Transient Weakening of Earth's Magnetic Shield Probed by a Cosmic Ray Burst.
The GRAPES-3 tracking muon telescope in Ooty, India measures muon intensity at high cutoff rigidities (15-24 GV) along nine independent directions covering 2.3 sr. The arrival of a coronal mass ejection on 22 June 2015 18:40 UT had triggered a severe G4-class geomagnetic storm (storm). Starting 19:00 UT, the GRAPES-3 muon telescope recorded a 2 h high-energy (∼20  GeV) burst of galactic cosmic rays (GCRs) that was strongly correlated with a 40 nT surge in the interplanetary magnetic field (IMF). Simulations have shown that a large (17×) compression of the IMF to 680 nT, followed by reconnection with the geomagnetic field (GMF) leading to lower cutoff rigidities could generate this burst. Here, 680 nT represents a short-term change in GMF around Earth, averaged over 7 times its volume. The GCRs, due to lowering of cutoff rigidities, were deflected from Earth's day side by ∼210° in longitude, offering a natural explanation of its night-time detection by the GRAPES-3. The simultaneous occurrence of the burst in all nine directions suggests its origin close to Earth. It also indicates a transient weakening of Earth's magnetic shield, and may hold clues for a better understanding of future superstorms that could cripple modern technological infrastructure on Earth, and endanger the lives of the astronauts in space.
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Mohanty
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Arunbabu
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Aziz
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Dugad
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Gupta
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Hariharan
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Jagadeesan
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Jain
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Morris
Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India†.
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Rao
Graduate School of Science, Osaka City University, 558-8585 Osaka, Japan†.
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Hayashi
Graduate School of Science, Osaka City University, 558-8585 Osaka, Japan†.
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Kawakami
College of Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan†.
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Oshima
College of Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan†.
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Shibata
Bose Institute, 93/1, A.P.C. Road, Kolkata 700009, India†.
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Raha
Indian Institute of Science Education and Research, Pune 411021, India†.
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Subramanian
Faculty of Engineering, Aichi Institute of Technology, Toyota City, Aichi 470-0392, Japan†.
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Kojima