dominated by closed magnetic flux tubes, open field geometries, or a predominately open geometry with a few closed flux tubes embedded.     

Local-area helioseismology as a diagnostic tool for solar variability

Dynamical and thermal variations of the internal structure of the Sun can affect the energy flow and result in variations in irradiance at the surface. Studying variations in the interior is crucial for understanding the mechanisms of the irradiance variations. “Global” helioseismology based on analysis of normal mode frequencies, has helped to reveal radial and latitudinal variations of the solar structure and dynamics associated with the solar cycle in the deep interior. A new technique, - “local-area” helioseismology or heliotomography, offers additional potentially important diagnostics by providing three-dimensional maps of the sound speed and flows in the upper convection zone. These diagnostics are based on inversion of travel times of acoustic waves which propagate between different points on the solar surface through the interior. The most significant variations in the thermodynamic structure found by this method are associated with sunspots and complexes of solar activity. The inversion results provide evidence for areas of higher sound speed beneath sunspot regions located at depths of 4–20 Mm, which may be due to accumulated heat or magnetic field concentrations. However, the physics of these structures is not yet understood. Heliotomography also provides information about large-scale stable longitudinal structures in the solar interior, which can be used in irradiance models. This new diagnostic tool for solar variability is currently under development. It will require both a substantial theoretical and modeling effort and high-resolution data to develop new capabilities for understanding mechanisms of solar variability.     

Solar and late-type dwarfs

The Einstein Observatory and the IUE satellite have provided the observational basis for a major restructuring in theories of coronal formation for late-type stars. For the first time, coronal and transition region emission from a large sample of low mass (1 M_o) dwarf stars has been directly observed, with the unexpected result that essentially all such stars are x-ray emitters. The Sun, which was previously assumed to be typical, is now known to be at the low end of the x-ray luminosity function for solar-type stars. K- and M-dwarfs are observed to have nearly the same luminosity distributions as G-dwarfs and all of these spectral types have a large spread in x-ray luminosity.Observationally, there is a strong correlation between the strength of coronal emission in stars with outer convective zones and the rotation rates of these stars. At the present time we have only the beginnings of a satisfactory theoretical explanation for this correlation; although we are beginning to understand the connection between coronal emission strength and the magnetic field, we do not yet understand the stellar dynamo which generates the magnetic field. Studies of the coronal emission of stars may lead to a better understanding of stellar dynamos.     

Spectral analysis of 3D MHD models of coronal structures

We study extreme-ultraviolet emission line spectra derived from three-dimensional magnetohydrodynamic models of structures in the corona. In order to investigate the effects of increased magnetic activity at photospheric levels in a numerical experiment, a much higher magnetic flux density is applied at the photosphere as compared to the Sun. Thus, we can expect our results to highlight the differences between the Sun and more active, but still solar-like stars. We discuss signatures seen in extreme-ultraviolet emission lines synthesized from these models and compare them to observed signatures in the spatial distribution and temporal evolution of Doppler shifts in lines formed in the transition region and corona. This is of major interest to test the quality of the underlying magnetohydrodynamic model to heat the corona, i.e. currents in the corona driven by photospheric motions (flux braiding).     

LISA: Heliocentric formation design for the laser interferometer space antenna mission

The LISA (Laser Interferometer Space Antenna) mission has been selected by the European Space Agency’s Science Programme Committee as the third large-class mission of the Cosmic Vision Programme, addressing the science theme of the Gravitational Universe. With a planned launch date in 2034, LISA will be the first ever space-borne Gravitational Wave observatory, relying on laser interferometry between three spacecraft orbiting the Sun in a triangular formation. Airbus is currently leading an industrial Phase A system study on behalf of the European Space Agency. The paper will address the astrodynamics challenges associated with the LISA constellation design, driven by tight requirements on the geometric quality metrics of the near equilateral formation.     

Helioseismology and the solar cycle

Helioseismic measurements stretching back for about three decades have clearly shown that the acoustic modes are sensitive to solar-cycle changes. 2006 marks the first full 11-year cycle of continuous observations from the Global Oscillation Network Group (GONG), and 10 years of observations with the Michelson Doppler Imager (MDI) aboard the SOHO spacecraft. For the first time, global helioseismology has traced the migrating zonal flow pattern of the torsional oscillation deep within the convection zone, while local helioseismology has revealed the changing pattern of meridional flows over the cycle. The frequencies, lifetimes, and amplitudes of acoustic modes all show variations that closely track the surface distribution of magnetic activity and may provide insight into the excitation and damping of the oscillations. Efforts to trace magnetic field and sound speed or density changes in the solar interior have proved more challenging.     

Heliospheric implications of structure in the interstellar medium

Equilibrium models of diffuse interstellar material (ISM) near the Sun show a range of cloud densities, ionization, and temperatures which are consistent with data, although the local ISM must be inhomogeneous over ∼2 pc scales. The ISM close to the Sun has properties that are consistent with the sheetlike warm neutral (and partially ionized) gas detected in the Arecibo Millennium Survey. Local interstellar magnetic fields are poorly understood, but data showing weak polarization for nearby stars indicate dust may be trapped in fields or currents in the heliosheath nose region. Implications of this dust capture are widespread, and may impact the interpretation of the cosmic microwave background data. Observations of interstellar H⁰ inside of the solar system between 1975 and 2000 do not suggest any variation in the properties or structure of local interstellar H⁰ over distance scales of ∼750 AU to within the uncertainties.     

The structure of the solar corona as observed by the Solar Extreme ultraviolet Rocket Telescope and Spectrograph

Data from the Solar Extreme-ultraviolet Rocket Telescope and Spectrograph (SERTS) have been used to address a number of important scientific problems. The primary strength of the SERTS data is the fact that this spectral range is rich with emission lines. Over 270 lines are seen in the SERTS active Sun spectrum, from 57 different ions. For example, multiple (≥4) lines are observed for all ionization states of iron from Fe IX to Fe XVII. Temperatures and densities have been derived for a number of active and quiet Sun regions, the coronal magnetic field strength has been estimated for both a plage region and an active region.     

Magnetohydrodynamics in solar and space physics

Because of its proximity, our Sun provides a unique opportunity to perform high resolution observations of its outer layers throughout the whole electromagnetic spectrum. We can also theoretically model most of the fascinating physical phenomena taking place on the Sun, as well as their impact on the solar system.     

Heating rates in X-ray flares: From solar microflares to stellar events

Soft X-ray solar and stellar flares appear in the coronae of solar-like stars due to abrupt release of energy accumulated in magnetic fields. To build a quantitatively correct model of a flare we need to know how much energy is released in flares of different sizes and strengths. Here we estimate and compare the energy release rate in flares as different as microflares occurring over the quiet Sun and strong stellar events in RS CVn systems. We find one simple scaling law which describes flares differing one from another by 10 orders of magnitude in the amount of emission measure.     

Solar cycles: A tutorial

The Sun is the nearest stellar and astrophysical laboratory, available for detailed studies in several fields of physics and astronomy. It is a sphere of hot gas with a complex and highly variable magnetic field which plays a very important role. The Sun shows an unprecedented wealth of phenomena that can be studied extensively and to the greatest detail, in a way we will never be in a position to study in other stars. Humans have studied the Sun for millennia and after the discovery of the telescope they realized that the Sun varies with time, i.e., solar activity is highly variable, in tune scales of millennia to seconds. The study of these variabilities helps us to understand how the Sun works and how it affects the interplanetary medium, Earth and the other planets. Solar power varies substantially and greatly affects the Earth and humans. Solar activity has several important periodicities, and quasi-periodicities. Knowledge of these periodicities helps us to forecast, to an extent, solar events that affect our planet. The most prominent periodicity of solar activity is the one of 11 years. The actual period is in fact 22 years because the magnetic field polarity of the Sun has to be taken into account. The Sun can be considered as a non-linear RLC electric circuit with a period of 22 years. The RLC equivalent circuit of the Sun is a van der Pol oscillator and such a model can explain many solar phenomena, including the variability of solar energy with time. Other quasi-periodicities such as the ones of 154 days, the 1.3, 1.7 to 2 years, etc., some of which might be harmonics of the 22 year cycle are also present in solar activity, and their study is very interesting and important since they affect the Earth and human activities. The period of 27 days related to solar rotation plays also a very important role in geophysical phenomena. It is noticeable that almost all periodicities are highly variable with time as wavelet analysis reveals. It is very important for humans to be in a position to forecast solar activity during the next hour, day, year, decade and century, because solar phenomena affect life on Earth and such predictions will help politicians and policy makers to better serve their countries and our planet.     

Theory and application of stellar seismology

Observation of multiperiodic variability in stars offers a powerful tool to probe their internal structure, rotation and magnetism. This follows from the possibility of identification of observed frequencies with those calculated for realistic stellar models. In spherical stars oscillations are described in terms of two-dimensional discrete sets of acoustic- and gravity-modes. In realistic cases there is no universal relation among the frequencies. Thus, each frequency observed is an independent observable. In recent years comparisons between observed and theoretical frequencies were made for the sun and various variable stars. The most dramatic discrepancy occurs for double-mode Cepheids. Magnetism and/or rotation induce fine structure in the frequency spectrum. The width and structure of multiplets depend on the size of these effects weighted in a specific way for each mode. Recently, oscillation data were used to determine the behavior of the angular velocity inside the sun. For stars only some average rotation rates have been determined in this way.     

用于深空探测的400 MHz带宽Chirp变换谱分析仪的设计与实现

Chirp变换频谱仪（CTS）具有低功耗、高稳定性等优点,在深空探测领域中具有独特的优势。罗塞塔（Rosetta）彗星探测器搭载的180 MHz带宽Chirp变换频谱分析仪,是迄今为止唯一成功完成空间任务的后端外差式实时频谱分析仪。基于Chirp变换谱分析的原理,设计构建了数字展宽线技术与声表面波压缩线技术相结合的400 MHz带宽Chirp变换谱分析仪。完成了数字展宽线与模拟声表面波压缩线的优化匹配设计,使系统的分辨率达到了理论值100 kHz。进一步采用调频信号和多频率点信号对CTS系统进行了测试验证。      

基于长椭球面波函数去卷积方法的高动态范围太阳射电成像

利用综合孔径射电望远镜对太阳进行观测时,通过对图像中存在的明亮扩展源进行准确建模并移除,可以更好地观测视场内的微弱源并提高图像的动态范围。在射电天文领域,主要利用CLEAN算法对图像中的明亮源进行移除,以显示微弱的背景。然而,使用图像像素作为基函数的CLEAN算法的固有限制导致其对扩展源的建模效果较差。为了克服这种限制,将基于长椭球面波函数（Prolate Spheroidal Wave Functions,PSWF）的去卷积方法应用于太阳射电成像。PSWF最优正交基由脏图中的感兴趣区域（Region of Interest,ROI）和UV覆盖共同决定。为了验证该方法的有效性,基于PSWF正交基对均匀圆环阵观测的太阳射电图像进行去卷积,并从动态范围和保真度两个方面定量化对比了CLEAN算法和基于PSWF正交基方法的性能。基于PSWF正交基去卷积方法剩余脏图中的微弱源更接近真实情况且动态范围更高。