Eruptions from coronal hole bright points: Observations and non-potential modelling

Context. We report on the third part of a series of studies on eruptions associated with small-scale loop complexes named coronal bright points (CBPs). Aims. A single case study of a CBP in an equatorial coronal hole with an exceptionally large size is investigated to expand on our understanding of the formation of mini-filaments, their destabilisation, and the origin of the eruption triggering the formation of jet-like features recorded in extreme ultraviolet (EUV) and X-ray emission. We aim to explore the nature of the so-called micro-flares in CBPs associated with jets in coronal holes and mini coronal mass ejections in the quiet Sun. Methods. Co-observations from the Atmospheric Imaging Assembly (AIA) and Helioseismic Magnetic Imager (HMI) on board the Solar Dynamics Observatory as well as GONG Hα images are used together with a non-linear force free field (NLFFF) relaxation approach, where the latter is based on a time series of HMI line-of-sight magnetograms. Results. A mini-filament (MF) that formed beneath the CBP arcade about 3−4 h before the eruption is seen in the Hα and EUV AIA images to lift up and erupt triggering the formation of an X-ray jet. No significant photospheric magnetic flux concentration displacement (convergence) is observed and neither is magnetic flux cancellation between the two main magnetic polarities forming the CBP in the time period leading to MF lift-off. The CBP micro-flare is associated with three flare kernels that formed shortly after the MF lift-off. No observational signature is found for magnetic reconnection beneath the erupting MF. The applied NLFFF modelling successfully reproduces both the CBP loop complex as well as the magnetic flux rope that hosts the MF during the build-up to the eruption. Conclusions. The applied NLFFF modelling is able to clearly show that an initial potential field can be evolved into a non-potential magnetic field configuration that contains free magnetic energy in the region that observationally hosts the eruption. The comparison of the magnetic field structure shows that the magnetic NLFFF model contains many of the features that can explain the different observational signatures found in the evolution and eruption of the CBP. In the future, it may eventually indicate the location of destabilisation that results in the eruptions of flux ropes.

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On the partial eruption of a bifurcated solar filament structure

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2902 ◽

2020 ◽

Vol 500 (1) ◽

pp. 684-695

Author(s):

Aabha Monga ◽

Rahul Sharma ◽

Jiajia Liu ◽

Consuelo Cid ◽

Wahab Uddin ◽

...

Keyword(s):

Magnetic Field ◽

Extreme Ultraviolet ◽

Free Field ◽

Small Scale ◽

Data Sets ◽

Imaging Data ◽

Filament Structure ◽

Filament Channel ◽

Solar Filament ◽

Using Data

ABSTRACT The partial eruption of a filament channel with bifurcated substructures is investigated using data sets obtained from both ground-based and space-borne facilities. Small-scale flux reconnection/cancellation events in the region triggered the pile-up of ambient magnetic field, observed as bright extreme ultraviolet (EUV) loops in close proximity to the filament channel. This led to the formation of a V-shaped cusp structure at the site of interaction between the coalesced EUV loops and the filament channel, with the presence of distinct plasmoid structures and associated bidirectional flows. Analysis of imaging data from SDO/AIA further suggests vertical splitting of the filament structure into two substructures. The perturbed upper branch of the filament structure rose up and erupted with the onset of an energetic GOES M1.4 flare at 04:30 ut on 2015 January 28. The estimated twist number and squashing factor obtained from non-linear force free-field extrapolation of the magnetic field data support the vertical split in the filament structure with high twist in the upper substructure. The loss in equilibrium of the upper branch due to torus instability implies that this is a potential triggering mechanism for the observed partial eruption.

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Photospheric magnetic structure of coronal holes

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935918 ◽

2019 ◽

Vol 629 ◽

pp. A22 ◽

Cited By ~ 6

Author(s):

Stefan J. Hofmeister ◽

Dominik Utz ◽

Stephan G. Heinemann ◽

Astrid Veronig ◽

Manuela Temmer

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Coronal Hole ◽

Magnetic Structure ◽

Coronal Holes ◽

Line Of Sight ◽

Magnetic Elements ◽

Background Magnetic Field ◽

The Individual ◽

The Magnetic Field

In this study, we investigate in detail the photospheric magnetic structure of 98 coronal holes using line-of-sight magnetograms of SDO/HMI, and for a subset of 42 coronal holes using HINODE/SOT G-band filtergrams. We divided the magnetic field maps into magnetic elements and quiet coronal hole regions by applying a threshold at ±25 G. We find that the number of magnetic bright points in magnetic elements is well correlated with the area of the magnetic elements (cc = 0.83 ± 0.01). Further, the magnetic flux of the individual magnetic elements inside coronal holes is related to their area by a power law with an exponent of 1.261 ± 0.004 (cc = 0.984 ± 0.001). Relating the magnetic elements to the overall structure of coronal holes, we find that on average (69 ± 8)% of the overall unbalanced magnetic flux of the coronal holes arises from long-lived magnetic elements with lifetimes > 40 h. About (22 ± 4)% of the unbalanced magnetic flux arises from a very weak background magnetic field in the quiet coronal hole regions with a mean magnetic field density of about 0.2−1.2 G. This background magnetic field is correlated to the flux of the magnetic elements with lifetimes of > 40 h (cc = 0.88 ± 0.02). The remaining flux arises from magnetic elements with lifetimes < 40 h. By relating the properties of the magnetic elements to the overall properties of the coronal holes, we find that the unbalanced magnetic flux of the coronal holes is completely determined by the total area that the long-lived magnetic elements cover (cc = 0.994 ± 0.001).

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Variations of the Coronal Radiation in X-ray Related to Coronal Holes, Active Region Loop Systems, Bright Points

International Astronomical Union Colloquium ◽

10.1017/s0252921100024660 ◽

1994 ◽

Vol 143 ◽

pp. 159-171

Author(s):

Ester Antonucci

Keyword(s):

Magnetic Field ◽

Spatial Distribution ◽

Large Scale ◽

Coronal Holes ◽

Active Regions ◽

Coronal Magnetic Field ◽

Small Scale ◽

X Ray ◽

Coronal Structures

The coronal features observed in X-ray emission, varying from the small-scale, short-lived bright points to the large-scale, long-lived coronal holes, are closely associated with the coronal magnetic field and its topology, and their variability depends strongly on the solar cycle. Here we discuss the spatial distribution of the coronal structures, the frequency distribution of the brightness variations in active regions, and the role of magnetic reconnection in determining the variability of the coronal features, on the basis of the new observations of the soft X-ray emission recently obtained with the Yohkoh satellite and the NIXT experiment.

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Flux rope, hyperbolic flux tube, and late extreme ultraviolet phases in a non-eruptive circular-ribbon flare

Astronomy and Astrophysics ◽

10.1051/0004-6361/201629654 ◽

2017 ◽

Vol 604 ◽

pp. A76 ◽

Cited By ~ 23

Author(s):

Sophie Masson ◽

Étienne Pariat ◽

Gherardo Valori ◽

Na Deng ◽

Chang Liu ◽

...

Keyword(s):

Magnetic Field ◽

Flux Tube ◽

Extreme Ultraviolet ◽

Topological Analysis ◽

Perfect Match ◽

Free Field ◽

Flux Rope ◽

Null Point ◽

Second Phase ◽

Flare Loops

Context. The dynamics of ultraviolet (UV) emissions during solar flares provides constraints on the physical mechanisms involved in the trigger and the evolution of flares. In particular it provides some information on the location of the reconnection sites and the associated magnetic fluxes. In this respect, confined flares are far less understood than eruptive flares generating coronal mass ejections. Aims. We present a detailed study of a confined circular flare dynamics associated with three UV late phases in order to understand more precisely which topological elements are present and how they constrain the dynamics of the flare. Methods. We perform a non-linear force-free field extrapolation of the confined flare observed with the Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly (AIA) instruments on board Solar Dynamics Observatory (SDO). From the 3D magnetic field we compute the squashing factor and we analyse its distribution. Conjointly, we analyse the AIA extreme ultraviolet (EUV) light curves and images in order to identify the post-flare loops, and their temporal and thermal evolution. By combining the two analyses we are able to propose a detailed scenario that explains the dynamics of the flare. Results. Our topological analysis shows that in addition to a null-point topology with the fan separatrix, the spine lines and its surrounding quasi-separatix layer (QSL) halo (typical for a circular flare), a flux rope and its hyperbolic flux tube (HFT) are enclosed below the null. By comparing the magnetic field topology and the EUV post-flare loops we obtain an almost perfect match between the footpoints of the separatrices and the EUV 1600 Å ribbons and between the HFT field line footpoints and bright spots observed inside the circular ribbons. We show, for the first time in a confined flare, that magnetic reconnection occurred initially at the HFT below the flux rope. Reconnection at the null point between the flux rope and the overlying field is only initiated in a second phase. In addition, we showed that the EUV late phase observed after the main flare episode is caused by the cooling loops of different length which have all reconnected at the null point during the impulsive phase. Conclusions. Our analysis shows in one example that flux ropes are present in null-point topology not only for eruptive and jet events, but also for confined flares. This allows us to conjecture on the analogies between conditions that govern the generation of jets, confined flares or eruptive flares.

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A Model of Solar X-ray Bright Points and Ephemeral Active Regions

Australian Journal of Physics ◽

10.1071/ph790671 ◽

1979 ◽

Vol 32 (6) ◽

pp. 671 ◽

Cited By ~ 2

Author(s):

JH Piddington

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Large Scale ◽

Flux Rope ◽

Active Regions ◽

Activity Cycle ◽

Solar Magnetic Fields ◽

Flux Tubes ◽

X Ray ◽

Field System

Solar ephemeral active regions may provide a larger amount of emerging magnetic flux than the active regions themselves, and the origin and disposal of this flux pose problems. The related X-ray bright points are a major feature of coronal dynamics, and the two phenomena may entail a revision of our ideas of the activity cycle. A new large-scale subsurface magnetic field system has been suggested, but it is shown that such a system is neither plausible nor necessary. The emerging magnetic bipoles merely represent loops in pre-existing vertical flux tubes which are parts of active regions or the remnants of active regions. These loops result from the kink (or helical) instability in a twisted flux tube. Their observed properties are explained in terms of the flux-rope theory of solar fields. The model is extended to some dynamical effects in emerging loops. Further observations of ephemeral active regions may provide important tests between the traditional and flux-rope theories of solar magnetic fields.

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Statistical Analysis and Catalog of Non-polar Coronal Holes Covering the SDO-Era Using CATCH

Solar Physics ◽

10.1007/s11207-019-1539-y ◽

2019 ◽

Vol 294 (10) ◽

Cited By ~ 11

Author(s):

Stephan G. Heinemann ◽

Manuela Temmer ◽

Niko Heinemann ◽

Karin Dissauer ◽

Evangelia Samara ◽

...

Keyword(s):

Magnetic Field ◽

Coronal Hole ◽

High Speed ◽

Extreme Ultraviolet ◽

Coronal Holes ◽

Central Meridian ◽

Solar Dynamics Observatory ◽

Speed Solar Wind ◽

Solar Dynamics ◽

Hole Boundary

Abstract Coronal holes are usually defined as dark structures seen in the extreme ultraviolet and X-ray spectrum which are generally associated with open magnetic fields. Deriving reliably the coronal hole boundary is of high interest, as its area, underlying magnetic field, and other properties give important hints as regards high speed solar wind acceleration processes and compression regions arriving at Earth. In this study we present a new threshold-based extraction method, which incorporates the intensity gradient along the coronal hole boundary, which is implemented as a user-friendly SSW-IDL GUI. The Collection of Analysis Tools for Coronal Holes (CATCH) enables the user to download data, perform guided coronal hole extraction and analyze the underlying photospheric magnetic field. We use CATCH to analyze non-polar coronal holes during the SDO-era, based on 193 Å filtergrams taken by the Atmospheric Imaging Assembly (AIA) and magnetograms taken by the Heliospheric and Magnetic Imager (HMI), both on board the Solar Dynamics Observatory (SDO). Between 2010 and 2019 we investigate 707 coronal holes that are located close to the central meridian. We find coronal holes distributed across latitudes of about ${\pm}\, 60^{\circ}$±60∘, for which we derive sizes between $1.6 \times 10^{9}$1.6×109 and $1.8 \times 10^{11}\mbox{ km}^{2}$1.8×1011 km2. The absolute value of the mean signed magnetic field strength tends towards an average of $2.9\pm 1.9$2.9±1.9 G. As far as the abundance and size of coronal holes is concerned, we find no distinct trend towards the northern or southern hemisphere. We find that variations in local and global conditions may significantly change the threshold needed for reliable coronal hole extraction and thus, we can highlight the importance of individually assessing and extracting coronal holes.

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How to Estimate the Far-Side Open Flux Using STEREO Coronal Holes

Solar Physics ◽

10.1007/s11207-021-01889-z ◽

2021 ◽

Vol 296 (9) ◽

Author(s):

Stephan G. Heinemann ◽

Manuela Temmer ◽

Stefan J. Hofmeister ◽

Aleksandar Stojakovic ◽

Laurent Gizon ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Coronal Hole ◽

Transition Region ◽

Coronal Holes ◽

Magnetic Flux Density ◽

Solar Dynamics Observatory ◽

Aging Effects ◽

Flux Transport ◽

Open Flux

AbstractGlobal magnetic field models use as input synoptic data, which usually show “aging effects” as the longitudinal $360^{\circ }$ 360 ∘ information is not obtained simultaneously. Especially during times of increased solar activity, the evolution of the magnetic field may yield large uncertainties. A significant source of uncertainty is the Sun’s magnetic field on the side of the Sun invisible to the observer. Various methods have been used to complete the picture: synoptic charts, flux-transport models, and far side helioseismology. In this study, we present a new method to estimate the far-side open flux within coronal holes using STEREO EUV observations. First, we correlate the structure of the photospheric magnetic field as observed with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory (HMI/SDO) with features in the transition region. From the 304 Å intensity distribution, which we found to be specific to coronal holes, we derive an empirical estimate for the open flux. Then we use a large sample of 313 SDO coronal hole observations to verify this relation. Finally, we perform a cross-instrument calibration from SDO to STEREO data to enable the estimation of the open flux at solar longitudes not visible from Earth. We find that the properties of strong unipolar magnetic elements in the photosphere, which determine the coronal hole’s open flux, can be approximated by open fields in the transition region. We find that structures below a threshold of $78\%$ 78 % (STEREO) or $94\%$ 94 % (SDO) of the solar disk median intensity as seen in 304 Å filtergrams are reasonably well correlated with the mean magnetic flux density of coronal holes (cc$_{\mathrm{sp}} = 0.59$ = sp 0.59 ). Using the area covered by these structures ($A_{\mathrm{OF}}$ A OF ) and the area of the coronal hole ($A_{\mathrm{CH}}$ A CH ), we model the open magnetic flux of a coronal hole as $|\Phi _{\mathrm{CH}}| = 0.25 A_{\mathrm{CH}}~\mathrm{exp}(0.032 A_{\mathrm{OF}})$ | Φ CH | = 0.25 A CH exp ( 0.032 A OF ) with an estimated uncertainty of 40 to $60\%$ 60 % .

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Evolution of magnetic field corresponding to X-ray brightening events in coronal holes and quiet Sun

Proceedings of the International Astronomical Union ◽

10.1017/s174392131300241x ◽

2012 ◽

Vol 8 (S294) ◽

pp. 155-156 ◽

Cited By ~ 1

Author(s):

Zhenghua Huang ◽

Maria Madjarska ◽

Gerry Doyle ◽

Derek Lamb

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Coronal Holes ◽

Magnetic Behaviour ◽

Field Structure ◽

Flux Emergence ◽

X Ray ◽

Quiet Sun ◽

Cancellation Rate ◽

The Magnetic Field

AbstractWe study the magnetic field structure and evolution for 26 X-ray brightening events in coronal holes and quiet Sun regions, including bright points and jets. We found that all brightening events are associated with bipolar regions and caused by magnetic flux emergence followed by cancellation. The emission fluctuations seen in the X-ray bright points are associated with reoccurring magnetic cancellation in the footpoints. An X-ray jet presents similar magnetic behaviour in the footpoints but its magnetic flux cancellation rate is much higher than in the bright point. Comparing coronal holes and the quiet Sun, we do not find differences in their corresponding magnetic field behavior.

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Small-scale Magnetic Flux Ropes with Field-aligned Flows via the PSP In-situ Observations

10.5194/egusphere-egu21-8043 ◽

2021 ◽

Author(s):

Yu Chen ◽

Qiang Hu ◽

Lingling Zhao

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Flux Rope ◽

Local Magnetic Field ◽

Small Scale ◽

Magnetic Flux Rope ◽

Magnetic Flux Ropes ◽

Flux Ropes ◽

Field Lines

<p>Magnetic flux rope, formed by the helical magnetic field lines, can sometimes remain its shape while carrying significant plasma flow that is aligned with the local magnetic field. We report the existence of such structures and static flux ropes by applying the Grad-Shafranov-based algorithm to the Parker Solar Probe (PSP) in-situ measurements in the first five encounters. These structures are detected at heliocentric distances, ranging from 0.13 to 0.66 au, in a total of 4-month time period. We find that flux ropes with field-aligned flows have certain properties similar to those of static flux ropes, such as the decaying relations of the magnetic fields within structures with respect to heliocentric distances. Moreover, these events are more likely with magnetic pressure dominating over the thermal pressure and occurring more frequently in the relatively fast-speed solar wind. Taking into account the high Alfvenicity, we also compare these events with switchbacks and present the cross-section maps via the new Grad-Shafranov type reconstruction. Finally, the possible evolution and relaxation of the magnetic flux rope structures are discussed.</p>

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Coronal Magnetic Field Measurements along a Partially Erupting Filament in a Solar Flare

The Astrophysical Journal ◽

10.3847/1538-4357/ac2f99 ◽

2021 ◽

Vol 923 (2) ◽

pp. 213

Author(s):

Yuqian Wei ◽

Bin Chen ◽

Sijie Yu ◽

Haimin Wang ◽

Ju Jing ◽

...

Keyword(s):

Magnetic Field ◽

Solar Flare ◽

Magnetic Flux ◽

Extreme Ultraviolet ◽

Flux Rope ◽

Solar Array ◽

Owens Valley ◽

Microwave Source ◽

Flux Ropes ◽

The Magnetic Field

Abstract Magnetic flux ropes are the centerpiece of solar eruptions. Direct measurements for the magnetic field of flux ropes are crucial for understanding the triggering and energy release processes, yet they remain heretofore elusive. Here we report microwave imaging spectroscopy observations of an M1.4-class solar flare that occurred on 2017 September 6, using data obtained by the Expanded Owens Valley Solar Array. This flare event is associated with a partial eruption of a twisted filament observed in Hα by the Goode Solar Telescope at the Big Bear Solar Observatory. The extreme ultraviolet (EUV) and X-ray signatures of the event are generally consistent with the standard scenario of eruptive flares, with the presence of double flare ribbons connected by a bright flare arcade. Intriguingly, this partial eruption event features a microwave counterpart, whose spatial and temporal evolution closely follow the filament seen in Hα and EUV. The spectral properties of the microwave source are consistent with nonthermal gyrosynchrotron radiation. Using spatially resolved microwave spectral analysis, we derive the magnetic field strength along the filament spine, which ranges from 600 to 1400 Gauss from its apex to the legs. The results agree well with the nonlinear force-free magnetic model extrapolated from the preflare photospheric magnetogram. We conclude that the microwave counterpart of the erupting filament is likely due to flare-accelerated electrons injected into the filament-hosting magnetic flux rope cavity following the newly reconnected magnetic field lines.

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