STUDY OF THE MOST HARMFUL SOLAR ENERGETIC PARTICLE FOR
SHIELDING NEXT HUMAN SPACE FLIGHTS
Bryan Yamashiro
Department of Physics & Astronomy
University of Hawai‘i at Mānoa
Honolulu, HI, 96822
ABSTRACT
Solar events such as solar flares and coronal mass ejections eject immense magnitudes
of energetic particle flux daily toward the Earth. Particle detectors in space and on Earth are
constantly monitoring these potentially harmful particles, aiming for a better understanding of
the mechanisms that drive the particles to extreme energies. Utilizing powerful particle
detectors in space, this study analyzes the highest energetic solar storms for deeper insight of
the behavior of energetic solar particles. Various solar event parameters and methods of
analysis are employed to portray an array of results for two major solar events. These results
support models for predicting high-energy solar events that are hazardous to society and more
importantly, human life on Earth and in space.
INTRODUCTION AND OVERVIEW
Heliophysics is the study of the Sun and its interaction with Earth and the Solar System.
The Sun exhibits 11-year solar cycles during which its magnetic field becomes very
complicated. Periods of fewer and smaller sunspots and solar flares are called solar minima,
and conversely periods of larger, more prominent, and more numerous sunspots are called solar
maxima [1]. Sunspots are the source regions of solar activity, such as flares and coronal mass
ejections (CMEs), thus solar activity increases during solar maxima. Solar flares and CMEs
emit large amounts of photons, particles, and plasma into the Solar System.
Solar flares and CMEs occur almost daily. Solar flares are intense bursts of radiation
at the surface of the Sun. Solar flares are classified with the letters A, B, C, M, and X, from the
least energetic to the most energetic events, which are assigned according to their peak X-ray
flux. CMEs are expulsions of plasma and magnetic fields into the Heliosphere that travel with
velocities rang- ing from 400 to 2,500 km/s [2]. Both phenomena are initiated from the release
of magnetic energy associated with sunspots, and results in the ejection of photons and highenergy solar energetic particles (SEPs) [3].
History has shown effects of large solar storms; the largest recorded being the Carrington
Event of 1859. This event was powerful enough to create auroras at equatorial latitudes, away
from the magnetic poles. An event similar to the Carrington Event could be highly detrimental
in modern society; the world’s high-tech infrastructure could grind to a halt [4]. The massive
amount of particle interference could cause radio disruptions, GPS and satellite failures, and
trigger large-scale blackouts. The radiation exposure from energetic particles is also a danger
to human life, especially in unprotected space outside the atmosphere onboard the International
Space Station (ISS), and will be a significant problem for long-duration manned space flight
missions.
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PARTICLE DETECTION
Spacecraft detectors are continuously monitoring solar activity, measuring photons and
particles emitted by solar flares and CMEs. SEPs, having mass and being electromagnetically
charged, move along a path dictated by the interplanetary magnetic field filling the Heliosphere.
The journey of these particles to Earth can take from less than thirty minutes to a few days
depending on the particle energy, intensity of the solar event, and the location of the event on
the Sun. Particle detectors such as the Solar and Heliospheric Observatory (SOHO) [6], the
Geostationary Operational Environmental Satellite-13 (GOES-13) [5], the ISS installed Alpha
Magnetic Spectrometer (AMS-02) [3] and many more, capture and measure data from solar
emissions. The data from these spacecraft allow for analysis of SEPs at specific points in time
and location.
The AMS-02 particle detector is primarily directed at high-energy physics studies such as
dark matter and cosmic rays, however it is also the largest solar energetic particle spectrometer
ever flown. AMS-02 provides observations of solar protons and helium from a few hundred to a
few thousand MeV, including precise measurements of intensity, spectra, and anisotropy and their
temporal evolution. This very high-energy range is poorly understood at present due to a lack of
precision measurements; however it is also the most potentially dangerous to astronauts [7].
AMS-02 has observed 18 different SEP events from May 2011 to February 2014. The specific
events are important since observation by AMS-02 requires SEPs to sustain extremely highenergy ranges above 125 MeV at low Earth orbit.
RELATION TO NASA GOALS
One of NASA’s main strategic objectives is to understand the Sun and its interactions with
Earth and the Solar System, including space weather [8]. A deeper knowledge of Heliophysics is
being stressed by agencies such as NASA, with a focus on monitoring and predicting changes
on the Sun [9]. For all reasons, it is imperative for the success of the next human space missions
to identify methods of predicting large solar storms. The goal of the AMS-02 group at the
University of Hawai‘i at Mānoa is to provide an understanding of the flux variation in cosmic rays
due to the solar modulation and to study the most energetic SEP events emitted by the Sun.
METHOD
SEP events were studied in detail focusing on characteristics such as particle intensity,
maximum particle energies, frequency and duration of events, and their origin locations on the
Sun. Full energy spectrums of SEP events were plotted with the data from the three satellites
SOHO, GOES-13, and AMS-02. From the start of the AMS-02 mission in May 2011 to the
middle of 2014, AMS-02 collected particle data for 18 major solar events, which determined
events suitable for research.
Excess particles were measured in the proton-counting rate for days associated with flares
and provide plots of SEPs measured by AMS-02. SEP flux data was collected from GOES-13
and SOHO for dates of interest, available online and can be downloaded from their respective
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websites. A C++ program was modified to graph data from each of these events while using
the CERN graphing program, ROOT [10].
Flare Date
Flare Class Flare Start CME Velocity AMS-02 Max
Time
(km/s)
Energy (MeV)
August 9, 2011
X6.9
February 25, 2014 X4.9
07:48
00:39
1610
2147
910
1220
TABLE I. The table includes initial parameters of the two SEP events. Flare class, start time of
the flare, corresponding CME velocity, and max AMS-02 observed velocities are included to
distinguish between the two events.
The main research study involved two high-class SEP events detected by AMS-02. The
events correspond to the August 9, 2011 X6.9 class flare and the February 24, 2014 X4.9 class
flare. These flares were chosen due to the high intensity, which makes the SEP flux increase
well defined. Another prime aspect of the flares was the timing, as both occurred in the middle
of the day, which allowed for a full analysis due to the presence of the rise and fall of SEP flux
propagation.
FIG.1. Full SEP proton flux propagation of particles for an entire day. Various vertical lines
represent the different time intervals used for analyzing the SEP event. The top graphs
represent flux from GOES while the bottom graphs portray SOHO proton flux.
The spectra in figure 1 showed the incoming proton flux for an entire day, each
horizontal line represented different energy bins. Higher horizontal lines represented lower
energy bins, and contrarily lower horizontal lines represented higher energy bins. Higher energy
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particles were less abundant than lower energy particles seen in the GOES flux. Spectra showed
consistent flux until the start of the SEP events, where defined increases in flux were observed.
Figure 1 includes time intervals for proton flux for the two events from GOES and SOHO shows
the flare start time (first vertical line), time of first fit for high energies (second vertical line),
time of first fit for low energies (third vertical line), and the time at which the highest energy
SEP flux was consistent with zero (fourth vertical line). The SEP event of February 25, 2014,
as shown for GOES and SOHO, had a more gradual flux increase and an extended SEP signal
at high energies compared to August 9. The flux was integrated in 10 - 30 minute intervals to
generate the SEP spectra. SEP spectra was generated by using background subtraction, which
was done by taking the particle flux of the SEP event and subtracting the particle flux of the
previous day. For the two SEP events, the previous days were quiet, without solar activity;
therefore the spectra represented the event exclusively. SOHO measures protons from 4 to 53
MeV and GOES measures from 0.8 to 500 MeV, allowing the creation of spectra over a broad
energy range.
FIG.2. The far left graphs shows SEP proton flux spectra including both GOES and SOHO
data. Following graphs were power law fits for SOHO (second column plots), GOES at low
energy (third column plots), and GOES at high energy (fourth column plots) in defined energy
ranges.
Converse to a method of having a full day interval (0:00-23:59), the time intervals
were optimized for in-depth flare analysis. The complete SEP proton spectra are shown in
figure 2 for a single time interval for August 9 and February 25. There were higher proton flux
magnitudes of low energy particles and fewer high energy particles indicated by the plots. The
spectra were divided into low and high-energy ranges for this analysis and each energy range
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was fit with a power law. In figure 2, the last histograms for both events represent the split
between the low and high-energy range regions. For the lower energy portion of the spectrum,
SOHO and GOES at low energy were fit between 0.09 - 0.4 GV. GOES at high energy was fit
between 0.3 - 2.0 GV for the higher energy portion of the spectrum. The spectra were divided
into two energy regions because spectral breaks are often observed in this energy range and
appear to be present for these two events. Spectral breaks are important in power law
observations, providing information about acceleration mechanisms that drive SEP events.
RESULTS
Data analysis was completed for 18 SEP events dating from 2011 to 2014 that were
determined energetic enough by AMS-02. For each event, data was gathered from online
graphical user interfaces of NASA satellites, SOHO and GOES-13, 15. Using the sets of data,
proton and X-ray flux were plotted for every event. X-ray flux was plotted with data from the
GOES-15 satellite detectors. The X-rays were graphed with two wavelengths that the on board
detector measures in, 0.05-0.4 nm to 0.1-0.8 nm seen in figure 3. Each X-ray showing time
versus flux showed sharp peaks of the initial phenomena of each SEP event. X-ray data is
important since detected rays reach Earth quicker than energetic particles, allowing for
predictions before particle arrival.
FIG.3. X-ray flux for the August 9, 2011 SEP event. The largest spike in flux represents the
X6.9 class flare and the associated detection time.
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Proton flux was plotted with a low energy range detector (SOHO), and a low to high
energy range detector (GOES-13) on individual histograms. Each event consisted of multiple
time intervals that saw an increase in proton flux, and all of the various intervals were plotted for
every event. These individual graphs were then combined in a single spectrum to portray the low
to high-energy range proton flux spectrum of the two different satellite detectors. The data
showed similar trends and a correlation between the varying detectors.
Slopes for power law fits were recorded for each time interval and plotted for August 9
and February 25 using generated SEP flux spectra in figure 4. Each slope was plotted versus
the interval start time. To search for a trend in the evolution of the event, a line was fit to the
slopes. Less time intervals were recorded for August 9, 2011 than February 25, 2014 since the
high energy SEP proton flux decreased to zero more quickly. SEP flux decreasing to zero meant
that the activity of the SEP event returned to the normalized background state. The power-law
slopes measured in the low energy range were consistent within the error bars between SOHO
and GOES, however SOHO was somewhat steeper than GOES.
Each fit showed SEP event characterization, and having a generalized flare fit allowed
for predictions for SOHO, GOES at low energy, and GOES at high energy. SOHO had a slope
of −4.73−4 ± 8.69−5 , GOES at low energy had a slope of −3.95−4 ± 8.07−5 , and GOES at
high energy had a slope of −1.54−5 ± 2.01−5 , for the August 9, 2011 SEP event. The
February event and the August event again are both listed in Table 2. Each generalized fit
showed negative slopes and a certain magnitude. The linear fits to the power law slopes show a
steepening of the low energy region of the SEP spectrum for both events. The linear fit to the
high-energy part of the SEP spectrum on August 9 results in a slope consistent with zero,
indicating that the spectrum is not changing. February 25, however, does show a steepening in
the high-energy part of the spectrum.
FIG.4. Spectral indicies found by power law fits corresponding to the defined time intervals for
each event. The top graphs were generated from the low energy range data from SOHO and
GOES. The two bottom graphs are illustrated using the high energy range data from GOES.
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Flare Date
August 9, 2011
Energy Range
SOHO Low Energy
GOES Low Energy
GOES High Energy
February 25, 2014 SOHO Low Energy
GOES Low Energy
GOES High Energy
Slope
−4.73−4 ± 8.69−5
−3.95−4 ± 8.07−5
−1.54−5 ± 2.01−5
−3.94−5 ± 1.81−6
−4.79−5 ± 6.38−6
−2.23−5 ± 2.75−6
TABLE II. Respective generalized fits for the two SEP events. Each energy range region
included correlating slopes for SOHO and GOES.
CONCLUSIONS
The spectra created from GOES and SOHO data proved compatibility for both the
August 2011 and February 2014 events. This phenomenon was seen in the power law fits as
both satellite detectors showed a difference between the error bars in the low-energy spectral
fits. Differences between the two events occurred at the higher energy range as deviations
were off by almost a magnitude. The discrepancy led to a model to find trends between the
event parameters and a potential prediction catalog.
The X4.9 flare, although smaller than the X6.9 flare, had a higher CME velocity and
max AMS-02 energy. Even with the higher observed energy and velocity the maximum SEP
flux was lower, showing that the higher flare class event generated more SEPs regardless of
event parameters. The steeper high energy range slope was observed in the X4.9 flare,
promoting a prediction that CME velocity and max AMS-02 energies support a more elevated
fit slope. The steep slope represented fewer high-energy SEPs since the difference in flux
would be greater. Higher SEP flux from the August 2011 event resulted in more high-energy
SEPs relative to the February 2014 event even with far lower CME velocities and max AMS-02
observed energies.
ACKNOWLEDGEMENTS
I would like to thank the Hawai‘i NASA Space Grant Consortium for providing me with
the opportunity to conduct scientific research. The knowledge I attained from this research
period will allow me to transition into my future aspirations. Along with the program, I would
like to thank Dr. Veronica Bindi for cultivating my knowledge in advanced physics and
providing valuable instruction throughout the entirety of my research.
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