ESA Science & Technology - Publication Archive

Context. Over the past 40 years, helioseismology has been enormously successful in the study of the solar interior. A shortcoming has been the lack of a convincing detection of the solar g modes, which are oscillations driven by gravity and are hidden in the deepest part of the solar body – its hydrogen-burning core. The detection of g modes is expected to dramatically improve our ability to model this core, the rotational characteristics of which have, until now, remained unknown.

Aims. We present the identification of very low frequency g modes in the asymptotic regime and two important parameters that have long been waited for: the core rotation rate, and the asymptotic equidistant period spacing of these g modes.

Methods. The GOLF instrument on board the SOHO space observatory has provided two decades of full-disk helioseismic data. The search for g modes in GOLF measurements has been extremely difficult because of solar and instrumental noise. In the present study, the p modes of the GOLF signal are analyzed differently: we search for possible collective frequency modulations that are produced by periodic changes in the deep solar structure. Such modulations provide access to only very low frequency g modes, thus allowing statistical methods to take advantage of their asymptotic properties.

Results. For oscillatory periods in the range between 9 and nearly 48 h, almost 100 g modes of spherical harmonic degree 1 and more than 100 g modes of degree 2 are predicted. They are not observed individually, but when combined, they unambiguously provide their asymptotic period equidistance and rotational splittings, in excellent agreement with the requirements of the asymptotic approximations.

[Remainder of abstract truncated due to character limitations]

[ESA Brochure BR-328] SOHO, the ESA-NASA Solar and Heliospheric Observatory, is studying the Sun, from its deep core to the hot and dynamic outer atmosphere, the solar wind and solar energetic particles. This brochure presents some science highlights marking twenty years of the mission.

During its sungrazing perihelion passage, comet ISON appeared in the field of view of the SUMER spectrometer and allowed unique observations at far-ultraviolet wavelengths with high spatial and temporal resolution. We report results of these observations completed on November 28, 2013, when the comet was only 2.82 R_Sun away from the Sun. Our data show the arrow-shaped dust tail in Ly-alpha emission trailing behind the predicted position of the nucleus, but offset from the trajectory. We interpret the emission as sunlight that is scattered at micron-sized dust particles. We modeled the dust emission and dynamics to reproduce the appearance of the tail. We were unable to detect any signature of cometary gas or plasma around the expected position of the nucleus and conclude that the outgassing processes must have stopped before the observation started. Moreover, the model we used to reproduce the observed dust tail needs a sharp fall-off of the dust production hours before perihelion transit. We compare the radiances of the disk and the dust tail for an estimate of the dust column density and tail mass.

Sunspots are regions where strong magnetic fields emerge from the solar interior and where major eruptive events occur. These energetic events can cause power outages, interrupt telecommunication and navigation services, and pose hazards to astronauts. We detected subsurface signatures of emerging sunspot regions before they appeared on the solar disc. Strong acoustic travel-time anomalies of an order of 12 to 16 seconds were detected as deep as 65,000 kilometers. These anomalies were associated with magnetic structures that emerged with an average speed of 0.3 to 0.6 kilometer per second and caused high peaks in the photospheric magnetic flux rate 1 to 2 days after the detection of the anomalies. Thus, synoptic imaging of subsurface magnetic activity may allow anticipation of large sunspot regions before they become visible, improving space weather forecast.

Flares are powerful bursts of energy released by relatively poorly understood processes that take place in the atmospheres of stars. However, although solar flares, from our own Sun, are the most energetic events in the solar system, in comparison to the total output of the Sun they are barely noticeable. Consequently, the total amount of radiant energy they generate is not precisely known, and their potential contribution to variations in the total solar irradiance incident on the Earth has so far been overlooked. In this work, we identify a measurable signal from relatively moderate solar flares in total solar irradiance data. We find that the total energy radiated by flares exceeds by two orders of magnitude the flare energy radiated in the soft-X-ray domain only, indicating a major contribution in the visible domain. These results have implications for our understanding of solar-flare activity and the variability of our star.

Abstract: We develop a model for estimating solar total irradiance since 1600 AD using the sunspot number record as input, since this is the only intrinsic record of solar activity extending back far enough in time. Sunspot number is strongly correlated, albeit nonlinearly with the 10.7-cm radio flux (F _10.7), which forms a continuous record back to 1947. This enables the nonlinear relationship to be estimated with usable accuracy and shows that relationship to be consistent over multiple solar activity cycles. From the sunspot number record we estimate _10.7 values back to 1600 AD. _10.7 is linearly correlated with the total amount of magnetic flux in active regions, and we use it as input to a simple cascade model for the other magnetic flux components. The irradiance record is estimated by using these magnetic flux components plus a very rudimentary model for the modulation of energy flow to the photosphere by the subphotospheric magnetic flux reservoir feeding the photospheric magnetic structures. Including a Monte Carlo analysis of the consequences of measurement and fitting errors, the model indicates the mean irradiance during the Maunder Minimum was about 1 ± 0.4 W /m² lower than the mean irradiance over the last solar activity cycle.

During the solar minimum of 2008, the value of total solar irradiance at 1 AU (TSI) was more than 0.2 Wm/2 lower than during the last minimum in 1996, indicating for the first time a directly observed long-term change. In contrast, chromospheric indices and hence solar UV irradiance do not exhibit a similar change. Comparison of TSI with other activity parameters indicates that only the open solar magnetic field, B_R, observed from satellites at 1 AU show a similar long-term behaviour. The values at the minima correlate well and the linear fit provides a direct physical relationship between TSI and B_R during the minimum times. This correlation allows an unambiguous reconstruction of TSI back in time, provided the open solar magnetic field can be determined from e.g. geomagnetic indices or cosmogenic radionucleides. Since the solar UV irradiance has no long-term trend, the mechanism for the secular change of TSI must differ from the effect of surface magnetism, as manifested by sunspots, faculae, and network which indeed explain well the intra-cycle variability of both total and spectral irradiance. The long-term trend of TSI is most probably caused by a global temperature change of the Sun that does not influence the UV irradiance in the same way as the surface magnetic fields.

p-mode oscillations in solar-like stars are excited by the outer convection zone in these stars and reflected close to the surface. The p modes are trapped inside an acoustic cavity, but the modes only stay trapped up to a given frequency [known as the acoustic cut-off frequency (v_ac)] as modes with larger frequencies are generally not reflected at the surface. This means that modes with frequency larger than the acoustic cut-off frequency must be travelling waves. The high-frequency modes may provide information about the physics in the outer layers of the stars and the excitation source and are therefore highly interesting as it is the estimation of these two phenomena that cause some of the largest uncertainties when calculating stellar oscillations. High-frequency modes have been detected in the Sun, in beta-Hydri and in alpha-Cen A and alpha-Cen B by smoothing the so-called echelle diagram and the large frequency separation as a function of frequency has been estimated. The large frequency separation has been compared with a simple model of the acoustic cavity which suggests that the reflectivity of the photosphere is larger at high frequency than predicted by standard models of the solar atmosphere and that the depth of the excitation source is larger than what has been estimated by other models and might depend on the order n and degree l of the modes.

Solar flares are large explosions on the Sun's surface caused by a sudden release of magnetic energy. They are known to cause local short-lived oscillations traveling away from the explosion like water rings. Here we show that the energy in the solar acoustic spectrum is correlated with flares. This means that the flares drive global oscillations in the Sun in the same way that the entire Earth is set ringing for several weeks after a major earthquake such as the 2004 December Sumatra-Andaman one. The correlation between flares and energy in the acoustic spectrum of disk-integrated sunlight is stronger for high-frequency waves than for ordinary p-modes which are excited by the turbulence in the near-surface convection zone immediately beneath the photosphere.

The solar gravity modes have not been conclusively detected in the Sun as yet due to their small surface amplitudes. They have been actively searched for because they directly probe the solar burning core (below 0.2 solar radius). Using data from the Global Oscillation at Low Frequency instrument we detect a periodic structure, which is in agreement with the period separation predicted by the theory for gravity dipole modes. The detailed study of this structure, compared to simulations including the best physics of the Sun determined through the acoustic modes, would favor a faster core rotation rate than in the rest of the radiative zone.

Variations in the Sun's total energy output (luminosity) are caused by changing dark (sunspot) and bright structures on the solar disk during the 11-year sunspot cycle. The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years. In this Review, we show that detailed analysis of these small output variations has greatly advanced our understanding of solar luminosity change, and this new understanding indicates that brightening of the Sun is unlikely to have had a significant influence on global warming since the seventeenth century. Additional climate forcing by changes in the Sun's output of ultraviolet light, and of magnetized plasmas, cannot be ruled out. The suggested mechanisms are, however, too complex to evaluate meaningfully at present.

Since its launch on 2 December 1995, SOHO has revolutionised our understanding of the Sun. It has provided the first images of structures and flows below the Sun's surface and of activity on the far side. SOHO has revealed the Sun's extremely dynamic atmosphere, provided evidence for the transfer of magnetic energy from the surface to the outer solar atmosphere, the corona, through a 'magnetic carpet', and identified the source regions of the fast solar wind. It has revolutionised our understanding of solarterrestrial relations and dramatically improved our space weather-forecasting by its continuous stream of images covering the atmosphere, extended corona and far side. The findings are documented in an impressive number of scientific publications: over 2500 papers in refereed journals since launch, representing the work of over 2300 individual scientists. At the same time, SOHO's easily accessible, spectacular data and fundamental scientific results have captured the imagination of the space science community and the general public alike. As a byproduct of the efforts to provide real-time data to the public, amateurs now dominate SOHO's discovery of over 1100 Sungrazing comets.

We present observations of four flares that occurred during coordinated observations between the Coronal Diagnostic Spectrometer (CDS) on board SOHO and the Domeless Solar Telescope (DST) at Hida Observatory. We studied the evolution of relative Doppler velocities in the flare kernels by using He I (3.5x10⁴ K), O V (2.2x10⁵ K), and Mg IX (1.0x10⁶ K) spectra obtained with high time cadence (42 s) SOHO CDS observations and the Halpha monochromatic images obtained with the DST. We found that the transition region plasma of O V showed strong downward velocities up to 87 km s^-1 simultaneously with the downflows in the lower temperature chromospheric emissions in He I and Halpha during the impulsive phase of all four flares. From these results we suggest that the downflows in the transition region and the chromosphere are a common feature in the impulsive phase of flares. For the Mg IX line we did not detect any significant change in velocity, which suggests that the 10⁶ K plasma was close to the intermediate temperature between the upflowing plasma (10⁷ K) and the downflowing plasma (10⁴-10⁵ K). These are important for understanding the dynamics of the solar atmosphere in response to the sudden energy deposition of a flare.

There is a controversy about how features protruding laterally from filaments, called barbs, are magnetically structured. On 2004 August 3, we observed a filament that had well-developed barbs. The observations were performed using the 10 inch refractor of the Big Bear Solar Observatory. A fast camera was employed to capture images at five different wavelengths of the Halpha line and successively record them on the basis of frame selection. The terminating points of the barbs were clearly discernable in the Halpha images without any ambiguity. The comparison of the Halpha images with the magnetograms taken by SOHO MDI revealed that the termination occurred above the minor polarity inversion line dividing the magnetic elements of the major polarity and those of the minor polarity. There is also evidence that the flux cancellation proceeded on the polarity inversion line. Our results together with similar other recent observations support the idea that filament barbs are cool matter suspended in local dips of magnetic field lines, formed by magnetic reconnection in the chromosphere.

The origin of the solar wind in solar coronal holes has long been unclear. We establish that the solar wind starts flowing out of the corona at heights above the photosphere between 5 megameters and 20 megameters in magnetic funnels. This result is obtained by a correlation of the Doppler-velocity and radiance maps of spectral lines emitted by various ions with the force-free magnetic field as extrapolated from photospheric magnetograms to different altitudes. Specifically, we find that Ne⁷⁺ ions mostly radiate around 20 megameters, where they have outflow speeds of about 10 kilometers per second, whereas C³⁺ ions with no average flow speed mainly radiate around 5 megameters. Based on these results, a model for understanding the solar wind origin is suggested.

Using an absorption cell, we measured the Doppler shifts of the interstellar hydrogen resonance glow to show the direction of the neutral hydrogen flow as it enters the inner heliosphere. The neutral hydrogen flow is found to be deflected relative to the helium flow by about 4 degrees. The most likely explanation of this deflection is a distortion of the heliosphere under the action of an ambient interstellar magnetic field. In this case, the helium flow vector and the hydrogen flow vector constrain the direction of the magnetic field and act as an interstellar magnetic compass.

Since its launch on 2 December 1995, the joint ESA/NASA SOHO mission has provided a wealth of information about the Sun, from its interior, through the hot and dynamic atmosphere, to the solar wind and its interaction with the interstellar medium. Analysis of the helioseismology data from SOHO has provided the first images of structures and flows below the Sun's surface and has shed new light on a number of structural and dynamic phenomena in the solar interior, such as the absence of differential rotation in the radiative zone, subsurface zonal and meridional flows, and sub-convection-zone mixing. Evidence for an upward transfer of magnetic energy from the Sun's surface toward the corona has been established. The ultraviolet imagers and spectrometers have revealed an extremely dynamic solar atmosphere where plasma flows play an important role. Electrons in coronal holes were found to be relatively ``cool', whereas heavy ions are extremely hot and have highly anisotropic velocity distributions. The source regions for the high speed solar wind have been identified and the acceleration profiles of both the slow and fast solar wind have been measured. SOHO has also revolutionized our space weather forecasting capabilities by providing a continuous stream of images of the dynamic atmosphere, extended corona, and activity on the far side of the Sun. At the same time, SOHO's easily accessible images and movies have captured the imagination of the science community and the general public alike. This article summarizes some of the key findings from 8 years of SOHO.

The roots of the SOHO mission and the story leading to the comprehensive observatory that the spacecraft is today are summarised here. Now fully operational in its halo orbit around the first Lagrangian point (L1) between the Earth and the Sun, SOHO is providing the international scientific community with the unique opportunity, and also challenge, of understanding the Sun and heliosphere as one complex, global system. It is a superb tool with which to investigate our daylight star and its circumstellar environment, from the Sun's centre, through its visible surface and tenuous corona, and out into the heliosphere to distances corresponding to more than ten times the orbital distance of the Earth.

Since its launch on 2 December 1995, the Solar and Heliospheric Observatory (SOHO) has provided an unparalleled breadth and depth of information about the Sun, from its interior, through the hot and dynamic atmosphere, out to the solar wind. Analysis of the helioseismology data from SOHO has shed new light on a number of structural and dynamic phenomena in the solar interior, such as the absence of differential rotation in the radiative zone, subsurface zonal and meridional flows, sub-convection-zone mixing, a possible circumpolar jet, and very slow polar rotation. The ultraviolet imagers and spectrometers have revealed an extremely dynamic solar atmosphere in which plasma flows play an important role. Evidence for an upward transfer of magnetic energy from the Sun's surface toward the corona has been found. Electrons in coronal holes have been found to be relatively 'cool', whereas heavy ions are extremely hot and have highly anisotropic velocity distributions. The source region for the high-speed solar wind has been identified and the acceleration profiles of both the slow and fast solar wind have been measured.

The SOHO mission is a major element of the joint ESA/NASA International Solar Terrestrial Programme (ISTP). ESA was responsible for the spacecraft's procurement, integration and testing. NASA provided the launcher, launch services and ground-segment system and is responsible for in-flight operations following the launch on 2 December 1995. The SOHO mission operations are therefore conducted under a NASA/Goddard Space Flight Center (GSFC) contract with Allied-Signal Technology Corporation (ATSC). Following the spacecraft's in-orbit checkout and the transit from low Earth orbit to its operational halo orbit around the Lagrangian point (L1) between the Earth and the Sun, the SOHO mission was declared fully operational in April 1996. SOHO then completed its two-year primary mission and entered an extended-mission phase in May 1998. On 25 June 1998, all contact with SOHO was lost.