Minimum Of Solar Cycle Research Articles

The Additional Criterion for the Determination of the Time of Minimum of a Solar Cycle

The sunspot number is becoming an increasingly insufficiently reliable parameter for the determination of the time of minimum of a solar cycle during the prolonged and deep minimum of the 23rd solar cycle. Moreover, the sunspot number does not quantitatively reflect physical processes and is a practically conventional qualitative “noisy” parameter. Introduction of an additional criterion for the determination of the time of minimum of a solar cycle is becoming particularly topical due to the upcoming common descent of the level of the 2-secular cycle, when the amplitude of sunspot activity variation will sequentially decrease during several subsequent cycles (after the 23rd cycle). We propose the adoption of the smoothed minimal level of the total solar irradiance (TSI) as an additional physically justified criterion for the determination of the time of minimum of a solar cycle during the minimum of sunspot activity. The minimal level of the monthly average values of the TSI smoothed for 13 months when the last two of its values exceed the preceding value at the point of minimum will additionally indicate the time of minimum of a cycle. The additional criterion has been successfully used for the determination of the time of minima of the preceding 21st and 22nd cycles.

Forecast of Maximum Sunspot Number of Cycle 24

It is important for space weather to forecast the maximum sunspot number of the coming cycle. Sunspot number is an indicator of solar activity and occurrence of space storms increases in proportion to solar activity. We found that there is a negative correlation between occurrence number of no-spot days near solar minimum and maximum sunspot number of the next cycle. Using this relation, we made a forecast of the maximum sunspot number of Cycle 24. The maximum sunspot number of Cycle 24 is lower than that of Cycle 23 of 120.8 according to our forecast. Low solar activity of Cycle 24 is also suggested by low geomagnetic activity near minimum and long cycle length of Cycle 23.

Solar-Cycle Characteristics Examined in Separate Hemispheres: Phase, Gnevyshev Gap, and Length of Minimum

Research results from solar-dynamo models show the northern and southern hemispheres may evolve separately throughout the solar cycle. The observed phase lag between the hemispheres provides information regarding the strength of hemispheric coupling. Using hemispheric sunspot-area and sunspot-number data from Cycles 12 - 23, we determine how out of phase the separate hemispheres are during the rising, maximum, and declining period of each solar cycle. Hemispheric phase differences range from 0 - 11, 0 - 14, and 2 - 19 months for the rising, maximum, and declining periods, respectively. The phases appear randomly distributed between zero months (in phase) and half of the rise (or decline) time of the solar cycle. An analysis of the Gnevyshev gap is conducted to determine if the double-peak is caused by the averaging of two hemispheres that are out of phase. We confirm previous findings that the Gnevyshev gap is a phenomenon that occurs in the separate hemispheres and is not due to a superposition of sunspot indices from hemispheres slightly out of phase. Cross hemispheric coupling could be strongest at solar minimum, when there are large quantities of magnetic flux at the Equator. We search for a correlation between the hemispheric phase difference near the end of the solar cycle and the length of solar-cycle minimum, but found none. Because magnetic flux diffusion across the Equator is a mechanism by which the hemispheres couple, we measured the magnetic flux crossing the Equator by examining magnetograms for Solar Cycles 21 - 23. We find, on average, a surplus of northern hemisphere magnetic flux crossing during the mid-declining phase of each solar cycle. However, we find no correlation between magnitude of magnetic flux crossing the Equator, length of solar minima, and phase lag between the hemispheres.

The “Floor” in the Interplanetary Magnetic Field: Estimation on the Basis of Relative Duration of ICME Observations in Solar Wind During 1976 – 2000

Measurement of the floor in the interplanetary magnetic field and estimation of the time-invariant open magnetic flux of the Sun require knowledge of closed magnetic flux carried away by coronal mass ejections (CMEs). In contrast with previous papers, we do not use global solar parameters to estimate such values: instead we identify different large-scale types of solar wind for the 1976 – 2000 interval to obtain the fraction of interplanetary CMEs (ICMEs). By calculating the magnitude of the interplanetary magnetic field B averaged over two Carrington rotations, the floor of the magnetic field can be estimated from the B value at a solar cycle minimum when the number of ICMEs is minimal. We find a value of 4.65±0.6 nT, in good agreement with previous results.

Anomalously low solar and geomagnetic activities in 2007

It has been indicated that 2007 is the year with clearly defined indications typical of a deep solar cycle minimum. In this case this year has no analogs for the entire period of regular solar and geomagnetic parameters concerning very many parameters. The specific behavior of different solar and magnetic activity indices in 2007 has been indicated in comparison with such a behavior of similar characteristics during other periods.

Evolution of the large-scale magnetic field over three solar cycles

AbstractObservations of the large-scale magnetic field show that the current extended solar cycle minimum is different from the three previous well-observed minima. The weaker polar fields increase the relative influence of middle and low-latitude flux patterns on the configuration of the corona and heliosphere. A much larger portion of the open flux originates in equatorial coronal holes. Even though the mean magnetic field of the Sun as a star is the weakest since measurements began, the sector structure of the interplanetary field, though smaller in magnitude, reached fairly high latitude until 2009. The emergence of active regions through the cycle and transport of flux from low to high latitudes also show quite different patterns, providing insight into the dynamo that drives the cycle. Long records of synoptic observations provide a rich source of information about solar activity that must be maintained.

The Solar Wind Quasi-Invariant Observed by STEREO A and B at Solar Minimum 2007 and Comparison with Two Other Minima

The solar wind quasi-invariant (QI) is defined as the ratio of the solar wind magnetic energy density to the plasma kinetic energy density (i.e., the inverse square of the Alfven Mach number). Previous work has found this quantity to be a good proxy for solar activity, correlating very well with the sunspot number at various heliospheric distances. It has the advantage of being locally determined from in situ measurements and can thus function as a heliospheric index of solar activity. Using STEREO A, STEREO B, and Wind data we obtain the distribution of QIs during the current solar activity minimum (March to December 2007). (1) We investigate whether this minimum is indeed weaker than previous ones by comparing this QI distribution with those during two other solar activity minima: 1995, using Wind data, and 1974, using Helios data at 1 AU. We find that, on average, QI(2007) is lower than during the previous two minima, indicating weaker solar activity. It also implies weaker MHD effects in solar wind flow around planetary magnetospheres, which, in turn, alters the solar wind’s interaction with them. (2) In all three solar cycle minima considered we find that the QI distributions are reasonably well represented by a log-normal distribution, for which we give the respective mean and standard deviations. These values are used in comparing the QIs over the three solar minima.

An optimal method for determining the time of the minimum of the 23rd solar cycle, July 2008

The time of the minimum of the solar cycle is usually determined by the minimum of the average monthly sunspot number smoothed over 13 months, i.e., with a large time delay, not earlier than eight months after the event. A new optimal method which allows one to establish the time of the minimum of the cycle as early as four months after the event is proposed. In the new method, the indicator of the time of a cycle minimum is the time of reaching the minimum level of average monthly values of the solar constant after which four succeeding values of this constant are larger than the pre-ceding minimum level. It is shown that the minimum of the past 23rd solar cycle took place in July, and the new 24th cycle started in August 2008.

Effects of the Weak Polar Fields of Solar Cycle 23: Investigation Using OMNI for the STEREO Mission Period

The current solar cycle minimum seems to have unusual properties that appear to be related to weak solar polar magnetic fields. We investigate signatures of this unusual polar field in the ecliptic near-Earth interplanetary magnetic field (IMF) for the STEREO period of observations. Using 1 AU OMNI data, we find that for the current solar cycle declining phase to minimum period the peak of the distribution for the values of the ecliptic IMF magnitude is lower compared to a similar phase of the previous solar cycle. We investigate the sources of these weak fields. Our results suggest that they are related to the solar wind stream structure, which is enhanced by the weak polar fields. The direct role of the solar field is therefore complicated by this effect, which redistributes the solar magnetic flux at 1 AU nonuniformly at low to mid heliolatitudes.

Solar cycle minimum measurements of the solar extreme ultraviolet spectral irradiance on 14 April 2008

[1] NASA sounding rocket 36.240 was launched from White Sands Missile Range in New Mexico on 14 April 2008. The primary instrument in this payload was the prototype extreme ultraviolet (EUV) Variability Experiment (EVE). The Multiple EUV Grating Spectrograph (MEGS) channels of EVE measure the solar irradiance from 6 to 106 nm at 0.1 nm spectral resolution. While the main purpose of this rocket flight is to provide the fifth underflight calibration for the Solar EUV Experiment (SEE), the importance of this rocket observation is the first observation of the solar EUV irradiance at high spectral resolution during solar cycle minimum conditions. The higher spectral resolution measurements will help resolve outstanding concerns about the previous solar soft X-ray irradiance results made with broadband photometers shortward of 27 nm, and the measurements provide the most accurate reference for the solar cycle minimum as part of the international Whole Heliosphere Interval (WHI) campaign.

The burst of solar and geomagnetic activity in August–September 2005

Abstract. During the August–September 2005 burst of solar activity, close to the current solar cycle minimum, a significant number of powerful X-ray flares were recorded, among which was the outstanding X17.0 flare of 7 September 2005. Within a relatively short period (from 22 August to 17 September) two severe magnetic storms were also recorded as well as several Forbush effects. These events are studied in this work, using hourly mean variations of cosmic ray density and anisotropy, derived from data of the neutron monitor network. During these Forbush effects the behavior of high energy cosmic ray characteristics (density and anisotropy) is analyzed together with interplanetary disturbances and their solar sources, and is compared to the variations observed in geomagnetic activity. A big and long lasting (~6 h) cosmic ray pre-decrease (~2%) is defined before the shock arrival on 15 September 2005. The calculated cosmic ray gradients for September 2005 are also discussed.

Seasonal features in the response of the mesopause temperature and emission intensities to solar activity variations

The seasonal dependences of the response of the hydroxyl ((6–2) band) and molecular oxygen O2(b1Σg+) ((0–1) band) emission intensities, temperature, and density indicator in the region of the hydroxyl emission maximum (87 km) to solar activity have been obtained based on the spectral observations of the mesopause emissions at Zvenigorod observatory during 2000–2007. The ratio of the OH (7–3) and (9–4) band intensities, characterizing the behavior of the vibrational temperature, has been used as an indicator of density. It has been established that the response of the studied mesopause characteristics to solar activity is positive in all seasons. In winter the response is maximal in the intensities and temperature and is minimal in the density indicator. The main mechanisms by which solar activity affects the mesopause characteristics have been considered. The behavior of the internal gravity waves with periods of 0.33–7 h depending on solar activity has been studied. It has been noted that these waves become more active at a minimum of the 11-year solar cycle.

Stratospheric and solar cycle effects on long‐term variability of mesospheric ice clouds

Model results of mesospheric ice layers and background conditions at 69°N from 1961 to 2008 are analyzed. The model nudges to European Centre for Medium‐Range Weather Forecasts data below ∼45 km. Greenhouse gas concentrations in the mesosphere are kept constant. At polar mesospheric cloud (PMC) altitudes (83 km) temperatures decrease until the mid 1990s by ‐0.08 K/yr resulting in trends of PMC brightness, occurrence rates, and, to a lesser extent, in PMC altitudes (−0.0166 km/yr). Ice layer trends are consistent with observations by ground‐based and satellite instruments. Water vapor increases at PMC heights and decreases above due to increased freeze‐drying caused by the temperature trend. Temperature trends in the mesosphere mainly come from shrinking of the stratosphere and from dynamical effects. A solar cycle modulation of H2O is observed in the model consistent with satellite observations. The effect on ice layers is reduced because of redistribution of H2O by freeze‐drying. The accidental coincidence of low temperatures and solar cycle minimum in the mid 1990s leads to an overestimation of solar effects on ice layers. A strong correlation between temperatures and PMC altitudes is observed. Applied to historical measurements this gives negligible temperature trends at PMC altitudes (∼0.01–0.02 K/yr). Strong correlations between PMC parameters and background conditions deduced from the model confirm the standard scenario of PMC formation. The PMC sensitivity on temperatures, water vapor, and Ly‐α is investigated. PMC heights show little variation with background parameters whereas brightness and occurrence rates show large variations. None of the background parameters can be ignored regarding its influence on ice layers.

A significant decrease of the fundamental Schumann resonance frequency during the solar cycle minimum of 2008-9 as observed at Modra Observatory

A significant decrease of the fundamental Schumann resonance frequency during the solar cycle minimum of 2008-9 as observed at Modra ObservatoryThe Schumann resonances (SR) are electromagnetic eigenmodes of the resonator bounded by the Earth's surface and the lower ionosphere. The SR frequency variability has been studied for more than 4 decades. Using data from the period 1988 to 2002,Sátori et al. (2005)showed that the SR fundamental mode frequency decreased on the 11-year time scale by 0.07 - 0.2 Hz, depending on which component of the field was used for estimation and likely also on the location of the observer. A decrease by 0.30 Hz from the latest solar cycle maximum to the minimum of 2009 is found in data from Modra Observatory. This extraordinary fall of the fundamental mode frequency can be attributed to the unprecedented drop in the ionizing radiation in X-ray frequency band. Although the patterns of the daily and seasonal variations remain the same in the solar cycle minimum as in the solar cycle maximum, they are significantly shifted to lower frequencies during the minimum. Analysis of the daily frequency range suggests that the main thunderstorm regions during the north hemisphere summer are smaller in the solar cycle minimum than in the maximum.

Electron properties of high‐speed solar wind from polar coronal holes obtained by Ulysses thermal noise spectroscopy: Not so dense, not so hot

We present radio observations of Ulysses' third fast latitude scan near the 2007 solar activity minimum of cycle 23. We deduce in situ measurements of the electron density and temperature using the method of quasi‐thermal noise spectroscopy. We study the large‐scale properties of the fast solar wind coming from polar coronal holes and compare our results to those obtained during Ulysses' first fast scan in 1994–1995 near the minimum of cycle 22. The fast solar wind in both hemispheres is less dense and cooler by about 19% and 13% respectively, as compared to the last solar minimum.

Long-Term Study of Active Spacecraft Potential Control

Emitters based on the liquid metal ion source principle have been operating on the Cluster spacecraft between 2000 and 2004, in order to control the spacecraft potential. The resulting reduction of positive spacecraft potential reduces perturbations to the plasma measurements on board. Ion currents up to 40 muA have been applied, which reduced the energy band in which photoelectrons disturb the plasma electron measurements to values close to the lower detection limit of the instrument. The experience with this method, meanwhile, covers both the maximum and minimum of the present solar cycle and allows one to study the variations of photoemission and other long-term trends and their effects on the measurements. A long-term trend of the controlled spacecraft potential is indeed observed. In addition, it appears that reconstruction of the uncontrolled spacecraft potential from the controlled one is possible if certain conditions are fulfilled. Spacecraft potential control can thereby improve the plasma measurements while still allowing one to estimate the total plasma density from the residual potential.

Solar Cycle Forecasting

Predicting the behavior of a solar cycle after it is well underway (2–3 years after minimum) can be done with a fair degree of skill using auto-regression and curve fitting techniques that don’t require any knowledge of the physics involved. Predicting the amplitude of a solar cycle near, or before, the time of solar cycle minimum can be done using precursors such as geomagnetic activity and polar fields that do have some connection to the physics but the connections are uncertain and the precursors provide less reliable forecasts. Predictions for the amplitude of cycle 24 using these precursor techniques give drastically different values. Recently, dynamo models have been used directly with assimilated data to predict the amplitude of sunspot cycle 24 but have also given significantly different predictions. While others have questioned both the predictability of the solar cycle and the ability of current dynamo models to provide predictions, it is clear that cycle 24 will help to discriminate between some opposing dynamo models.

3D evolution of solar magnetic fields and high-speed solar wind streams near the minimum of solar cycle 23

AbstractThe numerical method developed by Veselovsky & Ivanov (2006), together with magnetograms of the Sun obtained at the photospheric level were used to calculate the coronal magnetic field with open, closed and intermittent topology during March-December 2007. The results of the modelling are compared with stereoscopic images and movies of the corona observed by EUV telescopes onboard STEREO and SOHO spacecraft. The sources of the permanent and transient high speed solar wind streams as well as the sector structure and the heliospheric plasma sheet observed at the Earth's orbit by the ACE and STEREO spacecraft are discussed.

Seasonal and solar cycle variability in F‐region vertical plasma drifts over Ouagadougou

F‐region vertical drifts were made using ionosonde for the Africa equatorial station Ouagadougou (12°N, 1.5°W; 5.9°N dip) from 3 a of data during January 1987 to December 1989 for solar cycle minimum, medium, and maximum conditions (F10.7 = 85, 141, and 214, respectively) under geomagnetic quiet‐time. The variations are found to be dominated by the characteristics morning peak and evening prereversal enhancement (PRE) velocities. Seasonal and solar cycle effects are prominent near the dusk sector with an increase of PRE from solar minimum to maximum. The average equinoctial evening prereversal enhancement increases by almost a factor of three from low to high flux. On the average, the values of daytime and nighttime ionosonde‐derived vertical drifts are smaller by about a factor of four than the magnitude often mentioned in publication for equatorial regions from other experimental techniques. In addition, the morning peak velocities maximize between about 0830–0930 LT with typical values of 12–18 m/s. In contrast, PRE have largest amplitude between about 1700–2200 LT with typical values of 5–10 m/s. The morning reversal times do not reveal any dependence on season and solar cycle; but the most probable time of occurrence is around 0530 LT. The evening reversal times are in excellent agreements for the three levels of solar activity periods apart from June solstice that exhibit considerable variations. The most likely time of occurrence is near 2000 LT. Of importance, the link between the onset / inhibition parameters of postsunset irregularities over Ouagadougou indicate anti‐correlations with solar variability.

Latitude Distribution of Polar Magnetic Flux in the Chromosphere Near Solar Minimum

The distribution of magnetic flux elements as a function of latitude in the polar solar caps at a time close to solar cycle minimum is explored. High-sensitivity line-of-sight magnetograms acquired by the Synoptic Optical Long-term Investigations of the Sun (SOLIS) vector spectromagnetograph (VSM) in the chromospheric line Ca II 854.2 nm are utilized. The area density distribution of the magnetic flux averaged over months is found to decrease close to the solar poles. This trend is more pronounced when considering only flux elements with relatively large size (larger than ≈5'' × 5''). The flux density of the latter is relatively flat from the edge of the polar cap up to latitudes of 75°-80° and decreases by more than 50% up to the solar pole. The density of smaller flux features is more uniformly distributed. Although the density decrease is still present, it is less pronounced. Hence, we believe there are two populations of flux elements in the polar caps. The small flux elements are probably produced uniformly across the polar area, in contrast to the large flux elements that are the product of magnetic fields of decaying active regions. The latter are concentrated by solar convection and carried up to high latitudes by differential rotation and meridional circulation. This result is important in studying meridional flows that transport the magnetic flux from lower to higher solar latitudes resulting, in particular, to the solar-cycle-related polar reversal. It is also of importance in studying polar structures contributing to the fast solar wind, such as polar plumes.