How the thermospheric circulation affects the ionospheric F2-layer

\ PERGAMON

Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

How the thermospheric circulation a}ects the ionospheric F1!layer H[ Rishbeth Department of Physics and Astronomy\ University of Southampton\ Southampton SO06 0BJ\ U[K[ Accepted 19 April 0887

Abstract After a historical introduction in Section 0\ the paper summarizes in Section 1 the physical principles that govern the behaviour of the ionospheric F1!layer[ Section 2 reviews the physics of thermospheric dynamics at F!layer heights\ and how the thermospheric winds a}ect the neutral chemical composition[ Section 3 discusses the seasonal\ annual and semiannual variations of the quiet F1 peak at midlatitudes\ while Section 4 deals with storm conditions[ The paper concludes by summing up the state of understanding of F1!layer variations and reviewing some important principles that apply to ionospheric studies generally[ Þ 0887 Elsevier Science Ltd[ All rights reserved[

0[ Introduction The _rst suggestion of the ionosphere appears to have been made by Gauss "0728# who said] {{It may indeed be doubted whether the seat of the proximate causes of the regular and irregular changes which are hourly taking place in this ðter! restrial magneticŁ force\ may not be regarded as external in reference to the Earth [ [ [ But the atmo! sphere is no conductor of such ðgalvanicŁ currents\ neither is vacant space[ But our ignorance gives us no right absolutely to deny the possibility of such currents^ we are forbidden to do so by the enig! matic phenomena of the Aurora Borealis\ in which there is every appearance that electricity in motion performs a principal part||[ In a remarkably forward looking paper\ Stewart "0772# speculated on causes of the daily geomagnetic variations\ and concluded the most likely cause to be electric currents generated by dynamo action in the upper atmosphere[ Considering that the electron was not discovered till 0786\ Stewart could have no knowledge of the physics of these currents\ and it seems reasonable to regard the physical

 Corresponding author[ Tel[] 9933 0 692 481962^ Fax] 9933 0 692 482809^ E!mail] hrÝphys[soton[ac[uk

theory of the ionosphere as starting with the commentary by Lodge "0891#] {{Re Mr Marconi|s Results in day and night Wire! less Telegraphy] The observed e}ect\ which if con! _rmed is very interesting\ seems to me to be due to the conductivity [ [ [ of air\ under the in~uence of ultra!violet solar radiation [ [ [ No doubt electrons must be given o} from matter [ [ [ in the solar beams^ and the presence of these will convert the atmosphere into a feeble conductor[|| The systematic study of the ionosphere really began with the measurement of the heights of the re~ecting layers by Breit and Tuve "0814# and Appleton and Bar! nett "0814#\ followed by the discovery of the existence of at least two separate ionized layers "Appleton\ 0816# "subsequently named the E and F layers# and the intro! duction of the term {ionosphere| by Watson!Watt "0818#[0

0 At this point\ it may be noted that the term {region| "D\ E\ F# denotes parts of the atmosphere\ the D:E boundary being at 89 km height and the E:F boundary at 049 km[ The term {layer| refers to ionization within a region\ e[g[ the F0 and F1 layers lie within the F region\ and E1 and Es layers\ as well as the normal E layer\ lie within the E region[ Although there are many occasions on which the terms {region| and {layer| are inter! changeable\ this is not always the case\ and the o.cial distinction is worth preserving[ The term {ionosphere| was proposed\ appar! ently independently\ by Watson!Watt and by Appleton in 0815[

S0253Ð5715:87:, ! see front matter Þ 0887 Published by Elsevier Science Ltd[ All rights reserved PII] S 0 2 5 3 Ð 5 7 1 5 " 8 7 # 9 9 9 5 1 Ð 4

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H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

The subject developed in three interrelated ways] the magneto!ionic theory of radio wave propagation\ the use of the ionosphere for communications\ and ionospheric physics[ Scientists in several Western countries con! tributed to all these aspects[ As recounted by Wilkes "0886#\ the basis of magneto!ionic theory was well estab! lished by Eccles\ Larmor and Lorentz at the time iono! spheric research really began in the middle twenties\ the theory being then applied to the ionosphere by physicists and mathematicians[ The theory of the formation of ion! izing layers in the atmosphere was developed in Germany by Lassen "0815#\ in America by Hulburt "0817#\ in Denmark by Pedersen "0818# and in Britain by Chapman "0820#\ who produced the de_nitive work on the subject[ It may seem unfair that Appleton and Chapman sub! sequently received more credit than the others mentioned above\ but there is a valid reason[ Only in their case did the early work lead to continuing research that spanned many decades[ Key landmarks in the establishment of ionospheric physics in particular\ and the wider _eld of solar!ter! restrial physics in general\ occurred in the early thirties] 0820 0820 0820 0820

Regular ionospheric sounding began at Slough Chapman|s theory of ionized layers Chapman|s theory of airglow Chapman!Ferraro] Solar particles and the Earth|s magnetic _eld*First idea of magnetosphere< 0821 Start of Kp geomagnetic index 0822 Swept!frequency ionograms at Slough[ After World War II ionospheric physics took new directions\ which combined theoretical atomic physics with the old radio tradition[ The new theories gave a basic understanding of important topics] the chemistry of the production and loss processes "Bates and Massey\ 0835\ 0836^ Nicolet\ 0838#\ electrical conductivity "Cow! ling\ 0834^ Maeda\ 0841^ Baker and Martyn\ 0841^ Chap! man\ 0845#\ electrodynamics and the E!layer {dynamo| and F!layer {motor| "Martyn\ 0842^ Maeda\ 0844#\ and the di}usion of ions and electrons through the neutral air "Ferraro\ 0834^ Yonezawa\ 0844\ 0845^ Martyn\ 0845#[ Major advances in knowledge came from the Inter! national Geophysical Year "IGY 0846Ð0847# and the fol! low!up research programmes that continued through the International Quiet Sun Year "IQSY 0853Ð0854#[ The salient features were] , Worldwide experiments , Rockets and satellites , Solar radiation measured , World Data Centres , Start of computer age , Incoherent scatter radar , Topside sounding , Laboratory aeronomy , The data explosion , Theoretical modelling

Through these advances the worldwide ionosphere was better explored\ and the structure and composition of the neutral air began to be known[ Better knowledge of the neutral atmosphere paved the way to better under! standing of the ionosphere\ and to the converse idea that the ionized plasma*the ions and electrons*can act as a {tracer| for the ambient neutral atmosphere\ in that ionospheric measurements can be used to derive infor! mation about the neutral air[ In this way ionospheric science became integrated into the larger science of aeron! omy and indeed the wider _eld of solar!terrestrial physics[ The negative side of this evolutionary process was that the {physics| and {communications| sides of ionospheric science tended to diverge[ By the seventies a new understanding of the ionosphere was well established "Rishbeth\ 0863#[ All the main iono! spheric layers are created by solar ionizing radiation and*particularly the F1!layer*are in~uenced by the global thermospheric circulation[ The driving forces of this circulation are _rst\ and most important\ heating due to solar photon radiation^ second\ the solar wind\ energy from which appears in the high latitude ionosphere in the form of electric _elds or energetic particles^ and third\ tides and waves transmitted upwards from the middle atmosphere[ As a succinct if oversimpli_ed description\ suggested by Giraud and Petit "0867#\ the ionosphere|s vertical structure depends on the solar spectrum\ its lati! tude structure on the geomagnetic _eld[

1[ General principles of F!layer physics 1[0[ Basic equations The behaviour of the ionospheric plasma and the neu! tral thermosphere is subject to the equation of state "per! fect gas law# and the general conservation equations for mass\ momentum and energy] Continuity equation] "Density change#  "Production# −"Loss#−"Transport# Equation of motion] "Acceleration#  "Force# −"Drag#−"Transport# Heat equation] "Temp change#  "Heating# −"Cooling#−"Conduction# In the F1!layer the continuity equation for the electron density "or electron concentration# N takes the well! known form

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

1N:1t  q−bN−div"NV#

0276

"0#

where q is the production rate\ b is the linear loss coe.cient\ and V is the plasma drift velocity[ More detailed discussion can be found in ionospheric textbooks "e[g[ Rishbeth and Garriott\ 0858^ Rees\ 0878^ Harg! reaves\ 0881#[ Kelley "0878# deals with the plasma physics that governs the small!scale structure of the ionosphere\ which is not described in this paper[

1[1[ The quasi!equilibrium F1 layer The F1 layer is usually in {quasi!equilibrium| in the sense that 1N:1t is smaller than other terms in the con! tinuity eqn "0#[ To recapitulate brie~y the rules that apply to the behaviour of the F1 peak] "a# By day\ at heights below and up to the F1 peak\ the production and loss terms are roughly in balance] q depends mainly on the atomic oxygen concentration ðOŁ\ b depends mainly on the molecular nitrogen con! centration ðN1Ł with some contribution from molec! ular oxygen ðO1Ł[ The steady!state electron density is given by N ½ q:b ½ IðOŁ:"k?ðO1Ł¦kýðN1Ł#

"1#

Fig[ 0[ The steady!state F1 layer[ Left] Idealized log N"h# pro_les of electron density versus height[ The height of the peak is determined by a balance between di}usion and loss\ eqn "2#[ In the presence of an upward vertical drift W\ the pro_le is displaced from the full curve to the dashed curve and the peak rises by Dh\ eqn "3#[ Right] Sketches showing how a horizontal wind U blowing equatorward "above# or poleward "below#\ produces a _eld!aligned ion drift V  U cos I\ the vertical component of which is given by W  V sin I  U cos I sin I[ The thin sloping lines show the direction of the geomagnetic _eld "dip angle I#[

W  U sin I cos I "see right!hand sketches in Fig[ 0#[ This upward drift raises the peak and increases the peak electron density NmF1\ which changes approxi! mately in accordance with the value of the ratio q:b at the displaced level of the peak[ Very roughly\ the change in peak height is given by

where k?\ ký are rate coe.cients and I is proportional to the ~ux of solar ionizing radiation\ which varies with the solar cycle[ Both q and b decrease with the upward decrease of gas concentration\ but the ratio q:b\ which depends on the atomic:molecular ratio of the neutral air\ increases upwards so N increases too[ "b# The upward increase of N stops because\ at great heights\ gravity controls the ion distribution[ The F1 peak lies at the height where the transport terms are comparable to the production and loss terms\ i[e[ where chemical control gives way to di}usive "gravi! tational# control[ If D is the coe.cient of di}usion of the ions through the neutral air "inversely pro! portional to the ion!neutral collision frequency# and H is the atmospheric scale height\ the peak lies at the height at which

Opposite e}ects are produced by a poleward wind[ "e# Vertical drift can also be produced by a zonal elec! trostatic _eld E[ The drift velocity is E×B:B1\ and its vertical component is upward for eastward E\ down! ward for westward E[ This {electromagnetic drift| is most e}ective at equatorial latitudes\ because at mid! latitudes the vertical drift speed is greatly reduced by the reaction of the neutral air\ the so!called {ion!drag e}ect| "Dougherty\ 0850#[ "f# Above the peak\ the plasma distribution is gravi! tationally controlled\ with N decreasing exponentially upwards with the plasma scale height[

b ¹ D:H1

The sketches of Fig[ 0 illustrate the N"h# distribution that conforms to rules "aÐf#[

"2#

"c# The height of the peak "known as hmF1# tends to lie at a _xed pressure!level in the atmosphere\ i[e[ a _xed value of the reduced height z de_ned in Section 1[2[ "Garriott and Rishbeth\ 0852#[ "d# The height of the peak can be shifted by a neutral air wind or an electric _eld[ A horizontal wind U blowing towards the magnetic equator drives the ions and electrons up geomagnetic _eld lines "dip angle I# at speed U cos I\ of which the vertical component is

Dh ½ WH:D ½ "H:D#U sin I cos I

"3#

1[2[ Real hei`ht and reduced "pressure!level# hei`ht The rates of ionospheric processes "production\ loss and di}usion# are largely controlled by the density of neutral constituents[ Their variation with height may conveniently be expressed in terms of {pressure!levels|\ a concept that is commonplace in meteorology[ The ver! tical distance in which the air pressure decreases by a factor of e is the atmospheric scale height

0277

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

H  kT:m`  RT:M`

"4#

where T is gas temperature\ ` the acceleration due to gravity\ m and M are the mean molar mass of the air expressed in kg and in atomic mass units\ and k and R are Boltzmann|s constant and the universal gas constant "so M:m  R:k#[ It is convenient to de_ne a dimen! sionless parameter\ {reduced height|\ which represents the number of scale heights above a selected base level h9[ {Reduced height| z is related to {real height| h "km# by the relations

wave activity is strong enough to cause nonlinear {break! ing| with consequent mixing[ An increase of M at the lower boundary leads in turn to an increase of the molar mass versus reduced height\ M"z#\ at all greater heights[ Shimazaki "0861# pointed out that such mixing is less e}ective than vertical mass motion in in~uencing ther! mospheric composition\ but this point is not further pur! sued here[

2[ Neutral!air winds and plasma drifts

h

g

z  "h−h9#:H  "dh:H#[

"5#

2[0[ Upper atmosphere winds

h9

Conversely\ the real height of a given pressure!level z is found by inverting eqn "5# to give h

g

h"z#  h9¦Hz  h9¦ Hdz[

"6#

h9

The linear relations in eqns "5# and "6# hold if H is independent of height\ but the integral forms "in which the integration runs from height h9 "z  9# up to the height in question# are required if H varies with height[ The base level h9 may be chosen in di}erent ways[ In Chapman theory "Chapman\ 0820# h9 is measured from the height of peak production for overhead sun\ but in computations that involve thermospheric structure it may be useful to take h9 at the mesopause\ i[e[ the base of the thermosphere[ 1[3[ Composition in a static thermosphere Well above the turbopause\ each major constituent is said to be in {di}usive equilibrium|\ meaning that it is distributed with its own scale height\ so that eqns "5# and "6# hold for each constituent separately[ Exceptions to this situation should be noted] _rst\ up to about 019 km the distributions of O and O1 are partly controlled by photochemistry\ and second\ departures from {di}usive equilibrium| occur if there are strong vertical motions "Section 2[3#\ as especially happens during magnetic storms[ If the composition\ which may be speci_ed either by the mean molar mass M or by the molecular:atomic concentration ratio\ remains _xed at the lower boundary h9\ then it remains _xed at any _xed pressure!level z even if the temperature changes\ i[e[ the relation M"z# is unchanged^ see Appendix[ But since a temperature change a}ects the real height of a given pressure!level above the lower boundary\ according to eqn "6#\ it chan! ges the composition at a _xed height h[ At the lower boundary h9\ the mean molar mass "or the molecular:atomic ratio# can be raised by increased turbulence[ This may happen\ for example\ if gravity

The thermosphere is a vast heat engine driven by energy from solar\ auroral and interplanetary sources\ with tidal and wave input from the underlying middle atmosphere[ The heat inputs from these sources produce horizontal gradients of temperature and pressure\ as shown schematically in Fig[ 1[ The pressure gradients drive horizontal winds "King and Kohl\ 0854^ Kohl and King\ 0856# with typical speeds of 49 m s−0 at F!layer heights[ These winds\ together with the associated vertical upcurrents and downcurrents\ form a global circulation that carries energy away from the heat sources and lib! erates it elsewhere[ Vertical cross!sections of this cir! culation are sketched in Fig[ 2"A\ B#\ for {quiet| and {storm| conditions[ The sketches represent averages over local time\ so they show {prevailing winds|[ A daytime {snapshot| of the circulation would look quite di}erent\ with the horizontal winds diverging from the vicinity of the Sun|s latitude\ and a nighttime {snapshot| would show the winds converging in the winter hemisphere[ The sket! ches C\ D are discussed later in Section 3[4[ The wind velocity U resulting from the horizontal pres! sure gradients depends on the Coriolis force due to the Earth|s rotation "angular velocity V#\ on the molecular viscosity of the air\ and on the {ion!drag| due to collisions between air molecules and the ions[ Ion!drag exists because the ions\ being constrained by the geomagnetic _eld\ cannot move freely with the wind[ Omitting the viscosity term\ the equation of motion for the horizontal wind is dU:dt  F−1V×U−KN"U−V#

"7#

where F is the driving force due to the horizontal gradient of air pressure p\ and is given by F  −"0:r#9horizp

"8#

where r is air density\ K is a collision parameter and KN is the neutral!ion collision frequency[ Molecular viscosity is important in the F region because it smooths out the vertical variation of wind velocity\ so that dU:dh : 9 at great heights[ Lower down in the thermosphere\ however\

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

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Fig[ 1[ Sketch map of quiet day wind patterns at F1!layer heights at northern summer "June#\ in local time and geographic latitude[ Heavy dashed curves represent the auroral ovals[ The dash!dot curve represents the terminator "sunriseÐsunset line#[ Thin dashed curves represent both isobars and isotherms[ Points D9 show the locations of maximum and minimum temperature[ Arrows represent approximate wind directions[

Fig[ 2[ Sketches of the prevailing "local!time!averaged# meridional circulation in the midday thermosphere at solstice\ with the Sun  displaced from the geographic equator Eq[ Dark squares AO represent the dayside auroral ovals[ Arrows represent upwelling and downwelling\ thin lines indicate the general direction of the meridional air ~ow[ The main feature is the summer!to!winter ~ow\ driven by solar heating and reinforced by the e}ect of the summer auroral oval[ Heating in the winter auroral oval drives a subsidiary circulation[ The dashed arrows showing winds blowing into the polar caps are speculative[ Upwelling "decreased O:N1 ratio# occurs in low latitudes and the summer hemisphere^ downwelling "increased O:N1 ratio# occurs at high midlatitudes where the two circulations meet[ Sketch "A# represents quiet conditions\ sketch "B# represents storm conditions with more active auroral ovals and stronger circulations[ Sketches "C\ D# show the quiet!day situation for two longitudes\ one near to the winter magnetic pole where the downwelling is at a moderate geographic latitude^ the other far from the winter magnetic pole "but containing the summer magnetic pole#\ where the winter downwelling takes place at a high geographic latitude remote from the Sun[

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H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

small!scale velocity gradients are not smoothed out by viscosity[ The wind direction depends on the ratio of Coriolis force to ion!drag[ This may be seen by considering special steady!state cases of eqn "7# with dU:dt  9[ If Coriolis force is dominant and ion!drag is small\ as in the lower ionosphere\ the wind blows at right angles to the pressure gradient[ Then at geographic latitude f U  F:"1V V sin f# "U_F#

"09#

This is the situation familiar in weather maps for the lower atmosphere\ with the {geostrophic| wind blowing along the isobars of constant pressure[ In accordance with Buy Ballot|s Law\ the wind blows clockwise around pressure {highs| "anticyclones# and anticlockwise around {lows| in the northern hemisphere\ and in the opposite sense in the southern hemisphere[ A very di}erent situ! ation exists in the daytime F layer\ where ion!drag is large and the wind is almost parallel to the pressure!gradient force\ so that U  F:"KN sin I# "U>F#[

"00#

In general\ both ion!drag and Coriolis force are sig! ni_cant and the wind is inclined to F at the angle c  arc tan "1V V sin f:KN#[ To illustrate this\ the sche! matic wind vectors shown in Fig[ 1 are nearly parallel to the gradients of temperature "and pressure# by day\ but are de~ected by Coriolis force at night[ As the direction of the driving force F changes continually with local time\ the wind direction changes too\ but always lags behind its steady!state direction because of inertia "Rishbeth\ 0861#[ The ion velocity V in eqn "7# is caused mainly by electrostatic _elds\ as in "e# of Section 1[1[ The drifting ions accelerate the air horizontally by ion!neutral colli! sions\ and if V is large\ the ion!drag term in eqn "7# acts as a driving force for neutral air winds[ This happens especially in the polar ionosphere\ where strong electric _elds originate from the magnetosphere "and ultimately from the solar wind#[ For di}erent reasons\ electric _elds also play an important role in the equatorial F!region^ see Kelley "0878#[ In polar latitudes\ the winds have a dominant day!to! night pattern\ driven partly by the large!scale mag! netospheric electric _eld "Maeda\ 0866# and partly by solar heating which produces the day!to!night pressure variations "Kohl and King\ 0856#[ As in the midlatitude wind system\ the direction of the cross!polar wind is determined by the ratio of Coriolis force to ion!drag[ Localized heating in the auroral oval produces local pres! sure gradients\ which drive smaller scale convective winds away from the auroral oval\ as indicated rather specu! latively in the sketches of Figure 2[ But\ since the sketches represent a local!time average\ they do not show the

cross!polar wind just mentioned\ because its 13!hour average is small or even zero[ As discussed by Kohl et al[ "0857# and others\ the wind system shown in Fig[ 1 a}ects the height and electron density of the midlatitude F1!layer\ in the manner described in "d# of Section 1[1 and shown in Fig[ 0[ The _eld!aligned drift V\ shown in the _gure\ varies with local time^ it is essentially downward by day when the wind is mostly equatorward\ and upward at night when the wind is mostly poleward[ Note that U in eqn "3# represents the wind component parallel to the magnetic meridian\ which is inclined to the geographic meridian at the declination angle D[ It follows that the phase of the local time vari! ation of the wind!induced drift depends on D[ As shown by Kohl et al[ "0858#\ this explains the {declination e}ect| in NmF1 found by Eyfrig "0852#[ 2[1[ Neutral air continuity The neutral air is subject to equations of continuity and energy\ as well as the equation of motion eqn "7#[ Production and loss processes are unimportant for the major constituents of the neutral air\ so the continuity equation for the concentration n reduces to 1n:1t  −div"nU#[

"01#

The pressure distribution and the wind velocity con! tinually adjust themselves to satisfy eqn "01#\ because any convergence of horizontal wind produces a {pile!up| or accumulation of air and the resulting increase of pressure modi_es the winds[ In the F!region\ this situation may be caused by a localized enhancement "or depletion# of electron density\ which increases "decreases# the ion!drag and hence slows "quickens# the wind\ as in the idealized case studied by Dickinson et al[ "0860#[ This rapid readjustment of pressure gradients and wind speed has some resemblance to the way in which the voltages across the components of an electric circuit depend on their impedances[ The {geostrophic| winds described by eqn "00# are nearly divergence!free\ and are ine}ective in removing the horizontal pressure di}erences that drive them[ This is the situation in the lower atmosphere\ where pressure {highs| and {lows| can persist for days[ If the winds were directed across the isobars instead of along them\ the {highs| and {lows| would disappear in a matter of minutes[ 2[2[ Vertical winds In the real thermosphere\ air motion is three!dimen! sional\ and any divergence "or convergence# of the hori! zontal winds is at least partly balanced by upward "or downward# winds\ so the magnitude of div "nU# is smaller than it would otherwise be[ Vertical winds are important for the energy balance\ because air is heavy and raising

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

it involves doing work against gravity\ while downward air motion releases energy[ To give a numerical illus! tration] to raise the whole neutral F!region atmosphere from 049 km upwards at the typical vertical wind speed of 0 m s−0 requires a power input of order 0 mW m−1\ which is a signi_cant fraction of the daytime solar input[ For further discussion\ see Smith "0887#[ The vertical wind velocity may be regarded as the sum of two components "Dickinson and Geisler\ 0857\ Rish! beth et al[\ 0858# Uz  WB¦WD[

"02#

The {barometric| component represents the rise and fall of constant pressure!levels\ due to thermal expansion or contraction[ It may be expressed as WB  "1h:1t#p[

"03#

Consider _rst a simple one!dimensional situation\ in which the expansion or contraction involves only vertical up or down motion\ with no horizontal ~ow[ Bearing in mind that the pressure at any point in the atmosphere is just the weight of the column of air above it\ the amount of air above a given pressure!level must be constant\ and the air does not move with respect to the pressure!levels "assuming that the vertical acceleration ð `#[ As men! tioned in Section 1[3 and shown in the Appendix\ such {barometric| motion does not change the chemical com! position at any given pressure!level[ The more realistic three!dimensional situation includes both vertical and horizontal winds[ Since the neutral air in the thermosphere is neither produced nor destroyed\ the upward motion must be accompanied by horizontal divergence of air at great heights\ and horizontal con! vergence of air at the bottom of the thermosphere[ To balance this divergence and convergence\ the air must move through the pressure!levels with a vertical velocity\ called the {divergence velocity| and given by WD  −"0:r`# "dp:dt#[

0280

This parameter may be useful in studying the relation between vertical motion and composition changes[ To summarize the general rules that relate ionospheric behaviour to vertical air motions\ bearing in mind that the electron density N depends on the ðO:N1Ł ratio] , Barometric motion "positive or negative WB# changes the ðO:N1Ł ratio and the mean molar mass at a _xed height\ but not at a _xed pressure!level[ , Upwelling "positive WD# decreases the ðO:N1Ł ratio and increases the mean molar mass\ so tends to decrease N^ , Downwelling "negative WD# increases the ðO:N1Ł ratio and decreases the mean molar mass\ so tends to increase N[ These e}ects are in addition to any changes of com! position at the lower boundary h9 due to increased tur! bulence or gravity wave activity "Section 1[3#[ The composition changes\ produced by vertical motions\ are propagated horizontally by the global wind system sketched in Figs 1 and 2A[ The global transport of air is determined by the {prevailing| "local!time averaged# wind\ which at midlatitudes is typically 29 m s−0 directed from summer to winter[ As 29 m s−0 corresponds to about 1499 km per day\ the prevailing wind carries air from summer to winter midlatitudes in a few days[ The prevailing zonal wind\ also of order 29 m s−0 at midla! titudes\ carries the air from west to east in the winter hemisphere\ east to west in the summer hemisphere\ by about 1499 km per day "which\ depending on latitude\ is roughly 29> of longitude or two hours of local time#[ Superimposed on the prevailing winds is the day!to!night oscillation\ with a typical meridional and zonal wind amplitude of 49 m s−0 which causes a daily excursion of 249 m s−0×"0 day:1p# ¼ 799 km[ Although not negli! gible\ this oscillation is smaller in scale than the summer! to!winter prevailing motion\ so its e}ect on the large! scale transport of air should be relatively minor[1

"04# 3[ Quiet!day variations of the midlatitude F1 peak

2[3[ Effect of vertical and horizontal winds on neutral air composition Unlike the {barometric| motion described above\ the {divergence| component does change the chemical com! position at a _xed pressure!level z[ Evaluation of its e}ect is complicated by the fact that the composition varies with height even in a static thermosphere\ because each major constituent has its own scale height[ To meet this di.culty\ Rishbeth et al[ "0876# de_ned a parameter P which is height!independent in a static atmosphere with two components\ atomic oxygen and molecular nitrogen] P  17 ln ðOŁ−05 ln ðN1Ł¦01 ln T

"05#

where the square brackets indicate gas concentrations[

3[0[ The observed {anomalies| Figure 3 shows four solar cycles of solar!terrestrial data[ The two upper panels show the international rela! tive sunspot number R "formerly called the Zurich sun! spot number#\ which dates back to 0638\ and the solar radio ~ux density at 09[6 cm wavelength\ which dates back to 0837[ The centre panel\ showing the intensity of the interplanetary magnetic _eld\ is not particularly

1 It is interesting that this daily excursion resembles the dis! tance scale of 499Ð0999 km over which F!layer behaviour is thought to be correlated[

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H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

Fig[ 3[ 34 years of monthly mean data "Willis et al[\ 0883#[ Top to bottom] Relative Zurich "or international# sunspot number RZ or RI^ solar decimetric ~ux density at 09[6 cm wavelength^ strength of the interplanetary magnetic _eld measured by the IMP!7 probe in the vicinity of the Earth^ noon ionospheric critical frequencies foF1\ foF0 and foE at Slough "41N# and Port Stanley "41S# "MHz#[ Courtesy] Rutherford Appleton Laboratory[

relevant to the present discussion[ The two lower panels show the monthly median noon critical frequencies foF1 recorded at two stations at equal and opposite latitudes\ Slough and Port Stanley[ It is immediately obvious that\ at both stations\ foF1 has a pronounced solar!cycle vari! ation^ that Slough foF1 has a dominant annual variation^ and that Port Stanley foF1 has a dominant semiannual variation[ More detailed inspection shows that greatest foF1 occurs in winter at Slough but at the equinoxes at Port Stanley^ and that at solar minimum there is a perceptible semiannual variation at Slough[2 These features\ which are observed to di}erent degrees at other places\ have come to be known as {F1 layer anomalies|[ The term {anomaly| originally meant any departure from {solar!controlled| behaviour\ in which the critical frequency foF1 "proportional to zNmF1# varies regularly with the solar zenith angle x\ as it does in the well!known Chapman layer[ The term is also applied

2 Historically\ this may have been the earliest anomaly to be reported[ The _rst complete year of Slough data to be analysed was in 0822Ð0823\ near solar minimum "Appleton and Naismith\ 0824#[ The seasonal anomaly only became prominent with the increase of solar activity in 0824Ð0825[

to the well!known {equatorial F1!layer anomaly| "not discussed in this paper#[ The midlatitude F1!layer ano! malies may be characterized as follows] Winter or seasonal ano! maly Annual or non!seasonal anomaly Semiannual anomaly

Greatest NmF1 "or foF1# in winter Greatest NmF1 "or foF1# in December Greatest NmF1 "or foF1# at equinox

Torr and Torr "0862# constructed global maps that show regions where noon foF1 is greatest in summer\ or at equinox\ or in winter[ A simpli_ed version of their map is shown in Fig[ 4[ The three maps represent a very high solar maximum "0847#\ a moderate solar maximum "0858#\ and solar minimum "0853#[ Leaving aside the polar regions\ to which the present discussion does not apply\ the most obvious feature is the belt of strong seasonal anomaly "winter maximum# at high northern midlatitudes[ This feature is most pronounced in the Eur! opean:North American sector but extends\ more weakly\ over most of the temperate zone in the northern hemi! sphere[ A much smaller region of seasonal anomaly exists in the Australasian sector[ Low latitudes and southern midlatitudes show a semiannual variation "maximum at

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

equinox#[ At lower solar activity\ there are regions of summer maximum "i[e[ no anomaly# in equatorial and southern latitudes[ It is noticeable that the di}erent regions tend to be delineated by magnetic rather than geographic latitude[ The method of plotting\ however\ does not clearly distinguish areas of no data\ and its reliability is clearly a}ected by the very uneven dis! tribution of stations over the globe[ The annual anomaly "Berkner et al[\ 0825^ Yonezawa\ 0860# is related to the observation that\ taking the world as a whole\ the overall level of NmF1 appears to be greater in December than in June[ It can alternatively be described by saying that the {{seasonal anomaly is greater in the northern hemisphere than the southern||[ This e}ect may be discerned in Fig[ 4\ in that the regions of summer maximum "i[e[ no seasonal anomaly# are mainly south of the equator[ It should be mentioned that there is also a {winter anomaly| in the D!region "e[g[ Las³tovic³ka\ 0861#\ taking the form of large electron densities on indi! vidual days or groups of days superimposed on an enhanced winter background[ This pattern is di}erent from the more consistent winter:summer di}erences in the F1!layer[ The D!region winter anomaly is associated with chemical changes "enhancements of neutral NO#\ but these do not seem to be linked to the thermospheric composition changes that cause the F!layer anomaly "Section 3[3#[ 3[1[ Geometrical explanations In principle\ the simplest explanations of the anomalies are {geometrical|\ i[e[ related to the shape of the Earth|s orbit\ or to some other factor that a}ects the ~ux of ionizing radiation on the Earth|s upper atmosphere[ In these terms the annual anomaly might appear the easiest to explain\ because of the 2) variation in SunÐEarth distance[ Simple as it is\ this explanation is not necessarily complete\ because the annual variation of NmF1 seems to be rather more than would be caused by the 5) di}er! ence in the ~ux density of solar ionizing radiation received by the Earth "Yonezawa and Arima\ 0848^ Yonezawa\ 0860#[ However\ the phase is correct\ so in this sense the annual variation may not be an {anomaly| at all[ The same might be said of the semiannual variation at the geographic equator\ where the noon solar zenith angle does vary semiannually[ For the semiannual variation outside the tropics\ other explanations must be sought[ One idea "Burkard\ 0840# is that the Sun|s ionizing radiation is emitted aniso! tropically\ so that the ~ux received by the Earth depends on its heliographic latitude\ which attains its greatest values of 26> in early March and September\ but this idea has not been accepted[ 3[2[ Thermal explanations The earliest theory of the seasonal anomaly was simple[ Appleton "0824# suggested that the upper atmosphere

0282

would be hotter\ and therefore more expanded\ in sum! mer than in winter[ It had to be supposed that\ going from winter to summer\ the thermal expansion of the F! layer "which tends to decrease NmF1# more than com! pensates for the decrease of solar zenith angle x "which would increase NmF1#[ It is now known that the ther! mosphere at F!layer heights is indeed hotter in summer than in winter\ but only by about 19) at midlatitudes\ quite inadequate to explain the observed seasonal anom! aly[ Furthermore\ since thermal expansion just redis! tributes the ionization\ it cannot account for the observed fact that the height!integrated total electron content ÐN dh is greater in winter than in summer[ Another sugges! tion was that ionization ~ows along geomagnetic _eld lines from the hotter summer ionosphere to the cooler winter one "e[g[ Rothwell\ 0852#\ but this process is much too slow to be e}ective[ 3[3[ Chemical explanations As an alternative idea\ it was suggested by Rishbeth and Setty "0850# that the seasonal anomaly is caused by changes in the chemical composition "i[e[ the ato! mic:molecular ratio# of the neutral air[ This idea arose from a detailed study of the rate of increase dN:dt of F1! layer electron density just after F!layer sunrise[ At this time\ dN:dt depends largely on the production rate q\ which depends on the ðOŁ concentration\ "a# of Section 1[1[ Rishbeth and Setty "0850# and Wright "0852# realized that a change in the atomic:molecular ratio would account not only for the anomaly in sunrise dN:dt but also for the anomaly in noon NmF1\ though the relative importance of the production and loss terms would be di}erent at sunrise and noon[ Later work "Rishbeth et al[\ 0884# showed that the loss term is quite important even near sunrise\ so the change in the coe.cient b\ which depends on the ðN1Ł and ðO1Ł concentration "eqn 1#\ also contributes to the seasonal sunrise anomaly[ Transport processes are of lesser importance[ Through the associ! ated changes of scale height\ the composition changes also account for the fact that the increase of N starts earlier\ with respect to ground sunrise\ in winter than in summer[ At that time\ the atomic:molecular ratio at F!layer heights was not well known\ nor was there any evidence as to whether it changes with season[ Johnson "0853# and King "0853# suggested\ by qualitative arguments\ that the atomic:molecular ratio is in~uenced by summer!to! winter transport[ Subsequently Duncan "0858# took this idea further\ by suggesting that both storm e}ects and seasonal e}ects in the F1!layer are due to composition changes\ which in turn are caused by the vertical and horizontal winds associated with the global ther! mospheric circulation[ The experimental evidence came years later from inco! herent scatter radar data "Waldteufel\ 0869^ Cox and

0283

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Evans\ 0869^ Alcayde et al[\ 0863# and from rocket and satellite experiments "O}ermann\ 0863^ Prolss and von Zahn\ 0863^ Mauersberger et al[\ 0865#[ Duncan|s idea that the composition changes are caused by a global circulation was further developed by Mayr et al[ "0867#\ and was eventually con_rmed by global modelling\ which evaluated the seasonal composition changes induced by the prevailing summer!to!winter meridional wind "Fuller!Rowell and Rees\ 0872#[ This work basically explained the existence of the seasonal anomaly\ but not the fact that its strength varies with latitude and longitude in a way that appears to be geomagnetically controlled[ Nor does it have any obvious connection with the semi! annual e}ect\ so this question must now be explored[ 3[4[ Explainin` the semiannual effect None of the possible causes considered in Sections 3[0Ð 3[3 o}er an obvious explanation of the F1!layer semi! annual e}ect\ which remained a puzzle[ Possible clues are suggested by other semiannual phenomena in the upper atmosphere\ as reviewed by Ivanov!Kholodnii "0862#] "0# the equinoctial maxima of density of the neutral ther! mosphere\ _rst detected by observations of the atmo! spheric e}ects on satellite orbits "Paetzold and Zschorner\ 0850#^ "1# the equinoctial peaks in geomagnetic activity^ "2# the semiannual oscillations in the lower or middle atmosphere^ "3# the semiannual e}ect in the height hmF1\ found by Becker "0856#[ Of these "3# seems likely to be related to "0# because hmF1 tends to follow _xed pressure!levels\ "c# of Section 1[1^ "1# is unpromising for the simple reason that mag! netic activity normally depresses NmF1\ so would not produce equinoctial maxima of NmF1 "though it may well be a complicating factor in any other explanation#^ and the complexity of mesosphere:thermosphere dynami! cal coupling makes "2# a formidable problem to inves! tigate[ However\ Millward et al[ "0885# advanced a theory that depends on none of these\ but invokes only dynami! cal processes within the thermosphere itself\ and seems capable of explaining at least the main features of the seasonal and semiannual variations of NmF1[ Recently\ Fuller!Rowell "0887# has developed a theory of the ther! mospheric semiannual variation\ "0 above#\ based on the idea that\ as a global average\ the thermosphere is more mixed at solstice than at equinox[ The theory of Millward et al[ "0885# considers the interface of the solar!driven\ low:midlatitude ther! mospheric circulation with the magnetospherically! driven high latitude circulation[ The latitude of this inter! face depends on longitude[ The high latitude circulation\ being geomagnetically controlled\ extends to relatively

low geographic latitudes in the vicinity of the magnetic poles\ i[e[ in the North Atlantic "North American:West European# sector in the North and the AustralasianÐ Indian Ocean sector in the South[ These will be called {near!pole| sectors "{pole| here meaning magnetic pole#[ The key to the explanation is the geographic latitude of the {downwelling region| in the winter hemisphere "as shown in Fig[ 2#\ where the ðO:N1Ł ratio is enhanced as explained in Section 2[3[ Because of the interaction between the two circulations\ the downwelling is greatest a few degrees equatorward of the auroral oval[ In the {near!pole| sectors mentioned above\ on the dayside "Fig[ 2 C#\ this occurs at moderately high geographic latitudes "roughly 49Ð54>#[ At these latitudes\ the solar zenith angle at midwinter noon is 62Ð77> which\ though large\ does give enough photoionization to produce a high NmF1 in the oxygen!rich winter thermosphere[ But in regions remote from the magnetic poles\ here called {far!from! pole| sectors\ i[e[ the Asian and South Atlantic sectors\ the situation is di}erent "Fig[ 2 D#[ The downwelling region is at such high geographic latitude as to be in twilight or even in darkness at noon\ so there is insu.cient ionizing radiation to produce a large electron density despite the large ðO:N1Ł ratio\ and therefore NmF1 is smaller than in the {near!pole| sectors[ A contributory factor is the meridional wind\ which is equatorward during the day\ and therefore tends to reduce NmF1 through the e}ect of vertical drift U sin I cos I "Fig[ 0#[ At a given geographic latitude\ this e}ect is stronger in the {far!from!pole| sectors\ because there the dip angle I is smaller\ and over large areas of the midlatitude ionosphere the factor sin I cos I is not much below its greatest value of 9[4 at I  34>[ In addition\ the horizontal wind speed U tends to increase with distance away from the convergent downwelling region\ which would reinforce the dip angle e}ect[ How does this explain the semiannual e}ect< Consider the changes going from midwinter towards equinox[ Because of the seasonal change in the circulation pattern\ the ðO:N1Ł ratio at midlatitudes also decreases\ which tends to decrease NmF1[ But the noon solar zenith angle decreases too\ which tends to increase NmF1[ At the F1 peak\ the production rate q depends strongly on x when x is large "around 89>#\ but is rather insensitive to x when x is smaller\ i[e[ less than about 79> "This follows from the properties of the Chapman function\ bearing in mind that the F1 peak lies well above the peak of q\ except at grazing incidence\ when x is near 89>#[ In the {far!from!pole| sectors\ where the downwelling region is in darkness at winter noon and NmF1 is low\ as explained above\ NmF1 increases from winter to equinox because the e}ect of decreasing x exceeds the e}ect of decreasing ðO:N1Ł[ But in the {near!pole| sectors\ the downwelling region is in daylight even in winter\ so NmF1 is high and the reverse applies] the decrease of x has less e}ect than the composition change\ so NmF1 decreases from winter to equinox[

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

Going from equinox noon to summer noon\ x is not particularly large anywhere at midlatitudes\ and NmF1 is insensitive to its changes[ In this case\ the changes in ðO:N1Ł have a greater e}ect than changes in x in any longitude sector\ so NmF1 decreases generally from equi! nox to summer[ Combined with the winter!to!equinox changes described above\ this leads to a predominantly seasonal "winter:summer# variation in the {near!pole| sec! tors and a semiannual variation in the {far!from!pole| sectors[ At lower middle latitudes\ the semiannual e}ect pre! dominates more generally[ The solar!cycle changes in the domains of the seasonal and semiannual e}ects\ as shown in Fig[ 4\ have yet to be explained\ but may be connected with changes in the strengths of the global circulation[ Further study is needed as to the relative importance of the solar zenith angle and the ðO:N1Ł ratio in producing seasonal and semiannual variations[ It is thought that F1!layer seasonal and solar cycle variations may also be in~uenced by changes in photochemical conditions*for example by O¦ ions in metastable states\ and by the vibrational excitation of N1\ both of which modify the value of the loss coe.cient b in eqn "1# "Torr et al[\ 0879#[ These questions have yet to be _nally settled[ 3[5[ Summary] Solar and `eoma`netic control of the quiet! day thermospheric circulation

0284

more molecular in summer\ and becomes less molec! ular towards equinox\ i[e the mean molecular mass decreases[ The noon solar zenith angle increases towards equinox\ but its e}ect in the F1!layer is small since x has moderate values\ no more than about 59>[ Hence the net e}ect in all longitude sectors is] Summer : equinox] "compo# × "zenith angle# "b# Now consider winter at about 49> geographic\ where x × 69> at noon\ in a longitude sector near the mag! netic pole[ The noon sector is relatively close to the auroral oval\ so downwelling and the resulting e}ect on composition are strong[ The e}ect of the poleward wind is moderate] U is small\ I large\ cos I sin I ½ 9[14 Winter : equinox] "compo# × "zenith angle# so NmF1] "winter# × "equinox# × "summer# The predominant variation of NmF1 is seasonal[ "c# Now consider winter at about 49> geographic\ in longitude sectors far from the magnetic poles[ The noon sector is a long way from the auroral oval\ so downwelling and composition e}ects are small[ The e}ect of the poleward wind is strong] U is large\ I small\ cos I sin I ½ 9[4 Winter : equinox] "compo# ³ "zenith angle# so NmF1] "winter# ³ "equinox# × "summer#

The explanations of the midlatitude seasonal and semi! annual e}ects\ given in Sections 3[3 and 3[4\ may be summed up as follows with reference to the schematic circulations shown in Fig[ 2 "C\D#[ The notation "X# is used as a shorthand to denote {{the change in NmF1 resulting from the change of X||[ For the quiet!day vari! ations\ the key features of the circulation are] Summer midlatitudes] Upwelling : decreased ðO:N1Ł Winter midlatitudes] Downwelling : increased ðO:N1Ł The greatest downwelling occurs about 4> equatorward of auroral oval[ The horizontal "meridional# wind U is weak near the downwelling\ but stronger at lower lati! tudes[ By day\ the meridional wind U is poleward and therefore creates a downward drift of U sin I cos I which tends to depress NmF1\ as explained in Section 1[1 "d#[ This is secondary in importance to the composition e}ects\ but it does a}ect the shape of the local time variation of NmF1[ The wind e}ect is longitude depen! dent because U depends on the proximity to the auroral oval\ while I depends on magnetic latitude[ The in~uence of winds is not considered in detail here\ but has to be included in detailed modelling of the F1 layer[ "a# Consider the transition from summer to equinox[ At midlatitudes\ the thermosphere at F1!layer heights is

The predominant variation of NmF1 is semiannual[

4[ Storm variations of the midlatitude F1 peak 4[0[ {Positive| and {ne`ative| storm effects In the early days of ionospheric research\ it was noticed that geomagnetic disturbance is accompanied or quickly followed by marked changes in the F1!layer[ Sometimes\ especially in winter\ these changes take the form of increases of NmF1\ but more often there are severe decreases of NmF1 "Appleton and Ingram\ 0824^ Berkner et al[\ 0828#\ especially in summer and at equinox[ The phenomena came to be known collectively as {F!layer storms|\ the terms {positive| and {negative| being generally used to denote whether NmF1 is increased or decreased from its usual quiet!day value during the {main phase| of the ionospheric storm[ The {main phase|\ lasting typically 13Ð25 h\ is preceded by an {initial phase| lasting a few hours[ The {initial phase| is usually {positive|\ i[e[ NmF1 is enhanced\ whether the subsequent {main phase| e}ect be {positive| or {negative|[ Note that the terms {initial phase| and {main phase| are also used in connection with the variations of the geomagnetic _eld during a storm\ as observed by ground magnetometers\ though the geo! magnetic storm phases are generally shorter than the

0285

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H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

corresponding F1!layer phases[ Many processes are involved in F1!layer storm phenomena\ including atmo! spheric waves and electric _elds transmitted from high latitudes^ see for example the comprehensive review by Prolss "0884#[ 4[1[ Composition effects in ne`ative storms Using the newly developed photochemical ideas\ as set out in Section 1[1\ Seaton "0845# advanced the idea that negative F!layer storm e}ects are due to an increased loss coe.cient b\ resulting from increased concentrations of neutral molecular gases "though\ at that time\ it was thought that the loss processes involved only O1\ rather than N1#[ Duncan "0858#\ linking this explanation of negative storms to the photochemical theory of the F1! layer season anomaly "Section 3[3#\ proposed that both seasonal and storm e}ects are induced by a global ther! mospheric circulation\ primarily driven by the solar input but strongly modi_ed by high latitude {storm| inputs[ Subsequent authors developed this idea in terms of a thermospheric {storm circulation| superimposed on the solar!driven {quiet!day| circulation "e[g[ Mayr and Volland\ 0861^ Matuura\ 0861#\ and Reddy "0863# found evidence for a {return circulation| in the lower ther! mosphere during a storm[ Prolss "0879# reviewed the experimental evidence for composition changes during storms\ as found by satellite!borne instruments[ As in the case of the quiet!day seasonal changes\ it was some years before the idea that the {storm circulation| drives composition changes could be tested by full global modelling[ The detailed modelling was done by Burns et al[ "0878\ 0880\ 0884# and Fuller!Rowell et al[ "0883\ 0885#[ The e}ect of vertical motions was especially stud! ied by Burns et al[ "0884#\ but the full theory of the relationship between vertical motions and composition changes has yet to be established[ Fuller!Rowell et al[ "0883# further developed the {storm circulation| idea\ envisaging that a {composition bulge| "a region of enhanced molecular:atomic ratio# forms as a result of the storm heat inputs[ Once formed\ the {bulge| migrates in latitude and local time under the in~uence of the ther! mospheric wind system which\ within a day after the start of the storm\ usually returns more!or!less to the quiet! day pattern[ As pointed out in Section 2[3\ the diurnal amplitude of this migration is only of order 0999 km[ Detailed modelling by Fuller!Rowell et al[ "0885# and Field et al[ "0887# suggests that both {negative| and {posi!

0286

tive| main phase e}ects in NmF1 are broadly explained in terms of composition changes[ Very roughly\ if {s| and {q| denote {storm| and {quiet day| mean values] "06# NmF1s:NmF1q ¼ ðO:N1Łs:ðO:N1Łq[ To investigate this relationship\ and following the method introduced by Rodger et al[ "0878#\ Field and Rishbeth "0886# derived values of storm:quiet ratio "NmF1s:NmF1q# for 42 stations and extracted the DC Þ averaged over local time\ the AC peak!to! mean value N mean amplitude N of the variation with local time t\ and the local time ¼t of its maximum\ as in the equation = f "t−t¼#[ "07# ln "NmF1s:NmF1q#  N Þ ¦N The function f is arbitrary in shape\ but it has unit amplitude and zero mean and attains its maximum at zero time[ These AC:DC results are derived from many di}erent storms over the period 0846Ð0889\ and thus represent a substantial averaging of storm behaviour[ Despite the averaging\ this is a useful way of showing how the main phase e}ect varies with latitude and season[ The curves in Fig[ 5 show the DC amplitude N Þ computed for the 42 stations used by Field and Rishbeth "0886#\ for Þ is generally negative in summer and at three seasons[ N equinox "i[e[ short dashes in the southern hemisphere\ long dashes in the northern hemisphere\ and the full curve in both hemispheres# but is positive in winter "long dashes in the southern hemisphere\ short dashes in the northern hemisphere#[ The _gure shows the {storm seesaw e}ect| as the curves of N Þ swing from northern solstice through equinox to southern solstice[ Although the individual data points are rather scattered\ they mostly follow the {seesaw| trend at middle and low latitudes[ Field and Rishbeth "0886# showed that this trend is consistent with the composition changes given by the empirical MSIS!89 model of the neutral thermosphere "Hedin\ 0880#\ which is based on experimental data from many sources[ Field et al[ "0887# showed that the storm:quiet ratio of NmF1 follows quite closely the storm:quiet ratio of ðO:N1Ł[ This supports the idea that the positive main phase e}ects\ more common in winter\ are produced by composition changes\ and so are the negative main phase e}ects in summer and at equinox[ 5[ Conclusion 5[0[ {State of the ionosphere| This review has described how the quiet!day and storm variations of the F1 layer are controlled by the global

300000000000000000000000000000000000000000000000000000000 0 Fig[ 4[ Annual and semiannual e}ects in noon critical frequency foF1\ simpli_ed from Torr and Torr "0862#[ The shading shows {Max[ Summer| domains where "summer foF1 × equinox foF1#\ i[e[ {no seasonal anomaly|\ and {Max[ Winter| domains where "winter foF1 × equinox foF1#\ i[e[ {seasonal anomaly| exists[ No shading means foF1 is greatest at equinox\ i[e[ {semiannual anomaly|[ The {Max[ Winter| domains are divided according to whether "winter foF1*equinox foF1# is less than 1 MHz "A# or greater than 1 MHz "B#[ Symbols mark the positions of the magnetic dip poles "crosses on black spots#[

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Fig[ 5[ {Storm seesaw|[ The local!time "DC# average N Þ of the ratio "NmF1s:NmF1q# plotted against magnetic latitude\ based on ionosonde data for 42 stations\ 0846Ð0889[ The curves are averages drawn through the data points of Fig[ 3 of Field and Rishbeth "0886#[ Long dashes] northern solstice "MayÐAug#^ full curve] equinox "Mar\ Apr\ Sept\ Oct#^ short dashes] southern solstice "NovÐ Feb#[ The mean deviation of the data points is about 29[0[

quiet day and storm circulations in the thermosphere[ The control mainly comes about through composition changes of the neutral air\ which are induced by the circulation and largely determine the electron and ion density[ It is well said that the F!layer is dominated locally by photochemistry and globally by dynamics[ Ionospheric science is at the heart of the topical sub! jects of space weather\ predictions and forecasts[ Even the quiet ionosphere is heavily in~uenced by the Sun "i[e[\ the observed solar cycle variations and 16!day vari! ations#^ the disturbed ionosphere is in~uenced also by the solar wind and interplanetary magnetic _eld\ which modulate the energy sources of the storm circulation[ Future work will also consider interactions "which may turn out to be two!way# between the ionosphere and the weather\ as well as possible ionospheric e}ects of surface topography "oceans and mountains#\ events such as earthquakes and thunderstorms^ and the intriguing ques! tion of long!term global change in the upper atmosphere[ 5[1[ Guidin` principles for ionospheric research As a tailpiece\ here are some general ideas that may be found useful in thinking about the ionosphere*or\ for that matter\ many other natural systems[ First\ always consider the scales of time and distance that are involved[ The kinds of questions to ask are {{How long does ða processŁ take<||^ {{Over what distance does it operate<||^ {{How fast does it move<|| To help decide whether a particular explanation makes sense\ try com! paring a typical speed of motion with the ratio "distance scale 6 time scale# for the process under discussion[ Second\ remember that ionospheric parameters vary

much more rapidly vertically than they do horizontally[ For large scale structure\ vertical scales "tens of km# ð horizontal scales "099Ð0999 km#[ However\ ver! tical motions are generally slower] e[g[ for wind systems\ vertical speeds "½2 m s−0# ð horizontal speeds "½099 m s−0#[ But\ despite horizontal speeds being faster\ vertical motions are more important for two reasons] "a# the transport terms in the basic conservation equations "Sec! tion 1[0#\ which involve gradients and divergences\ are usually dominated by vertical motions^ "b# vertical motions\ which involve gravity\ are more energetic than horizontal motions\ of which the energetics are associated only with inertia and frictional "collisional# processes[ Third\ it is di.cult to derive absolute values of iono! spheric parameters\ such as production and loss rates\ from observations of electron density[ The reason is that the ionospheric layers\ even the F1!layer\ are almost always close to equilibrium\ in the sense that the "d:dt# terms in the conservation equations are much smaller than others[ For example\ measuring the F1!layer elec! tron density may give quite a good value of the ratio q:b\ but neither parameter is separately well determined[ This is true even for apparently non!steady situations such as sunrise\ sunset and eclipses\ and explains why so much early work gave unsatisfactory numerical results\ even though it did contribute to physical understanding[ Fourth\ a converse proposition] ionospheric modelling is often insensitive to adopted values of input parameters\ which means that observations can often be _tted by quite a wide range of numerical parameters[ A good _t does not necessarily mean that the chosen values of parameters are accurate[ Fifth\ it is commonplace that better space or time res!

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

olution in experiments leads to new science[ For example\ much of the gross structure\ particularly in active systems like the equatorial and auroral ionospheres\ depends on the small!scale physics[ Future research will be increas! ingly directed towards this microstructure and the associ! ated plasma science[ Sixth\ many aspects of the science depend on the ready availability of data[ Even {boring| routine data are most important in experimental {campaign!based| science\ and are vital for systematic and long term research[ Hence the importance of routine solar!terrestrial monitoring data\ such as Kp\ F09[6\ and R\ and archives such as the World Data Centres and national data and analysis centres[ See Willis et al[ "0883#[ Seventh\ use MKSA "SI# units; The old!fashioned cgs units have served well for atomic processes\ but in the author|s opinion they are burdensome whenever elec! tricity or magnetism is involved\ and the tiresome factors of c obscure the dimensional relationships between the physical parameters[ Finally\ the ionosphere is a vital part of the solar! terrestrial system^ and although its basic mechanisms appear to be known\ surprises are possible in any living and active area of science; Ionospheric science is alive and well\ and still has importance for space and terrestrial communications\ as it approaches the start of its second century in 1991[

Acknowledgements This paper is based on a Tutorial Lecture given at the National Science Foundation CEDAR Workshop at Boulder\ Colorado\ on 09 June 0886[ The author thanks the National Science Foundation for _nancial support[ Thanks are due to I[ Muller!Wodarg for help with two of the diagrams[

Appendix To prove that composition is invariant at a `iven pressure! level Consider an atmosphere in di}usive equilibrium at a uniform temperature T\ consisting of atomic oxygen "gas 0# and molecular nitrogen "gas 1#[ Let p0 and p1\ n0 and n1 denote the partial pressures and concentrations of these gases[ Their scale heights are H0  RT:05` and H1  RT:17`\ where R  universal gas constant\ and `  acceleration due to gravity which\ for the sake of simplicity\ is assumed independent of height[ Consider a base level {O|\ situated at the ground or at another level in the atmosphere\ at which the partial pressures are constant\ namely p09 and p19\ so the total pressure is p9  p09¦p19[ From the perfect gas law\ p09  kT[ n09 and

0288

p19  kT[ n19 where k  Boltzmann|s constant and the gas concentrations are n09 and n19[ Let the ðO:N1Ł ratio at level {O| be denoted by r9 so that r9  n09:n19  p09:p19[ Now consider another constant pressure!level {A|\ four oxygen scale heights or\ equivalently\ seven nitrogen scale heights above level {O|[ Introduce the {reduced height| z as in eqn "5#\ which can be de_ned for each gas separately "z0 for O\ z1 for N1# as well as for the whole atmosphere[ Then\ at the level {A|\ z0  3 and z1  6\ and the partial pressures are p0A  p09 exp "−3#\ p1A  p19 exp "−6#

"08#

and\ since the temperature T is the same as at level {O|\ the concentrations are n0A  n09 exp "−3#\ n1A  n19 exp "−6#

"19#

The total pressure at {A| is pA  p0A¦p1A and the ðO:N1Ł ratio is rA  n0A:n1A  "n09:n19# exp "6−3#  r9 exp "2#

"10#

Consider too a further constant pressure!level {B|\ at eight oxygen scale heights "z0  7# or equivalently fourteen nitrogen scale heights "z1  03# above level {O|\ and also at temperature T[ The partial pressures and con! centrations at {B| are p0B  p09 exp "−7#\ p1B  p19 exp "−03#

"11#

n0B  n09 exp "−7#\ n1B  n19 exp "−03#

"12#

The total pressure at {B| is pB  p0B¦p1B and the ðO:N1Ł ratio is rB  n0B:n1B  "n09:n19# exp "03−7#  r9 exp "5#

"13#

Now assume that the temperature is no longer constant\ but varies in an arbitrary way with height\ the temperature being T9 at level {O|\ TA at level {A|\ and TB at level {B|[ The levels {A| and {B| are still de_ned by their pressures pA and pB[ Whatever the temperature\ the scale heights H0 and H1 are always in the ratio 6:3 at any level in the atmosphere[ Applying eqn "5#\ in its general form in which the temperature varies with height\ it remains true that z0  3 and z1  6 at {A| and z0  7 and z1  03 at {B|[ Hence eqns "08# and "11# still hold for the partial pressures at these levels\ but "using the perfect gas law# the gas concentrations at {A| are altered in the ratio "TA:T9# and at {B| in the ratio "TB:T9#\ and are now n0A  n09"T9:TA# exp "−3#\ n1A  n19"T9:TA# exp "−6# "14# n0B  n09"T9:TB# exp "−7#\ n1B  n19"T9:TB# exp "−03# "15# As the temperature ratios disappear in deriving the ðO:N1Ł ratios\ the latter are the same as in the isothermal case\ eqns "10# and "13#[

0399

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

The same argument applies to any pressure level above {O|[ The ðO:N1Ł ratio depends only upon reduced height above {O| and of course on its value r9 at level {O|[ This conclusion is not a}ected if there are more than two gases "provided each has its own scale height and thermal di}usion is negligible# or if allowance is made for the variation of ` with height[

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0390

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0391

H[ Rishbeth:Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0274Ð0391

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