High-frequency type II radio emissions associated with shocks driven by coronal mass ejections

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. A8, PAGES 16,979-16,991, AUGUST 1, 1999 High-frequency type II radio emissions associated with shocks driven by coronal mass ejections M. J. Reiner l Raytheon ITSS, Lanham, Maryland M. L. Kaiser Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland Abstract. We investigate the causal relationship between type II radio emissions in the frequency range from 1 to 14 MHz observed by the WAVES experiment on the Wind spacecraft and metric type II bursts observed by ground-based radio telescopes. We do this by extrapolating, to lower radio frequencies, fits to the frequency drift rates of metric type II bursts. These fits, which describe the shock propagation through the solar corona, are based on coronal density laws derived from white light observations. In order to establish whether the type II radio emissions were generated from the same shock, we explicitly chose events for which the type II radio emissions in the frequency range from 1 to 14 MHz were temporally separated from the associated metric type II bursts. The analyses indicate that some type II radio events observed by WAVES may not be low-frequency continuations of the apparently associated metric type II bursts. We conclude that two temporally separated shocks are required to account for the radio observations for these events, one shock being temporally well associated with the flare maximum. In two cases the observed GOES X-ray flux confirms the presence of two closely spaced solar events. We argue that the type II radio emissions observed by Wind/WAVES are likely generated by coronal mass ejection-driven shocks that form as low as 2 Rs in the solar corona. 1. Introduction frequency (1-14 MHz) radio receivers, this critical gap between space-based and ground-based observations has been According to a widely accepted view, metric type II and interplanetary type II radio emissions are of different origin. largely filled. Gopalswamy et al. [1998] studied 34 metric Metric type II bursts are believed to be generated by flare type II radio bursts observed from ground-based observatories and found that none of these bursts extended into the associated (blast-wave) coronal shocks [Gergely et al., 1983; Wagner and MacQueen, 1983; Nelson and Melrose, 1985; frequency range below 14 MHz observed by the Wind/WAVES radio receivers. While these results were Gopalswamy et al., 1998], whereas interplanetary (kilometric wavelength) type II radio bursts are generated by consistent with an independent origin for metric and interplanetary shocks driven by coronal mass ejections interplanetary radio bursts, in fact, no interplanetary type II (CMEs) [Cane et al., 1987]. Recently, this view has been radio emissions were observed by WAVES during this challenged by Cliver [1999] and Cliver et al. [1999], who interval of study [Cliver, 1999]. More recently, Kaiser et al. have revived the view that both metric and interplanetary type [1998] studied an event on April 7, 1997, that did generate radio emissions in the frequency band between 1 and 14 II radio bursts are generated by CME-driven shocks. This MHz. They argued that some of these radio emissions were latter view, if correct, would require that CME-driven shocks generated by a blast-wave coronal shock, which also form very low in the solar corona (<1.1 Rs from the Sun center, where Rs = 1 solar radius = 696,000 km). produced a metric type II burst, and some were produced by a The study of the relationship between metric and CME-driven shock. This analysis provided the first evidence interplanetary type II bursts has been previously hindered by that blast-wave shocks could propagate beyond about 4 Rs, the frequency gap from a few megahertz to MHz that is, to frequencies < 3 MHz. On the other hand, the radio emission associated with the CME-driven shock was observed between space-based and ground-based observations. With the launch of the Wind spacecraft, which includes highat 4.5 MHz, the highest frequency reported for CMEassociated type II emission [Maliston et al., 1976]. However, this April 7 event also produced no interplanetary type II radio emissions at frequencies below 1 MHz that could be 1 Also at NASA Goddard Space Flight Center, Greenbelt, Maryland. definitely associated with a CME-driven interplanetary shock. The purposes of this paper are (1) to establish that the Copyright 1999 by the American Geophysical Union. Wind/WAVES observations, at least for some cases, require the presence of two temporally separated shocks in the solar Paper number 1999JA corona and (2) to determine the likely origin (blast-wave or /99/1999JA $09.00 CME-driven shock) of the radio emissions in the frequency 16,979

2 16,980 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS regime (decameter/hectometer wavelengths) made available by the high-frequency receivers on the Wind spacecraft. We establish which of the two solar events may be involved in generating the type II radio emissions observed in the provide evidence that some of the type II radio emissions previously unexplored frequency range from 1 to 14 MHz, we observed by Wind/WAVES are not low-frequency continuations of the apparently associated metric type II radio have selected events for which the decametric/hectometric type II radio emissions observed by WAVES were temporally bursts. Our arguments are based on the timings and observed separated from the associated metric type II bursts. As we frequency drift rates of these type II radio emissions. Some radio emissions in the 1-14 MHz range were generated by CME-driven shocks that must have formed very low in the corona, on the basis of the continuity between these radio shall show, these decametric/hectometric type II events are not likely low-frequency continuations of the associated metric type II bursts, suggesting that a (blast-wave) shock associated with the flare cannot easily also account for the emissions and those observed at kilometric wavelengths, type II radio emissions observed by WAVES. which are known to be generated by CME-driven shocks [Cane et al., 1987]. We present evidence that CME-driven shocks can generate radio emissions at radio frequencies as high as 14 MHz, corresponding to heliocentric distances of about 2 Rs. For the purposes of this investigation, we adopt the view that metric type II radio bursts are generated by blast-wave shocks associated with flares and argue that the To establish the causal relationship between a metric type II burst and a decametric/hectometric type II burst, it is not correct just to compare their beginning and end times. To establish their true causal relationship, we believe that it is necessary to incorporate the physics of the shock dynamics through the solar corona. We do this as simply and as best as we can, given the uncertainties inherent in the physics of the solar corona. The shock propagation through a given corona Wind/WAVES radio observations are consistent with this (described by a coronal density model) uniquely determines view. It is beyond the scope of the present paper to unequivocally establish whether metric type II bursts are generated from blast waves associated with flares or from CME-driven shocks. This will require, among other things, complete coronagraph data, which were not available for the events studied in this work. If, in fact, it tums out that metric type!i radio bursts are generated by CME-driven shocks as recently suggested by Cliver et al. [1999], then our results may also be consistent with this view, provided that there are multiple CMEs, or at least multiple CME components, that occur some 20 min or so apart. the observed frequency drift rate for a type II radio burst, and vice versa. Therefore we use the frequency drift rate measured for the metric type II to determine the shock dynamics within the context of a given coronal density model. This fit to the frequency drift yields the shock speed. We then allow this same shock to propagate forward in time through the same model corona until it reaches coronal plasma densities that correspond to the radio frequencies observed for the Wind/WAVES type II events. We say that the metric and decametric/hectometric type II bursts are causally related if and only if their frequency drift rates match, that is, are a continuation of each other for a given shock dynamics. This 2. Instrumentation then indicates that they were generated from the same shock propagating through the same corona. The WAVES instrument on the Wind spacecraft includes several radio receivers that cover the frequency range from khz to MHz [Bougeret et al., 1995]. The instruments used in the present analysis were the 3.1. August 18-19, 1998, Type II Radio Event Plates la and lb show dynamic spectra of radio emissions superheterodyne (step-tuned) receivers: RAD1, which covers observed by the Wind/WAVES RAD1 and RAD2 radio the frequency range from 20 to 1040 khz at 32 discrete frequencies (selected from 256 frequency channels), and RAD2, which sweeps 256 frequency channels from to MHz in s with a frequency resolution of 50 khz. The bandwidths of the RAD 1 and RAD2 receivers are 3 receivers on August 18-19, The RAD2 dynamic spectrum in Plate l a ranges from 2 to MHz for the 2 hour period from 2200 to 2400 UT on August 18. The vertical axis is plotted as inverse frequency. The intensity of the radio emission is indicated by the color, red being the most intense. and 20 khz, respectively. The RAD 1 receivers are connected The combined RAD1 and RAD2 dynamic spectrum in Plate to a dipole antenna (50 m elements) in the spacecraft spin plane and a dipole antenna (5.28 m elements) along the spacecraft spin axis, and the RAD2 receivers are connected to lb ranges from 286 khz to MHz for the time period from 2200 UT on August 18 to 0300 UT on August 19, The vertical axis is also plotted as inverse frequency. The a dipole antenna (7.5 m elements) in the spacecraft spin plane dynamic spectra in Plates l a and lb show intense type III in addition to the spin axis antenna. The spin axis of the Wind spacecraft is approximately perpendicular to the ecliptic radio bursts and weaker type II radio emissions associated with a major solar event. The frequency of the observed type plane. The Wind spacecraft, which was launched in II radio emissions decreased from- 7 MHz to 280 khz November of 1994, executes complex orbits that include excursions to the Lagrange point (L 1) and series of near-earth passes. During the time of the observations presented here Wind was in the solar wind, >50 RE upstream from Earth. between 2305 UT on August 18 and 0300 UT on August 19. The observed radio emissions were associated with a longduration X-ray (LDE) X4.9, lb solar flare at N33E87 from 2213 to 2350 UT on August 18, with maximum at 2216 UT (from Solar Geophysical Data (SGD) reports). A metric type 3. Analysis II radio burst was reported by the Culgoora radiospectrograph between 2216 and 2221 UT with clear fundamental and During major solar events it often happens that a flare and harmonic bands. The fundamental band drifted from 350 to 35 the lift-off of a CME occur nearly simultaneously. This makes MHz; the harmonic drifted from 1200 to 45 MHz. The it difficult to distinguish which of these coronal disturbances measured frequency drift rate gave an estimated shock speed, is responsible for the observed type II radio emissions. To based on the (unmodified) Newkirk coronal density model

3 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS 16,981 metric type II 22:35 UT : _. / 22:40 UT (a) MHz 0. lo H F H F :00 23:00 Time (UT) August 18, :00 2 khz O OOO (b) 1 ooo0 0. o oo O.00 OO0 OO0 22:00 23:00 24:00 01:00 02:00 Time (UT) August 18-19, :00 Plate 1. (a) Dynamic spectrum of the Wind/WAVES radio data on August 18, 1998, in the frequency range from 2 to MHz showing type II and type III radio emissions. The ordinate scale is the inverse of the observing frequency. The various curves are explained in the text. (b) Dynamic spectrum of the Wind/WAVES radio data on August 18-19, 1998, again plotted as inverse frequency versus time and showing the weak interplanetary type II radio emissions that are organized along straight lines. The nearly vertical features are intense type III radio bursts. The dynamic spectrum was purposely overexposed to bring out the weak type II radio emissions. Note that the format of the I/f versus time plot tends to artificially broaden the bandwidth of the lower-frequency emission so that a narrow band feature at lower frequencies appears relatively broadband on this dynamic spectrum.

4 16,982 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS m etric type II September 20, 1998 I I,,, I,,,, I,,, I , (a) :30 03:00 03:30 04:00 04:30 metric type II.,, 0.00 I 0.10 'rime(ut) September 20, 1998 I :00 MHz 10 N 0.20,,, F H F :00 04:00 Time (UT) 2 05:00 0 I intensity (db) relative to background Plate 2. (a) Conventional dynamic spectrum of the Wind/WAVES radio data on September 20, 1998, in the frequency range from to MHz showing type II and type III radio emissions. (b) Dynamic spectrum of the Wind/WAVES radio data on September 20, 1998, from 2 to MHz, plotted as inverse frequency versus time and showing slow frequency-drifting type II radio emissions. The various curves are explained in the text.

5 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS 16,983 I metric type II May 19, 1998 I t! 3.8 q 09:30 09:45 10:00 10:15 10:30 10:45 11:00 11:15 11:30 Tirne(UT) May 19, 1998, UHz loo O.lO o. 12 x, o GMT (HRS) 0 I intensity (db) relative to background Plate 3. (a) Conventional dynamic spectrum of the Wind/WAVES radio data on May 19, 1998, in the frequency range from to MHz showing type II and type III radio emissions. (b) Dynamic spectrum of the Wind/WAVES radio data on May 19, 1998, from 5 to MHz, plotted as inverse frequency versus time and showing slow frequency-drifting type II radio emissions. The various curves are explained in the text.

6 16,984 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS metric type II, i November 3, 1997, I i! (a) 4.0 I :45 05:15 Time(UT) 05:30 05:45 06:00 metric type II 0.00 November 3, 1997 MHz IO O. lo H H F F 5 05:00 06:00 Time (UT) 0 I intensity (db) relative to background Plate 4. (a) Conventional dynamic spectrum of the Wind/WAVES radio data on November 3, 1997, in the frequency range from to MHz showing type II and type III radio emissions. (b) Dynamic spectrum of the Wind/WAVES radio data on November 3, 1997, from 5 to MHz, plotted as inverse frequency versus time and showing slow frequency-drifting type II radio emissions. The various curves are explained in the text.

7 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS 16,985 [Newkirk, 1967], of 1700 km/s (N. Prestage, private communication, 1998). The time of the metric type II is indicated by the red horizontal bar over the top of the dynamic spectrum in Plate la. Because of loss of contact with the Solar and Heliospheric Observatory (SOHO), no visual CME was recorded for this event, nor were there any reported disappearing filaments on the Sun. Nevertheless, a CME was very likely to have been associated with this very energetic LDE X-ray event [Sheeley et al., 1983; Harrison, 1995]. We provide independent evidence from the radio data below for the presence of a CME in the interplanetary medium. The horizontal bands running across the upper regions of the dynamic spectrum in Plate l a are man-made terrestrial emissions [Kaiser et al., 1996]. The (nearly vertical) rapid frequency-drifting radio emissions from 2216 to UT are type III radio bursts associated with the flare/cme event. These intense radio emissions are produced by streams of suprathermal electrons propagating along open magnetic field lines at speeds from 0.1 to 0.3c (c = 3 x 108 km/s), hence the rapid frequency drift [Suzuki and Dulk, 1985]. The more slowly frequency-drifting radio emissions, drifting from -7 MHz (1/f = 0.14) to 2.2 MHz (1/f = 0.45) between 2305 and 2337 UT, are decametric/hectometric type II radio emissions observed in this previously unexplored frequency range and associated with this flare/cme event. Type II radio emissions are believed to be generated at the local plasma frequency (and its harmonic) by a shock propagating through the corona or interplanetary medium [Nelson and Melrose, 1985]. The observed frequency drift results from the decrease in the plasma density with increasing heliocentric distance, and the frequency drift rate is directly related to the speed of the shock. We wish to determine if the decametric/hectometric type II radio emissions, shown in Plate l a, are a lowfrequency continuation of the associated (ground-based) metric type II radio burst (not shown) or are of an independent origin. If the type II radio emissions observed in RAD2 were a low-frequency continuation of the observed metric type II burst, then we would expect that the frequency drift, based on the Newkirk coronal density-distance scale that was used in fitting the frequency drift of the Culgoora metric type II burst [Prestage et al., 1994], would extrapolate through the type II radio emissions observed in RAD2. The solid curves in Plate la show the extrapolation of the metric type II frequency drift, for fundamental and harmonic emissions, assuming a constant shock speed of 1700 km/s and an onset time of 2216 UT at 350 MHz for the fundamental. This fit to the frequency drift of the metric type II burst, based on the Newkirk coronal density model, clearly does not simultaneously fit the type II radio emissions observed in RAD2. Other density-distance scales, for example, Saito [1970], Saito et al. [1977], Guhathakurta et al. [1996], and Leblanc et al. [1999], yield similar results. The type II radio emissions observed in RAD2 occur significantly later than expected if they were a lowfrequency continuation of the reported metric type II burst. In order to make the derived coronal density model frequency drift, with an onset time of 2216 UT, intersect the type II radio emissions observed in RAD2 at all, we would have to reduce the shock speed to km/s. However, the frequency drift rate would then no longer simultaneously fit the metric type II drift rate. Alternatively, one could uniformly enhance the coronal density model by about 11 times. But this again would yield a frequency drift curve that did not simultaneously fit the metric type II frequency drift rate. Introducing a significant amount of deceleration in the shock dynamics also does not help. These results therefore suggest that the observed radio emissions from 7 to 2.2 MHz between 2305 and 2337 UT on August 18 are not a lowfrequency continuation of the associated metric type II burst. On the other hand, the WAVES RAD 1 receiver detects slow frequency-drifting interplanetary type II radio emissions decreasing from- 1 MHz to 280 khz from 0016 to 0205 UT on August 19. These sporadic radio emissions are visible on the radio dynamic spectrum in Plate lb, which combines the RAD 1 and RAD2 data, with frequency ranging from 286 khz to MHz. These slow sporadic frequency-drifting radio emissions, originating in the interplanetary medium, presumably were generated by a CME-driven shock as it propagated through the interplanetary medium [Cane et al., 1987; Reiner et al., 1998a]. (The nearly vertical saturated radio emissions are again type III radio bursts. We had to saturate them in order to make visible the weaker, slow frequency-drifting, sporadic type II emissions.) The reason for introducing the inverse frequency (l/j) versus time dynamic spectra is now clear. In the interplanetary medium the plasma density n is known to decrease roughly as 1/R 2 [Bougeret et al., 1984]. Since the plasma frequency scales as v/n, radio emissions generated by a CME-driven shock, propagating at a constant speed, will be organized along straight lines on this dynamic spectrum; the slope of the line depends on the shock speed and on whether the emission is at the fundamental or harmonic of the local plasma frequency, and the intercept of the line depends on the CME lift-off time [Reiner et al., 1998a, b]. Indeed, a linear extrapolation through the decametric/hectometric and kilometric radio emissions has an intercept at UT, suggesting that the CME lift-off time occurs significantly later than the onset time of the metric type II burst. It can be argued, however, that the 1/R 2 plasma density falloff in the interplanetary medium is not valid for heliocentric distances close to the Sun. To remedy this problem, we used a number of different coronal density models, including Newkirk [1967], Saito [1970], Saito et al. [1977], and Leblanc et al. [1999], all of which describe the coronal plasma density falloff near the Sun. We assumed that these coronal density models could also be extrapolated out into the interplanetary medium. The dashed curves in Plates l a and 1 b show the resulting frequency drift rate for the Saito et al. [1977] coronal density model for fundamental and harmonic emissions. We chose to use the Saito et al. [1977] density model because it was designed to fit the coronal plasma density farther from the Sun than was the Saito [ 1970] model. We adjusted the initial time and the shock propagation speed until this Saito et al. density model yielded frequency drift rate curves that simultaneously fit the type II radio emissions observed in both RAD1 and RAD2. The resulting fit implies that (1) the radio emissions observed in RAD2 are the high-frequency continuations of the radio emissions observed in RAD1 and (2) the radio emissions observed in RAD2 were generated at the harmonic, while the lower- frequency sporadic radio emissions in RAD1 were generated at the fundamental of the plasma frequency. (The radio emissions drifting from 3 to 2.6 MHz between 2325 and 2333 UT lie along another frequency drift curve originating from the same initial time and corresponding to radio emissions from another region of the shock front at a somewhat lower

8 16,986 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS plasma density [Reiner et al., 1998a]). The best fit onset time was 2235 UT on August 18, and the (shock) speed was 960 km/s, significantly less than the estimated speed of the shock generating the metric type II burst. Thus the shock dynamics defined by this coronal density model frequency drift rate also indicates that the solar lift-off time of the CME-driven shock producing the type II radio emissions observed by RAD 1 and RAD2 occurred significantly later than the 2216 UT onset time of the reported metric type II burst. By readjusting the onset times and shock speeds, as well as the overall density scale for the Leblanc et al. model, we also fitted the other coronal density models to the type II emissions observed by Wind/WAVES. We found that this always required the onset time to be significantly later ( UT) than the metric type II onset time (2216 UT). We therefore again conclude that the type II radio emissions observed below 7 MHz were not causally related to the metric type II radio burst. The type II radio emissions observed in RAD2 must have been generated by a shock associated with a CME that lifted off from the Sun between 2230 and 2240 UT, well after the peak time of the solar flare. The fact that the CME-driven type II radio emissions were observed to such high frequencies ( -7 MHz) indicates that the CME-driven shock, in this case, formed very low in the corona (<4 Rs,). In fact, there are some unclassified features below 100 MHz on the Culgoora dynamic spectrum at about 2235 UT which could be a highfrequency continuation of the RAD2 radio emissions (N. Prestage, private communication, 1998). If related, these radio emissions would indicate that the CME-driven shock may have formed at heliocentric distances < 2 Rs.. If the reported metric type II burst were caused by the same CME-driven shock, we would expect its frequency drift to lie along the same frequency drift (dashed) curves in Plates l a and lb, but it does not. We therefore must conclude that the metric type II burst was generated by a separate solar disturbance. Since the onset of the metric burst coincides with the peak time of the flare, it is natural to conclude that the metric type II burst was generated by a disturbance (such as a blast-wave shock) associated with the flare. The GOES A X-ray flux also indicates that there were two temporally separated solar events associated with this major flare/cme event. The 5 min average GOES X-ray flux (obtained from the SPIDR Web site) for this event is shown in Figure 1 for the 12 hour period from 1800 UT on August 18 to 0600 UT on August 19. The X-ray flux near 2200 UT shows two peaks. The first peak appears to be significantly more impulsive than the second peak, which decays very gradually over 8 hours or so. The onsetime of the metric type II burst and the projected CME lift-off time (2235 UT) are shown by the short vertical bars. These two times correspond very well to the times of the two peaks in the X-ray flux. The association of the metric type II burst with the more impulsive X-ray event may be an indication of its origin from a (blast-wave) shock associated with the flare. The second X-ray event is more like the LDE X rays usually associated with CMEs [Sheeley et al., 1983]. To our knowledge, such multiple peaks in the X-ray data have not been previously discussed. The high-frequency radio observations discussed above offer a possible explanation for the two peaks as being related to the onset of the metric type II burst and the lift-off of the CME, respectively September 20, 1998, Type II Radio Event Plate 2a shows a conventional dynamic spectrum (frequency versus time) for the 2.5 hour period from 0230 to 0500 UT on September 20, 1998, covering the entire RAD2 frequency range from MHz to MHz. During this time interval, there was a M1.8 X-ray solar flare from 0233 to 0328 UT, with a maximum at 0251 UT (SGD). A metric type II radio burst was reported from 0241 to 0255 UT drifting from 140 to 20 MHz. The time of the metric type II burst is again indicated by the horizontal bar over the top of the dynamic spectrum. The measured frequency drift rate implied an estimated shock speed of 1000 km/s (SGD). Although there again was no visual CME because of the outage of SOHO, it is very likely that there was a CME associated with this major flare event [Harrison, 1995]. metric type 1I t August 18-19, x ff 10-6 LDE X-ray _f C M GOES A 10-7 ' ' ' ' ',,,,,, B 18:00 24:00 06:00 Time (UT) Figure 1. The 5 min average GOES A X-ray flux in W/cm 2 from 1800 UT on August 18 to 0600 UT on August 19, Note the double peaks in the X-ray flux associated with the onset time of the metric type II burst and the derived solar lift-off time of the coronal mass ejection.

9 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS 16,987 Associated with the onset of this major flare event, the dynamic spectrum in Plate 2a shows some intense, rapid frequency-drifting type III radio bursts from 0235 to 0305 UT. In addition, there are some slow frequency-drifting type II emission features decreasing from - 14 to 2 MHz from about 0402 to 0445 UT. These latter radio emissions, observed in RAD2, occurred well after the end time (0255 UT) of the metric type II burst. Although no type II radio emissions for this event were observed below 2 MHz, this does not mean that there was no CME. Indeed, we will argue below that the observed decametric/hectometric radio emissions were generated by a disturbance that lifted off from the Sun some tens of minutes after the time of the flare, and it is natural to identify this disturbance as a CME. Plate 2b displays the corresponding 1/f versus time dynamic spectrum for the RAD2 data for the same time period but from 2 to MHz. The solid curves in Plate 2b show the extrapolation of the frequency drift of the fundamental and harmonic radio emissions with a start time of 0241 UT (the onset time of the metric type II burst at 140 MHz) and with a shock speed of 1000 km/s, calculated using the $aito [1970] coronal density model. (The Newkirk model gives essentially identical results in this frequency region). As expected, these low-frequency extrapolations of the frequency drift do not pass near the decametric/hectometric type II radio emissions observed by Wind/WAVES, suggesting that these latter radio emissions cannot be the low-frequency continuation of the reported metric type II radio burst. On the other hand, a linear extrapolation through the frequency-drifting decametric/hectometric type II radio emissions, on the I/f versus time dynamic spectrum, suggests that a disturbance causing the type II radio emissions lifted off from the Sun well after the end time of the metric type II bursts at 0255 UT. Indeed, the dashed curves on Plate 2b show the calculated frequency drift using the $aito et al. [1977] density model with an onset time of 0324 UT and a (shock) speed of 500 km/s. These curves provide a reasonable fit to the frequency-drifting fundamental and harmonic type II radio emissions observed in RAD2. Hence the Wind/WAVES radio observations again imply that a second disturbance, presumably a CME, lifts off from the Sun significantly later (- 30 min) than the time of the solar flare maximum (0251 UT). The 5 min average GOES A X-ray flux corresponding to this event is shown in Figure 2. The data shown are from 1800 UT on September 19 to 0600 UT on September 20. The X-ray flux near 0300 UT again shows a double peak. In this case, neither peak is especially impulsive (compare Figure 1). The onset times of the metric type II burst and the derived CME lift-off time (0324 UT), shown by the vertical bars, correspond in time very well with the two peaks in the X-ray flux. This again suggests that the two temporally separated metric and decametric/hectometric radio events are manifest in the observed X-ray flux May 19, 1998, Type II Radio Event The conventional dynamic spectrum in Plate 3a displays a two hour period from 0930 to 1130 UT on May 19, 1998, over the RAD2 frequency range from to MHz. During this time interval, there was a B7.9 X-ray solar flare from 1010 to 1018 UT, with maximum at 1015 UT, and a reporte disappearing filament beginning at 0926 UT (SGD). A metric type II radio burst (indicated by the horizontal bar over Plate 3a) was reported from to 1015 UT in the frequency range from 30 to 90 MHz, with an estimated speed of 1000 km/s. The SOHO Large Angle and Spectrometric Coronagraph (LASCO) reported a large CME already in progress at <1400 UT, after a planned data gap (C. St. Cyr, personal communication, 1998). The dynamic spectrum in Plate 3a shows rapid frequencydrifting type III radio bursts around 1000 UT associated with this solar event. In addition, there were slow frequencydrifting type II emission features decreasing from - 14 to 6 MHz from about 1005 to 1110 UT. No type II radio emissions for this event were observed below 6 MHz. These radio I... X Sept. 20, 1998 ß metric type 1I -- I 10-6 C GOES A I0-7,,,,,, I,,,,, '... B 18: :00 06:00 Time (UT) Figure 2. The 5 min average GOES A X-ray flux in W/cm 2 from 1800 UT on September 19 to 0600 UT on September 20, Note the double peaks in the X-ray flux associated with the onsetime of the metric type II burst and the derived solar lift-off time of the CME.

10 16,988 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS emissions, observed in RAD2, overlap with the time of the metric type II burst, suggesting that this radio emission may be the low-frequency continuation of the metric type II radio emissions. However, there are again some difficulties with this simple expectation, as we now illustrate. Plate 3b shows a I/f versus time dynamic spectrum of the frequency drift curve provides a reasonable fit to the two episodes of type II radio emissions, as well as to the harmonic emissions near 12 MHz at UT. A somewhat better fit can be obtained by using a somewhat earlier lift-off time. The important point here is that in order to account for the observed frequency drift of these type II radio emissions, it is RAD2 data from 0900 to 1130 UT and from 5 to necessary to assume a significantly earlier onset time for the MHz. The solid curves in Plate 3b show the extrapolated causal agent than the onset time of the metric type II burst or frequency drift of the fundamental and harmonic radio emissions with a start time of 0951 UT (the onsetime of the its associated flare. Since there was a disappearing filament reported in metric type II at MHz) and with a shock propagation progress near 0920 UT, it is likely that this time corresponds speed of 1000 km/s, calculated using the Saito [1970] coronal to the solar lift-off time of a CME. The likely scenario in this density model. Although the parameters could be adjusted case is that a driven shock formed ahead of the CME when it somewhato better fit the observed type II radio emissions was at about 2.1 Rs and this CME-driven shock produced the from 12 to 10 MHz (I/f ), the frequency drift fundamental type II radio emissions beginning at-12 MHz at calculated using these parameters cannot simultaneously fit the type II radio emissions observed from 8 to 6 MHz (1/f = ) between 1025 and 1040 UT. Furthermore, as 1005 UT. If this scenario is correct, then this is the highest frequency at which a CME-driven shock has ever been confirmed to produced type II radio emissions. Note that can be most readily seen in Plate 3a, the emissions observed unlike the previous example, in this case the li off of the between 1033 and 1042 UT clearly occur simultaneously at both the fundamental (6-7 MHz) and the harmonic ( MHz) of the plasma frequency, which must also be simultaneously fit by the frequency drift. The two episodes of type II radio emission observed in RAD2 appear to be connected by a straight line on the 1/f versus time dynamic spectrum that extrapolates back to UT at infinite frequency (i.e., the Sun's "surface"), well CME preceded the (blast-wave) shock that generated the metric type II radio burst. Also note that in this case the onset of the type II burst (0951 UT) also preceded the flare onset (1010 UT) by some 20 min. Unfortunately, since there was no flare patrol at this time, there was no visual flare which might explain this discrepancy. It is significanthat the onset of the type III radio bursts was at-0950 UT, suggesting an earlier onset for the solar activity. before the onset time of the metric type II burst and flare. The above scenario is also consistent with the Thus, to account for the frequency drift implied by these SOHO/LASCO observations. A CME lifting off at-0924 UT episodes of type II radio emission, it is necessary to assume and traveling at -300 km/s would be at -8 Rs by 1400 UT, that the coronal disturbance generating these emissions lifted off from the Sun at UT. The dashed (fundamental and harmonic) curves on Plate 3b show the frequency drift that was obtained from the Saito [1970] coronal density model assuming an onsetime of 0924 UT and a shock speed of 300 km/s. The shock dynamics implied by this calculated that is, in the field of view of the C3 coronagraph on LASCO. The 5 min average GOES A X-ray flux corresponding to this event is shown in Figure 3. The data shown are for May 19. In this case, there was only a very weak X-ray peak temporally associated with the onset time of the metric type II burst and flare. Although the GOES May 19, 1998 X 10-5 M 10-6 C GOF,S '7,,,,,,,, Figure 3. The 5 min average GOES Jk X-ray flux in W/cm 2 from 0000 to 2400 UT on May 19, The onsetime of the metric type II burst and the derived CME lift-off time are also shown.

11 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS 16, A X-ray shows no peak near 0924 UT, there was a very weak precursor peak in the / X-ray flux at-0930 UT (SGD). This precursor X ray is likely related to the disappearing filament and to the solar lift-off of the CME [Harrison, 1986] November 3, 1997, Type II Radio Event Plate 4a shows a conventional dynamic spectrum for a 1.5 hour period from 0430 to 0600 UT on November 3, 1997, covering the RAD2 frequency range from to MHz. During this time interval, there was a C8.6, SB solar flare at S20W13 from 0434 to 0502 UT, with a maximum at 0437 UT (SGD). Metric type II radio bursts were reported from 0437 to 0450 UT with an estimated speed of 700 km/s and from 0450 to 0457 UT with an estimated speed of 500 km/s. They are again indicated by the horizontal bar over the dynamic spectrum. The SOHO/LASCO coronagraph reported a CME at 0528 UT (C. St. Cyr, personal communication, 1998). Aside from some rapid frequency-drifting type III radio bursts at UT, the dynamic spectrum in Plate 4a shows slow frequency-drifting type II emission features decreasing from-11.5 to 5 MHz from 0522 to 0600 UT. At-0543 UT, there is a clear indication of radiation at both the fundamental emissions observed at about 0543 UT. The corresponding curves (not shown) for the first metric type II with onset at 0437 UT and speed of 700 km/s do not intersect either of these episodes of type II radio emissions observed by RAD2. Again, a linear extrapolation, on the 1/f versus time dynamic spectrum, connecting these two episodes of decametric/hectometric type II radio emission, to infinite frequency, suggests a solar lift-off time of the disturbance causing the type II radio emissions that is well before the onsetime of the metric type II bursts at 0437 UT. The dashed curves in Plate 4b show the calculated frequency drift using the $aito [1970] density model with an onsetime of 0406 UT and a speed of 210 km/s. This frequency drift curve provides a reasonable simultaneous fit to both episodes of the type II radio emissions as well as the harmonic emission at-13 MHz at 0543 UT. Hence the observations imply that again the CME lifts off from the Sun significantly earlier (-30 min) than the solar flare or metric type II burst. This scenario is also consistent with the LASCO observations of the CME made at-0530 UT. A CME lifting off at 0406 UT and propagating at 210 km/s would travel a distance of 1.5 Rs between 0406 and 0530 UT. This would put the CME at-2.5 Rs, well within the field of view of the C2 coronagraph. Furthermore, assuming the Saito [1970] densitydistance scale, the type II radio emissions at MHz at 0530 UT should occur at 2.5 Rs, which is again consistenthe LASCO observations. The 5 min average GOES A X-ray flux (-6.5 MHz) and harmonic (-13 MHz). These radio emissions observed in RAD2 occurred well after (>30 min) the end time of the metric type II bursts, so that it is again not obvious whether they are causally related. No type II radio emissions for this event were observed below 5 MHz. corresponding to this event is shown in Figure 4. The data shown are from 0000 to 0800 UT on November 3. In this case Plate 4b shows a 1/f versus time dynamic spectrum of the the onset time of the metric type II burst again corresponds RAD2 data for the same time period but from 5 to very well with the time of the flare. However, in this case, MHz. The solid curves in Plate 4b show the expected there is no obvious X-ray peak associated with the expected frequency drift rate of the fundamental and harmonic radio lift-off time of the CME. emissions with a start time of 0450 UT (the onset time of the second metric type II at-90 MHz) and with a shock speed of 500 km/s, calculated using the $aito [1970] coronal density 4. Discussion and Conclusions model. These curves do not fit the frequency drift implied by the two episodes of the radio emission from 11 to 7.5 MHz at For four independent solar events we have provided evidence from the Wind/WAVES radio data for the presence about 0525 UT and the fundamental and harmonic radio of two temporally separated shocks producing type II radio 10-4 Nov. 3, 1997 x 10-- Est. C1VIE liftoff-- l I -- metric type m 10-6 C GOES A ] 10-17, I I I,,, I I... B 00:00 04:00 08:00 Time (TIT) Figure 4. The 5 min average GOES X-ray flux in W/cm 2 from 0000 to 0400 UT on November 3, The onsetime of the metric type II burst and the derived CME lift-off time are also shown.

12 16,990 REINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS emissions. We did this by demonstrating that the lowfrequency extrapolation of the metric type II frequency drift did not match the observed frequency drift of the type II radio surprising, though, is that such slow shocks would be effective in generating radio emissions. However, we know very little about the conditions necessary for generating radio emissions observed by Wind/WAVES. These extrapolations emissions in this previously unexplored region of the were based on white-light coronal density laws. We therefore electromagnetic spectrum. We believe that at kilometric concluded that the solar lift-off times of the disturbances wavelengths there is growing evidence that the ambient producing the decametric/hectometric radio emissions were not related to the onsetimes of the associated metric type II conditions in the interplanetary medium may be as important for the generation of type II radio emissions as the speed of radio bursts. For the four events presented here, we concluded the shock [Reiner et al., 1998a, b]. An alternativ explanation that the origin of the type II radio emissions in the frequency for the low CME speeds derived here is that these radio range from 1 to 14 MHz must have been shocks associated with CMEs, whose solar lift-off times were found to differ significantly from the times of the flares or metric type II bursts. For two cases we deduced that the CME lift-off, emissions may have been generated in enhanced density regions, so that we should be using density-enhanced coronal models. If the coronal densities were uniformly enhanced by a factor of 5, we would have to increase the shock speed to estimated from the fit to the drift rate of the type II emissions, >400 km/s to get the same fit to the Wind/WAVES type II preceded the flare-associated (blast-wave) shock, while in two other cases it followed the coronal shock. frequency drift rates. Our conclusions depend somewhat on the derived coronal We believe that it is significanthat in a comprehensive density models. It should be clear, however, that simply statistical study of flare and CME associations, Harrison [1995] found that sometimes the CME lift-off preceded the uniformly enhancing the density model will not change our results. To maintain the observed frequency drift rates with an flare and other times it followed the flare by tens of minutes. enhanced coronal density model, we have to compensate by We see here this same behavior reflected in the metric and decametric/hectometric radio data. This is consistent with our inference that the metric type II burst is generated by a blastincreasing the speed parameters for both the metric and decametric/hectometric type II radio emissions. Thus the extrapolated frequency drift curves will be the same. The only wave shock associated with a flare, while the way for the decametric/hectometric radio emissions presented decametric/hectometric type II radio emissions are often generated by CME-driven shocks. The disturbance speeds as derived from the frequency drift rates also reinforce this conclusion. The speeds are summarized in Table 1. In every case presented here, the disturbance speed derived from the frequency drift for the decametric/hectometric type II emissions was always significantly less than the disturbance speeds derived for the metric type II burst, using essentially the same coronal density laws. This is what would be expected if the metric type II here to be the low-frequency continuations of the associated metric type II radio bursts is for the radial dependence of the coronal density models, extrapolated to the frequency range from 1 to 14 MHz, to be grossly in error. We believe that this is unlikely because a variety of independently derived coronal density models yielded similar frequency drift curves. Also, because we sometimes found the type II emissions to overlap with and sometimes occur significantly after the metric type II, the radial dependence of the coronal density laws would have to differ significantly from one event to the next. bursts were generated from a flare-associated blast wave and For the two cases where the onset of the disturbances the decametric/hectometric type II radio emissions were generated from a CME-driven shock [Wagner and MacQueen, 1983]. Note that introducing enhance densities causing the type II emissions in RAD2 occurred after the flare, we observed two peaks in the X-ray flare that corresponded precisely to the two onset times. Detailed in the coronal models will effectively increase the speeds examination of the soft X-ray data indicates that such double associated with both the metric and decametric/hectometric type II radio emissions. peaks are very rare for such large solar events. This is consistent with the rarity of the Wind/WAVES type II events For the two cases where the lift-off of the CME preceded that are significantly displaced in time from the flare or metric the flare, we derived surprisingly low speeds ( kin/s) for the CME-driven shock. Nevertheless, these speeds may still be higher than the sound speed in the corona (-170 km/s), type II burst. Thus the radio data may provide a clue as to the significance of these double peaks when observed in the X- ray flux. The relationship between radio events and double so that a shock can still form ahead of the CME. These speeds peaked X-ray flux will be further explored in a future paper. are also consistent with the analysis of Hundhausen et al. [1994]. They found that the average and median speeds of CMEs were 349 and 285 km/s, respectively. What is rather In all cases considered here, the flare related (blast-wave) shock presumably did not generate radio emissions reaching frequencies below 14 MHz, indicating that they are normally Table 1. Disturbance Speeds As Derived From the Frequency Drift Rates Date Metric Type II Speed, km/s Decametric/Hectometric Type II Speed km/s Read as August 18, 1998.

13 KEINER AND KAISER: HIGH-FREQUENCY TYPE II RADIO EMISSIONS 16,991 extinguished by -2 Rs (Gopalswamy et al, [1998]; however, also see Kaiser et al. [1998]). Our results also suggesthat most of the type II radio emissions that we observe in the frequency range 1-14 MHz are generated by CME-driven shocks and not by (blast-wave) shocks associated with metric type II bursts and their associated flares. We further demonstrated that for all these events the CME-driven type II radio emissions began at very high frequencies, indicating that a CME-driven shock can form very low in the corona, at heliocentric distances perhaps as low as 2 Rs. In two of the cases reported here, namely, August 18 and September 20, 1998, the Culgoora daily dynamic spectra revealed unclassified emissions presumably of solar origin in the frequency range of 18 to -75 MHz just at the times when the Saito [1970] frequency drift curves through the WAVES data would have intersected the Culgoora frequency range. These unclassified emissions might be the very first indications of unorganized CME-associated emissions at very low solar altitudes. The radio emissions observed in the frequency range from 1 to 14 MHz by the Wind/WAVES instrument are rather rare. Not surprisingly, they are usually associated with major flare events on the Sun, but not always. From November 1994 to April 1997 no decametric/hectometric radio emissions were observed in this frequency region [Gopalswamy et al., 1998]. As the Sun approaches solar maximum, the frequency of occurrence of type II radio emissions in this frequency range has steadily increased. From April 1997 through September 1998 some 35 radio events (-2/month) have been catalogued in this frequency range. Sometimes they are associated with metric type II radio bursts and sometimes not. Of these only about four showed wide temporal separation between the metric type II burst and the decametric/hectometric type II radio emissions. It is natural to speculate that for those cases where there is an apparent coincidence of the observed decametric/hectometric type II radio emissions and the metric type II burst they may still involve two distinct shock mechanisms for the generation of the respective type II radio emissions. We may have managed here to "catch" four type II events where the differences in the causal shocks were manifest. Acknowledgments. We gratefully acknowledge fruitful discussions with E. Cliver, particularly regarding the X-ray data. We thank the SOHO/LASCO team for the use of their preliminary quick-look CME list and the SPIDR Web site for the GOES X-ray data. This work was supported, in part, by the NSF grant ATM The Wind/WAVES experiment is a collaboration of NASA/Goddard Space Flight Center, the Observatoire of Paris-Meudon and the University of Minnesota. Janet G. Luhmann thanks Edward W. Cliver and another referee for their assistance in evaluating this paper. References Bougeret, J.-L., J. H. King, and R. Schwenn, Solar radio bursts and in situ determination of interplanetary electron density, Sol. Phys., 90, , Bougeret, J.-L., et al., WAVES: The radio and plasma wave investigation on the WIND spacecraft, Space Sci. Rev., 71, , Cane, H. V., N. R. Sheeley Jr., and R. A. Howard, Energetic interplanetary shocks, radio emission, and coronal mass ejections, d. Geophys. Res., 92, , Cliver, E. W., Comment on "Origin of coronal and interplanetary shocks: A new look with Wind spacecraft data" by N. Gopalswamy et al., d. Geophys. Res., 104, , Cliver, E. W., D. F. Webb, and R. A. Howard, On the origin of solar metric type II bursts, Sol. Phys., in press, Gergely, T. E., M. R. Kundu, and E. Hildner, A coronal transient associated with a high-speed type II burst, Astrophys. J., 268, , Gopalswamy, N., M. L. Kaiser, R. P. Lepping, S. W. Kahler, K. Ogilive, D. Berdichevsky, T. Kondo, T. Isobe, and M. Akioka, Origin of coronal and interplanetary shocks: A new look with Wind spacecraft data, d. Geophys. Res., 103, , Guhathakurta, M., T. E. Holzer, and R. M. MacQueen, The largescale density structure of the solar corona and the heliospheric current sheet, Astrophys. d., 458, , Harrison, R. A., Solar coronal mass ejections and flares, Astron. Astrophys., 162, , Harrison, R. A., The nature of solar flares associated with coronal mass ejection, Astron. Astrophys., 304, , Hundhausen, A. J., J. T. Burkepile, and O. C. St. Cyr, Speeds of coronal mass ejections: SMM observations from 1980 and , J. Geophys. Res., 99, , Kaiser, M. L., M.D. Desch, J.-L. Bougeret, R. Manning, and C. A. Meetre, Wind/WAVES observations of man-made radio transmissions, Geophys. Res. Lett., 23, , Kaiser, M. L., M. J. Reiner, N. Gopalswamy, R. A. Howard, O. C. St. Cyr, B. J. Thompson, and J.-L. Bougeret, Type II radio emissions in the frequency range from 1-14 MHz associated with the April 7, 1997 solar event, Geophys. Res. Lett., 25, , Leblanc, Y., G. A. Dulk, and J.-L. Bougeret, Tracing the electron density from the corona to 1 AU, Sol. Phys., 183, , Malitson, H. H., J. Fainberg and R. G. Stone, Hectometric and kilometric solar radio emission observed from satellites in August 1972, Space Sci. Rev., 19, , Nelson, G. J. and D. B. Melrose, Type II bursts, in Solar Radiophysics, edited by D. J. McLean and N. R. Labrum, Cambridge Univ. Press, New York, Newkirk, G., Jr., Structure of the solar corona, Annu. Rev. of Astron. Astrophys., 5, , Prestage, N. P., R. G. Luckhurst, B. R. Peterson, C. S. Bevins, and C. G. Yuile, A new radiospectrograph at Culgoora, Sol. Phys., 150, , Reiner, M. J., M. L. Kaiser, J. Fainberg, and R. G. Stone, New method for studying remote type II radio emissions from CMEdriven shocks, J. Geophys. Res., 103, 29,651-29,664,1998a. Reiner, M. J., M. L. Kaiser, J. Fainberg, J.-L. Bougeret, and R. G. Stone, On the origin of radio emissions associated with the January 6-11, 1997, CME, Geophys. Res. Lett., 25, , 1998b. Saito, K., A non-spherical axisymmetric model of the solar K corona ofthe minimum type, Ann. Tokyo Astron. Obs., Ser. 2, 12, , Saito, K., A. I. Poland, and R. H. Munro, A study of the background corona near solar minimum, Sol. Phys., 55, , Sheeley, N. R., Jr., R. A. Howard, M. J. Koomen, and D. J. Michels, Associations between coronal mass ejections and soft X-ray events, Astrophys. d., 272, , Suzuki, S. and G. A. Dulk, Bursts of type III and type V, in Solar Radiophysics, edited by D. J. McLean and N. R. Labrum, , Cambridge Univ. Press, New York, Wagner, W. J., and R. M. MacQueen, The excitation of type II radio bursts in the corona, Astron. Astrophys., 120, , M. L. Kaiser, Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, MD M. J. Reiner, NASA Goddard Space Flight Center, Code 690.2, Greenbelt, MD (reiner urap.gsfc.nasa.gov) (Received October 2, 1998; revised March 12, 1999; accepted March 15, 1999.)