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.     

Active region properties and irradiance variations

Total Solar Irradiance (TSI) has been measured for more than three decades. These observations demonstrate that total irradiance changes on time scales ranging from minutes to years and decades. Considerable efforts have been made to understand the physical origin of irradiance variations and to model the observed changes using measures of sunspots and faculae. In this paper, we study the short-term variations in TSI during the declining portion and minimum of solar cycle 22 and the rising portion of cycle 23 (1993–1998). This time interval of low solar activity allows us to study the effect of individual sunspot groups on TSI in detail. In this paper, we indicate that the effect of sunspot groups on total irradiance may depend on their type in the Zürich classification system and/or their evolution, and on their magnetic configuration. Some uncertainties in the data and other effects are also discussed.     

The integrated sun as a (magnetic) variable star

Although the source of the solar magnetic cycle is not yet fully understood, it is clear that observed solar variations result primarily from surface manifestations of magnetic field associated active regions. Globally, and at depth in the solar interior, variations of convective efficiency following from magnetic field variations may cause luminosity and diameter changes. The waxing and waning of sunspots and faculae in the photosphere lead to luminosity variations on time scales of days to months, or longer. In the chromosphere magnetically controlled plage leads to a variation of UV flux and line emission such as Ca II H and K. The coronal structure is dominated by background magnetic fields and experiences factor of 2 variations of total mass over the solar cycle. Relative modulation characteristics are remarkably similar from the different atmospheric levels with their unique activity types.     

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.     

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.     

Flux emergence,flux imbalance,magnetic free energy and solar flares

Emergence of complex magnetic flux in the solar active regions lead to several observational effects such as a change in sunspot area and flux embalance in photospheric magnetograms. The flux emergence also results in twisted magnetic field lines that add to free energy content. The magnetic field configuration of these active regions relax to near potential-field configuration after energy release through solar flares and coronal mass ejections. In this paper, we study the relation of flare productivity of active regions with their evolution of magnetic flux emergence, flux imbalance and free energy content. We use the sunspot area and number for flux emergence study as they contain most of the concentrated magnetic flux in the active region. The magnetic flux imbalance and the free energy are estimated using the HMI/SDO magnetograms and Virial theorem method. We find that the active regions that undergo large changes in sunspot area are most flare productive. The active regions become flary when the free energy content exceeds 50% of the total energy. Although, the flary active regions show magnetic flux imbalance, it is hard to predict flare activity based on this parameter alone.     

Energy balance in solar active regions: The dip of April, 1985

The presence of a solar active region affects the luminosity of the sun. Sunspots directly produce “dips” in the total solar irradiance approximately proportionally to their projected area, while faculae produce excess energy. These effects were discovered during the solar maximum period of 1980, and in this paper we examine the sunspot effect during solar minimum. We examine the “dip” due to an active region in April, 1985, as observed in the total solar irradiance by the ACRIM instrument on the Solar Maximum Mission. These data (obtained after the spacecraft repair in May, 1984) have simple variations, relative to those observed in 1980, because of the reduced level of activity approaching solar minimum. We find that the PSI index of projected sunspot area as defined in 1980 appears to describe this “dip” satisfactorily.     

Reconstruction of the solar UV irradiance back to 1974

The variability of the solar UV irradiance has strong effects on the terrestrial atmosphere. In order to study the solar influence for times when no UV observations are available, it is necessary to reconstruct the variation of the UV irradiance with time on the basis of proxies. We present reconstructions of the solar UV irradiance based on the analysis of space-based and ground-based magnetograms of the solar disk going back to 1974. With COde for Solar Irradiance (COSI) we calculate solar intensity spectra for the quiet Sun and different active regions and combine them according to their fractional area on the solar disk, whereby their time-dependent contributions over the solar cycle lead to a variability in radiation. COSI calculates the continuum and line formation under conditions which are out of local thermodynamic equilibrium (non-LTE). The applied temperature and density structures include the chromosphere and transition region, which is particularly important for the UV. The reconstructions are compared with observations.     

Solar magnetic feature detection and tracking for space weather monitoring

We present an automated system for detecting, tracking, and cataloging emerging active regions throughout their evolution and decay using SOHO Michelson Doppler Interferometer (MDI) magnetograms. The SolarMonitor Active Region Tracking (SMART) algorithm relies on consecutive image differencing to remove both quiet-Sun and transient magnetic features, and region-growing techniques to group flux concentrations into classifiable features. We determine magnetic properties such as region size, total flux, flux imbalance, flux emergence rate, Schrijver’s R-value, R^∗ (a modified version of R), and Falconer’s measurement of non-potentiality. A persistence algorithm is used to associate developed active regions with emerging flux regions in previous measurements, and to track regions beyond the limb through multiple solar rotations. We find that the total number and area of magnetic regions on disk vary with the sunspot cycle. While sunspot numbers are a proxy to the solar magnetic field, SMART offers a direct diagnostic of the surface magnetic field and its variation over timescale of hours to years. SMART will form the basis of the active region extraction and tracking algorithm for the Heliophysics Integrated Observatory (HELIO).     

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.     

Observations of the interactions of heavy ions from Comet P/Giacobini-Zinner with the solar wind

During the encounter between the ICE spacecraft and Comet Giacobini-Zinner, intense fluxes of energetic heavy ions were observed at distances up to 4 × 10⁶ km from the comet. These ions were observed with steep energy spectra and highly anisotropic angular distributions, and are consistent with a composition comprising mainly ions from the water group. The flux versus time profiles have a general fall-off with increasing distance from the comet, but are modulated by both changes in the magnetic field direction and the solar wind velocity, the magnetic field variations being mainly responsible for variations on a time scale of minutes, and the solar wind velocity variations being responsible for much larger time-scale modulations, such as the inbound/outbound asymmetry of the intensity profile. In this paper we present correlated observations of heavy ions, the solar wind velocity and the magnetic field direction, and compare the observations of the ions with the theoretical predictions for their variations with distance from the comet, with the solar wind velocity and with the magnetic field direction.     

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.     

Comparison of heliospheric conditions near the earth during four recent solar maxima

Statistical properties of the daily averaged values of the solar activity (sunspot numbers, total solar irradiance and 10.7 cm radio emission indices), the solar wind plasma and the interplanetary magnetic field parameters near the Earth’s orbit are investigated for a period from 1964 to 2002 covering the maxima of four solar cycles from 20th to 23rd. Running half-year averages show significant solar cycle variations in the solar activity indices but only marginal and insignificant changes in comparison with background fluctuations for heliospheric bulk plasma and magnetic field parameters. The current 23rd cycle maximum is weaker than 21st and 22nd maxima, but slightly stronger than 20th cycle in most of solar and heliospheric manifestations.     

太阳光球磁场特征物理量在质子事件短期预报中的应用

利用SOHO/MDI全日面纵向磁图, 计算了三个描述太阳活动区磁场复杂性和非势性的特征物理量, 即纵向磁场最大水平梯度Bz, 强梯度中性线长度L, 孤立奇点数目η. 为检验太阳光球磁场特征在质子事件短期预报中是否有效, 采用BP神经网络方法, 建立了基于这三个磁场特征物理量简单的太阳质子事件短期(24h)预报模型. 模 型在对2002年和2003年连续两年的样本检测中, 有很高的准确率(2002年和2003年 分别为90 %, 87.54 %)和较高的 质子事件报准率(2002年和2003年分别为60 %, 75 %),从而为光球磁场特征物理量作为质子事件预报的有效因子提供了依据.      

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.     

Variations of mid-term periodicities in solar activity physical phenomena

In this work we make an analysis of significant periodicities shown by phenomena linked to solar activity such as coronal hole area, radio emission in the 10.7 cm band and sunspots. We use the wavelet method that gives information in the frequency and time domains. Of particular interest are the mid-term periodicities (1–2 yrs). Over the whole period, coronal holes and radio variations show an important annual variation and a quasi-biannual periodicity. The increase in these variations is most important around the years of maximum solar activity. When the time series are separated in low and high frequencies, the latter are modulated by the general solar cycle. Although somewhat shifted in frequency, these periodicities might well correspond with those found in cosmic ray intensity, solar magnetic flux and other terrestrial and interplanetary phenomena as a wavelet coherence analysis of these series with the solar magnetic flux reveals.     

Observational study of magnetic chirality of solar active regions

We present the evolution of magnetic field and relationship with the magnetic (current) helicity in solar active regions from a series of photospheric vector magnetograms obtained at Huairou Solar Observing Station near Beijing, and also longitudinal magnetograms by MDI of SOHO, white light and 171 Å images by TRACE and soft X-ray images by Yohkoh.The conclusions in the analysis of the formation process of complex and delta magnetic configuration in some super active regions are the following: (1) The magnetic shear and gradient provide the non-potentiality of the magnetic field of active regions reflecting the existence of electric current. (2) Some of large-scale delta active regions could be due to the emergence of highly sheared non-potential magnetic flux bundles from the subatmosphere with amount of magnetic helicity, in addition to the emergence of twisted magnetic ropes. (3) We also present some results on the study of the magnetic (current) helicity in solar active regions.     

太阳辐照度下降的有效黑子模式和太阳辐照度可变性的有效光球活动区模式

本文作者提出了描述太阳辐照度下降的新模式--有效黑子模式;对1980年4月1日至7月16日的北京天文台黑子照片进行了保守测量;就有效黑子模式计算出的太阳辐照度与同一时期"太阳峰年使者"卫星观测到的太阳总辐照度进行了比较,两者在数值上非常接近,并且呈甚佳线性相关;进而提出了描述太阳辐照度可变性的新模式--有效光球活动区模式;从而在上述基础上提出了今后应继续的工作。      

Fine-scale structure of solar magnetic fields

Seeing limitations of the earth's atmosphere have prevented us from spatially resolving most of the basic magnetic flux elements on the sun, since their sizes are all well below one sec of arc (excluding sunspots). No space experiment to overcome this limitation has yet been performed, but the first step will be taken with Spacelab 2.Direct mapping of the circular polarization in spectral lines provides us with information on the morphology and evolution of the partially resolved magnetic structures. In reviewing recent results, special attention is payed to the question of flux disappearance, since it is fundamental for understanding the solar cycle, and depends on a knowledge of the fine-scale structures.The strong-field (kG) nature of the photospheric flux was revealed more than a decade ago using polarization recordings in pairs of spectral lines. A breakthrough in the use of spectral information to deduce the properties of the spatially unresolved magnetic fluxtubes has recently been achieved through the conversion of a Fourier transform spectrometer (FTS) into a polarimeter for Zeeman-effect recordings. We first use the FTS data to illustrate the diagnostic contents of the line-ratio technique, and then indicate how a statistical approach with 400 Fe I lines has recently been applied. In particular we discuss the implications of the observed Stokes V asymmetries for fluxtube dynamics.Finally the ongoing search for a small-scale “turbulent” magnetic field of mixed polarities is described. Observational limits derived from direct magnetograms, spectral line broadening, and the Hanle effect are illustrated.     

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.