On the Response of the Climate System to Solar Forcing

The climate response to changes in radiative forcing depends crucially on climate feedback processes, with the consequence that solar and greenhouse gas forcing have both similar response patterns in the troposphere. This circumstance complicates significantly the attribution of the causes of climate change. Additionally, the climate system displays a high level of unforced intrinsic variability, and significant variations in the climate of many parts of the world are due to internal processes. Such internal modes contribute significantly to the variability of climate system on various time scales, and thus compete with external forcing in explaining the origin of past climate extremes. This highlights the need for independent observations of solar forcing including long-term consistent observational records of the total and spectrally resolved solar irradiance. The stratospheric response to solar forcing is different from its response to greenhouse gas forcing, thus suggesting that stratospheric observations could offer the best target for the identification of the specific influence of solar forcing on climate.     

Resonance in a coupled solar-climate model

The 11–year solar activity cycle is magnetic in origin and is responsible for small changes in solar luminosity and the modulation of the solar wind. The terrestrial climate exhibits much internal variability supporting oscillations with many frequencies. The direct effect of changing solar irradiance in driving climatic change is believed to be small, and amplification mechanisms are needed to enhance the role of solar variability. In this paper we demonstrate that resonance may play a crucial role in the dynamics of the climate system, by using the output from a nonlinear solar dynamo model as a weak input to a simplified climate model. The climate is modelled as oscillating about two fixed points (corresponding to a warm and cold state) with the weak chaotically modulated solar forcing on average pushing the solution towards the warm state. When a typical frequency of the input is similar to that of the chaotic climate system then a dramatic increase in the role of the solar forcing is apparent and complicated intermittent behaviour is observed. The nonlinear effects are subtle however, and forcing that on average pushes the solution towards the warm state may lead to increased intervals of oscillation about either state. Owing to the intermittent nature of the timeseries, analysis of the relevant timeseries is shown to be non-trivial.     

Solar Forcing of Climate. 2: Evidence from the Past

The nature of the climatic response to solar forcing and its geographical coherence is reviewed. This information is of direct relevance for evaluating solar forcing mechanisms and validating climate models. Interpretation of Sun-climate relationships is hampered by difficulties in (1) translating proxy records into quantitative climate parameters (2) obtaining accurate age assessments (3) elucidating spatial patterns and relationships (4) separating solar forcing from other forcing mechanisms (5) lacking physical understanding of the solar forcing mechanisms. This often limits assessment of past solar forcing of climate to identification of correlations between environmental change and solar variability. The noisy character and often insufficient temporal resolution of proxy records often exclude the detection of high frequency decadal and bi-decadal cycles. However, on multi-decadal and longer time scales, notably the ∼90 years Gleisberg, and ∼200 years Suess cycles in the ¹⁰Be and ¹⁴C proxy records of solar activity are also well presented in the environmental proxy records. The additional ∼1500 years Bond cycle may result from interference between centennial-band solar cycles. Proxy evidence for Sun-climate relations is hardly present for Africa, South America and the marine realm; probably more due to a lack of information than a lack of response to solar forcing. At low latitudes, equatorward movement of the ITCZ (upward component of the Hadley cell) occurs upon a decrease in solar activity, explaining humidity changes for (1) Mesoamerica and adjacent North and South American regions and (2) East Africa and the Indian and Chinese Monsoon systems. At middle latitudes equatorward movement of the zonal circulation during solar minima probably (co-)induces wet and cool episodes in Western Europe, and Terra del Fuego as well as humidity changes in Southern Africa, Australia, New Zealand and the Mediterranean. The polar regions seem to expand during solar minima which, at least for the northern hemisphere is evident in southward extension of the Atlantic ice cover. The forcing-induced migration of climate regimes implies that solar forcing induces a non linear response at a given location. This complicates the assessment of Sun-climate relations and calls for nonlinear analysis of multiple long and high resolution records at regional scale. Unfortunately nonlinear Sun-climate analysis is still a largely barren field, despite the fact that major global climate configurations (e.g. the ENSO and AO) follow nonlinear dynamics. The strength of solar forcing relative to other forcings (e.g. volcanism, ocean circulation patterns, tides, and geomagnetism) is another source of dynamic responses. Notably the climatic effects of tides and geomagnetism are hitherto largely enigmatic. Few but well-dated studies suggest almost instantaneous, climatic deteriorations in response to rapid decreases in solar activity. Such early responses put severe limits to the solar forcing mechanisms and the extent of this phenomenon should be a key issue for future Sun-climate studies.     

The Influence of Total Solar Irradiance on Climate

To estimate the effect of the solar variability on the climate, two estimates of the solar intensity variations during the last three centuries have been used as forcing in numerical simulations. The model employed to carry out the experiments was the same coupled global ocean-atmosphere model used in a number of studies to assess the effect of the anthropogenic greenhouse gases on climate. The near surface temperature and the tropospheric temperature distribution shows a clear response to the variability of the solar input. Even the thermohaline circulation reacts on the large amplitudes in the forcing. In the stratosphere, the response pattern is similar as in the observations, however, the 11-year cycle found in the forcing data does not excite an appreciable response. This might be due to the missing parameterisation of the increase in the UV-radiation at the solar cycle maximum and the connected increase of the stratospheric ozone concentration.     

The Orbital Forcing of Climate Changes on Mars

Solar variability influences the climate of a planet by radiatively forcing changes over a certain timescale; orbital variations of a planet, which yield similar solar forcing modulations, can be studied within the same scientific context. It is known for Earth that obliquity changes have played a critical role in pacing glacial and interglacial eras. For Mars, such orbital changes have been far greater and have generated extreme variations in insolation. Signatures associated with the presence of water ice reservoirs at various positions across the surface of Mars during periods of different orbital configurations have been identified. For this reason, it has been proposed that Mars is currently evolving between ice ages. The advent of climate tools has given a theoretical frame to the study of orbitally-induced climate changes on Mars. These models have provided an explanation to many puzzling observations, which when put together have permitted reconstruction of almost the entire history of Mars in the last 10 million years. This paper proposes to give an overview of the scientific work dedicated to this topic.     

The Effect of Solar UV Irradiance Variations on the Earth's Atmosphere

The response of the lower and middle atmosphere to variations in solar irradiance typical of those observed to take place over the 11-year activity cycle has been investigated. The effects on radiative heating rates of changing total solar irradiance, solar spectral irradiance and two different assumptions concerning stratospheric ozone have been studied with a radiative transfer code. The response in the stratosphere depends on the changes specified in the ozone distribution which is not well known. A general circulation model (GCM) of the atmosphere up to 0.1 mbar (about 65 km) has been used to study the impacts of these changes on the thermodynamical structure. The results in the troposphere are very similar to those reported by Haigh99 using a quite different GCM. In the middle atmosphere the model is able to reproduce quite well the observed seasonal evolution of temperature and wind anomalies. Calculations of radiative forcing due to solar variation are presented. These show that the thermal infrared component of the forcing, due to warming of the stratosphere, is important but suggest a near balance between the longwave and shortwave effects of the increased ozone so that ozone change may not be important for net radiative forcing. However, the structure of the ozone change does affect the detailed temperature response and the spectral composition of the radiation entering the troposphere.     

Solar Variability Over the Past Several Millennia

The Sun is the most important energy source for the Earth. Since the incoming solar radiation is not equally distributed and peaks at low latitudes the climate system is continuously transporting energy towards the polar regions. Any variability in the Sun-Earth system may ultimately cause a climate change. There are two main variability components that are related to the Sun. The first is due to changes in the orbital parameters of the Earth induced by the other planets. Their gravitational perturbations induce changes with characteristic time scales in the eccentricity (～100,000 years), the obliquity (angle between the equator and the orbital plane) (～40,000 years) and the precession of the Earth’s axis (～20,000 years). The second component is due to variability within the Sun. A variety of observational proxies reflecting different aspects of solar activity show similar features regarding periodic variability, trends and periods of very low solar activity (so-called grand minima) which seem to be positively correlated with the total and the spectral solar irradiance. The length of these records ranges from 25 years (solar irradiance) to 400 years (sunspots). In order to establish a quantitative relationship between solar variability and solar forcing it is necessary to extend the records of solar variability much further back in time and to identify the physical processes linking solar activity and total and spectral solar irradiance. The first step, the extension of solar variability, can be achieved by using cosmogenic radionuclides such as ¹⁰Be in ice cores. After removing the effect of the changing geomagnetic field, a 9000-year long record of solar modulation was obtained. Comparison with paleoclimatic data provides strong evidence for a causal relationship between solar variability and climate change. It will be the subject of the next step to investigate the underlying physical processes that link solar variability with the total and spectral solar irradiance.     

Solar Variability and the Earth's Climate: Introduction and Overview

Numerous attempts have been made over the years to link various aspects of solar variability to changes in the Earth's climate. There has been growing interest in this possible connection in recent years, spurred largely by the need to understand the natural causes of climate change, against which the expected global warming due to man's activities will have to be detected. The time scale of concern here is that of decades to centuries, and excludes the longer millennial scale in which orbital variations play a dominant role. The field has long been plagued by the lack of an acceptable physical mechanism by which solar variability can affect climate, but the discovery of variability in the Sun's total irradiance (the solar ``constant' of meteorology) by spacecraft instruments has pointed to a direct mechanism. Other less direct mechanisms that have been suggested involve variations in the Sun's ultraviolet flux and in the plasma outflow of the solar wind. The purpose of this paper is to summarize the current state of the field, emphasizing the proposed mechanisms as an introduction to the more detailed papers that follow. The particular case of sea-surface temperature data will be used as an illustration.     

Regional Response of the Climate System to Solar Forcing: The Role of the Ocean

A coupled climate model is used to explore the regional response of the climate system to solar forcing, with emphasis on the role of the ocean. It is shown that both the transient and the equilibrium response of surface temperature to changes in total solar irradiation is smaller over ocean than over land because of the ocean’s large heat capacity and the feedback involving evaporation. Furthermore, the advection of temperature anomalies and changes in ocean currents have an impact on the timing and the geographical distribution of the response. Nevertheless, at regional scales, the response to the forcing is embedded within the large internal variability of the system making the detection and analysis of the forced response difficult. Furthermore, this forced response could imply both changes in the mean state of the system as well as in its variability.     

Solar Influences on Dynamical Coupling Between the Stratosphere and Troposphere

We use a simplified atmospheric general circulation model (AGCM) to investigate the response of the lower atmosphere to thermal perturbations in the lower stratosphere. The results show that generic heating of the lower stratosphere tends to weaken the sub-tropical jets and the tropospheric mean meridional circulations. The positions of the jets, and the extent of the Hadley cells, respond to the distribution of the stratospheric heating, with low latitude heating displacing them poleward, and uniform heating displacing them equatorward. The patterns of response to the low latitude heating are similar to those found to be associated with solar variability in previous observational data analysis, and to the effects of varying solar UV radiation in sophisticated AGCMs. In order to investigate the chain of causality involved in converting the stratospheric thermal forcing to a tropospheric climate signal we conduct an experiment which uses an ensemble of model spin-ups to analyse the time development of the response to an applied stratospheric perturbation. We find that the initial effect of the change in static stability at the tropopause is to reduce the eddy momentum flux convergence in this region. This is followed by a vertical transfer of the momentum forcing anomaly by an anomalous mean circulation to the surface, where it is partly balanced by surface stress anomalies. The unbalanced part drives the evolution of the vertically integrated zonal flow. We conclude that solar heating of the stratosphere may produce changes in the circulation of the troposphere even without any direct forcing below the tropopause. We suggest that the impact of the stratospheric changes on wave propagation is key to the mechanisms involved.     

The impact of solar forcing on the variability in a coupled climate model

The impact of variations in solar irradiance on the variability of climate is still a topic of debate. Herein we assess the response of a coupled General Circulation Model (GCM) of intermediate complexity to an estimate of the solar variability since 1700 and to a series of idealized sinusoidal solar forcings. On the continental to global scale and averaged over periods longer than 30 years, the solar-induced variability dominates internal variability in the annual global mean surface air temperature. Locally and on the regional scale, the internal variability dominates. The dominant patterns of natural variability and explained variance are not affected by a variable solar forcing, the spectra however are sensitive. The control run shows a preferred decadal time scale of 18 year in a sea surface temperature mode associated with the North Atlantic Oscillation. The preferred decadal time scale disappears for a variable solar forcing. This is caused by small changes in oceanic circulation resulting in subsurface oceanic modes with modified structure and time scale.     

The Role of Dynamics in Solar Forcing

This paper reviews the solar influence on climate through stratospheric dynamical processes. There are two possible processes, both being a consequence of the wave-mean flow interaction in the upper stratosphere. One involves changes in the vertical propagation of planetary waves and the resultant tropospheric circulation change in the extratropics of the winter hemisphere. The other involves change in the global meridional circulation in the stratosphere and associated convective activity change in the tropics. These processes have been discussed on an 11-year solar cycle, but they are also applicable for centennial-scale solar influence on climate.     

High-energy solar-particle events

Conclusion Recent high-energy solar-particle events have been illustrated and described individually. The manner in which the observations have been used to develop and test concepts of the hydromagnetic behaviour of the solar plasma in space has been outlined. Some of the events have been analyzed in more detail than hitherto. The results seem to support the concept that the magnetic field of a sunspot tends to erupt, or to balloon, into space in the form of a great tongue or sack attached to the sun near the sunspot. Supersonic shock processes also must be involved in the propagation of disturbances in planetary space, and the fluxes of galactic and of solar particles may change in the vicinity of such shocks, but alone shocks do not appear to be capable of explaining all the observed effects.     

Co2 and Climate: a Geologist's View

Climate is discussed as an integral part of System Earth, determined by a complex interplay of numerous geological, biological and solar processes. The historical and geological record of changing climate and atmospheric CO₂ pressure does not support the current popular vision that this greenhouse gas is the dominant climate controlling agent. When empirically ante post tested against past global climate changes, the forecasts of the climate models mainly based on forcing by atmospheric CO₂ are not borne out. On the other hand, recent studies show that solar variability rather than changing CO₂ pressure is an important, probably the dominant climate forcing factor.     

Solar Variability and the Search for Corresponding Climate Signals

Instrumental and paleodata from the last centuries are investigated to get circumstantial evidence for external influences on the Earth's climate machine. Such influences could be of extraterrestrial and/or anthropogenic origin. Anthropogenic influences are separated from solar on superdecadal time scales and on a hemispheric level using a non-linear regression model. The function to be explained is the northern hemispheric temperature. The model contains two forcing components explicitly: A parameterized anthropogenic component, which describes the aggregated effect of greenhouse gases, aerosols and other anthropogenic climate impacts. A solar component, which describes the solar variability history. The solution of the regression model allows, under certain assumptions, a functional separation of the variability components and provides an estimation of their relative contributions to global warming during the last 140 years.     

Cubesat solar sail 3-axis stabilization using panel translation and magnetic torquing

A Cubesat mission with a deployable solar sail of 5 meter by 5 meter in a sun-synchronous low earth orbit is analyzed to demonstrate solar sailing using active attitude stabilization of the sail panel. The sail panel is kept parallel to the orbital plane to minimize aerodynamic drag and optimize the orbit inclination change caused by the solar pressure force normal to the sail surface. A practical control system is proposed, using a combination of small 2-dimensional translation of the sail panel and 3-axis magnetic torquing which is proved to have sufficient control authority over the gravity gradient and aerodynamic disturbance torques. Miniaturized CMOS cameras are used as sun and nadir vector attitude sensors and a robust Kalman filter is used to accurately estimate the inertially referenced body rates from only the sun vector measurements. It is shown through realistic simulation tests that the proposed control system, although inactive during eclipse, will be able to stabilize the sail panel to within ±2° in all attitude angles during the sunlit part of the orbit, when solar sailing is possible.     

皮卫星姿态估计及粗太阳敏感器标定算法研究

粗太阳敏感器是一种由多个光电池片配置组成的模拟式太阳敏感器,它结构简单、资源要求少,在皮卫星平台应用较为广泛。光电池光伏输出特性是粗太阳敏感器测量的基础,使用余弦关系近似等效会引入很大的误差。通过实际测试和深入分析,建立了更高精度的粗太阳敏感器输出特性等效数值模型,并以此为基础,推导了卫星姿态估计及粗太阳敏感器标定的联合在轨实时算法。算法采用多级耦合结构,由1个扩展卡尔曼滤波器(EKF)和6个卡尔曼滤波器组成,同时估计卫星姿态、卫星角速度以及卫星6个面共30个标定参数。仿真表明,和常规的EKF姿态估计算法相比,联合算法的运算量只增加了一半,而估计精度却提高了一个量级。     

航天器太阳圆面速度差/太阳视方向组合导航

针对航天器，尤其是深空探测器的自主导航问题，提出了一种新的太阳圆面速度差天文量测信息，该信息利用太阳的较差自转所造成的太阳圆面各点速度不同的特性，是探测器当前位置的函数，其几何本质是一个探测器的位置圆锥。在此基础上，基于太阳圆面速度差和太阳视方向互补的特性，提出了一种太阳圆面速度差/太阳视方向组合导航新方法，将太阳圆面速度差与太阳视方向2种量测量结合起来，实现了量测量之间的优势互补，进一步提高了导航性能。以太阳探测器为例进行了仿真，仿真结果表明相比较于单独用太阳圆面速度差或太阳视方向的导航方法，基于太阳圆面速度差/太阳视方向的组合导航方法精度分别提升了10.2%和16.0%。此外，还分析了光谱仪精度、采样周期和光谱仪数量对导航性能的影响，为深空探测自主导航提供了新的理论与方法。     

A new method of single celestial-body sun positioning based on theory of mechanisms

Considering defects of current single celestial-body positioning methods such as discon-tinuity and long period, a new sun positioning algorithm is herein put forward. Instead of tradi-tional astronomical spherical trigonometry and celestial coordinate system, the proposed new positioning algorithm is built by theory of mechanisms. Based on previously derived solar vector equations (from a C1R2P2 series mechanism), a further global positioning method is developed by inverse kinematics. The longitude and latitude coordinates expressed by Greenwich mean time (GMT) and solar vector in local coordinate system are formulated. Meanwhile, elimination method of multiple solutions, errors of longitude and latitude calculation are given. In addition, this algo-rithm has been integrated successfully into a mobile phone application to visualize sun positioning process. Results of theoretical verification and smart phone’s test demonstrate the validity of pre-sented coordinate’s expressions. Precision is shown as equivalent to current works and is acceptable to civil aviation requirement. This new method solves long-period problem in sun sight running fix-ing and improves applicability of sun positioning. Its methodology can inspire development of new sun positioning device. It would be more applicable to be combined with inertial navigation systems for overcoming discontinuity of celestial navigation systems and accumulative errors of inertial nav-igation systems.     

Signature of the 11-Year Cycle in the Upper Atmosphere

In the last 45 years I have studied the thermal structure of the atmosphere from the thermosphere down to the stratosphere, and found evidence of its variability in relationship with the change of solar irradiation during the 11-year solar cycle. I would review, in the light of recent model results, the measurements which I had made since the 1960s and which, for some of them, did not find any explanation at the time of their publication. The data were obtained by two different techniques, rockets and lidars and correspond to different regions of the atmosphere from the upper thermosphere to the stratosphere. The expectation was until recently that the atmosphere should be warmed by an increase of solar flux in the course of the solar cycle due to the increase of UV flux. It has been shown to be the case in the tropical stratosphere and at all latitudes in the upper thermosphere. But, at high and mid latitudes and at other altitudes, the reverse situation was found to exist and, until recently, this cooling observed in parts of the atmosphere with increasing solar flux had never been simulated by models. In addition to reviewing our own data, the paper will present recent results using other dataset which support our observations. It is only recently that we succeeded with a model able to tune the forcing by planetary waves at the tropopause level and thus reproduce such behaviour.