Langmuir probe is one of the fundamental techniques for measuring properties of plasmas. We have developed two different types of plasma probes for the rocket and satellite experiments. One is the cylindrical Langmuir Probe that measures the electron density and temperature from its current-voltage characteristics in thermal plasmas, and the other is the electron temperature probe which directly gives the information of the ambient electron temperatures.
The sounding rocket, KSR-II (Korean Scientific Rocket-II) was launched on Jun 11, 1998 at 10:00 KST from Tae-An peninsula (37˚N, 126˚E). Two instruments, Langmuir probe and the electron temperature probe, successfully measured the electron density, electron temperature and the floating potential at altitudes from 73km to 130km. While the electron temperature measurement is not easy in this region since the temperature is very low and the contamination effect of the probe may cause a problem, we were able to obtain a reasonable electron temperature profile. The results show that electron density increases sharply at about 90km, and forms a peak at 102km. The density profile is roughly consistent with IRI (International Reference Ionosphere) 95-model or PIM (Parameterized Ionospheric Model) except that the peak density appears at 110km in the model and model density is slightly lower than the observed one. Electron temperature obtained from the electron temperature probe fluctuated between 200˚K and 700˚K, an effect presumably coming from the wake developed by Langmuir probe, and it increased more or less with the altitude, which is consistent with the Langmuir probe results.
The Korea's third satellite, KITSAT-3 was launched on May 26, 1999. The orbit of the satellite is 730 km altitude, sun-synchronous with a fixed local time of descending node near 12:00. The dimension of the satellite in stowed configuration is approximately 495×614×852㎣. The floating potential of KITSAT-3 was measured with an electron temperature probe. A series of attitude maneuvering was performed to rotate the satellite in the dayside. The electron temperature probe on KITSAT-3 detected significant variations of the floating potential during the rotation. When the telemetry data for the electrical currents and voltages from the solar arrays are examined, significant correlation between the electrical current and the floating potentials was found. The variations of the floating potential are interpreted in terms of negative charging of the satellite relative to the ambient plasmas. From numerical simulations, we found that the floating potential for KITSAT-3 were negatively charged mainly due to the operation of solar arrays.
KOMPSAT-1 was launched on December 21, 1999 into 685 km altitude sun-synchronous polar orbit with the descending node at 22:50 local time. Ionospheric Measurement Sensor (IMS) composed of a Langmuir Probe (LP) and the Electron Temperature Probe (ETP) was developed and monitored the variation of the nighttime local ionosphere along the satellite track. While the main mission of KOMPSAT-1 is to provide cartographic images with its electro-optical camera, the unique orbital characteristic of the spacecraft is suitable for the global ionospheric study at fixed local time and altitude. Preliminary result shows that the quiet time KOMPSAT-1 electron density reasonably follows the IRI model in the low latitude region, while the temperature is higher than the IRI. The density enhancement and the depression of the temperature are localized along the geomagnetic equator. Also, IMS was able to measure the electron density and temperature variation during the progress of the Bastille event storm on July 15, 2000. During the main phase of the storm KOMPSAT-1 detected near 350˚E a very deep and extensive trough of electron density centered around the geomagnetic equator. The electron density dropped sharply by more than an order of magnitude from $=sim4\times10^5 cm^{3}$ to less than $\sim2\times10^4 cm^{3}$, with the trough region extended over 1400 km along the satellite track. The electron temperature was seen increased everywhere, except in the trough region where reliable temperatures were not obtained due to its low density. Later in the recovery phase, KOMPSAT-1 observed severe distortion still persisted near 230˚E, with enhanced density in the southern hemisphere. Together with DMSP observations, we estimate the size of the observed trough to be at least 5500 km in the longitudinal direction. The trough was caused by the enhanced eastward electric field which lifted the equatorial F-region ionosphere upward.
Four plasma instruments are currently under development for KAISTSAT-4 which is scheduled for launch in 2003. They are the Solid-State Telescope (SST), Elecro-Static Analyzer (ESA), Langmuir Probe (LP), and the Scientific Magnetometer (SM). These instruments will respectively allow in-situ detection of high energy and low energy components of plasmas, and together with the Far-ultraviolet IMaging Spectrograph (FIMS), will provide micro-scale physics of Earth's polar ionosphere with detailed spectral information.
