Type II Radio Bursts with High and Low Starting Frequencies

We report on the detailed analysis of i) differences between the properties of type IIs with various starting frequencies (high: ≥100 MHz; low: ≤50 MHz; mid: 50 MHz ≤f≤ 100 MHz) and ii) the properties of CMEs and flares associated with them. For this study, we considered a sample of type II radio bursts observed by Culgoora radio spectrograph from January 1998 to December 2000. The X-ray flares and CMEs associated with these events are identified using GOES and SOHO/LASCO data. The secondary aim is to study the frequency dependence on other properties of type IIs, flares, and CMEs. We found that the type IIs with high starting frequencies have larger drift rate, relative drift rate, and shock speed than the type IIs with low starting frequencies. The flares associated with high frequency type IIs are of impulsive in nature with shorter rise time, duration and delay between the flare start and type II start times than the low frequency type IIs. There is a distinct power – law relationship between the flare parameters and the starting frequencies of type II bursts, whereas the trend in the CME parameters shows low correlation. While the mean speed of CMEs is larger for the mid-frequency group, it is nearly the same for the high and low frequency groups. On the other hand, the percentage of CME association (90%) is larger for low frequency type IIs than for the high frequency type IIs (75%).     

Coronal Shocks Associated with Impulsive and Decaying Phases of Solar Flares

We have analyzed a set of 147 metric Type II radio bursts observed by Culgoora radio spectrograph from November 1997 to December 2006. These events were divided into two sets: The first subset contains Type II events that started during the impulsive phase of the associated solar flares and the second subset contains those starting during the decaying phase of flares. Our main aim is to differentiate the metric Type IIs, flares and coronal mass ejections (CMEs) of these two subsets. It is found that while Type II burst characteristics of both subsets are very similar, there are significant differences between flare and CME properties for these two subsets. Considering all analyzed relationships between the characteristics of Type IIs, flares and CMEs in these two Type II subsets, we conclude that most of the coronal shocks causing metric Type II bursts are driven by CMEs, but that a fraction of events are probably ignited by solar flares.     

Re-Evaluation of the Flare–Type II–CME Association

We have re-evaluated the association of type II solar radio bursts with flares and/or coronal mass ejections (CMEs) using the year 2000 solar maximum data. For this, we consider 52 type II events whose associations with flares or CMEs were absent or not clearly identified and reported. These events are classified as follows; group I: 11 type IIs for which there are no reports of GOES X-ray flares and CMEs; group II: 12 type IIs for which there are no reports of GOES X-ray flares; and group III: 29 type IIs for which the flare locations are not reported. By carefully re-examining their association from GOES X-ray and H, Yohkoh SXT and EIT-EUV data, we attempt to answer the following questions: (i) if there really were no X-ray flares associated with the above 23 type IIs of groups I and II; (ii) whether they can be regarded as backside events whose X-ray emission might have been occulted. From this analysis, we have found that two factors, flare background intensity and flare location, play important roles in the complete reports about flare–type II–CME associations. In the above 23 cases, for more than 50% of the cases in total, the X-ray flares were not noticed and reported, because the background intensity of X-ray flux was high. In the remaining cases, the X-ray intensity might be greatly reduced due to occultation. From the H flare data, Yohkoh SXT data and EIT-EUV data, we found that ten cases out of 23 might be frontside events, and the remaining are backside events. While the flare–type II association is found to be nearly 90%, the type II–CME association is roughly around 75%. This analysis might be useful to reduce some ambiguities regarding the association among type IIs, flares and CMEs.     

Multiple Type II Solar Radio Bursts

We report on the detailed analysis of a set of 38 multiple type II radio bursts observed by Culgoora radio spectrograph from January 1997 to July 2003. These events were selected on the basis of the following criteria: (i) more than one type II were reported within 30 min interval, (ii) both fundamental and harmonic were identified for each of them. The X-ray flares and CMEs associated with these events are identified using GOES, Yohkoh SXT, SOHO/EIT, and SOHO/LASCO data. From the analysis of these events, the following physical characteristics are obtained: (i) In many cases, two type IIs with fundamental and harmonic were reported, and the time interval between the two type IIs is within 15 min; (ii) The mean values of starting frequency, drift rate, and shock speed of the first type II are significantly higher than those of the second type II; (iii) More than 90% of the events are associated with both X-ray flares and CMEs; (iv) Nearly 75% of the flares are stronger than M1 X-ray class and 50% of CMEs have their widths larger than 200^∘ or they are halo CMEs; (v) While most of the first type IIs started within the flare impulsive phase, 22 out of 38 second type IIs started after the flare impulsive phase. Weak correlations are found between the starting and ending frequencies of these type II events. On the other hand, there was no correlation between two shock speeds between the first and the second type II. Since most of the events are associated with both the flares and CMEs, and there are no events which are only associated with multiple impulsive flares or multiple mass ejections, we suggest that the flares and CMEs (front or flank) both be sources of multiple type IIs. Other possibilities on the origin of multiple type IIs are also discussed.     

Type II bursts in Meter and Decameter – Hectometer Wavelength Ranges and Their Relation to Flares and CMEs

Statistical analysis of the relationship between type II radio bursts appearing in the metric (m) and decameter-to-hectometer (DH) wavelength ranges is presented. The associated X-ray flares and coronal mass ejections (CMEs) are also reported. The sample is divided into two classes using the frequency-drift plots: Class I, representing those events where DH-type-II bursts are not continuation of m-type-II bursts and Class II, where the DH-type-II bursts are extensions of m-type-II bursts. Our study consists of three steps: i) comparison of characteristics of the Class I and II events; ii) correlation of m-type-II and DH-type-II burst characteristics with X-ray flare properties and iii) correlation of m-type-II and DH-type-II burst characteristics with CME properties. We have found no clear correlation between properties of m-type-II bursts and DH-type-II bursts. For example, there is no correlation between drift rates of m-type-II bursts and DH-type-II bursts. Similarly there is no correlation between their starting frequencies. In Class I events we found correlations between X-ray flare characteristics and properties of m-type-II bursts and there is no correlation between flare parameters and DH-type-II bursts. On the other hand, the correlation between CME parameters and m-type-II bursts is very weak, but it is good for CME parameters and DH-type-II bursts. These results indicate that Class I m-type-II bursts are related to the energy releases in flares, whereas DH-type-II bursts tend to be related to CMEs. On the contrary, for Class II events in the case of m-type-II and DH-type-II bursts we have found no clear correlation between both flare and CMEs.     

Interplanetary type II radio bursts and their association with CMEs and flares

We study the characteristics of the CMEs and their association with the end-frequency of interplanetary (IP)-type-II bursts by analyzing a set of 138 events (IP-type-II bursts–flares–CMEs) observed during the period 1997–2012. The present analysis consider only the type II bursts having starting frequency \(< 14~\mbox{MHz}\) to avoid the extension of coronal type IIs. The selected events are classified into three groups depending on the end-frequency of type IIs as follows, (A) Higher, (B) Intermediate and (C) Lower end-frequency. We compare characteristics of CMEs, flares and type II burst for the three selected groups of events and report some of the important differences. The observed height of CMEs is compared with the height of IP type IIs estimated using the electron density models. By applying a density multiplier (\(m\)) to this model, the density has been constrained both in the upper corona and in the interplanetary medium, respectively as \(m= 1 \mbox{ to } 10\) and \(m = 1 \mbox{ to } 3\). This study indicates that there is a correlation between the observed CME height and estimated type II height for groups B and C events whereas this correlation is absent in group A. In all the groups (A, B & C), the different heights of CMEs and type II reveal that the type IIs are not only observed at the nose but also at the flank of the CMEs.     

An association of CMEs with solar flares detected by Fermi γ-ray burst monitor during solar cycle 24

By performing certain spatial and temporal criteria, we obtained 492 CME events simultaneously associated with GBM solar flare events (hereafter, GBM-flare–CME) from the total number 5123 Gamma-ray Burst Monitor (GBM) solar flares and 15228 Coronal Mass Ejections (CMEs) detected during the solar cycle 24 (2008–2019). Among these 492 events, which represent about 9.6% of the total number of the detected GBM flares, there are just 381 events (77.4%) representing the CMEs associated with the flares that are detected instantly by both GBM and RHESSI detectors. We found no significant distinction in the results after applying the spatial criteria compared with those arising from applying the temporal criteria only.Actually, all CMEs are ejected within the flare's preflare and the impulsive phases only. From our results, we conclude that the GBM flares whose long duration are most frequently associated with faster and wider CMEs and vice versa. In addition, the longer the flare's duration, the more interval time between the start time of GBM solar flare and CME's ejection time through a linear correlation [Mean Interval = 0.464 × Duration (min)] with a correlation coefficient equals 0.93. We conclude also that, the highly probable, γ-ray emitting flares (detected by GBM only) have a shorter duration and time interval than X-ray flares (detected also by RHESSI). As well as the GBM - CMEs events, without RHESSI associated CMEs are faster and wider than those associated with RHESSI events.     

Coronal Mass Ejections Associated with Slow Long Duration Flares

It is well known that there is a temporal relationship between coronal mass ejections (CMEs) and associated flares. The duration of the acceleration phase is related to the duration of the rise phase of a flare. We investigate CMEs associated with slow long duration events (LDEs), i.e. flares with the long rising phase. We determined the relationships between flares and CMEs and analyzed the CME kinematics in detail. The parameters of the flares (GOES flux, duration of the rising phase) show strong correlations with the CME parameters (velocity, acceleration during main acceleration phase, and duration of the CME acceleration phase). These correlations confirm the strong relation between slow LDEs and CMEs. We also analyzed the relation between the parameters of the CMEs, i.e. a velocity, an acceleration during the main acceleration phase, a duration of the acceleration phase, and a height of a CME at the end of the acceleration phase. The CMEs associated with the slow LDEs are characterized by high velocity during the propagation phase, with the median equal to 1423 km?s^?1. In half of the analyzed cases, the main acceleration was low (a<300 m?s^?2), which suggests that the high velocity is caused by the prolonged acceleration phase (the median for the duration of the acceleration phase is equal 90 minutes). The CMEs were accelerated up to several solar radii (with the median ≈?7 R _⊙), which is much higher than in typical impulsive CMEs. Therefore, slow LDEs may potentially precede extremely strong geomagnetic storms. The analysis of slow LDEs and associated CMEs may give important information for developing more accurate space-weather forecasts, especially for extreme events.     

Solar Flares and Coronal Mass Ejections: A Statistically Determined Flare Flux – CME Mass Correlation

In an effort to examine the relationship between flare flux and corresponding CME mass, we temporally and spatially correlate all X-ray flares and CMEs in the LASCO and GOES archives from 1996 to 2006. We cross-reference 6733 CMEs having well-measured masses against 12 050 X-ray flares having position information as determined from their optical counterparts. For a given flare, we search in time for CMEs which occur 10 – 80 minutes afterward, and we further require the flare and CME to occur within ± 45° in position angle on the solar disk. There are 826 CME/flare pairs which fit these criteria. Comparing the flare fluxes with CME masses of these paired events, we find CME mass increases with flare flux, following an approximately log-linear, broken relationship: in the limit of lower flare fluxes, log (CME mass)∝0.68×log (flare flux), and in the limit of higher flare fluxes, log (CME mass)∝0.33×log (flare flux). We show that this broken power-law, and in particular the flatter slope at higher flare fluxes, may be due to an observational bias against CMEs associated with the most energetic flares: halo CMEs. Correcting for this bias yields a single power-law relationship of the form log (CME mass)∝0.70×log (flare flux). This function describes the relationship between CME mass and flare flux over at least 3 dex in flare flux, from ≈ 10⁻⁷ – 10⁻⁴ W m⁻².     

X-ray emission characteristics of flares associated with CMEs

We present the study of 20 solar flares observed by “Solar X-ray Spectrometer (SOXS)” mission during November 2003 to December 2006 and found associated with coronal mass ejections (CMEs) seen by LASCO/SOHO mission. In this investigation, X-ray emission characteristics of solar flares and their relationship with the dynamics of CMEs have been presented. We found that the fast moving CMEs, i.e., positive acceleration are better associated with short rise time (< 150 s) flares. However, the velocity of CMEs increases as a function of duration of the flares in both 4.1–10 and 10–20 keV bands. This indicates that the possibility of association of CMEs with larger speeds exists with long duration flare events. We observed that CMEs decelerate with increasing rise time, decay time and duration of the associated X-ray flares. A total 10 out of 20 CMEs under current investigation showed positive acceleration, and 5 of them whose speed did not exceed 589 km/s were associated with short rise time (< 150 s) and short duration (< 1300 s) flares. The other 5 CMEs were associated with long duration or large rise time flare events. The unusual feature of all these positive accelerating CMEs was their low linear speed ranging between 176 and 775 km/s. We do not find any significant correlation between X-ray peak intensity of the flares with linear speed as well as acceleration of the associated CMEs. Based on the onset time of flares and associated CMEs within the observing cadence of CMEs by LASCO, we found that in 16 cases CME preceded the flare by 23 to 1786 s, while in 4 cases flare occurred before the CME by 47 to 685 s. We argue that both events are closely associated with each other and are integral parts of one energy release system.     

Origin of Coronal Shocks without Mass Ejections

We present an analysis of all the events (around 400) of coronal shocks for which the shock-associated metric type IIs were observed by many spectrographs during the period April 1997– December 2000. The main objective of this analysis is to give evidence for the type IIs related to only flare-blast waves, and thus to find out whether there are any type II-associated coronal shocks without mass ejections. By carefully analyzing the data from multi-wavelength observations (Radio, GOES X-ray, Hα, SOHO/LASCO and SOHO/EIT-EUV data), we have identified only 30 events for which there were actually no reports of CMEs. Then from the analysis of the LASCO and EIT running difference images, we found that there are some shocks (nearly 40%, 12/30) which might be associated with weak and narrow mass ejections. These weak and narrow ejections were not reported earlier. For the remaining 60% events (18/30), there are no mass ejections seen in SOHO/LASCO. But all of them are associated with flares and EIT brightenings. Pre-assuming that these type IIs are related to the flares, and from those flare locations of these 18 cases, 16 events are found to occur within the central region of the solar disk (longitude ≤45^∘). In this case, the weak CMEs originating from this region are unlikely to be detected by SOHO/LASCO due to low scattering. The remaining two events occurred beyond this longitudinal limit for which any mass ejections would have been detected if they were present. For both these events, though there are weak eruption features (EIT dimming and loop displacement) in the EIT images, no mass ejection was seen in LASCO for one event, and a CME appeared very late for the other event. While these two cases may imply that the coronal shocks can be produced without any mass ejections, we cannot deny the strong relationship between type IIs and CMEs.     

Statistical Study of the CME-Solar Flares Associated Events

The aim of this paper is studying the relation between the coronal mass ejections (CMEs), and their associated solar flares. I used the CMEs data (obtained from CME catalogue) which observed by SOHO/LASCO, during the Solar Cycle 23rd (1996–2006), during this period I selected 12,433 CME records. Also I used the X-ray flares data which provided geostationary operational environmental satellite (GOES), during the same interval in the 1–8 Å GOES channel, the recorded flare events are 22,688. I filtered these CMEs and solar flare events to select 529 CME-Flare events. I found that there is a moderate relation between the solar flare fluxes and their associated CME energies, where R = 58 %. In addition I found that 61 % of the CME-Flare associated events ejected from the solar surface after the occurrence of the associated flare. Furthermore I found that the CME-Flare relation improved during the period of high solar activity. Finally, I examined the CME association rate as a function of flare longitude and I found that the CME association rate of the total 529 selected CME-Flare events are mostly disk-Flare events.     

An Investigation of Solar Maximum Metric Type ii Radio Bursts: do two Kinds of Coronal Shock Sources Exist?

A detailed statistical investigation of solar Type II radio bursts during the last solar maximum period 1999–2001 has been made to address the question if there exist two kinds of coronal shock sources. For this, the Type II bursts were classified into two classes: (i) those associated with flares only (Class I); and (ii) those associated with flares and CMEs (Class II) according to their temporal association. While the properties of all the type IIs agree in general with the common range of values, the properties of the shocks of the two classes differ slightly. For example, while the duration and shock speed for Class II are higher than those of Class I, the ending frequency for Class II is significantly lower. We have also examined in detail the physical association with other solar and interplanetary activities (Type IV bursts, Long Duration Events, Wind/WAVES deca-hectometric Type IIs, and interplanetary shocks) using the data in 2000. As a result, we have found noticeable differences between these two classes in terms of the following physical characteristics: First, the associations of these activities for Class II are much higher than those of Class I. Second, the correlation values between the flare parameters and the Type II properties for Class II are significantly smaller. Third, observed double Type IIs exist in only Class II events. The above results suggest that there are two kinds of coronal shocks or, rather, two general classes of coronal shock sources.     

Automated Prediction of CMEs Using Machine Learning of CME – Flare Associations

Machine-learning algorithms are applied to explore the relation between significant flares and their associated CMEs. The NGDC flares catalogue and the SOHO/LASCO CME catalogue are processed to associate X and M-class flares with CMEs based on timing information. Automated systems are created to process and associate years of flare and CME data, which are later arranged in numerical-training vectors and fed to machine-learning algorithms to extract the embedded knowledge and provide learning rules that can be used for the automated prediction of CMEs. Properties representing the intensity, flare duration, and duration of decline and duration of growth are extracted from all the associated (A) and not-associated (NA) flares and converted to a numerical format that is suitable for machine-learning use. The machine-learning algorithms Cascade Correlation Neural Networks (CCNN) and Support Vector Machines (SVM) are used and compared in our work. The machine-learning systems predict, from the input of a flare’s properties, if the flare is likely to initiate a CME. Intensive experiments using Jack-knife techniques are carried out and the relationships between flare properties and CMEs are investigated using the results. The predictive performance of SVM and CCNN is analysed and recommendations for enhancing the performance are provided.     

On the origin of solar metric type II bursts

The vast majority of solar flares are not associated with metric Type II radio bursts. For example, for the period February 1980–July 1982, corresponding to the first two and one-half years of the Solar Maximum Mission, 95% of the 2500 flares with peak >25 keV count rates >100 c s^–1lacked associated Type II emission. Even the 360 largest flares, i.e., those having >25 keV peak count rates >1000 c s^–1, had a Type II association rate of only 24%. The lack of a close correlation between flare size and Type II occurrence implies the need for a 'special condition' that distinguishes flares that are accompanied by metric Type II radio bursts from those of comparable size that are not. The leading candidates for this special condition are: (1) an unusually low Alfvén speed in the flaring region; and (2) fast material motion. We present evidence based on SMM and GOES X-ray data and Solwind coronagraph data that argues against the first of these hypotheses and supports the second. Type II bursts linked to flares within 30° of the solar limb are well associated (64%; 49/76) with fast (>400 km s^–1) coronal mass ejections (CMEs); for Type II flares within 15° of the limb, the association rate is 79% (30/38). An examination of the characteristics of 'non-CME' flares associated with Type IIs does not support the flare-initiated blast wave picture that has been proposed for these events and suggests instead that CMEs may have escaped detection. While the degree of Type II–CME association increases with flare size, there are notable cases of small Type II flares whose outstanding attribute is a fast CME. Thus we argue that metric Type II bursts (as well as the Moreton waves and kilometric Type II bursts that may accompany them) have their root cause in fast coronal mass ejections.     

Lasco and eit Observations of the Bastille day 2000 Solar Storm

The period of 10–14 July 2000 saw a large number of energetic solar events ending with a very energetic flare that was associated with a large solar energetic particle event and a fast halo coronal mass ejection (CME) that produced the largest geomagnetic disturbance since 1989. This paper tries to summarize the complex coronal activity observed during this period, in order to establish a background for a number of papers in this topical issue. The GOES X-ray data are presented. Data animations of observations from EIT and LASCO C2 and C3 are presented on the accompanying CD-ROM. The observations around the time of the three X-class flares are considered. EIT observations of the Bastille Day flare show coronal brightening followed by dimming. LASCO had good data coverage for all three events. For one of the flares, no coronal response was seen. The other two flares are associated with halo CMEs. The timing suggests that the start of the flares and CMEs are simultaneous to approximately 30 min. Analysis of the LASCO and EIT images following the Bastille Day flare show the arrival of energetic particles at SOHO at approximately 10:41 UT on 14 July. Individual features of these CMEs have been tracked and the height–time plots used to estimate the dynamics of the CMEs. The initial speed and deceleration of the halo CMEs estimated from the fitting of height–time plots are compared with the in-situ observations at L1. The three flares are identified as the solar sources of three shocks observed at 1 AU. Finally, it is stressed that global heliospheric effects during periods of exceptional activity should consider a cumulative scenario rather than events in isolation.     

Characteristics of CMEs associated with solar flares and DH type II radio bursts based on source position

We studied the characteristics of Coronal Mass Ejections (CMEs) associated with solar flares and Deca-Hectometric (DH) type II radio bursts, based on source position during 23rd solar cycle (1997–2007). We classified these CME events into three groups using solar flare locations as, (i) disk events (0–30^∘); (ii) intermediate events (31–60^∘) and (iii) limb events (61–90^∘). Main results from this studies are, (i) the number of CMEs associated with solar flares and DH-type IIs decreases as the source position approaches from disk to limb, (ii) most of the DH CMEs are halo (72%) in disk events and the number of occurrence of halo CMEs decreases from disk to limb, (iii) the average width and speed of limb events (164^∘ and 1447 km s⁻¹) are higher than those of disk events (134^∘ and 1035 km s⁻¹) and intermediate events (146^∘ and 1170 km s⁻¹) and (iv) the average accelerations for disk, intermediate and limb events are −8.2 m s⁻², −10.3 m s⁻² and −4.5 m s⁻² respectively. These analysis of CMEs properties show more dependency on longitude and it gives strong evidence for projection effect.     

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 Coronagraphs 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 H, 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-à-vis flare blast waves/ejecta.     

A Statistical Study on CMEs Associated with DH-Type-II Radio Bursts Based on Their Source Location (Limb and Disk Events)

We have statistically studied the 344 Coronal Mass Ejections (CMEs) associated with flares and DH-type-II radio bursts (1??C?14 MHz) during 1997??C?2008. We found that only 3?% of the total CMEs (344) compared to the general population CMEs (13208) drives DH-type-II radio bursts (Gopalswamy in Solar Eruptions and Energetic Particles, AGU Geophys. Monogr. 165, 207, 2006). Out of 344 events we have selected 236 events for further analysis. We divided the events into two groups: i) disk events (within 45° from the disk center) and ii) limb events (beyond 45° but within 90° from the disk center). We find that the average CME speed of the limb events (1370?km?s^?1) is three times, while for the disk events (1055?km?s^?1) it is two times the average speed of the general population CMEs (433?km?s^?1). The average widths of the limb events (129°) and disk events (116°) are two times greater than the average width of the general population CMEs (58°). We found a better correlation between the CME speed and width (correlation coefficient R=0.56) for the limb events than that of the disk events (R=0.47). The shock speed of the CMEs associated with DH-type-II radio bursts is found by applying Leblanc, Dulk, and Bougeret??s (Solar Phys. 183, 165, 1998) electron density model; the disk events are found to have an average speed of 1190 km?s^?1 and that of the limb events is 1275 km?s^?1. From this study we compare the CME properties between limb and disk events. The properties like CME speed, width, shock speed, and correlation between CME speed and width are found to be higher for limb events than disk events. The results in disk events are subject to projection effects, and this study stresses the importance of these effects.     

Characteristics of flares producing metric type II bursts and coronal mass ejections

We attempt to study the origin of coronal shocks by comparing several flare characteristics for two groups of flares: those with associated metric type II bursts and coronal mass ejections (CMEs) and those with associated metric type II bursts but no CMEs. CMEs accompany about 60% of all flares with type II bursts for solar longitudes greater than 30°, where CMEs are well observed with the NRL Solwind coronagraph. H flare areas, 1–8 Å X-ray fluxes, and impulsive 3 cm fluxes are all statistically smaller for events with no CMEs than for events with CMEs. It appears that both compact and large mass ejection flares are associated with type II bursts. The events with no CMEs imply that at least many type II shocks are not piston-driven, but the large number of events of both groups with small 3 cm bursts does not support the usual assumption that type II shocks are produced by large energy releases in flare impulsive phases. The poor correlation between 3 cm burst fluxes and the occurrence of type II bursts may be due to large variations in the coronal Alfvén velocity.Sachs/Freeman Associates, Inc., Bowie, MD 20715, U.S.A.