The frequency composition of the Madden-Julian oscillation. An indication of the solar activity contribution to the global atmospheric circulation and the Earth's rotation

In the series of the solar activity indices, the zonal component of the atmospheric momentum (AAM), and the Earth rotation variations (UT1-TAI), power spectra concentrations at 40, 50, and 60–80 day period are detected. This result indicates the solar origin of the Madden-Julian oscillation in atmospheric circulation and the 50-day oscillation of the Earth's rotation.     

Power Spectrum of Cosmic-ray Fluctuations During Consecutive Solar Minimum and Maximum Periods

Power spectral analysis of cosmic-ray intensity recorded by eight stations was carried out over a wide range of frequencies from 2.3 × 10^–8 Hz to 5.8 × 10^–6 Hz (2–500 days) during the period 1964–1995. Spectrum results of large-scale fluctuations have revealed the existence of a broad peak near 250–285 days and a narrower peak at 45–50 days during the studied epochs as a stable feature in all neutron monitors covering a wide rigidity range. The cosmic-ray power spectrum displayed significant peaks of varying amplitude with the solar rotation period (changed inversely with the particle rigidities) and its harmonics. The amplitudes of 27-day and 13.5-day fluctuations are greater during the positive-polarity epochs of the interplanetary magnetic field (qA>0) than during the qA<0 epochs. The comparison of cosmic-ray power spectra during the four successive solar activity minima have indicated that at the low-rigidity particles the spectrum differences between the qA>0 and qA<0 epochs are significantly large. Furthermore, the spectrum for even solar maximum years are higher and much harder than the odd years. There are significant differences in the individual spectra of solar maxima for different cycles.     

Thirteen-day periodicity and the center-to-limb dependence of UV,EUV, and X-ray emission of solar activity

Periodicity in the 13–14 day range for full-disk UV fluxes comes mainly from episodes of solar activity with two peaks per rotation, produced by the solar rotational modulation from two groups of active regions roughly 180° apart in solar longitude. Thirteen-day periodicity is quite strong relative to the 27-day periodicity for the solar UV flux at most wavelengths in the 1750–2900 Å range, because the rapid decrease in UV plage emission on average with increasing solar central angle shapes the UV variations for two peaks per rotation into nearly a 13-day sinusoid, with deep minima when the main groups of active regions are near the limb. Chromospheric EUV lines and ground-based chromospheric indices have moderate 13-day periodicity, where the slightly greater emission of regions near the limbs causes a lower strength relative to the 27-day variations than in the above UV case. The lack of 13-day periodicity in the solar 10.7 cm flux is caused by its broad central angle dependence that averages out the 13-day variations and produces nearly sinusoidal 27-day variations. Optically thin full-disk soft X-rays can have 13-day periodicity out of phase with that of the UV flux because the X-ray emission peaks when both groups of active regions are within view, one group at each limb, when the optically thick UV flux is at a rotational minimum. The lack of 13-day periodicity in the strong coronal lines of Fexv at 284 Å and Fexvi at 335 Å during episodes of 13-day periodicity in UV and soft X-ray fluxes shows that the active region emission in these strong lines is not optically thin; resonant scattering is suggested to cause an effective optical depth near unity in these hot coronal lines for active regions near the limb.     

Comparisons of the NOAA-11 SBUV/2, UARS SOLSTICE, and UARS SUSIM Mg II Solar Activity Proxy Indexes

A NOAA-11 SBUV/2 Mgii solar activity proxy index has been created for the period February 1989 through October 1994 from the daily discrete mode solar irradiance data using an algorithm that utilizes a thorough instrument characterization. This product represents a significant improvement over the previously released NOAA-11 SBUV/2 sweep mode-based Mgii data set. As measured by the NOAA-11 Mgii index, the amplitude of solar rotational activity declined from approximately 4–7% peak-to-peak near the maximum of solar cycle 22 in 1989–1991 to roughly 1% peak-to-peak by late-1994. Corresponding to this decrease, the 27-day averaged NOAA-11 Mgii index decreased by 5.8% over this period. The NOAA-11 Mgii data set is compared with coincident data sets from the UARS SOLSTICE and SUSIM instruments. The impact of differences in instrument resolution and observation platform are examined with respect to both the absolute value and temporal variations of the Mgii index. Periodograms of the three indexes demonstrate comparable solar variation tracking. Between October 1991 and October 1994 predominate power occurs near 27 days, with secondary maxima in the power spectra near 29 and 25 days. Overall, there is low power near 13.5 days during this period. Dynamic power spectral analysis reveals the quasi-periodic and quasi-stationary nature of the middle UV variations tracked by the Mgii index, and periods of significant power near 13.5 days in mid-1991 and late-1994 through mid-1995.     

Cyclic Changes in Solar Rotation Inferred From Temporal Changes in the Mean Magnetic Field

Time–frequency variability of the solar mean magnetic field (SMMF) was studied, based on a continuous wavelet analysis. The rotational modulation of the SMMF dominates the wavelet spectrum at 27–30 and 13.5-day time scales. The rotational variation, in turn, is amplitude-modulated by the quasi-biennial periodicity in the SMMF. This is caused by magnetic field eruptions. Rigidly rotating modes appear in the time–longitude distribution of the large-scale magnetic field that is plotted from a deconvolution of the SMMF time series with a Carrington period. These rotational modes coexist and transform into one another over an 11-yr cycle. The modes with periods of 27.8–28.0 days dominate the phase of activity rise, whereas the 27-day rotational mode dominates the declining phase of the 11-yr cycle. The rotational modes with periods of 29–30 days occurred episodically. Most of the features in the time–longitude distribution of the SMMF are identifiable with those in similar diagrams of the solar background magnetic fields. They represent a combined effect of the background magnetic fields from both hemispheres. Eruptions of magnetic fields lead to dramatic changes in the picture of solar rotation and correlate well with the polarity asymmetry in the SMMF signal. The polarity asymmetry in the SMMF time series exhibits both long-term changes and a 22-yr cyclic behaviour, depending on the reversals of the global magnetic field in cycles 20–23.     

Rotation of Plasma in Sunspots

In this paper we present the results of a sunspot rotation study using Abastumani Astrophysical Observatory photoheliogram data for 324 sunspots. The rotation amplitudes vary in theinebreak 2–64° range (with maximum at 12–14°), and the periods around 0–20 days (with maximum atinebreak 4–6 days). It could be concluded that sunspot rotations are rather inhomogeneous and asymmetric, but several types of sunspots are distinguished by their rotational parameters.During solar activity maximum, sunspot average rotation periods and amplitudes slightly increase. This can be affected by the increase of sunspot magnetic flux tube depth. So we can suppose that sunspot formation during solar activity is connected to a rise of magnetic tubes from deeper layers of the solar photosphere, strengthening the processes within the tube and causing variations in rotation.There is a linear relation between tilt-angle oscillation periods and amplitudes, showing higher amplitudes for large periods. The variations of those periods and especially amplitudes have a periodical shape for all types of sunspots and correlate well with the solar activity maxima with a phase delay of about 1–2 years.     

Variations in the solar rotation rate derived from Ca+ K plage areas

Daily calcium plage areas for the period 1951–1981 (which include the solar cycle 19 and 20) have been used to derive the rotation period of the Sun at latitude belts 10–15 ° N, 15–20 ° N, 10–15 ° S, and 15–20 ° S and also for the entire visible solar disk. The mean rotation periods derived from 10–20 ° S and N, total active area and sunspot numbers were 27.5, 27.9, and 27.8 days (synodic), respectively. A power spectral analysis of the derived rotation rate as a function of time indicates that the rotation rate in each latitude belt varies over time scales ranging from the solar activity cycle, down to about 2 years. Variations in adjacent latitude belts are in phase, whereas those in different hemispheres are not correlated. The rotation rates derived from sunspot numbers also behave similarly though the dependence over the solar cycle are not very apparent. The total plage areas, integrated over the entire visible hemisphere of the Sun shows a dominant periodicity of 7 years in rotation rate, while the other time scales are also discernible.     

Periodicities of solar irradiance and solar activity indices,I

Using a standard FFT time series analysis, our results show an 8–11 months periodicity in the solar total and UV irradiances, 10.7 cm radio flux, Ca-K plage index, and sunspot blocking function. The physical origin of this period is not known, but the evidence in the results exclude the possibility that the observed period is a harmonic due to the FFT transform or detrending. Periods at 150–157 and 51 days are found in those solar data which are related to strong magnetic fields. The 51-day period is the dominant period in the projected areas of developing complex sunspot groups, but it is missing from the old decaying sunspot areas. This evidence suggests that the 51-day period is related to the emergence of new magnetic fields. A strong 13.5-day period is found in the total irradiance and projected areas of developing complex groups. This confirms those results (e.g., Donnelly et al., 1983, 1984; Bai, 1987, 1989) which show that active centers are located 180 deg apart from each other.Our study also shows that the modulation of various solar data due to the 27-day solar rotation is more pronounced during the declining portion of solar cycle than during the rising portion. This arises from that the active regions and their magnetic fields are better organized and more long-lived during the maximum and declining portion of solar cycle than during its rising portion.     

Solar irradiance between 120 and 400nm and its variations

The solar ultraviolet irradiance measurements in the 120–400 nm wavelength range are reviewed and compared showing still important discrepancies between the irradiance values deduced from the most recent observations.The possible variations of the solar ultraviolet irradiances with the 27-day rotation period of the Sun and with the 11-year activity cycle are presented and discussed on the basis of the available irradiation fluxes obtained during the rising phase of solar cycle 21.The spectral features of both kinds of variation are clearly related to the solar atmospheric layer from which the corresponding radiation is emitted.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Rotational Modulation of Microwave Solar Flux

Time series data of 10.7 cm solar flux for one solar cycle (1985–1995 years) was processed through autocorrelation. Rotation modulation with varying persistence and period was quite evident. The persistence of modulation seems to have no relation with sunspot numbers. The persistence of modulation is more noticeable during 1985–1986, 1989–1990, and 1990–1991. In other years the modulation is seen, but its persistence is less. The sidereal rotation period varies from 24.07 days to 26.44 days with no systematic relation with sunspot numbers. The results indicate that the solar corona rotates slightly faster than photospheric features. The solar flux was split into two parts, i.e., background emission which remains unaffected by solar rotation and the localized emission which produces the observed rotational modulation. Both these parts show a direct relation with the sunspot numbers. The magnitude of localized emission almost diminishes during the period of low sunspot number, whereas background emission remains at a 33% level even when almost no sunspots may be present. The localized regions appear to shift on the solar surface in heliolongitudes.     

Dynamics, evolution, and structure of Uranus' brightest cloud feature

The brightest cloud feature ever observed on Uranus at near-infrared wavelengths was detected on 14 and 15 August 2005, in images obtained with the NIRC2 instrument and adaptive optics (AO) at the 10-m Keck II telescope. The feature has been tracked forward and backward in time, and appears to have existed almost certainly from 5 November 2004 (possibly as early as 11 July 2004) through 29 October 2005. It appears to exhibit two modes of oscillation in latitude and longitude. The slow oscillation period is too long to be completely characterized by the observations; its period is most likely near 448 days, but might be as long as 753 days. The slow oscillation is consistent with the zonal mean wind profile when a superimposed more rapid oscillation is accounted for. The slow oscillation, possibly associated with a Rossby wave, was centered at 30.2° N and had a latitude amplitude of 0.6°–0.7°. Its rapid oscillation had an amplitude of 1.2° in latitude and a likely period near 0.68455 days, which is consistent with an inertial oscillation at the observed latitude. The multi-component structure of the bright features has evolved over time, as has its vertical structure. Its brightness maximum was due to a combination of cloud particles being lofted to higher altitudes, some rising from 400–500 to 300 mb, and by its effective cloud fraction (or equivalent cloud area) increasing by a factor of 5 or more. In the K′ band (2.2 μm) the differential integrated brightness due to this bright complex increased to 13% of the total light reflected by Uranus on 15 August 2005, rising from about 2% a month earlier and declining to 0.7% two months later. It has not been seen in 2006 observations.     

Similar periodicities in the range 12 to 150 days in solar,ionospheric and atmospheric time series

Data series for the same time interval of characteristic solar parameters (sunspot number R; flux at 2.8 GHz), ionospheric parameters (critical frequency of the E-region) and atmospheric parameters (stratospheric and tropospheric temperatures T) have been analysed by the maximum-entropy method, in order to study the occurrence of periodicities in those parameters in the range from 12 to 150 days. Digital filtering of the most pronounced of the detected periods (mainly in the range between 19 to 33 days) shows a similar but not identical feature in the time interval 1974–1978. It is demonstrated that sunspot number and solar radio flux at 2.8 GHz behave in a similar way on the average, and at periods greater than 20 days. Although a number of similar periods occurred in solar, ionospheric and atmospheric parameters, cross-correlation estimations only show a relationship between periods in solar and ionospheric data, but none between solar data and stratospheric and tropospheric temperatures; exception: T (35 km) correlates with R at 12.3 days. The most obvious correlation was found between the critical frequency of the E layer and the solar flux at 2.8 GHz at a frequency of approximately 1/23 days^–1.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

The 27-Day and 22-Year Cycles in Solar and Geomagnetic Activity

We study the evolution of the longitudinal asymmetry in solar activity through the wave packet technique applied to the period domain of 25 – 31 days (centered at the 27-day solar rotation period) for the sunspot number and geomagnetic aa index. We observe the occurrence of alternating smaller and larger amplitudes of the 11-year cycle, resulting in a 22-year periodicity in the 27-day signal. The evolution of the 22-year cycle shows a change of regime around the year 1912 when the 22-year period disappears from the sunspot number series and appears in the aa index. Other changes, such as a change in the correlation between solar and geomagnetic activity, took place at the same time. Splitting the 27-day frequency domain of aa index shows an 11-year cycle for higher frequencies and a pure22-year cycle for lower frequencies, which we attribute to higher latitude coronal holes. This evidence is particularly clear after 1940, which is another benchmark in the evolution of the aa index. We discuss briefly the mechanisms that could account for the observed features of the 22-year cycle evolution.     

Periodicities in the Time Series of Solar Coronal Radio Emissions and Chromospheric uv Emission Lines

A spectral analysis of the time series of daily values of ten solar coronal radio emissions in the range 275–1755 MHz, the 2800 MHz radio flux, several UV emission lines in the chromosphere and in the transition region, and sunspot number, for six successive intervals of 132 days each, during June 1997–July 1999 (26 months) showed that the spectral characteristics were not the same for all intervals. Details are presented for Interval 1, where there was no 27-day oscillation, and Interval 2, where there was a strong 27-day oscillation. In every interval, periodicities were remarkably similar in most of these indices, indicating that the solar atmosphere (chromosphere and corona) rotated as one block, up to a height of 150000 km. Above this height, the periodicities became obscure. Near the solar surface, sunspots showed extra or different periodicities, some of which vanished at low altitudes. For the 27-day feature as also for the long-term rise during 1996–1998, the maximum percentage changes were for radio emissions near 1350–1620 MHz.     

Coronal rotation dependence on the solar cycle phase

A study of the green corona rotation rate, during the period 1970–1974, confirms that the differential rotation degree varies systematically through a solar cycle and that the corona rotates in an almost rigid manner before sunspot minimum. During the first two years, 1970–1971, the differential rotation degree, characteristic of high solar activity periods is detected. While during the years of declining activity, 1972–1974, a drastic decrease of the differential rotation degree occurs and the green corona rotates almost rigidly, as the coronal holes observed in the same period. These conclusions are valid only for the rotation of coronal features with lifetime of at least one solar rotation.     

Characteristics of enhanced and low-amplitude cosmic-ray diurnal variation

The occurrence of a large number of high- and low-amplitude cosmic-ray diurnal wave trains during the two solar cycles (20 and 21) over the years 1965–1990 has been examined as a function of solar activity. The high-amplitude days with the time of maximum in the 18:00 hr corotation direction do not indicate any significant correlation with solar activity. But, the low-amplitude days are inversely correlated with solar activity and the time of maximum shifts to earlier hours ( 15:00 hr direction). The slope of the power-specrum density roughly characterized by power spectral index n in the high-frequency range 3.5 x 10^–5 Hz to 8.3 x 10^–4 Hz (time scales of 20 min to 8 hr) is different for the two classes of events. A suggestion is made that the enhanced and low-amplitude cosmic-ray diurnal variations are produced by different types of interplanetary magnetic field distributions.     

The peculiarities of rotation of the solar polar regions

The sidereal rotation rate of the high-latitude solar regions is examined using long-lived photospheric polar faculae. The observations were carried out with the photoheliograph of Kislovodsk Mountain Station of the Pulkovo Observatory from 1982 to 1986. The following facts have been established: (a) There is a differential rotation of the polar faculae close to the maximum of solar activity, while the amount of latitude gradient of solar rotation decreases towards the sunspot minimum; (b) small differences of rotation in the northern and southern hemispheres of the Sun are observed; (c) some deviations of differential rotation curves constructed for each Carrington rotation from the mean curve of differential rotation are revealed. The total amplitude of the maximum positive and negative excesses is about 40–50 m s^–1. The positive surplus velocities of solar rotation (the amplitude of which is about 20–25 m s^–1) move in the form of a wave from heliographic latitudes 40° with a velocity of 1.6 m s^–1. The latitude width of this flow is B 15°. This wave of abnormally high velocity starts in the year of minimum solar activity and reaches the pole 11 years later. The picture is symmetrical relative to the equator.     

Time Variability of Rotation in Solar Convection Zone From soi-mdi

The variation of rotation in the convection zone over a period of two years from mid-1996 is studied using inversions of SOI–MDI data. We confirm the existence of near-surface banded zonal flows migrating towards the equator from higher latitudes, and reveal that these banded flows extend substantially beneath the surface, possibly to depths as great as 70 Mm (10% of the solar radius). Our results also reveal apparently significant temporal variations in the rotation rate at high latitudes and in the vicinity of the tachocline over the period of study.     

Present state of the study of 160-minutes solar oscillation

Global oscillation of the Sun with a period of 160 rain were first discovered in 1974 and since observed in Crimea during the last 6 years; they were confirmed, in 1976–1979, by Doppler measurements at Stanford (Scherrer et al., 1980) and quite recently by observations of Fossat and Grec at the south geographic pole. The average amplitude of the oscillation is about 0.5 m s^-1. The phase shows remarkable stability at the period 160.010 min and good agreement between different sites on the Earth; therefore, this oscillation should now be recognized as definitely of solar origin. It is probably accompanied by synchronous fluctuations in the IR brightness and radio-emission of the Sun, and exhibits a dependence of the amplitude on the phase of solar rotation (with a peak of power at 27.2 days).In agreement with results of the Birmingham group and the South Pole observation we also find evidence in favour of a discrete spectrum within the 5 min global oscillations of the Sun, with the average splitting of about 69.5 Hz in frequency.Strict gas-dynamical equations being solved in the adiabatic approximation for a polytropic sphere n = 3 display the pattern of radial oscillations with wave packets separated by 120 m time-intervals filled with high frequency (and split by 117 Hz) oscillations implying a similarity with the observed pattern.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

Variations in solar sub-surface rotation from GONG data 1995–1998

We have completed an analysis of the first 35 GONG Months (1 GM = 36 days) covering the last solar minimum and the rising phase of cycle 23. The mode parameters have been estimated from 33 time series, each of 3-GM duration, with centers spaced by 1 GM. We report on the temporal evolution of the rotational splitting coefficients up to 15th order. The coefficients do not correlate well with any surface magnetic flux measure yet considered, but we find small but significant trends in their temporal evolution. Inverting the coefficients for two-dimensional rotation information and looking at deviations from the mean produces a picture of a systematic zonal flow migrating towards lower latitudes during the rising phase of the cycle. This flow is probably associated with the torsional oscillation. Similar trends are seen in the 1986–1990 BBSO data.