Decrease In Solar Activity Research Articles

Improved VLBI astrometry of OH maser stars

Aims. Accurate distances to evolved stars with high mass loss rates are needed for studies of many of their fundamental properties. However, as these stars are heavily obscured and variable, optical and infrared astrometry is unable to provide enough accuracy. Methods. Astrometry using masers in the circumstellar envelopes can be used to overcome this problem. We have observed the OH masers of a number of Asymptotic Giant Branch (AGB) stars for approximately 1 year with the Very Long Baseline Array (VLBA). We have used the technique of phase referencing with in-beam calibrators to test the improvements this technique can provide to Very Long Baseline Interferometry (VLBI) OH maser astrometric observations. Results. We have significantly improved the parallax and proper motion measurements of the Mira variable stars U Her, S CrB and RR Aql. Conclusions. It is shown that both in-beam phase-referencing and a decrease in solar activity during the observations significantly improves the accuracy of the astrometric observations. The improved distances to S CrB ($418^{+21}_{-18}$ pc) and RR Aql ($633^{+214}_{-128}$ pc) are fully consistent with published P – L relations, but the distance to U Her ($266^{+32}_{-28}$ pc) is significantly smaller. We conclude that for sources that are bright and have a nearby in-beam calibrator, VLBI OH maser astrometry can be used to determine distances to OH masing stars of up to ~2 kpc.

A delayed climatic response to solar forcing at 2800 cal. BP: multiproxy evidence from three Irish peatlands

Multiproxy palaeohydrological records from three raised bogs in Northern Ireland indicate that a major shift to wetter/cooler climatic conditions postdated the rapid decrease in solar activity at 2800 cal. BP by ~100 years. This event is bracketed by two wiggle-match radiocarbon-dated cryptotephra layers in each profile, enabling a high degree of chronological precision. These replicated data corroborate previous findings based on Irish peat humification profiles, and may indicate spatial complexity in the climatic response to solar activity between oceanic and continental areas.

The Cosmic Ray Increases At 35 and 60 Kyr BP

Concentrations of 10Be in ice cores and marine sediments exhibit 2 peaks with significant enhancements at 35,000 and 60,000 BP. This radioisotope is produced in the upper atmosphere by spallation of cosmic-ray protons and secondary neutrons on atmospheric nitrogen and oxygen. Previously suggested explanations for the increases include geomagnetic field reversals, a decrease in solar activity, and a supernova explosion. We propose an alternative explanation which involves a change in the galactic environment of the solar system. The structure of the heliosphere is investigated for a period when the Sun enters a cold, dense, unmagnetized interstellar cloud. Under these conditions, the heliosphere contracts to 25% its present size, significantly affecting galactic cosmic ray modulation and increasing anomalous cosmic ray fluxes. A tenfold increase in anomalous cosmic ray flux and a twofold increase in galactic cosmic ray intensity at Earth are possible in this high-density case if heliosheath modulation is reduced. We show that this increase in galactic cosmic ray intensity could be responsible for the peaks in 10Be records.

Formation of the Anomalies of Hydrometeorological Fields on the South Coast of Crimea in the Years of Decreasing Solar Activity in Its 11-Year Cycles

We study the anomalies of some hydrometeorological fields in the Atlantic-European sector and on the South Coast of Crimea (SCC) in the years of decrease in solar activity in its even and odd 11-year cycles. It is shown that, in the first half of summer, the values of the sea level and surface temperature at the SCC averaged over the years of decrease of an even cycle are lower than for the same years of an odd cycle. As a result, we make a conclusion that there exists a tendency to the formation of quasi-22-year variability of some hydrometeorological characteristics near the SCC. We also discover a trend to the antiphase variability of the number of upwellings and the surface temperature at the SCC in the years of decrease in solar activity in its even and odd 11-year cycles.

Climatic change in Chile at around 2700 BP and global evidence for solar forcing: a hypothesis

Palaeoecological and geological evidence for changing atmospheric circulation patterns in Chile indicates that equatorward and poleward shifts of the Southern Westerlies (Pacific precipitation source) were an important factor during Weichselian and Holocene climate change. We focus on the evidence and possible causes of considerable climate change in the Holocene at around 2700 BP, which was associated with a steep rise in atmospheric radiocarbon content, indicating an abrupt decrease in solar activity. Climate change may have been caused by the lowering of solar irradiation through two amplifying factors, namely (1) increased cosmic ray intensity, stimulating cloud formation and precipitation, and (2) reduced solar UV intensity, causing a decline of stratospheric ozone production and cooling as a result of less absorption of sunlight. A decrease in the latitudinal extent of Hadley Cell circulation may have occurred with concomitant equatorward relocation of mid-latitude storm tracks, which brought about northward movement of vegetation belts and advance of glaciers.

VHF ionospheric scintillations near the equatorial anomaly crest: solar and magnetic activity effects

The paper presents a study of solar and magnetic activity effects on VHF ionospheric scintillations recorded during three and half years at Bhopal, a station near the northern crest of the equatorial anomaly in India. During E- (equinox) and D- (winter) months, scintillations occur mainly in the pre-midnight period whereas during J- (summer) months their occurrence is larger in the post-midnight period. Very intense scintillations (>20 dB) mainly occur in the pre-midnight period, and in the post-midnight period, the scintillations are generally moderate (5–10 dB) or weak (<5 dB). The nocturnal scintillation occurrence decreases with the decrease in solar activity from 1989 to 1992. Monthly mean scintillation occurrence changes according to solar activity during E- and D-months but not so during J-months. The effects of magnetic activity on scintillations vary with season and, in general, inhibit the scintillation occurrence in the pre-midnight period and enhance it a little in the post-midnight period, especially after 0300 hours IST (Indian Standard Time). For most of the severe magnetic storms in which Dst goes below −125 nT and the recovery phase starts in the post-midnight to dawn local time sector, strong post-midnight scintillations, which sometimes extend for several hours beyond the local sunrise, are observed.

Solar and Magnetic Activity Control on the VHF Ionospheric Scintillations at Low Latitude

Solar and magnetic activity effects on VHF night-time ionospheric scintillations recorded during three and half year at Bhopal, a station near the northern crest of the equatorial anomaly in the Indian region have been presented. The night time diurnal scintillation occurrence decreases with decrease in solar activity. Monthly mean scintillation occurrence changes according to solar activity during E- and D- months but not so during J-months. The magnetic activity shows seasonal effect on scintillations and, in general, inhibits the scintillation occurrence in pre-midnight period and enhances little in post-midnight period especially after 03h IST. For most of the severe magnetic storms in which D st goes below −125 nT and recovery phase starts in post-midnight — dawn local time sector, strong post-midnight scintillations which sometimes extend for several hours beyond the local sunrise, are observed.

Filament Disappearances During the Period of September 1991 through September 1994

Continuous full-disk Hα images recorded by the Big Bear Solar Observatory (BBSO) from 1 September 1991 to 19 September 1994 (the first three years of Yohkoh mission) were digitized and analyzed. The data set consists of nearly 10000 Hα images, one every half hour for the period when the BBSO was observing. Two statistical studies of the disappearing solar filaments based on this set of data are made: (1) The disk latitude distribution of all larger disappearing filaments with a minimum length of 70 arc sec, including the time of their disappearance. Of the 1095 such filaments, 439 disappeared during our continuous observations, 314 disappeared during the BBSO night gap, 162 disappeared during data gap (more than 94 hours) and 180 rotated beyond the west limb. If we plot latitudes as a function of time for all these disappeared filaments, it shows a uniform distribution in latitude. However, if we plot the distribution of larger disappeared filaments (200 arc sec or above), then the butterfly trend appears – position of filaments tends to drift to lower latitude as solar activity decreases. (2) The disk distribution of all detectable disappearing filaments, large and small, for the 9-months period, January 1994 to September 1994. We find that the size distribution of 351 collected disappeared filaments follows a power law with a power index of −1.40.

History and forecast of solaractivity

Abstract From a new reconstruction of the radiocarbon production rate in the atmosphere we obtain a long history of maximum Wolf sunspot numbers. Based on this reconstruction as well as on the history of other indicators of solar activity (10Be, aurora borealis), we derive a long-period trend which together with the results of spectral analysis of maximum Wolf numbers series (1506–1993) form a basis for prediction of solar activity up to 2100. The resulting trigonometric trend points to an essential decrease in solar activity in the coming decades.

POLE-TO-POLE SOLAR WIND DENSITY FROM ULYSSES RADIO MEASUREMENTS

We present new in situ measurements of solar wind electron density as a function of heliolatitude. The data were obtained on Ulysses during its fast transit from south solar pole to north solar pole, at heliocentric distance about 1.5 AU, near the 1996 solar activity minimum. The density is measured accurately using the method of quasi-thermal noise spectroscopy with the Ulysses radio experiment, at a higher time resolution than the particle analysers on board. At low heliolatitudes (22° S to 21° N) the histogram of our data shows three main classes of flows with densities centered at 3.5, 7, and 12 cm-3, close to the values previously found by near-ecliptic space probes, in the region where fast coronal hole wind alternates with slower streamer belt wind. Poleward of 22° latitude where Ulysses encountered fast wind coming from coronal holes, the histogram of our data shows a single class of flow centered at 2.9 cm-3 with a roughly normal distribution. We find a density nearly independent of latitude, with the mean density from the south coronal hole 10% larger than that from the north, which may stem from a genuine north/south asymmetry and/or from the small decrease in solar activity during the time of the observations. We finally compare the data with some analytical models.

Cold Periods During the Last Millennium

Studies of temperature change during the last millennium in China and in other regions of the globe were reviewed. Cold periods were identified for East Asia, USSR, Europe, North America, Polar Region and Southern Hemisphere. Most of them were grouped into the same time intervals: first half of12th, second half of 13th, second half of 15th, 17th and 19th century. Temperatures in the last two cold periods were 0.5°C-1.0°C lower than the average of the 20th century. The first three cold periods were less cold and less uniform in geographical distribution and in temporal variations. Therefore, the first three cold periods may be regarded as the transition from the Medieval Warm Period to the Little Ice Age. Long-term variations of solar activity and volcanism were compared with the changes of temperatures. The second to the fourth cold periods seem to relate to the Wolf, Sporer, and Maunder Minima of solar activity. The intensification of explosive volcano eruptions in early15th and 17th century may also have contributed to the occurrence of the third arid fourth cold periods. However, only a slight increase of volcanism and a weak decrease in solar activity in the early 19th century can hardly fully intepret the severe cold period throughout the 19th century. Perhaps neither one nor both of the aforementioned factors can be responsible for all of the five cold periods identified during the last millennium. It is suggested that the changes in cryosphere, biosphere and the oceans may interact with the atmosphere, controlling (together with the external factors) the occurrence of the cold periods. Of course, anthropogenic factors should also be considered, especially for the last one and a half centuries. Finally, factor or factors other than those aforementioned, such as changes in orbital parameters, may also take part in regulation of the climate for the last millennium.

Time-dependent cosmic-ray modulation: Role of drifts and interaction regions

The combination of gradient, curvature and heliospheric neutral sheet (HNS) drifts with interaction regions (IRs) occurring in the inner heliosphere and merged interaction regions (MIRs) in the outer heliosphere would enable one to successfully simulate complete 11 and 22-yr cycles with comprehensive, global modulation models. This is possible because both drifts and MIRs happen on a large scale. With this scenario, drifts are primarily responsible for long-term modulation during low to moderate solar activity for both magnetic cycles. In terms of the “tilt angle”, α, of the wavy neutral sheet this happens when α ≤ (35 ± 5)°. With moderate to high solar activity MIRs become progressively more important, but still in combination with drifts, to eventually dominate during maximum solar activity when global drift effects cannot occur any longer. The rate at which solar activity decreases (increases) before (after) minimum modulation will then become crucial in determining when and what process begins to dominate.

Contribution of Ultraviolet Irradiance Variations to Changes in the Sun's Total Irradiance

The sun's total irradiance decreased from 1980 to mid-1985, remained approximately constant until mid-1987, and has recently begun to increase. This time interval covered the decrease in solar activity from the maximum of solar cycle 21 to solar minimum and the onset of cycle 22. The sun's ultraviolet irradiance also decreased during the descending phase of cycle 21 and, like the total irradiance, is now increasing concurrently with the increase in cycle 22 activity. Although only 1 percent of the sun's energy is emitted at ultraviolet wavelengths between 200 and 300 nanometers, the decrease in this radiation from 1 July 1981 to 30 June 1985 accounted for 19 percent of the decrease in the total irradiance over the same period.

Ozone reduction in the 1980's: A model simulation of anthropogenic and solar perturbations

An interpretation of global ozone changes deduced from satellite data obtained since 1979 is presented, based on two‐dimensional model simulations. The study shows that a depletion in total ozone of the order of 2 percent and a reduction in ozone density near 40 km of 7 to 12 percent over the 1979–1986 period are consistent with the observed increase in trace gas densities (chlorofluorocarbons, methane, nitrous oxide, carbon dioxide) and the simultaneous decrease in solar activity during this period. The model also suggests that ozone variations of solar and anthropogenic origins between 1979 and 1986 were of similar magnitude but that the ozone response to trace gas emissions increases substantially with latitude while the solar signal in ozone is present lower in the atmosphere and is nearly independent of latitude.

An assessment of possible ozone‐solar cycle relationship inferred from NIMBUS 4 BUV data

The total ozone and the ozone mixing ratio at various pressure levels in the stratosphere measured from the NIMBUS 4 backscattered ultraviolet (BUV) experiment over a 7‐year period (1970–1977) comprise a very comprehensive data base available to study the possible effects of solar variability on stratospheric ozone. It is shown that with the decrease in solar activity from 1970 to 1976, the globally averaged ozone inferred from NIMBUS 4 data decreases from about 10–12% in the upper stratosphere to about 1–3% in the lower stratosphere. This systematic decrease at all pressure levels is modulated by a quasi‐periodic oscillation of about a 2‐year period. Both the systematic decrease and the quasi‐periodic oscillations in ozone seem to be correlated with the conventional indices of solar activity suggesting a solar UV‐ozone relationship. Despite this apparent correlation, it is difficult to account for the observed changes at various pressure levels with our current understanding of the photochemical models and the solar UV flux variations over a solar cycle. The agreement between the observed and the calculated profiles of ozone mixing ratio is considerably improved if the BUV data are corrected for the changes in the instrument sensitivity using Umkehr observations as a reference.

The structure of the heliospheric current sheet: 1978–1982

The structure of the heliospheric magnetic field changes substantially during the 11‐year sunspot cycle. We have calculated its configuration for the period 1976–1982 by using a potential field model, continuing our earlier study near solar minimum in 1976–1977 (Hoeksema et al., 1982). In this paper we concentrate on the structure during the rising phase, maximum, and early decline of sunspot cycle 21, from 1978 to 1982. Early in this interval there are four warps in the current sheet (the boundary between interplanetary magnetic field toward and away from the sun) giving rise to a four‐sector structure in the interplanetary magnetic field observed at earth. The location of the current sheet changes slowly and extends to a heliographic latitude of approximately 50°. Near maximum the structure is much more complex, with the current sheet extending nearly to the poles. Often there are multiple current sheets. As solar activity decreases, the structure simplifies until, in most of 1982, there is a single, simply shaped current sheet corresponding to a two‐sector interplanetary magnetic field structure in the ecliptic plane. The sun's polar fields, not fully measured by magnetographs such as that at the Stanford Solar Observatory, substantially influence the calculated position of the current sheet near sunspot minimum. We have determined the strength of the polar field correction throughout this period and include it in our model calculations. The lower latitude magnetic fields become much stronger as the polar fields weaken and reverse polarity near maximum, decreasing the influence of the polar field correction. The major model parameter is the radius of the source surface, the spherical surface at which the field lines become radial. Correlations of interplanetary magnetic field polarity observed by spacecraft with that predicted by the model calculated at various source surface radii indicate that the optimum source surface radius is not significantly different from 2.5 RS during this part of the solar cycle.

Banded Corals: Changes in Oceanic Carbon-14 During the Little Ice Age

Radiocarbon analyses and stable isotope measurements are presented foro recent cores of banded corals from the Florida Straits. These values provide a record of variations in the ratio of carbon-14 to carbon-12 in the dissolved inorganic carbon in the surface waters of the Gulf Stream from A.D. 1642 to 1800. An increase in the carbon-14/carbon-12 ratio of 7 per mil for coral growth during the early 1700's was most likely induced by an increase in the carbon-14/carbon-12 ratio of 20 per mil in the atmospheric carbon dioxide that occurred at about 1700. The ratios of oxygen 18 to oxygen-16 in these coral bands show a small decrease of a water temperature ( approximately 1 degrees C) during the latter part of the Little Ice Age (1700 to 1725). These results support the hypothesis that the increase in atmospheric carbon-14 at about 1700, and possibly the temperature change as well, was caused by a decrease in solar activity (Maunder sunspot minimum).

A Discussion on the physics of the solar atmosphere - Initial results from the high altitude observatory white light coronagraph on Skylab — a progress report

The frequent, periodic observations by the white light coronagraph allow an examination of coronal variations over a broad range of temporal scales. Examples of the slowest and most rapid variations are presented. An example of extremely slow coronal variations is the gradual evolution-to a large equatorial stream er-in association with a marked decrease in solar activity, as the total magnetic flux in one hemisphere decreased. Another example is given of a long-lived quasi-stable coronal streamer, apparently associated with a stable filament channel; comparison of this streamer with coronal potential magnetic field computations show little correlation. The remainder of the paper summarizes some results on coronal transients - the most rapid variations observed. Characteristic mass and energies involved in mass ejection transients, their temporal and spatial distributions, their associations with surface phenomena and possible interplanetary signatures, and finally their role in coronal evolution are briefly noted.

On the ionization transport in the low-latitude ionosphere

The variations of plasma drift velocity measured at Jicamarca with Thomson scatter radar are studied in connection with the variation of ƒoF2 and the correlations of ƒoF2's between different stations in low latitude. It is inferred that the sunset peak of drift velocity shows similar annual variation for both 75°W and 120°E longitudes, and becomes small as the solar activity decreases. The post-sunset peak of ƒoF2 at subtropical region is caused by anomalously large upward drift at sunset near the Equator, and the high h m in December is ascribable to the integrated effect of large and long sunset bulge of upward drift velocity. There is also an evidence of neutral wind which affects the correlation between ƒoF2's of two stations and produces the longitudinal effect.

Studies of the Horizontal Drifts in the F-Region of the Ionosphere at Ahmedabad

The paper describes the results of F-region drift measurements at Ahmedabad during different periods between 1957 and 1966, and compliments the results of E-region drifts published in an earlier paper (Rastogi 1969). During any of the season and epoch the mean drift speed is almost the same for the E or the F-region at Ahmedabad. The F-region drift speeds are found to be slightly greater during the night than during the day-time. The day-time speeds (F) are greater in winter than in summer while no consistent seasonal variation is evident for the night-time speed. The most probable direction changes from N-W to S-E direction seasonally from winter to summer or with the decrease of solar activity. The diurnal wave of E-W component for E or F-region drift are similar at Ahmedabad- Waltair or Delhi, but the diurnal wave of N-S components at Ahmedabad is opposite in phase to that in Waltair or Delhi. Ahmedabad seems to lie close to the region of transition of low and high latitude ionospheric wind system.