Journal of Atmospheric and Solar-Terrestrial Physics 99 (2013) 129–133

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Journal of Atmospheric and Solar-Terrestrial Physics

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Space radiation environment in low earth orbit during solar-activityminimum period from 2006 through 2011

H. Koshiishi n, H. Matsumoto

Aerospace Research and Development Directorate, Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba 305-8505, Japan

a r t i c l e i n f o

Article history:

Received 22 March 2012

Received in revised form

29 July 2012

Accepted 5 September 2012Available online 17 September 2012

Keywords:

Space environment

Space weather

Radiation belt

Low earth orbit

26/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.jastp.2012.09.004

esponding author. Tel.: þ81 50 3362 4508.

ail address: koshiishi.hideki@jaxa.jp (H. Kosh

a b s t r a c t

The Technical Data Acquisition Equipment on board the Advanced Land Observing Satellite had been

operated in low earth orbit at 700 km altitude from 2006 through 2011 in order to evaluate space

radiation environment, especially the proton environment and the electron environment in the

radiation belts, during solar-activity minimum period. The activation of the electron environment in

the inner radiation belt along with the 24th solar-activity cycle started in the beginning of 2010, 1 year

after the beginning of the 24th solar-activity cycle itself in the end of 2008. The electron environment in

the outer radiation belt was almost always modulated by solar wind variations; however, it showed

very low activities in the beginning of 2010 which was the same time when the lowest activities were

seen in the inner radiation belt. On the other hand, the proton environment in the inner radiation belt

showed a slight increase as solar activity went lower, and had a peak also in the beginning of 2010, the

same time when there was maximum galactic cosmic ray flux. 1-year delay of the response of space

radiation environment around the Earth is suggested to be because the beginning of the 24th solar-

activity cycle was very quiet as compared with the several former solar-activity cycles.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Long-term measurement of space environment is an essentialissue to understand its variation along with solar-activity cyclefor reliable design and safety operation of space missions.Especially, measurement in space is a unique method to knowdirectly how space environment responds to solar and geomag-netic activities. Consecutive measurement by a single instrumentis also an important point to avoid calibration problems. How-ever, long-term measurement in space using a single instrumentis a rare work because of limitation of spacecraft lifetime.

In this paper, measurement of space radiation environment inlow earth orbit using a single instrument carried out from 2006through 2011 is reported, which covers a half of solar-activitycycle. This period is well known as peculiar deep solar-activityminimum so that space radiation environment around the earthas consequences of solar and geomagnetic activities is consideredto be also peculiar as compared with those seen in the severalformer solar-activity cycles.

ll rights reserved.

iishi).

2. Measurements

The ALOS satellite (Advanced Land Observing Satellite: so-calledDaichi in Japanese) was developed by the Japan Aerospace Explora-tion Agency (JAXA) to contribute to the fields of mapping, disastermonitoring, and resource surveying. This satellite was launched on24 January 2006, and had been operated until 12 May 2011 forabout 5 years. The orbital parameters of this satellite were 700 kmaltitude, 981 inclination, and 46 days recurrent period (see JAXAHome Page). The TEDA (Technical Data Acquisition Equipment) wason board this satellite to evaluate space radiation environment inlow earth orbit for the safe operation of this satellite as well as forthe development of precise space radiation environment model (seeSEES Home Page).

The TEDA consisted of the Light Particle Telescope (LPT) formeasurement of energetic electrons, protons, and alpha-particles,and the Heavy Ion Telescope (HIT) for observation of energetic ions.The LPT was composed of 8 solid-state detectors. The aperture angleand the geometric factor were 21.51 and 0.071 cm2 sr, respectively.The LPT had a capability to measure electrons in the energy rangefrom 0.1 MeV to 10 MeV, protons from 1 MeV to 250 MeV, and alpha-particles from 6 MeV to 250 MeV with 1-s temporal resolution. TheLPT could also distinguish nuclei of hydrogen and helium isotopes.The HIT was composed of 2 position-sensitive detectors and 16 solid-state detectors. The aperture angle and the geometric factor were 451and 25 cm2 sr, respectively. The HIT had a capability to measure

H. Koshiishi, H. Matsumoto / Journal of Atmospheric and Solar-Terrestrial Physics 99 (2013) 129–133130

ions (helium–iron) from 5 MeV/nucleon to 155 MeV/nucleon. Boththe LPT and the HIT adopted the DE�E method to distinguishnucleus and energy of incident particles (e.g., Goulding and Harvey,1975). The center of the field of view of both the LPT and the HITwas 451 inclined from the zenith in the opposite direction of thesatellite movement.

In this study, energy ranges at around 10 MeV for protons and1 MeV for electrons are chosen to evaluate space radiationenvironment, because particles in these energy ranges have thehighest population among particles that can penetrate spacecraftwalls. Analyzed period is also selected such as from September2006 through February 2011 where measurement had beencarried out stably. Additionally, proton flux and electron flux atGeo-Stationary orbit are used, which were obtained by the SDOM(Standard Dose Monitor) on board the DRTS satellite (Data RelayTest Satellite: so-called Kodama in Japanese) (see JAXA HomePage, SEES Home Page).

3. Results

Figs. 1 and 2, respectively, illustrate the geographic distribu-tion of proton flux and electron flux averaged over the analyzed

Fig. 1. Geographic distribution of proton flux in the energy range from

Fig. 2. Geographic distribution of electron flux in the energy range from

period except strongly disturbed period by solar and geomagneticevents. The distributions are, in principle, determined by thegeomagnetic cut-off rigidity distribution except in the SAA (SouthAtlantic Anomaly) region that corresponds to the inner radiationbelt. Both proton flux and electron flux become higher as therigidity goes lower. The distribution of electron flux also showsthe horn region that corresponds to the foot point of the outerradiation belt. In low earth orbit, the SAA region is the dominantcontributor to both the proton environment and the electronenvironment. However, in case of solar and geomagnetic events,the proton environment in the polar region and the electronenvironment in the horn region are seriously disturbed.

These results are obtained from the measurement in low earthorbit carried out near the bounce loss cone; however, these canexhibit the whole aspects of trapped particles in the radiationbelts, because time scale of these particles to reach equilibriumwhen supplied is much shorter than solar-activity cycle so thatthese particles are considered to be coherent for wide L-shellrange in long-averaged measurement in this study (e.g., Imhofet al., 1991; Kanekal and Baker, 2001).

Figs. 3 and 4, respectively, demonstrate the L–t diagram ofproton flux and electron flux during the analyzed period. Protonflux in the inner radiation belt at around L¼1.5–2 was almost

7 MeV to 18 MeV averaged over September 2006–February 2011.

0.6 MeV to 1.2 MeV averaged over September 2006–February 2011.

Fig. 3. L–t diagram of proton flux in the energy range from 7 MeV to 18 MeV during September 2006–February 2011. Resolutions of diagram are 1 day in horizontal axis

and 0.1 in vertical axis. Data are averaged over these resolutions.

Fig. 4. L–t diagram of electron flux in the energy range from 0.6 MeV to 1.2 MeV during September 2006–February 2011. Resolutions of diagram are 1 day in horizontal

axis and 0.1 in vertical axis. Data are averaged over these resolutions.

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stable all through the analyzed period; however, it showed aslight increase as solar activity became lower. The precipitation ofsolar energetic protons was seen in December 2006, which wascaused by the X-class flares followed by the coronal massejections (e.g., Matthews et al., 2011; Zolotukhina et al., 2011).

Electron flux in the outer radiation belt at around L¼4–6 hadbeen disturbed both from several solar and geomagnetic eventsand from periodic high-speed solar wind. However, electron fluxin the outer radiation belt became smaller as solar activitybecame lower, almost disappeared in the beginning of 2010,and enhanced as solar activity rose again. Electron flux in theinner radiation belt at around L¼1.5–2 also became smaller sincethe last injection of electrons from the outer radiation belt in the23rd solar-activity cycle in December 2006, where electrons inthe outer radiation belt showed inward diffusion and filled theslot region, and enhanced again in April 2010 when the firstinjection of electrons from the outer radiation belt in the 24thsolar-activity cycle occurred. The width of the slot region alsobecame wider between two injections.

These tendencies are more obviously shown in Fig. 5.Fig. 5(a) depicts the monthly averaged F10.7 solar radio flux.The 24th solar-activity cycle seemed to begin at the end of 2008

(several results based on sunspot observations report almost thesame beginning of the 24th solar-activity cycle from the middle of2008 to the beginning of 2009 (e.g., Hathaway, 2011; Owens et al.,2011)). On the other hand, Fig. 5(b) depicts the monthly averagedKp index. The bottom of geomagnetic activity appeared in the endof 2009, 1 year after the solar-activity minimum.

Fig. 5(c) plots the proton flux at Geo-Stationary orbit. The lastsolar energetic proton event in the 23rd solar-activity cycle was seenin December 2006, which disturbed the proton environment in thepolar region. Fig. 5(d,e), respectively, plots the monthly averagedheavy ion count in the polar region and the proton flux in the SAAregion. Both temporal variations had the anti-correlation with solar-activity cycle, and the simultaneous maximum in the beginning of2010, the same time reported in several results obtained fromcosmic ray observations on the ground (e.g., Ahluwalia andYgbuhay, 2011; Moraal and Stoker, 2010). These results are con-sistent with the suggestion that high-energy protons trapped in theinner radiation belt are considered to be originated from theinteraction between galactic cosmic rays and atmospheric atoms(e.g., Hess and Killeen, 1966; Selesnick et al., 2007), in addition tothe fact that the reduction of these protons caused by the scatteringprocess into the atmospheric loss cone is less effective during solar-

Fig. 5. (a) Monthly averaged F10.7 solar radio flux. (b) Monthly averaged Kp index. (c) Proton flux in the energy range from 8 MeV to 18 MeV at Geo-Stationary orbit with

1-min temporal resolution. (d) Monthly averaged heavy ion count in the polar region. (e) Monthly averaged proton flux in the energy range from 7 MeV to 18 MeV in the

South Atlantic Anomaly region. (f) Monthly averaged electron flux in the energy range from 0.6 MeV to 1.2 MeV in the South Atlantic Anomaly region. (g) Electron flux in

the energy range from 0.6 MeV to 1.2 MeV at Geo-Stationary orbit with 1-min temporal resolution. (h) Electron flux in the energy range from 0.6 MeV to 1.2 MeV in the

horn region with 1-min temporal resolution. All plots are during September 2006–February 2011.

H. Koshiishi, H. Matsumoto / Journal of Atmospheric and Solar-Terrestrial Physics 99 (2013) 129–133132

activity minimum period (e.g., Jentsch and Wibberenz, 1980;Miyoshi et al., 2000). Large enhancement in December 2006 asshown in Fig. 5(d) was due to the precipitation of solar energeticprotons as mentioned earlier.

Fig. 5(f) presents the monthly averaged electron flux in theSAA region, having the correlation with solar-activity cycle. Theminimum appeared in the beginning of 2010, because the injec-tion of electrons from the outer radiation belt into the inner

H. Koshiishi, H. Matsumoto / Journal of Atmospheric and Solar-Terrestrial Physics 99 (2013) 129–133 133

radiation belt had not happened since the last influence of solarand geomagnetic events in December 2006. Fig. 5(g,h), respec-tively, presents the electron flux at Geo-Stationary orbit and theelectron flux in the horn region. Both temporal variations had theperiodic modulation caused by high-speed solar wind that flowedfrom recurrent coronal holes on the solar surface, and thesimultaneous minimum in the beginning of 2010, the same timewhen there was minimum electron flux in the SAA region.Amplitude of modulation in the horn region was smaller thanthat at Geo-Stationary orbit due to the differences in the pitch-angle distribution.

4. Summary

The TEDA on board the ALOS satellite had measured the protonenvironment and the electron environment in low earth orbit duringsolar-activity minimum period. The activation of the electronenvironment in the inner radiation belt along with the 24th solar-activity cycle started in the beginning of 2010, 1 year after thebeginning of the 24th solar-activity cycle itself in the end of 2008.The electron environment in the outer radiation belt was almostalways modulated by solar wind variations; however, it showedvery low activities in the beginning of 2010 which was the sametime when the lowest activities were seen in the inner radiationbelt. Large enhancement in the electron environment in the outerradiation belt sometimes occurred due to strong geomagneticstorms, some of which showed inward diffusion and filled the slotregion, resulting in sources of the electron environment in the innerradiation belt. On the other hand, the proton environment in theinner radiation belt showed a slight increase as solar activity becamelower, and had a peak also in the beginning of 2010 , the same timewhen there was maximum galactic cosmic ray flux.

In contrast, long-term measurement in the 23rd solar-activitycycle showed that both the electron environment in the outerradiation belt and the proton environment in the inner radiationbelt had the simultaneous variations with the sunspot numbervariation (Li et al., 2001). Thus, 1-year delay of the response ofspace radiation environment around the Earth is peculiar, and issuggested to be because the beginning of the 24th solar-activitycycle was very quiet as compared with the several former solar-

activity cycles, and because variations in space radiation environ-ment were consequences of solar and geomagnetic activities.

Acknowledgments

F10.7 solar radio flux and Kp index are provided from SpacePhysics Interactive Data Resource (SPIDR) operated by NationalGeophysical Data Center (NGDC).

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