Geomagnetic Activity Aa Index Research Articles

Estimation of the Occurrence Probability of Extreme Geomagnetic Storms by Applying Extreme Value Theory to Aa Index

AbstractIn this work, the occurrence probability of extreme geomagnetic storms is estimated by applying extreme value theory to the geomagnetic activity Aa index. The Aa index has 172 years observation time span, which is much longer than other geomagnetic indices, and thus is more suitable for analysis for rarely occurred extreme geomagnetic storms. We use two newly developed extreme value theory methods, block maxima method and peak over threshold, and find that the extreme geomagnetic storm that happened in March 1989 may happen once per century. This result implies that we should pay more attention to such extreme geomagnetic storms that can cause space weather hazards.

Persistent long-term (1944–2015) ionosphere-magnetosphere associations at the area of intense seismic activity and beyond

Analysis of the earthquakes catalogues since 1944 reveals the area of the peak global earthquake occurrence in the Pacific Ocean southwards from the magnetic equator, in particular, at Australia. In the present study a long series of geomagnetic aa indices gathered from two antipodal magnetic observatories at Melbourne (Australia) and Greenwich (UK) are compared with the monthly-hourly critical frequency, foF2, from the nearby ionosonde measurements at Canberra and Slough (Chilton) and Moscow (control site) for 1944–2015. The annual percentage occurrence of the positive ionosphere storms W index (pW+) and negative index (pW−) is determined. It is found that the occurrence of the ionosphere plasma depletion pW− of the instant foF2 as compared to the monthly median is well correlated with the aa index at all three sites (cc>0.85). The positive storm signatures of the plasma density enhancement pW+ show high correlation with the geomagnetic activity aa index at Slough (cc=0.68) and Moscow (cc=0.92) but drastic difference of missing correlation at Canberra (cc=0.06). It has been suggested that the frequent earthquake occurrence over Australia may produce the persistent significant ionosphere plasma enhancements at Canberra which disrupts balance between the ionosphere-magnetosphere activities.

Cardiovascular mortality in northwestern Russia in relation with geomagnetic disturbances

The aim of the study was to identify the impact of natural (geomagnetic disturbances), socioeconomic and man-made factors on mortality from cardiovascular diseases (CVD) in Northwestern Russia. Data from 9057 CVD deaths in Kirovsk town (Kola Peninsula, 67.6 N, 33.7 E) were analyzed with respect to seasons of the year from 1948 to 2010. Total male CVD deaths showed three significant peaks in May, September and winter. The May maximum was dominant for all males, except for the oldest age (≥70 years) group and seemed to be caused by emotional and physical effects of increased hormones in spring. Total female CVD deaths showed two significant maxima in autumn and winter and one insignificant spring peak. Seasonal distribution of the moderate (50<Ap<100 nT) magnetic had a bimodal pattern with spring and autumn peaks. This bimodal pattern of seasonal variation is also present in the female and male oldest age (≥70 years) groups. Multi-Taper-Method (MTM) spectral analysis revealed periods of ~9-10 years, ~4.7 years and ~2.3 years at confidence level of ≥90%. These periods seem to coincide with the main cycles of variations of the aa-index of geomagnetic activity. There is no significant association of air pollution and CVD mortality at the region.

On the statistical relationship between solar activity and spontaneous social processes

The starting times of mass spontaneous social movements have been compared with temporal changes in solar activity (Wolf numbers) and in the Aa index of geomagnetic activity. It is shown that relatively high values of solar and, hence, geomagnetic activity are typical (on average) of a set of years when social cataclysms began. In addition, the relationship between social activity and geomagnetic activity is expressed somewhat more strongly than with solar activity. Heliogeomagnetic activity itself is not, however, the cause of social conflicts, as is evidenced by the weakness of the statistical relationship and the fact that the time intervals of an extremely large number of social conflicts (the decades of the 1800s, 1910s, and 1990s) occur during periods of a reduced mean level of solar and geomagnetic activity. From an averaged statistical model of the solar-geomagnetic influence on social activity and the current status and forecast of the 24th solar cycle, we can assume that heliogeomagnetic factors will contribute to an increased level of sociopolitical activity at least until the end of 2014 and, possibly, a little longer.

The Centennial Gleissberg Cycle and its association with extended minima

AbstractThe recent extended minimum of solar and geomagnetic variability (XSM) mirrors the XSMs in the nineteenth and twentieth centuries: 1810–1830 and 1900–1910. Such extended minima also were evident in aurorae reported from 450 A.D. to 1450 A.D. This paper argues that these minima are consistent with minima of the Centennial Gleissberg Cycles (CGCs), a 90–100 year variation observed on the Sun, in the solar wind, at the Earth, and throughout the heliosphere. The occurrence of the recent XSM is consistent with the existence of the CGC as a quasiperiodic variation of the solar dynamo. Evidence of CGCs is provided by the multicentury sunspot record, by the almost 150 year record of indexes of geomagnetic activity (1868 to present), by 1000 years of observations of aurorae (from 450 to 1450 A.D.) and millennial records of radionuclides in ice cores. The aa index of geomagnetic activity carries information about the two components of the solar magnetic field (toroidal and poloidal), one driven by flares and coronal mass ejections (related to the toroidal field) and the other driven by corotating interaction regions in the solar wind (related to the poloidal field). These two components systematically vary in their intensity and relative phase giving us information about centennial changes of the sources of solar dynamo during the recent CGC over the last century. The dipole and quadrupole modes of the solar magnetic field changed in relative amplitude and phase; the quadrupole mode became more important as the XSM was approached. Some implications for the solar dynamo theory are discussed.

Northern Hemisphere patterns of phase coherence between solar/geomagnetic activity and NCEP/NCAR and ERA40 near-surface air temperature in period 7–8 years oscillatory modes

Abstract. Beginning from the 1950's, Paluš and Novotná (2009) observed statistically significant phase coherence among oscillatory modes with the period of approximately 7–8 years detected in monthly time series of sunspot numbers, geomagnetic activity aa index, North Atlantic Oscillation (NAO) index and near-surface air temperature from several mid-latitude European stations. Focusing on geographical distribution of the phenomenon we study Northern Hemisphere patterns of phase coherence between solar/geomagnetic activity and NCEP/NCAR and ERA40 near-surface air temperature. Both the reanalysis datasets provide consistent patterns of areas with marked phase coupling between solar/geomagnetic activity and climate variability observed in continuous monthly data, independent of the season, however, confined to the temporal scale related to the oscillatory periods about 7–8 years.

Recovery of data on interplanetary medium parameters based on the aa index of geomagnetic activity

The average annual values of the electric field and parameters of the solar wind and IMF from our time to 1868 have been estimated based on the statistical relation between the aa index of geomagnetic activity and the interplanetary medium parameters. This estimation indicates that the relative variations during the 20th century were observed in the electric field (25 ± 3%), the IMF vector component transverse with respect to the velocity (16 ± 3%), and the solar wind plasma velocity (9 ± 1%, 37 ± 4 km/s). The modulus of the IMF vector radial component increased by 9.0 ± 2.5% during this period.

Phase-coherent oscillatory modes in solar and geomagnetic activity and climate variability

Oscillatory modes with the period of approximately 7–8 yr were detected in monthly time series of sunspot numbers, geomagnetic activity aa index, NAO (North Atlantic Oscillation) index and near-surface air temperature from several mid-latitude European locations. Instantaneous phases of the modes underwent synchronization analysis and their statistically significant phase coherence, beginning from 1950s, has been observed. Thus the statistical evidence for a coupling between solar/geomagnetic activity and climate variability has been obtained from continuous monthly data, independent of the season, however, confined to the temporal scale related to oscillatory periods about 7–8 yr.

Predicting maximum sunspot number in solar cycle 24

A few prediction methods have been developed based on the precursor technique which is found to be successful for forecasting the solar activity. Considering the geomagnetic activity aa indices during the descending phase of the preceding solar cycle as the precursor, we predict the maximum amplitude of annual mean sunspot number in cycle 24 to be 111 ± 21. This suggests that the maximum amplitude of the upcoming cycle 24 will be less than cycles 21-22. Further, we have estimated the annual mean geomagnetic activity aa index for the solar maximum year in cycle 24 to be 20.6 ± 4.7 and the average of the annual mean sunspot num- ber during the descending phase of cycle 24 is estimated to be 48 ± 16.8. Predictions of solar and geomagnetic activities are important for various purposes, including the operation of low-earth orbiting satellites, operation of power grids on Earth, and satellite communication systems. Various techniques, namely, even/odd behaviour, precursor, spectral, climatology, recent climatology, neural networks have been used in the past for the prediction of solar activity. Many investigators (Ohl 1966; Kane 1978, 2007; Thompson 1993; Jain 1997; Hathaway & Wilson 2006) have used the 'precursor' technique to forecast the solar activity. Ohl (1966) noted that the geomagnetic activity in the declining phase of a sunspot cycle is found to be well correlated with the sunspot maximum of the next cycle. Using the geomagnetic activity aa index as the precursor, Jain (1997) predicted the maximum annual mean sunspot number for cycle 23 to be 166.2 which is found to be higher than the observed value of 120. In fact, he did not propose the error estimate in the predicted number otherwise it could have been within error limits. In this view, we predict the maximum amplitude of solar cycle 24 using the pre- cursor technique described by Jain (1997) in this paper. Section 2 describes about the acquisition of data of annual mean sunspot number and the geomagnetic activity aa index used in the current investigation. In section 3, we explain the analysis and results obtained. We briefly discuss and conclude our results in section 4 in the light of other investigations.

Restoration of information about the interplanetary and solar magnetic fields based on the aa index of geomagnetic activity

The method for searching statistical relations between different solar wind parameters and the aa index of geomagnetic activity, which formed the basis for restoring the average annual values of some of these parameters (indicating that the solar coronal magnetic field increased by not more than 10% during the last century), has been proposed.

Solar and geomagnetic activity on a long time scale: Reconstructions and possibilities for predictions

Using two mathematical methods based on the wavelet transform and nonlinear dynamics, we reconstructed the behavior of the aa-index of geomagnetic activity in the past. Two versions of the series are provided: for the last 400 years and on an almost 1000-year time scale. We consider typical values of the aa-index at grandiose extrema of solar activity. The same high level of geomagnetic activity as that observed in the last 50 years is shown to have also taken place in the early 12th and late 14th centuries. We suggest an extended time series of A-indices of the large-scale solar magnetic field. On the 400-year time scale, we confirmed that the large-scale magnetic field develops earlier than the magnetic fields of active regions. Ohl’s prediction method was verified on the same time scale.

The interannual modulation of the solar and geomagnetic activity relationships

The interannual modulation of the geomagnetic activity caused by solar activity is investigated by means of the mutual spectral analysis. The frequency characteristics of connection between the Wolf numbers W 0 and the aa index of geomagnetic activity for the time interval 1890–1990 are analysed. Interannual and seasonal variations in the corresponding coherence spectra CH(T) for the quasi-decennial, quasi-pentennial, quasi-triennial and quasi-biennial cycles are revealed. The particular properties of the mean parameters for individual months are outlined.

Strength and phase of the solar dynamo during the last 12 cycles

We use the aa index of geomagnetic activity recorded for 130 years to study the strength and phase properties of the solar dynamo. We split the monthly averaged aa into two parts: one part proportional to the sunspot number and the other part proportional to the residual. We argue that the first part is a proxy for the solar dynamo's large‐scale mean toroidal magnetic field. The residual has the same periodicity as the sunspot cycle and has closely related amplitude, but is shifted in phase. We relate this residual to the poloidal field generated from the toroidal field in the dynamo process. The changes in both components demonstrate that there was a long‐term trend in the strength and relative phases of the toroidal and poloidal components of the mean‐field dynamo. The changes in the relative phase of the toroidal and poloidal fields suggest that the distributions of the differential rotation and helicity within the Sun's convection zone have changed over timescales of 50 years.

Variable Solar Forcing and Climate Changes

Numerous studies of interrelations between solar activity and global climate changes report contradictory conclusions. The topic as such is too complex, and manifestations of the studied relationship appear to differ in time and space, and sometimes are even of the opposite sense, In this study the data on air temperature and precipitation totals from Hurbanovo, one of the oldest meteorological observatories in Europe, are used to study their evolution within the interval 1871–1995, covering solar cycles 12–22, The variability of the meteorological elements mentioned is compared with that of the sunspot number and aa index of geomagnetic activity. The sensitivity of climate changes to variable solar forcing is presented as a comparison of extreme (maximum/minimum) activity conditions. Harmonic components with periods close to the length of the solar secular and solar magnetic cycles were found in climate evolution profiles.

Prediction of the Amplitude of Sunspot Cycle 23

A few prediction methods have been developed using the ‘precursor’ techniques and are found to be successful. On the basis of geomagnetic activity aa indices during the descending phase of the preceding cycle, we have established an expression which predicts the maximum annual mean sunspot number in cycle 23 to be 166.2. This indicates that cycle 23 would be a highly active and historic cycle. The average geomagnetic activity aa index during the ascending phase of cycle 23 would be about 24.9, comparable to 22.2 and 24.8 in cycles 21 and 22, respectively. This further indicates that during the ascending phase of cycle 23 energetic two-ribbon flares will be produced so as to give rise to strong proton events.

Geomagnetic response of the Earth to crossings of the heliospheric current sheet

We performed a superposed epoch analysis of the aa index of geomagnetic activity with respect to the Earth's crossing of the heliospheric current sheet during periods when quiet and fluctuating activities are detected from 1976 to 1985. We found a statistically significant increase of the geomagnetic activity during boundary crossings when the polarity of the interplanetary magnetic field changes from minus to plus.

The geomagnetic response to the Earth's crossing of the heliospheric current sheet

We have performed a superposed epoch analysis of the aa index of geomagnetic activity with respect to the Earth's crossing of the heliospheric current sheet during periods when quiet and fluctuating activities are detected from 1976 to 1985. We found a statistically significant increase of the geomagnetic activity during sector boundary crossings and that a pattern of quiet followed by fluctuating activity is associated in about 70% of the cases with sector boundary crossings.

Geomagnetic and solar wind cycles, 1900–1975

Recent research has established the reality of the 80 to 100‐year cycle (long cycle) in solar‐terrestrial phenomena that for many years has been suspected to occur. This paper investigates some of the systematic changes that took place during the most recent long cycle. The variations of one hundred years of data on the aa index of geomagnetic activity are reinvestigated. Since the solar wind drives geomagnetic activity the results of the study are interpreted in terms of the solar wind. It is shown that the 11‐year solar cycle as expressed by the number of sunspots (the sunspot cycle) is very different from the 11‐year solar cycle as expressed by the solar wind and geomagnetics (the solar wind cycle). With an objective technique, using only sunspot numbers and not involving any judgement as to the transient or recurrent nature of any geomagnetic activity, the solar wind cycle is decomposed into the sun of two equally strong periodic variations, each having the period of the sunspot cycle, but differing in phase. One term in the decomposition, the R component, is chosen to have the phase and relative amplitude of the sunspot cycle. The other term in the decomposition, the I component, is found to be almost, but not quite, 180° out of the phase with R and to have very closely related amplitudes. The source of the R component is shown to be sporadic or short‐lived solar events and that of the I component is shown to be long lived solar features such as coronal holes. The amplitudes are interpreted as being due to solar wind parameters, probably 1BZ1V². The solar wind observed at earth oscillates rhythmically between the two sources. The amplitude of both the oscillations increased smoothly and steadily from 1900 to 1960, corresponding to the ascending branch of the long cycle of solar activity which has been established elsewhere as occuring in the solar wind with an average period since the Middle Ages of 87 years. From these results it is concluded that there is a very strong relationship between coronal conditions at the sites of origin of long lived streams of solar wind and the corresponding sites of sporadic and/or short lived events 5 or 6 years later. Moreover, there is a remarkable and orderly evolution of these conditions over at least 60 years. The interruption of the smooth evolution in 1960 suggests we are now in the declining phase of the long cycle, which is to be expected since the average cycle period is 87 years and the last minimum was in the first decade of this century.