Optical observations of the growth and day-to-day variability of equatorial plasma bubbles

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113,, doi:10.1029/2007ja012661, 2008 Optical observations of the growth and day-to-day variability of equatorial plasma bubbles J. J. Makela 1 and E. S. Miller 1 Received 19 July 2007; revised 26 October 2007; accepted 20 December 2007; published 19 March 2008. [1] A new narrow-field ionospheric imaging system, the Portable Ionospheric Camera and Small-Scale Observatory, has been installed at the Cerro Tololo Inter-American Observatory near La Serena, Chile (geographic 30.17 S, 289.19 E; geomagnetic 16.72 S, 0.42 E). We present observations of the naturally occurring nightglow emission at 630.0 nm on three consecutive nights demonstrating the day-to-day variability in the occurrence of equatorial plasma bubbles or depletions. On two nights, large-scale undulations with a wavelength on the order of 300 600 km are observed in the emission regions magnetically connected to the bottomside of the equatorial F layer. We demonstrate that at each crest of these large-scale waves, zero, one, or multiple depletions may grow. Thus, the presence of a large-scale wave on the bottomside alone is not sufficient for irregularity growth. This variability is presumably due to the presence, or lack, of small-scale seed waves or some other mechanism needed to increase the instability growth rate past the critical threshold. Citation: Makela, J. J., and E. S. Miller (2008), Optical observations of the growth and day-to-day variability of equatorial plasma bubbles, J. Geophys. Res., 113,, doi:10.1029/2007ja012661. 1. Introduction [2] Optical observations of the low-latitude, nighttime ionosphere/thermosphere system have been carried out in various locations over the past several decades and have greatly increased our understanding of the dynamics of this region of the Earth s atmosphere. A primary focus of these optical studies over the years has been the low-latitude phenomenon commonly referred to as equatorial spread F [see Makela, 2006, and references therein]. This family of instabilities is generated at, or near, the magnetic equator in the postsunset ionosphere and grows in altitude with time. As they grow in altitude, the instabilities expand poleward along the magnetic field lines, affecting a large region of the low-latitude ionosphere. These regions appear as depletions in optical intensity and in the airglow literature are commonly referred to as equatorial plasma bubbles (EPBs), or simply depletions. Equatorial spread F is an important manifestation of space weather at low latitudes since critical satellite-based navigation and communication systems are severely degraded because of scintillation effects during these periods. A major push of the space weather community has been to gain a better understanding of the properties of these instability regions, leading to better prediction or mitigation schemes. [3] The major advantage of using optical observations versus other techniques for studying EPBs (e.g., radar, GPSderived parameters, in situ satellite measurements) is that 1 Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA. Copyright 2008 by the American Geophysical Union. 0148-0227/08/2007JA012661 the two-dimensional spatial/temporal properties of the EPBs in the local ionosphere can be directly observed. Apart from scanning radar measurements, such as those made using the ALTAIR UHF/VHF radar [e.g., Tsunoda et al., 1979; Hysell et al., 2006] or the VHF Equatorial Atmosphere Radar (EAR) [e.g., Yokoyama and Fukao, 2006], optical imaging is the only single-instrument technique that can observe these properties. This is especially true of EPBs that occur later in the evening, as the small-scale (several meter) irregularities required to produce coherent radar backscatter decay, while the large-scale depletions imaged by optical instruments remain until sunrise, when solar production resumes and the depletions are filled in. [4] In this paper, we present the first optical observations made with the Portable Ionospheric Camera and Small- Scale Observatory (PICASSO) at the Cerro Tololo Inter- American Observatory (CTIO) near La Serena, Chile (geographic 30.17 S, 289.19 E; geomagnetic 16.72 S, 0.42 E). Observations from three consecutive nights are presented and demonstrate the day-to-day variability in the growth of EPBs. We highlight the advantage of having a wide longitudinal field of view for simultaneously observing multiple wavelengths of bottomside undulations similar to those seen in ALTAIR radar data observations of electron density prior to the onset of equatorial spread F [Tsunoda and White, 1981; Hysell et al., 2006]. Thus, we demonstrate the capability of optical observations in studying properties of EPB generation and growth hitherto only possible using scanning radars. Such a capability is an important step in space weather research because optical observations are relatively inexpensive to carry out in comparison to radar observations. Furthermore, optical instruments can more easily be operated for long periods of time, allowing for 1of7

Figure 1. (a) Field of view mapped to an assumed emission height of 250 km. (b) Field of view mapped to apex coordinates. The contours show the amount of geometric blurring caused by looking across multiple magnetic field lines as described by Tinsley [1982]. studies of the long-term trends of EPB occurrence and variability. 2. Instrumentation [5] The PICASSO narrow-field imager installed at CTIO is oriented northward such that the optical axis is approximately parallel to the geomagnetic field at F-region altitudes, as suggested by Tinsley [1982]. Although the magnetic latitude of CTIO is slightly lower than the ideal locations (17 to 23 magnetic) determined by Tinsley [1982], this location provides a better view of the magnetic field lines connected to the bottomside of the equatorial ionosphere. This is an important advantage when observing the potential seed waves and early development of ionospheric instabilities, as discussed below. [6] To select a specific wavelength for study, PICASSO employs a five-position filter wheel. We concentrate on data collected using only the 630.0-nm filter in this paper. This + emission is due to the dissociative recombination of O 2 [Link and Cogger, 1988], and the peak of the volume emission rate occurs below the peak in the ionospheric electron density. The assumed emission heights used in studies employing the 630.0-nm emission range from 250 to over 300 km. On the basis of modeling of this emission, using NRLMSISE-2000 [Picone et al., 2002], IRI2000 [Bilitza, 2001], and the solar conditions representative of this time, we have assumed an emission height of 250 km. The model shows that this assumed height varies by approximately 10 km over the course of the evening. The departure of the actual airglow layer from this height, in addition to the layer having a finite thickness, contributes to an error when analyzing the spatial scales present in the images. On the basis of further modeling, we have found that the spatial scales change by as much as 15% for a 50-km departure from the assumed airglow layer. This effect is purely geometric and affects the lower elevation angles most acutely. [7] Given the field of view of the imager system and an assumed emission layer height, images may be projected to geographic or geomagnetic coordinates. Furthermore, assuming that the magnetic field lines are perfect conductors in complete isolation, images from the geomagnetic coordinate system may be mapped along the magnetic field lines (using the Altitude Adjusted Corrected Geomagnetic Coordinate System (AACGM) magnetic field model) to the field line apexes at the geomagnetic equator. Geographic and apex coordinates for the PICASSO field of view at Cerro Tololo are shown in Figures 1a and 1b, respectively. As can be seen, the field of view covers over 18 in magnetic longitude, or approximately 2000 km. This is a slightly larger zonal field of view than that covered by the ALTAIR radar observations of Hysell et al. [2006] and several times larger than that covered by the EAR radar. This is advantageous when studying the zonal characteristics of the growth of EPBs and their relationship to the large-scale wave structure (LSWS) reported by Tsunoda and White [1981] and Tsunoda [2005], which has a nominal wavelength of 400 km. [8] The charge-coupled device (CCD) used by PICASSO is an array of 2184 1472 pixels and is cooled to 30 Cto reduce the noise caused by dark current (a dominant noise source in the images). Exposures are 90 s, and dark images are taken frequently to remove noise and read-out biases. For this work, the images are binned on chip by a factor of 3 to improve SNR, yielding a spatial resolution of approximately 0.5 km at the center and approximately 1.0 km at the edges. In addition to the optical resolution of the imager, the observation geometry limits the effective resolution by blurring features observed across magnetic field lines. Following Tinsley [1982], the spatial misalignment in kilometers due to both dip and declination is plotted over the entire field of view in Figure 1b. Without further interpretation, the spatial misalignment should be considered only a relative measure of the resolution deterioration. 3. Data Presentation 3.1. General Observations [9] We present data collected by the PICASSO system at CTIO for three consecutive nights: 14 15, 15 16, and 16 17 September 2006. Observing conditions were excellent during this period, with clear skies and the Moon down. Approximately 7 to 9 h of data at 630.0 nm were collected on each night, corresponding to approximately 200 to 250 images on each night. These nights were chosen as mag- 2of7

Figure 2. Keogram representation of 630.0-nm optical data collected on three consecutive nights. LT equals UT minus 4 h. netically quiet conditions were prevalent. The only minor activity seen in Kp (Kp >2) occurred after the last observing period on 17 September 2006. Therefore, the period is one in which we can study day-to-day variability of the occurrence of EPBs during the region s spread F season [e.g., Gentile et al., 2006] without having to consider any stormtime effects. [10] A summary of the data collected on these three nights is presented in Figure 2. To create Figure 2, the images from the entire night are processed with an asymptotically optimal blind estimation (denoising) technique to further improve the signal-to-noise ratio similar to that described by Atkinson et al. [2006]. Stars are also removed from the original images by using a point suppression algorithm [Garcia et al., 1997]. Each individual 630.0-nm image is then flat fielded before it is projected to apex coordinates. After processing and projecting, the intensities from each individual image are taken for all apex altitudes at a magnetic longitude equal to the observatory s to create a Keogram representation. The image intensities are reported in terms of analog-to-digital units (ADUs, or counts) rather than in absolute intensities because of the lack of a standard calibration light source. However, absolute calibration is not required for this study because we are interested in the relative spatial extent of the features in the airglow, not the emission chemistry. [11] Although we lose much of the spatial information present in the images by creating this Keogram representation, it serves as a good overview of activity on each night. Several general characteristics of the nighttime 630.0-nm emission are seen in these three plots. Although the background intensities vary significantly on each night, brighter intensities are observed closer to sunset because of the recombination of the postsunset ionosphere. A secondary intensity maximum is also seen on each night near or after local midnight. The striking features, and the subject of this paper, are the vertical depletions, or EPBs, seen on 14 15 and 15 16 September 2006 but notably lacking on 16 17 September 2006. 3.2. Night of 14 15 September 2006 [12] On the night of 14 15 September 2006, four nearly evenly spaced, fully-developed depletions (labeled B-E) are seen in Figure 2 between 2200 and 0130 LT, implying an average temporal spacing of about 70 min. Each depletion is an individual depletion, by which we mean that there is a single depleted channel puncturing from the bottomside to the topside of the F region (although that single channel 3of7

may bifurcate above the bright F region, as seen in the last depletion at 0130 LT). [13] Four individual frames from 14 15 September 2006 are presented in Figure 3 with the four depletions seen in the Keogram of Figure 2 labeled. It is clear that the wide longitudinal field of view allows us to simultaneously track multiple depletions, up to three at a time. This is a great advantage when trying to understand the properties of these structures, as assumptions about spacings and dynamics do not need to be made, as the depletions remain in the field of view for several hours at a time. [14] The first image presented, from 14 September 2006 at 1956 LT, has contours of constant intensity overlaid on the image. These show that there are two uplifts separated by approximately 600 km at this time. The uplift labeled A is much larger than uplift B, and as seen in Animation 1 1, several depletions develop out of this region over the next hour before drifting out of the field of view of the imager. These depletions in uplift A grow upward at a velocity on the order of 100 m/s, although this value has an uncertainty of at least 50 m/s due to the spatial blurring discussed with Figure 1b. These depletions develop as they approach the edge of the imager s field of view, and so we do not analyze them further in this paper. [15] By studying data from the remainder of night, we find that the spacings of the four single depletions (B-E) on 14 15 September 2006 range from 300 to 600 km and remain approximately constant as each set of depletions drifts past CTIO. Assuming that the depletions are drifting at the same velocity as the background ionosphere, this implies that the entire ionosphere in this region has approximately the same eastward velocity, approximately 80 ± 20 m/s over the course of this night. Comparisons to satellite data [Makela et al., 2005] have shown that under quiet conditions, this assumption holds. This assumption is also used to study the larger spatial scales separating the structures since the depletions that are observed later in the night (B-E) drift into our field of view fully formed, having developed earlier in the evening to the west. However, the wider longitudinal field of view of PICASSO allows us to directly observe the spacing between individual depletions, which was not always the case in the earlier studies from Hawaii. 3.3. Night of 15 16 September 2006 [16] On the night of 15 16 September 2006, we observe two discrete packets of depletions, the first around 0000 LT and the second at 0200 LT. In contrast to the previous night, the earlier packet has multiple depleted channels puncturing the F region. Similar to the previous night, there is still a clear tendency for regions of instability (the depleted regions) to be separated by large regions where the ionosphere is stable (the period between 0030 and 0200 LT). We refer to the depletions as being regions of instability in the sense that only radio waves traveling through the depleted regions experience scintillation effects [e.g., Kelley et al., 2002; Ledvina and Makela, 2005]. Radio waves that pierce the ionosphere just outside of the depletions are generally not affected. 1 Animations 1 3 are available in the HTML. [17] The occurrence of multiple depleted channels implies that there may be multiple scales at work during the seeding of the instabilities. The large-scale wavelength of 400 600 km on this night determines regions that can and cannot become unstable. Within the regions that are primed for instability growth by this large-scale wave, there must be a smaller-scale seed that enhances the growth rate to the point where it crosses a threshold and the instability grows into the elongated depletions observed with PICASSO. In the case of Figure 4 (bottom), the two large depletions within the unstable region denoted by 3 are separated by approximately 150 km, or about one third of the large-scale wavelength discussed above. [18] Not every region that is primed by the large-scale wave will necessarily grow into a full-fledged EPB. This is shown in Figure 4 where we present images from early in the evening of 15 16 September 2006. We have again included contours of constant intensity to emphasize variations in the bottomside of the ionosphere which we claim are indicators of the large-scale wave. Three uplifts in the bottomside are seen, separated by approximately 400 600 km. Unlike the previous night s large uplift labeled A, as time progresses the uplifts labeled 1 and 2 do not develop further during the 2 h they remain in the imager s field of view. On the other hand, the uplift labeled 3 grows into the large depletion seen in Figure 2 to pass the observing site at 0000 LT. Animation 2 gives additional clarity to this description. 3.4. Night of 16 17 September 2006 [19] In Figure 5 we present two images taken from 16 17 September 2006, the night on which no large-scale depletions were observed. Interestingly, there is a large-scale undulation seen in the image at 2011 LT with a significantly longer wavelength than seen on the previous nights and a smaller amplitude than seen on 14 15 September 2006. Careful examination of the raw data (as presented in Animation 3) suggests that there are structures on the bottomside in this region; however, no large-scale depletions are seen to grow through the F region. As the growth rate of the instability is expected to be largest for steep gradients at higher altitude in the equatorial ionosphere [Kelley, 1989], the shallow gradient seen on the bottomside of the ionosphere (compared to 15 16 September 2006) combined with the lack of a strong uplift (compared with 14 15 September 2006) could account for the lack of depletion development on this night. 4. Discussion and Conclusions [20] We have presented the first results from a new narrow-field imaging system located in Chile. The three nights examined illustrate the day-to-day variability associated with equatorial irregularities during the region s spread F season. In two cases, large-scale depletions are seen to occur in a quasiperiodic fashion, persisting into the early morning. It is interesting to note the similarities between the Keograms shown here and the composite images presented by Makela and Kelley [2003, Figure 2] for observations made using the Cornell Narrow-field Imager (CNFI) on the Haleakala Volcano on Maui, Hawaii. In the Hawaii example, four packets are seen, fairly evenly spaced with a 4of7

Figure 3. Individual apex-mapped images from the night of 14 15 September 2006. Individual depletions are labeled (A E) to help in tracking them from frame to frame and for comparison with Figure 2. Note that each image has been processed with a histogram equalization routine, so one cannot compare intensities from image to image. wavelength of approximately 620 km. From the results of 14 15 and 15 16 September 2006, we have shown that it is possible to have both single and multiple plumes occur within a single wavelength of this large-scale wave. [21] We suggest that our observations of the spatial distribution of the developing and developed depletions should directly correspond to the underlying distribution of any seeding mechanism. That is, a seed mechanism that has a given spatial wavelength should develop into depletions with the same wavelength. The periodicity of the depletions suggests that some mechanism may be ordering the postsunset ionosphere into regions that are and are not Figure 4. Individual, apex-mapped images from the night of 15 16 September 2006. Contours are provided on the top two images to emphasize the labeled undulations seen on the bottomside of the F layer. Figure 4 (bottom), from later in the night, shows multiple depletions that have grown out of the single uplifted region denoted by 3. Note that each image has been run through a histogram equalization routine, so one cannot compare intensities from image to image. Because of the change in intensity levels on this night, different contour levels are used than in Figure 3. 5of7

Figure 5. Individual, apex-mapped images from the night of 16 17 September 2006. Contours are provided to emphasize the bottomside of the F layer. The same contour levels are used as in Figure 4. conducive to instability growth. Other studies have reported periodicity of equatorial irregularity regions ranging from 200 to 700 km [e.g., Rottger, 1973; Hysell et al., 1990], attributing the periodicity to the possibility of gravity wave seeding. Such a seeding could account for the discrete regions of instability that we have observed from Hawaii and now from Chile. However, recent modeling results suggest preferred horizontal wavelengths for gravity waves penetrating to the thermosphere between 40 and 250 km, smaller than the wavelengths observed here [Vadas and Fritts, 2004]. [22] The recent ALTAIR results from Hysell et al. [2006] show the occurrence of large-scale (200 km) undulations on the bottomside in which smaller-scale waves (30 km) develop into full-blown spread F structures. Kudeki et al. [2007] present nonlocal modeling results showing that the fastest growing mode has a horizontal wavelength of 20 40 km. Thus, the distance between individual depletions within a single uplift region seen in our 15 16 September 2006 data, approximately 150 km, is larger than that quoted by the Hysell et al. [2006] and Kudeki et al. [2007] studies. However, it is important to notice that the width of each individual depletion, which should correspond more closely to the actual width of the underlying unstable region, is approximately one-half this value, or 75 km. [23] It should be noted that not every night with depletions exhibits the clear periodicity shown on 14 15 and 15 16 September 2006. In our ongoing observations from Chile (not shown), we have also observed isolated structures as well as more densely packed clusters of depletions, suggesting that a large-scale seed wave may not be strictly necessary for the development of EPBs. These cases will be studied in more detail in future work as we strive to come to a better understanding of the causes for the different characteristics seen. [24] In this paper, we have studied new optical observations made with the PICASSO narrow-field imaging system located at CTIO. The images obtained with this system show that multiple scales can play a role in the development of equatorial plasma bubbles. Specifically, we see evidence for large-scale undulations, on the order of 300 600 km in the cases presented here, in which bubbles are seen to develop. We suggest that these may be the optical signature of the LSWS discussed by Tsunoda [2005]. The smallerscale perturbations (75 km) operate within the regions primed by the large-scale undulations, likely increasing the growth rate past a critical threshold and determining where individual bubbles develop. Future work will focus on a more in-depth analysis of the scale sizes seen in the optical data and comparing the information gleaned from this data set to other complementary data sets collected in the region (e.g., GPS scintillation monitors and the Jicamarca radar). [25] Acknowledgments. We thank Ian Atkinson for providing code to denoise the images presented in this paper and the staff at the Cerro Tololo Inter-American Observatory for their support in the maintenance of the PICASSO instrument. We also thank Scientific Solutions, Inc. for providing the infrastructure to operate PICASSO while at CTIO. This work was supported by grants from the National Science Foundation (ATM- 0517641) and the Naval Research Laboratory (N00173-05-1-G904). [26] Amitava Bhattacharjee thanks Joseph Huba and another reviewer for their assistance in evaluating this paper. References Atkinson, I., F. Kamalabadi, S. Mohan, and D. L. Jones (2006), Asymptotically optimal blind estimation of multichannel images, IEEE Trans. Image Process., 15(4), 992 1007. Bilitza, D. (2001), International Reference Ionosphere 2000, Radio Sci., 36(2), 261 275. Garcia, F., M. Taylor, and M. Kelley (1997), Two-dimensional spectral analysis of mesospheric airglow image data, Appl. Opt., 36(29), 7374 7385. Gentile, L. C., W. J. Burke, and F. J. Rich (2006), A climatology of equatorial plasma bubbles from DMSP 1989 2004, Radio Sci., 41, RS5S21, doi:10.1029/2005rs003340. Hysell, D. L., M. C. Kelley, W. E. Swartz, and R. F. Woodman (1990), Seeding and layering of equatorial spread F by gravity waves, J. Geophys. Res., 95(A10), 17,253 17,260. Hysell, D., M. Larsen, C. Swenson, A. Barjatya, T. Wheeler, T. Bullett, M. Sarango, and R. Woodman (2006), Rocket and radar investigation of background electrodynamics and bottom-type scattering layers at the onset of equatorial spread F, Ann. Geophys., 24(5), 1387 1400. Kelley, M. C. (1989), The Earth s Ionosphere: Plasma Physics and Electrodynamics, Int. Geophys. Ser., vol. 43, 487 pp., Academic, New York. 6of7

Kelley, M. C., J. J. Makela, B. M. Ledvina, and P. M. Kintner (2002), Observations of equatorial spread-f from Haleakala, Hawaii, Geophys. Res. Lett., 29(20), 2003, doi:10.1029/2002gl015509. Kudeki, E., A. Akgiray, M. Milla, J. Chau, and D. Hysell (2007), Equatorial spread F initiation: Postsunsent vortex, thermospheric winds, gravity waves, J. Atmos. Sol. Terr. Phys., 69(17 18), doi:10.1016/j.jastp. 2007.04.012. Ledvina, B. M., and J. J. Makela (2005), First observations of SBAS/ WAAS scintillations: Using colocated scintillation measurements and all-sky images to study equatorial plasma bubbles, Geophys. Res. Lett., 32, L14101, doi:10.1029/2004gl021954. Link, R., and L. L. Cogger (1988), A reexamination of the O I 6300-Å nightglow, J. Geophys. Res., 93(A9), 9883 9892. Makela, J. J. (2006), A review of imaging low-latitude ionospheric irregularity processes, J. Atmos. Sol. Terr. Phys., 68(13), 1441 1458, doi:10.1016/j.jastp.2005.04.014. Makela, J. J., and M. C. Kelley (2003), Field-aligned 777.4-nm composite airglow images of equatorial plasma depletions, Geophys. Res. Lett., 30(8), 1442, doi:10.1029/2003gl017106. Makela, J. J., M. C. Kelley, and S.-Y. Su (2005), Simultaneous observations of convective ionospheric storms: ROCSAT-1 and ground-based imagers, Space Weather, 3, S12C02, doi:10.1029/2005sw000164. Picone, J. M., A. E. Hedin, D. P. Drob, and A. C. Aikin (2002), NRLMSISE-00 empirical model of the atmosphere: Statistical comparisons and scientific issues, J. Geophys. Res., 107(A12), 1468, doi:10.1029/2002ja009430. Rottger, J. (1973), Wave-like structures of large-scale equatorial spread-f irregularities, J. Atmos. Terr. Phys., 35(6), 1195 1206. Tinsley, B. A. (1982), Field aligned airglow observations of transequatorial bubbles in the tropical F region, J. Atmos. Terr. Phys., 44(6), 547 557. Tsunoda, R. T. (2005), On the enigma of day-to-day variability in equatorial spread F, Geophys. Res. Lett., 32, L08103, doi:10.1029/2005gl022512. Tsunoda, R. T., and B. R. White (1981), On the generation and growth of equatorial backscatter plumes: 1. Wave structure in the bottomside F layer, J. Geophys. Res., 86(A5), 3610 3616. Tsunoda, R., M. Baron, J. Owev, and D. Towle (1979), Altair: An incoherent scatter radar for equatorial spread F studies, Radio Sci., 14(6), 1111 1119. Vadas, S., and D. Fritts (2004), Thermospheric responses to gravity waves arising from mesoscale convective complexes, J. Atmos. Sol. Terr. Phys., 66(6 9), 781 804, doi:10.1016/j.jastp.2004.01.025. Yokoyama, T., and S. Fukao (2006), Upwelling backscatter plumes in growth phase of equatorial spread F observed with the Equatorial Atmosphere Radar, Geophys. Res. Lett., 33, L08104, doi:10.1029/ 2006GL025680. J. J. Makela and E. S. Miller, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, 316 CSL, Urbana, IL 61801, USA. (jmakela@uiuc.edu, esmiller@uiuc.edu) 7of7