Deep River Neutron Monitor Research Articles

COSMOGENIC RADIONUCLIDES AND NOBLE GASES IN THE WETHERSFIELD (1982) CHONDRITE

The Wethersfield (1982) chondrite was assayed for a suite of cosmogenic radionuclides shortly after fall. Data are reported for 7Be, 22Na, 26Al, 46Sc, 48V, 51Cr, 54Mn, 56Co, 57Co, and 60Co. A comparison is made with predicted results based on a scaling to the Deep River Neutron Monitor. Noble gases were also assayed in a sub‐sample. The cosmic ray exposure age is estimated to be 45 million years.

Latitudinal extent of the heliospheric current sheet and modulation of galactic cosmic rays

The possible relation between changes in the maximum heliographic latitude of the heliospheric current sheet during solar maximum and cosmic ray modulation has been investigated. A causal relation between the intensity of galactic cosmic rays and the inclination or maximum latitude of the current sheet is an important feature of the gradient drift model. The maximum latitudinal extent in the northern and southern solar hemispheres during 1976–1982 was determined from current sheet contours at the solar source surface (2.5 solar radii) derived by Stanford University by extrapolating the photospheric magnetic fields into the corona using potential theory. The maxima in the north and south were found to be reasonably symmetrical about the solar equator except for an interval of about one year near sunspot maximum when the sun's polar caps were reversing. It was therefore possible to take the difference between the maximum latitudes in the north and south as a measure of current sheet inclination. The latitude difference so derived showed the evolution of the current sheet to high latitudes near sunspot maximum with a subsequent return to lower inclinations, but with reversed polarity, following maximum. In addition to this general behavior, a number of episodes occurred in which the latitude difference changed significantly over intervals of one year or less. When the changing latitude differences from 1976 through 1982 were compared with Deep River neutron monitor observations (corresponding to cosmic rays with E ≥ 1 GeV), a significant correlation was found. Changes in maximum latitude also correlate well with cosmic ray measurements being made at lower energies (E > 200 MeV) by Pioneer 10 between 10 and 30 AU. The correlation is improved by introducing a delay corresponding to the time required for the solar wind to propagate from the sun to the spacecraft. The sensitivity of the cosmic ray intensity to changes in the current sheet is significantly different before and after the reversal in the polar cap and heliospheric magnetic field, the sensitivity being enhanced after the reversal when the field is inward in the north. The existence of this correlation and its sensitivity to the polarity of the solar heliospheric field provide significant support for the gradient drift model of cosmic ray modulation.

Interplanetary Cosmic Ray Intensity: 1972ndash;1984 and Out to 32 AU

Using counting rates of a number of Geiger‐Mueller tubes in the University of Iowa instruments on Pioneer 10 (P10) and Pioneer 11 (P11), we have measured the interplanetary intensity of cosmic rays (Ep > 80 MeV) over a complete solar activity cycle and out to a heliocentric distance of over 32 AU. Reliable free‐space values of the background corrections due to the radioisotope thermal generators were determined during passage of Pioneer 11 under the rings of Saturn and in to 1.34 Rs (planetary radii) during the September 1, 1979 encounter. These improved corrections and ones inferred therefrom for Pioneer 10 have supplanted earlier estimates and have been applied to all data. During the period 1972–1984, intensity vs. time curves have a flat, more‐or‐less constant maximum during 1975, 1976, and 1977; a brief (∼8 months) minimum in mid‐1981; and another brief minimum centered around early January 1983. The ratio of maximum to minimum intensity (normalized to the same radial distance) for the solar activity cycle is ≈ 2.7. The temporal correlation of intensity at P10 with that at P11 is good, after allowance for propagation delay, as is that with data from the Deep River Neutron Monitor. The max/min intensity ratio for the latter is ≈ 1.25. As noted by us and others in previous studies, the decline in intensity after the prolonged maximum occurred primarily by an irregular succession of stepwise decreases with subsequent slow recovery such that successive decreases overlapped cumulatively. The logarithms of the ratios of the counting rates of a GM tube on P10 to that of a matched GM tube on P11 (after allowance for propagation delay) were plotted against the differences in heliocentric radial distance Δr. Three such independent plots were made, one for each of a matched pair of tubes. The three least‐squares‐fitted trend lines through the entire body of data for each pair of detectors yield integral radial gradients G of +2.00 (±0.07), +2.07 (±0.05), and +2.12 (±0.06) percent per AU. Our adopted mean value with estimated uncertainty for all known causes is G = +2.06 (±0.20)%/AU for the radial range 1 < r < 32 AU. The reciprocal of G, namely 50 AU, is the scale length of the heliosphere. By direct observation, the modulating influence of the sun is still clearly present at 32 AU. Hence the radius of the heliospheric modulation region rB > 32 AU. Various arguments, no one of which is convincing, suggest that rB lies between 50 and 100 AU.

Cosmogenic nuclides in recently fallen meteorites: Evidence for galactic cosmic ray variations during the period 1967–1978

Cosmogenic radionuclides were measured on 48 fragments of 24 meteorites which fell between 1967 and 1978. Nondestructive gamma counting techniques were used to obtain data on 7Be, 46Sc, 48V, 51Cr, 54Mn, 56Co, 57Co, 58Co and 60Co on at least some of the samples. Sodium 22 and 26Al measurements are reported on all 48 samples. In addition, new rare gas data and exposure ages are reported for the meteorites Guibga, Gorlovka, Dhajala, Louisville, Acapulco, Jilin, Kabo, Alta‐Ameen, and Canon City. The cosmogenic radioisotope and rare gas data are interpreted in terms of a time dependent modulation of galactic cosmic rays spanning one full 11 year sun spot cycle. Special attention is given to the data on 22Na, 46Sc, 54Mn, and 48V with either 26Al or 22Ne/21Ne used to provide a shielding correction. The shielding normalized data using the 26Al method appear to correlate well with calculated production rates scaled against the Deep River neutron monitor. The data for the four isotopes are consistent with a production rate variation of a factor of 2.5–3 between solar maximum and solar minimum for sun spot cycle 20. These data demonstrate that the production rates of cosmic ray‐produced nuclides in meteorites vary considerably according to modulation by the 11‐year solar cycle and support the concept that variations of solar‐modulated, cosmic ray flux of similar magnitude have occurred over much longer time periods.

The annual and semiannual variations of the cosmic radiation

We determine the annual and semiannual harmonics in the Deep River Neutron Monitor counting rate for the years 1960–1975. A new Fourier analysis technique is used to eliminate solar cycle variations, and we discuss the statistical errors in the determination of the harmonics. The annual and semiannual waves change markedly from year to year. The yearly harmonic has an average amplitude ∼0.6% with a maximum in early March, corresponding to a southward anisotropy of ∼5%/AU perpendicular to the solar equatorial plane. The semiannual harmonic shows no phase coherence and its average amplitude is only marginally significant, ≲0.2%.

The effects of fluctuation and noise on the neutron monitor diurnal anisotropy, 3. Total vector anisotropy

The day‐to‐day variations of the total vector anisotropy observed by neutron monitors are discussed. A model is developed to explain these variations in terms of counting‐statistics noise, convection in the solar wind, scattering of cosmic rays in the interplanetary magnetic field, and particle drifts. Both the amplitude and the phase variations of the Deep River neutron monitor diurnal anisotropy are shown to agree with the model's predictions. A novel prediction, confirmed by the observations, is the tendency for afternoon hour anisotropies to be larger than evening hour anisotropies.

The diurnal anisotropy and field-aligned diffusion of cosmic rays

From the hourly counting rate of the Deep River neutron monitor we have calculated the magnitude and phase of the diurnal anisotropy, and from them we have inferred the magnitude and direction of cosmic ray diffusion on a daily basis. These data are compared with observed interplanetary magnetic field data, and we conclude that the diffusion is field aligned (within observational uncertainties) on essentially all days which are separated from sector boundaries. Days in which diffusion does not appear field aligned are characterized either by sector boundaries or by poor statistical accuracy.

The response of the deep river neutron monitor to an anisotropic flux of solar cosmic rays

The response of a neutron monitor to an anisotropic flux of solar cosmic rays is a function of the viewing direction of the detector through the magnetosphere, as defined by the asymptotic cone of acceptance, and the direction of the solar cosmic ray flux anisotropy. Since the Deep River, Canada, neutron monitor data are used extensively for monitoring the cosmic radiation intensity and are often used as a standard in the analyses of cosmic ray data, we have calculated the response of this monitor throughout the day to a high-energy (>450 MeV) anisotropic solar cosmic ray event propagating along the ecliptic plane. This technique can be applied to any ground-based neutron monitor.

Cosmic ray scintillations: 3. The low-frequency limit and observations of interplanetary scintillations

Statistically significant broad-band fluctuations, or 'scintillations', in the high-energy (about 1 GeV) cosmic ray intensity observed by neutron monitors are interpreted. The scintillations are caused by fluctuations in the interplanetary magnetic field. The theory of the scintillations is presented for the low-frequency limit, below .0001 Hz, including the effects of the earth's rotation on the fluxes observed by the neutron monitors. The observations and the theory are in good agreement. The shapes and amplitudes of the observed spectra and, in particular, a broad enhancement in the power spectrum of the Deep River neutron monitor flux near 1 cpd are related to the interplanetary magnetic field power spectrum and the cosmic ray anisotropy.

Time dependent worldwide distribution of atmospheric neutrons and of their products: 1. Fast neutron observations

The time dependent worldwide distribution of atmospheric fast neutrons has been determined in balloon and aircraft measurements from 1964 to 1971. The 1- to 10-Mev neutron spectrum was measured with a phoswich detector employing seven channels of pulse height analysis. Solar modulation effects were greatest near the high-latitude transition maximum, where the flux varied by more than a factor of 2 from solar minimum to the deepest Forbush decrease. Near solar maximum and during Forbush decreases the relation between the neutron flux in the upper atmosphere and the counting rate of the Deep River neutron monitor deviated from a single-valued function. The differential neutron spectrum between 1 and 10 Mev can be represented, within the resolution of the detector, by a power law N(E) = AE−n, where n = 1.17−0.20+0.12 near the transition maximum, n = 1.08−0.20+0.13 at 3- to 5-g/cm² atmospheric depth, and n at sea level is larger than these values and dependent on terrain. The spectral index remains the same to ±0.1 over the solar cycle at fast neutron energies. The fast neutron data are self-consistent to ±7% from 2 to 300 g/cm² over the range of cutoff rigidity and solar cycle variations. The characteristics of the fast neutrons as outlined here serve as a basis for checking a Monte Carlo calculation of the entire neutron distribution and its products.

Quiet-time electron increases: A measure of conditions in the outer solar system

One possible explanation for the increases in the intensity range of 3- to 12-Mev interplanetary electrons that McDonald et al. (1972) have labeled as ‘quiet-time electron increases’ is discussed. It is argued that the electrons in quiet-time increases are galactic in origin but that the observed increases are not the result of any variation in the modulation of these particles in the inner solar system. It is suggested instead that quiet-time increases may occur when more electrons than normal penetrate a modulating region that lies far beyond the orbit of the earth. The number of electrons penetrating this region may increase when field lines that have experienced an unusually large random walk in the photosphere are carried by the solar wind out to the region. As evidence of this increased random walk, it is shown that five solar rotations before most of the quiet-time increases occur there is an extended period when the amplitude of the diurnal anisotropy (measured by the Deep River neutron monitor) is relatively low. A delay time of five rotations implies that the proposed modulating region lies at ∼30 AU from the sun if the average solar-wind speed is assumed to be constant at ∼400 km/sec over this distance. The implications for the correlation between periods of low-amplitude diurnal anisotropy and quiet-time increases on interplanetary conditions out to ∼30 AU and some possible models for the proposed modulating region are also considered.

Variation of the anisotropy of the cosmic radiation within the sector structure of the interplanetary magnetic field

In a study of the diurnal-type variation, a wide-band filter has been applied to the pressure-corrected intensity recorded by the Deep River neutron monitor. The period December 1963 to February 1964 has been chosen for the analysis because IMP 1 provided information on the sector structure of the interplanetary field at this time.A superimposed epoch analysis of the filtered time series shows that there is a systematic change in the observed anisotropy through the sectors. If it is assumed that the variation is independent of energy across a sector, then the change can be explained by a streaming of the cosmic radiation away from the sun at the leading edge and towards it at the trailing edge. This interpretation is consistent with the observed distribution of field strength and solar wind velocity.

Solar modulation of galactic cosmic rays near solar minimum (1965)

The results of the continuous monitoring of the intensity of cosmic rays (of energy > 50 MeV) with identical G-M counter telescopes flown in satellites IMP I, II, and III and OGO-I are presented along with the differential spectrum studies obtained from balloon flights at Fort Churchill and from satellites. A comparison of the time behavior of the G-M counter data with Deep River neutron monitor data suggests the presence of a "hysteresis" type of behavior due to spectral changes occurring near solar minimum. The existence of this "hysteresis" suggests that the radial gradient of cosmic rays near the earth could be much smaller than the ~ 10%/AU obtained by O'Gallagher and Simpson (1967) and O'Gallagher (1967) at higher energies. The long-term intensity variation of cosmic rays seems to follow the Ap index rather closely in phase, in contrast to sunspot numbers which display a pronounced phase difference with cosmic-ray intensity. The differential spectra of protons and He nuclei have been analyzed in terms of two different models for the propagation in the interplanetary medium. The modulations indicated by the present data seem to disagree with a diffusion coefficient proportional to βR where β and R are the velocity and rigidity of the particle respectively (Jokipii 1966).

Analysis of balloon observations during the April 1960, solar cosmic-ray events

A series of balloon flights was conducted at Minneapolis, Minnesota, during the solar cosmic-ray events from April 1, 1960, through April 30, 1960. The largest event on April 1 was recorded by a balloon at high altitude, and for a power-law spectrum of the solar protons the exponent γ was determined to be 2.4. This spectrum, when extrapolated, agrees with the rates measured by the earth satellite Explorer 7 for the same event. It is shown that at Minneapolis a partial geomagnetic cutoff was in effect that limited the low-energy particles detected at balloon levels. Comparison of the simultaneous readings of the Pioneer 5 deep space probe confirms this picture. A relationship is established between the Deep River neutron monitor and various balloon instruments at high altitude, and the proportionality is determined during a large Forbush decrease. The event on April 28 was also detected at Minneapolis with balloons, and the data are consistent with time variations in the local geomagnetic cutoff. The paper includes many details of the construction of the ion chambers, counters, and telemetry system used for the balloon monitoring program at Minnesota.

Forbush decrease of the flux of heavy primary nuclei of cosmic rays on May 12 and July 12, 1959

The time variation of the flux of heavy nuclei with Z≥3 of primary cosmic radiation has been measured during seven balloon flights on May 6–12, 1959, and on July 10–12, 1959. Six of these flights were made from Minneapolis and one from Churchill, Canada. During a quiet day on May 6, 1959, the flux of these nuclei at the top of the atmosphere was 21.2±2.3 particles/m2·sec·sr. After the Forbush decrease on May 12, 1959, the value was 10.0±1.4 particles/m2·sec·sr, showing a reduction of 53±9 per cent of the primary heavy nuclei flux. On July 10–11, the flux of Z≥3 nuclei at the top of the atmosphere was measured as 17.5±1.7 particles/m2·sec·sr before the Forbush decrease and 10.0±1.8 particles/m2·sec·sr after the Forbush decrease on July 12. The reduction was 43±9 per cent during this event. The Deep River neutron monitor showed a reduction of counting rates of 12 and 7.5 per cent respectively during these two Forbush decrease events.