On the Analogy between the Zebra Patterns in

On the Analogy between the Zebra Patterns in Radio Emission from the Sun and the Crab Pulsar V. V. Zheleznyakov, V. V. Zaitsev, and E. Ya. Zlotnik*

More than ten different models have been proposed for ZPs; most of them include some emission of electrostatic plasma waves at the upper hybrid frequency at the double plasma resonance (DPR). ωUH = (ωPe^2 + ωBe^2)^1/2 = sωBe The DPR mechanism explains only general properties of ZP, and when a tentative is made to explain some unusual features some problems arise. Fiber bursts differ from ZP stripes only by a constant negative frequency drift, and one of the first models explained the radio emission (t) of fiber bursts by the coalescence of plasma waves (l) with whistlers (w), l + w t (Kuijpers, 1975) In Chernov (1976; 1990), the mechanism l + w t was proposed as an unified model in which the formation of ZPs in the emission and absorption spectra was attributed to the oblique propagation of whistlers, while the formation of stripes with a stable negative frequency drift (the fiber bursts) was explained by the ducted propagation of waves along a magnetic trap. This model explains occasionally observed transformation of the ZP stripes into fibers and vice versa. Calculations show that the DPR-based mechanism fails to describe the generation of a large number of ZP stripes in any coronal plasma model.

Altitude dependence of the plasma frequency in accordance with the barometric law (heavy line) and altitude profiles of the electron cyclotron harmonics s (light lines) in the solar corona. For the electron temperature Te = 1. 2 ∙ 10^6 K and initial frequency f. P 0 = 3800 MHz at an altitude of h. B 0 = 20 000 km, 34 DPR levels form between 2600 to 3800 MHz in the plasma layers (from Laptuhov and Chernov, 2010).

Fig. 4. A giant interpulse from the Crab pulsar, (strong linearly polarized) observed and processed in exactly the same way as the main pulses. Total intensity time resolution 25. 6 ns; dynamic spectral resolution 19. 5 MHz and 25. 6 ns. (Eilek & Hankins, 2007)

SPATIALLY AND SPECTRALLY RESOLVED OBSERVATIONS OF A ZEBRA PATTERN IN A SOLAR DECIMETRIC RADIO BURST Bin Chen 1, 2, T. S. Bastian 2, D. E. Gary 3, and Ju Jing 4 (Astrophys. J. 736, 64, 2011) 1 Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA The most cited and used model for ZP is mechanism at DPR, especially after the paper of Chen et al. (2011) where the authors allegedly confirm this mechanism using positional observation by the system FASR. In this connection, it is necessary to note that the model with whistlers was there mistakenly rejected. In the whistler model, when we estimate Vgr = 2. 5 x 10^9 cm s-1 , that’s value in the quasi-longitudinal propagation (along the magnetic trap, it is not Vproj as authors used ). And Vproj = Vgr cos α 1 , then for big whistler angle propagation (cos α 1~0. 1) you could receive the same value of tan α 1 , as for tan α 2 (see Fig. 6). So, no problem with the whistler model. The high speed of spatial drift of the zebra sources (~0. 1 c) can be related with more high density gradient across the magnetic trap. Then, Chen et al. (2011) used convenient idealistic models for plasma density and magnetic field with exponential dependence, instead of very known barometric formula and dipole dependence for B (or model of Dulk and Mc. Lean, 1978). An attempt at the determination of the displacement of source during one stripe of zebra in phenomenon of 14. 12. 2006 with the aid of the system FASR did not succeed because of the insufficient time resolution, ~ 20 ms approximately in the same time interval. Thus, for the future, it is desirable to observe more long lasting zebra with FASR to verify this.

Simplified source model (Fig. 14 from Chen et al. (2011) using positional observation by FASR ), supplemented by the scheme of vectors of the group velocity of whistlers

New models of zebra-patterns • • • The continuous discussions stimulate the developments of new models. Treumann et al. (2011) proposed new mechanism of ZP, the ion-cyclotron maser. Thanks to the special delta-shaped distribution function of the accelerated ions, the ioncyclotron maser generates a number of electromagnetic ion-cyclotron harmonics which modulate the electron maser emission. A part of the accelerated relativistic protons passes along the magnetic field across the trapped loss-cone electron distribution. The modulation of the loss-cone will necessarily cause a modulation of the electron cyclotron maser. Locally this produces the typical “Zebra” emission/absorption bands. However this mechanism can work in the strong magnetic field, when fpe/fce < 1. Karlicky et al. (2013) continued the development of the model of Kuznetsov (2006) for fiber bursts: fiber bursts can be explained by the propagating fast sausage magnetoacoustic wave train. Then Karlicky (2013) extended a similar model for ZP: the magnetoacoustic waves with density variations modulate the radio continua, and this modulation generates zebra effects. It should be noted that close model was examined earlier in three works by Laptuhov and Chernov (2006; 2009; 2012). Yurovsky (2011) continued a series of works about the formation ZP due to the refraction (interference mechanism) of rays on the heterogeneities in the corona. However the author doesn't refer to the previous works (Ledenev et al. 2006), and carried out the simplified simulation of ZP stripes which is well coinciding with those observed.

• In contrast to all the above models, only • • the model with the whistlers allows to explain many special features the zebra- structure: - the oscillatory frequency drift and the frequency splitting of stripes, - a change in the spatial drift of radio source synchronously with the frequency drift of stripes in the spectrum, - the millisecond superfine structure of stripes. • It should be noted that the relative significance of several recent possible mechanisms remains uncertain. • Simultaneous or consecutive appearance the zebra- structure in different frequency ranges is obviously connected with the dynamics of flare processes.

2. Last observations Fig. 1. Zebra- structure and pulsation during 13 sec registered by the SBRS/Huairou after the second flare ejection. The numerous stripes of zebra- structure reveal the superfine structure in the form of millisecond spikes. Radio emission is not polarized.

Fig. 5. Two frames from the movie of SDO/AIA 171 Å showing the beginning of the second ejection (24. 02. 2011) at 07: 37: 02 UT ( on the left) and its continuation at 07: 38: 48 UT (on the right). In the right frame the ejection has form of a magnetic reconnection with a X- point. The thickness of the loop bases under X- point is of ≤ 1".

New events with ZP. 11 April 2013 event was simultaneously observed in 4 groundbased observatories and SDO/AIA

Fig. 5. The 1 December 2004 event. Fiber bursts are immerged into zebra pattern. Zebra stripes consist of spikes, however fiber bursts are almost continuous (uninterrupted). It is very important to know whether both elements have the same radio source?

• Now, we hope on the progress of the solar radio spectral imaging observations. The Chinese spectral radioheliograph (CSRH) is a new generation solar radio telescope which will be the largest and most advanced radio imaging telescope for solar corona in the world. It can provide true imaging spectroscope with high temporal, spatial, and spectral resolutions, covering decimeter and centimeter wavelengths. • In decimeter range (0. 4 – 2 GHz) 40 antennas with diameter of 4. 5 m CSRH I will provide space resolution 10. 3’’ at 2 GHz, and CSRH II with 60 antennas with diameter of 2 m will provide space resolution 1. 4’’ at 15 GHz (Yan et al. 2012; Yan et al. 2013). • New possibilities we also wait from upgraded SSRT; 96 antenna radioheliograph in the range 4 – 8 GHz will construct one image at one frequency each 0. 1 – 1 c (Lesovoi et al. 2013).

• Conclusions • We demonstrated several events with radio fine structures in which the positional observations could be a determining factor for the selection of the radio emission mechanism. • It is very important to compare the source sizes of continuum and different fine structures, and to know whether the radio source is moving (shock wave) or not. • Radio sources of fiber bursts and ZP in the whistler model must have moving sources, and the spatial drift of ZP stripes should change synchronously with changes of frequency drift in the dynamical spectrum. • In the DPR model the ZP source must be rather stationary.