F-region storms and thermospheric circulation

JoumaZ of Atmospheric

Physics, Vol.37,pp.1055to 1064. PergamonPress, 1975. Printed in NorthernIreland

and Termstrial

F-region storms and thermospheric circulation H. RISHBETH S.R.C. Appleton

Laboratory,

Ditton Park, Slough, SL3 9JX, U.K.

Abstract-This paper puts together various current ideas on the effects of magnetic storms on the ionospheric F-layer, principally at midlatitudes. A major factor is a large-scale circulation in the thermosphere, with equatorward winds driven by the heating resulting from joule dissipation and particle precipitation, different from the quiet-day pattern associated with the diurnal bulge. At midlatitudes this circulation produces upward drift in the F-region which tends to increase NmF2; but it also transports, from lower heights and higher latitudes, air with an enhanced molecular gas concentration which tends to decrease iVmF2. Recovery from the midlatitude storm proceeds as molecular diffusion restores the gas composition to normal. Though in principle the circulation can cause both ‘positive’ and ‘negative’ PO-layer effects at midlatitudes, in practice other factors such as electromagnetic drift and ionosphere-magnetosphere plasma flux have important roles, and special considerations apply at low latitudes. Recent satellite experiments provide an excellent opportunity for investigating the storm circulation theory. 1. INTRODUCTION Over

the years

geomagnetic spheric This

the scientific

storms

problems

posed

and the accompanying

phenomena

applies

These

have

proved

particularly

very

to the

by

iono-

difficult.

F-region

are not

the ionosphere.

changed and the underlying

between

the

a brief

INGRAM,

1935), changes

(SEATON, and and

1956),

electromagnetic

SATTO, 1959) KING,

and

1967).

responsible

of chemical

None

each has a part to play. a

coherent

picture

emerging. but

a good

important

F-region

to outline

air motion

mainly

by (1962)

term

for

the

this

circulation’.

the main

is the

large-scale

in the

thermosphere,

auroral-zone

first

drew

whole Five

main

in turn in Section

attention.

A

aspects

2(a)-(e),

Energy

There

elements

geomagnetic

A very

to

various storms.

driven

mainly

by electric

origin

(COLE, heat

mainly

in the E-region.

‘storm

1962).

source,

(ii) Precipitation aurora1 being

zone

probably

sphere

(b) The resulting

lower latitudes.

; (c) The large-scale

heating

of the thermo-

sphere

(iii) equatorward

winds

that

are

set up;

5 6

Some

particles

of

more

the

the

high

during

they comprise: currents,

fields of magnetospheric is thought

its

energy

energetic (HARTZ of

input

to be the is deposited

particles,

and

significant

‘drizzle’

particle

entering

to

enhanced

BRICE,

to

harder

at higher

the

the 1967)

thermo-

particles

at

latitudes

of

via the magnetospheric

cusp or

cleft.

(d) Accompanying

changes

of

chemical

(iv)

com-

position; (e) A

than

inputs are

This and

‘splash’

(a) The input of energy from the magnetosphere; uneven

Section

and Section

rrtarms

Briefly

major

are as follows:

to the storm

by aurora1 electrojet

which

with

F-region

4 discusses

not.

phenomena

that

driven

of it, dealt

are

energy

ionosphere,

(i) Joule heating

of

convenient

is the

that

input during

are

latitude

pattern

heating

phenomenon

2(a)

support.

way.

3), Section

9. THE STORM CIRCULATION

is now

experimental

magnetosphere.

is a brief conclusion.

though

by experiment,

even in a qualitative

neutral COLE

of

electro-

of the salient

attributable

others

deals with some further

can be solely

storms

seem

and

out over the last few years,

deal

element

(KOHL

F-region

enhanced

and

resume (Section

that

circulation

But thanks to numerous

of

seems timely

of this picture,

winds

by

ionosphere

phenomena

phenomena

(MAEDA

phenomena,

It is not yet fully proved

has

It

carried

air

of these

for all the storm

pieces of work

drifts

neutral

storm

composition

influence

drifts and changes in the flow of plasma

Following

and

(APPLETON

that

partioularly

affected,

such

increases

factors

storms the magnetosphere

is

many possible causes of which have been proposed,

as temperature

only

is structurally magnetic

effects,

the During

At sub-auroral

precipitation possible

thermosphere.

return

flow

at the

base

of the

inner

latitudes,

thermal

magnetosphere.

input excites 1066

or

midlatitude

low energy particle

conduction

If sufficiently red arcs.

from

the

intense,

this

1066

H. RISEBETH

The midlatitude red arcs are believed to be an energy sink for the DR ring current (COLE, 1966), in which case (iv) differs in its time history from (i) and (ii). It seems probable that (i) is the most intense auroral-zone source, rather than (ii), and it is worth noting that the electric fields that drive the electrojets also have dynamical effects (Section 2(c)). 2 (b) Increasea of thermoepheric temperature The energy input causes heating in the auroral zone which is quickly spread to lower latitudes by gravity waves and winds. The resulting thermal expansion of the air causes the density increases detected by satellite drag (JACCHIA, 1959; JACCHU et al., 1967). There is still some difficulty in interpreting the drag observations in terms of a progressive spreading, since the time-lag between maximum IQ and maximum density appears to be latitude-independent, being about 6 hr (ROEMER, 1971). Satellite-borne interferometers, observing the spectrum of the 630 nm dayglow, did however detect auroral-zone temperature bulges which became intensified and then spread to low latitudes during a storm (BLAMONT and LUTON, 1972). 2(c) Winds of the storm circulation Given the existence of an energy input at aurora1 latitudes, significant under quiet conditions and exceeding the local solar XUV input during storms, the idea of the storm circulation arose quite naturally. ROBLE and DICKINSON (1970) studied theoretically, in some detail, the convection pattern set up by the heat source producing a red arc, (iv) of Section 2(a). The storm circulation was discussed more generally by DUNCAN (1969), VOLLAND and MAYR (1971), M~vn and VOLLAND (1972), OBAYASHI and MATWRA (1972) and HAYS et al. (1973). See Fig. 1. On being heated, the air expands upwards, creating horizontal pressure differences that cause air to flow outwards and away from the heated region. These motions effectively remove heat from the high-latitude sources and distribute it elsewhere. In its upward expansion, the air undergoes some adiabatic cooling but, aocording to the calculations of ROBLE and DICKINSON (1970), the air is nevertheless hotter than normal as it flows equatorwards. The strong winds produce some heating, because of ion-neutral collisions (STWBE and CHIWDRA, 1971). In the descending part of the circulation, at middle

and low latitudes, the air undergoes adiabatic compression which heats it further. There exists good observational evidence for the strong equatorward winds at midlatitudes during a storm. SMITH (1968) and STOFFRE~EN (1972) observed such winds (of over 200 m s-1) by the vapour-trail technique, while ARMSTRONG (1969) and HAYS and ROBLE (1971) utilized the doppler shift of the 630 nm nightglow: Hays and Roble reported wind speeds of around 300 m s-1 during a red arc occurrence and also during an aurora1 substorm. Not all observations of equatorward winds were made during storms, but then the thermospheric wind models predict equatorward winds at night even at quiet times. The wind pattern must in any practical case be far more complicated than the simple models so far devised. Both the heat sources and the atmospheric structure have complex spatial and temporal variations with much localized detail. The winds and pressure gradients must constantly be interacting and adjusting to each other, as the flow of air must be spatially continuous. Since the thermospheric wind speeds are largely controlled by the F-region ion-density, the pressure gradients quickly become matched to localized ionospheric features, in particular the ‘troughs’ (DICKINSON et al., 1971). COLE (1971) and REES (1971) pointed out that the aurora1 electric fields also cause air motion, via the ion-drag interaction. This will modify the winds produced by the heating, though the results of ROTHWELL et al. (1974) suggest that the ion-drag contribution is a minor one. 2(d) Chemical change3 in the neutral air In the thermosphere generally the molecular/ atomic ratio of the neutral gas decreases upwards. Hence the upflow of air in high latitudes lifts into the F-region air with a greater molecular/atomic ratio than is normally found there. This ‘molecule enriched air’ is transported to lower latitudes within a few hours by the equatorwards winds. The arrival of this air in midlatitudes strongly influences the ionosphere, as discussed in Section 4(a). The detection of such changes of composition by the neutral mass-spectrometer aboard OGO-6 (TAEUSCHet al., 1971) provided important evidence for the storm circulation theory. The main effect observed is a considerable enhancement of N, concentration in middle and high latitudes, at heights of 500 km or so. The 0 concentration is less changed, so there is an increase in the N,/O ratio. MAPR and VOLLAND (1972) found the data

F-region storms and therxnosphericcirculation to be more consistent with an auroral-zone heat source at around 160 km height than one at around 100 km. This favours source (i) of Section 2(a) (joule heating) rather than source (ii) (particles). The storm also influences the helium distribution in the thermosphere. As a side-effect, the changes of composition at midlatitudes may cause further temperature increases beyond those directly produced by the auroral-zone heating. This is because of the large part played by atomic oxygen in cooling the thermosphere : with a reduced fraction of 0, a given solar XW inputassumed unchanged during the storm-gives a higher thermospheric temperature (CHANDRA and HERMAN,1969). An alternative way by which changes of composition might occur during a storm was proposed by KING (1966). Waves travelling from the aurora1 zone are presumed to create turbulence and mixing in the midlatitude thermosphere, thereby raising the molecular/atomic ratio. Such mixing would probably be strongest at heights where the fractional pressure oscillation in the wave becomes large (Ap/w 1) so that nonlinear effects set in and the wave ‘breaks’. Unlike the circulation, the wave process doe8 not involve the transport of air from the aurora1 zone, only the transmission of waves. On the whole the circulation seems to provide the more effective way of changing composition. 2(e) The downward and return parts of the circulation

Figure 1 show8 a descending motion of air at middle and low latitudes, and a return poleward flow at the base of the thermosphere, but these features are far from establiehed. The equatorward transport of ‘molecule-enriched air’ must certainly perturb the vertical distribution of 0 and N, at midlatitudes, and normal conditions must be regained by vertical diffusion. The diffusion probably has a time constant of l-2 days, a typical time scale for storm recovery. Detailed diffusion calculation8 would be very difficult for any actual case, because of the large amount of data on temperature and particle distributions needed to set the boundary conditions. A8 already mentioned, the downward motion play8 a part in the thermodynamic8 of the storm circulation. Moreover, OBAYA~HI and M~Yurrn~ (1972) pointed out that the downward transport of atomic oxygen would increase the rate of recombination of atoms to molecules, and linked this to the

1057

ZONE

NEUTRAL-AIR

WIND -

WIND-PRODUCED

.

ION DRIFT

e3

Fig. 1. Idealized sketch of the storm oirculation in the thermosphere. There is experimental evidence for the strong equatomard winds at midlatitudes but the descending motion8 and the return flow (envisaged to be at the base of the thermosphere) are SpWLdatiVe, as are winds poleward of the aurora1zone. (RISHSETH, 1974) enhancement of OH emission following magnetia 8tOMXlS(K.H.ASSoVsEY,1968). The return flow was placed at about the 90-100 km level by Obayashi and Matuura. It must be present if the circulation i8 in a steady state, but since a storm is a transient phenomenon, any temporary accumulation of air in low latitude8 could subsequently be dispersed by an adjustment of the daily ebb and flow of thermospheric air. If real, the return flow (being located Borne scale height8 below the mean level of the outward flow) would have a net speed of only about 1 m s-l, i.e. 1 per cent of that of the outward flow, and would thus be difilcult to detect.

8. A BESUlUE OF F-RF&fOI’JSTORM EFFEOTS The P-region displays much day-to-day variability, SO a ‘storm’ is not always easy to define. Marked perturbation8 sometimes occur even in magnetically quiet time8 (e.g. SMITH et al., 1968). However, some well-marked P-region phenomena do Usually accompany severe magnetic 8toim8: for recent reviews 8ee RAJAHAH et al. (1971), SOMAYAJULU (1971) and KANE (1973). The present article purposely glO88eSover the complexities of P-region storm8 but a few outstanding facts are 8Ulllnlarized here.

1058

H. RISHBETH

Following a marked

the start

increase

and total the

columnar

‘positive

there Then

day,

typically

at midlatitudes is

the

storm may

obvious

effect.

and

‘negative

‘positive’

throughout,

in low latitudes.

illustrates mena.

the

salient

However,

statistical stormtime

midlatitude vary

in terms

1959).

a

and

FZ-layer

storm

Though

the

to geomagnetic

individual On or

changeover

field

time.

days,

greater

the

well-known

than

of

on

quiet

of hmF2

in a storm.

As

of

regards

substantially

may

temper-

to increase

with the neutral

electron

increase

changed

night,

tends

The

at midlatitudes

20 km

At

PP-layer

Ti

in concert

T,,.

Fe-peak

than

may occur, particularly

the ion temperature

temperature

the

days.

gas

temperature

at night

but is not

by day (Section

4. MECHAEISMS OF F-REQIOE

Dst

obtained and storms

very useful, the

M

Several

authors have attributed

F-region mitted

storm from

1967, 1971;

effects

higher

5(b)).

1971).

The

induced

F-region

DUNCAN

(1972)

oval

effects

from

gave

of

Detailed

assuming support

it is also necessary

to

(c)).

the

of

aurora1

circulation

of the circulation

to consider

be due to electric

4(b),

with

storms is discussed in Section 4(a),

to and from the overlying I2

studies

propagation

In what follows, the application but

MATVURA

1974).

idea to F-region might

discussed

and

a source on the dayside

general

(DAVIES,

1966,

compositional were

OBAYASHI

model

trans-

KING,

JONES and RISHBETH,

and JONES (1973).

disturbances

and waves (e.g.

by a circulation

(1969),

theoretical

theory

1970;

STORMS the midlatitude

to winds latitudes

EVANS,

changes

a

00.

local

studies

no more

during a storm, largely

by

I2

the

which

on a

Stonfwd l37ON, I22’W)

00

to

variations,

Detailed

height

increases

greater

facts

characteristics

is at midlatitudes

so

some

In particular,

storms are valuable.

storm

(hmF2)

may conveniently

stations

and

universal

early

pheno-

phase

obscure

phase have various

and

atures,

storm

can

are linked

there

which

and

positive

effect

example

variations,

data from many

(e.g. MATSUSHITA,

begins,

may

time

sets in and

widely,

of the

and DS local-time

by averaging

nr

of

follows.

and the same is true

basis the observations

be summarized

decreases often

phase’ is common

particular

storms

negative

the

generally

a

the

phase,

at midlatitudes

phase’,

being

2 shows

this is

important

fairly

Figure

nT: early

and at equinox,

and

But in winter

be no

be of key importance.

the storm

This ‘negative in summer

most

treatment

which

sharp

of NmF2

decrease

statistical

NmF2

recovery

12-24 hr after

lasts for 1-3 days.

is often

this

occur

a sharp

there density

content

During

however

by

marked

electron

phase’.

may

NrnF2;

of a storm,

of peak electron

Section

fields

what

and

magnetosphere

4(d)

deals

effects

plasma

with

flow

(Sections

low-latitude

storm phenomena. 4(a)

Positive

and negative effects attributable

to the

sto9-m circulation The storm ciple

25% May

26th Haj

27th May I%7

Fig. 2. Total electron content measured with the aid of a geostationary satellite during the storm of 25-27 May 1967 at Stanford and Auckland, showing a typical initial phase (positive) and main phase (negative in summer, positive in winter). The broken lines show monthly average values of total content. The magnetic disturbance parameter a, peaks early in the storm. (After K. L. Jones)

for

circulation

theory

major

midlatitude

two

the initial

increase

depression

below

be produced U blowing component field lines, height

and

normal

equatorward

values.

The former wind.

in the magnetic

U sin I cos I.

downward

diffusion

so that

the Fe-peak

hmF2

is increased.

may be said to ‘drive

effects:

and its subsequent

dip angle I, produces

of drift,

in prin-

storm

by the equatorward

at a place with the normal

of NmF2

accounts

can

A wind meridian,

an upward

This drift retards of plasma

along

forms at a greater

the F2-layer

(Thus

the wind

upwards’,

but

P-regionstormsand thermospheric circulation the downward plasma diffusion velocity actually exceeds the wind-produced velocity, except possibly under transient conditions (RISHBETH,1970)). This mechanism cause8 large increases only in the daytime when photoionization is taking place. JONES (1971) studied a particular type of positive storm event, which occurs only if the onset of the storm’s main phase (defined as the time when the magnetic &t(H) goes negative) takes place between about 08 and 12 LT. Equatorward winds due to the auroral-zone heating are the probable cause of these events (JONESand RISHBETH, 1971). Moreover, the shape of the Fl-layer cusp or ionograms obtained during positive storms is consistent with this interpretation (KING, 1971). However, some types of positive storm event require a different explanation (Sections 4(b, c)). In due course the equatorward winds introduce air with increased molecular/atomic ratio into the midlatitude F-region. As described by SEATON (1956) and RISHBETH (1962), there results an increase of Fe-layer loss coefficient, some reduction of the photoionization rate, and consequent reduction of NmF2. On this picture the onset of the negative phase depends on the transit time of the ‘molecule enriched air’, and how far the negative storm effect penetrates to low latitudes depends on the strength of the storm circulation. Again PI-layer ionograms for negative storms have been studied in detail by KING (1971), who finds the composition change theory to fit the observations better than other possible explanations. However, it should be noted that the aspect of the Fl-layer on ionograms depends very much on the ratio NmF2INmF1, and it is not obvious that ionograms give reliable information on the chemical processes when the FB-layer is very distorted, and certainly not in the extreme storm-produced ‘B-condition’ when NmF2 < NmFl. The temperature changes may produce some further distortions of the Fl and FZ-layer, but they are probably less striking than those due to composition changes (Section 5(b)). As pointed out by DUNCAN (1969), the composition changes due to the storm circulation are superimposedon the ones that cause the quiet-day seasonal anomaly in the F-region. It is not yet clear why, in these terms, there is a prevalence of positive storm effects in the winter hemisphere. Perhaps it is only some strong storms, when negative effects do occur even in the winter hemisphere, that the molecular gas lifted up in aurora1 latitudes does spread to midlatitudes.

1059

More complete observations of composition are required to settle this point. 4 (b) Electric field effects durhg

storms

Strong electric fields and currents certainly exist in the high latitude ionosphereduring storms so they must naturally be consideredas a possible cause of ionospheric effects elsewhere. But it is notoriously difficult to determine from magnetograms how much of a perturbation is due to ionospheric currents and how much to currents in the magnetosphere or solid earth. Even if storms do produce electric fields at midlatitudes, their FZ-layer effects are limited because (to recall some well-known considerations): (1) The drift velocity E x BIBS, due to an electric field E acting normal to the geomagnetic induction B, is practically nondivergent; (2) The drift velocity is horizontal if E is meridional, and (becauseB is quite steeply inclined even at midlatitudes) is mainly horizontal if E is zonal; horizontal drifts are rather ineffective in changing the electron density. (3) Ion-drag tends to nullify the vertical component of drifts whose time scale exceeds the ‘ion-drag time constant’ 7 (the reciprocal of the neutral-ion collisionfrequency). By day 7 < 1 h and electric fields have only transient effects, but at night 7 > 1 h and appreciableeffects may occur, such as those due to magnetic bays (ROOSTER,1969). TANAKA and HIRAO (1973) have discussed the time constants associated with electric field effects. These are two striking storm phenomena at mid-latitudes that seem to be due to electric fields, and in which the limitations mentioned above are overcome in various ways. The first of these, identi6ed by THOMAS(1968), occurs in the first few hours of a storm. Marked decreases of NmF2 sometimes occur almost simultaneously at widely separated stations, and therefore cannot be due to any wave or wind propagated from high latitudes (Fig. 3). Electric fields seem the obvious cause of these disturbances: a field induced by the changing external ring currentis the most obvious possibility, but Thomas found it inadequate by an order of magnitude. The examples given by Thomas occurred when there were marked peaks in the magnetic AE index, indicating that substorms were in progress and strong electric fields presumably existed. Just how these fields produce similar effects at widely separated stations deserves study. From the

H. RISEBETH

1060 f FORT 800

-

600

;

MONMOUTH

1 GRAND ;

BAHAMA

I

I

I

6

9

12

!

15

1973). These phenomena are prevalent in North America, and Evans shows that the accompanying FZ-layer drift velocities are different from those of the wind-produced increases described in Section 4(a). TANAIU and H~AO (1973) attribute these evening increases to eastward electric fields. But a different mechanism involving westward electricfields, describedbelow in Section 4(c), may cause positive storm effects at later hours, around midnight and in the early morning (PARK, 1970).

.800

4(c) Flour of plasma between the ionosphere and mugnetosphe~e A typical geomagnetic flux tube, with its base in the midlatitude ionosphere, contains as much magnetospheric plasma (say above 600 km) as FL’-layerplasma. Thus interchangeof plasma with the magnetosphere can be important to the F&layer (PIDDINGTON, 1964). It is established HUANCAYO that magnetospheric flux tubes are gradually 800 II 13 IO filled with ionosphericplasma during quiet periods 15 and rapidly emptied during storms (PARE, 1970). 600 09 The emptying might result from a downward flux of plasma into the ionosphere (PARK, 1971) 400 or a strengthening of the magnetospherioconvection, which peels off part of the corotating plasma& : 200 sphere and presumably disperses the plasma to the outer magnetosphere (CHAPPELL et al., 1970). I I I I I 0 4 6 If the downward flux is great enough it could 9 12 15 FREOUENCY (MHz) produce some nighttime positive storm effects Fig. 3. Ionograms at three stetioua neer the 76’W mentioned in Section 4(b). The mechanism is meridian-Fort Moumouth, 40”N; Grand Bahmma, quite complicated: it depends on a westward 27’N; Huancayo, 12”S-duriug the storm of 4 Sepelectric field which compresses the plasma by tember 1967. A strong deoreese of foP2 occurred at moving it rapidly equatorward into a region of dl three stations around 10-11 LT. The lifting of the leyer, evidentin the virtualheights,was confirmed stronger magnetic induction B, at the same time by the computedreal heightshmF2. (THOXAS,1968). producing a downward motion. But the same downward drift, given by (---E/B) cos1, acts also at the FZ-peak, where it tends to reduce accompanying increases of hmF.2, it seems that the layer has been lifted, and the ionograms NmF2. Ion-drag and the 0+-H+ diffusive indicate a considerable distortion. These aspects barrier complicate the situation further. It seems are not easily explained in terms of electrio probable that under daytime conditions NmF2 fields, because of the limitations mentioned above, will probably be decreasedby the downward drift, and detailed studies of the phenomenon would be but at night when the layer is high this mechanism useful. Thomas observed that the decreases are might well cause an increaseof NmF2. As the magnetosphereempties, the plasmapause followed by a quick recovery if they occur by day but not when they occur at night : this suggests and trough move equatorwardsand this can cause that the recovery could simply be due to solar marked dscreuaeeof FZ-layer electron density, photoionization. especially at night (TAYLOR, 1973; MENDILLO The other phenomenon, also tending to occur et al., 1974). This may bring about the onset of early in the storm, comprises large evening in- the negative storm effect, as in the case of the creases of total content and NmF.2, accompanied June 1965 storm studied by OBAYSHI (1972). by increases of the total magnetio field observed Subsequently, the negative phase is continued at the ground (PAPAG~S et al., 1971; EVANS, through the action of the composition changes

F-region storms and thermosphericcirculation With all these conflicting described earlier. mechanisms, F-region storms are complex affairs. Even so, some observations can be hard to interpret, such as the Arecibo data discussed by NELSON and COGGER(1971). 4(d) Storm mechanisms in the equatorial F-layer During storms the latitudinal trough in NmF2the Appleton anomaly-is poorly developed or completely absent, and NmF2 and the total electron content are often greatest at the magnetic equator (KING et al., 1967; RUSH et al., 1969; RASTOUI et al., 1972). On occasions, however, an enhancement of the anomaly has been found (RAGHAVARAO and SIVARAMEN, 1975). The diurnal variation of NmF2 at equatorial latitudes is also unusual, with a prevalence of positive storm effects (MARTYN, 1963; MATSUSHITA, 1959), and storm effects generally are often not symmetrical about

the

magnetic

equator

(RAJARAM et al.,

1971). At least four processes are likely to make major contributions to equatorial storm effects. First, the winds of the storm circulation, though not necessarily symmetrical with respect to magnetic latitude, must tend to converge on the magnetic equator. Hence the field-aligned (U cos I) drifts that they produce oppose the poleward diffusion of plasma along field lines. Thus the formation of the crests of the equatorial anomaly is hindered (BURGE et al., 1973). In fact, calculations by RUSH (1972) show that appropriately-phased winds could have such an effect; they increase the mean lifetime of the plasma by keeping it at a height where the loss coefficient /I is small, and retarding its diffusion to lower heights where fi is large. As a consequence, there must be a considerable increase in the total amount of plasma in the equatorial F24ayer, which should show up in measurements of total electron content nT. Second, the convergence of the winds produces some plasma compression (which must operate at midlatitudes too). If within a north-south distance (2) of 6000 km, say, the equatorward wind speed U decreases from 300 m s-l to zero, the compression is N-’

SN/Gt N -SlJ/Sx = 4. 1O-4 s-l.

This is a significant fraction of ,3 at the Fe-peak ( 10m4- 10-s s-l, depending on circumstances), so there will be some effect on N, though not on "T. 14

1061

Third, if the storm circulation is strong enough to bring air with increased molecular/atomic ratio into equatorial latitudes, this will tend to cause negative storm effects just as it does elsewhere. Fourth, electromagnetic drifts are important at low latitudes and if they are altered during storms, the development of the equatorial trough will be affected. Abnormal vertical drifts, which must be due to electric fields, have been measured at Jicamarca during storms, and drift data for a great storm in March 1970 were used in a successful synthesis of the observed N(h, t) distributions (WOODet al., 1972). This work confirms the importance of electric fields during storms, though it did not consider the latitude variations of N, which require further study. 5. FURTHER COESIDERATIONSAND PROBLEMS 6(a) The height of the Fe-layer The height of the F2-peak should be increased during storms because of (i) thermal expansion of the heated atmosphere as a whole; (ii) the upward drift, due to equatorwards winds, that cause positive phase increases of NmF2; (iii) the increase in loss coefficient /I, since this raises the height at which there is a balance between the effects of loss and plasma diffusion. By day the real increases in hmF2 during storms seem to be rather small, probably no more than about 20 km (as distinct from the large and totally misleading increases of virtual height). Hence there is a question as to whether the behaviour of hmF2 is consistent with these mechanisms. 5(b) The electron temperature T, Much information exists about charged particle temperatures during storms; a summary is given by SOMAYAJ~~XJ (1971). Many satellite data show an inverse relationship between T, and the electron and ion density N, probably because the heat loss of the electrons strongly depends on N. Thus when N is small, as at night, T, tends to be increased particularly where there is heating by particle precipitation or thermal conduction from the magnetosphere, e.g. in the South Atlantic magnetic anomaly. As regards ionospheric effects, the direct effect of increased T, or Ti on the N(h) profile is probably small, apart from thermal expansion. However, the rate of the interchange reaction between O+ and N,, on which /?depends, is believed to be a rapidly-increasing function of T,, the vibrational temperature of N, (THOMAS and NORTON, 1966),

1062

H. RI~EBETH

If, as is possible, TV is closely controlled by T,, there might be a cumulative feedback between increases of T, 8nd decreases of N, or vice verse. However, both EVANS (1970) and Kwa (1971), from ionospherio evidence, conclude that /I does not strongly depend on T,, so the increase of T, may be a consequence of the decreased electron density rather than a cause. Nevertheless, the interesting possibility remains that vibrationallyexcited N, could be produced during storms, e.g. in midlatitude red arcs, and diffuse to considerable distances from its source (WALKER et al., 1969); this could cause increases of @ which would however not be closely linked to local values of T#. 6(c) The top&de Numerous studies have been made of storm effects in the topside ionosphere (references given by RAJARAM et al., 1971, SOMAYAJULU,1971). TO 8 considerable extent the topside changes seem to reflect the changes in the underlying F2Jayer, e.g. increased temperatures produce increased plasma scale heights, and diffusion causes topside electron densities to respond to changes in NmF2. Are there serious discrepancies between topside and F2-18yer behaviour and, if so, whst c8n be learnt from them, e.g. about magnetosphereionosphere coupling? The absence of such discrepancies does not necessarily mean coupling is unimportant, since at great heights only 8 small departure of the N(h) distribution from its diffusive equilibrium form is needed to produce 8 large diffusive flux. B(d) Storm centresand magic houra There is much evidence that individu81 storms are most intense in localized regions, or ‘storm oentres’, 8nd that certain storm phenomene preferentially begin at particular ‘magic hours’ of local time (APPLETON and PIUUOTT, 1952; PIUGOTT, private communication). Whereas the magnetic SC seems fairly unimportant 843regards ionospheric effects, various LT and UT effects in storm phenomen8-e.g. the changeover between positive and negative phases-have been discovered. Some of these phenomena, m8y have a simple physical explanation; e.g. upward drifts due to winds only produce 18rge electron density inomes during dayfime, and electromagnetic drifts are most effective at night when the electron density, and hence ion-drag, is smdl. Otherwise, patterns of particle precipitation may well account for some ‘magi0 hour’ phenomena.

The ‘storm centrea’ may just be areas in the vicinity of the most intensive energy input, or other 8re8s where the ionosphere is disturbed by particle precipit8tion. The details of course change enormously from one storm to rtnother. 5(e) The E-hyw Excluding sporadic-E (which presents problems of its own) the midlatitude E-18yer is not grossly affected by magnetic disturb8nces, though there 8re detectrtble perturbations (SATO, 1967; BROWN and Wm, 1967). The neutml and ion composition in the norm8l E-layer is overwhelmingly molecular, and in these circumstances the electron density is not sensitive to small changes of atmospheric composition, so the storm effects seem more likely to be due to eleotromagnetic drifts. 6. coIVJLu6Iol? It does seem very important to est8blish as much as possible about the storm circulation, both observ8tion8lly and by theoretical caloulation. Recent satellites, such 8s Atmosphere Explorer, offer good opportunities for detailed studies. In partioul8r no good expl8n8tion h8.s yet emerged for the prevalence of positive storms in winter midlatitudes. Though this prevalence might result from 8 combination of the quiet-time and storm oiroul8tions in the thermosphere, the detailed meoh8nism is f8r from clear, 18rgely bec8use the ohasttoteristics of the 8verage quiettime circulation have not been established. The circulation is known to change ne8r the equinoxes: can m8gnetio disturbance trigger the changes? Relationships between F-18yer storm phenomena, and irglow emissions from the F-region, only seem well est8blished for low latitudes (WEILL and CERISTOPEE-GLAUME, 1967) and of course in midlatitude red 8rca (see THO~S, 1971). More could be learned about the F-region from airglow observations, pasticul8rly about winds. Another question of interest is whether F-region and D-region storm effects are linked. In the midlatitude D-region, besides the ‘primary effect’ during the magnetic main phase, an ‘after-effect’ appe8rs at midlatitudes sever81 d8ys 8fter the storm (BE~SE and THOU, 1968). This seems much longer than the suggested time scale of the thermospheric storm circulation, but are the two effects releted? The polar-c8p F-region has not been dealt with in this paper, but needs to be considered: does the storm circulation extend poleward of the 8uror8l zone?

Ir-region storm8 and thermospherio oiroulation

To recapitulate:

the broad

picture of P-region

storms seems to be roughly

8s follows.

zone

heating)

‘storm

heating

(largely

ciroul8tion’

thermospherio

winds.

of the ‘positive Fe-layer,

alters

sets up

the

pattern

a of

storm effects’ in the midlatitude

though

electromrtgnetic

of pl8sm8 from

other

positive

effects. can give

drifts

the maguetosphere The

contmction

negative

storm

APPLETON E. V. and IN~Q%AM L. J. AP~IJZTON E. V. 8nd Praaorr W. R. ANNS!I’XONclE. B. BELROSN J. S. and THOU L. BLAMONT J. E. and LTJTONJ. M. BROWN cf. M. and WYNNN R. Buzom J. D., EOOLBZID., Kma J. W. snd Ri%r~s R. CEAKDRA S. and HNRNAN J. R. CEAPPELL C. R., HARRIS K. K. and SHaRP a. w. Cola: K. D. COLIOK. D. COLE K. D. DAI~CS K. DICKINSON R. E., ROBLE R. G. and RIDLEY E. C. DUNOAN R. A. EVANS J. V. EVANS J. V. BIARTZT. R. and BRIOE N. M. ~YS P. B. and ROBL~ R. cf. HAYS P. B., JONES R. A. and REES M. H. JACCHIA L. G. JACCHIA L. G., SLO~NY J. and VERNIANI F. JONES K. L. JONES K. L. JONES K. L. and RI~IIBETE H. K&NE R. P. Ku.10 U. A. M. KINO a. A. M. KINQ a. A. M. K.mo J. W., REED K. C., OLATUNJI E. 0. and LE@3 A. J. KOHL H. and K~NQ J. W. KFUsSOVSKY V. I. MAEDA K. and SATO T. MARYYN D. F. Marsu.9111r~8. MAYR H. G. and VOLLAND H. MENDILLO M., KIOBUOICAB J. A. end HAJNB-HOBSEIN~H H. NELSON G. J. end COGIOERL. L. OBAYA~EI T. OBAYASHI T. and MA~~I-RA N. PAPAQIANNIS M. D., MNNDILLO M. and KLOBUOEAR J. A. PARK c. a. PABK C. a. PIDD~QTON J. H.

particukly

8t night;

negative storm effects are probably

These winds produce some

iuflow

plasmap8use

joule

which

the PZ-layer,

Aurorsl

1063

and c8use

of the

effects in

sition changes brought 8tiom fields

A

low-latitude

about by the storm circul-

combination

probably

but the main due to compo-

of

c&use the

winds storm

and

electric

effects

iu the

FB-layer.

_4ckno&&eme&---This article has benefited from disoussions with numerous oolleagues, end is published by pe&ion of the Director of the Appleton LabOratOry.

Nature, Lend. 186,648.

1936 1962 1969 1968 1972 1967 1973

J. atmos. terr. Phy8. 9,236. Planet. Space Scii 17,967. J. atmos. tew. Phvs. 80.1397. J. geophy8. Rea. 71.3634. Pbnet. Space Sci. 15,1677. J. atmoa. tew. Phys. 35,617.

1969 1970

Planet. Space Sci. 17,841. J. geophya. Res. 75,384s.

1962 1966 1971 1974 1971

Nature, Lond. 194, 42, 76. J. geophys. Res. 70, 1689. Planet. space sci. 19,69. J. geophy8. Rea. 79,606. J. ahnos. Sci. 99,128O.

1969 1970 1973 1967 1971 1973 1969 1967 1971 1973 1971 1973 1966 1967 1971 1967

J. atmoe. tew. Phy~. Sl, 69. J. geophy8. Rea. 75,4803. J. atmos. WT. Phye. 86,693. Planet. Space S&16, 301. J. !380&~8.RM. 76.6316. Pb&Bt. space sci. 21,669. Nature, Lond. 188, 1662. J. geophye. Rea. 79,1423. J. atmo8. tew. Pb8.83,379. J. dmO8. WT. Phy8. 86, 1616. J. &no& tew. p&18. 88, 391. An& C&bvhu8. 99.26. J. atmoe. &w. Ph&. 28,967. J. atmos. tew. Phi8. 99;161. J. atmoe. tew. Phv8.39.1223. J. atmoe. tew. P&8. 29; 1366.

1967 1968 1969 1963 1969 1972 1974

J. atmoe. tew. Phye. 29, 1046. Ann& C%phye. 9&1063. Proc. IRE 47,232. Proc. R. See. A919,l. J. $?tW$y8. Rea. 84,306. PM. Space Sei. 90,379. Planet. Space Sci. 22, 223.

1971 1972 1972

Planet. Svace Sci. 19, 761. Planet. S&e Sci. 20; 611. Solar TeweattGZ Phvsics, Pt.IV,

1971

(Edited bv E. R. DYNB p. 199. D: Reidoi, ‘Dordrooht). Planet. Space Sci. 19, 603.

1970 1971 1964

J. geophhys. Res. 75,424X J. gqhy8. Rea. 76,466O. Planet. Space. Sci. le. 663.

1064 RAQRAVARAO R. and SIVARAMAN M. R. RAJARAM G., DAS A. C. and RASTO~I R. G. RA~TO~I R. G., CHANDRA H., SEARMA R. P. ctnd RAJARAM G. REES D. RISHBETH H. RISHBETH H. RISHBETH H. ROBLE R. G. and DICKINSON R. E. ROEMER M.

H. RISHBETH

1971 1972 1971 1962 1970 1974 1970 1971

RUSH C. M. RUSH C. M., RUSH S. V., LYONS L. R. and VENEATESWARAN S. V. ROOSTER R. ROTIIWELL P., MOUNTFORDR. and MARTELLI G. SATO T. SEATON M. J. SMITH D. H., NELSON G. and PYKE J. R. SMITH L. B. SOMAYAJULU Y. V. STOFFRE~ENW.

1972 1969

STUBBE P. and CIXANDRAS. TAEUSCH D. R., CARICINANG. R. and REBER C. A. TANAXA T. and HIRAO K. TAYLOR G. N. THOMAS L. THOMAS L.

1971 1971

THOMAS L. and NORTON R. B. VOLLAND H. and MAYR H. G. WALKER J. C. G., STOLARSKIR. S. end NAVY A. F. WEILL G. and CHRISTOPEE-GLAUMEJ. WOODMXN R. F., S!~ERLINQD. L. and HANSON W. B.

1969 1974 1957 1956 1968 1968 1971 1972

(Submitted to) J. atmm. terr. Phys. (in press). An& G%phya. 87,469. Id. J. Radio Space Res. 1, 119. J. Brittih Inlerplalaetary Sot. 24, 233. Planet. Space Sci. 9,149. J. atmoe.terr. Phy~. 82, 413. Radio Sci. 9.183. Planed Sp& Sci. 19, 1489. Space Rmearch XI, p. 965. Akadamie-Verlag, Berlin. J. atmoa. terr. Phys. 84, 1403. Radio Sci. 4, 829. J. atmoa. terr. Phys. al, 765. J. atmo8. terr. Ph8. 86, 1916. J. Gwmagn. Gwekct. 9, 57. J. atmoa. tew. Phy8. 8, 122. Nature, Lond. 219, 1144. J. geophys. Rw. 78,4959. J. Sci. Ind. Rea. 80, 394. Magnetaspbre-Ionosphere Interactions (Edited by K. FOLKESTAD). p. 83. Universitetsforlaget, Oslo. Planet. Space Sci. 19, 731. J. geophya. Res. 76, 8318.

1966 1971 1969

J. atmos. terr. Phys. 35, 1443. J. atmoa. terr. Phys. 35, 647. J. atmoa. terr. Phia. 80; 1623. The Radiatina Atmosvhere (Edited bv B. M. M~CORMA~~,p. 106. R&&l, DordGcht. J. geophye. Rea. 71, 227. J. gwphyxq. Rw. 76, 3764. Ann& Cfdophys. 25, 831.

1967 1972

An& C%ophy8. 28, 317. Radio Sci. 7, 739.

1973 1973 1968 1971