Sources of intense geomagnetic storms over the rise of solar cycle 23

In this work we present a study of the triggers of intense geomagnetic storms since the launch of the WIND spacecraft, November 1995 until December 2001. Reviewing the signatures of solar wind flow, we looked for two different kinds of interplanetary events associated with intense geomagnetic storms: ejecta and corotating solar wind streams. We also looked for the solar origin related to both events. We provide a list of the solar–terrestrial events during the rising phase of this solar cycle. The paper includes statistical conclusions that shed light onto the paradigm of geomagnetic storms.     

Sources of intense geomagnetic storms over the rise of solar cycle 23

In this work we present a study of the triggers of intense geomagnetic storms since the launch of the WIND spacecraft, November 1995 until December 2001. Reviewing the signatures of solar wind flow, we looked for two different kinds of interplanetary events associated with intense geomagnetic storms: ejecta and corotating solar wind streams. We also looked for the solar origin related to both events. We provide a list of the solar–terrestrial events during the rising phase of this solar cycle. The paper includes statistical conclusions that shed light onto the paradigm of geomagnetic storms.     

The solar origin of geomagnetic storms

A recent study of associations between geomagnetic storms and solar phenomena has found more associations with solar flares than with coronal holes (Garcia and Dryer, 1987). This disagrees with observations of earthbound transients obtained from IPS imaging which showed that nearly all geomagnetically effective disturbances originated from coronal holes at low latitudes. The discrepancy has arisen because the former study failed to take into account the large angular extent of transient eruptions from coronal holes. It is highly probable that the intense geomagnetic storm of February 1986, discussed by Garcia and Dryer, was caused by a low-latitude coronal hole which was present at that time. This answers their question concerning moderately strong flares that apparently cause major storms, while much larger flares often do not; flares may sometimes be associated with eruptions from coronal holes, but only as peripheral events.     

A study of geomagnetic storms at Beijing in terms of solar wind

With the aid of the Akasofu's energy coupling function between the solar wind and the magnetosphere, we have made in this paper an analysis of about 20 geomagnetic storms recorded at Beijing during the period of years 1966 to 1972. There is a close correlation between the energy coupling function ? and the geomagnetic indices a_p and K_p. All in all an empirical formula as ? ～ 1?2 × 10¹⁷a_p has been found for the geomagnetic storms occurred in a low latitude station, i.e. Beijing of China. Comparisons of the horizontal component H_max (in γ) and ?(10¹⁸ erg s^?1) in Table 1 indicate that the development of storm main phase at Beijing depends very much on the ? values thus involved. Also, these are well illustrated for several individual storms as mentioned in the second section of the paper. In concluding this paper some brief discussions are made and included. It is hoped that geomagnetic observations in the middle and low latitudes from our vast country should make further contributions to the study of solar wind-magnetosphere coupling, including the Akasofu's energy coupling function.     

Ten cycles of solar and geomagnetic activity

Series of 110 years of sunspot numbers and indices of geomagnetic activity are used with 17 years of solar wind data in order to study through solar cycles both stream and shock event solar activity. According to their patterns on Bartels diagrams of geomagnetic indices, stable wind streams and transient solar activities are separated from each other. Two classes of stable streams are identified: equatorial streams occurring sporadically, for several months, during the main phase of sunspot cycles and both polar streams established, for several years, at each cycle, before sunspot minimum. Polar streams are the first activity of solar cycles. For study of the relationship between transient geomagnetic phenomena and sunspot activity, we raise the importance of the contribution, at high spot number, of severe storms and, at low spot number, of short lived and unstable streams. Solar wind data are used to check and complete the above results. As a conclusion, we suggest a unified scheme of solar activity evolution with a starting point every eleventh year, a total duration of 17 years and an overlapping of 6 years between the first and the last phase of both successive series of phenomena: first, from polar field reversal to sunspot minimum, a phase of polar wind activity of the beginning cycle is superimposed on the weak contribution of shock events of the ending cycle; secondly, an equatorial phase mostly of shock events is superimposed on a variable contribution of short lived and sporadic stable equatorial stream activities; and thirdly a phase of low latitude shock events is superimposed on the polar stream interval of the following cycle.     

Study of cosmic ray intensity in relation to the interplanetary magnetic field and geomagnetic storms for solar cycle 24

In the present study, we investigate the association of cosmic ray intensity (CRI) with various solar wind parameters (i.e. solar wind speed V, plasma proton temperature, plasma proton density), interplanetary magnetic field (IMF B), geomagnetic storms (GSs), averaged planetary A-index (Ap index) and sun spot number (SSN) for the period 2009–2016 (solar cycle 24) by using their daily mean average. To find the association of CRI with various solar wind parameters, GSs, IMF B, Ap index and SSN, we incorporate the analysis technique by superposed-epoch method. We have observed that CRI decreases with the increase in IMF B. Moreover the time-lag analysis has been performed by the method of correlation coefficient and observed a time lag of 0 to 2 day between the decrease in CRI and increase in IMF B. In addition, we show that the CRI is found to decrease in a similar pattern to disturbance storm time (Dst index) for most of the period of solar cycle 24. The high and positive correlation is found between CRI and Dst index. The CRI and Ap index are better anti-correlated to each other than CRI and IMF. CRI and SSN are positively correlated with each other. Solar wind parameters such as solar wind speed V is a CR-effective parameter while plasma proton temperature and plasma proton density are not CR-effective parameters. The indicated parameters such as Dst index, Ap index, IMF B and solar wind parameters such as solar wind speed V, plasma proton temperature, plasma proton density shows a kind of irregular variations for solar cycle 23 and 24 while CRI and SSN shows distinct behaviour for the two cycle.     

Passage of the solar current disk and major geomagnetic storms

It is shown that major geomagnetic storms (¦Dst¦ > 100) tend to develop at about the time of the passage of the solar current sheet or disk at the location of the Earth, provided this passage is associated with (1) a large impulsive increase of the IMF magnitude B, (2) a negative value of the IMF angle (Theta), and (3) an increasing solar wind speed. The passage occurs in association with the 27-day rotation of the warped current disk or a temporal up-down movement of the latter. The period in which ¦Dst¦/t< 0 during major storms coincides approximately with the period when the solar windmagnetosphere energy coupling function becomes 10¹⁹ erg s^–1. These conclusions do not depend on the phase of the sunspot cycle.These results may be interpreted as follows: A high speed solar wind flow, originating either from flare regions or coronal holes, tends to push the solar current disk to move upward or downward for either a brief period (1 3 days) or an extended period (2 weeks). A relatively thin region of a large IMF B > 10 is often present near the moving current disk. Waves are also generated on the moving current disk, and some of them cause large changes of . A high value of is found in the region of a large IMF B near the wavy solar current disk, where has a large negative value.     

A Neuro-Fuzzy modeling for prediction of solar cycles 24 and 25

The paper presents a Neuro-Fuzzy model to predict the features of the forthcoming sunspot cycles 24 and 25. The sunspot time series were analyzed with the proposed model. It is optimized based on Backpropagation scheme and applied to the yearly smoothed sunspot numbers. The appropriate number of network inputs for the sunspots data series is obtained based on sequential forward search for the Neuro-Fuzzy model. According to the model prediction the maximum amplitudes of the cycles 24 and 25 will occur in the year 2013 and year 2022 with peaks of 101±8 and 90.7±8, respectively. The correlation and error analysis are discussed to ensure the performance of the proposed Neuro-Fuzzy approach as a predictor for sunspot time series. The correlation coefficient between Neuro-Fuzzy model forecasted sunspot number values with the actual ones is 0.96.     

Solar activity during first six years of solar cycle 24 and 23: a comparative study

Attempt to look into the nature of solar activity and variability have increased importance in recent days because of their terrestrial relationships. In the present work we have attempted to compare the solar activity events during first six years (2008–2013) of the ongoing solar cycle 24 with first six years (1996–2001) of solar cycle 23. To that end, we have considered sunspot numbers, F10.7 cm solar flux, halo CMEs and geomagnetic storms as comparison parameters. Sunspot number during the year 2008–2013 varied from 0 to 96.7 while during the year 1996 to 2001 it was observed from 0.9 to 170.1. Solar radio flux (F10.7 cm index) varied from 65 to 190 during the years 2008–2013 while it was observed from 65 to 283 during the years 1996–2001. 197 cases of halo CMEs (width=360^°) in solar cycle 23 (1996–2001) and 177 cases of halo CMEs (width=360^°) in solar cycle 24 (2008–2013) are investigated. 287 and 104 geomagnetic storm cases (Dst varies between ?50 and ?350 nT) are analysed during the half period of solar cycle 23 and 24 respectively. Comparative results indicate that solar cycle 23 was more pronounced in comparison of solar cycle 24.     

Study of individual geomagnetic storms in terms of the solar wind

This paper summarizes a study of the development of a large number of geomagnetic storms in terms of the solar wind—magnetosphere energy coupling function ε, the AE and Dst indices. It is shown that the maximum magnitude of the main phase decrease (¦Dst¦) is determined primarily by the peak value of ε; for ε < 10¹⁹ erg s^?1, ~10¹⁹ erg s^?1, 10¹⁹–10²⁰ erg s^?1, ? 10²⁰ erg s^?1, the maximum values of ¦Dst¦ are < 50γ, ~50γ, ~100γ and ? 200γ, respectively. A few examples for different peak values of ε (and thus of ¦Dst¦) are presented and examined in detail. Substorm activity during storms is well controlled by ε.     

Association of CMEs with solar surface activity during the rise and maximum phases of solar cycles 23 and 24

The cyclical behaviors of sunspots,flares and coronal mass ejections(CMEs) for 54 months from 2008 November to 2013 April after the onset of Solar Cycle(SC) 24 are compared,for the first time,with those of SC 23 from 1996 November to 2001 April.The results are summarized below.(i) During the maximum phase,the number of sunspots in SC 24 is significantly smaller than that for SC 23 and the number of flares in SC 24 is comparable to that of SC 23.(ii) The number of CMEs in SC 24 is larger than that in SC 23 and the speed of CMEs in SC 24 is smaller than that of SC 23 during the maximum phase.We individually survey all the CMEs(1647 CMEs) from 2010 June to 2011 June.A total of 161 CMEs associated with solar surface activity events can be identified.About 45%of CMEs are associated with quiescent prominence eruptions,27%of CMEs only with solar flares,19%of CMEs with both active-region prominence eruptions and solar flares,and 9%of CMEs only with active-region prominence eruptions.Comparing the association of the CMEs and their source regions in SC 24 with that in SC 23,we notice that the characteristics of source regions for CMEs during SC 24 may be different from those of SC 23.     

A hydromagnetic theory of geomagnetic storms and auroras

A theory of geomagnetic storms, auroras and associated effects is further developed. It depends on motions in the Earth's exosphere or magnetosphere initiated by a combination of pressure and frictional drag of the solar wind and modified and extended by electric fields and currents in the ionosphere. Motion may be non-divergent, streamline flow opposed only by Lorentz forces in the ionosphere and not propagating to Earth, or divergent, non-streamline motion opposed by Lorentz forces in the Earth. The two types of motion are coupled in the E region where the former is identified with free flow of Hall current and the generation of non-streamline motion. The latter is identified with blockage of Hall current, the creation of a polarization field and hence the generation of streamline motion.

A theory of all components of a geomagnetic storm is given in terms of combinations of these motions, and their distant, ionospheric and earth currents. This includes a new theory of the preliminary reverse part of the DS field and the transition from the sudden commencement to the main phase of the DS field. It is extended to introduce briefly a theory of auroras based mainly on ionospheric drifts caused by the magnetospheric motions.     

Statistical relationship between solar wind conditions and geomagnetic storms in 1998-2008

Applying ACE data and pressure-corrected Dst index (Dst*), annual distributions of solar wind structures detected at L1 point (the first Lagrangian point between solar-terrestrial interval) and correlations between solar wind structures and geomagnetic storms in 1998-2008 have been studied. It was found that, within the Earth's upstream solar wind, the dominant feature was interplanetary coronal mass ejections (ICMEs), primarily magnetic clouds, during solar maximum period but corotating interaction regions (CIRs) at solar minimum. During rising and declining phases, solar wind features became unstable for the complicated solar corona transition processes between the maximum and minimum phases, and there was a high CIR occurrence rate in 2003, the early period of the declining phase, for the Earth's upstream solar wind was dominated by high-speed southern coronal-hole outflows at that time. The occurrence rate of sector boundary crossing (SBC) events was evidently higher at the late half of declining phase and minimum period. ICMEs mainly centered on the maximum period but CIRs on all the declining phase. The occurrence rate of ICMEs was 1.3 times of that of CIRs, and more than half of ICMEs were magnetic clouds (MCs). Half of magnetic clouds could drive interplanetary shock and played a crucial role for geomagnetic storms generation, especially intense storms (Dst*≤100 nT), in which 45% were jointly induced by sheath region and driving MC structure. Sixty percent of intense storms were totally induced by shock-driving MCs; moreover, 74% of intense storms were driven by magnetic clouds, 81% of them driven by ICMEs. Shock-driving MC was the most geoeffective interplanetary source for four fifths of it able to lead to storms and more than one-third to intense storms. The rest of intense storms (19%) were induced just by 3% of all detected CIRs, and most of CIRs (53%) were corresponding to nearly 40% moderate and small storms (−100 nT<Dst*≤−30 nT). The true sector boundary crossing (SBC) events actually had no obvious geoeffectiveness, just 6% of them corresponding to small storms.     

Radial deformation of the solar current sheet as a cause of geomagnetic storms

It is suggested that the solar current sheet, extending from a coronal streamer, develops a large-scale radial deformation, at times with a very steep gradient at the Earth's distance. The associated magnetic field lines (namely, the interplanetary magnetic field (IMF) lines) are expected to have also a large gradient in the vicinity of the current sheet. It is also suggested that some of the major geomagnetic storms occur when the Earth is located in the region where IMF field lines have a large dip angle with respect to the ecliptic plane for an extended period (6–48 h), as a result of a steep radial deformation of the current sheet.     

Solar radio events and geomagnetic storms

On the basis of a 24-hr patrol of solar radio noise established since the beginning of the International Geophysical Year, an identification is attempted of those solar flares which, on account of their associated radio responses, most probably were the cause of a geomagnetic storm. The cases for which we think the identification to be reliable are listed. It has appeared that great integrated intensity of the radio outburst at centimeter, decimeter and meter wavelengths is the primary criterion for identifying the solar flares responsible. Most of these giant radio outbursts, to which we assigned the “radio importance” figure 3 +, belong to the so-called type IV. Only a minor fraction of these events were accompanied by slow-drift bursts of type II. Of the importance 3 + radio outbursts about 60 per cent are clearly associated with the subsequent sudden commencement of a geomagnetic storm. Conversely, about 50 per cent of the sudden commencements of a storm can be related to an important radio event. Some reasons, why in a particular case the storm-outburst association may fail to exist, are mentioned.     

On the possibility of inferring the solar and interplanetary sector structure from statistics of geomagnetic storms and solar activity

Attention is drawn to the great statistical material on geomagnetic storms and solar activity, published mainly before the space age. By analyses of this material in connection with established correlations between geomagnetic activity and the interplanetary sector struc- ture, valuable information might be obtained that would significantly contribute to an increased understanding of solar and interplanetary sector magnetism.As an illustration of this, different analyses of solar-geomagnetic correlations have been considered in relation to the paper by Wilcox and Colburn (1972) on the observed sector struc- ture. Indications are found that (a) the interplanetary and solar sector pattern in the years 1919–1969 consisted of mainly 2 or 4 sectors per solar rotation, and (b) sector boundaries are related to bipolar magnetic regions on the Sun.     

The causes of recurrent geomagnetic storms

We studied the causes of recurrent geomagnetic activity by analyzing interplanetary magnetic field and plasma data from Earth-orbiting spacecraft in the interval from November 1973 to February 1974. This interval includes the start of two long sequences of geomagnetic activity and two corresponding corotating interplanetary streams. In general, the geomagnetic activity was related to an electric field which was primarily due to two factors: (1) the ordered, mesoscale pattern of the stream itself and (2) random, smaller-scale fluctuations in the southward component of the interplanetary magnetic field B_z. The geomagnetic activity in each recurrent sequence consisted of two successive stages. The first stage was usually the most intense and it occurred during the passage of the interaction region at the front of a stream. It was related to a V × B electric field which was large primarily because the amplitude of the fluctuations in B_z was large in the interaction region. It is suggested that these large amplitudes of B_z were primarily produced in the interplanetary medium by compression of ambient fluctuations as the stream steepened in transit to 1 A.U. The second stage of geomagnetic activity immediately following the first was associated with the highest speeds in the stream. It was, among other things, related to a V × B electric field which was large mainly because of the high speeds.     

Temporal variation of solar flare index during solar cycles 21–24

The present investigation attempts to quantify the temporal variation of Solar Flare Index(SFI)with other activity indices during solar cycles 21-24 by using different techniques such as linear regression,correlation,cross-correlation with phase lag-lead,etc.Different Solar Activity Indices(SAI)considered in this present study are Sunspot Number(SSN),10.7 cm Solar Radio Flux(F10.7),Coronal Index(CI)and MgⅡCore-to-Wing Ratio(MgⅡ).The maximum cycle amplitude of SFI and considered SAI has a decreasing trend from solar cycle 22,and cycle 24 is the weakest solar cycle among all other cycles.The SFI with SSN,F10.7,CI and MgⅡshows hysteresis during all cycles except for solar cycle 22 where both paths for ascending and descending phases are intercepting each other,thereby representing a phase reversal.A positive hysteresis circulation exists between SFI and considered SAI during solar cycles 22 and 23,whereas a negative circulation exists in cycles 21 and 24.SFI has a high positive correlation with coefficient values of 0.92,0.94,0.84 and 0.81 for SSN,F10.7,CI and MgⅡrespectively.According to crosscorrelation analysis,SFI has a phase lag with considered SAI during an odd-number solar cycle(solar cycles21 and 23)but no phase lag/lead during an even-numbered solar cycle(solar cycles 22 and 24).However,the entire smoothed monthly average SFI data indicate an in-phase relationship with SSN,F10.7 and MgⅡ,and a one-month phase lag with CI.The presence of those above characteristics strongly confirms the outcomes of different research work with various solar indices and the highest correlation exists between SFI and SSN as well as F10.7 which establishes that SFI may be considered as one of the prime activity indices to interpret the characteristics of the Sun’s active region as well as for more accurate short-range or long-range forecasting of solar events.     

The properties of solar active regions responsible for ground level enhancements during solar cycles 22 and 23

This is a study designed to analyze the relationship between ground level enhancements（GLEs）and their associated solar active regions during solar cycles 22and 23.Results show that 90.3%of the GLE events that are investigated are accompanied by X-class flares,and that 77.4%of the GLE events originate from super active regions.It is found that the intensity of a GLE event is strongly associated with the specific position of an active region where the GLE event occurs.As a consequence,the GLE events having a peak increase rate exceeding 50%occur in a longitudinal range from W20 to W100.Moreover,the largest GLE events occur in a heliographic longitude at roughly W60.Additionally,an analysis is made to understand the distributional pattern of the Carrington longitude of the active regions that have generated the GLE events.