arXiv subjects

L. Golub

Publications and source records attributed to L. Golub.

At least 19 recordsLinked to original sources

arXiv2025

The ASPIICS solar coronagraph aboard the Proba-3 formation flying mission. Scientific objectives and instrument design

We describe the scientific objectives and instrument design of the ASPIICS coronagraph launched aboard the Proba-3 mission of the European Space Agency (ESA) on 5 December 2024. Proba-3 consists of two spacecraft in a highly elliptical orbit around the Earth. One spacecraft carries the telescope, and the external occulter is mounted on the second spacecraft. The two spacecraft fly in a precise formation during 6 hours out of 19.63 hour orbit, together forming a giant solar coronagraph called ASPIICS (Association of Spacecraft for Polarimetric and Imaging Investigation of the Corona of the Sun). Very long distance between the external occulter and the telescope (around 144 m) represents an increase of two orders of magnitude compared to classical externally occulted solar coronagraphs. This allows us to observe the inner corona in eclipse-like conditions, i.e. close to the solar limb (down to 1.099 Rs) and with very low straylight. ASPIICS will provide a new perspective on the inner solar corona that will help solve several outstanding problems in solar physics, such as the origin of the slow solar wind and physical mechanism of coronal mass ejections.

astro-ph.SR↗

arXiv2023

The Multiview Observatory for Solar Terrestrial Science (MOST)

We report on a study of the Multiview Observatory for Solar Terrestrial Science (MOST) mission that will provide comprehensive imagery and time series data needed to understand the magnetic connection between the solar interior and the solar atmosphere/inner heliosphere. MOST will build upon the successes of SOHO and STEREO missions with new views of the Sun and enhanced instrument capabilities. This article is based on a study conducted at NASA Goddard Space Flight Center that determined the required instrument refinement, spacecraft accommodation, launch configuration, and flight dynamics for mission success. MOST is envisioned as the next generation great observatory positioned to obtain three-dimensional information of large-scale heliospheric structures such as coronal mass ejections, stream interaction regions, and the solar wind itself. The MOST mission consists of 2 pairs of spacecraft located in the vicinity of Sun-Earth Lagrange points L4 (MOST1, MOST3) and L5 (MOST2 and MOST4). The spacecraft stationed at L4 (MOST1) and L5 (MOST2) will each carry seven remote-sensing and three in-situ instrument suites, including a novel radio package known as the Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH). MOST3 and MOST4 will carry only the FETCH instruments and are positioned at variable locations along the Earth orbit up to 20° ahead of L4 and 20° behind L5, respectively. FETCH will have polarized radio transmitters and receivers on all four spacecraft to measure the magnetic content of solar wind structures propagating from the Sun to Earth using the Faraday rotation technique. The MOST mission will be able to sample the magnetized plasma throughout the Sun-Earth connected space during the mission lifetime over a solar cycle.

astro-ph.SR↗

arXiv2021

High resolution soft X-ray spectroscopy and the quest for the hot (5-10 MK) plasma in solar active regions

We discuss the diagnostics available to study the 5-10 MK plasma in the solar corona, which is key to understanding the heating in the cores of solar active regions. We present several simulated spectra, and show that excellent diagnostics are available in the soft X-rays, around 100 Angstroms, as six ionisation stages of Fe can simultaneously be observed, and electron densities derived, within a narrow spectral region. As this spectral range is almost unexplored, we present an analysis of available and simulated spectra, to compare the hot emission with the cooler component. We adopt recently designed multilayers to present estimates of count rates in the hot lines, with a baseline spectrometer design. Excellent count rates are found, opening up the exciting opportunity to obtain high-resolution spectroscopy of hot plasma.

astro-ph.SR↗

arXiv2020

LUCI onboard Lagrange, the Next Generation of EUV Space Weather Monitoring

LUCI (Lagrange eUv Coronal Imager) is a solar imager in the Extreme UltraViolet (EUV) that is being developed as part of the Lagrange mission, a mission designed to be positioned at the L5 Lagrangian point to monitor space weather from its source on the Sun, through the heliosphere, to the Earth. LUCI will use an off-axis two mirror design equipped with an EUV enhanced active pixel sensor. This type of detector has advantages that promise to be very beneficial for monitoring the source of space weather in the EUV. LUCI will also have a novel off-axis wide field-of-view, designed to observe the solar disk, the lower corona, and the extended solar atmosphere close to the Sun-Earth line. LUCI will provide solar coronal images at a 2-3 minute cadence in a pass-band centred on 19.5 nm. Observations made through this pass-band allow for the detection and monitoring of semi-static coronal structures such as coronal holes, prominences, and active regions; as well as transient phenomena such as solar flares, limb Coronal Mass Ejections (CMEs), EUV waves, and coronal dimmings. The LUCI data will complement EUV solar observations provided by instruments located along the Sun-Earth line such as PROBA2-SWAP, SUVI-GOES and SDO-AIA, as well as provide unique observations to improve space weather forecasts. Together with a suite of other remote-sensing and in-situ instruments onboard Lagrange, LUCI will provide science quality operational observations for space weather monitoring.

astro-ph.SR↗

arXiv2019

New deep coronal spectra from the 2017 total solar eclipse

Total eclipses permit a deep analysis of both the inner and the outer parts of the corona using the continuum White-Light (W-L) radiations from electrons (K-corona), the superposed spectrum of forbidden emission lines from ions (E-corona) and the dust component with F-lines (F-corona). By sufficiently dispersing the W-L spectrum, the Fraunhofer (F) spectrum of the dust component of the corona appears and the continuum Thomson radiation can be evaluated. The superposed emission lines of ions with different degrees of ionization are studied to allow the measurement of temperatures, non-thermal velocities, Doppler shifts and abundances. We describe a slit spectroscopic experiment of high spectral resolution for providing an analysis of the most typical parts of the quasi-minimum type corona observed during the total solar eclipse of Aug. 21, 2017 observed from Idaho, USA. Streamers, active region enhancements and polar coronal holes (CHs) are well measured using deep spectra. 60 spectra are obtained during the totality with a long slit, covering +/-3 solar radii in the range of 510 to 590nm. The K+F continuum corona is well exposed up to 2 solar radius. The F-corona can be measured even at the solar limb. New weak emission lines were discovered or confirmed. The rarely observed high FIP ArX line is recorded almost everywhere; the FeXIV and NiXIII lines are well recorded everywhere. For the first time hot lines are also measured inside the CH regions. The radial variations of the non-thermal turbulent velocities of the lines do not show a great departure from the average values. No significantly large Doppler shifts are seen anywhere in the inner and the middle corona. The wings of the FeXIV line show some non-Gaussianity.

astro-ph.SR↗

arXiv2018

Polar Coronal Plumes as Tornado-Like Jets

We examine the dynamical behavior of white light polar plume structures in the inner corona that are observed from the ground during total solar eclipses, based on their EUV hot and cool emission line counterparts observed from space. EUV observations from SDO/AIA of a sequence of rapidly varying coronal hole structures are analyzed. Evidence of events showing acceleration in the 1.25 Mk line of Fe XII at 193 A is given. The structures along the plume show an outward velocity of about 140 kms-1 that can be interpreted as an upwards propagating wave in the 304 A and 171 A lines; higher speeds are seen in 193 A (up to 1000 km/s). The ejection of the cold He II plasma is delayed by about 4 min in the lowest layer and more than 12 min in the highest level compared to the hot 193 A behavior. A study of the dynamics using time-slice diagrams reveals that a large amount of fast ejected material originates from below the plume, at the footpoints. The release of plasma material appears to come from a cylinder with quasi-parallel edge-enhanced walls. After the initial phase of a longitudinal acceleration, the speed substantially reduces and the ejecta disperse into the environment. Finally, the detailed temporal and spatial relationships between the cool and hot components were studied with simultaneous multi-wavelength observations, using more AIA data. The outward-propagating perturbation of the presumably magnetic walls of polar plumes supports the suggestion that Alfven waves propagate outwardly along these radially extended walls.

astro-ph.SR↗

arXiv2015

Simultaneous IRIS and Hinode/EIS observations and modelling of the 27 October 2014 X 2.0 class flare

We present the study of the X2-class flare which occurred on the 27 October 2014 and was observed with the Interface Region Imaging Spectrograph (IRIS) and the EUV Imaging Spectrometer (EIS) on board the Hinode satellite. Thanks to the high cadence and spatial resolution of the IRIS and EIS instruments, we are able to compare simultaneous observations of the \xxi~1354.08~Å~and \xxiii~263.77~Å~high temperature emission ($\gtrsim$ 10~MK) in the flare ribbon during the chromospheric evaporation phase. We find that IRIS observes completely blue-shifted \xxi~line profiles, up to 200 km s$^{-1}$ during the rise phase of the flare, indicating that the site of the plasma upflows is resolved by IRIS. In contrast, the \xxiii~line is often asymmetric, which we interpret as being due to the lower spatial resolution of EIS. Temperature estimates from SDO/AIA and Hinode/XRT show that hot emission (log($T$)[K] $>$ 7.2) is first concentrated at the footpoints before filling the loops. Density sensitive lines from IRIS and EIS give electron number density estimates of $\gtrsim$~10$^{12}$~cm$^{-3}$ in the transition region lines and 10$^{10}$~cm$^{-3}$ in the coronal lines during the impulsive phase. In order to compare the observational results against theoretical predictions, we have run a simulation of a flare loop undergoing heating using the HYDRAD 1D hydro code. We find that the simulated plasma parameters are close to the observed values which are obtained with IRIS, Hinode and AIA. These results support an electron beam heating model rather than a purely thermal conduction model as the driving mechanism for this flare.

astro-ph.SR↗

arXiv2015

Limb Event Brightenings and Fast Ejection Using IRIS Mission Observations

The Interface Region Imaging Spectrograph (IRIS) of the NASA small explorer mission provides significantly more complete and higher resolution spectral coverage of the dynamical conditions inside the chromosphere and transition region (TR) than has heretofore been available. Near the solar limb high temporal, spatial (0.3") and spectral resolution observations from the ultraviolet IRIS spectra reveal high-energy limb event brightenings (LEBs) at low chromospheric heights, around 1 Mm above the limb. They can be characterized as explosive events producing jets. We selected two events showing spectra of a confined eruption just off or near the quiet Sun limb, the jet part showing obvious moving material with short duration large Doppler shifts in three directions identified as macrospicules on slit-jaw (SJ) images in Si IV and He II 304 A. The events are analyzed from a sequence of very close rasters taken near the central meridian and the South pole limb. The processed SJ images and the simultaneously observed fast spectral sequences having large Doppler shifts, with a pair of red shifted elements together with a faster blue shifted element from almost the same position, are analyzed. Shifts correspond to velocities of up to 100 km/s in projection on the plane of the sky. The occurrence of erupting spicules and macrospicules from these regions is noticed from images taken before and after the spectra. The cool low first ionization potential (FIP) element simultaneous line emissions of the MgII h and k resonance lines do not clearly show a similar signature due to optical thickness effects but the Si IV broad-band SJ images do. The bidirectional plasma jets ejected from a small reconnection site are interpreted as the result of coronal loop-loop interactions leading to reconnection in nearby sites.

astro-ph.SR↗

arXiv2015

Homologous Helical Jets: Observations by IRIS, SDO and Hinode and Magnetic Modeling with Data-Driven Simulations

We report on observations of recurrent jets by instruments onboard the Interface Region Imaging Spectrograph (IRIS), Solar Dynamics Observatory (SDO) and Hinode spacecrafts. Over a 4-hour period on July 21st 2013, recurrent coronal jets were observed to emanate from NOAA Active Region 11793. FUV spectra probing plasma at transition region temperatures show evidence of oppositely directed flows with components reaching Doppler velocities of +/- 100 km/s. Raster Doppler maps using a Si IV transition region line show all four jets to have helical motion of the same sense. Simultaneous observations of the region by SDO and Hinode show that the jets emanate from a source region comprising a pore embedded in the interior of a supergranule. The parasitic pore has opposite polarity flux compared to the surrounding network field. This leads to a spine-fan magnetic topology in the coronal field that is amenable to jet formation. Time-dependent data-driven simulations are used to investigate the underlying drivers for the jets. These numerical experiments show that the emergence of current-carrying magnetic field in the vicinity of the pore supplies the magnetic twist needed for recurrent helical jet formation.

astro-ph.SR↗

arXiv2014

Prevalence of Small-scale Jets from the Networks of the Solar Transition Region and Chromosphere

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.

astro-ph.SR↗

arXiv2014

The Unresolved Fine Structure Resolved - IRIS observations of the Solar Transition Region

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.

astro-ph.SR↗

arXiv2014

On the prevalence of small-scale twist in the solar chromosphere and transition region

The solar chromosphere and transition region (TR) form an interface between the Sun's surface and its hot outer atmosphere. Here most of the non-thermal energy that powers the solar atmosphere is transformed into heat, although the detailed mechanism remains elusive. High-resolution (0.33-arcsec) observations with NASA's Interface Region Imaging Spectrograph (IRIS) reveal a chromosphere and TR that are replete with twist or torsional motions on sub-arcsecond scales, occurring in active regions, quiet Sun regions, and coronal holes alike. We coordinated observations with the Swedish 1-m Solar Telescope (SST) to quantify these twisting motions and their association with rapid heating to at least TR temperatures. This view of the interface region provides insight into what heats the low solar atmosphere.

astro-ph.SR↗

arXiv2014

Hot Explosions in the Cool Atmosphere of the Sun

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.

astro-ph.SR↗

arXiv2014

An Interface Region Imaging Spectrograph first view on Solar Spicules

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.

astro-ph.SR↗

arXiv2014

Observations of Subarcsecond Bright Dots in the Transition Region above Sunspots with the Interface Region Imaging Spectrograph

Observations with the Interface Region Imaging Spectrograph (IRIS) have revealed numerous sub-arcsecond bright dots in the transition region above sunspots. These bright dots are seen in the 1400Å and 1330Å slit-jaw images. They are clearly present in all sunspots we investigated, mostly in the penumbrae, but also occasionally in some umbrae and light bridges. The bright dots in the penumbrae typically appear slightly elongated, with the two dimensions being 300--600 km and 250--450 km, respectively. The long sides of these dots are often nearly parallel to the bright filamentary structures in the penumbrae but sometimes clearly deviate from the radial direction. Their lifetimes are mostly less than one minute, although some dots last for a few minutes or even longer. Their intensities are often a few times stronger than the intensities of the surrounding environment in the slit-jaw images. About half of the bright dots show apparent movement with speeds of $\sim$10--40~km~s$^{-1}$ in the radial direction. Spectra of a few bright dots were obtained and the Si~{\sc{iv}}~1402.77Å line profiles in these dots are significantly broadened. The line intensity can be enhanced by one to two orders of magnitude. Some relatively bright and long-lasting dots are also observed in several passbands of the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory, and they appear to be located at the bases of loop-like structures. Many of these bright dots are likely associated with small-scale energy release events at the transition region footpoints of magnetic loops.

astro-ph.SR↗

arXiv2014

Detection of supersonic downflows and associated heating events in the transition region above sunspots

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 Å, Si IV 1394 Å, and 1403 Å, 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.

astro-ph.SR↗

arXiv2014

High-resolution Observations of the Shock Wave Behavior for Sunspot Oscillations with the Interface Region Imaging Spectrograph

We present the first results of sunspot oscillations from observations by the Interface Region Imaging Spectrograph. The strongly nonlinear oscillation is identified in both the slit-jaw images and the spectra of several emission lines formed in the transition region and chromosphere. We first apply a single Gaussian fit to the profiles of the Mgii 2796.35 {\AA}, Cii 1335.71 {\AA}, and Si iv 1393.76 {\AA} lines in the sunspot. The intensity change is about 30%. The Doppler shift oscillation reveals a sawtooth pattern with an amplitude of about 10 km/s in Si iv. In the umbra the Si iv oscillation lags those of Cii and Mgii by about 3 and 12 s, respectively. The line width suddenly increases as the Doppler shift changes from redshift to blueshift. However, we demonstrate that this increase is caused by the superposition of two emission components. We then perform detailed analysis of the line profiles at a few selected locations on the slit. The temporal evolution of the line core is dominated by the following behavior: a rapid excursion to the blue side, accompanied by an intensity increase, followed by a linear decrease of the velocity to the red side. The maximum intensity slightly lags the maximum blueshift in Si iv, whereas the intensity enhancement slightly precedes the maximum blueshift in Mgii. We find a positive correlation between the maximum velocity and deceleration, a result that is consistent with numerical simulations of upward propagating magnetoacoustic shock waves.

astro-ph.SR↗

arXiv2014

Sparkling EUV bright dots observed with Hi-C

Observing the Sun at high time and spatial scales is a step towards understanding the finest and fundamental scales of heating events in the solar corona. The Hi-C instrument has provided the highest spatial and temporal resolution images of the solar corona in the EUV wavelength range to date. Hi-C observed an active region on 11 July 2012, which exhibits several interesting features in the EUV line at 193Å: one of them is the existence of short, small brightenings ``sparkling" at the edge of the active region; we call these EUV Bright Dots (EBDs). Individual EBDs have a characteristic duration of 25s with a characteristic length of 680 km. These brightenings are not fully resolved by the SDO/AIA instrument at the same wavelength, however, they can be identified with respect to the Hi-C location of the EBDs. In addition, EBDs are seen in other chromospheric/coronal channels of SDO/AIA suggesting a temperature between 0.5 and 1.5 MK. Based on their frequency in the Hi-C time series, we define four different categories of EBDs: single peak, double peak, long duration, and bursty EBDs. Based on a potential field extrapolation from an SDO/HMI magnetogram, the EBDs appear at the footpoints of large-scale trans-equatorial coronal loops. The Hi-C observations provide the first evidence of small-scale EUV heating events at the base of these coronal loops, which have a free magnetic energy of the order of 10$^{26}$ erg.

astro-ph.SR↗