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Relativistic electron fluxes and dose rate variations observed on the international space station
Journal of Atmospheric and Solar-Terrestrial Physics 99 (2013) 150–156
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Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Relativistic electron fluxes and dose rate variations observed on the international space station Ts.P. Dachev a,n, B.T. Tomov a, Yu.N. Matviichuk a, Pl.G. Dimitrov a, N.G. Bankov a, G. Reitz b, c ¨ , M. Lebert d, M. Schuster d G. Horneck b, D.P. Hader a
Space Research and Technology Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria DLR, Institute of Aerospace Medicine, K¨ oln, Germany c Neue Str. 9, 91096 M¨ ohrendorf, Germany d Friedrich-Alexander-University, Department of Biology, Cell Biology Division, Erlangen, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2012 Received in revised form 27 May 2012 Accepted 14 July 2012 Available online 4 August 2012
The paper presents observations of relativistic electron precipitations (REP) on the International Space Station (ISS) obtained by three Bulgarian-built instruments flown in 2001 and 2008–2010. The first data are from the Liulin-E094 instrument flown in May–August 2001 inside the US laboratory module of the ISS. Next the time profiles of the REP-generated daily fluences and the absorbed doses at the orbit of ISS during the period February 2008–August 2010 are analyzed in dependence of the daily Ap index and compared with the daily relativistic electron fluence with energies of more than 2 MeV measured by the GOES. The REP in April 2010 being the second largest in GOES history (with a 4 2 MeV electron fluence event) is specially studied. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Space radiation Radiation belt Relativistic electrons ISS
1. Introduction Relativistic electron precipitations (REP) have been observed for many years. First reports are by Brown and Stone (1972), Imhof et al. (1986, 1991). The most comprehensive study of longterm observations of REP was made by Zheng et al. (2006), using the 2–6 MeV electron data from the SAMPEX satellite during 1992–2004. Relativistic electrons enhancements in the outer radiation belt are one of the major manifestations of space weather (Zheng et al., 2006; Wrenn, 2009) near the Earth’s orbit. Their understanding is of significant importance from both a practical and a space radiation physics point of view. Electrons with energies of a few MeV can penetrate the spacecraft shielding and can deposit significant charge in the dielectric materials, which after electrostatic breakdown can damage sensitive electronic preamplifiers and whole systems of the spacecraft. A similar event happened with the Galaxy 15 spacecraft (Green et al., 2010), which stopped
n
Corresponding author. Tel.: þ359 878366225. E-mail addresses: [email protected] (Ts.P. Dachev), [email protected] (B.T. Tomov), [email protected] (Yu.N. Matviichuk), [email protected] (G. Reitz), ¨ [email protected] (G. Horneck), [email protected] (D.P. Hader), [email protected] (M. Lebert), [email protected] (M. Schuster). 1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2012.07.007
responding to ground commands at the beginning of the period studied by us on 5th of April 2010 at 09:48 UTC. The total dose of an astronaut, who is spending 6 h on Extra Vehicular Activity (EVA) inside the REP, has been estimated in the United States report of the Committee on Solar and Space Physics and Committee on Solar-Terrestrial Research (2000). The conclusion is that the dose will be large enough to exceed the astronaut’s shortterm limits for both skin and eyes. One of the recommendations (3b on page 37) is: ‘‘As soon as possible, JSC should install an electron dosimeter and an ion dosimeter outside the ISS that can return data in real time to the Space Radiation Analysis Group (SRAG) at the Johnson Space Center’’. To our knowledge such dosimeters are still not installed outside the ISS. There is an another more disturbing fact that there is no active control of the doses accumulated by the American astronauts and Russian cosmonauts during EVA. In 2001 we observed for the first time relativistic electrons in the US laboratory module of the ISS in the data of the mobile dosimetry unit no. 2 (MDU#2), which was a part of the LiulinE094 instrument (Dachev et al., 2002a; Reitz et al., 2005). Because of the relatively small dose rates the effect was not fully understood. It was part of a presentation (Dachev et al., 2002b) but was not published. In this paper we present more comprehensive study of the whole dynamics of the latitudinal distribution of the ISS radiation environment parameters, which are affected by middle-term leaving belt centered at L¼ 1.6, outer radiation belt signatures at 3 oLo5 and solar proton event (SPE) at L45. L is
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the McIlwain’s parameter (McIlwain, 1961; Heynderickx et al., 1996). Relativistic electrons were observed by us outside the FotonM2/M3 spacecraft in the periods 31 May–16 June 2005 and 14–29 September 2007 and outside the European Columbus module of the ISS in 2008 (Dachev et al., 2009). The relativistic electrons observed on ISS in 2008 were connected with the geomagnetic field disturbances in the period from 27 February to 7 May 2008. Here we present the full range of data obtained by two independent Liulin type instruments in the period between February 2008 and August 2010. The main idea of the analysis of more than 750 daily fluences on ISS in this period by the R3DE/R instruments, which were part of ESA EXPOSE-E/R facilities (Horneck et al., 1998; Rabbow et al., 2009, in press) is to underline that the REP events are common on ISS. Although the obtained doses do not pose extreme risks for the astronauts being on EVA they have to be considered as permanently observed source, which requires additional comprehensive investigations.
2. Material and methods Three different Liulin type instruments (Dachev et al., 2011a) were used in this study: (a) The Liulin-E094 instrument contains four Mobile Dosimetry Units (MDU) (Dachev et al., 2002a, 2006). It was a part of the experiment Dosimetric Mapping-E094 (Reitz et al., 2005) placed in the US Laboratory module of the ISS as a part of the Human Research Facility in May–August, 2001. The main purpose of this experiment was to understand the dose rate distribution inside the ISS; the obtained data were used for statistical validation of the high-charge and energy (HZE) transport computer (HZETRN) model (Wilson et al., 2007; Nealy et al., 2007; Slaba et al., 2011). (b) The R3DE instrument, which was part of the EXPOSE-E facility on the EuTEF platform outside the Columbus module of ISS in the period March 2008-September 2009 (Horneck et al., 1998; Dachev, 2009; Dachev et al., in press). (c) The R3DR instrument, which was part of the EXPOSE-R facility outside the Russian Zvezda module of ISS in the period March 2009–August 2010 (Dachev et al., 2012b). The experiments with R3DE/R spectrometers were performed after successful participation in the ESA announcements of opportunities, led by the German colleagues Dr. Gerda Horneck ¨ and Prof. Donat-P. Hader (Horneck et al., 1998). The R3DE/R spectrometers were jointly developed with the colleagues from ¨ the University of Erlangen, Germany (Streb et al., 2002; Hader et al., 2009). Liulin-E094 and R3DE/R instruments are low mass ( 100 g), small-dimensioned (76 76 34 mm) automatic devices which measure solar radiation in 4 channels and ionizing radiation in 256 channels. They are Liulin type energy deposition spectrometers (Dachev et al., 2011a). They are all mounted in small size aluminum boxes and the ionizing radiation is monitored using a semiconductor PIN diode detector (2 cm2 in area, 0.3 mm thick). The main measurement unit in the spectrometers is the amplitude of the pulse after the preamplifier generated by particles or quanta hitting the detector (Dachev et al., 2002a). The amplitude of the pulse is proportional by a factor of 240 mV MeV 1 to the energy loss in the detector and to the dose, respectively. By 12 bit ADC, using only the oldest 8 bit, these amplitudes are digitized and organized in a 256-channel spectrum. The dose D [Gy] by definition is 1 J deposited in 1 kg of matter. We calculate the absorbed dose by dividing the integrated
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energy deposition in the spectrum in Joules to the mass of the detector in kilograms. D¼K
255 X
iki Ai MD1
ð1Þ
i¼1
where MD is the mass of the detector in kg, ki is the number of pulses in channel ‘‘i’’, Ai is the amplitude in volts of pulses in channel ‘‘i’’, K.i.ki.Ai is the deposited energy (energy loss) in Joules in channel ‘‘i’’. K is a coefficient. All 255 deposited dose values, depending on the deposited energy for one exposure time, form the deposited energy spectrum. The energy channel number 256 accumulates all pulses with amplitudes higher than the maximal level of the spectrometer of 20.83 MeV. The methods for characterization of the type of incoming space radiation are described by Dachev (2009). The construction of the Liulin-E094 and R3DE/R boxes consists of 1.0 mm thick aluminum shielding before the detector. The total shielding of the detector is formed by additional internal constructive shielding of 0.1 mm copper and 0.2 mm plastic material. The total external and internal shielding before the detector of the devices is less than 0.41 g cm 2. The calculated stopping energy of normally incident particles to the detector is 0.78 MeV for electrons and 15.8 MeV for protons (Berger et al., 2012). This means that only protons and electrons with energies higher than the above mentioned values can reach the detector. The Liulin-E094 MDUs was situated at different places inside the American laboratory module and Node-1 of the ISS (Nealy et al., 2007), while the R3DE/R instruments were outside the Columbus and Russian Zvezda modules, respectively. The amplitudes of the fluxes and doses with the Liulin-E094 MDUs are smaller than the R3DE/R amplitudes because of the additional shielding by the walls of the ISS, but all radiation sources are clearly seen in both the locations.
3. Overview of the solar and particle data for the periods of Liulin observations on the ISS Fig. 1 presents the variations of the daily particle fluences and daily F10.7 solar radio flux obtained from DPD.txt and DSD.txt files, prepared by the U.S. Dept. of Commerce, NOAA, Space Environment Center http://www.swpc.noaa.gov/ftpmenu/index.html. The analysis of the proton daily fluence with more than 100 MeV in Fig. 1 shows a relatively good correlation with the solar activity. Because
Fig. 1. Variations of the measured data by GOES satellites for the ( 42 MeV electrons fluence), 4100 MeV protons fluence and solar radio flux (F10.7 cm) for the period 1997–2010.
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of the very low solar activity at the end of the solar cycle 23 no major SPE were observed for more than 3 yrs between 2007 and 2010. The GOES satellite daily electron fluence with energies more than 2 MeV shows variations over almost 4 orders of magnitude. There is no well-defined dependence on the solar activity. The daily electron fluence decreases first close to the solar maximum. At the end of 2009, when the solar activity reached its minimal value, the daily electron fluence also reached an absolute minimum. Two extremely high fluence events are well seen. The first occurred on 29 July 2004 with a value of 9.3 109 cm 2 day 1 sr 1, while the second one was on 7 April 2010 with a value of 5.4 109 cm 2 day 1 sr 1. Black horizontal lines and labels above them (bottom of Fig. 1) indicate the time spans in which our observations were made with Liulin-E094 in 2001 and with R3DE/R instruments in 2008–2010.
4. Liulin-E094 data analysis 4.1. Information about the solar and particle parameters dynamics during the measurements with Liulin-E094 We choose to present and analyze only a small time interval between 11 and 29 May 2001 from the whole available LiulinE094 data between 11 May and 25 July 2001. In this period the ISS was in an elliptical orbit with an average altitude of 401 km. Variations of the altitude were between 384 and 424 km. The inclination was 52.811. The F10.7 daily solar radio flux at the beginning was 140 10 22 J m 2 s 1 Hz 1and increased up to 170 10 22 J m 2 s 1 Hz 1 at the end of that interval. The geomagnetic activity was moderate with a maximal Kp index about 5 on 12th–13th of May and declining later. The magnetic storm was on 10 May with Dst about 80 nT in the early UT hours. All solar geomagnetic data are obtained using the http://cdaweb. gsfc.nasa.gov/ server (King and Papitashvilli, 2012). As seen from the http://cdaweb.gsfc.nasa.gov/ server (King and Papitashvilli, 2012) on 20 May 2001, the energetic particle environment of the Earth was relatively quiet with a small solar proton event with a value of 1.8 104 cm 2 day 1 sr 1 for more than 100 MeV proton daily fluence. The value of the proton flux with energies above 10 MeV measured by the Isotope Spectrometer on the Advanced Composition Explorer (ACE) satellite (Stone et al., 1998, http://cdaweb.gsfc.nasa.gov/) reached its maximum on 20 May at 10:00 UTC with 14 protons (cm 2 s 1 sr 1 MeV 1). The GOES42 MeV daily fluence reached a maximum of 1.5 108 cm 2 day 1 sr 1 particles on 17 May (see Fig. 1). The daily electron flux at 2–6 MeV in the outer radiation belt measured by the SAMPEX satellite (Baker et al., 1993) was best illustrated in the paper of Zhang et al., 2006 in Fig. 2, which shows that the relativistic electron population in the other outer radiation belt covered the whole time between the days of the year (DOY) 130 (11 May) and 148 (29 May). The equatorward boundary of the REP flux maximum moved slowly from a value of about L¼3.2 to L¼3.4. The poleward boundary was at L close to 5. An enhancement and widening of the REP location occurred on day 139 (17 May). Another clear increase in the flux is observed at L values between 1.2 and 2.2 centered at L¼1.6. This is a type of middle-term leaving belt in the slot region which occurred after DOY 100 and disappeared after DOY 270 as seen from the figure cited above. 4.2. Analysis of the L value distribution of the Liulin-E094, MDU#2 data Dachev (2009) showed that the dose to flux ratio can characterize the type of the predominant radiation source in the Liulin type instruments in the near Earth radiation field. In this paper we will
Fig. 2. Dynamics of the ISS internal radiation environment for the time period 11 May–27 May 2001 as measured by MDU#2 of Liulin-E094 instrument. The labels and the asterisks in panel 2b show the positions of the mentioned in the text radiation sources.
apply these ideas for the Liulin-E094, MDU#2 data set and will present that even with an instrument that was developed for monitoring the absorbed dose of astronauts, and that was situated behind the relatively thick shielding of the ISS walls, it is possible to characterize the energetic particle environment (Dachev et al., 2011c) including the relativistic electrons population. Fig. 2 presents (in three panels) data obtained with MDU#2, which for the period 11–29 May 2001 was situated in one of the less shielded places of the US laboratory module called ‘‘US lab—open rack overhead seat track in retention net’’. The battery operated cigarette box size MDU was very close to the US lab wall orientated with the detector toward the wall. Data presented by Dachev et al. (2006) are obtained with the same MDU but that paper deals mainly with the South Atlantic Anomaly (SAA) inner belt high energy proton distribution. About 14,400 points with 30 s resolution for the days 12, 20, 21, 22, 28 and 29 May 2001 were used. The MDU#2 dose rate (mGy h 1) and integral flux (cm 2 s 1) measurements, and the calculated dose to flux ratio (D/F), or specific dose (nGy cm2 particle 1) are plotted as a function of the McIlwain’s L-parameter (McIlwain, 1961; Heynderickx et al., 1996). L corresponds to the equatorial radius of a magnetic drift shell in the case of a dipole field. The orbital parameters of ISS used in this paper are calculated by the KADR-2 software (Galperin et al., 1980). Organizing the data in this way one can show the different particle populations and how they were distributed in the space around the Earth. The specific dose calculated for each data point provides information about the type and energy of the particles that contributed to the counts recorded for
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one measurement interval of 30 s (SI unit) in the detector. This is in accordance with the experimentally obtained dependences between the energy of the incoming protons and electrons and the dose to flux ratio (Heffner, 1971). The space environment region mostly composed of 20–300 MeV protons should be characterized by a specific dose in the range 1.12–3.2 nGy cm2 part 1 (the highest ratio values corresponding to the lowest proton energies), while a population of relativistic electrons with energies 1–10 MeV is characterized by a specific dose 0.4–0.45 nGy cm2 part 1 (e.g. Dachev, 2009). For Bremsstrahlung X-rays the dose to flux ratio is less than the electrons ratio. Three different radiation sources are visually distinguished from the data presented in Fig. 2b: Galactic cosmic rays (GCR), SAA (inner belt) protons and outer radiation belt (ORB) electrons. The GCR source is seen in the bottom area of the panels with dose rates between 0.5 and 10 nGy cm2 part 1. There is a distinct knee in the GCR distribution at L¼2.4. The GCR source produces the bunch of points around 1 nGy cm2 part 1 seen in Fig. 2a. The maximum at the left side of Fig. 2b and c is generated mainly by inner radiation belt protons in the region of the SAA for L values less than 1.6. This maximum reached the highest dose rate and flux values, but it is not further analyzed in this paper. Another source starts to contribute to the doses and fluxes in the range of L values from 1.6 to 2.4. This source, labeled with BRM, is recognized in Fig. 2b as a triangle with a larger density of points with values from 1–1.5 up to 5 cm 2 s 1. In Fig. 2a this source is seen as a small down looking maximum with values between 0.5 and 0.6 nGy cm2 part 1. When we plotted the locations of these events in geographic coordinates we found that they are situated as a belt at south–east direction from the SAA maximum. Our explanation of these very low specific depositions is that we recorded the Bremsstrahlung from the relativistic electrons outside the station. The electrons with energy of 10 MeV have only 22 mm range in aluminum (Daly et al., 1996) and are not able to penetrate all shielding materials and to reach the detector of the MDU. According to Zhang et al. (2006) at this L location is the middle-term leaving belt in the slot region. In the dose rate and flux panels we do not have enough resolution to distinguish between electrons and protons but in the dose to flux panel this belt is well recognized with the lowest dose to flux (D/F) values (Fig. 2a). In Fig. 2b we observe in the range of L values between 3.4 and 4.8 a number of points with flux values of up to 5 cm 2 s 1. These points do not have adequate high values of dose rates in the Fig. 2b. Our interpretation is that these higher flux points are the Bremsstrahlung signatures of outer radiation belt relativistic electrons because their D/F ratio seen in Fig. 2a is between 0.6 and 0.8 nGy cm2 part 1. The high latitude region L45 of Fig. 2b can be easily divided in two parts: the lower part with higher density of points is from GCR, while the upper part with flux values above 2 cm 2 s 1 is from the SPE protons observed on 20 May. The considerations taken above are confirmed with the shapes of the deposited energy spectra from the MDUs, which are different for the different radiation sources and can be found on-line in Fig. 1 in the paper published by Dachev (2008). The values shown in Fig. 2a with the abbreviation ALB having specific dose deposititon higher than 3 nGy cm2 part 1 at low L values was most probably generated by direct registration of albedo neutrons (Nakamura, 2008). According to the ‘‘neutron induced nuclear counter effect’’ introduced for the Hamamatsu PIN diodes type S2744-08 (used also in the Liulin-E094 MDUs) by Zhang et al. (2011) neutrons could be observed in all channels of the spectrum of the Liulin-E094 MDUs. The observed daily REP dose rates for 3.4 oLo4.8 were calculated using the assumption that the average dose rate from GCR is 7 mGy h 1 and that the dose rates above this value are build up by relativistic electrons and/or Bremsstrahlung. The
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highest value of 3.22 mGy day 1 was found on 20 May 2001, while the lowest on 29 May with value of 0.27 mGy day 1. In conclusion, the analysis of the Liulin-E094 MDU#2 data shows that even with an instrument, which was developed as a personal dosimeter, it is possible to distinguish between different radiation sources inside the ISS and to obtain an adequate picture of their variations.
5. R3DE data analysis The R3DE instrument was situated outside the Columbus module of the ISS and had only its own shielding of less than 0.4 g cm 2 (Dachev, 2009). Our first understanding that we did measure fluxes and doses from REP with energies above 0.78 MeV was developed on the base of the R3DE data. Later we revised our Foton-M2/M3 data and prepared and published the results (Dachev et al., 2009; Damasso et al., 2009), which present mainly the Foton-M2/M3 REP data. The R3DE data cover only the period 20 February–28 April, 2008, while this paper analyses all available R3DE data between February 2008 and September 2009. The selection procedure, which allows us to distinguish between three different radiation sources (GCR, SAA and ORB) seen by the R3DE instrument, is based on the analysis of the dose to fluxes values (Dachev, 2009) and on the methodology described in Bankov et al. (2010), which is based on a polynomial line dividing different sources in dependence of the L value. Fig. 3 accumulates in the bottom panel all available R3DE data of averaged daily fluences of relativistic electrons (ISS Flu) and daily absorbed dose rates (ISS AD). In the period between 1 September 2008 and 12 November 2012 there are no data from R3DE instrument. The number of different single 10-seconds measurements, which was used in the process of averaging the fluence data, was between 1 and 238. On average this number was 36.8 measurements per day. The places of averaged ISS fluence data were distributed over a relatively wide range of L
Fig. 3. Results for the measured with R3DE instrument on ISS daily 40.78 MeV fluence (ISS Flu) and the dose rate (ISS AD) deposited by it for the whole operational time between February 2008 and September 2009. These data are compared with the daily GOES-11 satellite 42 MeV (GOES) fluence and the daily global Ap index.
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values between 3.4 and 5.6, on average at L¼ 4.4. These measurements were compared with the GOES-11 daily fluence data at L¼6.6 for energies above 2 MeV and the daily global Ap index (upper panel). All curves represent the moving average over 2 points of the raw data. As seen from the upper panel in Fig. 3 the period between February 2008 and August 2009 is characterized by declining solar and magnetic activity, respectively, which is the main reason for the declining REP activity. Really the period can be divided into two parts. The first one finished in August 2008 when a real decreasing trend is observed in all parameters seen in Fig. 3. The second one starts after November 2008 when the daily global Ap index stays almost at very low levels. In this period the R3DE data are also practically horizontal lines. Only the GOES-11 daily fluence data continue to decrease. A close look at the data in Fig. 3 confirms that each increase of the Ap or each new magnetic storm increased the fluence recorded on both satellites, GOES and ISS (Zheng et al., 2006). The daily R3DE absorbed dose followed very closely the daily fluence because of a strong linear dependence between them as seen from formulae (1). When we plotted the daily absorbed dose in dependence of the daily fluence we revealed a straight line with a coefficient of linear regression R2 ¼0.9984 and a slope equal to 0.3104 nGy cm2 part 1. According the Heffner’s formulae (Heffner, 1970; Dachev, 2009) this slope value corresponds to a mean REP energy equal to 2.1 MeV.
6. R3DR data analysis The R3DR instrument was situated outside the Russian Zvezda module of the ISS. The available data covered two main periods: March–June 2009 and December 2009–August 2010. In the period between 24 June 2009 and 1 January 2010 there are no data from the R3DR instrument. Fig. 4 is designed in the same way as Fig. 3. Except the large data gap between July and January 2009 there are two smaller periods of data gaps in January–February 2010 and in the middle of March 2010. A close look on the GOES-11 daily fluence
Fig. 4. Results for the measured with R3DR instrument on ISS daily 40.78 MeV fluence (ISS flu) and the dose rate (ISS AD) deposited by it for the whole operational time between March 2009 and August 2010. These data are compared with the dayly GOES-11 satellite 42 MeV (GOES) fluence and the daily global Ap index.
data for these periods shows that except in the middle of March 2010 the fluence stays at very low levels as seen in Fig. 1. The R3DR instrument had the same shielding of its own as the R3DE instrument but the surrounding shielding was lower and this increased strongly the inner radiation belt energetic proton population (Dachev et al., 2011b). Here we have the opportunity to compare the outer belt electron fluences and dose rates obtained by the R3DE/ R instruments for the period March–June 2009 and to conclude that the energetic electron population also increased because of the smaller surrounding shielding. For example the R3DR daily fluences in the middle of March 2009 reached values of 105 cm 2 day 1 (see Fig. 4), while the R3DE fluences were only 3 104 cm 2 day 1 (Fig. 3). The same happened with the daily absorbed doses. The fact that the doses were distributed in dependence of the 3D mass distribution on the ISS supported again the conclusions by the Committee on Solar and Space Physics and Committee on SolarTerrestrial Research (2000), that only internal radiation monitoring is not sufficient for the astronauts/cosmonauts during EVAs. The results of measurements made during Expedition 4–6 on the ISS by the Canadian instrument EVA Radiation Monitoring (EVARM) (Thomson and Nielsen, 1999), which used tiny metal oxide semiconductor field effect transistor (MOSFET) dosimeters, also noted that ‘‘shielding from the station significantly affected the dose of radiation each badge received’’ ,http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/ 20070014542_2007014665.pdf. Personal dosimeters with abilities to distinguish between different radiation sources and to measure doses from REP were highly recommended by the mentioned Committee in 2000 but are still not implemented during EVAs on ISS. The most interesting period in Fig. 4 began on April 2010 and covered all data till 20 August. The R3DR and GOES-11 daily relativistic electron fluences almost explosively increased on 6 and 7 April as seen on Figs. 4 and 5. The consequences of the space weather processes, which provoked this increase can be described as follows (Space Weather Highlights, 5–11 April 2010, SWO PRF 1806, 13 April 2010, http://www.swpc.noaa.gov/): (1) Solar activity was at very low levels with isolated low-level B-class flares. (2) A halo coronal mass ejection (CME) is observed on 03/0954 UTC. (3) About 2 days later a shock was observed at ACE at 05/0756 UTC, which led to a sudden impulse on Earth at 05/0826 UTC (38 nT was observed at the Boulder magnetometer). (4) The ACE satellite observed wind speeds between 720– 800 km s 1 behind the shock, with Bz reaching values around 15 nT.
Fig. 5. Variations of the measured by R3DR instrument data for the ( 40.78 MeV electron fluence) and accumulated ORB, GCR, SAA and Tot (Total) dose rates together with the daily Ap index for the period 1 April–6 May 2010.
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(5) The activity continued on 6 April with predominantly active to minor storm levels, as well as an interval of minor to major storm levels observed between 06/0000–0600 UTC. (6) Wind speeds at ACE decreased to about 550 km s 1 at 06/2100 UTC. Although the created magnetic storm on 6 April was moderate (daily Ap¼49, minimal Dst¼ 72 nT at about noon), the second largest values in history of GOES fluences of electrons with energies 42 MeV were measured (see Figs. 1 and 4). The increase in the GOES-11 fluence of electrons with energies more than 2 MeV was by 4.5 orders of magnitude, while the R3DR40.78 MeV fluences increased less than 4 orders of magnitude. Till the end of measurements with the R3DR instrument at 20 August 2010, a few more smaller Ap maxima were observed and they were reflected by very similar responses on the GOES satellite, while the ISS data correlation was much smaller. Fig. 5 highlights the dynamics of the accumulation of the R3DR doses generated by ORB electrons, SAA protons, GCR particles and the sum of them (Tot). The ORB daily fluence divided by 500 is also presented. The daily Ap index is plotted using the right side axes. It is seen that the GCR accumulated dose had about 73 mGy day 1 as lowest daily rate. Another parameter, which had an almost linear accumulation, are the SAA inner radiation belt proton doses with an average daily dose rate of 409 mGy day 1. The accumulated ORB daily dose rate by electrons with energies greater than 0.78 MeV was a function of the daily fluence and showed a nonlinear dependence with a maximum in the period 5–10 April 2010. The largest daily increase occurred on 7 April with 2300 mGy day 1 generated by a fluence increase with more than 4 million electrons per day with energies above 0.78 MeV. This enhancement was in response to the magnetic storm on 6 April and reached its maximum during the recovery phase of it. Further, at every small disturbance of Ap on 12, 15 and 23 April, the R3DR fluence rose up, but the fastest changes of Ap occurred on 6 April and on 2 May. The geomagnetic conditions on 2–3 May 2010 were similar in magnitude, but the response of the R3DR daily fluence during the 3 May storm was about 1.5 orders of magnitude less than the 5–6 April response. We do not have any explanation of the reasons for these large differences in the responses and we hope that our experimental data will support the theoretical study of the REP physics. Using the same procedure as described for the R3DE instrument with plotting the absorbed dose in dependence of the fluence we found that the slope was higher and reached 0.3303 nGy cm 2 part 1. According the Heffner’s formulae (Heffner, 1970; Dachev, 2009) this value of the slope corresponds to a mean electron energy equal to 4.7 MeV.
7. Discussion and conclusions The paper presents observations of REP on the ISS obtained by three Bulgarian-built instruments flown in 2001 and during 2008–2010. The 2001 data cover a relatively small time interval of 18 days in May 2001, while the 2008–2010 data covered, with few exceptions, almost the whole time between February 2008 and August 2010. The three instruments being behind different shielding registered in all cases REP but the deposited dose rates varied over 3 orders of magnitude. The smallest were the daily dose rates observed with MDU#2 of the Liulin-E094 instrument, situated inside the US laboratory module of the ISS in May 2001. They did not exceed 3–4 mGy day 1. The relativistic electrons dose rates values measured with R3DE instrument being outside the ISS were higher and reached values of about 200 mGy day 1. The highest values of 2347 mGy day 1 were observed by the R3DR instrument outside the Russian ‘‘Zvezda’’ module on 7 April 2010
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during the second largest REP in GOES history. The comparison between the relativistic electron dose rate data measured by R3DE and R3DR show that during the periods of simultaneous operation the R3DR dose rates are higher than the R3DE dose rates because the R3DR instrument was situated in a less shielded surrounding. This allows the conclusion that the astronauts being on EVA will collect highly variable dose rates during REP in dependence of their position around the station. Only active personal dosimeters (Dachev et al., 2010) will be able to measure these large variations. We attempted to correlate the amplitudes of GOES and ISS relativistic electrons fluences and found a relatively small coefficient of linear regression R2 ¼ 0.5397, which means that there is no direct connection between both locations in the magnetosphere. This can be explained by the fact that the GOES satellites occupy a relatively small range of L values, while the ISS crosses all latitudes between L¼3 and L¼6.3. The analysis of the absorbed daily doses obtained by the R3DE/R instruments shows that during the quiet geomagnetic conditions they are at very low levels of a few to a few tens of mGy day 1 and do not pose any serious risk for the astronauts being on EVA where they are shielded only by their space suits, which have a shielding characteristics similar to our instruments of 0.4–0.5 g cm 2 (Benton et al., 2006; Cucinotta et al., 2003). In the case of very high daily relativistic electron fluxes as on 7 April 2010 the daily absorbed dose increased up to 2300 mGy day 1, which is much higher than the other daily sources of GCR (80–90 mGy day 1) and inner belt protons (400–500 mGy day 1). During this period, three EVAs were performed by the STS-131 astronauts on 9, 11 and 13 April 2010 (http://www.nasa.gov/mission_pages/shuttle/shuttle missions/sts131/main/index.html). The accumulated doses (mainly from ORB and GCR) calculated from our R3DR measurements during the ca. 6 h EVAs were between 440 and 300 mGy; these values pose no extreme danger to the health of the astronauts because the daily average absorbed dose rates reported by Reitz et al. (2005) inside of the ISS vary in the range 74–215 mGy day 1. On the other hand the relativistic electrons did not have enough energy to penetrate into the body of the astronauts and therefore deposited their dose mainly in the skin and eyes. For the first time we made an attempt to calculate the mean energy of the electrons reaching the detector of the R3DE/R instruments and found that the R3DR data, which are collected during intense REP conditions in 2010, show higher energies, which is reasonable. In conclusion, we would like to mention that the R3DE/R, low mass, dimension and price instruments, proved their ability to characterize the outside ISS radiation environment in a case of relativistic electron precipitations. This was achieved mainly with the analysis of the deposited energy spectra, which was obtained at each measurement cycle of 10 s. The main conclusion of the presented data is that REP events are common on the ISS. Although the obtained doses do not pose extreme risks for astronauts being on EVA they have to be considered as a permanently observed source, which requires additional comprehensive investigations.
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Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Relativistic electron fluxes and dose rate variations observed on the international space station Ts.P. Dachev a,n, B.T. Tomov a, Yu.N. Matviichuk a, Pl.G. Dimitrov a, N.G. Bankov a, G. Reitz b, c ¨ , M. Lebert d, M. Schuster d G. Horneck b, D.P. Hader a
Space Research and Technology Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria DLR, Institute of Aerospace Medicine, K¨ oln, Germany c Neue Str. 9, 91096 M¨ ohrendorf, Germany d Friedrich-Alexander-University, Department of Biology, Cell Biology Division, Erlangen, Germany b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2012 Received in revised form 27 May 2012 Accepted 14 July 2012 Available online 4 August 2012
The paper presents observations of relativistic electron precipitations (REP) on the International Space Station (ISS) obtained by three Bulgarian-built instruments flown in 2001 and 2008–2010. The first data are from the Liulin-E094 instrument flown in May–August 2001 inside the US laboratory module of the ISS. Next the time profiles of the REP-generated daily fluences and the absorbed doses at the orbit of ISS during the period February 2008–August 2010 are analyzed in dependence of the daily Ap index and compared with the daily relativistic electron fluence with energies of more than 2 MeV measured by the GOES. The REP in April 2010 being the second largest in GOES history (with a 4 2 MeV electron fluence event) is specially studied. & 2012 Elsevier Ltd. All rights reserved.
Keywords: Space radiation Radiation belt Relativistic electrons ISS
1. Introduction Relativistic electron precipitations (REP) have been observed for many years. First reports are by Brown and Stone (1972), Imhof et al. (1986, 1991). The most comprehensive study of longterm observations of REP was made by Zheng et al. (2006), using the 2–6 MeV electron data from the SAMPEX satellite during 1992–2004. Relativistic electrons enhancements in the outer radiation belt are one of the major manifestations of space weather (Zheng et al., 2006; Wrenn, 2009) near the Earth’s orbit. Their understanding is of significant importance from both a practical and a space radiation physics point of view. Electrons with energies of a few MeV can penetrate the spacecraft shielding and can deposit significant charge in the dielectric materials, which after electrostatic breakdown can damage sensitive electronic preamplifiers and whole systems of the spacecraft. A similar event happened with the Galaxy 15 spacecraft (Green et al., 2010), which stopped
n
Corresponding author. Tel.: þ359 878366225. E-mail addresses: [email protected] (Ts.P. Dachev), [email protected] (B.T. Tomov), [email protected] (Yu.N. Matviichuk), [email protected] (G. Reitz), ¨ [email protected] (G. Horneck), [email protected] (D.P. Hader), [email protected] (M. Lebert), [email protected] (M. Schuster). 1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2012.07.007
responding to ground commands at the beginning of the period studied by us on 5th of April 2010 at 09:48 UTC. The total dose of an astronaut, who is spending 6 h on Extra Vehicular Activity (EVA) inside the REP, has been estimated in the United States report of the Committee on Solar and Space Physics and Committee on Solar-Terrestrial Research (2000). The conclusion is that the dose will be large enough to exceed the astronaut’s shortterm limits for both skin and eyes. One of the recommendations (3b on page 37) is: ‘‘As soon as possible, JSC should install an electron dosimeter and an ion dosimeter outside the ISS that can return data in real time to the Space Radiation Analysis Group (SRAG) at the Johnson Space Center’’. To our knowledge such dosimeters are still not installed outside the ISS. There is an another more disturbing fact that there is no active control of the doses accumulated by the American astronauts and Russian cosmonauts during EVA. In 2001 we observed for the first time relativistic electrons in the US laboratory module of the ISS in the data of the mobile dosimetry unit no. 2 (MDU#2), which was a part of the LiulinE094 instrument (Dachev et al., 2002a; Reitz et al., 2005). Because of the relatively small dose rates the effect was not fully understood. It was part of a presentation (Dachev et al., 2002b) but was not published. In this paper we present more comprehensive study of the whole dynamics of the latitudinal distribution of the ISS radiation environment parameters, which are affected by middle-term leaving belt centered at L¼ 1.6, outer radiation belt signatures at 3 oLo5 and solar proton event (SPE) at L45. L is
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the McIlwain’s parameter (McIlwain, 1961; Heynderickx et al., 1996). Relativistic electrons were observed by us outside the FotonM2/M3 spacecraft in the periods 31 May–16 June 2005 and 14–29 September 2007 and outside the European Columbus module of the ISS in 2008 (Dachev et al., 2009). The relativistic electrons observed on ISS in 2008 were connected with the geomagnetic field disturbances in the period from 27 February to 7 May 2008. Here we present the full range of data obtained by two independent Liulin type instruments in the period between February 2008 and August 2010. The main idea of the analysis of more than 750 daily fluences on ISS in this period by the R3DE/R instruments, which were part of ESA EXPOSE-E/R facilities (Horneck et al., 1998; Rabbow et al., 2009, in press) is to underline that the REP events are common on ISS. Although the obtained doses do not pose extreme risks for the astronauts being on EVA they have to be considered as permanently observed source, which requires additional comprehensive investigations.
2. Material and methods Three different Liulin type instruments (Dachev et al., 2011a) were used in this study: (a) The Liulin-E094 instrument contains four Mobile Dosimetry Units (MDU) (Dachev et al., 2002a, 2006). It was a part of the experiment Dosimetric Mapping-E094 (Reitz et al., 2005) placed in the US Laboratory module of the ISS as a part of the Human Research Facility in May–August, 2001. The main purpose of this experiment was to understand the dose rate distribution inside the ISS; the obtained data were used for statistical validation of the high-charge and energy (HZE) transport computer (HZETRN) model (Wilson et al., 2007; Nealy et al., 2007; Slaba et al., 2011). (b) The R3DE instrument, which was part of the EXPOSE-E facility on the EuTEF platform outside the Columbus module of ISS in the period March 2008-September 2009 (Horneck et al., 1998; Dachev, 2009; Dachev et al., in press). (c) The R3DR instrument, which was part of the EXPOSE-R facility outside the Russian Zvezda module of ISS in the period March 2009–August 2010 (Dachev et al., 2012b). The experiments with R3DE/R spectrometers were performed after successful participation in the ESA announcements of opportunities, led by the German colleagues Dr. Gerda Horneck ¨ and Prof. Donat-P. Hader (Horneck et al., 1998). The R3DE/R spectrometers were jointly developed with the colleagues from ¨ the University of Erlangen, Germany (Streb et al., 2002; Hader et al., 2009). Liulin-E094 and R3DE/R instruments are low mass ( 100 g), small-dimensioned (76 76 34 mm) automatic devices which measure solar radiation in 4 channels and ionizing radiation in 256 channels. They are Liulin type energy deposition spectrometers (Dachev et al., 2011a). They are all mounted in small size aluminum boxes and the ionizing radiation is monitored using a semiconductor PIN diode detector (2 cm2 in area, 0.3 mm thick). The main measurement unit in the spectrometers is the amplitude of the pulse after the preamplifier generated by particles or quanta hitting the detector (Dachev et al., 2002a). The amplitude of the pulse is proportional by a factor of 240 mV MeV 1 to the energy loss in the detector and to the dose, respectively. By 12 bit ADC, using only the oldest 8 bit, these amplitudes are digitized and organized in a 256-channel spectrum. The dose D [Gy] by definition is 1 J deposited in 1 kg of matter. We calculate the absorbed dose by dividing the integrated
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energy deposition in the spectrum in Joules to the mass of the detector in kilograms. D¼K
255 X
iki Ai MD1
ð1Þ
i¼1
where MD is the mass of the detector in kg, ki is the number of pulses in channel ‘‘i’’, Ai is the amplitude in volts of pulses in channel ‘‘i’’, K.i.ki.Ai is the deposited energy (energy loss) in Joules in channel ‘‘i’’. K is a coefficient. All 255 deposited dose values, depending on the deposited energy for one exposure time, form the deposited energy spectrum. The energy channel number 256 accumulates all pulses with amplitudes higher than the maximal level of the spectrometer of 20.83 MeV. The methods for characterization of the type of incoming space radiation are described by Dachev (2009). The construction of the Liulin-E094 and R3DE/R boxes consists of 1.0 mm thick aluminum shielding before the detector. The total shielding of the detector is formed by additional internal constructive shielding of 0.1 mm copper and 0.2 mm plastic material. The total external and internal shielding before the detector of the devices is less than 0.41 g cm 2. The calculated stopping energy of normally incident particles to the detector is 0.78 MeV for electrons and 15.8 MeV for protons (Berger et al., 2012). This means that only protons and electrons with energies higher than the above mentioned values can reach the detector. The Liulin-E094 MDUs was situated at different places inside the American laboratory module and Node-1 of the ISS (Nealy et al., 2007), while the R3DE/R instruments were outside the Columbus and Russian Zvezda modules, respectively. The amplitudes of the fluxes and doses with the Liulin-E094 MDUs are smaller than the R3DE/R amplitudes because of the additional shielding by the walls of the ISS, but all radiation sources are clearly seen in both the locations.
3. Overview of the solar and particle data for the periods of Liulin observations on the ISS Fig. 1 presents the variations of the daily particle fluences and daily F10.7 solar radio flux obtained from DPD.txt and DSD.txt files, prepared by the U.S. Dept. of Commerce, NOAA, Space Environment Center http://www.swpc.noaa.gov/ftpmenu/index.html. The analysis of the proton daily fluence with more than 100 MeV in Fig. 1 shows a relatively good correlation with the solar activity. Because
Fig. 1. Variations of the measured data by GOES satellites for the ( 42 MeV electrons fluence), 4100 MeV protons fluence and solar radio flux (F10.7 cm) for the period 1997–2010.
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of the very low solar activity at the end of the solar cycle 23 no major SPE were observed for more than 3 yrs between 2007 and 2010. The GOES satellite daily electron fluence with energies more than 2 MeV shows variations over almost 4 orders of magnitude. There is no well-defined dependence on the solar activity. The daily electron fluence decreases first close to the solar maximum. At the end of 2009, when the solar activity reached its minimal value, the daily electron fluence also reached an absolute minimum. Two extremely high fluence events are well seen. The first occurred on 29 July 2004 with a value of 9.3 109 cm 2 day 1 sr 1, while the second one was on 7 April 2010 with a value of 5.4 109 cm 2 day 1 sr 1. Black horizontal lines and labels above them (bottom of Fig. 1) indicate the time spans in which our observations were made with Liulin-E094 in 2001 and with R3DE/R instruments in 2008–2010.
4. Liulin-E094 data analysis 4.1. Information about the solar and particle parameters dynamics during the measurements with Liulin-E094 We choose to present and analyze only a small time interval between 11 and 29 May 2001 from the whole available LiulinE094 data between 11 May and 25 July 2001. In this period the ISS was in an elliptical orbit with an average altitude of 401 km. Variations of the altitude were between 384 and 424 km. The inclination was 52.811. The F10.7 daily solar radio flux at the beginning was 140 10 22 J m 2 s 1 Hz 1and increased up to 170 10 22 J m 2 s 1 Hz 1 at the end of that interval. The geomagnetic activity was moderate with a maximal Kp index about 5 on 12th–13th of May and declining later. The magnetic storm was on 10 May with Dst about 80 nT in the early UT hours. All solar geomagnetic data are obtained using the http://cdaweb. gsfc.nasa.gov/ server (King and Papitashvilli, 2012). As seen from the http://cdaweb.gsfc.nasa.gov/ server (King and Papitashvilli, 2012) on 20 May 2001, the energetic particle environment of the Earth was relatively quiet with a small solar proton event with a value of 1.8 104 cm 2 day 1 sr 1 for more than 100 MeV proton daily fluence. The value of the proton flux with energies above 10 MeV measured by the Isotope Spectrometer on the Advanced Composition Explorer (ACE) satellite (Stone et al., 1998, http://cdaweb.gsfc.nasa.gov/) reached its maximum on 20 May at 10:00 UTC with 14 protons (cm 2 s 1 sr 1 MeV 1). The GOES42 MeV daily fluence reached a maximum of 1.5 108 cm 2 day 1 sr 1 particles on 17 May (see Fig. 1). The daily electron flux at 2–6 MeV in the outer radiation belt measured by the SAMPEX satellite (Baker et al., 1993) was best illustrated in the paper of Zhang et al., 2006 in Fig. 2, which shows that the relativistic electron population in the other outer radiation belt covered the whole time between the days of the year (DOY) 130 (11 May) and 148 (29 May). The equatorward boundary of the REP flux maximum moved slowly from a value of about L¼3.2 to L¼3.4. The poleward boundary was at L close to 5. An enhancement and widening of the REP location occurred on day 139 (17 May). Another clear increase in the flux is observed at L values between 1.2 and 2.2 centered at L¼1.6. This is a type of middle-term leaving belt in the slot region which occurred after DOY 100 and disappeared after DOY 270 as seen from the figure cited above. 4.2. Analysis of the L value distribution of the Liulin-E094, MDU#2 data Dachev (2009) showed that the dose to flux ratio can characterize the type of the predominant radiation source in the Liulin type instruments in the near Earth radiation field. In this paper we will
Fig. 2. Dynamics of the ISS internal radiation environment for the time period 11 May–27 May 2001 as measured by MDU#2 of Liulin-E094 instrument. The labels and the asterisks in panel 2b show the positions of the mentioned in the text radiation sources.
apply these ideas for the Liulin-E094, MDU#2 data set and will present that even with an instrument that was developed for monitoring the absorbed dose of astronauts, and that was situated behind the relatively thick shielding of the ISS walls, it is possible to characterize the energetic particle environment (Dachev et al., 2011c) including the relativistic electrons population. Fig. 2 presents (in three panels) data obtained with MDU#2, which for the period 11–29 May 2001 was situated in one of the less shielded places of the US laboratory module called ‘‘US lab—open rack overhead seat track in retention net’’. The battery operated cigarette box size MDU was very close to the US lab wall orientated with the detector toward the wall. Data presented by Dachev et al. (2006) are obtained with the same MDU but that paper deals mainly with the South Atlantic Anomaly (SAA) inner belt high energy proton distribution. About 14,400 points with 30 s resolution for the days 12, 20, 21, 22, 28 and 29 May 2001 were used. The MDU#2 dose rate (mGy h 1) and integral flux (cm 2 s 1) measurements, and the calculated dose to flux ratio (D/F), or specific dose (nGy cm2 particle 1) are plotted as a function of the McIlwain’s L-parameter (McIlwain, 1961; Heynderickx et al., 1996). L corresponds to the equatorial radius of a magnetic drift shell in the case of a dipole field. The orbital parameters of ISS used in this paper are calculated by the KADR-2 software (Galperin et al., 1980). Organizing the data in this way one can show the different particle populations and how they were distributed in the space around the Earth. The specific dose calculated for each data point provides information about the type and energy of the particles that contributed to the counts recorded for
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one measurement interval of 30 s (SI unit) in the detector. This is in accordance with the experimentally obtained dependences between the energy of the incoming protons and electrons and the dose to flux ratio (Heffner, 1971). The space environment region mostly composed of 20–300 MeV protons should be characterized by a specific dose in the range 1.12–3.2 nGy cm2 part 1 (the highest ratio values corresponding to the lowest proton energies), while a population of relativistic electrons with energies 1–10 MeV is characterized by a specific dose 0.4–0.45 nGy cm2 part 1 (e.g. Dachev, 2009). For Bremsstrahlung X-rays the dose to flux ratio is less than the electrons ratio. Three different radiation sources are visually distinguished from the data presented in Fig. 2b: Galactic cosmic rays (GCR), SAA (inner belt) protons and outer radiation belt (ORB) electrons. The GCR source is seen in the bottom area of the panels with dose rates between 0.5 and 10 nGy cm2 part 1. There is a distinct knee in the GCR distribution at L¼2.4. The GCR source produces the bunch of points around 1 nGy cm2 part 1 seen in Fig. 2a. The maximum at the left side of Fig. 2b and c is generated mainly by inner radiation belt protons in the region of the SAA for L values less than 1.6. This maximum reached the highest dose rate and flux values, but it is not further analyzed in this paper. Another source starts to contribute to the doses and fluxes in the range of L values from 1.6 to 2.4. This source, labeled with BRM, is recognized in Fig. 2b as a triangle with a larger density of points with values from 1–1.5 up to 5 cm 2 s 1. In Fig. 2a this source is seen as a small down looking maximum with values between 0.5 and 0.6 nGy cm2 part 1. When we plotted the locations of these events in geographic coordinates we found that they are situated as a belt at south–east direction from the SAA maximum. Our explanation of these very low specific depositions is that we recorded the Bremsstrahlung from the relativistic electrons outside the station. The electrons with energy of 10 MeV have only 22 mm range in aluminum (Daly et al., 1996) and are not able to penetrate all shielding materials and to reach the detector of the MDU. According to Zhang et al. (2006) at this L location is the middle-term leaving belt in the slot region. In the dose rate and flux panels we do not have enough resolution to distinguish between electrons and protons but in the dose to flux panel this belt is well recognized with the lowest dose to flux (D/F) values (Fig. 2a). In Fig. 2b we observe in the range of L values between 3.4 and 4.8 a number of points with flux values of up to 5 cm 2 s 1. These points do not have adequate high values of dose rates in the Fig. 2b. Our interpretation is that these higher flux points are the Bremsstrahlung signatures of outer radiation belt relativistic electrons because their D/F ratio seen in Fig. 2a is between 0.6 and 0.8 nGy cm2 part 1. The high latitude region L45 of Fig. 2b can be easily divided in two parts: the lower part with higher density of points is from GCR, while the upper part with flux values above 2 cm 2 s 1 is from the SPE protons observed on 20 May. The considerations taken above are confirmed with the shapes of the deposited energy spectra from the MDUs, which are different for the different radiation sources and can be found on-line in Fig. 1 in the paper published by Dachev (2008). The values shown in Fig. 2a with the abbreviation ALB having specific dose deposititon higher than 3 nGy cm2 part 1 at low L values was most probably generated by direct registration of albedo neutrons (Nakamura, 2008). According to the ‘‘neutron induced nuclear counter effect’’ introduced for the Hamamatsu PIN diodes type S2744-08 (used also in the Liulin-E094 MDUs) by Zhang et al. (2011) neutrons could be observed in all channels of the spectrum of the Liulin-E094 MDUs. The observed daily REP dose rates for 3.4 oLo4.8 were calculated using the assumption that the average dose rate from GCR is 7 mGy h 1 and that the dose rates above this value are build up by relativistic electrons and/or Bremsstrahlung. The
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highest value of 3.22 mGy day 1 was found on 20 May 2001, while the lowest on 29 May with value of 0.27 mGy day 1. In conclusion, the analysis of the Liulin-E094 MDU#2 data shows that even with an instrument, which was developed as a personal dosimeter, it is possible to distinguish between different radiation sources inside the ISS and to obtain an adequate picture of their variations.
5. R3DE data analysis The R3DE instrument was situated outside the Columbus module of the ISS and had only its own shielding of less than 0.4 g cm 2 (Dachev, 2009). Our first understanding that we did measure fluxes and doses from REP with energies above 0.78 MeV was developed on the base of the R3DE data. Later we revised our Foton-M2/M3 data and prepared and published the results (Dachev et al., 2009; Damasso et al., 2009), which present mainly the Foton-M2/M3 REP data. The R3DE data cover only the period 20 February–28 April, 2008, while this paper analyses all available R3DE data between February 2008 and September 2009. The selection procedure, which allows us to distinguish between three different radiation sources (GCR, SAA and ORB) seen by the R3DE instrument, is based on the analysis of the dose to fluxes values (Dachev, 2009) and on the methodology described in Bankov et al. (2010), which is based on a polynomial line dividing different sources in dependence of the L value. Fig. 3 accumulates in the bottom panel all available R3DE data of averaged daily fluences of relativistic electrons (ISS Flu) and daily absorbed dose rates (ISS AD). In the period between 1 September 2008 and 12 November 2012 there are no data from R3DE instrument. The number of different single 10-seconds measurements, which was used in the process of averaging the fluence data, was between 1 and 238. On average this number was 36.8 measurements per day. The places of averaged ISS fluence data were distributed over a relatively wide range of L
Fig. 3. Results for the measured with R3DE instrument on ISS daily 40.78 MeV fluence (ISS Flu) and the dose rate (ISS AD) deposited by it for the whole operational time between February 2008 and September 2009. These data are compared with the daily GOES-11 satellite 42 MeV (GOES) fluence and the daily global Ap index.
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values between 3.4 and 5.6, on average at L¼ 4.4. These measurements were compared with the GOES-11 daily fluence data at L¼6.6 for energies above 2 MeV and the daily global Ap index (upper panel). All curves represent the moving average over 2 points of the raw data. As seen from the upper panel in Fig. 3 the period between February 2008 and August 2009 is characterized by declining solar and magnetic activity, respectively, which is the main reason for the declining REP activity. Really the period can be divided into two parts. The first one finished in August 2008 when a real decreasing trend is observed in all parameters seen in Fig. 3. The second one starts after November 2008 when the daily global Ap index stays almost at very low levels. In this period the R3DE data are also practically horizontal lines. Only the GOES-11 daily fluence data continue to decrease. A close look at the data in Fig. 3 confirms that each increase of the Ap or each new magnetic storm increased the fluence recorded on both satellites, GOES and ISS (Zheng et al., 2006). The daily R3DE absorbed dose followed very closely the daily fluence because of a strong linear dependence between them as seen from formulae (1). When we plotted the daily absorbed dose in dependence of the daily fluence we revealed a straight line with a coefficient of linear regression R2 ¼0.9984 and a slope equal to 0.3104 nGy cm2 part 1. According the Heffner’s formulae (Heffner, 1970; Dachev, 2009) this slope value corresponds to a mean REP energy equal to 2.1 MeV.
6. R3DR data analysis The R3DR instrument was situated outside the Russian Zvezda module of the ISS. The available data covered two main periods: March–June 2009 and December 2009–August 2010. In the period between 24 June 2009 and 1 January 2010 there are no data from the R3DR instrument. Fig. 4 is designed in the same way as Fig. 3. Except the large data gap between July and January 2009 there are two smaller periods of data gaps in January–February 2010 and in the middle of March 2010. A close look on the GOES-11 daily fluence
Fig. 4. Results for the measured with R3DR instrument on ISS daily 40.78 MeV fluence (ISS flu) and the dose rate (ISS AD) deposited by it for the whole operational time between March 2009 and August 2010. These data are compared with the dayly GOES-11 satellite 42 MeV (GOES) fluence and the daily global Ap index.
data for these periods shows that except in the middle of March 2010 the fluence stays at very low levels as seen in Fig. 1. The R3DR instrument had the same shielding of its own as the R3DE instrument but the surrounding shielding was lower and this increased strongly the inner radiation belt energetic proton population (Dachev et al., 2011b). Here we have the opportunity to compare the outer belt electron fluences and dose rates obtained by the R3DE/ R instruments for the period March–June 2009 and to conclude that the energetic electron population also increased because of the smaller surrounding shielding. For example the R3DR daily fluences in the middle of March 2009 reached values of 105 cm 2 day 1 (see Fig. 4), while the R3DE fluences were only 3 104 cm 2 day 1 (Fig. 3). The same happened with the daily absorbed doses. The fact that the doses were distributed in dependence of the 3D mass distribution on the ISS supported again the conclusions by the Committee on Solar and Space Physics and Committee on SolarTerrestrial Research (2000), that only internal radiation monitoring is not sufficient for the astronauts/cosmonauts during EVAs. The results of measurements made during Expedition 4–6 on the ISS by the Canadian instrument EVA Radiation Monitoring (EVARM) (Thomson and Nielsen, 1999), which used tiny metal oxide semiconductor field effect transistor (MOSFET) dosimeters, also noted that ‘‘shielding from the station significantly affected the dose of radiation each badge received’’ ,http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/ 20070014542_2007014665.pdf. Personal dosimeters with abilities to distinguish between different radiation sources and to measure doses from REP were highly recommended by the mentioned Committee in 2000 but are still not implemented during EVAs on ISS. The most interesting period in Fig. 4 began on April 2010 and covered all data till 20 August. The R3DR and GOES-11 daily relativistic electron fluences almost explosively increased on 6 and 7 April as seen on Figs. 4 and 5. The consequences of the space weather processes, which provoked this increase can be described as follows (Space Weather Highlights, 5–11 April 2010, SWO PRF 1806, 13 April 2010, http://www.swpc.noaa.gov/): (1) Solar activity was at very low levels with isolated low-level B-class flares. (2) A halo coronal mass ejection (CME) is observed on 03/0954 UTC. (3) About 2 days later a shock was observed at ACE at 05/0756 UTC, which led to a sudden impulse on Earth at 05/0826 UTC (38 nT was observed at the Boulder magnetometer). (4) The ACE satellite observed wind speeds between 720– 800 km s 1 behind the shock, with Bz reaching values around 15 nT.
Fig. 5. Variations of the measured by R3DR instrument data for the ( 40.78 MeV electron fluence) and accumulated ORB, GCR, SAA and Tot (Total) dose rates together with the daily Ap index for the period 1 April–6 May 2010.
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(5) The activity continued on 6 April with predominantly active to minor storm levels, as well as an interval of minor to major storm levels observed between 06/0000–0600 UTC. (6) Wind speeds at ACE decreased to about 550 km s 1 at 06/2100 UTC. Although the created magnetic storm on 6 April was moderate (daily Ap¼49, minimal Dst¼ 72 nT at about noon), the second largest values in history of GOES fluences of electrons with energies 42 MeV were measured (see Figs. 1 and 4). The increase in the GOES-11 fluence of electrons with energies more than 2 MeV was by 4.5 orders of magnitude, while the R3DR40.78 MeV fluences increased less than 4 orders of magnitude. Till the end of measurements with the R3DR instrument at 20 August 2010, a few more smaller Ap maxima were observed and they were reflected by very similar responses on the GOES satellite, while the ISS data correlation was much smaller. Fig. 5 highlights the dynamics of the accumulation of the R3DR doses generated by ORB electrons, SAA protons, GCR particles and the sum of them (Tot). The ORB daily fluence divided by 500 is also presented. The daily Ap index is plotted using the right side axes. It is seen that the GCR accumulated dose had about 73 mGy day 1 as lowest daily rate. Another parameter, which had an almost linear accumulation, are the SAA inner radiation belt proton doses with an average daily dose rate of 409 mGy day 1. The accumulated ORB daily dose rate by electrons with energies greater than 0.78 MeV was a function of the daily fluence and showed a nonlinear dependence with a maximum in the period 5–10 April 2010. The largest daily increase occurred on 7 April with 2300 mGy day 1 generated by a fluence increase with more than 4 million electrons per day with energies above 0.78 MeV. This enhancement was in response to the magnetic storm on 6 April and reached its maximum during the recovery phase of it. Further, at every small disturbance of Ap on 12, 15 and 23 April, the R3DR fluence rose up, but the fastest changes of Ap occurred on 6 April and on 2 May. The geomagnetic conditions on 2–3 May 2010 were similar in magnitude, but the response of the R3DR daily fluence during the 3 May storm was about 1.5 orders of magnitude less than the 5–6 April response. We do not have any explanation of the reasons for these large differences in the responses and we hope that our experimental data will support the theoretical study of the REP physics. Using the same procedure as described for the R3DE instrument with plotting the absorbed dose in dependence of the fluence we found that the slope was higher and reached 0.3303 nGy cm 2 part 1. According the Heffner’s formulae (Heffner, 1970; Dachev, 2009) this value of the slope corresponds to a mean electron energy equal to 4.7 MeV.
7. Discussion and conclusions The paper presents observations of REP on the ISS obtained by three Bulgarian-built instruments flown in 2001 and during 2008–2010. The 2001 data cover a relatively small time interval of 18 days in May 2001, while the 2008–2010 data covered, with few exceptions, almost the whole time between February 2008 and August 2010. The three instruments being behind different shielding registered in all cases REP but the deposited dose rates varied over 3 orders of magnitude. The smallest were the daily dose rates observed with MDU#2 of the Liulin-E094 instrument, situated inside the US laboratory module of the ISS in May 2001. They did not exceed 3–4 mGy day 1. The relativistic electrons dose rates values measured with R3DE instrument being outside the ISS were higher and reached values of about 200 mGy day 1. The highest values of 2347 mGy day 1 were observed by the R3DR instrument outside the Russian ‘‘Zvezda’’ module on 7 April 2010
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during the second largest REP in GOES history. The comparison between the relativistic electron dose rate data measured by R3DE and R3DR show that during the periods of simultaneous operation the R3DR dose rates are higher than the R3DE dose rates because the R3DR instrument was situated in a less shielded surrounding. This allows the conclusion that the astronauts being on EVA will collect highly variable dose rates during REP in dependence of their position around the station. Only active personal dosimeters (Dachev et al., 2010) will be able to measure these large variations. We attempted to correlate the amplitudes of GOES and ISS relativistic electrons fluences and found a relatively small coefficient of linear regression R2 ¼ 0.5397, which means that there is no direct connection between both locations in the magnetosphere. This can be explained by the fact that the GOES satellites occupy a relatively small range of L values, while the ISS crosses all latitudes between L¼3 and L¼6.3. The analysis of the absorbed daily doses obtained by the R3DE/R instruments shows that during the quiet geomagnetic conditions they are at very low levels of a few to a few tens of mGy day 1 and do not pose any serious risk for the astronauts being on EVA where they are shielded only by their space suits, which have a shielding characteristics similar to our instruments of 0.4–0.5 g cm 2 (Benton et al., 2006; Cucinotta et al., 2003). In the case of very high daily relativistic electron fluxes as on 7 April 2010 the daily absorbed dose increased up to 2300 mGy day 1, which is much higher than the other daily sources of GCR (80–90 mGy day 1) and inner belt protons (400–500 mGy day 1). During this period, three EVAs were performed by the STS-131 astronauts on 9, 11 and 13 April 2010 (http://www.nasa.gov/mission_pages/shuttle/shuttle missions/sts131/main/index.html). The accumulated doses (mainly from ORB and GCR) calculated from our R3DR measurements during the ca. 6 h EVAs were between 440 and 300 mGy; these values pose no extreme danger to the health of the astronauts because the daily average absorbed dose rates reported by Reitz et al. (2005) inside of the ISS vary in the range 74–215 mGy day 1. On the other hand the relativistic electrons did not have enough energy to penetrate into the body of the astronauts and therefore deposited their dose mainly in the skin and eyes. For the first time we made an attempt to calculate the mean energy of the electrons reaching the detector of the R3DE/R instruments and found that the R3DR data, which are collected during intense REP conditions in 2010, show higher energies, which is reasonable. In conclusion, we would like to mention that the R3DE/R, low mass, dimension and price instruments, proved their ability to characterize the outside ISS radiation environment in a case of relativistic electron precipitations. This was achieved mainly with the analysis of the deposited energy spectra, which was obtained at each measurement cycle of 10 s. The main conclusion of the presented data is that REP events are common on the ISS. Although the obtained doses do not pose extreme risks for astronauts being on EVA they have to be considered as a permanently observed source, which requires additional comprehensive investigations.
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