Bruno SEP MEASUREMENTS IN LEO BY THE PAMELA EXPERIMENT Dr. Alessandro Bruno - INFN and University of Bari, Italy

Fig1: Time variations in the intensity (1.57 - 5.70 GV) of protons, He nuclei, electrons and

positrons, during the Forbush Decrease event associated with the 13 Dec 2006 CME [5].

[1] O. Adrian et al. (2015), ApJ 801 L3.

The PAMELA [1] and the AMS-02 [2] space experiments represent the state-of-the-art of the investigation of the charged Cosmic-Ray (CR) radiation in the near-Earth environment. The former was launched into a semi-polar (70 deg inclination) and elliptical (350–610 km altitude) orbit on June 2006 onboard of the Resurs-DK1 Russian satellite; the latter was installed in May 2011 on the ISS. Both the instruments are composed by several subdetectors, with the core constituted by a magnetic spectrometer, providing accurate particle identification and rigidity measurement. While the mission temporal coverage and geometric factor are limited in comparison to ground-based detectors, PAMELA and AMS-02 are able to directly measure the spectral shape and the composition of CR fluxes. The high-precision data collected at low energies are significantly improving our understanding of the solar modulation effects on CRs, allowing the investigation of the long- and short-term CR variations between solar cycles 23 and 24 [3]. In particular, PAMELA measured the temporal evolution of different CR species (p, He, e-, e+), founding evidence of particle charge-sign dependent modulation effects. In addition, PAMELA is providing comprehensive observations of SEP events during the solar cycles 23 and 24, including energetic spectra and pitch angle distributions in a wide interval (>80 MeV), bridging the low energy data by in-situ spacecrafts and the GLE data by the worldwide network of neutron monitors. Major PAMELA’s results include the first direct evidence of magnetosheath effects on SEPs [4]. Similar results are being achieved by AM-02 experiment at relatively higher energies, due to the higher geomagnetic cutoff related to the ISS orbit. Space- and ground-based measurements can be combined with data from NMs in order to model the directional distribution of solar events, estimating the omnidirectional density and weighted anisotropy. Finally, PAMELA and AMS-02 are performing detailed observations of geomagnetic storms and Forbush Decrease (FD) effects induced by CME events. Complementing the integrated fluxes measured by NMs, PAMELA and AMS-02 provide information on the dependency of FD effects on particle composition and energy. As an example, Fig.1 reports the variations in the intensity of the different CR species (1.57 - 5.70 GV), during the FD event associated with the 13 December 2006 CME [5].

[1] O. Adriani et al. (2014), Physics Reports, 544, 4, 323–370.
[2] M. Aguilar et al. (2013), Phys. Rev. Lett., 110, 141102.
[3] M. Boezio et al. (2015), Proc. 34rd Intl. Cosmic Ray Conf., PoS(ICRC2015)037.
[4] O. Adrian et al. (2015), ApJ 801 L3.
[5] M. Mergé et al. (2013), Proc. 33rd Intl. Cosmic Ray Conf., 1215.
Paul will consolidate the below contributions


Marek Zreda
Cosmic-ray neutron method for measuring area-average soil moisture

Marek Zreda, U. Arizona

Rationale for development

Soil moisture is the most important part of the water and energy cycle (Fig. 1). It plays a critical role in weather and seasonal climate forecasting, and in linking water, energy, and biogeochemical cycles over land. It should be measured at the scale that is useful for land-surface processes and hydrology (100 m – 1 km). Conventional methods measure soil moisture either at a point (eg, time-domain reflectometry) or over large areas (eg, satellite microwave instruments). They have to be upscaled or downscaled, respectively, to provide data at the useful scale, which is impractical and unreliable. The recently developed cosmic-ray method (Zreda et al., 2008, 2012; Desilets et al., 2010) has a hectometer footprint (Desilets and Zreda, 2013; Köhli et al., 2015), and is, therefore a good scale integrator of soil moisture for land-surface and hydrological studies.

The cosmic-ray method (Zreda et al., 2008, 2012; Desilets et al., 2010) takes advantage of the extraordinary sensitivity of cosmogenic low-energy, moderated neutrons of energy between 1 eV and 1000 eV to hydrogen present in materials at the land surface (Fig. 1). Most neutrons on earth are cosmogenic. Primary cosmic-ray protons collide with atmospheric nuclei and unleash cascades of energetic secondary neutrons that interact with terrestrial nuclei and produce fast (evaporation) neutrons at the land surface. The fast neutrons that are produced in air and soil travel in all directions within the air-soil-vegetation continuum, and in this way an equilibrium concentration of neutrons is established. The equilibrium is shifted in response to changes in the water present above and below the land surface, for example in soil. Adding water to soil results in more efficient moderation of neutrons by the soil, causing a decrease of fast neutron intensity above the soil surface, where the measurement is made. Removing water from the soil has the opposite effect. The resultant neutron intensity above the land surface is inversely proportional to soil water content (Zreda et al., 2008, 2012).

Low-energy cosmogenic neutrons are measured using proportional counters (Knoll, 2000), which are sensitive to thermal neutrons (median energy of 0.025 eV), shielded by a layer of plastic that shifts the energy sensitivity of the counter to neutrons of the desired energy (>1 eV). The cosmic-ray probe (Fig. 2) is powered using a solar panel paired with a rechargeable battery, and is equipped with an Iridium satellite modem or a cellular modem for real-time telemetry. It can be operated almost anywhere in the world, except areas with insufficient day light. A stationary neutron probe of this type is implemented in the Cosmic-ray Soil Moisture Observing System (Fig. 3), or COSMOS (Zreda et al., 2012; cosmos.hwr.arizona.edu); therefore, it is sometimes called “COSMOS probe”. A mobile COSMOS detector is a bigger version of the stationary probe, additionally equipped with a GPS system (Chrisman and Zreda, 2013).

The measured neutron, normalized for variations in pressure, humidity and incoming neutron intensity (Zreda et al., 2012), is converted to soil moisture using the response function, such as that developed by Desilets et al. (2010). Other local measurements needed for the conversion are atmospheric pressure, temperature and water vapor. Additionally, the knowledge of temporal variations of the intensity of high-energy cascade neutrons is necessary to assess the strength of the source function for low-energy evaporation neutrons. Those data are generated from measurements with neutron monitors. The feasibility of soil-moisture monitoring using the cosmic-ray method relies on the availability of real-time neutron monitor data.

Most of the work on the development and applications of the cosmic-ray soil moisture method was funded by the US National Science Foundation (NSF). The development work was funded through grants from the Hydrology program between 2001 and 2010 (grants EAR-0126241, EAR-0636110). The installation of COSMOS network in 2009-2013 was supported by a grant from the Mid-size Infrastructure program administerd by the Atmospheric Sciences program (grant ATM-0838491). Additional relevant work on cosmic-ray neutron variations in space and time was funded by the Geochemistry program between 1999 and 2004 (grants EAR-0001191 and EAR-0126209).

Chrisman B., and Zreda M. (2013): Quantifying mesoscale soil moisture with the cosmic-ray rover. Hydrology and Earth System Sciences 17, 5097-5108.

Desilets D. et al., 2010. Nature's neutron probe: Land-surface hydrology at an elusive scale with cosmic rays. Water Resources Research 46, W11505.

Desilets D., and Zreda M., 2013. Footprint diameter for a cosmic-ray soil moisture probe: Theory and Monte Carlo simulations. Water Resources Research 49, 3566-3575.

Knoll, G.F., 2000. Radiation detection and measurement. Wiley, New York, 802 p.

Köhli M. et al., 2015. Footprint characteristics revised for field-scale soil moisture monitoring with cosmic-ray neutrons. Water Resources Research 51, 5772–5790.

Zreda M. et al., 2008. Measuring soil moisture content non-invasively at intermediate spatial scale using cosmic-ray neutrons. Geophysical Research Letters 35, L21402.

Zreda, M. et al., 2012. COSMOS: the COsmic-ray Soil Moisture Observing System. Hydrology and Earth System Sciences 16, 4079-4099.

Figure 1. Left: Water and energy mass balance at the land surface; red labels indicate pools and fluxes of water measurable using cosmic-ray neutrons. Right: Cosmic-ray neutron interactions with air and soil. Tracks of two neutrons are shown in the lower panel. Neutron n1 was absorbed in soil and removed from the pool of neutrons measurable by cosmic-ray probe above the surface; neutron n2 went back to the atmosphere and is measurable there. These tracks are copied onto the left panel (red lines).

Figure 2. Cosmic-ray soil moisture probe installed at Marshall Lake, Colorado, USA. For description of the components, see Fig. 9 in Zreda et al. (2012).

Contributed by: Kyle Copeland, Ph.D.*

*U.S. Federal Aviation Administration, Civil Aerospace Medical Institute, Aerospace Medical Research Division, Protection and Survival Research Laboratory, Numerical Sciences Research Team. Mail Route AAM-630, 6500 S. MacArthur Blvd. Oklahoma City, OK 73169, USA.

Background

Ionizing radiation exposure is an unavoidable part of daily life. Even our bodies are radioactive, due to the radioactive chemicals from which we are made. Ionizing radiation dose in humans is usually reported in gray (Gy) or sievert (Sv). The gray is the unit of absorbed dose, the average energy per unit mass deposited by the radiation. Deterministic effects of radiation exposure, i.e., those effects which are caused by cell killing of large numbers of cells, are traditionally considered in terms of gray. The sievert is the name used when the absorbed dose is weighted for health effects which are probabilistic in nature, i.e., stochastic effects. This weighting can be based on type of radiation, the tissue being irradiated, or both. The most common use of sievert is the quantity effective dose, which attempts to weight all radiation exposures with respect to cancer inductions, genetic effects, and length of life lost, and quality of remaining life (ICRP, 2007).

Figure 1, available from the Japanese Ministry of Education, Culture, Sports, Sciences and Technology (MEXT) shows typical ionizing radiation doses from activities that occur in daily life. The global average dose per person from natural sources is about 2.4 mSv (or 2400 Sv: 1000 Sv = 1 mSv) each year, just from living on Earth, with most of the dose coming from radon exposure; for the U.S., the estimate is 3.1 mSv (National Council on Radiation Protection and Measurements [NCRP], 2009). Those living at unusually high altitudes or other places with unique living conditions (e.g., in the mountains of Brazil, a coral atoll, etc.) can get a few times more or less than the average dose. Most of the natural variation is the result of shielding afforded by the Earth's atmosphere to galactic cosmic radiation (GCR) and variation in local radon levels, but radioactivity in the soil can also be very important.

While their doses from manmade sources are low, aircrews are among the highest occupationally exposed populations. Both the NCRP and The U.N. Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) ranks aircrew as among the most exposed occupations, at 3 mSv per year, almost all from GCR, roughly equaling their off-duty natural background exposure (UNSCEAR, 2000; NCRP, 2009). As aircraft cruise altitudes continue to increase, this dose will rise. According to the U.S. Bureau of Labor Statistics (BLS), there were about 210,000 crewmembers (pilots and flight attendants) in the U.S. labor force in 2014 (BLS, 2016).

There are several potential sources of radiation in aviation, but most of it usually comes from GCR. A decent approximation for USA based commercial flights within the Northern hemisphere of a few hours or more is about 5 Sv per hour of flight time. Doses per hour on shorter flights and on flights away from the poles, such as trans-equatorial flights, are lower. Equivalently scaled doses from other sources are extremely rare:

-While a nuclear accident or spillage of radioactive cargo could cause a large exposure to aircrew or passengers. Such nuclear accidents are very rare and few flights carry radioactive cargo. Radioactive cargo is transported in well-shielded packages spaced far apart from each other to minimize exposure to aircraft occupants. Estimated annual dose: < 0.13 mSv per year (Warner-Jones et al., 2003).

Figure 1. Radiation in daily life (MEXT, 2016).

-Solar ionizing radiation is almost always orders of magnitude weaker than galactic cosmic radiation, the exception being during rare, intense solar particle events (SPEs). Because the even the most intense solar particle spectrum is much softer than the GCR spectrum, they typically only significantly increase doses on high-latitude flights where the ions with rigidities of a few GV or less are permitted access to the atmosphere. A study of the SPEs that occurred from 1986-2007 by Copeland et al. (2008) found that even at 18 km altitude, solar proton events with dose rates at high latitudes that exceed 20 Sv/h have occurred less than once per year, on average. The events roughly followed solar activity in frequency. Events resulting in possible in-flight exposures exceeding the 1 mSv recommended limit for pregnant crewmembers at 12 km (40,000 ft.) and above are extremely rare, occurring about once a decade.

-Lightning and terrestrial gamma-ray flashes (TGFs) are associated with large thunderstorms, which aircraft avoid if it is possible. About 1 in 1000 flights is struck by lightning, mostly at low altitudes, such as during approach maneuvers. While a dose of up to 30 mSv has been postulated (Dwyer et al., 2010) for a worst case exposure from a TGF, they are associated with only a small fraction of lightning strokes (best evidence is for about 1 in 1000 or less) and are thought to occur only above about 10-15 km. Most lightning only produces insignificant amounts of soft X-rays.

GCR is relatively constant. It cannot be practically shielded against and effective dose increases as altitude increases. At the levels of exposure found in commercial aviation today, a few millisieverts of occupational exposure per year is a reasonable estimate of occupational exposure for crewmembers.

As indicated by the threshold doses in Table 1, the potential exposures from all the exposures listed above are well below the dose levels that can cause deterministic effects. It is the stochastic effects (Table 2) that are of concern. Though the odds are low, cancer induction is the most likely effect. It should also be noted that these risk estimates are for large populations, not individuals. There is too much variation in sensitivity and too little knowledge of cancer induction pathways to estimate risk for any particular individual.

Once it reaches the atmosphere, a cosmic ray, whether of solar or galactic origin, creates a shower of secondary particles. The shower may reach all the way through the atmosphere to the ground if the initiating particle has enough energy. For purposes of calculating dose rates at altitudes all the way down to sea level, good information about the incoming particle spectrum is required and neutron monitor data currently play a vital role in unfolding the spectrum. As an example, in CARI-7, the FAA’s latest GCR flight dose calculation software, neutron monitor data are used in two places. The data are used both for Forbush modulation, and also within the GCR models to drive solar modulation. Because of the well-known inverse relation between solar activity and GCR flux, using NM data to estimate solar modulation is common practice for GCR model developers (e.g., O'Brien et al., 2003; O'Neill, Golge, and Slaba, 2015). The BO14 GCR models add satellite data and sunspot numbers as additional sources of solar modulation data, while the ISO model is driven by Wolf sunspot numbers and does not capture short-term variations in local GCR flux (O'Neill, Golge, and Slaba, 2015; ISO, 2004). For GCR spectral variation over the course of a day or more a single 18-tube monitor can be used to drive a model with enough accuracy to match in-flight measurements very well. While the HEPAD instruments on NOAA's GOES series satellites could also provide data on these time scales with reasonable accuracy, to capture shorter term variations, such as are needed for studies comparing hourly-averaged instrument measurements to theoretical calculations, only NMs can currently provide the needed short-term accuracy. Multiple monitors/more tubes are useful to improve counting statistics and reliability and are needed to gain detailed spectral information about short term variations such as Forbush decreases and solar particle events (SPEs).

During an SPE, the cosmic ray spectrum incident at any given location in the atmosphere is more difficult to model well. This is because the particle spectrum is constantly changing significantly (even on timescales as short as 1 minute), and because isotropy of the incident flux is often a poor assumption for the first few hours of an event. Data from many monitors, with many tubes each, is needed to maximize accuracy of event assessments. This is particularly true at the start of events, when high-energy particle fluxes are usually highest. For SPEs, a world grid of neutron monitors, in both hemispheres (N and S), is needed to provide a really good picture. Monitors at different altitudes at the same location are also useful. These data from Inuvik, Goose Bay, and Deep River monitors show the problem (Figure 2). Deep River is farthest south and has a slightly higher geomagnetic cutoff, so it could be argued that it's reduced count rate increase relative to that at Inuvik is the result of the softness of the SPE spectrum, but both Goose Bay and Inuvik are near-sea-level stations sited close enough to the magnetic pole that (after correction for local conditions) they would respond essentially identically if event flux were isotropic. Monitors at the same geomagnetic latitude but different longitudes respond differently when SPE flux is highly anisotropic.

Figure 2. Relative increase in count rate relative to GCR background during the SPE of 24-25 May, 1990, at three northern-latitude neutron monitors (Bartol Research Institute, 2015).

Using data from multiple high-latitude near-sea-level monitors, along with lower latitude monitors and higher altitude monitors provides information on the shape of the SPE particle spectrum with rigidities above 1 GV, a part of the solar cosmic ray spectrum that satellites do not measure well. Count rates from GOES satellite instruments provide a picture of the low to intermediate energy proton flux, but very limited information the high energy flux spectrum, only enough to make an educated guess at the shape of the spectrum.