Radio Science. Estimate of a D region ionospheric electron density profile from MF radio wave observations by the S rocket

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1 RESEARCH ARTICLE Key Points: Observed the MF radio wave propagation characteristics in the ionospheric D region The polarized mode waves propagation characteristics obtained by analyzing the observed waveform Estimated the electron density profile in the ionosphere from the wave propagation characteristics Correspondence to: Y. Ashihara, ashihara@elec.nara-k.ac.jp Citation: Ashihara, Y., K. Ishisaka, and T. Miyake (2016), Estimate of a D region ionospheric electron density profile from MF radio wave observations by the S rocket, Radio Sci., 51, 40 46, doi:. Estimate of a D region ionospheric electron density profile from MF radio wave observations by the S rocket Y. Ashihara 1, K. Ishisaka 2, and T. Miyake 2 1 Department of Electrical Engineering, Nara College, National Institute of Technology, Yamatokoriyama-shi, Japan, 2 Faculty of Engineering, Toyama Prefectual University, Imizu-shi, Japan Abstract The S rocket, launched at 11:20 (JST) on 16 January 2007, was equipped with a radio receiver to observe the medium-frequency (MF) radio wave propagation characteristics in the ionosphere. The radio receiver measured the intensity and the waveform of the radio wave at 873 khz from the NHK Kumamoto broadcasting station. The polarized mode waves intensity characteristics were obtained by analyzing the observed waveform. In this study, the S rocket-observed polarized mode waves propagation characteristics are analyzed in order to estimate the electron density profile in the ionospheric D region. These observations become better measurement approach because the electron density profile in the ionospheric D region is difficult to be observed by other equipment such as a Langmuir probe. A Langmuir probe can measure in the ionospheric D region; however, the absolute values may be off by the influence of wake effects around the sounding rocket. It is demonstrated that the propagation characteristics of the polarized mode waves can be successfully used to derive the electron density profile in the ionospheric D region. Received 5 NOV 2015 Accepted 28 DEC 2015 Accepted article online 5 JAN 2016 Published online 25 JAN American Geophysical Union. All Rights Reserved. 1. Introduction In general, there are two approaches to observing the electron density profile in the ionosphere: through remote sensing techniques such as the ionosonde [e.g., Hall and Hansen, 2003] and the medium-frequency (MF) radar [e.g., Murayama et al., 2000] and by in situ observation, using the sounding rocket. The ionosonde and the MF radar provide continuous observation data and obtain the electron density vertical profile. However, it is not easy to measure the very low electron density in the lower ionospheric zones, such as the ionospheric D region. In situ observation, in contrast, is most effective. For example, the Langmuir probe is the most popular method to measure electron densities by sounding rocket, although it is difficult to provide data for the low-altitude ionospheric D region. Because the absolute values of electron densities may be off by the influence of wake effects around the sounding rocket. In the present study, the electron density in the lower ionosphere is derived by observing the radio wave propagation characteristics with radio receivers on board a sounding rocket. As a method of using the radio wave propagation characteristics, for example, there are Faraday rotation method [Mechtly et al., 1967; Bennett et al., 1972] and VLF propagation characteristics observation [Kintner et al., 1983]. Friedrich and Torkar [2001] presented a low-altitude ionospheric model based on many results of sounding rocket experiments. Previously, the electron density was estimated by calculating the propagation characteristics of the radio waves transmitted from a ground-based broadcasting station during several rocket experiments [Nagano and Okada, 2000], as well as during a S rocket experiment [Ashihara et al., 2006]. In the case of S rocket experiment, the electron density was estimated in the lower E region, in altitudes from 89 to 110 km. However, it could not be determined in the D region below the altitude of 89 km because the radio waves RMS intensity characteristics did not attenuate below that level. The S rocket was launched at 11:20 (JST) on 16 January The solar zenith angle is 35.5, and the solar flux F 10.7 = A MF radio receiver was installed on the rocket to derive the electron density in the ionospheric D region by analyzing the propagation characteristics of the L and R (left- and right-hand polarized) mode waves. As the propagation characteristics of the polarized mode waves are affected in the very low electron density region, they can be used to derive the electron density profile in the ionospheric D region. In this study, the observed wave intensities of the polarized mode waves are presented, and the electron density profile in the ionospheric D region is estimated. ASHIHARA ET AL. ESTIMATION OF A D REGION ELECTRON DENSITY 40

2 Figure 1. Trajectory of S rocket. NHK Kumamoto second channel broadcast station is located in 185 km to the north from the rocket launch site (Uchinoura Space Center (USC)). 2. Experimental Procedure 2.1. The S Sounding Rocket Experiment The S sounding rocket was launched from the Uchinoura Space Center (31.25 N, E) in Japan at 11:20 (JST) on 16 January The MF receiver (MFR) was installed on the rocket to observe the MF radio wave propagation characteristics in the ionosphere. The observed MF radio waves were transmitted in the NHK Kumamoto second channel, whose frequency is 873 khz. The MFR sensors contain a dipole antenna, which was extended 61.5 s after the launch at the altitude of 68.3 km. The rocket reached the apogee of 138 km at 184 s after the launch. Accordingly, the MFR observed the radio wave from the point of ascent at 68.3 km to the splashdown of the rocket. The rocket trajectory is presented in Figure 1. Figure 2 shows the block diagram of the MFR. The sensor is a dipole antenna consisted of two monopole antennas which length is 1 m: the tip-to-tip length of the dipole antenna is 2 m. Both the monopole antennas are deployed to the centrifugal direction of the rocket spin at 68.3 km. The MFR embraces a superheterodyne as a detection method, and it has two property outputs: the RMS intensity (Int-out) and the waveform (WF-out) of the radio wave. The Int-out detects an envelope intensity of an AM radio wave, which time constant is 1 ms. And the Int-out contains only intensity data; all the polarized mode waves are mixed in the magnetized plasma, which is defined by the cold plasma theory [Sen and Wyller, 1960; Budden, 1985]. The WF-out waveform is used to identify the polarized mode waves [Kimura, 1967], which are converted from 873 khz to 100 Hz. The Int-output and WF-output signals are transmitted to a pulse code modulation (PCM) digital telemeter on board the rocket as a common instrument. The PCM telemeter transmits the digital PCM data: Int-out is 8 bit/200 sps, WF-out is 8 bit/400 sps, to the ground receiving station. The left-hand and the right-hand polarized mode waves are mostly used to the Faraday rotation experiment. The Faraday rotation experiment leverages the phase variation of between the two propagation mode waves. Though in this study, we leverage the attenuation intensities of the two propagation mode waves, and we only use the waveform (WF-out) data to separate the polarized mode waves. ASHIHARA ET AL. ESTIMATION OF A D REGION ELECTRON DENSITY 41

3 Figure 2. Block diagram of the MF radio wave receiver (MFR) Separating the Polarized Mode Waves In the ionosphere, the radio waves propagate in two characteristic modes, i.e., the left-handed (counterclockwise) and the right-handed (clockwise) modes, following a geomagnetic vector. The rocket spin axis is almost counterparallel to the direction of the geomagnetic field in the rocket experiment. Figure 3 shows the rocket rotation configuration, the geomagnetic vector, and the polarization modes. The rocket is spinning clockwise, parallel to the rotation direction of the left-handed mode wave electric field vector. The frequency of the left-handed mode wave is f c -ΔS, whereas the frequency of the right-handed mode wave is f c +ΔS, where f c is the MF radio wave carrier frequency and ΔS is the rocket spin frequency. The Doppler frequency shift, which is due to the relative velocity between the rocket and the radio wave, is added to the above frequencies. The Doppler frequency is given as ΔD = 1 2π k V (1) where k is the wave number and V is the rocket velocity. Here the upgoing wave Doppler shift is defined as ΔD u, and the downgoing wave Doppler shift is ΔD d.asδd u is different from ΔD d, the Doppler shift can be used to separate the upgoing from the downgoing waves on the basis of the observed radio waves (Figure 4). Therefore, four different mode waves (R u, R d, L u, and L d ) can be distinguished by the spectrum analysis of WF-out Full-Wave Analysis The theoretical radio wave propagation characteristics can be calculated using the full-wave analysis method [Nagano et al., 1975]. The full-wave method performs better than other simulation methods in analyzing the propagation characteristics of waves in the ionospheric anisotropic plasma. The radio waves propagation characteristics in the lower ionosphere are affected by the electron density and the electron neutral collision frequency, and the absorption is proportional to the product of the electron density and the collision frequency. However, the electron density is more variable than the collision frequency; thus, measuring the electron density is more important than establishing the collision frequencies. The collision frequencies Figure 3. Bottom view of the rocket rotation configuration, geomagnetic vector, and polarization modes. ASHIHARA ET AL. ESTIMATION OF A D REGION ELECTRON DENSITY 42

4 Figure 4. Frequency diagram of characteristic mode waves observed by the rocket s MF receiver. can establish with the product of the atmospheric pressures and a constant factor K m =6.4 [Thrane and Piggott, 1966; Gregory and Manson, 1967], and we calculated the electron neutral collision frequency from the MSIS-E-90 Atmosphere Model [Hedin, 1991]. Therefore, the electron density profile can be approximated by absorbing the radio waves observed in the rocket experiment [Nagano and Okada, 2000]. 3. Results The Int-out and WF-out observations during the rocket s ascent are presented in Figure 5. Then the rocket velocity is 1248 m/s at 68 km and 1087 m/s at 88 km. Figure 5a shows the radio wave intensities. The vertical axis corresponds to the altitude, and the horizontal axis indicates the relative radio wave intensity, compared with the maximum intensity, in db. The dipole antennas were deployed at an altitude of 68.3 km. The radio waves are received from approximately 68.3 to 83 km and suddenly attenuate between 83 and 84 km. From 68.3 to 75 km, the profile radio wave intensity varies with a 0.8 km spatial interval because of the dipole antenna s rotation with the rocket spinning. Between approximately 75 and 83 km, the radio wave intensity is gradually modified independent of the rocket spinning. This is probably because of the standing wave formed by the upgoing and downgoing waves reflected at 84 km. The WF-out frequency spectrum appears in Figure 5b. At 68 km, the spread spectrum during 96 khz to 101 Hz is observed, but this is a noise caused by deploying the dipole antenna. And there are two Figure 5. (a) Observed intensity, (b) WF-out frequency spectrum, (c) intensities of the L u (full line) and the R u +L d (dashed line) mode waves, during the rocket s ascent. L: left-handed (counterclockwise) polarized mode waves, R: right-handed polarized mode waves, u: upgoing mode waves, d: downgoing mode waves. ASHIHARA ET AL. ESTIMATION OF A D REGION ELECTRON DENSITY 43

5 Figure 6. (a) Electron density profile calculated by the IRI-2001 model. (b) Comparison between the observed and the calculated wave intensity. (c) Comparison between observed (bold lines) and the calculated (narrow lines) intensities of four polarization mode waves, during the rocket s ascent. The observed intensities correspond to the L u (red line) and the R u + L d (blue line). The calculated intensities from full-wave analysis are L u, R u, L d,andr d. pronounced components at frequencies approximately 97 Hz and 99 Hz, which correspond to the left-handed upgoing mode wave (L u ) and the composite right-handed upgoing and left-handed downgoing mode wave (R u + L d ), respectively. The rocket spin frequency (ΔS) is 0.8 Hz. The Doppler shift, during the rocket s ascent (ΔD u ), is approximately 1 Hz and during the rocket s descent (ΔD d ) approximately +1 Hz. This is because the right-handed upgoing mode wave (R u ) and the left-handed downgoing mode wave (L d ) cannot be separated clearly in the rocket experiment. Figure 5c shows the respective intensities for the L u (full line) and the R u + L d (dashed line). The L u intensity is constant at altitudes below 81 km and then decreases more than 40 db in altitudes from approximately 81 to 84 km. The intensity of the R u + L d gradually decreases between 68 and 75 km, due to the Ru propagation characteristics. Then, it increases again from 75 to 81 km, reflecting the L d propagation characteristics. Comparing the radio wave intensities (Figure 5a) with the polarized mode wave propagation characteristics (Figure 5c) at altitudes below 83 km, the radio wave intensity is attenuated by several db, whereas the intensity of the R u + L d mode waves clearly varies around 20 db. 4. Discussion 4.1. Radio Wave Propagation Characteristics Calculated From the IRI Electron Density Profile The S rocket was equipped with a fast Langmuir probe to observe the electron density profiles above 95 km. However, the MFR is aim to estimate the electron density profile in the ionospheric D region below 95 km. Therefore, the radio wave propagation characteristics are calculated using the International Reference Ionosphere (IRI)-2001 model [Bilitza, 2001]. Figure 6 presents the IRI-2001 model and the calculated and observed radio wave intensities. The comparison between the observed and the calculated radio wave intensities (Figure 6b) indicates that the calculated intensity gradually diverges from the observed intensity as the altitude increases. Furthermore, between 74 and 81 km, the observed R u and L d differ from the calculated R u and L d (Figure 6c). Consequently, the IRI-2001 model did not adequately predict the electron density profile in the ionospheric D region during the flight period of S rocket Radio Wave Propagation Characteristics Calculated From the Corrected Electron Density Profile The IRI electron density profile was corrected by fitting the observed intensity profiles to the calculated ones. Figure 7 shows the corrected electron density profile as well as the calculated and the observed radio wave ASHIHARA ET AL. ESTIMATION OF A D REGION ELECTRON DENSITY 44

6 Figure 7. (a) Corrected electron density profile, (b) comparison between the observed and the calculated wave intensity, and (c) comparison between the observed and the calculated intensity profiles of the polarized mode waves during the rocket s ascent. intensities. The IRI-2001 model calculated intensity increases very slowly from 68 to 84 km. The corrected electron density is almost constant at cm 3 between 68 and 82 km and then increases steeply, from 100 cm 3 to more than 10,000 cm 3 at altitudes between 82 and 84 km. This steep increase is not predicted by the IRI-2001 model (Figure 7a). Between 75 and 85 km, the calculated radio wave intensity is modulated by a standing wave (Figure 7b). The standing wave is composed of the upgoing wave and the downgoing wave, which are reflected by the steep increase of the estimated electron density at 84 km. The observed radio wave also deviates from the calculated profile between 75 and 83 km due to the standing wave and the rocket spinning. The trend, however, of the standing wave is consistent with the calculated results. The observed (bold lines) and calculated (thin lines) intensities of the different polarized mode waves are presented in Figure 7c. The observed L u agrees with the calculated L u below 81 km. The observed R u and L d are consistent with the calculated R u at altitudes from 68 to about 75 km and consistent with the calculated L d from about 75 to about 81 km. On the basis of the above analysis, the estimated electron density profile (Figure 7a) is most realistic for the ionospheric D region. The corrected electron density profile is higher than the IRI-2001 modeled profile for altitudes from 68 to 82 km. Acknowledgments I wish to thank T. Okada for instructive advice on a system design for this experiment. This experiment had supported by the sounding rocket project, Japan Aerospace Exploration Agency (JAXA). Data used for this analysis may be obtained by contacting the corresponding author, Y. Ashihara (ashihara@elec.nara-k.ac.jp). 5. Conclusions In the present experiment, an MFR is installed on the S rocket to observe the MF radio wave propagation characteristics in the ionospheric D region. The MFR s waveform output (WF-out) provides significant information on the polarized mode spectrum in the magnetized plasma. Here the polarized mode waves are separated by waveform frequency. Below 83 km, the radio wave intensity is attenuated in several db, whereas the polarized mode waves intensity peaks at 20 db, exhibiting clear variations. Furthermore, the observed intensities are compared with the theoretical values calculated according to the full-wave method. The calculated intensities, with the IRI-2001 model, are different from the observed intensities. The calculated electron density profile is corrected to conform to the observed intensities. It is demonstrated that the propagation characteristics of the polarized mode waves can adequately estimate the electron density profile in the ionospheric D region. References Ashihara, Y., K. Ishisaka, T. Okada, T. Miyake, Y. Murayama, and I. Nagano (2006), Estimation of electron density profile in the lower ionosphere at winter nighttime by rocket observations of LF and MF radio waves, IEICE Trans. Commun., J89-B(10), Bennett, F. D. G., J. E. Hall, and P. H. G. Dickinson (1972), D-region electron densities and collision frequencies from Faraday rotation and differential absorption measurements, J. Atmos. Terr. Phys., 34, Bilitza, D. (2001), International Reference Ionosphere 2000, Radio Sci., 36, Budden, K. G. (1985), The Propagation of Radio Waves, Cambridge Univ. Press, Cambridge, U. K. ASHIHARA ET AL. ESTIMATION OF A D REGION ELECTRON DENSITY 45

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