Foil chaff ejection systems for sounding rocket measurements of neutral winds in the mesopause region

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1 An Introduction to Space Instrumentation, Edited by K. Oyama and C. Z. Cheng, Foil chaff ejection systems for sounding rocket measurements of neutral winds in the mesopause region Yoshiko Koizumi-Kurihara 1, Junichi Kurihara 2,Yasuhiro Murayama 3, and Koh-Ichiro Oyama 4 1 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Japan 2 Graduate School of Science, Hokkaido University, Japan 3 National Institute of Information and Communications Technology, Japan 4 Plasma and Space Science Center, National Cheng Kung University, Taiwan The foil chaff technique is a simple in-situ technique to measure neutral winds in the mesopause region. In order to conduct foil chaff experiments by sounding rockets, two types of foil chaff ejection systems have been developed in Japan. The high resolution in altitude of the obtained neutral wind data by the foil chaff technique provides useful information for various studies on the dynamics in the mesopause region, when combined with simultaneous measurements of other geophysical parameters. Key words: Neutral wind, mesosphere and lower thermosphere, foil chaff technique, sounding rocket. 1. Introduction There are various techniques to measure neutral winds in the mesosphere and lower thermosphere (MLT), all of which make use of tracers: (1) the detection of Doppler shift in radar echo from turbulence or meteor trails (Murayama et al., 1999), (2) optical sensing of Doppler shift of airglow emissions (Killeen et al., 2006), (3) optical tracking of injected smoke or luminous chemicals (Larsen, 2002), and (4) radar tracking of dropped reflecting targets (falling sphere (Schmidlin et al., 1991), parachute, or metal foils). The artificial tracers in (3) and (4) are released from a rocket flying to the MLT region. In situ measurements of neutral winds in the lower thermosphere mostly made use of chemical release techniques. A trail of released lithium or barium gets luminous by resonant scattering of sunlight, and these chemicals have been used at twilight. Trimethyl aluminum (TMA) emits light by reacting with oxygen and this chemiluminescent technique can be used anytime during the night. The motion of these luminous trails is tracked with spaced cameras on the ground. In the minutes after release, irregularities in the trail serve as reference points for triangulation. In some experiments, the chemical is released intermittently with a solenoid valve to more easily identify the reference points. Heights and velocities of the reference point can be determined with a precision of the order of 100 m. In addition to the limitations in observing time, clear skies over at least two camera sites are required for the measurement using chemical release techniques. Radar-reflecting targets can be tracked by a ground-based radar at any time of the day or night under any sky conditions. As the tracer falls, a wind profile is obtained as a function of height. Each technique allows to access a different height range, and the height resolution mainly depends on the descending speed of the tracer. For example, an in- Copyright c TERRAPUB, flatable falling sphere has descending speeds exceeding the speed of sound (>100 m/s at km). Metal foils, on the other hand, often referred to as foil chaff, have descending speeds of the order of 10 m/s atthesame height range. The foil chaff technique was developed by H.-U. Widdel of the former Max-Planck-Institut für Aeronomie, Germany (Widdel, 1985, 1987, 1990). The foil chaff technique yields not only the horizontal neutral wind velocity but also the vertical wind velocity (Widdel, 1987), turbulent energy dissipation rates, eddy diffusion coefficients (Wu and Widdel, 1989), and vertical wave number spectra (Murayama et al., 1999; Wu and Xu, 2006). Foil chaff experiments have been conducted utilizing micro-rockets in the MLT region. These so-called meteorological rockets are usually launched nearly simultaneously with sounding rockets and the foil chaff technique is used to measure neutral winds at approximately the same time as the sounding rocket measurements of atmospheric and ionospheric parameters. It is important to measure the other parameters exactly simultaneously with the neutral wind for a better understanding of transient phenomena, such as short period gravity waves and turbulence. For that purpose, two types of foil chaff ejection systems for the Japanese sounding rocket S-310 were developed: a spring type and a differential pressure type (Koizumi et al., 2004). The S-310 rocket is a single-stage sounding rocket having 310 mm diameter, which can lift kg scientific payloads up to a height of km. As the first experiment with the two new ejection systems, the sounding rocket S was launched on January 10, Both types of the ejection system were flown, and the ejection of the foil chaff and the subsequent radar tracking were successful. In the second experiment, which was conducted on February 6, 2002, only the spring type ejection system was flown, the foil chaff was successfully ejected, but the radar could not track the foil chaff due to technical problems. The third and fourth experiments were successfully conducted with the spring 41

2 42 Y. KOIZUMI-KURIHARA et al.: FOIL CHAFF EJECTION SYSTEMS Table 1. Record of sounding rocket experiments using the foil chaff ejection system. Sounding rocket Date Type of ejection system References S Jan 2000 Spring and differential pressure Koizumi et al. (2004) S Feb 2002 Spring N/A S Jan 2004 Spring Koizumi et al. (2009) S Feb 2008 Spring N/A Fig. 1. Photograph of the foil chaff and the cutting machine. type. The sounding rocket experiments conducted with the two foil chaff ejection systems in Japan are summarized in Table 1. In this paper, we present the method of the foil chaff technique, the details of the foil chaff ejection systems installed on the sounding rocket, and the results obtained by the foil chaff experiments. 2. Method of Measurement Neutral wind measurements using air motion tracers such as falling sphere, foil chaff, and chemical release, responding instantly to a change in neutral wind velocity, are simple in-situ techniques. In order to obtain a sufficient height resolution, the tracer should descend with a speed less than afew tens of meters per second. This requires a tracer of a very low mass-to-area ratio (m/a), which cannot be achieved by inflatable falling spheres. In the case of foil chaff, the m/a isonly a few grams per square meter. Foil chaff with a thicknesses of 1 µm (m/a = 1.7 g/m 2 )isapplicable for measurement in the height range of km. Thus, the foil chaff technique is an optimal method for neutral wind measurements in the MLT region (Widdel, 1991). Plastic foil with Aluminum evaporation deposition is usually used as foil chaff, with an area of 25 mm 5mmand 1 µm thickness. The foils are cut to the proper length from a roll with an automatic cutting machine (Fig. 1). The length of the foil chaff should be chosen so that they resonate as half-wave dipoles at the wavelength of the tracking radar, and the width should be comparable to or smaller than the mean free path of the air at the observational height. The minimum number of foil chaff to be ejected depends on the characteristics of the tracking radar. When the target area S is known, which can be tracked by the radar at a given distance, the minimum number of foil chaff required for measurements is given by 5.81 S/λ2, where λ is the radio wavelength of the radar (in m). For S = 1m 2,aminimum of 2,500 foils is required for a C-band radar with a wavelength of 5 cm. In practice, one would choose a larger number of foil chaff using a safety factor of Figure 2 shows the concept of the foil chaff measurement of neutral winds. Approximately 20,000 foils are ejected from a rocket at an altitude of about 100 km, and they subsequently form a foil chaff cloud, which has the initial diameter of presumably about 1 km. The chaff cloud is expected to follow the ambient wind during descent while changing the size and the shape. The location of the foil chaff cloud is tracked by radar until the chaff cloud breaks up. The horizontal velocities and the speed of descent are obtained by differentiating the cloud s positions with respect to time. In the experiments utilizing micro-rockets, the foil chaff is released at around 100 km altitude, which is near the apogee of the micro-rockets. When released at the apogee,

3 Y. KOIZUMI-KURIHARA et al.: FOIL CHAFF EJECTION SYSTEMS 43 Fig. 2. The method of neutral wind measurement using foil chaff. Fig. 4. Chaff section for the S-310. Arrows show the directions of ejection (Koizumi et al., 2004). Fig. 3. Height profiles of the motion of 1 µm chaff released at 90, 100, and 110 km altitudes from the S-310 during the ascent (top) and descent (bottom) (Koizumi et al., 2004). the foil chaff exhibits no vertical velocity component. In the case of the S-310 sounding rocket, however, the initial vertical velocity of the foil chaff is nonzero depending on the altitude where the foil chaff is released. A typical vertical velocity of the S-310 rocket around 100 km altitude is about 1.2 km/s for the ascent and descent. Thus the chaff cloud released from the S-310 rocket at 100 km has an initial velocity of about 1.2 km/s aswell and slowly approaches the terminal velocity due to air drag. When the chaff cloud starts to descend with the terminal velocity, the radar starts tracking. Therefore it is necessary to predict the behavior of the chaff cloud before the experiment. The chaff cloud velocity in the vertical direction v is given by the equation of motion: m dv = mg 1 dt 2 C Dρv 2 l 2 (1) where m is the mass of the foil chaff, g is the acceleration of gravity, ρ is the air density, l is the foil chaff characteristic length (= width length).the coefficient of drag C D is given by 8cM C D = (2) (ρσl + CM)v where c is the thermal velocity, M is the mass of the air molecule, and σ is the molecular collision cross-section. The constant C is taken to be 4 for the elastic momentum exchange. The time to attain the terminal velocity of the foil chaff depends on the initial velocity and the altitude of the release. Using the foil chaff parameters (above) and the COSPAR International Reference Atmosphere 1986 (CIRA-86) (Rees et al., 1990), height profiles of the motion of a foil piece have been computed for different altitudes of the release during the ascent as well as the descent of the S-310 rocket. Figure 3 show the results for a 1 µm thick foil chaff released at the initial velocity of 1.2 km/s during the ascent and the descent, respectively. When the foil chaff is released at 90 or 100 km during the ascent of the rocket, the chaff ascends about 2 and 20 km, respectively. More than 100 sec is needed to achieve the terminal velocity after a foil chaff release above 100 km during ascent. If the chaff cloud is released during descend, it takes only a few tens of seconds to attain the terminal velocity. Therefore, the foil chaff was released during the descent in all of the previous experiments. 3. Instrumentation 3.1 Chaff section for the S-310 rocket The two ejection systems called spring type and differential pressure type system were developed for the S-310

4 44 Y. KOIZUMI-KURIHARA et al.: FOIL CHAFF EJECTION SYSTEMS Fig. 5. Foil chaff ejection system (spring type) (Koizumi et al., 2004). Fig. 6. Foil chaff ejection system (differential pressure type) (Koizumi et al., 2004). rocket (Koizumi et al., 2004). Both systems have two cylinders holding the foil chaff. Lock plates on the cylinders are unlocked at a prefixed time after launch, and the cylinders are ejected in opposite directions in order to release the foil chaff symmetrically around the rocket. In the case of micro-rocket experiments, a single payload section is occupied by the foil chaff. Since several instruments are installed in the payload section of a S-310 sounding rocket, a chaff section was newly developed to install the foil chaff ejection systems independently from the rest of the payload. The chaff section is placed between the payload section and the rocket motor. Figure 4 shows a photograph of the chaff section housing a two-tiered ejection system that can be combined with different types of ejection systems. The four windows of the chaff section are opened alternately in order to release the foil chaff symmetrically. The symmetrical window alignment also helps to keep the structural strength of the sounding rocket. 3.2 Spring type ejection system Figure 5a shows a cross section of the spring type ejection system. The foil chaff is kept in two inner split cylinders. Each cylinder can accommodate about 5,000 pieces of the foil chaff. The length and diameter of the cylinders are 125 and 54 mm, respectively. A shaft is attached to the cap of the inner cylinder on the ejection side. The opposite end of the shaft is locked with a lock plate, which is tied with a stainless steel wire to prevent a release of the inner split cylinder. When the rocket reaches the desired altitude, the stainless steel wire that holds the inner split cylinders is cut by a wire cutter, and the big coil spring pushes the cylinder outwards. As soon as the cylinder is ejected, the cylinder is split into two parts by an inner plate spring and releases the foil chaff. Therefore, in preflight tests, it is important to confirm that the cylinder is pushed out faster than split by the plate spring. The stainless steel wire must withstand an aerodynamic heating due to the rocket motion at high speeds. Microswitches are placed at the side of the cylinder to ensure a successful ejection of the cylinder. Figure 5b shows a photograph of the spring type ejection system. This type of ejection system is mechanically complex but has the advantages that the ejection of the foil chaff is independent of the ambient atmospheric conditions and that this type is more convenient to handle than the differential pressure type. 3.3 Differential pressure type ejection system The differential pressure type ejection system is shown in Fig. 6a. This type consists of a cylinder and a center shaft with a cap at one end. This cap seals the air-tight container and a sponge is attached to the shaft in order to

5 Y. KOIZUMI-KURIHARA et al.: FOIL CHAFF EJECTION SYSTEMS 45 Fig. 7. (left) Trajectory of the foil chaff cloud projected on the horizontal plane. The x and y axes indicate eastward and northward distances from the tracking radar, respectively. (right) Altitude of the chaff cloud with time after launch. Fig. 8. Profiles of (left) zonal and (middle) meridional wind velocities and (right) descending speed. sweep out the foil chaff. The shaft is locked by a lock plate and a stainless steel wire. The foil chaff is filled in the cylinder at the ground pressure. The cylinder has a length of 179 mm and a diameter of 50 mm, and thus can keep the same amount of foil chaff as the spring type ejection system. At the desired height, the wire is cut and the cap is pushed out due to the difference in pressure between the inside of the cylinder and the outside ambient atmosphere. Since the differential pressure is 1 atm, the cap with an area of 18 cm 2 is pulled out by a force of about 18 kgf just before ejection. Therefore, this type of ejection system uses a hard lock plate made of stainless steel. Microswitches at the side of each cap ensure a successful release of the foil chaff. Figure 6b shows a photograph of the differential pressure type ejection system. This system is mechanically simple but requires the pressure in the cylinder to be kept to the ground pressure. 4. Results Obtained by the Foil Chaff Experiments The WAVE2000 and WAVE2004 campaigns were carried out at Kagoshima in Japan using rocket-borne and groundbased instruments to study the formation process of waves in airglow structures from both, dynamical and chemical aspects. In the WAVE2000 campaign, the first experiment using the two new foil chaff ejection systems was successfully carried out with both types of ejection systems on the S sounding rocket, and horizontal and vertical velocity profiles were obtained. A prominent small-scale feature around an altitude of 90.5 km appeared in both, the horizontal and the vertical chaff motions (Koizumi et al., 2004). In the WAVE2004 campaign, the foil chaff experiment was performed using only the spring type ejection system on the S rocket. Wind shear layers of >40 m/s/km were located around the altitudes of 89 and 95 km, and the profile of the Richardson number reveals the existence of dynamically unstable layers at about the same height region (Koizumi et al., 2009). Between these experiments, the S experiment was performed but radar tracking failed due to technical problems. As an example of the results obtained by the foil chaff experiments, Fig. 7 shows the horizontal and vertical traces of the chaff cloud derived from the radar data obtained in the latest experiment with the S rocket. The horizontal trace indicates a complicated motion of the chaff cloud and the vertical trace shows a jump of a few kilometers at sec after launch. Since this jump is much larger than the typical size of the chaff cloud at this height range, this part of the trace was not used for the analysis. The observed foil chaff positions were averaged over an 800-m height interval taking the spread of the chaff cloud into account, and the horizontal and vertical velocities were obtained by differentiating the average positions with respect to time. The resultant horizontal wind velocity and descending speed profiles are shown in Fig. 8, indicating a northward wind above 93 km and a southward wind below 92 km. The fact that there is no vertical shear in the zonal wind at km altitudes is consistent with the result that the observed height of the sporadic E layer is above 95 km (Kurihara et al., 2010). These results demonstrate that the foil chaff technique and the newly developed ejection systems are a valuable tool for simultaneous observations of neutral winds and other parameters in the MLT region. Acknowledgments. The authors express their acknowledgements to Hans-Ulrich Widdel (deceased 23 Aug. 1999) from the German Max-Planck-Institut für Aeronomie for his technical information on chaff experiments. The authors also express their thanks to the rocket launching crew, and related institutions for their cooperation. The cutting machine was provided through the courtesy of Prof. Kristian Schlegel (Max-Planck-Institut für Aeronomie, renamed Max-Planck-Institut für Sonnensystemforschung in 2004). The foil chaff ejection system was manufactured by AD Co. Ltd. References Killeen, T. L., Q. 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