Horizontal structure of sporadic E layer observed with a rocket borne magnesium ion imager

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2009ja014926, 2010 Horizontal structure of sporadic E layer observed with a rocket borne magnesium ion imager J. Kurihara, 1,2 Y. Koizumi Kurihara, 3 N. Iwagami, 4 T. Suzuki, 5 A. Kumamoto, 5 T. Ono, 5 M. Nakamura, 6 M. Ishii, 6 A. Matsuoka, 3 K. Ishisaka, 7 T. Abe, 3 and S. Nozawa 1 Received 24 September 2009; revised 2 June 2010; accepted 21 June 2010; published 16 December [1] To study the spatial structure of midlatitude sporadic E (E s ) layers, the ultraviolet resonant scattering by magnesium ions (Mg + )inane s layer was observed during the evening twilight with the Magnesium Ion Imager (MII) on the sounding rocket launched from the Uchinoura Space Center in Kagoshima, Japan. The in situ electron density measured by an onboard impedance probe showed that the E s layer was located at an altitude of 100 km during both the ascent and descent of the flight. Simultaneous observation with a ground based ionosonde at Yamagawa identified the signature of horizontally patchy structures in the E s layer. The MII successfully scanned the horizontal Mg + density perturbations in the E s layer and found that they had patchy and frontal structures. The horizontal scale and alignment of the observed frontal structure is generally consistent with a proposed theory. To our knowledge, this is the first observation of the two dimensional horizontal structure of Mg + in an E s layer. Citation: Kurihara, J., et al. (2010), Horizontal structure of sporadic E layer observed with a rocket borne magnesium ion imager, J. Geophys. Res., 115,, doi: /2009ja Introduction [2] Midlatitude sporadic E (E s ) layers have been extensively studied for nearly half a century (see reviews by Whitehead [1989], and Mathews [1998]), but their spatial structure is still not well understood. The spatial structure is closely linked to quasiperiodic (QP) echoes [Yamamoto et al., 1991] and instabilities in E s layers [Tsunoda et al., 2004]. Horizontally patchy structures of electron density distribution in E s layers were originally proposed to account for the partial transparency of E s layers obtained from ionogram data [Whitehead, 1972], and cross sectional profiles of such a patchy E s layer were subsequently observed with the incoherent scatter (IS) radar [Miller and Smith, 1975, 1978]. Recently, Hysell et al. [2004] found patchy plasma structures in an E s layer collocated with QP echo events by using common volume incoherent and coherent radar observations. They also found large scale waves with a wavelength of 1 Solar Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan. 2 Now at Graduate School of Science, Hokkaido University, Sapporo, Japan. 3 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan. 4 Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan. 5 Graduate School of Science, Tohoku University, Sendai, Japan. 6 National Institute of Information and Communications Technology, Koganei, Japan. 7 Department of Information Systems Engineering, Toyama Prefectural University, Imizu, Japan. Copyright 2010 by the American Geophysical Union /10/2009JA about 30 km propagating southwest in the E region by radar imaging. However, the two dimensional horizontal structure of an E s layer has never been directly observed, except for the visualization by artificial airglow induced by HF radio waves [Djuth et al., 1999; Kagan et al., 2000, 2002; Bernhardt et al., 2003]. [3] The most widely accepted mechanism for the formation of a midlatitude E s layer is explained in the wind shear theory, which states that the E s layer is formed by the convergence of metallic ions owing to the effect of vertical wind shear. Among the metallic ions observed by rocket borne mass spectrometers in the E region [Kopp, 1997; Grebowsky et al., 1998], Mg + is one of the dominant species, along with Fe +. For example, Roddy et al. [2004, 2007] observed that the relative ion composition of an E s layer between 101 and 107 km was 84% (77%) Fe +, 13% (21%) Mg +, and 2.3% (2.5%) NO + on the ascent (descent) of a sounding rocket flight. Therefore, the spatial structure of Mg + distribution in an E s layer is expected to reflect the spatial structure of the E s layer. [4] The intense resonance line emissions of Mg + at and nm have been used for global remote sensing of upper atmospheric Mg + from satellites. However, these ultraviolet (UV) emissions cannot be observed from the ground because of strong absorption by the stratospheric ozone. Anderson and Barth [1971] measured Mg + emissions with a rocket borne spectrometer from just below the E region during the evening twilight and found indications of an E s layer spatial structure. Valenzuela et al. [1981] observed Mg + emissions with a balloon borne TV camera system which reached an altitude of 42.7 km, and they found a striated structure in their images when the UV shadow was cast over 1of6

2 Figure 1. Schematic diagram of the MII observation. the E region in the evening twilight. These two pioneering studies suggested the usefulness of Mg + imaging for understanding the spatial structure of E s layers, but a two dimensional horizontal structure of Mg + in an E s layer had not yet been demonstrated. [5] To investigate the spatial structure of E s layers, a sounding rocket was launched from the Uchinoura Space Center (USC) in Kagoshima, Japan, during the evening twilight. The Magnesium Ion Imager (MII) on the rocket successfully scanned the spatial structure of Mg + emissions from an E s layer at an altitude of 100 km. In addition, in situ electron density measurements of the E s layer were simultaneously performed by an onboard impedance probe, and the characteristics of the E s layer were obtained from a ground based ionosonde. To our knowledge, this is the first observation of the two dimensional horizontal structure of Mg + in an E s layer. 2. Instrumentations and Observations [6] The MII is a rocket borne photometer that consists of an interference filter, lens, and multianode photomultiplier tube (PMT). The interference filter has a center wavelength of nm and a bandwidth (full width at half maximum) of 16.3 nm. This means the measured intensity may include components of not only the resonant scattering by Mg + at and nm but also the Rayleigh scattering by neutral molecules. The multianode PMT has an eightchannel linear array. The instantaneous field of view (FOV) is for each channel and 1 10 for a total of eight channels. The line of sight is tilted at 150 with respect to the rocket spin axis. With the spin motion of the rocket, the MII scans a ring shaped area with a width of 10 (an inner diameter of 50 and an outer diameter of 70 ) rearward from the rocket, and the FOVs of eight channels are aligned concentrically in the scanning area. Figure 1 shows a schematic of the MII observation. The scanning area moves in accordance with changes in the position and attitude of the rocket. [7] The sounding rocket S carrying the MII was launched from the USC (31.25 N, E) at 1814:40 JST (UT + 9 hr) on 6 February The appearance of the E s layer at an altitude around 100 km was confirmed by ionosonde observations at Yamagawa (31.20 N, E). The time of the launch was preliminarily determined by a numerical simulation to maximize the observed intensity ratio of the Mg + resonant scattering to Rayleigh scattering. The solar produced E region plasma should have almost entirely recombined at that time because the solar zenith angles were 95.3 at Yamagawa and at the rocket trajectory. The rocket was launched geographically southeastward from the USC and reached its apogee of km at about 196 sec after the launch. The spin period of the rocket was about 1.4 sec during the MII observation. The FOV of the MII is calculated from the position and attitude data of the rocket. The attitude of the sounding rocket is estimated by correlating the geomagnetic data collected with the Digital Fluxgate magnetometer (DFG) on the rocket with the geomagnetic field derived from the International Geomagnetic Reference Field model by assuming that the precession of the rocket spin axis is a circular motion. Although the usual precession radius for the S 310 type sounding rocket is degrees, the S sounding rocket had the unexpectedly large precession radius of 25 degrees. The FOV of the MII mapped at a 100 km altitude was found to be located mainly westward from the rocket trajectory because of the unusually tilted spin axis of the rocket. As a result, measured intensities in the western part of the scanning area exceeded the measurement limit, owing to strong Rayleigh scattering, and meaningful data are confined to an arcshaped portion at the eastern edge of the ring shaped scanning area. Except for sec after the launch, the measurement suffered from contamination by stray light due to the unusual attitude of the rocket. Intensity data taken from higher altitudes above 125 and 145 km in the ascent and descent, respectively, are used for imaging of the horizontal structure. [8] Figure 2 is an intensity distribution measured with the innermost Channel 8 of the MII at around sec after the launch as a function of a spin phase angle. The spin phase angle is defined as an angle between the MII line of sight and antisunward directions in a plane perpendicular to the rocket spin axis. The measured intensity distribution basically has a minimum at the antisunward direction corresponding to zero spin phase angle. Perturbations on the intensity distribution can be readily seen by comparing with the 21 spin running averages as shown in Figure 2. The 21 spin running average is about one fifth of the total sample time. Figure 3 shows Figure 2. Intensity distribution measured with Channel 8 of the MII at around sec after the launch as a function of a spin phase angle and the 21 spin running average. 2of6

3 Figure 3. Temporal variation of relative intensity distributions in the MII scanning area mapped at a 100 km altitude. T is the time from launch, and the blue rectangles indicate the same longitudinal area. Dashed lines are the rocket trajectory mapped at 100 km. temporal variation of the relative intensity distributions in the MII scanning area mapped at an altitude of 100 km. The relative intensities were derived by dividing the measured intensities by the 21 spin running averages. Perturbations with amplitudes of 20% are found in the relative intensity distributions. It is noteworthy that the pattern of these perturbations seemed to be fixed and unchanged during observation. Figure 4 shows the composite image of relative intensity distributions on a averaging grid. Although the fine structures in the relativeintensity distributions in Figure 3 are smeared out by the averaging, the perturbations still remain in Figure 4. When the composite image is mapped at 90 or 110 km altitudes, the perturbations also remain but slightly weaken. This result indicates that these perturbations were centered around a 100 km altitude where the E s layer was located, because the twilight UV radiation was significantly attenuated below 90 km and Mg + was completely in the UV shadow below 80 km. In other words, this result is consistent with the assumption that Mg + is converged into a thin E s layer around 100 km. If the Rayleigh scattering component of the measured intensity has no perturbation, the perturbations in relative intensity are caused by perturbations in the Mg + density. In that case, the amplitude of the relative intensity perturbations provides a lower limit to the amplitude of Mg + density perturbations because the intensity ratio of Mg + resonant scattering to Rayleigh scattering is unknown. The amplitude of Mg + density perturbation is therefore at least 20%. [9] Figure 5 shows altitude profiles of the electron density observed by the impedance probe during rocket ascent and descent. Although modulations caused by the spin motion of the rocket are more apparent during descent, the altitude profiles are generally similar for the ascent and descent. The E s layer has a single layer structure at an altitude of 100 km in both profiles. The peak electron densities of the E s layer are and cm 3 for the ascent and descent, respectively. This difference in peak electron densities Figure 4. Map showing a composite image of relative intensity distributions. Points labeled A and D are footprints of the sounding rocket at a 100 km altitude during ascent and descent, respectively. 3of6

4 within the E s layer [Maruyama et al., 2006]. Around the time of the rocket launch, the highest and lowest electron densities estimated from f o E s and f b E s were and cm 3, respectively. This range encompasses the peak electron densities observed with the impedance probe. The height of the E s layer also coincided with the altitudes of peak electron densities. Although the footprint of the rocket at a 100 km altitude during descent was 200 km away from the site of the ionosonde at Yamagawa, impedance probe observations were consistent with the ionosonde observations. Since the intensity of the Mg + emissions observed with the MII was integrated along the MII line of sight, the amplitude of the relative intensity perturbations cannot be compared directly with the ratio of the highest to lowest electron densities estimated from the ionogram. However, on the basis of the highest and lowest electron densities in Figure 6, ionosonde observation identified the signature of a horizontally patchy structure in the E s layer over Yamagawa. Figure 5. Altitude profiles of electron density observed with an impedance probe on the rocket during ascent and descent. between ascent and descent is relatively small compared to similar observations with the impedance probe, in which they sometimes differ by a factor of 4 [Yamamoto et al., 1998]. The footprints of the rocket at a 100 km altitude during ascent and descent are separated from each other by 130 km, and they are only km away from the MII scanning area as shown in Figure 4. This result implies that the E s layer was spread over a broad region that included the MII scanning area. [10] There is another broad layer centered around 130 km in both profiles in Figure 5. If this layer is an intermediate layer, which is commonly observed in the midlatitude ionosphere, the layer could be formed by vertical shear in the neutral wind and composed mainly of metallic ions including Mg + [Roddy et al., 2004, 2007]. However, the peak electron density of this layer is only two times higher than the electron density at 135 km in the upper side of the layer and the electron densitiesin the lower side of the layer are very low ( 10 3 cm 3 ). These features indicate that the broad layer around 130 km is likely to be a vanishing E layer after sunset rather than an intermediate layer. Therefore, Mg + in this layer provides a negligible contribution to the MII observation. [11] Figure 6 presents time variations of the critical ( f o E s ) and blanketing ( f b E s ) frequencies and the height of the E s layer as derived from ionogram data at Yamagawa. The radio waves at frequencies between f o E s and f b E s are partially reflected by the E s layer, and the difference between the two frequencies quantifies the degree of layer transparency. In the model of the horizontally patchy E s layer, f o E s and f b E s give the highest and lowest electron densities 3. Discussion [12] The horizontal structure of E s layers is attributable to dynamical, electrodynamical, and meteor induced processes. Miller and Smith [1978] found a wavelike electron density structure in the E s layers observed with the IS radar at Arecibo, and they concluded that the wavelike structure can be generated by gravity waves and Kelvin Helmholtz (K H) instabilities. To explain QP echoes, Woodman et al. [1991] proposed multiple E s layers periodically distorted by gravity waves. On the basis of in situ neutral wind measurements, Larsen [2000] suggested that the K H billow structure embedded in the strong shear flow can lead to plasma instabilities in the E s layer. Cosgrove and Tsunoda [2002] showed that E s layer instability has an azimuthal dependence which the growth rate maximizes when the phase fronts align from northwest to southeast (NW SE) in the northern hemisphere. Maruyama et al. [2003] reported that spontaneous echoes in ionograms persisted for several tens of minutes during the Leonid meteor shower and con- Figure 6. Time variations of critical ( f o E s ) and blanketing ( f b E s ) frequencies (top) and height of the sporadic E layer (bottom) derived from ionogram data at Yamagawa. Right axis at the top provides an electron density corresponding to the frequency on the left axis. 4of6

5 cluded that these echoes could be attributed to meteorinduced E s patches. [13] The present results demonstrate that various horizontal structures temporarily coexist in a large scale twodimensional image of the E s layer. The composite image of relative intensity perturbations in Figure 4 appears to have a frontal structure elongated in the NW SE direction and a patchy structure contained within the frontal structure. The width of the frontal structure and the diameter of the patchy structure are km. It is important to note, however, that the perturbations have an upper limit to the horizontal scale for imaging. Since relative intensity perturbation is derived using 21 spin running averages, which correspond to an area approximately 60 km long and 20 km wide, the larger scale structures are removed from the image. Furthermore, the whole composite image is elongated in the NW SE direction because the location of each image is changing in accordance with rocket trajectory. Therefore, it is difficult to observe wavelike structures, especially those that propagate northeast or southwest. As these horizontal scale limits for MII observation are mainly due to the unusual attitude of the sounding rocket, much larger images with larger scale structures will be obtained in future experiments. [14] The theory of multiple distorted E s layers proposed by Woodman et al. [1991] has not been verified by observations. There are other theories that suggest that altitude modulation of an E s layer by gravity waves is responsible for the generation of QP echoes [Tsunoda et al., 1994]. If an E s layer is sinusoidally modulated in altitude and not in plasma density, the downward looking MII would not be able to detect the modulation. However, if the E s layer is deeply modulated or distorted in altitude enough to alter the column plasma density along the line of sight of the MII, the altitude distortion can be detected even in a single layered structure. In that case, positive perturbations are observed to be aligned along the horizontal wavefront of the distorted E s layer, and horizontal spacing of the positive perturbations corresponds to horizontal wavelength of the gravity wave that modulates the E s layer. The frontal structure in Figure 4 is represented by positive perturbations. While wavelike structures are not recognized in the composite image, the horizontal spacing of the frontal structure is >30 km, which is reasonable for a horizontal wavelength of gravity waves frequently observed in this altitude region. [15] The strong wind shear that contributes to the formation of the E s layer also causes neutral density perturbations by the K H billow around the height of the E s layer [Bernhardt, 2002]. The neutral density perturbations may produce perturbations in the Rayleigh scattering component in the MII observation. However, the estimated Rayleigh scattering component is distributed in a wide altitude range of km at the time of the rocket flight, while the Mg + resonant scattering is concentrated in the E s layer. Therefore, it is unlikely that the observed Rayleigh scattering component is significantly influenced by the neutral density perturbations by the K H billow with a vertical extent of a few kilometers. In general, neutral density perturbations with large vertical scales would have large horizontal scales, and thus the vertically widespread Rayleigh scattering component would show perturbations with horizontal scales larger than those observed by the MII. On the other hand, the Mg + resonant scattering can be strongly modulated by the Mg + density perturbations if the K H billow is formed at the height of the E s layer. The horizontal spacing of striated structures in the plasma density formed by the neutral K H billow is about eight times the vertical scale of the wind shear [Larsen, 2000]. If the vertical scale of the wind shear is >4 km, the frontal structure in the composite image in Figure 4 is possible with the neutral K H billow. [16] The azimuthal dependence of the E s layer instability is supported by many observations [Tsunoda et al., 2004], and NW SE structures with km horizontal wavelength were dominantly observed. The horizontal wavelength of the E s layer instability depends on the background wind profile that is modulated possibly by gravity waves and on the polarization electric field that is modulated by gravity waves at F region through electrodynamical coupling [Cosgrove, 2007]. As discussed above, sinusoidal altitude modulation of an E s layer cannot be detected by the MII, because the modulation makes no significant perturbation in the column plasma density. Numerical simulations have shown that the E s layer instability modulates the horizontal and vertical distribution of the plasma density in a plane perpendicular to the NW SE direction [Cosgrove and Tsunoda, 2003]. If the density modulation is well developed by the E s layer instability, the Mg + density perturbation aligned in a NW SE direction can be detected by the MII. Altitude distortions merely by gravity waves and density modulations by the K H billows have no such directional preference. 4. Conclusions [17] The present study provides the first observation of the two dimensional horizontal structure of Mg + in an E s layer. The E s layer was located at an altitude of 100 km and exhibits the features of the horizontally patchy structure. The horizontal Mg + density perturbations in the E s layer also have a patchy structure within a frontal structure, with amplitudes of at least 20%. The horizontal scale of the observed frontal structure is generally consistent with the proposed theories, and the E s layer instability is the most distinctive among the theories because the NW SE alignment of the observed frontal structure is consistent with the preferred azimuthal orientation of the E s layer instability. These results demonstrate the usefulness of Mg + imaging for understanding the spatial structure of E s layers. In future experiments, simultaneous observation with another imager that would be used to measure only the Rayleigh scattering component, or with a high resolution spectrometer to separate the Mg + resonant scattering component from the Rayleigh scattering component, would be useful for determining the absolute amplitude of Mg + density perturbations in E s layers. A combination of the radio tomographic imaging using a rocket beacon technique [Bernhardt et al., 2005] with Mg + horizontal imaging would be beneficial for studying the three dimensional structure of E s layers. [18] Acknowledgments. We thank all the staff of the Institute of Space and Astronautical Science for conducting the successful rocket experiment. One of the authors (J. K.) was supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. [19] Zuyin Pu thanks Russell Cosgrove and three other reviewers for their assistance in evaluating this paper. 5of6

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Suzuki, Graduate School of Science, Tohoku University, Aramaki, Aoba ku, Sendai, Miyagi , Japan. J. Kurihara, Graduate School of Science, Hokkaido University, Kita 10 Nishi 8, Kita ku, Sapporo, Hokkaido , Japan. (kurihara@mail. sci.hokudai.ac.jp) S. Nozawa, Solar Terrestrial Environment Laboratory, Nagoya University, Furo cho, Chikusa ku, Nagoya, Aichi , Japan. 6of6