CME Speed Research Articles

Near-Earth Interplanetary Coronal Mass Ejections and Their Association with DH Type II Radio Bursts During Solar Cycles 23 and 24

We analyse the characteristics of interplanetary coronal mass ejections (ICMEs) during Solar Cycles 23 and 24. The present analysis is primarily based on the near-Earth ICME catalogue (Richardson and Cane, 2010). An important aspect of this study is to understand the near-Earth and geoeffective aspects of ICMEs in terms of their association (type II ICMEs) versus absence (non-type II ICMEs) of decameter-hectometer (DH) type II radio bursts, detected by Wind/WAVES and STEREOS/WAVES. Notably, DH type II radio bursts driven by a CME indicate powerful MHD shocks leaving the inner corona and entering the interplanetary medium. We find a drastic reduction in the occurrence of ICMEs by 56% in Solar Cycle 24 compared to the previous cycle (64 versus 147 events). Interestingly, despite a significant decrease in ICME/CME counts, both cycles contain almost the same fraction of type II ICMEs (~47%). Our analysis reveals that, even at a large distance of 1 AU, type II CMEs maintain significantly higher speeds compared to non-type II events (523 km/s versus 440 km/s). While there is an obvious trend of decrease in ICME transit times with increase in the CME initial speed, there also exists a noticeable wide range of transit times for a given CME speed. Contextually, Cycle 23 exhibits 10 events with shorter transit times ranging between 20-40 hours of predominantly type II categories while, interestingly, Cycle 24 almost completely lacks such "fast" events. We find a significant reduction in the parameter $V_{ICME} \times B_{z}$, the dawn to dusk electric field, by 39% during Solar Cycle 24 in comparison with the previous cycle. Further, $V_{ICME} \times B_{z}$ shows a strong correlation with Dst index, which even surpasses the consideration of $B_{z}$ and $V_{ICME}$ alone. The above results imply the crucial role of $V_{ICME} \times B_{z}$ toward effectively modulating the geoeffectiveness of ICMEs.

Quantifying errors in 3D CME parameters derived from synthetic data using white-light reconstruction techniques

Current efforts in space weather forecasting of CMEs have been focused on predicting their arrival time and magnetic structure. To make these predictions, methods have been developed to derive the true CME speed, size, position, and mass, among others. Difficulties in determining the input parameters for CME forecasting models arise from the lack of direct measurements of the coronal magnetic fields and uncertainties in estimating the CME 3D geometric and kinematic parameters after eruption. White-light coronagraph images are usually employed by a variety of CME reconstruction techniques that assume more or less complex geometries. This is the first study from our International Space Science Institute (ISSI) team “Understanding Our Capabilities in Observing and Modeling Coronal Mass Ejections”, in which we explore how subjectivity affects the 3D CME parameters that are obtained from the Graduated Cylindrical Shell (GCS) reconstruction technique, which is widely used in CME research. To be able to quantify such uncertainties, the “true” values that are being fitted should be known, which are impossible to derive from observational data. We have designed two different synthetic scenarios where the “true” geometric parameters are known in order to quantify such uncertainties for the first time. We explore this by using two sets of synthetic data: 1) Using the ray-tracing option from the GCS model software itself, and 2) Using 3D magnetohydrodynamic (MHD) simulation data from the Magnetohydrodynamic Algorithm outside a Sphere code. Our experiment includes different viewing configurations using single and multiple viewpoints. CME reconstructions using a single viewpoint had the largest errors and error ranges overall for both synthetic GCS and simulated MHD white-light data. As the number of viewpoints increased from one to two, the errors decreased by approximately 4° in latitude, 22° in longitude, 14° in tilt, and 10° in half-angle. Our results quantitatively show the critical need for at least two viewpoints to be able to reduce the uncertainty in deriving CME parameters. We did not find a significant decrease in errors when going from two to three viewpoints for our specific hypothetical three spacecraft scenario using synthetic GCS white-light data. As we expected, considering all configurations and numbers of viewpoints, the mean absolute errors in the measured CME parameters are generally significantly higher in the case of the simulated MHD white-light data compared to those from the synthetic white-light images generated by the GCS model. We found the following CME parameter error bars as a starting point for quantifying the minimum error in CME parameters from white-light reconstructions: Δθ (latitude)=6°-3°+2°, Δϕ (longitude)=11°-6°+18°, Δγ (tilt)=25°-7°+8°, Δα(half-angle)=10°-6°+12°, Δh (height)=0.6-0.4+1.2R⊙, and Δκ (ratio)=0.1-0.02+0.03.

Dependence of Energetic Storm Particle Heavy Ion Peak Intensities and Spectra on Source CME Longitude and Speed

We examine variations in energetic storm particle (ESP) heavy ion peak intensities and energy spectra at CME-driven interplanetary shocks. We focus on their dependence with heliolongitude relative to the source region of their associated CMEs, and with CME speed, for events observed in Solar Cycle 24 at the STEREO-A, STEREO-B, and/or ACE spacecraft. We find that observations of ESP events at 1 au are organized by longitude relative to their CME solar source. The ESP event longitude distribution also showed organization with CME speed. The near-Sun CME speeds (V i ) for these events ranged from ∼560 to 2650 km s−1 while the average CME transit speeds to 1 au were significantly slower. The angular width of the events had a clear threshold at V i of ∼1300 km s−1, above which events showed significantly larger angular extension compared to events with speeds below. High-speed events also showed larger heavy ion peak intensities near the nose of the shock compared to the flanks while their spectral index was smaller near the nose and larger near the flanks. This organization for events with V i < 1300 km s−1 was not as clear. These ESP events were observed over a narrower range of longitudes though the heavy ion peak intensities still appeared largest near the nose of the shock. Their heavy ion spectra showed no clear organization with longitude. These observations highlight the impact of spacecraft position relative to the CME source longitude and V i on the properties of ESP events at 1 au.

Estimating the Transit Speed and Time of Arrival of Interplanetary Coronal Mass Ejections Using CME and Solar Flare Data

The dependence of Interplanetary Coronal Mass Ejections’ (ICMEs) transit speed on the corresponding Coronal Mass Ejections’ (CMEs) initial speed is investigated. It is shown that the transit speed and transit time depend not only on the CME’s initial speed, but also on the longitude of the solar source. The longitudinal dependence of the expected transit speeds and times are obtained from the analysis of 288 CMEs, associated with solar flares, observed from 1995 to 2020. A model, estimating the transit and maximum speeds, as well as the time of arrival of an ICME to Earth, based on the initial CME speed and the longitude of the associated solar flare has been created. It is shown that taking into account the longitude of the solar source in addition to the initial CME speed significantly improves the quality of the model, especially for events in the central part of the solar disk (E10°–W10°). The simplicity of the described model makes it accessible to a wide range of users and provides opportunities for further improvement as the statistics and the number of input parameters increase.

Assessing the spectral characteristics of band splitting type II radio bursts observed by CALLISTO spectrometers

Metric type II radio bursts are usually early indicators of CME-driven shocks and other space weather phenomena in the solar corona. This paper presents a detailed investigation of the spectral properties of band splitting type II radio bursts and their association with sunspot number. Using type II radio bursts in a frequency range [20 MHz–200 MHz] observed by CALLISTO during 2010–2017, it was discovered that the analyzed type II shock height, magnetic field strength, CME/shock speed, and Alfvén speed synchronize with the trajectory of the solar cycle 24. Also, the study revealed that the onset of the declining phase of solar cycle 24 has the highest electron density, whiles the frequency of type II bursts depicts a bimodal distribution during the study period, with peaks in 2012 and 2015. Further, a good correlation (with correlation factor R = 0.87) was obtained between the estimated CME/shock speeds from the dynamic spectra and the associated CME speeds from SOHO/LASCO. Moreover, the study confirmed a significant correlation (R= ∼0.8) between the absolute drift rates and the plasma frequency. Additionally, the study explored that ∼ 60% of the type II radio bursts considered in this study emanated from the western longitudes. Hence, the findings emphasize that the temporal dynamics of the physical conditions of band splitting type II radio bursts are essential parameters for space weather monitoring and forecasting.

Analysis of Type II radio burst relationship with CME driven shocks

Type II radio bursts are known to be the signatures of coronal shocks. In this paper we examine the relationship between 129 type II bursts in the frequency range 35 – 450 MHz observed at Culgooora observatory during May 2002 – October 2015 and the associated CMEs. We apply Newkirk (1961) density model to determine the formation height of type IIs. We find that in 109/129 cases, type II bursts were preceded/ succeeded by CMEs. The CME associated type II events in which the CME height is above the type II burst source are categorized as group I events (91/129 cases). 91% of the bursts in this group are also associated with flares and 58% of these bursts originate during decaying phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.33.The CME associated type IIs in which the CME height is below the type II source are categorized as group II (18/129 cases). CME driven shock could have been the exciter of these type II bursts.88% of this group events are associated with flares and 62% of these bursts originate during the rising phase of the flare. The correlation between CME speed and type II shock speed for limb events in this group is 0.96. In 20/129 cases of our data set, type II bursts are not associated with CME and are categorized as group III. 90% of the bursts in this group are associated with flares. 77% of the bursts in the group are originating in the decaying phase of flares. Poor temporal association (9/69 cases) between type IIs and flares of X class during this period. Our results suggest that inspite of temporal association with metric type II bursts, majority of the CME driven shocks (84%) are not successful in exciting type II bursts in 35-450 MHz domain. The type II bursts temporally correlated with CMEs and likely to have been excited by CMEs (type II height &gt; CME height) are originating during the rising phase of the flares in majority of the events. In case of type II bursts temporally correlated with CMEs supposedly not excited by the CMEs (type II height &lt; CME height) ,majority of them are originating in the decaying phase of flares.

Analysis of Type II and Type III Radio Bursts Associated with SEPs from Non-Interacting/Interacting Radio-Loud CMEs

We analyze radio bursts observed in events with interacting/non-interacting CMEs that produced major SEPs (Ip > 10 MeV) from April 1997 to December 2014. We compare properties of meter (m), decahectometer (DH) type II and DH type III bursts, and time lags for interacting-CME-associated (IC) events and non-interacting-CME-associated (NIC) events. About 70% of radio emissions were observed in events of both types from meters to kilometers. We found high correlations between the drift rates and mid-frequencies of type II radio bursts calculated as the mean geometric between their starting and ending frequencies for both NIC and IC-associated events (correlation coefficient R2 = 0.98, power-law index e = 1.68 ± 0.16 and R2 = 0.93, e = 1.64 ± 0.19 respectively). We also found a correlation between the frequency drift rates of DH type II bursts and space speeds of CMEs in NIC-associated events. The absence of such correlation for IC-associated events confirms that the shock speeds changed in CME-CME interactions. For the events with western source locations, the mean peak intensity of SEPs in IC-associated events is four times larger than that in NIC-associated SEP events. From the mean time lags between the start times of SEP events and the start of m, DH type II, and DH type III radio bursts, we inferred that particle enhancements in NIC-associated SEP events occurred earlier than in IC-associated SEP events. The difference between NIC events and IC events in the mean values of parameters of type II and type III bursts is statistically insignificant.

Improving Solar Wind Forecasting Using Data Assimilation

AbstractData Assimilation (DA) has enabled huge improvements in the skill of terrestrial operational weather forecasting. In this study, we use a variational DA scheme with a computationally efficient solar wind model and in situ observations from STEREO‐A, STEREO‐B and ACE. This scheme enables solar‐wind observations far from the Sun, such as at 1 AU, to update and improve the inner boundary conditions of the solar wind model (at 30 solar radii). In this way, observational information can be used to improve estimates of the near‐Earth solar wind, even when the observations are not directly downstream of the Earth. This allows improved initial conditions of the solar wind to be passed into forecasting models. To this effect, we employ the HUXt solar wind model to produce 27‐day forecasts of the solar wind during the operational lifetime of STEREO‐B (November 01, 2007–September 30, 2014). In near‐Earth space, we compare the accuracy of these DA forecasts with both non‐DA forecasts and simple corotation of STEREO‐B observations. We find that 27‐day root mean square error (RMSE) for STEREO‐B corotation and DA forecasts are comparable and both are significantly lower than non‐DA forecasts. However, the DA forecast is shown to improve solar wind forecasts when STEREO‐B's latitude is offset from the Earth, which is an issue for corotation forecasts. And the DA scheme enables the representation of the solar wind in the whole model domain between the Sun and the Earth to be improved, which will enable improved forecasting of CME arrival time and speed.

On the Sources and Sizes of Uncertainty in Predicting the Arrival Time of Interplanetary Coronal Mass Ejections Using Global MHD Models

AbstractAccurate predictions of the properties of interplanetary coronal mass ejection (ICME)‐driven disturbances are a key objective for space weather forecasts. The ICME's time of arrival (ToA) at Earth is an important parameter, and one that is amenable to a variety of modeling approaches. Previous studies suggest that the best models can predict the arrival time to within an absolute uncertainty of 10–15 h. Here, we investigate the main sources of uncertainty in predicting a CME's ToA at Earth. These can be broken into two main categories: (a) the initial properties of the ejecta, including its speed, mass, and direction of propagation and (b) the properties of the ambient solar wind into which it propagates. To estimate the relative contribution to ToA uncertainties, we construct a set of numerical experiments of cone‐model CMEs, where we vary the initial speed, mass, and direction at the inner radial boundary. Additionally, we build an ensemble of 12 ambient solar wind solutions using realizations from the ADAPT model. We find that each component in the chain contributes between ±2.5 and ±7 h of uncertainty to the estimate of the CME's ToA. Importantly, different realizations of the synoptic produce the largest uncertainties. This suggests that estimates of ToA will continue to be plagued with intrinsic uncertainties of ±10 h until tighter constraints can be found for these boundary conditions. Our results suggest that there are clear benefits to focused investigations aimed at reducing the uncertainties in CME speed, mass, direction, and input boundary magnetic fields.

On properties of coronal mass ejection-solar flare linked events

In the present paper, the relation between the CMEs and their linked solar flares is examined during the entire solar cycle 23. During this solar cycle and under certain temporal and spatial conditions, 521 CMEs linked with solar flares are selected. These selected CMEs are separated into two groups according to their time of detection, 254 Before Flare-ECME events (BF-CMEs) and 267 After Flare–ECME events (AF-CMEs). The results showed that the spatial separation angle between the CMEs and their linked flares has a peak between ± 25°. Also, it was found that there is a trend of decreasing the CME acceleration with the increasing of CME speed. Besides we obtained that the AF-CMEs are on average more massive and energetic than BF-CMEs. Finally, we found that the CME Mass-Width relation of AF-CMEs is more correlated to their associated flare events than BF-CMEs.

The Statistical Analysis of Characteristics of Coronal Mass Ejection During the Descending Phase of Solar Cycle 23 and 24

The characteristics of CMEs we studied are angular width, linear speed, and acceleration for all categories of CMEs such as narrow (W ≤20°), intermediate (20°&lt; W&lt;200°), wide (W ≥ 200°) and linear speed &lt;500 km/s during the descending phase of solar cycle 23 and 24 and compared them. We have found that there are 1951 narrow CMEs during solar cycle 23 that is 1.9 times greater than in solar cycle 24 (1047). On the other side, the number of intermediate CMEs during solar cycle 24 (1571) is 1.14 times more than solar cycle 23 (1162). We observed no noticeable difference between the number of wide CMEs of solar cycle 23 (29) and 24 (36). The angular width of CMEs during the descending phase of solar cycle 23 and solar cycle 24, predominately distributed around 100-600. The fascinating result is that the angular width distributions for the descending phase of solar cycles are approximately identical. On comparing the results of linear speed of both solar cycle, we can say that, (i) 93.7% (1729) and 87.7% (908) of narrow CMEs, (ii) 97% (1328) and 94% (1479) of intermediate CMEs and (iii) 44% (13) and 42% (15) of wide CMEs have speed of &lt;500 km s-1, respectively. Mostly the fractions of narrow and intermediate CMEs decline sharply at the speeds greater than 500 km s-1. The maximum speed observed during the 23rd cycle is 1994 km/s (wide CME) and the 24th cycle is 3163 km/s (wide CME) respectively. It was noticed that the speed of the 24th solar cycle CME is higher than the 23rd solar cycle CME. The major fraction of CMEs has acceleration in the range of -20 to 20 km s-2, all types of CMEs. The narrow and intermediate CMEs mostly show acceleration while wide CMEs show deceleration.

Temporal and Periodic Variation of the MCMESI for the Last Two Solar Cycles; Comparison with the Number of Different Class X-ray Solar Flares

In this study we compared the temporal and periodic variations of the Maximum CME Speed Index (MCMESI) and the number of different class (C, M, and X) solar X-Ray flares for the last two solar cycles (Cycle 23 and 24). To obtain the correlation between the MCMESI and solar flare numbers the cross correlation analysis was applied to monthly data sets. Also to investigate the periodic behavior of all data sets the Multi Taper Method (MTM) and the Morlet wavelet analysis method were performed with daily data from 2009 to 2018. To evaluate our wavelet analysis Cross Wavelet Transform (XWT) and Wavelet Transform Coherence (WTC) methods were performed. Causal relationships between datasets were further examined by Convergence Cross Mapping (CCM) method. In results of our analysis we found followings; 1) The C class X-Ray flare numbers increased about 16 % during the solar cycle 24 compared to cycle 23, while all other data sets decreased; the MCMESI decreased about 16 % and the number of M and X class flares decreased about 32 %. 2) All the X-Ray solar flare classes show remarkable positive correlation with the MCMESI. While the correlation between the MCMESI and C class flares comes from the general solar cycle trend, it mainly results from the fluctuations in the data in case of the X class flares. 3) In general, all class flare numbers and the MCMESI show similar periodic behavior. 4) The 546 days periodicity detected in the MCMESI may not be of solar origin or at least the solar flares are not the source of this periodicity. 5) C and M Class solar flares have a stronger causative effect on the MCMESI compared to X class solar flares. However the only bidirectional causal relationship is obtained between the MCMESI and C class flare numbers.

Study of Coronal Mass Ejections Succeeding the Associated X-Ray and γ-Ray Burst Solar Flares

This study is dedicated to the investigation of the characteristics of CMEs following the associated X-ray and γ-ray burst solar flares. Investigated 14786 CME events and 5092 Gamma Burst Monitor (GBM) solar flare events recorded during the solar period 2008–2017, found 503 (about 10%) CME events associated with GBM post-flare events (hereafter, GBM post-flare—CME). All of these 503 events (100%) are associated with solar flares detected simultaneously in both GBM and RHESI (γ -ray solar flare possibly associated with X-ray). The associated CMEs with GBM post-flare events are wider than non-associated CME events. These results indicate that, as the flare’s flux increases, the width of the associated CME increases. The Gamma burst solar flares accelerate CMEs, but with less extent than do non-associated or associated with X-ray solar flare only events. The GBM post-flare—CME associated events have a mean speed near the solar wind average speed approximately (which is less than speed of CMEs associated with X-ray solar flares only) and faster than non-associated events. The dominant CME initial speed of the GBM post-flare—CME associated events is ~ 300 (Km/s). The CME mean mass of the GBM post-flare—CME associated events indicate that the CMEs occurred after the solar flare is on average more massive than other CMEs. Found the relationship between the mass of the GBM post-flare—CME associated events and the CME width to be on the form: (CME Mass) = − 8.6 × 10+14 + 2.9 × 10+13 × (CME width).

Application of a Modified Spheromak Model to Simulations of Coronal Mass Ejection in the Inner Heliosphere

AbstractThe magnetic fields of interplanetary coronal mass ejections (ICMEs), which originate close to the Sun in the form of a flux rope, determine their geoeffectiveness. Therefore, robust flux rope‐based models of CMEs are required to perform magnetohydrodynamic (MHD) simulations aimed at space weather predictions. We propose a modified spheromak model and demonstrate its applicability to CME simulations. In this model, such properties of a simulated CME as the poloidal and toroidal magnetic fluxes, and the helicity sign can be controlled with a set of input parameters. We propose a robust technique for introducing CMEs with an appropriate speed into a background, MHD solution describing the solar wind in the inner heliosphere. Through a parametric study, we find that the speed of a CME is much more dependent on its poloidal flux than on the toroidal flux. We also show that the CME speed increases with its total energy, giving us control over its initial speed. We further demonstrate the applicability of this model to simulations of CME‐CME collisions. Finally, we use this model to simulate the 12 July 2012 CME and compare the plasma properties at 1 AU with observations. The predicted CME properties agree reasonably with observational data.

Characteristics of SEPs during solar cycles 21–24

The study of the solar energetic particle events (SEPs) and their association with solar flares and other activities are very crucial to understand the space weather. Keeping this in view, in this paper, we present the study of the SEPs (intensity equal to or greater than 10 pfu) during the solar cycle 21 to 24 (1976-2017) in > 10 MeV energy channels associated with solar flares. For our analysis, we have used the data from different instruments onboard SOHO satellite. We have examined the flare size, source location, CMEs characteristics of associated SEPs. About 31% and 69% of the SEPs were originated from the eastern and western solar hemisphere respectively. The average CME speed and width were 1238 km/s and 253 deg respectively. About 58 % SEPs were associated with halo CMEs and 42% of SEPs associated with CMEs width varying from 10 deg to 250 deg respectively.

Peak Temperatures of Large Solar X-Ray Flares and Associated CME Speeds and Widths

Abstract We recently repeated an earlier analysis by Garcia showing that large (≥M3.0) solar X-ray flares associated with solar energetic particle (SEP) events have significantly lower peak X-ray flux ratios R = (0.04–0.5 nm)/(0.1–0.8 nm), proxies for flare peak temperatures, than those without SEP events. As we expect SEP events to be produced by shocks ahead of fast coronal mass ejections (CMEs), a smaller R for an X-ray flare of a given peak flux Fp should also be more likely to be accompanied by a fast (Vcme &gt; 1000 km s−1) CME. We confirm this expectation, examine the role played by the ratios R in correlations between Fp and CME speeds Vcme, and then compare CME widths W, Vcme, and R with each other. We consider an apparent conflict between a global scaling model of eruptive events showing Vcme scaling with higher R and our confirmation that the Garcia analysis implies that faster CMEs are associated with flares of lower R. The R values are examined for 16 large flares of the well-studied AR 12192, for which nearly all flares had no associated CMEs. Those flares share the same high values of R as other active region (AR) flares with no CMEs. We also find that small (&lt;M3.0) flares of filament eruptions leading to SEP events share the lower R values of larger flares with fast CMEs.

Identifying Critical Input Parameters for Improving Drag‐Based CME Arrival Time Predictions

AbstractCoronal mass ejections (CMEs) typically cause the strongest geomagnetic storms, so a major focus of space weather research has been predicting the arrival time of CMEs. Most arrival time models fall into two categories: (1) drag‐based models that integrate the drag force between a simplified CME structure and the background solar wind and (2) full magnetohydrodynamic models. Drag‐based models typically are much more computationally efficient than magnetohydrodynamic models, allowing for ensemble modeling. While arrival time predictions have improved since the earliest attempts, both types of models currently have difficulty achieving mean absolute errors below 10 hr. Here we use a drag‐based model ANTEATR (Another Type of Ensemble Arrival Time Results) to explore the sensitivity of arrival times to various input parameters. We consider CMEs of different strengths from average to extreme size, speed, and mass (kinetic energies between and erg). For each scale CME, we vary the input parameters to reflect the current observational uncertainty in each and determine how accurately each must be known to achieve predictions that are accurate within 5 hr. We find that different scale CMEs are the most sensitive to different parameters. The transit time of average strength CMEs depends most strongly on the CME speed, whereas an extreme strength CME is the most sensitive to the angular width. A precise CME direction is critical for impacts near the flanks but not near the CME nose. We also show that the Drag‐Based Model has similar sensitivities, suggesting that these results are representative for all drag‐based models.

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.

Cosmic Ray Modulation with the Maximum CME Speed Index During Solar Cycles 23 and 24

We analyzed modulation of cosmic-ray intensities (CRIs) during Solar Cycles 23 and 24 by using the international sunspot numbers (ISSN) and the maximum CME speed index (MCMESI) as proxies for solar activity. Temporal variations, cross-correlations, and hysteresis patterns of CRI, MCMESI, and ISSN data were investigated. As a result, we concluded that the MCMESI better describes solar modulation of the CRI as compared to the ISSN. This is mainly because the correlation between CRI and ISSN is caused by the general cyclic trend of solar activity, while the correlation between the CRI and the MCMESI is mainly due to short-term fluctuations related to Forbush decreases. In contrast to the ISSN, there is no time lag between the CRI and the MCMESI variations.

Interplanetary Coronal Mass Ejections During Solar Cycles 23 and 24: Sun–Earth Propagation Characteristics and Consequences at the Near-Earth Region

In this article, we present a statistical study probing the relation between interplanetary coronal mass ejections (ICMEs) observed at 1 AU and their corresponding coronal mass ejections at the near-Sun region. The work encompasses the ICME activity that occurred during Solar Cycles 23 and 24 (1996 – 2017) while presenting an overall picture of ICME events during the complete Solar Cycle 24 for the first time. The importance of this study further lies in comparing two subsets of ICMEs, i.e. magnetic clouds (MCs) and ejecta (EJ), to explore how the observed structures of ICMEs at 1 AU could be associated with the properties of CMEs during their launch at the Sun. We find that, although Solar Cycle 24 saw a significant reduction in the number of ICME events compared to the previous cycle, the fraction of MCs was much higher during Cycle 24 than Cycle 23 (60% versus 41%). In general, the ICME transit-time decreases with the increase in the CME initial speed, although a broad range of transit times were observed for a given CME speed. We also find that the high-speed ICMEs ( ${\gtrsim}\,500~\mbox{km}\,\mbox{s}^{-1}$ ) form a distinct group in terms of the deficit in their transit times when compared with low-speed events ( ${\lesssim}\, 500~\mbox{km}\,\mbox{s}^{-1}$ ), which means that high-speed ICMEs acquire a much higher internal energy from the source active regions during the initiation process that effectively overcomes the aerodynamic drag force while they transit in the interplanetary medium. The CME propagation from the Sun to the near-Earth environment shows both an overall positive and negative acceleration (i.e. deceleration), although the acceleration is limited to only low-speed CMEs that are launched with a speed comparable with or less than the mean solar wind speed ( ${\approx}\, 400\,\mbox{--}\,450~\mbox{km}\,\mbox{s}^{-1}$ ). Within a given cycle, the similarities of MC and EJ profiles with respect to the CME–ICME speed relation as well as interplanetary acceleration support the hypothesis that all CMEs have a flux rope structure and that the trajectory of the CMEs essentially determines the observed ICME structure at 1 AU.