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The EUV high-throughput spectroscopic telescope (EUVST) onboard the Solar-C mission has the high spatial (0.4′′) resolution over a wide wavelength range in the vacuum ultraviolet. To achieve high spatial resolution under a design constraint given by the JAXA Epsilon launch vehicle, we further update the optical design to secure margins needed to realize 0.4′′ spatial resolution over a field of view of 100′′×100′′. To estimate the error budgets of spatial and spectral resolutions due to installation and fabrication errors, we perform a sensitivity analysis for the position and orientation of each optical element and for the grating parameters by ray tracing with the Zemax software. We obtain point spread functions (PSF) for rays from 9 fields and at 9 wavelengths on each detector by changing each parameter slightly. A full width at half maximum (FWHM) of the PSF is derived at each field and wavelength position as a function of the perturbation of each optical parameter. Assuming a mount system of each optical element and an error of each optical parameter, we estimate spatial and spectral resolutions by taking installation and fabrication errors into account. The results of the sensitivity analysis suggest that budgets of the total of optical design and the assembly errors account for 15% and 5.8% of our budgets of the spatial resolution in the long wavelength and short wavelength bands, respectively. On the other hand, the grating fabrication errors give a large degradation of spatial and spectral resolutions, and investigations of compensators are needed to relax the fabrication tolerance of the grating surface parameters.

Solar-C (EUVST) is the next Japanese solar physics mission to be developed with significant contributions from US and European countries. The mission carries an EUV imaging spectrometer with slit-jaw imaging system called EUVST (EUV High-Throughput Spectroscopic Telescope) as the mission payload, to take a fundamental step towards answering how the plasma universe is created and evolves and how the Sun influences the Earth and other planets in our solar system. In April 2020, ISAS (Institute of Space and Astronautical Science) of JAXA (Japan Aerospace Exploration Agency) has made the final down-selection for this mission as the 4th in the series of competitively chosen M-class mission to be launched with an Epsilon launch vehicle in mid 2020s. NASA (National Aeronautics and Space Administration) has selected this mission concept for Phase A concept study in September 2019 and is in the process leading to final selection. For European countries, the team has (or is in the process of confirming) confirmed endorsement for hardware contributions to the EUVST from the national agencies. A recent update to the mission instrumentation is to add a UV spectral irradiance monitor capability for EUVST calibration and scientific purpose. This presentation provides the latest status of the mission with an overall description of the mission concept emphasizing on key roles of the mission in heliophysics research from mid 2020s.

Aims. We present the design and pre-launch performance of the Solar Orbiter Heliospheric Imager (SoloHI) which is an instrument prepared for inclusion in the ESA/NASA Solar Orbiter mission, currently scheduled for launch in 2020.Methods. The goal of this paper is to provide details of the SoloHI instrument concept, design, and pre-flight performance to give the potential user of the data a better understanding of how the observations are collected and the sources that contribute to the signal.Results. The paper discusses the science objectives, including the SoloHI-specific aspects, before presenting the design concepts, which include the optics, mechanical, thermal, electrical, and ground processing. Finally, a list of planned data products is also presented.Conclusions. The performance measurements of the various instrument parameters meet or exceed the requirements derived from the mission science objectives. SoloHI is poised to take its place as a vital contributor to the science success of the Solar Orbiter mission.

Solar-C EUVST (EUV High-Throughput Spectroscopic Telescope) is a solar physics mission concept that was selected as a candidate for JAXA competitive M-class missions in July 2018. The onboard science instrument, EUVST, is an EUV spectrometer with slit-jaw imaging system that will simultaneously observe the solar atmosphere from the photosphere/chromosphere up to the corona with seamless temperature coverage, high spatial resolution, and high throughput for the first time. The mission is designed to provide a conclusive answer to the most fundamental questions in solar physics: how fundamental processes lead to the formation of the solar atmosphere and the solar wind, and how the solar atmosphere becomes unstable, releasing the energy that drives solar flares and eruptions. The entire instrument structure and the primary mirror assembly with scanning and tip-tilt fine pointing capability for the EUVST are being developed in Japan, with spectrograph and slit-jaw imaging hardware and science contributions from US and European countries. The mission will be launched and installed in a sun-synchronous polar orbit by a JAXA Epsilon vehicle in 2025. ISAS/JAXA coordinates the conceptual study activities during the current mission definition phase in collaboration with NAOJ and other universities. The team is currently working towards the JAXA final down-selection expected at the end of 2019, with strong support from US and European colleagues. The paper provides an overall description of the mission concept, key technologies, and the latest status.

The main characteristics of Solar-C_EUVST are the high temporal and high spatial resolutions over a wide temperature coverage. In order to realize the instrument for meeting these scientific requirements under size constraints given by the JAXA Epsilon vehicle, we examined four-dimensional optical parameter space of possible solutions of geometrical optical parameters such as mirror diameter, focal length, grating magnification, and so on. As a result, we have identified the solution space that meets the EUVST science objectives and rocket envelope requirements. A single solution was selected and used to define the initial optical parameters for the concept study of the baseline architecture for defining the mission concept. For this solution, we optimized the grating and geometrical parameters by ray tracing of the Zemax software. Consequently, we found an optics system that fulfills the requirement for a 0.4” angular resolution over a field of view of 100" (including margins) covering spectral ranges of 170-215, 463-542, 557-637, 690-850, 925-1085, and 1115-1275 A. This design achieves an effective area 10 times larger than the Extreme-ultraviolet Imaging Spectrometer onboard the Hinode satellite, and will provide seamless observations of 4.2-7.2 log(K) plasmas for the first time. Tolerance analyses were performed based on the optical design, and the moving range and step resolution of focus mechanisms were identified. In the presentation, we describe the derivation of the solution space, optimization of the optical parameters, and show the results of ray tracing and tolerance analyses.

We examine the different element abundances exhibited by the closed loop solar corona and the slow speed solar wind. Both are subject to the First Ionization Potential (FIP) Effect, the enhancement in coronal abundance of elements with FIP below 10 eV (e.g. Mg, Si, Fe) with respect to high FIP elements (e.g. O, Ne, Ar), but with subtle differences. Intermediate elements, S, P, and C, with FIP just above 10 eV, behave as high FIP elements in closed loops, but are fractionated more like low FIP elements in the solar wind. On the basis of FIP fractionation by the ponderomotive force in the chromosphere, we discuss fractionation scenarios where this difference might originate. Fractionation low in the chromosphere where hydrogen is neutral enhances the S, P and C abundances. This arises with nonresonant waves, which are ubiquitous in open field regions, and is also stronger with torsional Alfven waves, as opposed to shear (i.e. planar) waves. We discuss the bearing these findings have on models of interchange reconnection as the source of the slow speed solar wind. The outflowing solar wind must ultimately be a mixture of the plasma in the originally open and closed fields, and the proportions and degree of mixing should depend on details of the reconnection process. We also describe novel diagnostics in ultraviolet and extreme ultraviolet spectroscopy now available with these new insights, with the prospect of investigating slow speed solar wind origins and the contribution of interchange reconnection by remote sensing., 21 pages, accepted by ApJ

Remote observations of the solar photospheric light scattered by electrons (the K-corona) and dust (the F-corona or zodiacal light) have been made from the ground during eclipses1 and from space at distances as small as 0.3 astronomical units2–5 to the Sun. Previous observations6–8 of dust scattering have not confirmed the existence of the theoretically predicted dust-free zone near the Sun9–11. The transient nature of the corona has been well characterized for large events, but questions still remain (for example, about the initiation of the corona12 and the production of solar energetic particles13) and for small events even its structure is uncertain14. Here we report imaging of the solar corona15 during the first two perihelion passes (0.16–0.25 astronomical units) of the Parker Solar Probe spacecraft13, each lasting ten days. The view from these distances is qualitatively similar to the historical views from ground and space, but there are some notable differences. At short elongations, we observe a decrease in the intensity of the F-coronal intensity, which is suggestive of the long-sought dust free zone9–11. We also resolve the fine-scale plasma structure of very small eruptions, which are frequently ejected from the Sun. These take two forms: the frequently observed magnetic flux ropes12,16 and the predicted, but not yet observed, magnetic islands17,18 arising from the tearing-mode instability in the current sheet. Our observations of the coronal streamer evolution confirm the large-scale topology of the solar corona, but also reveal that, as recently predicted19, streamers are composed of yet smaller substreamers channelling continual density fluctuations at all visible scales. Observations of the solar corona by the Parker Solar Probe reveal evidence for the predicted dust-free zone and confirm that streamers comprise smaller substreamers that channel continuous multiscale density fluctuations.

The techniques for stray light analysis, optimization and testing are described for two space telescopes that observe the solar corona: the Solar Orbiter Heliospheric Imager (SoloHI) that will fly on the ESA Solar Orbiter (SolO), and the Wide Field Imager for Solar Probe (WISPR) that will fly on the NASA Parker Solar Probe (PSP) mission. Imaging the solar corona is challenging, because the corona is six orders of magnitude dimmer than the Sun surface at the limb, and the coronal brightness continues to decrease to ten orders of magnitude below the Sun limb above 5° elongation from Sun center. The SoloHI and WISPR instruments are located behind their respective spacecraft heat shield. Each spacecraft heat shield does not block the instrument field of view above the solar limb, but will prevent direct sunlight entering the instrument aperture. To satisfy the instrument stray light attenuation required to observe the solar corona, an additional set of instrument baffles were designed and tested for successive diffraction of the heat shield diffracted light before entering the telescope entrance pupil. A semi empirical model of diffraction was used to design the baffles, and tests of the flight models were performed in flight like conditions with the aim of verifying the rejection of the design. Test data showed that the baffle systems behaved as expected. A second source of stray light is due to reflections of the sunlight off of the spacecraft structures and towards the instruments. This is especially the case for SoloHI where one of the spacecraft 8m tall solar arrays is located behind the telescope and reflects sunlight back onto the instrument baffles. The SoloHI baffle design had to be adjusted to mitigate that component, which was achieved by modifying their geometry and their optical coating. Laboratory tests of the flight model were performed. The test data were correlated with the predictions of a ray tracing model, which enabled the fine tuning of the model. Finally, end-to-end ray tracing was used to predict the stray light for the flight conditions.

The Wide-field Imager for Solar PRobe Plus (WISPR) is the sole imager aboard the Solar Probe Plus (SPP) mission scheduled for launch in 2018. SPP will be a unique mission designed to orbit as close as 7 million km (9.86 solar radii) from Sun center. WISPR employs a 95∘ radial by 58∘ transverse field of view to image the fine-scale structure of the solar corona, derive the 3D structure of the large-scale corona, and determine whether a dust-free zone exists near the Sun. WISPR is the smallest heliospheric imager to date yet it comprises two nested wide-field telescopes with large-format (2 K × 2 K) APS CMOS detectors to optimize the performance for their respective fields of view and to minimize the risk of dust damage, which may be considerable close to the Sun. The WISPR electronics are very flexible allowing the collection of individual images at cadences up to 1 second at perihelion or the summing of multiple images to increase the signal-to-noise when the spacecraft is further from the Sun. The dependency of the Thomson scattering emission of the corona on the imaging geometry dictates that WISPR will be very sensitive to the emission from plasma close to the spacecraft in contrast to the situation for imaging from Earth orbit. WISPR will be the first ‘local’ imager providing a crucial link between the large-scale corona and the in-situ measurements.

Solar Probe Plus (SPP) is a NASA mission developed to visit and study the sun closer than ever before. SPP is designed to orbit as close as 7 million km (9.86 solar radii) from Sun center. One of its instruments: WISPR (Wide-Field Imager for Solar Probe Plus) will be the first ‘local’ imager to provide the relation between the large-scale corona and the in-situ measurements.

Very high angular resolution ultraviolet telescope (VAULT2.0) is a Lyman-alpha (Ly[Formula: see text]; 1216[Formula: see text]Å) spectroheliograph designed to observe the upper chromospheric region of the solar atmosphere with high spatial ([Formula: see text]) and temporal (8[Formula: see text]s) resolution. Besides being the brightest line in the solar spectrum, Ly[Formula: see text] emission arises at the temperature interface between coronal and chromospheric plasmas and may, hence, hold important clues about the transfer of mass and energy to the solar corona. VAULT2.0 is an upgrade of the previously flown VAULT rocket and was launched successfully on September 30, 2014 from White Sands Missile Range (WSMR). The target was AR12172 midway toward the southwestern limb. We obtained 33 images at 8[Formula: see text]s cadence at arc second resolution due to hardware problems. The science campaign was a resounding success, with all space and ground-based instruments obtaining high-resolution data at the same location within the AR. We discuss the science rationale, instrument upgrades, and performance during the first flight and present some preliminary science results.

Comprehensive measurements of magnetic fields in the solar corona have a long history as an important scientific goal. Besides being crucial to understanding coronal structures and the Sun’s generation of space weather, direct measurements of their strength and direction are also crucial steps in understanding observed wave motions. In this regard, the remote sensing instrumentation used to make coronal magnetic field measurements is well suited to measuring the Doppler signature of waves in the solar structures. In this paper, we describe the design and scientific values of the Waves and Magnetism in the Solar Atmosphere (WAMIS) investigation. WAMIS, taking advantage of greatly improved infrared filters and detectors, forward models, advanced diagnostic tools and inversion codes, is a long-duration high-altitude balloon payload designed to obtain a breakthrough in the measurement of coronal magnetic fields and in advancing the understanding of the interaction of these fields with space plasmas. It consists of a 20 cm aperture coronagraph with a visible-IR spectro-polarimeter focal plane assembly. The balloon altitude would provide minimum sky background and atmospheric scattering at the wavelengths in which these observations are made. It would also enable continuous measurements of the strength and direction of coronal magnetic fields without interruptions from the day-night cycle and weather. These measurements will be made over a large field-of-view allowing one to distinguish the magnetic signatures of different coronal structures, and at the spatial and temporal resolutions required to address outstanding problems in coronal physics. Additionally, WAMIS could obtain near simultaneous observations of the electron scattered K-corona for context and to obtain the electron density. These comprehensive observations are not provided by any current single ground-based or space observatory. The fundamental advancements achieved by the near-space observations of WAMIS on coronal field would point the way for future ground based and orbital instrumentation.

The SECCHI Heliospheric Imager (HI) instruments on-board the STEREO spacecraft have been collecting images of solar wind transients, including coronal mass ejections, as they propagate through the inner heliosphere since the beginning of 2007.

The accurate determination of the center-to-limb variation of the quiet solar spectrum is of fundamental importance to our understanding of both spatially resolved solar spectral radiance as well as full-disk spectral irradiance. Previous studies have examined the center-to-limb variation at various spectral resolutions using both observations and calculations. Here we derive the center-to-limb variation near Mg II at 2800 A from observations made by the HRTS-9 rocket-spectrograph at both high spatial (1'') and spectral (0.2 A) resolution. This region of the solar spectrum is important because of its impact on the terrestrial atmosphere and its use in generating the Mg II index. The initial part of this paper presents the correction methods used to determine the center-to-limb variation, including the determination of the vignetting properties of the HRTS-9 instrument. This latter correction accounts for the intensity variation along the slit axis of the observed spectrogram. The resulting center-to-limb variation is employed in a model of solar spectral irradiance near Mg II. Using these results and calibrated solar spectra, the absolute intensity calibration is determined for the HRTS-9 spectra. An important aspect of the high-resolution center-to-limb variation is the presence of many spectral features illustrating the differences between the wings and cores of the numerous Fraunhofer lines in the 2765-2875 A region. Also, both the Mg I and Mg II lines in this spectral region have broad, smooth wings that vary differently than the nearby line-blanketed continuum. The results of this study will provide a good source of comparison for detailed models of the quiet solar spectrum.

We present spectral line wavelengths, identifications, and intensities in the 171-211 and 245-291 A ranges from five solar plasma regions recorded by the Extreme-Ultraviolet Imaging Spectrometer (EIS) on Hinode. The recorded data were emitted from a quiet region, two active areas on the solar disk, a limb region, and a region 20'' above the limb. The line list contains 500 lines of which 55% were identified with previously known transitions. Although the EIS spectral coverage is limited to two ranges approximately 40 A wide, the identified lines belong to a total of 56 ions from 15 elements.

During relatively quiet solar conditions throughout the spring and summer of 2007, the SECCHI HI2 white-light telescope on the STEREO B solar-orbiting spacecraft observed a succession of wave fronts sweeping past Earth. We have compared these heliospheric images with in situ plasma and magnetic field measurements obtained by near-Earth spacecraft, and we have found a near perfect association between the occurrence of these waves and the arrival of density enhancements at the leading edges of high-speed solar wind streams. Virtually all of the strong corotating interaction regions are accompanied by large-scale waves, and the low-density regions between them lack such waves. Because the Sun was dominated by long-lived coronal holes and recurrent solar wind streams during this interval, there is little doubt that we have been observing the compression regions that are formed at low latitude as solar rotation causes the high-speed wind from coronal holes to run into lower speed wind ahead of it.

The SECCHI HI2 white-light imagers on the STEREO A and B spacecraft show systematically different proper motions of material moving outward from the Sun in front of high-speed solar wind streams from coronal holes. As a group of ejections enters the eastern (A) field of view, the elements at the rear of the group appear to overrun the elements at the front. (This is a projection effect and does not mean that the different elements actually merge.) The opposite is true in the western (B) field; the elements at the front of the group appear to run away from the elements at the rear. Elongation/time maps show this effect as a characteristic grouping of the tracks of motion into convergent patterns in the east and divergent patterns in the west, consistent with ejections from a single longitude on the rotating Sun. Evidently, we are observing segments of the "garden-hose" spiral made visible when fast wind from a low-latitude coronal hole compresses blobs of streamer material being shed at the leading edge of the hole.

A wavelength calibration of solar lines observed by the high resolution EUV Imaging Spectrometer (EIS) on the Hinode satellite is reported. Spectral features of the quiet sun and of two mildly active areas were measured and calibrated. A listing of the stronger observed lines with identification of the leading contributor ions is presented. 41 lines are reported, with 90% identified. Wavelength precisions (2{sigma}) of {+-}0.0031 Angstroms for the EIS short band and {+-}0.0029 Angstroms for the EIS long band are obtained. These lines, typical of 1-2x10{sup 6} K plasmas, are recommended as standards for the establishment of EIS wavelength scales. The temperature of EIS varies by about 1.5 C around the orbit and also with spacecraft pointing. The correlation of these temperature changes with wavelength versus pixel number scale changes is reported.

Coronal dimming has been a signature used to determine the source of plasma that forms part of a coronal mass ejection (CME) for many years. Generally dimming is detected through imaging instruments such as SOHO EIT by taking difference images. Hinode tracked active region 10930 from which there were a series of flares. We combined dimming observations from EIT with Hinode data to show the impact of flares and coronal mass ejections on the region surrounding the flaring active region, and we d iscuss evidence that the eruption resulted in a prolonged steady outflow of material from the corona. The dimming region shows clear structure with extended loops whose footpoints are the source of the strongest outflow ( � 40 km s � 1 ). This confirms that the loops that are disrupted during the event do lose plasma and hence are likely to for mp art of the CME. This is the first time the velocity of the coronal plasma has been measured in an extended dimming region away from the flare core. In addition there was a weaker steady outflow from extended, faint loops outside the active region before the eruption, which is also long lasting. These were disturbed and the velocity increased following the flare. Such out flows c ould be the source of the slow solar wind.

The EUV Imaging Spectrometer (EIS) on Hinode will observe solar corona and upper transition region emission lines in the wavelength ranges 170 – 210 A and 250 – 290 A. The line centroid positions and profile widths will allow plasma velocities and turbulent or non-thermal line broadenings to be measured. We will derive local plasma temperatures and densities from the line intensities. The spectra will allow accurate determination of differential emission measure and element abundances within a variety of corona and transition region structures. These powerful spectroscopic diagnostics will allow identification and characterization of magnetic reconnection and wave propagation processes in the upper solar atmosphere. We will also directly study the detailed evolution and heating of coronal loops. The EIS instrument incorporates a unique two element, normal incidence design. The optics are coated with optimized multilayer coatings. We have selected highly efficient, backside-illuminated, thinned CCDs. These design features result in an instrument that has significantly greater effective area than previous orbiting EUV spectrographs with typical active region 2 – 5 s exposure times in the brightest lines. EIS can scan a field of 6×8.5 arc min with spatial and velocity scales of 1 arc sec and 25 km s−1 per pixel. The instrument design, its absolute calibration, and performance are described in detail in this paper. EIS will be used along with the Solar Optical Telescope (SOT) and the X-ray Telescope (XRT) for a wide range of studies of the solar atmosphere.

We compare the morphology of active region structures observed in the 171 A (T ~ 9 × 105 K) and Lyα (T ~ 2 × 104 K) lines. The coronal data were obtained by the Transition Region and Coronal Explorer (TRACE) in support of the Very High Angular Resolution Ultraviolet Telescope (VAULT) sounding rocket launch, which acquired subarcsecond resolution images of an active region in the Lyα line, on 1999 May 7. Using a pair of calibrated, nearly simultaneous images, we find that: (i) a very good correlation exists between the Lyα and 171 A intensities in the TRACE moss regions, (ii) we can identify several identical structures in some (but not all) moss areas, and (iii) the correlations are greatly reduced at the footpoints of the 171 A large-scale loops. We derive a lower limit for the Lyα emission measure, under the assumption of effectively optically thin emission, and compare it to the 171 A emission measure. As in previous studies, we find an excess of Lyα material compared to the amount expected for a thermal conduction-dominated corona-chromosphere transition region, even for structures that appear to be identical in the two wavelengths. This result implies that some other mechanism besides classical heat conduction from the corona must contribute to the observed Lyα intensities. The observations do not support the idea of a physically distinct cool loop component within active regions.

The variation of the solar spectrum between 2000 and 4000 A is a major component of the total irradiance variability of the Sun. Recent analyses suggest that variations in the solar ultraviolet flux at these wavelengths account for about 30% of the total solar irradiance variability. Most estimates of solar spectral irradiance variability in the ultraviolet are based on the ratio of the intensity of features such as solar plages and sunspots to the intensity of the quiet Sun. These ratios are referred to as contrast factors. To a large degree, contrast factors at ultraviolet wavelengths have not been measured. We present measurements of the average intensities of plage, sunspot, and quiet-Sun regions in the spectral range between 2765 and 2885 A, derived from high spatial and spectral resolution spectra obtained during the ninth rocket flight of the High Resolution Telescope and Spectrograph in 1995. From these average spectra, plage and sunspot contrast factors are determined. To our knowledge, these are the first contrast factors at these wavelengths derived from measured solar intensities. These spectra show a large contrast in the Mg I, Mg II, and Si I Fraunhofer lines and a much smaller contrast (1.01 ± 0.015 to 1.04 ± 0.04) in the line-blanketed continuum. Contrast factors are also determined for three intensity levels of the quiet Sun as well as for a single sunspot. Many fine spectral features in the contrast factors can be attributed to weak Fraunhofer lines of Cr II, Fe I, Fe II, and Mg I.

The Very-high-resolution Advanced ULtraviolet Telescope (VAULT) experiment was successfully launched on 7 May 1999 on a Black Brant sounding rocket vehicle from White Sands Missile Range. The instrument consists of a 30 cm UV diffraction limited telescope followed by a two-grating, zero-dispersion spectroheliograph tuned to isolate the solar Lα emission line. During the flight, the instrument successfully obtained a series of images of the upper chromosphere with a limiting resolution of ∼ 0.33 arc sec. The resulting observations are the highest-resolution images of the solar atmosphere obtained from space to date. The flight demonstrated that sub-arc second ultraviolet images of the solar atmosphere are achievable with a high-quality, moderate-aperture space telescope and associated optics. Herein, we describe the payload and its in-flight performance.

The LASCO C1 coronagraph on the SOHO satellite observes the solar corona from 1.1 to 3.0 R ⊙ , and contains a Fabry-Perot interferometer which can image the corona in the 1.8 million K Fe XIV green line. We designed an observing program with reduced spatial coverage and reduced profile coverage at only three wavelengths to study coronal heating in off-limb structures at high temporal resolution. We illustrate the observations from 31 March 1997 of a bright loop system above an active region off the northeast limb.

White-light measurements made by the Mk III Mauna Loa K-coronameter and the SOHO LASCO C2 and C3 coronagraphs, extending from 1.15 to 30 Ro, have been combined to show that the boundaries of polar coronal holes, as determined by measurements of path-integrated density, extend approximately radially into interplanetery space.

We use Fe XIV 5303 A green line images obtained by the Large Angle Spectrometric Coronagraph (LASCO) on board SOHO to search for variability in the quiescent solar corona in the shortest observable timescales. The observing program obtained Fe XIV images of a small area of the inner corona every 2 minutes over a period of 1 hr. We present results from two executions of this program taken several months apart. The most obvious variability observed in the two sequences is in the form of quasi-steady brightening on timescales of at least an hour. Of particular interest are two compact loops that are observed to vary significantly during the course of the observations. Superposed on the long-term brightening in these loops are statistically significant variations on timescales of about 30 minutes. In both loops, the overall brightening is greatest at the apparent loop tops, where the intensity increases by at least 25%. In one loop there appears to be a flow up one of the legs of the loop. We place these observations in context with earlier observations of coronal variability, and we discuss the energy requirements for the observed brightening. Emission measures computed from Fe XIV intensities measured within the two brightening loops are over an order of magnitude lower than those typically found for active regions on the solar disk, which suggests significantly lower average densities. For one of the loops, we measure densities in the range ne = (5.0-7.4) × 108 cm-3. Lower than average densities are expected for the two loops, given that they reach more than 1 pressure scale height above the solar limb.

We present in this Letter the first coordinated radio occultation measurements and ultraviolet observations of the inner corona below 5.5 Rs, obtained during the Galileo solar conjunction in January 1997, to establish the origin of the slow solar wind. Limits on the flow speed are derived from the Doppler dimming of the resonantly scattered componentof the oxygen 1032 A and 1037 A lines as measured with the UltraViolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). White light images of the corona from the Large Angle Spectroscopic Coronagraph (LASCO) on SOHO taken simultaneously are used to place the Doppler radio scintillation and ultraviolet measurements in the context ofcoronal structures. These combined observations provide the first direct confirmation of the view recently proposed by Woo and Martin (1997) that the slow solar wind is associated with the axes, also known as stalks, of streamers. Furthermore, the ultraviolet observations also show how the fast solar wind is ubiquitous in the inner corona, and that a velocity shear between the fast and slow solar wind develops along the streamer stalks., 15 pages, LaTex, 6 jpg figures, accepted Aug. 28, 1997 for publication in the ApJ Letters

We employ coronal extrapolations of solar magnetograph data to interpret observations of the white-light streamer structure made with the LASCO coronagraph in 1996. The topological appearance of the streamer belt during the present minimum activity phase is well described by a model in which the Thomson-scattering electrons are concentrated around a single, warped current sheet encircling the Sun. Projection effects give rise to bright, jet-like structures or spikes whenever the current sheet is viewed edge-on; multiple spikes are seen if the current sheet is sufficiently wavy. The extreme narrowness of these features in polarized images indicates that the scattering layer is at most a few degrees wide. We model the evolution of the streamer belt from 1996 April to 1996 September and show that the effect of photospheric activity on the streamer belt topology depends not just on the strength of the erupted magnetic flux, but also on its longitudinal phase relative to the background field. Using flux transport simulations, we also demonstrate how the streamer belt would evolve during a prolonged absence of activity.

Images of the green line corona made with the LASCO C1 coronagraph on SOHO are analyzed by applying current-free extrapolations to the observed photospheric field. The Fe XIV λ5303 emission is shown to be closely related to the underlying photospheric field strength. By modeling the observed intensity patterns as a function of latitude and height above the solar limb, we derive an approximate scaling law of the form nfoot ∝ Bfoot0.9, where nfoot is the density of the green line-emitting plasma and Bfoot is the average field strength at the footprints of the coronal loop. The observed high-latitude enhancements in the green line corona are attributed to the poleward concentration of the large-scale photospheric field. The strongest such enhancements occur where the high-latitude unipolar fields become reconnected to active region flux at lower latitudes; the global emission pattern rotates quasi-rigidly at the rate of the dominant active region complex. The validity of the current-free approximation is assessed by comparing the topology of the observed and simulated green line structures.

Time-lapse sequences of white-light images, obtained during sunspot minimum conditions in 1996 by the Large Angle Spectrometric Coronagraph on the Solar and Heliospheric Observatory, give the impression of a continuous outflow of material in the streamer belt, as if we were observing Thomson scattering from inhomogeneities in the solar wind. Pursuing this idea, we have tracked the birth and outflow of 50-100 of the most prominent moving coronal features and find that: 1. They originate about 3-4 R☉ from Sun center as radially elongated structures above the cusps of helmet streamers. Their initial sizes are about 1 R☉ in the radial direction and 0.1 R☉ in the transverse direction. 2. They move radially outward, maintaining constant angular spans and increasing their lengths in rough accord with their speeds, which typically double from 150 km s-1 near 5 R☉ to 300 km s-1 near 25 R☉. 3. Their individual speed profiles v(r) cluster around a nearly parabolic path characterized by a constant acceleration of about 4 m s-2 through most of the 30 R☉ field of view. This profile is consistent with an isothermal solar wind expansion at a temperature of about 1.1 MK and a sonic point near 5 R☉. Based on their relatively small initial sizes, low intensities, radial motions, slow but increasing speeds, and location in the streamer belt, we conclude that these moving features are passively tracing the outflow of the slow solar wind.

We present the first observations of the initiation of a coronal mass ejection (CME) seen on the disk of the Sun. Observations with the EIT experiment on SOHO show that the CME began in a small volume and was initially associated with slow motions of prominence material and a small brightening at one end of the prominence. Shortly afterward, the prominence was accelerated to about 100 km s-1 and was preceded by a bright loop-like structure, which surrounded an emission void, that traveled out into the corona at a velocity of 200–400 km s-1. These three components, the prominence, the dark void, and the bright loops are typical of CMEs when seen at distance in the corona and here are shown to be present at the earliest stages of the CME. The event was later observed to traverse the LASCO coronagraphs fields of view from 1.1 to 30 R. Of particular interest is the fact that this large-scale event, spanning as much as 70 deg in latitude, originated in a volume with dimensions of roughly 35″ (2.5 × 104 km). Further, a disturbance that propagated across the disk and a chain of activity near the limb may also be associated with this event as well as a considerable degree of activity near the west limb.

The newly developed C1 coronagraph as part of the Large-Angle Spectroscopic Coronagraph (LASCO) on board the SOHO spacecraft has been operating since January 29, 1996. We present observations obtained in the first three months of operation. The green-line emission corona can be made visible throughout the instrument's full field of view, i.e., from 1.1 R⊙ out to 3.2 R⊙ (measured from Sun center). Quantitative evaluations based on calibrations cannot yet be performed, but some basic signatures show up even now: (1) There are often bright and apparently closed loop systems centered at latitudes of 30° to 45° in both hemispheres. Their helmet-like extensions are bent towards the equatorial plane. Farther out, they merge into one large equatorial ‘streamer sheet’ clearly discernible out to 32 R⊙. (2) At mid latitudes a more diffuse pattern is usually visible, well separated from the high-latitude loops and with very pronounced variability. (3) All high-latitude structures remain stable on time scales of several days, and no signature of transient disruption of high-latitude streamers was observed in these early data. (4) Within the first 4 months of observation, only one single ‘fast’ feature was observed moving outward at a speed of 70 km s-1 close to the equator. Faster events may have escaped attention because of data gaps. (5) The centers of high-latitude loops are usually found at the positions of magnetic neutral lines in photospheric magnetograms. The large-scale streamer structure follows the magnetic pattern fairly precisely. Based on our observations we conclude that the shape and stability of the heliospheric current sheet at solar activity minimum are probably due to high-latitude streamers rather than to the near-equatorial activity belt.

The Naval Research Laboratory is developing next generation CMOS imaging arrays for the Solar Orbiter and Solar Probe Plus missions. The device development is nearly complete with flight device delivery scheduled for summer of 2013. The 4Kx4K mosaic array with 10micron pixels is well suited to the panoramic imaging required for the Solar Orbiter mission. The devices are robust (

The Solar Probe Plus (SPP) mission scheduled for launch in 2018, will orbit between the Sun and Venus with diminishing perihelia reaching as close as 7 million km (9.86 solar radii) from Sun center. In addition to a suite of in-situ probes for the magnetic field, plasma, and energetic particles, SPP will be equipped with an imager. The Wide-field Imager for the Solar PRobe+ (WISPR), with a 95° radial by 58° transverse field of view, will image the fine-scale coronal structure of the corona, derive the 3D structure of the large-scale corona, and determine whether a dust-free zone exists near the Sun. Given the tight mass constrains of the mission, WISPR incorporates an efficient design of two widefield telescopes and their associated focal plane arrays based on novel large-format (2kx2k) APS CMOS detectors into the smallest heliospheric imaging package to date. The flexible control electronics allow WISPR to collect individual images at cadences up to 1 second at perihelion or sum several of them to increase the signal-to-noise during the outbound part of the orbit. The use of two telescopes minimizes the risk of dust damage which may be considerable close to the Sun. The dependency of the Thomson scattering emission of the corona on the imaging geometry dictates that WISPR will be very sensitive to the emission from plasma close to the spacecraft in contrast to the situation for imaging from Earth orbit. WISPR will be the first ‘local’ imager providing a crucial link between the large scale corona and the in-situ measurements.

The SoloHI instrument for the ESA/NASA Solar Orbiter mission will track density fluctuations in the inner heliosphere, by observing visible sunlight scattered by electrons in the solar wind. Fluctuations are associated with dynamic events such as coronal mass ejections, but also with the “quiescent” solar wind. SoloHI will provide the crucial link between the low corona observations from the Solar Orbiter instruments and the in-situ measurements on Solar Orbiter and the Solar Probe Plus missions. The instrument is a visible-light telescope, based on the SECCHI/Heliospheric Imager (HI) currently flying on the STEREO mission. In this concept, a series of baffles reduce the scattered light from the solar disk and reflections from the spacecraft to levels below the scene brightness, typically by a factor of 10 12 . The fluctuations are imposed against a much brighter signal produced by light scattered by dust particles (the zodiacal light/F-corona). Multiple images are obtained over a period of several minutes and are summed on-board to increase the signal-to-noise ratio and to reduce the telemetry load. SoloHI is a single telescope with a 40⁰ field of view beginning at 5⁰ from the Sun center. Through a series of Venus gravity assists, the minimum perihelia for Solar Orbiter will be reduced to about 60 Rsun (0.28 AU), and the inclination of the orbital plane will be increased to a maximum of 35⁰ after the 7 year mission. The CMOS/APS detector is a mosaic of four 2048 x 1920 pixel arrays, each 2-side buttable with 11 µm pixels.

The LASCO experiment on board the SOHO satellite has been making observations of the solar corona out to 30 Ro since first light on 29 December 1995, and routinely since 15 May 1996. Over 120 coronal mass ejections have been observed to date. LASCO represents a significant advance over previous coronagraphs in many ways, but principally in an expanded field of view, increased sensitivity and increased dynamic range. The satellite, orbiting about the Lagrangian point, L1, is in continual sunlight, providing the opportunity to view the corona continuously, uninterrupted by orbit night as is common with near-Earth orbits. While the CMEs observed by LASCO are similar to those observed with previous coronagraphs, there are several new aspects: (1) Many are accompanied by a global response of the solar corona, (2) Many show acceleration to the edge of the field, (3) Disconnection is a frequent occurrence, (4) CMEs are occurring more frequently than had been expected at this minimum phase of the solar activity cycle, and (5) CMEs undergo extensive internal evolution as they move outward.

The Large Angle Spectrometric Coronagraph (LASCO) C1 coronagraph on board the Solar and Heliospheric Observatory (SOHO) is designed to image the corona from 1.1 to 3.0 R☉. The resolution of C1 is defined by the size of its CCD pixels, which correspond to 56, and not by the diffraction limit of the optical system, which may be as small as 3''. The resolution of C1 can be improved using the technique of "dynamic imaging"—the process of acquiring successive images of the same scene using subpixel displacements of the steerable primary mirror. We developed a technique we call the fractional pixel restoration (FPR) algorithm that utilizes these observations to construct an image with improved resolution. Simulations were used to test this algorithm and to explore its limitations. We also applied the direct co-addition and FPR algorithms to laboratory preflight images of a wire mesh grid. These results show that the resolution of the C1 coronagraph can be significantly enhanced, even in the presence of noise and modest differences between successive images. In some cases, the results can even reach the diffraction limit of the telescope.

The primary science objective of the Coronal Suprathermal Particle Explorer (C-SPEX) is to investigate the spatial and temporal variations of coronal suprathermal particle populations that are seeds for acceleration to solar energetic particles (SEPs). It is understood that such seed particle populations vary with coronal structures and can change responding to solar flare and coronal mass ejection (CME) events. Models have shown that higher densities of suprathermal protons can result in higher rates of acceleration to high energies. Understanding the variations in the suprathermal seed particle population is thus crucial for understanding the variations in SEPs. However, direct measurements are still lacking. C-SPEX will measure the variation in the suprathermal protons across various coronal magnetic structures, before/after the passage of CME shocks, in the post-CME current sheets, and before/after major solar flares. Understanding the causes for variation in the suprathermal seed particle population and its effect on the variation in SEPs will also help build the predictive capability of SEPs that reach Earth. The CSPEX measurements will be obtained from instrumentation on the International Space Station (ISS) employing well-established UV coronal spectroscopy techniques.

We describe a simple visible-light stray-background-reducing baffle, suitable for use on a stabilized interplanetary platform. The design is a corrallike enclosure with five concentric walls. The baffle reduces direct sunlight and reflections from illuminated portions of the spacecraft by a factor of 10(-12), provided that all these lie beyond at least a hemisphere centered on the viewing aperture. With this condition these bright sources do not directly illuminate within the outermost wall of the corral, and diffraction over the wall tops is the dominant mechanism by which light reaches the corral interior. We present design calculations for such a corral, as well as a laboratory measurement confirming the basic design assumption.

The Very high Angular resolution ULtraviolet Telescope (VAULT) is a sounding rocket payload built to study the crucial interface between the solar chromosphere and the corona by observing the strongest line in the solar spectrum, the Ly-a line at 1216 {\AA}. In two flights, VAULT succeeded in obtaining the first ever sub-arcsecond (0.5") images of this region with high sensitivity and cadence. Detailed analyses of those observations have contributed significantly to new ideas about the nature of the transition region. Here, we present a broad overview of the Ly-a atmosphere as revealed by the VAULT observations, and bring together past results and new analyses from the second VAULT flight to create a synthesis of our current knowledge of the high-resolution Ly-a Sun. We hope that this work will serve as a good reference for the design of upcoming Ly-a telescopes and observing plans., Comment: 28 pages, 11 figures

The Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) is a five telescope package, which has been developed for the Solar Terrestrial Relation Observatory (STEREO) mission by the Naval Research Laboratory (USA), the Lockheed Solar and Astrophysics Laboratory (USA), the Goddard Space Flight Center (USA), the University of Birmingham (UK), the Rutherford Appleton Laboratory (UK), the Max Planck Institute for Solar System Research (Germany), the Centre Spatiale de Leige (Belgium), the Institut d’Optique (France) and the Institut d’Astrophysique Spatiale (France). SECCHI comprises five telescopes, which together image the solar corona from the solar disk to beyond 1 AU. These telescopes are: an extreme ultraviolet imager (EUVI: 1–1.7 R⊙), two traditional Lyot coronagraphs (COR1: 1.5–4 R⊙ and COR2: 2.5–15 R⊙) and two new designs of heliospheric imagers (HI-1: 15–84 R⊙ and HI-2: 66–318 R⊙). All the instruments use 2048×2048 pixel CCD arrays in a backside-in mode. The EUVI backside surface has been specially processed for EUV sensitivity, while the others have an anti-reflection coating applied. A multi-tasking operating system, running on a PowerPC CPU, receives commands from the spacecraft, controls the instrument operations, acquires the images and compresses them for downlink through the main science channel (at compression factors typically up to 20×) and also through a low bandwidth channel to be used for space weather forecasting (at compression factors up to 200×). An image compression factor of about 10× enable the collection of images at the rate of about one every 2–3 minutes. Identical instruments, except for different sizes of occulters, are included on the STEREO-A and STEREO-B spacecraft.

The Heliospheric Imager (HI) is part of the SECCHI suite of instruments on-board the two STEREO observatories launched in October 2006. The two HI instruments provide stereographic image pairs of solar coronal plasma and coronal mass ejections (CME) over a field of view ranging from 13 to 330 R 0 . The HI instrument is a combination of two refractive optical systems with a two stage multi-vane baffle system. The key challenge of the instrument design is the rejection of the solar disk light by the front baffle, with total straylight attenuation at the detector level of the order of 10 -13 to 10 -15 . Optical systems and baffles were designed and tested to reach the required rejection. This paper presents the pre-flight optical tests performed under vacuum on the two HI flight models in flight temperature conditions. These tests included an end-to-end straylight verification of the front baffle efficiency, a co-alignment and an optical calibration of the optical systems. A comparison of the theoretical predictions of the instrument response and performance with the calibration results is presented. The instrument in-flight photometric and stray light performance are also presented and compared with the expected results.

The laboratory end-to-end testing of the Extreme-Ultraviolet Imaging Spectrometer (EIS) for the Solar-B satellite is reported. A short overview of the EIS, which observes in two bands in the extreme-ultraviolet wavelength range, is given. The calibration apparatus is described, including details of the light sources used. The data reduction and analysis procedure are outlined. The wavelength calibration using a Penning source to illuminate the aperture fully is presented. We discuss the aperture determination using a radiometrically calibrated hollow-cathode-based source. We then give an account of the predicted and measured efficiencies from consideration of the efficiencies of individual optical elements in first order, an account of efficiencies out of band when radiation incident in one band is detected in the other, and efficiencies in multiple orders. The efficiencies measured in first order for in band and out of band are compared with the predictions and the sensitivity, and its uncertainties are derived. Application of the radiometric calibration is discussed.

The ISAS/JAXA Solar-B mission includes an Extreme-UV Imaging Spectrometer (EIS). It detects photons in the wavelength ranges 17 - 21 nm and 25 - 29 nm which include emission lines from several highly ionised species that exist at temperatures log T = 4.7, 5.6, 5.8, 5.9 and 6.0 - 7.3 K. Instrument throughput is increased substantially by the use of multilayer coatings optimized for maximum reflectance in the two selected wavelength bands. The use of back-illuminated CCDs provides significantly enhanced quantum efficiency over that previously available from microchannel plate systems. In this paper we will describe the design and operation of the instrument and present its performance parameters e.g. spectral and spatial resolution and sensitivity. Preliminary results of recent calibration measurements will be described. The role of EIS in the Solar-B mission will be illustrated with reference to the anticipated observing strategy for the first three months of the mission which will be outlined.

We present a detailed review of the calibration of the LASCO C3 coronagraph on the SOHO satellite. Most of the calibration has been in place since early in the mission and has been utilized to varying degrees as required by specific analysis efforts. However, using observational data from the nearly decade-long database of LASCO images, we have re-evaluated and improved many aspects of the calibration. This includes the photometric calibration, vignetting function, geometric distortion, stray light, and exposure and observation times. Using this comprehensive set of corrections we have generated and made available a set of calibrated coronal images along with a set of periodic background images to ease the accessibility and use of the LASCO database.

This paper describes an instrument for imaging spectroscopy of ultraviolet (UV) line emission from the solar corona, in the 0.3-1.2x102 nm wavelength range. The optical design for this Ultraviolet Spectro-Coronagraph (UVSC) is an externally occulted, off-axis Gregorian telescope where the secondary mirror is a Toroidal Varied Line-Space (TVLS) grating. A field stop with multiple slits is at the prime focus of the telescope’s mirror. This multi-slit field stop is the entrance aperture for the spectrograph. The slits select a number of strips in the field-of-view (FOV) with enough separation to minimize the spectral overlap of the UV lines dispersed by the TVLS grating. This type of gratings allows for a much larger stigmatic FOV (i.e., 3° x 4°) in both the spatial and spectral direction than that of the Toroidal Uniform Line-Space (TULS) gratings. The complete imaging of the FOV is obtained by interpolating the slit images along the spectral dispersion direction. As an example, this paper discusses the possible use of a UVSC instrument on HERSCHEL, a NASA sounding-rocket payload, and on Solar Orbiter (SOLO), an ESA mission. HERSCHEL includes the Sounding CORona Experiment (SCORE) that comprises a UV Coronagraphic Imager (UVCI) for narrow-band (i.e., λ/Δλ≈10) imaging of the HeII, 30.4 nm, line. How a spectroscopic capability (i.e., λ/uλ ≈0.3-1 x 10 4 ) would enhance the HERSCHEL science is discussed. The SOLO mission is planned for launch in 2013. Its orbital profile will bring the spacecraft as close to the Sun as 0.22 A.U. Also SOLO would represent an ideal and unique platform for a compact UVSC instrument (i.e., ≈ 1-m length) capable of obtaining simultaneous imaging and spectroscopy of the UV corona. The expected optical performances are presented for a UVSC/SOLO optimised for the OVI doublet, 103.2/103.7 nm.

The SIde-Looking Coronagraph (SILC) is one of the solar remote-sensing instruments proposed for the payload of the Solar Orbiter mission. The Solar Orbiter is a mission selected in September 2000 by the European Space Agency (ESA) for the definition study phase. The Solar Orbiter will describe elliptic orbits with a large range of heliocentric distance, from 0.21 to 0.6 AU (astronomical units), that is a factor 3 for the geometric conditions and will reach heliographic latitudes as high as 38 degrees. Furthermore, the spacecraft will have offset pointing capability so as to target any point of the solar disk. These constraints (in addition to the severe thermal environment) lead us to propose an externally occulted coronagraph entirely protected from direct sunlight by remaining in the shadow of the spacecraft and looking sideways. The optical design follows the general principles of an externally-occulted coronagraph adapted to the side-looking concept. Although SILC loses the full spatial coverage of the corona, it can observe the inner part of the corona (down to 1.5R) during the whole mission and compensate the off-pointing of the spacecraft in the two directions. The performances, resulting from ray-tracing calculations, are presented here together with the expected stray light level.

The measured efficiencies of two flight gratings and the reflectances of two flight mirrors developed for the Extreme-Ultraviolet Imaging Spectrometer (EIS) for the Japanese Solar-B mission are presented. Each optic has two sectors with Mo/Si multilayers that refelct the 17 - 21 nm and 25 - 29 nm wavebands at normal incidence. The efficiencies that were measured using monochromatic synchrotron radiation are in good agreement with the calculated efficiencies.

A hot, magnetized plasma such as the solar corona has the property that much of the physics governing its activity takes place on remarkably small spatial and temporal scales, while the response to this activity occurs on large scales. Observations from SMM, TRACE, SOHO and Yohkoh have shown that typical solar active regions have loops ranging in temperature from 0.5 to 10 MK, and flares up to 40MK. The spatial and temporal domains involved have been heretofore inaccessible to direct observations from Earth, so that theory has relied heavily on extrapolations from more accessible regimes, and on speculation. The RAM Solar-Terrestrial Probe consists of a set of carefully selected imaging and spectroscopic instruments that enable definitive studies of the dynamics and energetics of the solar corona.

The Very high Angular Resolution ULtraviolet Telescope experiment was successfully launched on May 7, 1999 on a Black Brant sounding rocket vehicle from White Sands Missile Range. The instrument consists of a 30 cm UV diffraction limited telescope followed by a double grating spectroheliograph tuned to isolate the solar Lyman (alpha) emission line. During the flight, the instrument successfully obtained a series of images of the upper chromosphere with a limiting resolution of approximately 0.33 arc-seconds. The resulting observations are the highest resolution images of the solar atmosphere obtained from space to date. The flight demonstrated that subarc-second ultraviolet images of the solar atmosphere are achievable with a high quality, moderate aperture space telescope and associated optics. Herein, we describe the payload and its in- flight performance.