Advances in Space Research 35 (2005) 1789–1794 www.elsevier.com/locate/asr Observations of ‘‘zebra’’ pattern in cm-range with spatial resolution Alexander T. Altyntsev a, Alexey A. Kuznetsov a a,* , Natalya S. Meshalkina a, Yihua Yan b Department of Radioastrophysics, Institute of Solar-Terrestrial Physics SB RAS, Lermontov Street 126, Irkutsk 664033, Russia b National Astronomical Observatories, Beijing, China Received 28 September 2004; received in revised form 6 January 2005; accepted 10 January 2005 Abstract We present the results of the ﬁrst observations of the solar microwave burst with ﬁne spectral structure of zebra type at the frequency about 5.7 GHz. The burst has been detected simultaneously by the Siberian Solar Radio Telescope and by the spectropolarimeter of the National Astronomical Observatory of China. Zebra pattern consisted of three parallel stripes with complex frequency drift. The degree of circular polarization of emission reached 100%, the polarization sense corresponded to the extraordinary wave (X-mode). We have determined the plasma parameters in the emission source: plasma density about 1011 cm3, magnetic ﬁeld strength 60–80 G. We argue that in the given event the most probable mechanism of the zebra pattern generation is non-linear coupling of harmonics of Bernstein modes. 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Solar ﬂares; Microwave emission; Zebra pattern; Non-linear processes 1. Introduction Observations of radio bursts with ﬁne spectral structure of zebra type, seen as a number of parallel bright stripes in the dynamic spectrum, give unique means for the local measurements of the magnetic ﬁeld in the solar corona. In the meter and decimeter waves, the bursts with zebra pattern are observed rather frequently. Until now, the highest frequency at which a zebra pattern has been observed is 3.8 GHz (Chernov et al., 2003). There are several approaches to the interpretation of zebra-like ﬁne structure (Rosenberg, 1972; Aurass et al., 2003; Zlotnik et al., 2003; Chernov, 1976; LaBelle et al., 2003. We present the results of the ﬁrst observations of the zebra pattern burst at the frequency about 5.7 GHz. The burst occurred during the solar ﬂare on January 05, 2003 * Corresponding author. Tel.: +7 3952 437299. E-mail addresses: [email protected] (A.T. [email protected] (A.A. Kuznetsov). Altyntsev), in AR 0243. It was observed simultaneously by the Siberian Solar Radiotelescope (SSRT) and by the spectropolarimeter of the National Astronomical Observatory of China (NAOC). 2. Observation techniques The dynamic spectra of zebra structures were received by the Solar Radio Broadband Fast Dynamic Spectrometers (5.2–7.6 GHz) at the Huairou Solar Observing Station of NAOC. The reception band of the NAOC spectropolarimeter individual frequency channel is 20 MHz, and the temporal resolution is 5.9 ms. The spatial characteristics of the microwave sources were recorded by the SSRT. The SSRT is a crossed radio interferometer, consisting of two lines of antennas, the east-west (EW), and the north-south (NS), operating in the 5.67–5.79 GHz frequency range. Radio maps of the solar disk are recorded at intervals of 3–5 min. The 0273-1177/$30 2005 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2005.01.018 1790 A.T. Altyntsev et al. / Advances in Space Research 35 (2005) 1789–1794 investigations of the ﬁne temporal structure of ﬂare bursts are based on the data, recorded by the EW and NS arrays, which provide one-dimensional images (scans) of the solar disk every 14 ms simultaneously. The SSRT beam width is down to 1500 and depends on the array direction and the observation local time. We used also the data from Nobeyama spectropolarimeters, SOHO/MDI magnetograms, SOHO/EIT images in the soft ultraviolet radiation. 3. Observations The radio emission proﬁles of the ﬂare are shown in Fig. 1. The background microwave burst has a relatively low polarization degree. The spike-like pulses with duration of several seconds were observed at the frequencies below 5.7 GHz. At 3.8 GHz the right-hand polarized pulses were registered for 12 min. We managed to detect two events with zebra pattern in the dynamic spectrum. Zebra structures were observed 3 min prior to the background burst maximum. One can see the highly polarized bursts at 3.8 GHz at the same time. The NAOC dynamic spectra in 2.6– 3.8 GHz frequency range show that these bursts were short-duration wide-band pulses, without zebra structure. The brighter zebra pattern is shown in Fig. 2. The dynamic spectrum contains three bright stripes that had no frequency drift ﬁrstly. Then, the drift towards lower frequencies began, and the duration of this (decreasing) branch is about 2.5 s. One can notice the second (increasing) branch with duration up to 2 s, although this part of the burst is expressed more poorly. Thus, the spectrum has the U-like shape. The mean frequency of the event can be estimated as 5.6 GHz. The frequency gap between adjacent bright stripes is about 0.16 GHz and this value is the same during the whole burst. The instantaneous bandwidth of stripes is about 0.06 GHz. We note, that all the zebra stripes started their drift simultaneously, with the accuracy within 50 ms, which shows that the emission of all stripes has the same source. The zebra structure is recorded by both instruments in the right handed mode only. Thus, the zebra pattern circular polarization degree reaches 100%. Another observed zebra pattern consisted of four stripes without frequency drift, the frequency separation Fig. 1. The temporal proﬁles of the radio emission ﬂuxes of right (continuous curve) and left (dotted curve) handed circular polarization, recorded by the Nobeyama and Huairou spectropolarimeters (magnitude in sfu) during the ﬂare on January 05, 2003. Vertical lines mark the zebra pattern times. A.T. Altyntsev et al. / Advances in Space Research 35 (2005) 1789–1794 1791 Fig. 2. The dynamic spectrum with zebra pattern (at the top, the darker areas correspond to the emission higher intensity) and the time proﬁles, recorded by the NAOC spectropolarimeters and the SSRT linear arrays at the same frequencies. between emission stripes was 0.22 GHz. Like in the ﬁrst event, the ﬁne structures were observed only in the rightpolarized channel. Unfortunately, the determination of source spatial parameters in this case is less accurate, so we will not discuss it further. The magnetogram of the ﬂare region is shown in Fig. 3 (left panel). The magnitude of the ﬁeld in the leading spot of the S-polarity reaches 875 G; the spot of the N-polarity is displaced 4500 to the north-east. The contours in Fig. 3 show the emission brightness in the UV (195 Å) at the ﬂare peak. The UV contours show a loop connecting the two sites with the magnetic ﬁelds of opposite direction (sources 1 and 2). The distance between them is about 3500 . The crossing of the straight lines points to the brightness centroid of the zebra source, which is located near the north-east loop foot, where the magnetic ﬁeld value (at the photosphere) is about 150 G of the N-polarity (source 1). Thus, the zebra emission polarization corresponds to the extraordinary wave (X-mode). This conclusion is conﬁrmed by the coincidence of the polarization signs of the zebra pattern and the background burst. The background microwave burst is most probably caused by the gyro- synchrotron emission mechanism, which produces preferably extraordinary wave. The structure of the microwave sources at 5.7 GHz is shown in Fig. 3 (right panel). There is a correspondence of sources 1 and 2 to the sources with the right and left polarization, accordingly. The brightness center of the background ﬂare burst is close to source 1. The same structure of the background burst is seen in the NoRH radio maps at 17 GHz. The straight dashed lines (right panel) deﬁne the bands around the straight lines shown on the magnetogram (left panel). Their widths are equal to the half widths of the corresponding interferometer beams, and can be considered as the upper estimate of the error in the source location. Thus, the zebra source is located within the parallelogram formed by the straight dashed lines. The overlapping with the magnetogram shows that the magnetic ﬁeld inside the parallelogram has N-polarity. In Fig. 3, we also show the magnetic ﬁeld lines, calculated in the potential ﬁeld approximation using the MDI magnetogram. The calculation method was developed by Rudenko (2001). In Fig. 4 the one-dimensional distributions of the radio brightness (scans), recorded by the EW array, 1792 A.T. Altyntsev et al. / Advances in Space Research 35 (2005) 1789–1794 Fig. 3. Left: The magnetogram (halftone) and the UV emission map (contour) of AR 0243. The crossed straight lines show the directions of scanning by SSRT interferometers. Zebra pattern source is situated at the point of intersection of these lines. Some reconstructed magnetic ﬁeld lines are also shown. The source of zebra pattern most probably lies on the line drawn as thick. Right: The structure of the microwave emission at 5.7 GHz. The brightness temperature in intensity is shown by grayscale. The dashed lines correspond to the contours in polarization. The straight dashed lines mark the bands on the solar disk, the emission from which is recorded during the zebra pattern observation. Fig. 4. The one-dimensional distributions (intensity vs. coordinate) of the microwave emission (scans) and polarization degree, recorded by the SSRT/EW array. Top panel shows the scans of the background burst in diﬀerent polarizations and the scans of the diﬀerent stripes of zebra emission; bottom panel shows the burst polarization degree. The background burst scan is determined as the scan at the 06:06:12.274 moment, when the zebra stripes went out the SSRT receiving band. The zebra scans are obtained by subtracting the background scan from the scans, corresponding to the zebra brightening. A.T. Altyntsev et al. / Advances in Space Research 35 (2005) 1789–1794 are shown in the intensity and polarization. The relative scans of the zebra source (with subtracted background) are shown for the two consecutive moments, corresponding to recording the zebra pattern diﬀerent stripes. It is evident, that the diﬀerent stripe sources spatially coincide, and they are displaced a few arcsec to the east relative to the brightness center of the background burst. The zebra source size is less than 1000 or 7000 km, and is signiﬁcantly smaller than the background burst size. At the background burst polarization scan the positions of 130 and 16500 correspond to the loop footpoints, and one can see diﬀerent polarization signs at these points. Note that we show the relative zebra scans only for the right polarized channel. The leftpolarized emission level after background subtraction is reduced to zero, without any ﬁne structures. Using the GOES X-ray observations, we can estimate the plasma temperature in the microwave emission source as 1.1 · 107 K, and the plasma density (from the integral emission measure) as n . 7.2 · 1010 cm3. This means that the emission is generated at the frequency close to the double plasma frequency. In our opinion, the model of non-linear interaction of Bernstein waves is the most suitable to explain the polarization, spectral and temporal parameters of the observed zebra pattern. In this case, the frequency interval between adjacent bright stripes is almost equal to the electron cyclotron frequency Dm = (eB)/(2pmc) in the emission source. Therefore, the observations of a zebra pattern provide a direct method for magnetic ﬁeld magnitude measurement. In the event under consideration, we managed to detect the zebra patterns twice with frequency intervals of 220 and 160 MHz (Fig. 2), respectively. These values correspond to the magnetic ﬁeld strength B of 60–80 G in the zebra source. One of the magnetic lines connecting sources 1 and 2 is marked by the thick line in Fig. 3 (left panel). Its length is 130,000 km and its height is about 45,000 km. It is remarkable that the point of this line with the magnetic ﬁeld value of 60 G is close to the visible position of the zebra source. So, we believe that this site, marked by the cross in Fig. 3 (left panel), corresponds to the zebra source location. The height of this point above the photosphere is about 14,000 km. We also note that there were three ﬂuctuations of microwave burst intensity with period about 170 s during the ﬂare (Fig. 1). These ﬂuctuations can be caused by the MHD waves that propagate along the loop with Alfven velocity. 4. Interpretation There are several approaches to interpretation of the bursts with the ﬁne structure of zebra type, which were developed with reference to bursts in the meter and deci- 1793 meter ranges. In the observed event, the two factors are of main importance when determining the emission mechanism: 100% polarization degree, corresponding to extraordinary wave. Small source size. The small source size follows not only from the direct measurements, but also from the high synchronism of the frequency drifts of diﬀerent stripes. Indeed, it is seen in Fig. 2 that zebra stripes have no frequency drift ﬁrstly. Then, at 06:06:11.6, all zebra stripes start their drift simultaneously, with possible delay not more than 50 ms (note that here, we discuss the onset of frequency drift, not the onset of the emission itself). The change of emission frequency requires the change of source parameters (such as plasma density and magnetic ﬁeld); this change must be simultaneous in all parts of the emission source. In the magnetized plasma the time scales of variation of plasma density and magnetic ﬁeld are determined by the Alfven velocity (for the MHD waves which are the fastest disturbances aﬀecting the above mentioned plasma parameters). Thus for plasma density 1011 cm3, magnetic ﬁeld B 6 200 G and time delay Dt 6 50 ms we obtain source size r < 100 km. In our opinion, among the mechanisms that have been proposed for an interpretation of zebra patterns, the process of non-linear coalescence of Bernstein waves (Zheleznyakov and Zlotnik, 1975a,b; Mollwo and Sauer, 1977) seems to be the most suitable. It suggests, that all emission stripes are generated in the same compact source. In the observed microwave zebra pattern (Fig. 2) the ratio of frequency interval between adjacent zebra stripes to the emission frequency is Dx/x . 1/35, which corresponds to the coalescence of cyclotron harmonics with numbers about 17–18. In this process, the polarization corresponds to the extraordinary wave (Mollwo and Sauer, 1977). The numerical modeling carried out by Haruki and Sakai (2001) shows that the polarization degree of the electromagnetic waves (in the X-mode sense) can reach 100% for coupling of Bernstein modes. The investigation by Willes (1999) shows that the eﬃciency of the non-linear interaction is suﬃcient to provide radio emission with the observed intensity; it is also shown that the polarization sign depends on the speciﬁc conditions, but in most cases it corresponds to the extraordinary wave. The polarization degree can be as high as 60% in the X-mode sense. However, these investigations correspond to the conditions that somewhat diﬀer from the conditions in the observed ﬂare loop. Thus, we can conclude that the emission mechanism of the observed microwave zebra pattern is not clearly determined yet: the coalescence of Bernstein modes is the most probable model of the existing 1794 A.T. Altyntsev et al. / Advances in Space Research 35 (2005) 1789–1794 ones, but the further investigation of this process is necessary (and, possibly, a new mechanism has to be proposed). Acknowledgements This work was supported by the RFBR (02-02-39030, 03-02-16229, 04-02-39003 and 04-02-31012), Russia Department of Education (E02-3.2-489), Integratsiya project (I0208), MOST (G2000078403), NSFC (10225313 and 10333030), and CAS. References Aurass, H., Klein, K.-L., Zlotnik, E.Ya., Zaitsev, V.V. Solar type IV burst spectral ﬁne structures. I. Observations. Astron. Astrophys. 410, 1001–1010, 2003. Chernov, G.P. Microstructure in continuous emission of type IV meter bursts. Modulation of continuous emission by wave packets of whistlers. Soviet Astron. 20, 582–589, 1976. Chernov, G.P., Yan, Y.H., Fu, Q.J. A superﬁne structure in solar microwave bursts. Astron. Astrophys. 406, 1071–1081, 2003. Haruki, T., Sakai, J.-I. Electromagnetic wave emission from a dynamical current sheet with pinching and the coalescence of magnetic islands in solar ﬂare plasmas. Astrophys. J. 552, L175– L179, 2001. LaBelle, J., Treumann, R.A., Yoon, P.H., Karlicky, M. 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