Coronal holes,solar wind streams,and recurrent geomagnetic disturbances: 1973–1976

Observations of coronal holes, solar wind streams, and geomagnetic disturbances during 1973–1976 are compared in a 27-day pictorial format which shows their long-term evolution. The results leave little doubt that coronal holes are related to the high-speed streams and their associated recurrent geomagnetic disturbances. In particular, these observations strongly support the hypothesis that coronal holes are the solar origin of the high-speed streams observed in the solar wind near the ecliptic plane.Visiting Scientist, Kitt Peak National Observatory, Tucson, Arizona.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Short-term variations of the galactic cosmic ray intensity: 1964–1967

Variations of the ground-level nucleonic intensity and of indices of solar photospheric, chromospheric, and coronal activity at time scales of less than forty days show strong 27-day recurrent structure. Correlation functions for these time series suggest an origin for short-term modulation in impulsive solar activity. Flare-associated shocks originating in long-lived active zones fixed in solar longitude produce recurrent cosmic ray variations during 1964–1967, and corotating solar wind disturbances are of secondary importance to short-term modulation.     

Solar-cycle phenomena in cosmic-ray intensity: Differences between even and odd cycles

Cosmic-ray intensity data for the period 1964–1985 covering two solar cycles are used to investigate the solar activity behaviour in relation to cosmic-ray modulation. A detailed statistical analysis of them shows a large time-lag of about one and half years between cosmic-ray intensity and solar activity (as indicated by sunspot number, solar flares and high-speed solar-wind streams) during the 21st solar cycle appearing for a first time. This lag indicates the very high activity level of this solar cycle estimating the size of the modulating region to the unambiguous value of 180 AU. The account of the solar-wind speed in the 11-year variation significantly decreases the modulation region of cosmic-rays to the value of 40 AU.A comparison with the behaviour of the previous solar cycle establishes a distinction between even and odd solar cycles. This is explained in terms of different contributions of drift, convection and diffusion to the whole modulation mechanism during even and odd solar cycles.     

Hysteresis,time lag,and relation between solar activity and cosmic rays during solar cycle 24

We study galactic cosmic ray (GCR) modulation during solar cycle 24. For this study we utilize neutron monitor data together with sunspot number (SSN) and 10.7 cm solar radio flux (SRF) data. We plot hysteresis curve between the GCR intensity and SSN, and GCR intensity and SRF. We performed time-lag correlation analysis to determine the time lag between GCR intensity and solar activity parameters. The time lag is determined not only for the whole solar cycle, but also during the two polarity states of the heliosphere (A<0 and A>0) in solar cycle 24. We notice differences in time lags during two polarity epochs of the solar cycle. We discuss these differences in the light of existing modulation models. We compare the results of this very weak solar activity cycle with the corresponding results reported for the previous comparatively more active solar cycles.     

Cosmic-ray intensity versus solar soft X-ray background in cross-correlation analysis

The relationship between the galactic cosmic ray modulation (CR) and the non-flare coronal level, as given by the solar soft X-ray background (XBG), is investigated from 1 July 1968 to 30 June 1980 on a daily basis. The stationarity problem of a multivariate time series, as well as the role of the short- and medium-term corona variability are faced. From them it is found that the CR/XBG relation is variable during the considered heliomagnetic semicycle, while CR and XBG are highly anticorrelated on a long-time scale (12-month averages). The CR/XBG relationship during the declining phase of solar activity shows a moderately strong anticorrelation, on short- and medium-term time scales (coefficient up to –0.77 for 27-day running averages), went towards insignificant values in the minimum phase and is only partially reconstructed during the rise of the following solar cycle. During the solar activity maximum of cycle 20 the cosmic-ray modulation is only related to the short-term coronal fluctuations (no other time scales are significant, supporting the reliability of the so-called `Gnevyshev gap' in solar parameters).     

Short and long-term variations in the cosmic ray intensity and their connection with the 5303 Å coronal line intensity in solar cycle No. 20

The cosmic ray modulation in the period 1965–70 is investigated by the comparison of the intensity data of groundbased stations with different response to primaries. The socalled step-like modulation, already observed by other authors, is found to be produced by the overlapping between the quasi-stationary solar cycle modulation and the Forbush decrease events. Moreover a good correlation between the cosmic-ray variance (Forbush decrease index) and the 5303 coronal intensity at middle heliolatitudes (17.5°–42.5°) is found, while the quasi-stationary solar cycle modulation is well correlated with the 5303 intensity near the solar equator (0°–17.5°). The different time behaviour of the solar activity at different heliolatitudes causes the step-like modulation.     

Ephemeral active regions during solar minimum

Ephemeral active regions (ER) identified on Kitt Peak daily full-disk magnetograms from April through November 1975 were analyzed and compared with larger active regions during the same interval. The 1975 ER were also compared with ER data from 1970, 1973, 1976, and 1977. ER were found to vary approximately with the sunspot cycle. However, a minimum in the number of ER occurred at least one year prior to sunspot minimum. All evidence to date indicates that the early ER minimum was due to the rise of solar cycle 21 primarily in the form of ER. ER were statistically identified as belonging to both outgoing solar cycle 20 and incoming cycle 21 by maxima in their distribution in latitude and by their statistically dominant orientation as a function of latitude. From the identification of ER with specific solar cycles and the persistent presence of high latitude ER maxima since 1970, it is suggested that the outgoing and incoming solar cycles may co-exist on the sun longer than the 0–3 year period of overlap between successive cycles already known from the properties of large sunspot-producing active regions.Presently associated with Solar Physics Research Corporation, Tucson, Arizona and Visiting Astronomer at Kitt Peak National Observatory, operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The residual cosmic ray modulation at the 1954 solar minimum

Analysis of the anomalous solar diurnal variation in cosmic rays at the 1954 solar minimum yields an estimate of the residual solar modulation present at that time. The data are consistent with a modulation parameter M in the range 0.6 < M 1.5 GV. Reasons are given for preferring a value in the upper half of this range.     

A pictorial comparison of interplanetary magnetic field polarity,solar wind speed,and geomagnetic disturbance index during the sunspot cycle

Observations of interplanetary magnetic field polarity, solar wind speed, and geomagnetic disturbance index (C9) during the years 1962–1975 are compared in a 27-day pictorial format that emphasizes their associated variations during the sunspot cycle. This display accentuates graphically several recently reported features of solar wind streams including the fact that the streams were faster, wider, and longer-lived during 1962–1964 and 1973–1975 in the declining phase of the sunspot cycle than during intervening years (Bame et al., 1976; Gosling et al., 1976). The display reveals strikingly that these high-speed streams were associated with the major, recurrent patterns of geomagnetic activity that are characteristic of the declining phase of the sunspot cycle. Finally, the display shows that during 1962–1975 the association between long-lived solar wind streams and recurrent geomagnetic disturbances was modulated by the annual variation (Burch, 1973) of the response of the geomagnetic field to solar wind conditions. The phase of this annual variation depends on the polarity of the interplanetary magnetic field in the sense that negative sectors of the interplanetary field have their greatest geomagnetic effect in northern hemisphere spring, and positive sectors have their greatest effect in the fall. During 1965–1972 when the solar wind streams were relatively slow (500 km s^-1), the annual variation strongly influenced the visibility of the corresponding geomagnetic disturbance patterns.Visiting Scientist, Kitt Peak National Observatory, Tucson, Arizona.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

Numerical studies of azimuthal modulations of the solar wind with magnetic fields

The evolution of azimuthal modulation of the solar wind is examined by numerical integration of the basic equations governing supersonic, super-Alfvénic, polytropic one fluid flow in the equatorial plane of the Sun with radial as well as azumuthal magnetic fields. The computations are started by introducing various sets of finite amplitude perturbations stationary with respect to the Sun. It is shown that for a properly chosen set of simultaneous perturbations of the radial velocity and temperature, quantitative agreement with the averaged observational results at 1 AU can be obtained, including the advance of the density modulation with respect to those of velocity and temperature. It is further shown that modulations at 1 AU depend on the relative azimuthal positions between the Sun and the Earth.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Stokes II — A new polarimeter for solar observations

A Stokes polarimeter has been built at the High Altitude Observatory to obtain line profiles in both linear and circular polarization in solar spectral lines. These measurements are interpreted using the theory of radiative transfer in the presence of a magnetic field to obtain vector magnetic fields on the solar disk and using the theory of resonance scattering and the Hanle effect to obtain vector magnetic fields in prominences. The polarimeter operates on the Sacramento Peak Observatory 40 cm coronagraph. It is an extensively modified and improved version of an earlier instrument.Polarization modulation is achieved by two KD^*P Pockels cells at the coronagraph prime focus and demodulation is by a microprocessor. The instrument control and data handling is done by a minicomputer. Silicon photodiode 128 element line array detectors have replaced the two photomultipliers used on the earlier instrument. This gives a speed increase of a factor of 50.A polarization scrambler provides a chop to a reference beam of unpolarized light by time scrambling the polarization of the solar beam. This device improves sensitivity to polarizations less than 0.01%. The polarization measurements are photon noise limited in most cases. This noise is 0.1% for a typical three second observation which is about one gauss on the longitudinal field and 10 gauss on the transverse field.The National Center for Atmospheric Research is sponsored by The National Science Foundation.     

A tidal theory for the origin of the solar nebula

There are two angular momentum (AM) problems associated with the formation of stars in general and the solar system in particular. The first is how to dispose of the AM possessed by turbulent protostellar clouds. Two-dimensional calculations of the gravitational infall of rotating gas clouds by several authors now indicate that stars are formed in groups or clusters rather than as single entities. Added evidence comes from observation of probable regions of star formation and young clusters, plus the fact that most stars are presently members of binaries or other multiples. Thus the first problem is solved by postulating the fragmentation of massive clouds with most of the AM ending up in the relative orbits. These clusters are notoriously unstable and evolve with the ejection of single stars like the Sun.The second problem is the uneven distribution of AM with mass in the solar system. It turns out that the collapse time for the majority of the infalling material is comparable to the time necessary for significant dynamical interaction of the protostellar fragment with its neighbors. It is found here through calculations utilizing very simplified numerical models that the last few tens of percent of infalling material can easily have sufficient AM transferred to it by the tidal action of passing protostars to form a solar nebula and ensure alignment of the solar spin. The most important parameter is the degree of central condensation: fragments without several tenthsM in a central core tend to be torn apart by encounters, or at least stimulated into binary fission. A stabilizing central mass maintains its identity and acquires a rotating envelope of material.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Improvements in the Determination of ISS Ca <Emphasis Type="SmallCaps">ii</Emphasis> K Parameters

Measurements of the ionized Ca ii K line are one of the major resources for long-term studies of solar and stellar activity. They also play a critical role in many studies related to solar irradiance variability, particularly as a ground-based proxy to model the solar ultraviolet flux variation that may influence the Earth’s climate. Full disk images of the Sun in Ca ii K have been available from various observatories for more than 100 years and latter synoptic Sun-as-a-star observations in Ca ii K began in the early 1970s. One of these instruments, the Integrated Sunlight Spectrometer (ISS) has been in operation at Kitt Peak (Arizona) since late 2006. The ISS takes daily observations of solar spectra in nine spectra bands, including the Ca ii K and H lines. We describe recent improvements in data reduction of Ca ii K observations, and present time variations of nine parameters derived from the profile of this spectral line.     

FIRST RESULTS FROM VIRGO,THE EXPERIMENT FOR HELIOSEISMOLOGY AND SOLAR IRRADIANCE MONITORING ON SOHO

First results from the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and Heliospheric Observatory) are reported. The observations started mid-January 1996 for the radiometers and sunphotometers and near the end of March for the luminosity oscillation imager. The performance of all the instruments is very good, and the time series of the first 4–6 months are evaluated in terms of solar irradiance variability, solar background noise characteristics and p-mode oscillations. The solar irradiance is modulated by the passage of active regions across the disk, but not all of the modulation is straightforwardly explained in terms of sunspot flux blocking and facular enhancement. Helioseismic inversions of the observed p-mode frequencies are more-or-less in agreement with the latest standard solar models. The comparison of VIRGO results with earlier ones shows evidence that magnetic activity plays a significant role in the dynamics of the oscillations beyond its modulation of the resonant frequencies. Moreover, by comparing the amplitudes of different components ofp -mode multiplets, each of which are influenced differently by spatial inhomogeneity, we have found that activity enhances excitation.     

Intermediate‐term variations in solar radius during solar cycle 23

In this study, we look for the mid‐term variations in the daily average data of solar radius measurements made at the Solar Astrolabe Station of TUBITAK National Observatory (TUG) during solar cycle 23 for a time interval from 2000 February 26 to 2006 November 15. Due to the weather conditions and seasonal effect dependent on the latitude, the data series has the temporal gaps. For spectral analysis of the data series, thus, we use the Date Compensated Discrete Fourier Transform (DCDFT) and the CLEANest algorithm, which are powerful methods for irregularly spaced data. The CLEANest spectra of the solar radius data exhibit several significant mid‐term periodicities at 393.2, 338.9, 206.5, 195.2, 172.3 and 125.4 days which are consistent with periods detected in several solar time series by several authors during different solar cycles. The knowledge relating to the origin of solar radius variations is not yet present. To see whether these variations will repeat in next cycles and to understand how the amplitudes of such variations change with different phases of the solar cycles, we need more systematic efforts and the long‐term homogeneous data. Since most of the periodicities detected in the present study are frequently seen in solar activity indicators, it is thought that the physical mechanisms driving the periodicities of solar activity may also be effective in solar radius variations (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Piston shock and Forbush effect

A model of a piston shock produced by a sharp jump in the velocity of the solar wind with a helical magnetic field is suggested to explain the origin of the Forbush decrease of cosmic rays with a hard energy spectrum at solar activity minimum and, in some cases, at solar activity maximum. The contact velocity, the compression ratio, the contact position, the ratio of the magnetic field near the contact to the seed field, the maximum momentum of the particles subjected to modulation, and the modulation depth for the neutron and muon cosmic ray components have been calculated. The theoretical calculations are compared with experimental data.     

The Maunder Minimum and the Sun as the Possible Source of Particles Creating Increased Abundance of the <Superscript>14</Superscript>C Carbon Isotope

The Maunder Minimum was the time during the second part of the 17th century, nominally from 1645 to 1717 AD, when unusually low numbers of sunspots were observed. On the basis of numerous recorded observations of auroras in the early 18th century, the end of the Minimum could be regarded as around 1700, but details of sunspot observations by Jan Heweliusz (Heweliusz, Machina Coelestis, 1679), John Flamsteed and Philippe de La Hire in 1684 allow us to interpret the Maunder Minimum as the period without a significant cessation of activity. This Minimum was also recognized in ¹⁴C data from trees which grew during the second part of 17th century. The variation in the production rate of radioactive carbon isotope ¹⁴C is due to modulation of the cosmic ray flux producing it by the changing level of solar activity and solar magnetic flux. Stronger magnetic fields in the solar wind make it more difficult for cosmic rays to reach the Earth, causing a drop in the production rate of ¹⁴C. However, more detailed analyses of ¹⁴C data indicate that the highest isotope abundances do not occur at the time of sunspot minima, as would be expected on the basis of modulation of the cosmic ray flux by the solar magnetic field, but two years after the sunspot number maximum. This time difference (or phase delay) can be accounted for if in fact there are both solar and non-solar cosmic ray contributions. Solar flares could also contribute high-energy particles and produce ¹⁴C and are generally not most frequent at the time of the highest sunspot numbers in the cycle.     

Heliospheric Modulation of Galactic Cosmic Rays During Solar Cycle 23

Galactic cosmic rays (GCRs) encounter an outward-moving solar wind with cyclic magnetic-field fluctuation and turbulence. This causes convection and diffusion in the heliosphere. The GCR counts from the ground-based neutron monitor stations show intensity changes that are anti-correlated with the sunspot numbers with a lag of a few months. GCRs experience various types of modulation from different solar activity features and influence space weather and the terrestrial climate. In this work, we investigate certain aspects of the GCR modulation at low cut-off rigidity (R _c≈1 GV) in relation to some solar and geomagnetic indices for the entire solar cycle 23 (1996?–?2008). We separately study the GCR modulation during the ascending phase of cycle 23 including its maximum (1996?–?2002) and the descending phase including its minimum (2003?–?2008). We find that during the descending phase, the GCR recoveries are much faster than those of the solar parameters with negative time-lag. The results are discussed in light of modulation models, including drift effects and previous results.     

Solar Variability Induced in a Dynamo Code by Realistic Meridional Circulation Variations

In this work we use an already-published method to infer a variation profile for the solar meridional circulation over the last 250 years. We feed this variation profile into a numerical dynamo code, and we reconstruct a sunspot time series that acts as a proxy for solar cycle activity. We perform three simulations with slightly different parameters, and the results are compared with the observational data. The medium and large correlation coefficients between reconstructed and observational time series seem to indicate that variations in meridional circulation play an important role in the modulation of solar activity.     

High- And Low-Latitude Solar Activities: Who Leads Whom In Time Phase?

Using the data from observations of polar faculae by the National Astronomical Observatory of Japan from July 1951 to December 1998, we investigate whether there is a time lag between high-latitude solar activity and low-latitude solar activity. The cross-correlation analysis of the smoothed monthly numbers of the polar faculae with the smoothed monthly sunspot numbers shows that, high-latitude solar activity should lead low-latitude solar activity in time phase. The periodic characteristics of both of them also indicate that high-latitude activity evidently leads low-latitude activity.