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Dependence of mesospheric Na and Fe distributions on electron density at Arecibo

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Earth, Planets and Space201567:146

  • Received: 2 March 2015
  • Accepted: 2 September 2015
  • Published:

The Erratum to this article has been published in Earth, Planets and Space 2015 67:202


We present case studies of the mesospheric alkali and non-alkali metals, Na and Fe, along with electron concentrations [Ne] obtained from measurements made at Arecibo on nights of 17–18 and 18–19 March 2004. The background mesospheric conditions as recorded by an airglow all-sky imager displayed ripple- and band-type structures on these nights. Both of the metals display detailed structures within their neutral sporadic layer but are more pronounced in Na than for Fe. A sporadic-E (Es) with electron concentrations [Ne] exceeding 3000 electrons cm−3 is accompanied by a strong Na enhancement and a weak sporadic Fe (Fes) layer around 95-km altitude. The concentration of Fe+ and Na+ is estimated to be close to 600 and 30−3, respectively, within the sporadic-E layer. In order to investigate ion-neutral coupling, a correlative analysis was performed in two altitude regions. Similar features are seen between neutrals and electrons in the 96–100-km altitude range, while within the altitude range of 80–90 km, an opposite behavior is seen. A comparative study between neutral layers below 90 km often referred to as the main or permanent layer and sporadic activity above 90 km reveals different characteristics for alkali and non-alkali metal. Fe concentrations in the main layer are higher than in Fes resulting in a density ratio of less than 1 determined from two layers of 3 km thickness centered at 97 and 87 km. For the case of Na, the ratio exceeds 1 during Es activity on both the nights.

The case studies discussed in this work facilitate our understanding of different factors that can influence the sporadic activity in alkali and non-alkali metals. In a region dominated by ion-molecule chemistry, temperature fluctuations that can be induced by wave activity will have more impact on Na than for Fe within their layers depending on altitude.


  • Mesospheric layers
  • Sporadic-E
  • Ion-neutral coupling
  • ISR


A variety of interesting phenomena occur in the mesospheric region of the Earth’s atmosphere that includes sporadic-E (Whitehead, 1989), polar mesospheric summer echoes (Rapp and Lübken, 2004), and deposition of meteoric metals (Gadsden, 1969; Plane, 2003). The existence of metals in the Earth’s mesosphere was first established using airglow studies (Brown, 1973). With the advent of laser technology, several groups reported temporal and altitudinal distributions of various metals like Na, K, Ca, and Fe using ground-based lidar systems located at different latitudes (Alpers et al., 1990,1994; Tepley et al., 2003; Collins et al., 1994; Clemesha et al., 2004; Sridharan et al., 2009; Gardner et al., 2011; Raizada et al., 2011; Yuan et al., 2012). This led to the development of modeling efforts to understand mesospheric chemistry and the role of meteor input in the distribution of metals (Plane, 2003). One of the most abundant metals in meteoroids as well as in the mesosphere is atomic Fe. Upper atmospheric atomic Fe has been studied from different locations (Granier et al. 1989, Alpers et al., 1990; Kane and Gardner, 1993; Chu et al., 2006; Raizada and Tepley, 2002, Huang et al., 2013). Raizada and Tepley (2003) found significant latitudinal differences in the Fe distribution. Observations of Fe at South Pole and Rothera have shown seasonal differences in its distribution (Gardner et al., 2011). In addition to the main or permanent metal layer, earlier studies have shown the occurrence of thin regions of enhanced concentrations occasionally, generally referred to as sporadic layers in the neutrals. A strong link between these sporadic layers and sporadic-E has been suggested in many earlier studies (Kirkwood and von Zahn, 1991; Gardner et al., 1993; Alpers et al., 1994; Collins et al., 2002; Williams et al., 2007; Zhou et al., 2008). Sporadic-E is often patchy in nature, i.e., the electron concentrations display spatial variations and hence result in inhomogeneity in the atmosphere (Miller and Smith, 1975). Observations of Fe by Alpers et al. (1990) from Andoya, Norway (69° N) revealed frequent occurrences of Fes within 15 h. A more comprehensive comparison of Na and Fe layers at mid-latitudes showed sporadic activity to be very prominent for Fe but not so for Na (Kane and Gardner, 1993). The latitudinal variation in the occurrence of Nas layers is evident from the following studies: Nagasawa and Abo (1995) reported frequent observations of sporadic Na layers above Tokyo (35.6° N). Yi et al. (2002) studied the seasonal variability of Nas layers over Wuhan (30.5° N), China and found that their occurrence rate maximized in summer. Their study revealed broader Nas widths as compared to low and high latitudes. Recent work by Wang et al. (2012) have shown that at 40° N double layers in Na occurred between 105 and 130 km with about 17 such events per 319 of total observations. Later on, Dou et al. (2013) analyzed Na lidar data from four sites (40° N, 31° N, 30° N, 19° N) representing mid- and low latitudes in China and found that the occurrence rate of Na sporadic layers were similar at locations less than 31° N but about 3 time smaller at 40° N. Nas layers are common at low and high latitudes (Batista et al., 1989; Kwon et al., 1988; Hansen and von Zahn, 1990).

The occurrence of both sporadic-E and neutral layers has been linked to several processes that include neutralization, the presence of wind shears, and advection (Gardner et al., 1993; Tepley et al., 2003; Plane, 2004; Raizada et al., 2011,2012). Recently, Yuan et al. (2014) related the seasonal variations of Nas and Es layers to the convergence of metal ions in summer, while ion diffusion appeared to dominate in wintertime. Zhou et al. (2008) investigated the seasonal and local time variations of Fes and their relation to sporadic-E at Arecibo (18° N). They reported three nights of simultaneous observations of Fe and electron concentrations, where good correlations were seen in the occurrence of Fes with Es layers. A correlative analysis between Na and Fe layers observed in China (30° N) revealed that the lower sides of these metal layers track each other extremely well, while the densities at the top side do not follow each other (Yi et al., 2008). They found that Na layer is broader and extends to higher altitudes; its upper boundary is about 5.2 km higher, while the lower boundary is ~0.6 km lower than Fe. Also, the temporal behavior of the upper boundary displayed some differences and was attributed to be likely due to sporadic layers. Recently, Yue et al. (2013) extended this study for the same metals observed at Arecibo with similar results showing that altitudinal dependence of Na and Fe correlations are global in nature. Both the studies did not compare the similarities or differences in sporadic activities, which is one of the goals in the current study.

Even though several studies of metal layers have been performed from different locations that have been mentioned in the above paragraphs, they were focused on other aspects of the metallic layers. In this study, we examine two cases of Na and Fe distributions along with simultaneous electron concentrations that were obtained using co-located resonance fluorescence lidars (RFLs) and incoherent scatter radar (ISR) at Arecibo (18.35° N, 66.75° W). The examples used in this investigation offer an excellent opportunity to improve our understanding related to metal layer response to sporadic-E and hence provide better insight about ion-neutral coupling. This study aims to understand the role played by various atmospheric constituents and other factors that may be responsible for differences observed in metallic layer.


The RFL system used for the measurements reported in this work consists of a single state of the art, Nd:YAG laser that pumps two identical dye lasers. The pumping was performed by splitting the second harmonic at 532 nm with 14 W of average power in each beam at 50-Hz repetition rate. The two dye lasers were tuned to generate resonance wavelengths of Na and Fe at 589 and 372 nm, respectively. We used the rhodamine 610 dye to generate the fundamental, 589-nm emission for Na, while the generation of the UV emission for 372 nm required a sum-frequency technique. So, for the second dye laser, we combined rhodamine 590 and 610 dyes for a dye fundamental at 572 nm and then sum frequency mixed with the IR residual from the Nd:YAG at 1064 nm to generate photons at 372 nm. The average power at this wavelength was ~1 W while at 589 nm, it was ~4 W. The backscattered photons were collected using an f/15 Cassegrain type telescope with a 0.8 m diameter. An optical fiber with a core diameter of 1.5 mm directed the light to a dichroic filter that splits the incoming beam to the detectors and narrow band interference filters of ~1-nm bandwidth at FWHM. EMI 9863-350B and Hamamatsu R943-02 photomultiplier tubes were used to collect the light at 372 and 589 nm, respectively. Other instrumental details can be found in the earlier work (Raizada and Tepley, 2002; 2003; Raizada et al., 2004; Tepley et al., 2003; Raizada et al., 2011; 2012).

The range resolution for all the lidar data presented here is 300 m. The temporal resolution for the Na and Fe datasets was ~1 min, and it was ~2 min for Ne.

The electron concentrations were derived from standard Barker-coded power profile measurements that were made during one of the topside ISR runs at Arecibo (González and Sulzer 1996). The technique utilizes a 13-baud, 4 μs/baud coded pulse, yet we oversampled the data by a factor of 2 to yield a bin size of 300 m (Ioannidis and Farley 1972).

The background atmospheric conditions over Arecibo were monitored by using The Penn State Allsky Imager (PSASI), which records nighttime airglow at three different optical wavelengths (557.7, 630.0, 777.4 nm) that are emitted from different layers of the ionosphere. The 557.7-nm airglow comes from the upper mesosphere (~96 km) or E-region and its intensity is proportional to column-integrated [O]3 (cube of concentration of atomic oxygen) in the emission layer. Since atomic oxygen is one of the major sources of ions and electrons in the lower E-region of the ionosphere, this emission can be used to monitor both neutral phenomena (such as gravity waves) and electrodynamics near this altitude. More details on PSASI, airglow, and ionospheric phenomena revealed by it can be found in Seker et al. (2007).

To obtain accurate information from the all-sky images, each image was processed in the following way. First, the images are spatially calibrated using star locations to find the geographic directions. Then, the stars are removed using a median filter and contrast is enhanced to better show the phenomena of interest (gravity waves). Finally, based on the location and coverage of the imager and height of the emission layer, the images are un-warped (to correct for the fish-eye lens effect) and then mapped to geographical coordinates.


Co-located ISR and RFL measurements corresponding to the different metals offer an excellent way to understand mesospheric processes. Data pertaining to an alkali and a non-alkali meteoric metal measured at Arecibo on two consecutive nights, 17–18 and 18–19 March 2004, are discussed. These two cases represent sporadic-E events that are accompanied by a strong enhancement in the neutral Na but relatively weaker in Fe. We discuss these two nights in the following sections.

17–18 March 2004

Figure 1 displays the temporal and altitudinal distribution of electron, neutral Fe, and Na concentrations in the top, middle, and lower panels for 17 March 2004. The upper panel exhibits electron concentrations that are shown from 21:00 to 30 h LT (06:00 AST). This day was characterized by the presence of a very strong sporadic-E during the early part of the evening, with a descending layer from above 105 km that merges with a weak Es at 97 km after 22:00 h AST. The false color scale in this figure has been saturated at values lower than maximum to provide a better contrast for weaker features. The maximum strength of the higher altitude Es descending from 105 km at 18 h (AST) was ~7 × 104 cm−3, while the lower layer at 93 km exhibited peak densities ~1.9 × 104 cm−3. The concentrations in both layers decreased by a factor of 7 and 4 in the upper and the lower regions, respectively, after 01:00 AST. The undulations observed in the electron distribution above 95 km are seen as “C” type structures, which are similar to those observed in Na distributions at another location (Clemesha, 2004). They appear to have periodicities varying between 0.5 and 1 h and descend at a rate of ~0.4 m/s. Such downward phase progressions have been attributed to tidal descending layers (Mathews and Bekeny, 1979; Mathews, 2001) and also observed earlier in the neutrals (Gardner et al., 1993, Raizada et al., 2012). The altitude-time-concentration plots of Fe and Na in the middle and the lower panel of Fig. 1 reveal bifurcated distributions of these metals with structures similar to the Ne distributions for both metals yet are stronger for Na than for Fe at altitudes exceeding 95 km. Such structures including billows have been observed in the Na distribution from Arecibo and other locations where these were attributed to wind shears and dynamical instabilities (Clemesha 2004; Sarkhel et al., 2012; Sarkhel et al., 2015a, b).
Fig. 1
Fig. 1

Altitudinal and temporal variations of Ne (upper panel), neutral Fe (middle panel), and Na (lower panel) are shown for the night of 17–18 March 2004. Descending C-shaped structures (best seen in the Na distribution) are strong in both Ne and Na but weaker in the case of Fe

The quantitative relation between neutrals and plasma is further illustrated in Fig. 2. This figure displays the correlation coefficients (r) calculated for Na/Ne, Fe/Ne, and Na/Fe for two different altitude ranges that have been determined for a 5- or 4-km altitude interval that corresponds to about 25 or 20 data points at each time. In the upper altitude range (96–100 km), ion-neutral molecule chemistry dominates while the lower part (85–90 km) is governed by neutral chemistry. The upper panel displays the correlation between Na and Ne that is shown as a solid line for 96–100 km, while the dashed line represents the temporal variation for Fe and Ne in the same altitude range. The neutralizations of both Na+ and Fe+ have negative temperature dependence, which can explain similar correlations of the neutrals with ions. Similarly, the middle panel shows the correlation coefficients calculated for the 85–90-km height interval. The lowest panel displays r determined only for the neutrals, i.e., Na and Fe, with solid/dashed lines showing upper/lower altitudes regions. However, between 85- and 90-km height intervals, both the neutrals behave similarly with Ne after 1:00 AM (LT) resulting in similar correlations with the latter. This is most likely due to the influence of neutral sporadic layer descending down to lower altitudes during the later part of the night. The early part of the night displays anti-correlation between Fe and Ne as compared to Na. This might be due to larger neutralization rates of Fe+ to Fe as compared to Na, which will result in enhanced neutral densities in the case of former in presence of sporadic-E. This also explains anti-correlation between Na and Fe in 85–90-km region before midnight. The similar correlation between neutrals and Ne within the sporadic layer located at higher altitudes (96–100 km) indicates that Na and Fe are responding in a similar way to Ne. However, such correlation studies do not provide information regarding the strength of this neutral sporadic relative to the main layer.
Fig. 2
Fig. 2

This figure displays the temporal variation of correlation coefficient, r, in two altitude regions, one encompassing the upper part of the sporadic layer occurring between 96–100 km, while the other represents the 85–90-km region. The top panel displays r between Na and Ne (solid line) and Fe and Ne (dashed line) for the upper portion of the layer. The middle panel is similar to the above except for the 85–90-km region, while the lower panel represents r between neutrals only. The solid/dashed lines display the correlation between Na and Fe for the upper/lower part of the sporadic layers

The differences observed in the neutral sporadic activity are further elucidated in Fig. 3 that compares the strength of the Na and Fe distributions within a region centered at 97 ± 1.6 km relative to 87 ± 1.6 km. The higher altitude region largely coincides with the occurrence of sporadic-E, while the lower altitude region represents the main layer. To determine the intensity of the neutral sporadic event relative to the permanent or main layer, the average column concentrations in these two altitude regions are calculated and their ratios are evaluated. The ratio of densities exceeding 1.0 indicates that the strength of the neutral layer at Es altitudes is strong relative to the main layer. Figure 3 illustrates that the Na ratio at the two altitudes is greater than 1 before 26 h (02:00 AST) and decreased to lower values during the later part of the night as the layer descended. However, the neutral Fe behaves differently with ratios of less than 1.0 throughout the night. This shows quantitatively that the sporadic Fe (Fes) is less pronounced than Nas.
Fig. 3
Fig. 3

This figure exhibits the ratio of concentration between the sporadic and the main layer for both Na (solid line) and Fe (dashed line). The ratio is determined by calculating the average densities in two regions of 3 km thickness centered around 97 and 87 km, representing the sporadic and main layers

Figure 4 exhibits the temporal variation of column abundance in the sporadic layers for Fe, Na, and Ne in the altitude range of 92–100 km. The left ordinate shows the neutral abundances while the Ne values are shown on the right axis. Both the neutral and electron concentrations reveal higher values associated with sporadic-E during the early part of the night, diminishing to lower values later. Larger values of Fe column abundances are consistent with the high content of this metal in chondrite meteoroids (Brown, 1973).
Fig. 4
Fig. 4

This plot displays the temporal variation of column abundance of Ne (dashed-dotted, red), Fe (dashed line, blue), and Na (solid line). Note that the metal abundances are displayed on the left Y-axes while the right ordinate represents Ne values for the night of 17–18 March 2004 at Arecibo

18–19 March 2004

Figure 5 displays the variation of electron concentrations, Fe and Na for the second night and is marked by Es activity with electron concentrations ~1.5 × 104 cm−3 occurring around 23 h (AST) centered at 95 km. However, this increase in Ne occurs suddenly and is not remnant of a strong descending layer during earlier times, which was the case for the previous night. The enhancement in Ne appears at 98 km and the layer moves down to 92 km, disappearing below this altitude level. This layer also bifurcates around 95 km into another layer that lasts for about 3 h more. Yet, the lack of a strong Fes corresponding to sporadic-E is noteworthy. Also, Fe concentrations in the main layer below 90 km are higher than those above 95 km, and this feature is consistent with the previous night. However, neutral sporadics are very well correlated with Es activity in the upper altitude range of 96–100 km as depicted in Fig. 6, which is similar to Fig. 2. On this night, advection of a strong sporadic layer causes a correlation coefficient (r) to exceed 0.6 after 22:30 h (AST), as shown in the upper panel. The anti-correlations between neutrals and Ne before 22:30 h (LT) followed by positive r values have been observed in the case of other metals (Raizada et al., 2012). They attributed the positive correlations to the advection of the layers while the neutralization can lead to anti-correlation since decrease in Ne results in increase in the neutrals. The middle panel shows the temporal variation of correlation coefficients between Ne and Na (Fe) shown as solid (dashed) lines in the 85–90-km altitude region. This demonstrates that an increase in Na concentrations is associated with the increase in Ne, while an opposite trend is seen for Fe. Since on this night sporadic-E occurred above 90 km, the electron densities are not enough to cause appreciable neutralizations in the case of Fe. Their neutral concentrations would be determined by meteoric deposition balanced by the loss rate through the formation of stable compound like FeOH etc. in the lower part of the layer. Earlier studies by Zhou et al. (2008) have pointed out that a sporadic-E with electron concentrations exceeding 4000 electrons/cc can result in a strong Fes layer after sunset. In absence of any strong Es activity below 95 km, Fe and Ne display anti-correlation. The third panel displays r between neutrals calculated for the upper layer (96–100 km, solid line) and the lower region (85–90 km, dashed line). It is observed that Na and Fe exhibit a very good correlation with values ~1.0 except for small durations that are separated by about 1–2-h time intervals in the upper altitude regions. In the 85–90-km region, the distribution of both neutrals is anti-correlated with each other between 23:00–24:00 h and will be discussed later. Figure 7, which shows the average density ratios of Na and Fe at two different altitude regions, corroborates that the sporadic neutral activity in the case of non-alkalis is weaker as compared to alkali metals. These ratios are found by taking the average densities at altitudes centered at 97 ± 1.6 and 87 ± 1.6 km, following the procedure that was used in Fig. 3. The results shown in this figure reveal that during strong Es activity, Na concentrations at higher altitudes exceed the values at lower levels, making the density ratios greater than 1.0 during the event. However, the enhancement in sporadic Fe concentration did not exceed that in the main layer for this night, as was also the case of the previous night.
Fig. 5
Fig. 5

This is similar to Fig. 1 but for the adjacent night. A good correspondence between neutral sporadic neutral and Es layers is seen. However, Fes appears to be weaker than Nas as compared to their main layers occurring below 90 km

Fig. 6
Fig. 6

This is similar to Fig. 2, except for the next night. Refer to the text for details

Fig. 7
Fig. 7

This figure shows the temporal variation of density ratio for Na (solid line) and Fe (dashed line) at two altitude levels calculated in a similar way as in Fig. 3. The ratio exceeds 1 for the case of Na during the sporadic event and is less than 1 for Fe throughout the night of 18–19 March 2004

The temporal variations of Na, Fe, (left ordinate) and Ne (right ordinate) column abundances determined from the Arecibo data for the night of 18 March 2004 are shown in Fig. 8. The observed primary peak in the Ne distribution coincides well with that seen in the Fe distribution around 23:20 h (AST). Also, the Na and Ne abundances display oscillations with a period of about an hour, as that observed in Fe. These aspects are discussed in the next section.
Fig. 8
Fig. 8

This plot is similar to Fig. 4 except for the consecutive night. The Ne abundance (dash-dot, red) displays a peak at around 23:10 h (AST) that coincides with the Na concentration maximum (solid line, left ordinate)

Additionally, to investigate the wave activity on these two nights, we further show the O(1S) airglow images at 557.7 nm originating near 96-km altitude, obtained using the PSASI imager. Examples of images taken on 17–18 March and 18–19 March 2004 are shown in Fig. 9a, b. The detailed characteristics of the waves in the mesosphere on these nights are described in the “Discussion” section.
Fig. 9
Fig. 9

All-sky images of the O(1S) 557.7-nm airglow emission obtained using the PSU imager located at Arecibo: a a typical image taken on the night 17–18 March at 02:49 h (AST) showing wave activity with a superimposed pink line that indicates the phase progression; b an image representing data taken at 02:51 h (AST) for the consecutive night 18–19 March 2004


This work investigates two cases based on consecutive nights of lidar and ISR data obtained from Arecibo. Earlier studies have associated Es layers with neutral distribution mainly through ion-molecule chemistry (Gardner et al., 1993; Collins et al., 2002; Raizada and Tepley, 2002; Zhou et al., 2008). The unique set of observations presented here illustrates the differences in neutral distribution and offers an excellent opportunity to understand the processes that can be responsible for such different behaviors observed for alkali and non-alkali metals. In this regard, it is important to compare the average/peak column abundances of Na and Fe in the main layer and also within Es.

Table 1 lists the average and maximum column abundances of Na and Fe in the lower (82–92 km) and top (93–100 km) parts of the metal layers that were observed on the two nights reported herein, that is, 17–18 and 18–19 March 2004. Also given in the table are the Ne abundances in sporadic-E above 92 km only, since there were no substantial layers below this altitude. It is interesting to note that the peak Ne abundances were about 3 times higher on 17 March relative to the next night. However, the average abundances of Ne are similar on both nights, analogous to the average neutral abundances observed in the 93–100-km altitude region. It can be seen that Na abundance in the top part is about 1.4 times stronger than in the lower region, while for Fe an opposite behavior is observed with the upper layer about 1.2 times weaker than the latter on these nights. These abundances agree well with the earlier observations (Raizada and Tepley, 2003; Kane and Gardner, 1993; Zhou et al., 2005). Considering that both Fe and Fe+ are dominant species in the nighttime mesosphere (Brown, 1973; Kopp 1997), a strong Fes activity compared to Na should be observed. Rocket measurements have revealed Fe+ and Mg+ to be the most abundant ions in the E-region (Kopp 1997). Ca+ is the only ion that has been measured from ground-based lidars since all other metal ions have their resonance wavelengths in the UV part of the spectrum and are highly absorbed by ozone (Granier et al. 1989, Gardner et al., 1993; Gerding et al., 2000; Tepley et al., 2003; Raizada et al., 2011; 2012). The ratio of Ca+/Ne concentrations and abundances inferred using in situ data obtained from Wallops Island rocket measurements is ~1–2 % and ~4.27 × 10−3, respectively (Kopp 1997). This agrees well with ground-based airglow and lidar measurements from Arecibo (Tepley et al., 1981, 2003; Raizada et al., 2012). Assuming that the fraction of Fe and Na ions in the E layer do not change drastically at Arecibo and Wallops, the fraction of these ions and abundances can be inferred using electron density information from ISR data. Thus, it is possible to estimate the Fe+ and Na+ abundances for the nights presented here by using electron density profiles obtained using ISR data from Arecibo and prior in situ measurements of these ions. Table 2 compares the average abundances of Na+ and Ca+ normalized to Fe+ that occur in carbonaceous chondrites with those obtained using rockets and estimated using ground-based electron concentrations from Arecibo. This table reveals that the average abundance of Fe ions over Arecibo is about 10 times larger than Na+ within a sporadic-E layer during nighttime. However, a limitation of this technique arises from the fact that the in situ experiments were not performed from the same location or time as the ground-based experiments. Because Fe+ has a much larger ion mass than the molecular ions [NO+, O2 +, N2 +] or other metallic ions, it is possible that its concentration can be estimated by fitting the incoherent scatter spectra (Behnke and Vickrey, 1975; Tepley and Mathews, 1985). We hope to be able to verify the earlier conclusions regarding Fe+ with future ISR experiments.
Table 1

Average and peak abundances for Na, Fe, and Ne on two nights at Arecibo


Altitude (km)

Na abundance (109 × cm−2)

Fe abundance (109 × cm−2)

Ne abundance (109 × cm−2)







17 March 2004















18 March 2004















Table 2

Comparing the relative abundances of ions of chondrites in the MLT region

Relative abundance



ISR (Arecibo)






6.3 × 10−2

7.4 × 10−2

9.1 × 10−2


6.8 × 10−2

2.8 × 10−2

3.9 × 10−2

Earlier studies from Arecibo have shown a good correlation between sporadic-E and Fes layers (Raizada and Tepley, 2002; Zhou et al., 2008). The current study shows a good correlation between neutral sporadic and Es layers in the altitude range of 96–100 km. The neutralizations of both Na+ and Fe+ have negative temperature dependence, which can explain such behavior. Between 85–90-km altitude intervals, the correlation between Na and Fe is similar when the sporadic-E descends to lower altitudes; otherwise these metals display different response to Ne. The simultaneous occurrence of sporadic neutral and Es layers also results in good correlation between Na and Fe but results in poor correlation in absence of such coincidence. Yue et al. (2013) investigated altitudinal correlations between Na and Fe and found negative correlation coefficients (r) occurring in a narrow middle region encompassed by positive r values at the top and bottom parts of the layers. They demonstrated that the layer structure for the case of Na and Fe is mainly determined by chemistry, while the correlation between the two species is governed by gravity wave-induced density fluctuations. However, they determined r using the average data for all the night and to obtain its temporal variation, a sliding window of 1 h was used. Additionally, the work presented by Yue et al. (2013) did not study the influence of electron concentration on the neutrals due to the lack of ISR data. Since in this study, we aim at investigating the ion-neutral coupling, the correlations have been determined for specific altitude regions based on chemical differences. Later, we determine the chemical lifetimes of Fe and Na ions to shed more light on correlations.

The lidar gives the temporal variation of a particular metal along the line of sight and its altitudinal variation is often referred to as “lidargrams” (Clemesha et al., 2004). Thus, both radar and lidar sample the line of sight volume of the atmosphere and do not provide any information regarding the spatial variability in horizontal dimensions. Collins et al. (2002) has discussed this aspect in detail. Earlier measurements have revealed structured sporadic-E with sizes ~300 m, limited by the range resolution of the radar (Miller and Smith, 1975). Later on, Hysell et al. (2012) found irregular sporadic-E with periodic structuring that was attributed to shear instability in the neutral flow. Though the medium can be inhomogeneous, the altitudinal/temporal resolution profiles at a given location can be useful to determine the variations, which can be caused either by dynamics or chemistry. A good correlation between metals is often observed apart from inhomogeneity in the atmosphere. The evidence comes from similarity between the lower boundary of Na and Fe layers as observed in the data presented in this study (Figs. 1 and 4), which is consistent with earlier observations (Yi et al., 2008; Yue et al. 2013). This suggests that local dynamics plays a significant role in the temporal variation of Na and Fe on any given night. The influence of Es through ion-neutral coupling is evident from high correlations between the metals and Ne in the presence of sporadic-E. In the absence of strong Es, particularly between 85- and 90-km regions, Na and Fe display anti-correlation but changes to good correlation as the sporadic-E layers descend to lower altitudes (Fig. 2). This elucidates a different sensitivity of Na and Fe to electron concentrations. Also, the concentration ratios evaluated for both the neutral in the upper altitudes and within the main layer reveal that the latter is stronger in the case of Fe relative to its sporadic, which is an opposite behavior to the alkali metal, Na, on the two nights reported here. The possible reasons for this different sporadic activity in the neutral Fe and Na are discussed below.

Earlier studies have shown that the structure and distribution of different mesospheric metals are governed both by dynamics and chemistry (Plane, 2003; Gerding et al., 2000; Yue et al. 2013). To investigate the dynamical activity on these nights, we use the PSASI imager that reveals wave characteristics on these two nights. It is to be noted that imagers are well-suited instruments that provide information about gravity waves, MSTIDS (Makela et al., 2010). Figure 9a displays a GW event on the night of 17–18 March 2004 as measured from the O(1S) 557.7-nm emission. This night is characterized by ripple-type (smaller wavelength, faster speed) and band-type (larger wavelength, slower speed) gravity waves (GWs) that were seen from 02:00 until 05:00 AST. Two separate GWs were observed around 03:00 AST. The first GW packet appeared southwest of the Arecibo and was aligned along the northwest-southeast direction and moved slightly westward of the north direction, disappearing toward the northwest of Arecibo. The horizontal wavelength, speed, and period of the GWs were measured as ~25 km, 2.5 km/min (150 km/h or ~40 m/s), and 10 min, respectively. These values are typical for E-region band-type (large-scale, slow speed) GWs. The second GW packet appeared southeast of Arecibo and propagated northward. The horizontal wavelength, speed, and period of the GWs were measured as ~10 km, 1.5 km/min (90 km/h or 25 m/s), and ~7 min, respectively. This wavelength value is typical for E-region ripple-type GWs (Chung et al., 2003).

Figure 9b shows the wave activity, again observed at 557.7 nm, obtained on the night of 18–19 March 2004. This night is also characterized by both band-type and ripple-type GWs. Ripple-type GWs were dominant, especially during 02:30–03:30 h AST and all-sky band-type GWs were dominant during 04:00 h AST. The ripple-type GWs appeared northeast of Arecibo, were aligned along northeast-southwest, and moved toward the northwest, disappearing north of the site. The horizontal wavelength, speed, and period of the GWs were measured as ~20 km, 2.5 km/min (150 km/h or ~40 m/s), and 8 min, respectively. The band-type GWs covered the whole sky, were aligned northwest-southeast, and propagated southwestward. The horizontal wavelength, speed, and period of the band-type GWs were measured as ~14 km, 1 km/min (60 km/h or ~17 m/s), and 14 min, respectively.

Thus, the PSASI imager data reveal that both nights had similar wave structure, which can also cause temperature perturbations. The occurrence of ripple-type structures seen on both the nights suggests that the atmospheric conditions are conducive for dynamical instabilities, which are likely to occur in the presence of strong horizontal shears. As discussed in the above paragraph, the ripple-type structures appear to propagate in meridional direction with a component in the zonal direction. This implies the possibility of existence of large horizontal wind shears that are required for Es formation (Whitehead, 1989). This is consistent with the ISR observations of sporadic-E layer on both the nights. Recent studies by Fytterer et al. (2014) revealed that the amplitude of 8-h Es maximizes during equinox at low and mid-latitudes in both hemispheres.

Previous studies have shown that the variability in mesospheric temperatures during sporadic events is ~30 K over Arecibo (Friedman et al., 2003). Climatological studies have revealed that tides resulted in a 20-K variation in mesospheric temperature in March over Arecibo (Friedman and Chu, 2007). Such temperature fluctuations can influence the mesospheric chemistry, which can affect the strength of sporadic neutral layers as a result of ion-neutral coupling. The Na dayglow intensity variations observed for a few days from Trivandrum (India), a low-latitude station, have been attributed to chemistry (Hossain et al., 2014). Another factor mentioned previously that can affect the strength of neutral sporadic activity can be due to inhomogeneity in the medium or advection, but that alone cannot explain why the minor species like Na show stronger sporadic layer as compared to one of the major metals like Fe relative to their main or permanent layers. Thus, to gain better insight, we compare the lifetimes of Na and Fe ions against neutralization based on previous studies (Cox and Plane, 1997; Helmer et al., 1998; Collins et al., 2002; Woodcock et al., 2006). Collins et al. (2002) have shown that Na+ neutralization decreases sharply below 100 km as compared to Fe+. At altitudes around 95 km, ion-molecule chemistry dominates, with the Na layer being influenced by the formation of ligands like Na+.CO2, Na+.N2. These cluster ions undergo dissociative recombination with electrons resulting in their neutralization. However, Na+.N2 can react with atomic O to form NaO+, which can result in Na+ formation through its reaction with another atomic O. Thus, the overall rate of neutralization is controlled by the altitude-dependent ratio of [CO2]:[O], which increases below 100 km. In the case of Fe+, Vondrak et al. (2006) have shown the importance of Fe+.N2, Fe+.O2, and FeO+ ions in governing their chemical lifetimes through dissociative recombination with electrons. Since formation of Fe+.N2, Fe+.O2 involves a third body (i.e., they are termolecular reaction), their rates increase below 85 km while FeO+ dominates above this altitude. We calculate the neutralization lifetimes of Na+ and Fe+ based on the chemistry discussed in Collins et al. (2002) and Vondrak et al. (2006), respectively. The reactions rates have been taken from their work, while the MSIS-90 model has been used to infer neutral densities like O2 and N2. Minor species like CO2 and H2O are taken from Swider (1996), while O3 profiles are obtained using the CIRA 1986 model. Figure 10 displays the altitude variation of lifetimes of Fe+ and Na+ against neutralization. This figure reveals that Fe ions have a longer lifetime compared with Na+, which will result in a more rapid neutralization of the latter resulting in stronger neutral Na sporadic layers compared with the former. Also shown is the sensitivity of these ions to neutral temperature and to minor species. Both Na+ and Fe+ are sensitive to temperature changes but have different altitudinal dependence. Na ion lifetimes exhibit a ±23–24 % variation for a ±10 % change in temperature, which does not change with altitude. Such temperature change produces a variable response in Fe+ lifetimes (~23–1 %), i.e., larger at lower altitudes as compared to higher altitudes. Thus, temperature-induced variability in Fe+ lifetimes is going to be greater than 10 % below 100 km and decreases above this altitude. Plane et al. (1999) showed that since Na+ chemistry involves ligand formations, which have square dependence on atmospheric density, they are more sensitive to its change as compared to Fe ions. The latter also displays more sensitivity to Ne changes relative to Na ions. A ±10 % change in Ne concentrations results in negligible variations in the latter, while it causes about 9–10 % changes in the former metal. This is consistent with the earlier work by Zhou et al. (2008) showed a dependence of strong Fes activity on Ne exceeding 4000 electrons cm−3. On the nights reported here, a weak Fes was observed corresponding to sporadic-E with electron less than 4000 electrons cm−3. Similarly, increasing CO2 by 10 % causes a ~9 % decrease in Na+ lifetimes, while an increase of 10 % in ozone has negligible influence on Fe+ lifetimes at all altitudes. The changes in the concentrations of minor species can occur due to advection or horizontal transport, however that will cause minor variability in the neutralization lifetimes of Na and Fe ions. But these are more sensitive to temperature variations that can result from GWs, dynamical instabilities, ripple-type structures that were observed on these nights. Hence, any fluctuation in this parameter will influence the neutralization process. Earlier studies have found the Na abundance to be sensitive to temperature changes (Zhou et al., 1993). Temperature perturbations can result in compressional or dissipative heating depending on the ambient conditions.
Fig. 10
Fig. 10

The left panel shows the altitude variation of electron density that represents a typical variation of the sporadic-E layer for the nights shown in this work. The middle panel displays the lifetimes of Na ions against neutralization (solid line) with superimposed dashed lines showing the change in the lifetime due to a ±10 % increase in the temperature. Also displayed is the dashed-dotted line showing the change due to a 10 % increase in CO2 concentration. The right panel exhibits Fe+ chemical lifetimes (solid line). The superimposed lines (thick dashed, dashed-dotted, thin-dashed) display the new lifetimes due to 10 % changes in temperature, ozone, and Ne values, respectively

The differences in lifetime sensitivity to electron concentrations manifest in correlation seen in the Figs. 2 and 6. The anti-correlation between Na and Fe in the lower part (85–90 km) on both the nights can possibly be attributed to longer Fe+ lifetimes and higher sensitivity to Ne concentrations relative to Na.

This work suggests that the stronger Na sporadic layer can result from the faster neutralization process, which is more sensitive to temperature changes compared with Fe at altitudes above 100 km. Our data suggests that the stronger structures in the case of Na compared with Fe within sporadic neutral layer are more likely to be the manifestation of temperature-induced lifetime variability. Thus, it appears that Na acts as better tracer for studying gravity wave-induced fluctuations at higher altitudes.


Two adjacent nights of simultaneous Na, Fe, and Ne data from Arecibo were studied to examine their characteristics. Fe+, being one of the most abundant ions in the upper mesosphere, makes its neutral species counterpart an interesting element to be studied, in particular when electron concentrations and measurements of other metals are available. An estimation of the concentration of ions within sporadic-E events observed at Arecibo during nighttime reveals that the Fe+ and Na+ concentrations are ~600 and 30−3, respectively. The strength of the neutral sporadic layers is determined by evaluating the ratios of the concentrations above 90 km to that of the main layer. This value is larger for Na compared with Fe, indicating that sporadic activity in Na is stronger than the more abundant metal, Fe on the two nights reported here. Thus, this suggests that each metal responds differently to Es layers. Several factors like dynamics and chemistry can contribute to the differences in sporadic activity. Airglow images obtained at 557.7 nm indicate similar dynamical activity on the two nights with ripple-type structures suggesting the occurrence of dynamical instabilities. This indicates the presence of large shears in the horizontal winds that are required for Es formation. The sporadic-E activity observed in the ISR data supports this scenario. Inhomogeneity alone cannot account for a weaker sporadic Fe layer relative to its main layer, as this is one of the dominant species in the mesosphere. A comparison of lifetimes of these two metals reveals a faster neutralization for the alkali metal (Na), which is more sensitive to temperature changes than for changes in minor species concentrations. To summarize, the weaker, or less pronounced, sporadic events in Fe are most likely due to slower neutralization rates along with their lesser sensitivity to temperature changes.

Future work should focus on understanding the climatological distribution of simultaneous measurements of Na and Fe. Such studies will facilitate the appreciation of the role of dynamics and chemistry that can influence the structure of the metals layers and contribute to their seasonal as well as latitudinal and altitudinal variability.




We acknowledge the National Science Foundation (NSF) grants AGS-1243063, AGS-1241436 to SRI International, USA and AGS-1243133 to Miami University, USA that allowed us to carry out the research for this paper. J. D. Mathews’ and part of S. Sarkhel’s component were supported by a NSF grant AGS 1241407 to The Pennsylvania State University, USA. S. Raizada is extremely grateful to Raúl García for providing technical support. The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968) and in alliance with Ana G. Méndez—Universidad Metropolitana and the Universities Space Research Association.

Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Authors’ Affiliations

Space and Atmospheric Sciences, Arecibo Observatory, SRI International, Arecibo, PR, USA
Electrical and Computer Engineering Department, Miami University, Oxford, OH, USA
Department of Physics, Indian Institute of Technology Roorkee, Roorkee, 247667, Uttarakhand, India
Radar Space Sciences Laboratory, The Pennsylvania State University, 323 Electrical Engineering East, University Park, PA, USA
Radar Systems Engineering Department Radar, Intelligence ASELSAN Inc, Ankara, Turkey


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