What is Causing the Deficit of High-Energy Solar Particles in Solar Cycle 24?

Mewaldt, R. A.; Li, Gang; Hu, Junxiang; Cohen, C. M. S.

doi:10.22323/1.301.0111

Cited by 6 publications

(11 citation statements)

References 19 publications

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“…Furthermore, it has to be noted that the vast majority of these major SEP events did occur in solar cycle 23 clearly underlining the unusual weak character of cycle 24 (Richardson et al, ). Similar effects have been also observed for the contribution to the fluence in previous studies (Mewaldt et al, , ).…”

Section: Long‐term Data Analysissupporting

confidence: 90%

Revising More Than 20 Years of EPHIN Ion Flux Data—A New Data Product for Space Weather Applications

Kühl

Heber

2019

Space Weather

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Solar energetic particle events and galactic cosmic rays are important aspects of space weather. Investigating them requires consistent measurements of electrons and ions over long time periods, that is, over more than a solar cycle. The Electron Proton Helium INstrument onboard the SOlar and Heliospheric Observatory is operational since 1995 and was designed to measure electrons in the energy range of 0.3 to 10 MeV as well as protons and helium ions in the energy range from 4 to 50 MeV per nucleon in four different coincidence channels with 1-min resolution. Early in the mission and in 2017 two of six detectors became noisy and had to be switched of reducing the number of coincidence channels from 4 to 2, and thereby significantly reducing the energy resolution of the instrument. In order to restore the original count rate channels we present here a new data analysis applying the so called dE∕dx − dE∕dx method. A data set with different temporal resolutions from 1 min to a day has been generated for the whole ongoing SOlar and Heliospheric Observatory mission from 1995 to 2018. The resulting data sets are successfully validated against measurements from instruments close to Earth. Studies with regard to long-term variation of the measured flux with special emphasis on the contribution of individual solar energetic particle events to the fluence observed over more than two solar cycles are presented here.

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Section: Long‐term Data Analysissupporting

confidence: 90%

Revising More Than 20 Years of EPHIN Ion Flux Data—A New Data Product for Space Weather Applications

Kühl

Heber

2019

Space Weather

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“…While some of the largest events in cycle 24 were not directed toward Earth, for example, 23 July 2012 and 1 September 2014, even including the STEREO observations in the top 10 list (Table ) does not change the conclusion that the conditions of cycle 23 yielded larger SEP events (on average a factor of 2.4 bigger). As suggested by Mewaldt et al () this may be a result of the lower interplanetary density of suprathermal seed particles, combined with a lower magnetic field strength in cycle 24 (see also Gopalswamy, Akiyama, et al, ; Vainio et al, ).…”

Section: Discussionmentioning

confidence: 81%

“…Several aspects of cycle 24 have been shown to be significantly weaker than those of cycle 23, including the solar wind speed and the magnetic field strength (see, e.g., McComas et al, ). The number of large SEP events is no exception (Gopalswamy, Akiyama, et al, ; Mewaldt et al, ), with the decrease in GLE events being particularly strong (two in cycle 24 vs. 16 in cycle 23; gle.oulu.fi). In this paper we focus not only on the most recent GLE event on 10 September 2017, comparing its properties to the GLE events of cycle 23 and the other cycle 24 event of 17 May 2012, but also present data from the extreme SEP events observed by STEREO and discuss the top 10 SEP events of cycles 23 and 24.…”

Section: Introductionmentioning

confidence: 99%

The Ground‐Level Enhancement Event of September 2017 and Other Large Solar Energetic Particle Events of Cycle 24

Cohen

Mewaldt

2018

Space Weather

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The 10 September 2017 solar energetic particle (SEP) event was the largest since June 2015 and one of only two ground-level enhancement (GLE) events of solar cycle 24. GLE events are subset of large SEP events (~15% of events identified by Space Weather Prediction Center) with particularly hard spectra, making them a substantial space weather hazard to space-based instrumentation and exposed astronauts. We present analysis of the 10 September 2017 event and compare it to the other cycle 24 GLE events, to those of cycle 23, and also to two extreme SEP events observed by Solar Terrestrial Relations Observatory (STEREO). We find the 10 September 2017 event had a broken power law spectrum typical of GLE events but was softer than average at high energies. However, it was hard at low energies with a relatively high break energy, which led to 100-MeV proton fluences within a factor of 4.5 of the largest cycle 23 GLE events. The composition was nominal, except for a low Fe/O ratio, which has also been seen in large SEP events this cycle, but is somewhat atypical of the cycle 23 GLE events. The extreme events seen by STEREO exhibited very hard high-energy spectra, with one event producing~80-MeV proton fluences larger than the largest cycle 23 GLE event. However, even including STEREO events, the top 10 largest cycle 24 events are, on average, 2.4 times smaller than the top 10 of cycle 23 based on >10-MeV proton fluences.

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“…What are the ultimate causes for that? [54] identified that the present cycle had fewer fast CMEs and a weaker magnetic field. Such effects are amplified by the nonlinear nature of the shock acceleration process: with lower suprathermal proton injection, there was weaker turbulence generation.…”

Section: Spectra (5)mentioning

confidence: 78%

“…The issue of why the present Solar Cycle 24 produced so few strong SEP events, with a much lower SEP fluence than the two preceding solar cycles (see Figure 8), was addressed by [54]. During strong events, SEP spectra have broken power laws (changing at E break ) and a high-energy rollover (at ∼ E max ).…”

Section: Spectra (5)mentioning

confidence: 96%

Solar-Heliospheric Physics

Ruffolo¹

2017

Proceedings of 35th International Cosmic Ray Conference — PoS(ICRC2017)

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This rapporteur paper summarizes the 99 presentations on solar-heliospheric physics at the 35th International Cosmic Ray Conference. These spanned a dizzying array of physical phenomena, especially time-dependent phenomena associated with solar storms, the 27-day solar rotation, the ∼11-year sunspot cycle, and the ∼22-year solar magnetic cycle. There was also work on composition-dependent effects and cosmic ray anisotropy involving transport effects. Topics included ground-and space-based instrumentation, solar energetic particles from solar storms (ranging from keV-range seed particles to GeV-range accelerated particles), interplanetary transport of energetic particles, Earth's radiation environment (including space weather and other effects of cosmic rays at Earth), the sun shadow in TeV-range Galactic cosmic rays (GCRs), and solar modulation of GCRs with the sunspot cycle and solar magnetic cycle.

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What is Causing the Deficit of High-Energy Solar Particles in Solar Cycle 24?

Cited by 6 publications

References 19 publications

Revising More Than 20 Years of EPHIN Ion Flux Data—A New Data Product for Space Weather Applications

Revising More Than 20 Years of EPHIN Ion Flux Data—A New Data Product for Space Weather Applications

The Ground‐Level Enhancement Event of September 2017 and Other Large Solar Energetic Particle Events of Cycle 24

Solar-Heliospheric Physics

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