Coronal Mass Ejections Acceleration Research Articles

Coronal Shocks of November 1997 Revisited: The Cme?Type II Timing Problem

We re-examine observations bearing on the origin of metric type II bursts for six impulsive solar events in November 1997. Previous analyses of these events indicated that the metric type IIs were due to flares (either blast waves or ejecta). Our point of departure was the study of Zhang et al. (2001) based on the Large Angle and Spectrometric Coronagraph’s C1 instrument (occulting disk at 1.1 R0) that identified the rapid acceleration phase of coronal mass ejections (CMEs) with the rise phase of soft X-ray light curves of associated flares. We find that the inferred onset of rapid CME acceleration in each of the six cases occurred 1–3 min before the onset of metric type II emission, in contrast to the results of previous studies for certain of these events that obtained CME launch times ∼25–45 min earlier than type II onset. The removal of the CME-metric type II timing discrepancy in these events and, more generally, the identification of the onset of the rapid acceleration phase of CMEs with the flare impulsive phase undercuts a significant argument against CMEs as metric type II shock drivers. In general, the six events exhibited: (1) ample evidence of dynamic behavior [soft X-ray ejecta, extreme ultra-violet imaging telescope (EIT) dimming onsets, and wave initiation (observed variously in Hα, EUV, and soft X-rays)] during the inferred fast acceleration phases of the CMEs, consistent with the cataclysmic disruption of the low solar atmosphere one would expect to be associated with a CME; and (2) an organic relationship between EIT dimmings (generally taken to be source regions of CMEs) and EIT waves (which are highly associated with metric type II bursts) indicative of a CME-driver scenario. Our analysis indicates that the broad (∼90○ to halo) CMEs observed in the outer LASCO coronagraphs for these impulsive events began life as relatively small-scale structures, with angular spans of ∼15○ in the low corona. A review of on-going work bearing on other aspects (than timing) of the question of the origin of metric type II bursts (CME association; connectivity of metric and decametric-hectometric type II shocks; spatial relationship between CMEs and metric shocks) leads to the conclusion that CMEs remain a strong candidate to be the principal/sole driver of metric type II shocks vis-a-vis flare blast waves/ejecta.

A Study on the Acceleration of Coronal Mass Ejections

We present a statistical study on the acceleration of CMEs. This study is based on 23 CME events best observed by SOHO LASCO/C1 coronagraph, which observes the inner corona from 1.1 to 3.0 ) and acceleration duration T (in unit of min), which can be simply described as A=10000/T.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

A Study of the Kinematic Evolution of Coronal Mass Ejections

We report the kinematic properties of a set of three coronal mass ejections (CMEs) observed with the LASCO (Large Angle and Spectrometric Coronagraph) on the Solar and Heliospheric Observatory (SOHO) spacecraft, which showed characteristics of impulsive, intermediate, and gradual acceleration, respectively. The first CME had a 30 minute long fast acceleration phase during which the average acceleration was about 308 m s-2; this acceleration took place over a distance of about 3.3 R☉ (from 1.3 to 4.6 R☉, height measured from disk center). The CME characterized by intermediate acceleration had a long acceleration phase of about 160 minutes during which the average acceleration was about 131 m s-2; the CME traveled a distance of at least 4.3 R☉, reaching a height of 7.0 R☉ at the end of the acceleration phase. The CME characterized by gradual acceleration had no fast acceleration phase. Instead, it displayed a persistent weak acceleration lasting more than 24 hr with an average acceleration of only 4.0 m s-2 throughout the LASCO field of view (from 1.1 to 30 R☉). This study demonstrates that the final velocity of a CME is determined by a combination of acceleration magnitude and acceleration duration, both of which can vary significantly from event to event. The first two CME events were associated with soft X-ray flares. We found that in the acceleration phase there was close temporal correlation both between the CME velocity and the soft X-ray flux of the flare and between the CME acceleration and derivative of the X-ray flux. These correlations indicate that the CME large-scale acceleration and the flare particle acceleration are strongly coupled physical phenomena occurring in the corona.

A High-Speed Erupting-Prominence CME: A Bridge Between Types

Several studies have indicated that there may be two distinct types of coronal mass ejections (CMEs); a high-velocity bright energetic type associated with flares, and a smaller slower less impressive type associated with erupting prominences. How valid is this distinction? We analyze a CME combining attributes of both types, a high-velocity bright CME associated with an erupting prominence. A study of this event and several others allows us to argue that the apparent differences separating the two types may be an observational effect. Our results are consistent with a single CME process for both flare-associated and filament-associated CMEs. This process consists of three stages. The initial stage is brought about by the emergence of new magnetic flux, which interacts with the pre-existing magnetic configuration and results in a slow rise of the magnetic structure, which later becomes the CME. The second stage is a fast reconnection phase with flaring and a sudden increase of the rise velocity of the magnetic structure. It also includes a rapidly increasing CME acceleration followed by a rapidly falling acceleration. The third stage or CME propagation stage shows only slow changes in the acceleration and finally the velocity becomes constant. LASCO observes only the third stage. The differences found between observed flare-associated and prominence-associated CME velocity behavior appear to be primarily due to the relative heights in the corona at which the erupting structures form.

Acceleration of coronal mass ejections

The acceleration of coronal mass ejections (CMEs) is examined focusing on three specific questions raised by observations: (1) what determines the height beyond which a CME exhibits no rapid acceleration, (2) why is the main acceleration of CMEs typically limited to below 2–3 solar radii, and (3) are distinct mechanisms required to explain the apparent bimodal distribution of speed‐height profiles. Using a theoretical model of CMEs based on a three‐dimensional (3‐D) magnetic flux rope, it is shown that the acceleration of CMEs exhibits a universal scaling law characterized by the critical height Z* = Sf/2 such that maximum acceleration is attained shortly after height Z of the flux‐rope apex exceeds Z*, where Sf is the footpoint separation distance. Theoretical analysis and observed CME dynamics show two distinct phases of acceleration: the “main” and “residual” acceleration phases. The main acceleration phase occurs for apex height Z ≤ Zm, where Zm is found to be ≃3 Z*, and the residual acceleration phase corresponds to Z > Zm. Thus the observed acceleration profile can be directly related to Sf. These results are explained in terms of the 3‐D geometry of a flux rope, its inductive properties, and the Lorentz self‐force. We have also constructed ensembles of flux‐rope profiles corresponding to varying amounts and durations of poloidal flux injection. We find that the resulting distribution of model speed‐height profiles is similar to that observed if an upper limit on the amount of injected flux is imposed. One mechanism is sufficient to account for the observed properties and distribution of CME acceleration. The theory is quantitatively tested against observed CMEs.

Launch of solar coronal mass ejections and submillimeter pulse bursts

The rapid solar spikes (100–500 ms) recently discovered at submillimeter waves bring new possibilities to investigate energetic processes near the solar surface that might have an important role in the launch and propelling of ionized mass away from the Sun. We present a study on the association between the launch time of coronal mass ejections (CMEs) observed by the LASCO instruments on the SOHO spacecraft and the onset of the new kind of rapid solar spikes (100–500 ms) observed at submillimetric waves (212 and 405 GHz) by the new Solar Submm‐wave Telescope (SST). We investigated six submm‐wave events, all found associated to CMEs. Seven related CME were identified. Five of them were associated with flares with large GOES class soft X‐rays, presenting distinct time histories and associations at other energy ranges, and two of them were related to flares behind the solar limb, with simultaneous related activity observed in the visible solar disk. Ultraviolet images from EIT on SOHO show some kind of small or large‐scale magnetic activity or brightening for all events. The extrapolation of apparent CME positions to the solar surface show that they occurred nearly coincident in time with the onset of submm‐wave pulses for all six events. These results suggest that pulse bursts might be representative of an important early signature of CMEs, especially for events beginning near the center of the solar disk, sometimes identified as “halo” CMEs. They lead to several challenging questions relative to the physical nature of the pulses and its association to the launch and acceleration of coronal mass ejections. Although these evidences may favor multiple rapid energy releases at the origin near the solar surface, they require further research in order to better understand both diagnostics and model descriptions.

Rapid Acceleration of a Coronal Mass Ejection in the Low Corona and Implications for Propagation

A high-velocity Coronal Mass Ejection (CME) associated with the 2002 April 21 X1.5 flare is studied using a unique set of observations from the Transition Region and Coronal Explorer (TRACE), the Ultraviolet Coronagraph Spectrometer (UVCS), and the Large-Angle Spectrometric Coronagraph (LASCO). The event is first observed as a rapid rise in GOES X-rays, followed by simultaneous conjugate footpoint brightenings connected by an ascending loop or flux-rope feature. While expanding, the appearance of the feature remains remarkably constant as it passes through the TRACE 195 A passband and LASCO fields-of-view, allowing its height-time behavior to be accurately determined. An analytic function, having exponential and linear components, is found to represent the height-time evolution of the CME in the range 1.05-26 R. The CME acceleration rises exponentially to approx. 900 km/sq s within approximately 20-min, peaking at approx.1400 m/sq s when the leading edge is at approx. 1.7 R. The acceleration subsequently falls off as a slowly varying exponential for approx.,90-min. At distances beyond approx. 3.4 R, the height-time profile is approximately linear with a constant velocity of approx. 2400 km/s. These results are briefly discussed in light of recent kinematic models of CMEs.

Energetic particle acceleration by coronal mass ejections

The current paradigm for the source of large, gradual solar energetic particle (SEP) events is that theyare accelerated in coronal/interplanetary shocks driven by coronal mass ejections (CMEs). Early studies established that there is a rough correlation between the logs of the CME speed and the logs of the SEP intensities. Here I review two topics challenging the basic paradigm, the recent discovery that CMEs are also associated with impulsive, high-Z rich SEP events and the search for gradual SEP sources other than CME-driven shocks. I then discuss three topics of recent interest dealing with the relationship between the shock or CME properties and the resulting SEP events. These are the roles that CME accelerations, interactions between fast and preceding slow CMEs, and widths of fast CMEs may play in SEP production.

A Statistical Study of Two Classes of Coronal Mass Ejections

A comprehensive statistical study is performed to address the question of whether two classes of coronal mass ejections (CMEs) exist. A total of 3217 CME events observed by SOHO/LASCO in 1996-2000 have been analyzed. We have examined the distributions of CMEs according to speed and acceleration, respectively, and investigated the correlation between speed and acceleration of CMEs. This statistical analysis is conducted for two subsets containing those CMEs that show a temporal and spatial association either with GOES X-ray solar flares or with eruptive filaments. We have found that CMEs associated with flares have a higher median speed than those associated with eruptive filaments and that the median speed of CMEs associated with strong flares is higher than that of weak-flare-associated CMEs. The distribution of CME acceleration shows a conspicuous peak near zero, not only for the whole data set, but also for the two subsets associated either with solar flares or with eruptive filaments. However, we have confirmed that the CMEs associated with major flares tend to be more decelerated than the CMEs related to eruptive filaments. The fraction of flare-associated CMEs has a tendency to increase with the CME speed, whereas the fraction of eruptive-filament-associated CMEs tends to decrease with the CME speed. This result supports the concept of two CME classes. We have found a possibility of two components in the CME speed distribution for both the CME data associated with flares larger than M1 class and the CME data related with limb flares. Our results suggest that the apparent single-peak distribution of CME speed can be attributed to the projection effect and possibly to abundance of small flares too. We also note that there is a possible correlation between the speed of CMEs and the time-integrated X-ray flux of the CME-associated limb flares.

Eruption and acceleration of flare‐associated coronal mass ejection loops in the low corona

Observations made by the EUV imaging telescope (EIT) and the Large‐Angle Spectrometric Coronagraph (LASCO) on board the Solar Heliospheric Observatory (SOHO) have been used to characterize the eruption and acceleration of flare‐associated coronal mass ejections (CMEs) in the low corona. For three well‐observed limb events we tracked CME loops back to preexisting but faint EUV‐emitting loops at heights of 100–250 Mm that initially brightened slowly and possibly increased slowly in height, apparently in response to filament activity and eruption in the associated active regions. Subsequent CME acceleration coincided with a rapid rise of the soft X‐ray flux, occurred between 100 and 350 Mm above the surface, and may have been as high as 0.5 km s−1 s−1, consistent with an impulsive acceleration of the CME to the speeds observed in subsequent white‐light observations. The existence of a delay of up to 30 min observed between initial filament eruption in H alpha and subsequent high acceleration of the CME in one event implies that there may have been two separate phases of magnetic reconnection, with the initial filament activity acting as a trigger for subsequent CME and energetic particle acceleration in the impulsive stage of the flare. The presence or absence of this impulsive phase may provide a basis for the two types of CMEs that have been discussed in the literature.

On the Temporal Relationship between Coronal Mass Ejections and Flares

The temporal relationship between coronal mass ejections (CMEs) and associated solar flares is of great importance to understanding the origin of CMEs, but it has been difficult to study owing to the nature of CME detection. In this paper, we investigate this issue using the Large Angle and Spectrometric Coronagraph and the EUV Imaging Telescope observations combined with GOES soft X-ray observations. We present four well-observed events whose source regions are close to the limb such that we are able to directly measure the CMEs' initial evolution in the low corona (~ 1-3 R☉) without any extrapolation; this height range was not available in previous space-based coronagraph observations. The velocity-time profiles show that kinematic evolution of three of the four CMEs can be described in a three-phase scenario: the initiation phase, impulsive acceleration phase, and propagation phase. The initiation phase is characterized by a slow ascension with a speed less than 80 km s-1 for a period of tens of minutes. The initiation phase always occurs before the onset of the associated flare. Following the initiation phase, the CMEs display an impulsive acceleration phase that coincides very well with the flares' rise phase lasting for a few to tens of minutes. The acceleration of CMEs ceases near the peak time of the soft X-ray flares. The CMEs then undergo a propagation phase, which is characterized by a constant speed or slowly decreasing in speed. The acceleration rates in the impulsive acceleration phase are in the range of 100-500 m s-2. One CME (on 1997 November 6, associated with an X9.4 flare) does not show an initiation phase. It has an extremely large acceleration rate of 7300 m s-2. The possible causes of CME initiation and acceleration in connection with flares are explored.

Early life of coronal mass ejections

Coronal mass ejections (CMEs) are large-scale magnetized plasma structures ejected from closed magnetic field regions of the Sun. White light coronagraphic observations from ground and space have provided extensive information on CMEs in the outer corona. However, our understanding of the solar origin and early life of CMEs is still in an elementary stage because of lack of adequate observations. Recent space missions such as Yohkoh and Solar and Heliospheric Observatory (SOHO) and ground-based radioheliographs at Nobeyama and Nancay have accumulated a wealth of information on the manifestations of CMEs near the solar surface. We review some of these observations in an attempt to relate them to what we already know about CMEs. Our discussion relies heavily on non-coronagraphic data combined with coronagraphic data. Specifically, we discuss the following aspects of CMEs: (i) coronal dimming and global disk signatures, (ii) non-radial propagation during the early phase, (iii) Photospheric magnetic field changes during CMEs, and (iv) acceleration of fast CMEs. The relative positions and evolution of coronal dimming, arcade formation, prominence eruption will be discussed using specific events. The magnitude and spatial extent of CME acceleration may be an important parameter that distinguishes fast and slow CMEs.

On space weather consequences and predictions

This paper focuses on the question of what needs to be predicted and what processes need to be understood to predict and forecast space weather conditions that are hazardous to current technology. The paper's aim is to see if we are working on the correct space parameters to permit prediction of those quantities that actually present hazards to current space technology. The paper is not intended to be encyclopedic. We conclude that although the sunspot number is a general proxy for many space hazards, there is surprisingly little direct need for its accurate prediction or for the prediction of solar flares as such. We also find that knowledge of Kp and other geomagnetic indices are rarely directly required. Important gaps in our knowledge exist concerning the variations of storm time electron, proton, and ion populations within the magnetosphere. Work is also required in predicting fast coronal mass ejections (CMEs) and in understanding the processes of CME initiation and acceleration within the corona and high energy solar particle acceleration and propagation.

Effects of reconnection on the coronal mass ejection process

This work investigates how magnetic reconnection affects the acceleration of coronal mass ejections (CMEs) and how the acceleration in turn affects the reconnection process. To model the CME process, we use a two‐dimensional flux rope model, which drives the ejection by means of a catastrophic loss of mechanical equilibrium. Our model provides a method for relating the motion of the ejected material to the reconnection rate in the current sheet created by the erupting field. In the complete absence of reconnection the tension force associated with the current sheet is always strong enough to prevent the flux rope from escaping from the Sun. However, our results imply that even a fairly small reconnection rate is sufficient to allow the flux rope to escape. Specifically, for a coronal density model that decreases exponentially with height we find that average Alfvén Mach number MA for the inflow into the reconnection site can be as small as MA = 0.005 and still be fast enough to give a plausible eruption. The best fit to observations is obtained by assuming an inflow rate on the order of MA ≈ 0.1. With this value the energy output matches the temporal behavior inferred for the long duration events often associated with CMEs. The model also suggests an explanation for the peculiar motion of giant X‐ray arches reported by Svestka et al. [1995, 1997]. X‐ray arches are the large loops associated with CMEs which are similar in form to “post”‐flare loops, but they have an upward motion that is often different. Instead of continually slowing with time, the arches move upward at a rate that remains nearly constant or may even increase with time. Here we show how the difference can be explained by reversal of the gradient of the coronal Alfvén speed with height.

On the Problem of the Relationship Between Solar Flares and Coronal Mass Ejections

Combination of SMM data with Mauna Loa and GOES measurements shows significant relationships between coronal mass ejections (CMEs) and solar flares, or, more precisely, long decay (duration) X-ray events (LDEs). This relationship is not understood in all its details. Although mass ejections are considered to be a more probable candidate for the role of a driver, there is a problem, namely, which of these two phenomena is cause and which is consequence. To understand this, one traditionally compares start times of CMEs and LDEs. We show that the study of the correlation between XUV line broadening and the origin of CME acceleration gives additional possibilities to solve this problem. In the EUV emission, such investigations might possibly be carried out in the framework of the SOHO program. In the soft X-rays, SMM and Yohkoh data are convenient, albeit not for the same event.