Time Variations Of Cosmic Rays Research Articles

Cycle 20 solar wind modulation of galactic cosmic rays: Understanding the challenge

It is well known that time variations of galactic cosmic rays for cycle 20 (1964–1976) challenge our understanding of the heliospheric dynamics and the physical processes involved in causing observed modulations. Several interesting questions have been raised over the years. We attempt to provide some plausible answers based on analyses of cosmic ray data obtained with detectors on the ground, on balloons, and on satellites; median rigidity of response of these detectors to the cosmic ray spectrum covers a range of rigidities (R) from 1 to 70 GV. In particular, we explore the observed correlation between hourly rates of neutron monitors and the time series of the product of solar wind velocity (V) and interplanetary magnetic field intensity (B) at Earth's orbit. Local convection is identified as a contributor to “steps” in cosmic ray time variations near Earth and in the outer heliosphere. The amplitude of 11‐year modulation varies as 1/R; data do not support the concept of “transition” rigidity for modulation. We argue that heliosheath‐related modulation effects are minimal at Earth's orbit at R > 3 GV. Heuristic considerations suggest that the space‐dependent part of the diffusion coefficient should vary as 1/B (not 1/B2). We relate modulation function to the product BV, suggesting it plays a significant role in the solar modulation of cosmic rays.

Peter Meyer

Peter Meyer

John Alexander Simpson

3 NOVEMBER 1916 * 31 AUGUST 2000 JOHN SIMPSON was an experimental physicist whose broad view of science, education, and the politics of nuclear energy played a significant role in those activities throughout the second half of the twentieth century. He worked on the Manhattan Project during World War II and was a founding member and the first chairman of the Atomic Scientists of Chicago, formally established 7 August 1945, the day after the atomic (nuclear) bomb burst at Hiroshima. This activity led to his participation in setting up the Bulletin of the Atomic Scientists the same year, with its familiar doomsday clock on the cover to show the estimated risk of nuclear annihilation during the cold war at any given time. Over the years the articles published in the Bulletin tracked the military and political implications of the bomb, commenting on possible rational courses of action. It was widely read and served as a guiding light through the long years of darkness. Henry Luce was so impressed with the concept of the Bulletin that he provided the Atomic Scientists of Chicago with two full pages in the 25 October 1945 issue of Life magazine to expound a reasoned argument presenting their case. Simpson joined the faculty of the University of Chicago in 1945 as an instructor in the physics department and immediately took leave of absence to work as an unofficial adviser to Senator Brian McMahon of Connecticut, an association leading to the McMahon Act of 1946, which placed nuclear energy in civilian hands, rather than leaving it as a military enterprise. The McMahon Act was a fundamental step in the history of governmental administration of nuclear energy. Simpson was continually aware of the future implications of the activities of the present. His efforts concerning the handling of nuclear energy were a direct consequence of this world view, and the ongoing success of his scientific work reflected it. He was a booster of his students and his colleagues and of the University of Chicago, and he would urge people to think about what they wanted to be doing ten years in the future in order to lay plans for getting there. He brought the present writer and others to Chicago and nurtured them through the fledgling years of their careers. When Simpson was appointed to the Compton Chair in physics in 1974 he used the funds that came with the chair to establish the Compton Lectures for the public. Junior staff members are honored by appointment as Compton Lecturer for a quarter. It is a substantial responsibility, and the lectures are inevitably excellent, with gratifying public response. In 1982 he established the Space Science Working Group in Washington, D.C., so that the space science laboratories around the country could communicate more effectively with NASA and Congress. He was the first chairman of the group, which came to be a useful means of conversing with the government. Simpson applied the same visionary approach to his scientific research, looking ahead even as he was in the midst of a scientific success arising from earlier planning. He took up his position as an active faculty member in the physics department at the University of Chicago in 1946, and became intrigued by Scott Forbush's discovery of modest time variations of cosmic rays. Cosmic rays had been studied since their existence was established in 1912. It was shown in 1929 that they are particles, rather than rays of some sort, and during the postwar years that they are made up of protons with a small number of heavier nuclei, all traveling at nearly the speed of light. They have tremendous energies, and they penetrate into the atmosphere, where they make hard collisions with the nuclei of the atoms of air, producing an energetic spray of downward-moving protons, neutrons, mesons, electrons, positrons, and gamma rays-cosmic shrapnel. At the bottom of the atmosphere mu mesons streak through the air, knocking out occasional electrons to create positive and negative ions. …

Cosmic rays in the past

A discussion is given of the status and potential of studying the time variation of cosmic rays over long timescales by high precision measurements of cosmogenic isotope content in natural archives such as tree rings, polar ice cores, stalagmites, oceanic and sea bottom sediments. Two unexpected and important experimental results will be central points of this paper: first, experimental confirmation of the hypothesis on cosmic ray generation during supernova explosions and secondly, heliomagnetic modulation of cosmic ray flux during the deep solar minimum.

Scott E. Forbush 1904”1984

Scott E. Forbush, a pioneer in cosmic ray research, was the quintessential geophysicist's geophysicist. Until, on the eve of his 80th birthday, he succumbed to pneumonia, he maintained an abiding interest in the continued reliable operation of the three remaining cosmic ray ionization chambers of the worldwide network that he had set up in the mid 1930s. No one could have predicted, when the first instrument at Cheltenham, Md. commenced operation in 1936, that Forbush was destined to discover most of the important multifarious time variations of cosmic rays that were accessible to his classic detectors

Rigidity responses of ionization chambers derived from cosmic-ray time variations

The coupling coefficient which connects the cosmic-ray variation in space with that observed on the ground is obtained for the case of the ionization chamber. The differential coefficients with respect to the zenith angles are averaged with weights corresponding to the zenith angle dependence of a spherically symmetric chamber at sea-level and mountain altitude. The resultant coefficients satisfactorily accord with the results of the neutron monitors and the multidirectional muon telescopes for the cases of great Forbush decreases.

BOOK REVIEW

Book review in this articleHandbuch der Physik/Encyclopedia of Physics, Ed.: S. FlüggeThe Aurora. By S.‐I. Akasofu, S. Chapman and A. B. Meinel.Time‐Variations of Cosmic Rays. By S. E. Forbush.Magnetic Properties of Rocks and Minerals. By T. Nagata.

Anisotropy of Cosmic Rays in the Galactic Arm

Many data on sidereal time variation of cosmic rays have been reported in the energy region mainly 1013 to 1018eV from air shower experiment at various places. These data show that both the amplitude and the time of maximum of sidereal daily variation change systematically with primary energy of air showers, that is, the time of maximum is at about 20 hour for energy less than 5×1015eV and turns to 12 hour in sidereal time for the higher energies. At the same time, amplitude shows a constant value of about 0.5% for the lower energy and it increases with the energy at the higher energy region. These tendencies that the sidereal variations change with the primary energy are statistically significant. Though Greisen suggested that data reported by many observers did not represent real anisotropy in consequence of statistical cause and atmospheric effect, it is thought that the energy dependent sidereal variation shows the existence of the anisotropy and this is interpreted as follows.On the assumption that a magnetic field of the order of the 10-5 gauss lies along a spiral arm of the Galaxy pointing to the outward-directed spiral arm and our solar system locates near the boundary of the arm, the above anisotropy can be explained by considering diffusion of cosmic rays in the arm. That is, the time of maximum in the lower energy region, 20 hour, corresponds to one of the direction of flow along the field lines, while the time of maxmum in the higher energy region, 12 hour, and also the energy dependence of the amplitude are expected as the results of the diffusion of cosmic rays due to the density gradient from the axis to the boundary of the arm.

Observations of solar flare radiation and modulation effects at balloon altitudes, July 1959

Observations of cosmic-ray time variations in the stratosphere were obtained over Berkeley, Calif. (USA), and Lindau/Harz (Germany) during the period of outstanding solar activity in July 1959. Whereas over Berkeley (geomagnetic latitude 44 deg N) only modulation of cosmic radiation by solar- controlled processes was observed, additional radiation accelerated in the vicinity of the sun (approximately 46% above normal) was observed over Lindau (geomagnetic latitude 52 deg N) on July 15. On the assumption that the excess radiation consisted almost entirely of protons, its energy spectrum had the form N(E)dE approximately E-4 dE and extended up to 400 Mev. According to conventional geomagnetic theory, individual protons of these energies are normally forbidden at geomagnetic latitude 52 deg N. Their presence over Lindau on July 15 indicates that they were admitted by some mechanism which temporarily lowers the cutoff rigidity at this latitude during a magnetic storm. There is some indication of further examples of excess radiation over Lindau at atmospheric depths down to 30 mm Hg on July 13, 21, 23, and 31.

Bariloche Cosmic-Ray Conference

The Cuarto Curso Interamericano de Rayos Cósmicos was held at San Carlos de Bariloche, Argentina, on January 19–31, 1959. The meeting was principally attended by South American investigators and consisted of a series of review papers and several original contributions. The principal topics discussed were cosmic-ray time variations and their possible relationships to solar and geophysical phenomena in general. There was also considerable discussion of corrections to be applied to long-term neutron and meson monitor records in order to permit the most effective use of worldwide IGY data.

Cosmic ray time variations produced by deceleration in interplanetary space

Cosmic ray time variations produced by deceleration in interplanetary space

Time variations of cosmic rays at Wellington, New Zealand

Time variations of cosmic rays at Wellington, New Zealand

Time variations of cosmic rays

While excluded as the source of all but 1/2%, at most, of the normal flux of cosmic rays, the Sun appears to be both directly and indirectly responsible for the variations in its intensity. The causes of the association of cosmic ray increases with solar flares and of the decreases with magnetic storms are discussed. Some evidence is noted indicating that the secular variation of the north-south difference in the diurnal may have the same period as that of the solar cycle.

Some time variations of cosmic rays

Cosmic ray observations made at Canberra have been examined for variations according to sidereal time and compared with an analysis of results from other localities. The analysis was made by noting annual changes in the phase and amplitude of the first harmonic of the apparent solar diurnal variation. A regular change of phase with a period of one year may be caused by a true sidereal diurnal variation. Such regular changes have been found in the results for several stations. However these changes do not occur simultaneously over the globe. The changes differ in phase by about 180° in the northern and southern hemispheres with a tropical station having an intermediate position. This suggests that the variations are not sidereal but are of a solar-seasonal nature. Changes in the amplitude of the solar diurnal variation display no obvious world-wide regularity in the results examined.

Symposium sôbre Raios Cosmicos, Rio de Janeiro, Agõsto 4–8, 1941

IN this volume are reprinted, in English and French, the papers given at a symposium on cosmic rays held in August 1941 at Rio de Janeiro, under the auspices of the Brazilian Academy of Sciences, on the occasion of the visit of a group of American physicists led by Prof. A. H. Compton. The scope of the conference is indicated by the following titles selected from some twenty papers: (1) "On the Fluctuations of Cosmic Rays", by A. H. Compton; a discussion of the sidereal time variation of cosmic rays. (2) "On the Latitude Effect of Cosmic Rays", by B. Gross. (3) "Cosmic Ray Studies in the Andes of Southern Peru", by N. Hilberry and A. H. Hilberry. This paper describes the determination of the variation in the frequency of extensive showers with altitude, up to 5,850 m. above sea-level, for two different counter arrays, and affords an interesting proof that most extensive air showers are initiated by high-energy electrons. (4) "Cloud Chamber Photographs at High Altitudes", by D. Hughes. (5) "The Latitude Effect for the Hard Component of Cosmic Rays and Evidence as to the Nature of the Primary Radiation", by W. P. Jesse. (6) "The Influence of a Solar Eclipse on the Cosmic Ray Intensity", by Y. Monteux, G. Occhialini and M. D. Souza Santos. (7) "On the Ultra-Soft Component of Cosmic Radiation", by G. Occhialini and M. Schonberg. (8) "Showers of Penetrating Particles under 30 m. of Clay", by P. A. Pompeia, M. D. Souza Santos and G. Wataghin. (9) "On the Production of Mesotrons at High Altitudes", by E. O. Wollan; this interesting paper gives the results of balloon flights to determine the multiple production of mesons as a function of altitude. No maximum is found up to a height corresponding to a pressure of 3 cm. of mercury.

Present Status of Solar and Sidereal Time Variation of Cosmic Rays

Present Status of Solar and Sidereal Time Variation of Cosmic Rays

Discussion on: “Time variations of cosmic rays” and summary of day's discussions

Discussion on: “Time variations of cosmic rays” and summary of day's discussions

Time variations of cosmic rays

Time variations of cosmic rays

A Critical Analysis for Sidereal Time Variations of Cosmic Rays on the Pacific

The problem of distinguishing a true sidereal diurnal variation in cosmic-ray intensity from a seasonal change in the amplitude of the solar diurnal variation is approached from the standpoint of the harmonic dial type of analysis. It is shown that the regular cyclic arrangement of the datum points taken in chronological order serves as a criterion for the presence of the sidereal time variation. Cosmic-ray data taken on the Pacific Ocean during 1936 by A. H. Compton and R. N. Turner are shown to contain no sidereal diurnal component larger than the experimental error.