arXiv subjects

S. Gissot

Publications and source records attributed to S. Gissot.

15 recordsLinked to original sources

arXiv2023

Beyond small-scale transients: a closer look at the diffuse quiet solar corona

Within the quiet Sun corona imaged at 1 MK, much of the field of view consists of diffuse emission that appears to lack the spatial structuring that is so evident in coronal loops or bright points. We seek to determine if these diffuse regions are categorically different in terms of their intensity fluctuations and spatial configuration from the more well-studied dynamic coronal features. We analyze a time series of observations from Solar Orbiter's High Resolution Imager in the Extreme Ultraviolet to quantify the characterization of the diffuse corona at high spatial and temporal resolutions. We then compare this to the dynamic features within the field of view, mainly a coronal bright point. We find that the diffuse corona lacks visible structuring, such as small embedded loops, and that this is persistent over the 25 min duration of the observation. The intensity fluctuations of the diffuse corona, which are within +/-5%, are significantly smaller in comparison to the coronal bright point. Yet, the total intensity observed in the diffuse corona is of the same order as the bright point. It seems inconsistent with our data that the diffuse corona is a composition of small loops or jets or that it is driven by discrete small heating events that follow a power-law-like distribution. We speculate that small-scale processes like MHD turbulence might be energizing the diffuse regions, but at this point we cannot offer a conclusive explanation for the nature of this feature.

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arXiv2023

Initial radiometric calibration of the High-Resolution EUV Imager ($\textrm{HRI}_\textrm{EUV}$) of the Extreme Ultraviolet Imager (EUI) instrument onboard Solar Orbiter

The $\textrm{HRI}_\textrm{EUV}$ telescope was calibrated on ground at the Physikalisch-Technische Bundesanstalt (PTB), Germany's national metrology institute, using the Metrology Light Source (MLS) synchrotron in April 2017 during the calibration campaign of the Extreme Ultraviolet Imager (EUI) instrument onboard the Solar Orbiter mission. We use the pre-flight end-to-end calibration and component-level (mirror multilayer coatings, filters, detector) characterization results to establish the beginning-of-life performance of the $\textrm{HRI}_\textrm{EUV}$ telescope which shall serve as a reference for radiometric analysis and monitoring of the telescope in-flight degradation. Calibration activities at component level and end-to-end calibration of the instrument were performed at PTB/MLS synchrotron light source (Berlin, Germany) and the SOLEIL synchrotron. Each component optical property is measured and compared to its semi-empirical model. This pre-flight characterization is used to estimate the parameters of the semi-empirical models. The end-to-end response is measured and validated by comparison with calibration measurements, as well as with its main design performance requirements. The telescope spectral response semi-empirical model is validated by the pre-flight end-to-end ground calibration of the instrument. It is found that $\textrm{HRI}_\textrm{EUV}$ is a highly efficient solar EUV telescope with a peak efficiency superior to 1 e$^-$.ph$^{-1}$), low detector noise ($\approx$ 3 e- rms), low dark current at operating temperature, and a pixel saturation above 120 ke- in low-gain or combined image mode. The ground calibration also confirms a well-modeled spectral selectivity and rejection, and low stray light. The EUI instrument achieves state-of-the-art performance in terms of signal-to-noise and image spatial resolution.

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arXiv2023

EUV fine structure and variability associated with coronal rain revealed by Solar Orbiter/EUI HRIEUV and SPICE

Coronal rain is the most dramatic cooling phenomenon of the solar corona and an essential diagnostic tool for the coronal heating properties. A puzzling feature of the solar corona, besides the heating, is its EUV filamentary structure and variability. We aim to identify observable features of the TNE-TI scenario underlying coronal rain at small and large spatial scales, to understand the role it plays in the solar corona. We use EUV datasets at unprecedented spatial resolution of ~240 km from EUI/HRIEUV and SPICE of Solar Orbiter from the spring 2022 perihelion. EUV absorption features produced by coronal rain are detected at scales as small as 260 km. As the rain falls, heating and compression is produced immediately downstream, leading to a small EUV brightening accompanying the fall and producing a "fireball" phenomenon. Just prior to impact, a flash-like EUV brightening downstream of the rain, lasting a few minutes is observed for the fastest events. For the first time, we detect the atmospheric response to the rain's impact on the chromosphere and consists of upward propagating rebound shocks and flows partly reheating the loop. The observed widths of the rain clumps are 500 +- 200 km. They exhibit a broad velocity distribution of 10 - 150 km s^-1, peaking below 50 km s^-1. Coronal strands of similar widths are observed along the same loops co-spatial with cool filamentary structure, which we interpret as the CCTR. Matching with the expected cooling, prior to the rain appearance sequential loop brightenings are detected in gradually cooler lines from corona to chromospheric temperatures. Despite the large rain showers, most cannot be detected in AIA 171 in quadrature, indicating that LOS effects play a major role in coronal rain visibility. Still, AIA 304 and SPICE observations reveal that only a small fraction of the rain can be captured by HRIEUV.

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arXiv2023

Temperature of Solar Orbiter/EUI quiet Sun small scale brightenings: evidence for a cooler component

Context: On 2020 May 30, small and short-lived EUV brightenings were observed in the Quiet Sun (QS) during a four minutes sequence by EUI/HRIEUV on board Solar Orbiter. Their physical origin and possible impact on coronal or Transition Region (TR) heating are still to be determined. Aims: Our aim is to derive the statistical thermal evolution of these events in order to establish their coronal or TR origin. Methods. Our thermal analysis takes advantage of the multithermal sensitivity of the Atmospheric Imaging Assembly (AIA) imager on board the Solar Dynamics Observatory (SDO). We first identified these HRIEUV events in the six coronal bands of AIA. We then performed a statistical time lag analysis, which quantifies the delays between the light curves from different bands. These time lags can give significant insights into the temperature evolution of these events. The analysis is performed taking into account the possible contribution to the results from the background and foreground emissions. Results: The events are characterized by time lags inferior to the AIA cadence of 12 s, for all nine couples of AIA bands analyzed. Our interpretation is the possible co-presence of events which reach or do not reach coronal temperatures ($\approx$ 1MK). We believe that the cool population dominates the events analyzed in this work.

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arXiv2023

First Perihelion of EUI on the Solar Orbiter mission

Context. The Extreme Ultraviolet Imager (EUI), onboard Solar Orbiter consists of three telescopes: the two High Resolution Imagers in EUV (HRIEUV) and in Lyman-α (HRILya), and the Full Sun Imager (FSI). Solar Orbiter/EUI started its Nominal Mission Phase on 2021 November 27. Aims. EUI images from the largest scales in the extended corona off limb, down to the smallest features at the base of the corona and chromosphere. EUI is therefore a key instrument for the connection science that is at the heart of the Solar Orbiter mission science goals. Methods. The highest resolution on the Sun is achieved when Solar Orbiter passes through the perihelion part of its orbit. On 2022 March 26, Solar Orbiter reached for the first time a distance to the Sun close to 0.3 au. No other coronal EUV imager has been this close to the Sun. Results. We review the EUI data sets obtained during the period 2022 March-April, when Solar Orbiter quickly moved from alignment with the Earth (2022 March 6), to perihelion (2022 March 26), to quadrature with the Earth (2022 March 29). We highlight the first observational results in these unique data sets and we report on the in-flight instrument performance. Conclusions. EUI has obtained the highest resolution images ever of the solar corona in the quiet Sun and polar coronal holes. Several active regions were imaged at unprecedented cadences and sequence durations. We identify in this paper a broad range of features that require deeper studies. Both FSI and HRIEUV operate at design specifications but HRILya suffered from performance issues near perihelion. We conclude emphasising the EUI open data policy and encouraging further detailed analysis of the events highlighted in this paper.

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arXiv2022

Prominence eruption observed in He II 304 Å up to $>6 R_\sun$ by EUI/FSI aboard Solar Orbiter

We report observations of a unique, large prominence eruption that was observed in the He II 304 Å passband of the the Extreme Ultraviolet Imager/Full Sun Imager telescope aboard Solar Orbiter on 15-16 February 2022. Observations from several vantage points (Solar Orbiter, the Solar-Terrestrial Relations Observatory, the Solar and Heliospheric Observatory, and Earth-orbiting satellites) were used to measure the kinematics of the erupting prominence and the associated coronal mass ejection. Three-dimensional reconstruction was used to calculate the deprojected positions and speeds of different parts of the prominence. Observations in several passbands allowed us to analyse the radiative properties of the erupting prominence. The leading parts of the erupting prominence and the leading edge of the corresponding coronal mass ejection propagate at speeds of around 1700 km/s and 2200 km/s, respectively, while the trailing parts of the prominence are significantly slower (around 500 km/s). Parts of the prominence are tracked up to heights of over $6 R_\sun$. The He II emission is probably produced via collisional excitation rather than scattering. Surprisingly, the brightness of a trailing feature increases with height. The reported prominence is the first observed in He II 304 Å emission at such a great height (above 6 $R_\sun$).

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arXiv2022

Automatic detection of small-scale EUV brightenings observed by the Solar Orbiter/EUI

Context. Accurate detections of frequent small-scale extreme ultraviolet (EUV) brightenings are essential to the investigation of the physical processes heating the corona. Aims. We detected small-scale brightenings, termed campfires, using their morphological and intensity structures as observed in coronal EUV imaging observations for statistical analysis. Methods. We applied a method based on Zernike moments and a support vector machine classifier to automatically identify and track campfires observed by Solar Orbiter/Extreme Ultraviolet Imager (EUI) and SDO/AIA. Results. This method detected 8678 campfires (with length scales between 400 km and 4000 km) from a sequence of 50 High Resolution EUV telescope (HRIEUV) 174Å images. From 21 near co-temporal AIA images covering the same field of view as EUI, we found 1131 campfires, 58% of which were also detected in HRIEUV images. In contrast, about 16% of campfires recognized in HRIEUV were detected by AIA. We obtain a campfire birthrate of 2*10-16m-2s-1. About 40% of campfires show a duration longer than 5 s, having been observed in at least two HRIEUV images. We find that 27% of campfires were found in coronal bright points and the remaining 73% have occurred out of coronal bright points. We detected 23 EUI campfires with a duration greater than 245 s. We found that about 80% of campfires are formed at supergranular boundaries, and the features with the highest total intensities are generated at network junctions and intense H I Lyman-α emission regions observed by EUI/HRILya. The probability distribution functions for the total intensity, peak intensity, and projected area of campfires follow a power law behavior with absolute indices between 2 and 3. This self-similar behavior is a possible signature of self-organization, or even self-organized criticality, in the campfire formation process.

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arXiv2022

The magnetic drivers of campfires seen by the Polarimetric and Helioseismic Imager (PHI) on Solar Orbiter

The Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter (SO) spacecraft observed small extreme ultraviolet (EUV) bursts, termed campfires, that have been proposed to be brightenings near the apexes of low-lying loops in the quiet-Sun atmosphere. The underlying magnetic processes driving these campfires are not understood. During the cruise phase of SO and at a distance of 0.523\,AU from the Sun, the Polarimetric and Helioseismic Imager on Solar Orbiter (SO/PHI) observed a quiet-Sun region jointly with SO/EUI, offering the possibility to investigate the surface magnetic field dynamics underlying campfires at a spatial resolution of about 380~km. In 71\% of the 38 isolated events, campfires are confined between bipolar magnetic features, which seem to exhibit signatures of magnetic flux cancellation. The flux cancellation occurs either between the two main footpoints, or between one of the footpoints of the loop housing the campfire and a nearby opposite polarity patch. In one particularly clear-cut case, we detected the emergence of a small-scale magnetic loop in the internetwork followed soon afterwards by a campfire brightening adjacent to the location of the linear polarisation signal in the photosphere, that is to say near where the apex of the emerging loop lays. The rest of the events were observed over small scattered magnetic features, which could not be identified as magnetic footpoints of the campfire hosting loops. The majority of campfires could be driven by magnetic reconnection triggered at the footpoints, similar to the physical processes occurring in the burst-like EUV events discussed in the literature. About a quarter of all analysed campfires, however, are not associated to such magnetic activity in the photosphere, which implies that other heating mechanisms are energising these small-scale EUV brightenings.

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arXiv2021

Capturing transient plasma flows and jets in the solar corona

Intensity bursts in ultraviolet (UV) to X-ray wavelengths and plasma jets are typical signatures of magnetic reconnection and the associated impulsive heating of the solar atmospheric plasma. To gain new insights into the process, high-cadence observations are required to capture the rapid response of plasma to magnetic reconnection as well as the highly dynamic evolution of jets. Here, we report the first 2 s cadence extreme-UV observations recorded by the 174 Å High Resolution Imager of the Extreme Ultraviolet Imager on board the Solar Orbiter mission. These observations, covering a quiet-Sun coronal region, reveal the onset signatures of magnetic reconnection as localized heating events. These localized sources then exhibit repeated plasma eruptions or jet activity. Our observations show that this spatial morphological change from localized sources to jet activity could occur rapidly on timescales of about 20 s. The jets themselves are intermittent and are produced from the source region on timescales of about 20 s. In the initial phases of these events, plasma jets are observed to exhibit speeds, as inferred from propagating intensity disturbances, in the range of 100 km s$^{-1}$ to 150 km s$^{-1}$. These jets then propagate to lengths of about 5 Mm. We discuss examples of bidirectional and unidirectional jet activity observed to have been initiated from the initially localized bursts in the corona. The transient nature of coronal bursts and the associated plasma flows or jets along with their dynamics could provide a benchmark for magnetic reconnection models of coronal bursts and jets.

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arXiv2021

Stereoscopy of extreme UV quiet Sun brightenings observed by Solar Orbiter/EUI

The 3D fine structure of the solar atmosphere is still not fully understood as most of the available observations are taken from a single vantage point. The goal of the paper is to study the 3D distribution of small-scale brightening events ("campfires") discovered in the EUV quiet Sun by the Extreme Ultraviolet Imager (EUI) aboard Solar Orbiter. We used a first commissioning data set acquired by the EUI's High Resolution EUV telescope on 30 May 2020 in the 174 Å passband and we combined it with simultaneous data taken by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory in a similar 171 Å passband. The two-pixel spatial resolution of the two telescopes is 400 km and 880 km, respectively, which is sufficient to identify the campfires in both data sets. The two spacecraft had an angular separation of around 31.5 degrees (essentially in heliographic longitude), which allowed for the 3D reconstruction of the campfire position. These observations represent the first time that stereoscopy was achieved for brightenings at such a small scale. Manual and automatic triangulation methods were used to characterize the campfire data. The height of the campfires is located between 1000 km and 5000 km above the photosphere and we find a good agreement between the manual and automatic methods. The internal structure of campfires is mostly unresolved by AIA; however, for a particularly large campfire, we were able to triangulate a few pixels, which are all in a narrow range between 2500 and 4500 km. The low height of EUI campfires suggests that they belong to the previously unresolved fine structure of the transition region and low corona of the quiet Sun. They are probably apexes of small-scale dynamic loops heated internally to coronal temperatures. This work demonstrates that high-resolution stereoscopy of structures in the solar atmosphere has become feasible.

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arXiv2021

Cosmic-ray flux predictions and observations for and with Metis on board Solar Orbiter

The Metis coronagraph is one of the remote sensing instruments hosted on board the ESA/NASA Solar Orbiter mission. Metis is devoted to carry out the first simultaneous imaging of the solar corona in both visible light (VL) and ultraviolet (UV). High-energy particles penetrate spacecraft materials and may limit the performance of on-board instruments. A study of galactic cosmic-ray (GCR) tracks observed in the first VL images gathered by Metis during the commissioning phase for a total of 60 seconds of exposure time is presented here. A similar analysis is planned for the UV channel. A prediction of the GCR flux up to hundreds of GeV is made here for the first part of the Solar Orbiter mission to study the Metis coronagraph performance. GCR model predictions are compared to observations gathered on board Solar Orbiter by the EPD/HET experiment in the range 10 MeV-100 MeV in the summer 2020 and with previous measurements. Estimated cosmic-ray fluxes above 70 MeV n$^{-1}$ have been also parameterized and used for Monte Carlo simulations aiming at reproducing the cosmic-ray track observations in the Metis coronagraph VL images. The same parameterizations can also be used to study the performance of other detectors. By comparing observations of cosmic-ray tracks in the Metis VL images with FLUKA Monte Carlo simulations of cosmic-ray interactions in the VL detector, it is found that cosmic rays fire a fraction of the order of 10$^{-4}$ of the whole image pixel sample. Therefore, cosmic rays do not affect sensibly the quality of Metis VL images. It is also found that the overall efficiency for cosmic-ray identification in the Metis VL images is approximately equal to the contribution of Z$>$2 particles. As a result, the Metis coronagraph may play the role of a proton monitor for long-term GCR variations during the overall mission duration.

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arXiv2021

Extreme UV quiet Sun brightenings observed by Solar Orbiter/EUI

The goal of this paper is to study the smallest brightening events observed in the EUV quiet Sun. We use commissioning data taken by the EUI instrument onboard the recently launched Solar Orbiter mission. On 2020 May 30, EUI was situated at 0.556AU from the Sun. Its HRIEUV telescope 17.4nm passband reached an exceptionally high two-pixel spatial resolution of 400km. The size and duration of small-scale structures is determined in the HRIEUV data, while their height is estimated from triangulation with the simultaneous SDO/AIA data. This is the first stereoscopy of small scale brightenings at high resolution. We observed small localised brightenings ("campfires") in a quiet Sun region with lengthscales between 400km and 4000km and durations between 10 and 200s. The smallest and weakest of these HRIEUV brightenings have not been observed before. Simultaneous HRILYA observations do not show localised brightening events, but the locations of the HRIEUV events correspond clearly to the chromospheric network. Comparison with simultaneous AIA images shows that most events can also be identified in the 17.1nm, 19.3nm, 21.1nm, and 30.4nm passbands of AIA, although they appear weaker and blurred. DEM analysis indicates coronal temperatures peaking at log(T)~6.1-6.15. We determined the height of a few campfires, which is between 1000 and 5000km above the photosphere. We conclude that "campfires" are mostly coronal in nature and are rooted in the magnetic flux concentrations of the chromospheric network. We interpret these events as a new extension to the flare/microflare/nanoflare family. Given their low height, the EUI "campfires" could be a new element of the fine structure of the transition region/low corona: apexes of small-scale loops that are internally heated to coronal temperatures.

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arXiv2020

The Solar Orbiter Science Activity Plan: translating solar and heliospheric physics questions into action

Solar Orbiter is the first space mission observing the solar plasma both in situ and remotely, from a close distance, in and out of the ecliptic. The ultimate goal is to understand how the Sun produces and controls the heliosphere, filling the Solar System and driving the planetary environments. With six remote-sensing and four in-situ instrument suites, the coordination and planning of the operations are essential to address the following four top-level science questions: (1) What drives the solar wind and where does the coronal magnetic field originate? (2) How do solar transients drive heliospheric variability? (3) How do solar eruptions produce energetic particle radiation that fills the heliosphere? (4) How does the solar dynamo work and drive connections between the Sun and the heliosphere? Maximising the mission's science return requires considering the characteristics of each orbit, including the relative position of the spacecraft to Earth (affecting downlink rates), trajectory events (such as gravitational assist manoeuvres), and the phase of the solar activity cycle. Furthermore, since each orbit's science telemetry will be downloaded over the course of the following orbit, science operations must be planned at mission level, rather than at the level of individual orbits. It is important to explore the way in which those science questions are translated into an actual plan of observations that fits into the mission, thus ensuring that no opportunities are missed. First, the overarching goals are broken down into specific, answerable questions along with the required observations and the so-called Science Activity Plan (SAP) is developed to achieve this. The SAP groups objectives that require similar observations into Solar Orbiter Observing Plans (SOOPs), resulting in a strategic, top-level view of the optimal opportunities for science observations during the mission lifetime.

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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.

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arXiv2013

On-Orbit Degradation of Solar Instruments

We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about On-Orbit Degradation of Solar and Space Weather Instruments that took place at the Solar Terrestrial Centre of Excellence (Royal Observatory of Belgium) in Brussels on 3 May 2012. The aim of this workshop was to open discussions related to the degradation observed in Sun-observing instruments exposed to the effects of the space environment. This article summarizes the various lessons learned and offers recommendations to reduce or correct expected degradation with the goal of increasing the useful lifespan of future and ongoing space missions.

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