Of Airglow Associated with Medium-Scale Traveling Ionospheric Disturbance

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Journal of Geophysical Research: Space Physics

RESEARCH ARTICLE Mesoscale ﬁeld-aligned irregularity structures (FAIs) 10.1002/2014JA020944 of airglow associated with medium-scale traveling Key Points: ionospheric disturbances (MSTIDs) • An evolution of medium-scale traveling ionospheric disturbances Longchang Sun1,2, Jiyao Xu1, Wenbin Wang3, Xinan Yue4, Wei Yuan1, Baiqi Ning5, Donghe Zhang6, (MSTIDs) is presented 1 • Mesoscale ﬁeld-aligned irregularity and F. C. de Meneses

structures (FAIs) (~150 km) of airglow 1 are firstly presented State Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, China, • We first find the mesoscale FAIs had a 2Graduate University of the Chinese Academy of Sciences, Beijing, China, 3High Altitude Observatory, National Center for northwestward relative velocity to Atmospheric Research, Boulder, Colorado, USA, 4COSMIC Program Office, University Corporation for Atmospheric Research, main-body MSTIDs Boulder, Colorado, USA, 5Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China, 6Department of Geophysics, Peking University, Beijing, China

Correspondence to: ﬁ L. Sun, Abstract In this paper, we report the evolution (generation, ampli cation, and dissipation) of optically [email protected] observed mesoscale ﬁeld-aligned irregularity structures (FAIs) (~150 km) associated with a medium-scale traveling ionospheric disturbance (MSTID) event. There have not been observations of mesoscale FAIs of

Citation: airglow before. The mesoscale FAIs were generated in an airglow-depleted front of southwestward propagating Sun, L., J. Xu, W. Wang, X. Yue, W. Yuan, MSTIDs that were simultaneously observed by an all-sky imager, a GPS monitor, and a digisonde around B. Ning, D. Zhang, and F. C. de Meneses Xinglong (40.4°N, 30.5° magnetic latitude), China, on 17/18 February 2012. A normalized cross-correlation (2015), Mesoscale field-aligned irregularity structures (FAIs) of airglow associated method has been used to obtain the velocities of mesoscale FAIs and MSTIDs. The mesoscale FAIs had an with medium-scale traveling ionospheric obvious northwestward relative velocity to main-body MSTIDs (about 87.0 m/s on average). The direction of this disturbances (MSTIDs), J. Geophys. relative velocity was roughly parallel to the depleted fronts. Furthermore, the evolution of the mesoscale Res. Space Physics, 120,9839–9858, doi:10.1002/2014JA020944. FAIs was mostly controlled by the intensity of the depleted fronts. Occurred in a highly elevated ionosphere that had a total electron content depletion associated with large negative airglow perturbations (25%), the Received 14 DEC 2014 mesoscale FAIs grew rapidly when they experienced southeastward wind, which had a speed of about 100 m/s Accepted 14 OCT 2015 and were measured by a Fabry-Perot interferometer. A northeastward polarization electric field within a Accepted article online 19 OCT 2015 fi Published online 12 NOV 2015 depleted airglow front can play a controlling role in the development of the mesoscale FAIs. The electric eld can significantly elevate the ionosphere and move the mesoscale FAIs northwestward by the E × B drift. The processes for the generation and development of the polarization electric field and the mesoscale FAIs, however, need further study.

1. Introduction Nighttime medium-scale traveling ionospheric disturbances (MSTIDs) are F region ionospheric wave-like perturbation structures. They are oriented from the northwest (NW) to the southeast (SE) in the Northern Hemisphere with a wavelength of a few hundred kilometers and travel southwestward (SW) with a phase velocity of approximately 100 m/s [Kelley and Miller, 1997; Mendillo et al., 1997; Miller et al., 1997]. There have been many observations of nighttime MSTIDs using various instruments, such as ionosondes [e.g., Bowman, 1990; Bowman and Hajkowicz, 1991], HF Doppler soundings [e.g., Waldock and Jones, 1987; Xiao et al., 2009], satellite beacons [e.g., Evans et al., 1983; Jacobson et al., 1995], incoherent scatter radars [e.g., Fukao et al., 1991; Miller et al.,1997],all-skyimagers[e.g.,Mendillo et al., 1997; Kubota et al., 2000, 2011; Shiokawa et al., 2013], and GPS receivers [e.g., Saito et al., 2001; Ding et al., 2011]. These observations indicate that MSTIDs are most likely atmospheric gravity waves propagating at thermospheric heights [Hines, 1960]. Statistical analyses using all-sky imagers—done by Garcia et al. [2000a] and Martinis et al. [2010] in Arecibo, Duly et al. [2013] in the Central Paciﬁc and South American sectors, Shiokawa et al. [2003a] and Narayanan et al. [2014] in Japan, Kubota et al. [2011] from the subauroral region of Alaska, and Shiokawa et al. [2013] in two high latitude stations of Europe and Canada—have shown the SW preferable propagation of nighttime MSTIDs in the Northern Hemisphere for all seasons and at all local times. In the Southern Hemisphere, a northwestward (NW) preferable propagation of nighttime MSTIDs was found [Candido et al.,

©2015. American Geophysical Union. 2008; Pimenta et al., 2008; Amorim et al., 2011]. In the statistical analyses cited above, Kubota et al. [2011] All Rights Reserved. recently showed that the SW propagating MSTIDs in Alaska are atmospheric gravity waves (AGWs), whereas

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other investigators showed that the SW (or NW) preference of MSTIDs in their analyses cannot be reasonably explained by the classical gravity wave theory [Hines, 1960]. In addition, Behnke [1979], Miller et al. [1997], Kelley et al. [2000], and Shiokawa et al. [2003b] found that strong eastward electric ﬁelds existed within MSTIDs. The electriﬁed characteristic of nighttime MSTIDs cannot be explained by the gravity wave theory either. The Perkins instability [Perkins, 1973] is generally believed to be the most likely mechanism for the nighttime MSTIDs (scale > 50 km), as it can successfully predict the azimuth of the wavefront (aligned from NW to SE in the Northern Hemisphere) of MSTIDs [Kelley and Miller, 1997; Yokoyama et al., 2008]. This instability occurs

when the perturbation wave vector (k) of the electric field E (where E = E0 + U × B, and E0, U, and B are the background electric field, neutral winds, and the geomagnetic field, respectively) or field-line integrated

Pedersen conductivity (Σp) perturbations lie between the direction of the current J (J = Σp E) and the geomagnetic east. Initial perturbations in both E and Σp can grow with time when the instability is triggered. The instability growth rate derived by Perkins [1973] is gsin2I sin α sin θ γ ¼ (1) < νin > Hn cos θ

where g, Hn, <νin>, θ, α, and I are the gravitational acceleration, neutral density scale height (about 50 km), height-integrated ion-neutral collision frequency, the angle of J with respect to the geomagnetic east, the angle of the perturbation wave vector (k) with respect to the geomagnetic east, and magnetic inclination, respectively. However, the growth rate (about 10 4 s 1) of the Perkins instability is too low, and the perturbation wave vector (k) predicted by the Perkins instability is not consistent with the propagation direction of the observed MSTIDs [Garcia et al., 2000a; Shiokawa et al., 2003b]. To overcome the low growth rate of the Perkins instability, Kelley and Fukao [1991] proposed that preexisting ionospheric undulations created by bottomside gravity waves could be further amplified by the Perkins instability. Later studies suggested that polarization electric fields associated with E layer/F layer coupling [Cosgrove et al.,2004;Yokoyama et al., 2009] and inter- hemispheric coupling between geomagnetic conjugated points [Otsuka et al., 2004; Shiokawa et al., 2005] could also increase the growth rate. In the Northern Hemisphere, Kelley and Makela [2001] suggested an additional polarization electric field in the direction parallel to the wave phase surfaces to explain the SW preference of propagation for MSTIDs, whereas Yokoyama et al. [2009] indicated that the SW preference could be explained by the coupling sporadic E (Es) patches in the E region. Field-aligned irregularity structures (FAIs) are another interesting phenomenon associated with MSTIDs at middle latitudes. They usually have a spatial scale from meters to about 100 km and occur in uplifted ionosphere [e.g., Bowman and Hajkowicz, 1991], turbulent upwelling [e.g., Fukao et al., 1991], or depleted airglow [e.g., Kelley et al., 2003, 2004; Otsuka et al., 2009] regions associated with propagating MSTIDs. FAIs also result in high-frequency spread F traces and radar echoes. From the ionograms, one can see that a series of multiple individual spread F traces caused by FAIs are superimposed on the main traces that result from main-body MSTIDs [Bowman and Hajkowicz, 1991]. In addition, the MU radar echo observations, reported by Fukao et al. [1991], showed that periodic patchy echo structures associated with 3 m scale irregularities moved westward with a velocity larger than 100 m/s in the ionospheric turbulent upwelling regions. Optical all-sky imagers also observed the existence of small-scale FAIs in the airglow-depleted regions of MSTIDs [Kelley et al., 2003, 2004]. Using concurrent observations from two all-sky imagers and the MU radar in Japan, Otsuka et al. [2009] showed that meter-scale FAIs oscillated in the northwestward direction almost parallel to the depleted wavefront of the airglow MSTIDs. FAIs with scales less than 50 km are generally believed to be most likely generated by the gradient drift instability [Kelley and Fukao, 1991; Saito et al., 2001, 2002; Kelley et al., 2004; Otsuka et al., 2009], which is driven by the background neutral winds or polarization electric fields within the MSTIDs. Other studies [Haldoupis et al., 2003; Kelley et al., 2003] proposed that polarization electric fields associated with Es patches could effectively map along magnetic field lines to the airglow-depleted regions in the F layer and cause FAIs with the same scale size (10–15 km) as the Es patches. However, not much attention has been paid to mesoscale FAIs with characteristic scales over 50 km. A study is necessary to show the existence of mesoscale FAIs and their correlations with main-body MSTIDs.

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In addition, more observations, particularly those that can reveal the full evolution process of mesoscale FAIs, are desired to further understand the possible physical mechanisms involved in them. In this paper, we present such observations. We speciﬁcally show the occurrence and development of several mesoscale FAIs associated with a MSTID event observed by an all-sky imager located at Xinglong [40.4°N, 117.6°E in the geographic coordinates, 30.5° magnetic latitude (MLAT), inclination (I) = 57.7°], China, during the night of 17/18 February 2012. In section 2, a brief description of the instruments is given. Section 3 describes the coordinated observations of the MSTID event by multi- instruments. In section 4, a discussion of the results is given. Conclusions of this study are given in section 5.

Figure 1. The locations of the observation sites. An all-sky airglow imager and a FPI instrument were installed at Xinglong (40.4°N, 2. Observations 117.6°E, 30.5°MLAT), China. The red solid circle at the center of the black rectangle denotes the location of Xinglong. The black rectangle As part of the multi-instrumental network box, which covers a geographic area of 110.0°–125.0°E in longitude of the Chinese Meridian project, an all-sky and 32.5°–47.5°N in latitude, denotes the projected range of airglow imager constructed by the State Key images used in this study. The yellow triangle gives the location of the Laboratory of Space Weather was deployed digisonde at Shisanling (40.7°N, 116.6°E, 30.7°MLAT). The black solid at Xinglong, Heibei Province, China. The diamond denotes the location of the GPS receiver at BJNM (40.2°N, fi 116.2°E, 30.2°MLAT). imager has a 630.0 nm optical lter to observe red line (OI 630.0 nm) airglow emission from the nighttime ionosphere. The geographic location of Xinglong is shown in Figure 1 by the red solid circle. Airglow OH emission is also measured by another imager that is similar to this one and has been used to study mesospheric gravity waves [Li et al., 2011, 2013]. The imager uses a Mamiya 24 mm/f4.0 fisheye lens with a 180° field of view. It has an eight-position, temperature stabilized filter wheel. The CCD detector is back illuminated, consisting of 1024 × 1024 pixels with a pixel depth of 16 bits. The physical dimension of the CCD array is 13.3 × 13.3 mm. The camera system is thermoelectrically cooled to 70°C (dark current 0.5 electrons/pixel/s). The red line (OI 630.0 nm) emission is filtered by an interference filter to observe nighttime MSTIDs. The bandwidth of the filter is 2.0 nm, and the center wavelength is 630.0 nm. The integration time is 5 min. The whole observational process is automated. In section 3, we show a MSTID event observed by this all-sky imager during the night of 17/18 February 2012. We also installed a Fabry-Perot interferometer (FPI) at the Xinglong Station in April 2010 to observe neutral winds at both the mesospheric and thermospheric heights (about 80 km and 250 km, respectively). The instrument was developed by the National Center for Atmospheric Research and had a similar design to the one currently being used in Resolute Bay, Canada [Wu et al., 2004]. By monitoring the Doppler shift and the broadening of different airglow emissions, we can obtain neutral winds and temperatures in five directions (zenith, east, west, north, and south) at an elevation angle of 45°. A detailed description of the instrument and the data processing method is given by Wu et al. [2004]. Wind observations from the FPI have been studied by Yuan et al. [2013] and Liu et al. [2014]. These observations were also compared with the HWM 07 model, and the discrepancy between the observation and model is discussed by Yuan et al. [2013]. In this study, we used the average of the north and south line-of-sight wind speeds to obtain the meridional winds. The zonal wind was obtained by averaging the line-of-sight winds from the west and east. The winds from the north and south (west and east) were very similar during the night of 17/18 February 2012.

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A digital portable ionosonde (DPS-4) deployed at Shisanling (40.7°N, 116.6°E, 30.7°MLAT) has also been used to study the MSTID event in this paper. The location of Shisanling is shown in Figure 1 by the yellow triangle. The instrument is managed by the Institute of Geology and Geophysics, Chinese Academy of Sciences. We use the ionosonde to examine spread F traces and the height variation of the ionosphere during the MSTID event. Concurrent ionospheric total electron content (TEC) observations by a GPS receiver from an international GPS observational station of Beijing NanMeng (BJNM, 40.2°N, 116.2°E, 30.2°MLAT) are also used in this paper. The location of BJNM is given in Figure 1. The GPS data were provided by the Peking University and were pro- cessed by the COSMIC Program Ofﬁce. GPS observations with elevation angles >20° are used for the analysis.

3. Results 3.1. Airglow Images From the All-Sky Imager Figures 2(i) and 2(ii) give time sequences (in 15 min interval) of airglow images that show the evolution of the MSTID event locally at the Xinglong station during the night of 17/18 February 2012. The unwarping process described by Garcia et al. [1997] has been applied for all raw images to remove the effects of compression and curving of the all-sky lens. We have mapped the images into a geographic range of about 110.0°–125.0°E in longitude and 32.5°–47.5°N in latitude by assuming that the airglow layer was located at about 250 km. A 2.5 h running mean was applied to obtain the airglow perturbation (%) field. All images are shown in the form of “Up-North, Down-South, Left-West, and Right-East.” The locations of Xinglong and Shisanling are also indicated in the images by red solid circles and yellow solid triangles, respectively. Note that at these stations, the relationship between local time (LT) and universal time (UT) is LT = UT + 8. In Figures 2(i) and 2(ii), from about 20:10–03:25 LT, nine successive regions (marked as “1,”“2,”“3,”“4,”“5,”“6,” “7,”“8,” and “9”) with negative (positive) perturbation intensity (%) illustrate the depleted (enhanced) fronts of the MSTID structures. All of these fronts oriented from the northwest (NW) to the southeast (SE) and continuously propagated southwestward at an angle of about 34° [see also Figure 3(a)] with the south in a northern latitude range of between about 47.5° and 32.5°N. The half wavelengths of fronts “1,”“2,”“3,”“4,” “5,” and “6” were about 300–500 km, whereas those of fronts “7,”“8,” and “9” were about 100 km. The maximum length (aligned from the NW to the SE) of fronts “3” and “4” was more than 1400 km. It is obvious that the maximum (minimum) perturbation intensity of the brightest front “4” (darkest fronts “3” and “5”) between 22:25 and 00:40 LT reached about 25% (25%) of the background intensity. However, the airglow perturbation intensity after 01:10 LT was only ±10% of the background intensity, and thus, we can only see weak perturbation structures. In Figure 3(b), we give additional information about the variation of raw background counts with local time. The background counts were obtained by averaging the zenith region of 10 × 10 pixels in raw images. Four fronts (fronts “1,”“2,”“3,” and “4”) can be identified between 21:00 and 01:00 LT when the MSTIDs passed over the zenith of Xinglong. However, the background counts were almost a constant between 01:00 and 04:00 LT when the perturbations of airglow structures were weak (<10%). This suggests that the background airglow intensity should not have much an effect on our ability to determine airglow perturbation intensity during this time. The intensity of airglow MSTIDs decreased after 01:00 LT. There were mesoscale structures occurring in the depleted front “3” between about 21:25 and 23:40 LT. After about 21:25 LT, they gradually evolved into band-like regions with alternate depleted and enhanced airglow intensities from the earlier weak airglow-depleted front “3” at the top right corner of the image at 21:25 LT. We can clearly see that seven parallel band-like structures, which are labeled as “c,”“d,”“e,”“f,”“g,”“h,” and “i” in Figure 3(a), were generated at 23:10 LT when they approached the zenith of the Xinglong. In Figure 3(c), we give a cross section marked as “i” (blue solid double arrows), which traverses to the band-like structures. The airglow perturbation variation (%) along this section [from NW (600 km) to SE (600 km)] caused by these special structures is given in Figure 3(d). The perturbation intensity and wavelength of these structures were estimated to be about 10% and 150 km, respectively. Thus, those structures were mesoscale and larger than previously reported FAIs in the literature. It is obvious that their fast development mostly occurred when the airglow intensity depletion of the main-body front “3” reached the maximum (about 25%). After 23:40 LT, they became less apparent when they propagated to the SW edge of the field of view (FOV).

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Figure 2. (i) Images (in 15 min cadence) of the MSTIDs from 20:10 to 00:55 LT, during the night of 17/18 February 2012. The observed images were mapped onto geographical coordinates, assuming that the airglow emission layer was at an altitude of about 250 km. The color scale shows the percentage perturbation of airglow intensity to the background of 2.5 h running average. The deep blue dotted lines marked as number “1,”“2,”“3,”“4,” and “5” denote several successive fronts of the MSTIDs. The red solid circles and yellow triangles denote the locations of Xinglong and Shisanling, respectively. (ii) Maps of the airglow perturbation (%) caused by the MSTIDs from 01:10 to 03:25 LT during the same night. Seven fronts (fronts “3,” “4,”“5,”“6,”“7,”“8,” and “9”) are denoted.

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Figure 3. Figure 3(a) shows the geometric relationship between the mesoscale FAIs in front “3” and the main-body MSTIDs at 23:10 LT. Seven band-like structures (labeled as “c,”“d,”“e,”“f,”“g,”“h,” and “i”) are seen within the depleted front “3.” Figure 3(b) shows the variation of background counts with local time by averaging a zenith region of 10 × 10 pixels in raw images. Figure 3(c) shows two cross sections (sections “i” and “ii”). The section “i” is along the wavefront “3” [(from NW (600 km) to SE (600 km)] and with the center location (0 km, red solid circle) at Xinglong. Section “ii” is along the propagating direction of MSTIDs [from NE (700 km) to SW (700 km)] and with the center location (0 km, yellow solid triangle) at Shisanling. Figure 3(d) gives the airglow perturbation (%) of the mesoscale FAIs along the section “i” in Figure 3(c).

There were similar mesoscale structures occurring in the depleted front “5” (e.g., 00:40–00:55 LT). They came into the FOV when the perturbation of airglow was the largest (25% of background counts). Their scales were comparable to those of the mesoscale structures in front “3.” After 00:55 LT, the mesoscale structures in front “5” became less apparent and gradually faded. Note that there were also similar mesoscale structures in front “9,” which could still be seen at later times (e.g., 03:25 LT). Hereafter, we will focus our study on mesoscale structures in front “3,” because mesoscale structures in other MSTID fronts were not as strong as those in front 3. As previously described in section 1, field-aligned irregularity structures (FAIs) can be generated in the propagating MSTID structures [Bowman and Hajkowicz, 1991; Fukao et al., 1991; Kelley et al., 2003, 2004; Otsuka et al., 2009]. The mesoscale structures mentioned above, which were also generated in airglow- depleted fronts of the propagating MSTIDs, should be classified as FAIs. Because their scales are in the range of mesoscale (>50 km), they should be the mesoscale FAIs of airglow. Images after 03:25 LT are not sufficiently clear for analysis. Thus, in this paper, we use images before 03:25 LT to analyze airglow perturbations.

3.2. The Relative Velocity of the Mesoscale FAIs In Figure 4, a two-dimensional normalized cross-correlation (ncc) method was used to determine the velocities of the mesoscale FAIs in front “3” and the main-body MSTIDs. This method was used by Makela and Kelley [2003] and Yao and Makela [2007] to investigate the velocity of plasma bubbles in equatorial regions. In Figures 4(a) and 4(b), the perturbation images (image “1” at 23:15 LT and image “2” at 23:20 LT) were con- verted to binary (0 and 1) images to make the structures that are relevant to our study clearer and sharper.

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Figure 4. Two regions [region “1” (green box) and region “2” (red box)] in Figure 4(a) were selected from image “1” at 23:15 LT for the normalized cross-correlation analysis with similar structures in image “2” at 23:20 LT in Figure 4(b). Figures 4(c) and 4(d) show the correlated results for the main-body MSTIDs in region “2” and the mesoscale FAIs in region “1” with similar structures in image “2,” respectively. The maximum correlated regions were enclosed by the red long-dotted curves.

Two regions [region “1” (green box) and region “2” (red box)] in image “1” were selected for correlation analysis with the similar structures in image “2.” The velocity of region “1” is the velocity of the mesoscale FAIs. The velocity of region “2” is the velocity of the main-body MSTIDs. We ﬁrst shifted regions “1” and “2” of image “1” and correlated them with the similar structures in image “2,” and then found the location of maximum correlation coefﬁcient. In this way, we obtained the offset between the structures enclosed by region “1” (region “2”)in image “1” and similar structures in image “2” in both zonal and meridional directions. The zonal and meridional

velocities (Vz and Vm) can then be obtained by dividing the offset with time. Figures 4(c) and 4(d) show the results of correlation for the main-body MSTIDs and the mesoscale FAIs, respectively. The maximum correlated regions were enclosed by the red long-dotted curves. The calculated velocity of the MSTIDs had a westward component with a speed of about 76.0 m/s and a southward component with a speed of 90.0 m/s, whereas the calculated westward and southward speeds of the mesoscale FAIs were about 106.7 m/s and 40.0 m/s, respectively. Thus, the mesoscale FAIs propagated southwestward with an angle of about 69.3° with the south. The MSTIDs propagated southwestward with an angle of about 37.9° with the south. It is worth pointing out here that the previously estimated angle between the propagation direction of front “3” and the south from the airglow structures was about 34.0°, which is somewhat smaller than the 37.9° angle calculated here by the ncc method. Thus, mesoscale FAIs and main-body MSTIDs were not moving at the same velocity. In Table 1, we also calculated the velocities of the mesoscale FAIs and the MSTIDs during other times and obtained the same results. The maximum correlation coefﬁcients, the zonal velocity, the meridional velocity, M MF M MF M MF and the total velocity of the MSTIDs (mesoscale FAIs) were denoted as Cmax (Cmax), Vz (Vz ), Vm (Vm ), and MFM MFM VMSTIDs (VMFAIs), respectively. dVz (dVm ) denotes the velocity difference between the mesoscale FAIs

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Table 1. Velocities of the Mesoscale FAIs and MSTIDs Calculated Using the ncc Method Time (LT) 23:05–23:10 23:10–23:15 23:15–23:20 23:20–23:25 23:25–23:30 23:30–23:35

M Cmax 0.92 0.92 0.92 0.94 0.94 0.94 MF Cmax 0.84 0.87 0.81 0.81 0.81 0.76 M Vz 56.0 m/s 56.0 m/s 76.0 m/s 70.0 m/s 60.7 m/s 53.0 m/s MF Vz 114.0 m/s 110.0 m/s 106.7 m/s 116.7 m/s 130.0 m/s 106.7 m/s MFM dVz 54.0 m/s 54.0 m/s 30.7 m/s 46.7 m/s 69.3 m/s 53.7 m/s M Vm 136.7 m/s 96.0 m/s 90 m/s 106.7 m/s 122.8 m/s 80.0 m/s MF Vm 30.0 m/s 30.0 m/s 40.0 m/s 13.3 m/s 56.7 m/s 30.0 m/s MFM dVm 106.7 m/s 66.0 m/s 50.0 m/s 80.0 m/s 66.1 m/s 50.0 m/s VMSTIDs 110.1 m/s 112.1 m/s 117.8 m/s 117.4 m/s 140.0 m/s 96.0 m/s VMFAIs 143.9 m/s 114.0 m/s 113.9 m/s 119.7 m/s 141.8 m/s 110.2 m/s dVMFM 119.6 m/s 85.3 m/s 58.7 m/s 92.6 m/s 95.7 m/s 73.4 m/s φMSTIDs w 22.3° 33.3° 37.9° 33.3° 29.4° 33.5° φMFM W 26.9° 39.3° 31.6° 30.3° 46.4° 47.0° ϕMSTIDs S 22.3° 33.3° 37.9° 33.3° 29.4° 33.5° ϕMFAIs S 75.3° 74.8° 69.3° 83.5° 66.4° 74.3°

and the MSTIDs in the zonal (meridional) direction. dVMFM is the total relative velocity of the mesoscale FAIs ϕMSTIDs ϕMFAIs relative to the main-body MSTIDs. s ( s ) denotes the angle between the propagating direction and φMSTIDs φMFM the south for the MSTIDs (mesoscale FAIs). w ( W ) denotes the angle between the wavefront of the MSTIDs (dVMFM) and the west. From Table 1, we can clearly see an obvious velocity difference between the mesoscale FAIs and the main- M body MSTIDs. The zonal velocity of the MSTIDs (Vz ) was obviously smaller than that of the mesoscale FAIs MF MF M MFM (Vz ). Vz was larger than Vz by about 30.0 m/s to 70.0 m/s (see dVz , positive eastward). However, M MF MF the meridional velocity of the MSTIDs (Vm) was obviously larger than that of the mesoscale FAIs (Vz ). Vz M MFM was smaller than Vm by about 50.0 m/s to 110.0 m/s (see dVz , positive northward). Thus, mesoscale FAIs and main-body MSTIDs were not moving at the same velocity. The mesoscale FAIs had a northwestward MFM φMFM – velocity (dV ) relative to the main-body MSTIDs. Having an angle ( W ) of about 26.9° 47.0° with respect to the west, dVMFM directed northwestward and varied from about 58.7 m/s to 119.6 m/s. Note that the φMSTIDs φMFM – – calculated w was close to W , especially during 23:05 23:10 and 23:20 23:25 LT. As described above, φMSTIDs φMFM MFM MFM w ( W ) denotes the angle between the wavefront of the MSTIDs (dV ) and the west, and dV denotes the relative motion of the mesoscale FAIs to the main-body MSTIDs. Therefore, the mesoscale FAIs moved northwestward in a direction that was almost parallel to wavefront “3.” To visualize the relative velocity between the mesoscale FAIs and the main-body MSTIDs, we used the M M calculated velocities of the main-body MSTIDs (Vm and Vz in Table 1) between 23:05 and 23:35 LT to predict the location of the mesoscale structure within the red curve at 23:05 LT in Figure 5. From Figure 5, we can clearly see the relative displacement between the predicted and the true locations of the mesoscale structure. This result clearly indicates that the mesoscale FAIs had an obvious northwestward relative motion. The average relative velocity was about 87.0 m/s.

3.3. GPS TEC Results In Figures 6(i) and 6(ii), we give simultaneous GPS TEC observations during the night of 17/18 February 2012. Three GPS satellites, PRN 30, 6, and 16, were used to trace the MSTID event. We have mapped the orbits of the ionospheric pierce points of GPS signals onto the all-sky images [Figure 6(i)]. In Figure 6(i), we use green, red, and deep blue solid lines to denote the orbits of PRN 30, 16, and 6 satellites, respectively. The green solid diamonds, blue solid circles, and red solid triangles denote the positions of the ionospheric piece points that corresponded to the times of the airglow images in Figure 6(i). In panels (a), (c), and (e) of Figure 6(ii), we give the slant TEC (STEC) of successive 3 days from 16th to 18th February for these three GPS satellites, respectively. Considering that no MSTIDs appeared during the nights of 16 and 18 February, and the orbits of the satellites were almost the same during those three nights, we chose the averaged STEC of 16 and 18 February as the background STEC for 17 February. The STEC disturbance (dSTEC) was calculated by subtracting the background STEC from the STEC of 17 February. The relative STEC disturbance (rdSTEC) was obtained by dividing by the background STEC. The resultant rdSTEC are plotted in panels (b), (d), and (f) of

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Figure 5. The expected locations of the band-like structure in the green box of Figure 4(a) (also enclosed by red curve at 23:05 LT) between 23:05 and 23:35 LT. The location is calculated using the MSTIDs’ velocities (Vz and Vm in Table 1) obtained from the normalized cross-correlation method.

Figure 6(ii) for PRN 30, 6, and 16 satellites, respectively. A negative rdSTEC value indicates that the TEC value is smaller than the background TEC, while a positive rdSTEC value means that the TEC value is higher than the background TEC. It is worth pointing out that in panels (a), (c), and (e) of Figure 6(ii), the STEC on 16 and 18 February had an obvious deviation from the STEC of 17 February. This deviation means that the averaged background STEC of 16 and 18 February was not at the same level as that of 17 February. The resultant rdSTEC shown in the panels (b), (d), and (f) of Figure 6(ii) was only a relative result. In this paper, we are only interested in the relative variation of rdSTEC, which is accurate enough to be used to compare with our airglow observational results. There were two types of rdSTEC structures in Figure 6(ii). The first was a large-scale structure with a quasi- period of about 2.0 h. The large-scale structure corresponded to the main-body MSTIDs. Four fronts (fronts “1,”“2,”“3,” and “4”) of the large-scale TEC structure, which are indicated in panels (b), (d), and (f) of Figure 6(ii), corresponded to the four fronts of the MSTIDs (fronts “1,”“2,”“3,” and “4”), marked by the blue dotted lines in the airglow images [Figure 6(i)]. The perturbation amplitude of these fronts was about 10% of the background STEC. The second was small-scale structures (between 22:45 and 23:40 LT) with a quasi- period of less than 15 min. They were associated with the mesoscale FAIs in front “3” of the MSTIDs shown in Figure 6(i). They appeared in the trough of the large-scale rdSTEC structure [see panels (d) and (f) in Figure 6(ii)]. The perturbation amplitude of them was less than 10% of the background STEC. Similarly, we give the observational results of the other two satellites, PRN 11 and PRN 19, in Figures 7(i) and 7 (ii). The carmine solid circles and blue-green solid triangles in four airglow images denote the orbits of PRN 11 and PRN 19, respectively. Airglow images at times of 01:05, 01:45, 02:25, and 03:05 LT were shown in the figure. Four fronts (fronts “4,”“5,”“6,” and “7”) can be identified. In panels (h) and (j) of Figure 7(ii), we also indicate these four fronts when the orbits of the satellites approximately passed them. A TEC wave with a period of about 2.0 h can be seen. However, the maximum perturbation intensity of the TEC wave was only

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Figure 6. (i) Green solid diamonds, blue solid circles, and red solid triangles denote the locations of the pierce points of three satellites, PRN 30, 6, and 16, at the times of the images. Deep blue dotted lines marked as “1,”“2,”“3,”“4,” and “5” indicate ﬁve successive fronts of the MSTIDs. (ii) Figures 6(a), 6(c), and 6(e) give the slant TEC (STEC) observed by PRN 30, 6, and 16 from 16 to 18 February. The relative STEC disturbance (rdSTEC) changes on 17/18 February are given in Figures 6(b), 6(d), and6(f).

±2.5% of the background STEC. The ±2.5% perturbation intensity was obviously smaller than the ±10% perturbation level detected by the PRN 30, 6, and 16 satellites. A possible reason is that the strength of the MSTIDs quickly decreased after midnight. This result is consistent with that from the airglow images.

3.4. Digisonde Observations In Figure 8, we give the ionograms observed by the DPS-4 digisonde at Shisanling during the night of 17/18 February 2012. The location of Shisanling is also shown in Figures 2(i) and 2(ii) by the yellow solid triangles.

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Figure 7. (i) The orbits of the pierce points of two satellites: PRN 19 (carmine solid lines) and PRN 11 (blue-green solid lines). The carmine solid circles (blue-green triangles) denote the orbits of the pierce points of the PRN 19 (PRN 11) satellite at 01:05, 01:45, 02:25, and 03:05 LT. (ii) The STEC (rdSTEC) changes for PRN 19 and PRN 11 satellite.

From these ionograms, we can see that spread F occurred during the entire night. At the earlier night (before 20:45 LT), strong Es occurred. Between 22:00 and 23:00 LT, satellite traces (ST) can be clearly identiﬁed. Tsunoda [2008] and Narayanan et al. [2012] suggested that ST can be usually regarded as a proxy for large- scale wave-like irregularity structures, which should be associated with the large-scale MSTIDs in our case. More studies on ST can be found from previous publications [e.g., Tsunoda, 2008; Fagundes et al., 2012; Narayanan et al., 2012; Lynn et al., 2013]. Mixed spread F (MSF) traces (simultaneous range and frequency spreading) [Candido et al., 2011] appeared between 23:00 and 00:45 LT when the depleted front “3” gradually passed over the digisonde. Between 00:45 and 06:30 LT, frequency spread F (FSF) still occurred. This means that irregular structures associated with the MSTIDs were continuously detected by the digisonde. However, the intensity of the spread F traces was obviously weaker than that of the MSF traces between 23:00 and 00:45 LT. Bowman [1995] pointed out that spread F traces depend on the magnitude of wave-like perturbations. This indicates that the ionospheric altitude variations caused by MSTID phase fronts after 00:45 LT were weaker. Figure 9(a) illustrates the NE-SW cross sections (keogram) of the airglow images shown in Figures 2(i) and 2(ii) during the same night. A NE-SW cross section (indicated by the blue solid double arrows marked as “ii”) normal to the wavefront “3” of the MSTIDs and traverse to the zenith of the Shisanling station was cut from the image (at 23:10 LT) in Figure 3(c). The keogram was obtained by stacking all NE-SW cross sections at successive times during the entire night (19:00–06:00 LT). The black solid line in Figure 9(a) denotes the zenith location of Shisanling. Between 20:30 and 03:00 LT, ﬁve fronts (fronts “1,”“2,”“3,”“4,” and “5” marked by vertical dotted lines) passed over Shisanling. In Figure 9(b), we give the minimum bottomside virtual height (h′f)of

the F2 layer (red line) on 17/18 February, along with the monthly averaged h′f (blue line) in February. A wave-like structure with a quasi-period of about 2.0 h from 21:00 to 01:00 LT on 17/18 February is evident.

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Figure 8. (a) The ionograms observed every 15 min by the digisonde at Shisanling from 20:00 to 01:45 LT, during the night of 17/18 February 2012. (b) The ionograms observed every 30 min from 02:00 to 06:30 LT.

Comparing h′f from 17/18 February with the monthly average, we can see that the wave-like structure during this night was caused by the passing MSTIDs. The bottomside ionospheric h′f was elevated (decreased) when depleted fronts “1” and “3” (bright fronts “2” and “4”) passed over Shisanling. The increase (decrease) in h′f associated with fronts “1” and “2” was about 20–25 km. Because of the mixed spread F at 23:00–00:45 LT, we cannot make an accurate estimate of the change of minimum h′f caused by front “3” and the mesoscale FAIs within the front. Nonetheless, mesoscale FAIs occurred in an altitude range of 320 to 475 km for h′f [see Figure 8 (a)]. The decrease in h′f caused by the bright front “4” was only about 10 km. The main reason for this is probably that its intensity was low when it passed over Shisanling. After 01:30 LT, the ionospheric h′f was almost a constant (about 300 km), when very weak airglow perturbation (<10%) passed over the digisonde [see also Figure 2(ii)].

3.5. Neutral Winds Observed by the FPI In Figures 9(c) and 9(d), we show the monthly averaged neural winds (blue lines) and the winds (red lines) during the night of 17/18 February at the Xinglong station. Those winds are from FPI measurements. The monthly averaged winds and the winds during that particular night had a very similar tendency. On 17/18

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Figure 9. (a) NE-SW sections (keogram) of the images shown in Figures 2(i) and 2(ii) during the night of 17/18 February 2012. (b) Monthly averaged minimum bottomside virtual height (h′f) of the F2 layer (blue line) in February and the h′f in the night of 17/18 February (red line). Successive ﬁve fronts (fronts “1,”“2,”“3,”“4,” and “5”) of the MSTIDs are also marked. Es, ST, MSF, and FSF stand for “sporadic E (Es),”“satellite traces,”“mixed spread F,” and “frequency spread F,” respectively. (c) Monthly averaged meridional winds (blue line) in February and the meridional wind on 17/18 February (red line). (d) Monthly averaged zonal winds (blue line) in February and the zonal wind on 17/18 February (red line).

February, the southward wind increased to about 87 m/s until 01:21 LT. The zonal wind blew eastward before 04:00 LT and kept an averaged value of about 93 m/s between 21:00 and 00:00 LT. The winds thus rotated clockwise. A parameter β in Table 2 gives the angle of neutral winds with respect to the geomagnetic east direction. This rotation of the neutral winds is a typical characteristic of thermospheric winds, which is driven by the pressure gradient force due to solar heating [Hedin et al., 1988].

4. Discussion In section 3, coordinated observations by an all-sky imager, a GPS monitor, and a digisonde have been used to investigate the evolution of a MSTID event and its associated mesoscale FAIs of airglow during the night of 17/18 February 2012. All of these measurements showed a wave-like perturbation structure caused by the MSTIDs with a quasi-period of about 2.0 h. There existed a positive correlation between the airglow intensity and the relative slant TEC disturbance (rdSTEC). The bright (depleted) fronts of the MSTIDs with positive

Table 2. Parameters Associated With the MSTIDs Observed at Xinglong on 17/18 February 2012 Time (LT) 21:21 22:21 23:21 00:21 01:21 02:21 03:21 04:21

Southward wind Us (m/s) 20 33 51 58 87 68 72 68 Eastward wind Ue (m/s) 97 99 99 82 83 56 46 19 Southeastward U (m/s) 99 104 111 100 120 88 85 71 β 18.3° 25.0° 33.9° 41.9° 52.9° 57.1° 64.0° 81.0° θ 71.7° 65.0° 66.1° 48.1° 37.1° 32.9° 26.0° 9.0° αMSTIDs 49.4° 49.4° 49.4° 49.4° 49.4° 49.4° 49.4° 49.4°

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(negative) perturbation intensities usually corresponded to increasing (decreasing) rdSTEC compared with the background values. This result is consistent with the previous study by Garcia et al. [2000b], who also showed a good spatial correlation between GPS TEC and 630 nm airglow band structures. The ionospheric height was also highly modulated by the passing MSTIDs. The bright airglow fronts were associated with a decrease in h′f, whereas the airglow-depleted fronts corresponded to an increase of the ionospheric height. A striking feature of the MSTID event is that it had a long lifetime of over 8 h (from about 20:00 to 06:00 LT). The MSTIDs propagated southwestward from over 47.5°N to lower than 32.5°N. More than nine fronts (six fronts with horizontal widths of 300–500 km) came into the FOV of the all-sky imager. Such large-scale MSTIDs are likely to come from high latitude regions. The SuperDARN HF radar in Hokkaido (43.5°N, 143.6°N, 36.5°MLAT), Japan, also showed a similar long lifetime, long distance propagation of nighttime MSTIDs from high latitudes (over 55.0°N) to the north of Hokkaido at middle latitudes [Ogawa et al.,2009].Recently,Shiokawa et al. [2013] showed that MSTID events at high latitude stations of Tromsø (69.6°N, 19.2°E, 67.1°MLAT) and Athabasca (54.7° N, 246.7°E, 64.5°MLAT) were responding to auroral electric fields. Some MSTIDs existed as plasma structures affected by auroral electric fields. However, the MSTIDs reported in this paper occurred during a geomagnetically very quiet night (Kp < 1 , ΣKp = 2+). Thus, auroral activity was unlikely the source of the MSTIDs. Another striking feature of this MSTID event is the occurrence of several mesoscale, periodic FAIs in some of the airglow-depleted fronts. These mesoscale structures were seen in depleted fronts “3” and “5,” and even front “9.” A common characteristic of these mesoscale structures in fronts “3” and “5” is that they appeared when the airglow perturbation was large (about 25% of the background count). In particular, the mesoscale FAI structures in front “3” became very strong and lasted for a long period of time. Thus, their perturbation signatures were evident in airglow images. We can clearly see that band-like structures were gradually generated from the initial weak depleted front “3,” then became more evident as front “3” increased its intensity (more depleted), and finally dissipated. This allows us, for the first time, to observe optically the full evolution of mesoscale FAIs of airglow associated with intense MSTIDs. Mesoscale FAIs of airglow with a characteristic scale length of ~150 km have not been reported before. Very strong MSF traces occurred when the depleted front “3” passed over the digisonde. Those MSF traces prevent us from accurately extracting the virtual height of the bottomside ionosphere (h′f) modulated by the mesoscale FAIs in front “3.” It is difficult to directly relate those special traces (between 23:00 and 00:45 LT) to the mesoscale FAIs. However, TIDs with shallow wave amplitudes of only a few kilometers in the height

of the F2 region had been detected by Bowman [1995]. Fukao et al. [1991] also found that the bottomside ionosphere can be strongly modulated by small-scale FAIs. In addition, in our observations, associated with the observed mesoscale FAIs, GPS TEC had oscillations with higher frequencies. Kelley et al. [2003] pointed out that intermediate-scale (1–50 km) ionospheric structures (a much smaller spatial scale than the mesoscale FAIs shown in this paper) and, particularly, the height variations of constant density contours can lead to MSF. Kelley et al. [2003] suggested that these oscillations are most likely due to plasma processes, since neutral atmospheric disturbances are heavily damped at the thermospheric height. Therefore, it is possible that those MSF traces have a close association with the mesoscale FAIs in front “3.” Signal reflection by the small-scale TEC structures or ionospheric oscillations associated with the mesoscale FAIs could be the main reason for the observed MSF. There is certainly a strong correlation between the mesoscale FAIs and the main-body MSTIDs. As described above, the growth of those mesoscale FAIs was largely controlled by the intensity of the MSTIDs. Similarly, Saito et al. [2001] found that the coherent echoes from small-scale FAIs were usually detected by the MU radar during the nights when TID activity was high. Saito et al. [2002] further showed that band-like structures of 3 m scale irregularities preferred to occur when TIDs were intense in amplitude and the ionosphere was uplifted. In addition, our analysis indicates that mesoscale FAIs had an obviously northwestward relative velocity with respect to the main-body MSTIDs, even when they occurred within the depleted front of the observed MSTIDs. This relative motion is very similar to the previously reported northwestward oscillation of small-scale FAIs within the depleted airglow regions by Otsuka et al. [2007, 2009], who pointed out that small-scale FAIs can be generated in an airglow-depleted region of the MSTIDs and oscillate northwestward in the airglow-depleted region because of the northeastward polarization electric field. Their results suggest that the northwestward relative motion of the mesoscale FAIs we observed may also be associated with a northeastward polarization electric field in the depleted airglow front “3.”

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According to the International Geomagnetic Reference Field model, the strength of the downward component of the geomagnetic ﬁeld at 300 km for Xinglong is about 4.0 × 104 nT. The averaged (maximum) velo-

city perturbation δvd was about 87 m/s (119.6 m/s) (see dVMFM in Table 1) in front “3,” which corresponded to about 25% in airglow intensity perturbation. The velocity perturbation in this event was comparable to the MU radar observations of about 82 m/s reported by Otsuka et al. [2009], which corresponded to a 30% airglow per-

turbation. If δvd was caused by a northeastward polarization electric fields δEd,wecan Figure 10. A sketch of the generation of the polarization electric field approximately estimate an averaged (maxi- “ ”“ ” in the depleted airglow region. Three airglow fronts (fronts 2, 3, mum) δEd of about 3.5 mV/m (4.8 mV/m) in “ ” “ ” and 4 ) are shown. The bright (dark) stripes in front 3 denote the front “3.” These values of the electric fields band-like regions associated with the mesoscale FAIs. The winds (U) are larger than satellite observations of and the field-aligned current J [J = Σp (U × B)] caused by U are also 1.2 mV/m given by Shiokawa et al. [2003b] drawn. δEd denotes the polarization electric field generated in the depleted front “3.” The perturbation wave vector kd is selected to be but comparable to the radar data of along δEd. δvd denotes the plasma drift velocity resulted from δEd by 3.0 mV/m at Arecibo [Miller et al., 1997]. the E × B drift. The MSF associated with front “3” made it difficult for us to estimate the ionospheric

uplift in the depleted airglow region. However, we can use the eastward component of the estimated δEd

to calculate the upward plasma velocity and ionospheric uplift caused by δEd through the E × B drift. Here “ ” φMSTIDs we assumed that front 3 had an angle ( w ) of 34.0° with the west [see Figure 3(a)]. The northeastward δEd normal to front “3” would have an angle of about 56.0° with the east. The average of the eastward component of δEd in our observations was estimated to be about 2.0 mV/m (δEd × cos56.0°). The magnitude of the northward component of the geomagnetic ﬁled at the height of 300 km for Xinglong is about 2.4 × 104 nT. The averaged value of the upward E × B plasma drift velocity is thus estimated to be about 80.0 m/s and thereby could produce a vertical plasma uplift of about 48 km in just 10 min. These estimates are in agree- ment with the measurements of Behnke [1979], who reported ionospheric uplifts of 30 to 100 km. They are also comparable with the estimated uplift of about 60 km in 10 min for an electric ﬁeld of typical 5.0 mV/m by Haldoupis et al. [2003] in their midlatitude spread F study. Thus, we can reasonably assume that the observed mesoscale FAIs in our case were associated with a large ionospheric uplift most likely caused by

a northeastward δEd by the E × B drift. In the following analysis, the Perkins instability will be considered as a possible mechanism for driving the main-body MSTIDs, because it can effectively amplify the initial polarization electric ﬁelds [Perkins, 1973; Kelley and Fukao, 1991; Shiokawa et al., 2003b] in the depleted regions of airglow MSTIDs. At the same time, we calculate the growth rate of the instability. Here we do not consider the effect of the background

electric ﬁeld (E0), which is expected to be small and antiparallel to U × B [Rishbeth, 1972; Otsuka et al., 2004]. Figure 10 gives a schematic picture to help further analyze the possibility of the Perkins instability in the main-body MSTIDs. This schematic picture is similar to Figure 10 in Otsuka et al. [2009]. Three fronts (fronts “2,”“3,” and “4”) are indicated. In the depleted front “3,” several band-like structures—represented by shading and bright stripes—denote the mesoscale FAIs we observed. The shaded (bright) stripes have depleted

(enhanced) airglow intensity. The polarization electric field δEd within front “3” is also illustrated. δvd denotes the velocity perturbation resulting from δEd by the E × B drift. U is the southeastward neutral wind measured by the FPI at Xinglong. The field-line integrated ionospheric current J [J = Σp (U × B)], which is caused by U,is also shown. The wave vector kd of perturbation electric field δEd in the depleted wavefront “3” of the main- body MSTIDs is also indicated. This selection of kd follows the previous works [e.g., Kelley and Makela, 2001;

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Shiokawa et al., 2003b; Otsuka et al., 2009], in which wave vector k was taken as the vector pointing in the

direction of δE within the depleted airglow regions. In this analysis, kd thus points to the northeast. In Table 2, we give several parameters at seven successive times. These parameters include the following:

FPI measured southward and eastward winds (Us and Ue) and the total southeastward wind U in the geographic coordinates; the angle β of U with respect to the magnetic east; the angle θ of U × B with respect to

the magnetic east; and the angles αMSTIDs between the wave vectors kd and the magnetic east. From β values, U rotated clockwise in the southeastward direction (from about 18.0° to 81.0° with respect to the magnetic east) between 21:21 and 04:21 LT. J was thus rotated clockwise (see θ) in the northeastward direc-

tion (from about 72.0° to 9.0° with the geomagnetic east). The angle αMSTIDs derived from airglow images was about 49.4° with respect to the magnetic east. As described in section 1, the Perkins instability could

be triggered only when θ > αMSTIDs, which means that the perturbation wave vector kd has to lie within the angle between J and the magnetic east. Thus, Table 2 shows that the Perkins instability could occur

(θ > αMSTIDs) before 00:21 LT and might not work after 00:21 LT (θ < αMSTIDs) for the main-body MSTIDs. This seems to be able to explain the fast development of the main-body MSTIDs before 00:21 LT and their dissipation after that time. The magnitude of the growth rate of the Perkins instability has been calculated using the method described by Shiokawa et al. [2003b]. We ﬁrst obtained the average of U (listed in Table 2) between 22:21 and 00:21 LT when both the mesoscale FAIs and the main-body front “3” quickly developed near the zenith of Xinglong.

The averaged U had a southward component (Us) of about 47 m/s and an eastward component (Ue) of about 93 m/s. Thus, the southeastward U would have an angle of about 27.6° with the geographical east. Because the declination is about 6.6° for Xinglong, the southeastward U had an angle (β) of about 34.2° with the geomagnetic east. The current J caused by the averaged U would have an angle (θ) of about 55.8° with

the geomagnetic east. In equation (1) of the Perkins instability growth rate, <νin> can be expressed as

< νin > ¼ ∫Neνindz=∫Nedz (2)

where Ne denotes the electron density of the F region and νin is the ion-neutral collision frequency and given by Rishbeth and Garriot [1969] 15 1=2 νin ¼ nKin Kin ¼ 2:610 M (3)

where n, Kin, and M is the neutral density, the rate of momentum transfer per unit volume, and the atomic number of oxygen (M = 16), respectively. Because we have no actual measurements of Ne and n to calculate <νin>, Ne and n are calculated using NRLMSISE-00 and IRI-2012, respectively. <νin> is integrated from the 1 height of 200 to 1000 km [Shiokawa et al., 2003b]. Substitute the calculated <νin> (<νin> = 662.7 s ), θ 2 (θ = 55.8°), α (αMSTIDs = 49.4°), g (g = 9.8 m/s ), and I (I = 57.7° at Xinglong) into equation (1), the growth rate is estimated to be about 4.0 × 10 8 s 1 for the main-body MSTIDs. The magnitude of the instability growth rate calculated here is too small, compared to that of about 10 4 s 1 calculated by Garcia et al. [2000a]. The very small growth rate, however, is consistent with the previous results of Shiokawa et al. [2003b, 2013] in Japan. This result suggests that other processes could be also involved in the development of the MSTID event. Tsunoda and Cosgrove [2001] and Cosgrove et al. [2004] suggested that the F layer and the sporadic E layer (Es) in the nighttime midlatitude ionosphere must be considered electrodynamically as a coupled system with the presence of the Hall polarization electric process in the Es layer [Haldoupis et al., 1996]. Otsuka et al. [2007] and Ogawa et al. [2009] also discussed the concurrent occurrence of Es and MSTIDs in Japan. Polarization electric fields associated with Es patches could map along magnetic field lines and result in the generation of MSTIDs in the F region. Simulation, performed by Yokoyama et al. [2009], showed that the E layer/F layer coupling mechanism can considerably increase the growth rate of the Perkins instability and successfully predict the southwestward propagating NW-SE aligned MSTID structure. In our case, strong Es occurred at earlier times during the night (Figure 8, before 20:45 LT) when the MSTIDs approached Shisanling. Es might have also occurred at further northern latitudes where the F region magnetic field lines cross the E region. Therefore, it is possible that E layer/F layer coupling occurred in this case. The polarization electric fields linked with the Es layer may have generated the main-body MSTIDs, in which

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the northeastward polarization electric fields would move the observed mesoscale FAIs northwestward by the E × B drift. It is also possible that polarization electric fields associated with the MSTIDs in the conjugated hemisphere seed the observed MSTIDs. Conjugated MSTIDs were previously reported by Otsuka et al. [2004] and Shiokawa et al. [2005] in the longitudinal sectors of Japan and Australia. Because our observation site is very close to Japan, the MSTIDs observed in China and Japan are very likely to have similar characteristics. Polarization electric fields associated with the MSTIDs in the Southern Hemisphere may have mapped along the magnetic field lines to the Northern Hemisphere and triggered the observed MSTIDs. More conjugate observations are desired to test this hypothesis. It is unclear whether the observed MSTID were seeded by gravity waves in higher latitude regions (over 47.5°N). The two mechanisms [Cosgrove et al., 2004; Otsuka et al., 2004] mentioned above can reasonably explain the observed MSTIDs as the result of an electrical coupling of polarization electric fields associated with the Es or the Southern Hemispheric conjugated MSTIDs. In these two processes, the observed MSTIDs can be generated under the condition of no seeding gravity waves. On the other hand, Kelley [2011] recently stressed that ubiquity of high latitude gravity waves, which are the result of the Joule heating effect even at low Kp (ΣKp = 2+ in our case) or in situ generation [Vadas et al., 2003], can be the sources of midlatitude MSTIDs. These high latitude waves can travel thousands of kilometers to midlatitudes. They are damped at midlatitudes except for those traveling in the direction that satisfies the condition for the Perkins instability. Once at midlatitude, these gravity waves can be further amplified by the E layer/F layer coupling effect and become strong electrified MSTIDs. It has been suggested that meter-scale FAIs in the F region are generated by the gradient drift instability [Fukao et al., 1991; Kelley and Fukao, 1991]. As discussed by Saito et al. [2002] and Otsuka et al. [2009], the effective electric field caused by the neutral wind has an eastward component, which can result in the gradient drift instability at the bottomside of the F layer, where an upward plasma density gradient can form.

In our case, the eastward component of Ee caused by the background neutral wind U (during 22:21–00:21 LT) was about 2.0 mV/m. In addition, Kelley and Fukao [1991] and Otsuka et al. [2009] pointed out that polarization electric ﬁelds within the MSTIDs can also drive the gradient drift instability. The eastward component

of the net electric field (Ee + δEd) in our case could be up to about 4.0 mV/m, which may generate the observed mesoscale FAIs by the gradient drift instability when an upward plasma density gradient can exist [Saito et al., 2002]. However, we still cannot exclude the possibility that the mesoscale FAIs are just the result of a directly upward coupling of small-scale gravity waves in the lower thermosphere [Kelley and Fukao, 1991] or polarization electric fields associated with the Es patches in the E layer [Kelley et al., 2003]. They can also be generated by other physical processes, such as the breaking of atmospheric gravity waves proposed by Bowman and Hajkowicz [1991]. More observations and analyses are needed to fully understand the physical mechanisms involved in the development and dissipation of mesoscale FAIs. We find the event presented here compares well with the documented “intense mid-latitude spread-F” event studied by Swartz et al. [2000]. Simultaneous coherent, incoherent scatter radars, and Cornell All- Sky imager at the Arecibo Observatory, showed kilometer-scale filaments generated in large ionospheric uplift (or airglow depletion) of alternating bright and dark NW-SE aligned MSTID airglow bands having scales over 100 km in their case. They believed that the general gradient drift instability possibly caused the formation of the filaments in their analysis. However, they did not know what caused those NW-SE aligned main-body band-like structures. We surprisingly find that the event they showed also occurred

on the night of 17/18 February but in 1998. The solar activity in their case (F10.7 =100.2)isalmostthesame as that in our case (F10.7 = 100.8). However, their event occurred during a very strong geomagnetic activity period (Kp = 7 ), whereas the event reported in this paper occurred on a geomagnetically very quiet night (Kp ≤ 1, ΣKp = 2+).

5. Conclusions In this paper, we show the morphological characteristics of a MSTID event and, more importantly, the occurrence of mesoscale FAIs within the MSTIDs that were simultaneously observed by an all-sky imager, a GPS

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monitor, and a digisonde near Xinglong, China, during the night of 17/18 February 2012. The main results are as follows: 1. The observed MSTIDs had a very long lifetime (over 8 h) and continuously propagated southwestward from higher latitude regions to Xinglong. More than nine wavefronts (six fronts with horizontal widths of 300–500 km) came into the FOV of the all-sky imager during the night. 2. A full evolution, including generation, amplification, and dissipation, of mesoscale (~150 km) FAIs of airglow associated with the MSTIDs was directly detected by the all-sky imager. This mesoscale FAIs and MSTID event occurred during a geomagnetically very quiet night with the values of the Kp index close to 0 (ΣKp = 2+). There have not been previous studies examining such mesoscale FAIs based on all-sky images. 3. A normalized cross-correlation method has been used to study the velocities of both the mesoscale FAIs of airglow and main-body MSTIDs. The results show that the mesoscale FAIs had an obvious northwestward relative velocity (about 87 m/s on average) with respect to the main-body MSTIDs. The direction of this relative velocity was almost parallel to the depleted front of the main-body MSTIDs where they occurred. 4. Airglow observations indicate that the evolution of the mesoscale FAIs was strongly controlled by the intensity of the main-body MSTIDs. The mesoscale FAIs were also observed to be associated with TEC depletion. They caused small-scale TEC oscillations with higher frequencies in the TEC-depleted region. They were quickly generated in a highly elevated ionosphere and associated with the maximum airglow perturbation of ~25%. The neutral winds were southeastward with an averaged eastward (southward) speed of about 93 m/s (47 m/s) when the mesoscale FAIs were observed. The main characteristics of the mesoscale FAIs are similar to those of small-scale FAIs detected by radio remote sensing methods. A northeastward polarization electric field within a depleted airglow front can play a very important role in the generation and development of the observed mesoscale FAIs. The electric fields can significantly elevate the ionosphere and move the mesoscale FAIs northwestward by the E × B drift. However, the physical processes driving the polarization electric fields and the mesoscale FAIs are still not clear. More observations and theoretical studies are needed to fully understand the physical processes of the mesoscale FAIs in MSTIDs reported in this paper.

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