Radio signatures of the origin and propagation of coronal mass ejections through the solar corona and interplanetary medium

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 106, NO. A12, PAGES 29,989-30,000, DECEMBER 1, 2001 Radio signatures of the origin and propagation of coronal mass ejections through the solar corona and interplanetary medium M. J. Reiner Raytheon ITSS, Lanham, Maryland M. L. Kaiser Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, Maryland J.-L. Bougeret Observatoire de Paris, Meudon, France Abstract. During a 16-day period from February 5 to 20, 2000, a series of decametric-tokilometric wavelength type II and type III radio events was observed by the WAVES radio experiment on board the Wind spacecraft. These radio events were related to observed coronal mass ejections (CMEs) and their associated flares. Each of the solar eruptive events was initiated by an intense, complex type III radio burst, which occurred within minutes of the liftoff on the CME. Some of the CMEs produce decametric-hectometric (D-H) type II radio emissions, which, when their frequency drift rates were sufficiently well defined, were used to provide a speed estimate. The complex type III and D-H type II radio emissions gave an indication of the presence of a CME well before the CME was first observed in the coronagraph images. This series of CMEs also generated interplanetary (kilometric) type II radio emissions that tracked the CMEassociated shock through the interplanetary medium and established the terrestrial connection. Thus the various radio emissions associated with these solar eruptive events provided a global view of each entire Sun-Earth connection event, from the initiation and liftoff of the CME at the Sun, to the propagation of the CME-associated shock through the solar corona and interplanetary medium, to its arrival at 1 AU. Finally, we show that simultaneous Wind/Ulysses observations of the interplanetary type II radio emissions on February 9-10 provide important information on the nature of the type II emission, on the type II source locations, and on the radiation characteristics of the type II emissions. For example, these simultaneous observations clearly indicate that the sporadic nature of the type II radiation was intrinsic to the radio source region. 1. Introduction unique means of identifying and studying the dynamics of CMEs in the same spatial range where they are observed by ground- For many years, metric type II radio bursts have been used as a based and space-based coronagraphs. Furthermore, it appears that proxy for coronal mass ejections (CMEs) propagating through the intense, complex type III-like radio emissions in this same low solar corona [see, e.g., Nelson and Melrose, 1985]. However, frequency range provide a means of identifying the presence of a there has been a long-standing controversy as to whether the CME by indicating that the generating electron beams are metric type II burst is caused by a CME-associated shock or propagating through a highly disturbed corona [Reiner and whether it is generated by blast-wave shocks associated with Kaiser, 1999a; Reiner et al., 2000b]. flares and is therefore not directly related to the CME [Cliver et Cane et al. [1987] provided convincing evidence that al., 1999, and references therein]. By systematically observing kilometric (interplanetary) type II radio bursts are the signatures solar radio signatures in the previously little explored frequency of CME-driven shocks propagating through the interplanetary window from 1 to 14 MHz, the WAVES radio experiment on medium. While the original works focused on identifying the board the Wind spacecraft has shed new light on this controversy physical characteristics of these type II emissions [Cane et al., [Gopalswamy et al., 1998, 1999; Cliver, 1999; Kaiser et al., 1998; 1982; Cane and Stone, 1984; Lengyel-Frey and Stone, 1989; Dulk et al., 1999]. It appears that much of the type II radio Lengyel-Frey, 1992], recent work has focused more on using emissions observed in this frequency range originate from the these interplanetary type II signatures to study the dynamics of CME-associated shocks [Reiner and Kaiser, 1999b; Reiner et al., 2000a]. Therefore these radio observations provide a new and CME-driven shocks propagating through the interplanetary medium [Reiner et al., 1997,1998]. It is generally observed in situ that fast CMEs (ejecta or 1 Also at NASA Goddard Space Flight Center, Greenbelt, Maryland. magnetic clouds) are usually preceded by interplanetary shocks [Sheeley et al., 1985]. For observations near 1 AU the Copyright 2001 by the American Geophysical Union. interplanetary shocks typically precede the CME by 8-12 hours. Paper number 2000JA It is therefor expected that the fast CMEs generally drive shocks, /01/2000JA at least out to 1 AU [Gosling 1990, 1993]. However, them may 29,989

2 29,990 REINER ET AL.: RADIO SIGNATURES OF CME'S be cases where the shock becomes detached from the CME to 940 khz, and a low-frequency radio receiver with 64 linearly driver, perhaps beyond 1 AU [e.g., Burlaga et al., 1981]. The spaced frequency channels from 1.25 to 48.5 khz. The electric precise relationship between the shock and CME is complex and sensors include a dipole antenna (35-m elements) in the not well understood or quantified; for some reviews, see spacecraft spin plane and a 7.5-m monopole antennalong the SteinolJkon [ 1985], Dryer [ 1994], and Hundhausen [ 1999]. When spacecraft spin axis. The Ulysses spacecraft is in a highly we speak of CME-driven shocks in this paper, we have in mind a elliptical orbit, out of the ecliptic plane, with aphelion shock that is continually driven by the CME, at least out to 1 AU. (perihelion) at-5.4 AU (-1.3 AU). During the time of the In this sense, the dynamics implied by the radio emissions, observations presented here, Ulysses was at-4 AU from the Sun produced by the shock, is directly applicable to the CME itself. and at-40 ø south latitude. The precise relationship between the CME and its associated shock is beyond the scope of this article. 3. Radio Signatures of the Origin and Propagation During a 16-day period from February 5 to 20, 2000, a series of CMEs Observed From February 5 to 20, 2000 of seven decametric-to-kilometric wavelength type II and type III radio events was observed that was related to a number of 3.1. Overview observed CMEs and their associated flares. In this paper we show that these radio signatures can be used to identify and track the progress of these CMEs all the way from the Sun to 1 AU. In particular, we show that for each observed halo CME event, intense, complex type III-like radio emissions provided the first radio indication of the presence of a CME in the corona, within minutes of the projected liftoff time and well before the CME was first observed by coronagraphs. In a number of cases, decametrichectometric (D-H) type II emissions were also observed. The frequency drift rates of the D-H type II bursts permitted an estimate of the shock speeds through the high corona. We also show that the various type II radio signatures at kilometric wavelengths can help to identify and track the progress of the CME-driven shock through the interplanetary medium. We then focus on one of these events (February 8, 2000) that was simultaneously observed by radio receivers on both the Wind and Ulysses spacecraft, and we discuss and analyze its dynamical and physical characteristics from its initiation on February 8, 2000, to the arrival of the shock at Earth on February 11, Instrumentation 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 superheterodyne (step-tuned) receivers: RAD2, which sweeps 256 frequency channels from to MHz in s with a frequency resolution of 50 khz; RAD1, which covers the frequency range from 20 to 1040 khz at 32 discrete frequencies (selected from 256 frequency channels), and a multichannel "thermal noise" receiver (TNR), which covers a frequency range from 4 to 245 The radio dynamic spectra in Plate 1, from the WAVES experiment on the Wind spacecraft, provide an overview of the radio emissions for a series of solar eruptive events observed from February 5 to 20, The lower dynamic spectra in each panel are consecutive 4-day plots of the intensity of the observed radio emissions (red being the most intense) plotted as a function of inverse frequency on the vertical axis and time along the horizontal axis. The frequency ranges from 20 khz to 13.8 MHz, and the total time period spans 16 days from February 5 to 20, The numerous intense (overexposed) vertical striations on the dynamic spectr are broadband type III radio bursts that are generated by beams of suprathermal electrons ejected from the Sun. These intense type III bursts are characterized by a rapid frequency drift. By contrast, the generally weaker, narrowband, sporadic, slowly frequency drifting emissions are type II radio emissions that are generated by CME-driven shocks propagating through the interplanetary medium. The type II radio emissions are generated the fundamental and/or harmonic of the plasma frequency in the source region, i.e., f(khz) = a /n(cm-3), where n is the plasma density and a = 9 (18) for fundamental (harmonic) emission. The reason for plotting the dynamic spectras inverse frequency versus time is that the plasma density in the interplanetary medium falls off as 1/R 2, n(cm 4) = no(cm'3)/r(au) 2, where no is the plasma density at 1 AU, so that the inverse of the radio frequency scales as R, the heliocentric distance. Assuming that the CME-driven shock propagates through the interplanetary medium at a constant speed, so that R = vo(t- to), we find that [Reiner et al., 1997] 1/f = [vo/(a /n0)] (t - to), khz with 96 frequency channels. The RAD 1 and TNR receivers where v0 is the shock speed, t is the time of the radio emissions at are connected to a dipole antenna (50-m elements) in the the shock, and to is the solar liftoff time of the CME which drives spacecraft spin plane and a dipole antenna (5.28-m elements) the shock. However, what is measured is the time of the radio along the spacecraft spin axis, and the RAD2 receivers are emissions at the spacecraft (Wind), so we finally have connected to a dipole antenna (7.5-m elements) in the spacecraft 1/f = [Vo/(a /n0)] {[tw- r w/c --tde ay(fi] to}, (1) spin plane in addition to the spin axis antenna. The spin axis of the Wind spacecraft is approximately perpendicular to the ecliptic where tw is the time of the radio emissions observed at the Wind plane. The Wind spacecraft, which was launched in November of spacecraft, r w is the distance from the radio source to Wind, c = 1994, executes complex orbits that include excursions to the 300,000 km s 4, and tdelay(f) is some possible, Lagrange point (L 1) and series of near-earth passes. During the propagation time delay, which may depend on the observing time of the observations presented here, Wind was in the solar frequency [Steinberg et al., 1984]. In describing the global wind, -100 RE upstream from Earth. propagation of the CME through the interplanetary medium The Ulysses radio receiver used in this investigation is a part (timescales of the order of days), we can ignore the small light of the Unified Radio and Plasma (URAP) wave investigation propagation time correction terms (order of minutes), in which [Stone et al., 1992]. It consists of a high-frequency case (1) reduces to superheterodyne radio receiver with 12 discrete, approximately logarithmically spaced fixed frequency channels ranging from 52 1/f = [Vo/(a 4n0)] (tw- to). (1') unknown

3 ß, ß RE1NER ET AL.: RADIO SIGNATURES OF CME'S 29, MHz X1.2 B flare NOAA 8858 East Bright Loop I + T) pe il Bur MHz M1.3, NO&A lb 8&q8 flare Halo C.,. I [2 s.oo 1 MHz $ii, ";...!..,¾-.I 1 MHz.; I i,.y,e II issions 13.8 MH _ :00 6 C7.4,2F flare C73 flare NOAA 8853 Halo CME NOAA 8858' 24:00 Halo CME 24:00 MI.7,1N flare NOAA 8858 Halo CME I. MHz: ", =_..,,, :la,,.. ' F3 pe 11 Burs.. }.x.pe Type I1 Burs, _03!,.04 00: MHz I 1MHz,.OO ',' I s',,', I.o Typ I ' sio 00: :00 MI.3,2N flare NOAA MH H 24:00 10 Material ejected :30 --L_. ß Shock a 1 AU ' - '"' I 24:00 A' -, ' ---- H r-a:,.-_-. - ',,. -.,, ',J, IE Shock at 1 I :oo 15 :oo Halo C Shock at 1 AU 1. 24: :00 Plate 1. Dynamic spectra of the Wind/WAVES radio data for the 16-day period from February 5 to 20, 2000, divided into four panels of 4 days. The insets show high-frequency and time resolution dynamic spectra, in the frequency band from I to 14 MHz, showing the intense, complex type III-like and decametric-hectometric (D-H) type II radio emissions associated with the solar eruptivevents that produced CMEs that propagated through the solar corona. The magnitude and time of the flare and the time of the first CME image are also indicated. The lower dynamic spectr are plotted as inverse frequency versus time. This has the advantage of organizing the lowfrequency type II radio emissions along straight lines originating from the CME liftoff time, thus providing a global view of each Sun-Earth connection event. The pink lines indicate the Sun-Earth connection times and frequencies, and the dashed white lines indicate the connection between the solar liftoff time and the slowly drifting interplanetary emissions

4 29,992 REINER ET AL.: RADIO SIGNATURES OF CME'S Equation (1') states that to directly reveal the dynamics of 3.2. Specific Event Description propagation of the CME-driven shock through the interplanetary This series of solar-terrestrial events was initiated by an east medium, it is preferable to plot the radio dynamic spectrum as a limb CME first seen at 1954 UT on February 5 (S. Plunkett, function of inverse frequency versus time, as we have done in ) that was Plate 1. In this case, we expect the type II radio emissions, associated with an X1.2 (3B) flare from 1917 to 1931 UT, with generated as the CME-driven shock propagates through the maximum at 1928 UT (SGD, 2000). The flare site was located in interplanetary medium, to be organized along straight lines that the active region NOAA 8858, which was then at N26øE52 ø. The originate at the CME liftoff time, if the shock propagates at a flare generated very complex and intense type III-like emissions constant speed. This organization is indicated by the white dashed in the frequency band from 1 to 13.8 MHz, which are lines in Plate 1 for the various radio events (we get two lines; one characteristic of the propagation of electrons through a highly corresponding to radio emissions at the fundamental (a = 9) and disturbed corona due to the presence of a CME [Reiner and another, of half the slope, corresponding to the harmonic (a = 18) Kaiser, 1999a]. These complex type III emissions are more of the plasma frequency). The slope of the lines in Plate 1 clearly illustrated in the inset in Plate 1 that plots the radio depends directly on the speed of the shock and inversely on the emission intensity on a linear scale between 1 and 13.8 MHz over plasma density in the radio source region. the time period of only 35 min, from 1915 to 1950 UT. These Each of the type II radio events shown in Plate 1 was complex type III-like emissions, which began at UT, have associated with a solar transient event detected by the Largea characteristic diminution in intensity near 7 MHz and have a Angle and Spectrometric Coronagraph (LASCO) [Brueckner et al., 1995] on the Solar and Heliospheric Observatory (SOHO) duration of some 20 min at 1 MHz. All other flares produced spacecraft. An X-class solar flare from NOAA 8858 near the during the day were C-class or less, and none of them generated eastern limb, with an associated CME, initiated the series of intense type III emissions in this frequency band. In events. During the subsequent 16-day interval, six partial or full addition to the type III-like emissions, some narrowband, slowly halo CMEs, not all from the same active region and not all frequency drifting hectometric type II emissions were observed at associated with major (X-class) flares, were reported (S. Plunkett, - 3 MHz at-1935 UT. These type II emissions were presumably generated by the CME-driven shock as it propagated through the ). Four CME-driven shocks were detected at 1 AU. The time high corona. Unfortunately, in this case the observed frequency (arrows) and magnitude of the associated flare for each radio drift of the type II was not sufficiently well defined to permit a event is indicated in Plate 1 at the top of each dynamic spectrum. determination of a speed through the high corona. Note that both Wind observed one additional spectacular type II radio event on the complex type III and hectometric type II radio emissions were February for which there was no major flare or halo CME observed well before the first CME image (1954 UT) and were reported; however, material was reported ejected from the Sun at therefore the earliest indicators of the liftoff of a CME. Finally, this time. some weak, narrowband kilometric type II radio emissions were Each of the type II radio events shown in Plate 1 was preceded detected at UT on February 7 at khz (1/f = by intense, complex type III-like radio emissions in the frequency ) as shown in the lower dynamic spectrum. band from 1 to 14 MHz. These complex type III-like bursts are Assuming that this kilometric type II emission occurred at the shown, using expanded timescales, in the insets above the inverse fundamental of the plasma frequency, the straight (dashed) lines drawn in Plate 1 show that this emission is indeed well connected frequency dynamic spectra. In each case the complex type III emissions indicate electrons propagating through a disturbed to the liftoff time of the CME at UT on February 5. The corona. For five of the seven complex type III radio events there shock associated with this east limb CME, however, did not encounter Earth. were also decametric-hectometric (D-H) type II radio emissions observed in this same frequency band, which were generated by From February 6 to 7 there were only small C-class solar shocks propagating through the high corona. These are also flares, and no halo CMEs were reported. Likewise, there was no indicated on the insete dynamic spectra. evidence for any type II radio emissions observed from metric to There were metric type II bursts reported for most of the kilometric wavelengths nor was there any intense, complex type events discussed here (Solar Geophysical Data (SGD), 2000), but III-like emissions observed in the MHz band. However, since it is not clear that all metric type II bursts are produced by on February 8 another major M1.3 (lb) flare was reported from CME-driven shocks or how they are related to the D-H type II 0842 to 0918 UT, with maximum at 0900 UT, from NOAA 8858, radio emissions, we will generally not include the metric type IIs which was now at N25øE26 ø. A halo CME, with an estimated in the analyses presented here. It should be mentioned that the speed of 715 km s '1, was also reported for this event (S. Plunkett, observed frequency drift rate of the metric type II burst can be ). used to estimate the speed of the coronal shocks that produce Correspondingly, both type II and type III radio emissions were them, however, different radio observatories use different density observed in the frequency band from 1 to 13.8 MHz. First, there models, which makes it difficult to interprethese derived speeds. were intense, complex type III-like emissions at about the time of Clearly, more joint observations in the metric and decametric the flare, shown in th e second inset in Plate 1, which plots the wavelength range are required in order to clarify the relationship radio intensity from 1 to 13.8 MHz from 0845 to 0925 UT. This between metric and decametric type IIs and between metric type complex type III event began at UT with a series of IIs and CMEs [e.g., see Reiner and Kaiser, 1999b; Reiner et al., relatively weak type III bursts. Th e intense, complex type III-like 2000a; Reiner et al., 2001 ]. emissions began at-0900 UT and lasted until UT at 1 In three cases the low-frequency type II radiation occurred MHz. This type III-like event had all the characteristics of events right up until the arrival time of the shock at 1 AU, where that involve the eruption of a CME on the Sun. There was a Langmuir waves, associated with electron beams, were also characteristic dimming of the emissions at- 7 MHz, and the detected, indicating that the spacecraft was in or very near the emissions at lower frequencies had characteristic very narrowtype II radio-emitting region of the shock [Bale et al., 1999]. band, short-duration, non-frequency-drifting emissions [Reiner

5 REINER ET AL.: RADIO SIGNATURES OF CME'S 29,993 and Kaiser, 1999a]. It has been suggested that these features are produced by electrons that were normally confined to helmet-like closed field structures in the low corona which, owing to the eruption caused by the liftoff of the CME, open to the interplanetary medium, so that the energetic electrons can escape through the disturbed corona [Reiner et al., 2000b]. Indeed, wellthe connection between the solar origin (oo frequency at UT on February 8) and the in-situ plasma frequency upstream of the shock at Wind. The plasma frequency just before the shock was usually low (17.5 khz), implying a very low plasma density (3.8 cm-3). Any type II radio emissions aligned along this (pink) line would imply type II radio emissions produced at the defined decimetric radio emissions were observed from to fundamental of the plasma frequency in this low-density region 0907 UT, indicating prolonged acceleration of electrons at the flare site. These type III-like emissions were followed by D-H type II emissions with a frequency drift from 11 to 6 MHz between 0905 and 0917 UT that presumably were produced by a shock propagating through the high corona. By using a coronal density model we can converthe observed frequencies to heights above the corona. The white-light K-corona density model of Saito along the shock front [Reiner et al., 1997]. The observed type II radio emissions associated with this event lie along the white dashed lines of smaller slope, suggesting that the type II radio emissions associated with the propagation of this CME were generated in higher-density regions along the shock front; the white dashed lines correspond to no = 10 cm '3.(It is also possible that this emission is generated at the fundamental downstream of the shock.) The kilometric type II radiation continued right up [1970], measured during solar minimum, gives a relationship until the arrival of the shock (0235 UT), indicating that the Wind between the coronal electron density and heliocentric distance. spacecraft was very close to the type II emitting region. This According to this model, radio emissions from 11 to 6 MHz correspond to heliocentric distances from 2.3 to 2.9 R (R = 696,000 km). Furthermore, using the Saito coronal density model, the observed type II frequency drift rate can be converted to a shock was followed- 5.5 hours later by complex ejecta. A more detailed description of the low-frequency type II emissions for this event is provided in section 4. This Sun-Earth connection (SEC) CME event produced only a very modest change in the shock propagation speed. We found that the best fit to the planetary geomagnetic activity index, Kp, which measures the frequency drift rate from 0905 to 0917 UT implied a shock speed maximum amount of fluctuations in the geomagnetic field of 620 km s -. These derived parameters are consistent with the averaged over several ground stations. The value of Kp rose to 4 radio-emitting shock being associated with the observed CME. at UT on February 11. The CME loop structure in the northeast was observed at 0954 The shock arrival time implied a Sun-to-Earth shock transit UT to be at -5 R (plane-of-the-sky distance), which is probably speed of 636 km s '. This value is remarkably close to our initial not significantly different from the true height. Therefore, at 0917 estimate of 620 km s - made from the measured frequency drift UT the CME, traveling at-620 km s 'l, would have been at- 3 R, rate of the radio emissions generated by the shock as it where the coronal density is -4.4x10 s cm -3, implying a radio propagated through the high corona on February 8. If we assume frequency of- 6 MHz, as is observed. that the CME at this time was moving at the transit speed of 636 This solution, however, is not unique. For example, if we were km s 'l, we can use Salto's [1970] coronal density model to to enhance Salto's [1970] model density by a factor of 5 estimate the time of liftoff of the disturbance causing this type II [Robinson and Stewart, 1985], as is often done for metric radio emission (assuming a constant speed all the way back to the observations, we would derive from the observed frequency drift Sun). The results of the fit of this model to the drift rate of the D- rate a shock speed of 1150 km s ' and the true height of the CME H type II emissions, shown by the white dashed lines in Plate 2a, shock at 0954 UT would then be -8 R. One would then have to imply a solar liftoff time of 0843 UT. This projected liftoff time invoke significant deceleration (-4.3 m s '2) of the shock in the is essentially the same as the time (0845 UT) of the onset of the interplanetary medium [see Gopalswarny et al., 2000] to account coronal waves observed by the Extreme Ultraviolet Imaging for the observed transit speed of 636 km s - (see below). For this event, however, there was no clear evidence for a deceleration of Telescope (EIT) (S. Plunkett, a magnitude from the observations of the type.ii radio bin/halocme_parse, 2000) which accompanied the observed halo emissions observed at the lower frequencies, corresponding to CME, suggesting that the D-H type II radio emissions were propagation through the interplanetary medium, as discussed indeed produced by the propagation of a CME-driven shock below. Since we have no a priori way of knowing neither the through the high corona. Note that the complex type III-like appropriate enhancement factor for the density model for a emissions commenced just minutes after the projected liftoff time particular event nor the true coronal height of the CME from the of this halo CME. Note also that both the complex type III-like coronagraph images, throughout this paper we take the approach and the D-H type II emissions were observed well before (- 30 of always applying the unmodified Saito density model to the min) the first CME image at 0930 UT. analysis of the radio and coronagraph images. The next intense, complex type III-like emissions in the 1 to The February 8 CME event also produced kilometric (interplanetary) type II radio emissions below 1 MHz as the CME-driven shock continued to propagate through the interplanetary medium. This weak narrowband emission was first observed at ki-iz (1/f = 0.008) at 2315 UT on February 8 and continued drifting to lower frequencies as indicated by the straight dashed lines from UT on February 8 in the lower dynamic spectrum in Plate 1. After-1730 UT on February 9 these type II emissions, which had then drifted down to - 50 khz (1/f = 0.02), suddenly became very intense. Finally, a shock was detected at Wind at 0235 UT on February 11, as is evident in 13.8 MHz band occurred at UT on February 9 as shown in the third inset in Plate 1 for the time period from 1925 to 2000 UT. This type III-like event was clearly associated with the next reported halo CME, with an estimated speed of 884 km s 'l, and associated with a C7.4 (2F) flare from 1915 to 2059 UT, with maximum at 2006 UT, from NOAA 8853, which was located at S17øW40 ø (SGD, 2000). Although this was not a major X-ray flare, it was the most intense flare produced during that day. Again, the intense type III-like emissions associated with this event, indicating the presence of a CME, were observed some 20 min before the first CME image at 1954 UT. Unlike the previous Plate 1 by the sudden jump in frequency of the quasi-thermal event, this one produced no observable D-H type II radio noise line [Meyer-Vernet, 1979]. The pink line in Plate 1 shows emissions in the frequency band between 1 and 13.8 MHz.

6 29,994 REINER ET AL.: RADIO SIGNATURES OF CME'S (a) 6.0 February 8, m 'l 10 9 m ' ' (b) :45 08:50' 08:55 09:00 09:05 09:10 09':15 time (UT) February 8-11, ':20 09':25 09:30 Ulysses (4.0 AU, 2 W',380S). :, : :,( Type II radio emissions ' I! ]l 08:00 16:00 24:01! 08:01! 16:0 24:00 is:ill). 16:00 24:00 08:0t! 16:1)0 24:01! ß 0.01 ' - j Wind 0.02 () o.05 i,.., ' uasi-lhermal,... noise line Type II radio emissions, }l ' st d 08:( 0 16:0 24:(10 :00 16:0 24: : 16: 24: 8: 16:00 24:( 8 9 time (UT) Plate 2. (a) Dynamic spectrum of the radio data from to MHz from 0845 to 0930 UT on February 8, 2000, showing the complex type III-like and type II radio emissions associated with the corresponding solar eruptivevent. The dashed white curves are obtained from a fit to the frequency drift rate of the D-H type II emissions obtained using the Saito's [1970] coronal density model. The fit corresponds to a speed of 636 km s-' and a projected solar liftoff time of 0843 UT. (b) Dynamic spectra of the radio data from 0800 UT on February 8 to 2400 UT on February 11 as observed at Ulysses (upper panel) and Wind (lower panel) showing simultaneous views of the sporadic, low-frequency type II radio emissions. These dynamic spectra are plotted as inverse frequency versus time in the frequency ranges from 17.5 to 940 khz for Ulysses and from 17.5 khz to MHz for Wind. Three episodes of sporadic type II emission simultaneously observed by Wind and Ulysses are indicated. Wind also observed some type II emissions that were not observed by Ulysses. The solid and dashed white lines again indicate the connection between the solar liftoff time and the fundamental (F) and harmonic (H) radio emissions generated along two different density region along the shock from. The solid line labeled F is the same as the corresponding pink line in Plate 1.

7 REINER ET AL.' RADIO SIGNATURES OF CME'S 29,995 The next complex type III-like emissions were observed shortly after this, at-0140 UT on February 10, shown in the fourth inset in Plate 1 for the time period from 0140 to 0215 UT. This event was associated with another halo CME, with an estimated speed of 845 km s -1, and associated with a C7.3 flare from 0140 to 0239 UT, with maximum at 0208 UT, from NOAA 8858, which was now located at-n31øe04 ø (SGD, 2000). This CME event did produce some decametric type II-like emissions near 14 MHz, but it is not clear if or how this emission may be related to the CME which was first observed at 0230 UT at -2.2 estimated height (-2.8 R ) of the observed LASCO CME at that time. There were some later type II emissions (not shown) at -2.7 MHz at-0445 UT, which if connected, would imply a speed of -950 km s ' and a CME liftoff time of-0406 UT. As shown in the lower dynamic spectrum in Plate 1, interplanetary type II emissions for this event started at 1130 UT on February 13 at -100 khz. These emissions are aligned along a straight line originating from the CME liftoff time and corresponding to emission at the harmonic of the plasma frequency, with no = 12 cm -3. Finally, complex eject arrived at Wind starting at-0713 UT on February 14, implying a CME transit speed of 817 km s '. (This ejecta does not appear to have been preceded by a shock.) The Sun-Earth connection is again indicated by the pink line. This CME presumably was responsible for the rise in the Kp index to between 5 and 6 after-1200 UT on February 14. The next intense, complex type III-like burst occurred at UT on February 13, shown in the sixth inset in Plate 1 for the time period from 1255 to 1330 UT. However, unlike the previous examples, there was no halo CME reported for this event nor were there any flares reported at this time. LASCO did, however, report material ejected from the Sun at-1354 UT (S. Ro. These two latter halo CME events apparently produced some low-frequency (kilometric) type II radio emissions only after they propagated well out into the interplanetary medium. Very intense, complex narrowband type II radio emissions were observed drifting down from -50 khz (1/f = 0.02) after UT on February 11. These intense type II radio emissions continued drifting to lower frequencies until a second in situ interplanetary shock was detected at the Wind spacecraft at 2333:54 UT on February 11. This shock was preceded by several minutes of intense Langmuir waves, suggesting that at this time the Wind spacecraft was in a beam of electrons accelerated by the shock and that these electrons likely produced at least some of the type II radio emissions. The shock was followed by clear ejecta material starting at-1200 UT on February 12, which may have been a magnetic cloud [Klein and Burlaga, 1982]. The presumed Sun-Earth connection for these two halo CMEs is again indicated by the pink lines. These CME events presumably produced a large jump in the Kp index to 5 by 0300 UT on February 12, and it remained high (maximum = 7) for the entire day. The geomagnetic index, Dst, describing the variations in the equatorial ring current, reached-170 nt at 1200 UT on February 12. Since there was no other shock observed near this time period, Plunkett, ). (The Astrophysikalisches Institut Potsdam (AIP) also observed a metric type II from 1321 to 1330 UT in the frequency range from 170 to 40 MHz, (A. Klassen, )). Nevertheless, this event apparently was associated with the very intense interplanetary type II radio emissions at- 200 khz (1/f = 0.005) observed from UT on February 15 and again drifting from -70 khz from 1000 UT on February 16 to -50 khz by 0200 UT on February 17, which is very conspicuous on the lower dynamic spectrum in Plate 1. It is seen that these type II emissions are very well aligned along straight lines that originated at the time of the intense type III-like burst on February 13, clearly demonstrating their association. Their it is reasonable to assume that the CME-driven shocks associated locations along two sets of lines of different slope, corresponding with the above two halo CMEs coalesced somewhere between to no = 18 cm '3 and 30 cm '3 with a speed of 500 km s 'l, indicate the Sun and 1 AU. The fact that at least some of the intense type II radio emissions observed after 0900 UT on February 11 do not drift along a straight line originating from either solar origin time suggests the interesting possibility that the two CME-driven shocks may have indeed interacted at this point in the that they were generated in two different density regions along the outwardly moving shock front. This event apparently produced no shock or ejecta at Earth. It is therefore possible that this was a backside CME event. This might also explain the unusual behavior of the type III-like emissions from 4 to -14 interplanetary medium, when they were at-0.7 AU. The MHz shown in the inset in Plate 1. propagation of one shock through the other shock could have produced a lateral motion of the type II radio source along the There were no intense, complex type III-like bursts in the frequency band from 1 to 13.8 MHz from February 14 to 16; shock front, and this could produce the observe drift of the type there were also no halo CMEs and no intense flares. Then at II radio emissions away from the straight (dashed) lines from the solar origin times, which correspond to a density of no = 10 cm -3 and a transit speed of 916 km s '. The next complex, type III-like burst occurred at-0400 UT on February 12 as shown in the fifth inset in Plate 1 for the time period from 0400 to 0435 UT. This event was again associated with a halo CME, with an estimated speed of 870 km s -1, and an M1.7 (1N) flare from 0351 to 0431 UT, with maximum at 0410 UT, from NOAA 8858, which was now located at N26øW23 ø (SGD, 2000). Again, this was the only intense type III-like burst in the MHz band, and all the other X-ray flares were weaker than C2 for that day. There was also some D-H type II emissions that drifted from -9 MHz, which occurred at about the same time as the first observed CME image at 0431 UT. For emission at 9 MHz the unmodified Saito [1970] density model implies a heliocentric distance of-2.4 R,, which is very near the UT on February 17 there was another intense, very complex type III-like burst, as shown in the last inset in Plate 1 for the time period from 2025 to 2100 UT. This burst was again associated with a halo CME, with an estimated speed of 550 km s 'l, and an M1.3 (2N) flare from 2017 to 2107 UT, with maximum at 2035 UT. from NOAA 8872, which was located at N29øE07 ø (SGD, 2000). Associated with this event there were also D-H type II emissions with a very well-defined frequency drift rate, which, again using the unmodified Saito [1970] model, implied a speed of 580 km s -. The coronal heights of the radio emissions and the CME again appear to correspond. The emission frequency at-2045 UT was-11 MHz, which for fundamental emission corresponds to a source height of 2.3 R,. Then assuming a CME speed of-550 km s - the CME would have traveled a distance of 2.1 Rz from 2045 to 2130 UT, which would put the CME at 4.4 R, when first observed by the LASCO

8 29,996 REINER ET AL.: RADIO SIGNATURES OF CME'S coronagraph, in agreement with observation. Again, the type III-like burst and the D-H type II emissions were both observed well before the first CME image at 2130 UT. burst that occurs at the same time at higher frequencies. As suggested by MacDowall [1989], this radio emission may have been generated as the type II shock passed through the electron As the CME-driven shock propagated through the beam that produced the type III emissions. Narrowband interplanetary medium, it produced some weak narrowband brightenings in the low-frequency "tails" of type III bursts are interplanetary type II emissions at ~220 khz from ~1730 UT on February 18. Finally, there was an in situ shock at the Wind spacecraft at 2104 UT on February 20. This in situ shock time suggested a CME transit speed of 573 km s -1, very close to our shock speed estimated from the D-H type II drift rate. The pink line again indicates the connection between the CME liftoff at the Sun and the in situ shock at Wind. The fact that interplanetary type II radio emissions are aligned along the white dashed lines of very commonly observed in the Wind data when type II shocks propagated through the interplanetary medium. The earlier type II radio emissions on February 9 and 10 are organized along straight lines (white dashed lines) with a smaller slope, indicating that these emissions originated along a radial line from the Sun where the plasma density along the shock front was somewhat higher (~7.7 cm -3 at 1 AU) than the plasma density along the radial line from the Sun to Earth (~3.8 cm -3 at 1 AU smaller slope again suggests that these radio emissions originated (solid lines); see section 3). Note in Plate 2b that the straight lines from a region of higher density, corresponding to no = 16 cm '3, along the shock front. This SEC CME event produced only a very modest increase in the Kp index to 4 at ~0600 UT on February 21. of the same slope pass directly through the sporadic emissions observed both by Wind and Ulysses. This is expected because the constant in (l') depends only on properties of the radio source and the shock dynamics: the propagation time differences between Wind and Ulysses are small compared to the timescales 4. Simultaneous Wind/Ulysses Observations of of the interplanetary shock propagation. The fact that the low-frequency, sporadic type II radio Low-Frequency Radio Emissions for the CME on emissions for this event were observed by both Wind and February 8, 2000 Ulysses, which were at very different locations in the heliosphere, Simultaneous Wind/Ulysses observations offer a unique opportunity to determine further information about the nature of the interplanetary type II radio emissions and the radio source locations [Hoang et al., 1998]. Of all the interplanetary type II events observe during this 16-day period, only the intense type II event on February 8-11 was simultaneously well observed by the Ulysses spacecraft, which at this time was ~4 AU from the Sun, 26 ø to the west of the Sun-Earth line, and -38 ø below the ecliptic plane. The interplanetary type II radio emissions associated with the February 8 event, simultaneously observed below 1 MHz by radio receivers on both the Wind and Ulysses spacecraft, are shown in the dynamic spectra in Plate 2b. The upper dynamic spectrum in the frequency range from 17.5 to 940 khz shows the intensity of the radio emissions observed at Ulysses from 0800 UT on February 8 to 2400 UT on February 11, The lower dynamic spectrum in Plate 2b shows the radio intensity observed at Wind over the same time period and in the frequency range from 17.5 khz to MHz. Over this time interval, both Wind arid Ulysses observed slowly frequency drifting, narrowband, sporadic type II radio emissions. The radio receivers on Wind observed these type II radio emissions to start at ~200 khz from UT on February 8 and to drift all the way down to 20 khz at the time of the shock at 0235 UT on February 11 (this is clearly visible in the Wind dynamic spectrum as a jump in the quasithermal noise line [Meyer-Vernet, 1979] at that time). The straight, solid white lines (corresponding to fundamental and harmonic) again connecthe CME liftoff time with the in situ plasma frequency at the time of the shock. As stated above, the fact that no type II radio emissions lie along the straight line corresponding to the fundamental of the plasma frequency indicates that the radio emissions were not produced in this lowdensity region along the Sun-Earth line at the fundamental. However, the sporadic emissions observed at 41 khz (1/f = 0.024) at 2000 UT on February 10 do lie along the solid white line corresponding to the harmonic of the plasma frequency, suggesting that this emission may originate along the Sun-Earth line at the harmonic of the plasma frequency. Note that this type II emission was observed in both the Wind and Ulysses dynamic spectra. This type II emission appears to be related to the type III suggests that the sporadic nature of the type II radio emissions intrinsic to the nature of the type II source; it is not likely to be due to beaming of the radiation in the source region. Since the radio receivers on both Wind and Ulysses have been carefully calibrated, we next compared the absolute intensities of the type II radiation observed at Wind and Ulysses. In order to make these comparisons, since the type II emissions are relatively weak, especially at Ulysses, we have carefully subtracted the background signal. For the three simultaneous episodes of the sporadic type II emissions observed at Wind and Ulysses, we have compared the measured intensity ratios. For example, plots of the radio intensity for the sporadic type II emissions observed at 40 ki-iz at ~2200 UT on February 9 are shown in Figure la. (The WAVES receiver on Wind has a bandwidth of 3 khz at 40 khz, whereas the bandwidth and frequency spacing of the Ulysses low-frequency receiver is 750 Hz. Therefore, to compare with the WAVES data, we combined the Ulysses frequency channels from 39.5 to 42.5 khz.) As expected, the intensity at Wind was much greater than that at Ulysses, owing mainly to Ulysses's greater distance from the type II radio source. The onset of the radio emissions was also delayed at Ulysses. The maximum radio intensities at Wind and Ulysses for this sporadic type II emission were found to be 75,858 solar flux units (sfu)(1 sfu = l0 '22 W m -2 Hz 4) and 178 sfu, respectively. The ratio of intensities was therefore 426. In order to understand this intensity ratio, we need to estimate where the type II radio source was located. The fact that the type II radio emissions are intrinsically sporadic allows us to accurately measure the relative time delay for the type II radiation observed between Wind and Ulysses. We can use this time delay in the onset of the sporadic type II radio emissions at Wind and Ulysses to estimate the location of the type II radio source in the interplanetary medium. The time delay between the observations of the type II emissions at ~2200 UT on February 9 between Wind and Ulysses was measured to be 27 min (see Figure la). In order to estimate the location of the radio source implied by this time delay, we assumed that the radio source was located somewhere on a Spherical shock front between the Sun and Earth. Since the transit speed of the February 8 shock was 636 km s 'l, at 2200 UT on February 9 the shock must have been at-0.57 AU.

9 REINER ET AL.: RADIO SIGNATURES OF CME'S 29,997 We then determined the location of a radio source on this spherical shock front such that the time difference of the radiation detected at Wind and Ulysses would be exactly 27 rain, that is, we ignored any possible anomalous time delays (see equation (1)). The geometry, in two dimensions, is illustrated in Figure 1 b. In this way, we found that the radio source was located somewhere along an are on the spherical shock front extending from 24 ø east of the Sun-Earth line and 6 ø north of the ecliptic plane to 90 ø east of the Sun-Earth line and 40 ø north of the ecliptic plane. Since the flare site was at N25øE26 ø, it is reasonable to assume the type II radio source to lie on the shock front at a point -24 ø east from the Sun-Earth line and-6 ø to the north of the ecliptic plane, as shown schematically in Figure lb. The precise source location will not significantly affect our conclusions below. This reasonable estimate of the radio source location also suggests that anomalous propagation time effects, if present, may be relatively small. Note that the source location was to the north of the ecliptic plane, whereas Ulysses was to the south, so that the radio emissions from the type II source must pass through the ecliptic plane to reach Ulysses. Having an estimate of the type II radio source location, we can compare the ratio of intensities with those expected for a point radio source. For a point source the ratios of intensities between Wind and Ulysses should be just the inverse of the ratio of the squares of the distances from the source to Wind and Ulysses. The distance from the estimated source location to Wind was AU while that to Ulysses was 3.77 AU. Thus the ratio of intensities, if the source intensity fell off as 1/R 2, would be a factor of 50. This is an order of magnitude less than the measured intensity ratio of 426 for the type II emissions observed at Wind (a) 5 ' Wind o I tirne(ut) I Ulysses ' 27 min II emission profijle tlrne(ut) February 9, 2000 (b) View from above the ecliptic plane IP Shock (4.0 AU, 26ow, v s = 636 km/s SUN N25øE26 flare site ø...' o.f.. 36o Rs= 0.57 ALD rsw source, f Figure 1. (a). Plot of the intensity (in log solar flux units (sfu) of the type II radio emission at 40 khz observed at Ulysses and Wind from 2000 to 2400 UT on February 9, In each case the background signal was subtracted. (b) Two dimensional view, from above the ecliptic plane, showing the relative geometry of Wind and Ulysses and the CME-driven shock at-2200 UT on February 9, The location of the type II radio source estimated from the relative time delay between Ulysses and Wind is also indicated.

10 29,998 REINER ET AL.: RADIO SIGNATURES OF CME'S and Ulysses. For other possible locations of the source along the The above results therefore imply that the apparent beaming arc on the spherical shock surface, this ratio decreases to as low reduced the intensity at Ulysses by as much as an order of as a factor of 15. These results suggest either that the source is not magnitude from that expected without beaming. From our a point source or that beaming or propagation effects may cause estimate of the type II source location, we can determine the the observed large intensity ratio between Wind and Ulysses. For actual angle between the normal to the shock at the location of the an extended source, however, we would expecthe fall off of the source and the vector to each spacecraft. We find that these intensity to be more like l/r, rather than 1/R 2. This would make angles do not greatly differ: 51 ø to Wind and 72 ø to Ulysses. the discrepancy greater. However, these angles do not take into account that the type II The second possibility is that beaming or propagation effects source beaming may be more strongly dependent on latitude, play an important role for this event. We have shown above that Ulysses being at a latitude of-38 ø at this time. In other words, the for this event the propagation effects did not appear to play an apparent beaming pattern may actually be "pancake" shaped. important role for the propagation times to Wind and Ulysses. If Although the above results suggesthat there may be a propagation effects significantly affected the intensities, owing, beaming pattern associated with the type II radiation, the for example, to some large-scale scattering or occulting medium complete characterization of the apparent beaming pattern of the between the source and Ulysses [Hoang et al., 1998], then we type II radiation for this event is not possible from just two point might expecthat other radio emissions at the same frequency measurements. We will have to make these comparisons for a (and therefore with the source at approximately the same large number of type II emission sourcesimultaneously observed location) would be affected in a similar way. To try to test this, between Wind and Ulysses to more completely characterize the we computed the intensity ratios for the other episodes of the possible apparent beaming patterns of the type II radiation. This sporadic type II emissions on February 10, shown in Plate 2b, will be done in future work. It is also possible that the Cassini between Wind and Ulysses. The results are summarized in Table spacecraft may, in the future, provide a third point measurement. 1. For the other two simultaneous observations of sporadic type II emissions at Wind and Ulysses, we found intensity ratios of Summary and 426, respectively, which are consistently high. For comparison, we next determined the intensity ratio between the Type II and type III radio emissions provide a global view of radio emissions observed in the low-frequency tail of a type III an entire Sun-Earth connection (SEC) event, from the initiation radio burst observed at UT on February 9, also at the and liftoff of the CME at the Sun, to the propagation of the CME frequency of 40 khz. We found this ratio to be 74, only a factor through the solar corona and interplanetary medium, to its arrival of 1.5 different from that expected for a point source at abouthe at 1 AU. same location (we were, however, not able to precisely locate the For a series of solar eruptive events observed from February 5 radio source for the type III emissions at 40 khz). We also to 20, 2000, each of the events began with intense, complex type determined the intensity ratios of the type III emissions between III radio bursts. These type III bursts occurred within minutes of Wind and Ulysses at high frequencies (940 khz), where we do the liftoff of the CME and were the first radio signature of an know the location of the corresponding radio source (very near eruptive event on the Sun, which provided an indication of a the Sun), and we found this ratio to be 27, compared to a factor of CME well before the CME was observed in the coronagraph 16 expected for a point source. Although from these arguments images. However, not every CME liftoff is associated with an we cannot completely rule out that there is for this particular intense, complex type III-like burst, but every intense, complex event some unique source of attenuation between the radio source type III-like burst that we have observed is associated with a and Ulysses, it is also possible that the observed large intensity CME. During the time period from February 5 to 20, 71 CMEs ratios are largely due to beaming of the type II radiation. were reported. Clearly, not all these CMEs are likely to be It has been known for some time that the intensity of the geoeffective. The radio observations as presented above may help radiation from solar radio bursts, observed over a wide frequency to select out those CMEs that are more likely to be geoeffective. range, depends on the observing angle, which is usually Again, not all CME events produce D-H type II radio interpreted as evidence for beaming of the observed radiation emissions, but when they do, if the frequency drift rate is [Caroubalos and Steinberg, 1974; Kaiser, 1975; Fitzenreiter et sufficiently well defined, this can provide a speed estimate which al, 1976; Suzuki and Sheridan, 1982; Steinberg et al., 1984]. can be directly compared to the plane-of-the-sky estimate of the Although previous investigations were concerned only with the LASCO CME speed. For the above eruptive events, D-H type II directivity patterns for type I and type III radio bursts, it is likely radio emissions, presumably caused by the propagation of a shock that the weaker type II radio emissions are also directive. What through the high solar corona, were observed for five of the can be directly measured, however, is the apparent beaming events. By fitting to the frequency drift rates of these D-H bursts, pattern of the radio emissions, since it is believed that refraction speed estimates of the corresponding propagating disturbance and scattering effects in the plasma medium will likely through the corona were made. These speed estimates agreed significantly modify the primary radiation pattern at the radio reasonably well with the LASCO plane-of-the-sky speed source. The finite extent of the radio source region also plays a estimates and with the transit speeds of the CMEs, suggesting that significant role [Reiner and Stone, 1989]. they are associated with the LASCO CMEs. The fact that these Table 1. Comparison of Wind and Ulysses radio fluxes Date Time, UT Frequency, khz S v, sfu Feb Feb Feb S½i, sfu Sv/ Sv

11 REINER ET AL.: RADIO SIGNATURES OF CME'S 29,999 speed estimates were made using an unmodified Saito [1970] density model is significant and needs to be explored further. The interplanetary type II radio emissions can provide a refinement of the speed estimate and an indication of Sun-Earth connection between a white-light CME and terrestrial consequence. As Plate 1 vividly illustrates during the period of solar maximum, when CMEs occur in rapid succession, the radio observations provide an important means of determining which terrestrial consequence belongs to which solar transient event. Although the radio tracking does not always uniquely define the SEC event, it provides additional information and clues about the complexities of the propagation of CMEs through the interplanetary medium. Finally, the simultaneous Wind/Ulysses observations can provide important information on the nature of the type II emission, on the type II source locations, and on the radiation characteristics. 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12 30,000 REINER ET AL.: RADIO SIGNATURES OF CME'S Steinolfson, R. S., Theories of shock formation in the solar atmosphere, in Collisionless Shocks in the Hellosphere: Reviews of Current Research, Geophys. Monogr. Ser., vol. 35, edited by B. T. Tsurutani and R. G. Stone, pp. 1-12, AGU, Washington, D.C., Stone, R. G., et al., The Unified Radio and Plasma Wave investigation, Astron. Astrophys. Suppl. Ser., 92, , Suzuki, S., and K. V. Sheridan, On the fundamental and harmonic components of low-frequency type III solar radio bursts, Proc. Astron. Soc. Aust., 4, , J.-L. Bougeret, Observatoire de Paris, Meudon, France. M. L. Kaiser, Lab. for Extraterrestrial Physics, NASA GSFC, Greenbelt, MD M. J. Reiner, NASA GSFC, Code 690.2, Greenbelt, MD (reiner urap.gsfc.nasa. gov) (Received July 10, 2000; revised September 6, 2000; accepted September 14, 2000.)