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Variation of Floating Potential in the Topside Ionosphere Observed by STSAT-1
Variation of Floating Potential in the Topside Ionosphere Observed by STSAT-1
Journal of Astronomy and Space Sciences. 2014. Dec, 31(4): 311-315
Copyright © 2014, The Korean Space Science Society
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
  • Received : November 11, 2014
  • Accepted : December 12, 2014
  • Published : December 15, 2014
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About the Authors
Junhyun Lee
School of Space Research, Kyung Hee University, Yongin, 446-701, Korea
Ensang Lee
Institute of Natural Sciences, Kyung Hee University, Yongin, 446-701, Korea
eslee@khu.ac.kr
Jaejin Lee
Korea Astronomy and Space Science Institute, Daejeon, 305-348, Korea
Khan-Hyuk Kim
School of Space Research, Kyung Hee University, Yongin, 446-701, Korea
Jongho Seon
School of Space Research, Kyung Hee University, Yongin, 446-701, Korea
Dong-Hun Lee
School of Space Research, Kyung Hee University, Yongin, 446-701, Korea
Ho Jin
School of Space Research, Kyung Hee University, Yongin, 446-701, Korea
Eung-Hyun Kim
Korea Aerospace Research Institute, Daejeon, 305-806, Korea
Hyun-Jin Jeon
Korea Aerospace Research Institute, Daejeon, 305-806, Korea
Seong-Bin Lim
Korea Aerospace Research Institute, Daejeon, 305-806, Korea
Taeyoun Kim
Korea Aerospace Research Institute, Daejeon, 305-806, Korea
Jaewoong Jang
Korea Aerospace Research Institute, Daejeon, 305-806, Korea
Kyung-Duk Jang
Korea Aerospace Research Institute, Daejeon, 305-806, Korea
Kwangsun Ryu
Satellite Technology Research Center, Korea Advanced Institute of Science and Technology, Daejeon, 305-701, Korea
Abstract
In this study, we investigated the effect of space plasmas on the floating potential variation of a low-altitude, polar-orbiting satellite using the Langmuir Probe (LP) measurement onboard the STSAT-1 spacecraft. We focused on small potential drops, for which the estimation of plasma density and temperature from LP is available. The floating potential varied according to the variations of plasma density and temperature, similar to the previously reported observations. Most of the potential drops occurred around the nightside auroral region. However, unlike the previous studies where large potential drops were observed with the precipitation of auroral electrons, the potential drops occurred before or after the precipitation of auroral electrons. Statistical analysis shows that the potential drops have good correlation with the temperature increase of cold electrons, which suggests the small potential drops be mainly controlled by the cold ionospheric plasmas.
Keywords
1. INTRODUCTION
When spacecraft are exposed to space plasmas, electrostatic potential can be built up due to the accumulation of electric charges on their surface or within their body, which is known as spacecraft charging ( Hastings & Garrett 1997 ). Spacecraft charging has been recognized from the beginning of the space era and extensively studied because it could cause damages on spacecraft.
The nature of the charging is mainly determined by the properties of space plasmas. It has been found that plasmas with high temperatures and low densities can produce large electrostatic potential on the surface of spacecraft ( Gussenhoven et al. 1985 ). Earth's radiation belts including the geosynchronous orbit (GEO) consist of such plasmas. DeForest (1972) identified large negative potentials of about 10 kV in eclipse and 1 kV in sunlight from Applied Technology Satellite-5 (ATS-5), which was located in the geosynchronous orbit. On the other hand, plasmas with low temperatures and high densities, which can be found in Earth's ionosphere, do not produce large potential. Thus, in the low Earth orbit (LEO), the potentials of a few V are usually observed ( Anderson et al. 1994 ). However, even in the LEO when spacecraft pass through auroral region potentials larger than ~100 V can be built up due to the precipitating energetic electrons ( Gussenhoven et al. 1985 , Wahlund et al. 1999 , Eriksson et al. 2006 ).
It has been reported that there exist certain relationships between the spacecraft potential and plasma density or temperature. When the temperature and spacecraft potential are sufficiently high and the temperature is larger than the potential level, the spacecraft potential is approximately proportional to the temperature of ambient plasma ( Lai & Della-Rose 2001 , Lai 2011 ). On the other hand, for low potentials it has been reported that a relationship can be found between the potential and plasma density ( Pedersen 1995 , Escoubet et al. 1997 , Ishisaka et al. 2001 ). The relationship has been used to indirectly estimate plasma density from the measured spacecraft potential in regions inaccessible to other plasma experiments.
In this study, we investigated the influence of the plasma environments on the potential drop observed by the STSAT-1 satellite in the LEO. We used the floating potential, plasma density, and plasma temperature measured by the Langmuir probe, and the electron flux with the energies between 100 eV and 20 keV measured by the Electrostatic analyzer. In Section 2, we describe about the STSAT-1 satellite and its instruments. In Section 3, we present the observational results and the statistical analysis of the relationship between the potential and plasma properties. Discussion and summary are given in Section 4.
2. INSTRUMENTATION
STSAT-1 was launched on 27 September 2003 into a sunsynchronous, polar orbit with the altitude of 680 km and inclination of 98°. The orbital period was 98.5 minutes. STSAT-1 passed through the equator at 22:00 MLT in the nightside. STSAT-1 has a dimension of 66 cm × 60 cm × 80 cm and the weight of 106 kg. The instruments onboard STSAT-1 consist of a Far-ultraviolet Imaging Spectrogragh (FIMS), an Electrostatic Analyzer (ESA), Solid State Telescopes (SST), a Langmuir Probe (LP) and a Scientific Magnetometer (SM).
In this study, the data from the ESA and LP were used ( Lee et al. 2005 , Lee et al. 2013 ). The ESA is a top-hat electrostatic analyzer with a field of view 180° × 4.2°. It measured the electron fluxes in the energy range from 100 eV to 20 keV, which was divided into 29 energy channels. A full energy sweep took 210 msec. Because STSAT-1 did not spin, but used three-axis stabilizing attitude control, the ESA could only have provided two-dimensional measurements in the angular range from 0° to 180° with the angular resolution of 22.5°. The LP measured the floating potential and the density and temperature of thermal electrons. The time resolution was 200 msec. The floating potential was used as a measure of spacecraft charging.
3. RESULTS
Fig. 1 shows a spacecraft charging event occurred on STSAT-1 on 5 April 2004. From the top panel, the figures represent floating potential, plasma temperature, and electron density. In the beginning of the observation, the floating potential was between -0.7 and 0 V with small fluctuations, and it suddenly reduced from -0.1 V to -1.4 V at 22:34 UT and recovered to the previous level. Along with the reduction of the floating potential, the electron density decreased an order of magnitude from ~6×10 4 cm -3 to ~6×10 3 cm -3 . Moreover, the variation of the floating potential is almost identical to the variation of the density. On the other hand, the plasma temperature largely increased more than 30 times from ~3×10 3 K to ~10 5 K when the floating potential decreased. However, the reverse of the variation of the temperature is also similar to the variation of the potential. Thus, both the density and temperature would have close relationship with the floating potential drop.
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Lager Image
Spacecraft charging event observed on 5 April 2004. The top panel represents the floating potential, the middle the plasma temperature, and the bottom the electron density.
Around the occurrence of the event, STSAT-1 was moving from the dayside to the nightside across the polar region in the northern hemisphere. Fig. 2 shows the trajectory of STSAT-1 during the same interval as in Figure 1 . The charging event occurred between 62.1° and 45.4° in the magnetic latitude (MLAT) (blue asterisks in Fig. 2 ), and the potential had the minimum at 56.3° MLAT.
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Lager Image
Trajectory of STSAT-1 from 22:20 UT to 22:40 UT on 5 April 2004 (red solid line) and the location of the floating potential drop (blue asterisks).
An interesting feature has been observed in electron fluxes in association with the floating potential drop. Fig. 3 shows the energy-time spectrogram of the energy flux of electrons with energies between 100 eV and 20 keV. Enhancement of the energy flux was observed from 22:33 to 22:35 UT prior to the floating potential drop. The enhancement of the energy flux is associated with the precipitation of electrons from the outer magnetosphere into ionosphere, which could result in auroras. At ~22:34 UT the energy flux increased very largely for energies between ~1 and 6 keV. The decrease of the floating potential started from about 22:34:20 UT, at which the electron flux rapidly decreased. The floating potential became minimum when the energy flux was almost disappeared.
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Lager Image
Spectrogram of energy flux of electrons (top) and floating potential (bottom) observed on 5 April 2004.
The other events show similar features to the 5 April event. Fig. 4 shows another example observed on 13 October 2004. In this event the floating potential dropped twice by ~0.8 V. Similar to the 5 April event the electron density decreased by ~1.7×10 4 cm -3 and ~2.1×10 4 cm -3 and the plasma temperature increased by ~2.1×10 4 K and 1.4×10 4 K for each floating potential drop, respectively. The energy flux of electrons also shows a similar tendency to the 5 April event ( Fig. 5 ). However, in this event the first potential drop occurred prior to the electron flux enhancement. The potential drop recovered to the undisturbed level as the electron flux increased and another drop occurred after the electron flux enhancement stopped. These observations imply that these potential drops are more closely associated with the density and temperature variations of cold plasmas than the precipitating electron fluxes with energies between ~100 eV and ~20 keV.
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Lager Image
The floating potential (top), plasma temperature (middle), and electron density (bottom) observed on 13 October 2004.
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Lager Image
Spectrogram of energy flux of electrons (top) and floating potential (bottom) observed on 13 October 2004.
In each event the variation of the floating potential at the potential drop looks very similar to the variations of the density and temperature. To examine if there exists any general relationship between them, we statistically analyzed the floating potential drop events. Fig. 6 shows the floating potential difference vs. the density difference ( Fig. 6(a) ) and the temperature difference ( Fig. 6(b) ). The difference of each quantity was estimated as the difference between the averages of the ambient values and the values around the minimum or maximum. The density difference shows poor correlation with the floating potential difference. In contrast, the temperature difference shows very good correlation with the floating potential difference. The correlation coefficient between the temperature and floating potential differences is ~0.92, and the relationship is estimated as follows:
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Lager Image
Relationships between the differences of the plasma temperature and floating potential (top) and between the differences of the electron density and floating potential (bottom).
ΔV=0.19 log(ΔT)-0.83,
when the temperature and potential were measured in K and V, respectively.
Because the variations of the density around the floating potential drops are very similar to the variations of the floating potential, we expected that the density difference also has good correlation with the floating potential difference. However, the density difference for each event produced different floating potential difference. We also examined the relationships between the peak values of the density and floating potential, the peak values of the temperature and floating potential, the peak value of the floating potential and density difference, and the peak value of the floating potential and temperature difference. However, there was no good correlation between these quantities.
4. DISCUSSION AND SUMMARY
In this paper, we examined the effect of space plasmas on the floating potential drop observed by the LP instrument onboard the STSAT-1 satellite. STSAT-1 is a low-altitude, polar-orbiting satellite, which passes through the topside ionosphere and auroral region. In the ionosphere, the floating potential is mainly determined by the cold ionospheric plasmas, which usually produces the potential larger than -10 V. In the auroral region, large potential drops from a few hundred V to a few kV can be produced by the precipitating energetic electrons. It should be noted that in this study, we used the cases where the floating potential drop was less than 5 V. When the potential drop is large, the LP instrument cannot correctly measure the temperature and density. Thus, in order to examine the effect of cold ionospheric plasmas on the floating potential drop, we only used the small potential drops of less than 5 V. In the other spacecraft that observed large potential drops, the potential drops were indirectly induced from the measurement of particles using the other instruments (e.g., Gussenhoven et al. 1985 ).
In our observaions, the floating potential drops were not much affected by the auroral precipitation. Moreover, the potential drops occurred before or after the auroral precipitation. This is different from the previous results in which large potential drops occurred when there was auroral precipitation (e.g., Gussenhoven et al. 1985 ). This might be because the cold electron density was large enough to reduce the effect of spacecraft charging by the precipitating auroral electrons. In our observations, the electron densities were larger than 10 3 cm -3 , while the large potential drops in the previous observations occurred when the electron densities were well below 10 3 cm -3 ( Yeh & Gussenhoven 1987 ).
The variation of the floating potential followed the variations of the plasma temperature and density. Interestingly, for the floating potential drop, there exists good correlation between the potential drop and temperature enhancement. On the other hand, there is only weak correlation between the potential drop and density decrease. Thus, although the potential tends to vary with both the temperature and density, the magnitude of the potential drop mainly depends on the magnitude of the temperature enhancement.
Acknowledgements
This work was in part supported by the Basic Science Research Program (NRF-2013R1A1A2010711), the Geo-KOMPSAT Development Program (NRF-2014M1A3A4A03048326) and BK21 Plus Program through the National Research Foundation of Korea funded by the Korean Government, and the grant CATER2012-5060 from the Center for Atmospheric Science and Earthquake Research in Korea.
References
Anderson PC , Hanson WB , Coley WR , Hoegy WR 1994 Spacecraft potential effects on the Dynamics Explorer 2 satellite JGR http://dx.doi.org/10.1029/93JA02104 99 3985 - 3997
DeForest SE 1972 spacecraft charging at synchronous orbit JGR http://dx.doi.org/10.1029/JA077i004p00651 77 651 - 659
Eriksson AI , Wahlund JE 2006 Charging of the Freja satellite in the auroral zone IEEE Trans. Plasma Sci. http://dx.doi.org/10.1109/TPS.2006.883373 34 2038 - 2045
Escoubet CP , Pedersen A , Schmidt R , Lindqvist PA 1997 Density in the magnetosphere inferred from ISEE 1 spacecraft potential JGR http://dx.doi.org/10.1029/97JA00290 102 17595 - 17609
Gussenhoven MS , Hardy DA , Rich F , Burke WJ , Yeh HC 1985 High-level spacecraft charging in the low-altitude polar auroral environment JGR http://dx.doi.org/10.1029/JA090iA11p11009 90 11009 - 11023
Hastings D , Garrett HB 1997 Spacecraft-Environment Interactions Cambridge University Press Cambridge, UK
Ishisaka K , Okada T , Tsuruda K , Hayakawa H , Mukai T 2001 Relationship between the Geotail spacecraft potential and the magnetospheric electron number density including the distant tail regions JGR http://dx.doi.org/10.1029/2000JA000077 106 6309 - 6319
Lai ST , Della-Rose DJ 2001 Spacecraft charging at geosynchronous altitude: New evidence of the existence of critical temperature J. Spacecr. Rockets http://dx.doi.org/10.2514/2.3764 38 922 - 928
Lai ST 2011 Fundamentals of spacecraft charging Univ. Princeton Press Princeton
Lee JC , Min KW , Ham JW , Kim HJ , Lee JJ 2013 Langmuir probe experiments on Korean satellites Curr. Appl. Phys. http://dx.doi.org/10.1016/j.cap.2012.12.011 13 846 - 849
Lee JJ , Parks GK , Min KW , Kim HJ , Park J 2005 Energy spectra of 170-360keV electron microbursts measured by the Korean STSAT-1 GRL http://dx.doi.org/10.1029/2005GL022996 32 (13)
Pedersen A 1995 Solar wind and magnetosphere plasma diagnostics by spacecraft electrostatic potential measurements Ann. Geophys. http://dx.doi.org/10.1007/s00585-995-0118-8 13 118 - 129
Wahlund JE , Wedin LJ , Carrozi T , Eriksson AI , Holback B 1999 Analysis of Freja charging events: Statistical occurrence of charging events;Eur. Space Agency Tech. Note, SPEE-WP130-TN
Yeh HC , Gussenhoven MS 1987 The statistical electron environment for defense meteorological satellite program eclipse charging JGR http://dx.doi.org/10.1029/JA092iA07p07705 92 7705 - 7715