Flare- and coronal mass ejection (CME)-associated type II bursts and related radio emissions

Transcription

1 , JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. A8, PAGES 18,225-18,234, AUGUST 1, 2000 Flare- and coronal mass ejection (CME)-associated type II bursts and related radio emissions Yolande Leblanc and George A. Dulk D partement de Recherche SpatiMe, CNRS, UMR 8632, Observatoire de Paris, Meudon, France Angelos Vourlidas Center for Earth Observing and Space Research, Institute for Computational Sciences and Informatics, George Mason University, Fairfax, Virginia Jean-Louis Bougeret D partement de Recherche SpatiMe, CNRS, UMR 8632, Observatoire de Paris, Meudon, France Abstract. We report on two events that occurred within 6 hours of each other on November 3, They were observed with ground-based spectrographs in the meter to decameter range and with the WAVES experiment on the Wind spacecraft at longer wavelengths. Complementary observations were made with Extreme Ultraviolet Imaging Telescope (EIT) and Large Angle and Spectrometric Coronagraph (LASCO) experiments on the Solar and Heliospheric Observatory (SOHO). The two events are very similar in many ways: both consist of type III bursts, type II shocks, shock-accelerated type III bursts proceeding from the type II to low frequencies, and type IV continuum. Flares and coronal mass ejections (CMEs) are associated with the two events. We trace the history of the two events from the time of the impulsive phase of the flare, which is coincident with the start time of the radio bursts and 10 to 30 min after the CME liftoff from the Sun. We derive the height-time progression of the type II shocks by using a coronal-solar wind density model and compare it with the progression of the CME in the plane of the sky. The results show that the speeds of the type II shocks in the low corona were high, km s -. Then, at a height of m2 Rs, the fast shocks decelerate and become slower shocks, <380 km s -. We discuss (1) the relationship between these type II shocks, flares and CMEs, including the apparent deceleration of the type II shocks, (2) the hypotheses of blast wave versus piston-driven shocks, (3) the acceleration of electrons at the shock front producing the shock-accelerated type III bursts, and (4) the acceleration of electrons that become trapped in the expanding loops of the CME and emit type IV continuum. 1. Introduction tal (F) or harmonic (H). Knowing whether it is F or H from the spectrum and using a coronal/solar wind Solar type II bursts are generated by energetic events density model, the frequency drift can be converted to in the solar corona that produce shock-excited plasma radial distance versus time, and the shock speed can emission at the fundamental and/or harmonic of the be derived. The average speed in the low corona, deplasma frequency. Sometimes they continue through rived from two large samples of Culgoora observations the corona and the solar wind. In the low corona they [Prestage et al., 1994], is 600 km s -, with the lowhave been observed with ground-based spectrographs, est speeds being 300 km s - and the highest being and many of their characteristics have been derived, 1500 km s - [Robinson, 1985b; Thompson, et al., 1996]. such as their speeds and their association with flares The knowledge of the speed is useful in estimating the and with coronal mass ejections (CMEs). time of arrival of the shock at the Earth. However, if the The emissions drift downward in frequency at a rate density model is not adequate or if the mode of emission that is related to the shock speed, the density grais not known, the derived speeds can be overestimated dient, and whether the emission is at the fundamen- Copyright 2000 by the American Geophysical Union. Paper number 2000JA /00 / 2000 JA $ ,225 or underestimated. The association of meter wave type II bursts with flares is well established. For > 90% of them a corresponding flare is recorded [Nelson and Melrose, 1985]. The association of type II bursts with CMEs is more

2 18,226 LEBLANC ET AL.' FLARE AND CME-ASSOCIATED TYPE II SHOCKS Iloo _15o 3O N 0.4 2O o lo 1.0,...,,i ii ii i i ii I 8 6 N , N loo 200 o UT 3 Nov I 1 06OO Plate 1. Dynamic spectrum of the first event of November 3, 1997, at 0437 UT. The right-hand scale, in solar radii, gives the approximate altitude where radiation at the fundamental of the plasma frequency is emitted; the scale is derived using the density model described in the text. The bottom plot, MHz, was recorded by the Culgoora radio spectrograph. Following two microwave impulsive bursts at > 200 MHz, there is type II burst with fundamental (F) and harmonic (H) components, followed by type IV continuum. The type II drift rate is high at the beginning, then lower. SA type III bursts emanate from the type II shock between 0440 and 0450 UT. The WAVES range below 13.8 MHz shows the SA type III bursts merging into a single, complex burst. After 0520 UT, there is a type II with F and H components and another SA type III burst at 0530 UT.

3 LEBLANC ET AL- FLARE AND CME-ASSOCIATED TYPE II SHOCKS 18,227 complex and controversial. Sheeley e! al. [1984] showed that 59% of CMEs observed by Solwind were not accom- panied with type II bursts, and for the inverse, 30% of type II bursts were not accompanied with CMEs. The remaining 41% of CMEs associated with type II bursts had speeds higher than 400 km s -1. The Solwind data were re-analyzed by Uliver et al. [1999, p. 89], who conclude that the lack of close correlation between flare size and type II occurrence implies the need for a "spe- cial condition" and that type II shocks have "their root cause in fast coronal mass ejections." The nature of type II shocks in the low corona and in the interplanetary space is also controversial. Uane [1983; 1997] suggested a two-shock model with the highfrequency type II being a flare-initiated blast wave and the low-frequency, interplanetary type II being a shock driven by a CME. It is not clear if the two shocks are independent and, if not, how a blast wave shock becomes a CME-driven shock. We present ground-based radio observations in the meter to decameter range that were obtained in the first With the WAVES receivers on the Wind spacecraft event by the Culgoora spectrograph [Prestage et al., observing the range from 13.8 MHz to 4 khz [Bougeret 1994], which covers the frequency range MHz, ½t al., 1995] and ground-based spectrographs observ- and in the second event by the Nansay decameter aring the higher frequencies, the frequency coverage is es- ray [Boischot et al., 1980] in the range MHz. In sentially complete for emissions arising anywhere from addition, we have used information from spectra made < 1.1 Rs to 1 AU. Simultaneous coronal observations available on the World Wide Web by the Ondrejov and were made by the Extreme Ultraviolet Telescope (EIT) Izmiran observatories and from Solar Geophysical Data: experiment [de la Boudinier et al. 1995] and the Large- spectral radio emissions. Angle and Spectrometric Coronagraph (LASCO) exper- Low-frequency observations were made aboard the iment [Brueckner et al., 1995] aboard the Solar and He- Wind spacecraft with the WAVES experiment, which liospheric Observatory (SOHO) spacecraft; these cover consists of long dipole antennas in the spin plane and the height range up to 30/i s. Thus we have an unique a short antenna along the spin axis, feeding into three opportunity to determine the characteristics of type II receivers covering the range from 4 khz to 13.8 MHz shocks in the corona and the solar wind. In particular, [Bougeret et al., 1995]. The Wind spacecraft is always we can examine some crucial points that are not under- near the Earth at 1 AU, in the ecliptic plane. Radio stood' (1) Is there a connection between coronal and waves are observed from remote locations, anywhere from near the Sun to > 1 AU. interplanetary shocks? Why do some type II shocks vanish in the low corona and some continue to 1 AU and beyond? Does this depend on the speed or the strength of the shock? Why is the type II radio radiation rarely continuous but patchy in the frequency-time plane? (2) What is the relationship between the CME and the type II shock? Are type II shocks always ahead of CMEs, as is the case for CME-driven shocks at 1 AU? Are blast wave shocks in front of or behind the leading edge of CMEs? How do the speeds of CMEs compare with the speeds of type II shocks? (3) What are the characteristics of electrons accelerated by shock waves and of the resultant radio radiation? In this paper we report on two radio events that occurred within 6 hours of each other on November 3, Each consisted of type III bursts, followed by a type II shock and a continuum associated with the type II. In the first event the type II burst was observed from 200 to 5 MHz, in the second one the burst was ob- served from 200 to 6 MHz. Shock-accelerated (SA) type III bursts, originating from the type II shocks, occurred during each of the events, and an inverted U burst occurred in the course of one of them. The two events were observed with the EIT and LASCO telescopes; in both cases the radio events were accompanied by relatively slow CMEs. Both were closely associated with flares. In section 2 we present the observational techniques. In section 3 we describe the radio observations of the first event, trace the height-time diagrams of the type II shocks by using a coronal-solar wind density model, and compare with the progression of the CME in the plane of the sky. In section 4 we repeat the process for the second event. In section 5 we discuss the similarity of the two events, the hypotheses of blast- and piston-driven waves, and the acceleration of electrons at the shock front with a model of the source regions. In section 6 we give our conclusions. 2. Observational Techniques 3. Observations' Event of November 3, 1997, at 0437 UT 3.1. Radio Observations Plate 1 shows the dynamic spectrum of the radio event in the range 1800 to 0.01 MHz, with only a small gap from 18 to 13.8 MHz. The right-hand scale gives the approximate radial distance where radiation at the fundamental of the plasma frequency is emitted; the scale was derived 'Using the density model of Leblanc et al. [1998], normalized to n cm - at 1 AU, the average value during the period of the events. We add to the model a term in r-16 to describe the coronal density at low heights [Baumbach, 1937]. At the bottom is the spectrum from the Culgoora spectrograph from 1800 to 18 MHz. It shows two type III bursts starting at 0434 UT, at the flash phase of the flare. The C8 X-ray flare, located at S20 W15, began at 0432 UT with a maximum at 0438 UT. The type IIIs were observed from 1200 to 120 MHz, followed by the type II shock starting at 0437 UT and by type IV continuum starting soon after the type II.

4 18,228 LEBLANC ET AL.' FLARE AND CME-ASSOCIATED TYPE II SHOCKS The type II burst consists of F and It components, between the frequency ranges of the type II harmonic clearly seen near 40 and 80 MHz at 0445 UT. The two and the FC-II suggests an association between the two, emissions drift downward in frequency very rapidly at with the FC-II also being at the harmonic of the plasma the beginning and then slow down significantly. Further frequency. If so, this is in contrast with the statement type II emission follows, with a lower drift rate. of Robinson [1985a] that FC-II emission at the fun- It is during the slowdown of the type II, from 0440 to damental UT in the range MHz, that several rapidly The top and middle plots of Plate i show the range drifting structures emanate from the main body, most from 13.8 to 0.01 MHz as recorded by WAVES. SA type pronounced toward lower frequencies. These are shock- III bursts originate from the type II shock and continue accelerated (SA) type IIi bursts. Similar, fast-drift to low frequencies. At m l MHz they merge into one structures originating from the "backbone" of the type burst that can be followed down to < 0.02 MHz. Type II II burst have been called "herringbones" [e.g., McLean, emission begins at 0520 UT at m12 MHz, drifting down 1985]. The unusually slow drift rate of type II bursts to m5 MHz. F and H co nponents are visible from 0535 containing herringbone structures has been pointed out to 0606 UT; the F component is considerably stronger. by Stewart and Magun [1980]. These authors explained The type II is patchy, as is often observed in type IIs at the slow drift as being due to the motion of part of the metric/decametric wavelengths, and is the general rule shock wave being nearly parallel to the solar surface at frequencies lower than m20 MHz. (thus with small density gradient) and the preferential The measured flux density of the F component of emission of SA type III bursts as being due to that part the type II is 2.5 x 104 solar flux units (SFU)(1 SFU of the shock traveling approximately transverse to open = W m -2 Hz - ) at 6.5 MHz, while that of the magnetic field lines. It component is 5 x 10 a SFU at 13 MItz. These flux Type IV continuum follows the type II burst in the densities are similar to those of other type II bursts in frequency range MHz. It was designated FC- this frequency range, some of which were observed to II by Robinson and Smerd [1975]. The correspondence 1 MItz and below [e.g., Dulk et al., 1999]. 10 L, s",'r UT ' 3.5 ' 3.0 rr ,.,,... O430, 0500 O53O 0600 UT 3 November 1997 Figure 1. Height-time diagram of the first ew:nt showing the progression of the type II burst (stars for both F and H components) and of the CME in the plane of the sky (squares). The frequency of the radio emissions has been converted into radial distance by using the coronal-solar wind density model of Leblanc et al. [1998] normalized to n, = 12.6 crn -a i AU. The solid line is a best fit, second-degree polynomial fit to the CME measurements. Top: The event with the CME observed out to 10 ils. Bottom: Detailed diagram showing the different speeds of the type II shock. ' - IO O 100

5 LEBLANC ET AL.' FLARE AND CME-ASSOCIATED TYPE II SHOCKS 18,229 Another SA type III burst, at m0530 UT, has a starting frequency of 4 MItz. The electrons producing it were accelerated by the type II shock, whose frequency at the time was 8 MHz Radial Distance Versus Time After measuring the frequency versus time of the type II emissions, we convert the frequency into radial distance by using the same coronal-solar wind density model used above, i.e., that of Leblanc et al. [1998] normalized to n cm -a at i AU, the average value during the period of the events. This normalization gives densities in the corona and solar wind that are 1.8 times larger than the background at solar minimum, and thus are more representative of the corona and solar wind at the time of these events. The results are shown in Figure 1, where type II emis- sions are indicated by stars for both the F and H components (the frequency of the latter having been divided by 2). At the beginning, in the frequency range MHz, the derived speed is 950 km s - and then the speed drops significantly to less than 100 km s - at 1.5 Rs. Then, for the part of the type II from 80 to 40 MItz the derived speed is 300 km s -. In the frequency range below 13.8 MHz (times after 0520 UT) the average speed is again m 300 km s -. The scatter of the data points is due to the burst being composed of severalanes with different frequencies Coronal Observations Because the timing of the EIT wave, the CME, the flare, and the type II burst is crucial, two of the au- thors (G.A.D. and A.V.) have independently measured the EIT and LASCO images. The two results are consistent. The EIT telescope recorded a large-scale wave moving away from the flare site, starting sometime between 0416 and 0432 UT. At 0432 UT, a wave was seen at 0.5 Rs from the active region. However, the precise initiation time is not known, so its association with the type II and CME is uncertain. Careful examination of the images from LASCO reveals a slow, faint CME traveling from m2.7 Rs at 0528 UT to 9.6 Rs at 1150 UT, when it was too faint to be further observed. The position angle (2290) is consistent with the position of the active center on the disc. We have overplotted the measurements of the leading edge (squares) on Figure i to compare with the radio positions of the type II shock. The CME points are not well fitted by a straight line: The curvature demonstrates significant deceleration, so we fit them with a second degree polynomial. The speed of the CME in the plane of the sky at the beginning was 380 km s -, dropping to m100 km s - by 9 Rs. The CME liftoff time is 0430 UT, estimated by extrapolating the second-degree polynomial to R = i Rs, i.e., when the CMEs appear above the limb. However, the estimated CME initiation at the active region, 250 from disk center, is 18 min earlier Type II-CME Comparison We point out that the radial distances of the CMEs are observed in the plane of the sky, while the type II shocks are presumably more or less radially above the active region, which was only 250 from disk center. The speeds in these two directions are not necessarily the same, although for halo transients, such as this one probably was, the two speeds may not be very different. However, the extrapolated CME liftoff times are independent of speed, so the comparison with type II initiations is meaningful. The data points of the type II and the CME on Figure 1 show that the initiation of the CME at the active region was 18 min earlier than its appearance at R Thus the CME commenced m25 min before the type II burst. The initial speed of the the type II, 950 km s - is much higher than its subsequent speed and that of the CME. Figure 1 suggests that the fast type II shock changed into a slower type II shock at m0443 UT. Subsequently, on Figure 1, the derived radial distance of the type II is lower than that of the CME. However, the exact physical relationship is uncertain because most of the type II shock was probably traveling radially outward from the active region near disk center, while the CME is measured in the plane of the sky. 4. Observations' Event of November 3, 1997, at 1025 UT 4.1. Radio Observations The second event was similar to the first one, with type III bursts followed by a type II burst and then a continuum FC-II. The event was associated with a flare located at S20 W15, nearly the same location as the earlier event. The M4 X-ray flare began at 1018 UT with a maximum at 1030 UT. A microwave impulsive burst was recorded by the Ondrejov observatory starting at 1025 UT, coincident with a group of type III bursts at lower frequencies. Type II bursts began at 1029 UT at m200 MHz, as recorded by the Izmiran spectrograph. Plate 2 (bottom) shows the dynamic spectrum of the radio event recorded by the Nan ay spectrograph from 75 to 25 MHz. It commences with intense type IIIs, followed by a fairly complicated type II with F and H components and several lanes with different slopes; at 75 MHz it started at 1029 UT. The FC-II continuum follows the harmonic emission, starting at 1048 UT. At lower frequencies the event was observed from 13.8 to 0.01 MHz by WAVES. The initial type III burst is the continuation of the bursts seen in the range MHz. After 1035 UT, there are $A type III bursts originat- ing from the type II burst, merging into one burst at ml MHz. At 13.8 MHz the type II burst starts at 1045 UT, and it continues down to 5 MHz at 1130 UT. It is continuous from 13.8 to 10 MHz, then discontinous, with weaker emissions at 9 and 5 MHz.

6 18,230 LEBLANC ET AL.' FLARE AND CME-ASSOCIATED TYPE II SHOCKS O z _ : o N _ 0.5-r- a UT IlO UT 3 November O 100 Figure 2. Height-time diagram of the event of November 3, 1997, at 1025 UT showing the progression of the type II burst (stars), the U burst (solid circle), the front of the CME (squares), and the distinct, lagging CME feature (triangles). The solid lines are best fit, second-degree polynomials to the CME data. Top: The overall event with the CME observed out to 22 Rs. Bottom: detailed diagram showing different speeds of the radio emissions: 900 km s - in the high frequency range, and 370 km s - in the lower range. The turning height of the inverted U burst is close to the progression line of the CME. An inverted U burst occurs at 1213 UT, with a turning frequency of 2 MHz. This burst was probably associated with a flare at 1211 UT at S19 W20, very close to the preceding one Radial Distance Versus Time As with the first event, we convert frequency into radial distance by using the same density model. The results are shown in Figure 2. The derived speed at high frequencies is 900 km s-, while at lower frequencies it was 340 km s-. The turning frequency of the inverted U burst corresponds to a radial distance of 4.7 Rs, as shown by a solid circle in Figure Coronal Observations An EIT image at 1021 UT shows a minor loop brightening, and the image at 1032 UT shows a wave at Rs from the active region. However, the wave moves to the north or northeast, while the CMEs propagate to the southwest. Therefore there may have been no relationship between the EIT wave and the CME liftoff. The LASCO telescope recorded a CME traveling out to > 22 -Rs, followed by another feature that was possibly an independant CME, but much more likely a trail- ing part of the same CME. Their position angles, 231 ø, were the same, consistent with the position of the active center, and as shown below, the liftoff times were nearly identical. On Figure 2 the CME front is indicated by squares, and the lower part is indicated by triangles. The front shows only slight evidence of deceleration, while the lower part shows somewhat more deceleration. The solid lines on Figure 2 are secondorder polynomials fitted to the data points. The initial speed of the CME front in the plane of the sky is 360 km s -, and that of the lower part is 210 km s - The extrapolated liftoff times at R = i Rs (when the CMEs appear at R = 1 Rs) are 1002 and 1010 UT. Extrapolating to the location of the active region 250 from disk center, these liftoff times are nearly the same, 0944 and 0940 UT, respectively Type II-CME Comparison For this event it is again clear that the liftoff time of the CME was min before the type II. As with

7 LEBLANC ET AL.' FLARE AND CME-ASSOCIATED TYPE II SHOCKS 18,231 U ø i lo 6 N N o 1.4 r O0 1 i UT 3 Nov 1997 Plate 2. Dynamic spectrum of the second event on November 3, 1997, at 1025 UT. The bottom plot shows the MHz part of the event as observed with the Nan ay radio spectrograph. The type III group is followed by the type II burst with F and H components and by type IV continuum. The top and middle plots show the first type III group, followed by SA type III bursts from 1035 to 1045 UT, originating from the shock front. The type II burst is visible down to 5 MHz. An inverted U burst is seen at 1213 UT.

8 18,232 LEBLANC ET AL.' FLARE AND CME-ASSOCIATED TYPE II SHOCKS the previous event, the initial speed of the type II shock was > 900 km s -1. At a certain distance, m2 Ils, the type II speed decreased to 300 km s-1. The turning height of the U burst, 4.7 -Rs, shown by the solid circle on Figure 2, is approximately on the line of the CME progression. To produce the U burst, energetic electrons were accelerated by a separate flare at 1211 UT and traveled along closed magnetic field lines of the CME whose tops were at 4.7 -Rs. An excellent example of an inverted U burst with a i MHz turning frequency, traveling along the loop of a CME, was presented by œeblanc et al. [1999a]. 5. Discussion 5.1. Relationship Between the Type II Shocks and CMEs model may be underdense if the corona above the active region was more dense than usual, or it may be overdense if the CME had evacuated much material of the corona. In the former case the type II shock height would be underestimated but very improbably enough to place the shock at the front of the CME. In the latter case the type II shock would be even farther behind the front of the CME Blast Wave and/or Piston-Driven Shocks? In these two events the speeds of the initial type II shocks were significantly lower after they reached Rs. This is marked by the sudden change in the slopes of type II data points in Figures 1 and 2. After the slope change, the shocks had speeds of km s -1, similar to the CME speeds. According to general belief, coronal type II shocks are due to blast waves initiated by the sudden release of energy of flares; the resulting shock waves do not travel to large distances in the corona. This contrasts with piston-driven shocks which are ahead of the CMEs that These two events were very similar in many ways: They occurred above the same active region at S20 W 15, at two different times corresponding to flare eruptions. Two distinct CMEs accompanied the events, and they propagated in the corona-interplanetary space drive them, where the shocks and CMEs propagate at with a speed of km s -1 measured in the plane similar speeds, the shocks slightly faster. The initiation of the sky. In both cases the extrapolation to the Sun of a CME-driven shock could be a flare accompanied by of the liftoff times of the CMEs are before the times of a CME. the flares and type II shock initiation. Thus the type II shocks were propagating in the wake of the CME. In the events discussed here the two type II shocks were observed only up to 3 s. In such cases the shocks The two events are "homologous" in several ways. can be attributed to blast waves which vanish in the This concept was applied to flare-associated type II bursts by Stewart and Hardwick [1969]. With these two events the concept can be further extended to flare- and CME-associated type II bursts. Both type II shocks were observed out to m3 Rs. The high corona. On the other hand, the two shocks were closely associated with CMEs. From these results we propose the following scenario' A rearrangement of the magnetic field in the corona on a large scale initiates a CME. The flare follows 20- derived speeds of the type II shocks in the low corona, 30 min later and initiates a type II shock in the low Rs, are km s -, much higher than the corona with a speed of m900 km s- i.e. much higher km s -1 speeds at higher altitudes, 2-3 -Rs. than the CME speed of m300 km s -1. Therefore the Deceleration of type II shocks has previously been re- type II shock propagates into the wake of CME, out to ported [Robinson, 1985b and references therein]. Robin- 2 -Rs. Indeed, Stewart [1980] and Gary ½t al. [1983] son noticed that m15% of type II shocks exhibit a deceleration and that these 15% are characterized by a high initial velocity. The present study is consistent with these results and extends them to lower frequencies. published detailed observations by the Culgoora radioheliograph and spectrograph, and SMM coronograph' They showed that the type II sources were well behind the front of the CME, certainly not ahead of it [see also With WAVES observations we can follow the shocks Wagner, 1983, MacQueen, 1980]. At 2 -Rs, the sudto higher altitudes and confirm the deceleration. The study of another example, April 7, 1997 [Leblanc ½t al., 1999b], shows that the shock speed was kms -1 in the low corona and then 600 km s -1 at a higher altitude. The sudden change in frequency drift rate, which we attribute to a change in shock velocity, could also result from a large and abrupt change of density gradient (not simply an abrupt change in density). It is possible that a density gradient ju,mp occurs at the front of a CME. However, this explanation is not in accord with Figures i and 2, where the type II shock is behind the front of the CME. As mentionned earlier, the density model we used to derive the radial distances of the type II shock is 1.8 den change in velocity in Figures i and 2 shows that the initial blast wave decelerated to approximately the speed of the CME. Perhaps these subsequent slow shocks were driven, not by the CME front, but by a slower moving, distinct CME feature such as that described by the triangles on Figure 2. A well-documented example where this occurred is given by Vourlidas ½t al. [1999]. We note that the two events were associated with relatively slow CMEs œ 380 km s-i, which is just below the Cliver ½t al. [1999] statistic that type IIs are associated with fast CMEs, defined as traveling faster than 400 km s-. In some events the flares, liftoff of the CMEs, and initiation of the type IIs occur nearly simultaneously. times that of Lcblanc et al. [1998], as was appropri- Then it is not possible to distinguish whether the shock ate for the corona/solar wind in November This is a blast wave or is CME-driven; it begins in the low

9 LEBLANC ET AL.: FLARE AND CME-ASSOCIATED TYPE II SHOCKS 18,233 corona and continues to 1 AU or beyond. This was the case of the November 4, 1997 event described by Dulk et at. [1999] Acceleration of Electrons at the Shock Front observations Acceleration of electrons at the shock front and escaping along open field lines: SA type III bursts. The two events show beautifully the continuity of SA type III bursts emanating from the radio emission of the shock front: They are observed from 2 Rs to close to i AU (Plates i and 2). It seems that these SA type III bursts are due to electron beams accelerated by the shock. There was no microwave emission at the same time that would indicate electron acceleration in the low corona. SA type III bursts similar to these were described by Cane el al. [1981] and Bougeret 6. Conclusion These events involve several different types of radio emission: type III and type II bursts, type IV continuum, and SA type III bursts emanating from the shock front and propagating to 1 AU. The radio observations cover the entire range Rs, and the simul- taneous coronal observations have revealed some new results on type II shock waves. One main result concerns the relationship between CME, flare, and type II shock. We have shown that they are closely related, with the CME liftoff occurring before the flare and initiation of the type II shock. At Electrons trapped in closed field lines: the beginning the shock front was behind the front CME Continuum plasma radiation. The flare contin- but with a higher speed. Then there may have been an uum FC-II started after the type II harmonic emission, interaction of the shock with a trailing part of the CME, with a similar frequency drift rate. The cutoff frequency where the rapidly drifting type II emission ceased and was 50 MHz for the first event and 35 MHz for the was replaced by slowly drifting emission. From then on, second event. The continuum lasted >2 hours. The the type II shock had a speed similar to the CME speed, continuum is plasma radiation, attributed by Robinson perhaps propagating as a CME-driven shock. The fact [1985a] to be at the fundamental, but here it is more that the CME and type II shock speeds are rather low closely related to the harmonic of the type II. The de- ( <400 km s-t), might explain why the radio emission rived heights are 1.3 to 1.7 Rs. was not observed beyond 3 Rs. According to Robinson [1985a and references there- The other results concern radio emissions to the type II shock. There is evidence of acceleration of electrons in], electrons are accelerated by the shock wave as it traverses slowly expanding CME loops lagging behind beams by the type II shock, producing SA type III the front. The electrons accelerated at the shock front bursts propagating from the corona to the interplanare trapped in these loops and radiate plasma radia- etary medium. The type IV continuum emission is protion, which starts near the time of the shock traversing the loops. This scenario is consistent with the present duced when electrons are trapped in expanding loops after being accelerated during the traversal of the type II shock. The observation of the U burst due to another flare in the same active region shows that fast streams of electrons can propagate along expanding arches to a large height in the corona. Acknowledgments. The WAVES experiment on Wind spacecraft is a joint project of the Observatoire de Paris, NASA/GSFC, and the Universities of Minnesota and Iowa. SOHO is an international collaboration between NASA and ESA and is part of the International Solar Terrestrial Physics Program. LASCO was constructed by a consortium of institutions: the Naval Research Laboratory (Washington, D.C.), the University of Birmingham (Birmingham, U.K.), et al. [1998]. the Max-Planck-Institut f/it Aeronomie (Katlenburg-Lindau, Germany), and the Laboratoire d'astronomie Spatiale (Mar- SA type III bursts are more complex than regular seille, France). We thank L. Denis, M. Aubier, and A. Lecatype III bursts, having several components, as seen in cheux for help in obtaining the Nan( ay decametric spectrum Plates 1 and 2. Below 1 MHz the components merge and N. Prestage for help in obtaining the Culgoora specinto a single type III burst. Thus, at i MHz the duratrum. Michel Blanc thanks Edward W. Cliver and two other tion is usually longer, and the intensity-time profile is more complex than for regular type III bursts. Reiner referees for their assistance in evaluating this paper. and Kaiser [1999] described examples of "type III-like" bursts observed from 13.8 to 0.1 MHz, some of them References being SA type III bursts. These authors find different Baumbach, S., Coronal densities from white hght echpse morphological characteristics above and below 7 MHz observations, Astron. Nachr., 263, 121, which are not evident in our examples. Rather, some Boischot, A., et aj., A new high gain, broadband, steerable of our SA type III bursts are seen to begin at the type array to study Jovian decametric emission, Icarus, 3, II shock, as high as 50 MHz. The SA type III bursts 399, Bougeret, J.-L., et al., WAVES: The radio and plasma wave intensify as they descend to lower frequencies, attain investigation on the Wind spacecraft, Space Sci. Rev., 71, peak flux at 1 MHz, and then the intensity decreases; 231, normal type III bursts behave similarly. 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