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Register a new userNorth-south asymmetry in Rieger-type periodicity during solar cycles 19-23
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Rieger-type periodicity has been detected in different activity indices over many solar cycles. It was recently shown that the periodicity correlates with solar activity having a shorter period during stronger cycles. Solar activity level is generally asymmetric between northern and southern hemispheres, which could suggest the presence of a similar behavior in the Rieger-type periodicity. We analyse the sunspot area/number and the total magnetic flux data for northern and southern hemispheres during solar cycles 19-23 which had remarkable north-south asymmetry. Using wavelet analysis of sunspot area and number during the north-dominated cycles (19-20) we obtained the periodicity of 160-165 days in the stronger northern hemisphere and 180-190 days in the weaker southern hemisphere. On the other hand, south-dominated cycles (21-23) display the periodicity of 155-160 days in the stronger southern hemisphere and 175-188 days in the weaker northern hemisphere. Therefore, the Rieger-type periodicity has the north-south asymmetry in sunspot area/number data during solar cycles with strong hemispheric asymmetry. We suggest that the periodicity is caused by magnetic Rossby waves in the internal dynamo layer. Using the dispersion relation of magnetic Rossby waves and observed Rieger periodicity we estimated the magnetic field strength in the layer as 45-49 kG in more active hemispheres (north during the cycles 19-20 and south during the cycles 21-23) and 33-40 kG in weaker hemispheres. The estimated difference in the hemispheric field strength is around 10 kG, which provides a challenge for dynamo models. Total magnetic flux data during the cycle 20-23 reveals no clear north-south asymmetry which needs to be explained in the future.
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Aims. In this paper, we investigate the temporal evolution and north-south (N-S) asymmetry in the occurrence of solar flares during cycle 21, 22, and 23, and compare the results with traditional solar activity indices. Methods. The flare activity is characterized by a soft X-ray (SXR) flare index, which incorporates information about flare occurrences during a selected interval along with the peak intensity of individual events. Results. The SXR flare index correlates well with other conventional parameters of solar activity. Further, it exhibits a significantly higher correlation with sunspot area over sunspot number, which suggests the variations in sunspot area to be more closely linked with the transient energy release in the solar corona. The cumulative plots of the flare index indicate a slight excess of activity in the northern hemisphere during cycle 21, while a southern excess clearly prevails for cycles 22 and 23. The study reveals a significant N-S asymmetry, which exhibits variations with the phases of solar cycle. The reliability and persistency of this asymmetry significantly increases when the data is averaged over longer periods, while an optimal level is achieved when data is binned for 13 Carrington rotations. The time evolution of the flare index further confirms evolution of dual peaks in solar cycles during the solar maxima and violation of Gnevyshev-Ohl rule for the pair of solar cycles 22 and 23. Conclusions. The SXR flare index in the northern and the southern hemispheres of the Sun exhibits significant asymmetry during the evolutionary phases of the solar cycle, which implies that N-S asymmetry of solar flares is manifested in terms of the flare counts as well as the intensity of flare events.
Solar activity undergoes a variation over time scales of several months known as Rieger-type periodicity, which usually occurs near maxima of sunspot cycles. An early analysis showed that the periodicity appears only in some cycles, and is absent in other cycles. But the appearance/absence during different cycles has not been explained. We performed a wavelet analysis of sunspot data from the Greenwich Royal Observatory and the Royal Observatory of Belgium during cycles 14-24. We found that the Rieger-type periods occur in all cycles, but they are cycle-dependent: shorter periods occur during stronger cycles. Our analysis revealed a periodicity of 185-195 days during the weak cycles 14-15 and 24, and a periodicity of 155-165 days during the stronger cycles 16-23. We derived the dispersion relation of the spherical harmonics of the magnetic Rossby waves in the presence of differential rotation and a toroidal magnetic field in the dynamo layer near the base of the convection zone. This showed that the harmonic of fast Rossby waves with m=1 and n=4, where m (n) indicate the toroidal (poloidal) wavenumbers, respectively, perfectly fit with the observed periodicity. The variation of the toroidal field strength from weaker to stronger cycles may lead to the different periods found in those cycles, which explains the observed enigmatic feature of the Rieger-type periodicity. Finally, we used the observed periodicity to estimate the dynamo field strength during cycles 14-24. Our estimations suggest a field strength of 40 kG for the stronger cycles, and 20 kG for the weaker cycles.
In this paper the N-S asymmetry of the soft X-ray flare index during the solar cycles 21, 22 and 23 has been analyzed. The results show the existence of a real N-S asymmetry which is strengthened during solar minimum. The slope of the regression lines fitted to the daily values of asymmetry time series has been found to be negative in all the three cycles. The yearly asymmetry curve can be fitted by a sinusoidal function with a period of eleven years. The power spectral analysis of daily asymmetry time series reveals the significant periods of around 28.26 days, 550.73 days and 3.72 years.
We report the temporal evolution of the excess brightness temperature above solar active regions (ARs) observed with the Solar Submillimeter Telescope (SST) at 212 ({lambda} = 1.4 mm) and 405 GHz ({lambda} = 0.7 mm) during Cycles 23 and 24. Comparison with the sunspot number (SSN) yields a Pearsons correlation coefficient R = 0.88 and 0.74 for 212 and 405 GHz, respectively. Moreover, when only Cycle 24 is taken into account the correlation coefficients go to 0.93 and 0.81 for each frequency. We derive the spectral index {alpha} between SST frequencies and found a slight anti-correlation with the SSN (R = -0.25); however, since the amplitude of the variation is lower than the standard deviation we cannot draw a definite conclusion. Indeed, {alpha} remains almost constant within the uncertainties with a median value approximate to 0 characteristic of an optically thick thermal source. Since the origin of the AR submillimeter radiation is thermal continuum produced at chromospheric heights, the strong correlation between the excess brightness temperature and the magnetic cycle evolution could be related to the available free magnetic energy to be released in reconnection events.
We present the characteristics of DH type II bursts for the Solar Cycles 23 and 24. The bursts are classified according to their end frequencies into three categories, i.e. Low Frequency Group (LFG; 20 kHz $leq$ $f$ $leq$ 200 kHz), Medium Frequency Group (MFG; 200 kHz $<$ $f$ $leq$ 1 MHz), and High Frequency Group (HFG; 1 MHz $<$ $f$ $le$ 16 MHz). We find that the sources for LFG, MFG, and HFG events are homogeneously distributed over the active region belt. Our analysis shows a drastic reduction of the DH type II events during Solar Cycle 24 which includes only 35% of the total events (i.e. 179 out of 514). Despite having smaller number of DH type II events in the Solar Cycle 24, it contains a significantly higher fraction of LFG events compared to the previous cycle (32% $versus$ 24%). However, within the LFG group the cycle 23 exhibits significant dominance of type II bursts that extend below 50 kHz, suggesting rich population of powerful CMEs travelling beyond half of the Sun-Earth distance. The events of LFG group display strongest association with faster and wider (more than 82% events are halo) CMEs while at the source location they predominantly trigger large M/X class flares (in more than 83% cases). Our analysis also indicates that CME initial speed or flare energetics are partly related with the duration of type II burst and that survival of CME associated shock is determined by multiple factors/parameters related to CMEs, flares, and state of coronal and interplanetary medium. The profiles relating CME heights with respect to the end frequencies of DH type II bursts suggest that for HFG and MFG categories, the location for majority of CMEs ($approx$65%-70%) is in well compliance with ten-fold Leblanc coronal density model, while for LFG events a lower value of density multiplier ($approx$3) seems to be compatible.
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