Solar Magnetic Polarity Research Articles

The geomagnetic semiannual anomaly on the four Dst‐fundamental observatories: Dependences with Sun‐Earth physical parameters

The semiannual anomaly (also known as semiannual variation) on the magnetic activity is a phenomenon that produces clear minima during March and September and maxima in June and December on the horizontal components of the geomagnetic field. This phenomenon has been known since the middle of the nineteenth century, but in spite of the accumulation of measurements and the development of three theoretical models, a conclusive physical explanation for it has not been developed. The usual approach to study the semiannual anomaly is by means of geomagnetic indices like the disturbance storm time, Dst, which is based on combining measurements registered on four magnetic observatories. This work follows a different approach based on the raw horizontal components registered at the four observatories. The analyses performed aimed to study and assess the impact of several external parameters, characteristics of the Sun‐Earth environment, on the semiannual anomaly. The influence of the global geomagnetic activity level, the solar activity level, the solar magnetic polarity, and the rising/declining phase of the solar radiation cycle is analyzed in detail. The most important finding is that the semiannual anomaly is always present and that none of the previously mentioned parameters significantly favor the development of it. A second result is the presence of a 27 day signal superposed to the semiannual anomaly which is significantly affected by the solar activity level.

CORRELATION BETWEEN THE 22-YEAR SOLAR MAGNETIC CYCLE AND THE 22-YEAR QUASICYCLE IN THE EARTH'S ATMOSPHERIC TEMPERATURE

According to the variation pattern of the solar magnetic field polarity and its relation to the relative sunspot number, we established the time series of the sunspot magnetic field polarity index and analyzed the strength and polarity cycle characteristics of the solar magnetic field. The analysis showed the existence of a cycle with about a 22-year periodicity in the strength and polarity of the solar magnetic field, which proved the Hale proposition that the 11-year sunspot cycle is one-half of the 22-year solar magnetic cycle. By analyzing the atmospheric temperature field, we found that the troposphere and the stratosphere in the middle latitude of both the northern and southern hemispheres exhibited a common 22-year quasicycle in the atmospheric temperature, which is believed to be attributable to the 22-year solar magnetic cycle.

Energetic Particles Measured in and out of the Ecliptic Plane During the Last Gnevyshev Gap

We analyzed the temporal variation of energetic particles measured by the Low Energy Telescope (LET), the Kiel Electron Telescope (KET), and the High Energy Telescope (HET) instruments aboard Ulysses and the Electron Proton Helium Instrument (EPHIN) aboard SOHO during the last solar magnetic field polarity reversal in 2001. We have found two periods with anomalous low fluxes during that time that are present both at low and high heliolatitudes. We compared the energetic particle fluxes with solar energetic phenomena that traditionally have been associated with solar energetic particle (SEP) events at 1 AU. Our results show that these periods are related to relative minima in the number of X-ray flares and CMEs. Since Ulysses scanned the whole latitude range from 80 °S to 80 °N, we conclude that this process affects the inner three-dimensional heliosphere globally.

North-south asymmetry in solar, interplanetary, and geomagnetic indices

Data of hourly interplanetary plasma (field magnitude, solar wind speed, and ion density), solar (sunspot number, solar radio flux), and geomagnetic indices (Kp, Ap) over the period 1970-2010, have been used to examine the asymmetry between the solar field north and south of the heliospheric current sheet (HCS). A persistent yearly north-south asymmetry of the field magnitude is clear over the considered period, and there is no magnetic solar cycle dependence. There is a weak N-S asymmetry in the averaged solar wind speed, exhibited well at times of maximum solar activities. The solar plasma is more dense north of the current sheet than south of it during the second negative solar polarity epoch (qA < 0). Moreover, the N - S asymmetry in solar activity (Rz) can be statistically highly significant. The sign of the average N - S asymmetry depends upon the solar magnetic polarity. The annual magnitudes of N - S asymmetry depend positively on the solar magnetic cycle. Most of the solar radio flux asymmetries occurred during the period of positive IMF polarity.

The effects of the solar magnetic polarity and the solar wind velocity on Bz-component of the interplanetary magnetic field

We have studied the effect of both solar magnetic polarity and the solar wind velocity on the Bz-component of the interplanetary magnetic field, IMFBz, for the minimum activity of the solar cycles 21, 22, 23 and 24. We made a statistical study of IMFBz in the first section which is considered as an extension of Lyatsky et al. (2003). They made a statistical study of IMFBz for two periods of minimum solar activity 22 and 23 related to 1985–1987 and 1995–1997 when the solar magnetic field had opposite polarity. Our results seem to be consistent with the results obtained by Lyatsky et al. (2003). We found that there is a dependence of IMFBz on the IMFBx and the solar magnetic polarity for the minimum periods of the selected four solar cycles. In addition, we found that there is a dependence of IMFBz on the solar wind velocity.

Solar magnetic polarity dependency of geomagnetic storm seasonal occurrence

[1] For nearly a century it has been known that the tendency for geomagnetic activity is, on average, higher at the equinoxes than at the solstices. Previous studies on semiannual geomagnetic activity were performed mainly for geomagnetic indices such as am, aa, U, and AL. Thus, we need to understand the seasonal variation of geomagnetic activity defined by the Dst index over the long term. It is also necessary to test the solar magnetic polarity dependence of geomagnetic activity. This paper is a statistical analysis of the geomagnetic storms defined by the Dst index. Our storm data consists of two sets of storm data for 5 years at each solar minimum during the four solar cycles (19–22) from 1962 through 1998 for two of each solar magnetic polarity. The storms are divided into two groups defined by Dst index (Dst(min) 70 nT; Dst(min) 90 nT). Monthly occurrences of these storms are compared. Storms of ∣Dst∣ > 90 nT and of ∣Dst∣ > 70 nT occurred 153 and 238 times, respectively, during the testing periods. Storms occurred more frequently during the spring and fall seasons for all solar cycle minima, regardless of solar magnetic polarity.

Solar Rotational Periodicities and the Semiannual Variation in the Solar Wind, Radiation Belt, and Aurora

The behavior of a number of solar wind, radiation belt, auroral and geomagnetic parameters is examined during the recent extended solar minimum and previous solar cycles, covering the period from January 1972 to July 2010. This period includes most of the solar minimum between Cycles 23 and 24, which was more extended than recent solar minima, with historically low values of most of these parameters in 2009. Solar rotational periodicities from 5 to 27 days were found from daily averages over 81 days for the parameters. There were very strong 9-day periodicities in many variables in 2005 – 2008, triggered by recurring corotating high-speed streams (HSS). All rotational amplitudes were relatively large in the descending and early minimum phases of the solar cycle, when HSS are the predominant solar wind structures. There were minima in the amplitudes of all solar rotational periodicities near the end of each solar minimum, as well as at the start of the reversal of the solar magnetic field polarity at solar maximum (∼ 1980, ∼ 1990, and ∼ 2001) when the occurrence frequency of HSS is relatively low. Semiannual equinoctial periodicities, which were relatively strong in the 1995 – 1997 solar minimum, were found to be primarily the result of the changing amplitudes of the 13.5- and 27-day periodicities, where 13.5-day amplitudes were better correlated with heliospheric daily observations and 27-day amplitudes correlated better with Earth-based daily observations. The equinoctial rotational amplitudes of the Earth-based parameters were probably enhanced by a combination of the Russell-McPherron effect and a reduction in the solar wind-magnetosphere coupling efficiency during solstices. The rotational amplitudes were cross-correlated with each other, where the 27-day amplitudes showed some of the weakest cross-correlations. The rotational amplitudes of the > 2 MeV radiation belt electron number fluxes were progressively weaker from 27- to 5-day periods, showing that processes in the magnetosphere act as a low-pass filter between the solar wind and the radiation belt. The Ap/Kp magnetic currents observed at subauroral latitudes are sensitive to proton auroral precipitation, especially for 9-day and shorter periods, while the Ap/Kp currents are governed by electron auroral precipitation for 13.5- and 27-day periodicities.

The space environment of Mercury at the times of the second and third MESSENGER flybys

The second and third flybys of Mercury by the MESSENGER spacecraft occurred, respectively, on 6 October 2008 and on 29 September 2009. In order to provide contextual information about the solar wind properties and the interplanetary magnetic field (IMF) near the planet at those times, we have used an empirical modeling technique combined with a numerical physics-based solar wind model. The Wang–Sheeley–Arge (WSA) method uses solar photospheric magnetic field observations (from Earth-based instruments) in order to estimate the inner heliospheric radial flow speed and radial magnetic field out to 21.5 solar radii from the Sun. This information is then used as input to the global numerical magnetohydrodynamic model, ENLIL, which calculates solar wind velocity, density, temperature, and magnetic field strength and polarity throughout the inner heliosphere. WSA-ENLIL calculations are presented for the several-week period encompassing the second and third flybys. This information, in conjunction with available MESSENGER data, aid in understanding the Mercury flyby observations and provide a basis for global magnetospheric modeling. We find that during both flybys, the solar wind conditions were very quiescent and would have provided only modest dynamic driving forces for Mercury's magnetospheric system.

Rigidity dependence of 11 year cosmic ray modulation: Implication for theories

We use data obtained with the global network of detectors, with median rigidity (Rm) of response covering a wide range (1 to 200 GV), to derive the rigidity dependence of the 11 year modulation of galactic cosmic rays (GCR) for 4 sunspot cycles (20 to 23). We find that observed rigidity dependence is represented by a power law with negative exponents; the exponents do not depend on the solar magnetic polarity. However, the dependence on GCR rigidity is significantly flatter than is expected from the Quasi‐linear Theory of modulation formulated by Jokipii.

Effect of volcanic activities and CO2 concentration on summer temperature in middle stratosphere, North Hemisphere

Successive filtration and comparison show that the stratosphere air temperature in 10 hPa-layer of the Northern Hemisphere (NH) in July continuously increases, which is associated to the increases in greenhouse gases mostly CO2, volcanic activities, and solar activity, demonstrating the follows. (a) The increase in CO2 concentration is largely consistent with that of the stratosphere air temperature in 10 hPa-layer of the NH in July. However, the increase in the air temperature is not in a linear pattern, during which several cooling events interrupt. The cooling events between late 1960s and late 1970s are remarkable ones and so is the one before mid 1990s. Analysis shows that these events are induced by volcanic activities and solar activity. (b) The CO2-free variation in the stratosphere air temperature in 10 hPa-layer of the NH is consistent with that of the solar magnetic index. The wave crests and wave troughs of the two curves are consistent in phase, and the curve of solar magnetic index leads the other slightly. In other words, when the solar magnetic pole is southward, a warming in the NH stratosphere corresponds; and on the contrary, the northward solar magnetic pole corresponds to a cooling event. The variation in solar magnetic polarity strongly impacts the variation in the stratosphere temperature.

The role of cyclic solar magnetic field variations in the long-term cosmic ray modulation

Updating the semi-empirical model of cosmic rays (CR) modulation proposed in our previous work has been discussed. In order to provide a description of long-term variations, in which the CR modulation would adequately reflect the complex interaction of global and local solar magnetic fields, we have supplemented the model with the following characteristics: the solar magnetic field polarity, the integral index, the partial indices, the tilt of the current sheet, and the index characterizing the X-ray flares. The role of each index in the CR modulation has been determined. In the multi-parameter description of long-term CR variations using the integral index or one of four partial indices, the best fit for the period 1977–1999 has been obtained for the integral index and the sector-odd index characterizing the inclined dipole. The discrepancy between the model and observations increases from the beginning of 2000. Therefore, the problematic features in the behavior and modeling of CR during cycle 23 have been discussed. It is suggested that the cycle-to-cycle decrease of the CR density in the minimum epochs of the past solar activity (SA) cycles could be explained by the decrease of the zone-odd index.

Galactic cosmic ray H and He nuclei energy spectra measured by Voyagers 1 and 2 near the heliospheric termination shock in positive and negative solar magnetic polarity cycles

Using data from the Voyager 1 and 2 spacecraft, we have followed the intensity variations of H, He and C + O nuclei between 1998 and 2008 and determined the spectra for H and He at the time of minimum modulation in 1998, when the solar magnetic polarity was positive and again in 2008 when the solar magnetic polarity was negative. At these times these data are representative of conditions near a heliospheric termination shock assumed to be located at ∼90 AU. Above ∼400 MeV/nuc for He nuclei the 11‐year solar modulation cycle observed at the Earth is not seen; instead there is a 22‐year variation. The negative polarity cycle intensities above ∼150 MeV/nuc are higher than those in the positive polarity cycle by a factor of 1.4–1.7 times for both H and He nuclei. Below ∼100 MeV/nuc the C nuclei intensities are similar in the two cycles to within ±10%. These observations are compared with theoretical calculations which also show a negative to positive polarity cycle intensity difference at higher energies, most likely associated with energy changes due to drifts near the termination shock, but the comparison suggests that improved estimates of the local interstellar spectra are required.

Possible link between multi-decadal climate cycles and periodic reversals of solar magnetic field polarity

The linkage between multi-decadal climate variability and activity of the sun has been long debated based upon observational evidence from a large number of instrumental and proxy records. It is difficult to evaluate the exact role of each of solar parameters on climate change since instrumentally measured solar related parameters such as Total Solar irradiance (TSI), Ultra Violet (UV), solar wind and Galactic Cosmic Rays (GCRs) fluxes are more or less synchronized and only extend back for several decades. Here we report tree-ring carbon-14 based record of 11-year/22-year solar cycles during the Maunder Minimum (17th century) and the early Medieval Maximum Period (9–10th century) to reconstruct the state of the sun and the flux of incoming GCRs. The result strongly indicates that the influence of solar cycles on climate is persistent beyond the period after instrumental observations were initiated. We find that the actual lengths of solar cycles vary depending on the status of long-term solar activity, and that periodicity of the surface air temperatures are also changing synchronously. Temperature variations over the 22-year cycles seem, in general, to be more significant than those associated with the 11-year cycles and in particular around the grand solar minima such as the Maunder Minimum (1645–1715 AD). The polarity dependence of cooling events found in this study suggests that the GCRs can not be excluded from the possible drivers of decadal to multi-decadal climate change.

Temporal and spectral variations of anomalous oxygen nuclei measured by Voyager 1 and Voyager 2 in the outer heliosphere

We have studied the temporal and spectral variations of anomalous oxygen nuclei at the Voyager 1 (V1) and 2 (V2) spacecraft in the outer heliosphere from 1990 to the present time in 2006 when V1 is now beyond the heliospheric termination shock. During this time period, the intensities increased from their lowest values in 1990–1991 up to a maximum in 1998–1999 and then decreased rapidly in 2000–2001 in time coincidence with the change in solar magnetic polarity from positive to negative. During the time period after 2001, significant changes in intensities and spectra are observed relative to the earlier period of positive solar magnetic polarity before 2001. It is found that the intensities of O above ∼10 MeV/nuc at V1 after ∼2002.0 were higher relative to the same galactic cosmic ray He intensity between 150–380 MeV/nuc than in the earlier time period. As a result, by 2006 these intensities were a factor ∼3 times those measured at the intensity maximum in 1998–1999 in the previous polarity cycle. The changes observed at V2 followed a similar pattern, but the relative intensity changes of O were a factor ∼2 times greater than those observed at V1. Also, above ∼10 MeV/nuc, the intensity changes at V1 and V2 were nearly energy independent, and the spectra at all times before and after the solar magnetic polarity change and at all modulation levels remained ∼E−3.0±0.2, possibly characteristic of a “source” spectrum. When V1 crossed the termination shock, no noticeable spectral or intensity changes were observed.

Wavelet image of the fine structure of the 11-year cycle based on studying cosmic ray fluctuations during cycles 20–23

A non-stationary transient oscillating process of the solar magnetic field polarity reversal of ≈3 years in duration has been established: a U-shaped dynamics in the wavelet representation of variations in the scintillation index of galactic cosmic rays (GCRs) (≈7, 13–14, and ≈7 solar rotations). The transient oscillating process of the field reversal is concluded with a sharp and deep decrease in the GCR intensity at the branch of 11-year cycle decline (1972, 1982, 1991, and 2003). The duration of the transient process inversely depends on the 11-year cycle amplitude. Retardation of relaxation oscillations during “weak” cycles (20 and 23) explains “anomalous” solar activity in 1972 and 2003. A decrease in the amplitude of the current cycle 23 is accompanied by an increase in its duration, which can mean that the 11-year cyclicity has become anomalous. The constancy of the energy released in a single cycle indicates that the 11-year cycle is the mechanism of energy regulation preventing the Sun from “overheating” at the critical temperature.

Effects of Different Solar Wind Speed Profiles in the Heliosheath on the Modulation of Cosmic‐Ray Protons

An asymmetric solar wind termination shock (TS) model is used to study the effects on the modulation of cosmic-ray protons for different scenarios of the solar wind speed (V) in the heliosheath. This two-dimensional model is applied using predictions for V in the heliosheath that were calculated with a time-dependent three-dimensional hydrodynamic model. Decreases stronger than the generally assumed V ∝ 1/r2 in the heliosheath are studied, as well as an extreme case, V ∝ r2, where r is the radial distance.The effect of the TS is enhanced under certain circumstances, and barrier-type modulation in the heliosheath also depends on the chosen V-profiles. Significant changes occur mostly for the A < 0 solar magnetic polarity cycle, at all distances in the equatorial plane, when the V is changed from an incompressible fluid (V ∝ 1/r2, ∇ V = 0) in the heliosheath, to V ∝ 1/r8, in a symmetrical model. For the asymmetrical case the TS is predicted to be more effective in the heliospheric nose than in the tail, especially for the A < 0 cycle during solar minimum conditions. The different profiles for V do not have a significant effect on the intensities inside the TS, but in the heliosheath the difference can be quite significant. It is found that V ∝ 1/r2 in the heliosheath is an oversimplification. The consequent effects of having ∇ V ≠ 0 in the heliosheath prove to be relevant for cosmic-ray modulation and acceleration, especially now that the Voyager 1 spacecraft encountered the TS and entered the heliosheath.

Solar Wind Forecasting with Coronal Holes

An empirical model for forecasting solar wind speed related geomagnetic events is presented here. The model is based on the estimated location and size of solar coronal holes. This method differs from models that are based on photospheric magnetograms (e.g., Wang–Sheeley model) to estimate the open field line configuration. Rather than requiring the use of a full magnetic synoptic map, the method presented here can be used to forecast solar wind velocities and magnetic polarity from a single coronal hole image, along with a single magnetic full-disk image. The coronal hole parameters used in this study are estimated with Kitt Peak Vacuum Telescope He I 1083 nm spectrograms and photospheric magnetograms. Solar wind and coronal hole data for the period between May 1992 and September 2003 are investigated. The new model is found to be accurate to within 10% of observed solar wind measurements for its best 1-month period, and it has a linear correlation coefficient of ∼0.38 for the full 11 years studied. Using a single estimated coronal hole map, the model can forecast the Earth directed solar wind velocity up to 8.5 days in advance. In addition, this method can be used with any source of coronal hole area and location data.

Time variations of cosmic ray electrons and nuclei between 1978 and 2004: Evidence for charge‐dependent modulation organized by changes in solar magnetic polarity and current sheet tilt

We have examined the 52‐day average intensities of 1.2‐GV electrons and positively charged nuclei of the same rigidity during the time period from 1978 to 2004. This time period covers a complete 22‐year solar cycle plus data from 2000 to 2004 that can be used to examine the repetitive nature of the variations. The 11‐year solar modulation changes both the electrons and nuclei intensities by similar overall factors ∼10 at 1.2 GV. Differences in the electron/nuclei modulation as exhibited by the e/nuclei ratios occur mainly between 1980–1984, 1990–1994, and 2000–2004. At these times the e/nuclei ratios increase or decrease by a factor ∼2 at 1.2 GV in close correspondence with the changes in solar magnetic polarity from positive to negative (1980) and negative to positive (1990). The e/nuclei ratio changes in 2000–2004 match those seen in 1980–1984, thus confirming the 22‐year nature of the e/nuclei variations.

Differences in the spectra of anomalous cosmic ray helium nuclei in two solar magnetic polarity cycles

We compare the spectra of anomalous He nuclei measured in the heliosphere by the IMP, Voyager, and Pioneer spacecraft at heliospheric distances ranging from 1 to over 80 AU in the time period from 1977 to 2003. Striking differences in the energy spectra of these nuclei are found between positive and negative solar magnetic polarity cycles. These differences are such that the spectra observed in negative polarity cycles appear to be greatly deficient in particles with energies of less than ∼15 MeV but have an excess intensity above &gt;25 MeV, relative to the spectra observed in positive cycles. These spectral differences are complex and may be related to both solar modulation effects inside of the heliospheric termination shock as well as differences occurring at or near the termination shock itself. At the modulation minimum in 1998 when V2 was at a distance of 56 AU and V1 at 71 AU, a high intensity of low‐energy anomalous He particles was observed with a peak in the spectrum at ∼5 MeV/nuc. In late 2001 after the solar magnetic polarity had changed from positive to negative and the 11‐year solar modulation had increased, the previous high intensities of the anomalous He nuclei seen in 1998 were greatly reduced at both V1 and V2, and the energy of the peak intensity had moved up to ∼20–25 MeV/nuc. However, in both polarity periods the highest‐energy part of this spectrum above ∼30 MeV/nuc is similar with a slope of −2.8 ± 0.2. The details of these complex spectral changes are described in this paper.

Observed north‐south asymmetries in the long‐term Bz mean average

Several publications have reported a north‐south asymmetry of the solar magnetic polarity in variations of the solar wind, geomagnetic activity, and polar coronal holes. The present study analyzes the polarity of the z‐component of the interplanetary magnetic field (IMF Bz), finding long‐term periods with a predominant polarity. IMF data was averaged using a 13‐month ‘boxcar’ long‐term averaging method that has been used to smooth the sunspot number for use in the McNish‐Lincoln forecasting method. Both the averaging technique and the McNish‐Lincoln forecasting method have also been used to forecast the average magnitude of Bz. However, this previous work did not examine the effect of polarity. The work reported here used the same averaging method to examine the long‐term average of Bz, but also considered the polarity. The resulting averages for the period 1963–2002 showed long periods where one polarity dominated the other. Examination of the data has led us to believe this predominance is caused by a shift of the heliomagnetic equator away from the heliographic equator.