BASTILLE DAY EVENT: A RADIO PERSPECTIVE

Transcription

1 BASTILLE DAY EVENT: A RADIO PERSPECTIVE M. J. REINER 1,M.L.KAISER 2, M. KARLICKÝ 3,K.JIŘIČKA 3 and J.-L. BOUGERET 4 1 NASA/GSFC, Greenbelt, MD and Center for Solar Physics and Space Weather, The Catholic University of America, Washington DC, U.S.A. ( reiner@urap.gsfc.nasa.gov) 2 Laboratory for Extraterrestrial Physics, NASA/GSFC, Greenbelt, MD 20771, U.S.A. 3 Astronomical Institute of the Academy of Sciences of the Czech Republic, CZ Ondřejov, Czech Republic 4 Observatoire de Paris, Meudon, France (Accepted 16 July 2001) Abstract. We describe the radio signatures that led up to and concluded the solar eruptive event of 14 July 2000 (Bastille Day Event). These radio signatures provide a means of remotely sensing the associated solar activity and transient phenomena. For many days prior to the Bastille Day Event kilometric Type III radio storm emissions were observed that were presumably associated with the active region NOAA These storm emissions continued until the X5.7 flare at 10 UT on 14 July 2000 that characterized the Bastille Day Event, then ceased abruptly. The Bastille Day Event itself produced very intense, complex, long-duration Type III-like radio emissions, which appear to have been associated with electrons generated (accelerated) deep in the solar corona. The coronal mass ejection (CME) associated with the Bastille Day Event generated decametric to kilometric Type II radio emissions as the CME propagated through the solar corona and interplanetary medium. The frequency drift of these Type II radio emissions are related to the dynamics of the propagating CME and indicate that the CME experienced significant deceleration as it propagated from the high corona into the interplanetary medium. 1. Introduction A variety of distinct radio signatures are associated with solar activity and with solar transient and eruptive events. Metric wavelength Type I and Type III radio storms are characterized by thousands of short-lived spikes that last for hours to several days (Kai, Melrose, and Suzuki, 1985). Kilometric wavelength Type III radio storms, which presumably are the low-frequency continuations of the metric storms, are generated by quasi-continuous beams of suprathermal electrons injected into the interplanetary medium (Lin, 1985). They have been found to be associated with active regions and to often last for several solar rotations (Fainberg and Stone, 1971; Bougeret, Fainberg, and Stone, 1984a; Kayser et al., 1987). The electrons generating these storms are believed to be accelerated in stable helmet streamer structures in the solar corona (Stewart and Labrum, 1972; Bougeret, Fainberg, and Stone, 1984a). The tracking of these storm radio sources at different frequencies permits one to remotely measure the solar wind speed and (possible) Solar Physics 204: , Kluwer Academic Publishers. Printed in the Netherlands.

2 124 M. J. REINER ET AL. acceleration as well as to determine the radial dependence of the coronal density (Fainberg and Stone, 1971; Bougeret, Fainberg, and Stone, 1983; 1984b). Transient solar phenomena, such as flares and CMEs, also produce transient radio signatures. Kilometric Type III radio bursts are usually associated with solar flares and are generated by suprathermal electrons that are injected into the interplanetary medium (Fainberg and Stone, 1974; Lin et al., 1981). Very large solar flares that are also associated with CMEs usually produce very intense, complex long-duration Type III-like radio emissions, which have characteristic features (Reiner and Kaiser, 1999; Reiner et al., 2000b). Coronal and interplanetary shock waves produce both complex Type III (SA) bursts (Cane and Stone, 1984) and Type II radio bursts (Wild, 1950; Malitson, Fainberg, and Stone, 1973; Cane et al., 1982). Although there is still some controversy concerning the origin of metric to hectometric wavelength (coronal) Type II radio bursts (Gopalswamy et al., 1998; Cliver, Webb, and Howard, 1999; Reiner et al., 2000a), it is now well established that the kilometric wavelength (interplanetary) Type II radio emissions result from shocks driven by CMEs as they propagate through the interplanetary medium (Cane, Sheeley, and Howard, 1987; Bale et al., 1999). These interplanetary Type II bursts can therefore be used to track CMEs through the interplanetary medium and thereby to study their dynamics (Reiner et al., 1998; Dulk, Leblanc, and Bougeret, 1999; Reiner et al., 2000a; Leblanc et al., 2000). The remote radio observations described above can therefore be used to detect, monitor and study solar activity and solar transient phenomena. The Type III radio bursts and Type III radio storms provide a monitor of solar activity; the complex Type III-like bursts and Type II radio emissions provide a means of monitoring and studying solar eruptive events. The Bastille Day Event of 14 July 2000 was the first very large solar transient event of solar cycle 23. It was characterized by a X5.7 flare and a halo (Earthdirected) CME. Since this event occurred near central meridian, it also produced significant geomagnetic phenomena at Earth after the arrival of the CME and its associated shock. The variations in the equatorial ring current produced a Dst minimum of 300 nt by 22 UT on 15 July. The planetary geomagnetic activity index, Kp, which measures the maximum amount of fluctuations in the geomagnetic field averaged over several ground stations, reached 9 by 18 UT on 15 July and remained at that level for some 9 hours. In this paper, we analyze the radio signatures produced in association with the Bastille Day Event. We first describe the characteristics of the kilometric Type III storm activity between 10 July and the Bastille Day Event on 14 July We show that the storm activity reached peak intensity just before the Bastille Day Event and then abruptly ceased. The Bastille Day Event was characterized by very complex Type III-like emissions which we suggest were generated by electrons generated deep in the solar corona at or near the flare site. We next analyze the decametric to kilometric Type II radio emissions and describe what their frequency

3 BASTILLE DAY EVENT 125 drift implies about the dynamics of the CME as it propagated through the solar corona and interplanetary medium. 2. Instrument Description Because of the decrease in solar plasma density with increasing heliocentric distance, there is generally a direct relationship between the observed radio frequency and the coronal height of the solar phenomena that produce these radio emissions. Ground-based radio observatories, observing above some 10s of MHz, provide remote radio sensing of solar phenomena in the low corona. Low-frequency radio instruments operating on spacecraft above Earth s ionosphere enable one to continue to detect and track solar radio phenomena from the high corona out through the interplanetary medium and eventually to 1 AU (Earth). The primary radio observations discussed here for the Bastille Day Event were obtained by the WAVES radio instrument on the WIND spacecraft. This instrument 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, and RAD1, which covers the frequency range from 20 to 1040 khz at 32 discrete frequencies (selected from 256 frequency channels) with a highest sampling rate of 45.8 s and a bandwidth of 3 khz. The RAD1 receivers are connected 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 a dipole antenna (7.5 m elements) in the spacecraft spin plane, in addition to the spin axis antenna. The WIND spacecraft was launched in November of During the time of the observations presented here WIND was passing near Earth during one of its distant prograde orbits. At the time of the Bastille Day Event the WIND spacecraft was at ( 1.65, 67.14, 6.31) GSE. 3. Description and Analysis of the Radio Signatures Associated with the Bastille Day Event In this section, we describe in detail the radio signatures that led up to and concluded the solar eruptive event of 14 July For at least 4 days prior to the Bastille Day Event, the active region NOAA 9077, which had rotated around the eastern solar limb, was injecting a quasi-continuous stream of suprathermal electrons along open magnetic field lines into the interplanetary medium. These electron streams were observed remotely as a kilometric Type III radio storm that lasted up until the X5.7 flare at 10 UT on 14 July 2000 that characterized the Bastille Day Event. Very intense, complex, long-duration Type III-like radio emissions,

4 126 M. J. REINER ET AL. which appear to have been associated with electrons generated (accelerated) deep in the solar corona, were associated with the X5.7 flare. The CME associated with the Bastille Day Event generated decametric to kilometric Type II radio emissions as the CME propagated through the solar corona and interplanetary medium. The frequency drift of these Type II radio emissions indicate that the CME experienced significant deceleration as it propagated from the high corona into the interplanetary medium SURVEY OF RADIO ACTIVITY LEADING UP TO THE BASTILLE DAY EVENT Figure 1 provides an overview of the WAVES radio data from 9 14 July 2000 (day ). For comparison, Figure 1(a) shows the 1-min averaged GOES-10 (1 8 Å) X-ray flux for the same time period. The color dynamic spectrum, Figure 1(c), shows the intensity of the radio emissions (1-min averages) plotted as a function of frequency (vertical axis) and time (horizontal axis). The frequencies plotted range from 20 khz to 1040 khz (linear scale). The narrow vertical streaks on the dynamic spectrum correspond to individual Type III radio bursts. As expected, there is a relatively good correlation between these Type III radio bursts and the GOES impulsive X-ray events shown in the upper plot. In addition to the individual Type III bursts which generally extend to very low frequencies (< 50 khz), there was also a quasi-continuous occurrence of weak, bursty Type III emissions for very nearly this entire time period. These quasicontinuous Type III emissions correspond to a kilometric Type III radio storm and consists of thousands of weak Type III bursts per day. These Type III storm bursts characteristically have a rather well-defined low-frequency cutoff from about 200 to 250 khz. Since the dynamic spectrum in Figure 1(c) was constructed from 1-min averaged data, the Type III storm bursts cannot be individually resolved but rather appear as an elevation in the observed radio intensity (lighter blue to yellow) on the dynamic spectrum. To better reveal the temporal behavior of these storm emissions, in Figure 1(b) we show a single frequency plot of the intensity of the radio emissions (in log (s.f.u.)) (1 s.f.u. = Wm 2 Hz 1 ) obtained from the full resolution ( 45.8 s) data at 940 khz. In this plot, the elevation of the storm radiation level above the galactic background as well as the fine time structure of the very bursty Type III storm emissions is evident. (The intense emissions (long vertical lines) correspond to individual Type III bursts observed during this same time period.) It is estimated that the site of the Bastille Day Event, active region NOAA 9077, rotated around the east limb of the Sun on about 7 July As the singlefrequency plot and dynamic spectrum in Figure 1 show, at that time the radiation at the higher frequencies was still at the galactic background level (e.g., about s.f.u. at 940 khz (dashed yellow line) this value includes a small amount of receiver noise). Soon after that, at about 12 UT on 9 July 2000 (DOY 191),

5 BASTILLE DAY EVENT 127 Figure 1. Overview of the WIND/WAVES radio data from 9 14 July (a) The GOES X-ray flux during this time period. (b) The intensity (log (s.f.u.)) of the radio emissions at 940 khz. (c) Dynamic spectrum of the radio intensity. The frequency range is from 20 to 1040 khz.

6 128 M. J. REINER ET AL. low-frequency Type III radio storm emissions, associated with this active region, became evident to the east of the Earth Sun line. The emission level of these Type III storm emissions gradually increased above the galactic background to an average level of about s.f.u. (factor of 1.8 increase) at 940 khz by 10 July 2000 (day 192). This gradual increase in the level of the storm radiation is expected since the Type III storm emissions are known to be beamed somewhat along the Archimedian spiral magnetic field lines (Bougeret, Fainberg, and Stone, 1984a). Thus as the active region and its associated Type III storm radio sources rotated to the west with the sun, the emission became more directly beamed toward Earth and therefore appeared to increase in intensity. This average storm level was maintained until about 15 UT on 10 July (from 2 to 4 UT on 10 July there was a temporary blockage of these solar storm radio emissions due to the passage of the WIND spacecraft through Earth s magnetopause). After about 15 UT on 10 July the storm emissions relaxed somewhat after some M-class X-ray flares from active region NOAA 9077 (see Figure 1(a)). Correspondingly, there was an increase in frequency and intensity of the individual Type III radio bursts. Beginning at about 7 UT on 11 July (DOY 193), the storm radiation level again dramatically increased but after a X1.0 flare at about 13 UT the storm emissions again decreased (but still remaining significantly above the galactic background). At about 2 UT on 12 July (DOY 194), the storm level increased again but then relaxed somewhat after the X1.9 flare from NOAA 9077 at 10:37 UT. Finally, starting at about 8 UT on 13 July (DOY 195) the storm level increased one last time reaching an average intensity level of about s.f.u. (now 2.6 times the background level). This high storm level was maintained for more then 12 hours, during which time there were no major flares from NOAA Then immediately after the X5.7 flare from NOAA 9077 at about 10 UT on 14 July, the Type III radio storm emission ceased simultaneously at all frequencies and the radiation level returned to the galactic background level (except for a number of weak individual Type III radio bursts). The single-frequency plots in Figure 2 show in greater detail the cutoff of the Type III storm emissions on 14 July. We show there the intensity of the radio emissions (in log (s.f.u.)) from 0 to 22 UT at two different radio frequencies, 940 and 540 khz, respectively. The elevated level and bursty nature (consisting of hundreds of weak individual Type III bursts) is evident on these plots. This Type III storm emission is essentially constant (at s.f.u.) up until the time of the very intense individual Type III radio burst associated with the X5.7 flare at about 10 UT on 14 July. This was the most intense Type III burst observed during this entire time period, reaching an intensity of s.f.u. at 940 khz. As suggested by the plots, the cutoff of the Type III storm emissions was simultaneous in all frequency channels, from 1 MHz to 100 khz. Immediately following the intense individual Type III radio burst at 10 UT, there was no longer any evidence of the bursty Type III storm emissions. There were, however, some weaker, smoother radio emissions that slowly drifted to lower frequencies with time. These are the kilometric Type II emissions to be discussed in Section 3.3 below. Then, except

7 BASTILLE DAY EVENT 129 Figure 2. Intensity of the radio emissions on 14 July 2000 at 940 and 540 khz.

8 130 M. J. REINER ET AL. Figure 3. (a) Dynamic spectra of the WIND/WAVES radio emission at the time of the Bastille Day flare. (b) IZMIRAN dynamic spectrum of the metric radio emissions. (c) Intensity versus time of the Ondřejov decimeter data at 3 GHz. (d) GOES X-ray flux. for some additional individual Type III bursts, the radiation level decreased to the corresponding galactic background level, s.f.u. at about 13 UT at 940 khz and s.f.u. at 15 UT at 540 khz. (The very bursty intense emissions, most evident between 16 and 19 UT, are the AKR emissions (Gurnett, 1975) from Earth.) 3.2. INTENSE, COMPLEX TYPE III LIKE EMISSIONS ASSOCIATED WITH THE BASTILLE DAY EVENT AND RELATIONSHIP TO DECIMETER EMISSIONS The Bastille Day Event was characterized by an X5.7 long-duration event (LDE) X-ray flare from 10:03 to 10:43 UT, with maximum at 10:24 UT, and a visual 3B flare from NOAA 9077 at N22 W07 beginning before 10:12 UT and lasting until 11:46 UT (Solar Geophysical Data). The GOES X-ray flux associated with the X5.7 flare is shown in Figure 3(d). The SOHO/LASCO coronagraph (Brueckner et al., 1995) observed a halo CME starting at 10:54 UT and SOHO/EIT (Delaboudinière et al., 1995) observed a wave at about 10:12 UT. There was also a major particle event (SEP) near Earth, starting at about 10:15 UT. Figure 3(a) shows an expanded plot, in dynamic spectral format, of the very intense, complex, long-duration Type III-like emissions associated with the Bastille

9 BASTILLE DAY EVENT 131 Day flare. The frequency range is from to MHz (linear scale) and the time period shown is from 10 to 11 UT. The data are from the (high-frequency) RAD2 receiver on Wind, which sweeps the entire frequency range every s. Although there are some weak Type III emissions as early as 10:09 UT, the intense Type III-like emissions, associated with the X5.7 flare, began about 10:23 UT. These Type III-like emissions extend to about 10:40 UT at MHz and to beyond 10:50 UT at MHz. Reiner and Kaiser (1999) noted that such complex Type III-like radio emissions, in the frequency range from 1 to 14 MHz, are usually associated with major flare/cme events and display characteristic features: they show no clear frequency drift (within the receiver sweep time ( s)) above about 7 MHz; they typically display a diminution in intensity in the frequency range centered at about 7 MHz; and they almost always have some associated very narrow-band (nearly) horizontal or very slowly drifting features below about 3MHz. Figure 3(b) shows the metric radio emissions in the frequency range from 25 to 270 MHz from the IZMIRAN radio observatory. Here we see what appears to be the high-frequency continuation of the intense Type III-like emissions seen in RAD2. In addition, the IZMIRAN radio observatory reported two metric Type II radio bursts from 10:18.9 to 10:26.3 UT from 175 to 35 MHz and from 10:28 to 10:34 UT from 150 to 45 MHz (Solar Geophysical Data). The low-frequency Type II-like radio emissions, from 10:30 to 10:50 UT in RAD2 (Figure 3(a)) are probably a continuation of the metric Type II radio burst from 10:18.9 UT to 10:26.3 UT. The decimeter radio emissions observed by the Ondřejov observatory (Jiřička et al., 1993), in the frequency range from 0.8 to 4.5 GHz, associated with this event exhibited broadband pulsations which grew in intensity (Karlický et al., 2001). At 10:24 UT there was a fast drift burst in the GHz range. This was followed by drifting pulsation continuum between 10:27 and 10:35 UT in the 0.8 to 1.5 GHz range. Reiner et al. (2000b), showed that the intensity profiles of many Type III-like radio emissions observed by WIND/WAVES often exhibit a remarkable similarity to the intensity profiles of the decimetric radio emissions (1 3 GHz), suggesting that they were generated by populations of electrons that were accelerated at about the same time and location. Figure 3(c) shows the intensity (in s.f.u.) of the decimetric radio emissions at 3 GHz. This intensity-time profile is remarkably similar to the intensity profile of the Type III-like emissions at MHz, as shown in the top panel (for comparison we superpose the MHz plot, shifted by 1 min the time that it takes the electrons to propagate from near the photosphere to the 14 MHz plasma level). The time duration of the decameter and decimeter emission profiles are essentially the same; even the minima and maxima in the emission profiles at MHz and 3 GHz seem to correspond.

10 132 M. J. REINER ET AL DECAMETRIC-KILOMETRIC TYPE II RADIO EMISSIONS ASSOCIATED WITH THE BASTILLE DAY EVENT The Bastille Day Event produced a halo (Earth-directed) CME. The initial projected speed of this CME, as measured from LASCO height-time plots, was between 1400 and 1900 km s 1 (Andrews, 2001). As the CME propagated through the solar corona and interplanetary medium, the shock driven by this CME produced low-frequency (decametric to kilometric) Type II radio emissions. Since the coronal and interplanetary plasma densities decrease monotonically with increasing heliocentric distance and since the Type II radio emissions are produced at the local plasma frequency (or its harmonic), the measured frequency drift rate of these Type II radio emissions provides information on the dynamics of the CME/shock. The Type II radio emissions associated with this event were rather complex and, because of the close proximity of WIND to Earth at that time, were often confused with radio emissions from Earth. This has made the analysis of the CME dynamics from the Type II radio data difficult for this event. Figure 4 shows dynamic spectral plots of the Type II radio emissions produced during and after the Bastille Day flare. In order to more clearly indicate the CME/shock dynamics implied by the frequency drift of these Type II radio emissions, we have plotted these radio dynamic spectra as inverse frequency versus time (Reiner et al., 1998), rather than as frequency versus time. The reason is that the radial dependence of the interplanetary plasma density is known, on average, to vary approximately as 1/r 2,wherer is the heliocentric distance (Bougeret, King, and Schwenn, 1984). Since the Type II radio emissions are generated at the plasma frequency and its harmonic and since the plasma frequency is directly proportional to the square root of the plasma density, f p (kh z) = 9 n(cm 3 ), it follows that the plasma frequency f p must, on average, scale as 1/r in the interplanetary medium. Thus, plotting the Type II radio intensity as 1/f versus time is essentially equivalent to plotting it as r versus time. Then, if the CME propagates through the interplanetary medium at a constant speed, radio emissions generated by the CMEdriven shock are expected to be organized along straight lines on this plot, since r = v(t t o ),wherer is the heliocentric distance of the shock, t is the time, t o is the solar liftoff time and v is the shock speed. Deviations from a straight line are indicative of acceleration or deceleration. The dynamic spectra in Figure 4 cover the frequency range from 40 khz to MHz and the time period from 10 UT on 14 July to 16 UT on 15 July. Figure 4 shows that the intense, complex Type III-like emissions, discussed in Section 3.2, were followed by the much more slowly frequency drifting Type II radio emissions that drifted from about 7 MHz at 10:40 UT to about 100 khz by 24 UT on 14 July as the CME propagated from the solar corona into the interplanetary medium. On these dynamic spectra the dynamic range had to be adjusted so as to bring out the weak Type II emissions by greatly overexposing the very intense solar Type III and Earth radio emissions. The CME and its corresponding shock arrived

11 BASTILLE DAY EVENT 133 Figure 4. (a c) WIND/WAVES dynamic spectra showing the Type II radio emissions generated by the CME after the Bastille Day flare. The curves are discussed in the text. (d) Height time and speed time plots corresponding to the solid white curves on the dynamic spectra.

12 134 M. J. REINER ET AL. at WIND at 14:35 UT on 15 July 2000 as indicated by the sudden jump in the quasi-thermal noise spectrum observed at low radio frequencies, implying a shock transit time of about 27.6 hours and a transit speed of about 1473 km s 1. Since the Type II radio emissions associated with this event occurred over a very wide frequency range, we show the 1/f versus time data on three different frequency and time scales in Figure 4. Figure 4(a) shows Type II radio emissions at high frequencies (14 MHz to 400 khz), which were generated by the CME-driven shock propagating from the high corona into the interplanetary medium between 10:54 UT and 14 UT on 14 July. These complex Type II radio emissions appear to be at both the fundamental and harmonic of the plasma frequency. The lowerfrequency component, which we interpret to be fundamental emissions (drifting along the lower white curves), is very diffuse with a wide bandwidth that appears to increase at lower frequencies. Unfortunately, at the lower frequencies this emission gets somewhat confused with the much more intense AKR emissions (Gurnett, 1975) from Earth. The weaker, higher-frequency component, which we interpret as harmonic emissions (drifting along the upper white curves) is more sporadic in nature. Figure 4(b) extends the radio range down to 67 khz and the corresponding Type II emissions were generated during the propagation of the CME-driven shock through the interplanetary medium to 24 UT on 14 July, when the CME was estimated to be at about 0.69 AU (= 149 R,whereR = km). On this dynamic spectrum we see the intense AKR emissions centered at about 250 khz between 14 UT and about 15 UT, which is superposed on the much weaker, diffuse and drifting Type II emissions. The AKR is easily distinguishable because of its bursty nature and by the fact that it does not drift in frequency. From about 15:40 UT to 18:40 UT, the AKR completely overwhelms all the other emissions in this frequency range. Finally, after about 19 UT, the weaker, diffuse Type II emissions re-appear at about 150 khz (below the typical low-frequency limit of the AKR emissions), which then drifted to 100 khz by the end of the day. The final panel, Figure 4(c), extends the frequency range down to 40 khz and the time to 16 UT on 15 July, just after the arrival of the CME at Earth. To determine how these frequency drifting Type II radio emissions relate to the dynamics of the CME, it is necessary to relate the observed radio frequencies to heights above the corona. To do this, we must use a density-distance model of the solar corona and interplanetary medium. We chose two density-distance relationships the Saito, Poland, and Munro (1977) model and the simple 1/r 2 falloff. The dotted (white) lines on Figure 4 were obtained assuming a 1/r 2 density falloff, a speed of 2400 km s 1, and a plasma density at 1 AU of 46 cm 3.(This corresponds to the plasma density in the region of the shock where the Type II emissions were generated such high density structures at 1 AU were observed during this time interval). The lower line is for fundamental emission and the upper for harmonic emission. It was further assumed that the CME was at 5 R at 10:54 UT. The parameters of this fit, however, are not unique. For example, we

13 BASTILLE DAY EVENT 135 get identical lines if we assume a speed of 2000 km s 1 and a density at 1 AU of 30 cm 3. However, the important point here is that, although these straight lines provide a reasonable fit to the frequency drift of the diffuse and sporadic Type II emissions at the high frequencies, they do not simultaneously fit the lowfrequency Type II emissions. The deviation of the observed low-frequency Type II emissions from these straight lines suggests that the propagating disturbance that generated these emissions experienced significant deceleration in the corona and interplanetary medium. We next try to determine the actual speed profile of the CME/shock through the interplanetary medium by requiring a best fit to the observed frequency drifting Type II emissions in Figure 4. Consistent with the low-frequency Type II radio data (Reiner et al., 1999), we make the simplifying assumption that the CME/shock accelerates at a constant rate a until time t a, i.e., v = v o + at, after which it moves at a constant speed v 1AU to 1 AU. The determination of the acceleration profile, i.e., a and t a, then depends on the initial speed v o, the shock speed v 1AU at 1 AU, and the transit time to 1 AU. Since in the present case the transit time to 1 AU and the shock speed at 1 AU are known, for a given initial CME speed we get a unique solution for the CME speed profile from the sun to 1 AU. Specifically, we started by assuming an initial CME speed of 2400 km s 1 at 10:54 UT. Then the known transit time (27.6 hours) and the speed of the CME shock at 1 AU ( 1100 km s 1 ) requires an acceleration of 22.8 m s 2 and a deceleration time of 15.9 hours. The corresponding speed profile is shown in Figure 4(d). It is then a simple matter to calculate the height time relationship that corresponds to this speed profile, as shown in the upper panel of Figure 4(d). In order to compare this CME speed profile with the frequency-drifting Type II emissions, we next convert the corresponding height time relationship in Figure 4(d) to a frequency time relationship, assuming a 1/r 2 falloff of the plasma density. Finally, we adjust the value of the density at 1 AU in order that this corresponding frequency versus time curve provide a best fit to the Type II frequency drift over the entire frequency range of the Type II emissions shown in Figure 4. The result of this procedure is shown by the solid (white) curves in Figure 4. These solid curves correspond to an initial CME speed of 2400 km s 1, an acceleration of 22.8 m s 2, a deceleration time of 15.9 hours, and a density at 1 AU of 46 cm 3. We repeated this procedure for other assumed initial speeds for the CME, in each case solving for the deceleration and deceleration time by using the contraints imposed by the known transit time and the shock speed at 1 AU, and again determined the best fit to the frequency drifting Type II data by finally adjusting the density at 1 AU. A summary of these calculations is provided in Table I. Each of these sets of parameters will give a somewhat different curve on the 1/f versus time dynamic spectra. As an example of how well these different speed profiles fit the frequency drift of the Type II, the black curves in Figure 4 show two other solutions. The black dashed curves are the result for a best fit assuming an initial CME speed of

14 136 M. J. REINER ET AL. TABLE I Deceleration parameters for different initial CME speeds. v o (km s 1 ) a(ms 2 ) t a (hours) n o (cm 3 ) km s 1 and the black dot-dashed curves are the result for a best fit assuming an initial CME speed of 1800 km s 1, with other pararmeters as shown in Table I. Finally, by comparing each of the possible solutions with the Type II emission data, we tried to pick out a set of parameters which gave a best overall fit to the frequency drift of the observed Type II emissions in Figure 4. This procedure, of course, will work best when we have a very long and clean period of Type II emissions. Unfortunately, for the Bastille Day Event, a number of factors made a unique and precise determination of the CME dynamics difficult: the Type II emissions for this event were very diffuse and somewhat indistinct, and we had very significant contamination from Earth radiations (AKR (Gurnett, 1975) at high frequencies and continuum emissions (Gurnett and Shaw, 1973) at low frequencies), which for example, prevented us from tracking the Type II emissions beyond 14 July. Therefore, at best we can only give a range of possible CME initial speeds and deceleration profiles which provide a reasonable fit to the observed Type II frequency drift for this Bastille Day Event. This range of solutions is from 1800 to 2800 km s 1. Clearly, if we had been able to track the Type II emissions significantly beyond 24 UT on 14 July, we would have been able to easily distinguish between these solutions. As it is, we cannot rule out these latter fits even though in our judgement an initial CME speed of about 2400 km s 1 seems to give a somewhat better overall fit to the observed Type II frequency drift. These frequency versus time curves do not precisely fit the Type II frequency drift at high frequencies, but this is expected since in the corona the falloff of the plasma density is more complex than r 2. To obtain some insight into how much this may effect our results, we also show a fit to the frequency drift of the Type II data obtained using the Saito, Poland, and Munro (1977) coronal density model. With this model there is perhaps some improvement at the high frequencies (dashed white curves in Figure 4). However, fitting the Type II data with the Saito et al. model also requires the introduction of significant deceleration. The parameters, which gave the fit shown, had an initial speed of 2400 km s 1, an acceleration of 22.8 m s 2, a deceleration time of 15.9 hours, a start time of 10.5 UT, and required a density enhancement factor of 3.

15 BASTILLE DAY EVENT Discussion In Section 3.1, we showed that after the X5.7 flare the kilometric Type III radio storm emissions ceased simultaneously at all frequencies. Presumably, the CME associated with this Bastille Day Event disrupted the stable helmet streamer structure, thereby closing off or destroying the magnetic field lines that were open to the interplanetary medium and along which the storm electrons propagated. It is significant that the metric noise storm, reported by Chertok et al. (2001), also displays a large-scale restructuring at this time. This behavior of the kilometric Type III radio storm emissions is very typical and has been observed numerous times during this and other solar maxima by radio receivers on ISEE-3 (cycle 21), Ulysses (cycle 22), and WIND (cycle 23). The type III radio storms that are associated with large active regions such as this one typically modulate in intensity in response to intense X-ray flares from the active regions associated with the storm. Then after a major flare (usually X-class) from the active region, the storm emissions will suddenly cease simultaneously at all frequencies. Presumably, the Type III radio storm provides a means for the active region to continuously release a small but constant amount of energy (Bougeret, Fainberg, and Stone, 1982). However, this energy release may often not be sufficient to release the tremendous energy which is building up in the active region, and which may then eventually be released during a major flare. It also very often happens that the Type III radio storms will begin to build up again many hours or days later, suggesting a reformation of a helmet streamer structure. In the present case, however, the storm emissions did not re-appear. (There was an intense Type III storm beginning on 20 July 2001 but this storm was on the east and therefore associated with a different active region.) The decimeter radio emissions for this event, described in Section 3.2, are characterized by drifting pulsation structures which are believed to correspond to radio emissions generated close to the reconnection process below the ejected plasmoid (Karlický et al., 2001; Yan et al., 2001). This reconnection process accelerates particles which can then escape into interplanetary space along open magnetic field lines. The remarkable similarity in the decimetric and decametric time profiles suggests to us that both the decimetric and decametric radio emissions were produced by populations of electrons that were accelerated at the same time and in the same spatial region. It is notable that the intense Type III-like radio emissions are seen at frequencies both higher and lower than the frequency of the metric Type II emissions and their presumed continuation to the decametric frequencies shown in Figure 3. This also suggests that the complex decametric Type III-like emissions were produced by electrons accelerated deep in the corona rather than by a shock (perhaps associated with the metric Type II bursts) that sometimes accelerate electrons higher up in the corona (the SA events) (Cane and Stone, 1984; Bougeret et al., 1998; Dulk et al., 2000). The fact that there appears to be an enhancement in the Type III

16 138 M. J. REINER ET AL. emissions at frequencies lower than those of the metric Type II burst may mean that the interaction of the coronal electrons with the Type II shock simply facilitated the generation of the Type III radio emissions (see, e.g., Bougeret et al., 1998). The observed frequency drift of the decametric to kilometric Type II radio emissions implied that the CME must have decelerated as it propagated through the corona and interplanetary medium. However, because of the rather limited time period over which we were able to track the Type II radio emissions, we were unable to obtain a unique speed profile for this CME. Clearly a crucial determining factor is to know when the deceleration ceased. In order to determine that we would have had to observe the Type II emissions to lower frequencies. Unfortunately, at these crucial lower frequencies, the radio emissions observed by WIND/WAVES, shown in Figure 4, were dominated by intense continuum emissions from Earth. The best that we can do under these circumstances is to argue that the observed frequency drift of the Type II emissions is consistent with a range of CME initial speeds between about 1800 and 2800 km s 1, with the corresponding deceleration profiles given in Table I, for example, suggesting a CME deceleration between 7 and40ms 2. Acknowledgements The WIND/WAVES experiment is a collaboration of NASA/Goddard Space Flight Center, the Observatoire of Paris-Meudon and the University of Minnesota. M.J.R acknowledges support, in part, from NSF grant ATM We thank Dr V. Fomichev for permission to use the IZMIRAN metric dynamic spectrum. The GOES X-ray data were obtained from the SPIDR Web site. References Andrews, M.: 2001, Solar Phys., this issue. Bale, S. D., Reiner, M. J., Bougeret, J.-L., Kaiser, M. L., Krucker, S., Larson, D. E., and Lin, R. P.: 1999, Geophys. Res. Lett. 26, Bougeret, J.-L., Fainberg, J., and Stone, R. G.: 1982, Interplanetary Radio Storms: 1-Extension of Solar Active Regions through the Interplanetary Medium, NASA TM-84940, Greenbelt, MD. Bougeret, J.-L., Fainberg, J., and Stone, R. G.: 1983, Science 222, 506. Bougeret, J.-L., Fainberg, J., and Stone, R. G.: 1984a, Astron. Astrophys. 136, 255. Bougeret, J.-L., Fainberg, J., and Stone, R. G.: 1984b, Astron. Astrophys. 141, 17. Bougeret, J.-L., King, J. H., and Schwenn, R.: 1984, Solar Phys. 90, 401. Bougeret, J.-L. et al.: 1995, Space Sci. Rev. 71, 231. Bougeret, J.-L. et al.: 1998, Geophys. Res. Lett. 25, 2513 (errata: Geophys. Res. Lett. 25, 4103). Brueckner, G. E. et al.: 1995, Solar Phys. 162, 357. Cane, H. V. and Stone, R. G.: 1984, Astrophys. J. 282, 339. Cane, H. V., Sheeley, Jr., N. R., and Howard, R. A.: 1987, J. Geophys. Res. 92, Cane, H. V., Stone, R. G., Fainberg, J., Steinberg, J. L., and Hoang, S.: 1982, Solar Phys. 78, 187.

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