Effects of the Polarity States of the Heliospheric Magnetic Field and Particle Drifts in Cosmic Radiation

Forbush decrease (FD) events recorded at the ground-based neutron monitors (NMs) during the period 1961 – 1999, have been selected and recovery characteristic of these events have been analyzed. The average profile of FDs observed during different polarity states of the heliosphere is obtained by superposed epoch analysis separately for the periods 1961 – 1969 (A < 0), 1971 – 1979 (A > 0), 1981 – 1989 (A < 0) and 1991 – 1999 (A > 0). Hourly count rate of neutron monitors of different cut-off rigidities have been utilized. The results are compared with model predictions including drifts. No marked difference is observed in the amplitudes of FDs during A < 0 and A > 0. Rigidity spectrum fitted with a power law yields the values of spectral exponent that are closer to values predicted by two-dimensional models including drifts. The recovery rate of FDs varies with the polarity of HMF and the rate is higher (recovery time smaller) during A > 0 than during A < 0 epoch, consistent with the model predictions including the drift effects in the HMF. This difference in recovery time of FDs during A > 0 and A < 0 polarity conditions provides experimental evidence that drift plays an important role in cosmic ray modulation.     

Spacecraft measurement of Forbush decreases in the cosmic radiation

The Forbush decrease in the cosmic radiation has been measured by a charged-particle monitor (E _p )> 50 MeV) on board the OGO-6 satellite. For the events of June 7–10, September 27–30, and November 21–December 6, 1969, the Forbush decrease totalled 4.6, 6, and 6% in amplitude, respectively, for the Mt. Washington neutron monitor (P _c = 1.24 GV), and 5.2, 13, and 16%, respectively, for the OGO-6 charged-particle monitor in the polar region (P _c < 0.3 GB). The depression in the OGO-6 charged-particle monitor was larger at higher geomagnetic latitudes than at lower latitudes. However, for the events of June 7–10 and November 21–December 6, 1969, the Forbush decrease totalled 20 and 15% in amplitude respectively for the Pioneer 8 cosmic-ray telescope (P _c > 0.4 GV), which was at the respective distances of 1.08 AU and 1 AU from the Sun. These results indicate that the Forbush decrease has greater effects on lower-energy charged particles, the magnitude of the effect also depending on the location of the detector with respect to the modulating region.The spacecraft data near Earth also showed that, for vertical cut-off rigidities P _c 1.8 GV, the total percentage decrease in the amplitudes of the Forbush decreases can be represented by –mP _c + k, where m and k are each constant for the particular Forbush decrease but which increase with increasing Mt. Washington neutron monitor monthly average rates, an indication of a flattening of the rigidity dependence of Forbush decreases towards maximum solar modulation.     

Variations of the Solar Differential Rotation Associated with Polarity Reversal

Variations of solar differential rotation have been studied using observations of solar quiescent Hα filaments obtained during 1965–1993 at the Abastumani Astrophysical Observatory. In both hemispheres of the Sun, propagation of a quasi-biennial pulse of residual rotation velocities of filaments was found. There is a pulse drift from high latitudes to the equator in the northern hemisphere in 1968–1970, 1979–1981, 1988–1990 and in the southern one in 1969–1971, 1979–1981, 1989–1991. Propagation of a pulse starts near the time of the polarity reversal of the circumpolar regions of the Sun. High-latitude double peaks of rapid motion were found in the northern hemisphere for cycle 20 and in the southern hemisphere for cycle 22. The relation of the appearance of suggested double pulse peaks of residual velocities with the threefold polarity changing of the circumpolar areas is suggested.     

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.     

Cosmic-Ray Decreases and Interplanetary Disturbances During the Solar Magnetic Polarity Reversal in Solar Cycle 20

Using the cosmic-ray intensity data recorded with ground-based monitors at Mt. Washington and Deep River, and with cosmic-ray telescopes on Pioneer 8 and 9 spacecraft as well as the 2-hour averages of the IMF (magnitude and direction) and the solar wind bulk speed and density at 1 AU, the cosmic-ray decreases and interplanetary disturbances, that occurred during the period of solar magnetic polarity reversal in solar cycle 20, were investigated.We observed a two-step Forbush decrease on 22–23 November 1969, and a Forbush decrease on 26 November 1969, which are respectively consistent with the model of Barnden (1973), and of Parker (1963) and Barnden (1973). Only one Forbush decrease event was observed in December 1969, a period during which there was a solar magnetic polarity reversal; the Forbush decrease was attributed to a long-lived corotating high-speed solar wind stream. This is indicative that at heliolongitudes from 43° E to 70° W of S–E radial, covered by the observations, the solar magnetic polarity reversal in solar cycle 20 was not carried by, nor related to, individual transient structures, and that the reversal most probably evolved gradually.     

The 22-Year Hale Cycle in Cosmic Ray Flux – Evidence for Direct Heliospheric Modulation

The ability to predict times of greater galactic cosmic ray (GCR) fluxes is important for reducing the hazards caused by these particles to satellite communications, aviation, or astronauts. The 11-year solar-cycle variation in cosmic rays is highly correlated with the strength of the heliospheric magnetic field. Differences in GCR flux during alternate solar cycles yield a 22-year cycle, known as the Hale Cycle, which is thought to be due to different particle drift patterns when the northern solar pole has predominantly positive (denoted as qA>0 cycle) or negative (qA<0) polarities. This results in the onset of the peak cosmic-ray flux at Earth occurring earlier during qA>0 cycles than for qA<0 cycles, which in turn causes the peak to be more dome-shaped for qA>0 and more sharply peaked for qA<0. In this study, we demonstrate that properties of the large-scale heliospheric magnetic field are different during the declining phase of the qA<0 and qA>0 solar cycles, when the difference in GCR flux is most apparent. This suggests that particle drifts may not be the sole mechanism responsible for the Hale Cycle in GCR flux at Earth. However, we also demonstrate that these polarity-dependent heliospheric differences are evident during the space-age but are much less clear in earlier data: using geomagnetic reconstructions, we show that for the period of 1905?–?1965, alternate polarities do not give as significant a difference during the declining phase of the solar cycle. Thus we suggest that the 22-year cycle in cosmic-ray flux is at least partly the result of direct modulation by the heliospheric magnetic field and that this effect may be primarily limited to the grand solar maximum of the space-age.     

ON THE POSSIBLE CHANGES OF THE SOLAR DIFFERENTIAL ROTATION DURING THE ACTIVITY CYCLE DETERMINED USING MICROWAVE LOW-BRIGHTNESS-TEMPERATURE REGIONS AND Hα FILAMENTS AS TRACERS

The solar rotation rate obtained using the microwave Low-brightness-Temperature Regions (LTRs) as tracers in the heliographic range ± 55° from the years 1979–1980, 1981–1982, 1987–1988, and 1989–1991 varied from 3% to 4% in medium latitudes, and below 1% at the equator. Using H filaments as tracers at higher latitudes from the years 1979, 1980, 1982, 1984, and 1987, the solar rotation rate variation was between 2% and 8%. This represents an upper limit on the rotation rate variation during the solar activity cycle. Such changes could be caused by short-lived, large-scale velocity patterns on the solar surface. The Sun revealed a higher rotation rate on the average during the maxima of the solar activity cycles 21 and 22, i.e., in the periods 1979–1980 and 1989–1991, respectively, which differs from the rotation rates (lower on the average) in some years, 1981–1982 and 1987–1988, between the activity maximum and minimum (LTR data). Simultaneous comparison of rotation rates from LTRs and H filament tracings was possible in very limited time intervals and latitude bands only, and no systematic relationship was found, although the rotation rates determined by LTRs were mostly smaller than the rotation rates determined by H filaments. The errors obtained by applying different fitting procedures of the LTR data were analyzed, as well as the influence of the height correction. Finally, the north–south asymmetry in the rotation rate investigated by LTRs indicates that the southern solar hemisphere rotated slower in the periods under consideration, the difference being about 1%. The reliability of all obtained results is discussed and a comparison with other related studies was performed.     

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.     

Energy spectrum and time variations of hard X-rays from Cyg X-1

Experimental results on the intensity, energy spectrum and time variations in hard X-ray emission from Cyg X-1 based on a balloon observation made on 1971, April 6 from Hyderabad (India) are described. The average energy spectrum of Cyg X-1 in the 22–154 keV interval on 1971 April 6 is best represented by a power law dN/dE=(5.41±1.53)E ^{–(1.92±0.10)} photons cm^–2s^–1 keV^–1 which is in very good agreement with the spectrum of Cyg X-1 derived from an earlier observation made by us on 1969 April 16 in the 25–151 keV band and given by dN/dE=(3.54±2.44)E ^{–(1.89±0.22)} photons cm^–2s^–1 keV^–1. A thermal bremsstrahlung spectrum fails to give a good fit over the entire energy range for both the observations. Comparison with the observations of other investigators shows that almost all balloon experiments consistently give a spectrum of E ^–2, while below 20 keV the spectrum varies fromE ^–1.7 toE ^–5. There is some indication of a break in the Cyg X-1 spectrum around 20 keV. Spectral analysis of data in different time intervals for the 1971 April 6 flight demonstrates that while the source intensity varies over time scales of a few minutes, there is no appreciable variation in the spectral slope. Analysis of various hard X-ray observations for long term variations shows that over a period of about a week the intensity of Cyg X-1 varies upto a factor of four. The binary model proposed by Dolan is examined and the difficulties in explaining the observed features of Cyg X-1 by this model are pointed out.     

Recent spectral variations of the Seyfert galaxy NGC 5548

The nucleus of the Seyfert 1.5 galaxy NGC 5548 was very faint and the intensity of the broad emission component of H was unusually low in March–April and in July 1990. Similar stages was found only twice in this decade, in 1979 and 1981, prior to the present one. The very broad components of He I and He II were not detected in 1990.The blueward edge of the broad component of H was much steeper than the redward one in 1990, in contrast to the profiles with the opposite asymmetry in the years 1979–1981. This result suggests that a main part of the broad component blueshifted in the recent several years. An upper limit of the displacement of the broad component of H occurred during the years from 1984 to 1990 was estimated to be about - 2000 km s^–1. If this variation of the radial velocity was due to an orbital motion of the exciting source in a binary system, the total mass of the system is about 6 × 10⁶ M . The luminosity of the nucleus is the same order of the Eddington limit of this total mass.     

New period and period variations of DX Aquarii

The new period (P=0 _. ^d 461700) of the eclipsing binary system DX Aqr has been presented, which is based on available times of minima. O–C diagram of DX Aqr has been presented for the first time, and the period variations present in the system have been analysed. In all five period increases and five period decreases are nothed, and four period increases and five period decreases have been discussed. The strongest period increase occurs between 1975 and 1976. The total period change in different portions of the O–C diagram ranges from 1.40×10^–4 d to 3.61×10^–6 d. Appreciable period fluctuations have been noted to have occurred in the time intervals, 1964–1965 and 1974–1975.     

Positions of sunspot groups and solar rotation

The question is studied whether the one-year solar oscillation found by V. F. Chistyakov for the years 1965–1973 can be traced in the observations of sunspots of 1874–1971 published by Greenwich Observatory. The result is negative. But the study leads to the following two conclusions: (1) The average observable centres of gravity of spot groups are variably displaced towards the central meridian or towards the limb, the time scale of this variability being of the order of 70 years. Thus the angular velocity should be determined from recurrent groups in transit of the central meridian only. (2) The angular velocity will be smaller when determined from older spots.Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16–19 September 1980, Scheveningen, The Netherlands.     

The unusual supernova 1987A

V andB light curves for supernova 1987A covering some 120 days from the outburst are here presented and discussed; they are shown to be rather atypical for a type II supernova. The absolute magnitude at maximum brightness is also analyzed, and after applying a correction for interstellar absorption we obtainM _{V, max, 0} =–16.1, andM _{B, max, 0} =–14.7; is is then concluded that 1987A is a supernova quite fainter than average. A comparison with other known supernova is made and some similarity is found with peculiar objects such as 1948B in NGC 6946, and probably, 1909 A in M 101.Research supported in part by SECyT and CONICOR.     

Cosmic ray intensity variations and two types of high speed solar streams

This study deals with the short-term variations of cosmic ray intensity during the interval 1973–78. Daily means of high latitude neutron and meson monitors from the same station and those of a low latitude neutron monitor have been analysed using the Chree method of superposed epochs. The zero epoch for the Chree analyses corresponds to the day of a substantial increase (V 200 km s^–1) in the solar wind speed to values of 550 km s^–1 and which persists at such high values for an interval of at least three days. The investigation reveals the existence of two types of cosmic ray intensity variations with distinctly different spectral characteristics. During the interval 1973–76, relative changes in the neutron and meson monitor rates are nearly equal indicating an almost flat rigidity spectrum of variation. During 1977–78, however, the spectrum acquires a negative spectral character similar to that observed for Forbush decreases. We suggest that events of the interval 1973–76 are essentially due to high speed streams associated with solar coronal holes and that events of the interval 1977–78 are due to fast streams from solar active regions with flare activity.     

The G Index of Interplanetary Scintillation Data and Its Relation to Forbush Decreases During 1991–1994

The purpose of this work is to analyze a global index of interplanetary scintillation, G, obtained from the g-maps of Cambridge Observatory, UK, and associate it with the occurrence of sudden decreases of the cosmic radiation, known as Forbush decreases (Fds), to determine their possible relation. For this purpose we perform a superposed epoch analysis of the Gindex, with respect to the occurrence of Forbush decreases, registered at Oulu Station, Finland, for the period 1991–1994. We found an increase in the value of Gcoinciding with the occurrence of the Forbush decrease, especially for those events with a fall greater than 10%. We conclude that the G index is a macroscopic parameter representative of the inner structure of the heliosphere, and has bearings on phenomena affecting the Earth's environment, as shown by Forbush decreases and possibly geomagnetic activities.     

71 GHz (4.2 mm) solar radio bursts in the period July 1967 to December 1969

The results of 2 1/2 years (July 1967 – December 1969) monitoring of solar radio bursts at 71 GHz ( = 4.2 mm) at the Radio and Space Research Station, Slough are presented. During this period only seven events were positively identified as 71 GHz bursts. One of these events (6 July, 1968) is among the largest solar bursts ever recorded anywhere in the microwave-millimetre wave band (47000 × 10^–22Wm^–2Hz^–1), and the associated magnetic field may possibly have exceeded 7200 G. Another event (27 March, 1969) has demonstrated that bursts at 71 GHz can be both intense (4700 × 10^–22Wm^–2Hz^–1) and complex. On other occasions, the absence of any detectable event at 71 GHz helps to define the high frequency spectrum of the burst, this being an important factor in determining the initial energy distribution of the electrons ejected by the associated flare. On one such occasion (21 March, 1969) the derived energy distribution index is 8, in contrast with the more usual values of 2–4.1969–1970 NCR-OAR Senior Post-Doctoral Research Associate at Air Force Cambridge Research Laboratories, L. G. Hanscom Field, Bedford, Mass., U.S.A.     

Large-scale structure of the interplanetary medium

We introduce a method for constructing large-scale (0.25 AU) interplanetary magnetic field lines using only solar wind velocity from well-separated appropriately located spacecraft. The technique is based on labeling the field lines at each spacecraft with their coronal connection longitudes calculated in the EQRH (extrapolated quasi-radial hypervelocity) approximation (Nolte and Roelof, 1973). Even though the EQRH approximation is most applicable to quasi-steady solar wind, we propose that it should also be satisfactorily accurate for moderately evolving conditions. For strongly evolving conditions (e.g., flare-associated plasma) we propose a straightforward correction based on the inferred coronal longitudinal velocity profile. To illustrate the multispacecraft EQRH technique, we perform a calculation in which the interplanetary field lines in a model evolving solar wind disturbance are deduced from model observations at separated spacecraft. Since the expected agreement is found, we use data from Pioneers 8 and 9 and Vela to construct field lines for an unusually quiet period (April 26–30, 1969) and for a flare-associated disturbance accompanied by a Forbush decrease (March 23–25, 1969). The deduced field lines (even though strongly distorted by the disturbance), order the onsets of the Forbush decrease at the separated spacecraft, and the interplanetary plasma and field structures correspond to equatorial structures apparent in H synoptic charts of chromospheric magnetic features.     

Magnetic Cycle Dependence of the Cosmic-Ray Diurnal Anisotropy

The two components of the solar diurnal variation observed with two detectors characterized by linearly independent coupling functions have been used to estimate the free space anisotropy vector during the period 1968–1995 using the least-squares method (LSM). The values of R_cshow 20-year magnetic cycle with the lowest values at solar activity minima for positive polarity (qA>0). A good correlation is obtained between R_cand the IMF magnitude. The amplitude of the radial anisotropy (A_R) shows 20-year magnetic cycle with the highest values around solar activity minima for qA>0 (1975–1976 and 1995), whereas that of the east-west (A) is minimum. This results in shifting the anisotropy vector to the earliest hours. The amplitude of the anisotropy is high around solar maxima and low around solar minima. It is also enhanced during the declining phase of solar activity (1971, 1984–1985, and 1991). Our results of the anisotropy have been used to calculate the cosmic-ray radial and transverse gradients. The value of the radial gradient exhibits a magnetic polarity dependence as well, with larger value during qA<0 than during qA>0.     

Evolution du spectre de la Nova Delphini 1967 dans le proche infrarouge en 1969

Spectra of Nova Del 1967 have been obtained in the photographic infrared during the period March to November 1969. The gradient in the region 1.3–1.7 µ^–1 has decreased from 3.06 in November 1968 to 2.57 in November 1969. The main spectral features occurring in the wavelength range 5800–9600 Å are described, while the profiles of some strong lines are displayed on Figures 2–6.

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Les observations ont été effectuées à l'Observatoire de Haute-Provence (CNRS).     

Analysis of the spectral features of a recent supernova 1987n

We present an attempt to analyse the spectra of SN 1987n in NGC 7606, covering a period of 10 days from the time of maximum brightness. The velocities in the rest frame of NGC 7606 and the depths of the spectral lines at maximum light are very close to those of SN 1981b in NGC 4536, slight differences being nevertheless present. A distance to NGC 7606 of 46±11 Mpc for anH ₀ value of 50 km s^–1 Mpc^–1 is inferred from Pskovskii's relation.The analysis carried out within a standard simple model of spectral synthesis involves a high uncertainty in the abundance determinations for the intermediate-mass elements quoted. This uncertainty arises on the one hand from the free choice of the excitation temperature and from the sensitivity to changes in the excitation temperature of the depths of the strongest lines of those elements and, on the other hand, from the impossibility of obtaining within this model an estimate of other abundances — He, O, Na, S — which have NLTE populations.The analysis developed in a new model based (as is the standard one) on Sobolev's approximation but allowing for a more realistic continuum treatment points to an important attenuation effect on the radiation in the lines, due to the continuum scattering, which can also affect abundance determinations.