Type III solar radio bursts in inhomogeneous interplanetary space observed by Geotail

Transcription

1 Radio Science, Volume 36, Number 6, Pages , November/December 2001 Type III solar radio bursts in inhomogeneous interplanetary space observed by Geotail Yoshiya Kasahara Department of Communications and Computer Engineering, Kyoto University, Kyoto, Japan Hiroshi Matsumoto and Hiro sugu Kojima Radio Science Center for Space and Atmosphere, Kyoto University, Uji, Japan Abstract. Characteristics of type III solar radio bursts are studied using high-frequency resolution of the SFA (sweep-frequency analyzer) of the Plasma Wave Instrument (PWI) on board the Geotail spacecraft. We often observe abnormal type III bursts, which have separated frequency bands or have prolonged tails at particular frequencies. These observations provide observational clues to detect density inhomogeneities in the upstream interplanetary medium. We propose possible models of interplanetary density structures which can account for some type III spectrum structures observed. 1. Introduction The type III burst is a radio emission which is characterized on dynamic spectra by a drift in time from high to low frequencies. Spaceborne observations have revealed type III kilometric radio bursts. It is generally accepted that type III bursts are generated by beams of energetic electrons which are ejected from the Sun and travel outward along open magnetic field lines from the Sun through the outer corona and interplanetary space [Gurnett and Frank, 1975; Linet al., 1981]. Type III bursts are thought to be electromagnetic radiation converted from local plasma oscillations (Langmuir waves) at the local plasma frequency fp and/or 2fp. The spectrum of a type III burst, therefore, shows a smooth decrease of frequency in time because of the outward motion of the exciter from high-density regions near the Sun to regions of lower density farther out. Both fundamental and harmonic components are often present in the type III bursts in the frequency range of meter wavelength, but these components of type III kilometric bursts are difficulto separate. Kellogg [1980] studied three bursts at frequencies higher than 20 khz; in two cases he found the emission to occur at the funda- Copyright 2001 by the American Geophysical Union. Paper number 2000RS / 01 / 2000RS mental frequency of fp, and in one case he found the burst to change from harmonic to fundamental radiation. Dulk et al. [1984, 1987, 1998] and Hoang et al. [1994] analyzed a large sample of kilometric bursts on ISEE 3 [Dulk et al., 1984, 1987], on Wind [Dulk et al., 1998], and on Ulysses [Hoang et al., 1994]; these authors found that at a given frequency most of the bursts radiate at the fundamental mode at the onset time, followed near or after the time of the peak emission by the harmonic emission generated at distance farther out from the Sun. Fainberg et al. [1972] examined the trajectory of the source region of type III burst based on comparison of the results from the direction-finding analyses using a spin modulation with a global density model which assumes a smooth power law relationship between the plasma density and the distance from the Sun. After that, many authors conducted similar direction findings using ISEE 3, Wind, Ulysses and other satellites [e.g., Steinberg et al., 1984; Reiner and Stone, 1986; Reiner et al., 1998]. Generally, the source region of type III burst expands in size as the heliocentric distance increases. Therefore spin modulations observed by satellite are not clearly seen below 0.1 MHz. Reiner and Stone [1986] developed a new direction-finding method called a local density approximation (LDA). The method uses the measured arrival directions and the measured frequency drift rates of the type III burst to determine the locations of the radio source at consecutive frequency 1701

2 1702 KASAHARA ET AL.: TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE levels. Reiner and Stone [1988, 1989] proposed e[ model of the radio emission region for kilometric type III bursts including spatial and temporal effects. Reiner et al. [1998] have recently derived trajectories of the generation points of type III bursts in three-dimensional interplanetary space using radio triangulation from Ulysses orbiting out of the ecliptic plane and Wind, which was located near the Earth.- Steinberg et al. [1984, 1985] and Dulk et al. [1984, 1987] have provided the evidence that propagation effects such as ducting, scattering, and group delays may modify the observed characteristics of the primary radio source. For example, Steinberg et al. [1984, 1985] investigated the occurrence region and the angular size of type III burst source and concluded that the angular size of the source is doubled by scattering of the radiation from interplanetary density inhomogeneities. As was reported by Steinberg et ed., type III bursts are important phenomena to investigate plasma environments in interplanetary space because the radiation of type III burst may reflect the results of interactions with the medium. The dynamic spectrum of type III burst carries some information on the interplanetary medium where the emission is generated and propagates under the influence of scattering and group delay, etc. In the present paper, we introduce some examples of unusual type III burst spectra observed by the Ple[sme[ We[ve Instrument (PWI) on boe[rd Geotail. They exhibit e[ dynamic spectrum with e[ continuum and smooth frequency variation from high to low frequency but show separated segments with frequency gaps and/or are sometimes associated with prolonged tails at particular frequencies. Similar abnormal spectra of type III bursts were sometimes observed by other satellites, and there has been some attempt to explain them in terms of density inho- mogeneities in the interplanetary medium. Lacombe and M ller-pedersen [1971] reported intensity enhancements in dece[meter type III bursts. They interpreted that these enhancements were caused by e[ shock wave in the upper solar corona. Dunckel [1976] studied narrowband enhancements in e[ series of kilometric type III bursts observed by OGO 3. The enhancements showed e[ decrease in frequency over e[ period of several hours, and the rate of this decrease was consistent with the movement of an interplan- etary shock. MacDowall [1989] discussed (1) lower cutoffs (CO) in which intensity of type III bursts is abruptly reduced and remains at the reduced level for all lower frequencies, (2) narrowband intensifications (NBI) that frequently occur on the high-frequency edge of e[ cutoff, and (3) narrowband intensity reductions below e[ cutoff. MacDowall [1989] proposed that these phenomena were caused by e[ pitch angle scattering of the beam electrons in the enhanced magnetic turbulence downstream of interplanetary (IP) shocks. The spectra of type III bursts introduced in the present paper are similar to those previously observed but not completely explained by the previously proposed mechanism. In this paper, we give e[ detailed description of the features of unusual type III bursts observed by Geotedl and discuss the differences from the type III bursts previously studied. Finally, we propose plausible models of interplanetary density structures which account for the spectrum structures of these type III bursts. 2. Instruments Geotail was launched in 1992 for investigating the structure and dynamics of the geomagnetic tail. The PWI on board Geotail measures plasma waves generated by a variety of plasma processes in the Earth's magnetosphere [Matsumoto et al., 1994]. The sweepfrequency analyzer (SFA), which is one of the PWI receivers, provides spectral information on plasma wave amplitudes over the frequency range from 24 Hz to 800 khz for the electric field and from 24 Hz to 12.5 khz for the magnetic field. The SFA consists of eight independent receivers covering five frequency bands for the electric fields and three frequency bands for the magnetic fields. Each receiver has a frequency resolution of 1/128 of the receiver frequency band while the time resolution is 8 s for the higher-frequency bands (bands 3, 4, and 5) and 64 s for the lower-frequency bands (bands I and 2). In the present paper, we analyze the detailed spectrum structures of type III bursts by taking advantage of high-frequency resolutions of the SFA in the two upper frequency bands. 3. Observation Type III bursts are frequently observed by Geotail, especially in the frequency range from 100 to 800 khz covered by the band 5 receiver of SFA with e[ frequency resolution of 5.4 khz. The low-frequency part of e[ type III burst sometimes spreads down to the frequency range of the band 4 receiver of SFA, which covers khz with e[ frequency resolution of 680 Hz. Spin modulation of the type III burst intensity is often recognized by the SFA although it becomes less distinctive in the low-frequency tail ob- served in band 4.

3 --,, ,.,..,-- GEOTAIL PWI SFA Date=940? ' I, A; B ii i i i iii i iii iiii i -216 Universal 1900 Time Plate 1. Dynamic spectra observed from 1800 to 2000 UT on July 1, Points "A" and "B" indicate a temporal delay of the type III burst around 280 khz and simultaneous appearance of frequency components around 240 and 150 khz, respectively. 800 I GEOTAIL PWI SFA Date=93111? , 'A o Universal Plate 2. Dynamic spectra observed from 0300 to 0400 UT on November 17, A typical type III bursts starts around 0320 UT, and discrete structures (DS) appear around 605 khz at 0333 UT (point "A") and 400 khz at 0337 UT (point "B") after the normal type III burst. Similar but less intense tails are also seen at 760, 520, and 300 khz. Time

4 1704 KASAHARA ET AL.: TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE 3.1. Event 1 Plate I shows a dynamic spectrum of the electric field component for the time period from 1800 to 2000 UT on July 1, During the whole period, Geotail was located in the solar wind near the stagnation point in front of Earth's bow shock (X - 19Re, Y = ORe, and Z = ORe in the geocentric solar magnetospheric (GSM) coordinate system, where Re is the radius of the Earth). A type III burst is observed from 1845 UT to 2000 UT drifting in time with decreasing frequency. The emissions between 200 and 700 khz are auroral kilometric radiation (AKR). It is remarkable that there exist two distinctive band gaps; one is found around 280 khz from 1851 to 1855 UT (point "A" in Plate 1), and another is at 210 khz around 1903 UT (point "B"). The former is a rather temporal discontinuity, and the lower frequency part of the type III appears with a time lag of 4 min. On the other hand, the latter is characterized by the simultaneous arrivals of frequency components of 240 and 150 khz. Reiner et al. [1992] reported type III bursts with banded spectrum structures in their low-frequency portions. They concluded that these banded emissions were generated in the vicinity of the spacecraft because they detected them at the fundamental, second harmonic, and possibly third harmonic of the local electron plasma frequency. Our case, shown in Plate I is, however, different from the ones reported by Reiner et al. [1992]. The local plasma frequency and its second harmonic are observed at 20 and 40 khz, respectively, so that no relation is found between the emission band gaps of the type III burst and the local electron plasma frequency Event 2 Another example of dynamic spectra observed from 0300 to 0400 UT on November 17, 1993, is illustrated in Plate 2. Geotail was located in the solar wind at the distant tail (GSM X -210Re, Y -15Re, and Z ORe). A typical type III burst starts at 800 khz around 0320 UT and smoothly drifts with time toward the lower frequency part. It is found that discrete structures (DS) appear around 605 khz at 0333 UT (point "A" in Plate 2) and 400 khz at 0337 UT (point "B") just after the normal type III burst. Similar but less intense tails are seen at 760, 500, and 320 khz. Both intense banded structures have less negative frequency drift than the preceding normal type III burst. The local electron plasma frequency at this location is 20 khz, and no relation is found between these DS and the local plasma frequency, or its harmonics. We applied a direction-finding method to the type III bursts using SFA data. The spin rate of Geotail is -. 3 s, and the time resolution of the SFA is 8 s for bands 4 and 5. Therefore a signal at each frequency is sampled three times during approximately 24 s with the spin angle interval of 240 ø. This means that the angular resolution for the beat period of 24 s is ø for a spinning electrical dipole antenna. Furthermore, we accumulate consecutive frequency channels within a finite bandwidth and fit them by the least squares method (I. Nagano et al., Remote sensing the magnetosheath by the spin modulation of terrestrial continuum radiation, submitted to Geophysical Research Letters, 2000). It is thus found that the azimuthal angle of type III burst at around 800 khz is in almost a sunward direction (.. 3 ø toward the west of the Sun at 750 khz) and becomes more oblique in the lower frequency range ( 15 ø toward the west of the Sun at 300 khz). This is consistent with the explanation that the source region distributes along the spiral structure of the open interplanetary mag- netic field line, where an energetic electron beam is released from the Sun. We checked the difference of arrival angles between the normal part of type III burst and discrete components; however, there are no distinctive differences in their features, except that the discrete part is a little closer to the Sun-aligned direction. 4. General Characteristics Since the solar rotation twists the magnetic field lines from the solar surface into the interplanetary medium, the resulting pattern of the magnetic field lines in interplanetary space should be an Archimedean spiral. Energetic electron beams ejected from the Sun are guided by the magnetic field lines, and they continue to excite Langmuir waves along their paths while they travel in interplanetary space. Type III bursts are electromagnetic waves converted from the Langmuir wave energy at the local fp and/or 2fp. Since the electron density of the solar wind varies as the inverse of the heliocentric distance squared, the frequency of type III bursts generated at each distance varies approximately as the inverse of the distance from the Sun. As was mentioned in the introduction, abnormal spectra of type III bursts were generally explained in terms of density inhomogeneities in the interplanetary medium by many authors [e.g., Lacombe and M ller-pedersen, 1971; Dunckel, 1976; MacDowall, 1989]. MacDowall [1989] made an explanation of the mechanism of cutoffs (CO) and narrowband intensifications (NBI) as follows: Initially, the electrons

5 -- _ KASAHARA ET AL.' TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE I IIIIll Jlllll I Frequency (khz) Figure 1. Intensity versus frequency overwrapped every 1 min from 1840 to 1915 UT on July 1, which travel outward from the Sun encounter the turbulent "wake" downstream of the shock, which causes a temporary increase in type III intensity (NBI). As a next step, the electron beam is scattered as the electrons move further into the wake, which causes reduction of the wave intensity (CO), and finally, the flux decrease of electrons with small pitch angles may cause a quasi-permanent reduction in the type III intensity at lower frequencies. The spectra of type III bursts introduced in the present paper are quite similar to those introduced by Mac- Dowall [1989]. In this section, we examine distinctive features of these events in detail and discuss whether or not the model proposed by MacDowall [1989] is applicable to our observation Spectrum In event I the type III burst has two CO as shown in Plate 1. Figure I shows electric field intensity versus frequency observed from 1845 to 1915 UT on July 1, Electric field intensities averaged every I rain are overwritten in the same graph; thus intensity along the type III burst shown in Plate 1 is represented by the upper envelope of data points. Sudden spikes emerging from the envelope at several frequencies are due to artificial interference. As shown in Figure 1, the intensity is maximum around 500 khz in the higher-frequency part of the type III burst. It gradually decreases in the lower frequency part, and the intensity in the first gap at 285 khz is reduced by,,8 db compared to the maximum intensity at 500 khz. The intensity increases again at 245 khz to nearly the same level as at 500 khz, followed by a reduction of 3 db in the second gap of,,210 khz. It is notable that the intensity peaks at the maximum value in the type III burst at 150 khz. As the frequencies of the two CO about 285 khz and 210 khz are not harmonic, it is sug- gested that they are not due to two emission modes at fp and 2fp in the same region but are independently produced in different regions. In addition, this type III burst has its peak intensity at a frequency lower than those of the two CO. This is inconsistent with the model in which electrons are scattered in the turbulent region, which causes quasi-permanent reduction of intensity in the lower frequency part of the type III burst. On the other hand, distinctive point of the type III burst in event 2 is that the preceding type III burst has a normal spectrum and does not have CO or NBI, but is followed by several discrete structures (DS) at particular frequencies of 605 khz, 400 khz, etc. Figures 2a and 2b show electric field intensity versus frequency observed from 0315 to 0330 UT and 0335 to 0345 UT, respectively. In Figures 2a and 2b a peak at 635 khz is due to artificial interference. Figure 2a corresponds to the intensity along the preceding normal spectrum, and the intensity is almost constant. In Figure 2b the peak intensity around 605 and 400 khz corresponds to the DS which are marked with symbols "A" and "B" in Plate 2, respectively. This feature also cannot be thoroughly explained by MacDowall's [1989] model Relationship Between the Type III Bursts and Interplanetary Plasma Conditions MacDowall [1989] estimated speed and location of an interplanetary shock which was simultaneously observed with abnormal type III bursts. He showed that CO and NBI occur in the downstream of the shock, and thus the decrease of the CO frequency in the series of type III bursts is closely correlated with the location of the shock front. In a similar way, we discuss the relationship between the unusual spectra of type III bursts and interplanetary conditions. For the case of event 2 we could find several type

6 1706 KASAHARA ET AL.' TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE (a) (b) Frequency (khz) Frequency (khz) Figure 2. Intensity versus frequency overwrapped every I min (a) from 0315 to 0330 UT and (b) from 0335 to 0345 UT on November 17, III bursts whose spectrum structures and amplitudes exhibit discontinuity in the data set of November 17, For example (not shown), a type III burst observed around 1306 UT has an intensity gap between 440 and 480 khz. The intensity increases again below 420 khz in the lower frequency tail, which is.followed by a DS which has a less negative frequency drift from 380 down to 320 khz. Another type III burst observed around 2220 UT has several inten- sity gaps at 450, 250, 135, and 95 khz and enhanced spectrum peaks between them. Figure 3 shows the transition of intensity gaps and peaks summarized in the series of type III bursts from November 17 to 19, In Figure 3, time from 0300 UT on November 17 to 0300 UT on November 19 and the inverse of observing frequency are displayed on the horizontal and vertical axes, respectively. Since the plasma density profile in the solar wind at heliodistance R varies approximately as R -2, all the spectrum features of each type II! burst are represented by a straight line in Figure 3 plotted as 1/f versus time. There are six type III burst spectrum features during this period; these are shown by solid lines in Figure 3. On the other hand, the Geotail spacecraft crossed a shock structure around 1315 UT on the next day (November 18, 1993) when the local fp suddenly increased from 30 to 60 khz (increase of 4 times for the electron density). The speed and density of the solar wind plasma observed by IMP 8 were not available during the corresponding time interval; we use the speed of plasma flow observed by LEP [Mukai et al., 1994] on board Geotail, which was found to increase from 300 to 400 km/s at the time of the shock passage around 1315 UT on November 18 and, finally, to peak at 600 km/s at 0200 UT on November 19. Since Geotail was located in the distant tail of the Earth during this whole period, this speed is assumed to be nearly the same in the solar wind. We estimate the location of the shock front by assuming that the shock moves outward with a constant speed and the average density at the shock

7 KASAHARA ET AL.' TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE 1707 xlo , I f o.oo ' ' I ' ' I ' ' I ' ' I ' ' I ' Universal Time Shock passe e on the GEO?AIL orbit FiEure 3. Transition of nten$ity gaps and peaks in a series of type III bursts from 0 00 UT on November 17 to 0300 UT on November 19. Solid lines indicate occurrence times and frequencies of type III bursts. Intensity peaks, discrete structures (DS), and gaps are indicated by circles, squares, and crosses, respectively. Dashed lines show the time variation of local fp at the shock front assuming that the shock moved with a speed of 400 km/s (dashed line with smaller slope) and 600 km/s (dashed line with larger slope). Dash-dotted lines indicate the corresponding 2fp at the shock front. (]) Intensity peaks [] Discrete structures x Gaps front varies approximately as R -2. In the calculation we neglect the corotation of the density structure measured at the spacecraft and propagation direction of the shock with respect to the Sun-spacecraft line for simplicity. Figure 3 shows the time variation of the local fp at the shock front assuming that the shock moved with a speed of 400 km/s (dashed line with lower slope) and 600 km/s (dashed line with larger slope). Dash-dotted lines indicate the corresponding 2fp at the shock front. As is shown in Figure 3, the frequency of each spectrum feature (intensity peak, DS, and gap) changes up and down irregularly, and no particular relationship is found among them. It is also noted that these intensity fluctuations in the type III bursts generally occur at much higher frequencies than fp at the shock front and, furthermore, many of them are at frequencies higher than 2fp. For the case of event I several unusual type III bursts were observed by Geotail from 0800 UT on June 29 to 2100 UT on July 1, We can also find that there is no relationship among frequencies of gaps and intensified components in a series of type III bursts. The speed of plasma flow was km/s according to the observation of LEP on board Geotail. No significant evidence of shock encounter was recognized by Geotail during this period, but Geotail observed in situ a shock structure 3 days before this period; from 2000 UT on June 25 to 1000 UT on June Discussion In section 4, the detailed dynamic spectra of type III bursts and corresponding conditions of the interplanetary plasma were investigated. Although these features are basically consistent with the interpretation that unusual spectra are caused by density inhomogeneities in interplanetary space, there still remain disagreements with the previous studies. MacDowall [1989] proposed that radiation of type III is temporarily enhanced in some cases and then decreases because the beam of the type III electrons is scattered in a turbulent "wake" downstream of the shock. Although a band gap at 285 khz from 1851 to 1855 UT on July 1, 1994 (event 1), which

8 1708 KASAHARA ET AL.: TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE x lo -s 9. oo _ o--.. s.oo o- _ O. O0 '/I, Universal Time u -215 Figure 4. Spectrum of type III burst in event 1 plotted as 1If versus time. The onset time of the burst is generally aligned along a solid line except for the frequency part around 240 khz (1If 4.0 -a (1/kHz)). is marked by symbol "A" in Plate 1, is quite similar to the case introduced by MacDowall [1989], this mechanism might not explain the spectrum of event 1. This is because the intensity of the type III burst is enhanced again in the lower frequency part, and therefore the scattering effect on the electron beam would not be large enough to reduce the intensity below 285 khz. An alternative mechanism for the frequency gap would be explained by an effect of occultation by overdense region such as plasma cloud. If an overdense region exists between radiation source and observation point, the radiation at frequencies lower than fp in the overdense region will be masked at the observation point. Actually, if we consider the time and inverse of frequency of the type III burst on July 1, 1994 (event 1), the onset time of the burst is generally represented by a straight line except the frequency part around 240 khz as shown in Figure 4. That is, two frequency parts of the type III burst above 285 khz and below 210 khz might be smoothly connected with time and frequency by nature, but the middle frequency part between 285 khz and 210 khz would be masked by an overdense region. Moreover, a change of directivity of the type III radiation due to an Archimedean spiral of the interplanetary magnetic field may also affect the enhancement of the type III burst in the lower frequency range, though we could not determine the detailed propagation direction because of the poor time resolution of SFA. One more mechanism should be needed to make a plausible explanation for the generation of enhanced bands and/or discrete structures with prolonged tails off from the standard spectrum of type III bursts such as the frequency part around 240 khz in event 1. If the electron beam encounters an overdense region such as a plasma cloud in the solar wind, the type III radiation would be generated at that time and frequency as suggested by MacDowall [1989]. In addition, however, propagation effect around the peak density would also play an important role in the formation of the type III spectrum at an observation point. Figure 5a shows a plausible model in which a profile of radiation frequency versus time in the front of a traveling electron beam along its path is indicated by a solid line and the corresponding spectrum of the type III burst at an observation point is indicated by a coarsely hatched region. Arrows in Figure 5a indicate the delay time from the source to an observation point. The radiation generated from-behind the plasma cloud at frequencies lower than the fp at the density peak in the cloud, however, will be trapped between the two reflection walls between the source region and the plasma cloud and cannot propagate through the latter as shown in Figure 5a. The crosses in Figure 5a mean that the wave is blocked by a "density wall" in the overdense region. On the other hand, the waves generated in and behind the cloud at frequencies just above the maximum fp in the cloud propagate slowly around the peak density in the plasma cloud because

9 KASAHARA ET AL.' TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE 1709 " I'A'.'.'.'.%... (b).'.'.'.'.'.'.'.- t'.'.'.' ;.'.'.'.'. 1'.'.'.'.' '.'.'.'.'.3 t'.'.'.'.v.'.'.'.'. VA'.'.'A'.'.'.'.'fi._ V ¾.:.:-:.:.:.". :.:.:.:.:.:.:.'., (.-.'.'.'.'.'..'.'.'.','.'.'.'. (.-...,...,,.'.'.'.'.'.'.'.".',;,'.;i :,:,"'. _ =.*_..._.,,._,_,, ,. " -' -'.'&:..'.:..%:.;:.½:.2-..2:.:.'..:.' : ' ""' ' '.:,';,'.'.'.'.'.'.'.'.'.2. :.'-? '5-' 2.' :i.:½';'; _--, Figure 5. Profile of radiation frequency versus time in the front of traveling electron beam along source (solid line) and corresponding spectrum of the type III bursts at an observation point (hatched region). (a) Density profile is assumed to include a overdense region along the electron beam trajectory. Arrows indicate delay time from the source region to an observation point. Crosses mean that the radiation from the source region is blocked by the "density wall" in the overdense region. (b) The blended spectrum of a nominal spectrum of type III burst and perturbed spectra as shown in Figure 5a. the group velocity become smaller, and therefore the dynamic spectrum is stretched horizontally (finely hatched region illustrated in Figure 5a). Therefore it is possible that a frequency component which undergoes strong retardation (finely hatched region), due to a nearly zero group velocity, arrives later than the lower frequency part (lower portion of coarsely hatched region) as shown in Figure 5a. The spectrum observed around 240 khz from 1855 to 1910 UT on July 1, 1994 (event 1), can be explained by this model shown in Figure 5a. In order to explain the spectrum structures of type III burst observed on November 17, 1993 (event 2), shown in Plate 2, more complicated density models are needed. For example, if we consider that the source size (i.e., the size of a region covered by energetic electron beams) of the type III burst is large enough for the overdense region to be smaller than the source size as shown in Figure 6, the spectrum of type III burst observed at an observation point "X" in Figure 6 will be composed of a mixture of normal type III burst and the unusual type III (Figure 5a), resulting in the spectrum shown in Figure 5b. The case introduced in Plate 2 can be explained if the interplanetary medium contains several small-size overdense clouds between the Sun and the Geotail location. As shown in Figure 3, the intensity fluctuations in a series of type III burst spectra occurred irregularly at much higher frequencies than the estimated fp at the shock front and thus were not directly correlated with the shock location. It is convincing evidence that multiple turbulent and overdense regions were scattered in interplanetary space after the passage of the shock. Inhomogeneities of interplanetary space in the unshocked region were also suggested by Lecacheux et al. [1989], and our observations would provide observational clues to detect density inhomogeneities in the upstream interplanetary medium. In the above discussion, we only consider the spectrum structures of type III bursts assuming that they are excited at the local fp in the source region for simplicity, but the radiation may occur either at the fundamental, the second harmonic, or both. For example, it has been demonstrated that the type III burst at a given frequency is the fundamental emission at the onset time, followed near or after the time of the peak emission by the harmonic emission gen- erated at distance farther out from the Sun [Dulk et al., 1984; Hoang et al., 1994]. Therefore the harmonic emission may not be affected even if the œundamental radiation is perturbed (block or delayed). It should be also noted that the magnetic field lines may deviate from the spiral shape in the overdense regions. This deviation may affect the directivity of the type III burst in the generation process, but the effect of propagation, which was discussed in the paper, is also important for the explanation of our observation. It is impossible for an observer at one point to estimate the size and location of plasma clouds or reconstruct the inhomogeneoustructure of interplanetary medium, because the plasma environment in interplanetary space varies both spatially and temporally. Although several attempts have been already

10 1710 KASAHARA ET AL.' TYPE III IN INHOMOGENEOUS INTERPLANETARY SPACE ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß., ß ß ß ß ß ß ß ß ß ß ß ß ß. ß ß ß ß ß ß ß ß ß ß ß ß. ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ii ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß. ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß... ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß ß x Figure 6. A possible inhomogeneous density model with multiple clouds in interplanetary medium. A dotted region indicates the region filled by type III electrons, and coarsely and finely hatched regions represent multiple clouds in the medium. "X" is the observing location, and straight lines from 1 to 6 are propagation paths of the type III burst. IMF, interplanetary magnetic field. performed the determination of the source trajec- Acknowledgments. We are grateful to I. Nagano, tory of type III bursts by multisatellite observations H. Takano, and S. Yagitani for providing a direction- IReinet et al., 1998], it is indispensable the near fu- finding tool applicable to the Geotail-SFA data sets and ture to perform simultaneous observations using two or more separated satellites. the other Geotail PWI members for their contribution to the PWI instrument and its analysis software. Geotail

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