Equatorial plasma bubble zonal velocity using nm airglow observations and plasma drift modeling over Ascension Island

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117,, doi: /2012ja017750, 2012 Equatorial plasma bubble zonal velocity using nm airglow observations and plasma drift modeling over Ascension Island Narayan P. Chapagain, 1,2 Michael J. Taylor, 1 Jonathan J. Makela, 2 and Timothy M. Duly 2 Received 21 March 2012; revised 2 May 2012; accepted 15 May 2012; published 27 June [1] We present OI (630.0 nm) airglow image data from Ascension Island (geographic: 7.9 S, 14.4 W; dip latitude: 16 S) in the southern Atlantic Ocean taken with the Utah State University all-sky CCD camera during 20 March to 7 April 1997 in order to study plasma bubbles occurring in the low-latitude nighttime ionosphere. The initial plasma bubble onset occurs in the early evening hours at 19:15 20:00 LST and is followed by eastward propagation with an average speed of m/s prior to local midnight, rapidly decreasing around the midnight and postmidnight periods. The Ascension results are compared with similar observations from Christmas Island in order to examine the longitudinal variations of EPB development and propagation. The observed EPB velocities from Ascension Island are also compared with the results of a plasma drift model. In a case study during the night of 4 5 April, the velocity reveals unusual latitudinal shear, up to 0.12 m/s/km, with a reversal to westward flow at low latitudes while eastward flow is maintained at higher latitudes. Consequently, the bubble rotates counterclockwise and tilts eastward, significantly away from alignment with the geomagnetic field lines. The westward reversal of the drift motion near the geomagnetic equator is most likely the result of a reversal in the F region dynamo or from a large increase in the altitude of the shear node in the F region plasma drift at the geomagnetic equator. Citation: Chapagain, N. P., M. J. Taylor, J. J. Makela, and T. M. Duly (2012), Equatorial plasma bubble zonal velocity using nm airglow observations and plasma drift modeling over Ascension Island, J. Geophys. Res., 117,, doi: /2012ja Introduction [2] The characteristics of the zonal plasma drift in the equatorial F region ionosphere were first described by Woodman [1972] and, later, Fejer et al. [1985, 1991] using data from the Jicamarca incoherent scatter radar (ISR). They reported a well-developed diurnal cycle of zonal plasma drift toward the west during the day reversing to eastward motion at night. Under solar minimum conditions, daytime drift speeds up to 50 m/s westward are typical, while the nocturnal eastward drift motion exhibits a significantly larger peak value of 120 m/s prior to local midnight, resulting in a net superrotation of ionospheric plasma. Related electric field studies have established that the plasma drift is usually eastward throughout the night in the ionosphere ( km altitude), reversing to westward near 06:00 LT [e.g., Fejer et al., 1991], although the reversal time depends on season, geomagnetic activity, and solar flux conditions. 1 Center for Atmospheric and Space Sciences, Utah State University, Logan, Utah, USA. 2 Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA. Corresponding author: N. P. Chapagain, Center for Atmospheric and Space Sciences, Utah State University, Logan, UT 84322, USA. (npchapagain@gmail.com) American Geophysical Union. All Rights Reserved /12/2012JA [3] The equatorial ionospheric plasma drifts along the geomagnetic zonal direction are driven by electric fields (in E B) produced by the neutral winds in the E and F regions of the ionosphere [e.g., Rishbeth, 1971; Heelis et al., 1974]. At night, E region plasma densities rapidly decay and the E region dynamo becomes negligible. Therefore, the F region dynamo dominates the nighttime plasma drifts [e.g., Coley and Heelis, 1989]. Model results indicate that a strong vertical electric field is set up, driving the plasma in the direction of the F region neutral winds with almost the same magnitude [Haerendel et al., 1992; Eccles, 1998]. Moreover, a recent study from coincident observations of equatorial plasma bubbles and the thermospheric neutral winds has shown consistent results between the zonal velocity of the bubbles and the neutral winds (mostly after 22:00 LT), suggesting that the F region dynamo is fully developed and the bubble velocity is indicative of the background plasma motion [Chapagain et al., 2012]. [4] Detailed observations of the postsunset bottomside equatorial ionosphere have established the existence of a strong vertical shear in the horizontal plasma (ion) flow at all longitudes, with plasma near the F region peak drifting rapidly eastward while plasma at lower altitudes drifts slowly westward [e.g., Kudeki et al., 1981; Eccles et al., 1999]. Fejer et al. [1985] have reported a strong shear in the zonal plasma drifts around the F peak where the eastward drift decrease with heights and become westward at lower altitudes, while at higher altitudes, the eastward drifts decrease 1of12

2 slowly with height. Such postsunset eastward and westward drifts are associated with downward and upward motions, respectively, resulting in a plasma flow vortex below the F layer peak [Kudeki and Bhattacharyya, 1999]. The altitude of the shear mainly depends on the vertical drifts of the plasma at sunset [Haerendel et al., 1992] and can persist for a few hours [Kudeki and Bhattacharyya, 1999; Hysell et al., 2005]. [5] Furthermore, irregularities originating near the geomagnetic equator in the postsunset bottomside ionosphere via the Rayleigh-Taylor instability (RTI) grow upwards into the F region ionosphere, developing into elongated magnetic North-South field-aligned structures of depleted plasma [e.g., Maruyama, 1988; Sahai et al., 1994]. These structures are readily detected in airglow emissions generated in the thermosphere/ionosphere as equatorial depletions or equatorial plasma bubbles (EPBs). They have zonal widths of a few tens to hundreds of km and extend along the geomagnetic meridian for hundreds to thousands of km. Ground-based optical observations of EPBs in the nighttime F region OI (630.0 nm) airglow emissions (peak altitude 250 km) have been used to investigate the plasma drift motion from a number of low-latitude sites [e.g., Weber et al., 1978; Mendillo and Baumgardner, 1982; Sahai et al., 1994; Taylor et al., 1997; Kelley et al., 2002; Pimenta et al., 2003b; Makela et al., 2004]. [6] Mendillo and Baumgardner [1982] presented early optical observations of EPBs from Ascension Island (geographic: 7.9 S, 14.4 W; dip latitude: 16 S) in the south Atlantic Ocean from which they derived drift velocities. The measurements were made during January February 1981 under high solar flux conditions and indicated that the airglow depletions occurred most often during the premidnight period (20:30 23:30 LT), drifting eastward with speeds that decreased from 190 m/s at 21:00 LT to 80 m/s at 01:00 LT. They further noted several cases of apparently twisting, overlapping, and bifurcating depletion structures. [7] In this paper, we present ground-based measurements of EPB optical signatures from Ascension Island made during solar minimum conditions. Estimates of their zonal drift velocity are made and compared with model results for the ambient plasma drift velocities over Ascension Island as well as results from the Jicamarca empirical model [Fejer et al., 2005]. Our results are contrasted with previous results from Mendillo and Baumgardner [1982] and compared with similar observations from Christmas Island (geographic: 2.1 N, W; dip latitude: 2.8 N) to examine the longitudinal variations of EPB development and propagation. We further report an unusual latitudinal shear in the motion of the bubble structures observed on the night of 4 5 April 1997, which appeared as a counterclockwise rotation. 2. Observations and Data Analysis [8] An all-sky CCD camera was deployed by Utah State University (USU) to Ascension Island in the south Atlantic region from 20 March to 7 April 1997 (17 nights) for comparative observations of mesospheric gravity wave and plasma depletions. The USU camera is a well-proven field instrument fitted with a sensitive back-thinned solid state pixels CCD array. Data obtained were 2 2binnedonchip, resulting in a zenith spatial resolution of 0.5 km. Sequential observations of the thermospheric OI (630.0 nm) airglow emission, as well as the mesospheric near infrared (NIR) hydroxyl (OH) Meinel broad band emissions ( nm) and the OI (557.7 nm) green line emissions, were made using exposure times of 120, 15, and 90 s, respectively. A background sky measurement was also recorded to discriminate between mesospheric and thermospheric structures and meteorological clouds. Further details of the all-sky instrument are given by Taylor et al. [1995]. [9] The imager was operated nightly during the campaign, but variable weather conditions and the rising moon in the early morning hours limited the observation period from early evening (20:00 LST) to shortly after local midnight (02:00 LST), with a maximum duration of 6 h. The time intervals between consecutive OI (630.0 nm) images alternated between 4 to 7 min, depending on the filter sequence being utilized. Well-defined plasma depletion structures were observed on 7 nights. On several occasions, the bubble structures were observed to overlap and bifurcate similar to the previous study reported by Mendillo and Baumgardner [1982] from Ascension Island. [10] Figure 1 is a map showing the location of Ascension Island in the south Atlantic Ocean, between South America and Africa. The dashed lines plot the dip latitudes at 10 S, 20 S, and 30 S. The open circle centered on Ascension Island represents the geographic field of view (FOV) of the camera with diameter 1500 km assuming an emission altitude of 250 km. This range covers the magnetic latitudes 10 S 24 S. An enlarged airglow image recorded from Ascension Island illustrates the typical bubble structures observed during the campaign. The dark band near the center of the image depicts a broad magnetic north-south aligned plasma bubble. [11] The raw images recorded from the camera were first calibrated using the known star field to determine the pixel scale size and orientation of image data. The stars were then removed from the data, which were subsequently unwarped [e.g., Garcia et al., 1997], and projected onto a km uniform spaced geographic grid at the assumed airglow emission altitude of 250 km. This technique is the same as applied to analyzing wave motions in mesospheric image data [e.g., Taylor and Garcia, 1995; Garcia et al., 1997]. The method used to measure the zonal velocity of the plasma bubble is similar to that proposed by Pimenta et al. [2001] and employed by Pautet et al. [2009] and Chapagain et al. [2011] to quantify bubble drift speeds over equatorial Brazil and Christmas Island, respectively. In each case, the velocities of individual bubbles were estimated using the embedded depletion structures visible from pairs of consecutive OI (630.0 nm) images separated in time by 12 min. The motion of individual bubbles was measured at various locations along a structure to determine the bubble s average velocity. This procedure was repeated for subsequent image pairs to determine the average drift velocity of the bubbles during the course of each night as a function of local solar time (LST). The velocities were calculated for all depletion structures within the images and then sorted by the 2of12

3 Figure 1. Map showing the location of Ascension Island, the field of view covered by the imager represented by the open circle assuming an emission height at 250 km, and an enlarged example of OI (630.0 nm) airglow depletion image. LST of the bubble location, assuming an emission altitude of 250 km. The difference between LST and Universal Time (UT) at Ascension Island is 56 min. 3. Model Calculation of Plasma Drift Velocity [12] To investigate the zonal drift velocities estimated from the optical measurements and their relationship to the ambient ionospheric plasma drift velocity, we utilize a model. The ambient ionospheric plasma zonal drift velocities are estimated using theoretical formulations for the ionospheric vertical electric field, which is responsible for the zonal plasma drift [Haerendel et al., 1992; Eccles, 1998]. The method for calculating the zonal drift velocities is described below. [13] The local Pedersen (s P ) and Hall (s H ) conductivities are given by [Kelley, 1989]: s P ¼ qn e jk e j B 1 þ ke 2 þ k i 1 þ ki 2 ð1þ Macmillan, 2000]. J en and J in are the electron-to-neutral and ion-to-neutral collision frequencies which are estimated as [Kelley, 1989;Sobral et al., 2009]: u en ¼ 5: ðn n Þ T e 1=2 u in ¼ 2: ðn n þ N i Þ A 1=2 ; where, A (= i M i N i / i N i ) denote mean neutral molecular mass in atomic unit, M i is atomic/molecular mass units, T and T e are neutral and electron temperatures, N n and N i are number densities (cm 3 ) of the neutrals and species i (e, O 2,N 2,O,and N), respectively. [14] Field line integrated Pedersen ( P ) and Hall ( H ) conductivities are determined by integrating along the geomagnetic field lines using IGRF. We further estimate the Pedersen (U 8 P ) and Hall (U L H ) conductivities weighted winds using the following field line integrated expressions: U8 P ¼ 1 Z ht s P u Z dr S P ha ð5þ s H ¼ qn e B ke 2 1 þ ke 2 k2 i 1 þ ki 2 ð2þ U H L ¼ 1 S H Z ht ha s P u q dr ð6þ where q is the charges of ions, N e is number density of electrons, B is the Earth s total magnetic field intensity, k e and k i are mobilities of electrons and ions, respectively, and are expressed by: k e ¼! ė ð3þ # en k i ¼! i. # in Here, w e and w i are the electron and ion gyrofrequencies, respectively, and are calculated as: w e = eb/m e and w i = qb/m i. The value of B is obtained from the International Geomagnetic Reference Field (IGRF) magnetic field model [Mandea and ð4þ where, ht (=90 km) and ha (=250 km) are the target (lower limit of altitude) and apex heights, respectively. We utilize a step size of 1 km in the integration along the magnetic field line, dr. Here, u z is the local zonal wind at a given place along the magnetic field line, and u q is component of the meridional wind perpendicular to the magnetic field line in the vertical direction. The zonal plasma drift velocity (v) was calculated as [Sobral et al., 2009]: v ¼ Uf P S H V p UL H 1 þ 3z 2 1=2 S P 1 z 2 3=2 ð7þ 3of12

4 Table 1. Onset Times of EPBs Above Ascension Island Obtained From Optical Images on 7 Nights During the Campaign UT Days Onset Time (LST) 20 Mar 19:22 27 Mar 19:26 29 Mar 19:58 30 Mar 19:58 31 Mar 19:38 4 Apr 19:12 5 Apr 19:27 where V p is the vertical drift velocity at the magnetic equator perpendicular to magnetic field, and z = sinl. The magnetic latitude, l, obtained from IGRF for Ascension Island is 9 Sat 250 km. [15] The neutral wind motions were obtained from the empirical Horizontal Wind Model, 1993 (HWM93) [Hedin et al., 1996]. The Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar-2000 model (NRLMSISE-00) [Picone et al., 2002] was used to derive the neutral densities (O 2,N 2, O, and N), and the International Reference Ionosphere version 2007 model (IRI07) [Bilitza and Reinisch, 2008] was used to obtain the electron (N e ) and ion (N i ) densities of the species (O 2,N 2, O, and N), as well as the electron temperature (T e ). The vertical plasma drift velocity (V p ) was obtained from the Scherliess and Fejer [1999] empirical model. The model calculations were made using the MATLAB programming language Apex Altitude [16] The apex altitude of the dipole field line that passes through the airglow emission layer is derived using the expression for the dipole field lines as explained by Sobral et al. [2009]: H apex ¼ R E þ H E cos 2 l R E ð8þ where R E (=6370 km) is the radius of the Earth, H E is the airglow emissions altitude (250 km), and l is the magnetic latitude. The apex altitudes mapped above the magnetic equator were computed using the above equation for the locations of the pixels within the FOV of the imager over Ascension Island at 250 km altitude in the range of geographic latitudes 0 S 14 S and longitudes 8 W 22 W. 4. Results [17] The image data from the seven days exhibiting EPBs during this campaign have been used to investigate the structures characteristics including their general morphology, temporal evolution, and zonal drift speeds Onset and Zonal Drift Velocity of EPBs [18] Table 1 summarizes the initial appearance (onset times) of the EPBs that were observed to grow within the FOV of the camera (1500 km diameter) over Ascension Island. During the 7 nights of the data exhibiting EPBs, depletion onset occurred in the early evening hours over a limited 45 min interval (19:15 20:00 LST) in the western region of the FOV, elongating along the meridional directions. These results are in good agreement with observations from Christmas Island [Chapagain et al., 2011] as discussed later. [19] The nocturnal plasma bubble zonal velocities were estimated using the embedded depletion structures visible in OI (630.0 nm) airglow images. As described in section 2, the unwarped images were used to determine an average drift velocity for each bubble. Figure 2 shows temporal variations of the plasma bubble zonal velocities over Ascension Island and the corresponding modeled drift velocity of the ambient plasma as calculated using equation (7). The data are presented for 6 nights of the campaign. The measurements were restricted from early evening to 02:00 LST due to the limitation in the camera operations caused by moonlight. During the night of March, a bubble appears for a few minutes in the early evening but was then occluded from further observations by a cloud. Therefore, the data during this night are excluded from the figure. The missing data points in the figure are mainly due to interference from clouds. The vertical bars on each plot represent one standard deviation in the measurement uncertainty in the estimates of EPB drift velocity and illustrates the scattering of the estimated data. In each plot, the value of Kp represents the average of the three hourly geomagnetic activity index (K p ) over a nine hour period from 13:30 LST to 22:30 LST. This period includes 6 h prior to local solar sunset during which EPB onset may be influenced by the geomagnetic activity [Fejer et al., 2005]. However, all days were geomagnetically quiet with only minor activity ( Kp < 3) except on March, when Kp was slightly larger than 3. During this night no significant difference in the observed EPB was evident as compared with magnetically quiet data. The average solar flux (F10.7 cm) during the measurements period was 75. [20] Figure 2 reveals a similar pattern of temporal variations of the ambient plasma drift model and observed bubble velocities. However, the magnitudes of the observed plasma bubble velocities, in general, are smaller than the calculated model results except around the peak values of the bubble velocities where their magnitudes are slightly larger than that of the model results. The velocities generally increase during the early evening periods, peaking at 21:00 22:00 LST, and decrease around and after the local solar midnight (except on March, where the velocity decreased steadily). However, on March, March, and 30 March to 1 April the data were limited due to cloudy skies. On 4 5 April the drift velocity exhibits large variations (as illustrated by the large values of standard deviations) around midnight and postmidnight periods, while on 5 6 April the velocity rapidly decreases (to 10 m/s) prior to 22:45 LST before the bubble faded out of the images. [21] Figure 3 summarizes the plasma bubble zonal velocities for these six nights of the campaign. The bold line with the filled circles shows the average velocity of all data sets as a function of local solar time with the vertical bars representing the standard deviation from this mean. The data, particularly during the premidnight period, indicate large day-to-day variability in the magnitude of the bubble velocities in the range of m/s. The average velocity peaks at 110 m/s around 22:15 LST and then decreases to a minimum value of 10 m/s at 02:00 LST. For comparison of our results with the only previous available measurements of the bubble velocities from the same 4of12

5 Figure 2. Local solar time variations of plasma bubble and model plasma drift zonal velocities from Ascension Island. The bubble velocities calculated from two successive images (time interval 8 15 min) during six nights of the campaign considering a reference altitude of airglow emissions at 250 km. The model drift velocities are plotted at time intervals of 30 min. site, we plot the zonal drift velocity of the airglow depletion from Ascension Island during solar maximum condition (as shown by the line plots with stars symbols) from Mendillo and Baumgardner [1982]. The results illustrate that the bubble velocities during low solar flux conditions are significantly smaller compared to the measurements obtained during high solar activity. [22] Figure 4 displays the average bubble zonal velocities observed from Ascension Island compared with the plasma drift model calculated using equation (7) (blue line, open circles), the local wind model (HWM93; green line, crosses), and Pedersen conductivity weighted winds model (U 8 P ) (black line) obtained using equation (5). The average bubble velocities are in generally good agreement with the Pedersen conductivity weighted wind model and also with the ambient plasma drift model results. However, the comparison to the local wind model in the early evening hours (prior to 21:30 LST) is less satisfying, with both the observed bubble velocity and the ambient plasma drift model results (maximum value of 100 m/s) significantly smaller than the peak value (150 m/s) of the local neutral winds model (HWM93). This shows the importance of considering the field line integrated quantities, especially during the early evening hours. However, around midnight and postmidnight periods, the bubble velocity becomes consistent with the local neutral winds model result. [23] To put our results into context with other measurements under similar low solar flux conditions, Figure 5 compares the average bubble zonal velocities from Ascension Island with recent results from optical measurements at Cajazeiras, Brazil (geographic: 6.87 S, W; geomagnetic: 7.11 S) during October December [Chapagain et al., 2012] and Christmas Island during September October 1995 [Chapagain et al., 2011] and previous optical observations from Alcantara, Brazil (geographic: 2.3 S, 44.5 W; dip latitude: 1.3 S) in October 1994 [Taylor et al., 1997]. The smooth curve represents the empirical model results derived from long-term Jicamarca radar measurements from 1970 to 2003, Peru (geographic: 12 S, 5of12

6 Atlantic sector sites (below to 10 m/s) during the midnight and postmidnight periods. Figure 3. The plasma bubble zonal velocities over Ascension Island during six nights of the campaign. Bold data points represent averaged velocity and vertical bars are the standard deviations from the mean. For comparison, the average depletion velocity (line plot with stars) from Mendillo and Baumgardner [1982] obtained during solar maximum condition is replotted W; dip latitude: 1 N) for low solar flux conditions (average F10.7 = 90) [Fejer et al., 2005]. [24] The average bubble velocity from Ascension Island is consistent with the recent results from Cajazeiras and previous studies from Alcantara, Brazil. All of the results from sites in South America and the Atlantic sector show good agreement with the Jicamarca model drift velocities around the peak values in the early evening ( m/s). However, these drift measurements begin to decrease approximately two hours before the decrease obtained from the model. In contrast, the Christmas Island results do not show a similar decrease and are significantly higher (above 75 m/s) than the results from the South American and 4.2. Comparison of Propagation Characteristics of EPB [25] The general characteristics of the EPB propagation observed during the campaign from Ascension Island are compared with the Christmas Island results. Figure 6 shows the keogram plot of the OI (630.0 nm) airglow emissions in the west-east direction (positive eastward) from Ascension Island (Figure 6, left) on 4 5 April 1997 compared to Christmas Island results (Figure 6, right) on September Both measurements were carried out under similar low solar flux conditions using the same USU CCD camera. These plots compare the evolution, development, and propagation of EPBs from Ascension Island and Christmas Island. They were spliced together by taking a magnetic zonal slice passing through the zenith of each unwarped image to create a time series showing zonal development of the EPB observed from both sites. The dashed lines in each plot indicate the central row used for creating the keograms. The curved band in the bottom left of the Christmas Island plot is due to contamination from the Milky Way. The dark bands (plasma bubbles) progressing from bottom left to top right (i.e., from west to east with time) show the onset and subsequent motion of EPBs, as indicated by the arrows. The slope of the dark bands gives the plasma bubble zonal velocity. The small values of the slopes seen in the Ascension Island plot indicate EBPs propagating with smaller drift velocities, while the bubbles observed from Christmas Island exhibit very large and nearly linear slopes, illustrating that EPBs were moving with fast and steady drift velocities throughout the course of the night Shear Velocity of Plasma Bubble [26] Figure 7 shows a sequence of processed images of prominent F region bubble structures recorded on 4 5 April Figure 4. Comparison of the average bubble zonal velocities from Ascension Island with the plasma drift model, local wind model, and Pedersen conductivity weighted winds model from HWM93. The vertical bars represent the standard deviations from the mean indicating the variability of the bubble velocities from the average value. Figure 5. Comparison of the average plasma bubble zonal velocities from Ascension Island with previous results from Cajazeiras, Brazil [Chapagain et al., 2012], Alcantara, Brazil (replotted) [Taylor et al., 1997], Christmas Island [Chapagain et al., 2011], and an empirical model of the plasma drift velocities derived from radar measurements (replotted) from Jicamarca, Peru [Fejer et al., 2005]. 6of12

7 Figure 6. Keogram plots of the nighttime OI (630.0 nm) airglow emissions in west-east (positive eastward) direction from (left) Ascension Island and (right) Christmas Island. The dark bands represent plasma bubbles and arrows indicate their direction of motions. and mapped into geographic coordinates. The ordinate axis on the left of each map indicates the magnetic latitudes, spanning from 8 Sto24 S, while the right axis indicates the geographic latitude, ranging from 15 S to the equator. The figure clearly illustrates the development and spatial characteristics of the bubble structures during the course of the night. Depletions initially appeared in the northwestern region of the image around 20:00 LST (top left map). They then grew in size and contrast into mature structures over the next hour (second row). The depletions were aligned along the magnetic field lines (perpendicular to the magnetic latitude) and propagated eastward as indicated by the white arrows (second and third rows). During the postmidnight period, the bubbles extended to higher latitudes (>20 S dip latitude; bottom row). Remarkably, during this period, the motion of the bubbles close to the equator reversed in direction to westward, while the extension of the bubble structures at higher latitudes continued moving eastward. This caused the bubble to appear to rotate counterclockwise in the mapped data (as indicated by arrows in the bottom row) resulting in an eastward tilt contrary to the well-established westward tilt documented in the literature [e.g., Mendillo and Tyler, 1983; Abalde et al., 2001; Makela and Kelley, 2003; Huang et al., 2010]. Regrettably, further observations of this unusual event were not available after 02:00 LST due to moon light contamination. [27] We estimated the apex altitudes (H apex ) mapped along the magnetic field lines above the magnetic equator using equation (8). The apex altitudes corresponding to geographic latitudes (2 S 14 S) and longitudes (8 W 22 W) covered by the FOV of the camera at Ascension Island are in the range of km as shown in Figure 8 (left). [28] Figure 8 (right) presents the plasma bubble zonal velocities as a function of local solar time during the night of 4 5 April, illustrating the shear motions of the bubble structure. The velocities v 1,v 2, and v 3 were computed across the top (0 S 5 S), middle (5 S 10 S), and the bottom (10 S 14 S) portions of the bubble structure from unwarped images corresponding to the apex altitudes mapping to km, km, and km, respectively. Prior to 23:30 LST, the bubble propagated with approximately the same velocity across all latitudes, while around and after local midnight (23:30 02:00 LST), the bubble exhibited a strong shear motion with different velocities at different latitudes (or apex altitudes). The velocity calculated closest to the magnetic equator, v 1, corresponding to the lowest apex altitudes, showed westward motion up to 20 m/s, while the other two calculated velocities, v 2 and v 3, indicate eastward motions with the magnitudes of m/s and m/s, respectively. This represents a maximum shear of 0.12 m/s/km at around 01:00 LST. Due to this shear motion, the bubble titled eastward contrary to the typical westward tilt commonly reported in the literature [e.g., Zalesak et al., 1982; Abalde et al., 2001; Makela and Kelley, 2003]. [29] To examine the shear velocity of the bubble structure in more detail, we plot zonal winds weighted by the Pedersen conductivity and the ambient zonal plasma drift model derived from the altitude of the assumed airglow emissions height of 250 km in Figure 8 (right). The observed bubble velocity reversal at the lowest latitudes occurs just before midnight (23:40 LST) while the weighted winds model reverts from east to westward two hours later (02:00 LST) and the ambient plasma drift reverses westward still another two hours later (04:00 LST). Unfortunately, no bubble measurements were recorded during this period due to the moon conditions. The local winds model (HWM93) (not shown here) during the same night exhibits eastward neutral flow throughout the night. The result illustrates that the eastward motion of the plasma bubble is closely consistent with the weighted neutral winds and plasma drift models, while the westward motion of the bubble structure at lower latitude of the FOV of the camera are contrary to the model results. [30] To investigate the shears observed in the EPB velocities in more detail, we present the plasma drifts model velocities at apex heights of 600 km, 900 km, and 1200 km above the magnetic equator in Figure 9. The results reveal no significant shear motion with respect to the change in apex altitudes in the modeled plasma drifts. The eastward drift velocities at around 01:00 LST were 30 m/s at 600 km, 38 m/s at 900 km, and 45 m/s at 1200 km 7of12

8 Figure 7. Sequence of unwarped all-sky airglow images showing spatial characteristics and time evolution of EPBs during 20:00 02:00 LST on April 4 5, The images have been projected onto km geographic grid at an assumed altitude of 250 km. The time separation between images is 0.5 h except for the first images, which is one hour. corresponding to a shear of only m/s/km compared to the observed EPB shear motion of 0.12 m/s/km. The magnitudes of the drift velocities after local midnight corresponding to higher apex altitudes (at 900 km and 1200 km) are on the same order of the observed bubble velocities in the local midnight and postmidnight sector. Furthermore, the model drifts at the lower apex altitude of 600 km do not exhibit any westward motion until 04:00 LST at which time all of the modeled plasma drifts switch direction, contrary to the observations. 5. Discussion [31] The initial onset times of postsunset EPBs from Ascension Island are in good agreement with the recent results (19:30 20:00 LST) reported by Chapagain et al. [2011] from optical measurements of EPBs over Christmas Island. The Ascension results presented here are also consistent with the onset times of initial ESF obtained from a climatological study of Jicamarca radar observations [Chapagain et al., 2009], which suggested that the average spread F onset time is about 19:30 LT. [32] The plasma bubble zonal drift velocities from Ascension Island increase during early evening hours and decrease around the local midnight and postmidnight period in a similar trend to previous studies reported from a number of sites in the equatorial region [e.g., Fejer et al., 1991; Taylor et al., 1997; de Paula et al., 2002; Martinis et al., 2003; Yao and Makela, 2007; Pautet et al., 2009, Haase et al., 2011; Chapagain et al., 2012]. Our results illustrate that the observed plasma bubble zonal velocity are consistent with modeled ambient plasma drift velocities, similar to what has been reported by Sobral et al. [2009] by comparing airglow depletion and GPS scintillation zonal velocities with the plasma drift model results. Such good agreement between the observed bubble velocities and the model results illustrate 8of12

9 Figure 8. (left) Apex mapping over Ascension Island for corresponding geographic latitudes and longitudes covered by the FOV of the CCD camera. (right) Plasma bubble velocities over Ascension Island on 4 5 April illustrating latitudinal shear velocities. The Pedersen conductivity weighted winds model and plasma drift model results are also plotted. that the ionospheric vertical current must become negligible during the night, and that the field line mapped vertical electric field drives the zonal plasma drift motion as suggested by Haerendel et al. [1992] and Eccles [1998]. [33] Figure 4 illustrates that the model results of both the Pedersen conductivity weighted winds and ambient plasma drifts match well with the observed average bubble drift velocities. During nighttime, the contribution from the vertical velocity (V p ) and Hall conductivity weighted winds (U L H ) to the vertical electric field that is expressed in terms of zonal plasma drift velocity (v) in equation (7) may be neglected [Eccles, 1998]. In this case, the plasma zonal velocity at the apex location equals the Pedersen conductivity weighted zonal winds [Martinis et al., 2003] and the zonal plasma drift velocity tends to be equal to that of the neutral wind at altitudes where the Pedersen conductivity is higher and ion drag is negligible [e.g., Sobral et al., 2009; Chapagain et al., 2012]. However, in the early evening hours, the experimental values of the bubble velocities are smaller than the local neutral wind model and the plasma drift model velocities because the plasma bubble is in its development phase and likely has different motions from the background plasma drift. These results are an excellent agreement with a recent study from the coincident observations of airglow depletions and thermospheric neutral winds by Chapagain et al. [2012] from the Brazilian sector. They have investigated the early evening discrepancy of EPB drifts from the neutral winds illustrating that the F region dynamo is not fully developed, while after about 22:00 LT, the agreement between the EPB drift velocity and the neutral winds suggests that the F region dynamo is fully activated and hence the drift velocity of the EPB indicates the background motion. [34] The local time dependence of the average bubble velocities from Ascension Island and Alcantara, Brazil are similar and follow the same trend of the model results as shown in Figure 5. However, the differences seen in the postmidnight results between the drift velocities measured in the South American and Atlantic sectors compared to Christmas Island are possibly due to the latitudinal and longitudinal variations of the zonal drift velocities as reported by previous studies [e.g., Martinis et al., 2003; Immel et al., 2004; Sahai et al., 2004; Liu et al., 2009; Pautet et al., 2009; Chapagain et al., 2011]. These studies have also reported that the plasma flows faster near the magnetic dip equator than at higher latitudes. Martinis et al. [2003] and Pimenta et al. [2003a] have also shown that the ion drag from the equatorial ionosphere anomaly can cause the thermospheric neutral winds and, therefore, the plasma drift Figure 9. Model plasma drift velocity on corresponding apex altitudes of 600 km, 900 km, and 1200 km on 4 5 April of12

10 velocities to decrease in the anomaly region. Faster flow at the magnetic equator, compared to off of the magnetic equator, is also consistent with the westward tilt with altitude/latitude typical of EPB observations. Since the sites used to estimate the drift velocities in the South American and Atlantic sectors presented in Figure 5 lie in the region of the equatorial anomaly while Christmas Island is at the magnetic equator, it is likely that this is the explanation for the discrepancies seen in the postmidnight bubble velocities reported here. Moreover, such longitudinal variability in the bubble velocity also illustrates the possibility of contribution to the bubble propagation by the localized changes in the F region dynamo. [35] Turning to the shear motion of the plasma bubble calculated for 4 5 April during 23:30 02:00 LST, we note that the bubbles exhibit a maximum value of 0.12 m/s/km around 01:00 LST. Mendillo and Tyler [1983] reported such shear velocities of 0.022, m/s/km, m/s/km, and m/s/km for apex altitudes of km. These shears are smaller than our Ascension result, but are consistent with the drift model results presented above of m/s/km. Using airglow measurements of EPBs made from Hawaii, Makela and Kelley [2003] have also estimated the bubble velocities corresponding to apex altitudes of 500 km, 700 km, and 900 km. They found a shear motion of 0.1 m/s/km at around 22:30 LT, which is consistent with our Ascension result. However, neither of the two previous reports of the shears showed westward velocities at the lowest altitudes. [36] The unusual shear motion of the EPB on 4 5 April results in the eastward tilt of the structure, as shown in Figure 2 (bottom panel), contrary to the trend of westward tilts commonly referred to the literature [e.g., Woodman and LaHoz, 1976; Abalde et al., 2001; Makela and Kelley, 2003]. The westward reversal of the plasma bubble velocity at low latitudes (i.e., low apex altitude) during quiet geomagnetic conditions is most likely the result of either a reversal in the F region dynamo or from an increase in the altitude of the shear node in the nighttime ionospheric plasma drift. The competing E and F region dynamos create the shear winds, which are directed in opposite directions near sunset [Haerendel et al., 1992]. At altitudes above the F peak, the field line integrated Pedersen conductivity is dominated by the F region, so the F region dynamo controls the drifts. On the other hand, the E region dynamo dominates below the F peak. The altitude of the shear rises and falls with the F region ionosphere. This results in the characteristic C shape observed in radar measurements of equatorial spread F structures [e.g., Woodman and LaHoz, 1976]. [37] The OI (630.0 nm) airglow layer is between 220 and 270 km and, therefore, the apex altitudes being observed typically lie above the shear node at the magnetic equator and the observed depletions drift eastward. However, if the shear node increases in altitude to lie above the airglow region, the observed depletion would move westward. Ascension Island lies in the region of equatorial anomaly and the airglow layer is located in the altitude region of the west-east shear drifts, which probably would be the reason for the westward bubble motion seen closer to the magnetic equator (lower apex altitudes). Taylor et al. [1997] have also reported the westward motion of the plasma bubbles from Alcantara, Brazil during geomagnetically moderate conditions and argued that the drift reversal was possibly due to the change in the altitudinal profile of the zonal drift, with a strong eastward flow at F region altitudes and weaker westward flow below the F ledge. Valladares et al. [2002] reported that latitudinal gradients in neutral winds could also be responsible for the effects observed in their comparisons of equatorial zonal winds and scintillations drifts. Winds do indeed have latitude dependence due to ion drag forces imposed by the latitude structure of the equatorial ionospheric anomaly and could be a contributing factor to the results presented here. Unfortunately, neutral wind measurements were not available for the period reported here. Sobral et al. [2009] have also reported shear in the plasma zonal velocities with heights from the comparison of airglow depletion and GPS zonal velocities. It is important to note that such altitudinal gradients (shears) could correspond to a latitudinal gradient in the plasma zonal velocities. [38] Heelis et al. [1974] also modeled the height variation of the east-west drifts. Their results show large shears in the zonal drifts at 23:00 and 24:00 LT, with the eastward drifts decreasing slowly with altitude above the F region peak, and westward drifts at lower altitudes. Watanabe and Kondo [2011] show DE-2 satellite data indicating that the shear in the zonal plasma drift velocity occurs below 600 km altitude over the dip equator with a maximum value of 100 m/s at 600 km. Our measurements of the shear motions (westward) also correspond to the same altitude range (apex altitudes km). Finally, Huba et al. [2009] have reported from simulation work using HWM93 and HWM07 that the dynamics and morphology of ESF bubbles depend strongly on the zonal neutral wind. They point out that the plasma bubble is distorted in the west-east direction because of the effect of the neutral wind and Pedersen conductivity. For the constant neutral wind case they present, the bubble is vertical up to 600 km and then has a westward tilt. They further explained using HWM07 that the bubble has an eastward tilt at low altitude (below 500 km) similar to our result of the eastward tilt of the bubble observations. [39] Several previous studies have reported the westward reversal of the plasma bubble zonal velocities being caused by significant changes in ionospheric conductivity when a Hall electric field is induced by prompt penetration electric field associated with storm development [e.g., Haerendel et al., 1992; Abdu et al., 2003; Sheehan and Valladares, 2004]. However, our present study occurs during magnetic quiet conditions and the westward reversal is at around local midnight. Sobral et al. [2011] have presented an example of westward reversal of a plasma bubble in a climatological study from Brazil and suggested that midnight is the center time of the westward events as is the case of our Ascension Island results. They also pointed out that eastward and westward traveling bubbles can coexist in the OI (630.0 nm) image and such occurrence is due to the motion of the different flux tubes embedded in the airglow emitting layers. 6. Conclusions [40] We have presented OI (630.0 nm) airglow measurements of plasma bubble motion from Ascension Island and compared them to model results of the ambient plasma drift velocities, Pedersen conductivity weighted neutral winds, and local neutral winds. Plasma bubble onsets occurred inside the FOV of the camera from 19:15 20:00 LST over Ascension Island. 10 of 12

11 [41] The plasma bubble zonal velocity exhibits significant nighttime variability with the average peak value of m/s prior to local midnight, decreasing as expected around midnight through the postmidnight period to a minimum value of 10 m/s. The calculation of the ambient plasma zonal drift velocities based on formalisms developed by Haerendel et al. [1992] and Eccles [1998] with the use of the HWM93, NRLMSISE-00, IRI07, and IGRF models is in good agreement with the experimental results from airglow measurements. [42] The result from a one-night case study (4 5 April 1997) illustrates clear evidence of the shear velocity of a bubble structure. The magnitude of the shear velocity is up to 0.12 m/s/km with reversal from westward drift (20 m/s) at lower latitudes and eastward drift (up to 45 m/s) at higher latitudes. Consequently, the bubble rotates counterclockwise and tilts eastward significantly. The actual cause of this shear motion is yet to be confirmed, however, the westward reversal of the plasma bubble motion at lower latitudes (low apex altitudes) in a quiet geomagnetic condition probably results from a reversal in the F region dynamo or an increase in the altitude of the shear node in the nighttime F region plasma drift to altitudes above the peak altitude of the OI (630.0 nm) emission. [43] To corroborate such a latitudinal dependence of the plasma depletions, optical data from multiple stations with latitudinal overlapping FOV are needed. Latitudinal measurements of the neutral winds as well as plasma velocities would also be needed. The ongoing Remote Equatorial Nighttime Observatory of Ionospheric Regions (RENOIR) project currently being undertaken in Brazil [Makela et al., 2009; Meriwether et al., 2011; Chapagain et al., 2012] and the latitudinal chain of Fabry-Perot interferometers currently operating in Peru will hopefully provide such measurements needed to study the latitudinal dependence of the motions in the nocturnal low-latitude ionosphere-thermosphere system. [44] Acknowledgments. These research measurements were made as part of a collaborative program initiated by E. J. Weber of the Air Force Research Laboratory (AFRF), Hanscom AFB, MA, to whom we are most grateful. The data analysis was performed as part of N. P. Chapagain s Ph. D. dissertation at Utah State University while the subsequent modeling study was supported under a NASA Living With a Star Heliophysics Postdoctoral Fellowship, administered by the University Corporation for Atmospheric Research. 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