arXiv subjects

Louise K. Harra

Publications and source records attributed to Louise K. Harra.

15 recordsLinked to original sources

arXiv2025

Surprisingly Large Doppler Shifts in Hinode EUV Imaging Spectrometer (EIS) Solar Spectra, Resulting from an Inconspicuous Small-scale Jet in EUV Images

Strong EUV lineshifts in solar spectra are generally indicative of highly dynamic and explosive events that are easily detected in comparable-wavelength EUV images, with the strongest such line shifts (several 100 km/s) occurring in solar flares. Here we present observations of exceptionally strong lineshifts detected in Hinode/EUV Imaging Spectrometer (EIS) spectra outside the time of a flare-like brightening, with 195 Ang blueshifts of ~200 km/s. Although the likely culprit is too weak to register in GOES Soft X-ray fluxes, EIS pinpoints the source at the edge of an active region. Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) images and Helioseismic and Magnetic Imager (HMI) magnetograms show a nondescript small-scale eruptive event at this location. We find this event likely to be an inconspicuous coronal jet, apparently triggered by converging/canceling magnetic flux patches, with plane-of-sky velocity ~159+-29 km/s. AIA and HMI observations of this faint transient feature, together with observations of a slightly brighter jetting event near the same location an hour earlier, suggest that the strong EIS Doppler shifts are indeed due to a coronal jet that is hard to detect in AIA images. These observations, together with other recent studies, show that EUV Doppler maps are a much more sensitive tool for detecting small-scale eruptions than are EUV images, and those eruptions are frequently triggered by magnetic flux cancelation episodes. Such-detected small-scale eruptions, that often produce small-scale coronal-jet-like features, might propagate into and help drive the solar wind.

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arXiv2024

Solar flares in the Solar Orbiter era: Short-exposure EUI/FSI observations of STIX flares

Aims: This paper aims to demonstrate the importance of short-exposure extreme ultraviolet (EUV) observations of solar flares in the study of particle acceleration, heating and energy partition in flares. This work highlights the observations now available from the Extreme Ultraviolet Imager (EUI) instrument suite on board Solar Orbiter while operating in short-exposure mode. Methods: A selection of noteworthy flares observed simultaneously by the Spectrometer Telescope for Imaging X-rays (STIX) and the Full Sun Imager of EUI (EUI/FSI) are detailed. New insights are highlighted and potential avenues of investigation are demonstrated, including forward-modelling the atmospheric response to a non-thermal beam of electrons using the RADYN 1D hydrodynamic code, in order to compare the predicted and observed EUV emission. Results: The examples given in this work demonstrate that short-exposure EUI/FSI observations are providing important diagnostics during flares. A dataset of more than 9000 flares observed by STIX (from November 2022 until December 2023) with at least one short-exposure EUI/FSI 174 Å image is currently available. The observations reveal that the brightest parts of short-exposure observations consist of substructure in flaring ribbons that spatially overlap with the hard X-ray emission observed by STIX in the majority of cases. We show that these observations provide an opportunity to further constrain the electron energy flux required for flare modelling, among other potential applications.

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arXiv2024

Localising pulsations in the hard X-ray and microwave emission of an X-class flare

Aims: This work aims to identify the mechanism driving pulsations in hard X-ray (HXR) and microwave emission during solar flares. Here, by using combined HXR and microwave observations from Solar Orbiter/STIX and EOVSA we investigate an X1.3 GOES class flare, 2022-03-30T17:21:00, which displays pulsations on timescales evolving from ~ 7 s in the impulsive phase to ~ 35 s later in the flare. Methods: The temporal, spatial and spectral evolution of the HXR and microwave pulsations during the impulsive phase of the flare are analysed. Images are reconstructed for individual peaks in the impulsive phase and spectral fitting is performed at high cadence throughout the first phase of pulsations. Results: Imaging analysis demonstrates that the HXR and microwave emission originates from multiple sites along the flare ribbons. The brightest sources and the location of the emission changes in time. Through HXR spectral analysis, the electron spectral index is found to be anti-correlated with the HXR flux showing a "soft-hard-soft" spectral index evolution for each pulsation. The timing of the associated filament eruption coincides with the early impulsive phase. Conclusions: Our results indicate that periodic acceleration and/or injection of electrons from multiple sites along the flare arcade is responsible for the pulsations observed in HXR and microwave. The evolution of pulsation timescales is likely a result of changes in the 3D magnetic field configuration in time related to the associated filament eruption.

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arXiv2023

Characterising fast-time variations in the hard X-ray time profiles of solar flares using Solar Orbiter's STIX

Aims: The aim of this work is to develop a method to systematically detect and characterise fast-time variations ($\gtrsim 1$s) in the non-thermal hard X-ray (HXR) time profiles of solar flares using high-resolution data from Solar Orbiter's Spectrometer/Telescope for Imaging X-rays (STIX). Methods: The HXR time profiles were smoothed using Gaussian Process (GP) regression. The time profiles were then fitted with a linear combination of Gaussians to decompose the time profile. From the Gaussian decomposition, key characteristics such as the periodicity, full width at half maximum (FWHM), time evolution, and amplitude can be derived. Results: We present the outcome of applying this method to four M and X GOES-class flares from the first year of Solar Orbiter science operations. The HXR time profiles of these flares were decomposed into individual Gaussians and their periods were derived. The quality of fit is quantified by the standard deviation of the residuals (difference between observed and fitted curve, normalised by the error on the observed data), for which we obtain $\leq 1.8$ for all flares presented. In this work, the first detection of fast-time variations with Solar Orbiter's STIX instrument has been made on timescales across the range of 4-128s. Conclusions: A new method for identifying and characterising fast-time variations in the non-thermal HXR profiles of solar flares has been developed, in which the time profiles are fit with a linear combination of Gaussian bursts. The opportunity to study time variations in flares has greatly improved with the new observations from STIX on Solar Orbiter.

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arXiv2022

A Statistical Comparison of EUV Brightenings Observed by SO/EUI with Simulated Brightenings in Non-potential Simulations

The High Resolution Imager (HRI_EUV) telescope of the Extreme Ultraviolet Imager (EUI) instrument onboard Solar Orbiter has observed EUV brightenings, so-called campfires, as fine-scale structures at coronal temperatures. The goal of this paper is to compare the basic geometrical (size, orientation) and physical (intensity, lifetime) properties of the EUV brightenings with regions of energy dissipation in a non-potential coronal magnetic field simulation. In the simulation, HMI line-of-sight magnetograms are used as input to drive the evolution of solar coronal magnetic fields and energy dissipation. We applied an automatic EUV brightening detection method to EUV images obtained on 30 May 2020 by the HRI_EUV telescope. We applied the same detection method to the simulated energy dissipation maps from the non-potential simulation to detect simulated brightenings. We detected EUV brightenings with density of 1.41x10^{-3} brightenings/Mm^2 in the EUI observations and simulated brightenings between 2.76x10^{-2} - 4.14x10^{-2} brightenings/Mm^2 in the simulation, for the same time range. Although significantly more brightenings were produced in the simulations, the results show similar distributions of the key geometrical and physical properties of the observed and simulated brightenings. We conclude that the non-potential simulation can successfully reproduce statistically the characteristic properties of the EUV brightenings (typically with more than 85% similarity); only the duration of the events is significantly different between observations and simulation. Further investigations based on high-cadence and high-resolution magnetograms from Solar Orbiter are under consideration to improve the agreement between observation and simulation.

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arXiv2022

Inconspicuous Solar Polar Coronal X-ray Jets as the Source of Conspicuous Hinode/EUV Imaging Spectrometer (EIS) Doppler Outflows

We examine in greater detail five events previously identified as being sources of strong transient coronal outflows in a solar polar region in Hinode/EUV Imaging Spectrometer (EIS) Doppler data. Although relatively compact or faint and inconspicuous in Hinode/Soft X-ray Telescope (XRT) soft-X-ray (SXR) images and in Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) EUV images, we find that all of these events are consistent with being faint coronal X-ray jets. The evidence for this is that the events result from eruption of minifilaments of projected sizes spanning 5000 -- 14,000 km and with erupting velocities spanning 19 -- 46 km/s, which are in the range of values observed in cases of confirmed X-ray polar coronal hole jets. In SXR images, and in some EUV images, all five events show base brightenings, and faint indications of a jet spire that (in four of five cases where determinable) moves away from the brightest base brightening; these properties are common to more obvious X-ray jets. For a comparatively low-latitude event, the minifilament erupts from near (<~few arcsec) a location of near-eruption-time opposite-polarity magnetic-flux-patch convergence, which again is consistent with many observed coronal jets. Thus, although too faint to be identified as jets a priori, otherwise all five events are identical to typical coronal jets. This suggests that jets may be more numerous than recognized in previous studies, and might contribute substantially to solar wind outflow, and to the population of magnetic switchbacks observed in Parker Solar Probe (PSP) data.

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arXiv2022

Spatial distribution of jets in solar active regions

Context. Solar active regions are known to have jets. These jets are associated with heating and the release of particles into the solar wind. Aim. Our aim is to understand the spatial distribution of coronal jets within active regions to understand if there is a preferential location for them to occur. Methods. We analysed five active regions using Solar Dynamics Observatory Atmospheric Imaging Assembly data over a period of 2-3.5 days when the active regions were close to disk centre. Each active region had a different age, magnetic field strength, and topology. We developed a methodology for determining the position and length of the jets. Results. Jets are observed more frequently at the edges of the active regions and are more densely located around a strong leading sunspot. The number of coronal jets for our active regions is dependent on the age of the active region. The older active regions produce more jets than younger ones. Jets were observed dominantly at the edges of the active regions, and not as frequently in the centre. The number of jets is independent of the average unsigned magnetic field and total flux density in the whole active region. The jets are located around the edges of the strong leading sunspot.

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arXiv2021

Matching temporal signatures of solar features to their corresponding solar wind outflows

The role of small-scale coronal eruptive phenomena in the generation and heating of the solar wind remains an open question. Here, we investigate the role played by coronal jets in forming the solar wind by testing whether temporal variations associated with jetting in EUV intensity can be identified in the outflowing solar wind plasma. This type of comparison is challenging due to inherent differences between remote-sensing observations of the source and in situ observations of the outflowing plasma, as well as travel time and evolution of the solar wind throughout the heliosphere. To overcome these, we propose a novel algorithm combining signal filtering, two-step solar wind ballistic backmapping, window shifting, and Empirical Mode Decomposition. We first validate the method using synthetic data, before applying it to measurements from the Solar Dynamics Observatory, and Wind spacecraft. The algorithm enables the direct comparison of remote sensing observations of eruptive phenomena in the corona to in situ measurements of solar wind parameters, among other potential uses. After application to these datasets, we find several time windows where signatures of dynamics found in the corona are embedded in the solar wind stream, at a time significantly earlier than expected from simple ballistic backmapping, with the best performing in situ parameter being the solar wind mass flux.

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arXiv2018

A Two-Sided-Loop X-Ray Solar Coronal Jet Driven by a Minifilament Eruption

Most of the commonly discussed solar coronal jets are of the type consisting of a single spire extending approximately vertically from near the solar surface into the corona. Recent research supports that eruption of a miniature filament (minifilament) drives many such single-spire jets, and concurrently generates a miniflare at the eruption site. A different type of coronal jet, identified in X-ray images during the Yohkoh era, are two-sided-loop jets, which extend from a central excitation location in opposite directions, along low-lying coronal loops more-or-less horizontal to the surface. We observe such a two-sided-loop jet from the edge of active region (AR) 12473, using data from Hinode XRT and EIS, and SDO AIA and HMI. Similar to single-spire jets, this two-sided-loop jet results from eruption of a minifilament, which accelerates to over 140 km/s before abruptly stopping after striking overlying nearly-horizontal loop field at about 30,000 km altitude and producing the two-sided-loop jet. Analysis of EIS raster scans show that a hot brightening, consistent with a small flare, develops in the aftermath of the eruption, and that Doppler motions (approx 40 km/s) occur near the jet-formation region. As with many single-spire jets, the magnetic trigger here is apparently flux cancelation, which occurs at a rate of approx 4x10^18 Mx/hr, comparable to the rate observed in some single-spire AR jets. An apparent increase in the (line-of-sight) flux occurs within minutes of onset of the minifilament eruption, consistent with the apparent increase being due to a rapid reconfiguration of low-lying field during and soon after minifilament-eruption onset.

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arXiv2018

The Triggering of the 29-March-2014 Filament Eruption

The X1 flare and associated filament eruption occurring in NOAA Active Region 12017 on SOL2014-03-29 has been the source of intense study. In this work, we analyse the results of a series of non linear force free field extrapolations of the pre and post flare period of the flare. In combination with observational data provided by the IRIS, Hinode and SDO missions, we have confirmed the existence of two flux ropes present within the active region prior to flaring. Of these two flux ropes, we find that intriguingly only one erupts during the X1 flare. We propose that the reason for this is due to tether cutting reconnection allowing one of the flux ropes to rise to a torus unstable region prior to flaring, thus allowing it to erupt during the subsequent flare.

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arXiv2018

Plasma evolution within an erupting coronal cavity

Coronal cavities have previously been observed associated with long-lived quiescent filaments and are thought to correspond to the associated magnetic flux rope. Although the standard flare model predicts a coronal cavity corresponding to the erupting flux rope, these have only been observed using broadband imaging data, restricting analysis to the plane-of-sky. We present a unique set of spectroscopic observations of an active region filament seen erupting at the solar limb in the extreme ultraviolet (EUV). The cavity erupted and expanded rapidly, with the change in rise phase contemporaneous with an increase in non-thermal electron energy flux of the associated flare. Hot and cool filamentary material was observed to rise with the erupting flux rope, disappearing suddenly as the cavity appeared. Although strongly blue-shifted plasma continued to be observed flowing from the apex of the erupting flux rope, this outflow soon ceased. These results indicate that the sudden injection of energy from the flare beneath forced the rapid eruption and expansion of the flux rope, driving strong plasma flows which resulted in the eruption of an under-dense filamentary flux rope.

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arXiv2016

Magnetic Properties of Solar Active Regions that Govern Large Solar Flares and Eruptions

Solar flares and coronal mass ejections (CMEs), especially the larger ones, emanate from active regions (ARs). With the aim to understand the magnetic properties that govern such flares and eruptions, we systematically survey all flare events with GOES levels of >=M5.0 within 45 deg from disk center between May 2010 and April 2016. These criteria lead to a total of 51 flares from 29 ARs, for which we analyze the observational data obtained by the Solar Dynamics Observatory. More than 80% of the 29 ARs are found to exhibit delta-sunspots and at least three ARs violate Hale's polarity rule. The flare durations are approximately proportional to the distance between the two flare ribbons, to the total magnetic flux inside the ribbons, and to the ribbon area. From our study, one of the parameters that clearly determine whether a given flare event is CME-eruptive or not is the ribbon area normalized by the sunspot area, which may indicate that the structural relationship between the flaring region and the entire AR controls CME productivity. AR characterization show that even X-class events do not require delta-sunspots or strong-field, high-gradient polarity inversion lines. An investigation of historical observational data suggests the possibility that the largest solar ARs, with magnetic flux of 2x10^23 Mx, might be able to produce "superflares" with energies of order of 10^34 erg. The proportionality between the flare durations and magnetic energies is consistent with stellar flare observations, suggesting a common physical background for solar and stellar flares.

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arXiv2015

Spectroscopic Signatures Related to a Sunquake

The presence of flare related acoustic emission (sunquakes) in some flares represents a severe challenge to our current understanding of flare energy transport processes. We present a comparison of new spectral observations from Hinode's EUV imaging Spectrometer (EIS) and the Interface Region Imaging Spectrograph (IRIS) of the atmosphere above a sunquake, and compare them to the spectra observed in a part of the flaring region with no acoustic signature. Evidence for the sunquake is determined using both time-distance and acoustic holography methods, and we find that, unlike many previous sunquake detections, the signal is rather dispersed, but that the time-distance and 6 and 7 mHz sources converge at the same spatial location. We also see some evidence for different evolution at different frequencies, with an earlier peak at 7 mHz than at 6 mHz. Using spectroscopic measurements we find that in this location at the time of the 7 mHz peak the spectral emission is significantly more intense, shows larger velocity shifts and substantially broader profiles than in the location with no sunquake, and that there is a good correlation between blue-shifted, hot coronal, hard X-ray (HXR) and red-shifted chromospheric emission, consistent with the idea of a strong downward motion driven by rapid heating by non-thermal electrons and the formation of chromospheric shocks. Exploiting the diagnostic potential of the Mg II triplet lines, we also find evidence for a single, large temperature increase deep in the atmosphere, consistent with this scenario. The time of the 6 mHz and time-distance peak signal coincides with a secondary peak in the energy release process, but in this case we find no evidence of HXR emission in the quake location, but very broad spectral lines, strongly shifted to the red, indicating the possible presence of a significant flux of downward propagating Alfven waves.

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arXiv2013

Measuring the Magnetic Field Strength of the Quiet Solar Corona Using "EIT Waves"

Variations in the propagation of globally-propagating disturbances (commonly called "EIT waves") through the low solar corona offer a unique opportunity to probe the plasma parameters of the solar atmosphere. Here, high-cadence observations of two "EIT wave" events taken using the Atmospheric Imaging Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) are combined with spectroscopic measurements from the Extreme ultraviolet Imaging Spectrometer (EIS) onboard the Hinode spacecraft and used to examine the variability of the quiet coronal magnetic-field strength. The combination of pulse kinematics from SDO/AIA and plasma density from Hinode/EIS is used to show that the magnetic-field strength is in the range ~2-6 G in the quiet corona. The magnetic-field estimates are then used to determine the height of the pulse, allowing a direct comparison with theoretical values obtained from magnetic-field measurements from the Helioseismic and Magnetic Imager (HMI) onboard SDO using PFSS and local-domain extrapolations. While local-scale extrapolations predict heights inconsistent with prior measurements, the agreement between observations and the PFSS model indicates that "EIT waves" are a global phenomenon influenced by global-scale magnetic field.

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arXiv2011

LEMUR: Large European Module for solar Ultraviolet Research. European contribution to JAXA's Solar-C mission

Understanding the solar outer atmosphere requires concerted, simultaneous solar observations from the visible to the vacuum ultraviolet (VUV) and soft X-rays, at high spatial resolution (between 0.1" and 0.3"), at high temporal resolution (on the order of 10 s, i.e., the time scale of chromospheric dynamics), with a wide temperature coverage (0.01 MK to 20 MK, from the chromosphere to the flaring corona), and the capability of measuring magnetic fields through spectropolarimetry at visible and near-infrared wavelengths. Simultaneous spectroscopic measurements sampling the entire temperature range are particularly important. These requirements are fulfilled by the Japanese Solar-C mission (Plan B), composed of a spacecraft in a geosynchronous orbit with a payload providing a significant improvement of imaging and spectropolarimetric capabilities in the UV, visible, and near-infrared with respect to what is available today and foreseen in the near future. The Large European Module for solar Ultraviolet Research (LEMUR), described in this paper, is a large VUV telescope feeding a scientific payload of high-resolution imaging spectrographs and cameras. LEMUR consists of two major components: a VUV solar telescope with a 30 cm diameter mirror and a focal length of 3.6 m, and a focal-plane package composed of VUV spectrometers covering six carefully chosen wavelength ranges between 17 and 127 nm. The LEMUR slit covers 280" on the Sun with 0.14" per pixel sampling. In addition, LEMUR is capable of measuring mass flows velocities (line shifts) down to 2 km/s or better. LEMUR has been proposed to ESA as the European contribution to the Solar C mission.

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