Solar cosmic rays

The first observations of solar cosmic rays were made simultaneously by many investigators at worldwide cosmic-ray stations in the periods of powerful chromospheric flares on February 28 and March 7, 1942. The discovery of these and the investigation of cosmic-ray solar-daily variations with maximum time near noon led some authors (Richtmyer and Teller, 1948; Alfvén, 1949, 1950) to a model of apparent cosmic-ray solar origin. We present here the results of the properties of solar cosmic rays from ground events (experimental and theoretical investigations). We also discuss important information from solar experimental data relating to these ground events observed in September and October 1989 and May 1990. Some experimental evidence of acceleration processes in associated phenomena with flares and long-term (solar cycle) variation of the average flux of solar cosmic rays is discussed as also cornal and interplanetary propagation, and that in the terrestrial magnetosphere. Note that the energy spectrum of solar cosmic rays varied very strongly from one flare to another. What are the causes of these phenomena? What is the nature of chemical and isotopic contents of solar cosmic rays? How can its changes occur in the energy spectrum and chemical contents of solar cosmic rays in the process of propagation? Is it possible to recalculate these parameters to the source? What makes solar cosmic rays rich in heavy nucleus and³He? The important data about electrons, positrons, gamma-quanta and neutrons from flares will be discussed in a subsequent paper (Dorman and Venkatesan, 1992). The question is: What main acceleration mechanism of solar flare and associated phenomena are reliable? These problems are connected with the more general problem on solar flare origin and its energetics. In Dorman and Venkatesan (1993) we will consider these problems as well as the problem of prediction of radiation hazard from solar cosmic rays (not only in space, but also in the Earth's atmosphere too).     

Interdisciplinary summary of solar/interplanetary events during August 1972

The solar/interplanetary events in early August 1972 are summarized in Section 1 (Introduction), Section 2 (August 1972 Events in the Solar Cycle 20), Section 3 (Evolution of Solar Active Region: McMath region No. 11976 and its flare-activity), Section 4 (Radio, X-ray, and Proton Characteristics of Four Major Solar Flares: F-1 at 0316 UT on 2 August, F-2 at 1958 UT on 2 August, F-3 at 0626 UT on 4 August, and F-4 at 1522 UT on 7 August), Section 5 (Interplanetary Shock Waves: observations of the shock waves generated from the four major solar flares at several points in interplanetary space, the Earth, Pioneer-9, Pioneer-10, etc.; interplanetary scintillations; shock trajectories in the heliosphere), Section 6 (Variations of Solar and Galactic Cosmic Rays: four solar proton events observed in the vicinity of the earth and at the Pioneer-9 location in the course of interplanetary disturbances; Forbush decreases of cosmic ray intensity; the spikeshaped variation in solar and galactic cosmic rays on 5 August), and Section 7 (Conclusions).     

Large Scale Modulation: View From the Earth

The current knowledge and ideas, obtained from groundlevel observations and concerning the solar modulation of cosmic rays, are reviewed. The following topics are discussed: observations of the cosmic ray modulation at the Earth and main characteristics of the accumulated experimental data; manifestations of the solar magnetic cycle in cosmic rays; the effect of hysteresis and its relation to the size of the heliosphere; the rigidity spectrum of long-term cosmic ray variations; the influence of the sporadic effects on long-term modulation; long-term variations of cosmic ray anisotropy and gradients; the place of groundlevel observations in current studies of cosmic ray modulation and their future prospects. Particular consideration is given to the correlation of long-term cosmic ray variations with different solar-heliospheric parameters, and to empirical models of cosmic ray modulation. This revised version was published online in August 2006 with corrections to the Cover Date.     

The relevance of molecular observations to gamma-ray astronomy

The interaction of cosmic rays with interstellar clouds may produce some of the observed gamma-ray sources. The use of molecular observations to estimate the cloud masses, which are used to derive cosmic-ray fluxes, is reviewed. Molecular diagnostics of high cosmic-ray ionization rates are discussed, and a detailed application of those diagnostics is summarised and presented as evidence that second-order Fermi acceleration is important in old supernova remnants and can produce cosmic rays of too low energy to induce gamma-ray emission.Proceedings of the XVIII General Assembly of the IAU: Galactic Astrophysics and Gamma-Ray Astronomy, held at Patras, Greece, 19 August 1982.Royal Society Jaffé Donation Fellow.     

Solar cosmic ray phenomena

This review attempts to present an integrated view of the several types of solar cosmic ray phenomena. The relevant large and small scale properties of the interplanetary medium are first surveyed, and their use in the development of a quantitative understanding of the cosmic ray propagation processes summarised. Solar cosmic ray events, in general, are classified into two phenomenological categories: (a) prompt events, and (b) delayed events. The properties of both classes of events are summarised. The properties considered are the frequency of occurrence, dependence on parent flare position, the time profile, energy spectra, anisotropies, particle species, velocity dispersions, etc. A single model is presented to explain the various species of delayed event. Thus the halo and core events, energetic storm particle events, EDP events and proton recurrent regions are suggested to be essentially of common origin. The association of flare particle events with electromagnetic phenomena, including optical, X-ray and microwave emissions is summarised. The conditions in a sunspot group, and solar flare that are considered to be conducive to cosmic ray acceleration processes are discussed. Considerable discussion is devoted to physical processes occurring near the Sun. Near Sun particle storage, and diffusion, and secondary injection processes that are triggered by a far distant solar flare are reviewed. In order to explain the considerable differences between aspects of the prompt and delayed events, we propose selective diffusion processes that only occur at early times in a solar flare. The type IV radio emissions at metric wave-lengths are suggested to yield direct evidence for the storage processes that are necessary to explain the properties of the delayed events, and also as yielding direct evidence of secondary injection processes. We conclude by briefly summarising the ionospheric effects of the solar cosmic radiation.     

Neutron Monitor Design Improvements

The original design by J. A. Simpson of the neutron monitor enabled continuous monitoring of the primary cosmic-ray flux by ground-based recordings of the nucleonic component with only a rather simple correction for atmospheric effects. Simpson (1957) extended the original pile to the 12 counter IGY neutron monitor which was deployed in a world wide network during the International Geophysical Year 1957/8. The desirability for monitors with higher counting rates became evident soon afterwards. Subsequently the NM64 super neutron monitor was designed by H. Carmichael for deployment in time for the International Quiet Sun Year 1964. Using unusually large ¹⁰BF₃ proportional counters made at Chalk River, Hatton and Carmichael (1964) studied comprehensively the experimental design of the NM64. Consequently the efficiency of neutron counters to record evaporation neutrons produced in the lead of a monitor increased from 1.9% for the IGY to 5.7% for the NM64, an increase of 3.3 times the counting rate per unit area of lead producer. During the years much attention was given to the neutron multiplicity spectrum in neutron monitors. This spectrum is related to the energy spectrum of the nucleonic component incident on the neutron monitor, but is only weakly dependent on the spectrum of galactic cosmic rays at the top of the atmosphere. Contrary to galactic cosmic rays, solar flare protons and neutrons are observed predominantly as single counts per interaction, in multiplicity 1, because of the softness of solar flare particle energy spectra. Neutron monitors have also been specially designed to record solar neutrons with increased sensitivity. Newly developed ³He counters with a largely reduced thermal neutron absorption mean free path should lead to improved efficiency in recording primary cosmic radiation. Design criteria are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

On The Stratospheric Measurements of Cosmic Rays

The results of measurements of cosmic-ray intensity in the Earth's atmosphere from 1957 up to now are presented. Balloon launchings performed several times a week at northern and southern polar latitudes as well as at midlatitudes yield a homogeneous data series for the study of the spatial distribution and temporal changes in the cosmic-ray intensity. The 11- and 22-year solar modulation, north-south asymmetry, energy hysteresis and a long-term trend in the cosmic-ray intensity are discussed. The fluxes and energy spectra of protons with energy > 100 MeV for about 100 solar energetic particle events in 1958-1992 are given. The role of cosmic rays in terrestrial atmospheric processes is briefly discussed.     

Solar modulation of galactic cosmic radiation

In this review an attempt is made to present an integrated view of the solar modulation process that cause time variation of cosmic ray particles. After briefly surveying the relevant large and small scale properties of the interplanetary magnetic fields and plasma, the motion of cosmic ray particles in the disordered interplanetary magnetic fields is discussed. The experimentally observed long term variations of different species of cosmic ray particles are summarised and compared with the theoretical predictions from the diffusion-convection model. The effect of the energy losses due to decelaration in the expanding solar wind are clearly brought out. The radial density gradient, the modulation parameter and their long term variation are discussed to understand the dynamics of the modulating region. The cosmic ray anisotropy measurements at different energies are summarised. At high energies (E 1 GeV), the average diurnal anisotropy is shown to be energy independent and along the 18.00 h direction consistent with their undergoing partial corotation with the sun. The average semi-diurnal anisotropy seems to vary with energy as E ⁺¹ and incident from a direction perpendicular to the interplanetary field line, consistent with the semi-diurnal component being produced by latitudinal gradients. Both the diurnal and semi-diurnal components are shown to be practically time invariant. On a day to day basis, however, the anisotropy characteristics such as the exponent of variation, the amplitude and the phase show very high variability which are interpreted in terms of convection and variable field aligned diffusion due to the redistribution of the galactic cosmic ray density following transient changes in the interplanetary medium. The anisotropy observation at low energies (E 100 MeV) are, however, not explained by the theory.The rigidity dependence and the anisotropies during short term variations such as Forbush decreases are discussed in terms of the proposed field models for the interplanetary field structure and are compared with the observed rigidity dependence of long term variations. The data pertaining to the 27 day corotating Forbush decreases and their association with enhanced diurnal variation are also presented. The relationship between the energetic storm particle events which are caused by the acceleration of particles in the shock fronts and the Forbush decreases which are caused by the exclusion of galactic particles by the enhanced field structure in the same fronts are clearly brought out. Thus the recurrent increases at low energies and recurrent decreases at high energies may both be caused by the field structure in the shock front. In conclusion, the properties of the very short period fluctuations (18–25 cph) are summarised.     

Topological defects and highest energy cosmic and gamma rays

In this paper we review the hypothesis that a substantial part of the cosmic ray flux observed above about 10¹⁹ eV may be produced by decaying or annihilating topological defects left over from phase transitions in the early universe at grand unification energy scales ( 10¹⁶ GeV). Possible signatures of cosmic ray producing defect models are discussed which could be tested experimentally in the near future. We thereby focus on model independent universal spectral properties of the predicted particle fluxes.     

Skylab measurements of low energy cosmic rays

In this review the present state of our knowledge on the properties of heavy ions in low energy cosmic rays measured in the Skylab mission and in other spacecrafts is summarised and the possible mechanisms of their origin are discussed. A brief review of the general features of the galactic and solar cosmic rays is given in order to understand the special features of the low energy heavy ions of cosmic rays. The results of the cosmic ray experiment in the Skylab show that in the low energy interval of 8–30 MeV/N, the abundances of oxygen, nitrogen, and neon ions, relative to carbon are enhanced by a factor of 5 to 2 as compared to high energy cosmic rays; while Mg, Si, S, and A are depleted. In 50–150 MeV/N energy interval the abundance of nuclei of Ca-Cr relative to iron-group (Z = 25–28) is found to be highly enhanced, as compared to high energy cosmic rays. Furthermore the observations of the energy spectra of O, N, and Ne ions and their fairly large fluences in the energy interval of 8–30 MeV/N below the geomagnetic cut off energy of 50 MeV/N for fully stripped nuclei at the Skylab orbit indicate that these heavy ions are probably in partly ionised states. Thus, it is found that the Skylab results represent a new type of heavy ion population of low energy cosmic rays below 50 MeV/N, in the near Earth space and their properties are distinctly different from those of high energy cosmic rays and are similar to those of the anomalous component in the interplanetary space. The available data from the Skylab can be understood at present on the hypothesis that low energy interplanetary cosmic ray ions of oxygen etc. occur in partly ionised state such as O⁺¹,O⁺², etc. and these reach the inner magnetosphere at high latitudes where stripping process occurs near mirror points and this leads to temporarily trapped ions such as O⁺³, O⁺⁴, etc. It is noted that the origin of these low energy heavy cosmic ray ions in the magnetosphere and in interplanetary space is not yet fully understood and new type of sources or processes are responsible for their origin and these need further studies.     

Cosmic Rays in Relation to Space Weather

A review of selected experimental results relevant for the use of cosmic ray records in Space Weather research is presented. Interplanetary perturbations, initiated in the solar atmosphere, affect galactic cosmic rays. In some cases their influence on the cosmic ray intensity results in data signatures that can possibly be used to predict geomagnetic storm onsets. Case studies illustrating the complexity of the cosmic ray effects and related geomagnetic activity precursors are discussed. It is shown that some indices for cosmic ray activity are good tools for testing the reliability of cosmic ray characteristics for Space Weather forecasts. A brief summary of the influence of cosmic rays on the ozone layer is also given. The use of cosmic ray data for Space Weather purposes is still in its infant stage, but suggestions for both case and statistical studies are made. This revised version was published online in August 2006 with corrections to the Cover Date.     

Space Weather and the Ground-Level Solar Proton Events of the 23rd Solar Cycle

Solar proton events can adversely affect space and ground-based systems. Ground-level events are a subset of solar proton events that have a harder spectrum than average solar proton events and are detectable on Earth’s surface by cosmic radiation ionization chambers, muon detectors, and neutron monitors. This paper summarizes the space weather effects associated with ground-level solar proton events during the 23rd solar cycle. These effects include communication and navigation systems, spacecraft electronics and operations, space power systems, manned space missions, and commercial aircraft operations. The major effect of ground-level events that affect manned spacecraft operations is increased radiation exposure. The primary effect on commercial aircraft operations is the loss of high frequency communication and, at extreme polar latitudes, an increase in the radiation exposure above that experienced from the background galactic cosmic radiation. Calculations of the maximum potential aircraft polar route exposure for each ground-level event of the 23rd solar cycle are presented. The space weather effects in October and November 2003 are highlighted together with on-going efforts to utilize cosmic ray neutron monitors to predict high energy solar proton events, thus providing an alert so that system operators can possibly make adjustments to vulnerable spacecraft operations and polar aircraft routes.     

High energy cosmic ray observations during August 1972

A series of spectacular cosmic ray events which included two relativistic solar particle enhancements and three major Forbush decreases were registered by ground-based cosmic ray monitoring stations beginning 4 August, 1972. These were associated with four major proton flare events on the Sun and with large interplanetary magnetic field disturbances and high velocity shock waves. This review attempts to discuss and interpret the high energy cosmic ray phenomena observed during this period in the light of the known behaviour of low energy particulate flux, interplanetary plasma and field observations and other associated solar and terrestrial effects recorded during this period.The first Forbush decrease event FD-1 occurred in the early hours of 4 August, exhibiting very strong north-south and east-west anisotropies. Immediately following the onset of FD-1, the first ground level solar particle enhancement occurred. This event, which had its onset almost 6 h after the flare event on 4 August, had a very steep rigidity spectrum. The major Forbush event of the series which had its onset at 2200 UT on 4 August, exhibited extremely interesting and complex behaviour, the prominent features of which are a precursory increase prior to the onset (PI-1), a large decrease (FD-2), the largest observed to date, followed immediately by an abrupt square wave like enhancement (PI-2). Interplanetary space during this entire period was highly disturbed by the presence of large low energy particulate fluxes and shock waves, at least one of which had a velocity exceeding 2000 km s^-1. Large north-south and east-west anisotropies existed throughout the event. Both FD-2 and PI-2 were characterized by almost the same rigidity spectrum, with a power law index of -1.2 ± 0.2, and a predominant anisotropy along the sunward direction. The square wave-like spike PI-2 during the recovery of FD-2 was associated with a similar abrupt change in low energy particle flux in space, as well as an abrupt decrease in the interplanetary magnetic field value from 50 to 10 .Based on the available particle, field and plasma observations, an unified model is presented to explain the Forbush event in terms of a transient modulating region associated with the passage of a narrow magnetic shock front. In this model, the reflection of particles from the approaching shock front account for the precursory increase PI-1. The main Forbush event is caused when the magnetic barrier at the shock front sweeps past the Earth. The square wave increase is due to the enhanced flux contained in the magnetic well just behind the shock front and bounded by magnetic discontinuities, which is explained as due to the transverse diffusion of particles into this region from the interplanetary space which have easy access to this region. In situ plasma, field and low energy particle observations are reviewed to support the model.Also Professor at Physical Research Laboratory, Ahmedabad 380009, India.     

The Energy Spectra and Anisotropies of Cosmic Rays

The existing paradigm of the origin of Galactic cosmic rays places strong supernovae shocks as the acceleration site for this material. However, although the EGRET gamma-ray telescope has reported evidence for GeV gamma rays from some supernovae, it is still unclear if the signal is produced by locally intense cosmic rays. Although non-thermal X-ray emissions have been detected from supernova remnants and interpreted as synchrotron emission from locally intense electrons at energies up to 100 TeV, the inferred source energy spectral slopes seem much steeper than the electron source spectrum observed through direct measurements. It remains the case that simple energetics provide the most convincing argument that supernovae power the bulk of cosmic rays. Two characteristics which can be used to investigate this issue at high energy are the source energy spectra and the source composition derived from direct measurements.     

The Evolution of the Anomalous Cosmic Ray Oxygen Spectra From 1995 to 1998: Ulysses Observations

Moraal and Steenberg (1999), showed that the peak energy in the anomalous cosmic ray spectra is independent of the radial distance up to a few AU away from the termination shock but dependent on the solar wind speed, the radius of the termination shock and the scattering strength. In this paper we will discuss the variation of the cosmic ray oxygen energy spectrum as measured by the Ulysses EPAC and the COSPIN/LET on board Ulysses. We found that the peak energy decreased from ∼5 MeV nucl⁻¹, when Ulysses was at high northern heliographic latitudes embedded in the fast solar wind to ∼3.5 MeV n⁻¹, in the streamer belt. The shift towards lower energy might also be caused by changing modulation although Voyager measurements indicate no variation of the ACR Oxygen spectrum at ∼60 AU. This revised version was published online in August 2006 with corrections to the Cover Date.     

Large solar flare radiation shielding requirements for manned interplanetary missions

As the 21st century approaches, there is an ever-increasing interest in launching manned missions to Mars. A major concern to mission planners is exposure of the flight crews to highly penetrating and damaging space radiations. Beyond the protective covering of the Earth's magnetosphere, the two main sources of these radiations are galactic cosmic rays and solar particle events. Preliminary analyses of potential exposures from galactic cosmic rays (GCR's) were presented elsewhere. In this Note, estimates of shielding thicknesses required to protect astronauts on interplanetary missions from the effects of large solar flare events are presented. The calculations use integral proton fluences for the February 1956, November 1960, and August 1972 solar particle events as inputs into the NASA Langley Research Center nucleon transport code BRYNTRN. This deterministic computer code transports primary protons and secondary protons and neutrons through any number of layers of target material of arbitrary thickness and composition. Contributions from target nucleus breakup (fragmentation) and recoil are also included. The results for each flare are presented as estimates of dose equivalent [in units of roentgen equivalent man (rem)] to the skin, eye, and bloodforming organs (BFO) behind various thicknesses of aluminum shielding. These results indicate that the February 1956 event was the most penetrating; however, the August 1972 event, the largest ever recorded, could have been mission- or life-threatening for thinly shielded (< or = 5 g/cm2) spacecraft. Also presented are estimates of the thicknesses of water shielding required to reduce the BFO dose equivalent to currently recommended astronaut exposure limits. These latter results suggest that organic polymers, similar to water, appear to be a much more desirable shielding material than aluminum.     

Neutron Monitor Response Functions

The neutron monitor provides continuous ground-based recording of the hadronic component in atmospheric secondary radiation which is related to primary cosmic rays. Simpson (1948) discovered that the latitude variation of the secondary hadronic component was considerably larger than the muon component suggesting the response of a neutron monitor is more sensitive to lower energies in the primary spectrum. The different methods of determining the neutron monitor response function of primary cosmic rays are reviewed and discussed including early and recent results. The authors also provide results from a new calculation (Clem, 1999) including angle dependent yield functions for different neutron monitor types which are calculated using a simulation of cosmic ray air showers combined with a detection efficiency simulation for different secondary particle species. Results are shown for IGY and NM64 configurations using the standard ¹⁰BF₃ detectors and the new ³He detectors to be used in the Spaceship Earth Project (Bieber et al., 1995). The method of calculation is described in detail and the results are compared with measurements and previous calculations. A summary of future goals is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

ATS-6 Solar Cosmic Ray and Trapped Particle Experiment

The Solar Cosmic Ray and Trapped Particle Experiment was designed to study the entry, propagation, and loss of solar cosmic rays and the acceleration and loss of trapped electrons and protons in the magnetosphere. Two orthogonal proton and alpha partical telescopes measure protons from 300 keV to 250 MeV and alphas from 2 MeV to 200 MeV. Electron spectrometers measure electrons from 50 keV to 1 MeV and are used in conjunction with the 300-keV to 1.2-MeV proton channels to study the injection of electrons and protons into the magnetosphere during substorms. Two solar cosmic ray events were observed during the first four months of operation. The first of these began on July 3, 1974, and is probably one of the more complicated events in recent years. There were numerous flares and sudden commencements as well as intense fluxes of low energy plasma with a severly perturbed magnetosphere. The second solar cosmic ray event was smaller and was associated with an isolated east limb flare. The first increase was observed on September 11, 1974.     

Modulation of Galactic Cosmic Rays at Solar Minimum

Cosmic ray particles respond to the heliospheric magnetic field in the expanding solar wind and its turbulence and therefore provide a unique probe for conditions in the changing heliosphere. During the last four years, concentrated around the solar minimum period of solar cycle 22, the exploration of the solar polar regions by the joint ESA/NASA mission Ulysses revealed the three-dimensional behavior of cosmic rays in the inner and middle heliosphere. Also during the last decades, the Pioneer and Voyager missions have greatly expanded our understanding of the structure and extent of the outer heliosphere. Simultaneously, numerical models describing the propagation of galactic cosmic rays are becoming sophisticated tools for interpreting and understanding these observations. We give an introduction to the subject of the modulation of galactic cosmic rays in the heliosphere during solar minimum. The modulation effects on cosmic rays of corotating interaction regions and their successors in the outer heliosphere are discussed in more detail by Gazis, McDonald et al. (1999) and McKibben, Jokipii et al. (1999) in this volume. Cosmic-ray observations from the Ulysses spacecraft at high heliographic latitudes are also described extensively in this volume by Kunow, Lee et al. (1999). This revised version was published online in August 2006 with corrections to the Cover Date.