Variation of the mm radio emission in the polar zones of the Sun. A.Riehokainen, J.Kallunki

Introduction In this work we considered the enhanced temperature regions (ETRs) in the polar zones of the Sun. These regions were discoverd by Babin et al.(1976) and Efanov et al.(1980) in the polar coronal holes. It was suprising, because at almost all other wavelenghts coronal holes are seen as dark areas. The radio enhancement is limited to a range of wavelengths (0.3 to 3 cm). At these wavelengths, most of the radio emission originates from the chromosphere and corona.The typical temperature enhancement range from 200 K to 600 K at 37 Ghz (in the Metsahovi radio map) and polar brightening near the poles in 17 Ghz maps (Nobeyama Radioheliograph) are about 40 % more intense than the quiet Sun brightness. Lifetimes of the ETRs are in the range from some tens of minutes to few hours, but for some ETRs it can last for several days (Riehokainen et al.1998). Special investigations carried out by various authors have suggested that ETRs could be connected with magnetic field configurations (Gopalswamy et al. 1999), diffuse brightenings, bright points (Pohjolainen et al. 1999) and polar faculae groups (Riehokainen et al. 1998; Gelfreikh et al. 2002). Equatorial coronal holes are also enhanced, and they have been associated with the H(alpha) intranetwork brightenings (Moran et al.2001). Nevertheless, no one- to-one correlations between these coronal features and ETRs has been found.

Introduction In last time some new studies of the polar brightening were made, among them: (Selhorst et al.2005) They showed that the intensity of discrete bright patches observed near the poles in 17 GHz maps, can only be explained by holes in the spicule forest. These regions without spicules probably reflect the presence of the polar faculae, which inhibit the appearence of spicules. (Rodriguez et al.2007) They showed that several thousand polar faculae were found in the polar caps, many more then assumed previously. Thus, this result confirmed (undirectly) the results obtained by Riehokainen et al.(1998) that the ETR correspond to several tens of polar faculae. This our result was obtained from the simultaneous observations of the ETRs at the Metsahovi radiotelescope and polar faculae at the Kislovodsk solar mountain station in Except this, they confirmed that the magnetic field strenght of the polar faculae may be 1.5 KGauss. (Gelfreikh et al.2006) They showed, that polar faculae have some typical periods of oscillation, namely: 3; 5;15-20; 40-60, >100 minutes.

Introduction Why the ETRs are bright? The coronal addition to the ETR’s enhancements was found to be not enough (Gopalswamy et al.1998) to explain the observable enhancement. Addition from the transition region also was not enough (Grebinskii 1987). This leads to the conclusion that almost all millimeter radio emission in coronal holes must originate in the chromoshere and probably have some counterparts with the brightness structures which are seen in CaII(k3). This idea was partly concerned in our previous works (Riehokainen et al 2003; 2005). We found that all ETRs were connected with two different types of bright structures which seen in CaII(k3). Also, the same structures have been seen in the SOHO/MDI magnetogramms.

Introduction In the first class, the nominal radio maximum coincides with the CaII(k3) maximum and the maximal magnetic field strenght within observational accuracy. In the second class, the nominal radio maxima and the local CaII(k3) brightest features and magnetogram maxima are offset from each other. In this case nominal radio maximum locates somewhere inside structure which looks like cell boundered by the bright features. Polar faculae groups are preferably situated at the cell’s boundaries or inside the cells (Riehokainen et al. 2001). So, mutial disposition of the ETR’s nominal maxima and polar faculae is in a good agreement with mentioned above two types of brightness structures. Connection between polar faculae cycle and ETRs cycle was also shown (Gelfreikh et al. 2002; Riehokainen et al. 1998)

Introduction Here we show one real example of the bright structure which we call as a first class. Brightness temperature enhancement ∆T is about 200 K, excess of brightness in CaII(k3) is about 30% and maximal value of the magnetic field is about 240 Gauss.

Introduction Here we show one real example of the bright structure which we call as a second class. Brightness temperature enhancement ∆T is about 200 K. Magnetic field background is positive with negative sources (-30 Gauss), but the nominal radio maximum is coincide with so-called cell structure which we seen in CaII(k3) and SOHO/MDI images.

Introduction Thus we showed that the ETRs are not only radio phenomena. Each ETR has complicated plasma structure at different heights in the solar atmosphere, at least in the photosphere and cromosphere (and only sometimes in the transition region and solar corona). Such plasma structures limited in space may have typical periods of resonance oscillations which depends on size and its internal structure. Almost all mentioned above works considered polar brightenings statically. But brightness of the ETRs is not a constant value, it changes with the time. For this reason in this study we tryed to find typical time for brightness variation of the ETRs. For that we used two methods. One is the direct tracking of the polar brightenings and another one is using the radio map at 37 GHz obtained with the Metsahovi radiotelescope. In both case solar observations continued during several hours. Unfortunately, finnish weather conditions are not very good (a lot of clouds) and in winter season we have short time interval for the solar observations. Therefore the total number of observations is about 25 in

Observations Radio observations were made at the Metsahovi Radio Telescope at 37 GHz in Telescope beam size at this frequency is 2.4 arcmin. Estimated quiet Sun level is 7800 K. In the temperature scale the resolution is better than 100 K and it is limited by short term changes in the atmospheric attenuation. Some examples of radio maps

Some examples of radio maps At this slide you can see 4solar radio maps obtained at the Metsahovi radio telescope at 37 Ghz. These solar maps were obtained during 3 days. Each map contains the ETR in the south polar zone of the Sun.

Results This plot shows the variation of the relative brightness for the ETR on 06/08/2007. Brightness temperature enhancement ∆T is in interval from 540 to 1100 K. Solar coordinates of this ETR:q(latitude)=78; fi(longitude)=-33. This ETR’s life-time is at least 4 hours, but typical time variation is in interval of minutes. This plot was obtained from the consecutive Metsahovi radio maps

Results This plot shows the variation of the relative brightness for the ETR on 13/08/2007. Brightness temperature enhancement ∆T is in interval from 350 to 600 K. Solar coordinates of this ETR: q(latitude)=68; fi(longitude)=9. The ETR’s life-time is at least 6 hours, but typical time variation is about one hour. This plot was obtained from the consecutive Metsahovi maps

Results Variations of T(enh) for two different ETRs observed 7 hours during the same day are shown here. Typical time for the brightness variation is minutes, but these variation are not stable. It is typical for all concerned ETRs, obtained from the Metsahovi maps. So, ETR’s brightness variation is an indicator of the dynamic process which have place in these regions.

Results Variation of brightness for these ETRs has relatively long typical time. Time interval between each two points in the curves is about 9 minutes. For this reason, in principle, we cannot find any short typical time for variation of brightness. Such kind of data is not very good for the analysing of typical time of the brightness variation and may be used only for the estimation relatively long time of the brightness variation. Nature of relatively long ( minutes) variations is not clear even for sunspots. For the polar ETRs it is very new and perfectly unknown field for the future investigations

Results (top) This plot shows tracking record of the ETR’s brightness near the North pole during 2.5 h on 22/05/2008. It is possible to see the variation of brightness with typical time about minutes. (bottom) The power spectrum obtained with FFT on time scale (with filter). 5-minute oscillations are visible clearly.

Results (top) This plot shows the record of the ETR’s brightness near the North pole during 2 h on 23/07/2008. It is possible to see the variation of brightness with typical time about minutes. (bottom) The power spectrum obtained with FFT on time scale (with filter). 3 and 5-minute oscillations are visible.

Results Five-minute oscillations were observed in the polar ETRs. These oscillations are the result of the solar global processes which have deal with the homogeneous structures along the solar surface. Three-minute oscillations were observed only in one polar ETR. Such kind of oscillations were observed usually only in the sunspot oscillations. Magnetic field of the sunspot could be about 2000 Gauss. But we believe that ETRs and polar faculae are closely connected to each other. Thus, we can suggest that magnetic field of the polar ETRs have, at least in some cases, a similar structures as the normal sunspots. As we noticed early, the polar ETRs oscillations are very new and absolutely unknown field for the future investigations.

Conclusions At least larger part of the ETRs show variation of the brightness with typical time minutes which indicate their dynamic origin, this phenomenon we will study in the future. We found that ETRs have 5-minute oscillations. We found that at least some ETRs have 3-minute oscillations, it means that magnetic field of these regions have some similar structures as normal sunspot. Unfortunately, we have analysed only two ETRs, for this reason we need to continue our investigation.