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Geometry of the observation. Three thick lines show the center of the radar beam with crosses marked at every 100 km range.
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Mid-latitude F-region field-aligned irregularities (FAIs) were studied by using the middle-and-upper atmosphere (MU) radar ultra-multi-channel system with the radar imaging technique. On 12 June 2006, F-region FAI echoes with a period of about one hour were observed intermittently. These echoes were found to be embedded in medium-scale traveling io...
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Context 1
... of the MU radar observations of the F-region are summarized in Table 1. Figure 2 shows the ge- ometry of the experiment. The beams used for the observa- tions were named beams 1, 2, and 3 from west to east, respec- tively. ...
Context 2
... ones, the contribution of electrons to the slant TECs is assumed to be entirely from a virtual thin layer at an altitude of 300 km. This is the same method as that developed by Saito et al. (1998). Figure 8 shows the temporal variations of TEC per- turbation along the line passing through the region observed by the MU radar which is drawn in Fig. 2 as an oblique dashed line. Oblique stripes from top left to bottom right indicate the occurrence of MSTIDs propagating southwest- ward. The amplitude was about ±1 TECU (10 16 m −2 ). The echoes with positive (negative) range rates were observed in the negative (positive) phase of MSTIDs. Figure 9 shows the relationship between FAI ...
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Citations
... Whereas, the mid-latitude sector was earlier considered to be a comparatively quiet region. Over the last few decades different ionospheric irregularities were detected over the midlatitude region thus leading to several studies examining the dynamic nature of the region (Behnke, 1979;Garcia et al., 2000;Hunsucker, 1982;Hocke and Schlegel, 1996;Kelley et al., 2000Kelley et al., ,2003Kotake et al., 2007;Otsuka et al., 2004Otsuka et al., , 2008Perkins, 1973;Saito et al., 2008;Shiokawa et al., 2003). In addition, observational studies by Afraimovich et al. (2011) and have shown that mid-latitude plasma irregularities can also cause significant GPS positioning error. ...
... This is mainly because the MSTIDs propagation direction changes from daytime in the south or southeast direction to the nighttime in southwest direction during winter. Some studies suggest that the nighttime MSTIDs are caused by the electrodynamical processes (i.e., Perkins instability associated with E-and F-region coupling processes) (Cosgrove et al., 2004;Makela & Otsuka, 2012;Otsuka et al., 2007Otsuka et al., , 2009Saito et al., 2008), and the daytime MSTIDs are generated by the atmospheric GWs (MacDougall et al., 2009). Here, we demonstrate that both nighttime and daytime MSTAD/TIDs follow similar statistical day-to-day trends over a season, implying it is likely that there is a shared source for daytime and nighttime. ...
This paper investigates the lower‐to‐upper atmosphere coupling at high latitudes (>60°N) during the northern winter months of 2012–2013 years, which includes a period of major Sudden “Stratospheric” Warming (SSW). We perform statistical analysis of thermosphere wind disturbances with periods of 30–70 min, known as the medium scale traveling atmospheric disturbances (MSTADs) in atomic oxygen green line (557.7 nm) near ∼120 km and red line (630.0 nm) emissions near ∼250 km observed from Scanning Doppler Imagers (SDIs) over Alaska. The SDI MSTADs observations (60°–75°N) are interpreted in conjunction with the previous daytime medium‐scale traveling ionospheric disturbance (MSTID) observations by SuperDARN midlatitudes (35°–65°N) radars in the F‐region ionosphere and western hemisphere, which confirm findings from the SDI instruments. Increases in MSTAD activity from SDIs show correlations with the increasing meridional planetary wave (PW) amplitudes in the stratosphere derived from MERRA2 winds. Furthermore, a detailed study of the lower atmospheric conditions from MERRA2 winds indicates that the lower atmospheric sources of MSTADs are likely due to the stratospheric generated Gravity Waves (GWs) and not orographic GWs. Favorable stratospheric propagation conditions and polar vortex disturbances resulting from the increased PW activity in the stratospheric region both appear to contribute to increased MSTAD activity in the thermosphere. Additionally, the results show that the MSTID activity from SuperDARN HF radars at mid latitudes during the January 2013 SSW is lower than the MSTAD activity in SDI winds at high latitudes.
... Specifically, the wavefront directions of nighttime MSTIDs are known to conform to backward-C shapes virtually straddling the dip equator (e.g., Otsuka et al., 2004). The causes of nighttime MSTID are dominantly attributed to electrodynamic mechanisms such as Perkins instability and its synergy with sporadic-E (Es) layers Park et al., 2010;Saito et al., 2008;Tsunoda, 2006), and day-time MSTID is related to atmospheric gravitational waves (AGWs; MacDougall et al., 2009;Jonah et al., 2017). ...
Nighttime medium-scale traveling ionospheric disturbances (MSTIDs) have been generally observed by ground-based instruments. However, they provide 2-dimensional images over only a limited field of view and are not distributed globally. The ground-based observations reported that MSTID wavefronts exhibit backward-C shapes virtually straddling the dip equator. In situ plasma density measurements onboard individual satellites could overcome the limited coverage of ground-based MSTID observations. But, most of those spacecrafts could obtain only 1-dimensional profiles of plasma density, which leaves uncertain whether the observed perturbations generally have the characteristic directivity of MSTIDs. This paper addresses this knowledge gap by statistically investigating nighttime perturbations in the mid-latitude topside ionosphere observed by tandem satellites, Swarm A and C. We cross-correlate the plasma density profiles observed by Swarm A and C. The correlation coefficient tends to increase as the two spacecraft move closer, allowing us to derive the disturbances' directivity whenever the Swarm A and C observations are correlated significantly. The directivity statistics agree well with the backward-C shape. Furthermore, the wavefront directions have clear dependence on magnetic latitudes while they are not as well aligned with local time, which is also consistent with previous reports on nighttime MSTIDs using ground-based observations and computer simulations. Additionally, we demonstrate that the nighttime MSTIDs can increase the topside Rate Of Total electron content Index above Swarm. All the above-mentioned results support that the nighttime mid-latitude perturbations observed by Swarm can be identified as MSTIDs on the whole, which is the most important finding of this paper.
... However, not all undulations are accompanied by FAIs, probably due to low amplitudes or out of view of the radar beam. Saito et al. (2002Saito et al. ( , 2008 and Otsuka et al. (2009) and references therein have also shown a summer night-time common occurrence of MSTIDs and FAIs over Japan, and the occurrence of FAIs is related to the intensity of MSTIDs. Same as Fig. 8 but for 11 June 2007 (DOY 162), Fig. 11 shows the time variation of E-and F-region TECPs in H eq at night-time, along the line from northeast to southwest indicated as the red dashed line in Fig. 1. ...
Observations and theoretical analysis on the night-time mid-latitude ionospheric irregularities support the postulation of frequently coupled E and F regions. In this paper, we attempt at asserting this notion while using total electron content (TEC) measurements. The TECs are from a dense GNSS receiver network over Japan with more than 1200 stations and a mean distance of ~ 25 km between receivers; thus, ideal for analyzing small-scale perturbations in ionospheric electron density. We take an ansatz that mid-latitude night-time plasma instabilities concentrate at E and F layers. Then the integrated three-dimensional density perturbations are parameterized with a double-thin-shell model. At each shell, perturbation components are assumed identical at any point within a given grid block. Two days with events of night-time medium-scale traveling ionospheric disturbances (MSTIDs), but with different amplitudes, were investigated. Results show that the newly developed technique can infer several horizontal characteristics on E–F coupled instabilities; the coexistence of northwest–southeast (NW–SE) aligned irregular structures in E and F regions is evident. Both E- and F-region irregularities share similar propagation parameters, a shred of clear evidence of strong coupling.
Graphical Abstract
... At present, the interferometry technique has been unanimously confirmed by MU radar, St. Croix radar, Gadanki VHF radar, Chung-li VHF radar, and Sanya VHF radar (Chu et al., 2013;Hysell et al., 2002Hysell et al., , 2004G. Li et al., 2014;Patra et al., 2014;Saito et al., 2008;Yamamoto et al., 1994). ...
In this work, the daytime E‐region field‐aligned irregularities (FAIs) observed by the Qujing (geographic 25.64°N, 103.70°E) Very High Frequency coherent backscatter radar was reported. The range spread and enhanced sporadic‐E layer (Es) during the same period of the unusual radar echoes were separately observed by two ionosondes located at the Huize and Qujing. The Huize (geographic 26.51°N, 103.61°E) ionosonde was roughly located underneath the radar scattering volume which measured enhanced and range spread Es layer during this period of the unusual radar echoes, indicating that the daytime E‐region FAIs were closely related to the localized density gradient that embedded within the coordinated Es layer. Fengyun‐4A (FY‐4A) satellite data show that a strong convective activity accompanied by gravity waves occurred near the radar scattering volume region. As a result, we proposed that the gravity waves driven by the convective activity in the lower atmosphere can modulate the Es layers and then generate the daytime FAIs through the polarization process.
... The nighttime MSTID is caused by the electrodynamical processes (i.e. Perkins instability associated with E-and F-region coupling processes) (Cosgrove et al. 2004;Makela and Otsuka 2012;Otsuka et al. 2007Otsuka et al. , 2009Saito et al. 2008), and the daytime MSTID is generated by the atmospheric GWs (MacDougall et al. 2009). In this study, we are focusing on the relationship between the daytime MSTIDs and GWs. ...
The total electron content (TEC) data derived from the GAIA (Ground-to-topside model of Atmosphere Ionosphere for Aeronomy) is used to study the seasonal and longitudinal variation of occurrence of medium-scale traveling ionospheric disturbances (MSTIDs) during daytime (09:00–15:00 LT) for the year 2011 at eight locations in northern and southern hemispheres, and the results are compared with ground-based Global Positioning System (GPS)-TEC. To derive TEC variations caused by MSTIDs from the GAIA (GPS) data, we obtained detrended TEC by subtracting 2-h (1-h) running average from the TEC, and calculated standard deviation of the detrended TEC in 2 h (1 h). MSTID activity was defined as a ratio of the standard deviation to the averaged TEC. Both GAIA simulation and GPS observations data show that daytime MSTID activities in the northern and southern hemisphere (NH and SH) are higher in winter than in other seasons. From the GAIA simulation, the amplitude of the meridional wind variations, which could be representative of gravity waves (GWs), shows two peaks in winter and summer. The winter peak in the amplitude of the meridional wind variations coincides with the winter peak of the daytime MSTIDs, indicating that the high GW activity is responsible for the high MSTID activity. On the other hand, the MSTID activity does not increase in summer. This is because the GWs in the thermosphere propagate poleward in summer, and equatorward in winter, and the equatorward-propagating GWs cause large plasma density perturbations compared to the poleward-propagating GWs. Longitudinal variation of daytime MSTID activity in winter is seen in both hemispheres. The MSTID activity during winter in the NH is higher over Japan than USA, and the MSTID activity during winter in the SH is the highest in South America. In a nutshell, GAIA can successfully reproduce the seasonal and longitudinal variation of the daytime MSTIDs. This study confirms that GWs cause the daytime MSTIDs in GAIA and amplitude and propagation direction of the GWs control the noted seasonal variation. GW activities in the middle and lower atmosphere cause the longitudinal variation.
... Since the post-midnight FAIs' zonal propagation velocities are similar to that of MSTIDs at middle latitudes, some post midnight FAIs may be mid-latitude FAIs simultaneously existing with MSTIDs [Otsuka, 2012] Taori et al. [2015] found EPBs and MSTIDs in the lower latitudes of the Indian sector at the same time, suggesting that MSTIDs may be one of their seeding mechanisms. Both the EPB and the MSF are capable of producing FAIs [Saito et al., 2008]. The EPBs tend to drift eastward, while the MSF structures tend to drift westward [Sivakandan et al. 2020] and even though EPBs can map up to mid-latitudes, the propagation path can be used to distinguish between EPBs and MSF. ...
We report the observation of plasma depletions/plumes in the F region ionosphere over a low to middle latitude transition region in the Indian sector. The observation of these plasma depletions is based on the data obtained in May 2019 through the all-sky airglow CCD imager installed in the campus of University of Kashmir, Srinagar (34.12 °N, 74.83 °E, magnetic latitude 25.91 °N). The depletions on the two consecutive nights of 05 and 06 May 2019 are aligned along the North-South (N-S) direction and drift westward. Several depletion bands along with some enhancement bands are seen in the 630-nm airglow images throughout the two nights. The observed structures show certain characteristics similar to Medium Scale Traveling Ionospheric Disturbances (MSTIDs) but these airglow features are not completely periodic. Further, in the observed depletion bands some East-West asymmetries are observed along with the structured tree-like branches of the airglow depletions. Some depletion bands even bifurcate leading to the inference that the structures are signatures of plasma irregularities rather than the usual MSTIDs observed in low-mid latitude transition region. The westward drift of the depletions especially during geomagnetic quiet times over this region makes this study significant since it offers a possible evidence that shows extension of spread F irregularities from the mid latitude region to the low-mid latitude transition region. In this paper, we point out some possible mechanisms related to the occurrence of plasma depletions at this region and their westward movement during geomagnetic quiet times.
... Using various ground based and satellite measurements such as Ionosonde, radar, Global Positioning System total electron content (GPS-TEC) and airglow imager etc., characteristics of electrified MSTIDs (Behnke, 1979;Bowman, 1990;Bowman & Monro, 1988;Ding et al., 2011;Otsuka et al., 2009;Tsugawa et al., 2007) and FAIs (Haldoupis et al., 2003;Hysell et al., 2016;Larsen et al., 2007;Mathews et al., 2001;Saito et al., 2008;Sun et al., 2015) are reported in the literature extensively. However, onset of MSTID or FAI in the FOV of an airglow imager are not reported to the best of our knowledge. ...
On a geomagnetic quiet night of October 29, 2018, we captured an observational evidence of the onset of dark band structures within the field‐of‐view of an all‐sky airglow imager operating at 630.0 nm over a geomagnetic low‐mid latitude transition region, Hanle, Leh Ladakh. Simultaneous ionosonde observations over New Delhi shows the occurrence of spread‐F in the ionograms. Additionally, virtual and peak height indicate vertical upliftment in the F layer altitude and reduction in the ionospheric peak frequency were also observed when the dark band pass through the ionosonde location. All these results confirmed that the observed depletions are indeed associated with ionospheric F region plasma irregularities. The rate of total electron content index (ROTI) indicates the absence of plasma bubble activities over the equatorial/low latitude region which confirms that the observed event is a mid‐latitude plasma depletion. Our calculations reveal that the growth time of the plasma depletion is ∼2 h if one considers only the Perkins instability mechanism. This is not consistent with the present observations as the plasma depletion developed within ∼25 min. By invoking possible Es layer instabilities and associated E‐F region coupling, we show that the growth rate increases roughly by an order of magnitude. This strongly suggests that the Cosgrove and Tsunoda mechanism may be simultaneously operational in this case. Furthermore, it is also suggested that reduced F region flux‐tube integrated conductivity in the southern part of onset region created conducive background conditions for the growth of the plasma depletion on this night.
... Besides, both EPB and MSF generate field aligned plasma irregularity structures (Fukao et al., 1988Kelley & McClure, 1981;Saito et al., 2008;Sekar & Chakrabarty, 2008;Tsunoda, 1980). In general, the EPBs drift mostly eastward (Nade et al., 2013;Paulino et al., 2011;Taori & Sindhya, 2014;Yao & Makela, 2007), while the MSF structures drift westward Kelley et al., 2003Kelley et al., , 2004Sun et al., 2015). ...
... This is an important question, as characterization of F region plasma irregularity structures at the transition region of low to mid latitudes remains unexplored. Earlier studies Saito et al., 2002;Saito et al., 2008) suggest that MSTIDs (airglow imaging) and mid-latitude field aligned irregularities (FAIs)/MSF (ionosonde and radar) structures have a few features in common like wave vector and traveling velocity, although the work of showed that only 15% of their total observation period, these two structures observed simultaneously over the Japanese sector. Therefore, the relationship between mid-latitude FAIs and MSTIDs phenomena has not been critically investigated. ...
An observational evidence of a unique plasma depletion event was captured by an O(¹D) 630.0 nm airglow imager on 13 June 2018 over a transition region of geomagnetic low‐mid latitude, Hanle, Leh Ladakh, India (32.77°N, 78.97°E; Mlat. ~24.1°N). The observed plasma depletion structures are tilted at an angle of 13° ± 2° west of the geomagnetic north and drifted toward west. Collocated Global Navigation Satellite System‐Total Electron Content measurements confirm that the structures are indeed associated with TEC depletions. Simultaneous ionosonde measurements from Delhi, India (28.70°N, 77.10°E; Mlat. ~20.2°N) shows spread‐F signatures confirming that these structures are associated with the ionospheric irregularities. Interestingly, radar observations over the geomagnetic low‐latitude station Gadanki, India (13.5°N; 79.2°E; Mlat. ~6.5°N) reveal the absence of equatorial plasma bubbles on this night. Therefore, these observations strongly suggest that the observed structures in the airglow images over Hanle are associated with mid‐latitude spread‐F (MSF). These MSF structures are possibly affected by the shear in the zonal plasma drift that forces the field aligned plasma irregularity structures to tilt toward west. These observations, for the first time, bring out the presence of MSF structures over geomagnetic low‐mid latitude transition region. It is suggested that the plasma distribution over low latitudes plays an important role in the occurrence of MSF structures over this transition region. Understanding the source and characteristics of the plasma irregularity structures over this transition region can help in understanding the spatio‐temporal evolution of global L‐band scintillation in a better way.
... Only fairly recently have aperture synthesis methods been applied to radar observations of the upper atmosphere and ionosphere (see, e.g., Hysell, 1996;Hysell et al., 2002Hysell et al., , 2004Hysell et al., , 2008Kudeki & Sürücü, 1991;Saito et al., 2006Saito et al., , 2008Sommer & Chau, 2016;Urco, Chau, Milla, et al., 2018;. The problem is similar to the one in radio astronomy with a few important differences. ...
Inverse methods involving compressive sensing are tested in the application of two‐dimensional aperture‐synthesis imaging of radar backscatter from field‐aligned plasma density irregularities in the ionosphere. We consider basis pursuit denoising, implemented with the fast iterative shrinkage thresholding algorithm, and orthogonal matching pursuit (OMP) with a wavelet basis in the evaluation. These methods are compared with two more conventional optimization methods rooted in entropy maximization (MaxENT) and adaptive beamforming (linearly constrained minimum variance or often “Capon's Method.”) Synthetic data corresponding to an extended ionospheric radar target are considered. We find that MaxENT outperforms the other methods in terms of its ability to recover imagery of an extended target with broad dynamic range. Fast iterative shrinkage thresholding algorithm performs reasonably well but does not reproduce the full dynamic range of the target. It is also the most computationally expensive of the methods tested. OMP is very fast computationally but prone to a high degree of clutter in this application. We also point out that the formulation of MaxENT used here is very similar to OMP in some respects, the difference being that the former reconstructs the logarithm of the image rather than the image itself from basis vectors extracted from the observation matrix. MaxENT could in that regard be considered a form of compressive sensing.
