Geophysical Research Letters

Simultaneous observations at Darwin of equatorial bubbles by ionosonde-based range/time displays and airglow imaging



[1] Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions.

1. Introduction

[2] Depletions of electron density in the equatorial night-ionosphere and their association with equatorial spread F have been studied by a variety of equipments for some 40 years [Woodman, 2009] but some aspects still remain ambiguous. In particular, different types of equipments respond to different aspects of a depletion while the depletions themselves differ in such properties as their occurrence and magnitude with longitude, season and the sunspot cycle [e.g., Sahai et al., 1988; Abdu, 2001; Chapagain et al., 2009]. Optical airglow observations have been established as a direct method of imaging such depletions [Makela, 2006].

[3] Both experiment and theory (the Raleigh/Taylor instability) have established that depletions are often spawned near the magnetic equator at the pre-sunset height rise peak and subsequent fall associated with the sunset terminator as it moves westward [Kelley, 2009]. The depletions commonly rise rapidly in height and as they rise, spread along the magnetic field lines and thus in geomagnetic latitude [Tsunoda, 1985]. Having formed, the depletions typically drift eastward with the flow of the background ionosphere. The horizontal drift velocity varies with height giving the depletion plume a typical slope to the west corresponding to lower drift velocities with increasing height and/or lower drift velocities at higher equatorial latitudes. This will give the projection of the depletion on the ground a reverse c-shaped curve with latitude as the base of the depletion moves faster to the east at the equator than higher parts of the depletion [Kelley et al., 2003]. The variability of the ionosphere is such that bubbles characteristics may differ greatly from night to night if indeed, the bubbles are present at all. Much effort has gone into trying to identify possible triggers of the Raleigh/Taylor instability and thus bubble formation [e.g., Abdu, 2001; Paulino et al., 2011].

[4] Spread F was originally named after an apparently continuous spread either in the time delay of the first hop trace of an ionogram (range spread) or in the critical frequency of the first hop trace (frequency spread) and both may be present. The earliest example of spread F was made at the equatorial site of Huancayo [Booker and Wells, 1938] and attributed to scattering from ionospheric irregularities.

[5] Satellite traces on ionograms appear as apparent copies of the ionogram first hop trace but at a greater time delay and usually appear to be moving either upward or downward with respect to the overhead trace. In fact, such satellite traces were early identified as being off-angle echoes from sloping electron density contours some distance from the undisturbed overhead ionosphere and moving towards or away from the ionosonde [McNicol et al., 1956; Wright, 1959; King, 1970]. Satellite traces were often found to develop into range-spread F. Occurrence rates and seasonal behaviour were seen to distinguish spread F in equatorial regions from middle latitude spread F [McNicol and Bowman, 1957; Wright, 1959]. Both range and frequency spread F were regarded as more important matters than satellite traces because of their detrimental effect on HF, trans-ionospheric radio transmissions and GPS navigation. The possible sources of the various types of spread F observed with ionosondes became increasingly muddled with time. The discovery of electron depletions in the low latitude ionosphere introduced a new and more persuasive source for the unique features of equatorial spread F.

[6] Ionosonde measurements from Darwin, Australia at the outer edge of the Equatorial Anomaly have previously established that the occurrence of equatorial spread F at this location is highly dependent on the sunspot cycle. At this site, Equatorial Spread F is only present at sunspot maximum when the ionosphere is at its densest and highest. The postulated depletions will reach to their greatest height at this time and thus can expand in latitude to be seen in overhead Darwin ionograms. However until observations reported in this paper were made between all-sky-images and the Darwin ionosonde, the correspondence between the type of spread F event discussed here and the presence of an overhead depletion bubble could not be directly established.

2. Observations

[7] All-sky imagers have operated at Darwin (12.4 S, 131.0 E; magnetic latitude 22 S) in Australia since October 2001 as part of the Optical Mesosphere Thermosphere Imagers (OMTIs) program [Shiokawa et al., 1999]. Two-dimensional images of 630.0-nm airglow intensity were obtained at Darwin every 6 min, 777.4-nm images obtained every 30 min. The exposure time was 165 s for both the 630.0-nm and 777.4-nm images.Otsuka et al. [2002], showed that bubble depletion mapped along field lines to Darwin. Shiokawa et al. [2004]described a burst of consecutive depletions also seen in night airglow observations made with the same Darwin equipment. In both cases, five-minute cadence ionograms were available from the Australian Ionospheric Prediction and Space Services ionosonde located at Darwin.

[8] A range-time display was developed to investigate variations in the base height of the night-time F2 ionosphere, initially using ionograms from Vanimo, New Guinea. Such displays have been previously used to follow variations in the base height of the equatorial F2 at night [Lynn et al., 2006]. A detailed paper describing the manner in which such range-time displays are produced and their application to ionogram observation and measurement is in preparation. Amongst other things, these displays also show off-angle reflections approaching and receding from the ionosonde at F2 heights. On individual ionograms, such reflections appear as moving satellite traces.

[9] Figure 1bshows an example of a range-time display based on five-minute ionograms received at a Darwin ionosonde. Multiple traces are seen approaching and receding from the ionosonde with the reflector passing over the ionosonde at 15:42 UT (00:42 LT). Strong range spread F is observed at this time as the satellite traces merge briefly with the overhead echo. Such characteristic signatures were often seen at night at all the low latitude Australian ionosonde sites where depletions could be expected (Vanimo, Port Moresby, Darwin and, very occasionally, Townsville). When present, events could occur at any time following the peak of the post-sunset height rise visible inFigure 1b, until a few hours before sunrise. Electron depletions in the ionosphere were suspected to be the source of such major off-angle reflections with the associated spread F to be expected as the bubbles passed overhead. Whether the satellite traces could be from ionospheric density depletions remained speculative until examples of optical depletions taken at Darwin became available for comparison [Otsuka et al., 2002; Shiokawa et al., 2004].

Figure 1.

(a) Darwin airglow depletion taken at 15:42UT [Otsuka et al., 2002]. (b) Range/time display showing the drift of the depletion seen by the Darwin ionosonde with the resultant satellite traces and strong range spread F as the bubble structure passes overhead. Times at which sunset (SS) and sunrise (SR) are marked as corresponding to solar zenith angles of 90.8° and 108.6° (i.e. ground level and 350 km height).

[10] Figure 1a shows the optical airglow depletion seen by Otsuka et al. [2002]at 15:42 UT on 12 November 2001. The ionosonde range-time plot for this night is shown inFigure 1band shows the typical signature of a moving reflector passing overhead simultaneously with the passage of the depletion. The time at which the optical depletion was photographed is shown on the ionosonde range-time plot as a solid black vertical line. This was the only optical depletion and ionosonde off-angle occurrence during this night so that there is no doubt that they represent aspects of the same phenomenon. SS1 and SS2 mark the times of sunset atχ = 90.8o (SS1 ground sunset) and χ = 98.5o(SS2 sunset at 350 km) with corresponding values at sunrise for SR2 and SR1.The depletion passed overhead just after midnight local time. The occurrence of satellite traces and range spread F are also indicated and can be compared with representative ionograms from the range-time display which are shown inFigure 2.

Figure 2.

Ionograms taken from Figure 1 showing (a) descending satellite traces between the first and second hop as the depletion approaches, (b) strong range spread F when the depletion is overhead and (c) rising satellite traces as the depletion moves away.

[11] In Figure 2a, the bubble approaches. The off-angle (satellite) traces are seen between the labeled first and second hop traces and are decreasing in range towards the first hop trace, as shown by the arrow.Figure 2bshows what happens when the approaching satellite traces reach the range of the overhead trace indicating the bubble is overhead. At this point the whole ionogram explodes into range-Spread F with multiple reflections from both the edge and interior of the moving bubble.Figure 2c is taken as the bubble is receding. Range spread F has ceased and satellite traces are again visible and moving to greater range as the distance to the bubble increases (shown by the arrow).

[12] Shiokawa et al. [2004], using the same airglow equipment, have published examples of multiple depletions seen at Darwin on the night of November 12, 2001. Three such depletions were seen as shown in the upper part of Figure 3. These three depletions were also seen by the Darwin ionosonde as shown in the range-time plot in the lower half ofFigure 3. Times at which the airglow depletion photos were taken are indicated with lines drawn to associate the individual airglow depletions with the corresponding moving ionosonde reflectors. In Figure 3 there are times when more than one depletion, as well as the overhead echo, was observed by the ionosonde. At such times, both approaching and receding satellite traces were present in the ionogram. The ionosonde field of view typically covered a horizontal range up to some ±500 km from the overhead position, which closely matches that of the optical images [Shiokawa et al., 2004]. However, an ionosonde can only see an off-angle reflection from an electron density gradient which is at right angles to the line-of-sight.

Figure 3.

The three optical depletions seen by Shiokawa et al. [1999] at Darwin and the corresponding range/time ionosonde events.

[13] It is important to notice that sporadic E was absent during the traversal of the depletions shown in Figures 1, 2 and 3. Multiple reflections between sporadic E, the ground and the F2 layer will also produce apparent satellite traces which will move slightly as the relative heights of the sporadic E and F2 change with time.

[14] Drift velocities for the four range-time events were estimated to lie in the range 80–120 m/s which is in good agreement with the value of ∼ 100 m/s given by bothOtsuka et al. [2002] and Shiokawa et al. [2004]. Such drift velocities are typical of those previously reported [e.g., Yao and Makela, 2007].

[15] The ionosonde observations alone do not allow the direction of travel to be determined. The particular associations shown in Figure 3 depend on the direction of drift of the three depletions. The airglow observations determined the drift was eastward. This is the usual direction of drift for such bubbles. Reversal of the eastward drift velocity has been reported associated with magnetic storms [Abdu et al., 2003].

3. Discussion

[16] As a depletion bubble rises, it often splits, forming a tree-like depletion structure. Higher components of the depletion structure map down magnetic field lines further from the magnetic equator in latitude. As a result, what is seen as a single hole in the base of the F2 region at the magnetic equator can appear as a complex north–south oriented group of depletions in the base of the ionosphere at distant sites such as Darwin. The splitting of the depletion structure with height is readily seen in the airglow observations shown inFigures 1 and 3. Off-angle backscatter from this complex structure appears in the ionosonde range-time display as multiple satellite traces surrounding full-blown range spread F.

[17] The off-angle reflections converge at the base of the ionospheric F2, as would be expected if the echoes come from the abrupt gradient in electron density contours at the edges of the bubble structure. There is often a slight upward kink in the base height reflection at this point which again is indicative of a hole extending upward. Trains of consecutive bubbles as shown inFigure 3 often occur. Makela et al. [2010] sees this as evidence for gravity wave seeding of the Raleigh/Taylor instability.

[18] This is not the first time that the relationship between spread F on ionograms and equatorial depletions has been identified [e.g., Weber, 1996; Sinha and Raizada, 2000] and also the possible relevance of satellite traces, [e.g., Weber and Buchau, 1978; Cabrera et al., 2010]. However the range-time events reported here can be seen as definitive in terms of their time sequence display. Such time sequences were observed at all low latitude Australian ionosonde sites when using the range-time display.

[19] Satellite ionogram traces, as mentioned previously, can also result from echoes received between sporadic E, the ground, and the F2 layer. These can only be distinguished by a tedious calculation of all possible travel-time combinations. It is better to avoid interpreting events when sporadic E is present. Satellite traces can also be produced by off-angle reflection from Travelling Ionospheric Disturbances (TIDs). TIDs can be expected to produce lesser electron density gradients than the edges of depletions and thus such satellite traces may involve smaller values of time retardation and closer association with variations in the height of the overhead trace.

[20] The four range-time events shown in this paper to be reflections from approaching and receding ionospheric electron density depletions are typical of such ionosonde events seen at night between sunset and sunrise. A question remains as to whether all such low latitude events seen in range-time ionosonde displays correspond to depletions. There is much evidence to support the belief that there are forms of spread F at low latitudes which are not related to depletions, particularly at the outer-most edge of the equatorial zone [Candido et al., 2011]. Further comparison between ionosonde and directly observed depletions by other types of instrument such as optical, radar and satellite can answer this question.


[21] The Editor thanks Peter Dyson and an anonymous reviewer for their assistance in evaluating this paper.