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Solar abundances have been historically assumed to be representative of cosmic abundances. However, our knowledge of the solar abundance of helium, the second most abundant element, relies mainly on models1 and indirect measurements through helioseismic observations2, because actual measurements of helium in the solar atmosphere are very scarce. Helium cannot be directly measured in the photosphere because of its high first ionization potential, and measurements of its abundance in the inner corona have been sporadic3,4. In this Letter, we present simultaneous global images of the helium (out to a heliocentric distance of 3R⊙ (solar radii)) and hydrogen emission in the solar corona during the minimum of solar activity of cycle 23 and directly derive the helium abundance in the streamer region and surrounding corona (out to 2.2R⊙). The morphology of the He+ corona is markedly different from that of the H corona, owing to significant spatial variations in helium abundance. The observations show that the helium abundance is shaped according to and modulated by the structure of the large-scale coronal magnetic field and that helium is almost completely depleted in the equatorial regions during the quiet Sun. This measurement provides a trace back to the coronal source of the anomalously slow solar wind observed in the heliosphere at the Sun–Earth Lagrangian point L1 in 2009, during the exceptionally long-lasting minimum of solar activity cycle 23. Global images of helium and hydrogen emission are used to directly derive the helium abundance out to 2.2R⊙. The helium abundance is shaped by the large-scale coronal magnetic field. Helium is almost completely depleted near the equator in the quiet Sun.

The second NASA Earth Venture Mission, Geostationary Carbon Cycle Observatory (GeoCarb), will provide measurements of atmospheric carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), and solar-induced fluorescence (SIF) from Geostationary Orbit (GEO). The GeoCarb mission will deliver daily maps of column concentrations of CO2, CH4, and CO over the observed landmasses in the Americas at a spatial resolution of roughly 10 x 10 km. Persistent measurements of CO2, CH4, CO, and SIF will contribute significantly to resolving carbon emissions and illuminating biotic processes at urban to continental scales, which will allow the improvement of modeled biogeochemical processes in Earth System Models as well as monitor the response of the biosphere to disturbance. This is essential to improve understanding of the Carbon-Climate connection. In this paper, we introduce the instrument and the GeoCarb Mission, and we demonstrate the potential scientific contribution of the mission through a series of CO2 and CH4 simulation experiments. We find that GeoCarb will be able to constrain emissions at urban to continental spatial scales on weekly to annual time scales. The GeoCarb mission particularly builds upon the Orbiting Carbon Obserevatory-2 (OCO-2), which is flying in Low Earth Orbit.

The Solar Dynamics Observatory provides multiwavelength imagery from extreme ultraviolet (EUV) to visible light as well as magnetic-field measurements. These data enable us to study the nature of solar activity in different regions of the Sun, from the interior to the corona. For solar-cycle studies, synoptic maps provide a useful way to represent global activity and evolution by extracting a central meridian band from sequences of full-disk images over a full solar Carrington rotation (≈ 27.3 days). We present the global evolution during Solar Cycle 24 from 20 May 2010 to 31 August 2013 (CR 2097 – CR 2140), using synoptic maps constructed from full-disk, line-of-sight magnetic-field imagery and EUV imagery (171 A, 193 A, 211 A, 304 A, and 335 A). The synoptic maps have a resolution of 0.1 degree in longitude and steps of 0.001 in sine of latitude. We studied the axisymmetric and non-axisymmetric structures of solar activity using these synoptic maps. To visualize the axisymmetric development of Cycle 24, we generated time–latitude (also called butterfly) images of the solar cycle in all of the wavelengths, by averaging each synoptic map over all longitudes, thus compressing it to a single vertical strip, and then assembling these strips in time order. From these time–latitude images we observe that during the ascending phase of Cycle 24 there is a very good relationship between the integrated magnetic flux and the EUV intensity inside the zone of sunspot activities. We observe a North–South asymmetry of the EUV intensity in high-latitudes. The North–South asymmetry of the emerging magnetic flux developed and resulted in a consequential asymmetry in the timing of the polar magnetic-field reversals.

We study the association of solar flares with coronal mass ejections (CMEs) during the deep, extended solar minimum of 2007-2009, using extreme-ultraviolet (EUV) and white-light (coronagraph) images from the {\it Solar Terrestrial Relations Observatory} (STEREO). Although all of the fast (v $>$ 900 km s$^{-1}$) {\it and} wide ($��>$ 100$\arcdeg$) CMEs are associated with a flare that is at least identified in GOES soft X-ray light curves, a majority of flares with relatively high X-ray intensity for the deep solar minimum (e.g. $\gtrsim$1 \times 10^{-6}$ W m$^{-2}$ or C1) are not associated with CMEs. Intense flares tend to occur in active regions with strong and complex photospheric magnetic field, but the active regions that produce CME-associated flares tend to be small, including those that have no sunspots and therefore no NOAA active-region numbers. Other factors on scales comparable to and larger than active regions seem to exist that contribute to the association of flares with CMEs. We find the possible low coronal signatures of CMEs, namely eruptions, dimmings, EUV waves, and Type III bursts, in 91%, 74%, 57%, and 74%, respectively, of the 35 flares that we associate with CMEs. None of these observables can fully replace direct observations of CMEs by coronagraphs., 22 pages, 9 figures, accepted by Solar Physics

We generated an event catalog with an automated detection algorithm based on the entire EUVI image database observed with the two Solar Terrestrial Relations Observatory STEREO-A and -B spacecraft over the first six years of the mission (2006\,--\,2012). The event catalog includes the heliographic positions of some 20\,000 EUV events, transformed from spacecraft coordinates to Earth coordinates, and information on associated GOES flare events (down to the level of GOES A5-class flares). The 304 \ang\ wavelength turns out to be the most efficient channel for flare detection (79\,%), while the 171 \ang\ (4\,%), 195 \ang\ (10\,%), and the 284 \ang\ channel (7\,%) retrieve substantially fewer flare events, partially due to the suppressing effect of EUV dimming, and partially due to the lower cadence in the later years of the mission. Due to the Sun-circling orbits of STEREO-A and -B, a large number of flares have been detected on the farside of the Sun, invisible from Earth, or seen as partially occulted events. The statistical size distributions of EUV peak fluxes (with a power-law slope of $\alpha_P = 2.5\pm0.2$) and event durations (with a power-law slope of $\alpha_T=2.4\pm0.3$) are found to be consistent with the fractal-diffusive self-organized criticality model. The EUVI event catalog is available on-line \url{this http URL} and may serve as a comprehensive tool to identify stereoscopically observed flare events for 3D reconstruction and to study occulted flare events.

The Atmospheric Imaging Assembly (AIA) provides multiple simultaneous high-resolution full-disk images of the corona and transition region up to 0.5 R ⊙ above the solar limb with 1.5-arcsec spatial resolution and 12-second temporal resolution. The AIA consists of four telescopes that employ normal-incidence, multilayer-coated optics to provide narrow-band imaging of seven extreme ultraviolet (EUV) band passes centered on specific lines: Fe xviii (94 A), Fe viii, xxi (131 A), Fe ix (171 A), Fe xii, xxiv (193 A), Fe xiv (211 A), He ii (304 A), and Fe xvi (335 A). One telescope observes C iv (near 1600 A) and the nearby continuum (1700 A) and has a filter that observes in the visible to enable coalignment with images from other telescopes. The temperature diagnostics of the EUV emissions cover the range from 6×104 K to 2×107 K. The AIA was launched as a part of NASA’s Solar Dynamics Observatory (SDO) mission on 11 February 2010. AIA will advance our understanding of the mechanisms of solar variability and of how the Sun’s energy is stored and released into the heliosphere and geospace.

The masses of coronal mass ejections (CMEs) have traditionally been determined from white-light coronagraphs (based on Thomson scattering of electrons), as well as from extreme ultraviolet (EUV) dimming observed with one spacecraft. Here we develop an improved method of measuring CME masses based on EUV dimming observed with the dual STEREO/EUVI spacecraft in multiple temperature filters that includes three-dimensional volume and density modeling in the dimming region and background corona. As a test, we investigate eight CME events with previous mass determinations from STEREO/COR2, of which six cases are reliably detected with the Extreme Ultraviolet Imager (EUVI) using our automated multi-wavelength detection code. We find CME masses in the range of m {sub CME} = (2-7) x 10{sup 15} g. The agreement between the two EUVI/A and B spacecraft is m{sub A} /m{sub B} = 1.3 +- 0.6 and the consistency with white-light measurements by COR2 is m{sub EUVI}/m{sub COR2} = 1.1 +- 0.3. The consistency between EUVI and COR2 implies no significant mass backflows (or inflows) at r < 4 R{sub sun} and adequate temperature coverage for the bulk of the CME mass in the range of T approx 0.5-3.0 MK. The temporal evolution of the EUV dimming allowsmore » us to also model the evolution of the CME density n{sub e} (t), volume V(t), height-time h(t), and propagation speed v(t) in terms of an adiabatically expanding self-similar geometry. We determine e-folding EUV dimming times of t{sub D} = 1.3 +- 1.4 hr. We test the adiabatic expansion model in terms of the predicted detection delay (DELTAt approx 0.7 hr) between EUVI and COR2 for the fastest CME event (2008 March 25) and find good agreement with the observed delay (DELTAt approx 0.8 hr).« less

Here we develop a novel three-dimensional (3D) reconstruction method of the coronal plasma of an active region by combining stereoscopic triangulation of loops with density and temperature modeling of coronal loops with a filling factor equivalent to tomographic volume rendering. Because this method requires only a stereoscopic image pair in multiple temperature filters, which are sampled within 1 minute with the recent STEREO/EUVI instrument, this method is about four orders of magnitude faster than conventional solar rotation-based tomography. We reconstruct the 3D density and temperature distribution of active region NOAA 10955 by stereoscopic triangulation of 70 loops, which are used as a skeleton for a 3D field interpolation of some 7000 loop components, leading to a 3D model that reproduces the observed fluxes in each stereoscopic image pair with an accuracy of a few percents (of the average flux) in each pixel. With the stereoscopic tomography we infer also a differential emission measure distribution over the entire temperature range of T 104-107, with predictions for the transition region and hotter corona in soft X-rays. The tomographic 3D model provides also large statistics of physical parameters. We find that the extreme-ultraviolet loops with apex temperatures of Tm 3.0 MK tend to be super-hydrostatic, while hotter loops with Tm 4-7 MK are near-hydrostatic. The new 3D reconstruction model is fully independent of any magnetic field data and is promising for future tests of theoretical magnetic field models and coronal heating models.

Using the stereoscopically derived three-dimensional (3D) geometry of 30 loops observed with STEREO EUVI (described in Paper I) we determine here the electron density profiles ne(s) and electron temperature profiles Te(s) from a triple-filter analysis of the stereoscopic images taken in the wavelengths of λ = 171, 195, and 284 A. The statistical results of our analysis of seven complete loops are: observed loop widths wobs = 2.6 ± 0.1 Mm, corresponding to effective loop widths of w = 1.1 ± 0.3 Mm if corrected for the instrumental point-spread function; loop flux ratios floop/ftotal = 0.11 ± 0.04; mean loop (DEM peak) temperatures Tp = 1.1 ± 0.2 MK; DEM temperature Gaussian widths σDEM = 0.35 ± 0.04 MK; temperature variations along loops σT/Tp = 0.24 ± 0.05; (resolution-corrected) loop base densities ne = (2.2 ± 0.5) × 109 cm−3; loop lengths of L = 130 ± 67 Mm; and all quantities are found to agree between STEREO A and B within a few percent. The temperature profiles T(s) along loops are found to be nearly constant, within the uncertainties of the background subtraction. The density profiles ne(s) are consistent with the gravitational stratification of hydrostatic loops, ne(h) = nbaseexp (− h/λT) , defined by the temperature scale heights λT and stereoscopically measured from the height profiles h(s) . The stereoscopic 3D reconstruction allows us for the first time to accurately measure the loop length L and to test loop scaling laws. We find that the observations are not consistent with equilibrium solutions, but rather display the typical overpressures of loops that have been previously heated to higher temperatures and cool down in a nonequilibrium state, similar to earlier EIT and TRACE measurements.

We present one of the first triangulations and 3D reconstructions of coronal loops, using the EUVI telescopes of the two STEREO A and B spacecraft. The first triangulation of coronal loops was performed in an active region, observed with STEREO A and B on 2007 May 9 with a spacecraft separation angle of αsep = 7.3°, at a wavelength of 171 A. We identify 30 loop structures (7 complete loops and 23 partial segments) and compute their 3D coordinates (x,y,z) (the full 3D coordinates are available as an electronic file). We quantify the height range, the stereoscopic height measurement errors, the loop plane inclination angles, and the coplanarity and circularity of the analyzed loops. The knowledge of the exact 3D geometry of a loop with respect to the observer's line of sight has important consequences for determining the correct vertical density scale height (used in hydrostatic models), the aspect angle of loop cross sections (used in inferring electron densities from optically thin emission measures), the absolute flow speeds (used in siphon flow models), the correct loop length (used in loop scaling laws), and the 3D vectors of the coronal magnetic field (used in testing theoretical magnetic field extrapolation models). The hydrodynamic and magnetic modeling of the analyzed loops will be described in subsequent papers.

Recent extreme ultraviolet (EUV) observations from SOHO have shown the common occurrence of flare-associated global coronal waves strongly correlated with metric type II bursts, and in some cases with chromospheric Moreton waves. Until now, however, few direct soft X-ray detections of related global coronal waves have been reported. We have studied Yohkoh Soft X-ray Telescope (SXT) imaging observations to understand this apparent discrepancy, and describe the problems in this paper. We have found good X-ray evidence for a large-scale coronal wave associated with a major flare on 6 May 1998. The earliest direct trace of the wave motion on 6 May consisted of an expanding volume within 20 Mm (projected) of the flare-core loops, as established by loop motions and a dimming signature. Wavefront analyses of the soft X-ray observations point to this region as the source of the wave, which began at the time of an early hard X-ray spike in the impulsive phase of the flare. The emission can be seen out to a large radial distance (some 220 Mm from the flare core) by SXT, and a similar structure at a still greater distance by EIT (the Extreme Ultraviolet Imaging Telescope) on SOHO. The radio dynamic spectra confirm that an associated disturbance started at a relatively high density, consistent with the X-ray observations, prior to the metric type II burst emission onset. The wavefront tilted away from the vertical as expected from refraction if the Alfven speed increases with height in the corona. From the X-ray observations we estimate that the electron temperature in the wave, at a distance of 120 Mm from the flare core, was on the order of 2–4 MK, consistent with a Mach number in the range 1.1–1.3.

The Interface Region Imaging Spectrograph (IRIS) reveals small-scale rapid brightenings in the form of bright grains all over coronal holes and the quiet Sun. These bright grains are seen with the IRIS 1330, 1400, and 2796 Å slit-jaw filters. We combine coordinated observations with IRIS and from the ground with the Swedish 1 m Solar Telescope (SST) which allows us to have chromospheric (Ca ii 8542 Å, Ca ii H 3968 Å, Hα, and Mg ii k 2796 Å) and transition region (C ii 1334 Å, Si iv 1403 Å) spectral imaging, and single-wavelength Stokes maps in Fe i 6302 Å at high spatial 0."33, temporal, and spectral resolution. We conclude that the IRIS slit-jaw grains are the counterpart of so-called acoustic grains, i.e., resulting from chromospheric acoustic waves in a non-magnetic environment. We compare slit-jaw images (SJIs) with spectra from the IRIS spectrograph. We conclude that the grain intensity in the 2796 Å slit-jaw filter comes from both the Mg ii k core and wings. The signal in the C ii and Si iv lines is too weak to explain the presence of grains in the 1300 and 1400 Å SJIs and we conclude that the grain signal in these passbands comes mostly from the continuum. Although weak, the characteristic shock signatures of acoustic grains can often be detected in IRIS C ii spectra. For some grains, a spectral signature can be found in IRIS Si iv. This suggests that upward propagating acoustic waves sometimes reach all the way up to the transition region. Reproduced with permission from the Astrophysical Journal. © IOP Publishing

We have investigated the relationship between magnetic activity and coronal structures using soft X-ray data from the Yohkoh soft X-ray telescope and magnetic field data from the Kitt Peak Solar Observatory for the period of 1991-2001 and EUV data from the Solar and Heliospheric Observatory EUV Imaging Telescope for 1996-2001. The data are reduced to Carrington synoptic maps, which reveal two types of migrating structures of coronal activity at low and high latitudes in the time-latitudinal distribution. The low-latitude coronal structures, migrating equatorward, correspond to photospheric sunspot activity, and the high-latitude structures migrating toward the poles reflect polar activity of the Sun. We present the following new results

The heating of the outer solar atmospheric layers, i.e., the transition region and corona, to high temperatures is a long standing problem in solar (and stellar) physics. Solutions have been hampered by an incomplete understanding of the magnetically controlled structure of these regions. The high spatial and temporal resolution observations with the Interface Region Imaging Spectrograph (IRIS) at the solar limb reveal a plethora of short, low lying loops or loop segments at transition-region temperatures that vary rapidly, on the timescales of minutes. We argue that the existence of these loops solves a long standing observational mystery. At the same time, based on comparison with numerical models, this detection sheds light on a critical piece of the coronal heating puzzle., Published in Science on October 17 and can be found on: http://www.sciencemag.org/content/346/6207/1255757.full?sid=23ef14f7-7090-4270-8eb2-35af3def0fa6 17 pages, 5 figures. Movies are available at http://folk.uio.no/viggoh/download/science_movies/

As the interface between the Sun's photosphere and corona, the chromosphere and transition region play a key role in the formation and acceleration of the solar wind. Observations from the Interface Region Imaging Spectrograph reveal the prevalence of intermittent small-scale jets with speeds of 80-250 km/s from the narrow bright network lanes of this interface region. These jets have lifetimes of 20-80 seconds and widths of 300 km or less. They originate from small-scale bright regions, often preceded by footpoint brightenings and accompanied by transverse waves with ~20 km/s amplitudes. Many jets reach temperatures of at least ~100000 K and constitute an important element of the transition region structures. They are likely an intermittent but persistent source of mass and energy for the solar wind., Figs 1-4 & S1-S5; Movies S1-S8; published in Science, including the main text and supplementary materials. Reference: H. Tian, E. E. DeLuca, S. R. Cranmer, et al., Science 346, 1255711 (2014)

The physical processes causing energy exchange between the Sun's hot corona and its cool lower atmosphere remain poorly understood. The chromosphere and transition region (TR) form an interface region between the surface and the corona that is highly sensitive to the coronal heating mechanism. High resolution observations with the Interface Region Imaging Spectrograph (IRIS) reveal rapid variability (about 20 to 60 seconds) of intensity and velocity on small spatial scales at the footpoints of hot dynamic coronal loops. The observations are consistent with numerical simulations of heating by beams of non-thermal electrons, which are generated in small impulsive heating events called "coronal nanoflares". The accelerated electrons deposit a sizable fraction of their energy in the chromosphere and TR. Our analysis provides tight constraints on the properties of such electron beams and new diagnostics for their presence in the nonflaring corona., Comment: Published in Science on October 17: http://www.sciencemag.org/content/346/6207/1255724 . 26 pages, 10 figures. Movies are available at: http://www.lmsal.com/~ptesta/iris_science_mov/

The solar atmosphere was traditionally represented with a simple one-dimensional model. Over the past few decades, this paradigm shifted for the chromosphere and corona that constitute the outer atmosphere, which is now considered a dynamic structured envelope. Recent observations by IRIS (Interface Region Imaging Spectrograph) reveal that it is difficult to determine what is up and down even in the cool 6000-K photosphere just above the solar surface: this region hosts pockets of hot plasma transiently heated to almost 100,000 K. The energy to heat and accelerate the plasma requires a considerable fraction of the energy from flares, the largest solar disruptions. These IRIS observations not only confirm that the photosphere is more complex than conventionally thought, but also provide insight into the energy conversion in the process of magnetic reconnection., Comment: published in Science 346, 1255726 (2014), 30 pages, 13 figures; for associated movie, see http://www2.mps.mpg.de/data/outgoing/peter/papers/2014-iris-eb/fig1-movie.mov

IRIS data allow us to study the solar transition region (TR) with an unprecedented spatial resolution of 0.33 arcsec. On 2013 August 30, we observed bursts of high Doppler shifts suggesting strong supersonic downflows of up to 200 km/s and weaker, slightly slower upflows in the spectral lines Mg II h and k, C II 1336 \AA, Si IV 1394 \AA, and 1403 \AA, that are correlated with brightenings in the slitjaw images (SJIs). The bursty behavior lasts throughout the 2 hr observation, with average burst durations of about 20 s. The locations of these short-lived events appear to be the umbral and penumbral footpoints of EUV loops. Fast apparent downflows are observed along these loops in the SJIs and in AIA, suggesting that the loops are thermally unstable. We interpret the observations as cool material falling from coronal heights, and especially coronal rain produced along the thermally unstable loops, which leads to an increase of intensity at the loop footpoints, probably indicating an increase of density and temperature in the TR. The rain speeds are on the higher end of previously reported speeds for this phenomenon, and possibly higher than the free-fall velocity along the loops. On other observing days, similar bright dots are sometimes aligned into ribbons, resembling small flare ribbons. These observations provide a first insight into small-scale heating events in sunspots in the TR., Comment: accepted by ApJL

We present observations of the magnetic field configuration and its transformation in six solar eruptive events that show good agreement with the standard bipolar model for eruptive flares. The observations are X-ray images from the Yohkoh soft X-ray telescope (SXT) and magnetograms from Kitt Peak National Solar Observatory, interpreted together with the 1-8 Angstrom X-ray flux observed by Geostationary Operational Environmental Satellites (GOES). The observations yield the following interpretations: (1) Each event is a magnetic explosion that occurs in an initially closed single bipole in which the core field is sheared and twisted in the shape of a sigmoid, having an oppositely curved elbow on each end. The arms of the opposite elbows are sheared past each other so that they overlap and are crossed low above the neutral line in the middle of the bipole. The elbows and arms seen in the SXT images are illuminated strands of the sigmoidal core field, which is a continuum of sheared/twisted field that fills these strands as well as the space between and around them; (2) Although four of the explosions are ejective (appearing to blow open the bipole) and two are confined (appearing to be arrested within the closed bipole), all six begin the same way. In the SXT images, the explosion begins with brightening and expansion of the two elbows together with the appearance of short bright sheared loops low over the neutral line under the crossed arms and, rising up from the crossed arms, long strands connecting the far ends of the elbows; and (3) All six events are single-bipole events in that during the onset and early development of the explosion they show no evidence for reconnection between the exploding bipole and any surrounding magnetic fields. We conclude that in each of our events the magnetic explosion was unleashed by runaway tether-cutting via implosive/explosive reconnection in the middle of the sigmoid, as in the standard model. The similarity of the onsets of the two confined explosions to the onsets of the four ejective explosions and their agreement with the model indicate that runaway reconnection inside a sheared core field can begin whether or not a separate system of overlying fields, or the structure of the bipole itself, allows the explosion to be ejective. Because this internal reconnection apparently begins at the very start of the sigmoid eruption and grows in step with the explosion, we infer that this reconnection is essential for the onset and growth of the magnetic explosion in eruptive flares and coronal mass ejections.

This contract is for the development and flight of an experiment to study the solar atmosphere with excellent spatial and temporal resolution; and reduction and analysis of the resultant data. After being launched into a near perfect orbit on 2 April 1998, the spacecraft and instrument remain in good condition and the resultant data are spectacular. Over 6.6 million images have now been taken. Observing highlights this month included several coordinations with CDS, studies of the quiet Sun with SUMER and NMI, coordinations with observers at the SPO Dunn Tower Telescope, and a variety of active region observations. Some of the latter were relatively unique in that they emphasized using the hottest (284A) channel of TRACE. We were informed of the results of the Senior Review Committee's evaluation of all Space Science on-orbit missions and the corresponding fiscal year budgets for TRACE. The budget for FY-02 is modestly less than is being spent in FY-01 and for the years beyond that it is much, much lower.

The analysis of X-ray solar flare spectra obtained by the Bent Crystal Spectrometer on board the Solar Maximum Mission satellite is presented. The ratio of the Ca XIX resonance line intensity to the nearby continuum is used to measure the calcium abundance relative to hydrogen (ACa). A description of the spectroscopic method of determining the absolute calcium abundance is given. Possible instrumental and solar effects that might influence the abundance estimates are evaluated. Over 5000 spectra from more than 100 flares are analyzed. We find a flare-to-flare variation for ACa that is not correlated with flare size, Hα importance, or with several other flare characteristics. For flares observed from two active regions, the observed value of ACa increases as a function of time. The average for all flares is ACa = (5.77 ± 1.41) × 10-6. A discussion of investigated correlations of derived ACa values with several flare characteristics is presented.

Solar EUV images recorded by the EUV Imaging Telescope (EIT) on SOHO have been used to evaluate temperature and density as a function of position in two largescale features in the corona observed in the temperature range of 1.0–2.0 MK. Such observations permit estimates of longitudinal temperature gradients (if present) in the corona and, consequently, estimates of thermal conduction and radiative losses as a function of position in the features. We examine two relatively cool features as recorded in EIT's Fe ix/x (171 A) and Fe xii (195 A) bands in a decaying active region. The first is a long-lived loop-like feature with one leg, ending in the active region, much more prominent than one or more distant footpoints assumed to be rooted in regions of weakly enhanced field. The other is a near-radial feature, observed at the West limb, which may be either the base of a very high loop or the base of a helmet streamer. We evaluate energy requirements to support a steady-state energy balance in these features and find in both instances that downward thermal conductive losses (at heights above the transition region) are inadequate to support local radiative losses, which are the predominant loss mechanism. The requirement that a coronal energy deposition rate proportional to the square of the ambient electron density (or pressure) is present in these cool coronal features provides an additional constraint on coronal heating mechanisms.

We present the measurement of magnetic field gradient in magnetic loops in the solar corona, based on the multi-wavelength Very Large Array observations of two transient microwave brightenings (TMBs) in the solar active region 7135. The events were observed at 2 cm (spatial resolution ∼ 2=) and 3.6 cm (spatial resolution ∼ 3=) with a temporal resolution of 3.3 s in a time-sharing mode. Soft X-ray data (spatial resolution ∼ 2.5=) were available from the Soft X-ray Telescope on board the Yohkoh satellite. The three-dimensional structure of simple magnetic loops, where the transient brightenings occurred, were traced out by these observations. The 2-cm and 3.6-cm sources were very compact, located near the footpoint of the magnetic loops seen in the X-ray images. For the two events reported in this paper, the projected angular separation between the centroids of 2 and 3.6-cm sources is about 2.3= and 3.1=, respectively. We interpret that the 2 and 3.6-cm sources come from thermal gyro-resonance emission. The 2-cm emission is at the 3rd harmonic originating from the gyro-resonance layer where the magnetic field is 1800 G. The 3.6-cm emission is at the 2nd harmonic, originating from the gyro-resonance layer with a magnetic field of 1500 G. The estimated magnetic field gradient near the footpoint of the magnetic loop is about 0.09 G km=1 and 0.12 G km=1 for the two events. These values are smaller than those observed in the photosphere and chromosphere by at least a factor of 2.

Prominence eruption is one of the most important solar phenomenon because of the possibility of using it as proxy of geoeffective solar disturbances. In a series of investigations using Nobeyama radio observations in conjunction with Yohkoh data we have found that there is more to a prominence eruption than the simple picture portrayed by Hα observations. The extent of coronal volume associated with the eruption is much larger than the prominence. We illustrate this with several examples and discuss the implication of the results for understanding the interplanetary manifestation of coronal mass ejections.

Solar spicules have eluded modelers and observers for decades. Since the discovery of the more energetic type II, spicules have become a heated topic but their contribution to the energy balance of the low solar atmosphere remains unknown. Here we give a first glimpse of what quiet Sun spicules look like when observed with NASA's recently launched Interface Region Imaging Spectrograph (IRIS). Using IRIS spectra and filtergrams that sample the chromosphere and transition region we compare the properties and evolution of spicules as observed in a coordinated campaign with Hinode and the Atmospheric Imaging Assembly. Our IRIS observations allow us to follow the thermal evolution of type II spicules and finally confirm that the fading of Ca II H spicules appears to be caused by rapid heating to higher temperatures. The IRIS spicules do not fade but continue evolving, reaching higher and falling back down after 500-800 s. Ca II H type II spicules are thus the initial stages of violent and hotter events that mostly remain invisible in Ca II H filtergrams. These events have very different properties from type I spicules, which show lower velocities and no fading from chromospheric passbands. The IRIS spectra of spicules show the same signature as their proposed disk counterparts, reinforcing earlier work. Spectroheliograms from spectral rasters also confirm that quiet Sun spicules originate in bushes from the magnetic network. Our results suggest that type II spicules are indeed the site of vigorous heating (to at least transition region temperatures) along extensive parts of the upward moving spicular plasma., Comment: 6 pages, 4 figures, accepted for publication in ApJ Letters. For associated movies, see http://folk.uio.no/tiago/iris_spic/

Transient microwave brightenings (TMBs) are small-scale energy releases from the periphery of sunspot umbrae, with a flux density two orders of magnitude smaller than that from a typical flare. Gopalswamy et al (1994) first reported the detection of the TMBs and it was pointed out that the radio emission implied a region of very high magnetic field so that the emission mechanism has to be gyroresonance or nonthermal gyrosynchrotron, but not free-free emission. It was not possible to decide between gyroresonance and gyrosynchrotron processes because of the low time resolution (30 s) used in the data analysis. We have since performed a detailed analysis of the Very Large Array data with full time resolution (3.3 s) at two wavelengths (2 and 3.6 cm) and we can now adequately address the question of the emission mechanism of the TMBs. We find that nonthermal processes indeed take place during the TMBs. We present evidence for nonthermal emission in the form of temporal and spatial structure of the TMBs. The fast time structure cannot be explained by a thermodynamic cooling time and therefore requires a nonthermal process. Using the physical parameters obtained from X-ray and radio observations, we determine the magnetic field parameters of the loop and estimate the energy released during the TMBs. The impulsive components of TMBs imply an energy release rate of 1.3 x 10^22 erg/s so that the thermal energy content of the TMBs could be less than 10^24 erg., 15 pages (Latex), 4 figures (eps). ApJ Letters in press (1997)

SOHO EIT spectroheliograms showing the polar coronal holes during the present sunspot minimum are compared with National Solar Observatory (Kitt Peak) magnetograms taken in Fe I λ8688 and Ca II λ8542. The chromospheric λ8542 magnetograms, obtained on a routine, near-daily basis since 1996 June, reveal the Sun's strong polar fields with remarkable clarity. We find that the Fe IX λ171 polar plumes occur where minority-polarity flux is in contact with flux of the dominant polarity inside each polar hole. Moreover, the locations of "plume haze" coincide approximately with the patterns of brightened He II λ304 network within the coronal hole. The observations appear to be consistent with mechanisms of plume formation involving magnetic reconnection between unipolar flux concentrations and nearby bipoles. The fact that minority-polarity fields constitute only a small fraction of the total magnetic flux within the polar holes suggests that plumes are not the main source of the high-speed polar wind.

We present fluxes, temperatures, and emission measures for nine solar flares, using data from both the Fe XXV and Ca XIX channels of the Bragg Crystal Spectrometer (BCS) experiment on the Yohkoh satellite and from the wide-band soft X-ray spectrometers on the GOES spacecraft. We also present hard X-ray fluxes from the Hard X-ray Telescope (HXT) on Yohkoh and the BATSE spectrometer on the Compton Gamma-Ray Observatory (CGRO). All events occurred during 1992 and ranged in size from GOES class C5 to M2. Three of the events occurred near the solar limb. For each flare we give two sets of plots. The first set shows flux, electron temperature, and emission measures for Fe XXV, Ca XIX, and GOES as functions of time. The second set of plots gives log electron temperature as functions of log (emission measure)1/2 for these three wavelength ranges; we refer to these plots as E1/2-T diagrams. Hard X-ray flux information is included in both sets of plots. Our observations indicate that (1) cooler plasmas are located along the legs of, or are evenly distributed along, the flaring loops, while hotter plasmas are concentrated near the loop tops, (2) peaks in temperature in each of the wavelength bands are closely associated with hard X-ray enhancements, and (3) the emission from both relatively hot and relatively cool flaring plasmas emanates from the same loop or from closely related loops.

We report on the observations of an X-ray coronal mass ejection (CME) with its three part structure: frontal loop, coronal cavity, and the eruptive prominence core. The prominence core was observed in microwaves, and the frontal loop was observed in X-rays. A coronal volume much larger than that occupied by the prominence seems to be affected by the eruption. Formation of an arcade structure was also observed beneath the erupting prominence. X-ray enhancement at the arcade persisted for several hours similar to long decay events. At the apex of the arcade there was a bright knot, which we interpret as the reconnection region from which the filament gets detached. We determined the trajectories of the frontal loop and the prominence core and found them to have very different characteristics. The CME showed an extremely small acceleration, while the prominence had a linear motion in the beginning followed by an exponential rise. However, during the several hours of simultaneous observation, the prominence did not catch up with the frontal loop. We determined the evolution of the CME mass, which increased by a factor of 4 during our observations. We discuss the implications of the observations in the general context of coronal mass ejections.

The Extreme Ultraviolet Imaging Telescope (EIT) on board the SOHO spacecraft has been operational since 2 January 1996. EIT observes the Sun over a 45 × 45 arc min field of view in four emission line groups: Fe IX, X, Fe XIIi, Fe xv, and He II. A post-launch determination of the instrument flatfield, the instrument scattering function, and the instrument aging were necessary for the reduction and analysis of the data. The observed structures and their evolution in each of the four EUV bandpasses are characteristic of the peak emission temperature of the line(s) chosen for that bandpass. Reports on the initial results of a variety of analysis projects demonstrate the range of investigations now underway: EIT provides new observations of the corona in the temperature range of 1 to 2 MK. Temperature studies of the large-scale coronal features extend previous coronagraph work with low-noise temperature maps. Temperatures of radial, extended, plume-like structures in both the polar coronal hole and in a low latitude decaying active region were found to be cooler than the surrounding material. Active region loops were investigated in detail and found to be isothermal for the low loops but hottest at the loop tops for the large loops.

The SOHO (SOlar and Heliospheric Observatory) satellite was launched on December 2nd 1995. After arriving at the Earth-Sun (L1) Lagrangian point on February 14th 1996, it began to continuously observe the Sun. As one of the instruments onboard SOHO, the EIT (Extreme ultraviolet Imaging Telescope) images the Sun's corona in 4 EUV wavelengths. The He II filter at 304 A images the chromosphere and the base of the transition region at a temperature of 5 − 8 × 104 K; the Fe IX–X filter at 171 A images the corona at a temperature of ∼ 1.3 × 106 K; the Fe XII filter at 195 A images the quiet corona outside coronal holes at a temperature of ∼ 1.6 × 106 K; and the Fe XV filter at 284 A images active regions with a temperature of ∼ 2.0 × 106 K. About 5000 images have been obtained up to the present. In this paper, we describe also some aspects of the telescope and the detector performance for application in the observations. Images and movies of all the wavelengths allow a look at different phenomena present in the Sun's corona, and in particular, magnetic field reconnection.

Soft X-ray observations by SMM and other spacecraft have shown that the abundance of certain elements in solar corona varies from flare to flare. In this study, observations made by the Yohkoh Bragg Crystal Spectrometer (BCS) in helium-like Ca XIX have been analysed, and Ca abundance determined for 177 flares observed during the first four years of the mission (1991–1995). The average abundance of Ca relative to H for all flares is 〈ACa〉 = (3.64±0.39) × 10−6. As with an earlier study of SMM data, the abundance is found to be enhanced compared to the photosphere ((2.24±0.10) × 10−6), and with only minor variation from flare to flare. However, the absolute value and range of values determined by this study is smaller than in the previous study; these differences are discussed.

Temperatures of coronal holes are estimated from several sets of soft X-ray images taken through various broad-band filters with the Soft X-ray Telescope (SXT) aboard Yohkoh . The effect of scattered X-rays from bright regions surrounding a temperature determination area, especially those from nearby active regions, is carefully removed with the point spread function derived from the post-launch data. An isothermal approximation is applied to thus corrected data. The temperatures of coronal holes near the disk center are found to be 1.8 – 2.4 × 10 6 K , which is almost the same as those derived for quiet regions. The emission measures in coronal holes are estimated to be 10 25.5–26.2 cm −5 , about ten times smaller than in quiet regions. We conclude that temperatures in coronal holes do not differ from those in quiet regions, and that the depression in soft X-ray intensity of coronal hole regions results from a lower density by a factor of 3 than quiet regions. We propose that the coronal hole component observed with the SXT is not the same one which is observed with the Skylab EUV instrument. An X-ray intensity from a coronal hole is independently confirmed by the eclipse observation on 1993 November 13, and consistent with intensities derived from the scattering correction.

We study the diagnostic capabilities of the high-resolution, Low-Energy Transmission Grating Spectrometer, LETGS, of NASA's planned Advanced X-ray Astrophysics Facility, AXAF, for optically thin stellar coronae. Spectra are simulated on the basis of isothermal and source loop models and are analyzed with particular emphasis on the extraction of the differential emission measure distribution. The AXAF-LETGS is shown to be particularly sensitive for plasma at temperatures between 0.5 and 15 MK. Emission from temperatures in excess of 20 MK can be observed, but the lack of strong spectral lines hampers accurate temperature determinations. We simulate spectra of close binaries to demonstrate the observability of the Doppler effects associated with orbital motions. We present lists of spectral lines that can be used for density diagnostics, and we simulate and compare various spectra at different electron densities.

With increasing solar activity since 2010, many flares from the backside of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI) on either of the twin STEREO spacecraft. Our objective is to estimate their X-ray peak fluxes from EUVI data by finding a relation of the EUVI with GOES X-ray fluxes. Because of the presence of the Fe xxiv line at 192 A, the response of the EUVI 195 A channel has a secondary broad peak around 15 MK, and its fluxes closely trace X-ray fluxes during the rise phase of flares. If the flare plasma is isothermal, the EUVI flux should be directly proportional to the GOES flux. In reality, the multithermal nature of the flare and other factors complicate the estimation of the X-ray fluxes from EUVI observations. We discuss the uncer- tainties, by comparing GOES fluxes with the high cadence EUV data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We conclude that the EUVI 195 A data can provide estimates of the X-ray peak fluxes of intense flares (e.g., above M4 in the GOES scale) with uncertainties of a factor of a few. Lastly we show examples of intense flares from regions far behind the limb, some of which show eruptive signatures in AIA images., Comment: 15 pages, 9 figures, accepted by Solar Physics

The Extreme-ultraviolet Imaging Telescope (EIT) will provide wide-field images of the corona and transition region on the solar disc and up to 1.5 R⊙ above the solar limb. Its normal incidence multilayer-coated optics will select spectral emission lines from Fe IX (171 A), Fe XII (195 A), Fe XV (284 A), and He II (304 A) to provide sensitive temperature diagnostics in the range from 6 × 104 K to 3 × 10 6 K. The telescope has a 45×45 arcmin field of view and 2.6 arcsec pixels which will provide approximately 5-arcsec spatial resolution. The EIT will probe the coronal plasma on a global scale, as well as the underlying cooler and turbulent atmosphere, providing the basis for comparative analyses with observations from both the ground and other SOHO instruments. This paper presents details of the EIT instrumentation, its performance and operating modes.

We have found several occurrences of slowly expanding giant arches in Yohkoh images. These are similar to the giant post-flare arches previously discovered by SMM instruments in the 80s. However, we see them now with 3–5 times better spatial resolution and can recognize well their loop-like structure. As a rule, these arches follow eruptive flares with gradual soft X-ray bursts and rise with speeds in the range of 1.1–2.4 km s−1 which keep constant for > 5 to 24 hours, reaching altitudes up to 250,000 km above the solar limb. These arches differ from post-flare loop systems by their (much higher) altitudes, (much longer) lifetimes, and (constant) speed of growth. One event appears to be a rise of a transequatorial interconnectiong loop.

The point spread function of the SXT telescope aboard Yohkoh has been measured in flight configuration in three different X-ray lines at White Sands Missile Range. We have fitted these data with an elliptical generalization of the Moffat function. Our fitting method consists of chi squared minimization in Fourier space, especially designed for matching of sharply peaked functions. We find excellent fits with a reduced chi squared of order unity or less for single exposure point spread functions over most of the CCD. Near the edges of the CCD the fits are less accurate due to vignetting. From fitting results with summation of multiple exposures we find a systematic error in the fitting function of the order of 3% near the peak of the point spread function, which is close to the photon noise for typical SXT images in orbit. We find that the full width to half maximum and fitting parameters vary significantly with CCD location. However, we also find that point spread functions measured at the same location are consistent to one another within the limit determined by photon noise. A 'best' analytical fit to the PSF as function of position on the CCD is derived for use in SXT image enhancemnent routines. As an aside result we have found that SXT can determine the location of point sources to about a quarter of a 2.54 arc sec pixel.

The NASA Interface Region Imaging Spectrograph (IRIS) mission is a Small Explorer (SMEX) satellite mission designed to study plasma dynamics in the “interface region” between the Sun’s chromosphere and corona with high spatial, spectral, and temporal resolution. The primary instrument is a dual Czerny-Turner spectrograph fed by a 20-cm Cassegrain telescope measuring near- and far-ultraviolet (NUV, FUV) spectral lines in the ranges 133-141 nm and 278- 283 nm. To determine the position of the slit on the solar disk, a slit-jaw imaging system is used. The NUV slit-jaw imaging system produces high spatial resolution images at two positions in the Mg II 280 nm spectral line complex using a birefringent Solc filter with two wide-band interference pre-filters for spectral order selection. The Solc filter produces a 0.36 nm full-width at half-maximum (FWHM) filter profile with low sidelobes and a peak transmission of 15% at 279.6 nm. The filter consists of two “wire grid’’ polarizers surrounding 8 quartz waveplates configured in a modified Solc “fan” rotational pattern. The elements are optically coupled using DC200 silicon-based grease. The NUV Solc filter is sealed in a windowed cell to prevent silicon contamination of the FUV channel. The design of the sealed cell and assembly of the filter into the cell were among the most challenging optomechanical aspects of the IRIS spectrograph system.

This paper discusses the design of the IRIS Small Explorer (SMEX) Cassegrain telescope, as well as its intended and measured performance. Lockheed Martin, along with SAO, Montana State University, and Stanford University are developing the IRIS instrument for a mission to examine the solar spectra in two bands, one centered on 1369 A, and the other centered on 2810 A. SAO led the design and construction of the telescope feed, with assistance from Lockheed and Montana State University. The telescope posed a number of implementation challenges, which are discussed here, including the fact that no effective filters exist to isolate the science spectra to the exclusion of the rest of the solar flux, making it necessary to allow full sunlight into the telescope.

We performed for the first time stereoscopic triangulation of coronal loops in active regions over the entire range of spacecraft separation angles ($\alpha_{sep}\approx 6^\circ, 43^\circ, 89^\circ, 127^\circ$, and $170^\circ$). The accuracy of stereoscopic correlation depends mostly on the viewing angle with respect to the solar surface for each spacecraft, which affects the stereoscopic correspondence identification of loops in image pairs. From a simple theoretical model we predict an optimum range of $\alpha_{sep} \approx 22^\circ-125^\circ$, which is also experimentally confirmed. The best accuracy is generally obtained when an active region passes the central meridian (viewed from Earth), which yields a symmetric view for both STEREO spacecraft and causes minimum horizontal foreshortening. For the extended angular range of $\alpha_{sep}\approx 6^\circ-127^{\circ}$ we find a mean 3D misalignment angle of $\mu_{PF} \approx 21^\circ-39^\circ$ of stereoscopically triangulated loops with magnetic potential field models, and $\mu_{FFF} \approx 15^\circ-21^\circ$ for a force-free field model, which is partly caused by stereoscopic uncertainties $\mu_{SE} \approx 9^\circ$. We predict optimum conditions for solar stereoscopy during the time intervals of 2012--2014, 2016--2017, and 2021--2023., Comment: Solar Physics, (in press), 22 pages, 9 figures

High-resolution observations with Hinode have shown that understanding the interface between the solar photosphere and the corona requires the ability to study the transition region by imaging plasma heated to around 500,000 K on spatial scales of ∼ 0.2 arc seconds and at a cadence of ∼5 seconds or less. And how this interface of the solar atmosphere ranges in temperatures from 104 — 107 K continues to be an area of active research in heliophysics. Although the much celebrated launch of the Atmospheric Imaging Assembly (AIA) is revealing activity of the Extreme Ultraviolet (EUV) corona and chromosphere, down to ∼1 arc seconds resolution with ∼10 seconds cadence, there is a continuing need to improve our observational capabilities at EUV wavelengths. Therefore, the next generation EUV imager for use in heliophysics will need to have the capability of improving performance in all three of these observational categories in order to obtain higher thermal, spatial and temporal resolution of the solar atmosphere. Herein, we present results that demonstrate our ability to estimate EUV performance as well as experimental data that show the viability of using SiC for fabricating the next-generation of space-based EUV telescopes.

The Solar Ultraviolet Imager (SUVI) is one of several instruments being fabricated for use on board the upcoming Geostationary Operational Environmental Satellites, GOES-R and -S platforms, as part of NOAA's space weather monitoring fleet. SUVI is a Generalized Cassegrain telescope that employs multilayer coatings optimized to operate in six extreme ultraviolet (EUV) narrow bandpasses centered at 93.9, 131.2, 171.1, 195.1, 284.2 and 303.8 A. Over the course of its operational lifetime SUVI will image and record full disk, EUV spectroheliograms approximately every few minutes, and telemeter the data to the ground for digital processing. This data will be useful to scientists and engineers wanting to better understand the effects of solar produced EUV radiation with the near-Earth environment. At the focus of the SUVI telescope is a thin, back-illuminated CCD sensor with 21 μm (2.5 arc sec) pixels. At the shortest EUV wavelengths, image degradation from mirror surface scatter effects due to residual optical fabrication errors dominate the effects of both diffraction and geometrical aberrations. Discussed herein, we present a novel forward model that incorporates: (i) application of a new unified surface scatter theory valid for moderately rough surfaces to predict the bidirectional reflectance distribution function (BRDF) produced by each mirror (which uses optical surface metrology to determine the power spectral density, PSD, that characterizes the "smoothness" of an optical surface); (ii) use of the BRDF for each mirror at each EUV wavelength, in tandem with the optical design, to calculate the in-band point spread function (PSF); (iii) use of the PSF to calculate the fractional ensquared energy in the focal plane of SUVI; (iv) comparison of BRDF measurements taken at 93.9 A with the forward model predictions and (v) final prediction of the in-band, total system responsivity.

The Japanese Yohkoh satellite is now in orbit observing the sun with a set of x-ray imagers and x-ray and gamma-ray spectrometers. The data from this successful mission provide new information on solar flares and the sun's corona. This paper discusses the Yohkoh observations and presents a sample of the first scientific results from the mission.

The design of the 4 telescopes that make up the Solar Dynamics Observatory Atmospheric Imaging Assembly (SDOAIA) is described. This includes the optical design, optical mounting system, front aperture filters, and launch protection system. SDO-AIA is a study of taking a difficult telescope design and making four of them. We describe the technical challenges associated with the telescope mounting, mirror mounting, and the front aperture filter design and launch protection.

All of the SOLAR-A telemetry data will be reformatted before distribution to the analysis computers and the various users. This paper gives an overview of the files which will be created and the format and organization which the files will use. The organization has been chosen to be efficient in space, to ease access to the data, and to allow for the data to be transportable to different machines. An observing log file will be created automatically using the reformatted data files as the input. It will be possible to perform searches with the observing log to list cases where instruments are in certain modes and/or seeing certain signal levels.

The predicted response of the Solar-A Soft X-ray Telescope (SXT) to various solar targets is presented. It is found that the SXT is very sensitive and should be able to observe active regions and flares at its nominal cadence with exposure times ranging from 0.0001 to 1 s. The SXT temperature diagnostic capabilities are presented for various combinations of the X-ray filters.

The diversity in the published values of coronal abundances is unsettling, especially as the range of results seems to be beyond the quoted uncertainties. Measurements of the relative abundance of iron and neon derived from soft X-ray spectra of active regions are presented. From a data base of over 200 spectra taken by the Solar Maximum Mission Flat Crystal Spectrometer, it is found that the relative abundance can vary by as much as a factor of about 7 and can change on timescales of less than 1 h.

A generalized method of calculating the distribution of the emission measure with temperature (DEM) for optically thin plasma has been developed. The method simultaneously uses line flux ratios in addition to line fluxes. When a ratio of lines from the same element is used, the resulting DEM is independent of this element's abundance. The method has been applied to derive the absolute abundances of iron in solar flares from X-ray spectra recorded by the Bent Crystal Spectrometer on SMM. The iron abundances have been found to vary between flares. The calcium abundances have also been calculated using the same method and are found to be in close agreement with the values derived from the line-to-continuum technique (Lemen et al., 1990 and Sylwester et al., 1990). The variation of iron and calcium abundances is compared. A correction to the ionization balance for iron is proposed.