Probable Value for the Next Sunspot Minimum

Predictions of sunspot cycle are important due to their space weather effects. Bhattet al.(2009) predicted sunspot cycle 24 (Maximum amplitude: 92.8±19.6; Timing:October 2012±4 months) using relative sunspot number (International Sunspot Number), and average geomagnetic activity indexaaconsidering 2008 as the year of sunspot minimum. Owing to the extended solar minimum till 2009, we re-examine our prediction model. Also, the newly calibrated international sunspot number reduces many discrepancies in the old dataset and is available from Solar Influences Data Center (SIDC) website. Considering 2009 as sunspot minimum year and newly calibrated international sunspot number, (i) The annual maximum amplitude of cycle 24 = 118.5±24.4 (observed = 113.3±0.1), (ii) A smoothed monthly mean sunspot number maximum in January 2014±4 months (observed in February 2014). Our prediction method appears to be a reliable indicator for the predictability of cycle 25.

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Abstract. Yearly averages of geomagnetic activity indices Ap for the years 1967–1984 are compared to the respective averages of ν2·Bs, where v is the solar wind velocity and Bs is the southward interplanetary magnetic field (IMF) component. The correlation of both quantities is known to be rather good. Comparing the averages of Ap with ν2 and Bs separately we find that, during the declining phase of the solar cycle, ν2 and during the ascending phase Bs have more influence on Ap. According to this observation (using Fourier spectral analysis) the semiannual and 27 days, Ap variations for the years 1932–1993 were analysed separately for years before and after sunspot minima. Only those time-intervals before sunspot minima with a significant 27-day recurrent period of the IMF sector structure and those intervals after sunspot minima with a significant 28-28.5-day recurrent period of the sector structure were used. The averaged spectra of the two Ap data sets clearly show a period of 27 days before and a period of 28–29 days after sunspot minimum. Moreover, the phase of the average semiannual wave of Ap is significantly different for the two groups of data: the Ap variation maximizes near the equinoxes during the declining phase of the sunspot cycle and near the beginning of April and October during the ascending phase of the sunspot cycle, as predicted by the Russell-McPherron (R-M) mechanism. Analysing the daily variation of ap in an analogue manner, the same equinoctial and R-M mechanisms are seen, suggesting that during phases of the solar cycle, when ap depends more on the IMF-Bs component, the R-M mechanism is predominant, whereas during phases when ap increases as v increases the equinoctial mechanism is more likely to be effective.Key words. Interplanetary physics · Magnetic fields · Solar wind plasma · Solar wind · magnetosphere interaction

The paper discusses the daily variation of the geomagnetic H-field averaged over the Five International Quiet days, Sq(H), at low latitude stations. The Sq(H) at a station near the dip equator during a low sunspot year represents an increase from about 0600 LT to 1600 LT and an almost constant value from 1600 LT throughout the night time. During a high sunspot year the H-field continues to decrease from noon to evening and through the night time up to sunrise, there being no period in the night when the field remains constant.The harmonic analysis of Sq(H) curves shows that the local time of maximum of the diurnal as well as the semidiurnal wave advances towards local noon with the increase of the average sunspot number. During minimum sunspot years the daily maximum in H occurs almost at 1100 LT while during maximum sunspot years the maximum occurs at about 1200 LT.The amplitudes (C1) of the diurnal and (C2) of the semidiurnal waves are shown to increase linearly with sunspot number. For any of the stations, the rate of increase of the diurnal component with sunspot number is about 2.5 times which is remarkably larger than the corresponding rate of about 1.8 times for the semidiurnal component, such that the ratio C1/C2 at the low latitude stations is about 1.7 in 1964 and about 2.0 in 1959.The monthly mean daily range, rSq(H), shows a predominant semiannual variation with equinoxial peaks at the equatorial stations and a predominant annual variation with summer maximum at the non-equatorial station Alibag. A linear increase of the annual mean daily range with Zurich mean sunspot number Rz is established, the rate of increase being larger towards the equator.The dependence of the diurnal range on the sunspot number is due to enhanced E-region ionization with increasing sunspot number. The delay in the time of diurnal maximum with increasing Rz, the semiannual variation of rSq(H) at equatorial stations and the annual variation of rSq(H) at nonequatorial stations are suggested to be due to the effects of E-region electric fields.

To facilitate the use of cosmic-ray intensity data from ionization chambers which are being sent to International Geophysical Year World Data Centers, and of those which have appeared in two publications of the Carnegie Institution of Washington, an improved correction for instrumental drift at Huancayo is derived and the reliability of the corrections for seasonal variations is discussed. From ionization chambers, the decrease of intensity from its maxima (near sunspot minima) is shown to lag a year or more behind the increase of solar activity following sunspot minima. This lag does not appear in the results obtained by Neher at high altitude and high latitude, nor in those obtained by Rose from a neutron monitor at Ottawa. The variability of daily means of cosmic-ray intensity (from monthly means) is in 1957 the largest observed during two complete solar cycles. Tables of monthly means corrected for seasonal wave are included, together with graphs of daily means at Huancayo for the period 1954–1957.

This paper reviews observations of the variation over long and short time scales of solar coronal flux as observed from ground-based optical observatories. This includes data from Fe X, Fe XIV, Ca XV and the white-light electron corona. The various parameters are compared and contrasted as indicators of global coronal structure and output. The overall envelope of Ca XV minimizes before and during sunspot minimum, begins its slow rise in phase with the solar cycle, but maximizes late with strong 150-day oscillations resulting in values of zero at the local minima. Within the sensitivity of the observations, the increase of the envelope from Cycle minimum to maximum is over a factor of 30. Excluding possible secondary maxima and ignoring strong 150-day oscillations, the general envelope of Fe X appears to indicate a slow decline in intensity following sunspot maximum. Earlier observations indicate a Cycle minimum-to-maximum variation of a factor of 2 to 3. Later observations indicate a factor of 4 to 5. The overall picture for Fe XIV indicates that it varies in phase with the sunspot cycle. However, multiple maxima occur. The amplitudes of all observed Cycles are probably approximately equal, and the increase from Cycle minimum to maximum is approximately a factor of 10. The variation of the polarized and unpolarized white light corona is in phase with the sunspot cycle, although it may have a slower decline to minimum than the sunspot number. The increase from Cycle minimum to maximum is a factor of 2 to 3. Notable periodicities seen in the emission-line corona are (approximately) 27, 150, 180, 220 and 340 days and 3.4 years.

During 1957, observations of the sun were made at Mount Wilson on 310 days. Not since 1941 have solar observations been possible on as few days in one year as in 1957. The total number of sunspot groups observed at Mount Wilson in 1957 was 855, the largest number ever observed here in one year. The previous record was 663 in 1947. Without question we are witnessing the greatest sunspot activity ever recorded. The only months before 1957 in which the mean Zurich Sunspot Number exceeded 200 were May 1778, with 238.9, December 1836, with 206.2, and May 1947, with 201 .3. In 1957, the provisional mean sunspot numbers from Zurich for June, September, October, November, and December were 205.6, 244.3, 262.9, 207.3, and 233.9 respectively, all higher than 201.3, that of the most active month in the last cycle. Prior to 1840 systematic daily counts of sunspots were not being made and therefore the numbers before that date have lower weight than the present ones. The southern hemisphere was the more active for the first time since sunspot minimum, having 460 groups compared with 395 in the northern hemisphere. Since 1840 the cycles have been alternately high and low, as can be seen in Figure 1. The very high maximum of 1947 was a severe test of this relationship, which if it were to continue, would require the present maximum to be much higher than that of 1947 and to surpass any known maximum. It now seems probable that the mean of the sunspot numbers for 1937 and 1957 will be higher than that for 1947. The number of groups at very high latitudes continued to be large in 1957. From 1878 to 1954 only twelve groups were observed on more than one day at a distance of forty or more degrees from the equator. During 1957, eleven such groups were observed, making a total of twenty-two since 1954. Group 12428, June 20^23, had the highest latitude, +51°. The exceptionally

Components of the 11- and 22-year variation of cosmic rays

An algebraic method for the reconstruction and potentially prediction of the solar dipole moment value at sunspot minimum (known to be a good predictor of the amplitude of the next solar cycle) was suggested in the first paper in this series. The method sums up the ultimate dipole moment contributions of individual active regions in a solar cycle: for this, detailed and reliable input data would in principle be needed for thousands of active regions in a solar cycle. To reduce the need for detailed input data, here we propose a new active region descriptor called ARDoR (Active Region Degree of Rogueness). In a detailed statistical analysis of a large number of activity cycles simulated with the 2 × 2D dynamo model we demonstrate that ranking active regions by decreasing ARDoR, for a good reproduction of the solar dipole moment at the end of the cycle it is sufficient to consider the top N regions on this list explicitly, where N is a relatively low number, while for the other regions the ARDoR value may be set to zero. For example, with N = 5 the fraction of cycles where the dipole moment is reproduced with an error exceeding ±30% is only 12%, significantly reduced with respect to the case N = 0, i.e. ARDoR set to zero for all active regions, where this fraction is 26%. This indicates that stochastic effects on the intercycle variations of solar activity are dominated by the effect of a low number of large “rogue” active regions, rather than the combined effect of numerous small ARs. The method has a potential for future use in solar cycle prediction.

Space‐based synthetic aperture radars can be detrimentally affected by electron density variations in the ionosphere, which cause signal phase variations resulting in image defocusing. In order to evaluate the significance of this problem, this paper determines how often the phase perturbations are large enough to cause defocusing. High cadence GPS TEC signals collected on Ascension Island during sunspot minimum and in Papua New Guinea over half a solar cycle have been analyzed. Both locations lie in the equatorial crest region. For a space‐based synthetic radar operating at 435 MHz, the average probability of defocusing varies from zero at quiet times, to around 1% between 18:00 and 24:00 at solar minimum, and to 45% between 18:00 and 24:00 at solar maximum. Corresponding values are given for radars operating at other frequencies. A method is developed to estimate such variations from the large database of S4 measurements.

The regression coefficients of the time‐weighted AL index (ALW) to the Dst index are calculated for each year from 1966 to 1994. It is found that the slope of the linear regression equation varies over a period of 11 years: The coefficient a1 in a regression equation, Dst = a0 + a1 ALW, is maximized near the sunspot maximum and is minimized near the sunspot minimum. The long‐term variations in the relationship between these indices and solar wind parameters are also examined. The dependence of Dst on the solar wind electric flux, BzV2, is nearly constant, while the values of AL for fixed values of BzV2 are greater at the solar minimum than at the solar maximum, where Bz and V denote the Z component of the interplanetary magnetic field and the solar wind speed, respectively. Possible causes that can account for these characteristics are discussed.

Since late 1980, a ground-based Fabry-Perot interferometer has been in operation near Kiruna, Sweden (20.4°E, 67.8°N). This instrument has recorded thermospheric neutral winds under a wide range of geophysical conditions which have occurred during six observing winters. These data cover the period from the time of maximum geomagnetic activity during the last solar cycle, 1980/82, to the 1985/86 period of generally quiet geomagnetic activity near sunspot minimum. A statistical analysis of the data from about 400 nights of observation provides a graphic description of the generally rapid time-dependence of thermospheric winds in the vicinity of the mean auroral oval to individual geomagnetic disturbances. The data also provide an excellent data base describing the auroral oval. The data describe ion convection patterns as they respond to variable geomagnetic activity, and also the mean distribution of O I 630 nm emission as a function of local time, latitude and geomagnetic activity. These results can be used to examine the geomagnetic input parameters to a global thermospheric model (for example the semi-empirical global models of magnetospheric convection) as required to bring the simulations of thermospheric circulation into overall improved agreement with the observations.

In the late 18th century speculation began about the physical causality of the aurora borealis and the beginning of the 19th century brought new insight, but still no clear picture. In this time Alexander von Humboldt became interested in geomagnetic and related fields and a systematic program of observations commenced at some places. Humboldt described parallel variations of the magnetic needle and the appearance of aurora (1806) and concluded that there was some link between them and this was supported by the observations of other scientists of the Gottinger Magnetischer Verein (set up by Carl-Friedrich Gau and Wilhelm Weber). However, the exact nature of the connection between geomagnetism and aurora was still a mystery. The next advance came through solar physics when Heinrich Schwabe discovered the 11 year cycle in sunspot activity. It was argued that a variable Sun must have some influence on geomagnetic events and shortly afterwards Rudolf Wolf noted a relationship between aurora and sunspots over nearly an 11 year period. His friend, Hermann Fritz, using Wolf’s auroral catalouge and his own data, confirmed a strong relationship between the appearance of aurora and the number of sunspots, with maximums in both phenomena coinciding. At sunspot minimum, auroras were not observed at middle latitudes. Thus, by the middle of the 19th century, the relationship between the Sun and aurora was well known. Further advances came through the spectroscopic observation of the Sun and auroral displays by Karl Zollner, Eugen Goldstein, Carl Vogel and Wilhelm Foerster and others.

Statistically significant correlations exist between the size (maximum amplitude) of the sunspot cycle and, especially, the maximum value of the rate of rise during the ascending portion of the sunspot cycle, where the rate of rise is computed either as the difference in the month-to-month smoothed sunspot number values or as the ‘average rate of growth’ in smoothed sunspot number from sunspot minimum. Based on the observed values of these quantities (equal to 10.6 and 4.63, respectively) as of early 1989, one infers that cycle 22's maximum amplitude will be about 175 ± 30 or 185 ± 10, respectively, where the error bars represent approximately twice the average error found during cycles 10–21 from the two fits.

We present two-dimensional observations of the quiet Sun at 73.8, 50.0, and 38.5 MHz obtained with the Clark Lake Radioheliograph during the sunspot minimum period of September 1986. The observed peak brightness temperatures during the entire period of sunspot minimum are found to be extremely low, lying in the range (0.6 × 105 K − 2.5 × 105 K). It is shown that these low values cannot be explained by the generally adopted models for N e and T e in a homogeneous corona. The effect of scattering by random density fluctuations is introduced in order to decrease the values of predicted T b . The value of peak T b is computed as a function of relative r.m.s. density fluctuations e = <ΔN e >/N e ; and it is found that ɛ should be in the range from 0.07 to 0.19, 0.1 to 0.25, and 0.15 to 0.35, respectively, at 38.5, 50.0, and 73.8 MHz, respectively, to explain the observed low brightness temperatures.