Impact of lithium releases on ionospheric electron density observed by impedance probe during WIND campaign
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB 2010
Received: 12 February 2010
Accepted: 2 July 2010
Published: 31 August 2010
This paper presents direct observation of the impact of the lithium releases on the ionospheric electron density during the WIND (wind measurement for ionized and neutral atmospheric dynamics study) campaign conducted on 2 September 2007 in Japan. The direct observation is unique in that the electron density enhancement was observed by using the NEI (number density of electrons by impedance probe) which can measure accurately the absolute value of the electron density, and the distance between the NEI and the LES (lithium ejection system) was very close (several tens of meters). Data analyses of the NEI on-board the sounding rocket S-520-23, which was launched from Uchinoura (31.3°N, 131.1°E) at 19:20 JST (JST = UT + 9 h), clarifies that lithium releases performed in the descending phase increased the electron density up to approximately 7 × 105 cm−3. A simple model calculation performed under the assumption that the increased electron density equals the photoionized lithium ion density indicates that the observed electron density enhancements cannot be explained by considering each lithium release as an instantaneous one, but rather by considering a convolution of very short-time intermittent releases. The model calculation is verified by comparison with the observation of the lithium resonance scattering light from the ground.
Key wordsIonosphere impedance probe electron density chemical release
It is important to investigate expansion processes of released materials and its impact on the background physical quantities in space, since chemical releases enable us to observe physical values such as the thermospheric neutral wind and the electric field which are difficult to be observed. This is one of the reasons that the chemical release is a subject which has been extensively studied for many years (e.g., Ma and Schunk, 1991; Schunk and Szuszczewicz, 1991). In the ionosphere, it has been well known that the electron density is dramatically affected by chemical releases (e.g., Lloyd and Haerendel, 1973; Ma and Schunk, 1991; Schunk and Szuszczewicz, 1991; Szuszczewicz et al., 1993). While many sophisticated simulations have been performed to date, some observations of the chemical ion density which accompanies release experiments have been reported to be roughly consistent with simple model calculations (e.g., Hunton, 1993; Szuszczewicz et al., 1996). Hunton (1993) reported that the barium ion density calculated from his model based on a spherically symmetric, collisionless and Gaussian neutral barium shell agreed reasonably well with that observed from the ion spectrometer (QIMS) on-board the CRESS satellite for the G-12 release experiment. In their experiment, the distance from the barium canister and QIMS was taken to be 4.2 km. Szuszczewicz et al. (1996) reported the in-situ observation of the artificial multi ions during the El Coqui rocket campaign in which the distance from the canisters to the onboard instruments was approximately several hundred meters. They found that the observations generally agreed with a simple spherical free expansion model except for multi-peak characteristics due to the ion gyro-motion. The distance from a release point to a sensor is one of the important parameters of the chemical release experiment because the distance concerns what time stage of the expansion process we can observe.
On 2 September 2007, a sounding rocket experiment referred to as the WIND (wind measurement for ionized and neutral atmospheric dynamics study) campaign was conducted in Japan to clarify the interaction process between the ionospheric plasma and the thermospheric neutral wind through a direct observation (Watanabe et al., in preparation). In the WIND campaign, a lithium release experiment was conducted in the descending phase to estimate the thermospheric neutral wind by observing the motion of the released lithium cloud from the ground (Yamamoto et al.,in preparation). While the primary purpose of this release experiment is to estimate the thermospheric neutral wind, it provides an opportunity to perform an in-situ observation of the impact of the lithium release on the ionospheric electron density. The direct observation during the WIND campaign was unique in terms of the method of the electron density observation and the distance from the sensor to the release point. The electron density enhancement was observed by using the NEI (number density of electrons by impedance probe) developed by Oya (1966) which can measure accurately the absolute value of the electron density, and the distance from the LES (lithium ejection system) to the NEI was very close (several tens of meters). There were no lithium release experiments in which the electron density was observed using the NEI with such a very close distance from the sensor to the release point. The purpose of this paper is to evaluate quantitatively a lithium release impact on the ionospheric electron density enhancement with such a close distance observed during the WIND campaign. In the following sections, we discuss the observed characteristics of the electron density enhancement after summarizing an overview of the lithium release experiment during the WIND campaign, and suggest that the observation can be reasonably explained by considering the convolution of very short-time intermittent releases using a simple spherical free expansion model.
The ionospheric electron density structure is strongly affected by the thermospheric neutral wind through their interaction, such as the ion-drag and the dynamo processes (e.g., Rishbeth, 2000; Heelis, 2004). At night especially, their interaction has been believed to be one of the key factors for controlling the ionospheric irregularities, such as the meso-scale travelling ionospheric disturbance (MSTID) in the mid-latitude ionosphere (e.g., Shiokawa et al., 2003) and the plasma bubble in the equatorial ionosphere (e.g., Abdu, 2001). To evaluate the interaction between the ionosphere and the thermospheric neutral wind through direct observation, the sounding rocket experiment referred to as the WIND campaign was conducted on 2 September 2007 in Japan.
Specification for the NEI instrument on-board S-520-23.
BeCu, 1.2 cm dia., ∼1.0 m length
300 kHz–12 MHz
Dynamic Range of Electron Density
∼103−2 × 106 el/cm3
Dynamic Range of Equivalent Capacitance
Sweep Repetition Period
The LES consists of three canisters with each canister containing solid lithium of 130 g. Each canister was designed to gasify and release the lithium individually by thermite process within 10±5 s at approximately 10 s after receiving an ejection signal. The released lithium resonance scattering light of a 670.8 nm wavelength was observed using CCD imagers synchronously from Shionomisaki (33.5°N, 135.8°E), Miyazaki (31.8°N, 131.4°E), Uchinoura (31.3°N, 131.1°E) and Amami (28.5°N, 129.7°E). The sampling rate and exposure time are 8 s and 4 s for the time period from 373 s to 478 s after the launch, 20 s and 15 s for the period from 494 s to 1229 s, and 60 s and 30 s for the period from 1299 s and 3344 s, respectively. The details of the LES system, ground observation of the lithium resonance scattering light and the other on-board instruments are provided in Yamamoto et al (in preparation) and Watanabe et al. (in preparation).
3. NEI Observation
In the following section, we suggest that these observed electron density peaks are caused by the lithium releases by performing a simple model calculation and by comparison with the luminosity profile obtained from the ground observation of the lithium resonance scattering light.
4. Analysis and Discussion
4.1 Deduction of electron density variation
Figure 4(b) indicates that the lithium releases increased the electron density up to approximately 7 × 105 cm−3, and each enhancement lasts approximately 10 to 20 s. The electron density peaks are almost synchronized with L1, L2 and L3, however, the density peak at around 210 km is not synchronized with any scheduled lithium releases. Although Szuszczewicz et al. (1996) reported that a single chemical release can generate plural peaks depending on gyro-motion periods of ions, the density peak at around 210 km is not due to the gyro-motion effect because it was observed approximately 10 s after L1, and the gyro-motion periods of ions are less than 1 s. The density peak at around 210 km appears to imply that an unexpected lithium release occurred.
4.2 Estimation of altitudinal lithium releases profile
4.3 Model calculation and comparison
Both the analysis results of the electron density and luminosity profiles indicate that the unexpected lithium release occurred between L1 and L2. Hereinafter, we refer to this unexpected lithium release as L1.5. The existence of L1.5 prevents us from investigating the relationship between the electron density enhancement and the lithium releases based on the scheduled time sequence of the lithium releases. Instead, we calculate an altitude profile of the vaporized lithium from the electron density enhancement Δne using a simple lithium expansion model under the assumption that ×ne equals the photoionized lithium density. By comparing an altitude profile of the vaporized lithium calculated from the model described below and that of the luminosity shown in Fig. 5(b), we suggest that Δne can be reasonably explained by considering a convolution of very short-time intermittent releases.
To evaluate quantitatively the lithium release impact on the ionospheric density enhancement during the WIND campaign, we analyzed the c(f) data obtained from the NEI which can measure accurately the absolute value of the electron density. It was clarified that the lithium releases performed in the descending phase increased the electron density up to approximately 7 × 105 cm−3. This is a first observational result showing the impact of the lithium releases on the ionospheric electron density at the very close distance of several ten meters from the sensor to the release point. To investigate the relationship between the observed electron density enhancement and the lithium releases, we developed a convolution model of the simple free expansion. By using this convolution model, we found that the peak altitudes of the vaporized lithium estimated from the electron density enhancement agreed well with those estimated from the ground observation of the lithium resonance scattering light. Based on this good agreement, it is suggested that the free expansion model is effective at the distance of several ten meters from the release point and the effect of superposition of releases cannot be ignored. On the other hand, it was found that the electron density was increased at the altitude range where the sun light did not directly reach. Clarifying the mechanism of the electron density enhancement in this altitude region is a subject which remains to be investigated further.
The sounding rocket experiment was conducted by the Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (ISAS/JAXA) as international project. We thank all members of the rocket experiments. The impedance probe was manufactured by System Keisoku Co., Ltd. The activities of the three authors (T. Ono, A. Kumamoto, and T. Suzuki) are supported by the Global COE Program “Global Education and Research Center for Earth and Planetary Dynamics” at Tohoku University.
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