Radar results
Figure 1 shows the range-time-intensity (RTI) maps of the F-region echoes detected by AMISR-14 on each night of the observation campaign. The RTI maps are for the beam pointed vertically and show that F-region echoes occurred on most nights after July 22. In particular, vertically developed echoing structures that started late in the night were observed on July 24–25, July 27–28, and August 3–4. Coincidentally, the observations were made during moon phases that are adequate for nighttime airglow measurements. The moon phase varied from Third Quarter on July 26 to New on August 2.
The RTI maps in Fig. 1 showed echoing structures starting much later than typical ESF. Typical ESF echoes start between 19:00 and 20:00 LT and last until about 23:00–24:00 LT, depending on season and solar flux conditions (Hysell and Burcham 2002; Chapagain et al. 2009; Smith et al. 2015). The echoing structures observed on July 24–25, July 27–28, and August 3–4 first appeared around 22:00 LT, 23:45 LT, and 22:45 LT, respectively. The structures of July 27–28 and August 3–4 lasted several hours after local midnight. The late appearance of echoes provide an opportunity to use airglow measurements to investigate the F-region conditions in the magnetic equatorial region leading to ESF development. Two-dimensional AMISR-14 observations using beams pointed in multiple directions in the magnetic equatorial plane indicate that the vertically developed echoing structures associated with fully formed ESF events formed around the radar site. Processing of the airglow measurements show good images with mostly clear skies (no clouds) for the three nights cited above.
On geomagnetic activity during observations
Figure 2 shows an expanded version of the RTIs for July 24–25, July 27–28, and August 3–4. In order to provide information about geomagnetic conditions under which the measurements were made, Fig. 2 also shows the temporal variation of the auroral electrojet (AE) index below each RTI map. During and after geomagnetically disturbed conditions, equatorial electric fields can deviate severely from their typical pattern mostly due to prompt penetration and disturbance dynamo effects (Fejer 2011; Fejer et al. 2017). They could drive upward plasma drifts in the nighttime sector, when drifts are typically downward, and destabilize the equatorial F-region. Short time scale (from minutes to 1–2 hours) variations are caused by prompt penetration electric fields of magnetospheric origin, which drive upward plasma drifts from about sunrise to sunset and downward drifts at night. Longer lasting (few hours) plasma drift variations occurring a few hours after the onset of high latitude magnetic disturbances, as indicated by the AE index for example, are due to disturbance dynamo effects driven by enhanced energy deposition into the high altitude ionosphere (Blanc and Richmond 1980; Scherliess and Fejer 1997). Disturbance dynamo effects drive downward plasma drifts during the day and upward drifts at night and can last up to about 30 hours after geomagnetic active times (Fejer et al. 2017).
In order to provide more information about the F-region conditions over Jicamarca during our measurements, we also show the height of the F-region peak (hmF2) and the virtual height of the bottomside F-region (h’F) as estimated by a collocated digisonde. The digisonde measurements show clear but modest increases in h’F and hmF2 around 21:00 LT on July 24 and 27. A sudden increase in h’F and hmF2 can also be noticed a few minutes prior to detection of the first echoes on July 27. This rapid rise in F-region height is likely to be caused by prompt penetration electric fields associated with the surge in auroral activity that can be seen in the AE index starting around 23:00 LT.
The digisonde measurements show only a small increase in h’F and hmF2 around 20:00 LT on August 3 compared to previous days. While weaker upward F-region uplifts were observed on this day, the layer did not return to lower heights (h’F ∼ 250km) as it was observed on the other 2 days. This behavior could have been driven by disturbance electric fields associated with large AE indices observed between 16:00 LT on August 2 and 14:00 LT on August 3 (not shown here) and the AE surge around 19:30 LT on August 3.
It will be shown, later on this report, that irregularity drifts were westward at the beginning of the ESF event on this night, which confirms the effects of disturbance electric fields.
For the sake of completeness, Fig. 3 shows the temporal variation of the AE index for all the days of our observation campaign. Note that we show the AE index starting at 00:00 LT of the day when observations started. It serves to show that high-latitude geomagnetic disturbances were present during the period of the campaign, which could have driven low-latitude disturbance electric fields. These fields could have provided conditions that were favorable for the development of at least some of the June solstice ESF events observed in our campaign. Comparing Figs. 1 and 3, we also find that ESF was observed even during geomagnetically quiet days as well. For instance, the AE index shows quiet conditions on July 30 and July 31 when pre- and post-midnight F-region echoes were observed by AMISR, respectively.
Airglow results
Figures 4, 5, and 6 summarize our comparisons between airglow features and radar observations for July 24–25, July 27–28, and August 3–4, respectively. The top panel in each figure shows the RTI map (vertical beam) for the night being analyzed. Analyses of the multi-beam observations made by AMISR-14 indicate that the observed plumes developed near the radar site and, therefore, are driven by local conditions. An example of the two-dimensional evolution of a plume, as observed by AMISR-14, is shown (Figs. 7 and 8) and discussed later in the text. The bottom panels show the 630 nm airglow images for the times indicated by vertical red lines on the RTI map. The airglow emission is mapped to geographic latitude versus longitude coordinates assuming a mean emission height of 250 km. While this height might not be accurate, it does not affect our analysis and interpretation of the observations. In addition to the two-dimensional images (gray tones), we also show the mean zonal variation of the airglow intensity, around the latitude of the observation site (∼− 12° N), as a red solid line curve. This curve has been added to the images to help the reader to identify the intensity fluctuations we refer to throughout the text. The location of the radar site is indicated by a green “+” sign in the center of the airglow images.
Event 1: July 24–25, 2016
Figure 4 shows our radar-airglow comparisons for July 24–25, 2016. The most striking feature of the first airglow images (top two rows) is the appearance of a faint dark band (region of low airglow emission) aligned in the north-south (NS) direction. We interpret this band as being produced by a small amplitude electron density perturbation in the bottomside F-region. The band precedes the appearance of ESF radar echoes.
At 21:08 LT, this faint dark band is located to the west of the radar site, around −79.5° E. At 21:48 LT, the band is located at − 78° E. Therefore, the sequence of images indicates an eastward motion of approximately 72 m/s.
By 21:56 LT, the single dark band becomes more structured. Two thin dark bands, spaced by less than 150 km in the zonal direction, can now be distinguished in the images (between approximately − 76° and − 78° E). The increase in the amplitude of the airglow dark bands suggests the development of ESF plasma depletions (radar plumes).
The sequence of observations can be interpreted in terms of the processes described by Tsunoda (1983). The initial faint dark band is produced by a small amplitude electron density perturbation in the bottomside F-region. This density perturbation is, presumably, a result of the bottomside height modulation (upwelling) such as those observed by the ALTAIR radar (Tsunoda 1983). With time, the airglow perturbation grows in amplitude indicating an amplification of the bottomside density perturbation. Two strong dark bands start to be observed around 21:56 LT. They are optical signatures of ESF structures that developed in the initial upwelling. The easternmost dark band would be the optical signature of a primary ESF depletion. The second dark band would be the result of a secondary plume that is known to develop in the western wall of the initial bottomside upwellings caused by a wind-driven gradient drift instability. A close look at the RTI map for that night confirms the appearance of a primary plume followed by a secondary echoing structure. The tilt of the bottom portion of the scattering layer in the RTI map also supports the inferences above.
Event 2: July 27–28, 2016
Figure 5 now shows the airglow observations for the night of July 27–28. The format of Fig. 5 is the same of Fig. 4. In this case, the first airglow images show two initial faint dark bands that are, again, well aligned in the NS direction and spaced by about 3° (350 km) in longitude. For instance, at 22:43 LT, one band is located to the west of the radar site, around − 78° E. The other band is located to the east, at about − 75° E.
Like the case of July 24–25, the bands move in the eastward direction. By 23:23 LT, the two bands seemed to have merged producing the single, low-intensity airglow band right above the radar site. Also, the airglow intensity over the entire field of view of the all-sky camera seems to have decreased. This is consistent with the sudden uplift of the F-region shown by the digisonde around 23:15 LT (see h’F for July 27 in Fig. 2). The amplitude of the airglow band also increased.
The radar observations, however, show that no echoes are detected by the vertical beam until about 23:35 LT.
Event 3: August 3–4, 2016
Figure 6 shows our third example of airglow-radar observations. It shows results of measurements made on August 3–4, 2016.
Like in the previous cases, the first images show the occurrence of two faint airglow perturbations. For instance, at 21:41 LT, one perturbation is located around − 77° E and the other is located at around − 73° E. The zonal spacing between the faint airglow depletions is about 4° or 460 km. In this case, one of the perturbations (the easternmost) is already located right above the radar site while no radar echoes were observed. This provides additional experimental evidence indicating that the faint airglow perturbations are not produced by a well-developed ESF structure (bubble or plume) but, like in the previous cases, are optical signatures of ESF precursors. While cases of ionospheric depletions without radar echoes are possible (e.g., Saito et al. 2008), most generally meter-scale irregularities are generated during the turbulent, development phase of plasma bubbles (e.g., Rodrigues et al. 2004). Furthermore, our interpretation of the airglow observations as signatures of ESF precursors is supported by the collocated GPS measurements of the ionospheric total electron content (TEC). Depletions in TEC are also only observed after about 23:00 LT when radar echoes reach the main F-region and topside as discussed later in this report. Measurements of TEC depletions are examined in more detail in the “Discussion” section.
For this event, the sequence of images show a weak westward motion of the airglow perturbations. The observations made on this night followed a surge in high-latitude geomagnetic disturbances as shown in Fig. 2, and as suggested earlier, observations could have been made under disturbed dynamo conditions (Blanc and Richmond 1980; Fejer et al. 2017). Westward motion of the ionospheric irregularities are known to occur during disturbed dynamo (Abdu et al. 2003; Paulino et al. 2010), and therefore, our observations confirm the occurrence of low-latitude disturbances.