Solar flux and its variations

We present data on the solar irradiance as derived from a number of sources. An attempt was made to bring these data onto a uniform scale. The results are presented in Table IV and Figure 6. Summation of fluxes at all wavelengths yields a figure of 1357.826 W m^-2 for the solar constant. Estimates are made of the solar flux variations due to flares, active regions (slowly varying component), solar rotation and the 11-year cycle.Solar activity does not produce a significant variation in the value of the solar constant. Nevertheless, variations in the X-ray and extreme ultraviolet portions of the solar flux may be several orders of magnitude during solar activity, especially at times of major flares. It is well established that these short wavelength flux enhancements cause significant changes in the terrestrial ionosphere.     

Solar Surface Magnetism and Irradiance on Time Scales from Days to the 11-Year Cycle

The uninterrupted measurement of the total solar irradiance during the last three solar cycles and an increasing amount of solar spectral irradiance measurements as well as solar imaging observations (magnetograms and photometric data) have stimulated the development of models attributing irradiance variations to solar surface magnetism. Here we review the current status of solar irradiance measurements and modelling efforts based on solar photospheric magnetic fields. Thereby we restrict ourselves to the study of solar variations from days to the solar cycle. Phenomenological models of the solar atmosphere in combination with imaging observations of solar electromagnetic radiation and measurements of the photospheric magnetic field have reached high enough quality to show that a large fraction (at least, about 80%) of the solar irradiance variability can be explained by the radiative effects of the magnetic activity present in the photosphere. Also, significant progress has been made with magnetohydrodynamic simulations of convection that allow us to relate the radiance of the photospheric magnetic structures to the observations.     

Reconstruction of Past Solar Irradiance

Accurate measurements of solar irradiance started in 1978, but a much longer time series is needed in order to uncover a possible influence on the Earth's climate. In order to reconstruct the irradiance prior to 1978 we require both an understanding of the underlying causes of solar irradiance variability as well as data describing the state of the Sun (in particular its magnetic field) at the relevant epochs.Evidence is accumulating that on the time-scale of the solar cycle or less, variations in solar irradiance are produced mainly by changes in the amount and distribution of magnetic flux on the solar surface. The main solar features contributing to a darkening of the Sun are sunspots, while active-region faculae and the network lead to a brightening. There is also increasing evidence for secular changes of the solar magnetic field and the associated of solar brightness variability. In part the behavior of sun-like stars is used as a guide of such secular changes.Under the assumption that solar irradiance variations are due to solar surface magnetism on all relevant time scales it is possible to reconstruct the irradiance with some reliability from today to around 1874, and with lower accuracy back to the Maunder minimum. One major problem is the decreasing amount and accuracy of the relevant data with age. In this review the various reconstructions of past solar irradiance are presented and the assumptions underlying them are scrutinized.     

Modelling solar irradiance variations: Comparison with observations,including line-ratio variations

Solar irradiance variations show a strong temporal and spectral dependence. The progression of the Sun through its activity cycle as well as solar rotation are mirrored in the irradiance variations. The spectral dependence is such that the variations are several magnitudes larger in the EUV than in the visible or infrared.We present a simple 3-component model that is based on the assumption that changes in the solar flux are exclusively due to changes in spot and facular coverage. We compare our model to observations of the spectral solar irradiance variations.Despite its simplicity, we find that the agreement between our model and the observations is surprisingly good. We also explore the reliability and the limitations of our approach by comparing observations of the solar facular contrast and of the changes in spectral line depths with our calculations.     

Photospheric and Subphotospheric Dynamics of Emerging Magnetic Flux

Magnetic fields emerging from the Sun’s interior carry information about physical processes of magnetic field generation and transport in the convection zone. Soon after appearance on the solar surface the magnetic flux gets concentrated in sunspot regions and causes numerous active phenomena on the Sun. This paper discusses some properties of the emerging magnetic flux observed on the solar surface and in the interior. A statistical analysis of variations of the tilt angle of bipolar magnetic regions during the emergence shows that the systematic tilt with respect to the equator (the Joy’s law) is most likely established below the surface. However, no evidence of the dependence of the tilt angle on the amount of emerging magnetic flux, predicted by the rising magnetic flux rope theories, is found. Analysis of surface plasma flows in a large emerging active region reveals strong localized upflows and downflows at the initial phase of emergence but finds no evidence for large-scale flows indicating future appearance a large-scale magnetic structure. Local helioseismology provides important tools for mapping perturbations of the wave speed and mass flows below the surface. Initial results from SOHO/MDI and GONG reveal strong diverging flows during the flux emergence, and also localized converging flows around stable sunspots. The wave speed images obtained during the process of formation of a large active region, NOAA 10488, indicate that the magnetic flux gets concentrated in strong field structures just below the surface. Further studies of magnetic flux emergence require systematic helioseismic observations from the ground and space, and realistic MHD simulations of the subsurface dynamics.     

Oscillations and Waves in Solar Spicules

Since their discovery, spicules have attracted increased attention as energy/mass bridges between the dense and dynamic photosphere and the tenuous hot solar corona. Mechanical energy of photospheric random and coherent motions can be guided by magnetic field lines, spanning from the interior to the upper parts of the solar atmosphere, in the form of waves and oscillations. Since spicules are one of the most pronounced features of the chromosphere, the energy transport they participate in can be traced by the observations of their oscillatory motions. Oscillations in spicules have been observed for a long time. However the recent high-resolution and high-cadence space and ground based facilities with superb spatial, temporal and spectral capacities brought new aspects in the research of spicule dynamics. Here we review the progress made in imaging and spectroscopic observations of waves and oscillations in spicules. The observations are accompanied by a discussion on theoretical modelling and interpretations of these oscillations. Finally, we embark on the recent developments made on the presence and role of Alfvén and kink waves in spicules. We also address the extensive debate made on the Alfvén versus kink waves in the context of the explanation of the observed transverse oscillations of spicule axes.     

On the Slow Solar Wind

A theory for the origin of the slow solar wind is described. Recent papers have demonstrated that magnetic flux moves across coronal holes as a result of the interplay between the differential rotation of the photosphere and the non-radial expansion of the solar wind in more rigidly rotating coronal holes. This flux will be deposited at low latitudes and should reconnect with closed magnetic loops, thereby releasing material from the loops to form the slow solar wind. It is pointed out that this mechanism provides a natural explanation for the charge states of elements observed in the slow solar wind, and for the presence of the First-Ionization Potential, or FIP, effect in the slow wind and its absence in fast wind. Comments are also provided on the role that the ACE mission should have in understanding the slow solar wind. This revised version was published online in June 2006 with corrections to the Cover Date.     

Total solar irradiance measurements by ERB/Nimbus-7. A review of nine years

The advent of reliable extraterrestrial solar irradiance measurements from satellites has supplied the impetus for new research in solar physics and solar-terrestrial relationships. The records for the principal experiments now extend beyond nine years. The Nimbus-7 measurements began in November 1978 and the Solar Maximum Mission (SMM) results started in February 1980. Both the ERB experiment of Nimbus-7 and the ACRIM experiment of SMM are still operational as of this writing (June 1988). We describe the nine-year Nimbus-7 total solar irradiance data set and compare it with similar data sets from the SMM and other satellite solar monitoring programs. Long-term downward trends of less than 0.02 % per year had been noted during the decaying portion of solar cycle 21 with indications that a leveling off and possible reversal of the trend was being experienced as we enter the new cycle. It had been demonstrated that fluctuations in the data over shorter periods corresponded to solar activity, from a primary discovery of irradiance depletions in times of building large sunspot groups to more subtle effects on the scale of solar rotation. Studies of the frequency spectra of the measurements have advanced the interest in helioseismology or mode analysis. Studies of photospheric activity have advanced by modelling of the sunspot blocking and photospheric brightening versus the measured irradiance. The theories are being extended to longer time-scales which indicate that solar irradiance is higher near solar cycle maximum, as defined by activity, and somewhat lower during the period between cycles. While measurements of total solar irradiance, the solar constant, alone cannot be employed to answer all of the questions of solar physics or helioclimatology, these long-term, high-precision data sets are valuable to both disciplines. The continuation of such measurements to more meaningful, longer time-scales should have a high priority in the international space community.     

Plasma-physical aspects of the solar cycle

Mass motions below the photosphere drive the solar cycle which is associated with variations in the magnetic field structure and accompanying phenomena. In addition to semi-empirical models, dynamo theories have been used to explain the solar cycle. The emergence of magnetic field generated by these mechanisms and its expansion into the corona involves many plasma physical processes. Magnetic buoyancy aids the expulsion of magnetic flux. The corona may respond dynamically or by continually adjusting to a quasi-static force-free or pressure-balanced equilibrium. The formation and disruption of current sheets is significant for the overall structure of the coronal magnetic field and the physics of quiescent prominences. The corona has a fine structure consisting of magnetic loops. The structure and stability of these are important as they are one of the underlying elements which make up the corona.     

The Solar Wind in the Outer Heliosphere

The solar wind evolves as it moves outward due to interactions with both itself and with the circum-heliospheric interstellar medium. The speed is, on average, constant out to 30 AU, then starts a slow decrease due to the pickup of interstellar neutrals. These neutrals reduce the solar wind speed by about 20% before the termination shock (TS). The pickup ions heat the thermal plasma so that the solar wind temperature increases outside 20–30 AU. Solar cycle effects are important; the solar wind pressure changes by a factor of 2 over a solar cycle and the structure of the solar wind is modified by interplanetary coronal mass ejections (ICMEs) near solar maximum. The first direct evidences of the TS were the observations of streaming energetic particles by both Voyagers 1 and 2 beginning about 2 years before their respective TS crossings. The second evidence was a slowdown in solar wind speed commencing 80 days before Voyager 2 crossed the TS. The TS was a weak, quasi-perpendicular shock which transferred the solar wind flow energy mainly to the pickup ions. The heliosheath has large fluctuations in the plasma and magnetic field on time scales of minutes to days.     

Solar Ultraviolet Bursts

The term “ultraviolet (UV) burst” is introduced to describe small, intense, transient brightenings in ultraviolet images of solar active regions. We inventorize their properties and provide a definition based on image sequences in transition-region lines. Coronal signatures are rare, and most bursts are associated with small-scale, canceling opposite-polarity fields in the photosphere that occur in emerging flux regions, moving magnetic features in sunspot moats, and sunspot light bridges. We also compare UV bursts with similar transition-region phenomena found previously in solar ultraviolet spectrometry and with similar phenomena at optical wavelengths, in particular Ellerman bombs. Akin to the latter, UV bursts are probably small-scale magnetic reconnection events occurring in the low atmosphere, at photospheric and/or chromospheric heights. Their intense emission in lines with optically thin formation gives unique diagnostic opportunities for studying the physics of magnetic reconnection in the low solar atmosphere. This paper is a review report from an International Space Science Institute team that met in 2016–2017.     

The Morphology of the Solar Upper Atmosphere During the Sunspot Minimum

The solar upper atmosphere (SUA) is defined as the volume above the photosphere occupied by plasmas with electron temperatures, T _e, above 2×10⁴ K. Until the Skylab era, only little was known about the morphology of the SUA, while the quality of the spectroscopic observations was continually improving. A spherically symmetric atmosphere was assumed at that time, in which the temperature increased with height. With advances in the observational techniques, it became apparent that the morphology of the SUA was very complex even during the minimum of the magnetic activity cycle. In particular, spectroscopic measurements with high spectral and spatial resolution, which were made in the light of ultraviolet emission lines representing a variety of temperatures, led to the conclusion that most of the radiation from the solar transition region could not be explained by assuming a continuous chromosphere-corona interface, but rather by a region of unresolved fine structures. Recent observational results obtained by modern instruments, such as the Extreme-ultraviolet Imaging Telescope (EIT), the Large Angle Spectroscopic Coronagraph (LASCO), and the Solar Ultraviolet Measurements of (SUMER) spectrograph on the Solar and Heliospheric Observatory (SOHO), as well as the Transition Region and Coronal Explorer (TRACE), and their interpretations will be presented in this review of our understanding of the morphology of the SUA.     

Elemental Abundances in the Solar Upper Atmosphere Derived by Spectroscopic Means

The composition of the solar photosphere is believed to be uniform. Indeed a quantity that does not vary with solar surface location or with a particular solar feature, i.e., no observational evidence is available to indicate that the photospheric composition near the solar equator is different from the photospheric composition near the solar poles or that the photospheric composition in quiet regions is different from the composition in active regions. In contrast, the composition of the solar upper atmosphere is not well defined. Solar composition work in recent decades has brought the recognition that there are systematic differences between the composition of the corona and the photosphere and revealed evidence for spatial and time variability in the composition of various coronal features. We review the spectroscopic techniques used and the progress that was made in recent years in deriving the plasma compositions of various solar upper atmosphere structures. This revised version was published online in August 2006 with corrections to the Cover Date.     

FIP Effect in the Solar Upper Atmosphere: Spectroscopic Results

Recent spectroscopic measurements from instruments on the Solar and Heliospheric Observatory (SOHO) find that the coronal composition above a polar coronal hole is nearly photospheric. However, similar SOHO observations show that in coronal plasmas above quiet equatorial regions low-FIP elements are enhanced by a factor of ≈ 4. In addition, the process of elemental settling in coronal plasmas high above the solar surface was shown to exist. Measurements by the Ulysses spacecraft, which are based on non-spectroscopic particle counting techniques, show that, with the exception of He, the elemental composition of the fast speed solar wind is similar to within a factor of 1.5 to the composition of the photosphere. In contrast, similar measurements in the slow speed wind show that elements with low first ionization potential (FIP < 10 eV) are enhanced, relative to the photosphere, by a factor of 4-5. By combining the SOHO and Ulysses results, ideas related to the origin of the slow speed solar wind are presented. Using spectroscopic measurements by the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument on SOHO the photospheric abundance of He was determined as 8.5 ± 1.3% (Y = 0.248). This revised version was published online in June 2006 with corrections to the Cover Date.     

Probing Solar Subsurface Magnetic Fields

Helioseismology uses solar p-mode oscillations to probe the structure of the solar interior. The modifications of p-mode properties due to the presence of solar magnetic fields provide information on the magnetic fields in the solar interior. Here we review some of results in helioseismology on the magnetic fields in the solar convection zone. We will also discuss a recent result on the magnetic fields at the base of the convection zone. This revised version was published online in August 2006 with corrections to the Cover Date.     

Active Region Dynamics

New methods of local helioseismology and uninterrupted time series of solar oscillation data from the Solar and Heliospheric Observatory (SOHO) have led to a major advance in our understanding of the structure and dynamics of active regions in the subsurface layers. The initial results show that large active regions are formed by repeated magnetic flux emergence from the deep interior, and that their roots are at least 50 Mm deep. The active regions change the temperature structure and flow dynamics of the upper convection zone, forming large circulation cells of converging flows. The helioseismic observations also indicate that the processes of magnetic energy release, flares and coronal mass ejections, might be associated with strong (1–2 km/s) shearing flows, 4–6 Mm below the surface.     

Coronal holes and solar magnetic fields

Since 1972, nearly continuous observations of coronal holes and their associated photospheric magnetic fields have been made using a variety of satellite and ground-based equipment. The results of comparisons of these observations are reviewed and it is demonstrated that the structure and evolution of coronal holes is basically governed by the large-scale distribution of photospheric magnetic flux. Non-polar holes form in the decaying remnants of bipolar magnetic regions in areas with a large-scale flux imbalance. There is strong indirect evidence that the magnetic field in coronal holes is always open to interplanetary space but not all open-field regions have associated coronal holes. The well-observed declining phase of the last solar cycle was characterized by stable magnetic field and coronal hole patterns which were associated with recurrent, high-speed wind streams and interplanetary magnetic field patterns at the Earth. The ascending phase of the current cycle has been characterized by transient magnetic field and coronal hole patterns which tend to occur at high solar latitudes. This shift in magnetic field and coronal hole patterns has resulted in a less obvious and more complicated association with high-speed wind streams at the Earth.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.Visiting Scientist, Kitt Peak National Observatory.     

Measurements of solar total irradiance and its variability

The development of electrically self calibrated cavity pyrheliometric instrumentation that occurred in the early 20th century provided the technological base for experiments to detect variability of the solar total irradiance. Experiments from ground based observatories, aircraft and balloons during the 1st half of the 20th century were unable to achieve sufficient accuracy or long term precision to unambiguously detect irradiance variations of solar origin. Refinements in pyrheliometric technology during the 1960's and 1970's and the accessibility of extended experimental opportunities above the Earth's atmosphere in recent years have provided the first direct observations of solar total irradiance variability and provided the cornerstone observations of a long term database on solar irradiance. A program of solar irradiance monitoring has evolved to sustain the database over at least 22 years, corresponding to a single cycle of solar magnetic activity, and the shortest well identified cycle of climate variation. Direct links between total irradiance variations, solar magnetic activity and the solar global 5 min oscillation phenomena have been derived from recent space flight observations by the SMM/ACRIM I experiment.     

Origin of the Solar Wind: Theory

A theory is presented for the origin of the solar wind, which is based on the behavior of the magnetic field of the Sun. The magnetic field of the Sun can be considered as having two distinct components: Open magnetic flux in which the field lines remain attached to the Sun and are dragged outward into the heliosphere with the solar wind. Closed magnetic flux in which the field remains entirely attached to the Sun, and forms loops and active regions in the solar corona. It is argued that the total open flux should tend to be constant in time, since it can be destroyed only if open flux of opposite polarity reconnect, a process that may be unlikely since the open flux is ordered into large-scale regions of uniform polarity. The behavior of open flux is thus governed by its motion on the solar surface. The motion may be due primarily to a diffusive process that results from open field lines reconnecting with randomly oriented closed loops, and also due to the usual convective motions on the solar surface such as differential rotation. The diffusion process needs to be described by a diffusion equation appropriate for transport by an external medium, which is different from the usual diffusion coefficient used in energetic particle transport. The loops required for the diffusion have been identified in recent observations of the Sun, and have properties, both in size and composition, consistent with their use in the model. The diffusive process, in which reconnection occurs between open field lines and loops, is responsible for the input of mass and energy into the solar wind. This revised version was published online in August 2006 with corrections to the Cover Date.