arXiv subjects

Sabrina L. Savage

Publications and source records attributed to Sabrina L. Savage.

At least 19 recordsLinked to original sources

arXiv2025

A sudden fine-scale bright kernel captured by Hi-C Flare in 11 MK emission during an M1.6-class solar flare's post-maximum phase

On April 17, 2024, the third successful Hi-C sounding rocket flight, Hi-C Flare, recorded coronal images in Fe XXI 129 A emission from 11 MK plasma during the post-maximum phase of an M1.6-class solar flare, achieving unprecedented spatial (~300 km) and temporal (1.3 s) resolutions. The flare started at 21:55 UT, peaked at 22:08 UT, and lasted ~40 minutes. Hi-C observed for over five minutes (22:15:45 to 22:21:25), starting roughly eight minutes after flare maximum. A sudden compact bright burst - 875 +/- 25 km wide, lasting 90 +/- 1.3 s, exhibiting a proper motion of ~50 km/s, and splitting into two toward the end - occurs near the foot of some post-flare loops. Its size and brightness are reminiscent of flare-ribbon kernels during a flare's rapid rise phase, kernels marking sites of sudden heating and hot plasma upflow, making its occurrence during the late phase surprising. Such isolated brightenings in a flare's post-maximum phase are rare, and have not been previously reported. The kernel was detected in all SDO/AIA channels. Its 1600 A light curve peaked ~50 s earlier than its 131 A light curve, similar to that of flare-ribbon kernels, albeit with a smaller delay of ~25 s, during the impulsive phase of the flare. In SDO/HMI magnetograms, the kernel sits in unipolar positive magnetic flux near an embedded clump of negative flux. Although localized magnetic reconnection within the kernel (a microflare) cannot be ruled out for its cause, the observations favor the localized brightening being an isolated, exceptionally late flare-ribbon kernel, resulting from an exceptionally late burst of the flare's coronal reconnection.

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arXiv2024

Determining the nanoflare heating frequency of an X-ray Bright Point observed by MaGIXS

Nanoflares are thought to be one of the prime candidates that can heat the solar corona to its multi-million kelvin temperature. Individual nanoflares are difficult to detect with the present generation instruments, however their presence can be inferred by comparing simulated nanoflare-heated plasma emissions with the observed emission. Using HYDRAD coronal loop simulations, we model the emission from an X-ray bright point (XBP) observed by the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS), along with nearest-available observations from the Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) and X-Ray Telescope (XRT) onboard Hinode observatory. The length and magnetic field strength of the coronal loops are derived from the linear-force-free extrapolation of the observed photospheric magnetogram by Helioseismic and Magnetic Imager (HMI) onboard SDO. Each loop is assumed to be heated by random nanoflares, whose magnitude and frequency are determined by the loop length and magnetic field strength. The simulation results are then compared and matched against the measured intensity from AIA, XRT, and MaGIXS. Our model results indicate the observed emissions from the XBP under study could be well matched by a distribution of nanoflares with average delay times 1500 s to 3000 s, which suggest that the heating is dominated by high-frequency events. Further, we demonstrate the high sensitivity of MaGIXS and XRT to diagnose the heating frequency using this method, while AIA passbands are found to be the least sensitive.

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arXiv2023

Future High-Resolution and High-Cadence Observations for Unraveling Small-Scale Explosive Solar Features

Solar coronal jets are frequently occurring collimated ejections of solar plasma, originating from magnetically mixed polarity locations on the Sun of size scale comparable to that of a supergranule. Many, if not most, coronal jets are produced by eruptions of small-scale filaments, or minifilaments, whose magnetic field reconnects both with itself and also with surrounding coronal field. There is evidence that minifilament eruptions are a scaled-down version of typical filament eruptions that produce solar flares and coronal mass ejections (CMEs). Moreover, the magnetic processes building up to and triggering minifilament eruptions, which is often flux cancelation, might similarly build up and trigger the larger filaments to erupt. Thus, detailed study of coronal jets will inform us of the physics leading to, triggering, and driving the larger eruptions. Additionally, such studies potentially can inform us of smaller-scale coronal-jet-like features, such as jetlets and perhaps some spicules, that might work the same way as coronal jets. We propose a high-resolution (~0.1 pixels), high-cadence (~5 seconds) EUV-solar-imaging mission for the upcoming decades, that would be dedicated to observations of features of the coronal-jet size scale, and smaller-scale solar features produced by similar physics. Such a mission could provide invaluable insight into the operation of larger features such as CMEs that produce significant Space Weather disturbances, and also smaller-scale features that could be important for coronal heating, solar wind acceleration, and heliospheric features such as the magnetic switchbacks that are frequently observed in the solar wind.

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arXiv2022

The First Flight of the Marshall Grazing Incidence X-ray Spectrometer (MaGIXS)

The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) sounding rocket experiment launched on July 30, 2021 from the White Sands Missile Range in New Mexico. MaGIXS is a unique solar observing telescope developed to capture X-ray spectral images, in the 6 - 24 Angstrom wavelength range, of coronal active regions. Its novel design takes advantage of recent technological advances related to fabricating and optimizing X-ray optical systems as well as breakthroughs in inversion methodologies necessary to create spectrally pure maps from overlapping spectral images. MaGIXS is the first instrument of its kind to provide spatially resolved soft X-ray spectra across a wide field of view. The plasma diagnostics available in this spectral regime make this instrument a powerful tool for probing solar coronal heating. This paper presents details from the first MaGIXS flight, the captured observations, the data processing and inversion techniques, and the first science results.

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arXiv2022

Parallel plasma loops and the energization of the solar corona

The outer atmosphere of the Sun is composed of plasma heated to temperatures well in excess of the visible surface. We investigate short cool and warm (<1 MK) loops seen in the core of an active region to address the role of field-line braiding in energising these structures. We report observations from the High-resolution Coronal imager (Hi-C) that have been acquired in a coordinated campaign with the Interface Region Imaging Spectrograph (IRIS). In the core of the active region, the 172 A band of Hi-C and the 1400 A channel of IRIS show plasma loops at different temperatures that run in parallel. There is a small but detectable spatial offset of less than 1 arcsec between the loops seen in the two bands. Most importantly, we do not see observational signatures that these loops might be twisted around each other. Considering the scenario of magnetic braiding, our observations of parallel loops imply that the stresses put into the magnetic field have to relax while the braiding is applied: the magnetic field never reaches a highly braided state on these length-scales comparable to the separation of the loops. This supports recent numerical 3D models of loop braiding in which the effective dissipation is sufficiently large that it keeps the magnetic field from getting highly twisted within a loop.

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arXiv2022

A publicly available multi-observatory data set of an enhanced network patch from the Photosphere to Corona

New instruments sensitive to chromospheric radiation at X-ray, UV, Visible, IR, and sub-mm wavelengths have become available that significantly enhance our ability to understand the bi-directional flow of energy through the chromosphere. We describe the calibration, co-alignment, initial results, and public release of a new data set combining a large number of these instruments to obtain multi-wavelength photospheric, chromospheric, and coronal observations capable of improving our understanding of the connectivity between the photosphere and the corona via transient brightenings and wave signatures. The observations center on a bipolar region of enhanced network magnetic flux near disk center on SOL2017-03-17T14:00-17:00. The comprehensive data set provides one of the most complete views of chromospheric activity related to small scale brightenings in the corona and chromosphere to date. Our initial analysis shows strong spatial correspondence between the areas of broadest width of the Hydrogen-$α$ spectral line and the hottest temperatures observed in ALMA Band 3 radio data, with a linear coefficient of $6.12\times 10^{-5}$Å/K. The correspondence persists for the duration of co-temporal observations ($\approx 60$ minutes). Numerous transient brightenings were observed in multiple data series. We highlight a single, well observed transient brightening along a set of thin filamentary features with a duration of 20 minutes. The timing of the peak intensity transitions from the cooler (ALMA, 7000 K) to hotter (XRT, 3 MK) data series.

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arXiv2022

Further Evidence for the Minifilament-Eruption Scenario for Solar Polar Coronal Jets

We examine a sampling of 23 polar-coronal-hole jets. We first identified the jets in soft X-ray (SXR) images from the X-ray telescope (XRT) on the Hinode spacecraft, over 2014-2016. During this period, frequently the polar holes were small or largely obscured by foreground coronal haze, often making jets difficult to see. We selected 23 jets among those adequately visible during this period, and examined them further using Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) 171, 193, 211, and 304 Ang images. In SXRs we track the lateral drift of the jet spire relative to the jet base's jet bright point (JBP). In 22 of 23 jets the spire either moves away from (18 cases) or is stationary relative to (4 cases) the JBP. The one exception where the spire moved toward the JBP may be a consequence of line-of-sight projection effects at the limb. From the AIA images, we clearly identify an erupting minifilament in 20 of the 23 jets, while the remainder are consistent with such an eruption having taken place. We also confirm that some jets can trigger onset of nearby "sympathetic" jets, likely because eruption of the minifilament field of the first jet removes magnetic constraints on the base-field region of the second jet. The propensity for spire drift away from the JBP, the identification of the erupting minifilament in the majority of jets, and the magnetic-field topological changes that lead to sympathetic jets, all support or are consistent with the minifilament-eruption model for jets.

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arXiv2020

A Solar Magnetic-fan Flaring Arch Heated by Non-thermal Particles and Hot Plasma from an X-ray Jet Eruption

We have investigated an M1.3 limb flare, which develops as a magnetic loop/arch that fans out from an X-ray jet. Using Hinode/EIS, we found that the temperature increases with height to a value of over 10$^{7}$ K at the loop-top during the flare. The measured Doppler velocity (redshifts of 100$-$500 km s$^{-1}$) and the non-thermal velocity ($\geq$100 km s$^{-1}$) from Fe XXIV also increase with loop height. The electron density increases from $0.3\times10^{9}$ cm$^{-3}$ early in the flare rise to $1.3\times10^{9}$ cm$^{-3}$ after the flare peak. The 3-D structure of the loop derived with STEREO/EUVI indicates that the strong redshift in the loop-top region is due to upflowing plasma originating from the jet. Both hard X-ray and soft X-ray emission from RHESSI were only seen as footpoint brightenings during the impulsive phase of the flare, then, soft X-ray emission moves to the loop-top in the decay phase. Based on the temperature and density measurements and theoretical cooling models, the temperature evolution of the flare arch is consistent with impulsive heating during the jet eruption followed by conductive cooling via evaporation and minor prolonged heating in the top of the fan loop. Investigating the magnetic field topology and squashing factor map from SDO/HMI, we conclude that the observed magnetic-fan flaring arch is mostly heated from low atmospheric reconnection accompanying the jet ejection, instead of from reconnection above the arch as expected in the standard flare model.

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arXiv2020

Is the High-Resolution Coronal Imager Resolving Coronal Strands? Results from AR 12712

Following the success of the first mission, the High-Resolution Coronal Imager (Hi-C) was launched for a third time (Hi-C 2.1) on 29th May 2018 from the White Sands Missile Range, NM, USA. On this occasion, 329 seconds of 17.2 nm data of target active region AR 12712 was captured with a cadence of ~4s, and a plate scale of 0.129''/pixel. Using data captured by Hi-C 2.1 and co-aligned observations from SDO/AIA 17.1 nm we investigate the widths of 49 coronal strands. We search for evidence of substructure within the strands that is not detected by AIA, and further consider whether these strands are fully resolved by Hi-C 2.1. With the aid of Multi-Scale Gaussian Normalization (MGN), strands from a region of low-emission that can only be visualized against the contrast of the darker, underlying moss are studied. A comparison is made between these low-emission strands with those from regions of higher emission within the target active region. It is found that Hi-C 2.1 can resolve individual strands as small as ~202km, though more typical strands widths seen are ~513km. For coronal strands within the region of low-emission, the most likely width is significantly narrower than the high-emission strands at ~388km. This places the low-emission coronal strands beneath the resolving capabilities of SDO/AIA, highlighting the need of a permanent solar observatory with the resolving power of Hi-C.

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arXiv2019

Hi-C 2.1 Observations of Small-Scale Miniature-Filament-Eruption-Like Cool Ejections in Active Region Plage

We examine 172 Ang ultra-high-resolution images of a solar plage region from the Hi-C 2.1 ("Hi-C") rocket flight of 2018 May 29. Over its five-minute flight, Hi-C resolves a plethora of small-scale dynamic features that appear near noise level in concurrent Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) 171 Ang images. For ten selected events, comparisons with AIA images at other wavelengths and with the Interface Region Imaging Spectrograph (IRIS) images indicate that these features are cool (compared to the corona) ejections. Combining Hi-C 172 Ang, AIA 171 Ang, IRIS 1400 Ang, and H$α$, we see that these ten cool ejections are similar to the H$α$ "dynamic fibrils" and Ca ii "anemone jets" found in earlier studies. The front of some of our cool ejections are likely heated, showing emission in IRIS 1400 Ang. On average, these cool ejections have approximate widths: $3''.2 \pm 2''.1$, (projected) maximum heights and velocities: $4''.3 \pm 2''.5$ and $23 \pm 6$ km/s, and lifetimes: $6.5 \pm 2.4$ min. We consider whether these Hi-C features might result from eruptions of sub-minifilaments (smaller than the minifilaments that erupt to produce coronal jets). Comparisons with SDO's Helioseismic and Magnetic Imager (HMI) magnetograms do not show magnetic mixed-polarity neutral lines at these events' bases, as would be expected for true scaled-down versions of solar filaments/minifilaments. But the features' bases are all close to single-polarity strong-flux-edge locations, suggesting possible local opposite-polarity flux unresolved by HMI. Or, it may be that our Hi-C ejections instead operate via the shock-wave mechanism that is suggested to drive dynamic fibrils and the so-called type I spicules.

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arXiv2019

Hi-C 2.1 Observations of Jetlet-like Events at Edges of Solar Magnetic Network Lane

We present high-resolution, high-cadence observations of six, fine-scale, on-disk jet-like events observed by the High-resolution Coronal Imager 2.1 (Hi-C 2.1) during its sounding-rocket flight. We combine the Hi-C 2.1 images with images from SDO/AIA, and IRIS, and investigate each event's magnetic setting with co-aligned line-of-sight magnetograms from SDO/HMI. We find that: (i) all six events are jetlet-like (having apparent properties of jetlets), (ii) all six are rooted at edges of magnetic network lanes, (iii) four of the jetlet-like events stem from sites of flux cancelation between majority-polarity network flux and merging minority-polarity flux, and (iv) four of the jetlet-like events show brightenings at their bases reminiscent of the base brightenings in coronal jets. The average spire length of the six jetlet-like events (9,000$\pm$3000km) is three times shorter than that for IRIS jetlets (27,000$\pm$8000km). While not ruling out other generation mechanisms, the observations suggest that at least four of these events may be miniature versions of both larger-scale coronal jets that are driven by minifilament eruptions and still-larger-scale solar eruptions that are driven by filament eruptions. Therefore, we propose that our Hi-C events are driven by the eruption of a tiny sheared-field flux rope, and that the flux-rope field is built and triggered to erupt by flux cancelation.

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arXiv2019

Fine-scale explosive energy release at sites of prospective magnetic flux cancellation in the core of the solar active region observed by Hi-C 2.1, IRIS and SDO

The second Hi-C flight (Hi-C2.1) provided unprecedentedly-high spatial and temporal resolution ($\sim$250km, 4.4s) coronal EUV images of Fe IX/X emission at 172 Å, of AR 12712 on 29-May-2018, during 18:56:21-19:01:56 UT. Three morphologically-different types (I: dot-like, II: loop-like, III: surge/jet-like) of fine-scale sudden-brightening events (tiny microflares) are seen within and at the ends of an arch filament system in the core of the AR. Although type Is (not reported before) resemble IRIS-bombs (in size, and brightness wrt surroundings), our dot-like events are apparently much hotter, and shorter in span (70s). We complement the 5-minute-duration Hi-C2.1 data with SDO/HMI magnetograms, SDO/AIA EUV images, and IRIS UV spectra and slit-jaw images to examine, at the sites of these events, brightenings and flows in the transition-region and corona and evolution of magnetic flux in the photosphere. Most, if not all, of the events are seated at sites of opposite-polarity magnetic flux convergence (sometimes driven by adjacent flux emergence), implying likely flux cancellation at the microflare's polarity inversion line. In the IRIS spectra and images, we find confirming evidence of field-aligned outflow from brightenings at the ends of loops of the arch filament system. In types I and II the explosion is confined, while in type III the explosion is ejective and drives jet-like outflow. The light-curves from Hi-C, AIA and IRIS peak nearly simultaneously for many of these events and none of the events display a systematic cooling sequence as seen in typical coronal flares, suggesting that these tiny brightening-events have chromospheric/transition-region origin.

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arXiv2019

The High-Resolution Coronal Imager, Flight 2.1

The third flight of the High-Resolution Coronal Imager (Hi-C 2.1) occurred on May 29, 2018, the Sounding Rocket was launched from White Sands Missile Range in New Mexico. The instrument has been modified from its original configuration (Hi-C 1) to observe the solar corona in a passband that peaks near 172 Angstrom and uses a new, custom-built low-noise camera. The instrument targeted Active Region 12712, and captured 78 images at a cadence of 4.4 sec (18:56:22 - 19:01:57 UT; 5 min and 35 sec observing time). The image spatial resolution varies due to quasi-periodic motion blur from the rocket; sharp images contain resolved features of at least 0.47 arcsec. There are coordinated observations from multiple ground- and space-based telescopes providing an unprecedented opportunity to observe the mass and energy coupling between the chromosphere and the corona. Details of the instrument and the data set are presented in this paper.

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arXiv2019

Signatures of the non-Maxwellian $κ$-distributions in optically thin line spectra. II. Synthetic Fe XVII--XVIII X-ray coronal spectra and predictions for the Marshall Grazing-Incidence X-ray Spectrometer (MaGIXS)

We investigated the possibility of diagnosing the degree of departure from the Maxwellian distribution using the Fe XVII - Fe XVIII spectra originating in plasmas in collisional ionization equilibrium, such as in the cores of solar active regions or microflares. The original collision strengths for excitation are integrated over the non-Maxwellian electron $κ$-distributions characterized by a high-energy tail. Synthetic X-ray emission line spectra were calculated for a range of temperatures and $κ$. We focus on the 6-24 A spectral range to be observed by the upcoming Marshall Grazing-Incidence X-ray Spectrometer MaGIXS. We find that many line intensity ratios are sensitive to both $T$ and $κ$. Best diagnostic options are provided if a ratio involving both Fe XVII and Fe XVIII is combined with another ratio involving lines formed within a single ion. The sensitivity of such diagnostics to $κ$ is typically a few tens of per cent. Much larger sensitivity, of about a factor of two to three, can be obtained if the Fe XVIII 93.93 A line observed by SDO/AIA is used in conjuction with the X-ray lines. We conclude that the MaGIXS instrument is well-suited for detection of departures from the Maxwellian distribution, especially in active region cores.

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arXiv2017

An exploration of heating mechanisms in a supra-arcade plasma sheet formed after a coronal mass ejection

We perform a detailed analysis of the thermal structure of the region above the post-eruption arcade for a flare that occurred on 2011 October 22. During this event, a sheet of hot plasma is visible above the flare loops in the 131 Å bandpass of the Atmospheric Imaging Assembly (AIA) on the {\it Solar Dynamics Observatory (SDO)}. Supra-arcade downflows (SADs) are observed traveling sunward through the post-eruption plasma sheet. We calculate differential emission measures using the AIA data and derive an emission measure weighted average temperature in the supra-arcade region. In areas where many SADs occur, the temperature of the supra-arcade plasma tends to increase, while in areas where no SADs are observed, the temperature tends to decrease. We calculate the plane-of-sky velocities in the supra-arcade plasma and use them to calculate the potential heating due to adiabatic compression and viscous heating. Ten of the 13 SADs studied have noticeable signatures in both the adiabatic and the viscous terms. The adiabatic heating due to compression of plasma in front of the SADs is on the order of 0.1 - 0.2 MK/s, which is similar in magnitude to the estimated conductive cooling rate. This result supports the notion that SADs contribute locally to the heating of plasma in the supra-arcade region. We also find that in the region without SADs, the plasma cools at a rate slower than the estimated conductive cooling, indicating additional heating mechanisms may act globally to keep the plasma temperature high.

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arXiv2016

Miniature loops in the solar corona

Magnetic loops filled with hot plasma are the main building blocks of the solar corona. Usually they have lengths of the order of the barometric scale height in the corona that is 50 Mm. Previously it has been suggested that miniature versions of hot loops exist. These would have lengths of only 1 Mm barely protruding from the chromosphere and spanning across just one granule in the photosphere. Such short loops are well established at transition region temperatures (0.1 MK), and we investigate if such miniature loops also exist at coronal temperatures (>1 MK). We used extreme UV imaging (EUV) observations from the High-resolution Coronal Imager (Hi-C) at an unprecedented spatial resolution of 0.3" to 0.4". Together with EUV imaging and magnetogram data from the Solar Dynamics Observatory (SDO) and X-Ray Telescope (XRT) data from Hinode we investigated the spatial, temporal and thermal evolution of small loop-like structures in the solar corona above a plage region close to an active region and compared this to a moss area within the active region. We find that the size, motion and temporal evolution of the loop-like features are consistent with photospheric motions, suggesting a close connection to the photospheric magnetic field. Aligned magnetograms show that one of their endpoints is rooted at a magnetic concentration. Their thermal structure, as revealed together with the X-ray observations, shows significant differences to moss-like features. Considering different scenarios, these features are most probably miniature versions of hot loops rooted at magnetic concentrations at opposite sides of a granule in small emerging magnetic loops (or flux tubes).

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arXiv2016

Hi-C Observations of Sunspot Penumbral Bright Dots

We report observations of bright dots (BDs) in a sunspot penumbra using High Resolution Coronal Imager (Hi-C) data in 193 Å and examine their sizes, lifetimes, speeds, and intensities. The sizes of the BDs are on the order of 1\arcsec\ and are therefore hard to identify in the Atmospheric Imaging Assembly (AIA) 193 Å images, which have 1.2\arcsec\ spatial resolution, but become readily apparent with Hi-C's five times better spatial resolution. We supplement Hi-C data with data from AIA's 193 Å passband to see the complete lifetime of the BDs that appeared before and/or lasted longer than Hi-C's 3-minute observation period. Most Hi-C BDs show clear lateral movement along penumbral striations, toward or away from the sunspot umbra. Single BDs often interact with other BDs, combining to fade away or brighten. The BDs that do not interact with other BDs tend to have smaller displacements. These BDs are about as numerous but move slower on average than Interface Region Imaging Spectrograph (IRIS) BDs, recently reported by \cite{tian14}, and the sizes and lifetimes are on the higher end of the distribution of IRIS BDs. Using additional AIA passbands, we compare the lightcurves of the BDs to test whether the Hi-C BDs have transition region (TR) temperature like that of the IRIS BDs. The lightcurves of most Hi-C BDs peak together in different AIA channels indicating that their temperature is likely in the range of the cooler TR ($1-4\times 10^5$ K).

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arXiv2012

Re-interpretation of Supra-Arcade Downflows in Solar Flares

Following the eruption of a filament from a flaring active region, sunward-flowing voids are often seen above developing post-eruption arcades. First discovered using the soft X-ray telescope aboard Yohkoh, these supra-arcade downflows (SADs) are now an expected observation of extreme ultra-violet (EUV) and soft X-ray coronal imagers and spectrographs (e.g, TRACE, SOHO/SUMER, Hinode/XRT, SDO/AIA). Observations made prior to the operation of AIA suggested that these plasma voids (which are seen in contrast to bright, high-temperature plasma associated with current sheets) are the cross-sections of evacuated flux tubes retracting from reconnection sites high in the corona. The high temperature imaging afforded by AIA's 131, 94, and 193 Angstrom channels coupled with the fast temporal cadence allows for unprecedented scrutiny of the voids. For a flare occurring on 2011 October 22, we provide evidence suggesting that SADs, instead of being the cross-sections of relatively large, evacuated flux tubes, are actually wakes (i.e., trailing regions of low density) created by the retraction of much thinner tubes. This re-interpretation is a significant shift in the fundamental understanding of SADs, as the features once thought to be identifiable as the shrinking loops themselves now appear to be "side effects" of the passage of the loops through the supra-arcade plasma. In light of the fact that previous measurements have attributed to the shrinking loops characteristics that may instead belong to their wakes, we discuss the implications of this new interpretation on previous parameter estimations, and on reconnection theory.

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