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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS2005

On the Relation Between Spotless Days and the Sunspot Cycle

Spotless days are examined as a predictor for the size and timing of a sunspot cycle. For cycles 16-23 the first spotless day for a new cycle, which occurs during the decline of the old cycle, is found to precede minimum amplitude for the new cycle by about approximately equal to 34 mo, having a range of 25-40 mo. Reports indicate that the first spotless day for cycle 24 occurred in January 2004, suggesting that minimum amplitude for cycle 24 should be expected before April 2007, probably sometime during the latter half of 2006. If true, then cycle 23 will be classified as a cycle of shorter period, inferring further that cycle 24 likely will be a cycle of larger than average minimum and maximum amplitudes and faster than average rise, peaking sometime in 2010.

Wilson, Robert M.↗
NASA NTRS1983

X-ray bright points and the sunspot cycle - Further results and predictions

X-ray images obtained during two rocket flights near the maximum of sunspot cycle 21 now allow the study of the variation of X-ray bright point number over an eleven-year period covering the maxima of the last two cycles. The new data are consistent with the earlier conclusion that the temporal variation of bright point and sunspot number are out of phase. The quantities are related through a power law with a negative exponent of 2/3.

Davis, J. M.↗
NASA NTRS1986

Measurement of radial and latitudinal gradients of cosmic ray intensity during the decreasing phase of sunspot cycle 21

The cosmic ray radial and latitudinal gradients during the 1981-1984 decreasing phase of sunspot cycle 21 are investigated based on data from Voyagers 1 and 2, with a detector threshold of not less than 70 MeV/nuc, and IMP-8, with a detector threshold of not less than 35 MeV/nuc. During the interval, the heliolongitudinal separation between the Voyager spacecraft changed from about 4 to 26 deg, and comparison of the 26-day means of the cosmic ray intensities obtained show that the data is consistent on the average with a long-term zero latitudinal gradient. Comparison at 1 AU of Voyager and IMP-8 data illustrate that the radial gradient decreased over this period at the rate of about 0.4 percent per AU per year, reaching a value of about 2.0 percent/AU between 16 and 22 AU, and a value of about 0.6 percent/AU between 16 and 22 AU, a pattern which would locate both Voyagers during 1977 outside the principal cosmic ray modulation at solar minimum within 22 AU.

Venkatesan, D.↗
NASA NTRS1992

Coronal holes and solar wind streams during the sunspot cycle

Complementary synoptic observations of the Sun and interplanetary space have been obtained nearly continuously for more than two sunspot cycles and have led to new ideas about the origin of the solar wind. These observations show an inverse correlation between wind speed at Earth and magnetic flux tube expansion in the corona, with fast wind originating from slowly diverging tubes and vice versa. Although this result is consistent with the Skylab-era concept that fast wind originates from the center of a large isolated coronal hole, it implies that the wind may be even faster at the facing edges of like-polarity holes where the flux-tubes converge as they begin their outward extension. Thus, very fast wind ought to originate from the high-latitude edges of the circumpolar holes soon after sunspot maximum and from the mid-latitude necks of the polar-hole lobes during the declining phase of the cycle. The observed inverse correlation may be understood physically in terms of a model in which Alfven waves boost the wind to high speed provided that the wave energy flux is distributed approximately uniformly at the coronal base.

Sheeley, N. R., Jr.↗
NASA NTRS1974

The spiral interplanetary magnetic field - A polarity and sunspot cycle variation

Spacecraft observations near the earth of the average direction of the interplanetary magnetic field during the sunspot maximum year 1968 showed a deviation from the spiral field of Parker's classical description. The included angle between the average field direction when the field polarity was away from the sun and the average direction when the field polarity was toward the sun was 168 deg, rather than 180 deg as predicted by Parker. This effect appears to have a sunspot cycle variation.

Svalgaard, L.↗
NASA NTRS1991

Chaos in the sunspot cycle - Analysis and prediction

The variability of solar activity over long time scales, given semiquantitatively by measurements of sunspot numbers, is examined as a nonlinear dynamical system. First, a discussion of the data set used and the techniques utilized to reduce the noise and capture the long-term dynamics inherent in the data is presented. Subsequently, an attractor is reconstructed from the data set using the method of time delays. The reconstructed attractor is then used to determine both the dimension of the underlying system and also the largest Lyapunov exponent, which together indicate that the sunspot cycle is indeed chaotic and also low dimensional. In addition, recent techniques of exploiting chaotic dynamics to provide accurate, short-term predictions are utilized in order to improve upon current forecasting methods and also to place theoretical limits on predictability extent. The results are compared to chaotic solar-dynamo models as a possible physically motivated source of this chaotic behavior.

Mundt, Michael D.↗
NASA NTRS1974

The spiral interplanetary magnetic field: A polarity and sunspot cycle variation

Spacecraft observations near the earth of the yearly average direction of the interplanetary magnetic field during the sunspot maximum year 1968 showed a deviation from the spiral field. The angle between the average field direction when the field polarity was away from the sun and the average direction for toward polarity was 168 deg, rather than 180 deg. This effect appears to have a sunspot cycle variation.

Svalgaard, L.↗
NASA NTRS2014

Deciphering Solar Magnetic Activity. I. On the Relationship Between the Sunspot Cycle and the Evolution of Small Magnetic Features

Sunspots are a canonical marker of the Sun's internal magnetic field which flips polarity every ~22 yr. The principal variation of sunspots, an ~11 yr variation, modulates the amount of the magnetic field that pierces the solar surface and drives significant variations in our star's radiative, particulate, and eruptive output over that period. This paper presents observations from the Solar and Heliospheric Observatory and Solar Dynamics Observatory indicating that the 11 yr sunspot variation is intrinsically tied to the spatio-temporal overlap of the activity bands belonging to the 22 yr magnetic activity cycle. Using a systematic analysis of ubiquitous coronal brightpoints and the magnetic scale on which they appear to form, we show that the landmarks of sunspot cycle 23 can be explained by considering the evolution and interaction of the overlapping activity bands of the longer-scale variability.

Sun: interior↗
NASA NTRS1995

Long-term total solar irradiance variability during sunspot cycle 22

Total solar irradiance measurements from the 1984-1993 Earth Radiation Budget Satellite (ERBS) active cavity radiometer and 1978-1993 Nimbus 7 transfer cavity radiometer spacecraft experiments are analyzed to detect the presence of 11-, 22-, and 80-year irradiance variability components. The analyses confirmed the existence of a significant 11-year irradiance variability component, associated with solar magnetic activity and the sunspot cycle. The analyses also suggest the presence of a 22- or 80-year variability component. The earlier Nimbus 7 and Solar Maximum Mission (SMM) spacecraft irradiance measurements decreased approximately 1.2 and 1.3 W/sq m, respectively, between 1980 and 1986. The Nimbus 7 values increased 1.2 W/sq m between 1986 and 1989. The ERBS irradiance measurements increased 1.3 W/sq m during 1986-1989, and then decreased 0.4 W/sq m (at an annual rate of 0.14 W/sq. m/yr) during 1990-1993. Considering the correlations between ERBS, Nimbus 7, and SMM irradiance trends and solar magnetic activity, the total solar irradiance should decrease to minimum levels by 1997 as solar activity decreases to minimum levels, and then increase to maximum levels by the year 2000 as solar activity rises. The ERBS measurements yielded 165.4 +/- 0.7 W/sq m as the mean irradiance value with measurement accuracies and precisions of 0.2% and 0.02%, respectively. The ERBS mean irradiance value is within 0.2% of the 1367.4, 1365.9, and 1366.9 W/sq m mean values for the SMM, Upper Atmosphere Research Satellite (UARS), and Space Shuttle Atmospheric Laboratory for Applications and Science (ATLAS 1) Solar Constant (SOLCON) active cavity radiometer spacecraft experiments, respectively. The Nimbus 7 measurements yielded 1372.1 W/sq m as the mean value with a measurement accuracy of 0.5%. Empirical irradiance model fits, based upon 10.7 -cm solar radio flux (F10) and photometric sunspot index (PSI), were used to assess the quality of the ERBS, Numbus 7, SMM, and the UARS irradiance data sets and to identify irradiance variability trends which may be caused by drifts or shifts in the spacecraft sensor responses. Comparisons among the fits and measured irradiances indicate that the Nimbus 7 radiometer response shifted by a total of 0.8 W/sq m between September 1989 and April 1990 and that the ERBS and UARS radiometers each drifted approximately 0.5 W/sq m during the first 5 months in orbit.

Lee, Robert B., III↗
NASA NTRS1991

Observations of intermediate- and high-degree p-mode oscillations during sunspot cycles 21 and 22

Extensive time series of resolved solar images have been available for helioseismological investigations since the late 1970's. The paper presents examples of the frequencies, power levels, modal energies, and widths of solar intermediate-degree and high-degreee p-modes obtained at the Mt. Wilson Observatory's 60-Foot Solar Tower Telescope during the rising phase of the current sunspot cycle. It is shown that the inclusion of frequency splittings from the high-degree p-modes has made it possible to demonstrate that the sun's internal equatorial angular velocity is not constant with radius but varies systematically with radius throughout the solar convection zone and below.

Rhodes, E. J.↗
NASA NTRS1989

Average properties of bipolar magnetic regions during sunspot cycle 21

The statistical properties of some 2700 bipolar magnetic regions (BMRs) with magnetic fluxes equal to or greater than 3 x 10 to the 20th Mx, which erupted during 1976-1986, are examined. The analysis shows the following: (1) the average flux per BMR declined between 1977 and 1985; (2) the average tilts of BMRs relative to the east-west line increase toward higher latitudes; (3) weaker BMRs had larger root-mean-square tilt angles than stronger BMRs at all latitudes; and (4) over the interval 1976-1986, BMRs with their leading poles equatorward of their trailing poles contributed a total of 4 times as much flux as BMRs with inverted tilts, but the relative amount of flux contributed by BMRs with inverted or zero tilts increased as the sunspot cycle progressed.

Wang, Y.-M.↗
NASA NTRS1988

Three-halves law in sunspot cycle shape

Evidence is provided of a nonlinear effect consisting of a 3/2 power law in annual mean sunspot numbers since 1700. With correction for this nonlinearity, it has been possible to reproduce theoretically the well-known Waldmeier relation between rise time and maximum sunspot number, in addition to other features of the 11 yr sunspot curve such as a break in the decline of strong 11 yr cycles. Eighteenth century sunspot numbers are found to fit theoretical semicycle shapes well. The present model suggests an upward travelling 22 yr wave launched by a reciprocating magnetic oscillator deep in the sun.

Bracewell, R. N.↗
NASA NTRS2006

Geomagnetic Activity Indicates Large Amplitude for Sunspot Cycle 24

The level of geomagnetic activity near the time of solar activity minimum has been shown to be a reliable indicator for the amplitude of the following solar activity maximum. The geomagnetic activity index aa can be split into two components: one associated with solar flares, prominence eruptions, and coronal mass ejections which follows the solar activity cycle and a second component associated with recurrent high speed solar wind streams which is out of phase with the solar activity cycle. This second component often peaks before solar activity minimum and has been one of the most reliable indicators for the amplitude of the following maximum. The size of the recent maximum in this second component indicates that solar activity cycle 24 will be much higher than average - similar in size to cycles 21 and 22 with a peak smoothed sunspot number of 160 plus or minus 25.

Hathaway, David H.↗
NASA NTRS1982

Structure of the heliospheric current sheet in the early portion of sunspot cycle 21

The structure of the heliospheric current sheet on a spherical source surface of radius 2.35 solar radii has been computed via the use of a potential field model during the first year and a half after the last sunspot minimum. The solar polar magnetic field that is not fully observed in conventional magnetograph scans was included in the computation. The computed heliospheric current sheet had a quasi-stationary structure consisting of two northward and two southward maxima in latitude per solar rotation. The extent in latitude slowly increased from about 15 deg near the start of the interval to about 45 deg near the end. The magnetic field polarity (away from the sun or toward the sun) at the subterrestrial latitude on the source surface agreed with the interplanetary magnetic field polarity observed or inferred at the earth on 82 percent of the days. The interplanetary field structure observed at the earth at this time is finely tuned to the structure of low-latitude fields on the source surface.

Hoeksema, J. T.↗
NASA NTRS1989

Evolution of the sun's polar fields during sunspot cycle 21 - Poleward surges and long-term behavior

Longitudinally averaged observations of the photospheric field during 1976-1986 are analyzed using a flux transport model. The way in which source eruptions, supergranular diffusion, and meridional flow collaborate to produce strong, highly concentrated polar fields near sunspot minimum is clarified as follows: (1) widespread eruptions of individual bipolar magnetic regions, with their leading polarity flux equatorward of their trailing polarity flux, collectively establish a large-scale separation of polarities in latitude; (2) the low-latitude, leading polarity flux diffuses across the equator and merges with its opposite hemisphere counterpart; and (3) meridional flow carries the resulting surplus of trailing polarity flux to the poles, and concentrates it there against the spreading effect of diffusion. Episodic 'surges' of flux to the poles are induced by fluctuations in the source eruption rate. Simulations indicate that relatively weak, trailing polarity surges may occur even in a steady flow field. However, in order to account for the giant surges of alternating polarity and the resulting oscillations in the polar field strength observed during 1980-1982, both accelerated flow and enhanced eruption rates are required.

Wang, Y.-M.↗
NASA NTRS2008

On the Relationship between Solar Wind Speed, Earthward-Directed Coronal Mass Ejections, Geomagnetic Activity, and the Sunspot Cycle Using 12-Month Moving Averages

For 1996 .2006 (cycle 23), 12-month moving averages of the aa geomagnetic index strongly correlate (r = 0.92) with 12-month moving averages of solar wind speed, and 12-month moving averages of the number of coronal mass ejections (CMEs) (halo and partial halo events) strongly correlate (r = 0.87) with 12-month moving averages of sunspot number. In particular, the minimum (15.8, September/October 1997) and maximum (38.0, August 2003) values of the aa geomagnetic index occur simultaneously with the minimum (376 km/s) and maximum (547 km/s) solar wind speeds, both being strongly correlated with the following recurrent component (due to high-speed streams). The large peak of aa geomagnetic activity in cycle 23, the largest on record, spans the interval late 2002 to mid 2004 and is associated with a decreased number of halo and partial halo CMEs, whereas the smaller secondary peak of early 2005 seems to be associated with a slight rebound in the number of halo and partial halo CMEs. Based on the observed aaM during the declining portion of cycle 23, RM for cycle 24 is predicted to be larger than average, being about 168+/-60 (the 90% prediction interval), whereas based on the expected aam for cycle 24 (greater than or equal to 14.6), RM for cycle 24 should measure greater than or equal to 118+/-30, yielding an overlap of about 128+/-20.

Wilson, Robert M.↗