Coronal Mass Ejections Expansion Research Articles

Estimating the lateral speed of a fast shock driven by a coronal mass ejection at the location of solar radio emissions

Fast coronal mass ejections (CMEs) can drive shock waves capable of accelerating electrons to high energies. These shock-accelerated electrons act as sources of electromagnetic radiation, often in the form of solar radio bursts. Recent findings suggest that radio imaging of solar radio bursts can provide a means to estimate the lateral expansion of CMEs and associated shocks in the low corona. Our aim is to estimate the expansion speed of a CME-driven shock at the locations of radio emission using 3D reconstructions of the shock wave from multiple viewpoints. In this study, we estimated the 3D location of radio emission using radio imaging from the Nançay Radioheliograph and the 3D location of a CME-driven shock. The 3D shock was reconstructed using white-light and extreme ultraviolet images of the CME from the Solar Terrestrial Relations Observatory, Solar Dynamics Observatory, and the Solar and Heliospheric Observatory. The lateral expansion speed of the CME-driven shock at the electron acceleration locations was then estimated using the approximate 3D locations of the radio emission on the surface of the shock. The radio bursts associated with the CME were found to reside at the flank of the expanding CME-driven shock. We identified two prominent radio sources at two different locations and found that the lateral speed of the shock was between $ $ and $ second $ at these locations. Such a high speed during the early stages of the eruption already indicates the presence of a fast shock in the low corona. We also found a larger ratio between the radial and lateral expansion speed compared to values obtained higher up in the corona. We estimated for the first time the 3D expansion speed of a CME-driven shock at the location of the accompanying radio emission. The high shock speed obtained is indicative of a fast acceleration during the initial stage of the eruption. This acceleration leading to lateral speeds in the range of $ second $ is most likely one of the key parameters contributing to the presence of metric radio emissions, such as type II radio bursts.

Direct In Situ Measurements of a Fast Coronal Mass Ejection and Associated Structures in the Corona

We report on the first direct in situ measurements of a fast coronal mass ejection (CME) and shock in the corona, which occurred on 2022 September 5. In situ measurements from the Parker Solar Probe spacecraft near perihelion suggest two shocks, with the second one decayed, which is consistent with more than one eruption in coronagraph images. Despite a flank crossing, the measurements indicate unique features of the young ejecta: a plasma much hotter than the ambient medium, suggestive of a hot solar source, and a large plasma β implying a highly non-force-free state and the importance of thermal pressure gradient for CME acceleration and expansion. Reconstruction of the global coronal magnetic fields shows a long-duration change in the heliospheric current sheet (HCS), and the observed field polarity reversals agree with a more warped HCS configuration. Reconnection signatures are observed inside an HCS crossing as deep as the sonic critical point. As the reconnection occurs in the sub-Alfvénic wind, the reconnected flux sunward of the reconnection site can close back to the Sun, which helps balance magnetic flux in the heliosphere. The nature of the sub-Alfvénic wind after the HCS crossing as a low Mach-number boundary layer (LMBL) leads to in situ measurements of the near subsonic plasma at a surprisingly large distance. Specifically, an LMBL may provide favorable conditions for the crossings of the sonic critical point in addition to the Alfvén surface.

Unraveling the Thermodynamic Enigma between Fast and Slow Coronal Mass Ejections

Coronal mass ejections (CMEs) are the most energetic expulsions of magnetized plasma from the Sun that play a crucial role in space weather dynamics. This study investigates the diverse kinematics and thermodynamic evolution of two CMEs (CME1: 2011 September 24 and CME2: 2018 August 20) at coronal heights where thermodynamic measurements are limited. The peak 3D propagation speed of CME1 is high (1885 km s−1) with two-phase expansion (rapid and nearly constant), while the peak 3D propagation speed of CME2 is slow (420 km s−1) with only a gradual expansion. We estimate the distance-dependent variations in the polytropic index, heating rate, temperature, and internal forces implementing the revised FRIS model, taking inputs of 3D kinematics estimated from the graduated cylindrical shell model. We find CME1 exhibiting heat release during its early-rapid acceleration decrease and jumps to the heat-absorption state during its constant acceleration phase. In contrast to CME1, CME2 shows a gradual transition from the near-adiabatic to the heat-absorption state during its gradually increasing acceleration. Our analysis reveals that although both CMEs show differential heating, they experience heat absorption during their later propagation phases, approaching the isothermal state. The faster CME1 achieves an adiabatic state followed by an isothermal state at smaller distances from the Sun than the slower CME2. We also find that the expansion of CMEs is primarily influenced by centrifugal and thermal pressure forces, with the Lorentz force impeding expansion. Multiwavelength observations of flux-ropes at source regions support the FRIS-model-derived findings at initially observed lower coronal heights.

Three-dimensional relation between coronal dimming, filament eruption, and CME

Context.Coronal dimmings are localized regions of reduced emission in the extreme-ultraviolet (EUV) and soft X-rays formed as a result of the expansion and mass loss by coronal mass ejections (CMEs) low in the corona. Distinct relations have been established between coronal dimmings (intensity, area, magnetic flux) and key characteristics of the associated CMEs (mass and speed) by combining coronal and coronagraphic observations from different viewpoints in the heliosphere.Aims.We investigate the relation between the spatiotemporal evolution of the dimming region and both the dominant direction of the filament eruption and CME propagation for the 28 October 2021 X1.0 flare/CME event observed from multiple viewpoints in the heliosphere by Solar Orbiter, STEREO-A, SDO, and SOHO.Methods.We present a method for estimating the dominant direction of the dimming development based on the evolution of the dimming area, taking into account the importance of correcting the dimming area estimation by calculating the surface area of a sphere for each pixel. To determine the propagation direction of the flux rope during early CME evolution, we performed 3D reconstructions of the white-light CME by graduated cylindrical shell modeling (GCS) and 3D tie-pointing of the eruptive filament.Results.The dimming evolution starts with a radial expansion and later propagates more to the southeast. The orthogonal projections of the reconstructed height evolution of the prominent leg of the erupting filament onto the solar surface are located in the sector of the dominant dimming growth, while the orthogonal projections of the inner part of the GCS reconstruction align with the total dimming area. The filament reaches a maximum speed of ≈250 km s−1at a height of about ≈180 Mm before it can no longer be reliably followed in the EUV images. Its direction of motion is strongly inclined from the radial direction (64° to the east, 32° to the south). The 3D direction of the CME and the motion of the filament leg differ by 50°. This angle roughly aligns with the CME half-width obtained from the CME reconstruction, suggesting a relation between the reconstructed filament and the associated leg of the CME body.Conclusions.The dominant propagation of the dimming growth reflects the direction of the erupting magnetic structure (filament) low in the solar atmosphere, though the filament evolution is not directly related to the direction of the global CME expansion. At the same time, the overall dimming morphology closely resembles the inner part of the CME reconstruction, validating the use of dimming observations to obtain insight into the CME direction.

The Evolution of Ion Charge States in Coronal Mass Ejections

We model the observed charge states of the elements C, O, Mg, Si, and Fe in the ejecta of coronal mass ejections (CMEs). We concentrate on “halo” CMEs observed in situ by the Advanced Composition Explorer/Solar Wind Ion Composition Spectrometer to measure ion charge states, and also remotely by the Solar Terrestrial Relations Observatory when in near quadrature with the Earth, so that the CME expansion can be accurately specified. Within this observed expansion, we integrate equations for the CME ejecta ionization balance, including electron heating parameterized as a fraction of the kinetic and gravitational energy gain of the CME. We also include the effects of non-Maxwellian electron distributions, characterized as a κ function. Focusing first on the 2010 April 3 CME, we find a somewhat better match to the observed charge states with κ close to the theoretical minimum value of κ = 3/2, implying a hard spectrum of nonthermal electrons. Similar but more significant results come from the 2011 February 15 event, although it is quite different in terms of its evolution. We discuss the implications of these values, and of the heating required, in terms of the magnetic reconnection Lundquist number and anomalous resistivity associated with CME evolution close to the Sun.

On the relationship between expansión angle of earth-directed CMES and soft X-ray emission from their related flare

In space weather, to study the impact of Earth-directed coronal mass ejections (CME) in our terrestrial environment, one of the most important parameters is the propagation speed of these disturbances. We present an improvement of the 3D CME Geometrical Propagation-Expansion Description (3D-CGPED) model developed in previous work to increase the simple that we can use in CME arrival time predictions. This 3D model estimates the arrival time of Earth-directed CMEs at Earth by including a 3D geometry for the CME propagation and expansion in interplanetary space. Since the 3D-CGPED model computes the expansion of the CME based on the radial distance of the CME front, only travel times for CMEs with welldefined shapes seen by coronographs can be estimated. In the present work, we found an empirical relationship between the expansion angle of CMEs with well-defined shapes and the start-to-peak SXR fluence of their associated flares. We applied this relationship in the 3D-CGPED model to obtain the expansion angle for 8 CMEs with an irregular shape. We found similar window errors in arrival time predictions compared to the previous work. This result allows us to complement the 3D-CGPED model to include not only regular shapes but also irregular ones for CMEs observed by coronographs in future works.

Evolution of the Radial Size and Expansion of Coronal Mass Ejections Investigated by Combining Remote and In Situ Observations

A fundamental property of coronal mass ejections (CMEs) is their radial expansion, which determines the increase in the CME radial size and the decrease in the CME magnetic field strength as the CME propagates. CME radial expansion can be investigated either by using remote observations or by in situ measurements based on multiple spacecraft in radial conjunction. However, there have been only few case studies combining both remote and in situ observations. It is therefore unknown if the radial expansion in the corona estimated remotely is consistent with that estimated locally in the heliosphere. To address this question, we first select 22 CME events between the years 2010 and 2013, which were well observed by coronagraphs and by two or three spacecraft in radial conjunction. We use the graduated cylindrical shell model to estimate the radial size, radial expansion speed, and a measure of the dimensionless expansion parameter of CMEs in the corona. The same parameters and two additional measures of the radial-size increase and magnetic-field-strength decrease with heliocentric distance of CMEs based on in situ measurements are also calculated. For most of the events, the CME radial size estimated by remote observations is inconsistent with the in situ estimates. We further statistically analyze the correlations of these expansion parameters estimated using remote and in situ observations, and discuss the potential reasons for the inconsistencies and their implications for the CME space weather forecasting.

Type II radio bursts and their association with coronal mass ejections in solar cycles 23 and 24

Context. Meter-wavelength type II solar radio bursts are thought to be the signatures of shock-accelerated electrons in the corona. Studying these bursts can give information about the initial kinematics, dynamics, and energetics of coronal mass ejections (CMEs) in the absence of white-light observations. Aims. We investigate the occurrence of type II bursts in solar cycles 23 and 24 and their association with CMEs. We also explore whether type II bursts might occur in the absence of a CME. Methods. We performed a statistical analysis of type II bursts that occurred between 200 and 25 MHz in solar cycles 23 and 24 and determined the temporal association of these radio bursts with CMEs. We categorized the CMEs based on their linear speed and angular width and studied the distribution of type II bursts with fast (≥500 km s−1), slow (< 500 km s−1), wide (≥60°), and narrow (< 60°) CMEs. We explored the dependence of type II bursts occurrence on the phases of the solar cycle. Results. Our analysis shows that during solar cycles 23 and 24, 768 and 435 type II bursts occurred, respectively. Of these, 79% were associated with CMEs in solar cycle 23, and 95% were associated with CMEs in solar cycle 24. However, only 4% and 3% of the total number of CMEs were accompanied by type II bursts in solar cycle 23 and 24, respectively. Most of the type II bursts in both cycles were related to fast and wide CMEs (48%). We also determined the typical drift rate and duration for type II bursts, which is 0.06 MHz s−1 and 9 min. Our results suggest that type II bursts dominate at heights ≈1.7 − 2.3 ± 0.3 R⊙. A clear majority have an onset height around 1.7 ± 0.3 R⊙ assuming the four-fold Newkirk model. Conclusions. The results indicate that most of the type II bursts had a white-light CME counterpart, but a few type II bursts lacked a clear CME association. There were more CMEs in cycle 24 than in cycle 23. However, cycle 24 contained fewer type II radio bursts than cycle 23. The onset heights of type II bursts and their association with wide CMEs reported in this study indicate that the early lateral expansion of CMEs may play a key role in the generation of these radio bursts.

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.

Observations of a Flare-ignited Broad Quasiperiodic Fast-propagating Wave Train

Large-scale extreme-ultraviolet (EUV) waves are frequently observed as an accompanying phenomenon of flares and coronal mass ejections (CMEs). Previous studies mainly focused on EUV waves with single wave fronts that are generally thought to be driven by the lateral expansion of CMEs. Using high spatiotemporal resolution multi-angle imaging observations taken by the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory, we present the observation of a broad quasiperiodic fast-propagating (QFP) wave train composed of multiple wave fronts along the solar surface during the rising phase of a GOES M3.5 flare on 2011 February 24. The wave train transmitted through a lunate coronal hole (CH) with a speed of ∼840 ± 67 km s−1, and the wave fronts showed an intriguing refraction effect when they passed through the boundaries of the CH. Due to the lunate shape of the CH, the transmitted wave fronts from the north and south arms of the CH started to approach each other and finally collided, leading to a significant intensity enhancement at the collision site. This enhancement might hint at the occurrence of interference between the two transmitted wave trains. The estimated magnetosonic Mach number of the wave train is about 1.13, which indicates that the observed wave train was a weak shock. Period analysis reveals that the period of the wave train was ∼90 s, in good agreement with that of the accompanying flare. Based on our analysis results, we conclude that the broad QFP wave train was a large-amplitude fast-mode magnetosonic wave or a weak shock driven by some nonlinear energy release processes in the accompanying flare.

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.

Investigating the Cross Sections of Coronal Mass Ejections through the Study of Nonradial Flows with STEREO/PLASTIC

Abstract The solar wind, when measured close to 1 au, is found to flow mostly radially outward. There are, however, periods when the flow makes angles up to 15° away from the radial direction, both in the east–west and north–south directions. Stream interaction regions (SIRs) are a common cause of east–west flow deflections. Coronal mass ejections (CMEs) may be associated with nonradial flows in at least two different ways: (1) the deflection of the solar wind in the sheath region, especially close to the magnetic ejecta front boundary, may result in large nonradial flows; and (2) the expansion of the magnetic ejecta may include a nonradial component, which should be easily measured when the ejecta is crossed away from its central axis. In this work, we first present general statistics of nonradial solar wind flows as measured by STEREO/PLASTIC throughout the first 13 yr of the mission, focusing on solar cycle variation. We then focus on the larger deflection flow angles and determine that most of these are associated with SIRs near solar minimum and with CMEs near solar maximum. However, we find no clear evidence of strongly deflected flows, as would be expected if large deflections around the magnetic ejecta or ejecta with elliptical cross sections with large eccentricities were common. We use these results to develop a better understanding of CME expansion and the nature of magnetic ejecta, and point to shortcomings in our understanding of CMEs.

Temperature and Thermal Energy of a Coronal Mass Ejection

Due to the scarcity of UV–EUV observations of coronal mass ejections (CMEs) far from the Sun (i.e., at heliocentric distances larger than 1.5 Rsun) our understanding of the thermodynamic evolution of these solar phenomena is still very limited. This work focuses on the analysis of a slow CME observed at the same time and in the same coronal locations in visible light (VL) by the MLSO Mark IV polarimeter and in the UV Lyman-α by the SOHO UVCS spectrometer. The eruption was observed at two different heliocentric distances (1.6 and 1.9 Rsun), making this work a test case for possible future multi-slit observations of solar eruptions. The analysis of combined VL and UV data allows the determination of 2D maps of the plasma electron density and also the plasma electron temperature, thus allowing the quantification of the distribution of the thermal energy density. The results show that the higher temperatures in the CME front are due to simple adiabatic compression of pre-CME plasma, while the CME core has a higher temperature with respect to the surrounding CME void and front. Despite the expected adiabatic cooling, the CME core temperatures increased between 1.6 and 1.9 Rsun from 2.4 MK up to 3.2 MK, thus indicating the presence of plasma heating processes occurring during the CME expansion. The 2D distribution of thermal energy also shows a low level of symmetry with respect to the CME propagation axis, possibly related with the CME interaction with nearby coronal structures. This work demonstrates the potential of UV and VL data combination and also of possible future multi-slit spectroscopic observations of CMEs.

Multi-spacecraft observations of the structure of the sheath of an interplanetary coronal mass ejection and related energetic ion enhancement

Context. Sheath regions ahead of coronal mass ejections (CMEs) are large-scale heliospheric structures that form gradually with CME expansion and propagation from the Sun. Turbulent and compressed sheaths could contribute to the acceleration of charged particles in the corona and in interplanetary space, but the relation of their internal structure to the particle energization process is still a relatively little studied subject. In particular, the role of sheaths in accelerating particles when the shock Mach number is low is a significant open research problem. Aims. This work seeks to provide new insights on the internal structure of CME-driven sheaths with regard to energetic particle enhancements. A good opportunity to achieve this aim was provided by multi-point, in-situ observations of a sheath region made by radially aligned spacecraft at 0.8 and ∼1 AU (Solar Orbiter, the L1 spacecraft Wind and ACE, and BepiColombo) on April 19−21, 2020. The sheath was preceded by a weak and slowly propagating fast-mode shock. Methods. We apply a range of analysis techniques to in situ magnetic field, plasma and particle observations. The study focuses on smaller scale sheath structures and magnetic field fluctuations that coincide with energetic ion enhancements. Results. Energetic ion enhancements were identified in the sheath, but at different locations within the sheath structure at Solar Orbiter and L1. Magnetic fluctuation amplitudes at inertial-range scales increased in the sheath relative to the solar wind upstream of the shock, as is typically observed. However, when normalised to the local mean field, fluctuation amplitudes did not increase significantly; magnetic compressibility of fluctuation also did not increase within the sheath. Various substructures were found to be embedded within the sheath at the different spacecraft, including multiple heliospheric current sheet (HCS) crossings and a small-scale flux rope. At L1, the ion flux enhancement was associated with the HCS crossings, while at Solar Orbiter, the ion enhancement occurred within a compressed, small-scale flux rope. Conclusions. Several internal smaller-scale substructures and clear difference in their occurrence and properties between the used spacecraft was identified within the analyzed CME-driven sheath. These substructures are favourable locations for the energization of charged particles in interplanetary space. In particular, substructures that are swept from the upstream solar wind and compressed into the sheath can act as effective acceleration sites. A possible acceleration mechanism is betatron acceleration associated with a small-scale flux rope and warped HCS compressed in the sheath, while the contribution of shock acceleration to the latter cannot be excluded.

Investigating Width Distribution of Slow and Fast CMEs in Solar Cycles 23 and 24

Coronal Mass Ejections (CMEs) are highly dynamic events originating in the solar atmosphere, that show a wide range of kinematic properties and are the major drivers of the space weather. The angular width of the CMEs is a crucial parameter in the study of their kinematics. The fact that whether slow and fast CMEs (as based on their relative speed to the average solar wind speed) are associated with different processes at the location of their ejection is still debatable. Thus, in this study, we investigate their angular width to understand the differences between the slow and fast CMEs. We study the width distribution of slow and fast CMEs and find that they follow different power law distributions, with a power law indices (α) of –1.1 and –3.7 for fast and slow CMEs respectively. To reduce the projection effects, we further restrict our analysis to only limb events as derived from manual catalog and we find similar results. We then associate the slow and fast CMEs to their source regions, and classified the sources as Active Regions (ARs) and Prominence Eruptions. We find that slow and fast CMEs coming from ARs and PEs, also follow different power laws in their width distributions. This clearly hints toward a possibility that different mechanisms might be involved in the width expansion of slow and fast CMEs coming from different sources.These results are also crucial from the space weather perspective since the width of the CME is an important factor in that aspect.

A geometrical description for interplanetary propagation of Earth-directed CMEs based on radiative proxies

ABSTRACT We present a 3D geometrical model to describe the propagation and expansion of coronal mass ejections (CMEs) in the interplanetary space based on radiative proxies to be implemented in previous procedures that use SXR and microwave emissions to estimate the Earth-directed CME propagation speed. We carefully selected a sample of 45 well-defined CME-ICME events to evaluate our model. We computed this 3D geometrical model for each event as a tool to improve the arrival time predictions based on radiative proxies. We conducted a different analysis for each radiative proxy: SXR emission and microwave emission at 9 GHz, and we compared the results separately with the observations by the Wind spacecraft. In general, the results showed that the implementation of our 3D geometrical model improves the predictions and provides an important complement to the arrival time prediction method based on radiative proxies, especially for CME events whose source origin were located at helio longitudes far from the central meridian at least 10°. Improvements for this tool based on observations by Parker Solar Probe and Solar Orbiter must be developed in the future work.

Moving solar radio bursts and their association with coronal mass ejections

Context. Solar eruptions, such as coronal mass ejections (CMEs), are often accompanied by accelerated electrons that can in turn emit radiation at radio wavelengths. This radiation is observed as solar radio bursts. The main types of bursts associated with CMEs are type II and type IV bursts that can sometimes show movement in the direction of the CME expansion, either radially or laterally. However, the propagation of radio bursts with respect to CMEs has only been studied for individual events. Aims. Here, we perform a statistical study of 64 moving bursts with the aim to determine how often CMEs are accompanied by moving radio bursts. This is done in order to ascertain the usefulness of using radio images in estimating the early CME expansion. Methods. Using radio imaging from the Nançay Radioheliograph (NRH), we constructed a list of moving radio bursts, defined as bursts that move across the plane of sky at a single frequency. We define their association with CMEs and the properties of associated CMEs using white-light coronagraph observations. We also determine their connection to classical type II and type IV radio burst categorisation. Results. We find that just over a quarter of type II and half of type IV bursts that occurred during the NRH observing windows in Solar Cycle 24 are accompanied by moving radio emission. All but one of the moving radio bursts are associated with white–light CMEs and the majority of moving bursts (90%) are associated with wide CMEs (> 60° in width). In particular, all but one of the moving bursts corresponding to type IIs are associated with wide CMEs; however, and unexpectedly, the majority of type II moving bursts are associated with slow white–light CMEs (< 500 km s−1). On the other hand, the majority of moving type IV bursts are associated with fast CMEs (> 500 km s−1). Conclusions. The observations presented here show that moving radio sources are almost exclusively associated with CMEs. The majority of events are also associated with wide CMEs, indicating that strong lateral expansion during the early stages of the eruption may play a key role in the occurrence of the radio emission observed.

A comparison of CME expansion speeds between solar cycles 23 and 24

We report on a comparison of the expansion speeds of limb coronal mass ejections (CMEs) between solar cycles 23 and 24. We selected a large number of limb CME events associated with soft X-ray flare size greater than or equal to M1.0 from both cycles. We used data and measurement tools available at the online CME catalog (https://cdaw.gsfc.nasa.gov) that consists of the properties of all CMEs detected by the Solar and Heliospheric Observatory’s (SOHO) Large Angle and Spectrometric Coronagraph (LASCO). We found that the expansion speeds in cycle 24 are higher than those in cycle 23. We also found that the relation between radial and expansion speeds has different slopes in cycles 23 and 24. The cycle 24 slope is 45% higher than that in cycle 23. The expansion speed is also higher for a given radial speed. The difference increases with speed. For a 2000 km/s radial speed, the expansion speed in cycle 24 is ∼48% higher. These results present additional evidence for the anomalous expansion of cycle 24-CMEs, which is due to the reduced total pressure in the heliosphere.

Research progress on coronal extreme ultraviolet waves

<p indent="0mm">The hot, tenuous coronal plasma medium can support the propagation of various kinds of magnetohydrodynamic (MHD) waves, such as Alfvén waves, fast and slow magnetosonic waves. These MHD waves are important for understanding the enigmatic problems of coronal heating and the acceleration of solar winds, as well as the basic physical property of the solar atmosphere and the physics behind solar eruptions. In this review, we mainly introduce two types of fast-propagating extreme ultraviolet (EUV) magnetosonic waves in the corona, namely, the large-scale (global) EUV waves and the quasi-periodic fast-propagating (QFP) magnetosonic waves. EUV waves are large-scale propagating disturbances in the corona; they are intimately related to violent solar eruptions such as flares, coronal mass ejections (CME), and radio type II bursts. In history, large-scale propagating disturbances were firstly discovered in the chromosphere in 1960s, and they were called Moreton waves. In theory, the dense chromosphere can not support the fast propagating of Moreton waves. Therefore solar physicists explained Moreton waves as the chromosphere responses of fast magnetosonic or shock waves in the corona, although people did not detected corona waves in that era due to the lack of coronal observations. Until the 1990s, the long-expected similar large-scale fast-propagating coronal disturbances called EUV waves were observed by the Extreme ultraviolet Imaging Telescope (EIT) onboard the Solar and Heliosphereic Observatory (SOHO). EUV waves were initially thought to be fast mode magnetosonic waves driven by flare pressure pulses, and can be regarded as the coronal counterparts of Moreton waves. However, latter observations raised questions about their driving mechanism and physical nature. For their driving source it was unclear that EUV waves are driven by flare pressure pulses or CMEs. For their physical nature it was unclear that EUV waves are true MHD waves or apparent motions caused by reconfiguration of large-scale coronal magnetic fields or other mechanisms. Thanks to the high spatiotemporal resolution and multi-angle observations provided by the Solar Terrestrial Relations observatory (STEREO) and the Solar Dynamics Observatory (SDO) in recent years, we have achieved a deeper and complete understanding about the generation and physical properties of EUV waves. A common consensus reached in recent years is that at least two types of EUV waves can be detected during the eruption of a CME. One is a fast mode magnetosonic wave or shock at a speed ranged from several hundred to more than one thousand km/s, which corresponds to the coronal counterpart of a chromosphere Moreton wave; the other one whose physical nature is unclear, propagates following the fast one with a low speed generally below 500 km/s. For the driving sources of EUV waves, the majority of high resolution observations showed that they are driven by the lateral expansion of CMEs, and a few studies suggested that they can also be excited by other mechanisms such as flare pressure pulses, sudden loop expansion caused by ambient solar eruptions, coronal jets, and expanding motions of unwinding helical structure of filaments. The high spatiotemporal resolution observations taken by the SDO discovered a new type of fast mode waves called QFP waves, which have multiple wavefronts and generally propagate along magnetic field lines with a speed in the range of several hundred to more than 2000 km/s, and their periods are often similar to the quasi-periodic pulsations in the associate flares. Here we present a summary of the recent research progress about the two types of EUV waves in the corona, and try to point out the key and difficult issues, as well as the possible research topics in the future.

Inconsistencies Between Local and Global Measures of CME Radial Expansion as Revealed by Spacecraft Conjunctions

Abstract The radial expansion of coronal mass ejections (CMEs) is known to occur from remote observations, from the variation of their properties with radial distance, and from local in situ plasma measurements showing a decreasing speed profile throughout the magnetic ejecta (ME). However, little is known on how local measurements compare to global measurements of expansion. Here, we present results from the analysis of 42 CMEs measured in the inner heliosphere by two spacecraft in radial conjunction. The magnetic-field decrease with distance provides a measure of their global expansion. Near 1 au, the decrease in their bulk speed provides a measure of their local expansion. We find that these two measures have little relation with each other. We also investigate the relation between characteristics of CME expansion and CME properties. We find that the expansion depends on the initial magnetic-field strength inside the ME, but not significantly on the magnetic field inside the ME measured near 1 au. This is indirect evidence that CME expansion in the innermost heliosphere is driven by the high magnetic pressure inside the ME, while by the time the MEs reach 1 au, they are expanding due to the decrease in the solar-wind dynamic pressure with distance. We also determine the evolution of the ME tangential and normal magnetic-field components with distance, revealing significant deviations as compared to the expectations from force-free field configurations as well as some evidence that the front half of MEs expand at a faster rate than the back half.