Magnetic field and turbulent velocities in the solar corona

3. Discussion and Conclusions

[18] The values presented above of the nonthermal velocities for the same formations in the corona are significantly different not only according to various authors but also according to our observations carried out during different eclipses. No doubts, these results manifest in some sense real differences in physical conditions in various places in the corona and their variability with time. However, (during eclipse observations) the variability from one occasion to another probably is partly related to systematic errors in taking into account the background. However, it is important that in each particular observation the nonthermal velocity in the vicinity of a quiet prominence is always less than in the undisturbed corona, whereas in a coronal hole the velocity is always higher than in quiet corona. The result we obtained during the 1999 eclipse is especially valuable because of the scatter of the observational results obtained by different authors. We determined the values of x for all three different structural formations in the corona within the same system, this fact reliably confirming the obtained regularity.

[19] The CH region and the coronal cavity around a quiet prominence are characterized by the density depleted as compared to the quiet corona [Waldmeier, 1970; Wilhelm et al., 1998]. The depleted density can lead to an increase of nonthermal velocities [Banerjee et al., 1998], but this is not observed in the cavity around a quiet prominence. The observations show that x is increased in CH, but reduced in the cavity around a quiet prominence as compared to the ambient environment. Therefore the depleted density cannot completely explain high nonthermal velocities also in CH. Until now the physical nature of the additional nonthermal broadening of spectral lines is not fully clear. Observations show that the nonthermal velocities are isotropic and processes causing them have short timescale and small space-scale nature. The most plausible mechanism is thought to be magnetohydrodynamic turbulence [Chae et al., 2000; Delone et al., 2003; Dere and Mason, 1993; Dmitruk and Gomez, 1997; Doyle et al., 1998; Gomez et al., 2000; Voitenko and Goossens, 2002]. The magnetic field in coronal cavity around a quiet prominence is relatively stable, whereas in the CH that is unstable.

[20] Harvey et al. [1982] studied variations in the magnetic field within CH during 1975-1980. They noted that the mean magnetic field strength in the maximum of solar activity lies within the range of 3-36 G, whereas in the activity minimum it does not exceed 1-7 G. Studying recurrent CHs around solar activity maximum, Harvey et al. [1982] noted that frequent emergences of the new flux change the boundaries of the coronal holes so strongly, that CH look topologically different during the sequent rotations of the Sun. Such activity is responsible for strong changes in fluxes, dimensions, and average strength of the field observed in some coronal holes from one rotation to another [Harvey et al., 1982]. Bilenko and Kononovich [1999] showed that in the epoch of rising solar activity in 1996-1999, the total magnetic flux in the region of coronal holes was by a factor of 2-3 higher than in the adjacent undisturbed coronal regions.

[21] Wang et al. [1997] have found that the most dark parts of a coronal hole coincide with the area in the magnetograms in the l sim 8542 Å (the chromospheric level) where there are small fluxes of contaminating polarity or where there are isolated spots of the main polarity. Malanushenko and Stepanyan [2001] studied the case when during several days an emergence of a weak magnetic field ( <7 G) was observed in the CH in the form of separated formations. At the beginning the total area of these formations grew slowly, then the rate of the growth of this area and the magnetic field strength increased sharply. The same authors note frequent cases when on the Sun image in the l sim 10,830 Å line the activating region within the coronal hole is surrounded by a light rim. They interpret this fact as a manifestation of a magnetic flux output. Malanushenko [2002] noted that CH is destroyed when the magnetic flux formed inside the hole comes to its boundaries. Malanushenko [2002] concludes that the coronal hole existence itself is related to changes in magnetic flux.

[22] The relation of the value of the turbulent velocity to variations in the magnetic field is obvious from the comparison of the values of nonthermal velocities for the coronal region around a quiet prominence, in quiet (undisturbed) corona and in a coronal hole:

[23] The question is still left: What is the cause and what is the result. Does the turbulence increase lead to an increase in the magnetic field (as was shown by Parker [1982], or vice versa changes in the fields lead to plasma turbulization? Parker [1982] stated that there is no indication that the magnetic field influence turbulence in general. At the same time, the analysis of the observational data presented above demonstrates that there are such indications.

[24] It is known that the rate of the coronal hole rotation differs from the rotation rate of the background magnetic field. The change in differential rotation with solar activity of the coronal hole with an open magnetic configuration [Stepanyan and Malanushenko, 2001] is stronger than of the background magnetic field. While observing a coronal hole in the center of the solar disk in the H_a line only point-like structures are seen and there is no fibriles. This shows that the magnetic field in the coronal hole is radial. Observations fulfilled in the Crimean Observatory (Ukraine) in 1999 demonstrated that five minutes oscillations were suppressed at the photospheric and chromosphere levels of the CHs in comparison with quiet regions on the Sun outside CH [Malanushenko, 2002]. The suppression of the oscillations in the coronal hole manifests that the roots of CH are in underphotospheric layers. Therefore the CHs seem to be solar formations originating under the photospheric layer and stretching up to the chromosphere. As it was concluded by Malanushenko [2002] the CH are related to the fluxes of the magnetic field the source of which is located deeper than the source of the background magnetic field.

[25] So it is natural to suggest that not changes in these deep fields are caused by the turbulence of the upper atmosphere, but rising up of deep fields lead to the increase of the turbulence in the transition region and corona.

[26] Thus, comparing the nonthermal velocities in the coronal plasma around quiet prominences, in quiet (undisturbed) coronal regions, and in coronal holes to the magnetic situation, we can conclude that the turbulence difference is caused by differences of the magnetic field in the considered formations. The turbulent increase in CH with changeable (by flux, topology and strength) magnetic field with open configuration is possibly due to enhanced wave dissipation as in comparison with closed-field quiet regions of the corona or coronal cavities around quiet prominences with relatively stable magnetic configuration.