T. Ataç

Periodic behavior of solar flare index during solar cycles 20 and 21

Short-term periodicities of solar activity were studied with the flare index by using Discrete Fourier Transform for the time interval 1966–1986. Two noticeable periodicities (18.5 and 5 months) have been found. The existence of these periodicities comparing with the early findings is discussed.

Effects of hysteresis in solar cycle variations between flare index and cosmic rays

Abstract We study the hysteresis effect between the solar flare index and cosmic ray intensity for the past 37 years from January 1, 1965 to December 31, 2001 on a daily basis. We show that smoothed time series of flare index and the daily Calgary Galactic Cosmic Ray intensity values exhibit significant solar cycle dependent differences in their relative variations during the studied period. The shapes of these differences vary from cycle to cycle. So we investigate the momentary time lags between the two time series for the odd and even cycles.

Flare Index During the Rising Phase of Solar Cycle 23

A brief description and results of the flare index (FI) as a measure of solar activity for cycle 23 are given. The calculation of the daily flare index was determined using the final solar flare tables from the National Geophysical Data Center A. The final data of FI are presented in graphical form over the rising activity phase of solar cycle 23. Daily calculated values are available for general use in Kandilli observatorys and NGDCs anonymous ftp servers. The pattern of similar activity indices that arise under different physical conditions during the rising activity phase are compared with the flare index. The north-south asymmetry in the daily flare index data was studied.

Confirmation of the 25.5-day fundamental period of the Sun using the north-south asymmetry of the flare index

Short-term periodicities of solar activity were studied. To perform the study, a north-south asymmetry time series was constructed by using the northern and the southern hemisphere flare index values for solar cycle 22. The statistical significance of this time series was calculated. It indicates that in most of cases the asymmetry is highly significant during cycle 22. Power spectral analysis of this time series reveals a periodicity around 25.5 days, which was announced before as a fundamental period of solar activity (Bai and Sturrock, 1991). To investigate the time agreement between the two hemispheres, the phase distribution was studied and a phase shift of about 0.5 was found. An activity trend from the north to the south was found.

Examination of the solar cycle variation of foF2 by using solar flare index for the cycle 21

Abstract For solar cycle 21 (1976 – 1986) the variation of monthly mean values of noon-time f oF2 at Slough, Rome, and Manila are examined by using solar flare index and geomagnetic Ap index. A single regression analysis for dependence of f oF2 on solar flare index shows better matching: Moreover, less hysteresis effect is seen when we use solar flare index instead of other solar indices. Thus, for making prediction, one needs to take into account just the solar flare index and not the solar flare index and geomagnetic Ap index simultaneously.

Effects of Hysteresis in Solar Cycle Variations Between Flare Index and Some Solar Activity Indicators

ABSTRACT We show that smoothed time series of some solar activity indicators exhibit significant hysteresis, i.e. they have solar cycle dependent differences in their relative variations during the past three solar cycles 20, 21, and 22. This implies that the shapes and the directions of these differences vary from cycle to cycle, while a saturation effect exists at extreme phases. The choice of activity indicator versus flare index may strongly influence the resulting hysteresis in a given solar cycle. Therefore these observed hysteresis patterns may not give evidence that this is a normal feature of solar variability.

The Ionospheric foF2 Data over Istanbul and their Response to Solar Activity for the Years 1964–1969 and 1993

A Polish-made vertical ionosonde (VI) has been operated at the Kandilli Observatory in Istanbul, for almost one year (May 1993 - April 1994) as part of the COST 238, PRIME Project, The critical frequencies were obtained for every half-hour interval. The data obtained during this campaign, on the descending branch of solar cycle 22, and the data measured earlier in Istanbul for cycle 20 were analysed and the characteristic behaviour of the F2 region ionosphere over Istanbul has been determined. This is a unique data set for this area. Several markers of the solar cycle activities in terms of the daily relative sunspot numbers, F10.7 cm solar radio flux and solar flare index, and the magnetic daily index of Ap were then used to seek the possible influence of the solar and ionospheric activities on the critical frequencies observed in Istanbul. It was found that the solar flare index, as a solar activity index, was more reliable in determining quiet ionospheric days. It is shown that the minimum and maximum time values of the solar activity are more convenient for ionospheric prediction and modelling.

On the SID durations

This Letter is based on analysis of data on Sudden Ionospheric Disturbances (SID) listed inSolar Geophysical Data. The mean durations of 1272 SIDs taken from the SGD lists were calculated with respect to their associated X-ray flares. In general, all of our calculated mean durations showed a large scatter in the mean values. The comparison of our values by the estimated Solar Geophysical Data Center SID durations indicated an apparent discrepancy in the mean duration relative to that estimated by SGD for the case of low levels of SID importance. In our opinion this apparent discrepancy suggests a need for a correction in SGDs assumptions in the importance range from −1 to +1.

Evidence of the fundamental periodicity in the flare index between the years 1966-2002†

The short-term periodicities of the flare index (FI) which is roughly proportional to the total energy emitted by the flare, are investigated in detail using Fourier transform (FT) for the full-disc and for the northern and the southern hemispheres of the Sun separately over the epoch of almost 4 cycles (1966-2002) which covers 13392 days. Figure 1 shows the power spectra of the three time series of FI calculated for the 289-771 nHz (15-40 days) range with 0.24 nHz interval. The uncertainty in each frequency is ± 0.4 nHz due to the 36.5-year data length. The dashed lines indicate the false alarm probability significance levels. In this figure, there are several pronounced power peaks whose significances are enough high. These are at 25.6, 27.0, 30.2 days for the full-disc, 27.0 days for the northern hemisphere and 27.5 and 33.8 days for the southern hemisphere. Since commonly used FT is not able to disclose the possible changes in the periodicities over the period studied, the wavelet transform (WT) was applied to search for temporal variability. The wavelet transform results show that the occurence of periodicities of flare index power is highly intermittent in time. All three FI time series show that the major periods, obtained by Fourier transform are localised in short time intervals (typically up to half of a year). A comparison of the results of the FT and the time-period WT of the flare index time series has clarified the importance of different periodicities, whether they are or are not the harmonics of the basic ones, as well as the temporal location of their occurrence. Because we found that the modulation of the flare index due to the 27-day solar rotation is more pronounced during the declining portion of solar cycle than during the rising portion. Our results show that 27-day modulation seems 27.5-day in the southern hemisphere of the Sun. There is a possibility that some parts of an active region move rapidly forward or backward with respect to the mean rotation rate of the Sun, to give spacings different from 27 days. This can be checked only by an actual image comparison. The fundamental period proposed by Bai and Sturrock (1991) was reported as 25.5day. In our analysis, we found this value as 25.6 days. This periodicity operates mainly during maximum phases of solar cycles only short time intervals. However, Ziȩba et al. (2001) found the rotation rate of the “active longitudes” in the rising phase of cycle 23 as equal to 26.0±0.3 days. Recently, Bai (2003) studied longitude distributions of major flares in a coordinate system rotating about the solar axis, using the rotation period as a free parameter. And he concluded that, the idea of 25.5 days is a fundamental period of the Sun is well supported by data. However, its clock mechanism is still unknown. The differences between the results of the different authors can be explain that because of the non-sinusoidal nature of the variation of some activities (e.g., square waveform shape

On the short term variations of solar activity during solar cycle 21

Short-term variations of the last solar activity cycle were studied by the flare and coronal indices using Gleissberg method. Systematic short-term variations are found from their course during the 21st solar activity cycle. Comparison of their autocorrelograms constructed by the new set of data obtained from the magnitude of the fluctuations showed us the existence of the phase shift between the temporal variations of the two indices.