Upward-moving irregularities in the subequatorial ionosphere

Press.Printedin Northern Ireland Soemalof Atm~phe~~andTerre&&lPhysics,1DT1,f’ol.33,pp. 16~i-l6tl~.Pergsmon

Upward-moving irregularities in the subequatorial ionosphere J. M. FAYNOT, P. VISTAand J. WALTER GRI-CNET, 92 Issy-les-Moulinectux, France (Received 24 November

1970; i?~ revised form 32 Marclb 1971)

Abstract-Upwesd-rising equatorial strata, differing from other h’(f) structures, &re described. The rising motion, analysed by Nfh) profiles, can be explained at ES?-region levels by Hell drift from the daytime wxd-eest electric field. Day-to-day and seasonal variations are discussed.

of direct evidence for vertical motion in the equatorial belt, where strong dynamo and motor effects act independently, has long been a difficulty of Martyn’s ionospheric fountain model. Etecently, WOODMANand BAL~LEY (1969) have shown cases of perfect correlation between the E-region cross-magnetic field motion V,E and the F-region upward velocity V,F and RASTOGI (1970) discovered rising irregularities on sequences of ionograms at Thumba. Similar events were found independently in 1969 at Fort-Archambault (Chad) by one of us and some corresponding cases have been subsequently traced at Ouagadougou (Upper Volta). In this paper, we describe the African rising irreglllarities, discuss their bearing on the vertical motion Vi,F, and estimate some values for the corresponding east-west electric field.

THE LACK

2. CHARACTERISTICS FOR 1969 2.1 Virtua,l-trace morphology We describe a new type of ionospheric irregularity observed in day-time at very low magnetic latitudes (Ouagadougou, # = 4*5”N, Fort-Archambault, # = 1.5’S) during high sunspot activity periods. On the African ionosonde map of Fig. 1, only Ouagadougou and Fort-Archambauft are close enough to the dip equator to consistently observe the rising irregularities. The phenomenon seems to be restricted to a belt: -+6” north and south of the dip equator. It appears on the ionograms (Fig. 2) as a thin layer of vertical thickness 2-10 km, drifting upwards from E-region to upper F-region levels, without changing shape. At Fort-Archambault, the relative ionic density increase at F-region levels remains less than or equal to 5 per cent. At Ouagadougou, 15 per cent excess densities are frequent (as on our example of Fig. 3), extreme cases reaching 30 per cent near f,P.Z levels. We will use symbol Y to qualify these rising strata. Several partial or complete Y events can be observed on a single day at irregular intervals. Y strata can occur with equal probability at any time between 0800 and 1700 LT, but only on certain days of steady and rather quiet magnetic activity. They reach the PZ-layer peak after its morning maximum at both stations. The whole ascent of a given stratum lasts 2-3 hr at Fort-Archambault and 1631

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J. M. FAYNOT,P. VILA and J. WALTER

Fig. 1. African ionosondes map, with magnetic latitude lines: + = 0, thick lines: + = &6’, thin lines.

3-4 hr at Ouagadougou. In all cases however, the irregularity remains discrete; it cannot be confused with other stratifications: --foE’.Z oblique-propagation types would never show the two magnetoionic components give such indentical thin layer shapes. ----PIG stratification types, associated with the depleted ledge at the inner boundary of the ‘Arch’ of enhanced ionization, have 3-5 times larger scales in space and time. The close similarity of the o and x traces further shows that, even in the neighbourhood of the irregularity, horizontal stratification will grant truly zenithal N(h) profiles. 2.2 N(h) analysis of the rise True height analysis is not complicated by the presence of the irregularities, due to their limited extent on the virtual traces. Several series of N(iL) profiles heve been computed by the lamination method of DOUPNIK and SCHMERLING (1965). In all these cases, the altitude-time slope dh/dt flattens out noticeably near foFl levels. Figure 4 shows an example of true-height variation, scaled on the lower-irregularity cusp of the f-plot Fig. 3. Three different altitude zones are

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Upward-moving

irregularities in the subequatorial ionosphere

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Fig. 3. f-plot

for Ouagadougou, 25 October 1969, 0700-1400 LT. lower tips of bars mark the two ordinary-trace cusps.

Upper and

LT

Fig. 4. Altitude vs. LT variation for the sequence of Fig. 3.

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J, M. FAYNOT, P. VILA and J. WGTER

consistently found as follows: Through E and 1Tt’llevels, I’, ranges typically from 10 to 25 m see-l. (2) Beyond f,P1, V, seems to decrease across the 180-220 km altitude zone, then to rise rapidly again. (On our example, this height interval lies between 210 and 250 km,) Altitude variations through these levels probably correspond to changes in local production and maintenance of the irregularity; these have to fuRi1 certain conditions for a third phase to be reached. (3) Up to maximum 27.2levels, V, remains constant, with values from 20 to 60 m see-l, depending on the day (35 m see-” on our example).

(1)

2.3 Seasonal a& day-to-day occurrence patter% Table 1 summarizes the seasonal distribution of rising irregularities at FortArchambault. These, mostly observed during low magnetic activity periods (Ap Q 10) do not generally occur on the same days as the slant type of sporadic-E, Table 1. Occurrence of risers and slant sporadic-E events, in percentage of days

Periods

Average duration of event (hr)

Y (per cent)

March-May May 254une 21 July August

93 <2 92 92

6 29 33 57

Aug. 21-Nov. 15 Nov. 16-Feb. 15


Type

25 80

ESS (per cent)

Magnetic season

95 61 30 7

(a) Spring Pm-summer (b) Summer Post-summer

29 35

(c) Autumn (d) Winter

noted ‘Ess’, which is associated with less ‘quiet’ current systems (AP 2 10): During the African magnetic ‘Spring’ equinox (March-May), Y cusps are practically absent (6 per cent of all days), while 15 per cent of all days show EM. (b) After the Summer solstice, both phenomena appear with Y events on 33 per cent of all days, Ess sequences only on 12 per cent and Ess alone on more than 30 per cent of all days. (4 From end of August to mid-November, groups of Y-only and &s-only days alternate, the short Y events being confined to low ionosphere levels. ((1)In northern winter (December-February), 80 per cent of the days show Y cusps; only on 28 per cent do Ess precede Y, while isolated Y appear on 70 per cent of all days.

(4

To sum up the variations of Fort-Archambault

F-region Y events in 1969:

-They occur when previous magnetic activity is relatively weak and has been slowly varying over the preceding 45 hr (The diurnal f,PZ variation then usually shows a morning bite-out, as the F2 peak density crests clear rapidly away towards the tropic).

Upward-moving irregularities in the subequatorial ionosphere -There is a striking belated transition in November equinox and the Y-crowded December solstice. In the next section, we concentrate on the F-region on the rising process and the seasonal occurrence.

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between a low Y-occurrence processes, merely touching

2. DISCUSSION 3.1

Hall clrijt hypothesis

The whole ascent of a given stratum lasts from 1 to 3 hr, according to FZ-layer thickness and to the mean value of YZ. During the high occurrence solstice intervals of steady magnetic activity, the irregularities appear on the average equally likely at any time from 0800 to 1700 LT. Thus they exactly coincide with the main phase of the equatorial electrojet. On allf-plots of the type given in Fig. 3, they reach FL? maximum levels after the fOFZ morning peak phase. DUNFORD’S (1970) analysis indicates a 2-hr time-constant between the rise in E-region electric field E, and the resulting F2 maximum density decrease. Neglecting ion-neutral collisions, the Hall drift V=EXB

(1)

B2 where B, the magnetic irregularity reaches the motion is produced by Table 2 gives a few (1) on material already for the magnetic field work-based on more absolute values for the

field vector, must be well established by the time our F2 peak. It therefore seems probable that the irregularity the upward Hall drift. examples of the absolute evaluations of E, from equation analysed, assuming a provisional value H = 0.342 Gauss Our results agree with Dunford’s correlation intensity. disturbed periods-and have the advantage of giving prevailing E, electric field component.

Table 2. E, electric field W-E

component from &‘-region, noon values, in mV m-l Ouagadougou

26 28 25 15

June 1969 June 1969 October 1969 November 1969

0.7 0.3 1.1 0.55

Fort-Archambault I.5 0.8 -

The diurnal occurrence pattern of the rising irregularities agrees with the fountain ionization transport model. This is confirmed by the fact that whenever they are conserved over the 250 km level, the irregularities are seen to remain remarkably uniform in velocity and shape up to the peak altitude h,F2. Woodman and Balsley find exactly the same uniform velocities during quiet or steadilyFrom the ionogram studies in Africa (VILA, 1971), varying magnetic periods. we know that steady conditions of ionic transport are not met every day. The day-to-day occurrence pattern of our irregularities, which reproduces that of the periods of steady magnetic activity, further confirms the Hall drift hypothesis.

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J. M. FAYNOT, P. VILA and J. WALTER

3.2 Seasonal occurrence Several different processes might be responsible for the prolonged Septembermid-November period of low occurrence: Lack: of fo~~at~o~ at ~-reg~ou leveb ~a~uet~~a~~~cou~e~ted with the s~uator~a~ ~ouos~he~e. This is only possible if E-region perturbations, perfectly symmetrical relative to the magnetic equator (e.g. the Lorentz-coupled gravity waves of CIIIMONAS,1970) induce opposite effects across the two equipollent horizontal components of the magnetic field at conjugate points. (On the other hand, irregularities may result from gravity wave frontal perturbations, when less symmetrical boundaries of the solsticial electrojet initiate E-region irregularities, subsequently driven upwards by the Hall drift.) Local ~~p~~g-out of the ~~~eg~~a~~t~e~ by the neutral feud at F-reason EeveEs. This may be the case if, instead of migrat~g slowly across low latitudes (as is generally supposed) the neutral pressure bulge shifts back and forth during the day from 10% to 30’S of the African magnetic equator, causing unsteady diffusive ion flow along magnetic lines of force. Conversely, the December solstice maximum occurrence would agree with a pre-afternoon equilibrium between the steady northward neutral wind (pressure bulge tied to the 23” S latitude) and the founeain southward meld-aligned diffusion in the southern magnetic hemisphere. Lack of e~ec~T~c ~~~~e~t~onbetween ~~n~u~ateirregularities. According to this hypothesis, the boundary of the African electrojet central vein may be so asymmetric during equinox periods that field-aligned irregularities cannot appear at the magnetic equator. During the rest of the year however, irregularities form one boundary, maintaining connection with the other, might be seen moving through the magnetic equatorial ionosphere. To decide between the three formation hypotheses suggested in this discussion, it will be necessary to study the lifetimes and conservation processes of the rising irregularities between EZs and 220 km altitude levels (~LEMMOW et al., 1955). 4. CONCLUSION Rising equatorial irregularities have been identified and analysed on Mrican high sunspot period ionograms. Our study does not provide continuous diurnal variations, but allows absolute evaluation of the electric field west-east component E, from quiet-time Hall drift. The di~cult theoretical problem remains the irregular formation and maintenance through lower and upper PI-region levels. Three different mechanisms may account for the seasonal occurrence of the irregularities, gravity waves moving away from the Equator, diffusion under neutral wind steady flow, or symmetric regimes of electrojet boundaries. We hope to test these hypotheses by more detailed work on synchronous African data from tropical ionosondes, from a Centre National de la Recherche Scienti~que magnetic Survey and from E-region drift data at Fort-Archambault. wish to thank Dr. 33. RISEBETH for his encouraging comments. We are grateful to Mr. F. DU CASTELwho initiated the low-latitude studies of the Groupe de Rscherches Ionosph&iques in Africa, to Dr. P. N. MAYAUD who fostered the magnetic network in Central Africa and to Mr. J. HtiBLOT who founded the ionospheric station at Fort-Archambault~.

Acknotuledgemew%v-We

Upward-moving

irregularities in the subequatorial ionosphere

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This work woufd not have been possible without the sponsorship of the French Centre PJationaI de la Reeherche Scientgque under R.C.P. 169 contracts and without the support of the Office de Recherches Seientifiques des Territoires d’Outremer in financing and operating the magnetic network and the maintenance of the Fob-Archambault station. REFERENCES BAEJZEY B.and W~~DIUNR.F. CLE~MOWP.~.,JOH~SONM.A.~~~ WX~EKE~K. CO~ENR.,BOWLE~ K.L.and CALVERT W. DOUPN~KJ. R.and SCHMERLINQER. DUNFORD E. RASTOUI R.G. VILA P.

0

1962

3. Atmosph. Terr. Phys. 31, 865. The Physics of the ionosphere, Rep. Phys. Sot. p. 136. J. gwphys. Res. 3’7, 965.

1965 1970 1970 1971

J. Atmmph. Terr. Phys. 27, 917. J. Atrmsph. Terr. Phys. 32, 421. Nature, Lortd. 225, 258. Radio Sci. In press.

1969 1965