Recent advances in observations and modeling of the solar ultraviolet and X-ray spectral irradiance

There have been significant, recent advances in understanding the solar ultraviolet (UV) and X-ray spectral irradiance from several different satellite missions and from new efforts in modeling the variations of the solar spectral irradiance. The recent satellite missions with solar UV and X-ray spectral irradiance observations include the X-ray Sensor (XRS) aboard the series of NOAA GOES spacecraft, the Upper Atmosphere Research Satellite (UARS), the SOHO Solar EUV Monitor (SEM), the Solar XUV Photometers (SXP) on the Student Nitric Oxide Explorer (SNOE), the Solar EUV Experiment (SEE) aboard the Thermosphere, Ionosphere, Mesosphere, Dynamics, and Energetics (TIMED) satellite, and the Solar Radiation and Climate Experiment (SORCE) satellite. The combination of these measurements is providing new results on the variability of the solar ultraviolet irradiance throughout the ultraviolet range shortward of 200 nm and over a wide range of time scales ranging from years to seconds. The solar UV variations of flares are especially important for space weather applications and upper atmosphere research, and the period of intense solar storms in October–November 2003 has provided a wealth of new information about solar flares. The new efforts in modeling these solar UV spectral irradiance variations range from simple empirical models that use solar proxies to more complicated physics-based models that use emission measure techniques. These new models provide better understanding and insight into why the solar UV irradiance varies, and they can be used at times when solar observations are not available for atmospheric studies.     

Comparison of solar soft X-ray irradiance from broadband photometers to a high spectral resolution rocket observation

The solar soft X-ray (XUV; 1–30 nm) radiation is highly variable on all time scales and strongly affects the ionosphere and upper atmosphere of Earth, Mars, as well as the atmospheres and surfaces of other planets and moons in the solar system; consequently, the solar XUV irradiance is important for atmospheric studies and for space weather applications. While there have been several recent measurements of the solar XUV irradiance, detailed understanding of the solar XUV irradiance, especially its variability during flares, has been hampered by the lack of high spectral resolution measurements in this wavelength range. The conversion of the XUV photometer signal into irradiance requires the use of a solar spectral model, but there has not been direct validation of these spectral models for the XUV range. For example, the irradiance algorithm for the XUV Photometer System (XPS) measurements uses multiple CHIANTI spectral models, but validation has been limited to other solar broadband measurements or with comparisons of the atmospheric response to solar variations. A new rocket observation of the solar XUV irradiance with 0.1 nm resolution above 6 nm was obtained on 14 April 2008, and these new results provide a first direct validation of the spectral models used in the XPS data processing. The rocket observation indicates very large differences for the spectral model for many individual emission features, but the differences are significantly smaller at lower resolution, as expected since the spectral models are scaled to match the broadband measurements. While this rocket measurement can help improve a spectral model for quiet Sun conditions, many additional measurements over a wide range of solar activity are needed to fully address the spectral model variations. Such measurements are planned with a similar instrument included on NASA’s Solar Dynamics Observatory (SDO), whose launch is expected in 2009.     

New flare model using recent measurements of the solar ultraviolet irradiance

The solar photon output from the Sun, which was once thought to be constant, varies considerably over time scales from seconds during solar flares to years due to the solar cycle. This is especially true in the wavelengths shorter than 190 nm. These variations cause significant deviations in the Earth and space environment on similar time scales, which then affects many things including satellite drag, radio communications, atmospheric densities and composition of particular atoms, molecules, and ions of Earth and other planets, as well as the accuracy in the Global Positioning System (GPS). The Flare Irradiance Spectral Model (FISM) is an empirical model that estimates the solar irradiance at wavelengths from 0.1 to 190 nm at 1 nm resolution with a time cadence of 60 s. This is a high enough temporal resolution to model variations due to solar flares, for which few accurate measurements at these wavelengths exist. This model also captures variations on the longer time scales of solar rotation (days) and solar cycle (years). Daily average proxies used are the 0–4 nm irradiance, the Mg II c/w, F10.7, as well as the 1 nm bins centered at 30.5 nm, 121.5 (Lyman Alpha), and 36.5 nm. The GOES 0.1–0.8 nm irradiance is used as the flare proxy. The FISM algorithms are given, and results and comparisons are shown that demonstrate the FISM estimations agree within the stated uncertainties to the various measurements of the solar Vacuum Ultraviolet (VUV) irradiance.     

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.     

Total solar irradiance and climate

The solar radiation is the fundamental source of energy that drives the Earth’s climate and sustains life. The variability of this output certainly affects our planet. In the last two decades an enormous advance in the understanding of the variability of the solar irradiance has been achieved. Space-based measurements indicate that the total solar irradiance changes at various time scales, from minutes to the solar cycle.Climate models show that total solar irradiance variations can account for a considerable part of the temperature variation of the Earth’s atmosphere in the pre-industrial era. During the 20th century its relative influence on the temperature changes has descended considerably. This means that other sources of solar activity as well as internal and man-made causes are contributing to the Earth’s temperature variability, particularly the former in the 20th century.Some very challenging questions concerning total solar irradiance variations and climate have been raised: are total solar irradiance variations from cycle to cycle well represented by sunspot and facular changes? Does total solar irradiance variations always parallel the solar activity cycle? Is there a long-term variation of the total solar irradiance, and closely related to this, is the total solar irradiance output of the quiet sun constant? If there is not a long-term trend of total solar irradiance variations, then we need amplifying mechanisms of total solar irradiance to account for the good correlations found between total solar irradiance and climate. The latter because the observed total solar irradiance changes are inconsequential when introduced in present climate models.     

Reconstruction of solar UV irradiance

Understanding solar influence on the Earth’s climate requires a reconstruction of solar irradiance for the pre-satellite period. Considerable advances have been made in modelling the irradiance variations at wavelengths longer than 200 nm. At shorter wavelengths, however, the LTE approximation usually taken in such models fails, which makes a reconstruction of the solar UV irradiance a rather intricate problem. We choose an alternative approach and use the observed SUSIM UV spectra to extrapolate available models to shorter wavelengths.     

How much of the solar irradiance variations is caused by the magnetic field at the solar surface?

The contribution to total solar irradiance variations by the magnetic field at the solar surface is estimated. Detailed models of the irradiance changes on the basis of magnetograms show that magnetic features at the solar surface account for over 90% of the irradiance variations on a solar rotation time scale and at least 70% on a solar cycle time scale. If the correction to the VIRGO record proposed by Fröhlich & Finsterle (2001) is accepted, then magnetic features at the solar surface are responsible for over 90% of the solar cycle irradiance variations as well.     

Solar UV irradiance variation during cycles 22 and 23

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has been measuring solar UV irradiances since October 1991, a period which includes the decline of solar cycle 22 followed by the rise of cycle 23. Daily solar measurements include scans over the wavelength range 115–410 nm at 1.1 nm resolution. As expected, the measured time series of UV irradiances exhibit strong periodicities in solar cycle and solar rotation. For all wavelengths, the UV irradiance time series are similar to that of the Mg II core-to-wing ratio. During solar cycle 22, the irradiance of the strong Ly- line varied by more than a factor of two. The peak-to-peak irradiance variation declined with increasing wavelength, reaching 10% just below the Al edge at 208 nm. Between the Al edge and 250 nm the variation was 6–7%. Above 250 nm, the variation declines further until none is observed above 290 nm. Preliminary results for the first portion of cycle 23 indicate that the far UV below the Al edge is rising at about the same rate as the Mg II index while the irradiances in the Ly- emission line and for wavelengths longer than the Al edge are rising more slowly — even after accounting for the lower level of activity of cycle 23.     

SCIAMACHY solar irradiance observation in the spectral range from 240 to 2380 nm

The SCanning Imaging Absorption Spectrometer for Atmospheric CHartographY (SCIAMACHY) is part of the payload of ESA’s Environmental Satellite ENVISAT which was launched into a sun-synchronous polar orbit on 2002-03-01. It is the first spaceborne instrument covering a wavelength range from 240 to 2380 nm thus including ultraviolet, visible and near infrared spectral regions.The main purpose of SCIAMACHY is to determine the amount and distribution of a large number of atmospheric trace constituents by measuring the radiance backscattered from the Earth. In addition, several solar observations are performed with daily or orbital frequency.The presented results will cover the following topics: (a) comparison of the solar irradiance measured by SCIAMACHY with data from the instruments SOLSPEC/SOLSTICE/SUSIM and a solar spectrum derived by Kurucz; (b) comparison of the SCIAMACHY solar Mg II index with GOME and NOAA data; (c) correlation of the relative change of solar irradiance measured by SCIAMACHY with the sun spot index.The mean solar irradiance for each of the 8 SCIAMACHY channels agrees with the Kurucz data within ±2–3%. The presented analysis proves that SCIAMACHY is a valuable tool to monitor solar irradiance variations.     

Irradiance models

Measurements of solar irradiance have revealed variations at all the sampled time scales (ranging from minutes to the length of the solar cycle). One important task of models is to identify the causes of the observed (total and spectral) irradiance variations. Another major aim is to reconstruct irradiance over time scales longer than sampled by direct measurements in order to consider if and to what extent solar irradiance variations may be responsible for global climate change. Here, we describe recent efforts to model solar irradiance over the current and the previous two solar cycles. These irradiance models are remarkably successful in reproducing the observed total and spectral irradiance, although further improvements are still possible.     

Results of solar observations by the CORONAS-F payload

The CORONAS-F mission experiments and results have been reviewed. The observations with the DIFOS multi-channel photometer in a broad spectral range from 350 to 1500 nm have revealed the dependence of the relative amplitudes of p-modes of the global solar oscillations on the wavelength that agrees perfectly well with the earlier data obtained in a narrower spectral ranges. The SPIRIT EUV observations have enabled the study of various manifestations of solar activity and high-temperature events on the Sun. The data from the X-ray spectrometer RESIK, gamma spectrometer HELICON, flare spectrometer IRIS, amplitude–temporal spectrometer AVS-F, and X-ray spectrometer RPS-1 have been used to analyze the X- and gamma-ray emission from solar flares and for diagnostics of the flaring plasma. The absolute and relative content of various elements (such as potassium, argon, and sulfur) of solar plasma in flares has been determined for the first time with the X-ray spectrometer RESIK. The Solar Cosmic Ray Complex monitored the solar flare effects in the Earth’s environment. The UV emission variations recorded during solar flares in the vicinity of the 120-nm wavelength have been analyzed and the amplitude of relative variations has been determined.     

The role of the solar magnetic field systems in modulating the solar irradiance

Variations of indices that characterize various systems of the large-scale solar magnetic field (LSSMF) - magnetic field multipoles of different order, LSSMF energy index, index of the effective solar multipole, etc.- are compared with variations of the solar irradiance in different frequency ranges during 1978–1996. The role of the local and global magnetic fields in modulating the solar irradiance is investigated in various time intervals, in particular, in different phases of the 11-year solar cycle.     

Non-axisymmetrical distributions of solar magnetic activity and irradiance

Active longitudes play an important role in spatial organization of solar activity. These zones associated with complexes of solar activity may persist for 20–40 consecutive rotations, and may be caused by large-scale non-axisymmetrical components of the global magnetic field. These zones of the field concentrations are 20°–40° wide and during subsequent rotations tend to reappear at constant longitude or drift slightly eastward or westward. Since the magnetic field is the principle source of the variations of radiation on the solar surface the active longitudes affect the solar irradiance received at the Earth. In this paper I study connections between the active longitudes and irradiance variations using VIRGO/SOHO, KPO and WSO data, which covered the transition period from solar cycle 22 to cycle 23 and rising phase of cycle 23. The result of this investigation is that active longitudes are associated with increases of the total solar irradiance and are prime sources of enhanced EUV radiation and coronal heating.     

Reconstruction of solar irradiance using the Group sunspot number

We present a reconstruction of total solar irradiance since 1610 to the present based on variations of the surface distribution of the solar magnetic field. The latter is calculated from the historical record of the Group sunspot number using a simple but consistent physical model. Our model successfully reproduces three independent data sets: total solar irradiance measurements available since 1978, total photospheric magnetic flux from 1974 and the open magnetic flux since 1868 (as empirically reconstructed from the geomagnetic aa-index). The model predicts an increase in the total solar irradiance since the Maunder Minimum of about 1.3 Wm⁻².     

Thermospheric neutral density response to solar forcing

Recent measurements by the Solar EUV (Extreme Ultra Violet) Experiment (SEE) aboard the Thermosphere–Ionosphere–Mesosphere Energetics and Dynamics satellite (TIMED) provide solar EUV spectral irradiance with adequate spectral and temporal resolution, and thus the opportunity to use solar measurements directly in upper atmospheric general circulation models. Thermospheric neutral density is simulated with the NCAR Thermosphere–Ionosphere–Electrodynamic General Circulation Model (TIEGCM) using TIMED/SEE measurements and using the EUVAC solar proxy model. Neutral density is also calculated using the NRLMSISE-00 empirical model. These modeled densities are then compared to density measurements derived from satellite drag data. It is found that using measured solar irradiance in the general circulation model can improve density calculations compared to using the solar proxy model. It is also found that the general circulation model can improve upon the empirical model in simulating geomagnetic storm effects and the solar cycle variation of neutral density.     

Regional tropospheric responses to long-term solar activity variations

The influence of ∼200-year solar activity variations (de Vries cyclicity) on climatic parameters has been analyzed. Analysis of palaeoclimatic data from different regions of the Earth for the last millennium has shown that ∼200-year variations in solar activity give rise to a pronounced climatic response. Owing to a nonlinear character of the processes in the atmosphere–ocean system and the inertia of this system, the climatic response to the global influence of solar activity variations has been found to have a regional character. The regions where the climatic response to long-term solar activity variations is stable and the regions where the climatic response is unstable, both in time and space, have been revealed. It has also been found that a considerable lag of the climatic response and reversal of its sign with respect to the solar signal can occur. Comparison of the obtained results with the simulation predictions of the atmosphere–ocean system response to long-term solar irradiance variations (T > 40 years) has shown that there is a good agreement between experimental and simulation results.     

太阳光谱辐射强度变化对地球能量平衡的影响分析

目前太阳对地球能量平衡影响的研究大都是以太阳总辐射通量密度作为输入参数的. 本文以美国航空航天局（National Aeronautics and Space Administration,NASA）太阳辐射与气候实验项目的卫星实测数据为基础,对太阳上升相（2010年上半年）和下降相（2007年12月）期间太阳光谱变化对地球能量平衡的影响进行了研究. 结果表明,2010年上半年较强的太阳总辐射通量密度主要是由紫外及红外波段的能量增强引起的,其在200～400nm 和760～4000nm波段内的平均能量分别增加了0.11%和0.05%,而在 400～760nm可见光区的能量却呈减小趋势,平均减小量为0.05%. 通过对MLS2.2全球臭氧日数据进行再分析后发现,相对于2007年12月,2010年上半年平流层臭氧浓度也有所增加,其中在太阳紫外辐射呈现较大增强的2月和3月,其臭氧增量也相对较大,最大值分别出现在33km和40km处,值为0.6mL·m-3和0.62mL·m-3. 因此,可见光区能量减弱与平流层臭氧浓度增加的双重削弱作用致使虽然2010年上半年的太阳总辐射通量密度较大,但是到达对流层顶的太阳辐射却有所减小,最大减小量出现在3月,值为0.15W·m-2. 这一结果说明,太阳活动或总辐射通量密度的增强也有可能对地球对流层系统起到冷却作用.      

Scientific requirements for future spatially resolved white-light and broad-band high-cadence observations of the Sun

Several important issues are open in the field of solar variability and they wait their solution which up to now was attempted using critical ground-based instrumentations. However, accurate photometric data are attainable only from space. New observational material should be collected with high enough spatial and spectral resolution, covering the whole visible range of the electromagnetic spectrum as well infrared and ultraviolet to reconstruct the total solar irradiance: (1) the absolute contributions of different small-scale structural entities of the solar atmosphere from the white light flares and from micro-flares are still poorly known; (2) we do not know the absolute contributions of different structural elements of the solar atmosphere to the long-term and to the cyclic variations of the solar irradiance, including features of the polar regions of the Sun; (3) the variations of the chromospheric magnetic network are still poorly evaluated; (4) only scarce information is available about the spectral variations of different small-scale features in the high photosphere. Variability of the Sun in white light can be studied with higher spectral, spatial and time resolution using space-born telescopes, which are more appropriate for this purpose than ground based observatories because of better seeing conditions, no interference of the terrestrial atmosphere and a more precise calibration procedure. Scientific requirements for such observations and the possible experimental tools proposed for their solution. Suggested solar studies have broader astrophysical importance.     

Solar irradiance forecast and far-side imaging

This paper presents a new approach to forecasting short-term Lyα solar irradiance variations due to the presence and evolution of magnetically heated regions in the Sun’s outer atmosphere. This scheme is based on images of the solar disk at key wavelengths, currently Ca II K filtergrams, maps of backscattered solar Lyα from the interplanetary medium, and helioseismic images of large far-side active regions. The combination of these resources allows accurate forecasts of the UV solar irradiance several days in advance. The technique takes into consideration the evolution of recently observed activity on the Sun’s near surface as well as active regions on the Sun’s far side. The far-side helioseismic maps and the Lyα backscattering are very important, because of the long period of time features spend on the Sun’s far side compared with their typical evolution time and their relatively sudden appearance on the near side. We describe the basics of the forecasting technique and apply it to a case study that shows how the technique dramatically improves Lyα irradiance forecasting. An extension of the technique described here promises realistic forecasts of the entire FUV/EUV solar spectral irradiance spectrum.     

Distinct EUV minimum of the solar irradiance (16–40 nm) observed by SolACES spectrometers onboard the International Space Station (ISS) in August/September 2009

In the field of terrestrial climatology the continuous monitoring of the solar irradiance with highest possible accuracy is an important goal. SolACES as a part of the ESA mission SOLAR on the ISS is measuring the short-wavelength solar EUV irradiance from 16–150 nm. This data will be made available to the scientific community to investigate the impact of the solar irradiance variability on the Earth’s climate as well as the thermospheric/ionospheric interactions that are pursued in the TIGER program. Since the successful launch with the shuttle mission STS-122 on February 7th, 2008, SolACES initially recorded the low EUV irradiance during the extended solar activity minimum. Thereafter it has been observing the EUV irradiance during the increasing solar activity with enhanced intensity and changing spectral composition. SolACES consists of three grazing incidence planar grating spectrometers. In addition there are two three-signal ionisation chambers, each with exchangeable band-pass filters to determine the absolute EUV fluxes repeatedly during the mission. One important problem of space-borne instrumentation recording the solar EUV irradiance is the degradation of the spectrometer sensitivity. The two double ionisation chambers of SolACES, which could be re-filled with three different gases for each recording, allow the recalibration of the efficiencies of the three SolACES spectrometers from time to time.