The mid-latitude trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling

JournalofAtmospheric and TerrestriaIPhysics. Vol. 45, No. 5. pp. 315 -343, 1983.

0021 - 9169/83 $3.00 + 0.00 Pergamon Press Ltd.

Printed in G r eat Britain.

The mid-latitilde trough in the electron concentration of the ionospheric F-layer: a review of observations and modelling* R. J. MOFVETrand S. QUEGANt Department of Applied and Computational Mathematics, University of Sheffield, Sheffield SI0 2TN, U.K. (Receired in final form 16 November 1982)

Abstract--Experimental observations and theoretical modelling of the terrestrial mid-latitude trough are reviewed.The mid-latitude trough is considered as an F-layer phenomenon, and its relationshipsto the lightion trough in the topside ionosphere and to the plasmapause are discussed.The observed morphology of the mid-latitude trough is summarised.Recent evidenceon plasma temperatures in the trough is examined.The physical processes that may be important in the trough region are listed. Large-scalecomputational models that includesome of those processes are describedand the resultscompared vdth observations.Deficienciesin the models and possible future developments are mentioned. I. INTRODUCTION

As is evident from the title ofthe paper, attention is here focused on a distinctive feature of the electron distribution of the terrestrial F-layer. The terminology 'mid-latitude trough' is used to describe this feature. Such terminology is, however, differently understood by various workers; furthermore, terms that describe other features of the ionospheric and magnetospheric thermal plasma distribution may conflict with our proposed usage. A precise description is needed ofwhat is meant by the 'mid-latitude trough'. If the electron concentration (or total ion concentration) in the F-region is plotted as a function of latitude, there often occurs a marked depression at midlatitudes. The depression usually lies between invariant latitudes of 55~and 75~(corresponding to L values of 315), equatorward of the auroral oval. The occurrence of an abrupt poleward wall is caused by auroral ionization. The phenomenon is primarily a night-time one but has also been observed in the pre-dusk, dawn and noon time sectors. The trough is observed to extend into the topside F-region but care must be exercised in asserting that it is present at altitudes such as, say, 1000 km or 2500 km. As we shall see later, the mid-latitude trough is due to a depletion of the O + constituent ofthe ionospheric plasma. With increasing altitude, the electron concentration is increasinglydependent on the light-ion (H + and He +) concentrations and a depression in electron concentration may simply reflect

*A preliminary version of this paper was presented at the MIST meeting, University of Cambridge, April 1981. "]'Present address: Remote Sensing Group, Marconi Research L~boratories, Great Baddow, Chelmsford, Essex, U.K.

a depletion of H + rather than O +. It should be noted also that alatitudinal variation ofelectron temperature may distort the topside electron concentration behaviour relative to the trough behaviour around the F2-peak.Thus we attempt to restrict our description of observations to those where it is reasonably certain that the observed electron or ion concentration reflects the concentration of O § Althbugh the number of papers published on the bottomside trough is not large, these do indicate that the trough does persist to altitudes below the F2-peak. Statistical studies ofthe positions of the plasmapause and trough-like features in the topside ionosphere show that the two phenomena are related, but the bearing of these results on the F-layer midlatitude trough is not clear. Figu re 1(BRINTONet al., 1978) is a schematic diagram that places the mid-latitude trough in the perspective of the high-latitude F-region. The feature displayed as the mid-latitude trough was derived by BRIN'rONet al. from AE-C mass spectrometer measurements of F-region O + and other ions in the southern winter hemisphere. Other features shown are the auroral oval, the so-called 'high-latitude ionization hole' and a model convection pattern. Theory suggests that flux tubes of plasma flow antisunward over the polar cap (where they are open) and return on the flanks. The dawn--dusk electric field plus the corotation field give stagnation regions of slow. flow. The convection pattern shown is due to HEPPNER (1977). SPmO et al. (1978) have modified this pattern in the evening sector and in the dayside throat region. If the plasmapause is taken to be the boundary that separates those tubes of plasma that pass over the polar cap from those that do not, then the implication of Fig. 1 is that the trough lies just poleward ofthe plasmapause. It must be emphasized, however, that Fig. 1 is schematic: the convection pattern is superposed on the

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Interesting global pictures of the trough have been obtained using satellite beacon transmissions through the ionosphcre. A good example is Fig. 2. Thcse results, first given in conference procecdings by WAND and EVANS (1975), are reproduced in a review by EVANS (1977). The trough in Nm~ is clearly evident during the night-time hours arotmd 60 ~to 65 ~latitude. The trough -6 tends to move equatorward as the night progresses and is apparent in summer as well as in winter. Total electron content (TECO measurements can also reveal the presence of the mid-latitude trough. MENDILLO and KLOnUOIAR (1975) have examined T E C measurements obtained from the Faraday rotation of the ATS3 signal at a chain of stations near the 70~ longitude meridian. In Fig. 3 are plotted 0 contours of T E C as a function of local time and invariant latitude" the values shown are median values Fig. I. Schematic diagram of the average locations of the mid- for December 1971. The trough at night around 60 ~ latitude trough (and the high-latitude ionization hole), from latitude reflects mainly a trough in O + content. AE-C observations, relative to a representative plasma drift pattern (alter HEPP,~ER,1977),and the quiet-time auroral oval MENDILLO and KLOBUC~tAR (1975) also give T E C results for particular days to illustrate substorm and of Fr.LOSrEI.'~and Sr ARlC.ov(196 7). From BRiNroN et al.(1978). major magnetic storm effects. It is found that a storm drives the mid-latitude trough to lower latitudes and observed trough, without a detailed correspondence gives a large mid-latitude T E C enhancement at itamilton (A = 53~ equatorward of the trough necessarily being expected to hold. References to the discovery of the trough and other latitude. The positive phase signature in T E C ends early work may be found in a review by WRENS and earlier at high latitudes than at mid-latitudes. MENDtLLO and CHACKO(1977) have analysed ISIS 2 RAn'T (1975). But descriptions, quoted by WRE,":,'qand RAITI', Of trough features at 2500 km altitude, for topside sounder electron concentration profiles for a example, must be read with the above comments on restricted set of geophysical conditions. A total of 30 terminology in mind. At 2500 km altitude the electron individual satellite passes during a 21 day period in concentration measurements (MAHAJANand BRACE, December 1971 were used to determine the mid1969) are almost certainly measurements showing latitude trough's characteristic features under the light-ion trough behaviour (TAYLORand WALSrI,1972). conditions of midwinter in the northern hemisphere, The key theoretical paper by KNUDSElq(1974) reviews midnight local time and very low geomagnetic activity. other data on the trough; it is discussed later for its Results from an individual pass are shown in Fig. 4. The theoretical content. poleward wall of the trough is clearly evident but the In Section 2 recent observations [with an occasional trough is rather shallow and could be said to stretch at reference to work prior to the WRENN and RAITT(1975) the height of the F2-peak from 50 ~ latitude to greater review] of electron concentration or total electron than 67 ~ latitude. We see how tile pattern of N, values content troughs are examined and an attempt is made shifts as we examine greater heights in the topside. We to summarise what is known about the basic trough see also the difficulty in defining uniquely the 'trough morphology. Sections 3 and 4 discuss the observed centre' and other features. relationship of the trough to other properties of The numbers 1-8 placed on the hm~x and 450 km ionospheric structure, such as plasma temperature and curves in Fig. 4 were used by MENDILLOand OIACKO topside light-ion distribution, and the degree to which (1977) in an attempt to characterise trough features. these properties and the mid-latitude trough are The resulting latitude segments represent (i) the midobserved to be signatures of the plasmapause. In latitude gradient in N,, (it) the gradient in the' Section 5 the present state of theoretical studies of the equatorward wall of the trough, (iii) the trough trough is examined, and concluding remarks make up minimum, (iv) the poleward wall of the trough, (v) the auroral peak, (vi) the auroral peak decline and (vii) the the final section.

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polar gradient. While the equatorward edge of the trough and the trough minimum are latitudinally wide features (about 5~ and 10~ respectively), the poleward wall is sharp (about 1.5~ The electron concentration in the trough minimum has an average value of about 5 x I0" cm -3 at the F2-peak. The results obtained by MENDILLOand CHACKOare in good agreement with the statistical deductions on the trough's equatorward edge, minimum and poleward wall at the F2-peak given in the reports by HALCROW(1976) and F~aNnLOMand ttORAr~(1973). Both the position of the trough centre at 550 km altitude derived by TULUNAYand SAYERS(197 I) and the trough's minimum Ne value of 2.6 x 104 cm -3 at 400 km altitude from CHANand COLIN(1969) are also in good agreement with MENDILLO and CHACKO'S results. MENDILLO and CHACKO find.that the foot of the poleward wall at the F2-peak lies about 2.5 ~ equatorward of the statistical position of the equatorward edge of the band of diffuse aurora for low values of Kp (Lot et al., 1975; StlEEIIAN and CAROVILLANO, 1976). The centre of the FELDSTEIN (1966) oval lies about 3~ further to the north. The top of

the poleward wall (point 5 in the parameterisation, see Fig. 4) lies approximately 1~ equatorward of the diffuse aurora, a value consistent with the work of BATESet al. (1973), which reported a 1-2 ~ separation of the poleward wall from the visual aurora. Discussions of MENDILLOand CHACKO'Scomments on plasma temperatures in the trough and the relation of the trough to the light -ion trough are deferred to later sections. Figure 4 shows latitudinal profiles ofelectron concentration up to the altitude of the satellite (1400 km); MENDILLOand CtlACKO show that consideration of the mid-latitude trough as an O + depletion is obscured with increasing altitude by the variation in ion composition. Measurements of electron concentrations in the topside ionosphere by the ill situ probe on board the Ariel 4 satellite have been analysed by TULUNAY and GREBOWSKY (1975, 1978) and GREBOWSKY et al. (1976b). Criteria for the identification of features of the mid-latitude trough are similar to those used earlier in analysing Ariel 3 data (TuLONAY and GREBOWSKY, 1975, and references therein). Figure 5 displays examples of the mid-latitude trough, as observed by

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Ariel 4 at about 550 km altitude (GREBOWSKYet al., 1976b). The relation of the trough to the Explorer 45 results will be discussed later. Figure 5 demonstrates the variety of trough position and shape that may Occur.

TULUNAYand GRF.nOWSr:V(1978) have examined the movement ofmid-latitude trough features observed on Ariel 4 around midnight local time during the period 1 2 - 2 1 December 1971 ; in this period a magnetic storm occurred with the 3 h Kp index reaching a ma~ximum value of 7. With increasing magnetic activity both the low-latitude edge of the equatorward wall and the minimum point of the trough move to lower latitudes. Ariel 3 results (GREBOWSKYet at., 1974) show similar trough movements with varying Kp in the dusk sector; at dusk substorm-related variations of trough position

also occur. In the dawn sector movement of the trough is only apparent with the onset of a large storm. GREBOWSKVet al. (1974) also point out the existence of finely-structured concentration variations in the outer plasmasphere and in the mid-latitude trough region. TULUNAY and GREBOWSKY(1978) refer to "noon mid-latitude troughs". These appear during winter. From the Ariel 4 data (December 1971), the noon troughs occur at rather high invariant latitudes (between about 67 ~ and 75~ but Ariel 3 data from TULUNAY(1973)(1967--1968data) give an instance of an afternoon trough occurring at A < 65~ CnACKO (1978), using ISIS 2 topside sounder results for December 1971, found a trough near the noon meridian for quiet midwinter conditions at about 73 ~ invariant latitude. :

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Plasma density measurements that are relevant to the mid-latitude trough have been made from the ESRO 4 satellite (RAIrT et aL, 1975 ; WRENNand RAtTT, 1975 ; K6HNLEhNand RAITT, 1977). Observations were carried out from November 1972 until April 1974, a period of relatively quiet solar conditions (average F i o . 7 = 92 Janskys). The altitude of the satellite in polar orbit varied between 245 and 1177 km. It is not stated by K6HNLEXNand RArrr (1977) how many of the approximately 300 troughs analysed by them occur at altitudes where the presence of light ions may obscure the O + behaviour. The particular examples of troughs given by K6HNLEINand RAn-r come from the topside adjacent to the F2-peak. Such an example is reproduced in Fig. 6. The depletion in N, at 59~latitude is by about an order of magnitude. KOIINLEIN and RAtTTremark that due to disturbances the pattern seen in Fig. 6 is often changed and the trough becomes shallow with a much smaller N, decrease. Figure 7 is a plot of the ESRO 4 trough minimum position as a function of Kp in the dusk time sector. There is-good agreement between the northern and southern hemisphere results. The trough moves equatorward by just over 2 ~ per unit of K r The same result holds at other local times, it should be noted that most of the Kt, values lie between 2 and 5. For larger Kp values reference may be made to Ariel 3 data (GREBOWSKYet al., 1974). Ki311NLEINand RAITTfind that in April and May the trough is observed from around 19 h LT until 05 or 06 h LT. As local time progresses through the night, the trough moves to lower latitudes. KIStlNLEINand RAFrT summarise their findings on the invariant latitude, A-r, of the trough minimum by the formula (valid for

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where t is the time in h from midnight LT (positive after midnight and negative before midnight); the range of data is such that - 5 h ~< t ~< 5 h and the occurrence of events is heavily biassed to the post-midnight sector. Data obtained from instruments on board the AE-C satellite have proved a rich field for investigationsof the mid-latitude trough at F-region heights. The retarding potential analyser was used to measure ion drift velocities, as well as the ambient ion concentrations, during the period January 1974-November 1977. In Fig. 8 are displayed results from SPmo et aL (1978). The plot of total ion concentration, Ni, in the lower panel, shows a well-defined mid-latitude trough. From a value of 1.2 x I 0 s c m - 3 at 59" invariant latitude, N i decreases to a minimum value of 5.8x I0 ~ cm -~ at 63~. The poleward edge of the trough is abrupt, with N l increasing by a factor of 10 in 0.7 ~ of latitude. We have chosen to reproduce also the typical ion drift results in Fig. 8 because of the significance of the results to the high-latitude ion convection pattern (SPmo et al., 1978). In Section 5, the crucial rfle played by the convection pattern in the formation of the midlatitude trough is discussed. Four regions have been labelled by SPIROet al. (1978) in Fig. 8. Equatorward of the trough in region A the F-layer is observed to corotateeastward with the Earth. As the satellite passes poleward into region B, the measured ion drift velocity begins to depart from corotation. As the satellite passes into region C, Ni continues to decrease and the ion drift

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direction changes to generally westward. The ion drift speed increases throughout region C, maximising at the poleward edge of the trough. Poleward of the trough (region D) the ion drift speed again decreases as the satellite enters the auroral zone. It should be noted that, viewed from the nonrotating coordinate frame, both slow eastward and rapid westward drift are present within the trough and that/V i varies smoc~thly across the region where the flow reverses. These properties were found to be characteristic of troughs observed in the pre-midnight local time sector. SPmo (1978) has codified the various ion drift velocity signatures associated with the trough. SPIRO et al. (1978) also observed troughs when the Fregion was sunlit. In those cases the ion drift was usually directed westward throughout the trough with the change from eastward drift to westward drift occurring

at the low-latitude boundary of the trough rather than within the trough. The deduction by SPIROet al. is that the pre-dusk trough is formed on the nightside of the Earth and the depleted tubes of plasma are subsequently convected westward into the afternoon sector. SPtRO et al. find that AE-C observations of the midlatitude trough show great variability from one orbit to" the next. Several general characteristics of the trough concentration morphology are consistently observed, however, particularly in the pre-midnight MLT sector. The observations show the poleward edge of the trough to be much steeper than the equatorward edge. In the post-midnight sector the observed trough shapes exhibit much greater variation than in the evening and are more difficult to characterise. Usually, both the equatorv~ard and poleward edges are more gradual and

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R.J. Morrill and S. QUEGAN

more structured than their evening counterparts. The dependence of mid-latitude trough location on Occasionally, though, evening-like troughs are geomagnetic activity and on local time in the AE-C observations has been surveyed by SPIRO (1978). He observed as late as 2 h MLT. Ion drift within the pre-midnight trough is found that the trough moves equatorward with characterised by generally westward flow in a increasing K p value, at a rate ofabout 1.7~per unit of Kp coordinate frame that corotates with the Earth. as in the midnight MLT sector. This is in g o o d However, when viewed from a nonrotating frame, the agreement with Kt)HNLEIN and RAITT (1977). The low-latitude portion of the trough is frequently found trough moves equatorward as MLT progresses from to be drifting slowly eastward at a rate less than the mid-afternoon to about midnight. In the evening sector, velocity of the corotating atmosphere. In troughs for Kp about 3, this movement is about 0.8 ~ h - 1. In the observed after midnight, the measured ion drift afternoon sector, however, the trough lies at higher velocities seldom depart much from corotation with the invariant latitudes than is predicted by a formula that is Earth, although a few troughs have been observed that linear in MLT and in the postmidnight sector the have all the ion drift characteristics of the typical movement towards the equator may cease or even evening-like trough. Detailed frequency plots of the reverse. In Fig. 10 are displayed the best fits to the occurrence of velocity signatures, as a function of M LT trough location data found by SPIRO(1978). Results are and season, have been given by SPIRO (1978). shown for selected values of Kp. Rather large ion convection velocities have been The occurrence and position of the trough as observed on AE-C have been examined by SPIRO(1978). observed at sub-auroral latitudes (SMIDDYe t al., 1977; It is found that the mid-latitude trough is primarily a MAYNARD, 1978; SPIRO et al., 1978; RICH et al., 1980). night-time phenomenon. As pointed out above, Further examination of the AE-C data has been carried however, troughs are occasionally observed even when out by SPIROet al. (1979). Defining these ion drifts to be the mid-latitude ionosphere is not eclipsed. Troughs those sunward (westward) drifts occurring just are observed on more than 60yo of the satellite passes equatorward of the auroral zone with magnitude in that cross the interface between mid-latitudes and the excess of 500 m s -1, SPmo et al. (1979) find that auroral zone when the solar zenith angle is greater than approximately 85~ are observed in association with 90 ~. The occurrence probability falls to approximately well-defined mid-latitude troughs. Only about 20~, 20~o in the solar zenith angle range from 70 to 90 ~ In however, of AE-C mid-latitude troughs are acinvestigating daily, seasonal and magnetic activity companied by the rapid sub-auroral ion drifts. The variations of trough occurrence, SPIRO (1978) chose drifts are found predominantly between 18 h and 2 h those observations obtained when the solar zenith LT; the occurrence probability of the events is greater angle was greater than 90 ~. The chosen observations when the AE magnetic index is greater, indicating that were further restricted to the altitude range from 250 to the sub-auroral drifts are substorm related. SPIROet al. 600 km. This latter restriction ensures that the chosen (1978) had already concluded that rapid ion drifts in a troughs are indeed (F-region) mid-latitude troughs (see latitudinally narrow region are not primarily Section I above). responsible for the nocturnal mid-latitude troughs. The seasonal and daily occurrence probabilities In passing, it is worth noting that in S PIROet al. (1979) from AE-C observations found by SPIRO (1978) are (their Fig. 4) there is a beautiful example of a midshown in Fig. 9. Both northern hemisphere and latitude trough with an abrupt poleward wall, a fairly southern hemisphere data are included. For the steep equatorward wall and an almost flat minimum in summer data, the restriction to solar zenith angle the trough. This is in contrast to the results quoted in greater than 90 ~ means that the MLT is constrained to the current Fig. 6 (K6I INLEINand RAri-r, 1977) and Fig. lie between 19 h and 3 h. During winter and equinox, the 8 (SPIRO et al., 1978). data fail to show large variations with magnetic local Data on ion concentrations from the AE-C Bennett time (MLT) from late afternoon to early morning. ion mass spectrometer, on electron temperature from Troughs appear slightly more likely during equinox. In the electrostatic probes and on low-energy electron contrast, in the summer the probability peaks in the late fluxes, have been analysed by BRINTON et al. (1978). Most of the data presented were obtained during a evening sector. Spiro finds that the trough occurrence probability, in magnetically quiet period centred on the June 1976 both the pre-midnight and post-midnight time sectors, solstice and the southern winter F-region is studied. In is greater during periods of greater magnetic activity. the mid-latitude trough in electron concentration at However, even when the level ofmagnetic activity is low about 300 km altitude the dominant ion is found to be (Kp < 1), the probability exceeds 507o (provided that O + at all times. BRINTON et al. sorted the data into the solar zenith angle is greater than 90~ -" 9 ranges of maximum and minimum O § concentrations

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I

I

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I

2Z

I

I 2a

Kp=6 . . . .

I

i

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t,

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LOCAL TIME |hours)

Fig. 10. Empirical dependence o1"the position of the midlatitude trough minimum on magnetic local time, as deduced from AE-C data by Svmo (1978), for three values of the Kp index.

observed at a particular location. The mid-latitude trough is a persistent feature, extending round the nightside from 17 h to 5 h MLT (the limits of the observations). The ion concentration at the bottom of the trough is typically about 3 x 103 cm -3. The lowest concentrations were detected near dusk, with higher values near dawn. In their discussion of additional A E-C data, BRt,~'rON " et aL note that the nightside mid-latitude trough in the southern hemisphere is deepest near dusk around the September 1976 equinox and around the June 1977 solstice. Northern hemisphere obser~,ations during the same time periods reveal a consistent but different morphology from that observed at southern latitudes. The trough concentrations (for all three time periods studied) measured in the northern hemisphere are lowest in'lhe postmidnight sector, in contrast to the

324

QUEGAN

R . J . Morr~l'r and S.

(a) I01

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Fig. 11. Simultaneous measurements of ion composition, electron temperature and energetic electron flux obtained on four AE-C passes (see Fig. 12) through the winter F-region at high southern latitudes. From BRLN'ro.Net al. (1978).

A IDEG) kILT (HR) SZA (Ds UT (STCI

The mid-latitude trough in the electron concentration of the ionospheric F-layer dusk minimum of the southern hemisphere results. It may be noted that the longitude regions sampled in the two hemispheres by AE-C are usually different: usually western longitudes arc covered in the northern hemisphere and eastern longitudes in the southern hemisphere. In Fig. 1 a schematic diagram from BRINTOX et al. was reproduced. To demonstrate how data from individual satellite passes exhibit the ionospheric features deduced statistically, data from four orbits are shown in Fig. 11 (BP,tNTON et al., 1978) and the subsatcllite tracks of these orbits in M L T - A space are shown in Fig. 12 (BmNTON et al., 1978). BRINTON et al. have marked "main trough" on Figs 1 l(b), (c) and (d); the depression in O + concentration around 65 ~ invariant latitude (A) in Fig. l l(a) could also be considered a main or mid-latitude trough. Figure 11 gives unequivocal confirmation that the poleward wall of the mid-latitude trough is due to ionization by particles in the auroral oval. The feature labelled "hole" lies in the polar cap poleward of the auroral oval and is distinct from the mid-latitude trough. The electron temperature structure will be discussed in Section 3. DUDENEY (1981) has drawn attention to the limitations of the AE-C orbits for studies of the midlatitude trough. The satellite only sampled the invariant latitude range of interest (55-65 ~ in one defined longitude zone in each hemisphere, because of the offset between the geographic and geomagnetic poles. For these longitude zones, there is little difference between M L T and local time (LT). DUDENEY suggests that results such as those shown in Figs 9 and 10 reflect LT behaviour and that the MLT behaviour shown may not hold at other longitudes, i.e. there will be a longitude effect. DUI)ENEY adds further that SPIRO'S (1978)

325

formula for trough position (as a function of time) is a better fit to Halley Bay data when MLT in Spmo's formula is replaced by LT. HALCROW and NISBEr (1977) have derived an empirical model of the peak electron concentrations in the region of the northern mid-latitude trough. The model was obtained from measurements made by the satellites Alouette 1 and 2 and is in the form of a multiplicative modification factor to the CCIR (1966) peak electron concentration model. The model was expected to be of considerable use for radio-wave propagation calculations in the affected region. Since the mid-latitude trough arises from depletion of O +, the trough may be expected to be present also in the bottomside T-layer. Relatively little attention, however, has been paid to trough behaviour at heights below the F2-peak. LOCKWOOD(1980) provides a useful source of references to work on the bottomside trough. LOCKWOODuses the statistical model of I IALCROWand NISBEr (I 977) to show that the propagation ofh.f, radio waves over a long, west-east, sub-auroral path may give information on trough characteristics. A deduction on the F2opeak height in the trough will be mentioned later. Recently, RODGER and PINYOCK(1980) have briefly described daily, seasonal and solar cycle variations in the occurrence and shape of the trough from analyses of night-time ionosonde data from I t alley Bay, Antarctica (76~ 27~ geographic; L = 4.2). The data shown in Fig. 13 include earlier results from Ellsworth Station 9

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Fig. 13. Contours of plasma frequency (in MHz) observed in Antaritica. From RODGERand PZNNOCK(1980).

326

R.J. Movr-rrr and S. QUrGA,'q

(L = 4.5); these were obtained from information about the angle of arrival using ionograms (BOWMAN, 1969). Figure 13(A) shows steep gradients of electron concentration on both equatorward and poleward sides ofthe trough minimum. In contrast, in Fig. 13(B) (constructed by evaluating the electron concentration profile at 15 min intervals) the steep gradient on the equatorward side is absent. RODGER and PINNOCK (1980) have identified five different types of trough and summarised a set of conditions associated with each type (Table 1). Halley Bay ionograms for the months of April in 1958, 1962, 1965 and 1972 and the complete )'ear i'n 1978 were analysed by RODGrR and PIN,XOCr:.As Table I shows, the type of trough sequence observed depends on the local time of occurrence which in turn depends primarily on magnetic activity. The level of magnetic activity required for a particular type of trough sequence to be observed at Halley Bay is slightly lower at sunspot maximum than at sunspot minimum. The trough was observed on all nights during the months of April for which data were available, except under the quietest magnetic conditions (K e = 0). The relative frequency of occurrence of each type of trough crossing with season and solar cycle was found to be mainly dependent upon the magnetic activity variations for those epochs. These findings by RODGER and PINNOCK agree with later results by D trDEr,'EYet al. (1982). It may be noted that the data (Table I) show that sporadic E is present at disturbed times at Halley Bay during 16-23 h LT. This is evidence of hard particle precipitation at those times. 2.1. Summary of mid-latitude trouffh morphology

Even a cursory examination of the review of WRENN and RAtrr (1975) and the material presented above gives an impression of the complexity of the observed morphology of the mid-latitude trough. To quote from

AII.~IEI)et al. (1979), "Studies of the diurnal, seasonal, and altitude variation of the trough characteristics have led to somewhat confused results". It is worth quoting further: "There are several reasons for the differences in trough results. These include use of a limited data base, widely varying altitudes of the measurements, varying spatial resolutions of the measurements from a few km to nearly 1000 kin, difficulties in detecting dayside features due to photoelectrons, as well as varying criteria used to define the trough location". To these several reasons we may add two others. The first of these has already been addressed in Section 1. To illustrate our point about the definition of the mid-latitude trough, it is somewhat ironic that we refer again to AII.~tED et aL (1979). The abstract of that paper refers to the "main trough"; yet later in the paper (pp. 495-496) it i s stated that, "For the altitude range studied in this paper, the satellites are generally above the heavy/light ion transition on the night-side. In this case the total and light ion troughs coincide". Thus the (nightside) "main trough" referred to by Attx~ro et al. (1979) is in reality the light-ion trough. On the other hand, the "main trough" referred to, for example, by RODGER and PtNNOCK (1980) is indeed the F-region mid-latitude trough. AtI~tEI~et aL's (1979) paper gives valuable insight into the behaviour ofthe light-ion trouflz iri the 1969-1972 period. But it is understandable that the first sentence of the paper by LOCKWOOD(I 980) reads,"The majority ofobservations of the mid-latitude trough in the nocturnal F-region have been made using topside measurements (AitxlrD et al., 1979)...". The second reason [in addition to the list from AII.~ED et al. (1979)] for the apparent differences in reported trough results is the greater accuracy of the more recent generations of probes and mass spectrometers. These improvements in the accuracy of an individual measurement may supplement improved

Table 1. Summary of features of main trough types over llalley Bay (RODGERand Pl.~,xocr:, 1980) Type 1 Time of occurrence (local zone time) Magnetic activity (Kp) Time to cross Halley Bay (h) Storm lypes of sporadic E Conditions following trough

160(O2000

Type 2

2000-2300

Type 3

Type 4

Type 5

2300-0100

0100-0500

usually after 0100

~, 5

3-5

3-5

2

<2

2

3

3

4

Does not cross

Always Blackout ; high latitude ridges very disturbed

Usually Similar type to l but less severe

None Extensive spread--F

None Sunrise ends sequence

Seldom Very extensive spread--F blackout rare

The mid-latitude trough in the electron concentration of the ionospheric F-layer

327

midnight and post-midnight troughs: the latter show much greater variability and are more difficult to classify quantitatively. The local time of occurrence of the deepest part of the trough has been reported as dusk (BRINTONet at., 1978, southern hemisphere winter AEC data), midnight (FEINBLUMand HORAN, 1973) and early morning (BRINTO'N et al., 1978, northern (1) The mid-latitude trough is primarily a nightside hemisphere winter; WAGNERet al., 1973, using data for phenomenon, extending in a band from the dusk sector the Alaskan longitude sector obtained from groundto the dawn sector. It is most frequently observed when based and airborne ionosondes). TULUNAYand GREBOWSKY(1978), using Ariel 4 data, the solar zenith angle exceeds 90 ~ (Figs 1-3, 5 and 9.) Troughs of mid-latitude type have also been observed agreed with earlier deductions (SHARP,1966; TULUNAY and SAYERS,1971; FEINBLUMand HORAN, 1973) that in the noon sector. (2) The trough is most regularly observed during the trough width decreases when magnetic activity winter and equinox; in summer it is only observed near increases. KOn~LEXNand RAn-r (1977) remarked that due to magnetic disturbances the trough becomes local midnight (Figs 2 and 9). (3) The poleward edge of the trough, usually seen as a shallow with a much smaller electron density decrease. sharp increase in electron concentration, lies just It may be noted that most of the ESRO-4 data equatorward of the boundary of diffuse auroral presented were obtained with Kp < 5. In the variety of precipitation. (Figs 1, 4-6, 8, 11 and 13.) The poleward troughs presented by GREBOWSKYet al. (1976b) from May 1972 data (Fig. 5), the range of Kp values was not edge is usually steeper than the equatorward edge. (4) The latitude of the trough decreases through the indicated. The recently available information on I,mF2, the night. At quiet times there may be movement back towards higher latitudes in the dawn sector (Fig. 10). height of the F-layer peak concentration, is briefly (5) During periods of increased magnetic activity the summarised. MENDILLOand CHACKO(1977) (see also CttACKO, 1978) found that hmF2 decreased (with trough moves to lower latitudes (Fig. 7 and Table 1). (6) The occurrence of the trough and the validity of increasing latitude) across the midnight trough (Fig. 4) points (1)-(5) above do not depend markedly on solar from about 350 to about 300 km. This is contrary to the picture of the trough as a region of high temperature cycle variations. and consequently greater hmF2 (see, for example, There is no such consensus view on the width or TAYLOR, 1973). MENDILLOand CHACKOnoted again depth of the trough or the local time of the trough's the very low K v values associated with their results. deepest point. This may be partly due to the difficulty of RODGERand PINNOCK(1980) observed that the virtual quantifying trough width, shape or position; different height of the poleward edge of the trough was authors use different criteria. The trough shape varies remarkably constant over Halley Bay (360_+28 krn from the wide, structured valley of Fig. 4 (MENDILLO from 26 different occasions). Within the trough and CrIAeKO, 1977) to the smooth latitudinal decline to minimum, hmF2 increases relative to its value outside the foot of the poleward wall of Fig. 6 (KOtlNLEINand the minimum (Fig. 13). LOCKWOOD(1980) deduced that RAta'r, 1977) and Fig. 8 (SPmo et at., 1978), with several h~F2 inside the trough is greater than its value outside intermediate cases in Fig. 5 (GREROWSKYet al., 1976b) by between 30 and 80 km. Detailed data on ion drift velocity in the trough and Fig. 11 (BRINTONet at., 1978). Earlier studies of the mid-latitude trough, some of which were statistical region comes from SPtRO(1978) and SPmo et al. (1978, surveys of extensive data bases, reported deductions on 1979). An example is shown in Fig. 8. For troughs in the the shape of the trough ;in recently published work, the pre-midnight local time sector, both slow eastward and authors are less ready to be as forthright. TULUNAYand rapid westward drift are usually present within the SAVERS (1971), using Ariel 3 data with a spatial trough (as viewed from the nonrotating reference resolution of 1.7~ latitude, found that the trough is frame). In troughs observed after midnight, the narrowest near mid night. FEINnLtJMand HORT~N(1973), observed ion drift velocities seldom depart much from using Alouette 1 data with a spatial resolution of 1~ corotation with the Earth. Observations from AE-C by BRINTONet al. (1978) latitude have been quoted as reporting that the trough is narrowest in the morning sector. Svmo et al. (1978) and SPIRO (1978) bear out the assumption that the found that, from AE-C observations (January 1974- trough in N, is chiefly due to a depletion in the O + November 1977) with spatial resolution of about 0.4 ~ concentration. The ion N +, not readily separated latitude, there are distinct differences between pre- observ~ttionally from O +, may be present at

spatial resolution of the measurements. The degree of improvement is not easily established from the open literature but the point should be borne in mind when comparing older results with more recent results. We describe first the mid-latitude "trough features about which most workers agree. These are:

328

R.J. MorrwrT and S. QUEGAN

concentrations around 10~ of the O § concentrations (ScHuNK and RAlrr, 1980). TAYLOR et al. (1975) and GREBOWSKYet al. (1976a) have investigated troughs in the high-latitude ionosphere that they term "high-latitude troughs". Distinguishing features of these troughs are that they occur poleward of the usual topside light-ion trough and show depressions of all the measurable atomic ions (H +, He +, N + and O +) while the molecular ion (such as NO +) concentrations are often enhanced. The troughs frequently lie in or near the polar cap boundary. It is not now clear that these troughs need to be distinguished from mid-latitude O + troughs, since the latter are not always colocated With the topside light-ion trough (see Section 4). GREBOWSKYet al. (1976a) have suggested that a strong convection electric field may raise the ion temperature and deplete the O + concentration; the H + concentration may fall by virtue of its Chemical link to O +, and He + and N + may flow downwards more readily to the loss region, while NO + will be enhanced by the rapid O + depletion. Another suggestion by GREBOWSKY et al. (1976a) is that soft electron precipitation may play a r61e in ion production.

not been resolved(WILLIAMS et of., 1976; HASEGAWA and MIMA, 1978 ;LANZEROrrl et al.,1978 ;BURKE et al., 1979; SHEPHERDet of., 1980). The interaction of the ring current with the plasmaspheric electrons can proceed during magnetically quiet times, though not with sufficient intensity to produce an SAR arc. BURKEet al. (1979) found that for quiet times in the H + (and electron) trough at 2500 km altitude, the electron temperature was always sharply peaked. Below 1300 km, however, peaked T, distributions in the trough were not consistent features of the data. Observations from the Dyn.amics Explorer satellites have thrown further light on this topic (Geophys. Res. Letts., September 1982). Of the recent papers discussed in Section 2, two contain data on electron temperatures in the midlatitude trough. MENDILLO and CHACKO (1977) estimate that, for very quiet midwinter conditions, the sum of the electron and ion temperatures shows only a modest enhancement in the mid-latitude trough. On the other hand, BRINTON et al. (1978) use the AE-C electrostatic probe measurements of electron temperature, T~, and find that a band of enhanced T~ coincides statistically with the mid-latitude trough throughout the night. The relation of T~ to the observed O + 3. PLASMATEMPERATURES IN TIlE TROUGII concentration on individual passes is displayed in Fig. 1 I. Well-defined T~ peaks are seen in the trough region Heat flow from the magnetosphere is an important source ofthermal energy for the electron gas in the mid- in Figs 11(b) and (c), though the T~ peak may not latitude F-region (for review, see SCHUNK and NAGY, coincide with the minimum in O + density. The 1978). Within the plasmasphere this energy source afternoon mid-latitude troughs (Figs 1 la and d) have becomes more important with increasing latitude, the no T~peak associated with them. Either the conduction maximum downward electron heat flow occurring at heating was absent or the T~feature was obscured by the about the plasmapause. For a given topside electron neighbouring, relatively high values of T~ on the lowheat flux, lower electron concentrations will tend to latitude side of the trough. From recent calculations using a model of the highlead to greater electron temperatures because of the greater thermal energy available per particle and less latitude ionosphere (SoJKA et al., 1981a) that will be Coulomb coupling with the ions. Thus the region of the discussed in Section 5, SCHUNK and SOmA (1982) have mid-latitude trough may be expected to be a region of suggested that enhanced electron temperatures will elevated electron temperature. This is often the case have little direct effect on the ion temperature, Ti, at high (WRENNand RAITT, 1975; SCHUNK and NAGY, 1978). latitudes or in the trough region. A possible cause of A visual manifestation of the enhanced electron high values of Ti is the presence of large convection temperatures is the occurrence of stable auroral red electric fields, giving rise to rapid ion convection and so arcs. These SAR arcs may be observed during the rapid heating the ions by ion-neutral friction. To investigate recovery phase of magnetic storms and are characthis O § frictional heating, a reliable horizontal terised by enhanced 6300 A wavelength emission from neutral air wind velocity field is required (for a recent oxygen atoms excited by the hot ambient electrons in discussion, see ST-MAURICE and HANSON, 1982). the F-region (for review of earlier work, see RE~ and Another source of frictional heating (in this case from ROnLE, 1975). It has been suggested that additional ion-ion collisions) comes from rapid field-aligned flow energy is generated in the magnetosphere by the of H + ions (RAI'I'Tet al., 1977). interaction of the ring current with the plasmaspheric TmIERID~E (1976) has analysed Alouette I electron, electron gas during the storm recovery phase (COLE concentration profiles for the years 1962-1965. His 9 1965 ; CORNWALLet al., 1971). The details of the heating analysis gave (among other deductions) results for of the magnetospheric electron gas by this interaction plasma temperature (treated as the sum of electron and and the effectiveness of proton precipitation have still" ion temperature) in the topside ionosphere. His results

The mid-latitude trough in the electron concentration of the ionospheric F-layer for 400 km altitude and 1000 km altitude show an increase in plasma temperature with increasing latitude, with peaks occurring on both the dayside and nightside at a latitude of about 60 ~ From comments above, it remains to be determined-whether these temperature peaks are a consequence of increased effectiveness of heat flow from the magnetosphere or increased heating due to ring current-plasmasphere interaction. 4. OBSERVED RELATIONSIIIPOF TIlE TROUGll TO TIlE PLASMAPAUSEAND TilE LIGIIT-1ONTROUGII Two ways in which the plasmapause boundary may be defined were considered by RYCROFT(1975). I t can be defined as the dramatic, order of magnitude or more, decrease in electron concentration (with increasing distance from the Earth) within a fraction of an Earth radius in the equatorial plane. This definition will lead to a problem if there are two, or more, such decreases or the decrease is poorly defined. An alternative definition considered by RYCROrr is a theoretical one in which the plasmapause is the boundary between plasma that essentially corotates with the Earth and plasma that takes part in magnetospheric convection over the polar cap. This definition suffers the drawbacks that the surface cannot be observed directly and will usually vary with time. FOSTERet al. (1978) put forward a third possibility. In their study of possible plasmapause signatures in the ionosphere and magnetosphere, they found that plasmasheet electrons were observed just outside the equatorial plasmapause at both dawn and dusk. The low-latitude boundary of the plasmasheet electrons at 1400 km altitude could be used as a signature of the equatorial plasmapause position. The question then is how to define the 'plasmapause' and the ionospheric projection thereof. The concept of the plasmapause as a sharp gradient in the electron concentration in the equatorial plane is longstanding and familiar. We suggest, first, that this concept of the plasmapause be identified as the "equatorial plasmapause" (as used above when quoting from FOSTER et al., 1978) or "magnetospheric plasmapause". The idea is firmly fixed in the literature that the lightion trough in the topside ionosphere and the F-region mid-latitude trough are somehow related to the plasmapause. This idea is supported by statistical surveys. With hindsight, from the theoretical ideas about the r61e played by convection in the formation of the mid-latitude trough (Section 5 below) and from examination of the papers discussed later in this

329

section, it is clear that the L-shell mapping of the equatorial plasmapause into the ionosphere may bear no obvious relation to, say, the mid-latitude trough. Our second suggestion is to use RvcgoFr's (1975) second definition of the plasmapause to define the "ionospheric plasmapause", i.e. the ionospheric plasmapause is conceived to be the surface in the ionosphere separating plasma taking part in convection over the polar cap from that which does not. As RvcRov-'r (1975) pointed out, the equatorial plasmapause and the ionospheric plasmapause, as we have defined them, will tend to lie on the same L-shell after a prolonged period of magne'tic quiet (although in such a period the nightside convection pattern may become irregular). Usually, however, the equatorial plasmapause and the ionospheric plasmapause will not be L-shell aligned. The complete plasmapause surface from the ionosphere to the equatorial plane, bounding the relatively dense plasmasphere, will cut across L-shells with increasing altitude. Our suggestions for defining an ionospheric (or convection) plasmapause and an equatorial plasmapause are supported by model computations by GREBOWSKY et al. (1974). These authors assumed a magnetospheric spatially,constant equatorial electric field that varies in step with the K v index and calculated the resulting convection plasmapause motion. The equatorial plasmapause (i.e. the plasmapause in the electron concentration in the equatorial plane) was estimated by assuming the flux tubes to be filled by plasma if the tubes remain closed for longer than 5 consecutive days. The results of GREBOWSKYet al. (see their Fig. 6), show that, in the recovery period after a magnetic storm, the observed (Ariel 3) positions ofthe mid-latitude trough at dusk follow the convection plasmapause rather than the equatorial plasmapause--the latter lies at lower L-values. There are few simultaneous observations of the midlatitude trough and the electron concentration in the equatorial plane, although there are several studies of the light-ion trough and other parameters associated with the plasmapause. GREBOWSKYet al. (1976b) have compared Ariel 4 measurements of electron concentration at 600 km altitude and ISIS 2 measurements of electron concentration at 1400 km with measurements made by Explorer 45 at near-equatorial latitudes. Examples ofsuch comparisons are shown in Fig. 5. The arrows in Fig. 5 indicate where the d.c. electric probe on Explorer 45 became saturated; MAYNARD and CAUFFMAN (1973) estimated that such a saturation occurred when the ambient plasma concentration fell to less than about 60 cm -3. GREBOWSKYet al. argued that the value of 60 cm -3 is likely to lie close to the equatorial plasmapause. There is recent evidence from

330

R.J. Mor, Ll~ and S.

ISLE l thermal plasma profiles (ltoRwrrz et al., 1981), however, that after a magnetically quiet period of a few days an arbitrary concentration value on the radial Nc profile may not be a good indicator of where the equatorial plasmapause is likely to lie. With this reservation in mind (no indication of Kp values being given by GREBOWSKYet al.), we may consider results such as those of Fig. 5, where the trough passages took place within 1 h of UT and 1 h of MLT of the near-equatorial Explorer 45 probe saturation crossing. GREBOWSKY et al. found that in all the examples with a _ 1 h time window a depression ofthe electron concentration was detected along Ariel's orbit in the vicinity ofthe field line on which the equatorial N, was about 60 cm -3. The equatorial plasmapause field line, however, was not repeatedly associated with the same characteristic feature in each trough observation. Work is in progress (THOMASand S.~IITH, 1982) to correlate the position of the equatorial plasmapause, deduced from whistler observations, with the position of the mid-latitude trough observed at Halley Bay, Antarctica.. Preliminary results indicate that on 27 June 1980 (during a recovery period after a magnetic storm) the mid-latitude trough lay significantly poleward of the equatorial plasmapause in the pre-midnight MLT sector .

Three studies ofthe light-ion trough (MORGANet al., 1977 ; FOSTERet al., 1978 ; GREBOWS~:Yet at., 1978) will be considered together (since they do not directly concern the mid-latitude trough). Approximately simultaneous measurements in the topside ionosphere and measurements in the equatorial plane were examined. The results of the three papers may be summed up by the conclusion that the dynamics of plasma coupling between ionosphere and magnetosphere dominate the topside data and obscure the precise L-shell projection of the equatorial plasmapause. Direct comparison between mid-latitude trough behaviour and light-ion trough behaviour is notably scarce in the literature. TAYLORand WALSll(1972) used data from topside ionospheric observations of ion concentration by the OGO series of satellites. They found that during the daytime the light-ion trough typically occurs without any signature in the total ion (or electron) concentration. The absence of a topside total ion trough will usually correspond to the absence of a mid-latitude trough on the dayside, although the relationship of topside and peak N, may be obscured by 7~ effects. On the nightslde, the tt + decrease occurs in the vicinity of a much shallower depression in electron concentration ; the latter may. be presumed to be the "diffusive equilibrium extension of the F-region midlatitude trough into the topside.

QUEGAN

TAYLORand WAL$tl stress the point that the shallow N, trough and the N(H +') trough both occur in a region of rapidly changing ion composition. In the topside around I000 km altitude, It + is often the dominant ion equatorward of the topside troughs and O + the dominant ion polcward of the troughs. MARUBASH! (1970) had already noticed the rapid change of mean ion mass near 1000 km altitude by deducing electron scale heights from Alouette 1 topside soundings. This point must be borne in mind when using topside electron concentrations to make deductions about the topside extension of the mid-latitude trough. MENDILLO and" CIlACKO (1977) showed how altitude profiles of electron concentration can be used to demonstrate the approximate colocation of the midlatitude trough and the light-ion trough on the nightside. 5. TIIEORETICAL STUDIES

There is general agreement that the part of the ionosphere that lies equatorward of the mid-latitude trough region is usually little affected by the highlatitude convection electric field and lies in the inner plasmasphere.,Observation and theory also agree that the poleward wall of the trough is caused by ionization by precipitated particles in the auroral zone. In this section are discussed the physical processes that arc likely to cause the trough itself, large-scale models that incorporate some or all of these processes and what understanding is thereby brought to bear on the observations detailed in Sections 2-4 above. 5. I. Physical processes in the mid-latitude trough region

Free electrons are produced by photoionization of neutrals by solar e.u.v, radiation and electrons are lost by recombination of ions and electrons. These production and loss processes are important in the ionosphere at all latitudes. In considering qualitatively the additional factors likely to influence F-layer density at high latitudes, KNUDSEN (1974) made significant advances. In particular he suggested the crucial r61e that plasma convection may play in the maintenance of the polar ionosphere and in the formation of the midlatitude trough. He showed that convection of plasma across the polar cap (with velocities from 0.5 to 1.0 km s- ') in the antisunward direction was likely to maintain a tongue of ionization over the polar cap towards the nightside. The convection pattern was taken from KAVANAGttet al. (1968), corresponding to the 'tear drop' pattern in the equatorial plane. This pattern leads to a stagnation point around 18 h MLT with slow eastward slow on the nightside from dusk through midnight towards dawn (Fig. 14). KNUDSI-Net

The mid-latitude trough in the electron concentration of the ionospheric F-layer

331

12 MLT

I

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24 - - 1 5 - - ELECTRON CONCENTRATION ( I04 CM -3) - - ~ - - C O N V E C T I O N FLOW LINES ....

EXTRAPOLATED CONTOURS - 104" SOLAR ZENITH ANGLE

Fig. 14. Contours of NmF2 derived from the model of KNUDSENet al. (1977). The short-dashed portions of contours are extrapolated contours.

al. (1977) showed that photoionization by solar e.u.v. (rather than by par ticle precipitation in the cusp region) is capable of supplying the observed polar cap ionization levels. KNUDSEN (1974) suggested that the mid-latitude trough is formed as a result of the slow convective transport of plasma on the nightside. The usual recombination processes are sufficient to produce low electron concentrations. Other processes have been discussed" by SCttUNK et al. (1976). Plasma escape along magnetic field lines by the upward flow of H § in the polar wind acts to decrease the O + content and contributes to'the rate of decay of the F-layer. By itself, this process is too slow to account for the deep mid-latitude O § troughs. In combination with other processes, however, it may account for part of the ionization depression in the trough (ScHuNK et al., 1976). MURPttY et al. (I 976) have shown that replenishment of the protonosphere inside

t h e plasmasphere may depress F-layer electron concentrations by up to 15~o. Also, the upward flow of H + in both the polar wind and inside the plasmasphere is expected to be one of the factors that are important in producing the light-ion trough. At lower latitudes, downfiows of H § may help to maintain the night-time F-layer. The dependence of the rate of the O § loss reaction O + +N2 --' NO + + N

. (2)

on ion temperature, on enhanced N 2 concentration and on the vibrational state of the N 2 molecules is considered by SCmINK et al. (1976). Strong convection electric fields give rise to rapid plasma drift. When the relative ion-neutral velocity becomes significant, the resultant frictional heating raises the temperature of the ions. Experimental evidence on increased ion temperature from AE-C data has recently been published by ST-MAURICEand HANSON(1982). This T~

332

R.J. Morrell and S. QOEGAN

increase may lead to an increase in the rate coefficient of reaction (2). For a given ion-neutral relative velocity, this increase is not as marked in recent theoretical work on the rate coefficient (ST-MAURICEand TORR, 1978) as in earlier work (McFARLAND et at., 1973). Indeed, in the paper by SOJKA et al. (1981c) (see Section 5.2), where strong convection electric fields are used in the model, the influence ofelevated Ti on the decay ofO + appears to have been discounted, although the T~ values are discussed in connection with scale-height effects. Increased loss rate of O + may lead to significant amounts of NO + in the so-called "high-latitude troughs" (TAYLOR et al., 1975) (see Section 2.1). We recall that AE-C results for a magnetically quiet period showed O + to be the dominant ion in the mid-latitude trough (BRINTON et al., 1978). Enhanced molecular nitrogen concentrations during magnetically active periods in the sub-auroral zone have been reported by RAi-vr et al. (1975) and PROLSS and YON ZAnN (1974). These appear insufficient to explain the deeper mid-latitude troughs (WRENNand RAITT, 1975; SCHUNK et al., 1976) but again may contribute at times to the depression in electron concentration. The rate coefficient of reaction (2) is increased if the N 2 molecules are vibrationally excited. The enhanced electron temperatures often present in the trough region (see Section 3 above) are unlikely to cause significant vibrational excitation of N 2 (NEWTON and WALKER,1975) but excited N2 from the auroral zone could be transported into the trough region by diffusion and an equatorward neutral wind. SCHtJNK and BANKS (1975) show that the vibrating N 2 molecules can travel about 4 ~ of latitude if the equatorward transport speed is 100 m s-1, before quenching with atomic oxygen reduces their density. With no experimental measurements available, the influence of this process on midlatitude trough formation remains uncertain and has not been included in any of the recent large-scale models (see Section 5.2). There is considerable evidence for the presence of winds blowing equatorward from the nocturnal auroral oval. Ion-drag by the convecting plasma on the neutral air and the neutral air pressure gradients act in the same direction on the nightside (FULLER-ROWELL and REES, 1980; ROBLE et al., 1982; HEPPNER and MILLER, 1982). The neutral air winds can raise (or lower) the F-layer causing decreased (or increased) decay rates, an effect well-known in the normal moderate-latitude ionosphere. The winds have other effects, as mentioned above: the relative ion-neutral velocity determines the ion temperature in the F-region and the winds may transport vibrationally excited N 2 into the trough region.

Summarising, the physical processes that must be considered in models of the high-latitude ionosphere are [using the labels ofSctlUNK et al. (1976)] : ii) plasma convection; (ii) ion chemistry: how the rate of reaction (2) varies; (iii) plasma escape; (iv) neutral air winds. These are in addition to ionization production mechanisms by solar e.u.v, radiation and energetic particles (including the positioning of the convection pattern relative to the production regions), to the diffusive transport of ionization along magnetic field lines, and to the thermal balance of the ionosphere. 5.2. Large-sca?e coniputational models The idea put forward by KNOt)SEN(1974) (that slow convection plus recombination can explain the nightside trough) was pursued quantitatively by KNUDSENet al. (1977). The computational code used to solve the coupled continuity and momentum equations for the ions O +, O~ and N O § was based on the work of SCHUNK and WALKER (1973). Convecting tubes of plasma were followed and the equations solved in the altitude range 120-500 km. At the upper boundary a constant O + outflow velocity of 10 m s - 1 was assumed to simulate loss of O + as the polar wind. Production of O +, N~ and O~ ions by energetic particles i n the cleft and nightside auroral zone was included ; these particle production rates have been used by other workers (SOJKA et at., 1981a; QUEGAN et at., 1982). No computations were carried out by KNUDSEN et al. for plasma tubes that lie inside the plasmasphere and so do not partake in high-latitude convection over the polar cap. Results from KNUDSENet al. are shown in Fig. 14. Evident is the trough on the nightside, with concentrations as low as 3 x 103 c m - 3 thus vindicating in principle KNUDSEN'S(1974) idea. The constancy of the trough electron concentration with local time before dawn is due to an assumed constant flux of solar e.u.v, photons scattered into the night ionosphere. The ridge of ionization (with Ne approaching 106 cm -3) equatorward of 70 ~latitude is due to particle ionization in the auroral zone and is over-emphasized because of the assumed convection pattern. WATK1NS (1978) modelled the F-layer density at 300 km altitude at high latitudes. Figures 15 and 16 show some of his results. These demonstrate the occurrence of universal time (UT) effects on the ionization distribution. These effects arise when the geomagnetic pole is displaced from the geographic pole. According to WATKINS, his predicted UT effects are consistent with ionosonde observations at Thule, Greenland (Sa'ROMMAN and MAEHLUM, 1974). The convection" pattern used by Watkins was similar to that used by KNUDSEN et al. (1977). The mid-latitude trough is dearly seen in Figs 15 and 16, with concentrations as

The mid-latitude trough in the electron concentration of the ionospheric F-layer 12

9

x.-~.

TM"

:,

30

1

\'

r

oo,

oo

Fig. I 5. Contours of F-region electron concentralion (in units

of l0 s cfn -a) derived from the model of ~VATKINS(1978) for equinox conditions and 1730 UT.The large circle refers to 60~ invariant latitude. The two small bars either side of the circle denote the line of zero solar depression angle.

low as 3 x 103 cm -3. The M L T range of trough occurrence depends on the particular UT chosen. The relation of the trough to the plasmapause is not clear. WATKINS also investigated seasonal variations. Ilis model gave no mid-latitude trough in summer; the trough appears at equinox and is deeper during winter. A model incorporating vertical ion diffusion [in contrast to WA'rKIr~S(1978)'1 and the displacement of the geomagnetic and geographic poles [in contrast to KNtJDSEN et al. (1977)] has been presented by SOJKAet al. (I 981 a). Their convection model is based on the work

/

_

J/

3.0 zz5o

Fig. 16. Contours of f-region electron concentration (in units of 10~ era'; ~) derived from the model of WA'rKI~S(1978) for equinox conditions and 0530 UT. The MLT's are as given in Fig. t5.

333

of VOLLAND(1978) and includes the offset between the geographic and geomagnetic poles, the assumed tendency of plasma to corotate about the geographic pole and a dawn--dusk electric field mapped to a circular region in the ionosphere about a centre offset by 5~ in the antisunward direction from the magnetic pole. The radius of this circular region corresponds to 17~ of latitude and the electric potentials (and thus convection flows paths) are aligned parallel to the noon-midnight meridian within the circular region. Equatorward of the circle the potential diminishes radially, varying inversely as sin 4 0, where 0 is magnetic colatitude. In the ionospheric model of SOJKAet al., the ions are O+,NO+,O~',N~-,N + andHe + (but not H§ the altitude range is 160-800 km, with zero ion flux across the upper boundary. To obtain the results given by SOJKAet al. (1981a), a magnetospheric cross-tail potential of 20 kV was chosen to represent low magnetic activity conditions; the atmospheric conditions were appropriate to the winter solstice at sunspot minimum. The main result obtained by SOJKAet al. (1981a) was that high-latitude ionospheric features, such as the mid-latitude trough and UT effects, are a natural consequence of the competition between the various chemical and transport processes operating in the high-latitude ionosphere. This point is beautifully illustrated by the plates in SOJKA et al. (198 la), giving O § concentration contours at 300 km altitude. (Keen readers may note that the captions to the plates should be interchanged : see correction by SOJKA in April 1981 issue of d. geophys. Res.)

A further striking deduction by SoJr,Aet al. (1981 a) is the demonstration of the care required when interpreting satellite observations in the geographic or geomagnetic coordinate frames. SOJKAet al. simulated a circular orbit at 300 km altitude that traverses the polar region in the dawn-dusk plane and plotted the O § concentrations that would be observed along the satellite track in both the geographic inertial (GEl) frame and the magnetic local time (MLT) frame. T h e results are shown in Fig. 17, with the left panels corresponding to the GEl frame and the right panels to the MLT frame. Two simulated satellite trajectories were considered, one at 8 h UT and the other at 20 h LT~ The centre panel results show the importance of displaying and analysing data in the reference frame of the dominant or controlling process. In the midlatitude trough the concentrations vary systematically in the G E l frame, while in the auroral oval they vary systematically in the M LT frame. The bottom panels of Fig. 17 show the confusion that could arise from accumulating and displaying data from a whole day of satellite crossings of the polar region.

334

R. J. Mortal= and S. QUEGAN

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Fig. 17. Results from model calculations by SOJKAet al. (198 la), showing predictions from simulated satellite crossings of the northern polar cap in the dawn--dusk plane at 300 km altitude. The top panels show two representative crossings 12 h apart in both the geographic inertial frame (left panel) and the magnetic quasiinertial frame (right panel). The centre panels display the O + concentration along the satellite track for these crossings in the geographic inertial frame (left panel) and the magnetic quasi-inertial frame (right panel). The bottom panels display the O § concentration along the satellite track for one full day; each symbol represents data from a particular satellite crossing. Negative colatitude is in the dusk sector, positive colatitude in the dawn sector.

Two caveats to the SOJKAet al. (1981a) work must be added, however, one of these concerns the details of the U T effects and the other the details of the mid-latitude trough behaviour. In transforming from the geographic coordinate frame to the geomagnetic coordinate frame, SOJKA et al. (1979) state that corotation is U T independent in the geomagnetic frame. QUEGAN(1982) disputes this statement (work to be submitted in the open literature by QUEGANet al., 1983). SOJKA (1982)

contends that the U T dependence of corotation in the geomagnetic frame causes only a second-order effect, while he agrees in principle with the coordinate transformations derived by QUEGAN (1982). TO understand our second caveat it has to be noted that in the SOJKA et al. (1981a) results [and also in the SOmA et al. (1981C) resultsl, the mid-latitude trough seems to occur on tubes of plasma that follow convection paths inside the plasmasphere. This means

The mid-latitude trough in the electron concentration of the ionospheric F-layer that the ndgleci of H § in the computations is a serious omission, certainly as far as the mid-latitude trough is concerned; upflows ot" H § depress the F-layer O § concentration and downflows o f l I § may maintain the F-layer at night (MtrRt'nY et at., 1976). T.he neglect of meridional neutral air winds also has a marked effect on the mid-latitude trough, as noted by SOJKA et al. (1981a); this was rectified in later papers (SOJKA et at., 1981b, c). SOJrZA et al. (1981b) have presented the ion composition results predicted by the SOJI,:A et al. (1981 a) model. They find that O § is always dominant at 300 km altitude in the mid-latitude trough; in the morning sector mid-latitude trough, O~" is the dominant molecular ion only ira meridional neutral air wind isincluded. A good example of the UT dependence

335

of the mid-latitude trough and of ion concentrations is shown in Fig. 18 (SoJI,:A et al., 1981b). SOJKAet al. (1981c) repeated the SOJKAet al. (1981a) calculations but taking the case of strong convection (cross-tail potential of 90 kV; radius of polar cap of 18.5 ~ latitude; offset of centre of potential pattern of 7.5 ~ and using the auroral oval of COXtFORX (1972), applicable to a Kp value of 5, but still using the KNUDSEN et al. (1977) production rates in the oval. It should be noted that the atmospheric conditions are appropriate to solar maximum. SOJKAet al. (1981 c) find that the mid-latitude trough concentrations are lower for weak convection at solar ~ninimum (1981a) than for strong convection at solar m a x i m u m ; SOJKA et al. (1981c) also show themselves to be aware of the consequences of the mid-latitude trough lying at rather

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336

R.J. MOFFE'I'rand S. QUEGAN

low latitudes inside the plasmasphere (see second caveat above). We note that the discussion by SOJKAet al. (1981c) of enhanced T~over the polar cap as giving greater topside scale heights with no mention of significantly increased O § loss rates, implies that even with strong convection any increase in the O § loss rate is relatively insignificant. SOJKA et al. (1982a) investigated seasonal variations of the high-latitude Fregion for strong convection under sunspot maximum conditions. They concluded that the depth and latitude of the mid-latitude trough can exhibit appreciable seasonal variation, in qualitative agreement with the conclusions ofWATKI,XS(1978) for weak convection (see WATKINS' Fig. 7 rather than his abstract). The large-scale model developed by QUEGAN et al. (1982) is complementary to that of SOJKAet al. (198 l a). The ions included in the main study are O + and It + (although later calculations have included the molecular ions). In the high-latitude region outside the ionospheric plasmapause the altitude range is 1501400 km, with tt + field-aligned velocities at 1400 km altitude based on empirical data of HOFFMAN and DODSON'(1980). For the outer plasmasphere immediately adjacent to the plasmapause, the same modelling procedure is used as that for the highlatitude tubes of plasma. In the inner plasmasphere (L -%<3),distributions ofO § and H § are computed from the lower F-region to the equatorial plane. Across the ionospheric plasmapause the plasma convection, the atmospheric parameters and the neutral air wind are continuous. The two-cell convection pattern is based on the work of SPtRO et al. (1978) and SPIRO(1978), and is displayed in Fig. 19. Such a pattern is specifically designed to include several features apparent in AE-C ion drift data: a dayside throat, plasma flows on each side of the convection reversal boundary being parallel to that boundary, slow convection in the evening sector and the observed directions of flow in the evening sector mid-latitude trough region. Certain of these features are supported by incoherent scatter measurements made by WAND and EVANS (1981). The geomagnetic and geographic poles are taken to be coincident, thus excluding UT effects. In Fig. 19 the ionospheric plasmapause is considered to be the equipotential surface such that its projection lies between paths 5 and 6. Neutral air winds for high latitudes are based on the global model of FULLER-ROWELLand REES (1980); a model computation was performed that included the convection pattern of Fig. 19 as a momentum source for the neutral air. Results were given by QUEGAN el al. (1982) for equinox under sunspot minimum conditions. A latitudinal plot of F-layer peak and topside concentrations along the 2-14 h LT meridian is given in

1. 7OkV 2. -0-SkV 3. -9.5kV /..- 17OkV

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Fig. 19. Examples of convection paths of plasma at 300 km altitude in the northern hemisphere under the combined influence of the magnetospheric and corotation electric fields, depicted in the fixed Sun-Earth frame. The large dots indicate the starting points used in the calculation of the paths. The time between sdccessivedots is one hour, with the exception of the return to the starting point. The numbers on the paths correspond to the listed potentials of the paths. The polar cap boundary (not marked) is a circle of radius 15~ centred 5~ towards midnight from the geomagnetic pole. From QUEGAN et al. (1982).

Fig. 20. Evident at 2 h LT is the mid-latitude trough in N m F 2 , located at about 57 ~ invariant latitude just

equatorward of the ionospheric plasmapause. The tubes of plasma in the outer plasmasphere have travelled eastward at less than the corotation speed and have received little ionization from the protonosphere. The high-latitude tubes poleward of the plasmapause (paths 3, 4 and 5, for example, in Fig. 19) have left the polar cap after passing through the auroral oval. Those which left the polar cap in the late evening have travel times sufficiently long for ionization to decay as the tubes travel eastwards. Hence the poleward wall of the trough is not just a sudden jump. For 2 h LT the auroral oval in the model is located roughly in the latitude range 68-73 ~ At some other times, such as at 20 h LT, the poleward wall is very abrupt and coincides with the local equatorward edge of the auroral zone. The influence of the meridional component of the neutral air wind is seen in the behaviour in Fig. 20 of" h m F 2 , w i t h the F-layer generally higher on the nightside than on the dayside. Calculations that exclude the neutral air wind give deeper mid-latitude troughs on

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Fig. 20. Calculated variations with invariant latitude of h=F2, N=F2 and the O* and H + concentrations at l I00 km along the 020(01400 LT meridian. Each point plotted is a point on one of the convection paths ; no attempt is made to interpolate latitudinally between points on different paths. The plotted values at the three lowest latitudes correspond to tubes of plasma of the inner plasmasphere 2 days after an initial It § depletion ; the next two values correspond to tubes of plasma in the outer plasmasphere; all other values correspond to high-latitude tubes of plasma. The arrow labelled "PP'" indicates the latitude of the ionospheric plasmapause. Both the outer plasmasphere and the high-latitude tubes of plasma have reached steady state in a day-to-day sense. From QUrGANet al. (1982). the nightside. For example, at midnight LT the m i n i m u m N~F2 value is 3 x I0 t~ m -3 with winds but 5 x 109 m -3 without winds. In the topside ionosphere at 2 h LT the electron concentration tends to reflect the behaviour of NmF2, although the poleward wall of the

N,[ = N(O+)+N(II+)] trough is less steep. The light-ion trough that is formed equatorward of the ionospheric plasmapause parallels the behaviour of N,~F2. In the dayside-topside ionosphere (Fig. 20) the H + concentration shows a steady but gentle decline with increasing latitude from its value at L = 2; this light-ion trough is embedded in N=F2 behaviour and topside N(O +) behaviour that show no signature of the plasmapause. The latitudinal profile of H + flow (Q UEGANet al., 1982), corresponding

1o the results of Fig. 20, is shown in Fig. 21. On the dayside the increasing H § flow with increasing latitude produces the light-ion trough; on the nightside the N(H § m i n i m u m is linked to the NmF2 trough rather than determined by the H + flow characteristics. At this stage it is worth referring back to the processes listed (i)-(iv) at the end of Section 5.1 and asking to what extent these processes have been included in the models of SOJKA et al. (1981a, c) and QUEGANet al. (1982). Plasma convection patterns are the basis of both models, although the patterns are different. SOmA et al. (1981 a) appealed to the agreement found between their adopted convection pattern and incoherent scatter observations made at Chatanika and Millstone Hill (SoJKA et al., 1980).The QUEGANet al. (1982) convection pattern agrees with AE-C observations (SPtRO et al., 1978). It must be emphasised that in both models the

R. J. MoFrErr and S. QUEGAN

338

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Fig. 21. Calculated variation with invariant latitude of the II § field-aligned flux at 1100 km along the 02001400 LT meridian. The arrow labelled "PP" on Fig. 20 indicates the latitude of the ionospheric plasmapause. From QUEGANet al. (1982).

convection pattern is assumed to be steady "over a period of 1-2 days at high latitudes, and for longer periods in the outer plasmasphere in the QUEGANet al. model. Thus the variability of the convection is neglected, both in the short term (substorm effects) and in the longer term (the quiet period may occur during recovery from a major storm). The chief defect in the treatment of process (ii), ion chemistry as affected by the O § loss rate, is the neglect in the models of the possible presence of vibrationally excited N 2 molecules. Empirical and theoretical evidence on this point is lacking. An attempt to take account of plasma escape (and downflow from the protonosphere) has been made by QOEGAN et al. (1982). But quite strong provisos need to be registered. Plasma escape in the high-latitude region is based on one set of empirical data with limited coverage (HoFVMAN and DODSO,% 1980). Theoretical prediction of the magnitude of the polar wind outflow for given N ~ F 2 val'ues and atmospheric and magnetospheric conditions is one of the major unsolved problems of ionospheric and magnetospheric physics. Inside the plasmasphere the return flow of H § from the protonosphere at night depends on (among other factors) the plasma temperature altitudinal gradients, and these are poorly known. Progress has been made in assessing the rfle of process (iv), neutral air winds, but much remains to be done. The winds used by QOEGAN et al. (1982) are consistent with their convection pattern but not necessarily consistent with the calculated ion concentrations. The winds have little effect on the dusk trough

but help considerably to maintain N m F 2 in the nightside trough. SOJKA et al. (1981c) used modified mid-latitude winds. The r61e of auroral zone ionization and other auroral processes on the formation of the mid-latitude trough is inadequately dealt with in the models. SOJKA et al. (1981a) demonstrated that the way the auroral oval is positioned relative to the plasma convection pattern has an appreciable effect on the MLT extent of their calculated mid-latitude trough. QUEGAN et al. (1982) concurred with this conclusion. The difficulty is the great variability of the auroral processes and their positioning relative to the convection pattern and the consequent lack of empirical data for input to the ionospheric models. No attempt is made in the models to calculate the electron temperature self-consistently. Such a calculation is a major computational undertaking. Thus the models have nothing to say about the interesting T, effects observed in the mid-latitude trough (Section 3 above). Finally, we remark on the large computational effort that is required to incorporate UT dependence. SOJKA et al. (1981a) gave details of their computing housekeeping. Concentration values for O § N § N~, O~, NO § and He + were stored at 20 km altitude intervals between 160 and 800 km for each step along their 23 trajectory runs, each for 12 UT start times. The result was a data base containing about 22,000 altitude profiles for each of the six ion species considered. For mid-latitude trough calculations, these numbers could be reduced by omitting some of the ions and adding H +.

The mid-latitude trough in the electlon concentration of the ionospheric F-layer 5.3. Comparison of model predictions with obserred midlatitude trough morp.hology We first examine points (1)-(6) from Section 2.1, the summary of trough morphology. (1,2) The models show that the nightside occurrence of the mid-latitude trough is a consequence of long travel times for the plasma in the convection pattern while the ionosphere is in darkness. QtJIZGAN et al. (1982) confirm that the late afternoon occurrence of the trough (at latitudes poleward of the plasmapause) occurs as a consequence of the St,fRo et al. (1978) convection pattern, as already suggested by SrtRO et al. SOJKA et al. (1981a) show that, at certain universal times, a mid-latitude trough could form on the dayside in winter for weak convection. SOJKAet al. (1982a) find that, for strong convection at sunspot maximum, the trough appears throughout the night during winter but is restricted to the late night sector in summer. In the WArKtNS (1978) weak convection model, no trough appears in summer. (3) The shape and extent of the poleward edge of the trough is critically dependent on the positioning of and the intensity of ionization in the auroral zone ( S o J ~ et al., 1981a; QUEGANet at., 1982), and on the convection pattern. (4) The decrease in latitude of the trough through the night is a natural consequence of the convection pattern and may cease before dawn (QtJEGAN et at., 1982). (5) Stronger convection, associated with increased K~, values, causes the trotlgh to move to lower latitudes (SoJKA et at., 1981a,c). (6) The variation with solar cycle has not yet been investigated theoretically. Examples of model mid-latitude trough shapes are given in Fig. 18 (SoJKA et at., 1981b) and in Fig. 20 (QUEGAN et al., 1982). Variations of depth and width do occur in the model results but changes in the values of the input paranaeters to the models in their present form seem unlikely to give structured troughs such as those of Fig. 4 (MEYDILCO and CnACKO, 1977) or Fig. 5 (GREBOWSKYet al., 1976b). The model results appear to fit better the sinooth, unstructured trough of Fig. 6 (KOIINLEIN and RAIVr, 1977). The ion composition results of SOJKAetal. (1981 a, b) for weak convection at solar minimum show that O + is dominant at 300 km altitude in the mid-latitude trough. No detailed explanation of the "high-latitudeqrough" form (TAYLORet al., 1975) of the mid-latitude trough is yet available. In comparing their UT predictions with experimental results, SOJKA et al. (1981a) note that the presentation of AE data by BRIN'roN et al. (1978) includes results for many universal times. SoJr.A et al.

339

(1982b) make a more meaningful comparison of the SOJKA et al. (1981a) model predictions with experimental data obtained by DMSP (U.S. Defence Meteorological Satellite Program) F2 and F4 satellites. SOJKA et al. (1982b) point out that what they term a "diurnal variation" corresponds to what DUNCAN (1962) called the"UT control of the polar ionosphere". A diurnal variation is one in which stations at different longitudes observe different 24 h variations o f f oF2. SOJKA et al. (1982b) examined both the long term variation of the observed high-latitude ion concentration at 800 km altitude on a time scale of days, and orbit-by-orbit variations'at the same geomagnetic location in the northern winter hemisphere. Qualitative agreement is found between these observed variations and the diurnal variations predicted by the SOJKAet al. (1981a) results. 5.4. Interrelationships between the mid-latitude trough, the plasmapause and the light-ion trough 5.4.1. Mid-latitude trough: ionospheric plasmapause. In Table 2 are presented the latitude of the trough minim urn, as predicted by Q UEGANet al. (I 982), and the corresponding location of the ionospheric plasmapause. As pointed out by Svmo (1978), the evening bulge in the location of the plasmapause (at about 20 h LT in this model) is not reflected in the mid-latitude trough position. Prior to about 22 h LT, the trough minimum lies at considerably higher latitudes (than the plasmapause), where the convection pattern gives rise to decay ofplasma on certain tubes of plasma. After 22 h LT, in the model the trough seems more likely to lie just inside the plasmapause on outer plasmasphere tubes. It must be remembered, however (see above) that there is considerable uncertainty in the model due to the intensity and position of the auroral oval ionization. The prediction of the trough minimum position may depend on how much auroral ionization is picked up by tubes that lie poleward oftheionospheric plasmapause.

Table 2. Calculated position of the mid-latitude trough (QUFCANet al., 1982) LT (h) 1600 1800 2003 2200 2400 0200 0400 0600

Latitude of trough minimum !degrees)

Approximate latitude of ionospheric plasmapause (degrees)

75.5 73.0 67.5 61.5 58.5 56.5 56.0 56.0

61.0 62.0 64.0 62.5 59.0 57.0 56.5 57.0

340

R. J. Morr,-ll and S. QUEGAN

5.4.2. Light-ion trough : ionospheric plasmapause. In contrast to the mid-latitude trough in the F-layer, the light-ion trough is usually present on the dayside as well as on the nightside (e.g. TAYLOR and WALSn, 1972; ttOFI-,~,IANet al., 1974; ~,VRENNand RAITT, 1975). The QUEGANet al. (1982) model results give on the dayside a factor of 4 or 5 decrease in N(II +) at ll00 km from L = 2 to the region poleward of the ionospheric plasmapause. This decrease is due to the increasing importance of upward H + flow to the protonosphere with increasing latitude and L-value. To drive the upward H + flow, through the retarding O +, the tl § concentration is depressed in the topside ionosphere. QUEGANet al. (1982) state that results for a later day of calculation inside the plasmasphere (as the protonosphere is fuller) show that the decline in N(H § with increasing latitude becomes very gentle. Under the steady, quiet conditions adopted by QUrGAN et al., it appears that the ionospheric plasmapause can be identified fairly readily in daytime by a plateau in N(H +) poleward of the plasmapause position. Of course, itnmediately following a magnetic storm, the situation may be complicated by more rapid flow that is similar both inside and outside the plasmasphere (GREBOWSKYet al., 1978). Similarly, perturbations to the convection pattern will obscure the relatively simple picture presented by QUEGANet al. (1982). The behaviour of N(H +) at 1100 km altitude is generally more complicated on the nightside (QUEGAN et at., 1982). Inside the plasmasphere H § is the major ion at this altitude; sufficiently poleward of the ionospheric plasmapause O § becomes major. Thus the formation of the light-ion trough is influenced directly and significantly by the chemical link with the F-layer concentration of O +. This is in accordance with results from the ISIS 2 satellite for 1400 km altitude, published by HOFI:MANet al. (1974). Examination of the nightside N(H +) profiles for 1100 km altitude (QUEGANet al) shows the difficulty of finding a correspondence between the ionospheric plasmapause position and the light-ion trough. In practice deduction of the plasmapause position from an empirical night-time topside N(H +) profile is likely to be uncertain. 5.4.3. Arid-latitude trough: light-ion trough. This relationship can be inferred from the previous subsections. Here we just summarise the QUEGAN et al. (1982) model results. On the dayside at equinox there is no mid-latitude trough. On the nightside the light-ion decline corresponds to the equatorward wall of the mid-latitude trough for LT's between about 19 and 6 h. In the late afternoon and dusk sector the light-ion minimum corresponds to the mid-latitude trough minimum, but the light-ion decline at lower latitudes is similar to dayside N(H +) gradients.

5.4.4. Mid-latitude trough: equatorial plasmapause. If the equatorial plasmapause is field-aligned with the ionospheric plasmapause, then the remarks of subsection 5.4.1 apply. In general, of course, this will not be the case; there will be no obvious, direct connection between the mid-latitude trough and the equatorial plasmapause during a time of magneiic quieting as the protonosphere is replenished (GREBOWSKYet at., 1974). There may be a subtle connection, however, as the recovery proceeds. Equatorward convection drift on the nightside in the outer plasmasphere (caused perhaps by substorms) can drive plasma into the ionosphere a~ the tube volume is decreased. The additional plasma in the ionosphere can influence the mid-latitude trough behaviour (QUEGAN, 1982). 5.4.5. Light-ion trough: equatorial plasmapause. Several of the comments made above (in Section 5.4.2) apply here also. It is well established that with increasing L-value the thermal plasma in the upper reaches of the closed magnetic flux tubes of the plasmasphere is less likely to be in a state ofequilibrium (over a 24 h period). Thus with increasing latitude the signature of upward H + flow in the topside ionosphere will become more marked i.e., N(It +) at a fixed altitude is expected to decrease with latitude. The H § flow, however, may show no marked change as the L-value of the equatorial plasmapause is crossed. The dynamics of the plasma coupling between the ionosphere and magnetosphere obscures the L-shell projection of the equatorial plasmapause. 6. CONCLUDINGREMARKS Much remains to be learned observationally about the behaviour of the mid-latitude trough. In particular, the relationships between the trough behaviour and the convection electric field, the neutral air wind, the lightion trough and the plasmapause have not been clearly delineated. Detailed computational studies of the mid-latitude trough in the context of the high-latitude ionosphere and the plasmasphere have only recently been carried out. Substantial computing time is required for these studies. Given a particular steady configuration for the convection electric field, it is a major undertaking to calculate ion concentrations and temperatures, allowing for the offset between the geographic and geomagnetic poles and including self-consistently the coupling with the neutral air wind field. First results from the studies are encouraging since there is qualitative agreement with several of the observational features. Major limitation in the inputs to the" theoretical work are the uncertainties in tile convection electric field, in the intensity and positioning of the auroral particle source of ionization and in the

The mid-latitude trough in the electron concentration of the ionospheric F-layer abundance of excited nitrogen molecules in the trough region. These limitations will take some time to be resolved. In principle, the inclusion of a time-dependent convection electric field should be straightforward, even if computationally tedious. Subsform variations of convection on the time-scale of a few hours (see SPmO et at., 1981, and references therein) will perturb the nightside mid-latitude trough structure that is predicted by present models. Universal time effects may also play a r61e in producing tile variety of trough width and depth seen observationally. It appears likely, though, that the very wide trough observed in ISIS 2 data by MENDILLO and CHACKO(1977) does not serve as a good indicator of"baseline b e h a v i o u r ' ; rather, we speculate that the MENDILLO-CHACKO trough is due to the variation with latitude of ionization production and loss rates inside the plasmasphere in an extended period of very quiet midwinter conditions. The "highlatitude trough" (TAYLORet al., 1975) is another form of mid-latitude trough that appears to lie outside the scope of the present models. Summarising, it appears that our present understanding of the physical processes operating in the midlatitude trough region is probably sufficiently good to explain the most commonly observed mid-latitude troughs, viz. those seen at local times from late afternoon through to dawn. Slow convection in dark

regions gives rise to decay of ionization and the formation of the trough. Mid-latitude troughs observed in the noon local time sector in winter may be due simply to the variation of ion production with solar zenith angle. A third type of mid-latitude trough, also called the "high-latitude trough", may be related to the troughs sometimes seen in conjunction with rapid sub-auroral ion drifts. The ion composition in these latter troughs requires elucidation. An increased O § loss rate, caused by heating of the ions by rapid ion-neutral drift, does not seem to be a convincing candidate to explain these troughs (although clear evidence for ion heating in the presence ofrapid ion drifts is now available). It has been shown that the rapid sub-auroral ion drifts may be understood in terms of ionosphere-magnetosphere plasma coupling during substorms (SOuTItWDOD and WOLF, 1978 ; HAREL et at., 1981). Speculation has been made (SMIDDY et at., 1977 ; RIcll et at., 1980) that the same coupling process may have effects that lead to depletion of the F-layer. Acknowledgements--We are grateful to J. R. DUDENEY,A. S. RODGER and J. J. SOJKA for helpful correspondence and discussion and we thank R. J. Foo~'rr for useful comments on an earlier version of the paper. The willing efforts of JILL BRADLEY and LINDA WILKINSON are gratel'ull~ acknowledged. The work was supported in part by SERC Grant SGD/04367.

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