Cosmic Rays

Invited review talk on cosmic-ray anisotropy observations and their astrophysics impact at the 2nd LHAASO Symposium.

Mar 20, 2025

The Southern Wide-field Gamma-ray Observatory (SWGO) is an astrophysics project in the southern hemisphere. The scientific potential of a wide field of view and very high-duty cycle ground-based gamma-ray detectors has been demonstrated by the current generation instruments HAWC and ARGO, and this potential will be extended in the Northern Hemisphere by LHAASO. No such instrument currently exists in the Southern Hemisphere, where there is significant potential for mapping large-scale emissions and providing access to the entire sky for transient and variable multi-wavelength and multi-messenger phenomena. Access to the Galactic Center, in conjunction with the major facility CTA-South, motivates the establishment of such a gamma-ray observatory in the south. Additionally, there is substantial potential for cosmic ray studies, including anisotropy. The shared concept for the future observatory is as follows: A gamma-ray observatory based on ground-level particle detection, with close to 100% duty cycle and order steradian field of view. Located in Atacama Astronomical Park, Chile. At an altitude of 4770 m. Covering an energy range from 100s of GeV to the PeV scale. Based primarily on water Cherenkov detector units. With a high fill-factor core detector with an area considerably larger than HAWC and significantly better sensitivity, and a low-density outer array.

Feb 1, 2025

Introduction Observations of the time-dependent cosmic-ray Sun shadow have proven to be a valuable diagnostic tool for assessing solar magnetic field models. This project compares several years of IceCube data with solar activity and solar magnetic field models. For the first time, a quantitative comparison of solar magnetic field models with IceCube data at the event rate level is conducted. Additionally, we present an initial energy-dependent analysis compared to recent predictions. We utilize seven years of IceCube data for the moon and the Sun, comparing them with simulations at the data rate level. The simulations are performed under the geometrical shadow hypothesis for both the moon and the Sun and under a cosmic-ray propagation model influenced by the solar magnetic field for the Sun case. Our findings indicate a linearly decreasing relationship between Sun shadow strength and solar activity, which is preferred over a constant relationship at the 6.4σ level. We evaluate two commonly used coronal magnetic field models, both in conjunction with a Parker spiral, by modeling cosmic-ray propagation in the solar magnetic field. Both models predict a weakening of the shadow during periods of high solar activity, which is also observable in the data. We observe tensions with the data on the order of 3σ for both models, assuming only statistical uncertainties. The magnetic field model CSSS fits the data slightly better than the PFSS model. This finding generally aligns with earlier results from the Tibet AS-γ Experiment; however, the deviation of the data from the two models is not significant at this time. Regarding the energy dependence of the sun shadow, we find indications that the shadowing effect increases with energy during periods of high solar activity, which is consistent with theoretical predictions. In contrast, the Sun’s shadow varies seasonally, with IceCube detecting a significant deviation from the mean Sun shadow (χ²/ndof = 22.47/4, 3.8σ). This variation is likely linked to the Sun’s magnetic field changes, consistent with past Tibet observations at lower energies. A Spearman’s rank test suggests a 96% likelihood of correlation between sunspot number and the Sun shadow’s amplitude. However, further data are needed to confirm this relationship, given the limited observation periods and a weak solar cycle. Future studies should refine models of cosmic ray deflection by solar magnetic fields and improve point spread function treatments to enhance comparisons between observational data and simulations. Publication of IceCube research The investigations on the Sun’s cosmic-ray shadow’s time variabilities were published in The Astrophysical Journal. A more detailed study was published in Physical Review D where the Sun shadow’s time variability is associated with solar cycles, as opposed to the constant Moon cosmic-ray shadow, whose slow variability with time is only associated to the change in average Moon-Earth’s distance. Publication of numerical calculations The latest observational result was paired with a dedicated study of cosmic-ray particle trajectories propagating near the Sun, assuming different models of the solar corona’s magnetic field and its variability across the solar cycles. The report was published in Astronomy & Astrophysics.

Feb 1, 2025

IceCube Neutrino Observatory is an astrophysics project located at the geographic South Pole. The IceCube Neutrino Observatory is the first detector of its kind, designed to observe the cosmos from deep within the South Pole ice. The IceCube Collaboration is an international group of scientists responsible for scientific research. Encompassing a cubic kilometer of ice, IceCube searches for nearly massless subatomic particles called neutrinos. These high-energy astronomical messengers provide information to probe the most violent astrophysical sources, such as exploding stars, gamma-ray bursts, and cataclysmic phenomena involving black holes and neutron stars. The Antarctic Neutrino Observatory, including the surface array IceTop and the dense infill array DeepCore, was designed as a multipurpose experiment. IceCube collaborators address several big questions in physics, like the nature of dark matter and the properties of the neutrino itself. IceCube also observes cosmic rays that interact with the Earth’s atmosphere, which have revealed fascinating structures that are not presently understood. The IceCube Collaboration comprises approximately 350 physicists from 58 institutions in 14 countries. The international team is responsible for the scientific program, and many collaborators contributed to the design and construction of the detector. Exciting new research conducted by the collaboration is opening a new window for exploring our universe. The National Science Foundation (NSF) provided the primary funding for the IceCube Neutrino Observatory, with assistance from partner funding agencies around the world. The University of Wisconsin–Madison is the lead institution responsible for maintaining and operating the detector. Funding Agencies in each collaborating country support their scientific research efforts.

Feb 1, 2025

The High Altitude Water Cherenkov (HAWC) is an astrophysics project located in Mexico. HAWC is a facility designed to observe gamma rays and cosmic rays between 100 GeV and 100 TeV. TeV gamma rays are the highest energy photons ever observed — 1 TeV is 1 trillion electron volts (eV), about 1 trillion times more energetic than visible light! These photons are born in the most extreme environments in the known universe: supernova explosions, active galactic nuclei, and gamma-ray bursts. Cosmic rays are charged particles that achieve energies far beyond what we can create in man-made particle accelerators. (The highest energy cosmic ray ever observed was 300 million TeV.) The origin of such particles has been a mystery for over 100 years. Gamma rays are thought to be correlated with the acceleration sites of charged cosmic rays, so we observe them to help answer this and other cosmic questions. HAWC is located on the flanks of the Sierra Negra volcano near Puebla, Mexico, at an altitude of 4100 meters (13,500 feet). The detector has an instantaneous field of view covering 15% of the sky, and during each 24 hours, HAWC observes two-thirds of the sky. Using the HAWC Observatory, we are performing a high-sensitivity synoptic survey of the gamma rays from the Northern Hemisphere.

Feb 1, 2025

CREW-HaT stands for Cosmic Radiation Extended Warding using the Halbach Torus. NIAC Phase I 2022 The CREW-HaT project was awarded the NASA Innovative Advanced Concepts - NIAC Phase I award in 2022. The 21st century will be when human space exploration gets off the ground. NASA’s priority is to send humans back to the Moon in the next decade with the Artemis mission and travel to Mars in the following decade. In parallel, SpaceX and Blue Origin companies are developing the technology to make human access to space routine. However, achieving this goal is only possible if we can protect the humans we send to space from the damaging effects of cosmic rays and energetic solar radiation. The health risks to astronauts associated with chronic exposure to radiation in space include carcinogenesis, cardiovascular damage, and degradation of the central nervous system. Since the Earth’s magnetic field is responsible for protecting us on Earth’s surface, a logical solution to the problem would be to have a spacecraft bring along its equivalent magnetic field. Here, we propose CREW HaT, a new concept for a Halbach Torus (HaT), which consists of light, deployable, mechanically supported magnetic coils activated by a new generation of high-temperature superconducting tapes that have recently become available. This configuration produces an enhanced external magnetic field that diverts cosmic radiation particles, complemented by a suppressed magnetic field in the astronaut’s habitat. The Halbach torus geometry has never been explored in this context or studied in combination with modern superconductive tapes. It diverts over 50% of the biology-damaging cosmic rays (protons below 1 GeV) and higher energy high-Z ions. This is sufficient to reduce the radiation dose absorbed by astronauts to a level of <5% of the lifetime excess risk of cancer mortality levels established by NASA. Paper The concept was presented at the 51st International Conference on Environmental Systems (ICES) in Saint Paul, MN, in July 2022. Innovation NASA has announced future human space expeditions (the Artemis mission in the next decade and the Mars mission within the next decade). These expeditions will only be possible if we efficiently mitigate the effects of harmful cosmic radiation on astronauts. Using a lightweight and non-cost-prohibitive structure, one feasible solution is to surround the spacecraft with protective magnetic fields that divert cosmic radiation. The CREW HaT consists of a Halbach Torus (HaT), a novel arrangement comprising spatially rotating coils around the spacecraft. This configuration produces an enhanced external magnetic field (open magnetic field) to divert cosmic radiation particles, complemented by a reduced magnetic field in the astronauts’ habitat. CREW HaT is likely to be the most feasible solution for active shielding, which this proposal intends to demonstrate. The CREW HaT is a deployable device integrated with the spacecraft. It unfolds and generates a magnetic field that deflects cosmic radiation from solar wind particles and galactic cosmic rays. Recent innovations in high-temperature superconductors (e.g., ReBCO) enable the necessary high currents. Our innovation benefited from the support provided to us by the UW-Madison’s Discovery to Product. Potential & Benefits The CREW-HaT is superior to the existing benchmark active shielding technology previously proposed (MAARSS Soleinoidal Coil system by Westover et al., NIAC 2012). It dramatically reduces the health risk for astronauts because: It diverts over 50% of the biology-damaging cosmic rays (protons below 1 GeV) and higher energy high-Z ions. This is sufficient to reduce the radiation dose absorbed by astronauts to a level of <5% of the lifetime excess risk of cancer mortality levels established by NASA. It has an over 80% smaller support structure volume, thus reducing secondary radiation, X-rays, gamma-rays, and neutrons produced by cosmic rays impacting spacecraft. Previous magnetic topologies (e.g., the MAARSS system) did not fully address this problem. It suppresses the magnetic field in the astronauts’ habitat without the need for an additional compensation coil. This increases crew safety and prolongs the lifetime of the instrumentation without increasing weight and power consumption. This innovative concept provides the aerospace community with a solution to protect humans and instrumentation on satellites exposed to space radiation from catastrophic solar flares and cosmic ray particles. It can protect any spacecraft or surface installation on, for example, the Moon while making it possible for long-duration space trips to Mars.

Feb 1, 2025

Introduction The IceCube and the HAWC observatories have established themselves as leaders in studying Galactic cosmic-ray anisotropy in the TeV–PeV energy range. IceCube captures anisotropy amplitudes with high precision by mapping cosmic-ray arrival directions relative to an isotropic reference. The IceTop surface array detects showers above 500 TeV, while the deep in-ice array records muons down to 10 TeV, both closely aligning with primary cosmic-ray directions. HAWC gamma-ray array detects showers above 1-10 TeV. IceCube and HAWC’s continuous sky observation enhances measurement stability, enabling energy-dependent anisotropy studies and spherical harmonic expansion analysis. Recent findings highlight the dipole component’s amplitude and phase as indicators of cosmic-ray diffusion in interstellar plasma. The angular power spectrum at different energies reflects pitch angle scattering processes. IceCube has submitted results from 12 years (2011–2023) of cosmic-ray muon data, refining event selection for improved stability. High-resolution sky maps will explore temporal anisotropy variations and cross-check muon and shower data consistency. IceCube also analyzes the Compton-Getting effect for calibration and cosmic-ray spectral index measurement. Given individual experiments’ limited sky coverage, full-sky measurements via collaborations with HAWC, GRAPES-3, TALE, and KASCADE aim to provide a comprehensive view of anisotropy. These efforts will improve understanding of cosmic-ray diffusion and heliospheric influence on observed distributions. Publications This is the list of cosmic-ray anisotropy results published by the team: citation title DOI arXiv ApJ (2010) 718 L194 Measurement of the Anisotropy of Cosmic Ray Arrival Directions with IceCube 10.1088/2041-8205/718/2/L194 1005.2960 ApJ (2011) 740 16 Observation of Anisotropy in the Arrival Directions of Galactic Cosmic Rays at Multiple Angular Scales with IceCube 10.1088/0004-637X/740/1/16 1105.2326 ApJ (2012) 746 33 Observation of an Anisotropy in the Galactic Cosmic Ray arrival direction at 400 TeV with IceCube 10.1088/0004-637X/746/1/33 1109.1017 ApJ (2013) 765 55 Observation of Cosmic Ray Anisotropy with the IceTop Air Shower Array 10.1088/0004-637X/765/1/55 1210.5278 ApJ (2016) 826 220 Anisotropy in Cosmic-Ray Arrival Directions in the Southern Hemisphere with Six Years of Data from the IceCube Detector 10.3847/0004-637X/826/2/220 1603.01227 ApJ (2019) 871 96 All-Sky Measurement of the Anisotropy of Cosmic Rays at 10 TeV and Mapping of the Local Interstellar Magnetic Field 10.3847/1538-4357/aaf5cc 1812.05682 ApJ (2025) Observation of Cosmic-Ray Anisotropy in the Southern Hemisphere with Twelve Years of Data Collected by the IceCube Neutrino Observatory 2412.05046

Feb 1, 2025

Student Colloquium on cosmic-ray science at Loyola University Chicago, followed by Q&A with the students.

Feb 15, 2024

The outer heliosphere regulates cosmic ray penetration into near-Earth space, reduces space radiation, and makes life possible in our solar system. Voyager and IBEX in-situ and remote observations of the outer heliosphere and the distorted local interstellar magnetic are important for heliospheric modeling. TeV cosmic rays provide a new tool to study the heliosphere interstellar medium boundary.

Jul 1, 2023

Lightning talk at the US Antarctic Science Meeting.

Jun 20, 2023