Longitudinal Distribution of Solar Flares and Their Association with Coronal Mass Ejections and Forbush Decreases

Major Hα solar-flare events of high optical importance have been employed to study their heliographic distribution in longitude around the Sun for the period of 2001 to 2006. A statistical analysis was performed to obtain their relationship with halo/partial-halo CMEs and Forbush decreases (Fds) of cosmic-ray intensity. Our analysis indicates that 63% of the solar flares associated with halo CMEs and Fds occur in the western hemisphere and of 37% of such flares occur in the eastern hemisphere. Similarly, we found that nearly 60% of the solar flares associated with partial- halo CMEs and Fds occur in the western hemisphere and the rest (40%) occur in the eastern hemisphere. Finally, we conclude that the flares in association with CMEs and located in the western hemisphere of the solar disk are more effective in producing Fds. The magnitudes of Fds are observed to be higher when in association of halo CMEs. A slight excess in the eastern hemisphere is found for both the halo and partial-halo CMEs.     

Latitudinal Distribution of Solar Flares and Their Association with Coronal Mass Ejections

Major solar flare events have been utilised to study the latitudinal frequency distribution of solar flares in northern and southern hemispheres for the period of 1986 to 2003. A statistical analysis has been performed to obtain the correlation between Coronal Mass Ejections (CMEs) and Forbush decrease (Fds) of cosmic ray intensity. Almost the same flares distribution in both hemispheres is found in association with CMEs. In a further analysis, it is noted that a larger number of CME-associated solar flares located in the northern hemisphere are found to be more effective in producing Forbush decreases.     

Solar activity and the 11-year modulation of cosmic rays

The cosmic-ray intensity during the 18th and 19th solar cycles is examined in the light of Gnevyshev's suggestion of the presence of two maxima in each solar cycle. The 18th solar cycle (1944–54) has two prominent and widely separated cosmic-ray minima corresponding in phase with the two maxima in Bartel's A_p index. For the 19th solar cycle the existence of two minima is less prominent than for the 18th solar cycle. The maximum at higher solar latitudes is more effective in reducing cosmic-ray intensity than the maximum at the lower latitudes. A_p, however, has a larger maximum during the lower latitude solar maximum. A relation between A_p and cosmic-ray intensity is obtained. This relationship is shown to be consistent with Parker's solar-wind theory of the modulation of cosmic rays.     

Cosmic-Ray Modulation: An Empirical Relation with Solar and Heliospheric Parameters

Long-term variations of galactic cosmic rays were compared with the behavior of various solar activity indices and heliospheric parameters during the current solar cycle. This study continues previous works where the cosmic-ray intensity for the solar cycles 20, 21, and 22 was well simulated from the linear combination of the sunspot number, the number of grouped solar flares, and the geomagnetic index A _p. The application of this model to the current solar cycle characterized by many peculiarities and extreme solar events led us to study more empirical relations between solar-heliospheric variables, such as the interplanetary magnetic field, coronal mass ejections, and the tilt of the heliospheric current sheet, and cosmic-ray modulation. By analyzing monthly cosmic-ray data from the Neutron Monitor Stations of Oulu (cutoff rigidity 0.81 GV) and Moscow (2.42 GV) the contribution of these parameters in the ascending, maximum, and descending phases of the cycle was investigated and it is shown that a combination of these parameters reproduces the majority of the modulation potential variations during this cycle. The approach applied makes it possible to better describe the behavior of cosmic rays in the epochs of the solar maxima, which could not be done before. An extended study of the time profiles, the correlations, and the time lags of the cosmic-ray intensity against these parameters using the method of minimizing RMS over all the considered period 1996 – 2006 determines characteristic properties of this cycle as being an odd cycle. Moreover, the obtained hysteresis curves and a correlative analysis during the positive polarity (qA>0, where q is the particle charge) and during the negative polarity (qA<0) intervals of the cycle result in significantly different behavior between solar and heliospheric parameters. The time lag and the correlation coefficient of the cosmic-ray intensity are higher for the solar indices in comparison to the heliospheric ones. A similar behavior also appears in the case of the intervals with positive and negative polarity of the solar magnetic field.     

Influence of Magnetic Clouds on Variations of Cosmic Rays in November 2004

We investigate the effects of two magnetic clouds on hourly cosmic-ray intensity profiles in the Forbush decrease events in November 2004 observed by 47 ground-based neutron-monitor stations. By using a wavelet decomposition, the start time of the main phase in a Forbush decrease event can be defined, and then clearer definitions of initial phase, main phase, and recovery phase are proposed. Our analyses suggest that the main phase of this Fd event precedes the arrival time of the first magnetic cloud by about three hours, and the Fds observed at the majority (39/47) of the stations were found to originate from the sheath region as indicated by large fluctuations in magnetic field vectors at 19:00 UT on 7 November 2004, regardless of the station location. In addition, about 45% of the onset times of the recovery phase in the Forbush decreases took place at 04:00 UT on 10 November, independent of the station position. The results presented here support the hypothesis that the sheath region between the shock and the magnetic cloud, especially the enhanced turbulent magnetic field, results in the scattering of cosmic-ray particles, and causes the following Forbush decreases. Analysis of variation profiles from different neutron monitors reveals the global simultaneity of this Forbush decrease event. Moreover, we infer that the interplanetary disturbance was asymmetric when it reached the Earth, inclined to the southern hemisphere. These results provide several observational constraints for more detailed simulations of the Forbush decrease events with time-dependent cosmic-ray modulation models.     

Interplanetary Transients and Cosmic-Ray Anisotropy

Cosmic-ray intensity data recorded with the ground-based neutron monitor at Deep River have been investigated taking into account the associated interplanetary magnetic field and solar-wind plasma data during 1981 – 1994. A large number of days having abnormally high or low amplitudes for five or more successive days as compared to the annual average amplitude of diurnal anisotropy have been taken as high- or low-amplitude anisotropic wave-train events. The amplitude of the diurnal anisotropy of these events is found to increase on days with a magnetic cloud as compared to the days prior to the event, and it is found to decrease during the later period of the event as the cloud passes the Earth. The high-speed solar-wind streams do not play any significant role in causing these types of events. However, corotating solar-wind streams produce significant deviations in cosmic-ray intensity during high- and low-amplitude events. The interplanetary disturbances (magnetic clouds) are also effective in producing cosmic-ray decreases. Hα solar flares have a good positive correlation with both the amplitude and direction of the anisotropy for high-amplitude events, while the principal magnetic storms have a good positive correlation with both amplitude and direction of the anisotropy for low-amplitude events. The source responsible for these unusual anisotropic wave trains in cosmic rays has been proposed.     

Cosmic-ray variations related to solar,geomagnetic and interplanetary disturbances (23 March–7 April, 1976)

During the second interval of the Study of Travelling Interplanetary Phenomena (STIP, 20 March–5 May, 1976) a series of solar, interplanetary, geomagnetic and cosmic-ray events have occurred. These are surprising events, since this period falls into the minimum of the solar activity of the past solar cycle. The present analysis is concentrated on Forbush decreases, cosmic-ray increases, geomagnetic variations and the related solar wind disturbances recorded by the heliocentric satellites Helios-1, 2 and the geocentric IMP-8, in the period 23 March–7 April, 1976. The cosmic-ray enhancements on 26 March and 1 April were of geomagnetic origin and particularly expressed in middle latitude stations during the largeDst magnetic field depressions. The detected multiple Forbush decreases are related with the type IV solar flares, all produced by the same active region (McMath Plage 14143). The relative positions among the satellites Helios-1, 2, the Sun, and the Earth were very favorable in this period for studying these events, since Helios-1 approached the Sun to its perihelion and Helios-2 was lined-up with the Earth. Helios-2 detected two shock fronts on 30 March and 1 April, respectively, and Helios-1 detected a tangential discontinuity on 26 March. An attempt is made to relate these shock fronts with the erupted solar flares and Storm Sudden Commencements (SSC) recorded on the Earth and to estimate a lower limit of the deceleration distance of the involved shock waves.     

Unusual Cosmic Ray Variations During the Forbush Decreases of June 2015

Although the current Solar Cycle 24 is characterized by low solar activity, an intense geomagnetic storm (G4) was recorded in June 2015. It was a complex phenomenon that began on 22 June 2015 as the result of intense solar activity, accompanied by several flares and coronal mass ejections that interacted with the Earth’s magnetic field. A Forbush decrease was also recorded at the neutron monitors of the worldwide network, with an amplitude of 8.4%, and in its recovery phase, a second Forbush decrease followed, with an amplitude of 4.0% for cosmic rays of 10 GV obtained with the global survey method. The Dst index reached a minimum value of ?204 nT that was detected on 23 June 2015 at 05:00?–?06:00 UT, while the Kp index reached the value eight. For our analysis, we used hourly cosmic-ray intensity data recorded by polar, mid-, and high-latitude neutron monitor stations obtained from the High Resolution Neutron Monitor Database. The cosmic-ray anisotropy variation at the ecliptic plane was also estimated and was found to be highly complex. We study and discuss the unusual and complex cosmic-ray and geomagnetic response to these solar events.     

Precursor Effects in Different Cases of Forbush Decreases

Over the last few years, the pre-decreases or pre-increases of the cosmic-ray intensity observed before a Forbush decrease, called the precursor effect and registered by the worldwide neutron monitor network, have been investigated for different cases of intense events. The Forbush decreases presented in this particular study were chosen from a list of events that occurred in the time period 1967?–?2006 and were characterized by an enhanced first harmonic of cosmic-ray anisotropy prior to the interplanetary disturbance arrival. The asymptotic longitudinal cosmic-ray distribution diagrams for the events under consideration were studied using the “Ring of Stations” method, and data on solar flares, solar-wind speed, geomagnetic indices, and interplanetary magnetic field were analyzed in detail. The results revealed that the use of this method allowed the selection of a large number of events with well-defined precursors, which could be separated into at least three categories, according to duration and longitudinal zone. Finally, this analysis showed that the first harmonic of cosmic-ray anisotropy could serve as an adequate tool in the search for precursors and could also be evidence for them.     

Lifetimes of High-Degree <Emphasis Type="Italic">p</Emphasis> Modes in the Quiet and Active Sun

We study variations of the lifetimes of high-ℓ solar p modes in the quiet and active Sun with the solar activity cycle. The lifetimes in the degree range ℓ=300 – 600 and ν=2.5 – 4.5 mHz were computed from SOHO/MDI data in an area including active regions and quiet Sun using the time – distance technique. We applied our analysis to the data in four different phases of solar activity: 1996 (at minimum), 1998 (rising phase), 2000 (at maximum), and 2003 (declining phase). The results from the area with active regions show that the lifetime decreases as activity increases. The maximal lifetime variations are between solar minimum in 1996 and maximum in 2000; the relative variation averaged over all ℓ values and frequencies is a decrease of about 13%. The lifetime reductions relative to 1996 are about 7% in 1998 and about 10% in 2003. The lifetime computed in the quiet region still decreases with solar activity, although the decrease is smaller. On average, relative to 1996, the lifetime decrease is about 4% in 1998, 10% in 2000, and 8% in 2003. Thus, measured lifetime increases when regions of high magnetic activity are avoided. Moreover, the lifetime computed in quiet regions also shows variations with the activity cycle.     

Solar-cycle phenomena in cosmic-ray intensity: Differences between even and odd cycles

Cosmic-ray intensity data for the period 1964–1985 covering two solar cycles are used to investigate the solar activity behaviour in relation to cosmic-ray modulation. A detailed statistical analysis of them shows a large time-lag of about one and half years between cosmic-ray intensity and solar activity (as indicated by sunspot number, solar flares and high-speed solar-wind streams) during the 21st solar cycle appearing for a first time. This lag indicates the very high activity level of this solar cycle estimating the size of the modulating region to the unambiguous value of 180 AU. The account of the solar-wind speed in the 11-year variation significantly decreases the modulation region of cosmic-rays to the value of 40 AU.A comparison with the behaviour of the previous solar cycle establishes a distinction between even and odd solar cycles. This is explained in terms of different contributions of drift, convection and diffusion to the whole modulation mechanism during even and odd solar cycles.     

Short-Period Features of the Interplanetary Plasma and Their Evolution

The solar wind plasma exhibits many features of the solar surface passed on to the interplanetary medium as temporal variations due to the solar rotation. The yearly average values of solar wind velocity, and geomagnetic index A _p during 1965–1999 were found to exhibit long period evolution. They were found to peak around the declining phase of each solar cycle. While the solar wind velocity peaks around the second half of the declining phase, the IMF field strength increases around the first half of the declining phase of each solar cycle. The power spectrum of these parameters shows peaks around 37-day, 30-day, 27-day, 13.5-day, 9-day, and 7-day periods. The temporal evolution of the power spectrum of the solar wind plasma parameters and the geomagnetic activity index A _p are also studied in detail and presented with the help of contour graphs. These studies indicate that the strength of the quasi-periodicities in the interplanetary medium evolves with time.     

Short-term variations of cosmic-ray intensity and flare related data in 1981–1983

A statistical analysis of the cosmic-ray intensity (CR) daily means, registered at three Neutron Monitor stations with different cut-off rigidities (Deep River, Climax and Alma-Ata), as well as, of the solar hard X-ray flares fluence recorded by Venera-13, -14 space-probes, has been performed for the time interval 1981–1983. Various methods of time series spectrum analysis, such as Fast Fourier Analysis (FFT) and Maximum Entropy (MESA), accompanied by appropriate statistical tests, have been employed to detect periodicities, while the method of Successive Approximations (SA) is used independently in order to define the amplitude and the phase of each fluctuation. New short-term periodicities of 100, 70, 50 and 32 days, in addition to the known ones of 152, 27 and 14 days, appeared in cosmic ray data. During this particular time interval, similar spectral behaviour has been reported in the solar hard X-ray flares data. The influence of the solar hard X-ray flares variability in the energy range 50–500 keV, expressed by their fluence values, upon the cosmic-ray modulation, is discussed.     

Cosmic-ray modulation and long-duration solar flare events

The correlation between the long-term intensity variations of cosmic rays at neutron monitor energies and the LDE index measure of solar flares with long-lasting soft X-ray emissions is reported. Three subsequent solar cycles, 20–22, are taken into account and half-monthly data are analyzed. Possible explanation of this correlation is discussed in terms of the recent concepts of cosmic-ray modulation, in particular with merged interaction regions affecting the cosmic-ray intensity.     

Correction to: Solar Temperature Variations Computed from SORCE SIM Irradiances Observed During 2003 – 2020

Forbush decreases (FDs) are sharp reductions of the cosmic-ray (CR) intensity, following intense solar activity such as coronal mass ejections (CMEs) and their corresponding interplanetary shocks. In some cases, shocks create sudden storm commencements (SSCs) at the Earth’s magnetosphere with significant interest for space-weather studies. Preincreases and/or predecreases of CR intensity before the onset of FDs, known as precursory signals, have been widely examined by many authors. In this work, an attempt to define precursory signals that are not related to SSCs is presented. For the present analysis, CR data recorded by the ground-based Neutron Monitor Network as well as data on solar flares, CMEs, solar-wind speed, interplanetary magnetic field, and geomagnetic indices for the years 1969?–?2019 are used. To identify FDs that present precursors, the adopted criteria are mainly the FD amplitude (> 2%) and the equatorial CR anisotropy before the onset time (> 0.8%). The analysis of FDs and the study of their asymptotic-longitude CR distribution for precursors are based on the Global Survey Method and the Ring of Stations Method, respectively. Precursory signals are identified in 17 out of 27 events without SSCs.

     

Statistical Characteristics on SEPs,Radio-Loud CMEs,Low Frequency Type II and Type III Radio Bursts Associated with Impulsive and Gradual Flares

We have statistically analyzed a set of 115 low frequency (Deca-Hectometer wavelengths range) type II and type III bursts associated with major Solar Energetic Particle (SEP: E_p?>?10 MeV) events and their solar causes such as solar flares and coronal mass ejections (CMEs) observed from 1997 to 2014. We classified them into two sets of events based on the duration of the associated solar flares:75 impulsive flares (duration <?60 min) and 40 gradual flares (duration >?60 min).On an average, the peak flux (integrated flux) of impulsive flares?×?2.9 (0.32 J m^?2) is stronger than that of gradual flares M6.8 (0.24 J m^?2). We found that impulsive flare-associated CMEs are highly decelerated with larger initial acceleration and they achieved their peak speed at lower heights (??27.66 m s^?2 and 14.23 R_o) than the gradual flare-associated CMEs (6.26 m s^?2 and 15.30 R_o), even though both sets of events have similar sky-plane speed (space speed) within LASCO field of view. The impulsive flare-associated SEP events (R_t?=?989.23 min: 2.86 days) are short lived and they quickly reach their peak intensity (shorter rise time) when compared with gradual flares associated events (R_t?=?1275.45 min: 3.34 days). We found a good correlation between the logarithmic peak intensity of all SEPs and properties of CMEs (space speed: cc?=?0.52, SE_cc?=?0.083), and solar flares (log integrated flux: cc?=?0.44, SE_cc?=?0.083). This particular result gives no clear cut distinction between flare-related and CME-related SEP events for this set of major SEP events. We derived the peak intensity, integrated intensity, duration and slope of these bursts from the radio dynamic spectra observed by Wind/WAVES. Most of the properties (peak intensity, integrated intensity and starting frequency) of DH type II bursts associated with impulsive and gradual flare events are found to be similar in magnitudes. Interestingly, we found that impulsive flare-associated DH type III bursts are longer, stronger and faster (31.30 min, 6.43 sfu and 22.49 MHz h^?1) than the gradual flare- associated DH type III bursts (25.08 min, 5.85 sfu and 17.84 MHz h^?1). In addition, we also found a significant correlation between the properties of SEPs and key parameters of DH type III bursts. This result shows a closer association of peak intensity of the SEPs with the properties of DH type III radio bursts than with the properties DH type II radio bursts, atleast for this set of 115 major SEP events.

     

Long-term behavior of latitudinal cosmic-ray gradients

We have used neutron monitor data covering a wide range of energy over a period of 22 years (1966–1987), as well as sea-level multidirectional meson telescope data from Nagoya to examine the latitude effect of solar diurnal vectors and its dependence on the polarity of interplanetary magnetic field (IMF). By sorting the daily cosmic-ray data according to whether the IMF is toward (T) or away (A) from the Sun, the annual mean solar diurnal variations (amplitude and phase) for the T and A days were determined separately. Results showed a northward-pointing latitudinal gradient from neutron monitors of the most northerly latitudes, and a predominant southward gradient at high southerly latitudes. The resultant latitudinal cosmic-ray gradients are the sum of two gradients: a north-south symmetry gradient (occurring in minimum and maximum solar activity years), and a north-south asymmetry gradient (occurring during different phases of solar activity cycles). The difference vector (T - A) between the solar diurnal vector for two groups was calculated, which represents a good indicator for the resultant perpendicular gradient relative to the Earth. This difference vector shows a considerable change in phase for detectors located in the northern hemisphere of the Earth. On the other hand, there exists much less change in phase for detectors located in the southern hemisphere.     

The Asymptotic Longitudinal Cosmic Ray Intensity Distribution as a Precursor of Forbush Decreases

Identifying the precursors (pre-increases or pre-decreases) of a geomagnetic storm or a Forbush decrease is of great importance since they can forecast and warn of oncoming space weather effects. A wide investigation using 93 events which occurred in the period from 1967 to 2006 with an anisotropy A _xy >1.2% has been conducted. Twenty-seven of the events revealed clear signs of precursors and were classified into three categories. Here we present one of the aforementioned groups, including five Forbush decreases (24 June 1980, 28 October 2000, 17 August 2001, 23 April 2002, and 10 May 2002). Apart from hourly cosmic ray intensity data, provided by the worldwide network of neutron monitor stations, data on solar flares, solar wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field were used for the analysis of the examined cosmic ray intensity decreases. The asymptotic longitudinal cosmic ray distribution diagrams were plotted using the “ring of stations” method. Results reveal a long pre-decrease up to 24 hours before the shock arrival in a narrow longitudinal zone from 90° to 180°.     

Power Spectrum of Cosmic-ray Fluctuations During Consecutive Solar Minimum and Maximum Periods

Power spectral analysis of cosmic-ray intensity recorded by eight stations was carried out over a wide range of frequencies from 2.3 × 10^–8 Hz to 5.8 × 10^–6 Hz (2–500 days) during the period 1964–1995. Spectrum results of large-scale fluctuations have revealed the existence of a broad peak near 250–285 days and a narrower peak at 45–50 days during the studied epochs as a stable feature in all neutron monitors covering a wide rigidity range. The cosmic-ray power spectrum displayed significant peaks of varying amplitude with the solar rotation period (changed inversely with the particle rigidities) and its harmonics. The amplitudes of 27-day and 13.5-day fluctuations are greater during the positive-polarity epochs of the interplanetary magnetic field (qA>0) than during the qA<0 epochs. The comparison of cosmic-ray power spectra during the four successive solar activity minima have indicated that at the low-rigidity particles the spectrum differences between the qA>0 and qA<0 epochs are significantly large. Furthermore, the spectrum for even solar maximum years are higher and much harder than the odd years. There are significant differences in the individual spectra of solar maxima for different cycles.