Observations of the large scale magnetic field in the photosphere taken at the Wilcox Solar Observatory since 1976 up to 2005 have been analyzed to deduce its latitudinal and longitudinal structures, its differential rotation, and their variability in time. Basic topology of solar magnetic field was reconstructed in coordinate system rotating with different rate.

Employing the Poincaré index of isolated null-point in a vector field, we work out a mathematical method of searching 3D null-points in coronal magnetic fields. After introducing the theory of differential topology, we test the method by using the analytical model of Brown & Priest. The location of null-point identified by our method coincides precisely with the analytical solution. Finally we apply the method to search 3D null-points in coronal magnetic fields reconstructed based on observed magnetograms of two active regions (NOAA AR 10488 and AR 10720). We find that the 3D null-points seem to be a key element to the magnetic topology associated with flare occurrence.

We report the eruption of a small filament on 2001 September 28. Followed by a two-ribbon flare, the eruption was associated with a partial halo coronal mass ejection. During the eruption, coronal bipolar double dimmings were formed on the regions of opposite magnetic polarities. The optical counterparts of the coronal dimmings, i.e., the $H_{\alpha}$ dimmings, were clear and showed similar appearance and consistent development. A remarkable fact is that the two dimmings were preceded by impulsive EUV and $H_{\alpha}$ brightenings at the flare's rise phase, suggesting that the eruption can be explained by the tether-cutting model.

From Yohkoh and SoHO observations, the magnetic reconnection is considered as a main energy release mechanism of the solar active phenomena. In this study, we try to reproduce GOES X-ray light curves (1–8 $\AA$) of solar flares using a model incorporating the radiative and conductive cooling and the magnetic reconnection heating.

The Prototype Brazilian Decimetre Array (PBDA) consists of 5 element alt-azimuth mounted parabolic dishes of 4-m diameter, having baselines up to 216 m in East-West direction. We present initial solar observations carried out with the PBDA during the period 22$^{\hbox{\scriptsize{nd}}}$ November to 11$^{\hbox{\scriptsize{th}}}$ December, 2004. The frequency of observation was 1.6 GHz. The temporal and spatial resolution were 100 ms and 3 arcmin, respectively.

The propagation of surface and body linear MHD modes in a twisted magnetic flux tube embedded in a magnetically twisted plasma environment is considered. The dispersion relation for surface and body modes is derived assuming constant external twisted field. Analytic approximate solutions to the dispersion equation are found for the long and short wave length cases.

It was found, that in case the twisted component of the magnetic field in the environment of the flux tube is constant the index of Bessel functions $\nu$ in the corresponding dispersion relation is not integer. In the particular case of a homogen magnetic twist the total pressure is found to be constant across the boundary of the flux tube.

We use the spectral shapes of the EUV line profiles to study the plasma dynamics, acceleration and heating, in polar plumes (PP). We find that the observed profiles are reproduced fairly well when considering low plume wind speeds and velocity turbulence $(\alpha_S)$ at low altitudes followed by a rapid acceleration and heating of the plasma to reach the properties of inter-plumes (IP) by $\approx3-4~R_\odot$. We also find that plumes very close to the pole give narrow profiles at all heights that are not observed above $\approx2.5~R_\odot$. This suggests a tendency for plume footpoints to lie more than 10° away from the pole. High resolution magnetograms of SOLIS and EUV images support this hypothesis.

Longitudinal structure of the photospheric magnetic field over last tree solar cycles has been studied. The reconstruction of the longitudinal structure in the heliographic system rotating rigidly with Carrington rate was performed and compared with longitudinal distributions of random origin.

The Statistics analyses of the microwave type III bursts, coronal mass ejections (CMEs), H$\alpha$flares and relevant events observed with 5200–7600 MHz spectrograph at the Chinese National Astronomical Observatory during the 23rd solar cycle are carried out in this article. Some significant results are obtained from the relevant events. The radiation mechanism of that is also discussed from the observation characteristics.

We analyze the periodicities for 294 solar flares observed by Nobeyama Radioheliograph over 6 discrete frequencies of 1, 2, 3.75, 9.4, 17 and 35 GHz during the solar cycle 23 between 1998 and 2004. Using Fourier analysis, we pick up the shortest period ($T_{\rm{s}}$) for each event at each frequency channel. We find that $T_{\rm{s}}$ varies in the range of 0.25–0.62 s over 6 frequency channels.

Using the data from BBSO, YNAO and SOHO/MDI we studied the formation and evolution of a ring-filament which was located in NOAA AR09470 from May 21 to 24, 2001. We found that: (1) the ring-filament's formation was well associated with newly-emerging flux. (2) all the eruptions of the filament happened during the decay of the emerging bipoles.

The Kislovodsk series of the green (FeXIV 530.3 nm; KI$_{5303}$) and red (FeX 637.4 nm; KI$_{6374}$) corona has been used to calculate the mean monthly intensities at high latitude (45$^\circ$–90$^\circ$) zones for 1957–2002. The ratio KI$_{6374}$/KI$_{5303}$ was observed during the minimum sunspot activity. It was shown that this ratio increased by more than a factor 2 during the last 45 years at the high latitudes (45$^\circ$–90$^\circ$). This may be interpreted that the fraction of cool regions in the polar corona has more than doubled over these years. We suggest that this increase in the amount of cool regions is related to the increase in the area of the polar zones occupied by magnetic field of a single polarity at the solar minimum and possibly to a corresponding increase in the area occupied by polar coronal holes, while the magnetic field strength itself has not or barely increased.

The broad range of dynamics exhibited by plasma motions within the solar interior affects many aspects of the generation and transport of magnetic fields during the solar magnetic activity cycle. On the photosphere, such dynamics include the differential rotation, meridional flows, and a hierarchy of convection cells, and these fluid motions are observed to readily advect any small-scale magnetic fields embedded within them. While the effects of large-scale flows on the global activity cycle are well known, it is becoming increasingly apparent that small-scale dynamics can also affect global magnetic activity throughout the solar cycle. Such effects include variations in the strength of the magnetic dipole moment with time, and the timing of the reversals of the polar-cap flux. In this article, several aspects of this coupling between small and large scales will be illustrated, and the implications of such coupling on the solar activity cycle will be discussed.