Solar Modulation Effects Research Articles

Constraints on cosmic-ray positron excess and average pulsar parameters

Recent, accurate ratio measurements in cosmic rays allow us to distinguish among different estimates of secondary positron production in the interstellar medium (ISM), provided the effect of solar modulation and solar polarity are properly taken into account. Data above a few GeV indicate that a possible extra component of positrons could be required in addition to the secondaries. This positron excess is compatible with the hypothesis of pair production at the polar cap of mature pulsars. Assuming only pulsar contributions without any exotic contributions such as dark-matter annihilation, the average parameters of Galactic pulsars contributing to positron and electron interstellar fluxes were obtained. These parameter values are found near the peak of the distributions of the observed characteristics of radio pulsars. The studied gamma-ray pulsar sample is too small to make any conclusion. The expected ratio from the PAMELA experiment currently in orbit is reported in this paper. The GLAST mission will allow us to double-check our findings about the role of pair production at the pulsar polar cap and outer gap.

Numerical simulations with the MCNPX and LAHET Code Systems compared with direct measurement of neutron fluxes in terrestrial environments

Neutron fluxes as a function of geomagnetic latitude were investigated using both the MCNPX and LAHET Code Systems (LCS) and compared with Southern Hemisphere measurements made during 1998–2001. Preliminary calculations for neutron fluxes near sea level at various geomagnetic latitudes are consistent with the measured values using a cosmic-ray flux slightly lower than the effective proton flux associated with nuclide production in lunar samples over thousands to millions of years. This lower neutron flux could be due to the strong solar modulation effect during the measurement period.

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.

A search for the signature of microquasars in the cosmic ray iron spectrum measured by TIGER

In a recent paper Heinz and Sunyaev suggest that relativistic jets observed in microquasars might result in narrow features in the energy spectra of heavy cosmic rays with ≈1 to ≈10 GeV/nuc. They further argue that such features might be observable if there has been one or more microquasars nearby within the last few million years. We report preliminary results of a search for evidence of such features using data from a 32-day balloon flight of the Trans-Iron Galactic Element Recorder (TIGER). Although this flight took place near solar maximum, calculations of the broadening effects of solar modulation indicate that a narrow feature of sufficient intensity should still be observable. An energy spectrum for iron with high statistical significance has been derived from ≈100,000 Fe events in the energy range from about 2.5 to 10 GeV/nuc. Although our preliminary results do not reveal any obvious features, we will discuss the possibility of observing such features with TIGER and other instruments.

Voyager 1 Explores the Termination Shock Region and the Heliosheath Beyond

Voyager 1 crossed the termination shock of the supersonic flow of the solar wind on 16 December 2004 at a distance of 94.01 astronomical units from the Sun, becoming the first spacecraft to begin exploring the heliosheath, the outermost layer of the heliosphere. The shock is a steady source of low-energy protons with an energy spectrum approximately E(-1.41 +/- 0.15) from 0.5 to approximately 3.5 megaelectron volts, consistent with a weak termination shock having a solar wind velocity jump ratio r=2.6(-0.2)(+0.4). However, in contradiction to many predictions, the intensity of anomalous cosmic ray (ACR) helium did not peak at the shock, indicating that the ACR source is not in the shock region local to Voyager 1. The intensities of approximately 10-megaelectron volt electrons, ACRs, and galactic cosmic rays have steadily increased since late 2004 as the effects of solar modulation have decreased.

Addressing solar modulation and long-term uncertainties in scaling secondary cosmic rays for in situ cosmogenic nuclide applications

Abstract Solar modulation affects the secondary cosmic rays responsible for in situ cosmogenic nuclide (CN) production the most at the high geomagnetic latitudes to which CN production rates are traditionally referenced. While this has long been recognized (e.g., D. Lal, B. Peters, Cosmic ray produced radioactivity on the Earth, in: K. Sitte (Ed.), Handbuch Der Physik XLVI/2, Springer-Verlag, Berlin, 1967, pp. 551–612 and D. Lal, Theoretically expected variations in the terrestrial cosmic ray production rates of isotopes, in: G.C. Castagnoli (Ed.), Proceedings of the Enrico Fermi International School of Physics 95, Italian Physical Society, Varenna 1988, pp. 216–233), these variations can lead to potentially significant scaling model uncertainties that have not been addressed in detail. These uncertainties include the long-term (millennial-scale) average solar modulation level to which secondary cosmic rays should be referenced, and short-term fluctuations in cosmic ray intensity measurements used to derive published secondary cosmic ray scaling models. We have developed new scaling models for spallogenic nucleons, slow-muon capture and fast-muon interactions that specifically address these uncertainties. Our spallogenic nucleon scaling model, which includes data from portions of 5 solar cycles, explicitly incorporates a measure of solar modulation ( S ), and our fast- and slow-muon scaling models (based on more limited data) account for solar modulation effects through increased uncertainties. These models improve on previously published models by better sampling the observed variability in measured cosmic ray intensities as a function of geomagnetic latitude, altitude, and solar activity. Furthermore, placing the spallogenic nucleon data in a consistent time-space framework allows for a more realistic assessment of uncertainties in our model than in earlier ones. We demonstrate here that our models reasonably account for the effects of solar modulation on measured secondary cosmic ray intensities, within the uncertainties of our combined source datasets. We also estimate solar modulation variations over the last 11.4 ka from a recent physics-based sunspot number reconstruction derived from tree-ring 14 C data. This approximation suggests that spallogenic nucleon scaling factors in our model for sea level and high geomagnetic latitudes can differ by up to ∼ 10%, depending on the time step over which the model sunspot numbers are averaged. The potential magnitude of this difference supports our contention that incorporating long-term solar modulation into secondary cosmic ray scaling is important. Although millennial-scale solar modulation clearly requires further study, we believe it is reasonable at present to use our S value record for scaling spallogenic nucleons during the last 11.4 ka, and the weighted mean S value for that period of 0.950 for longer exposure times. By accounting for solar modulation effects on the global variations in nucleon and muon fluxes, these models thus provide a useful framework on which to base CN production rate scaling functions.

Solar cycle modulation of the Southern Annular Mode

The effect of the 11‐year solar cycle modulation on the Southern Annular Mode (SAM) in the Southern Hemisphere is examined through analysis of observational data from 1968 to 2001. It is found that the year‐to‐year variability of the October–November mean SAM differs significantly according to the solar activity. In high solar activity years, the SAM signal extends to the upper stratosphere during October to December and activity in the troposphere lasts until autumn, whereas in low solar activity years, the SAM signal is confined almost inside the troposphere from October to December and it disappears by January. This situation is very similar to that observed for the modulation of the winter mean North Atlantic Oscillation in the Northern Hemisphere. A possible mechanism of solar cycle modulation of the SAM is also discussed.

Differences in the spectra of anomalous cosmic ray helium nuclei in two solar magnetic polarity cycles

We compare the spectra of anomalous He nuclei measured in the heliosphere by the IMP, Voyager, and Pioneer spacecraft at heliospheric distances ranging from 1 to over 80 AU in the time period from 1977 to 2003. Striking differences in the energy spectra of these nuclei are found between positive and negative solar magnetic polarity cycles. These differences are such that the spectra observed in negative polarity cycles appear to be greatly deficient in particles with energies of less than ∼15 MeV but have an excess intensity above >25 MeV, relative to the spectra observed in positive cycles. These spectral differences are complex and may be related to both solar modulation effects inside of the heliospheric termination shock as well as differences occurring at or near the termination shock itself. At the modulation minimum in 1998 when V2 was at a distance of 56 AU and V1 at 71 AU, a high intensity of low‐energy anomalous He particles was observed with a peak in the spectrum at ∼5 MeV/nuc. In late 2001 after the solar magnetic polarity had changed from positive to negative and the 11‐year solar modulation had increased, the previous high intensities of the anomalous He nuclei seen in 1998 were greatly reduced at both V1 and V2, and the energy of the peak intensity had moved up to ∼20–25 MeV/nuc. However, in both polarity periods the highest‐energy part of this spectrum above ∼30 MeV/nuc is similar with a slope of −2.8 ± 0.2. The details of these complex spectral changes are described in this paper.

Observation by an air-shower array in Tibet of the multi-TeV cosmic-ray anisotropy due to terrestrial orbital motion around the Sun.

We report on the solar diurnal variation of the galactic cosmic-ray intensity observed by the Tibet III air shower array during the period from 1999 to 2003. In the higher-energy event samples (12 and 6.2 TeV), the variations are fairly consistent with the Compton-Getting anisotropy due to the terrestrial orbital motion around the Sun, while the variation in the lower-energy event sample (4.0 TeV) is inconsistent with this anisotropy. This suggests an additional anisotropy superposed at the multi-TeV energies, e.g., the solar modulation effect. This is the highest-precision measurement of the Compton-Getting anisotropy ever made.

Precise measurements of the cosmic ray antiproton spectrum with BESS including the effects of solar modulation

The Balloon Borne Experiment with a Superconducting Spectrometer (BESS) has measured the energy spectrum of cosmic-ray antiprotons between 0.18 and 4.20 GeV in eight flights between 1993 and 2002. Above about 1 GeV, models in which antiprotons are secondary products of the interactions of primary cosmic rays with the interstellar gas agree with the BESS antiproton spectrum. Below 1 GeV, the data show a possible excess antiproton flux compared to secondary model predictions, suggesting the presence of an additional source of antiprotons. The antiproton/proton ratios measured between 1993 and 1999, during the Sun’s positive-polarity phase, are consistent with simple models of solar modulation. However, results from the 2000 flight, following the solar magnetic field reversal, show a sudden increase in the antiproton/proton ratio and tend to favor a charge-sign-dependent drift model. To extend BESS measurements to lower energies, an evolutionary instrument, BESS-Polar, is under construction for polar flight in 2004.

Production of cosmogenic Be nuclei in the Earth's atmosphere by cosmic rays: Its dependence on solar modulation and the interstellar cosmic ray spectrum

Recent work by McCracken [2001] shows that 10Be production rates by cosmic rays on the polar plateau are little affected by geomagnetic field changes in the last few hundred years. Also, the 10Be observed in ice cores on the polar plateau probably originated at high latitudes and precipitated to the Earth in about 1 year, according to McCracken. As a result of this assumption, ice core records of 10Be concentration extending back several hundred years, including the Maunder minimum, have the potential to study the solar modulation of cosmic rays on a time scale extending back several hundred years. These ice core records indicate that the 10Be concentration at the time of the Maunder minimum was ∼2.0 times what it was during recent sunspot minima in 1965 and 1976. We have examined 10Be production in the atmosphere using new data related to the interstellar cosmic ray spectrum and the effects of solar modulation as determined from Voyager spacecraft data in the outer heliosphere. We have used the FLUKA Monte Carlo program along with new cross‐section data to calculate the production of nucleons and 10Be nuclei in the atmosphere. These calculations show that 10Be temporal variations are sensitive indicators of low‐energy solar modulation. Our calculations of 10Be production are able to reproduce well the factor ∼1.5–2.0 change in 10Be observed in the ice core data as a result of the 11‐year solar modulation. We are also able to show that starting as recently as the sunspot minimum of 1954, the cosmic ray intensity at the Earth was higher than it was during more recent minima. The cosmic ray intensity during these minima time periods represents the residual modulation between the Earth and interstellar space. The 10Be measurements are consistant with the fact that given the interstellar cosmic ray spectrum used in this analysis, this residual modulation was small or zero at the time of the Maunder minimum.

Monitoring and forecasting of the geomagnetic and radiation storms during the 23 rd solar cycle: Aragats regional space weather center

The Solar Environment Center located on Mt. Aragats will provide real-time monitoring and forecasting of the extreme Geomagnetic and Radiation Events. Two neutron monitors, a muon telescope and a Solar scintillation telescope operating on Aragats research stations will provide accurate detection of very low fluxes of the most energetic Solar particles and modulation effects of Solar events. The correlation of the Solar Energetic Phenomena with modulations of the Galactic Cosmic Ray flux and direct detection of the high-energy protons and neutrons emitted during Coronal Mass Ejection provide firm grounds for specification and forecasting of the effects of tremendous explosions on the Solar surface. Consequently ground observation of the variations of Cosmic Ray fluxes make it possible to issue alerts to avoid imminent hazard of upcoming Geomagnetic and Radiation Storms.

Precise measurements of atmospheric muon fluxes with the BESS spectrometer

The vertical absolute fluxes of atmospheric muons and muon charge ratio have been measured precisely at different geomagnetic locations by using the BESS spectrometer. The observations had been performed at sea level (30 m above sea level) in Tsukuba, Japan, and at 360 m above sea level in Lynn Lake, Canada. The vertical cutoff rigidities in Tsukuba (36.2°N, 140.1°E) and in Lynn Lake (56.5°N, 101.0°W) are 11.4 and 0.4 GV, respectively. We have obtained vertical fluxes of positive and negative muons in a momentum range from 0.6 to 20 GeV/ c with systematic errors <3% in both measurements. By comparing the data collected at two different geomagnetic latitudes, we have seen an effect of cutoff rigidity. The dependence on the atmospheric pressure and temperature, and the solar modulation effect have been also clearly observed. We also clearly observed the decrease of charge ratio of muons at low momentum side with at higher cutoff rigidity region.

Cosmogenic radionuclides in the recently fallen Kobe (CK4) meteorite.

Cosmogenic radionuclides in the Kobe chondrite (CK4) fell on September 26th, 1999, have been measured immediately after the fall (21 hours) using a ultra low-background Ge-detector. Nineteen cosmogenic nuclides including the very short-lived 24Na were detected and their activities place constrains on the exposure history of the meteorite and reflect effect of solar modulation of galactic cosmic rays during solar maximum. Two nuclides, 28Mg and 43K were possibly detected for the first time and signals probably due to 56Ni and 57Ni were present although statistical uncertainties were very large. On the other hand, low activities of 60Co (∼1 dpm/kg) and 26Al (∼38 dpm/kg) mainly suggest a small preatmospheric size (≤10 cm) of the Kobe meteorite.

Voyager and Pioneer spacecraft measurements of cosmic ray intensities in the outer heliosphere: Toward a new paradigm for understanding the global modulation process: 2. Maximum solar modulation (1990–1991)

In this paper we present interplanetary radial intensity profiles of five particle energies/species of cosmic rays measured by the IMP, Voyager 1, Voyager 2, and Pioneer 10 spacecraft at the time of maximum solar modulation in 1990–1991. These radial profiles are then compared with those measured in 1987 at a period of minimum modulation in order to examine the radial dependence of the 11‐year solar modulation effects. It is found that the radial intensity profiles observed in 1990–1991 can be described to first order by simple reduction factors of the radial profiles observed in 1987. These reduction factors range from ∼2.5 at the highest rigidities to ∼10 at the lowest rigidities measured here. These reduction factors describe the magnitude of the 11‐year modulation and imply that the intensities at the estimated location of the termination shock (TS), here taken to be at 88 AU, are reduced by a corresponding factor to those at the Earth at each energy. Therefore these TS intensities range from only 0.4 to 0.05 of the estimated IS intensities at the highest to lowest rigidities, respectively. This implies that most of the overall (11‐year and residual) modulation at this time and at these rigidities is occurring beyond the TS. Some of this 11‐year modulation (∼50% or less) occurs within this location. This part of the modulation is usually described in the present modulation models by invoking changing heliospheric current sheet tilt and changes in the drift and diffusion coefficients. However, overall, the 11‐year solar modulation, at least at the rigidities examined here, is most properly described as an outer heliospheric modulation phenomenon occurring mainly at or beyond the TS with a secondary (weaker) modulation occurring inside the TS. This is similar to the situation described by Webber and Lockwood [this issue], where the dominant modulation at times of minimum (residual) solar modulation also appears to occur in the outer heliosphere near to or beyond the TS. This large outer heliosphere modulation remains largely unexplained in present modulation theories. A new paradigm in solar modulation theory is required to more fully account for the large fraction of the 11‐year and/or residual solar modulation that is occurring in the outer heliosphere near to or beyond the TS.

Solar Activity and Regional Climate

We performed a statistical analysis of the data on summer temperature anomalies in northern Fennoscandia (8–1995 AD) and found that a 70–130-yr cycle is present in this series during most of the time period. A comparison of the reconstructed northern Fennoscandia temperature with different indicators of solar activity (Wolf numbers, the length of solar Schwabe cycle, extended bi-decadal radiocarbon series, and data on sunspots observed by naked eye) shows that the more probable cause of the periodicity is the modulation of regional northern Fennoscandia climate by the long-term solar cycle of Gleissberg. The effect of this century-scale solar modulation of the global Northern Hemisphere temperature is weaker.

Illuminating Dark Matter and Primordial Black Holes with Interstellar Antiprotons

Interstellar antiproton fluxes can arise from dark matter annihilating or decaying into quarks or gluons that subsequently fragment into antiprotons. Evaporation of primordial black holes also can produce a significant antiproton cosmic-ray flux. Since the background of secondary antiprotons from spallation has an interstellar energy spectrum that peaks at $\sim 2\gev$ and falls rapidly for energies below this, low-energy measurements of cosmic antiprotons are useful in the search for exotic antiproton sources. However, measurement of the flux near the earth is challenged by significant uncertainties from the effects of the solar wind. We suggest evading this problem and more effectively probing dark-matter signals by placing an antiproton spectrometer aboard an interstellar probe currently under discussion. We address the experimental challenges of a light, low-power-consuming detector, and present an initial design of such an instrument. This experimental effort could significantly increase our ability to detect, and have confidence in, a signal of exotic, nonstandard antiproton sources. Furthermore, solar modulation effects in the heliosphere would be better quantified and understood by comparing results to inverse modulated data derived from existing balloon and space-based detectors near the earth.

A New Estimate of the Local Interstellar Energy Density and Ionization Rate of Galactic Cosmic Cosmic Rays

Using data from the Voyager and Pioneer spacecraft at distances up to 60 AU from the Sun to help define the effects of solar modulation, we have made a new estimate of the local interstellar cosmic-ray spectra and the associated energy density and ionization rate. The energy density, including the low-energy electron contribution, is ~1.80 eV cm-3, equivalent energetically to that in a 7 μG magnetic field. The ionization rate from this same cosmic-ray population, including a significant contribution from low-energy electrons, is (3-4) × 10-17 s-1 (H atom). This is most likely a minimum value and is not too different from estimates of the typical ionization rate required to heat the interstellar medium and maintain the diffuse ionized gas range from (3-30) × 10-17 s-1 (H atom) as a function of locale in the Galaxy. This suggests that cosmic rays could be a significant contributor to interstellar heating. The cosmic-ray ionization rate will be more or less uniform throughout the Galactic disk and also perpendicular to the disk according to the scale height of cosmic rays which may be ~1 kpc, thus providing a source of ionization at high Z values. The recent observation of low-energy particle acceleration at the heliospheric termination shock, which produces local ionization rates comparable to those due to Galactic cosmic rays, raises the interesting possibility that "hot spots," where the ionization rate due to locally accelerated particles may exceed 10-16 s-1 (H atom), may occur near massive stars with more powerful stellar winds.

Boron cosmochemistry. Part II: Boron nucleosynthesis and condensation temperature

Abstract— The new B solar‐system abundance calculated by Zhai and Shaw (1994), 16.9 atoms/106 Si (or 606 atoms/1012 H) is used to reevaluate the different possibilities of LiBeB (except 7Li) nucleosynthesis. The revised abundances support two models: (1) Light elements were formed by continual bombardment of interstellar medium (ISM) by galactic cosmic rays (GCRs), but these galactic cosmic rays should contain a very intense low‐energy component, in the form of E−5 which cannot be observed near the Earth due to solar modulation effects; (2) Light elements are a mixture of two sources. In the first source, light elements were synthesized by continual bombardment of interstellar medium by galactic cosmic rays. In the second source, they were made by the interactions of C and O nuclei ejected from supernovae with the H and He in the surrounding gas. The first source constitutes ∼46% of total B.The Si‐normalized and CI‐meteorite‐normalized abundances of common and volatile elements in carbonaceous chondrites show a linear correlation with their condensation temperatures. Using this relationship and the normalized B abundances in CM, CO, and CV meteorites, we can estimate the B condensation temperature to be ∼910 K, which is similar to Ga.

Observations of cosmic-ray electrons and positrons using an imaging calorimeter

A ballon-borne magnet spectrometer system was flown for 5.5 hr at an altitude of more than 117,00 feet from Prince Albert, Saskatchewan (Canada), on 1989 September 5, when the Newark neutron monitor rate was 2952. The instrument was a modified version of the one used to observe antiprotons in 1979. The most significant modification was the addition of an imaging calorimeter, 7.33 radiation lengths thick. Inclusion of the calorimeter has significantly improved the ability to distinguish electrons and positrons from the other constituents of the cosmic rays. The absolute electron flux has been determined in the energy interval 1.3-26 GeV. The electron spectrum at the top of the atmosphere was found to be J<SUB>e-</SUB> = 177E<SUP>-(3.15+/-0.13</SUP>) electrons/ sq m/(sr s GeV) in the energy range 4.0-26 GeV. Below 4 GeV, the spectrum showed flattening, which is consistent with the effect of solar modulation. The e(+)/(e(+)+e(-)) ratio was found to be (0.11 +/- 0.03) in the energy range 5.2-13 GeV.