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.     

Detection of solar neutrons by ground-based neutron monitor

A review is presented of solar neutron observation by ground-based neutron monitors (NM), focusing on the five solar neutron events of 1980 June 7, 1980 June 21, 1980 November 6, 1982 November 26, and 1984 April 25 observed by the Tokyo NM. These events are analyzed by comparison with the time profiles of gamma-rays observed by the Gamma-Ray Spectrometer (GRS) on the Solar Maximum Mission (SMM) satellite and with the enhancements of counting rate observed at various NM stations in the solar neutron event of 1982 June 3.The energy range of solar neutrons observed by the NM is estimated in each event, based on some simple assumptions, using the gamma-ray data from the GRS and decay proton data from the ISEE-3 spacecraft. It is shown that these enhancements can be almost completely explained by the continuous emission of solar neutrons for several minutes at the flare. Finally, the effective detection and the newly found possibility to predict, in the short term, the occurrence time of a solar neutron event, and the plans for observation of solar neutrons by the ground-based NM stations are presented.     

Integration of Neutron Monitor Data with Spacecraft Observations: a Historical Perspective

Beginning in the early 1950s, data from neutron monitors placed the taxonomy of cosmic ray temporal variations on a firm footing, extended the observations of the Sun as a transient source of high energy particles and laid the foundation of our early concepts of a heliosphere. The first major impact of the arrival of the Space Age in 1957 on our understanding of cosmic rays came from spacecraft operating beyond the confines of our magnetosphere. These new observations showed that Forbush decreases were caused by interplanetary disturbances and not by changes in the geomagnetic field; the existence of both the predicted solar wind and interplanetary magnetic field was confirmed; the Sun was revealed as a frequent source of energetic ions and electrons in the 10–100 MeV range; and a number of new, low-energy particle populations was discovered. Neutron monitor data were of great value in interpreting many of these new results. With the launch of IMP 6 in 1971, followed by a number of other spacecraft, long-term monitoring of low and medium energy galactic and anomalous cosmic rays and solar and interplanetary energetic particles, and the interplanetary medium were available on a continuous basis. Many synoptic studies have been carried out using both neutron monitor and space observations. The data from the Pioneer 10/11 and Voyagers 1/2 deep space missions and the journey of Ulysses over the region of the solar poles have significantly extended our knowledge of the heliosphere and have provided enhanced understanding of many effects that were first identified in the neutron monitor data. Solar observations are a special area of space studies that has had great impact on interpreting results from neutron monitors, in particular the identification of coronal holes as the source of high-speed solar wind streams and the recognition of the importance of coronal mass ejections in producing interplanetary disturbances and accelerating solar energetic particles. In the future, with the new emphasis on carefully intercalibrated networks of neutron monitors and the improved instrumentation for space studies, these symbionic relations should prove to be even more productive in extending our understanding of the acceleration and transport of energetic particles in our heliosphere. This revised version was published online in August 2006 with corrections to the Cover Date.     

Public Access to Neutron Monitor Datasets

Over the last few years, great strides have been made in providing access to data, both archival and near-real-time, for researchers throughout the field of Space Science. Neutron monitor data, in particular, has for many decades enjoyed a unique history of world-wide collaborative efforts and the unrestricted sharing of datasets among researchers. This is in large part due to the nature of the measurements made by neutron monitors; an understanding of the time-varying, anisotropic galactic or solar cosmic ray spectrum in most cases requires that data from a large array of stations needs to be considered, and often that array must be global in scope. This paper will attempt to summarize the current availability of neutron monitor data, by (a) describing the current status of archival data and near-real-time data access to neutron monitor data, and (b) looking into the future, with an emphasis on the use of the World Wide Web and other electronic means as the source mechanism. Public outreach efforts using active neutron monitors will also be discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Neutron measurements in space

The experimental measurements of the neutron flux and energy spectrum in space since 1964 are reviewed and related to the theoretical predictions. A discussion of the neutron sources is presented. The difficulties associated with neutron measurements of both the atmospheric neutron leakage flux and solar neutrons are included. Particular emphasis is placed upon the neutron leakage flux and energy measurements at energies greater than about 1 MeV. The possibilities of CRAND as a source for the energetic trapped protons are discussed in light of recent measurements of the 10–100 MeV neutron flux. The current status of the solar neutron flux observations is also presented.The primary purposes of neutron measurements in space have been to determine the neutron leakage flux from the atmosphere of the Earth and the solar neutron flux. As a consequence of the inefficient methods for neutron detection and the difficulties of conducting the measurements in the presence of the galactic and solar cosmic-ray backgrounds, the experimental results are very conflicting. It is the purpose of this review to interpret and discuss recent neutron measurements. In order to understand these results the theoretical predictions of the neutron fluxes and energy spectra from possible neutron sources will be briefly presented. Since comparisons of the different neutron measurements depend critically upon the experimental techniques, we will briefly discuss neutron detection methods applicable to space measurements. The emphasis will be upon measurements since 1964 made outside the Earth's atmosphere, but considerable reference will be made to high energy neutron experiments conducted within the Earth's atmosphere at < 10g cm^-2 altitude. A review of earlier neutron measurements of terrestrial and solar neutrons has been made by Haymes (1965).     

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.     

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).     

Possible evidence for a rigidity-dependent release of relativistic protons from the solar corona

A model of the time evolving relativistic solar proton spectra for the 7 May 1978 ground level solar cosmic ray event is presented. This event, with associated cosmic ray neutron monitor increases of over 100% and containing relativistic particles with energies greater than 10 GeV/nucleon was characterized by an extreme anisotropy and a rapidly evolving spectrum, particularly during the initial phase. The observational data from cosmic ray neutron monitors viewing in the anti-Sun direction (180° away from the initial solar particle direction) indicates that a back scatter pulse of 4% of the primary pulse was observed at the Earth 20 min after the event onset. Previous attempts to model the solar particle spectrum found consistent and systematic differences between the theoretically calculated cosmic ray increase and the actual increase as observed by neutron monitors. In order to reconcile these differences, we have concluded that the observational data give evidence for a rigidity dependent release of relativistic solar protons from the solar corona during the very early stages of this event.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

Anomalous and Galactic Cosmic Rays at 1 AU During the Cycle 23/24 Solar Minimum

Anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum generally track galactic cosmic ray (GCR) intensities such as those measured by neutron monitors, albeit with differences between solar polarity cycles. The unusual cycle 23/24 solar minimum was long-lasting with very low sunspot numbers and significantly reduced interplanetary magnetic field strength and solar wind dynamic pressure and turbulence, but also featured a heliospheric current sheet tilt that remained high for an extended period. Peak ACR intensities did not recover to the maximum values reached during the last two A>0 solar minima and just barely reached the last A<0 levels. However, GCR intensities in 2009 (neutron monitor rates and also at ～200 MeV/nucleon) were the highest recorded during the last 50 years, indicating their intensities were not as heavily modulated during their transport from the outer heliosphere. This unexpected difference in the behavior of ACRs and GCRs remains unexplained, but suggests that either the ACR source intensity may have weakened since the last A<0 epoch, or perhaps that ACR intensities at 1 AU in the ecliptic may be more sensitive than GCRs to the higher tilt angle. This seems plausible if the ACR source intensity is greater at low latitudes during A<0 cycles, while the GCR distribution at the heliospheric boundary is more uniform in latitude. Shortly after an abrupt increase in the current sheet tilt angle in late 2009, both ACR and GCR intensities showed dramatic decreases, marking the end of solar minimum modulation conditions for this cycle.     

Energetic Particles and Corotating Interaction Regions in the Solar Wind

This paper reviews three important effects on energetic particles of corotating interaction regions (CIRs) in the solar wind that are formed at the leading edges of high-speed solar wind streams originating in coronal holes. A brief overview of CIRs and their important features is followed by a discussion of CIR-associated modulations in the galactic cosmic ray intensity, with an emphasis on observations made by spacecraft particle telescope ‘anti-coincidence’ guards. Such guards combine high counting rates (hundreds of counts/s) and a lower rigidity response than neutron monitors to provide detailed information on the relationship between cosmic ray modulations and CIR structure. The modulation of Jovian electrons by CIRs is then described. Finally, the acceleration of ions to energies of ～20 MeV/n in the vicinity of CIRs is reviewed.     

Cosmogenic Radionuclides as an Extension of the Neutron Monitor Era into the Past: Potential and Limitations

The cosmogenic radionuclides, ¹⁰Be, ¹⁴C and others, provide a record of the paleo-cosmic radiation that extends >10,000 years into the past. They are the only quantitative means at our disposal to study the heliosphere prior to the commencement of routine sunspot observations in the 17th century. The cosmogenic radionuclides are primarily produced by secondary neutrons generated by the galactic cosmic radiation, and can be regarded, in a sense, as providing an extrapolation of the neutron monitor era into the past. However, their characteristics are quite different from the man-made neutron monitor in several important respects: (1) they are sensitive to somewhat lower cosmic ray energies; (2) their temporal resolution is ～1 to 2 years, being determined by the rapidity with which they are sequestered in ice, biological, or other archives; (3) the statistical precision for annual data is very poor (～19%); however it is quite adequate (～5% for 22-year averages) to study the large variations (±40%) that have occurred in the paleo-cosmic ray record in the past between grand solar minima and maxima. The data contains “noise” caused by local meteorological effects, and longer-term climate effects, and the use of principal component analysis to separate these “system” effects from production effects is outlined. The concentrations of ¹⁰Be decreased by a factor of two at the commencement of Holocene, the present-day “interglacial”, due to a 100% increase in the ice accumulation rates in polar regions. The use of the ¹⁰Be flux to study heliospheric properties during the last glacial is discussed briefly.     

Modulation Near Solar Maximum at High Solar Latitudes Observations From the Ulysses Cospin High Energy Telescope

The three-dimensional structure of the solar maximum modulation of cosmic rays in the heliosphere can be studied for the first time by comparing observations from Ulysses at high solar latitudes to those from in-ecliptic spacecraft, such as IMP-8. Observations through mid-2000 show that changes in modulation remain well correlated at Earth and Ulysses up to latitudes of ∼60° south. The observed changes seem to be best correlated with changes in the inclination of the heliospheric current sheet. The spectral index of the proton spectra at energies <100 MeV in the ecliptic and at high latitudes remain roughly consistent with the T ⁺¹ spectrum expected from modulation models, while the spectral index of the helium spectrum at both locations has changed smoothly from the flat or even negative index spectra characteristic of anomalous component fluxes toward the T ⁺¹ galactic spectrum with increasing modulation. Intensities near the equator and at high latitude remain nearly equal, and latitudinal gradients for nucleonic cosmic rays thus remain small (<1% deg⁻¹) at solar maximum. In the most recent data fluxes of protons and helium with energies less than ∼100 MeV nucl⁻¹ measured by Ulysses are smaller than those measured at IMP-8, suggesting that the gradients may have switched to become negative toward the poles even before a clear reversal of polarity for the solar magnetic dipole has been completed. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Cosmic Ray Nucleonic Component: The Invention and Scientific Uses of the Neutron Monitor – (Keynote Lecture)

The invention of the neutron monitor pile for the study of cosmic-ray intensity-time and energy changes began with the discovery in 1948 that the nucleonic component cascade in the atmosphere had a huge geomagnetic latitude dependence. For example, between 0° and 60° this dependence was a ∼ 200–400% effect – depending on altitude – thus opening the opportunity to measure the intensity changes in the arriving cosmic-ray nuclei down to ∼1–2 GeV nucl⁻¹ for the first time. In these measurements the fast (high energy) neutron intensity was shown to be a surrogate for the nuclear cascade intensity in the atmosphere. The development of the neutron monitor in 1948–1951 and the first geomagnetic latitude network will be discussed. Among its early applications were: (1) to prove that there exists interplanetary solar modulation of galactic cosmic-rays (1952), and; (2) to provide the evidence for a dynamical heliosphere (1956). With the world-wide distribution of neutron monitor stations that are presently operating (∼ 50) many novel investigations are still to be carried out, especially in collaborations with spacecraft experiments. 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.     

A Brief History of Recurrent Solar Modulation of the Galactic Cosmic Rays (1937-1990)

This brief review of the pre-Ulysses era begins with the first measurements by ionization chambers in 1937 of a cosmic ray 27-day intensity variation that was believed to have its origin in recurrent variations of the geomagnetic field. However, with the introduction of neutron monitor analysis of the nucleonic component, it was shown in the 1940s and 1950s that this cosmic ray intensity variation arose from interplanetary dynamical phenomena. Beginning in the 1960s direct spacecraft investigations in the heliosphere with Pioneer-10, Pioneer-11, Voyager-1 and Voyager-2 proved that Corotating Interaction Regions were not only the source of the cosmic ray recurrent intensity modulation, but also the source of charged particles accelerated in corotating forward and reverse shocks associated with the corotating interaction regions.These early investigations, confined to low latitudes, have contributed to the understanding of solar phenomena, interplanetary dynamics, charge particle acceleration and the Sun-Earth convection.     

A cosmochemical view of cosmic rays and solar particles

The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above 15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z > 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.Miller Institute Professor, 1972–73.     

Relativistic solar proton events

Energetic solar flare particles contain rich information concerning mechanisms of particle acceleration on the Sun and subsequent transport through turbulent interplanetary space. Even the most energetic particles, in particular protons with kinetic energy above 500 MeV, may undergo coronal and interplanetary propagation effects, disturbing their accelerated injection spectrum after release from the solar flare. Relativistic solar proton events are recorded by neutron monitors at ground level. A detailed knowledge of the response of these ground-based detectors to the impact by a beam of protons on the top of the atmosphere is required to analyze these observations. The spectral index of arriving protons can be obtained from the response of the world-wide network of neutron monitors provided their directional anisotropy is known. The spectral index may also by determined from the relative enhancements in count rates of two similar detectors at different altitudes but similar asymptotic cones of acceptances, or from the relative enhancements of two detectors with different spectral sensitivities but at the same location of high latitude. Ground level enhancements from solar flare protons have been recorded at Sanae, Antarctica, since 1971 by two neutron monitors with different sensitivities to primary protons in the rigidity range from 1 GV to 5 GV. Spectral indexes of about 20 of these more energetic solar flare proton events have been determined from the two detector enhancements recorded at Sanae. These indexes do not show any increase (softening of the relativistic proton spectra) with increasing heliolongitude away from the preferred IMF connection region as was obtained for 20–80 MeV protons. Furthermore, most of the enhanced count rates show fluctuations larger than statistical, indicative of propagation in a mostly turbulent interplanetary magnetic field.     

Cosmic Rays and Earth – A Summary

Cosmic rays provide a diagnostic tool to analyze processes in interplanetary space and at the Sun. Cosmic rays also directly affect the terrestrial environment and serve as indicators of solar variability and non anthropogenic climatic changes on Earth at present and in the distant past. After the invention of the neutron monitor by John A. Simpson in 1948, an international network of cosmic ray detectors developed in a cooperative effort to examine temporal and spatial variations in our space environment. The resulting datasets represent the longest continuous, high time resolution series of particle radiation measurement in space science. At present, the neutron monitor network is complemented by spacecraft instrumentation to study solar-terrestrial correlated phenomena. This revised version was published online in August 2006 with corrections to the Cover Date.     

Martian Radiation EnvIronment Experiment (MARIE)

Measurements of radiation levels at Mars including the contributions of protons, neutrons, and heavy ions, are pre-requisites for human exploration. The MARIE experiment on the Mars-01 Odyssey spacecraft consists of a spectrometer to make such measurements in Mars orbit. MARIE is measuring the galactic cosmic ray energy spectra during the maximum of the 24^th solar cycle, and studying the dynamics of solar particle events and their radial dependence in orbit of Mars. The MARIE spectrometer is designed to measure the energy spectrum from 15 to 500 MeV/n, and when combined other space based instruments, such as the Advanced Composition Explorer (ACE), would provide accurate GCR spectra. Similarly, observations of solar energetic particles can be combined with observations at different points in the inner heliosphere from, for example, the Solar Heliospheric Observatory (SOHO), to gain information on the propagation and radial dependence in the Earth-Mars space. Measurements can be compared with the best available radiation environment and transport models in order to improve these models for subsequent use, and to provide key inputs for the engineering of spacecraft to better protect the human crews exploring Mars.     

Muon Observations

Muon observations are complementary to neutron monitor observations but there are some important differences in the two techniques. Unlike neutron monitors, muon telescope systems use coincidence techniques to obtain directional information about the arriving particle. Neutron monitor observations require simple corrections for pressure variations to compensate for the varying mass of atmospheric absorber over a site. In contrast, muon observations require additional corrections for the positive and negative temperature effects. Muon observations commenced many years before neutron monitors were constructed. Thus, muon data over a larger number of solar cycles is available to study solar modulation on anisotropies and other cosmic ray variations. The solar diurnal and semi-diurnal variations have been studied for many years. Using the techniques of Bieber and Chen it has been possible to derive the radial gradient, parallel mean-free path and symmetric latitude gradient of cosmic rays for rigidities <200 GV. The radial gradient varies with the 11-year solar activity cycle whereas the parallel mean-free path appears to vary with the 22-year solar magnetic cycle. The symmetric latitudinal gradient reverses at each solar polarity reversal. These results are in general agreement with predictions from modulation models. In undertaking these analyses the ratio of the parallel to perpendicular mean-free path must be assumed. There is strong contention in the literature about the correct value to employ but the results are sufficiently robust for this to be, at most, a minor problem. An asymmetric latitude gradient of highly variable nature has been found. These observations do not support current modulation models. Our view of the sidereal variation has undergone a revolution in recent times. Nagashima, Fujimoto and Jacklyn proposed a narrow Tail-In source anisotropy and separate Loss-Cone anisotropy as being responsible for the observed variations. A new analysis technique, more amenable to such structures, was developed by Japanese and Australian researchers. They confirmed the existence of the two anisotropies. However, they found that the Tail-In anisotropy is asymmetric and that both anisotropies had different positions from the prediction. Most 27-day modulations are observed at neutron monitor rigidities but not so readily at higher rigidities. An exception to this is the Isotropic Intensity Wave modulation observed in the early 1980s and again in 1991. This modulation is very strongly related to the heliospheric sector structure and implies a significantly different cosmic ray density on either side of the neutral sheet. The interpretation of most cosmic ray modulation phenomena requires good latitude coverage in both hemispheres. The closure of many muon observatories is a matter of concern. In the northern hemisphere a few new instruments are being constructed and spatial coverage is barely adequate. In the southern hemisphere the situation is far worse with the possibility that within a decade only the Mawson observatory in Antarctica will still be in operation. This revised version was published online in August 2006 with corrections to the Cover Date.