Flux Rope Coronal Mass Ejections Research Articles

Spectropolarimetric Radio Imaging of Faint Gyrosynchrotron Emission from a CME: A Possible Indication of the Insufficiency of Homogeneous Models

The geo-effectiveness of coronal mass ejections (CMEs) is determined primarily by their magnetic fields. Modeling of gyrosynchrotron (GS) emission is a promising remote sensing technique to measure the CME magnetic field at coronal heights. However, faint GS emission from CME flux ropes is hard to detect in the presence of bright solar emission from the solar corona. With high dynamic-range spectropolarimetric meter wavelength solar images provided by the Murchison Widefield Array, we have detected faint GS emission from a CME out to ∼8.3 R ⊙, the largest heliocentric distance reported to date. High-fidelity polarimetric calibration also allowed us to robustly detect circularly polarized emission from GS emission. For the first time in the literature, Stokes V detection has jointly been used with Stokes I spectra to constrain GS models. One expects that the inclusion of polarimetric measurement will provide tighter constraints on the GS model parameters. Instead, we found that homogeneous GS models, which have been used in all prior works, are unable to model both the total intensity and circular polarized emission simultaneously. This strongly suggests the need for using inhomogeneous GS models to robustly estimate the CME magnetic field and plasma parameters.

Internal magnetic field structures observed by PSP/WISPR in a filament-related coronal mass ejection

We investigated the coronal mass ejection (CME) related to an eruptive filament over the southwestern solar limb on December 8, 2022, at around 8 UT. We tracked localized density enhancements reflecting the magnetic structures using white-light data taken with the Wide-field Instrument for Solar PRobe (WISPR) aboard the Parker Solar Probe (PSP). We aim to investigate the 3D location, morphology and evolution of the internal magnetic fine structures of CMEs. Specifically, we focused on the physical origin of the features in the WISPR images, how the white-light structures evolve over time, and their relationship with the source region, filament, and the flux rope. The fast tangential motion of the PSP spacecraft during its perihelion permits a single event to be viewed from multiple angles in short times relative to the event's evolution. Hence, three-dimensional information of selected CME features can be derived from this single spacecraft using triangulation techniques. We grouped small-scale structures with roughly similar speeds, longitude, and latitude into three distinct morphological groups. We found twisted magnetic field patterns close to the eastern leg of the CME that may be related to "horns" outlining the edges of the flux-rope cavity. We identified aligned thread-like bundles close to the western leg, and they may be related to confined density enhancements evolving during the filament eruption. High density blob-like features (magnetic islands) are widely spread in longitude (sim 40°) close to the flanks and the rear part of the CME. We also note that the large-scale outer envelope of the CME, seen clearly from 1 AU, was not well observed by PSP. We demonstrate that CME flux ropes, apart from the blobs, may comprise different morphological groups with a cluster behavior; the blobs instead span a wide range of longitudes. This finding may hint at either the three-dimensionality of the post-CME current sheet (CS) or the influence of the ambient corona in the evolutionary behavior of the CS. Importantly, we show that the global appearance of the CME can be very different in WISPR (0.11--0.16 AU) and the instruments near 1 AU because of the shorter line-of-sight integration of WISPR.

Identifying footpoints of pre-eruptive and coronal mass ejection flux ropes with sunspot scars

Context. The properties of pre-eruptive structures and coronal mass ejections (CMEs) are characterized by those of their footpoints, the latter of which attract a great deal of interest. However, the matter of how to identify the footpoints of pre-eruptive structures and how to do so with the use of ground-based instruments still remains elusive. Aims. In this work, we study an arc-shaped structure intruding in the sunspot umbra. It is located close to the (pre-)eruptive flux rope footpoint and it is expected to help identify the footpoint. Methods. We analyzed this arc-shaped structure, which we call a “sunspot scar”, in a CME event on July 12, 2012, and in two CME events from observationally inspired magnetohydrodynamic simulations performed by OHM and MPI-AMRVAC. Results. The sunspot scar displays a more inclined magnetic field with a weaker vertical component and a stronger horizontal component relative to that in the surrounding umbra and is manifested as a light bridge in the white light passband. The hot field lines anchored in the sunspot scar are spatially at the transition between the flux rope and the background coronal loops and temporally in the process of the slipping reconnection which builds up the flux rope. Conclusions. The sunspot scar and its related light bridge mark the edge of the CME flux rope footpoint and particularly indicate the edge of the pre-eruptive flux rope footpoint in the framework of “pre-eruptive structures being flux ropes”. Therefore, they provide a new perspective for the identification of pre-eruptive and CME flux rope footpoints, as well as new methods for studying the properties and evolution of pre-eruptive structures and CMEs with photospheric observations only.

Modeling the propagation of coronal mass ejections with COCONUT: Implementation of the regularized Biot-Savart law flux rope model

Coronal mass ejections (CMEs) are rapid eruptions of magnetized plasma that occur on the Sun. They are known to be the main drivers of adverse space weather. The accurate tracking of their evolution in the heliosphere in numerical models is of the utmost importance for space weather forecasting. The main objective of this paper is to implement the regularized Biot-Savart law (RBSL) method in a new global corona model, called COCONUT. This approach has the capability to construct the magnetic flux rope with an axis of arbitrary shape. We present the implementation process of the RBSL flux rope model in COCONUT, which is superposed onto a realistic solar wind reconstructed from the observed magnetogram around the minimum of solar activity. Based on this, we simulate the propagation of an S-shaped flux rope from the solar surface to a distance of 25$R_ odot Our simulation successfully reproduces the birth process of a CME originating from a sigmoid in a self-consistent way. The model effectively captures various physical processes and retrieves the prominent features of the CMEs in observations. In addition, the simulation results indicate that the magnetic topology of the CME flux rope at around 20$R_ odot $ deviates from a coherent structure and manifests as a mix of open and closed field lines with diverse footpoints. This work demonstrates the potential of the RBSL flux rope model in reproducing CME events that are more consistent with observations. Moreover, our findings strongly suggest that magnetic reconnection during the CME propagation plays a critical role in destroying the coherent characteristics of a CME flux rope.

SoloHI observations of coronal mass ejections observed by multiple spacecraft

Context. The Solar Orbiter Heliospheric Imager (SoloHI) instrument of the Solar Orbiter mission is a next-generation heliospheric imager. New observations from SoloHI demonstrate the improved spatial and temporal resolution compared to previous observations of the heliosphere and corona. At perihelion, the field of view (FoV) of SoloHI covers the transition between the coronagraph (COR2) and heliospheric imager (HI1) Sun-Earth Connection Coronal and Heliospheric Investigation (SECCHI) suite. In this paper, we focus on an active solar period following the first Solar Orbiter science perihelion that resulted in a number of well-observed large coronal mass ejections (CMEs) in SoloHI data in March and April 2022. Specifically, we highlight a series of events produced by AR12795 between 28 March and 2 April and show overlapping observations with SECCHI/COR2 and HI1 and LASCO/C3. Aims. We compare the performance of the SoloHI instrument against similar observations from 1 au imagers. We describe CME observations, highlighting the unique structural features captured within the SoloHI FoV. These observations demonstrate that SoloHI will provide new insights into CME morphology and evolution from a unique vantage point. Methods. To provide a direct and relevant comparison, images from all the telescopes we used in the paper are presented in FoVs common to each and with minimal processing applied. The J-maps we used to highlight outflowing features are also presented to show that the CME kinematics can be tracked through the SoloHI FoV, and also to report how the rest of the Heliophysics Systems Observatory (HSO) can be used to support the SoloHI data. Results. The high-resolution SoloHI images of these eruptions, taken from ∼0.3 au, reveal a number of detailed structural CME features, including internal cavities or cores of the CME flux rope(s). They also show the surrounding material and associated sheath region of the compressed upstream solar wind plasma. Many features that could not have been observed by other instruments are highlighted and discussed. Conclusions. The SoloHI instrument is performing well and has already provided detailed observations of CMEs that can help us understand the details of the internal structure and magnetic field of CMEs. These new observations in combination with synoptic observations from 1 au offer new opportunities for CME propagation from the corona to the heliosphere.

A Time-efficient, Data-driven Modeling Approach for Predicting the Geomagnetic Impact of Coronal Mass Ejections

To understand the global-scale physical processes behind coronal mass ejection (CME)–driven geomagnetic storms and predict their intensity as a space weather forecasting measure, we develop an interplanetary CME flux rope–magnetosphere interaction module using 3D magnetohydrodynamics. The simulations adequately describe CME-forced dynamics of the magnetosphere including the imposed magnetotail torsion. These interactions also result in induced currents, which are used to calculate the geomagnetic perturbation. Through a suitable calibration, we estimate a proxy of geoeffectiveness—the Storm Intensity index (STORMI)—that compares well with the Dst/SYM-H index. Simulated impacts of two contrasting CMEs quantified by the STORMI index exhibit a high linear correlation with the corresponding Dst and SYM-H indices. Our approach is relatively simple, has fewer parameters to be fine-tuned, and is time efficient compared to complex fluid-kinetic methods. Furthermore, we demonstrate that flux rope erosion does not significantly affect our results. Thus our method has the potential to significantly extend the time window for predictability—an outstanding challenge in geospace environment forecasting—if early predictions of near-Earth CME flux rope structures based on near-Sun observations are available as inputs. This study paves the way for early warnings based on operational predictions of CME-driven geomagnetic storms.

Numerical Research on the Effect of the Initial Parameters of CME Flux-rope Model on Simulation Results. III. Different Initial Energy of CMEs

In numerical studies, the initial parameters of coronal mass ejections (CMEs) have great influence on the simulation results. In our previous work, it has been proved that when the initial velocity is constant, the initial total mass mainly determines the propagation of the CME. On this basis, we carry out further research from the perspective of CME initial energy. We introduced a graduated cylindrical shell model into a 3D interplanetary total variation diminishing magnetohydrodynamic model to study the effect of different parameters of CMEs on simulation results. In this paper, we simulate several CME cases with different initial parameters and study the simulation results with a different initial energy composition. Actually, in interplanetary space, the kinetic energy of the CME always plays a dominant role. In order to study the effect of the initial thermal energy and magnetic energy on the propagation process of the CME, in this simulation, we adjust the initial parameters to make the thermal energy and magnetic energy reach the same level as the kinetic energy or an even higher level. Our results show that the initial total energy of the CME basically determines its arrival time at Earth, which indicates that the kinetic energy, thermal energy, and magnetic energy have similar effects on the propagation of the CMEs. Moreover, when the total energy keeps constant, the decrease of initial density will lead to the enhancement of CME expansion, which may make the front of the CME reach Earth earlier.

Overexpansion-dominated coronal mass ejection formation and induced radio bursts

Aims. Coronal mass ejections (CMEs) are the most fascinating explosions in the Solar System. Their formation is still not fully understood, however. Methods. We investigated a well-observed CME on 2021 May 7 that showed a typical three-component structure and was continuously observed from 0 to 3 R⊙ by a combination of SDO/AIA (0–1.3 R⊙), PROBA2/SWAP (0–1.7 R⊙), and MLSO/K-Cor (1.05–3 R⊙). Furthermore, we compared the morphological discrepancy between the CME white-light bright core and the extreme-UV (EUV) blob. We finally explored the origin of various radio bursts that are closely related to the interaction of the CME overexpansion with a nearby streamer. Results. An interesting finding is that the height increases of the CME leading front and of the bright core are dominated by the overexpansion during the CME formation. The aspect ratios of the CME bubble and bright core, quantifying the overexpansion, are found to decrease as the SO/STIX 4–10 keV and GOES 1–8 Å soft X-ray flux of the associated flare increases near the peaks. This indicates that the flare reconnection plays an important role in the first overexpansion. The CME bubble even undergoes a second overexpansion, although it is relatively weak, which is closely related to the compression with a nearby streamer and likely arises from an ideal magnetohydrodynamics process. Moreover, the CME EUV blob is found to be relatively lower and wider than the CME white-light bright core, which may correspond to the bottom part of the growing CME flux rope. The interaction between the CME and the streamer leads to two type II radio bursts, one that is drifting normally and another that is stationary, which are speculated to be induced by two different sources of the CME-driven shock front. The bidirectional electrons shown in series of C-shaped type III bursts suggest that the interchange reconnection is also involved during the interaction of the CME and streamer.

Effect of the Heliospheric State on CME Evolution

The culmination of solar cycle 24 by the end of 2019 has created the opportunity to compare the differing properties of coronal mass ejections (CMEs) between two whole solar cycles: solar cycle 23 (SC 23) and solar cycle 24 (SC 24). We report on the width evolution of limb CMEs in SCs 23 and 24 in order to test the suggestion by Gopalswamy et al. that CME flux ropes attain pressure balance at larger heliocentric distances in SC 24. We measure CME width as a function of heliocentric distance for a significantly large number of limb CMEs (∼1000) and determine the distances where the CMEs reach constant width in each cycle. We introduced a new parameter, the transition height (hc) of a CME, defined as the critical heliocentric distance beyond which the CME width stabilizes to a quasi-constant value. Cycle and phase-to-phase comparisons are based on this new parameter. We find that the average value of hc in SC 24 is 62% higher than that in SC 23. SC 24 CMEs attain their peak width at larger distances from the Sun than SC 23 CMEs do. The enhanced transition height in SC 24 is new observational ratification of the anomalous expansion. The anomalous expansion of SC 24 CMEs, which is caused by the weak state of the heliosphere, accounts for the larger heliocentric distance where the pressure balance between CME flux rope and the ambient medium is attained.

Multispacecraft Remote Sensing and In Situ Observations of the 2020 November 29 Coronal Mass Ejection and Associated Shock: From Solar Source to Heliospheric Impacts

We investigate the source eruption, propagation and expansion characteristics, and heliospheric impacts of the 2020 November 29 coronal mass ejection (CME) and associated shock, using remote sensing and in situ observations from multiple spacecraft. A potential-field source-surface model is employed to examine the coronal magnetic fields surrounding the source region. The CME and associated shock are tracked from the early stage to the outer corona using extreme ultraviolet and white light observations. Forward models are applied to determine the structures and kinematics of the CME and the shock near the Sun. The shock shows an ellipsoidal structure, expands in all directions, and encloses the whole Sun as viewed from both SOHO and STEREO A, which results from the large expansion of the CME flux rope and its fast acceleration. The structure and potential impacts of the shock are mainly determined by its radial and lateral expansions. The CME and shock arrive at Parker Solar Probe and STEREO A. Only based on the remote sensing observations, it is difficult to predict whether and when the CME/shock would arrive at the Earth. Combining Wind in situ measurements and Wang–Sheeley–Arge-ENLIL simulation results, we confirm that the far flank of the CME (or the CME leg) arrives at the Earth with no shock signature. These results highlight the importance of multipoint remote sensing and in situ observations for determining the heliospheric impacts of CMEs.

What Is Unusual About the Third Largest Geomagnetic Storm of Solar Cycle 24?

AbstractWe report on the solar and interplanetary (IP) causes of the third largest geomagnetic storm (26 August 2018) in solar cycle 24. The underlying coronal mass ejection (CME) originating from a quiescent filament region becomes a 440 km/s magnetic cloud (MC) at 1 au after ∼5 days. The prolonged CME acceleration (for ∼24 hr) coincides with the time profiles of the post‐eruption arcade intensity and reconnected flux. Chen et al. (2019, https://doi.org/10.3847/1538-4357/ab3f36) obtain a lower speed since they assumed that the CME does not accelerate after ∼12 hr. The presence of multiple coronal holes near the filament channel and the high‐speed wind from them seem to have the combined effect of producing complex rotation in the corona and IP medium resulting in a high‐inclination MC. The Dst time profile in the main phase steepens significantly (rapid increase in storm intensity) coincident with the density increase (prominence material) in the second half of the MC. Simulations using the Comprehensive Inner Magnetosphere‐Ionosphere model show that a higher ring current energy results from larger dynamic pressure (density) in MCs. Furthermore, the Dst index is highly correlated with the main‐phase time integral of the ring current injection that includes density, consistent with the simulations. A complex temporal structure develops in the storm main phase if the underlying MC has a complex density structure during intervals of southward IP magnetic field. We conclude that the high intensity of the storm results from the prolonged CME acceleration, complex rotation of the CME flux rope, and the high density in the 1‐au MC.

Coronal Mass Ejections and Dimmings: A Comparative Study Using MHD Simulations and SDO Observations

Abstract Solar coronal dimmings have been observed extensively in recent years. Due to their close association with coronal mass ejections (CMEs), there is a critical need to improve our understanding of the physical processes that cause dimmings as well as their relationship with CMEs. In this study, we investigate coronal dimmings by combining simulation and observational efforts. By utilizing a data-constrained global magnetohydrodynamics model (Alfvén-wave solar model), we simulate coronal dimmings resulting from different CME energetics and flux rope configurations. We synthesize the emissions of different EUV spectral bands/lines and compare with SDO/AIA and EVE observations. A detailed analysis of the simulation and observation data suggests that the transient dimming/brightening are related to plasma heating processes, while the long-lasting core and remote dimmings are caused by mass-loss process induced by the CME. Moreover, the interaction between the erupting flux rope with different orientations and the global solar corona could significantly influence the coronal dimming patterns. Using metrics such as dimming depth and dimming slope, we investigate the relationship between dimmings and CME properties (e.g., CME mass, CME speed) in the simulation. Our result suggests that coronal dimmings encode important information about the associated CMEs, which provides a physical basis for detecting stellar CMEs from distant solar-like stars.

Constraining the CME Core Heating and Energy Budget with SOHO/UVCS

Abstract We describe the energy budget of a coronal mass ejection (CME) observed on 1999 May 17 with the Ultraviolet Coronagraph Spectrometer (UVCS). We constrain the physical properties of the CME’s core material as a function of height along the corona by using the spectra taken by the single-slit coronagraph spectrometer at heliocentric distances of 2.6 and 3.1 solar radii. We use plasma diagnostics from intensity ratios, such as the O vi doublet lines, to determine the velocity, density, temperature, and nonequilibrium ionization states. We find that the CME core’s velocity is approximately 250 km s−1, and its cumulative heating energy is comparable to its kinetic energy for all of the plasma heating parameterizations that we investigated. Therefore, the CME’s unknown heating mechanisms have the energy to significantly affect the CME’s eruption and evolution. To understand which parameters might influence the unknown heating mechanism, we constrain our model heating rates with the observed data and compare them to the rate of heating generated within a similar CME that was constructed by the MAS code’s 3D MHD simulation. The rate of heating from the simulated CME agrees with our observationally constrained heating rates when we assume a quadratic power law to describe a self-similar CME expansion. Furthermore, the heating rates agree when we apply a heating parameterization that accounts for the CME flux rope’s magnetic energy being converted directly into thermal energy. This UVCS analysis serves as a case study for the importance of multislit coronagraph spectrometers for CME studies.

Modeling a Coronal Mass Ejection as a Magnetized Structure with EUHFORIA

Abstract We studied an Earth-directed coronal mass ejection (CME) that erupted on 2015 March 15. Our aim was to model the CME flux rope as a magnetized structure using the European Heliospheric Forecasting Information Asset (EUHFORIA). The flux rope from eruption data (FRED) output was applied to the EUHFORIA spheromak CME model. In addition to the geometrical properties of the CME flux rope, we needed to input the parameters that determine the CME internal magnetic field like the helicity, tilt angle, and toroidal flux of the CME flux rope. According to the FRED technique geometrical properties of the CME flux rope are obtained by applying a graduated cylindrical shell fitting of the CME flux rope on the coronagraph images. The poloidal field magnetic properties can be estimated from the reconnection flux in the source region utilizing the post-eruption arcade method, which uses the Heliospheric Magnetic Imager magnetogram together with the Atmospheric Imaging Assembly (AIA) 193 Å images. We set up two EUHFORIA runs with RUN-1 using the toroidal flux obtained from the FRED technique and RUN-2 using the toroidal flux that was measured from the core dimming regions identified from the AIA 211 Å images. We found that the EUHFORIA simulation outputs from RUN-1 and RUN-2 are comparable to each other. Overall using the EUHFORIA spheromak model, we successfully obtained the magnetic field rotation of the flux rope, while the arrival time near Earth and the strength of the interplanetary CME magnetic field at Earth are not as accurately modeled.

Internal Structure of the 2019 April 2 CME

Abstract We present the first analysis of internal coronal mass ejection (CME) structure observed very close to the Sun by the Wide-field Imager for Solar PRobe (WISPR) instrument on board the Parker Solar Probe (PSP). The transient studied here is a CME observed during PSP’s second perihelion passage on 2019 April 2, when PSP was only 40 R ⊙ from the Sun. The CME was also well observed from 1 au by the STEREO-A spacecraft, which tracks the event all the way from the Sun to 1 au. However, PSP/WISPR observes internal structure not apparent in the images from 1 au. In particular, two linear features are observed, one bright and one dark. We model these features as two loops within the CME flux rope (FR) channel. The loops can be interpreted as bundles of field lines, with the brightness of the bright loop indicative of lots of mass being loaded into those field lines, and with the dark loop being devoid of such mass loading. It is possible that these loops are actually representative of two independent FR structures within the overall CME outline.

Multi-point analysis of coronal mass ejection flux ropes using combined data from Solar Orbiter, BepiColombo, and Wind

Context. The recent launch of Solar Orbiter and the flyby of BepiColombo opened a brief window during which these two spacecraft, along with the existing spacecraft at L1, were positioned in a constellation that allowed for the detailed sampling of any Earth-directed coronal mass ejection (CME). Fortunately, two such events occurred during this time period with in situ detections of an interplanetary coronal mass ejection (ICME) by Solar Orbiter on the 2020 April 19 and 2020 May 28. These two events were subsequently observed in situ by BepiColombo and Wind as well around a day later. Aims. We attempt to reconstruct the observed in situ magnetic field measurements for all three spacecraft simultaneously using an empirical magnetic flux rope model. This allows us to test the validity of our flux rope model on a larger and more global scale. It additionally allows for cross-validation of the analysis with different spacecraft combinations. Finally, we can also compare the results from the in situ modeling to remote observations obtained from the STEREO-A heliospheric imagers, which were able to capture the interplanetary evolution of the coronal mass ejections. Methods. We made use of the 3D coronal rope ejection model (3DCORE) in order to simulate the ICME evolution and reconstruct the measured flux rope signatures at the spacecraft positions. For this purpose, we adapted a previously developed approximate Bayesian Computation sequential Monte-Carlo (ABC-SMC) fitting algorithm for the application to multi-point scenarios. This approach not only allows us to find global solutions, within the limits of our model, but to also naturally generate error estimates on the model parameters and detect potential ambiguities. Results. We show that we are able to generally reconstruct the flux rope signatures at three different spacecraft positions simultaneously by using our model in combination with the flux rope fitting algorithm. For the well-behaved April 19 ICME, our approach works very well and displays only minor deficiencies. The May 28 ICME, on the other hand, shows the limitations of our approach for less clear ICME measurements or strongly deformed shapes. Unfortunately, the usage of multi-point observations for these events does not appear to solve inherent issues, such as the estimation of the magnetic field twist or flux rope aspect-ratios due to the specific constellation of the spacecraft positions, which all lie near the ecliptic plane. As our general approach can be used for any fast-forward simulation based model, we give a blueprint for future studies using more advanced ICME models.

The first coronal mass ejection observed in both visible-light and UV H I Ly-αchannels of the Metis coronagraph on board Solar Orbiter

Context.The Metis coronagraph on board Solar Orbiter offers a new view of coronal mass ejections (CMEs), observing them for the first time with simultaneous images acquired with a broad-band filter in the visible-light interval and with a narrow-band filter around the H ILy-αline at 121.567 nm, the so-called Metis UV channel.Aims.We show the first Metis observations of a CME, obtained on 16 and 17 January 2021. The event was also observed by the EUI/FSI imager on board Solar Orbiter, as well as by other space-based coronagraphs, such as STEREO-A/COR2 and SOHO/LASCO/C2, whose images are combined here with Metis data.Methods.Different images are analysed here to reconstruct the 3D orientation of the expanding CME flux rope using the graduated cylindrical shell model. This also allows us to identify the possible location of the source region. Measurements of the CME kinematics allow us to quantify the expected Doppler dimming in the Ly-αchannel.Results.Observations show that most CME features seen in the visible-light images are also seen in the Ly-αimages, although some features in the latter channel appear more structured than their visible-light counterparts. We estimated the expansion velocity of this event to be below 140 km s−1. Hence, these observations can be understood by assuming that Doppler dimming effects do not strongly reduce the Ly-αemission from the CME. These velocities are comparable with or smaller than the radial velocities inferred from the same data in a similar coronal structure on the east side of the Sun.Conclusions.The first observations by Metis of a CME demonstrate the capability of the instrument to provide valuable and novel information on the structure and dynamics of these coronal events. Considering also its diagnostics capabilities regarding the conditions of the ambient corona, Metis promises to significantly advance our knowledge of such phenomena.

Uncovering the process that transports magnetic helicity to coronal mass ejection flux ropes

Magnetic helicity, an intrinsic property of eruptive helical flux ropes (FRs) forming coronal mass ejections (CMEs), plays an important role in determining CME geoeffectiveness. In the solar atmosphere and heliosphere, helicity remains conserved in a closed volume. Considering this fact as a basis of our study, we perform a quantitative comparison between total magnetic helicity and twisted flux in interplanetary CMEs and those transported to CMEs via magnetic reconnection at low corona. At the source, twisted/poloidal flux (ϕpcme) of CMEs is directly estimated from total reconnection flux, and CME helicity (Hcme) is obtained by combining reconnection flux information with CME physical parameters. At 1 AU, the twisted/poloidal flux (ϕpmc) and helicity (Hmc) of CMEs are obtained from in situ observations. Considering uncertainties steaming from FR length, reconnection flux and CME physical parameter estimations, poloidal flux and helicity of CMEs at 1 AU are found to be highly relevant (ϕpmcϕpcme = 0.4–1.5, HmcHcme = 0.3–1) to low-corona magnetic reconnection at the wake of CMEs. This result remains unchanged despite CME association with pre-existing FRs. We show that a significant reduction in CME helicity during its heliospheric propagation may result from a high rate of FR erosion in the interplanetary medium. Our event analysis confirms that CME’s intrinsic magnetic properties are transported to CME FRs during magnetic reconnection at sheared coronal arcades. A one-to-one correspondence between the chirality of 1-AU CMEs and their pre-eruptive structures complies with the fact that the sense of field line rotations in FRs may remain unchanged during coronal reconnection at the source. By connecting intrinsic magnetic properties of FRs through Sun-Earth medium, this study provides important implications for the origin of geoeffectiveness in CMEs.

Investigation on the spatiotemporal structures of supra-arcade spikes

Context. The vertical current sheet (VCS) trailing coronal mass ejections (CMEs) is the key place at which the flare energy release and the CME buildup take place through magnetic reconnection. The VCS is often studied from the edge-on perspective for the morphological similarity with the two-dimensional “standard” picture, but its three-dimensional structure can only be revealed when the flare arcade is observed side on. The structure and dynamics in the so-called supra-arcade region thus contain important clues to the physical processes in flares and CMEs. Aims. We focus on supra-arcade spikes (SASs), interpreted as the VCS viewed side on, to study their spatiotemporal structures. By comparing the number of spikes and the in situ derived magnetic twist in interplanetary CMEs (ICMEs), we intend to check on the inference from the standard picture that each spike represents an active reconnection site and that each episode of reconnection adds approximately one turn of twist to the CME flux rope. Methods. For this investigation we selected four events, in which the flare arcade has a significant north-south orientation and the associated CME is traversed by a near-Earth spacecraft. We studied the SASs using high-cadence high-resolution 131 Å images from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. Results. By identifying each individual spike during the decay phase of the selected eruptive flares, we found that the widths of spikes are log-normal distributed. However, the Fourier power spectra of the overall supra-arcade extreme ultraviolet emission, including bright spikes, dark downflows, and the diffuse background, are power-law distributed in terms of either spatial frequency k or temporal frequency ν, which reflects the fragmentation of the VCS. We demonstrate that coronal emission-line intensity observations dominated by Kolmogorov turbulence would exhibit a power spectrum of E(k) ∼ k−13/3 or E(ν) ∼ ν−7/2, which is consistent with our observations. By comparing the number of SASs and the turns of field lines as derived from the ICMEs, we found a consistent axial length of ∼3.5 AU for three events with a CME speed of ∼1000 km s−1 in the inner heliosphere; but we found a much longer axial length (∼8 AU) for the fourth event with an exceptionally fast CME speed of ∼1500 km s−1, suggesting that when the spacecraft traversed its leg this ICME was flattened and its “nose” was significantly past the Earth.

Numerical Research on the Effect of the Initial Parameters of a CME Flux-rope Model on Simulation Results. II. Different Locations of Observers

Abstract In numerical studies of the initiation and propagation of coronal mass ejections (CMEs), it has been proven that the shape, size, and plasma parameters of CMEs could significantly affect simulation results and subsequent space weather predictions. In our previous research, we proposed a new way to initiate a CME based on the graduated cylindrical shell model, and studied the effect of different initial parameters of CMEs on the simulation results when the observer is aligned with the initial propagation direction of the CME. In this paper, we investigate the influence of the different initial parameters of CMEs on simulation results at the observational points with different longitudes and latitudes. Our results indicate that as long as the initial mass of the CME remains unchanged, the initial geometric thickness will have a different influence in the latitudinal and longitudinal directions. The deflection of the CMEs always occurs in both latitudinal and longitudinal directions, when the CMEs interact with the background solar wind structures, such as the corotating interaction region, in the heliosphere.