High-speed Coronal Mass Ejections Research Articles

A Statistical Study of Solar Filament Eruptions that Form High-speed Coronal Mass Ejections

Abstract Coronal mass ejections (CMEs) play a decisive role in driving space weather, especially the fast ones (e.g., with speeds above 800 km s−1). Understanding the trigger mechanisms of fast CMEs can help us gain important information in forecasting them. The filament eruptions accompanied with CMEs provide a good tracer in studying the early evolution of CMEs. Here we surveyed 66 filament-accompanied fast CMEs to analyze the correlation between the trigger mechanisms, namely either magnetic reconnection or ideal MHD process, associated flares, and CME speeds. Based on the data gathered from SDO, GONG, and STEREO, we find that (1) active region (AR) filament and intermediate filament (IF) eruptions show a higher probability for producing fast CMEs than quiet Sun (QS) filaments, while the probability of polar crown (PC) filament eruptions is zero in our statistic; (2) AR filament eruptions that produce fast CMEs are more likely to be triggered by magnetic reconnection, while QS filaments and IFs are more likely to be triggered by an ideal MHD process; (3) for AR filaments and IFs, it seems that the specific trigger mechanism does not have a significant influence on the resulting CME speeds, while for the QS filaments, the ideal MHD mechanism can more likely generate a faster CME; (4) comparing with previous statistical studies, the onset heights of filament eruptions and the decay indexes of the overlying field show some differences: for AR filaments and IFs, the decay indexes are larger and much closer to the theoretical threshold, while for QS filaments, the onset heights are higher than those obtained in previous results.

Особенности развития длительных потоков высокоэнергичного гамма-излучения на разных стадиях солнечных вспышек

We have studied properties of sustained gamma fluxes having quantum energies of >100 MeV at different stages of flares with 1-min temporal resolution (Fermi/LAT). 
 The most probable process of emergence of high-energy gamma-quanta during the impulsive phase of flares (6 events) has been confirmed. Acceleration of particles, produced by flare energy release (at dissipation of current sheet), occurs when they interact with a shock front of a coronal mass ejection (CME), which develops in the same active region at the same time. Nuclear interactions of accelerated protons (>500 MeV) with plasma ions lead further to the emergence of high-energy gamma-quanta. We have established that the interaction between a flare flux and a high-speed CME during the flare impulsive phase occurs within fairly limited periods — from 2 to 16 min. In the events considered, we have found a direct connection between maximum gamma flux F max (γ > 100 MeV) and CME velocity.
 High maximum values of gamma fluxes are typical of the flare impulsive phase: 3.5·10⁻⁴ cm⁻²s⁻¹ ≤ F max (γ > 100 MeV) ≤ 1.3·10⁻² cm⁻² s⁻¹. At the same time, the value F max (γ > 100 MeV) = 0.013 cm⁻²s⁻¹ was the highest for the events observed by Fermi/LAT from 2008 to 2017.
 During the development of CMEs moving with a supersonic speed, shock waves are formed which are the major power source of accelerated particles during the main phase of gradual flares. In some cases, however, the impact of shock waves on particle acceleration is the greatest in the short impulsive phase.
 To reveal parameters most effectively influencing the generation of high-energy gamma-ray emission, we have compared 17 flare events. The most significant parameter proved to be the time interval of joint action of flare process and CME shocks.
 We have established that during simultaneous development of flare process and CME attendant on the flare, the most efficient particle acceleration occurs which gives rise to maximum fluxes of high-energy gamma-quanta.

Features of development of sustained fluxes of high-energy gamma-ray emission at different stages of solar flares

We have studied properties of sustained gamma fluxes having quantum energies of >100 MeV at different stages of flares with 1-min temporal resolution (Fermi/LAT). 
 The most probable process of emergence of high-energy gamma-quanta during the impulsive phase of flares (6 events) has been confirmed. Acceleration of particles, produced by flare energy release (at dissipation of current sheet), occurs when they interact with a shock front of a coronal mass ejection (CME), which develops in the same active region at the same time. Nuclear interactions of accelerated protons (>500 MeV) with plasma ions lead further to the emergence of high-energy gamma-quanta. We have established that the interaction between a flare flux and a high-speed CME during the flare impulsive phase occurs within fairly limited periods — from 2 to 16 min. In the events considered, we have found a direct connection between maximum gamma flux F max (γ > 100 MeV) and CME velocity.
 High maximum values of gamma fluxes are typical of the flare impulsive phase: 3.5·10⁻⁴ cm⁻²s⁻¹ ≤ F max (γ > 100 MeV) ≤ 1.3·10⁻² cm⁻² s⁻¹. At the same time, the value F max (γ > 100 MeV) = 0.013 cm⁻²s⁻¹ was the highest for the events observed by Fermi/LAT from 2008 to 2017.
 During the development of CMEs moving with a supersonic speed, shock waves are formed which are the major power source of accelerated particles during the main phase of gradual flares. In some cases, however, the impact of shock waves on particle acceleration is the greatest in the short impulsive phase.
 To reveal parameters most effectively influencing the generation of high-energy gamma-ray emission, we have compared 17 flare events. The most significant parameter proved to be the time interval of joint action of flare process and CME shocks.
 We have established that during simultaneous development of flare process and CME attendant on the flare, the most efficient particle acceleration occurs which gives rise to maximum fluxes of high-energy gamma-quanta.

THE RELATIONSHIP BETWEEN THE PARTICLE INJECTION RATE AND THE DISPERSION OF THE SCATTERING ANGULAR DISTRIBUTION

Although non-linear diffusive shock acceleration could be simulated by some well-established models, the particle injection rate is a priori assumed to proceed from thermal particles to the superthermal population in some models that use numerical simulations. In order to investigate the effect of anisotropy in particle scattering on the particle injection rate, we present a dynamical Monte Carlo simulation of a bow shock from Earth with different dispersions in the pitch-angle scattering. Since the particle injection rate is intrinsically defined by the prescribed scattering law in a Monte Carlo approach, we can directly study the relationship between the particle injection rate and the dispersion of the particle scattering angular distribution. As a result, we find that the particle injection rate increases as the dispersion of the scattering angular distribution increases. The shock energy spectrum also becomes harder as the scattering angular distribution varies from a situation with anisotropy to one with isotropy. If the particle injection rate can dominate the shock strength, that may imply that a low-speed coronal mass ejection (CME) could drive a strong shock and a high-speed CME could drive a weak shock.

Origin of wide-band IP type II bursts

Context. Different types of interplanetary (IP) type II bursts have been observed, where the more usual ones show narrow-band and patchy emissions, sometimes with harmonics, and which at intervals may disappear completely from the dynamic spectrum. The more unusual bursts are wide-band and diffuse, show no patches or breaks or harmonic emission, and often have long durations. Type II bursts are thought to be plasma emission, caused by propagating shock waves, but a synchrotron-emitting source has also been proposed as the origin for the wide-band type IIs.Aims. Our aim is to find out where the wide-band IP type II bursts originate and what is their connection to particle acceleration.Methods. We analyzed in detail 25 solar events that produced well-separated, wide-band IP type II bursts in 2001–2011. Their associations to flares, coronal mass ejections (CMEs), and solar energetic particle events (SEPs) were investigated.Results. Of the 25 bursts, 18 were estimated to have heights corresponding to the CME leading fronts, suggesting that they were created by bow shocks ahead of the CMEs. However, seven events were found in which the burst heights were significantly lower and which showed a different type of height-time evolution. Almost all the analyzed wide-band type II bursts were associated with very high-speed CMEs, originating from different parts of the solar hemisphere. In terms of SEP associations, many of the SEP events were weak, had poor connectivity due to the eastern limb source location, or were masked by previous events. Some of the events had precursors in specific energy ranges. These properties and conditions affected the intensity-time profiles and made the injection-time-based associations with the type II bursts difficult to interpret. In several cases where the SEP injection times could be determined, the radio dynamic spectra showed other features (in addition to the wide-band type II bursts) that could be signatures of shock fronts.Conclusions. We conclude that in most cases (in 18 out of 25 events) the wide-band IP type II bursts can be plasma emission, formed at or just above the CME leading edge. The results for the remaining seven events might suggest the possibility of a synchrotron source. These events, however, occurred during periods of high solar activity, and coronal conditions affecting the results of the burst height calculations cannot be ruled out. The observed wide and diffuse emission bands may also indicate specific CME leading edge structures and special shock conditions.

Multiwavelength Study on Solar and Interplanetary Origins of the Strongest Geomagnetic Storm of Solar Cycle 23

We study the solar sources of an intense geomagnetic storm of solar cycle 23 that occurred on 20 November 2003, based on ground- and space-based multiwavelength observations. The coronal mass ejections (CMEs) responsible for the above geomagnetic storm originated from the super-active region NOAA 10501. We investigate the H-alpha observations of the flare events made with a 15 cm solar tower telescope at ARIES, Nainital, India. The propagation characteristics of the CMEs have been derived from the three-dimensional images of the solar wind (i.e., density and speed) obtained from the interplanetary scintillation data, supplemented with other ground- and space-based measurements. The TRACE, SXI and H-alpha observations revealed two successive ejections (of speeds ~350 and ~100 km/s), originating from the same filament channel, which were associated with two high speed CMEs (~1223 and ~1660 km/s, respectively). These two ejections generated propagating fast shock waves (i.e., fast drifting type II radio bursts) in the corona. The interaction of these CMEs along the Sun-Earth line has led to the severity of the storm. According to our investigation, the interplanetary medium consisted of two merging magnetic clouds (MCs) that preserved their identity during their propagation. These magnetic clouds made the interplanetary magnetic field (IMF) southward for a long time, which reconnected with the geomagnetic field, resulting the super-storm (Dst_peak=-472 nT) on the Earth.

Distribution and clustering of fast coronal mass ejections

[1] The purpose of this paper is to investigate the statistical properties of high-speed coronal mass ejections (fast CMEs), which play a major role in Space Weather. We study the cumulative distribution of the initial CME speeds applying a new, advanced statistical method based on the scaling properties of averages of maximal speeds selected in time intervals of fixed sizes. This method allows us for the first time to obtain a systematic statistical description of the fast CME speeds. Using this method, we identify a self-similar (power law) high-speed portion of the spectrum of the speed maxima in the range of speeds from about 700 km/s to 2000 km/s. This self-similar range of the speed distribution provides a meaningful definition of “the fast” CMEs and indicates that these CMEs are produced by a process that is the same across the range of scales. The investigation of the temporal behavior of the fast CME events indicates that the time intervals between fast CMEs are not independent, i.e., fast CMEs arrive in clusters. We characterize the fast CMEs clustering by the exponent θ called the extremal index, which is the inverse of the averaged number of CMEs per cluster. An independent correlation analysis of the tail of the CME distribution confirms and further quantifies the temporal dependence among the fast CME events. To illustrate the predictive capabilities of the method, we identify clusters in the time series of CMEs with speeds greater than 1000 km/s and calculate their statistical characteristics such as the size and duration of the clusters. The method used in this paper can be applied to many other extreme geophysical events.

Are halo coronal mass ejections special events?

We revisited the properties of wide coronal mass ejections (CMEs) called halo CMEs. Using the large LASCO/SOHO CMEs data set, from 1996 to 2004, we examined the statistical properties of (partial and full) halo CMEs and compare with the same properties of “normal” width (lower than 120°) CMEs. We found that halo CMEs have different properties than “normal” CMEs, which cannot be explained merely by the current geometric interpretation that they are seen as halos because they are traveling in the Sun Earth direction. We found that the CME width distribution is formed by, at least, three different populations: Two gaussians: a narrow and a medium distribution centered at ∼17° and ∼38°, respectively; the narrow population most likely corresponds to the “true” observed widths, whereas the medium width population is the product of projection effects. The third distribution corresponds to wider CMEs (80° < W < 210°) which behaves as a power law. Partial and full halo CMEs wider than these do not follow any particular distribution. This lack of regularity may be due to the small number of such events. In particular, we found (and test by a statistical approach) that the number of observed full halo CMEs is lower than expected. The CME speed follows a log‐normal distribution, except for the very low speed CME population, which follows a gaussian distribution centered at ∼100 km/s and is probably due to projection effects. When the CMEs are divided by width into nonhalo, partial halo, and full halo, we found that the peaks of the distributions are shifted toward higher speeds, ∼300, ∼400 and ∼600 km/s for nonhalo, partial halo, and full halo CMEs, respectively. This confirms that halo CMEs tend to be high speed CMEs. The acceleration of full halo CMEs tends to be more negative compared with nonhalo and partial halo CMEs. We introduce a new observational CME parameter: The final observed distance (FOD), i.e., the highest point within the coronograph field of view where a CME can be distinguished from the background. In other words, the highest CME altitude measured. The FOD for nonhalo CMEs decreases exponentially from ∼5 to ∼30 R⊙ in the LASCO field of view. On the other hand, the FOD of halo CMEs increase with distance. This means that it is more likely to see halo CMEs at large distances (from the Sun) than nonhalo CMEs. These halo CME properties may be explained if the white light wide enhancements (or halo) seen by coronographs correspond to a combination of an expanding (shock) wave which disturbs and/or compresses the ambient material and the CME material itself.