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Astron. Astrophys. 355, 1152-1159 (2000)

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4. Discussion

Suematsu et al. (1992) have suggested that spicules are generated by an impulsive upward force on the atmosphere such as shock wave propagating upwards and its seed perturbation might take place at a layer deeper than the chromosphere. We have studied here the effect of these giant macro-spicules or small scale jets on the coronal plasma, particularly the plume conditions. In the first part we have studied the temporal evolution of the macro-spicule in our time series and in the second part, with various power spectra analysis techniques, studied the dynamics of the plumes.

Loucif (1994) have classified the existence of two classes of macro-spicules in the high chromosphere-transition zone: (a)- `classical' macro-spicules, (b)- macro-spicules which evolve up to a critical height beyond which they eject a plasmoid. These plasmoids are impulsive, recurrent and reach the corona. Some of their velocities are supersonic; a succinct estimate of mass flux seems to confirm the in situ fast solar wind measurements realised by rocket flights.

Our observations suggest that we have encountered the `b' type, the more dynamic macro-spicule type, in our temporal series. Following Pike & Mason (1998) we suggest that macro-spicules have a rotating as well as a jet-like structure. Over a considerable time interval we were able to show the continued existence of two velocity components, which further indicate that the plasma is rotating. Kudoh & Shibata (1997) have proposed a theoretical model for spicules which is based on the concept of a rotating flux tube. They report MHD simulations of torsional Alfvén waves propagating through open magnetic flux tubes. They assume that the Alfvén waves are generated by random foot-point motions in the photosphere. Their model exhibits local enhancement of the O V emission. Our observed macro-spicule shows bright emission at its base. It has been detected not only in O V (250, 000 K) but also in Mg IX which is characteristic of a 1 million K plasma. We also find evidence of plasma acceleration between heights of 20-45 arc sec and then a plateau is reached. In the left side of the macro-spicule (see Fig. 4), we have found red-shifts of up to 130 km s-1. We do not know the non-radial nature of the macro-spicule out of the plane of the image, so we can not calculate the actual velocities involved in the high speed solar wind. But if we assume that the macro-spicule is rooted this side of the limb and it is tilted from the plane of the sky, then a small velocity translates to a much larger actual wind speed. These properties of our observed macro-spicule match very well with those of Pike & Harrison (1997).

Our observations further indicate the presence of a double sided jet, namely blue shifts in the limb and red shifts outside the limb. A closer inspection of the space time plot (see the extreme right-side of the macro-spicule in Fig. 2) shows weak signatures of fragmentation or ballooning along the length of the macro-spicule. Habbal & Gonzalez (1991) from their radio data have found evidence of the pinching off or ballooning at higher altitudes in the macro-spicules. Our observations are quite consistent with the unified model proposed by Shibata (1997). In his simulation a twisted or sheared magnetic flux emerges to reconnect with the overlying field. He found that as a result of reconnection between twisted and untwisted fields, shear Alfvén waves are generated and propagate along magnetic reconnected field lines. Since these Alfvén waves have large amplitudes, they excite large transversal motions (or spinning motion) of jets and exert a nonlinear magnetic pressure force to cause further acceleration of the jet, as originally suggested by Shibata & Uchida (1986). Moore et al. (1977) proposed that macro-spicules are associated with flaring bright points. It will be interesting to know whether macro-spicules are linked with bright points and X-ray jets. We hope that in the future co-ordinated observations with many instruments will be able to answer these questions.

High-cadence SOHO/EIT observations indicate that quasi-periodic fluctuations with periods 10-15 minutes are present in polar plumes (DeForest & Gurman 1998). They have also found that the fluctuations propagate outwards in the plumes at speeds 75-150 km s-1. Ofman et al. (1999) reported that the relative wave amplitude increases with height in the plumes out to about 1.2 [FORMULA]. They believe that these are slow magneto-acoustic waves propagating through the plumes. Using the white light channel (WLC) of UVCS, Ofman et al. (1997) have also reported density fluctuations in coronal holes on a time scale of 9.3 [FORMULA] 0.4 minutes. This may indicate the presence of compressional waves farther out in the corona, at 1.9 [FORMULA].

Our observations do indicate the very clear presence of long period oscillations with periods of 20-25 minutes. The power spectra obtained by Fourier and wavelet transforms have each established the existence of these long periods. Even from the light curves (see Fig. 9), and also from the contrast enhanced space time plots (see middle panel of Fig. 8) one can clearly see a 25 minute period. We interpret the waves as slow magneto-acoustic in nature. Ofman et al. (1999) have presented an analytical solution for the propagation of the magneto-acoustic waves in a gravitationally stratified, linear, one-dimensional model of the polar plumes. They find that for typical coronal conditions, 10-15 minute waves are propagating in the plume. Waves with periods longer than 70 minutes will become evanescent. We feel that with a different set of parameters, their calculations can also give 25 minute periods.

Recently, Kudoh & Shibata (1999) have presented a magnetohydrodynamic simulation of torsional Alfvén waves propagating along an open magnetic flux tube in the solar atmosphere. They show that if the Alfvén waves are generated by random motions (with velocities greater than 1 km s-1) at the photosphere, the transition region is lifted more than [FORMULA] 5000 km (i.e. a spicule is produced), the emission lines are broadened ([FORMULA] 20 km s-1) and as the Alfvén waves propagate upwards they produce slow or fast waves through nonlinear coupling. Part of the Alfvén waves are reflected back in the transition region and the remaining waves propagate upwards to the corona. They contribute a significant energy flux of [FORMULA] ergs s-1 cm-2 to heat the corona and also broaden the emission lines. Thus their simulation suggests that the non-thermal broadening, spicule formation and oscillations, all could be caused by Alfvén waves generated in the photosphere. This scenario is also consistent with our observations. Finally we remind the reader not to put too much emphasis on this one temporal sequence. We have several temporal series in polar plumes observed with CDS, whose analysis is in progress. We hope to present these results in a subsequent paper.

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000