Fast Halo Coronal Mass Ejections Research Articles

Refined Modeling of Geoeffective Fast Halo CMEs During Solar Cycle 24

AbstractThe propagation of geoeffective fast halo coronal mass ejections (CMEs) from solar cycle 24 has been investigated using the European Heliospheric Forecasting Information Asset (EUHFORIA), ENLIL, Drag‐Based Model (DBM) and Effective Acceleration Model (EAM) models. For an objective comparison, a unified set of a small sample of CME events with similar characteristics has been selected. The same CME kinematic parameters have been used as input in the propagation models to compare their predicted arrival times and the speed of the interplanetary (IP) shocks associated with the CMEs. The performance assessment has been based on the application of an identical set of metrics. First, the modeling of the events has been done with default input concerning the background solar wind, as would be used in operations. The obtained CME arrival forecast deviates from the observations at L1, with a general underestimation of the arrival time and overestimation of the impact speed (mean absolute error [MAE]: 9.8 ± 1.8–14.6 ± 2.3 hr and 178 ± 22–376 ± 54 km/s). To address this discrepancy, we refine the models by simple changes of the density ratio (dcld) between the CME and IP space in the numerical, and the IP drag (γ) in the analytical models. This approach resulted in a reduced MAE in the forecast for the arrival time of 8.6 ± 2.2–13.5 ± 2.2 hr and the impact speed of 51 ± 6–243 ± 45 km/s. In addition, we performed multi‐CME runs to simulate potential interactions. This leads, to even larger uncertainties in the forecast. Based on this study we suggest simple adjustments in the operational settings for improving the forecast of fast halo CMEs.

Analysis of Geoeffective Impulsive Events on the Sun During the First Half of Solar Cycle 24

A coronal mass ejection (CME) is an impulsive event that emerges rapidly from the Sun. We observed a quiet Sun without many spectacular episodes during the last decade. Although some fast halo and partial halo CMEs had taken place, among them was the backside CME on 23 July 2012. In this work, we verify the link between the variability of solar-wind, heliospheric and geomagnetic parameters and the transmission grid failures registered in southern Poland during 2010 – 2014 when many geomagnetic storms appeared, caused by halo and partial halo CMEs. We aim to apply three machine learning methods: Principal Components Analysis, Self-Organizing Maps, and Hierarchical Agglomerative Clustering to analyze sources on the Sun and the impacts of the intense geomagnetic storms in the first half of Solar Cycle 24. The conducted analyzes underline the importance of solar-wind proton temperature and point out other solar-wind and geomagnetic parameters independently indicated by all the methods used in this study.

The first gradual solar energetic particle event with an enhanced 3He abundance on Solar Orbiter

The origin of 3He abundance enhancements in gradual solar energetic particle (SEP) events remains largely unexplained. Two mechanisms have been suggested: the reacceleration of remnant flare material by coronal mass ejection (CME)-driven shocks in interplanetary space, and concomitant activity in the corona. We explore the first gradual SEP event with enhanced 3He abundance that was observed by Solar Orbiter. The event started on 2020 November 24 and was associated with a relatively fast halo CME. During the event, the spacecraft was at 0.9 au from the Sun. The event-averaged 3He/4He abundance ratio is 24 times higher than the coronal or solar wind value, and the timing of the 3He intensity was similar to that of other species. We inspected available imaging, radio observations, and the spacecraft magnetic connection to the CME source. The most probable cause of the enhanced 3He abundance apparently are residual 3He ions remaining from a preceding long period of 3He-rich SEPs on 2020 November 17–23.

Nonequilibrium Flux Rope Formation by Confined Flares Preceding a Solar Coronal Mass Ejection

Abstract We present evidence that a magnetic flux rope was formed before a coronal mass ejection (CME) and its associated long-duration flare during a pair of preceding confined eruptions and associated impulsive flares in a compound event in NOAA Active Region 12371. Extreme-ultraviolet images and the extrapolated nonlinear force-free field show that the first two (impulsive) flares, SOL2015-06-21T01:42, result from the confined eruption of highly sheared low-lying flux, presumably a seed flux rope. The eruption spawns a vertical current sheet, where magnetic reconnection creates flare ribbons and loops, a nonthermal microwave source, and a sigmoidal hot channel that can only be interpreted as a magnetic flux rope. Until the subsequent long-duration flare, SOL2015-06-21T02:36, the sigmoid’s elbows expand, while its center remains stationary, suggesting nonequilibrium but not yet instability. The “flare reconnection” during the confined eruptions acts like “tether-cutting reconnection” whose flux feeding of the rope leads to instability. The subsequent full eruption is seen as an accelerated rise of the entire hot channel, seamlessly evolving into the fast halo CME. Both the confined and ejective eruptions are consistent with the onset of the torus instability in the dipped decay index profile that results from the region’s two-scale magnetic structure. We suggest that the formation or enhancement of a nonequilibrium but stable flux rope by confined eruptions is a generic process occurring prior to many CMEs.

The Solar Event of 14\u2009\u2013\u200915 July 2012 and Its Geoeffectiveness

During Solar Cycle 24, which started at the end of 2008, the Sun was calm, and there were not many spectacular geoeffective events. In this article, we analyze the geomagnetic storm that happened on 15 July 2012 during the 602nd anniversary of the Polish Battle of Grunwald, thus we propose this event to be called the “Battle of Grunwald Day Storm”. According to NOAA scale, it was a G3 geomagnetic storm with a southward component of the heliospheric magnetic field, Bz, falling to −20 nT, minimum Dst index of −139 nT, AE index of 1368 nT, and Ap index of 132 nT. It was preceded by a solar flare class X1.4 on 12 July. This geomagnetic storm was associated with the fast halo coronal mass ejection at 16:48:05 UT on 12 July, first appearance in the Large Angle and Spectroscopic Coronagraph C2, with a plane-of-sky speed of 885 km s−1 and maximum of 1415 km s−1. This geomagnetic storm was classified as the fourth strongest geomagnetic storm of Solar Cycle 24. At that time, a significant growth in the failures of the Polish electric transmission lines was observed, which could have a solar origin.

Development of a formalism for computing in situ transits of Earth-directed CMEs – Part 2: Towards a forecasting tool

Abstract. Earth-directed coronal mass ejections (CMEs) are of particular interest for space weather purposes, because they are precursors of major geomagnetic storms. The geoeffectiveness of a CME mostly relies on its physical properties like magnetic field and speed. There are multiple efforts in the literature to estimate in situ transit profiles of CMEs, most of them based on numerical codes. In this work we present a semi-empirical formalism to compute in situ transit profiles of Earth-directed fast halo CMEs. Our formalism combines analytic models and empirical relations to approximate CME properties as would be seen by a spacecraft near Earth's orbit. We use our formalism to calculate synthetic transit profiles for 10 events, including the Bastille Day event and 3 varSITI Campaign events. Our results show qualitative agreement with in situ measurements. Synthetic profiles of speed, magnetic intensity, density, and temperature of protons have average errors of 10 %, 27 %, 46 %, and 83 %, respectively. Additionally, we also computed the travel time of CME centers, with an average error of 9 %. We found that compression of CMEs by the surrounding solar wind significantly increased our uncertainties. We also outline a possible path to apply this formalism in a space weather forecasting tool.

Low Geo‐Effectiveness of Fast Halo CMEs Related to the 12 X‐Class Flares in 2002

AbstractIt is generally accepted that extreme space weather events tend to be related to strong flares and fast halo coronal mass ejections (CMEs). In the present paper, we carefully identify the chain of events from the Sun to the Earth induced by all 12 X‐class flares that occurred in 2002. In this small sample, we find an unusual high rate (58%) of solar sources with a longitude larger than 74°. Yet all 12 X‐class flares are associated with at least one CME. The fast halo CMEs (50%) are related to interplanetary CMEs (ICMEs) at L1 and weak Dst minimum values (more than −51 nT), while five (41%) of the 12 X‐class flares are related to solar proton events (SPEs).We conclude that (i) all 12 analyzed solar events, even those associated with fast halo CMEs originating from the central disk region, and those ICMEs and SPEs were not very geo‐effective. This unexpected result demonstrates that the suggested events in the chain (fast halo CME, X‐class flares, central disk region, ICME, and SPE) are not infallible proxies for geo‐effectiveness. (ii) The low value of integrated and normalized southward component of the interplanetary magnetic field ( ) may explain the low geo‐effectiveness for this small sample. In fact, is well correlated to the weak Dst and low auroral electrojet activity. Hence, the only space weather impact at Earth in 2002 we can explain is based on at L1.

Comprehensive Analysis of the Formation of a Shock Wave Associated with a Coronal Mass Ejection

Abstract On 2014 November 1, a solar prominence eruption associated with a C2.7 class flare and a type II radio burst resulted in a fast partial halo coronal mass ejection (CME). Images acquired in the extreme ultraviolet (EUV) by the Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) and PROBA2/SWAP and in white light (WL) by Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph show the expansion of a bright compression front ahead of the CME. In this work, we present a detailed investigation of the CME-driven shock associated with this event following the early evolution of the compression front observed near the Sun up to the extended corona. Our aim is to shed light on the long-debated issue concerning the location and timing of shock formation in the corona. Through differential emission measure analysis, we derived, for the first time, the compression ratio across the expanding EUV front observed by AIA at different temperature ranges: higher compression ratios corresponded to higher plasma temperature ranges, as expected. Moreover, comparison between up- and downstream temperatures and those expected via adiabatic compression shows that no additional heating mechanisms occurred in the early front expansion phase, implying that the shock formed beyond the AIA field of view. Finally, the analysis of the associated type II radio burst, in combination with the inferred coronal density distribution, allowed us to identify a well-defined region located northward of the CME source region as the site for shock formation and to outline its kinematics in accordance with the evolution of the expanding front as obtained from the EUV and WL data.

Relationship of Halo CMEs and Solar Proton Events

Coronal Mass Ejections (CMEs) are important sources of Solar Proton Events (SPEs). Their speeds and source region locations have significant effects on the occurrence of SPEs. In this paper, all the halo CMEs observed in recent five years are statistically analyzed. The results show that the fast halo CMEs with small angular distances are more likely to produce SPEs, especially, those halo CMEs with a speed greater than 1200kms−1 and an angular distance less than 60°. Three fast halo CMEs with no SPEs caused are elaborately studied. The results show that the ejection direction of the CME's main body and the variation of interplanetary magnetic field also have important impacts on the occurrence of SPEs. Consequently, in the practical daily space environment forecasts, an accurate forecast for SPEs must take various factors into account, such as the eruption speed, source region location, the main-body ejection direction of CMEs, and the interplanetary environment, etc.

Radial distributions of magnetic field strength in the solar corona as derived from data on fast halo CMEs

In recent years, information about the distance between the body of rapid coronal mass ejection (CME) and the associated shock wave has been used to measure the magnetic field in the solar corona. In all cases, this tech-nique allows us to find coronal magnetic field radial profiles B(R) applied to the directions almost perpendicular to the line of sight. We have determined radial distributions of magnetic field strength along the directions close to the Sun–Earth axis. For this purpose, using the “ice-cream cone” model and SOHO/LASCO data, we found 3D characteristics for fast halo coronal mass ejections (HCMEs) and for HCME-related shocks. With these data we managed to obtain the B(R) distributions as far as ≈43 solar radii from the Sun’s center, which is approximately twice as far as those in other studies based on LASCO data. We have concluded that to improve the accuracy of this method for finding the coronal magnetic field we should develop a technique for detecting CME sites moving in the slow and fast solar wind. We propose a technique for selecting CMEs whose central (paraxial) part actually moves in the slow wind.

Радиальные распределения величины магнитного поля в солнечной короне, полученные с использованием сведений о быстрых гало-КВМ

In recent years, information about the distance between the body of rapid coronal mass ejection (CME) and the associated shock wave has been used to measure the magnetic field in the solar corona. In all cases, this tech-nique allows us to find coronal magnetic field radial profiles B(R) applied to the directions almost perpendicular to the line of sight. We have determined radial distributions of magnetic field strength along the directions close to the Sun–Earth axis. For this purpose, using the “ice-cream cone” model and SOHO/LASCO data, we found 3D characteristics for fast halo coronal mass ejections (HCMEs) and for HCME-related shocks. With these data we managed to obtain the B(R) distributions as far as ≈43 solar radii from the Sun’s center, which is approximately twice as far as those in other studies based on LASCO data. We have concluded that to improve the accuracy of this method for finding the coronal magnetic field we should develop a technique for detecting CME sites moving in the slow and fast solar wind. We propose a technique for selecting CMEs whose central (paraxial) part actually moves in the slow wind.

Halo coronal mass ejections during Solar Cycle 24: reconstruction of the global scenario and geoeffectiveness

Coronal mass ejections (CMEs), in particular Earth-directed ones, are regarded as the main drivers of geomagnetic activity. In this study, we present a statistical analysis of a set of 53 fast (V ≥ 1000 km·s−1) Earth-directed halo CMEs observed by the SOHO/LASCO instrument during the period Jan. 2009–Sep. 2015, and we then use this CME sample to test the forecasting capabilities of a new Sun-to-Earth prediction scheme for the geoeffectiveness of Earth-directed halo CMEs. First, we investigate the CME association with other solar activity features such as solar flares, active regions, and others, by means of multi-instrument observations of the solar magnetic and plasma properties, with the final aim of identifying recurrent peculiar features that can be used as precursors of CME-driven geomagnetic storms. Second, using coronagraphic images to derive the CME kinematical properties at 0.1 AU, we propagate the events to 1 AU by means of 3D global MHD simulations. In particular, we use the WSA-ENLIL+Cone model to reconstruct the propagation and global evolution of each event up to their arrival at Earth, where simulation results are compared with interplanetary CME (ICME) in-situ signatures. We then use simulation outputs upstream of Earth to predict their impact on geospace. By applying the pressure balance condition at the magnetopause and the coupling function proposed by Newell et al. [J Geophys Res: Space Phys 113 (2008)] to link upstream solar wind properties to the global Kp index, we estimate the expected magnetospheric compression and geomagnetic activity level, and compare our predictions with global data records. The analysis indicates that 82% of the fast Earth-directed halo CMEs arrived at Earth within the next 4 days. Almost the totality of them compressed the magnetopause below geosynchronous orbits and triggered a minor or major geomagnetic storm afterwards. Among them, complex sunspot-rich active regions associated with X- and M-class flares are the most favourable configurations from which geoeffective CMEs originate. The analysis of related Solar Energetic Particle (SEP) events shows that 74% of the CMEs associated with major SEPs were geoeffective, i.e. they triggered a minor to intense geomagnetic storm (Kp ≥ 5). Moreover, the SEP production is enhanced in the case of fast and interacting CMEs. In this work we present a first attempt at applying a Sun-to-Earth geoeffectiveness prediction scheme − based on 3D simulations and solar wind-geomagnetic activity coupling functions − to a statistical set of fast Earth-directed, potentially geoeffective halo CMEs. The results of the prediction scheme are promising and in good agreement with the actual data records for geomagnetic activity. However, we point out the need for future studies performing a fine-tuning of the prediction scheme, in particular in terms of the evaluation of the CME input parameters and the modelling of their internal magnetic structure.

The density compression ratio of shock fronts associated with coronal mass ejections

We present a new method to extract the three-dimensional electron density profile and density compression ratio of shock fronts associated with coronal mass ejections (CMEs) observed in white light coronagraph images. We demonstrate the method with two examples of fast halo CMEs (∼2000 km s−1) observed on 2011 March 7 and 2014 February 25. Our method uses the ellipsoid model to derive the three-dimensional geometry and kinematics of the fronts. The density profiles of the sheaths are modeled with double-Gaussian functions with four free parameters, and the electrons are distributed within thin shells behind the front. The modeled densities are integrated along the lines of sight to be compared with the observed brightness in COR2-A, and a χ2 approach is used to obtain the optimal parameters for the Gaussian profiles. The upstream densities are obtained from both the inversion of the brightness in a pre-event image and an empirical model. Then the density ratio and Alfvénic Mach number are derived. We find that the density compression peaks around the CME nose, and decreases at larger position angles. The behavior is consistent with a driven shock at the nose and a freely propagating shock wave at the CME flanks. Interestingly, we find that the supercritical region extends over a large area of the shock and lasts longer (several tens of minutes) than past reports. It follows that CME shocks are capable of accelerating energetic particles in the corona over extended spatial and temporal scales and are likely responsible for the wide longitudinal distribution of these particles in the inner heliosphere. Our results also demonstrate the power of multi-viewpoint coronagraphic observations and forward modeling in remotely deriving key shock properties in an otherwise inaccessible regime.

The Solar Energetic Particle Event of 2010 August 14: Connectivity with the Solar Source Inferred from Multiple Spacecraft Observations and Modeling

Abstract We analyze one of the first solar energetic particle (SEP) events of solar cycle 24 observed at widely separated spacecraft in order to assess the reliability of models currently used to determine the connectivity between the sources of SEPs at the Sun and spacecraft in the inner heliosphere. This SEP event was observed on 2010 August 14 by near-Earth spacecraft, STEREO-A (∼80° west of Earth) and STEREO-B (∼72° east of Earth). In contrast to near-Earth spacecraft, the footpoints of the nominal magnetic field lines connecting STEREO-A and STEREO-B with the Sun were separated from the region where the parent fast halo coronal mass ejection (CME) originated by ∼88° and ∼47° in longitude, respectively. We discuss the properties of the phenomena associated with this solar eruption. Extreme ultraviolet and white-light images are used to specify the extent of the associated CME-driven coronal shock. We then assess whether the SEPs observed at the three heliospheric locations were accelerated by this shock or whether transport mechanisms in the corona and/or interplanetary space provide an alternative explanation for the arrival of particles at the poorly connected spacecraft. A possible scenario consistent with the observations indicates that the observation of SEPs at STEREO-B and near Earth resulted from particle injection by the CME shock onto the field lines connecting to these spacecraft, whereas SEPs reached STEREO-A mostly via cross-field diffusive transport processes. The successes, limitations, and uncertainties of the methods used to resolve the connection between the acceleration sites of SEPs and the spacecraft are evaluated.

Solar and interplanetary activities of isolated and non-isolated coronal mass ejections

We report our results on comparison of two halo Coronal Mass Ejections (CME) associated with X-class flares of similar strength (X1.4) but quite different in CME speed and acceleration, similar geo-effectiveness but quite different in Solar Energetic Particle (SEP) intensity. CME1 (non-isolated) was associated with a double event in X-ray flare and it was preceded by another fast halo CME of speed = 2684 km/s (pre-CME) associated with X-ray flare class X5.4 by 1 h from the same location. Since this pre-CME was more eastern, interaction with CME1 and hitting the earth were not possible. This event (CME1) has not suffered the cannibalism since pre-CME has faster speed than post-CME. Pre-CME plays a very important role in increasing the intensity of SEP and Forbush Decrease (FD) by providing energetic seed particles. So, the seed population is the major difference between these two selected events. CME2 (isolated) was a single event. We would like to address on the kinds of physical conditions related to such CMEs and their associated activities. Their associated activities such as, type II bursts, SEP, geomagnetic storm and FD are compared. The following results are obtained from the analysis. (1) The CME leading edge height at the start of metric/DH type II bursts are ~2 R⊙/~4 R⊙ for CME1, but ~2 R⊙/~2.75 R⊙ for CME2. (2) Peak intensity of SEP event associated with the two CMEs are quite different: 6530 pfu for CME1, but 96 pfu for CME2. (3) The Forbush decrease occurred with a minimum decrease of 9.98% in magnitude for CME1, but 6.90% for CME2. (4) These two events produced similar intense geomagnetic storms of intensity of Dst index ~−130 nT. (5) The maximum southward magnetic fields corresponding to Interplanetary CME (ICME) of these two events are nearly the same, but there is difference in Sheath Bz maximum (−14.2, −6.9 nT). (6) The time-line chart of the associated activities of two CMEs show some difference in the time delay between the onsets of activities with respect to the onset of flare/CME.

The Origin of Solar Filament Plasma Inferred from In Situ Observations of Elemental Abundances

Abstract Solar filaments/prominences are one of the most common features in the corona, which may lead to energetic coronal mass ejections (CMEs) and flares when they erupt. Filaments are about 100 times cooler and denser than the coronal material, and physical understanding of their material origin remains controversial. Two types of scenarios have been proposed: one argues that the filament plasma is brought into the corona from photosphere or chromosphere through a siphon or evaporation/injection process, while the other suggests that the material condenses from the surrounding coronal plasma due to thermal instability. The elemental abundance analysis is a reasonable clue to constrain the models, as the siphon or evaporation/injection model would predict that the filament material abundances are close to the photospheric or chromospheric ones, while the condensation model should have coronal abundances. In this Letter, we analyze the elemental abundances of a magnetic cloud that contains the ejected filament material. The corresponding filament eruption occurred on 1998 April 29, accompanying an M6.8 class soft X-ray flare located at the heliographic coordinates S18E20 (NOAA 08210) and a fast halo CME with the linear velocity of 1374 km s−1 near the Sun. We find that the abundance ratios of elements with low and high first ionization potential such as Fe/O, Mg/O, and Si/O are 0.150, 0.050, and 0.070, respectively, approaching their corresponding photospheric values 0.065, 0.081, and 0.066, which does not support the coronal origin of the filament plasma.

Solar flare associated coronal mass ejections causing geo-effectiveness and Forbush decreases

In the present study, we have selected 35 halo Coronal Mass Ejections (CMEs) associated with solar flares, Geomagnetic Storms (GSs) and Forbush decrease (Fd) chosen from 1st January 2000 to 31st December 2007 (i.e., the descending phase of solar cycle 23) observed by the Large Angle Spectrometric Coronagraph (LASCO) on board the SOHO spacecraft. Statistical analyses are performed to look at the distribution of solar flares associated with halo CMEs causing GSs and Fd and investigated the relationship between solar flare and halo CME parameters with GSs and Fd. Forbush decrease is the phenomenon of rapid decrease in cosmic ray intensity following the CME. Our analysis indicates that during 2000 to 2007 the northern region produced 44 % of solar flares associated with halo CMEs, GSs, and Fd, whereas 56 % solar flares associated with halo CMEs, GSs, and Fd were produced in the southern region. The northern and the southern hemispheres between 10° to 20° latitudinal belts are found to be more effective in producing events leading to Fd. From our selected events, we found that about 60 % of super-intense storms (\(\mbox{Dst} \leq -200~\mbox{nT}\)) caused by halo CMEs are associated with X-class flares. Fast halo CMEs associated with X-class flares originating from 0° to 25° latitudes are better potential candidates in producing super-intense GSs than the slow halo CMEs associated with other classes of flares.

Study of large solar energetic particle events with halo coronal mass ejections and their associated solar flares

The purpose of the present study is to investigate the association of solar energetic particle (SEP) events with halo coronal mass ejections (CME) and with their associated solar flares during the period 1997–2014 (solar cycle 23 and 24). We have found that halo CMEs are more effective in producing SEP events. The occurrence probability and peak fluxes of SEPs strongly depend on the halo CMEs speed (V) as follows. The highest associations, 56% for occurrence probability and 90% for average peak fluxes, are found for the halo CMEs with V> 1400kms−1 but the lowest associations, 20% for occurrence probability and 5% for average peak fluxes, are found for halo CMEs with speed range 600 ≤V≤ 1000kms−1. We have also examined the relationship between SEP events and halo CME associated solar flares and found that 73% of events are associated with western solar flares while only 27% are with eastern solar flares. For longitudinal study, 0–20° belt is found to be more dominant for the SEP events. The association of SEP events with latitudinal solar flares is also examined in the study. 51% of events are associated with those halo CMEs associated solar flares which occur in the southern hemisphere of the Sun while 49% are with those solar flares that occur in the northern hemisphere of the Sun. Also, 10–20° latitudinal belt is found to be likely associated with the SEP events. Further, 45% of SEP events are associated with M-class solar flares while 44% and 11% are with X and C-class respectively. Maximum number of SEP events are found for the fast halo CME associated X- class solar flares (68%) than M and C- class solar flares.

OBSERVATIONS OF MAGNETIC FLUX-ROPE OSCILLATION DURING THE PRECURSOR PHASE OF A SOLAR ERUPTION

ABSTRACT Based on combined observations from the Interface Region Imaging Spectrograph (IRIS) spectrometer with the coronal emission line of Fe xxi at 1354.08 Å and SDO/AIA images in multiple passbands, we report the finding of the precursor activity manifested as the transverse oscillation of a sigmoid, which is likely a pre-existing magnetic flux rope (MFR), that led to the onset of an X class flare and a fast halo coronal mass ejection (CME) on 2014 September 10. The IRIS slit is situated at a fixed position that is almost vertical to the main axis of the sigmoid structure that has a length of about 1.8 × 105 km. This precursor oscillation lasts for about 13 minutes in the MFR and has velocities in the range of [−9, 11] km s−1 and a period of ∼280 s. Our analysis, which is based on the temperature, density, length, and magnetic field strength of the observed sigmoid, indicates that the nature of the oscillation is a standing wave of fast magnetoacoustic kink mode. We further find that the precursor oscillation is excited by the energy released through an external magnetic reconnection between the unstable MFR and the ambient magnetic field. It is proposed that this precursor activity leads to the dynamic formation of a current sheet underneath the MFR that subsequently reconnects to trigger the onset of the main phase of the flare and the CME.

Characteristics of the motion of coronal mass ejections and related shocks depending on the heliocentric distance

The 3D characteristics of the coronal mass ejection (CME) body and shock at different distances (R) from the Sun’s center have been determined for several fast halo CMEs based on data from the Large Angle and Spectrometric Coronagraph Experiment (LASCO): the positions of the CME body boundary and the related shock on their axes, the directions and velocities of these points, the distance between a shock and the CME body boundary ΔR(R), and the difference between the velocities of these structures ΔV(R) and their angular dimensions. It was shown that the character of variations in the positions and velocities of the CME body boundary and a shock differs at different distances. The dependences of parameter [ΔR/Rcme](R), where Rcme is the curvature radius of the CME body boundary on the CME axis, were constructed. The obtained dependence was compared with the ΔR/Rcme (MA(R)) dependence. Here MA is the Alfven Mach number. The relation of ΔR/Rcme to MA was obtained by Russell and Mulligan (2002) for shocks in the Earth’s orbit. A comparison of [ΔR/Rcme](R) performed in two ways makes it possible to conclude that shocks related to a CME body are piston-like, with the CME body as a piston, at least at a distance of R > 10R0 (R0 is the solar radius).