Downward transport of nighttime Es-layers into the lower E-region at Arecibo

Journalof Atmosphericand Terrestrial Physics,1974,Vol. 36, pp. 225-234. Pergamon Press. Printedin h’orthern Irel%~(l

Downward transport of nighttime Es-layers into the lower ~-region at Arecibo J. F. ROWE, JR.* Arecibo Observatory, Arecibo, Puerto Rico 00612, U.S.A. (Received 16 March 1973; in revised form. 31 May 1973) Abstract-Incoherent scatter observations at the Arecibo Observatory have shown that nighttime electron density enhancements are formed in the upper E-region and move down to merge with a stable Es-layer lying between 105-110 km. The phenomenon is strongest in winter months, and is accompanied by the presence of an underlying Es-layer near 90 km. T&se observations provide support for a nighttime ‘corkscrew’ mechanism which sweeps metallic ions down from higher altitudes and deposits them between 105-110 km. A statistical study of the wind shears necessary to maintain nighttime Es-layers suggests that strong layers lying

below 110 km are composed primarily of long-lifetime metallic ions.

TRE PROPERTIES of the nighttime E-layer have been described in detail by WAKAI (1971), and a collection of nearly 100 nighttime E-region density profiles from widely spread stations has been assembled and classified by KNIGHT (1972). This layer is centered at about 100 km and is some 30 km thick with electrondensitiesl~ng between 103and 104/cm3,and is often characterized by pronounced vertical structure. The region between the nighttime-E and the base of the P-layer is usually beyond the resolution limits of ground-based measurements since, on magnetically quiet nights, electron densities commonly fall to a few hundred per cm3. However, during periods of magnetic disturbance, this nighttime valley fills in to electron densities comparable to the lower E-layer (WAKAI, 1967).

Several mechanisms have been proposed to explain the persistence of the nocturnal E-layer throughout the night, and it is quite likely that all are operative at various times. The possibility of 0, ionization in the nighttime E-region by scattered solar Ly-,C-Iwas suggested by SWIDER (1965). YOUNG et at. (1968) detected Ly-@ in sufficient strength to maintain the night E-layer at lo3 cm-3, and TOHMATSU and WAKAI (1970) reported the foE symmetries about local midnight that such a mechanism could produce. Daytime rocket mass spectrometer observations of long-lifetime metallic ions in the B-region were reported by YOUNGet al. (1967). Layers of these ions can remain at night when the background of NO+ decays away, and can account for the serrated structure often seen in nighttime E-region profiles. Mechanisms involving downward transport of plasma into the E-layer have been described by COLE (1969) (J x B Lorentz forces), and CHIMONAS and AXFORD (1968) (trapping of ionization in descending wind profiles). This paper will present nighttime E-region incoherent scatter data from Arecibo in which layers of ionization were transported downward from the upper E-region and merged with sporadic E-layers lying between 105 and 110 km. Rocket observations of layer descent through the nighttime valley have been presented by SBXITH (1970).The interaction of plasma and measured neutral winds on this night has * Present a

address;

Lockheed Missiles and Space Co., Box 504, Sunnyvale, California 94088. “Z5

J. F. ROWE, JR.

226

been analyzed in detail by FUJITAKA and OC+AWA(1971), CONSTANTINIDESand BEDINGER (1971), and CHEN and HARRIS (1971). The data presented here are inferior in comparison since drift measurements are not available. This drawback is, however, partially compensated for by the continuous time coverage offered by the incoherent scatter technique. Owing to extensive equipment improvement in early 1972, the Arecibo Observatory is now capable of resolving the complex layer patterns and motions characterizing the night E-layer. This layer often exhibits profound changes in the time scale of several minutes, and in wintertime, consistently responds to the passage of pre-midnight disturbances. EXPERIMENT AND DATA REDUCTION For nighttime E-region studies, the Arecibo 436MHz 2*5-MW phase modulated with a 13-bit Barker-coded pulse of 6-psec baud 0.9 km height resolution with a vertical beam. The ionospheric echo the RF carrier frequency to recover the Barker-coded video envelope, transferred to the computer for off-line processing. Digital decoding in a device developed by IOANNIDIS and FARLEY (1972). The backscatter gain is given by

transmitter is length, giving is mixed with decoded, and is performed

G(r) =

B f”(0, 4, r) sin 0 d0 d$ (I) ss where g(0, 4, r) is the amplitude gain and 0, 4 and r are the vertical angle, polar angle and range, respectively, of an integration element. The ‘near field’ correction factor, G(r)/G( co), h as b een computed by SHEN and BRICE (private communication) from measurements made on a model of the new Arecibo line feed, and is shown in Fig. 1.

THE NEAR

ARECIBO FIELD

430

MHz

CORRECTION

HEIGHT (KM)

Fig. 1. The near field correction factor VS. height for the arecibo radar antenna (SHEN and BRICE, private communication)

Downward transport of nighttime Es-layers into the lower E-region at Arecibo

The radar equation

relevant to the incoherent

‘(‘I = K [l

227

scatter ion line (EVANS, 1969) is

N&9G(r) + a2(r)][l + T,(r)/TJr)

+ a”(r)]r” wattse

where P(r) is the echo power, N,(r) the electron density, and T,(r) and T,(r) the This expression was derived on the basis of a colelectron and ion temperatures. lisionless plasma. We shall assume that collisions do not appreciably modify the result. K is a constant of the radar, and is evaluated by normalizing the power profile to ionosonde data. The quantity a(T) is equal to 4n times the Debye length divided by the wavelength, a(T) =

276~ ii

(4

where L is the 430 MHz radar wavelength. The Debye length factor determines the distribution of power between the ion and electron components of the spectrum, and can usually be ignored in daytime E- and F-region work. The phase-coding process has detected electron densities of a few hundred per cubic centimeter. At these low density levels, Debye length effects on the incoherent scatter cross section are no longer insignificant, and must be accounted for. Equations (2) and (3) require a knowledge of electron and ion temperatures. Since the backscatter runs are made between the hours of 2100 and 0400 local time, it is assumed that the ionosphere is in thermal equilibrium, i.e. T, = Ti = T,. Neutral temperature, T,, is taken from the CIRA 1965 model atmosphere for 0000 hr local time and mean solar activity. The absolute electron density scale of the power profile is obtained from foF2 ionosonde data and equation (3) is inverted for N,. OBSERVATIONS A contour plot of electron densities taken at Arecibo (18”N geographic, 30”N magnetic) on the night of 29-30 November 1972 is shown in Fig. 2. The data is presented in profile form in Fig. 3. There is clear evidence of downward motion of plasma layers into the lower E-region where they merge with a sharp sporadic-E layer lying between 105 and 110 km. The second profile of Fig. 3 (2218) shows two well-defined, descending layers centered at 130 and 160 km. The lower layer continues downward to combine with a sharp layer at 108 km, while the upper layer remains nearly stationary at 140 km for 1 hr. Subsequently, at 2354 this layer resumes its descent, and by 0030 has merged with the underlying layer. A layer lies at 90 km through the sequence of events, and shows no significant vertical motion. Four nights of observations were made during the period November 197% January 1973, and on three of those nights, the transport phenomenon described above has been observed. Hence, it is taken as typical of the winter nighttime E-region at Arecibo latitudes. While some evidence of this phenomenon appears in the six summer nights of data presently available, the mechanism appears weak in comparison to the winter months. An underlying layer between 90 and 95 km is always present when downward layer transport is observed, but frequently appears during nights when no transport is seen.

228 170 -I-

160

IS0

I40

I30

z Fi c9 iii f

120

it0

100

90

60 i-I 2200

2360

0100

2400 AST

29-

30

NOV 1972

Fig. 2. Electron density contour plots at Areoibo, 29-30 November

1972.

DISCUSSION The data presented in the previous section are interpreted as evidence for the existence of a nighttime winter ‘corkscrew’ phenomenon as described by CHIMONAS and AXFORD (1969). Lines that are hypothesized to correspond to the trapping points of an internal gravity wave have been placed on Fig. 2. While large-scale transport is not associated with the central line, & layer does appear to form and descend to 140 km (Fig. 3), and the sporadic-E layer enhancement at 2345 near 105 km is consistent with the passage of a strong east-west shear through the ionization in this region. This presumed gravity wave exhibits s, period of 50 min with a vertical wavelength of 33 km. HINES (1964) has shown that such B wavelength becomes severely damped between 140-150 km by viscosity. From the early profiles of Fig. 3, the response of plasma to this disturbance decays rapidly upwards, and is barely detectable above 160 km, agreeing well with Hines’ theory. Dumping by these waves is observed in the data presented here to occur in the 105-110 height range, whereas the ‘corkscrew’ calculations of CHIMONAS and AXFORD (1968) have predicted dumping levels of 85-95 km for numerical computations.

200

200

LOG

LOG

N,lcd

N,/sm3

,’

,f’

,’

/ :

I’

/’

,’

/’

Pig. 3. Eleotron density profiles at Arecibo, 29-30 November 1072. Each profile is a 12-miu centered at the time shown. The dotted lines indicate hand smoothing of noise fluctuations, be oonsidered SXJindicative of electron densities.

HEIGMT (KM)

HElGliT (KM)

integration and should

J. F. ROWE, JR.

230

I BO

I

90

I

I

I

100

110

ItO

I

140

110

HEIGHT

(KM)

Fig. 4. Plots of normalized ion-neutral collision frequency (Y&D) vs altitude. The Arecibo measurements by Zamlutti, and Wand and Perkins assume NO+ as the ion species.

The difference is significant, and cannot be accounted for by the difference of magnetic field parameters in the calculations from those at Arecibo. Figure 4 is a plot of normalized collision frequencies from measurements made at Arecibo by WAND and PERKINS (1968) and ZAMLUTTI (private communication), as well as data derived from a JACCHIA model (1971) and BANKS’ (1966) vin collision formula. The normalized collision frequency curve used by Chimonas and Axford is seen to be from 7 to 14 km below these other curves at the dumping level (v/w N 50). These data and the observations presented here in suggest that the dumping level occurs above 100 km, and that the persistent Es-layer lying at about 105 km is fed by downward transport via the ‘corkscrew’ mechanism. The existence of metallic ion layers lying in the vicinity of 90 km has been reported at equatorial latitudes by AIKIN and GOLDBERG(1973), and in midlatitudes by NARCISI (1968). A statistical study of nighttime sporadic-E layer altitudes at Arecibo (to be published separately) has shown that the region from 90 to 93 km is a strongly preferred zone of sporadic-E layer location and it is considered likely that the persistent layer at 90 km in Fig. 3 represents an accumulation of metallic ions as reported elsewhere. Model studies of meteoric input of metallic ions (LEBEDINETS and SHUSHKOVA, 1970) show a maximum deposition rate near 107 km, very close to the dumping levels observed in the data presented here. Metallic ions lying above 110 km are

Downward transport of nighttime Es-layers into the lower E-region at Arecibo

231

subject to the sweeping effect of internal gravity waves, and it is likely that a transition zone for relative absence to presence of long-lifetime metallics occurs in this zone. While direct compositional measurements are not made in conjunction with the Arecibo nighttime E-region runs, it is possible to estimate the recombination coefficient with the aid of reasonable assumptions concerning wind shear statistics. Comparisons of the computed recombination with laboratory values of uKo+ permit inferences to be drawn concerning the presence of metallic ions. The relevant electron density continuity equation in the E-region is

GIN -=qat

aN”--.Nv+Dg

set-l

where cc is the effective recombination coefficient, q the ion production, and D the diffusion coefficient. At the peak of a steady-state Es-layer, 8Nla.z = aN/at = 0, and assuming horizontal stratification (a/ax = a/&j = 0), equation (4) can be solved for the gradient of vertical (positive upwards) plasma velocity:

q-aN2+Dg

av,--

N

a2

hmax

set-l

where h,,, is the height of the sporadic E-layer electron density maxima. Below about 130 km, eastward winds are more effective than northward winds in moving ionization vertically (CHEN and HARRIS, 1971), and to a good approximation

(6)

where I is the magnetic field dip angle (50” at Arecibo), U, is the geomagnetic eastward component of neutral wind, and v/w is the ratio of ion-neutral collision frequency to ion gyrofrequency. It is noted that equation (6) ignores electric fields and the associated gradient instability (BEER and MOORCROITT,1972a, b; KATO, 1972), and implicitly assumes that the observed layers are all generated by shears in the neutral wind (WHITEHEAD, 1960). Differentiating

equation (6) V -

au o -2 N a2

oosIC (vyE+ l$

u_d(~,~(~))

set-I.

(7)

0.

Rocket observations suggest that Es-layers tend to accumulate at the nodal point of ion vertical drift velocity (FUJITAKA and OGAWA, 1971) and since eastward winds are most effective in moving ionization vertically in the lower E-region, we assume from equation (6) that v, = U, = 0 at the layer peak and omit the second

232

JR.

J. F. Ram,

term in equation (7). Equations (5) and (7) can be combined to give an estimate of the shear term at the layer peak: V2

I+

-

y-d2+Dg

0

au, -2F

-

V

set-I.

(‘[

N

cosI

(8)

I

h max

0.)

The terms involving electron density can be estimated from the incoherent scatter data on the Es-layer, and the collision frequency and diffusion coefficient can be estimated with the use of a neutral model. The nighttime production function q is on the order of 1 cmB3 see-l, and is negligibly small compared to the recombination term in an NO+ layer of 104/cm3 peak density. The unknowns in equation (3) are au,/& and a. In Fig. 5, equation (8) has been

I-

z IdY

2 it 2 LL

9

H

0

,02YIIIYUY

I

WEAR

--A_---_---_---.

ROSERSERB 2IYIERYAW

?

::

*

(1972)

-_

----__

---_---_-_-_-_

w

9 !i 0

AVERABE SHEAR ---_---------_-

6

r

IO-

HEIGHT

(KM)

Fig. 5. Histogram of computed E-W wind shears necessary to maintain the observed nighttime Es-layers, assuming an NO+ recombination coefficient of 4 x lo-’ cm3 se+. Statistical average and maximum shear values as reported by ROSENBERG and ZIMMERMAN (1972) twe superimposed.

Downward transport of nighttime Es-layers into the lower

E-regionat Arecibo

233

used to compute the shear term of all Es-layers observed in ~ghttime E-region data collected during 17 nights of 1972 and 1973. Consistent with the laboratory measurements of BIoNDI (1968), an NO+ recombination coefficient of 4 x lo-’ cm3/sec has been used in the computations. ROSENBERGand ZIMMERMAN(1972) have given values of maximum and average east-west shears encountered in a vast collection of rocket flights. These values are superimposed over the data in Fig. 5. Above 110 km, the data coincide acceptably well. Below 110 km, the computed shear necessary to maintain the observed layers in the presence of NO+ re~ombinatio~l rises sharply, and by 95 km is an order of magnitude higher than the maxirnull~ shears encountered by Rosenberg and Zimmerman. The data can be brought into agreement by tapering the assumed recombination coefficient beginning at 110 km. Physically, this is indicative of large percentages of metallic ions in nighttime Es-layers below 110 km. The transition near 110 km appearing in a statistical treatment of Es-layers suggests that the corkscrew mechanism regularly operates in the nighttime E-region, sweeping meteoric ionization down from above this altitude. CONCLUSION

Contour plots show electron density enhancements moving down in altitude from above 140 km to 105-110 km, where they merge with sporadic E-layers. The consistency of these observations with the CHIMONAS and AXFORD (1968) ‘corkscrew’ theory is taken asevidence for that mechanism in the nighttime E-region at Arecibo. It is argued that the dumping level is actually over 10 km higher than presented in their paper. ~ombi~ng 1’7 nights of incoherent scatter profile data with statistical rocket wind shear data, model calculations further suggest that as a regular feature of the nighttime E-region at Arecibo, sporadic E-layers below (and not above) 110 km are primarily composed of long-lifetime metallic ions. rlcknowEedge?nent-The author thanks Dr. H. C. CARLSONfor his interest and criticism. The Arecibo Observatory is operated by Cornell University with support from the National Science Foundation. R~~ERE~~E~ AIKIN A.C.and GOLDBERG.% A. BASKS P. BEER T. and MOORO~OFTD. R. BEER T. and MOORCRO~FT D. R. BIONDI M. A. CREN W. M. and HARRIS R. D. CHIMONASG. and AXFORD W. I. COLE K.D. ~~ONSTA~TI~~DESE. and BEDINGER J.F. EvANsJ.V. FUJITAx K.and OGAWAT. HINES C. 0. IOANNIDIS 0.andF~x~1z~I3.T. KATO S. KNIGHT P. lA~~~~~~~~~V.N. and SWSHKOVAV.~.

1973 X966 1972a 1972b 1968 1971 1968 1969 1971 1969 1971 1964 1972 1972 1972

1970

J. geo~l~~~.Res. 78, 734. Planet. Space, Sci. 14, 1105. J. atmos. terr. Phys. 34. 2025. J. atmos. terr. Phys. 34, 2045. Can J. Phys. 47, 1711. J. atmos. terr. Phys. 33, 1193 J. geophys. Ree. 73, 111. PEaRet. Space Sci. 17, 1977. J. atop. ten+. Whys. 33, 461. Proc. IEEE

57, 496.

J. atmos. terr. Phys. 33, 667. J. geophys. Res. 69, 2847. Radio Sci. 7, 763. Radio Sci. 7, 417. J. atmos. terr. Phys. 34, 461. Plant. Space Sci. 18, 1659.

234

J. F. ROWE, JR.

NARCISI R. S.

1968

ROSENBERG N. W. and ZIMMERMAN S. P. SMITH L. G. SWIDER W. TOHMATSU T. and WAKAI N. WAKAI N. WAKAI N. WAND R. H. and PERKINS F. W. WHITEHEAD J. D. YOUNG J. M., JOHNSON C. Y. and HOLMES J. C. YOUNG J. M., CARRUTHERS G. R., HOLMES J. C., JOHNSON C. Y. and PATTERSON N. P.

1972 1970 1965 1970 1967 1971 1968 1960 1967

space Research VIII, p. 360. NorthHolland, Amsterdam. Radio Sci. 7, 377. J. atmos terr. Phys. 32, 1247. J. geophys. Res. 70, 4859 An&. geophys. 26, 209. J. geophys. Res. 72, 4507. J. Radio. Res. Labs. Japan 18, 245. J. geophys. Res. 13, 6370. Nature, Land. 188, 567. J. geophys. Res. 12, 1473.

1968

Science

Reference

is also made to the following

JACCHIA L. G.

unpublished 1971

160,990.

muterial: SAO Rept. No. 332, Smithsonian Institutional Astrophysical Observatory, Cambridge, Mass.