sporadic E layer event at Arecibo during AIDA-89

Lidar, radar and airglow observations of a prominent sporadic Nalsporadic E layer event at Arecibo during AIDAT. J. KANE,* C. S. GARDNER,*

Q, ZHOLJ,?#

J. D. MATHEWS?

and C. A. TEPLEY~

*Department of Electrical and Computer Engineering, Univpersity of Illinois at Urbana Champaign. U.S.A. ; t Communications and Space Sciences, The Pennsylvania State University. University Park, Pennsylvania, U.S.A. : 8 Arecibo Observatory. Cornell University, Arccibo. Puerto Rico, U.S.A.

Abstract-Sporadic Na (Na,) layer events were frequently identified during 160 h of lidar observations at Arecibo in January. March and April 1989. Most were accompanied by sporadic E (E,) layers. The most spectacular Na,,/E, event occurred on the night of 30~-31 March when both the Na and electron abundances between 90 and 100 km increased by approximately 700% during a period of 2.25 h starting at 2100 LST. The maximum Na density was almost 42,000 cm ‘. The vertical and temporal structure of the Na and electron densities were remarkably similar during the event. The ratio of the average Na enhancement to the electron density varied from a maximum of 3.5 Na atoms/electron at 98 km to about 0.5 Na atoms! electron below 94 km. Between 93 and 97 km the electron enhancement preceded the Na enhancement by I5 30 min. Above 97 km and below 93 km the Na and electron density variations were in phase. The data suggest that the E, layer triggered the release of Na from a reservoir. but the E, layer was not the source of the major Na, layer. Two minor Na, layers were observed between 101 and 107 km after midnight LST which were also accompanied by intense E, layers and enhancements of the O( ‘S) emission intensities. The abundances of these high altitude Na, layers were less than I % of the electron abundances. These Na, layers appear to be caused by the conversion of Na’ in the E, layer to Na through a set of clustering reactions involving N1, CO? and HZO.

campaign have shown that Na, layers can form rapidly over large geographical areas (GARDNER et cd., 1991) and the horizontal extent of some events can approach 2000 km (KANE ct ~1.. 1991). The first observations of sporadic Fe layers have recently been reported at Urbana (BILLS and GARDNER, 1990) and confirmed at Andoya (ALPBR c’t al., 1990), so it now appears that Na, may simply be one manifestation of a larger class of sporadic layering phenomena. Numerous Na, events were observed during 160 h of lidar measurements at Arecibo during AIDA-89. Most of these events were accompanied by E, layers. The most spectacular event occurred during the night of 30-31 March when both the Na and e abundances increased by almost 700% during a 2.25 h period. The Na and electron enhancements occurred over a wide altitude range (-90-100 km) and the peak Na density was nearly 42,000 cm ‘. In this paper we describe the characteristics of this Na,/E, event as well as two smaller Na, layers and their accompanying E, layers. both near 103 km. We present lidar and incoherent scatter radar measurements of the Na and electron density profiles as well as airglow observations of winds, temperatures, and emission strengths. The data are compared with several of the current theories of Na, layer formation.

1. INTRODUCTION

The erratic formation of dense narrow Na layers has been observed to occur frequently at low- and highlatitude sites. The thickness of these sporadic Na (Na,) layers are typically about 1 km FWHM and their maximum densities can exceed the normal Na densities by a factor of 10 or more. Formation periods vary from a few minutes to several hours and some events have been observed for more than 6 h. CLEMESHA PI d. (1980) were the first to report observations of Na, layers at Slo Paula, Brazil (23-S) in the late 1970s. Since then, observations have been reported from several other sites including Andoya, Norway (69”N) (VON ZAHN et al., 1987). Mauna Kea, Hawaii (20 N) (KWON pt cd., 1988), Longyearbyen, Svalbard (78 N) (GARDNER et al.. 1988), and Arecibo, Puerto Rico (I 8 N) ( BEATTY et cd., 1989). Simultaneous Na lidar and incoherent scatter radar measurements at Arecibo have established the connection between certain Na, events and sporadic E (&) layers. Recent airborne lidar and airglow measurements near Hawaii during the ALOHA-90

#The author is currently Cornell University. Arecibo.

at the Arecibo Puerto Rico.

Observatory. 499

T. J. KANE eI al.

500 2. EXPERIMENTAL

CONFIGURATION

The 430 MHz Incoherent Scatter Radar (ISR) located at Arecibo Observatory (18.3”N, 66.8”W) was used to observe the upper mesosphere and lower thermosphere during the AIDA campaign. While the ISR was operated such that it was cycled through several observing modes every few minutes the mode of importance here is the so-called power profile mode which is used to infer electron concentration. In this mode a 13 baud Barker coded (4 ps/baud) pulse of 52,11s duration and I .67 MW peak power was transmitted every 11 ms. This coding scheme yields a 600 m range resolution and individual profiles were formed by averaging over 1000 or 3600 pulses. All ISR obscrvations were made with the beam oriented at a 11.297’ zenith angle and progressively moved from 0” to 300” in azimuth. This results in the ISR observation ‘circle’ displaced about 20 km from the lidar beam as shown in Fig. 1. As a result of the interleaving of various experimental modes the pointing time and direction of the beam were quite complex and the time spacing between individual power profile determinations ranged from about 0.5 to 6 min.

_l?lv

15 NaLida Distance

(km)

Fig. I. Map illustrating the regions observed by the radar, lidar and airglow instruments near 100 km altitudes at Arecibo on the night of 30-31 March 1989.

Before processing for conversion to electron concentration, power profiles were first corrected for meteor and coherent echo (e.g., aircraft and ship returns) effects (GERMAN and MATHEWS, 1986). The noise baseline was then determined and subtracted while also converting each profile from relative power to signal temperature utilizing the noise calibration source sampled at the end of each profile. The profile was then converted to equivalent electron concentration using the incoherent scatter radar equation in a process described by MATHEWS (1986) and MATHEWSel al. (1982). In January 1989, the University of Illinois CEDAR lidar was installed in the optical laboratory at the Arecibo Observatory. The laboratory is located 350 m northwest of the center of the 305 m radar reflector. The system was operated as a Rayleigh and Na lidar during the months of January, March and April 1989. The lidar measured relative atmospheric density profiles from approximately 25-60 km altitude and Na density profiles from about 80-I 10 km altitude both with a vertical resolution of 37.5 m and a temporal resolution of 30 s. Approximately 160 h of lidar data were collected at Arecibo during 30 nights of obscrvations. For the measurements reported in this paper the laser beam was directed at zenith and had a divergence of approximately 1 mrad. At an altitude of 100 km, the beam diameter was approximately 100 m fullwidth at the em’ power points. The region illuminated by the lidar is marked on the map in Fig. I. The laser operated at the Na D, wavelength of 589 nm with an average output power of 5 W @ 200 pps. The receiving telescope objective was a 1.2 m diameter Fresnel lens so that the power-aperture product of the lidar was approximately 6 Wm*. The photon counts from 4500 laser shots were accumulated over a period of approximately 23 s to form a single lidar photon count profile. These profiles were formatted and stored on a hard disk during data acquisition. The data were then processed off-line using the standard analysis techniques to determine the Na density profiles (GARDNER rt ul., 1986). The absolute accuracy of the Na density measurement is limited to approximately f20% by uncertainties in the assumed calibration parameters. The accuracy of the relative Na density is limited by signal photon noise. For the typical signal levels associated with the highest resolution data reported here (i.e., 37.5 m and 30 s), the accuracy of the relative Na density is approximately 4% near the layer peak. Of course, the accuracy can be improved by reducing the vertical and temporal resolution through averaging. Other optical instruments operating at the Arecibo

Sporadic

Na/sporadic

Observatory this night included a Fabry-Perot interferometer (FPI), an Ebert-Fastie spectrophotometer (EFS), and a tilting-filter photometer. The atomic oxygen emission at 557.7 nm was observed with both the FPI and the photometer. The spectrometer scanned the spectral region between 832 and 872 nm which included the OH (6-2) Meinel and the (02(0-I) atmospheric bands. The variations in the background neutral temperature and emission strengths during the intense layering event were observed from these three emissions, which occur at different altitudes in the mesopause region. Wind measurements from the 557.7 nm line were also made at the upper altitude range of the Na layer. The Arecibo FPI is a single etalon, pressure scanned instrument with 15 cm diameter Spectrosil plates. The basic instrument and the observational method are described in detail by BURNSIDEet al. (198 1) in their work with the thermospheric atomic oxygen emission at 630.0 nm. For the AIDA campaign, however, the FPI was configured to be sensitive to measurements of the lower thermospheric 0(‘S) emission at 557.7 nm using an etalon separation of 3 cm, a 0.3 nm bandwidth prefilter, and an aperture size of 0.27 cm yielding a working finesse of 12. The focal length of the instrument is 122 cm and, together with the aperture diameter, defines a field-of-view of 2.25 mrad and a spot size of 225 m at 100 km altitude. The Fabry-Perot did not look in the vertical since measurements of the horizontal component of the neutral wind were desired. Instead, to maximize sensitivity to the mean winds and to estimate the horizontal gradients (BURNSIDEet al., 1981), observations were taken at an elevation angle of 30-, separating diametrically opposite look directions by 350 km at 100 km altitude. The look geometry included a zenith position and seven cardinal and off-cardinal directions (with the exception of an additional eighth direction that would have looked due west but was blocked by the lidar receiver). For each direction the 557.7 nm emission was scanned twice in wavelength (both increasing and decreasing pressure) usually sampling I2 points across the line with a total integration time of 14 s per spectra1 point. This yields about 2.8 min between line-of-sight measurements, and roughly 20 min resolution for the derivation of the full vector components from the expansion of all eight sky positions. Following BURNSIDEet al. (198 I), in the data analysis it is assumed that each component of the horizontal neutral wind velocity can be represented by a second order Taylor expansion, that is, with constant horizontal velocity gradients, about a point directly above the observatory. A harmonic analysis then gives the

E layer event

SO1

zonal (u) and meridional (v) velocities, and two parameters related to the deformation of the flow, du/d.u and du/dy, where x and y are the eastward and northward directions, respectively. The vertical wind velocity (w) is not measured directly but can be inferred from the product of the horizontal divergence of the horizontal wind velocity. and the neutral scale height. A spatially uniform vertical wind has no effect on the derived values of u and I:. However, systematic errors may result if the actual value of u’ is not constant over the area sampled by the instrument. but these errors are small since the horizontal wind components are generally much larger than the vertical component. Photometric observations of the 557.7 nm emission intensity were also made in the zenith with a higher time resolution (30 s) than for the FPI. This was accomplished with a programmable two point (line and background positions) tilt scan where equal sampling time was split between the two positions. The instrument had a field-of-view of 5” defining a spot size of roughly 9 km at 100 km altitude, as shown in Fig. 1. The relative error of the signal is less than 1% for emission strengths greater than about 100 R. The night of 30-31 March 1989 was not perfect with occasional clouds drifting over the observatory. Their passage, however, was relatively quick and did not seriously affect our ability to measure the trend of the 557.7 nm emission. For the instruments needing a longer integration time, such as the FPI and EFS, larger data gaps due to clouds are present. These questionable periods have been edited from the subsequent presentation. The EF grating spectrometer has a 100 cm focal length and was used in the first order for the nearinfrared emissions. It was also pointed in the zenith and was operated without its field reducing telescope in order to increase its optical throughput. The absence of this telescope results in a rectangular fieldof-view of 8.8 x 12.3 (at 860 nm) which is also shown in Fig. I for an altitude of 90 km, or about halfway between the expected altitude centroids for the OH(6-2) and 0,(0-l) emissions. Integrating for 3 s for each of the 240 spectral points in the spectrometer wavelength scan before recording on disk results in a 12 min temporal resolution for this instrument. The O2 temperature and intensity were derived by fitting temperature dependent synthetic spectra to the band (TEPLEY, 1985). Similarly for OH, a temperature was derived by approximating a Boltzmann distribution to the first four P, lines of the 6-2 band using the MIES (1974) transition probabilities. Further details of the spectrometer located at Arecibo can be found in MEKIWETHER (1979)

2314 LST

2100 LST

4 are several Na/e~- density profiles at various times during the formation (Fig. 3) and dissipation (Fig. 4) of the primary Na,, layer. It is commonly assumed that the ionization layers are composed primarily of singly ionized atoms so that the positive ion

and

density 30 March 1989 80" 0

siml

1KKnl

”

2m

30000

”

4rmc

Density (cm-3) Fig. 2. Na and electron density profiles measured at Arecibo on the night of 30-31 March 1989 just prior to the large Na,!E, event and at the time the Na, layer reached maximum density. The density scale applies to both the Na and em profiles.

3. EXPERIMENTAL

is approximately

equal

to the electron

density.

The structure of the Na and electron density profiles is remarkably similar and the Na enhancement occurs over a very wide altitude range from approximately 90 to 100 km. The Na and electron abundances between 90 and 100 km are plotted vs time in Fig. 5. Unfortunately this night was partly cloudy and lidar observations wcrc frequently interrupted because of weather. During those periods when the Ii&r field-ofview was obscured by clouds, the Na profiles were linearly interpolated. The interpolated regions are marked by the thick line segments at the top of Fig. 5. This approach is also used to denote the interpolated data in subsequent figures. The incoherent scatter radar is unaffected by clouds and obtained continuous electron density observations during the Na,/E, event. Later in the night Na enhancements occurred between 101 and 107 km near midnight and 0200 LST that were well-correlated with the formation of very dense E, layers and with photometric 557.7 nm observations. The Na and electron abundances between 101 and 107 km are plotted vs time in Fig. 6. The Na

DATA

The most spectacular Na,/E, event observed during AIDAoccurred on the night of 30-3 1 March 1989. The Na,, layer formed over a period of 2.25 h starting near 2100 LST during which time the Na abundance between 90 and 100 km increased by approximately 700%. The electron abundance in this same altitude region also increased by about 700%. Plotted in Fig. 2 are the Na and electron density profiles just prior to the formation of the Na, layer and at the time the Na, layer reached maximum abundance. Plotted in Figs 3

105D

105

1

2200LST

_

85

0

I 1Ocm

5m

801 0

5000

Density (cme3)

loo00

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2Ocm

Density (cmm3)

e-

85 801 0

5coO

1OOcxl

15m

I 2OOLlO 25coO

0

Density (cme3) Fig. 3. Na and e

profiles measured

2OOm

10000

30000

Density (cmm3) during

the formation

of the primary

Na,?/E, layers.

I 4OfxQ

sol

Sporadic

Najsporadic

3KlOO

40000

E layer event

’

0

20000

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Density (cmm3)

801 0

5000

1OOcm

15mO

2OOm.l

Density (cmm3)

I 25000

2Ocoo

Density (cmT3)

Density (cme3) Fig. 4. Na and em profiles measured

during

and em profiles measured at 0025 and 0220 LST are plotted in Fig. 7. Notice that these high altitude Na enhancements are approximately 100 times smaller than the electron enhancements. The spatial and temporal evolution of both the Na, and E, layers arc illustrated by the density contour plots in Fig. 8. The remarkable similarities between the Na and electron densities for both the major low-altitude and minor high-altitude events are clearly evident in this figure. The Na enhancements below 100 km are on average comparable to the electron enhancements. However, by examining Figs 2 and 3 it is seen that at some altitudes the Na enhancements are several times larger than the electron densities while at other altitudes the

the dissipation

of the primary

Na,/E,

layers

electron densities are larger. For example, the 2200 LST profiles plotted in Fig. 3 show four distinct layers in the Na and em profiles at 91,93,96 and 98 km. The Na densities are larger than the electron densities by a factor of 2-5 at 91 and 98 km while the electron densities are slightly larger at 93 and 96 km. The total electron abundance between 90 and 100 km reaches maximum at 2300 LST and decreases rapidly between 2300 and 2315 LST. However, the Na abundance almost doubles during this same period. By comparing the 2300, 2308 and 2314 LST profiles in Figs 2 and 3 it is seen that after 2300 LST the Na increase

”

c

I

0.12,

,250

~‘~~ 2.0 20

21

22

23

0

1

2

3

Time (LST) Fig. 5. Temporal variations of the Na and electron abundances between 90 and 100 km during the Na,/E, event on 30- 3 I March 1989.

21

22

23

Time (LST) Fig. 6. Temporal variations of the Na and electron abundances between 101 and 107 km and the 0(‘S) emission intensity on 30-31 March 1989. The electron abundance has been scaled by 1,‘lOO.

T. J.

504

KANE

el

ul.

(a) 0025LST

‘100

E

200

3Ml

400

500

Electron Densities 8

s .

0

0 .

Density Conto max: 70,000 cm-3 min: 4,000 cm-3 /-_--___--------____

102

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_

600

Density (cmm3 ) ‘lOB---

I

’ 0220 LST

94 Density Contours max: 20,000 cm3 min: 4,000 cm-3

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92’

’

21 0

400

500

’

’

’

22

.

23

8

00

.

’

01

a 02

’

03

Time (LST)

600

Density (cm -3 ) Fig. 7. Na and e- profiles measured at (a) 0025 LST and (b) 0220 LST on 31 March 1989. The electron density has been scaled by I/100.

106

is concentrated in the 93-98 km region with a very dense layer forming near 97 km. Plotted vs time in Fig. 9 are the average Na and electron densities in each 1 km interval between 91 and 99 km. The electron enhancements exceed the Na enhancements between 91 and 93 km while the Na enhancements are 4-5 larger than the electron enhancements between 96 and 99 km. The electron enhancements appear to lead the Na enhancements by approximately 30 min between 93 and 97 km. Below 93 and above 97 km the Na and electron enhancements appear to be in phase. Cross-correlation functions were calculated for the Na and etime series at each altitude and used to determine the time delay between the em and Na enhancements. The results are plotted in Fig. 10. A positive delay implies the electron enhancement leads the Na enhancement. Except for the small anomaly near 98 km, the electron enhancements precede the Na enhancements and the largest delay is almost 30 min at 95.5 km. The short term temporal variations in Na and electron densities are strikingly similar throughout the 90-100 km region. Notice the large increase in Na between 96 and 98 km starting at 2300 LST (Fig. 9). The ratio of the average Na enhancement between 2135 and 2315 LST (the formation period of the Na,Y event) to the average electron density during this same period is plotted vs altitude in Fig. 11. Near 98 km approximately 3.5 Na atoms were generated for each ion in the E, layer. This ratio drops to zero at 100 km

102

Na Densities max: 500 cm-3 min: 200 cm-3

---------------

--I

max: 42,000 cm-3

min: 5,000 cm-s 92 t

211

I

22

23

00

01

02

03

Time (LST) Fig. 8. Density contour plots of the three Na./E, events observed on 30-31 March 1989. Above 100 km the E, densities are considerably larger than the Na, densities while below 100 km the E, densities are slightly smaller than the Na, densities.

and to about 0.5 Na atom/ion below 94 km. Plotted vs time in Fig. 12 are the ratios of the Na and electron abundances between 90 and 100 km and between 101 and 107 km. The ratio varies dramatically with time particularly between 90 and 100 km. These results suggest that below 100 km the E, ions triggered the release of Na from some reservoir (perhaps dust and aerosols), but the E,v layer was not the source of Na. The characteristics of the two NaJE, events above 100 km are considerably different. The formation

505

Sporadic Na/sporadic E layer event

35 94 -95 km -

30 252015 -

96-97km

30 -

10 -

-

50

93 -94km

35

I

I

I

I

I I 95-96km

_

_

25-

,;KQhJ O20

21

Fig. 9. Temporal

22

23 0 Time (LST)

variations

1

of the average

2

3

20

21

22

23

Tie Na and electron 99 km.

and dissipation times (- 20 and - 60 min) of these two events are rapid and the Na enhancements are approximately 100 times smaller than the electron enhancements. By comparison, the 9GlOO km event formed over a period of 135 min, required almost 3 h to dissipate and the Na and electron enhancements were comparable. The mechanism responsible for the formation of the high altitude events is probably considerably different. Because the ion densities are approximately 100 times larger than the Na densities,

densities in each

0

1

2

3

@ST)

1km interval between 91 and

Na ions in the E,Ylayer may be the source of neutral Na for these high altitude enhancements. The airglow observations made on this night were passive, ground-based measurements of integrated column emission rates. Unlike an in-situ rocket measurement, or for that matter, observations with a ground-based lidar, information on the height variation of the observed emissions is impossible to obtain. Historically from rocket observations it is generally assumed that the OH and 0(‘S) emitting layers

506

T. J.

KANE

el

al. 4

go- loolan

i .

-10

0

10

20

I\

I,-j’~] 20

21

22

23

0

1

2

3

30

Time Delay (min) Fig. 10. Altitude profile of the time delay between the electron and Na enhancements plotted in Fig. 9. A positive delay implies that the electron enhancement precedes the Na enhancement.

12k 23

0

Tie

are each slightly greater than one scale height thick and have layer centers near 85-87 and 97-100 km, respectively. The 0, emission layer is generally believed to be twice as wide as the other two emissions with a maximum near 93-95 km altitude. The mesopause region is not expected to be static, however, and some movement in altitude could occur due to the dynamical interaction between wave activity in the region and the background chemistry. Despite these assumptions, an observed response of airglow emissions to the same activity that correlates with the Na,/E,, events would help determine the altitude of the emissions measured. This procedure is examined in more detail by HECHT et ul. (1993, this issue) for other days during the AIDA campaign. The nightly variation of the three mesospheric emissions is shown in Fig. 13. The OH and O2 emissions are generally anticorrelated, which was also demonstrated by NOXON (1978). The trends of the 0, and 0(‘S) emissions agree for the second half of the night while no apparent correlation exists for the two during

I

9oo

1 1

2

Na Density Enhancement/

3

J 4

e- Density

Fig. 11, Altitude profile of the ratio of the average enhancement to the average electron density during period 2135-2315 LST 30 March 1989.

Na the

1

2

3

(LST)

Fig. 12. Temporal variations of the ratio of the average Na enhancement to the average electron density between 90 and 100 km and between 101 and 107 km.

the premidnight period. What is remarkable is the excellent correspondence between the 0(‘S) emission and the higher altitude Na,/E,r event for this night (see Fig. 6). Again, the similarities of the O2 emission with 0(‘S) and the lidar/radar measurements after midnight suggest that the 0, emission layer was moving up in altitude, but as will be seen shortly, the temperature observations do not support this conclusion. Whatever the response to the event at the higher altitudes was, the behavior of the OH emission suggests that this layer was far removed from the bulk of the activity, which is also supported by Figs 3 and 4. That is, the airglow measurements also support the inference mentioned before that the lower and higher altitude layer events are driven by different mechanisms. Figure 14 shows the temperatures derived from the three mesospheric emissions. The top panel now shows the results from FPI observations. Unfortunately the FPI failed to operate after 0100 LST on this night so the nature of the winds or temperatures at 0(‘S) altitudes during the high altitude events is not known. Even though 0(‘S) temperatures were not available for comparison with those derived from 0, after midnight, it is believed that the O2 emission layer was not moving up in altitude because the O2 temperatures did not increase during this period as one would expect above the mesopause. What is apparent from Fig. 14 is that the 0, is colder than

Sporadic

Na/sporadic

both OH and 0(‘S) in the premidnight period (this is also the case for most of the entire AIDA campaign) implying that the mesopause is high, near 95 km, and more than twice as deep (about 4 K/km) than model predictions for these low latitudes. The 95 km mesopause height also corresponds well to the maximum time delay between e- and Na enhancements shown in Fig. 10 and the maximum in the sink function of Fig. 15 (to be discussed later), which may be significant for both the dynamics involved and for the temperature dependent chemistry of the region. This correspondence also suggests that the assumptions for the emission heights, in particular for Oz. have some validity.

E layer

cvcnt

507 Mesopause Region Temperatures T--r--r---m

4. INTERPRETATION OF THE MESOPAUSE REGION LAYERS

Strong evidence exists linking Na,\ and 6, layers. Assuming this relationship, various theories have been proposed to explain the formation of the Na,? layers. The suggested mechanisms can be classified into one of three categories : (I) Na,, and E, are both generated by the same phenomenon, (2) E, layers are the source of the Na in the Na,Ylayers or (3) E,, layers trigger the release of Na from a reservoir. While considerable attention has been given to the formation mechanisms, the dissipation of the Na, layers is even more

150 tt. 19 20 21 22 23 24

1

2

3

4

5

6

7

LST Fig. 14. The observed temperature variation corresponding to the emissions shown in Fig. 13. The 0(‘S) temperature in panel (a) was measured with a Fabry-Perot interferometer.

Mesopause Region Emission Rates 250;17--7-.

Fig. 15. Altitude profiles of the estimated Na source k,,.[M,,] and sink k,,[M,$,] distributions for the main Na,,;E, event.

o 19 20 21 22 23 24

1

LST

2

3

4

5

6

7

Fig. 13. Intensity variations of three different mesospheric airglow emissions measured in the zenith. The value for the P,(3) line of the OH(6-2) band shown in panel (c) is appro~i~tel~ 0. I of the total band intensity.

difficult to explain. Some Na,s layers have been observed to maintain narrow widths (h 1 km FWHM) for periods of 6 h or longer over horizontal distances approaching 2000 km (KANE ef ul., 1993). When the Na, layers eventually dissipate, there is a concomitant decrease in the total Na layer abundance. The Na, layers do not diffuse into the background Na layer but instead appear to be absorbed by a Na sink. The successful theory must explain not only how the Na, layers are generated. but also how their very narrow widths can be maintained for periods sometimes exceeding several hours over very large horizontal distances and how the dissipation mechanism affects only the Na,Ylayer and not the background Na layer. The Brazilian group was the first to suggest that Na, and E, layers are both generated by meteoric

508

T. J. KANE et al.

ablation (CLEMESHA et al., 1978, 1988; BATISTA et al., 1989). While this theory can explain some of the characteristics of Na, layers, it cannot explain why the ratio of Na to ions illustrated in Figs 9, 11 and 12 varies so dramatically with altitude and time for the 3&31 March event. If the ablation of a meteor generated both the Na, and E, layers, then the Na/eratio should be approximately constant or show a systematic variation with altitude and time. This is clearly not the case for the data reported here. BEATTY et al. (1989) point out that the vertical motions of the E, and Na,, layers should be different if they are constituents of the same meteor ablation trail because the ionized E, layer will be influenced by the Earth’s magnetic and electric fields while the neutral Na,y layer will not. Their observation of one event at Arecibo showed the E,, layer forming almost 1 h before the Na, layer and then both the E, and Na, layers moved upwards together. The meteor ablation theory attributes dissipation of the Na,, layers to horizontal advection but cannot explain how very thin layers of neutral Na can be maintained for hours without significant diffusion. Although it is generally agreed that the measured concentrations of Na+ in the mesopause region are far too small to generate rapidly the large amounts of Na needed for the more prominent Na,, layers (HANSEN and VON ZAHN, 1990; BATISTAet al., 1989), this mechanism will be discussed in more detail in the next section in relation to the two high altitude NaJE, events observed near 0025 and 0220 LST. The University Bonn group initially suggested that the Na, layers and the accompanying E, layers are formed by the impact of aurora1 particles on upper atmospheric dust and smoke particles (VON ZAHN et al., 1987; VON ZAHN and HANSEN, 1988). They argued that this process would evaporate and ionize metals which are adsorbed on the surfaces of the dust particles. The authors did not describe the mechanism for generating the thin layers of dust but the greatest weakness of this theory is that it cannot explain Na, layer formation at low latitudes. Because the Na, layers at Mauna Kea, Arecibo and Sgo Paulo are very similar in character to those observed at Andoya and Svalbard, it seems likely that they are all generated by the same mechanism. Deficiencies in the meteor ablation theory favored by the Brazilians and the aurora1 bombardment mechanism proposed by the Bonn group, led BEATTYet al. (1989) to suggest that the Na,, layers were formed by the interaction of the ions in the E, layer with a Na reservoir which triggered the release of Na. They theorize that the bombardment of upper atmospheric dust and smoke particles by the E, ions releases Na

cluster ions. Subsequent collisions of the E, ions with the Na cluster ions release neutral Na. This mechanism could work at both high and low latitudes and does not require the presence of a thin dust layer. Because Na is generated by collisions between the Na cluster ions and the Es ions, the Na,, layer would stay compressed into a narrow layer of approximately the same thickness as the E, layer. The motions of the Na,? and E, layers would be similar because the E, ions and the Na cluster ions would both be influenced by the Earth’s magnetic and electric fields. Dissipation of the Na,, layers can occur due to further ionization processes and subsequent reattachment of the ions to dust. Recently, VON ZAHN and MURAD (1990) proposed another mechanism in which NaHCO: is converted to neutral Na through a reaction that is facilitated by tf : high electron densities in the E, layer. The BEATTYet al. (1989) and VONZAHN and MURAD (1990) mechanisms are examples of category 3 mechanisms in which the E, layers trigger the release of Na from some reservoir. By appropriately modeling this type mechanism it is possible to infer some information about the Na source and sink distributions during the 3&3 1 March event. We assume a one-step process in which the Na,, layers are generated by the interaction of E, ions or electrons with a Na reservoir. Similarly, the Na, layers are dissipated by interaction of atomic Na with a sink. Let [M,, (z,t)] and [Mrk(z,t)] denote, respectively, the vertical and temporal distributions of the Na sources and sinks. In this case we can write ?[Na] ~~ =

?t

~,,~~.s,.l~~~l-~,~~~~~l~~~l (1)

where k,, and k,sLare the rate coefficients. The source and sink distributions as well as the Na and electron concentration can vary with altitude and time. Because [e-l is small and the Na layer is in equilibrium with all the sources and sinks prior to the Na, event, we may assume that [Msk] is negligible in the early phases of the event, so that I

?[Na]

One sees from this relationship that the rate of increase of [Na] is proportional to [e-l. Even if the E> layer occurred suddenly, the associated Na, layer would require some time to form. This might explain the lag between E, and Na, formation already seen in the data of Fig. 10. During the dissipation phase of the event when [e-l - 0. the source term is negligible so that ks,[M\,]

-1 a[Na] 2 [~a] at

Sporadic

Najsporadic

The Na and electron density data for the 30-31 March event were used with equations (2) and (3) to obtain estimates of the source and sink profiles scaled by their respective rate constants. The results are plotted in Fig. 15. At 2140 LST the source appears to be broadly distributed between 94 and 99 km with distinct peaks near 94.7 and 98.3 km. The maximum production rate is approximately 9 x lo-’ Na atoms per electron per second. At 2350 LST when the total electron abundance had dropped to about 25% of its maximum value, the sink distribution was broadly distributed between about 93.5 and 97.5 km with peaks near 94.7 and 96 km. The maximum Na dissipation rate was approximately 8 x IO ’ s ’ BEATTY rt al. (1989) suggest that atomic Na and Na cluster ions are released from the surfaces of upper atmospheric dust particles by collisions with the E, ions and the Na, layers are dissipated by reabsorption by the dust particles. Equilibrium is reached when the particles become saturated with Na and the E, layers dissipate. Under this scenario the source and sink distributions should be similar. The data plotted in Fig. 15 are at least qualitatively consistent with the Beatty et al. mechanisms. The differences in the source and sink profiles in Fig. I5 may be due to vertical transport during the 2 h 10 min between the measurements and to differences in the altitude distributions of k,, and k,, which probably depend upon temperature. Ground-based observations at Mauna Kea (KWON et al., 1988) and Sao Paulo (BATISTA ct al., 1989) and recent airborne observations near Hawaii (KANE et al., 1993; GARDNER et al., 1991), have shown that tides can play an important role in the vertical transport of Na, layers. The downward vertical movement of the upper peak in Fig. 15 is approximately 30 cm s ‘. Vertical wind amplitudes of this magnitude are reasonable for the semi-diurnal tide at mid-latitudes (KWON et al., 1987; FRANKE et d.. 1990). In the Beatty et al. mechanism the dust particles serve as both the source and sink for Na so that the distributions of [M,,] and [M,,] should be similar. Thus, the distributions plotted in Fig. I5 suggest that k,,,ik,, - 5-10 and this may explain why the dissipation periods of Na, events arc typically 225 times longer than the formation periods. It is important to note that the inferences regarding the possible source and sink distributions discussed above are only qualitative because we have assumed that the temporal variations in the Na and em profiles are caused entirely by chemical processes which can be modeled by equation (1). It is likely that some of the observed temporal variations, especially the rapid small scale changes, are caused by the horizontal advcction of the Na, and ,!?\ clouds over the observ-

E layer event

509

atory by the background winds. Our analysis is further complicated by the fact that the radar and lidar were not observing a common region of the atmosphere (see Fig. 1). Measurements from Hawaii obtained during the ALOHA-90 campaign show that some large Na, layers can form quickly over regions of several thousand km (KANE et al., 1993 ; GARDNER et al., 1991). The intensity and duration of the 3&31 March event suggests that it also covered a large geographical area. We believe the radar and lidar at Arecibo observed the formation of these Na, and E, layers and that the major temporal variations reflect the dynamics of the formation and dissipation mechanisms.

5. INTERPRETATION

OF THE LOWER REGION

THERMOSPHERE

LAYERS

The characteristics of the Na,?/E,, layers observed above 100 km after local midnight are considerably different from the major lower altitude event. Although the E, densities are several times larger than those observed below 100 km, the Na enhancements are very small. The total Na abundance between 101 and 107 km (- 10” cm->) is approximately 100 times smaller than the electron abundance (Fig. 6). Also the Na,, layer is considerably broader than the E,$ layer (- 3 km FWHM compared to _ 1 km FWHM) and is displaced slightly above the E, layer (Fig. 7). The mechanisms for the generating of these high altitude Na, layers appear to be considerably different from those responsible for the main Na,, event. The conversion of Na+ in E, layers to Na through charge exchange or clustering reactions has been discounted as a source of the larger lower altitude Na, events because unrealistically large quantities of Na+ are required (CLEMESHAet al., 1980 ; VON ZAHN and HANSEN, 1988). Data on metal ion abundances in E, layers are rare, but the existing in-situ measurements obtained with rocketborne ion mass spectrometers suggest that Nat abundances are typically a few percent of the total ion abundances. Thus the E, layers plotted in Fig. 7 are expected to have maximum Na+ densities of - 1000 cm-’ or larger. By assuming Na+ densities of 1000-3000 cm 3, HANSEN and VON ZAHN (1990) numerically modeled the conversion of Na+ to Na through a set of clustering reactions involving N,, CO, and H20. By comparing the numerical calculations with NaJE, observations at Andoya, they concluded that the characteristics of a large fraction of the smaller high altitude (> 95 km) Na, events they observed are compatible with the assumption that the Na, layers are caused by the conversion of Na+ in the

510

T. J. KANE et al.

E, layers to neutral Na. Hansen and von Zahn also point out that the E,>cloud is influenced by the wind shear while the Na,, layer is not. If the wind shear is associated with the downward phase progression of a gravity wave or tide, then as the E,y cloud moves downward it should leave behind a wake of Na atoms since the Na lifetime is much longer than the Na+ lifetime with respect to cluster ion formation. The differences between the E, and Na,\ profiles plotted in Fig. 7 may be caused by the wake effect predicted by HANSENand VON ZAHN (1990). During the event, the peak of the Na, layer was between 0.5 and 1 km higher than the E, peak and both moved downward between midnight and 0300 LST with a velocity of approximately 15 cm s ’ The high altitude Na,,/E, event also appears to be correlated with 0(‘S) emissions above Arecibo. The 0 emission intensity is plotted vs time in Fig. 6 along with the Na and electron abundances. We do not know what mechanism is responsible for the apparent link between the lower thermospheric sporadic E and Na layering and the intensity enhancements observed in the 0(‘S) emission. The correlation among these data is striking. Similar strong correlations between E, and 0(‘S) have been reported before (GORBUNOVA et al., 1982) but not for Na, layers. Perhaps the same wave activity that triggers the Na, layers through an interaction with E, simply disturbs the background airglow in phase at the height of the 0(‘S) layer. Below this altitude at the O2 emission layer height, or even near OH, the effect may not be as strong if the disturbing wave amplitude is weak. The meridional and zonal winds observed with the FPI for the first half of the night are shown in Fig. 16. These measurements are coincident with the first Na,/E, enhancement near midnight shown in Fig. 5 but not with the second event in the early morning. Although there is more scatter or variability in the zonal component than for the meridional in Fig. 16, there is a general trend for the mean winds to be changing from westward to eastward during the 6 h of operation. The meridional component, however, was more consistent northward, with a southward turning that corresponds to both the first 0(‘S) intensity enhancement (Fig. 13a) and the midnight NaJE,$ event (Fig. 6).

6. CONCLUSIONS

The data presented in this paper demonstrate conclusively the strong correlation between certain Na, and E,, events as well as airglow emissions. The mechanism responsible for the low-altitude NaJE, event

0(‘S) NeutralWind Components

50

II

L..’ ._

t B @ > -50

-1

, (A) v i

I

1

,,,,,

z

19 20 21 22 23 24 1 2

3

4

5

6 I

LST Fig. 16. The meridional and zonal wind components measured by observing the 0(‘S) 557.7 nm emission with a FabryyPerot interferometer. The winds are positive northward and eastward for panels (A) and (B). respectively.

between 90 and 100 km is clearly much different from the mechanism responsible for the two high-altitude events, which have Na densities of less than 1% of the electron densities, are most likely caused by the conversion of Na+ in the E,, layer to neutral Na. The Na enhancements for the main event are very large, at some altitudes exceeding the electron densities by a factor of 3 or more. The remarkable similarities between the vertical and temporal structure of the Na and electron densities and the large Na enhancements suggest that the E,, ions or associated electron cloud triggered the release of Na from a reservoir. The amount of released Na indicates that the reservoir was quite large for this case compared to Na, layers observed elsewhere. The simple source and sink models investigated in this paper suggest that both the Na reservoir and sink for this event were broadly distributed between about 93 and 99 km. Unfortunately, the data provided little insight into the physical and chemical characteristics of the Na reservoir and sink. The data are qualitatively compatible with both the dust/Na cluster ion model proposed by BEATTYet al. (1989) and the NaHCO, reservoir model proposed by VON ZAHN and MURAD (1990). Much has been learned about the characteristics of sporadic Na layers during the past several years. The phenomenon is restricted primarily to low (,< 25 ) and high (2 60 ‘) latitudes and as the data in this paper show, Na, is closely related to the occurrence of E, layers. Recent airborne lidar observations in Hawaii have shown that some Na, events can extend over

Sporadic Na/sporadic E layer event

horizontal distances approaching 2000 km (KANE et ul., 1993) and can be accompanied by enhancements in both OH Meinel band and 0, atmospheric band emission intensities and temperature (GARDNER et al., 1991). Tides also appear to play a crucial role in the

tion, aerosol and dust distributions and temperatures within the Na,/E, layers are especially important.

vertical transuort and Derhaus the formation of some Na, layers. Future piogress in understanding this puzzling phenomenon will require a multiplicity of observations including both remote sensing and insitu measurements. Measurements of the ion composi-

Acknol~ledgement.F-The authors would like to thank T. J. Beatty. R. E. Bills and C. A. Hostetler for their assistance with the lidar installation and data acquisition at Arecibo. The operation of the Illinois lidar at Arecibo was supported by NSF grant ATM 88-00513 as part of the CEDAR i%tiative. The work of the University of Illinois group was also supported in part by NSF grant ATM 88-l 1771, The Arecibo Observatory is operated by Cornell University under a cooperative agreement with the National Science Foundation.

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