Channel -55. Channel NRH 237 MHz. NRH 410 MHz

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1 Metric Radio Emission Associated With X-ray Plasmoid Ejections M. R. Kundu 1, A. Nindos 1;2, N. Vilmer 3, K.{L. Klein 3, K. Shibata 4, M. Ohyama 1 1 Astronomy Department, University of Maryland, College Park, MD Observatoire de Paris, Section de Meudon, DASOP, CNRS-UMR 8645, F Meudon, France. 4 Kwasan and Hida Observatories, Kyoto University, Yamashina, Kyoto , Japan. Abstract In this paper we report the rst detection of metric/decimetric radio emission associated with two soft X{ray plasmoid ejecta events that occurred during two limb ares observed by the Yohkoh SXT. In the rst event a loop started to rise slowly ( 10 km/sec) before the beginning of the hard X{ray impulsive phase of the are. At about the onset of the impulsive are, there was acceleration of the ejecta, resulting in a speed of 130 km/sec and nally to 200 km/sec. The associated radio emission was observed with the Nancay radioheliograph (NRH) in the frequency range of 230{450 MHz. It was an unpolarized continuum that lasted 8{10 min. The 410 MHz source was located close to the height where the plasmoid was last identied in the SXT images. In the second event an eruption resulted in the expansion of a large{scale loop{like feature and the development of two plasmoid ejecta which moved in dierent directions. The speed of the ejecta was 60{100 km/sec. In this event, the associated radio emission was a long{lasting (about 2 hours) continuum observed from 450 to 164 MHz. The onset of the low frequency emission was delayed with respect to the onset of the high frequency emission. In both cases the radio sources were located above the soft X-ray ejecta in the general direction of the prolongation of the ejecta movement. In both cases the radio emission is attributed to the radiation of non thermal electrons accelerated in close relationship with the propagation of the X-ray plasmoid. We discuss possible scenarios and we indicate that magnetic reconnection which leads to the production of the soft X{ray plasmoid ejecta may also provide the stage for the acceleration of these electrons. 1 Introduction The discovery of compact impulsive hard X-ray sources above the soft X-ray aring loops in some impulsive limb ares (Masuda 1994; Masuda et al. 1995) led to the revival of the reconnection model of solar ares. Specically, it is possible to 2 Current address: Section of Astrogeophysics, Physics Department, University of Ioannina, Ioannina, GR-45110, Greece. 1

2 interpret the above{the{loop{top hard X{ray source as due to magnetic reconnection occuring above the soft X{ray loop. Such magnetic reconnection process responsible for solar ares is intrinsic to some are models such as the Carmichael{Sturrock{ Hirayama{Kopp{Pneuman (CSHKP) model of ares (Carmichael 1964; Sturrock 1966; Hirayama 1974; Kopp & Pneuman 1976) which can account for the post{ are loop prominence systems, X{ray arcades as well as the creation of cusp{shaped magnetic structures. A consequence of impulsive ares resulting from magnetic reconnection process as in CSHKP model is that one expects a plasmoid ejection to take place above the soft X{ray aring loop. It turns out that such plasmoid ejecta have been found (Shibata et al. 1995) in all eight of Masuda's compact impulsive limb ares; the ejecta had apparent velocities of 50{400 km/sec. This nding led Shibata et al. (1995) to suggest that impulsive ares occur through a magnetic reconnection process, for example as proposed in CSHKP model. As a follow up of this suggestion, Ohyama & Shibata (1997,1998) studied several plasmoid ejection events and derived their physical parameters. They found that the acceleration of the ejected loop occurs just before or at about the onset of the impulsive phase of the are. The temperature, mass and apparent velocity of the ejected material are 6{14 MK, 2? gr and 200{500 km/sec respectively; the kinetic energy is smaller than its thermal energy, which also is smaller than that of the aring loop. It will clearly be of interest to nd if this acceleration process results in the production of nonthermal energetic electrons which may be responsible for radio burst emission. Since the height of X{ray plasmoid ejecta may reach values as large as 2? km high in the corona, it is logical to look for radio burst emission in the meter wavelength domain. The environment of the soft X{ray plasmoid ejections is quite dierent from the environments investigated in other studies of the relationship between soft X{ ray moving features and their associated radio emission. For example, Kundu et al. (1994, 1995a, 1995b) and Raulin et al. (1996a, 1996b) studied metric type III emission produced by beams of electrons associated with soft X{ray jets and aring X{ray bright points. On the other hand, Gopalswamy et al. (1997) studied a plasmoid which was produced after a soft X{ray eruption. Gopalswamy et al. (1997) speculated that a possible moving type IV burst was located in the vicinity of the X{ray plasmoid. Klein et al. (1999) studied a rapidly expanding or disrupting soft X{ray stucture which produced a plasma blob similar to the plasmoid ejecta of the 1993 November 11 event. The speed of that ejected blob was high (about 770 km/sec) and the shock waves that were produced were detected in type II metric emission. The best radio telescope for such observations is the Nancay (France) metric radioheliograph which operates at 5 frequencies: 164, (henceforth referred to as 237 MHz), 327, 410 and 435 MHz. For the time period of observations of the plasmoid ejections reported by Ohyama & Shibata (1997,1998), the Nancay radioheliograph (NRH; The Radioheliograph Group 1993) operated as two one{dimensional arrays, East{West and North{South. Its spatial resolution is frequency{dependent; 2

3 it also varies as a function of time in course of the year. In general, its spatial resolution ranges from approximately 1 0 to several minutes of arc. Among the dierent plasmoid ejections studied by Ohyama & Shibata (1997,1998), only two of them corresponded to time periods when the Nancay (France) metric radioheliograph was operating: (1) 1993 November 11, (2) 1993 February The 1993 November 11 event The X{ray plasmoid ejection was associated with a are which occurred near the eastern limb. The event was observed by Yohkoh soft X{ray telescope (SXT). Ohyama & Shibata (1997) studied the physical conditions and morphological evolution of the ejecta and are. Their Fig. 2 shows the evolution of the event as it appears in SXT images. They found that a loop which formed near the main aring loop between 10:55:49 and 11:04:21 UT, started to rise slowly at about 11:05 UT. The upward motion started after its footpoint brightened. Our gure 1 shows the height of the ejecta as a function of time and also the soft X{ray light curve (from GOES satellite) and the hard X{ray light curves from the L (14{23 kev) and M1 (23{33 kev) channels of the HXT (Hard X{ray Telescope) instrument on board Yohkoh. Fig. 1 shows that the loop started to rise slowly (at 10 km/sec, the \preare slow rise") long before the hard X{ray impulsive phase of the are. At about 11:15 UT, it suddenly accelerated to 130 km/sec just before or at about the onset of the impulsive phase (the \main rise"). After a couple of minutes it reached a velocity of 200 km/sec. The motion of the plasmoid ejecta could be followed in SXT partial frame images (PFIs) until about 11:26 UT, although with decreased intensity. At the same time the main aring loop also started to rise with a speed of about 8{10 km/sec. Its intensity increased until 11:26 UT. That loop could be followed in PFIs until about 11:30 UT and its projected height was always an order of magnitude smaller than the projected height of the ejecta. Ohyama & Shibata (1997) computed the plasma parameters of the ejected material. They found that it was already heated to 11:3 4 MK before the \main rise". The electron density of the ejecta (4:5 10 9? 1: cm?3 ) was larger than the typical density of the active region corona. The mass of the ejecta was about gr. Ohyama & Shibata (1997) suggested that magnetic reconnection may already be occurring in the preare phase, leading to heating of the ejected material and causing chromospheric evaporation. The 23{33 kev hard X{ray time prole of Fig. 1 shows an impulsive burst between 11:15 and 11:19 UT, with a maximum at 11:17:40 UT. Microwave emission from 950 MHz to 9 GHz was reported in Solar Geophysical Data Reports (597-II) during the same time interval, but no counterpart was detected by the NRH, i.e. at frequencies below 432 MHz. Hard X{ray and microwave emission, which reect the presence of energetic electrons in the low corona, occur at the time the soft X-ray plasmoid is observed to accelerate its ascent (see gure 1). While at energies above 3

4 23 kev the count rates fall to background after 11:19 UT, a gradual rise occurs in the 14{23 kev channel, together with the plasmoid's ascent to greater altitude. The soft X-ray ux continues to rise after the end of the hard X-ray burst above 23 kev, albeit more slowly than before. The soft X-ray ux would be expected to decay after the hard X-ray burst in the light of earlier studies involving hard X-rays 25 kev (Dennis and Zarro 1993). The slower rise observed in the present case is due to the superposition of a new episode of energy release, with a signature at h < 23 kev, on the decay of the impulsive soft X-ray emission. This new episode is associated with decimetric emission in a well dened frequency range 230{450 MHz (see Fig. 1, top panel for the 410 MHz NRH emission). Tremsdorf Observatory data indicate that there was no counterpart at 640 MHz. Hence the radio emission had a high frequency cuto between 432 and 640 MHz. The brightest radio emission occurs between 11:27 and 11:28 UT, during the decay of the 14{23 kev X-rays and close to the maximum of the soft X-ray GOES ux. In the frequency range between 230 and 450 MHz, the radio emission evolution from its onset to the time of maximum was similar at each frequency. But the lower frequency emission decayed more slowly. Analysis of the NRH data at 237 and 410 MHz with 0.2 s time resolution shows no signicant ne structure on time scales shorter than 10 s during the radio emission enhancement. The similar behavior over a band of 200 MHz and the absence of ne structure show that the emission, including the brightening between 11:27 and 11:28 UT, is a continuum. Its clear cuto towards lower frequencies suggests that the emission is coming from a well conned nonthermal electron population, while its roughly simultaneous onset with the second kev hard X{ray peak may indicate a common source of energetic electrons. No signicant circular polarization was observed by the NRH at any frequency. For the computation of the heliographic positions of the radio sources from the one-dimensional NRH scans, a pre-event scan was subtracted from the are data in order to better reveal the faint radio source. This was especially necessary for the observations with the north-south array, where the source under study was dicult to see due to the quiet Sun background emission at higher frequencies and the presence of a noise storm above the western limb at lower frequencies. Then the centroid positions and half widths of the radio sources were extracted from the one-dimensional scans by gaussian ts. The centroid position of the noise storm was scanned at 164 MHz between 11:00 and 11:30 UT to detect ionospheric gravity (164 MHz)2 f 2 waves. We applied a correction of 0:8 arcmin to the east-west positions at 11:27 UT, where f is the observing frequency. The north-south positions were not signicantly perturbed during the time interval of interest. The corrected onedimensional centroid positions and half widths were used for the computation of the heliographic positions of the radio sources. During the whole event the radio source size (full width at half maximum of the gaussian) was more than twice the beam dimension. This may be due to unresolved internal structure. The width of the crosses in Figure 2 is hence a measure of the extent of the sources, not of the antenna beam. 4

5 Channel Channel NRH 237 MHz NRH 410 MHz Figure 1: The time evolution of the 1993 November 11 event. Top panel: Grey{ scale representation of the one{dimensional brightness distribution as a function of time and position observed at 410 MHz near the eastern solar limb by the NRH. The vertical axis denotes distance from disk center (located in channel 0) along the terrestrial east-west direction (east is up). Integration time is 10 s. Second panel: The height of the plasmoid ejecta as a function of time. Squares, triangles and diamonds indicate data taken with full, half and quarter resolution, respectively (from Ohyama & Shibata 1997). Third panel: the soft X{ray ux time prole (from GOES satellite). The other two panels show the time proles of the hard X{ray emission observed with the HXT in the energy ranges of 14{23 kev and 23{33 kev. 5

6 The radio sources at all NRH frequencies are located above the eastern solar limb. The location of the radio sources observed between 435 and 327 MHz is in the general direction of the prolongation of the ejecta movement. The centroid positions and half widths of the radio sources at the time of brightest emission ( 11:27:30 UT) are plotted in Figure 2 on the SXT partial frame image taken at 11:18:19 UT. That image was the most suitable image to show the ejected plasmoid. The 237 MHz source is broader, presumably due to confusion in the north-south images with the noise storm above the western limb. Confusion with the noise storm source probably also shifts the centroid of the 237 MHz source southward, as seen in Figure 2. There is no signicant dierence of the source heights at 435, 410, and 327 MHz. The 237 MHz source seems to lie somewhat higher, but its intrinsic complexity suggests that its centroid position shown in Fig. 2 may be too uncertain to be used for the study of the dispersion of the source positions with frequency. The 410 MHz source is about 1: km above the photosphere, close to the height where the plasmoid is last identied in the SXT observations (Figure 1). The timing of the continuum emission as well as the location of the 410 MHz source suggest a strong relationship between the X-ray ejecta and the appearance of the radio emission, since the radio emission occurs at about the time when the X-ray structure reaches the height of the radio sources. 3 The 1993 February 17 event The second event that we studied was associated with a are that occurred close to the west limb. This event was presented by Shibata et al. (1994). The soft X{ray morphology of the event is presented in Fig. 3 while the time proles of the soft X{ray and hard X{ray emission as well as the height of the ejecta as a function of time and the associated radio emission are shown in Fig. 4. The top left panel of Fig. 3 shows a loop{like feature in the west{central part of the image. Its size was about km and it erupted after 10:35:22 UT. The eruption lasted until about 10:46 UT. The resulting expanding loop (labeled \L" in Fig. 3 and \Loop" in Fig. 4) initially seems to have an X{type or cusp{like structure. The expansion velocity was about 60{70 km/sec. At 10:38:22 UT, material is seen to erupt from the northwestern edge of the expanding loop{like feature. This ejecta is labeled \F" in Fig. 3 and \Front" in Fig. 4 and its velocity was about 100 km/sec. A secondary ejection feature occurred after 10:39 UT just above the main soft X{ray loop. This ejecta is labeled \B" in Fig. 3 and \Blob" in Fig. 4. The blob could be followed until about 10:46 UT and its velocity was about 70 km/sec. The size of the blob was about 10 4 km. The event was also observed in hard X{rays with the HXT (see Fig. 4). The 23{33 kev HXT observations show that the burst starts at about 10:35 UT, approximately at the time when the soft X{ray eruption started. The 23{33 kev hard X{ray emission has a maximum at about 10:36 UT and then decays smoothly until about 10:45 UT. At lower photon energies (14-23 kev) the initial 6

7 11-NOV-1993 Figure 2: SXT partial frame image observed at 11:18:19 UT with the AlMg \sandwich" lter and superposed centroid positions and half widths of the radio sources as measured at 11:27:24 UT at 410 MHz (solid line) and 327 MHz (dashed line), and at 11:27:30 UT at 237 MHz (dotted line). 7

8 Figure 3: SXT partial{frame images which show the 1993 February 17 ejecta. North is up and West to the right. decay is followed by a nearly constant count rate level from 10:40 to 10:45 UT, after which the emission decays until about 10:55 UT. Enhanced microwave emission is reported during the same time interval as the hard X-ray burst. The time evolution of the radio emission at 237 and 410 MHz is displayed in the grey-scale plots of Fig. 4 and in the ux density time proles of Fig. 5. The emission consists of a long{lasting broadband continuum (\storm continuum" or \stationary type IV burst") and superposed bursts near the western edge of the NRH eld of view. The total duration of the radio emission was about 2 hours. A group of type III/U bursts at 10:40 UT (Artemis spectrograph, Space Res. Dept. of Paris Observatory, courtesy of M.Poquerusse) occurred near central meridian, and was probably not related to the X-ray ejecta at the western limb. The continuum, not much brighter than the quiet Sun, consists of components with dierent temporal evolution (Fig. 4). At all frequencies the emission spreads westward (downward in the grey-scale plots) in the course of the event due to the brightening of a new source. The decimetric and metric radio emission starts only in the decay phase of the impulsive X-ray burst. Its onset is accompanied by a new rise of the soft X-ray 8

9 NRH 237 MHz NRH 410 MHz Figure 4: Same as in Fig. 1 for the 1993 February 17 event. The only exception is that we present the radio emission at two frequencies: 237 MHz and 410 MHz. In the panels that show the radio emission, the vertical axes represent the distance from disk center along the terrestrial east-west direction (east is up). The length of the vertical axis is about 14 0 at each frequency. Integration time is 10 s. The center of the solar disk is in channel 0. Only the western limb is shown. 9

10 GOES nm Relative flux density (linear scale) 164 MHz 237 MHz 327 MHz 410 MHz 10:40 11:00 11:20 Universal time (17-Feb-93) Figure 5: Flux density of the decimetric and metric radio emission in the region of the storm continuum sources (integration time: 10 s) and of the soft X-ray emission (integration time: 3 s). The soft X-ray time histories represent the ux (dashed line) and its time derivative (solid line) computed after smoothing over 15 points. At 237 MHz the solid line represents the lower source (i.e. the eastern source in the grey scale plot of Fig. 4) and the dotted line shows the higher source. The background shading distinguishes dierent intervals during the event as discussed in the text. 10

11 ux (see the GOES plots of gs. 4, 5). The NRH data showed no evidence for perturbations of the radio source positions by ionospheric gravity waves during the event. We extracted the one{dimensional source centroid positions and half widths by tting a gaussian or the sum of a gaussian and a straight line to the one{dimensional NRH data. In the scans made with the north-south array of the NRH, the weak continuum emission is dicult to separate from the quiet Sun. An average one-dimensional scan computed between 10:30 and 10:38 UT was subtracted in order to better reveal the continuum source. The sizes of the extracted sources (at half maximum) are typically three times the beam width of the arrays. The positions of the radio sources with respect to the soft X{ray ejecta are shown in Fig. 6. The ux density time history (Fig. 5) shows a sequence of events during which the storm continuum extends stepwise to successively lower frequencies. The dierent time intervals are marked by dierent background shadings in Fig. 5: The earliest emission occurs at decimetric waves from about 10:37 to 10:50 UT. Fig. 6 (top left panel) shows that the location of the 410 and 327 MHz sources is just above the ejecta. The sources are aligned in the direction of the soft X{ ray ejecta and show a dispersion of source heights that is typical of the plasma emission process for storm continuum radiation. The radiation is unpolarized at 410 and 327 MHz. The only low-frequency counterpart is a partially righthand polarized short burst at 237 MHz from a high coronal source (dasheddotted cross). In a second step (10:50-11 UT, Fig. 5) the storm continuum makes a more pronounced rise at 327 MHz, and starts at 237 MHz. The sources at 410, 327, and 237 MHz form a compact conguration (Fig.6, top right panel). Near 11:00 new rises in emission occur from 410 to 237 MHz. The dispersion of the source centroid positions tends to decrease from 10:44 to 11:12 UT. This suggests that the radio emission comes from a structure which is directed out of the plane of the sky such that dierent radio sources at dierent frequencies eventually have similar plane-of-the-sky positions. The strong 164 MHz bursts between 11:13 and 11:17 belong to a type III group according to Artemis spectrograph data. They come from a high coronal source far south of the storm continuum complex (Fig. 6, middle left panel, dotted cross). Less intense type III bursts occurred at a similar position earlier (near 10:57 UT: Fig. 5). At 164 MHz the storm continuum emission starts around 11:20 UT. The onset is masked in the time proles (Fig. 5) by the type III bursts. The ux density at 327 MHz and higher frequencies, which was decreasing at the end of the previous period, shows bursts superposed on a roughly constant background. This is also the case for the 237 MHz source seen previously. However, a new 11

12 10:44 UT 10:56 UT 11:12 UT 11:36 UT 11:52 UT 12:20 UT Figure 6: Centroid positions and half widths of radio sources during the 1993 February 17 event superposed on the Yohkoh-SXT partial frame image obtained at 10:42:52 UT with the AlMg lter. This is the SXT image where the expanding coronal structures are best seen. Centroid positions and half widths of radio sources are plotted as solid (410 MHz), dashed (327 MHz), dashed-dotted (237 MHz) and dotted lines (164 MHz). The time labels indicate the middle of the 4{minute intervals used for the determination of the radio source parameters. 12

13 237 MHz source brightens (dashed line in gure 5); the evolution of this new source is similar to the evolution of the 164 MHz source. The westward spread of the 237 MHz emission in the grey scale plots of Fig. 4 reects in fact the presence of the new source. Fig. 6 (middle right panel) shows that at 11:36 UT, the 164 MHz source is located above the others and the 237 MHz source appears broader than before in the east{west array scans due to the rise of the new emission. The two 237 MHz sources can be distinguished later on in the east-west scans, but form a single broad source in the north-south scans. At 11:52 UT (Fig. 6, bottom left panel) there is a group of sources from 410 to 237 MHz just above the expanding soft X-ray structures and another group (237 and 164 MHz) at greater projected height. The latter sources are about 10% right-hand circularly polarized. The two groups of sources may be associated with emission from two magnetic ux tubes which have dierent orientations with respect to the observer. The conguration is similar during the decay of the continuum (e.g. Fig.6, bottom right panel). Now the 410 MHz source can no longer be reliably distinguished from the quiet Sun in the northsouth scans of the NRH. Comparison of the radio time histories with the derivative of the soft X-ray ux in Fig. 5 suggests some relationship between the stepwise evolution of the storm continuum and the temporal evolution of energy release to the soft X-ray plasma. While the rst decimetric signatures of the continuum coincide with a post-impulsive soft X-ray brightening as indicated by the second peak in the derivative, the changes at 10:50 and 11:00 UT occur together with changes in the slope of the decaying soft X-ray ux. 4 Discussion In both events that we studied, the plasmoid ejecta were associated with metric/decimetric emission. In this range the radio emission started signicantly after the impulsive hard X-ray and microwave bursts. It was accompanied either by distinct rises of the soft X-ray emission or by changing slopes in its time history, suggesting new energy releases. In the two cases studied here, the radio sources are located above the soft X{ray ejecta in the general direction of the prolongation of the ejecta movement. We suggest that the radio emission comes from plasma emissions of non-thermal electrons produced in connection with the propagation of the plasmoid. Plasma emission is indeed the natural explanation of the frequency dispersion of source heights on 1993 February 17. Frequency dispersion of source heights is less pronounced for the 1993 November 11 event. However, the average density at the easternmost edge of the plasmoid derived from the analysis by the lter ratio technique (Hara et al. 1992) of SXT images obtained in the Al.1{lter at 11:25:57 UT and in the Be119{lter at 11:26:29 UT is 2: cm?3 in good consistency with the ambient electron 13

14 density required for fundamental plasma emission at 410 MHz, thus supporting the suggestion that the radio emission arises from plasma emission of electrons produced by the propagating plasmoid. To obtain this density estimate, we extrapolated the Be images linearly to the time of the Al.1 image. The resulting emission measures are then converted to densities assuming the lling factor to be unity and a range of thickness at each location comparable with the apparent width of the plasmoid. The computation of the emission measures are performed using a Fe/H abundance in the corona which is 4 times larger than the photospheric value (White et al. 2000). As stated above, we thus need energetic electrons of several tens of kev to produce the continuum radiation and therefore we must have some means of accelerating electrons to energies in excess of 10 kev. The observations at long decimetric/metric wavelengths show that this electron acceleration and injection to increasing coronal heights is linked in the present cases to the propagation of the plasmoid. The plasmoid associated events studied here, as well as another one analyzed by Kliem et al. (2000), appear to occur in an initially well conned structure. The prominent features in the SXT images are too dense ( cm?3 at the time of the impulsive hard X-ray emission) for the propagation of radio waves at frequencies below about 600 MHz. The clear low-frequency cuto of the microwave spectrum shows conversely that electrons accelerated during the impulsive phase are only injected into dense structures in the low corona. Radio emission at long decimetric and metric wavelengths is the signature of distinct later episodes of energy conversion that appears to occur at greater heights than the impulsive energy release and seems to be associated with the propagation of the plasmoid. This late coronal energy release is extended in time. This is shown by the association of the metric/decimetric radio emission with the gradual production of kev hard X-rays on 11 November 1993 and by the long duration of the storm continuum on 17 February As the lifetime of electrons of 10 kev to some tens of kev which likely generate this emission is a few tens of seconds or less (cf. Raulin and Klein 1994, Klein 1998) in the low corona, radiating electrons must be continuously produced during the time extended coronal energy release. However, as suggested by the observations of 17 February 1993, the delayed acceleration itself may consist of several distinct episodes. The radio emission proceeds indeed in successive steps of 10 { 20 minute duration, each of them extending to lower frequencies than the preceding one, and this evolution is associated with dierent behaviors of the slope of the soft X-ray time prole. The last of these episodes, when the storm continuum appears at 164 MHz, shows the emissions at 237MHz and 164 MHz in a distinct coronal structure while emissions at 410 MHz, 327 MHz, and also 237 MHz are still produced in the structure which was radiating earlier. The extension of the emission towards lower frequencies proceeds as if the accelerators acting during successive intervals were able to inject particles into a growing range of magnetic structures, starting with relatively compact loops in the low active region corona and extending to loops which lie successively farther out. This is physically dierent from e.g. a type II burst, whose frequency drift is ascribed to a shock wave propagating through regions 14

15 of decreasing ambient density. We note that the microwave spectra of the plasmoidassociated event of 5 October 1992 show similar evidence of a stepwise progression towards lower frequencies already during the impulsive phase (Figure 1 of Kliem et al. 2000), in general coincidence with distinct episodes of electron acceleration seen in hard X-rays. For the two events studied here, the close relationship between the time and location of the long decimetric radio source and the position of the plasmoid at the time of the rst appearance of the radio source strongly suggests that a possible injection of energetic electrons in a range of magnetic structures may be associated with the propagation of the plasmoid. This may be due either to the successive opening of or access to previously closed magnetic elds above the active region in the course of the plasmoid's ascent or to its interaction on its way outwards with increasingly larger scale magnetic eld lines creating new coronal sites of nonthermal electron production. There are however dierences in the behaviour of the radio emission at long decimetric and metric wavelengths during the three plasmoid-associated events studied so far: - no emission at all below 600 MHz (5 October 1992; Kliem et al. 2000; Aurass, pers. comm. 2000), - delayed emission lasting 10 min (11 November 1993) and 2 hrs (17 February 1993) (present study). One may speculate that the three events refer to a range of physical environments at great altitude, each with dierent ability to accelerate electrons. We note also that the moving soft X-ray structures (plasmoids and loops) are fastest (500 km s?1 ) for the metric/decimetric radio-silent event of 5 October 1992, and slowest (100 km s?1 ) for the long-duration event of 17 February This is further evidence that the acceleration process is not directly related with the macroscopic speed of the plasma structures, e.g. it is not due to the propagation of a large-scale shock wave. Regarding the two events studied here, the comparison of the soft X-ray behaviour gives some clues to understanding the dierence seen at radio wavelengths (total duration and temporal evolution) as well as some good arguments for the speculation that the dierent behaviours of reported plasmoid associated events may be due to the dierent physical environments. The total duration of the soft X-ray are is only about 1.5 hours for the 1993 November 11 event, but more than 4.5 hours on 1993 February 17. Only one ejecta is observed in the 1993 November 11 event while in the 1993 February 17 event there is a large{scale expanding loop{like feature and two separate ejecta (named \front" and \blob" in section 3) moving in dierent directions, as well as a system of post-are loops that are observed at increasing heights from 11:57 to 12:23 UT. The observations hence suggest that prolonged coronal electron acceleration is associated with prolonged plasma heating and that both processes may be associated with the ambient magnetic structure. Indeed, the observations on previous days of SXR connections between the aring active region of the 1993 February 17 event and neighbouring active regions suggest the presence of large scale loops in connection with this event: during the propagation outwards, the plasmoids may thus encounter a stronger pre-existing magnetic ux which could prevent fast propagation (the plasmoid associated with this event is the slowest one), but which could 15

16 also lead to many more and long lasting interactions. Klein et al. (1999) also reported decimetric/metric radio emission in association with the propagation of a fast plasma blob. However, contrary to the case studied here, the speed of the ejecta was quite high and the shock wave which was created resulted in the generation of a decimetric/metric type II burst. In the present events where the speeds of the plasmoid are much smaller the decimetric/metric radio emission produced are continua and no type II bursts are associated with the ejecta. This is probably due to the fact that the velocities of the ejecta in the events that we studied are smaller (about 100{200 km/sec in the 1993 November event and about 60{110 km/sec in the 1993 February 17 event) and no fast coronal magnetohydrodynamic shock can be produced. 5 Conclusions The two events that we studied provide the rst detections of long decimetric/metric radio emission associated with X{ray plasmoid ejections. The observations can be summarized as follows: impulsive HXR and microwave emissions are observed without counterparts at long dm-m waves. emission at long dm-m wavelengths is seen when the ejected structure attains the appropriate height in the corona. radio emission sources are located in the direction of propagation of the ejecta in close temporal and spatial relationship with the arrival of the ejected structure at appropriate heights. time-extended emission proceeds to lower frequencies in a stepward manner for the long duration event. Time-extended acceleration and injection of energetic electrons in a growing range of magnetic structures is thus linked to the propagation of the plasmoid. This coronal acceleration of electrons may arise when the plasmoid propagating outwards encounters and interacts with increasingly more extended magnetic eld lines thus creating new coronal sites of production of nonthermal electrons. The time extended production of nonthermal electrons, over an extended height range is not compatible with the acceleration by a single propagating exciter such as a rising point of reconnection or a coronal shock wave (whatever its nature). 6 ACKNOWLEDGMENTS This research at the University of Maryland was carried out with support from NSF grants ATM 96{12738, ATM 99{09809, INT 98{19917 and NASA grants NAG 16

17 5{8192 and NAG 5{7901. AN would like to thank Dr. S. M. White for valuable discussions and advice. KLK acknowledges data supplied by H. Aurass and M. Poquerusse. The Nancay Radio Observatory is funded by the French Ministry of Education, the CNRS and the Region Centre. 17

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