Periodicities in Forbush decreases and solar activity

We have studied Forbush decreases (FD) with solar flare data and major solar proton event (SPE) data (E > 10 MeV) for the years 1976–1986 and have found that FD solar flare data exhibit periods around 0.95, 2.4, and 4.75 years at >99% level of confidence (CL), while SPE data exhibit periods around 2.6 and 5.0 years (at >95% CL). Because of a common periodicity around 2.5 years, it is suggested that FD with the solar flare data and major SPE data, together with solar diameter and solar neutrino variations, behave similarly and may have a common origin.     

Characteristics of active-region sources of solar wind near solar maximum

Previous studies of the source regions of solar wind sampled by ACE and Ulysses showed that some solar wind originates from open magnetic flux rooted in active regions. These solar wind sources were labeled active-region sources when the open flux was from a strong field region with no corresponding coronal hole in the NSO He 10830 Å synoptic coronal-hole maps. Here, we present a detailed investigation of several of these active-region sources using ACE and Ulysses solar wind data, potential field models of the corona, and solar imaging data. We find that the solar wind from these active-region sources has distinct signatures, e.g., it generally has a higher oxygen charge state than wind associated with helium-10830 Å coronal-hole sources, indicating a hotter source region, consistent with the active region source interpretation. We compare the magnetic topology of the open field lines of these active-region sources with images of the hot corona to search for corresponding features in EUV and soft X-ray images. In most, but not all, cases, a dark area is seen in the EUV and soft X-ray image as for familiar coronal-hole sources. However, in one case no dark area was evident in the soft X-ray images: the magnetic model showed a double dipole coronal structure consistent with the images, both indicating that the footpoints of the open field lines, rooted deep within the active region, lay near the separatrix between loops connecting to two different opposite polarity regions.     

Characteristics of active-region sources of solar wind near solar maximum

Previous studies of the source regions of solar wind sampled by ACE and Ulysses showed that some solar wind originates from open magnetic flux rooted in active regions. These solar wind sources were labeled active-region sources when the open flux was from a strong field region with no corresponding coronal hole in the NSO He 10830 Å synoptic coronal-hole maps. Here, we present a detailed investigation of several of these active-region sources using ACE and Ulysses solar wind data, potential field models of the corona, and solar imaging data. We find that the solar wind from these active-region sources has distinct signatures, e.g., it generally has a higher oxygen charge state than wind associated with helium-10830 Å coronal-hole sources, indicating a hotter source region, consistent with the active region source interpretation. We compare the magnetic topology of the open field lines of these active-region sources with images of the hot corona to search for corresponding features in EUV and soft X-ray images. In most, but not all, cases, a dark area is seen in the EUV and soft X-ray image as for familiar coronal-hole sources. However, in one case no dark area was evident in the soft X-ray images: the magnetic model showed a double dipole coronal structure consistent with the images, both indicating that the footpoints of the open field lines, rooted deep within the active region, lay near the separatrix between loops connecting to two different opposite polarity regions.     

Global isothermal oscillations in the solar photosphere

Observational data on solar irradiance oscillations from the VIRGO (SOHO) and DIFOS-F (CORONAS-F) experiments are used to obtain stratifications of perturbed hydrogen concentrations that produce isothermal oscillations in the solar photosphere. The study reveals the nodes and antinodes of the oscillations in the solar photosphere. A simulation of long-period isothermal oscillations from the DIFOS data shows that the nodes and antinodes of Δn/n tend to shift towards lower photosphere layers with a decrease in the oscillation frequency.     

The phase variations of the solar cycle

It has previously been shown that the statistics of the phase fluctuation of the sunspot cycle are compatible with the assumption that the solar magnetic field is generated deep in the Sun by a frequency stable oscillator and that the observed substantial phase fluctuation in the sunspot cycle is due to variation in the time required for the magnetic field to move to the solar surface (Dicke, 1978, 1979). It was shown that the observed phase shifts are strongly correlated with the amplitude of the solar cycle. It is shown here that of two empirical models for the transport of magnetic flux to the surface, the best fit to the data is obtained with a model for which the magnetic flux is carried to the surface by convection with the convection velocity proportional to a function of the solar cycle amplitude. The best fit of this model to the data is obtained for a 12-yr transit time. The period obtained for the solar cycle is T = 22.219 ± 0.032 yr. It is shown that the great solar anomaly of 1760–1800 is most likely real and not due to poor data.     

Semi-empirical solar line blanketing

Statistical data for line blanketing in the solar atmosphere have been extracted from the Revised Rowland Table. The data are corrected for apparent observational incompleteness due to line crowding and blending and are placed in a convenient form for calculations. An attempt is made to interpret the data in terms of schematic representations of the energy levels and distribution of gf-values for the important absorbers. The analysis leads to predictions in good agreement with the forms of the statistical data and substantiates the assumption of observational incompleteness in the original data. The analysis suggests that the solar data may provide useful guidance in the blanketing of stellar atmospheres sufficiently close to solar type.A blanketing method is presented which makes use of a uniformly-smeared-line (single picket) procedure and which assumes LTE and a weak-line curve of growth. The blanketing coefficient is obtained in a simple manner from the corrected statistical data and is both depth and wavelength dependent. An alternative method utilizing a four-picket scheme and average absorption coefficient profiles is suggested. Comparison of solar models computed by the two methods shows good agreement and gives support for the smeared-line procedure. Comparison of the solar model with a non-LTE blanketed model of Athay also shows reasonable agreement.Publications of the Goethe Link Observatory, Indiana University, No. 110.     

Comparison of long-term trend of solar radius with sunspot activity and flare index

Results are presented from a study of solar radius measurements taken with the solar astrolabe at the TUBITAK National Observatory (TUG) over seven years, 2001–2007. The data series with standard deviation of 0.35 arcsec shows the long-term variational trend with 0.04 arcsec/year. On the other hand, the data series of solar radius are compared with the data of sunspot activity and H-α flare index for the same period. Over the seven year trend, we have found significant linear anti-correlations between the solar radius and other indicators such as sunspot numbers, sunspot areas, and H-α flare index. While the solar radius displays the strongest anti-correlation (−0.7676) with sunspot numbers, it shows a significant anti-correlation of −0.6365 with sunspot areas. But, the anti-correlation between the solar radius and H-α flare index is found to be −0.4975, slightly lower than others. In addition, we computed Hurst exponent of the data sets ranging between 0.7214 and 0.7996, exhibiting the persistent behavior for the long term trend. In the light of the strong correlations with high significance, we may suggest that there are a causal relationship between the solar radius and solar time series such as sunspot activity and H-α flare index.     

Frequencies of low-degree solar acoustic oscillations and the phase of the solar cycle

A study of the solar total irradiance data of the Active Cavity Radiometer Irradiance Monitor (ACRIM) on the Solar Maximum Mission (SMM) satellite shows a small but formally significant shift in the frequencies of solar acoustic (p-mode) oscillations between the epochs of maximum and minimum solar activity. Specifically, the mean frequency of the strongest p-mode resonances of low spherical-harmonic degree (l = 0–2) is approximately 1.3 parts in 10⁴ higher in 1980, near the time of sunspot maximum, than in 1985, near sunspot minimum. The observed frequency shift may be an 11-yr effect but the precise mechanism is not clear.     

Observation of hysteresis between solar activity indicators andp-mode frequency shifts for solar cycle 22

Using intermediate degreep-mode frequency data sets for solar cycle 22, we find that the frequency shifts and magnetic activity indicators show a “hysteresis” phenomenon. It is observed that the magnetic indices follow different paths for the ascending and descending phases of the solar cycle while for radiative indices, the separation between the paths are well within the error limits.     

A new observational strategy in search for solar g-modes

The aim of the present work is the detection of solar g-modes, by means of a new observational strategy based on the exploitation of their spatial and temporal properties. The basic data, obtained at the Observatorio del Teide in 1993, consists of daily solar velocity measurements taken continuously and sequentially at six different and symmetric positions on the solar disk. By correlating the time series resulting from the reduction process, from different solar disk positions and considering the geometrical properties of different modes (l,m) on the Sun's surface, some of these can be selectively eliminated or enhanced. Moreover, the main spectral features present in the resulting power spectra must have precise phase relations if they correspond to global solar g-modes. The severe constraints established by the above properties have been applied to the best observed series (summer 1993). As a result, a discrete series of peaks have been selected that fulfill all the imposed conditions and which can therefore be interpreted as being of solar origin.     

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.     

A two-component solar cycle

From a previous analysis of a long series of geomagnetic data, we came to the conclusion that, during 91.5% of the time, geomagnetic activity is controlled by the solar wind flow at the Earth's orbit.In this paper, we consider the flow of the solar wind plasma in a coronal field whose source is a dipole. The temporal evolution of the dipole source as well as any small scale evolution occurring in the associated coronal field topology can be closely monitored from the latitudinal distribution of the wind velocity.In the geomagnetic data series, the index Aa is closely linked to the wind velocity at the power 2.25. From this data set, we can reconstruct the behavior of the solar dipole field from 1868 onward.The main results of our analysis are as follows. The solar cycle has two distinct components, dipole and toroidal, of which the respective cycles are out of phase. The toroidal component is strongly linked, with a 5–6 yr delay, to the preceding dipole component. This finding is in contradistinction to the view that the dipole field is a result of the poleward migration of the decaying toroidal field. This result should contribute to improve our understanding of the Sun's cyclical behaviour.     

The solar wind velocity in the eleven-year cycle no. 20 and the solar radar cross-section

The annual average values of the solar wind velocity over the period 1962–1972 were investigated on the basis of data obtained from different space probes. The comparison of the pattern of the annual average solar wind velocities observed by the Vela and Pioneer 6 satellites indicates that the pattern presented by Gosling et al. (1971) is realistic. The long-range trend in the solar wind velocity during the 11-year cycle is governed by the number and intensity of irregularities occurring in the corona. These irregularities may represent motions of mass or some types of MHD shock waves and they are responsible for the increased heating of the corona which then in turn causes an increase in the values of the solar radar cross-section and of the solar wind velocity. A close relation is demonstrated between the monthly and annual average values of the solar wind velocity and of the cross-section.     

Response of peak electron densities in the martian ionosphere to day-to-day changes in solar flux due to solar rotation

Photochemical Chapman theory predicts that the square of peak electron density, N_m, in the dayside ionosphere of Mars is proportional to the cosine of solar zenith angle. We use Mars Global Surveyor Radio Science profiles of electron density to demonstrate that this relationship is generally satisfied and that positive or negative residuals between observed and predicted values of are caused by periods of relatively high or low solar flux, respectively.Understanding the response of the martian ionosphere to changes in solar flux requires simultaneous observations of the martian ionosphere and of solar flux at Mars, but solar flux measurements are only available at Earth. Since the Sun's output varies both in time and with solar latitude and longitude, solar flux at Mars is not simply related to solar flux at Earth by an inverse-square law. We hypothesize that, when corrected for differing distances from the Sun, solar fluxes at Mars and Earth are identical when shifted in time by the interval necessary for the Sun to rotate through the Earth–Sun–Mars angle.We perform four case studies that quantitatively compare time series of N_m at Mars to time series of solar flux at Earth and find that our hypothesis is satisfied in the three of them that used ionospheric data from the northern hemisphere. We define a solar flux proxy at Mars based upon the E_10.7 proxy for solar flux at Earth and use our best case study to derive an equation that relates N_m to this proxy. We discuss how the ionosphere of Mars can be used to infer the presence of solar active regions not facing the Earth.Our fourth case study uses ionospheric observations from the southern hemisphere at latitudes where there are strong crustal magnetic anomalies. These profiles do not have Chapman-like shapes, unlike those of the other three case studies. We split this set of measurements into two subsets, corresponding to whether or not they were made at longitudes with strong crustal magnetic anomalies. Neither subset shows N_m responding to changes in solar flux in the manner that we observe in the three other case studies.We find many similarities in ionospheric responses to short-term and long-term changes in solar flux for Venus, Earth, and Mars. We consider the implications of our results for different parametric equations that have been published describing this response.     

Rotational enhancement of Doppler measurements of solar and stellar hexadecapole oscillations

Rotational enhancement of the sensitivity of whole-disk Doppler observations of solar oscillations may permit the measurement of five-minute modes with l = 4. We estimate from superposed power spectra of artificial solar data that there might be identifiable power lying above the noise in the data acquired by Pallé et al. (1986), which could provide confirmation of the rotational splitting measured by Duvall and Harvey (1984).     

A search for solar neutrons during solar flares

The upper limit on the solar neutron flux from 1–20 MeV has been measured, by a neutron detector on the OGO-6 satellite, to be less than 5 × 10^–2 n cm^–2 s^–1 at the 95% confidence level for several flares including two flares of importance 3B and a solar proton event of importance 3B. The measurements are consistent with the models proposed by Lingenfelter (1969) and by Lingenfelter and Ramaty (1967) for solar neutron production during solar flares. The implied upper limit on the flux of 2.2 MeV solar gamma rays is about the same as the 2.2 MeV flux observed by Chupp et al. (1973).     

Integrated colors in the solar neighborhood

The bivariate spectral type-luminosity class distribution combined with thez-distribution and broad-band photometric data have been used in order to derive integrated colors in Johnson's UBVRIJKL system for the solar neighborhood.The frequency distribution of white dwarfs is also taken into account for theU-B, B-V colors.     

Solar neutrino flux,cosmic rays,and the solar activity cycle

It is suggested that the experimental data on the solar neutrino flux as measured by Davis and his collaborators from 1970 to 1982 vary with the solar activity cycle to a very high level of statistical significance for all the available tests of the hypothesis (e.g., (t-test, ²-test, run test, Wilcoxon-Mann-Whitney test) when the solar neutrino flux data are computed from the weighted moving averages of order 5. The above tests have also been applied to the data that have been generated by the Monte Carlo simulation with production rate and background rate parameters that are typical of those in the actual experiment. It is shown that the Monte Carlo simulated data do not indicate a variation within the solar cycle. Thus the moving average data strongly favours the variation within the solar activity cycle.     

Coronal sources of high-speed plasma streams in the solar wind during the declining phase of solar cycle 20

A correlative study is made between inferred solar sources of high-speed solar wind streams and extended white-light coronal features. The solar wind data used in the study consists of 110 co-rotating high-speed plasma streams observed from spacecraft at 1 AU in the period February 1971-December 1974; the coronal data consists of 144 equatorward extensions of polar coronal holes and 15 equatorial coronal holes, derived fromK-coronometer maps of the white-light corona during the same period. Of 110 observed solar wind streams 88 could directly be associated with an equatorward extension of a polar-cap coronal hole and 14 could be associated with a low-latitude equatorial coronal hole. In 8 cases no visible coronal feature was identified. Of 144 identified polar-cap extensions 102 were associated with a high-speed stream observed at 1 AU; 19 coronal features were related in time to data gaps in the solar wind measurements, while 38 features did not give rise to solar wind streams observed at Earth orbit. The probability of an association depended on the heliographic co-latitude of a polar hole extension, being 50% for a polar lobe extending down to 45° co-latitude and 100% for a polar coronal hole extending to 80° co-latitude or more.Paper presented at the 11th European Regional Astronomical Meeting of the IAU on New Windows to the Univese, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

A solar cycle variation of the interplanetary magnetic field

Measurements of the north-south (B _z component of the interplanetary field as compiled by King (1975) when organized into yearly histograms of the values of ¦B _z ¦ reveal the following. (1) The histograms decrease exponentially from a maximum occurrence frequency at the value ¦B _z ¦ = 0. (2) The slope of the exponential on a semi-log plot varies systematically roughly in phase with the sunspot number in such a way that the probability of large values of ¦B _z ¦ is much greater in the years near sunspot maximum than in the years near sunspot minimum. (3) There is a sparsely populated high-value tail, for which the data are too meager to discern any solar cycle variation. The high-value tail is perhaps associated with travelling interplanetary disturbances. (4) The solar cycle variations of B _z and the ordinary indicators of solar activity are roughly correlated. (5) The solar cycle variation of B _z is distinctly different than that of the solar wind speed and that of the geomagnetic Ap disturbance index.Now at the Aerospace Corporation, El Segundo, Calif. 90245, U.S.A.