Forbush Decreases Associated with Western Solar Sources and Geomagnetic Storms: A Study on Precursors

As suggested in many studies the pre-increases or pre-decreases of the cosmic ray intensity (known as precursors), which usually precede a Forbush decrease, could serve as a useful tool for studying space weather effects. The events in this study were chosen based on two criteria. Firstly, the heliolongitude of the solar flare associated with each cosmic ray intensity decrease was in the 50°?–70°W sector and, secondly, the values of the geomagnetic activity index, Kp _max, were ≥?5. Twenty five events were selected from 1967 to 2006. We have used data on solar flares, solar wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field in our detailed analysis. The asymptotic longitudinal cosmic ray distribution diagrams were plotted using the “Ring of Stations” method for all the events. The results reveal clear signs of precursors in 60 % of selected events.     

Transient perturbations and their effects in the heliosphere, the geo-magnetosphere, and the Earth’s atmosphere: Space weather perspective

We discuss the effects of certain dynamic features of space environment in the heliosphere, the geo-magnetosphere, and the earth’s atmosphere. In particular, transient perturbations in solar wind plasma, interplanetary magnetic field, and energetic charged particle (cosmic ray) fluxes near 1 AU in the heliosphere have been discussed. Transient variations in magnetic activity in geo-magnetosphere and solar modulation effects in the heliosphere have also been studied. Emphasis is on certain features of transient perturbations related to space weather effects. Relationships between geomagnetic storms and transient modulations in cosmic ray intensity (Forbush decreases), especially those caused by shock-associated interplanetary disturbances, have been studied in detail. We have analysed the cosmic ray, geomagnetic and interplanetary plasma/field data to understand the physical mechanisms of two phenomena namely, Forbush decrease and geomagnetic storms, and to search for precursors to Forbush decrease (and geomagnetic storms) that can be used as a signature to forecast space weather. It is shown that the use of cosmic ray records has practical application for space weather predictions. Enhanced diurnal anisotropy and intensity deficit of cosmic rays have been identified as precursors to Forbush decreases in cosmic ray intensity. It is found that precursor to smaller (less than 5%) amplitude Forbush decrease due to weaker interplanetary shock is enhanced diurnal anisotropy. However, larger amplitude (greater than 5%) Forbush decrease due to stronger interplanetary shock shows loss cone type intensity deficit as precursor in ground based intensity record. These precursors can be used as inputs for space weather forecast.     

Diurnal anisotropy of cosmic rays during intensive solar activity for the period 2001–2014

The diurnal variation of cosmic ray intensity, based on the records of two neutron monitor stations at Athens (Greece) and Oulu (Finland) for the time period 2001 to 2014, is studied. This period covers the maximum and the descending phase of the solar cycle 23, the minimum of the solar cycles 23/24 and the ascending phase of the solar cycle 24.These two stations differ in their geographic latitude and magnetic threshold rigidity. The amplitude and phase of the diurnal anisotropy vectors have been calculated on annual and monthly basis.From our analysis it is resulted that there is a different behaviour in the characteristics of the diurnal anisotropy during the different phases of the solar cycle, depended on the solar magnetic field polarity, but also during extreme events of solar activity, such as Ground Level Enhancements and cosmic ray events, such as Forbush decreases and magnetospheric events. These results may be useful to Space Weather forecasting and especially to Biomagnetic studies.     

On the Analysis of the Complex Forbush Decreases of January 2005

In this work an analysis of a series of complex cosmic ray events that occurred between 17 January 2005 and 23 January 2005 using solar, interplanetary and ground based cosmic ray data is being performed. The investigated period was characterized both by significant galactic cosmic ray (GCR) and solar cosmic ray (SCR) variations with highlighted cases such as the noticeable series of Forbush effects (FEs) from 17 January 2005 to 20 January 2005, the Forbush decrease (FD) on 21 January 2005 and the ground level enhancement (GLE) of the cosmic ray counter measurements on 20 January 2005. The analysis is focusing on the aforementioned FE cases, with special attention drawn on the 21 January 2005, FD event, which demonstrated several exceptional features testifying its uniqueness. Data from the ACE spacecraft, together with GOES X-ray recordings and LASCO CME coronagraph images were used in conjunction to the ground based recordings of the Worldwide Neutron Monitor Network, the interplanetary data of OMNI database and the geomagnetic activity manifestations denoted by K _p and D _st indices. More than that, cosmic ray characteristics as density, anisotropy and density gradients were also calculated. The results illustrate the state of the interplanetary space that cosmic rays crossed and their corresponding modulation with respect to the multiple extreme solar events of this period. In addition, the western location of the 21 January 2005 solar source indicates a new cosmic ray feature, which connects the position of the solar source to the cosmic ray anisotropy variations. In the future, this feature could serve as an indicator of the solar source and can prove to be a valuable asset, especially when satellite data are unavailable.     

The geomagnetic and cosmic-ray storm of May 25/26, 1967

A series of geomagnetic disturbances and cosmic ray variations caused by the McMath plage region 8818 in the latter half of May 1967 were examined. The systematic changes of the geomagnetic disturbances were observed as the relative location between the responsible flares and the earth changed during the half solar rotation period.The storm of May 25/26, 1967 was then studied in great detail on the basis of records from a number of magnetic and cosmic ray observatories. A large asymmetric main phase field in mid-and low-latitude regions (and thus an asymmetric ring current belt) grew rapidly during the first three successive polar magnetic substorms. The cosmic ray intensity variations during the storm consisted of the Forbush decrease and the ring current effect. The Forbush decrease had a marked north-south asymmetry during its developing phase.     

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.     

Study of Cosmic-Ray Intensity Variations Associated with Anomalous,Long-Duration High-Speed Solar Wind Streams in 2003

High-speed solar wind streams (HSWS) were identified for solar cycles 22 and 23 (up to 2004). Preliminarily, HSWS were classified in three groups according to their continuous period of occurrence. In the declining phase of solar cycle 23, 2003 is found to be anomalous, showing a very large number of HSWS events of long duration (> ten days). We have studied the effect of HSWS on the cosmic-ray intensity as well as their relationship with geomagnetic disturbance index Ap on yearly, daily, and hourly bases. The yearly average of solar-wind speed was also found to be maximum in 2003. Being within the declining phase of solar activity, the occurrence of solar flares in 2003 is quite low. In particular during HSWS, no solar flares have been observed. Associations with cosmic-ray changes do not support the notion that the HSWS are usually effective in producing significant cosmic-ray decreases. Out of 12 HSWS events observed during the period 2002 (December) to 2003, four events of significant cosmic-ray decreases at all the stations have been selected for further analysis. The cosmic-ray intensity has been found to decrease during the first phase of the event (first five days of HSWS) at all three neutron-monitor stations situated at different latitudes with different cutoff rigidities. The rigidity spectra of observed decreases in cosmic-ray intensity for these four cases have been found to be significantly different than that of Fds (Forbush decrease). In two cases the spectra are softer, whereas in the other two they are harder than that of Fds. However, if the average of all four events is considered together then the spectra of the decrease in cosmic rays during HSWS exactly match that of Fds. Such a result implies that initially individual events should be considered, instead of combining them together, as was done earlier. The Ap index is also found to generally increase in the first phase of the event. However, the four events selected on the basis of cosmic-ray decrease are not always associated with enhanced values of the Ap index. As such, the significance of our study is that further detailed investigations for much longer periods and on an event-by-event basis is required to understand the effect of coronal-hole-associated HSWS.     

May 2005 Halo CMEs and Galactic Cosmic Ray Flux Changes at Earth’s Orbit

The pressure corrected hourly data from the global network of cosmic ray detectors, measurements of the interplanetary magnetic field (IMF) intensity (B) at Earth’s orbit and its components B _x , B _y , B _z (in the geocentric solar ecliptic coordinates) are used to conduct a comprehensive study of the galactic cosmic ray (GCR) intensity fluctuations caused by the halo coronal mass ejection of 13 May 2005. Distinct differences exist in GCR timelines recorded by neutron monitors (NMs) and multidirectional muon telescopes (MTs), the latter respond to the high rigidity portion of the GCR differential rigidity spectrum. The Forbush decrease (FD) onset in MTs is delayed (～5 h) with respect to the onset of a geomagnetic storm sudden commencement (SSC) and a large pre-increase is present in MT data before, during, and after the SSC onset, of unknown origin. The rigidity spectrum, for a range of GCR rigidities (≤200 GV), is a power law in rigidity (R) with a negative exponent (γ=?1.05) at GCR minimum intensity, leading us to infer that the quasi-linear theory of modulation is inconsistent with observations at high rigidities (>1 GV); the results support the force field theory of modulation. At present, we do not have a comprehensive model for the FD explaining quantitatively all the observational features but we present a preliminary model listing physical processes that may contribute to a FD timeline. We explored the connections between different phases of the FD and the power spectra of IMF components but did not find a sustained relationship.     

Coronal Mass Ejections and Non-recurrent Forbush Decreases

Coronal mass ejections (CMEs) and their interplanetary counterparts (interplanetary coronal mass ejections, ICMEs) are responsible for large solar energetic particle events and severe geomagnetic storms. They can modulate the intensity of Galactic cosmic rays, resulting in non-recurrent Forbush decreases (FDs). We investigate the connection between CME manifestations and FDs. We used specially processed data from the worldwide neutron monitor network to pinpoint the characteristics of the recorded FDs together with CME-related data from the detailed online catalog based upon the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO) data. We report on the correlations of the FD magnitude to the CME initial speed, the ICME transit speed, and the maximum solar wind speed. Comparisons between the features of CMEs (mass, width, velocity) and the characteristics of FDs are also discussed. FD features for halo, partial halo, and non-halo CMEs are presented and discussed.     

Transient phenomena in cosmic ray intensity during extreme events

In the present work an analysis has been made of the extreme events occurring during July 2005. Specifically, a rather intense Forbush decrease was observed at different neutron monitors all over the world during 16 July 2005. An effort has been made to study the effect of this unusual event on cosmic ray intensity as well as various solar and interplanetary plasma parameters. It is noteworthy that during 11 to 18 July 2005 the solar activity ranged from low to very active. Especially low levels occurred on 11, 15, and 17 July whereas high levels took place on 14 and 16 July 2005. The Sun is observed to be active during 11 to 18 July 2005, the interplanetary magnetic field intensity lies within 15 nT, and solar wind velocity was limited to ∼500 kms-1. The geomagnetic activity during this period remains very quiet, the Kp index did not exceed 5, the disturbance storm time Dst index remains ∼-70 nT and no sudden storm commencement has been detected during this period. It is noted that for the majority of the hours, the north/south component of the interplanetary magnetic field, Bz, remains negative, and the cosmic ray intensity increases and shows good/high correlation with Bz, as the polarity of Bz tends to shift from negative to positive values, the intensity decreases and shows good/high anti-correlation with Bz. The cosmic ray intensity tends to decrease with increase of interplanetary magnetic field strength (B) and shows anti-correlation for the majority of the days. Published in Astrofizika, Vol. 51, No. 2, pp. 255–265 (May 2008).     

Galactic Cosmic Ray Modulation and the Last Solar Minimum

In this work the galactic cosmic ray modulation in relation to solar activity indices and heliospheric parameters during the years 1996??C?2010 covering solar cycle 23 and the solar minimum between cycles 23 and 24 is studied. A new perspective of this contribution is that cosmic ray data with a rigidity of 10 GV at the top of the atmosphere obtained from many ground-based neutron monitors were used. The proposed empirical relation gave much better results than those in previous works concerning the hysteresis effect. The proposed models obtained from a combination of solar activity indices and heliospheric parameters give a standard deviation <?10?% for all the cases. The correlation coefficient between the cosmic ray variations of 10?GV and the sunspot number reached a value of r=?0.89 with a time lag of 13.6±0.4 months. The best reproduction of the cosmic ray intensity is obtained by taking into account solar and interplanetary indices such as sunspot number, interplanetary magnetic field, CME index, and heliospheric current sheet tilt. The standard deviation between the observed and calculated values is about 7.15?% for all of solar cycle 23; it also works very well during the different phases of the cycle. Moreover, the use of the cosmic ray intensity of 10?GV during the long minimum period between cycles 23 and 24 is of special interest and is discussed in terms of cosmic ray intensity modulation.     

Short and long-term variations in the cosmic ray intensity and their connection with the 5303 Å coronal line intensity in solar cycle No. 20

The cosmic ray modulation in the period 1965–70 is investigated by the comparison of the intensity data of groundbased stations with different response to primaries. The socalled step-like modulation, already observed by other authors, is found to be produced by the overlapping between the quasi-stationary solar cycle modulation and the Forbush decrease events. Moreover a good correlation between the cosmic-ray variance (Forbush decrease index) and the 5303 coronal intensity at middle heliolatitudes (17.5°–42.5°) is found, while the quasi-stationary solar cycle modulation is well correlated with the 5303 intensity near the solar equator (0°–17.5°). The different time behaviour of the solar activity at different heliolatitudes causes the step-like modulation.     

Heliospheric Modulation of Galactic Cosmic Rays During Solar Cycle 23

Galactic cosmic rays (GCRs) encounter an outward-moving solar wind with cyclic magnetic-field fluctuation and turbulence. This causes convection and diffusion in the heliosphere. The GCR counts from the ground-based neutron monitor stations show intensity changes that are anti-correlated with the sunspot numbers with a lag of a few months. GCRs experience various types of modulation from different solar activity features and influence space weather and the terrestrial climate. In this work, we investigate certain aspects of the GCR modulation at low cut-off rigidity (R _c≈1 GV) in relation to some solar and geomagnetic indices for the entire solar cycle 23 (1996?–?2008). We separately study the GCR modulation during the ascending phase of cycle 23 including its maximum (1996?–?2002) and the descending phase including its minimum (2003?–?2008). We find that during the descending phase, the GCR recoveries are much faster than those of the solar parameters with negative time-lag. The results are discussed in light of modulation models, including drift effects and previous results.     

Solar modulation of galactic cosmic-ray intensity as seen by neutron monitors during 1990–1999 – I. Rigidity Dependence and Correlations with Solar Activity Parameters

We analyze the evolution of cosmic ray intensity detected by six neutron monitors located at high altitude from 1990 to 1999, that includes most of solar cycle 22 and the start of cycle 23. This set of neutron monitors covers a wide range of geomagnetic cutoff rigidities. We discuss the most significant characteristics of the cosmic ray modulation during the period as: the extraordinary decreases produced by the events of the first half of 1991, the significant two step evolution of the recovery phase of solar cycle 22 and the start of cycle 23. We also determine the rigidity dependence of the different phases of the modulation cycle. Cosmic ray intensity correlations with several solar activity parameters as sunspots, microwave flux at 10.7 cm and solar flares and with the intensity of the interplanetary magnetic field are studied.     

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.     

A new approximate coupling function: The case of Forbush decreases

We attempt the derivation of a new coupling function between ground level and primary cosmic rays for the case of Forbush decreases. The calculations for the new function are based on basic Quantum Field Theory theoretical tools, something that has not been attempted till now in other widely used cosmic ray coupling functions. The use of Quantum Field Theory calculations in cosmic rays events in general, is expected to be a suitable frame of work since it describes well the high energy particle interactions which result in variations to the total number of the particles involved through annihilation or creation of particles. The newly computed function is tested to the case of two events of Forbush decreases, February 2011 and March 2012, using data from the high resolution neutron monitor database. Results for the primary particle intensity values obtained from this function are compared directly to the corresponding ones from the use of the Dorman's widely accepted coupling function. The two sets are discussed in detail in order to deduce the possible suitability of Quantum Field Theory tools to cosmic ray events.     

Interplanetary magnetic clouds and cosmic ray modulation

We studied the cosmic ray intensity variation due to interplanetary magnetic clouds during an unusual class of low amplitude anisotropic wave train events. The low amplitude anisotropic wave train events in cosmic ray intensity have been identified using the data of ground based Deep River neutron monitor and studied during the period 1981–1994. Even though the occurrence of low amplitude anisotropic wave trains does not depend on the onset of interplanetary magnetic clouds, but the possibility of occurrence of these events cannot be overlooked during the periods of the interplanetary magnetic cloud events. It is observed that the solar wind velocity remains higher (> 300) than normal and the interplanetary magnetic field B remains lower than normal on the onset of the interplanetary magnetic cloud during the passage of low amplitude wave trains. It is also noted that the proton density remains significantly low during high solar wind velocity, which is expected. The north south component of interplanetary magnetic field Bz turns southward to one day before the arrival of cloud and remains in the southward direction after the arrival of a cloud. During these events the cosmic ray intensity is found to increase with increase of solar wind velocity. The superposed epoch analysis of cosmic ray intensity for these events during the onset of interplanetary magnetic clouds reveals that the decrease in cosmic ray intensity starts not at the onset of the cloud but after a few days. The cosmic ray intensity increases on arrival of the magnetic cloud and decreases gradually after the passage of the magnetic cloud.     

The G Index of Interplanetary Scintillation Data and Its Relation to Forbush Decreases During 1991–1994

The purpose of this work is to analyze a global index of interplanetary scintillation, G, obtained from the g-maps of Cambridge Observatory, UK, and associate it with the occurrence of sudden decreases of the cosmic radiation, known as Forbush decreases (Fds), to determine their possible relation. For this purpose we perform a superposed epoch analysis of the Gindex, with respect to the occurrence of Forbush decreases, registered at Oulu Station, Finland, for the period 1991–1994. We found an increase in the value of Gcoinciding with the occurrence of the Forbush decrease, especially for those events with a fall greater than 10%. We conclude that the G index is a macroscopic parameter representative of the inner structure of the heliosphere, and has bearings on phenomena affecting the Earth's environment, as shown by Forbush decreases and possibly geomagnetic activities.