Understanding Cosmic Rays and Their Interaction with Solar Activity

Cosmic rays, high-energy particles originating primarily from outside our solar system, constantly bombard Earth from all directions. These particles, mostly protons and atomic nuclei, travel at nearly the speed of light and carry clues about the universe's most violent events, such as supernovae explosions and black hole activities. Full name: Galactic Cosmic Rays (GCRs). However, their journey to Earth is not unimpeded. Solar modulation refers to the process by which the Sun's magnetic field and solar wind—a stream of charged particles emitted from the Sun—alter the flux and energy spectrum of these incoming cosmic rays. This modulation varies with the 11-year solar cycle, intensifying during solar maximum when sunspots and eruptions peak.

One striking manifestation of solar modulation is the Forbush decrease, named after physicist Scott Forbush who first observed it in the 1930s. During coronal mass ejections (CMEs)—massive bursts of plasma and magnetic fields from the Sun—the interplanetary magnetic field strengthens, creating barriers that scatter and reduce cosmic ray intensities at Earth. These decreases last days to weeks, followed by gradual recovery, providing a natural laboratory for studying particle propagation in turbulent magnetic environments.

Recent advancements in space-based detection have allowed unprecedented precision in observing these phenomena, particularly for lighter cosmic ray components like electrons and positrons. These leptons are especially sensitive to solar influences due to their lower rigidity and higher scattering rates compared to heavier nuclei.

China's DAMPE Mission: The Wukong Satellite at the Forefront

Launched on December 17, 2015, from the Jiuquan Satellite Launch Center, the Dark Matter Particle Explorer (DAMPE), affectionately known as 'Wukong' after the Monkey King from Chinese folklore, represents a pinnacle of China's space science endeavors. Orbiting at 500 km altitude, DAMPE features four sub-detectors: a plastic scintillator detector for charge measurement, a silicon-tungsten tracker for direction, a BGO calorimeter for energy, and a neutron detector. This setup enables identification and measurement of cosmic rays up to 100 TeV with exceptional resolution.

While primarily designed to hunt for dark matter signatures in cosmic ray electrons/positrons and gamma rays, DAMPE has excelled in heliospheric physics. Over nine years, it has collected billions of events, revealing fine structures in cosmic ray spectra and now, novel solar modulation effects.

Key Chinese Universities Powering the DAMPE Collaboration

The DAMPE mission is a testament to collaborative higher education in China, involving over 280 scientists from more than 30 institutions. Leading the charge are universities like the University of Science and Technology of China (USTC) in Hefei, which contributes expertise in particle physics and instrumentation; Purple Mountain Observatory (PMO) of the Chinese Academy of Sciences (CAS) in Nanjing, handling data analysis; and Nanjing University of Science and Technology. Other participants include Linyi University, Shandong University, Zhejiang University, and the Institute of High Energy Physics (IHEP, CAS).

• USTC: Key in calorimeter development and cosmic ray propagation models.
• PMO, CAS: Leads solar modulation studies and Forbush event analysis.
• IHEP, CAS: Tracker and silicon detector contributions.
• Linyi University: Data processing for low-energy electrons.
• Nanjing University: Theoretical modeling of heliospheric transport.

These institutions not only build hardware but foster PhD programs and postdocs in astroparticle physics, training the next generation amid China's push for space science self-reliance.

The Breakthrough Study on Forbush Decreases

Published in Physical Review Letters in early March 2026, the study titled "Diverse Properties of Electron Forbush Decreases Revealed by the Dark Matter Particle Explorer" analyzes eight Forbush decrease (FD) events from 2016 to 2024. Led by researchers from PMO and USTC, it marks the first precise measurement of electron-plus-positron fluxes (2-20 GeV) during FD decline and recovery phases.Read the full paper

Unlike ground-based neutron monitors that integrate all cosmic rays, DAMPE species-separates electrons/positrons, revealing their unique responses to solar disturbances.

Detailed Findings: Flux Variations and Phase Behaviors

The team identified FDs triggered by CMEs, measuring flux drops up to 30-50% at low energies. Key novelty: asymmetric decline and recovery. Decline phases show rapid, energy-dependent drops—steeper at lower energies due to enhanced diffusion barriers from CME shocks. Recovery is slower, modulated by magnetic turbulence decay.

For instance, during the 2017 September FD, 2-5 GeV fluxes dropped 40% within hours, recovering over 10 days. Higher energies (10-20 GeV) exhibited milder, symmetric changes, indicating rigidity-dependent scattering.

Photo by Lan Lin on Unsplash

Correlations with CME Physical Parameters

A pivotal discovery links FD properties to CME traits: speed, size, and magnetic field strength. Faster CMEs (>1000 km/s) cause deeper, sharper FDs; stronger B-fields prolong recovery. This correlation, quantified via Pearson coefficients (r ~ 0.7-0.9), validates models like diffusion-barrier suppression and offers empirical calibration for simulations.CGTN coverage

Implications for Space Weather and Astrophysics

These findings enhance space weather forecasting, critical for satellite operations, aviation, and astronauts. Forbush decreases signal geomagnetic storms, now predictable via CME-FD links. In astrophysics, they refine GCR propagation models, aiding dark matter searches by isolating solar effects from intrinsic spectra.

China's contribution underscores its rising prowess, with DAMPE data complementing AMS-02 and CALET.

Chinese Higher Education's Role in Space Research Excellence

Universities like USTC host DAMPE labs, offering BS-MS-PhD tracks in particle astrophysics. PMO affiliates with Nanjing University, integrating undergrad projects into satellite data analysis. This ecosystem produced the PRL paper's lead authors, many young faculty trained domestically.

• Training: Specialized cosmic ray courses at USTC, Zhejiang U.
• Funding: NSFC, CAS grants support 100+ PhDs.
• Impact: 50+ high-impact papers/year from DAMPE unis.

International Collaborations and Global Impact

DAMPE's 10% international members (INFN Italy, UniGe Switzerland) foster exchanges. Joint workshops at USTC advance heliophysics.

Future Prospects: Next-Generation Chinese Missions

Building on DAMPE, China plans HERD (High Energy cosmic-Radiation Detection) and e-ASTROGAM analogs. Universities gear up for LHAASO-2, ground-based cosmic ray array.

Prospects include AI-enhanced modulation modeling, multi-messenger astronomy.

Photo by Lan Lin on Unsplash

Career Opportunities in China's Cosmic Ray Research

With booming space programs, roles abound: postdocs at USTC (¥300k+/yr), faculty at PMO affiliates. Explore research positions or faculty openings in physics.