It is well-known that large wedges of depleted plasma often form in the equatorial ionosphere after sunset. Irregularities in and around these depletions cause scintillation on trans-ionospheric radio paths, as well as strong VHF backscatter. The ultimate stage of depletion growth is controlled by a collisional interchange instability. However, the initiation stages remain the subject of debate. Depletions formed in the first 1–3 hours after sunset are probably seeded by instability processes operating in the valley region from 150–250 km. However, depletions that form later in the evening do not benefit from the rapid pre-reversal enhancement of the zonal electric field nor the sheared flow of the evening vortex. We investigate evidence from airglow images and VHF coherent backscatter that a polarization electric field associated with an ionospheric instability process at middle latitudes may induce the formation of post-midnight depletions at the geomagnetic equator during geomagnetically-quiet periods.
 Irregularities in the post-sunset equatorial F-region ionosphere have been a source of immense scientific and engineering curiosity since their first reporting by Booker and Wells . Although the name “equatorial spread F” persists, it belies the fact that a complex system of drivers results in a variety of morphologies discovered through continued in-situ and remote monitoring. It is now understood that equatorial spread F is the observational result of a Rayleigh-Taylor-type instability that causes meridionally-elongated wedges of depleted plasma to rise from the bottom side of the F region (see reviews by Hysell  and Makela [2006, and references therein]). Another irregularity process operating within the depleted region is responsible for coherent VHF radar backscatter at the Bragg wavelength [e.g., Tsunoda, 1980]. Among the enduring unknowns regarding equatorial plasma depletions is the sequence of events linking their formation to the apparently unstructured equatorial ionosphere of the late afternoon. Although depletions tend to have a well-defined seasonal occurrence for a given longitude (July–October, March–May, for Hawaii [Makela et al., 2004]), the day-to-day variability remains elusive.
 Within the depleted regions, polarization electric fields of sufficient scale map efficiently along the geomagnetic field pushing depleted wedges into low and tropical latitudes, piercing layers of airglow emission. Imaging observations of the 630.0-nm (dissociative recombination of O2+) airglow have been conducted from Hawaii since December 2001 [Kelley et al., 2002]. These observations are supported by a 50-MHz radar on Christmas Island, Kiribati. On-going climatological study of airglow images and radar backscatter from these sites uncovered a minor peak in post-midnight depletion and backscatter activity in solar minimum years (2005–2009). Although post-midnight depletions are often associated with geomagnetic activity [Martinis et al., 2005], these events almost universally occurred under quiet conditions, prompting this investigation. A similar pattern of post-midnight spread F was reported at Hawaii in the climatology by Reber .
 In addition to depletions, mesoscale traveling ionospheric disturbances (MSTIDs) are frequently observed in the 630.0-nm airglow emission over Hawaii, particularly during solar minimum conditions. The MSTID is a propagating perturbation in the altitude of the electron density peak attributed variously to the Perkins instability [Perkins, 1973] and coupling to Es layers [Tsunoda and Cosgrove, 2001]. MSTIDs appear propagating from the northeast to the southwest with wavefronts aligned in the southeast-northwest direction. Like depletions, MSTIDs contain polarization electric fields that map into the conjugate hemisphere [Otsuka et al., 2004]. However, meter-scale field-aligned irregularities (FAIs) responsible for coherent backscatter are confined to the undulations and are not generally observed at high altitudes [Otsuka et al., 2009].
 In this letter, we present observational evidence of a relationship between mid-latitude structure and the development of post-midnight equatorial spread-F during geomagnetic quiet conditions. We suggest that the polarization electric field associated with the mid-latitude structure maps to the bottom-size equatorial F region, initiating the development of plasma depletions, as proposed by Tsunoda . We will first give a brief overview of the instrumentation, which is amply discussed elsewhere in the literature [e.g., Makela et al., 2009]. Then, we will present two case study events followed by a discussion of the events and their potential implications for understanding the seeding of equatorial plasma depletions in general.
 The instrumentation employed in this study comprises two airglow imaging systems located atop Mount Haleakala (geographic: 20.71°N, 203.83°E; geomagnetic 21.03°N, 271.84°E) and a 50-MHz radar at Christmas Island (CXI; geographic: 2.0°N, 202.6°E; geomagnetic: 3.1°N, 273.6°N). The Cornell Narrow-Field Imager (CNFI) has a narrow field-of-view oriented at a low elevation approximately parallel to the geomagnetic field lines at the altitude of the airglow layer (taken to be 250 km for the 630.0-nm emission). The Cornell All-Sky Imager (CASI) is oriented at zenith. By tracing the ray paths from each pixel on the CCD imaging sensors in these systems to the airglow layer height, two-dimensional maps of the airglow emission extending from 5°N to 28°N latitude are produced. The radar has two stationary beams that have been operational since 2002 (east) and 2003 (north). The east beam provides backscatter data slightly to the east of the CNFI field-of-view, while the north beam provides observations within the same geomagnetic volume as CNFI. That is, the range gate centroid locations of the north beam reside on geomagnetic field lines that intersect the airglow observed by CNFI north of the geomagnetic equator, as illustrated in Figure 1.
 During cataloging of the images from Haleakala from January 2002 to January 2009, a high incidence of MSTID structures was noted during the November–January period during the solar minimum years (from late 2005 onward). Furthermore, a radar climatology indicates a coincident increase in post-midnight backscatter. This correlation was not immediately apparent since separate statistics for MSTIDs and plasma depletions were not being kept. However, a few events provided impetus for further study. Of these, we have selected two examples to present here: 23 November (327) 2005 and 2 September (246) 2008. Both of these days are preceded by geomagnetically-quiet conditions (Kp < 3).
 The first example, from 23 November 2005, is striking: it is near the end of the normal plasma bubble season for Hawaii, the airglow data are of high contrast, and the 50-MHz backscatter continues until the radar stops taking data. A composite data summary of this event is presented in Figure 2a. Figure 2a (top) contains header information about the date, site, and geomagnetic activity for ±5 days from the date of interest. Figure 2a summarizes the airglow and radar data for the night, mapped to the field line apex for comparison using the International Geomagnetic Reference Field (IGRF) [Maus et al., 2005]. The vertical scale is the apex altitude and the horizontal scale is Universal Time (leads Hawaiian Standard Time, HST, by 10 hours). Figure 2a (top) is a keogram, except that each altitude bin at each time step corresponds to a range gate of the Christmas Island radar's north beam. This permits direct comparison of the range-time-intensity (RTI) map from the radar with the airglow images, which are shown overlaid in Figure 2a (middle). The dark bands (indicated by arrow) from upper left to lower right between 2300 and 0000 HST are the signature of an MSTID passing through the local ionosphere over Haleakala. Figure 2a (bottom) provides an estimate of the eastward drift velocity of features in the airglow corresponding to different apex altitudes using the technique of Yao and Makela . The black regions indicate locations where no features were tracked. The regions marked with ‘W’ have westward drift components due to the MSTID.
Figure 3 contains eight selected airglow frames from CNFI and CASI on 23 November 2005. The entire sequence of airglow images is available in the auxiliary material as Animation S1. The selected frames illustrate the appearance of a band structure propagating to the southwest as plasma bubbles appear at the equator and propagate to the east. The geomagnetic projections of the north and east beams of the Christmas Island radar along the flux tubes corresponding to each range gate are plotted as well. A large alternating band structure begins to appear in the all-sky images of the 630.0-nm emission around 2042 HST. The isointensity contours of the MSTID emerge in the airglow already elongated magnetically northwest to southeast and propagating to the southwest (about 50 degrees off magnetic north).
 At 2153 HST additional crests of the MSTID become apparent, suggesting a separation of 150 km between consecutive dark bands. Between 2259 and 2250 HST, the MSTID strucutre reaches all the way to the southern extent of CNFI's field-of-view, corresponding to geomagnetic apex heights of 325 km. This is well-correlated with first radar echoes occurring at 2250 HST. Bubbles begin to emerge around 2311 HST (see Animation S1, 0911 UT) and are becoming clear by the time of the last all-sky image at 2340 HST. At 0015 HST, both the plasma bubbles drifting to the east and the MSTID drifting to the southwest are visible in the same image. In the accompanying Animation S1, 12 frames illustrate seeding between 2250 HST (0850 UT) and 0015 HST (1015 UT). The bubbles grow in altitude (and latitude) and continue to drift east throughout the remainder of the night.
 In Figure 2a, there appears to be a break in the backscatter, which is indicated by the dashed line in Figure 2a (middle). This coincides with a shear in the drift velocity as the depletions grow into a region still affected by the MSTID instability. As the drift slows, secondary structures grow on the eastern walls of the depletions which are particularly evident in the 0216 HST frame from Figure 3.
Figure 2b illustrates the summary data for 2 September 2008. This night exhibits a bottom-type layer that formed at 2200 HST but did not yield any plumes before decaying around 2330 HST. In our experience, bottom-type layers never reform after descending in altitude. This has also been confirmed by Hysell and Burcham . However, at 2345 HST, coincident to the passage of an MSTID in the airglow (indicated by arrows), a packet of depletions emerges along with the attendant backscatter. Although they are not evident in the keogram, these depletions are visible in the airglow frames (not shown). Most of the backscatter disappears in conjunction with the lowering (brightening of the 630.0-nm emission) of the ionosphere around 0300 HST.
 The event from 2 September 2008 is interesting because the MSTID structure continues in the airglow until the backscatter vanishes around 0300 HST, which is still a few hours before local sunrise. This is in sharp contrast with the 23 November 2005 example in which the equatorial irregularities not only grow to high altitudes (latitudes), but the MSTID introduces a shear in the drift, apparently allowing the cascade to smaller-scale irregularities to continue until after the radar stopped collecting data at 0600 HST, which is 15 minutes before sunrise at the CXI site. The descent of the ionosphere at 0300 HST on 2 September 2008 (Figure 2b) appears to quench the cascade to smaller scales, corroborating the assertion of Saito et al. .
 Among the most compelling recent theories for the initiation of equatorial plasma depletions in the post-sunset ionosphere is a fast-growing mode operating on perturbations with westward-tilting wavefronts (with a maximum growth rate at 45°) in the equatorial plane put forth by Kudeki et al. . In the post-sunset equatorial ionosphere, this mode provides the “seed” necessary to initiate the gravitational Rayleigh-Taylor instability which ultimately produces depletions. However, after the F-region dynamo has been fully established later in the evening, the differential between the zonal neutral wind and the zonal plasma drift will be small, rendering the wind-driven instability ineffective. Thus, it is unlikely that our cases of actively developing depletions near midnight at solar minimum are controlled by this mechanism.
 However, with the coincidence of the passage of the MSTIDs, comes the possibility that the Ep internal to these structures is providing the seed on these nights. As has been described by Tsunoda and Cosgrove  and Cosgrove and Tsunoda , coupling between altitude-modulated sporadic-E (Es) layers in the mid-latitude ionosphere can extend into the F region, generating the polarization electric field characteristic of MSTIDs. Tsunoda  showed how this sort of coupling, from Es through the F layer, could act as a seed for equatorial depletions through the induced height perturbations caused by Ep × B drift. We contend that the data presented here is observational evidence for this mechanism operating in the equatorial ionosphere at solar minimum. On 23 November 2005, the spacing of the depletions is near identical to the projected wavelength (240 km × cos 50° ∼ 150 km; where 50° is the angle of MSTID propagation with respect to the magnetic equator) of the MSTID bands indicated at 2340 HST in Figure 3, lending credence to this interpretation.
 In this letter, we have presented two examples of post-midnight equatorial plasma depletions initiated by polarization electric fields from a mid-latitude process mapped into the equatorial ionosphere, as suggested by Tsunoda . Airglow images in conjunction with VHF coherent backscatter provide a four-dimensional (three spatial, one temporal) picture of the initiation of many post-midnight equatorial plasma depletions observed during quiet periods.
 The principle contribution of this work is to illustrate that seeding of the Rayleigh-Taylor instability responsible for the ultimate growth stage of equatorial plasma depletions need not be the sole providence of a particular process, rather the title goes to the highest bidder. That is, the bottom-side F-region gradient is susceptible to the Rayleigh-Taylor instability for much of the night. In the hours just after sunset and during periods of peak depletion activity, irregularities growing out of the evening vortex region are the fastest-growing, and therefore most-likely seeds. Later in the evening, when the ionospheric current system has approached steady state with the F-region dynamo firmly in control, perturbations may be created by electric fields mapped-in from off-equatorial sources.
 That depletions may be triggered by off-equator processes also underscores the practical importance of understanding the ionosphere at middle latitudes from a space weather perspective. Scintillations associated with equatorial plasma depletions have negative effects on the availability and accuracy of satellite-based communication and navigations systems. Users of these systems would benefit from enhanced prediction and mitigation schemes.
 There are at least a half-dozen more examples of coincident mid-latitude and equatorial events in our data from November and December 2008 alone. We anticipate performing a more detailed study, particularly seeking examples of MSTIDs with no depletions, as well as depletions that appear to have been formed around local midnight without forcing from middle latitudes.
 Airglow observations on Mt Haleakala are supported by the Air Force Office of Scientific Research under contract FA9550-05-0160494 to Cornell University. Christmas Island radar is operated with support from the Air Force Research Laboratory. The authors are grateful to Warner Ecklund, Roland Tsunoda, and Keith Groves, for providing access to the radar data. ESM acknowledges fruitful and spirited discussions with Erhan Kudeki about the growth and character of equatorial plasma depletions. Work at the University of Illinois at Urbana-Champaign (ESM and JJM) was supported by the Office of Naval Research under contract N00014-08-1-1136.