Structure of the July 1982 event in relation to the magnetosphere's response

A large Forbush-type decrease with an amplitude of 16–22% was observed by the world-wide network of cosmic-ray detectors during the period 13–14 July, 1982. Combined neutron-monitor measurements with interplanetary plasma and magnetic field data, auroral data, and Earth's magnetospheric data are used for the study of this event. It is suggested that this interesting event is probably a consequence of the dynamic interactions of the solar wind with the Earth's magnetosphere as it is obvious from the large magnetic storm which was recorded in the auroral electrojet indices.     

First Analysis of Ground-Level Enhancement (GLE) 72 on 10 September 2017: Spectral and Anisotropy Characteristics

Using data obtained with neutron monitors and space-borne instruments, we analyzed the second ground-level enhancement (GLE) of Solar Cycle 24, namely the event of 10 September 2017 (GLE 72), and derived the spectral and angular characteristics of associated GLE particles. We employed a new neutron-monitor yield function and a recently proposed model based on an optimization procedure. The method consists of simulating particle propagation in a model magnetosphere in order to derive the cutoff rigidity and neutron-monitor asymptotic directions. Subsequently, the rigidity spectrum and anisotropy of GLE particles are obtained in their dynamical evolution during the event on the basis of an inverse-problem solution. The derived angular distribution and spectra are discussed briefly.     

Time-evolution of cosmic-ray intensity modulation

Application of analyzing time-series into trigonometric series allows the investigation of cosmic-ray intensity variations in a wide periodicity range from a few months to 10 or even more years. By this technique, the amplitude and the phase of all observed fluctuations can be given. For this purpose, cosmic-ray data of five ground-based neutron-monitor stations for the time interval 1964–1985 have been analyzed.Two kinds of periodicities appeared in these data. The first one includes occurrences at periods greater than two years, as the ones of 10.41, 8.41, and 5.50 yr, which differ very little in amplitude from station to station but are similar in phase, and the second one includes periodicities smaller than two years (24, 12, 8, and 6 months) which are similar in all stations but appeared in variable time intervals.The possible origin of each observed variation due to a contribution either of cosmic-ray interaction in the upper atmosphere or to the solar dynamics is discussed.     

Stratospheric Measurements of Magnetospheric Electron Precipitation and Interplanetary Medium Conditions in Solar Activity Cycles 22–24

High-energy electrons precipitate into the atmosphere under the influence of disturbances of the interplanetary medium on the magnetosphere. Electrons captured from interplanetary space interact in the magnetosphere with waves, resulting in both acceleration and electron energy loss. Some high-energy electrons precipitate into the atmosphere where they generate bremsstrahlung X-rays, which can penetrate deep into the atmosphere to heights of the order of 20 km. The current 11-year cycle is characterized by weak solar activity and a small number of precipitations. The paper discusses the correlation between the parameters of the interplanetary medium and the magnetosphere with the number of precipitations recorded from 1987 to the present during regular measurements of ionizing radiation in the atmosphere in the Murmansk region.     

Application of diffusion — Convection model to diurnal anisotropy data

The diurnal anisotropy of cosmic-ray intensity observed over the period 1970–1977 has been analysed using neutron-monitor data of the Athens and Deep River stations. Our results indicate that the time of the maximum of diurnal variation shows a remarkable systematic shift towards earlier hours than normally beginning in 1971. This phase shift continued until 1976, the solar activity minimum, except for a sudden shift to a later hour for one year, in 1974, the secondary maximum of solar activity.This behavior of the diurnal time of maximum has been shown to be consistent with the convective- diffusive mechanism which relates the solar diurnal anisotropy of cosmic-rays to the dynamics of the solar wind and of the interplanetary magnetic field. Once again we have confirmed the field-aligned direction of the diffusive vector independently of the interplanetary magnetic field polarity. It is also noteworthy that the diurnal phase may follow in time the variations of the size of the polar coronal holes. All these are in agreement with the drift motions of cosmic-ray particles in the interplanetarty magnetic field during this time period.     

The role of the energy spectrum anisotropy in the variations of the cosmic-ray fluctuations power-spectrum before interplanetary medium disturbances

The variations in the form of the cosmic-ray fluctuation power spectrum as an interplanetary shock wave approaches the Earth have been calculated for different values of cosmic ray anisotropy. The relevant experimental estimates of the power spectra are inferred from the data of cosmic ray detection with the ground-based neutron monitors at cosmic-ray stations. A comparison between the theoretical and experimental estimates has demonstrated an important role of the cosmic ray anisotropy spectrum in the generation of the power spectrum as the latter is rearranged before the interplanetary medium disturbances.     

A search of cosmic-ray variations generated by pulsations of the heliosphere

Application of new statistical techniques to time series allow the investigation of cosmic-ray intensity variation in the periodicity range of 1 to 10 years. We can put significant levels to the existence of these oscillations and define their character as quasi-periodic and/or recurrent. Correlations between cosmic-ray intensity variations and solar activity changes during 1944–1979 are investigated. The two-year variation in cosmic rays is observed to be variable both in amplitude and phase, and not correlated with sunspot cyclic variations; but seems to depend on the magnetic polarity of the interplanetary medium. No significant evidence for the existence of longer period variations is obtained.     

PARTICLE ACCELERATION IN GEOSPACE AND ITS ASSOCIATION WITH SOLAR EVENTS

Particle acceleration is a prominent feature of the geomagnetic storm, which is the prime dynamic process in Geospace – the near-Earth space environment. Magnetic storms have their origin in solar events, which are transient disturbances of the solar atmosphere and radiation that propagates as variations of the solar wind fields and particles through interplanetary space to the Earth's orbit. During magnetic storms, ions of both solar wind origin and terrestrial origin are accelerated and form an energetic ring current in the inner magnetosphere. This current has global geomagnetic effects, which have both physical and technical implications. Recently, it has been shown that large magnetic storms, which exhibit an unusually energized ionospheric plasma component, are closely associated with coronal mass ejections (CMEs). This implies a cause/effect chain connecting solar events through CMEs and the solar wind with the acceleration of terrestrial ion populations which eventually constitute the main source of global geomagnetic disturbances. Here we present spacecraft observations related to storm-time particle acceleration and assess the observations within the framework of causes and effects of solar-terrestrial relationships.     

Time-lag of cosmic-ray intensity

The best correlation coefficient between the monthly cosmic-ray intensity of the Inuvik Station and various kinds of solar, interplanetary, and geophysical parameters has been found. It is calculated for different time-lags of cosmic-ray intensity with respect to these parameters. The maximum of these coefficients lead us to a useful empirical model for the 11-year cosmic-ray modulation.     

PHASE EVOLUTION OF SOLAR ACTIVITY AND COSMIC-RAY VARIATION CYCLES

Cycles of phase evolution of solar activity and cosmic-ray variations are reconstructed by means of the delay component method, which allows us to study the temporal behaviour of time lag between solar activity and cosmic-ray cycle phases. It is shown that the period of the late 20th cycle was very unusual. We have found a delay in the phase of the solar activity cycle with respect to that of cosmic rays and discuss the heliospheric conditions responsible for this delay.     

Simulation of the temporal variations of the galactic cosmic-ray intensity at neutron monitor energies

We show that the temporal variations of the integrated galactic cosmic-ray intensity at neutron monitor energies (approximately above 3 GeV) can be reproduced applying a semi-empirical 1-D diffusion-convection model for the cosmic-ray transport in interplanetary space. We divide the interplanetary region into `magnetic shells' and find the relative reduction that each shell causes to the cosmic-ray intensity. Then the cosmic-ray intensity at the Earth is reproduced by the successive influence of all shells between the Earth and the heliospheric termination shock. We find that the position of the termination shock does not significantly affect the cosmic-ray intensity although there are some differences between the results for a constant and a variable termination shock radius. We also reproduce the cosmic-ray intensity applying the analytical solution of the force-field approximation (Perko, 1987) and find that the results cannot fit the observed data. Our results are compared with the Climax (geomagnetic cut-off 3 GV) and Huancayo (geomagnetic cut-off 13 GV) neutron monitor measurements for almost two solar cycles (1976–1996).     

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.     

Study of the Cosmic-Ray Modulation During the Passage of ICMEs and CIRs

We compare the cosmic-ray response to interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) during their passage in near-Earth space. We study the relative importance of various structures/features identified during the passage of the ICMEs and CIRs observed during Cycle 23 (1995?–?2009). The identified ICME structures are the shock front, the sheath, and the CME ejecta. We isolate the shock arrival time, the passage of the sheath region, the arrival of ejecta, and the end time of their passage. Similarly, we isolate the CIR arrival, the associated forward shock, the stream interface, and the reverse shock during the passage of a CIR. For the cosmic-ray intensity, we utilize the data from high counting rate neutron monitors. In addition to neutron monitor data, we utilize near-simultaneous and same time-resolution data of interplanetary plasma and field, namely the solar-wind velocity, the interplanetary magnetic field (IMF) vector, and its variance. Further, we also utilize some derived interplanetary parameters. We apply the method of the superposed-epoch analysis. As the plasma and field properties are different during the passage of different structures, both in ICMEs and CIRs, we systematically vary the epoch time in our superposed-epoch analysis one by one. In this way, we study the role and effects of each of the identified individual structures/features during the passage of the ICMEs and CIRs. Relating the properties of various structures and the corresponding variations in plasma and field parameters with changes of the cosmic-ray intensity, we identify the relative importance of the plasma/field parameters in influencing the amplitude and time profiles of the cosmic-ray intensity variations during the passage of the ICMEs and CIRs.     

A Complete Catalogue of High-Speed Solar Wind Streams during Solar Cycle 23

High-speed solar wind streams (HSSWSs) are ejected from the Sun and travel into the interplanetary space. Because of their high speed, they carry out energetic particles such as protons and heavy ions, which leads to an increase in the mean interplanetary magnetic field (IMF). Since the Earth is in the path of those streams, Earth’s magnetosphere interacts with the disturbed magnetic field, leading to a significant radiation-induced degradation of technological systems. These interactions provide an enhanced energy transfer from the solar wind/IMF system into the Earth’s magnetosphere and initiate geomagnetic disturbances that may have a possible impact on human health. Solar cycle 23 was a particularly unusual cycle with many energetic phenomena during its descending phase and also had an extended minimum. We have identified and catalogued the HSSWSs of this cycle and determined their characteristics, such as their maximum velocity, beginning and ending time, duration, and possible sources. We identified 710 HSSWSs and compared them with the corresponding characteristics of the streams of previous solar cycles. For first time, we used the CME data to study the stream sources, which led to useful results for the monitoring and forecasting of space weather effects.     

Long-term modulation of the Galactic cosmic-ray fluctuation spectrum

We study the temporal behavior of the power spectra for Galactic cosmic-ray fluctuations during the last two solar cycles. We use the 5-min data for 1980–2002 corrected for the barometric effect from two widely separated high-latitude cosmic-ray stations, Tixie Bay and Oulu. The cosmicray fluctuation spectrum is shown to be subjected to a regular long-term modulation with a period of about 11 years in phase with the solar cycle, in accordance with the variations in the inertial part of the turbulence spectrum for the interplanetary magnetic field. Based on independent measurements, we confirm the previously detected cosmic-ray fluctuation power enhancement at the maximum of the 11-year solar cycle and its subsequent decrease at minimum solar activity using new, more extensive data sets. We reach the conclusion about the establishment of a new cosmic-ray modulation phenomenon that has not been described previously in scientific literature.     

On reproduction of long-term cosmic-ray modulation as seen by neutron monitor stations

The dependence of cosmic-ray intensity on 21st solar cycle phenomena has been studied using monthly cosmic-ray values from nine world wide Neutron Monitoring Stations.For this purpose the long-term cosmic-ray modulation is modelled by treating the most appropriate source functions among various solar, interplanetary and terrestrial activity indices as the input and the cosmic-ray intensity as the output of a linear system taking into account the corresponding time-lag. In this way the modulated galactic cosmic-ray intensity has been reproduced to a certain degree as the cosmic-ray variations follow the observations with a standard deviation of ~ 10%. Still remaining short-term variations in all stations with periods of 2.7 and 3.7 months can possibly be related to the galactic origin of cosmic-rays.The Simpson solar wind model improved by the spherically symmetric diffusion-convection theory can describe our proposed method.     

Study of the Geoeffectiveness and Galactic Cosmic-Ray Response of VarSITI-ISEST Campaign Events in Solar Cycle 24

We analyze and compare the geomagnetic and galactic cosmic-ray (GCR) response of selected solar events, particularly the campaign events of the group International Study of Earth-affecting Solar Transients (ISEST) of the program Variability of the Sun and Its Terrestrial Impact (VarSITI). These selected events correspond to Solar Cycle 24, and we identified various of their features during their near-Earth passage. We evaluated the hourly data of geomagnetic indices and ground-based neutron monitors and the concurrent data of interplanetary plasma and field parameters. We recognized distinct features of these events and solar wind parameters when the geomagnetic disturbance was at its peak and when the cosmic-ray intensity was most affected. We also discuss the similarities and differences in the geoeffectiveness and GCR response of the solar and interplanetary structures in the light of plasma and field variations and physical mechanism(s), which play a crucial role in influencing the geomagnetic activity and GCR intensity.     

An empirical model for the 11-year cosmic-ray modulation

An analysis of monthly data from nine world-wide neutron monitoring stations over the period 1965–1975 is carried out for the study of the long-term cosmic-ray modulation. In an attempt to gain insight into the relationships which exist between solar activity, high-speed solar wind streams and various terrestrial phenomena an empirical relation for the cosmic-ray modulation has been found. Accordingly the modulated cosmic-ray intensity is equal to the galactic cosmic-ray intensity corrected by a few appropriate solar, interplanetary and terrestrial activity indices which causes the disturbances in interplanetary space, multiplying with the corresponding time-lag of cosmic-ray intensity from each of these indices. This relation is well explained by a generalization of the Simpson solar wind model which has been proved by the spherically symmetric diffusion-convection theory.     

An Analysis of Polar Coronal Hole Evolution: Relations to Other Solar Phenomena and Heliospheric Consequences

We use observations of the green corona low-brightness regions to construct a time series of a polar coronal hole area from 1939 to 1996, covering 5 solar cycles. We then perform a power-spectral analysis of the monthly data time series. Several persistent significant periodicities appear in the spectra, which are related with those found in solar magnetic flux emergence, geomagnetic storm sudden commencements and cosmic-ray flux at Earth. Of particular importance are the peak at around 1.6–1.8 yr recently found in cosmic-ray intensity fluctuations, and the peak at around 1 yr, also identified in coronal hole magnetic flux variations. Additional interesting features are the peaks close to 5 yr, 3 yr and the possible peak at around 30 yr, that were also found in other solar and interplanetary phenomena. Our results stress the physical connection between the solar magnetic flux emergence and the interplanetary medium dynamics, in particular the importance of coronal hole evolution in the structuring of the heliosphere.     

A Comparative Analysis of Ground, Magnetospheric and Interplanetary Observations of Long Period Magnetic Oscillations

Continuous measurements of the geomagnetic field variations at ground stations are important to investigate several aspects of magnetospheric dynamics related to variations in the solar wind conditions which, ultimately, originate from the Sun. We present a comparative analysis of geomagnetic field measurements at several ground stations with simultaneous magnetospheric and interplanetary observations in order to understand the origin and characteristics of the observed fluctuations. The results suggest that long period geomagnetic field fluctuations can be directly driven by solar wind density fluctuations at the same frequencies via the modulation of the magnetopause current. We also discuss the possible occurrence of additional contributions related with cavity/waveguide resonances of the entire magnetosphere as well as those of resonance processes of the geomagnetic field lines.