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Effects of temperature variations of the upper atmosphere on the formation of ionospheric layers
Journal
of Atmosphericand TerrestrialPhysics,185X,Vol. 1. pp.278.to 285. Pergamonpress Ltd., London
Effects of temperature variations of the upper atmosphere on the formation of ionospheric layers F2 LAYER
F2 LAYER
ANOMALY-l?2 BEHAVIOUR
AND
DURING
Fl
SEPARATLON-
MAGNETIC
STORMS
D. LEPECHINSKY f,aboratoire National de Rrtdioelectricit6 196, Rue de Paris, Bagneux (Seine) (Reeeiwd 16 May
1951)
ABSTRACT S. CHAPMANS well known theory of ion production by photo-ionization of a gas with an exponentially decretxsing pressure considers the c&se of a constant temperature. In the present paper a study is made of the effect of an increase of the mean temperature on the distribution of the rate of Ffonproduction a,t various heights. It is shown tbrtt at s given height, this rate is extremely sensitive to temperature changes and that it may go through a sharp minimum at midday if it is assumed that the temperature rises regularly from morning to midday. The two main effects of a temperature increase being a general elevation of the region where ions are produced and a general decrease of the rate of ion production, an estimete is made of the percentage of temperature increase which one has to assume to explain t.hn higher summer altitude of the F2 layer at midday as compared with its winter vdue, on the one hand, and its slight and gradual elevation between 6 a.m. and midday on a summer day, on the other. The percentages found (based on Slough 1950 records) are 46% and 21% respectively. It is further shown that the splitting of the F region into .Fr and F2 may be due to a general temperature increase of the upper atmosphere. The “summer structure” of this region and the critical frequency drop of the J?slayer during magnetic storms may similarly be considered as an effect of a temperature increase of the gas submitted to the photo-ioniz&tion process. Finally it is pointed out th& the seasonal variation of the absorption of short waves may also find an explanation in a seasonal temperature variation. I.
INTRODUCTION
In a recent work [l] a graphical method hes been proposed for the determination of the rate of ion production and of the apparent recombination and attachment coefficients governing the formation of a layer the hourly critical frequency and height of which are known experimentally. The assumptions made were: fa) That the layer’s density distribution with height. is almost, parabolic; (b) That the layer formation obeys the law: ax --=~--lyw)~ at
(1) where : N is the electron density q the rate of electron production y (N) a function of N only, when [l] applies to a constant height above ground. An analysis t&s made of the daily variation of the electron density of the Ft layer at various levels (based on data published for Slough, March 1948) led to the following results : function y(X) values obtained for various levels at (I ) The electron “disappearance” different
hours,
passing through
plotted against corresponding values of N, fit fairly well parabolas the origin of the coordinates, thus justifying its expression by:
@hen
YW)=A,N2$_B,N*
278
(2)
Effwtsoft.~mperaturavaria&ms of the upper atmosphere DKI the formation of ionospheric &WS
(2) The rates af electron~rod~et~~~l q,‘as expected, wcsre found to reach their highestdailg value at noon at Ibe Ievelsstudied; they ranged fram 150 per cm9, aec_J (at 350 km) Lo2f0 per em3 . see-l (at 275 km), When applied to summer months this m&hod of investigation leads to on apparently anomalous hehaviour of the rate of electron production: the latter exhibits at some le~eIs a marked minimum at noon. S. S.BARAL and A. P. Ildrrr~[2] found the same anomaly in the daily variation of the &c;tron production rate during summer months for the pa and even for the E la,yer, ushtg another method. They point out that it may be a possible effect of Lida motions. Tt seemed interesting to us to examine closely whether the &as&al S. &%APMA'pJS layer f~~a~on thecay [SJ can explain this anomaly, when it is as~n~~d that the mean t~rnp~ratur~ of the upper atm~s~hare ~adu~~ rises from morning to noon during the long summer days. A study of the effects of temperature variations on layer formation has thus been nnd~rtak~~*
INCRXASXON THE RATE OP ION PROIXTCTIOW AT VARIOUSHEICHZ~~ It is well known that in his theory of photo-ionisation of a gas with an ex~~e~~~ll~ decreasing density, S. CHAPMAN assumed t.he latter to be at constant ~rnpe~~tnre T and t,o have a constant “‘Scale height” H given by: 11.
EFFECT
OF A TENWCRATURE
Jj zzz.!!r m!?’ where T is the teln~erature in degrees R m is the mo~eeular mass of the gas g the acceleration of gravity k the BOLTZNJWNconstant. Xow, if we suppose that the t.emperatnre becomes T’===oT, where a>f, the height of max~mnm ion productions given by:
(3)
h,=HlogA~,H
(4 where A is the absorption constant and @a the ground level density of the gas, becomes : ~~~~~*~~H~~~~. (4’1 On the other hand, the maximum rate of ion ~rodnct~on~ given by: 1, E _&FE Ee )
(5)
where ~3is the ionization constant and 8, the intensity of the ionizing ~adi&tio~~ outside the atmosphere, becomes when the temperature is a times higher: & = Is__%? and thus, we can write: and :
2*
aLle ’
w
D. LEPECHINSKY
This means that when the average temperature of the gas rises, t.he whole region where ionization t.akes place moves up to higher levels and t,hat at the same time the maximum rate of ion production decreases. In order to get a still better idea of the effect of a temperature increase on ion production we shall draw curves representing t,he rate of ion production as a function of height, for different zenith angles x, when the temperature is T’ = a T and compare them to those corresponding to temperature T. This comparison can be done directly only if unit lengths chosen for the abscissae on the one hand and for the ordinates on the other are the same. 1st us take I, (the rate of maximum ion production in a temperature T) as unit length for the abscissae and H (the scale height in a temperature T) as unit length for the ordinates. The rate of ion production in a temperature 7’ being given by: I = I,cxp[l
-z-(secx)ecZ].
(8)
where : z=h-ho
(9)
n T’ = a T, a rate I’ given by:
wf: shall have in a temperature I’=
I;,exp[l
---2’ --(secx)e-Z’].
where : tl = h hb (1.II .
w (9’)
We thus see that, when expressed in units I,, which we have chosen t.he rate I’ has to be represented by a length II/I,; but since I; = I,/a, because of [i], we have: I’._ I, -
’
n
exp [ 1 -- 2’ .- (fjfxX) e- “1
at t.he same time the rate I, corresponding to temperature ding to [8] by a length: I = cxp [l ---Z ---(secx)ePL]. 10
(8”) T is of course given accor(8”‘)
The following rule can thus be used for plotting our curves: “The value of the exponential exp [I --z -(secX)e“1 for z = z, divided by a gives the value of F/I, at the level a Hz, (or az,, when expressed in units “H “) above hb. At the same time the value of this exponential for z = z1 gives the value of l/1,, at the level Hz, (or z,, when expressed in units “H”) above h,. It is clear in these circumst.ances that our two families of curves can be plotted and compared only if we know what are the values of h, and hi, or at least, what is the value of the difference (hb-h,), which: according to [6]. and expressed in is given by: unit,s “H” nh,. -__ ho R (a-l)+aloga. (6’) H In Figure la three families of curves have been represented, corresponding respect.ively to temperatures T, T’= 1.5 T and T” = 2 T with the assumption that the ratio h,/H in (6’) is equal to 4.5 (this corresponds to a value of h, ranging from 200 to 300 km with an H from 45 to 65 km). 280
Effects
of temperature
variations
of the upper akmosphere on the formation
of ionospheric layers
Careful inspection of these curves shows at once how sensitive to temperature variations is the rate of ion production at a given height. Thus for instance at level z=O, for x=O”, a 50% increase of the temperature reduces this rate more than 30 times. It is further seen that the higher the temperature, the higher is the region where ions are produced and the thicker it is. Finally, as already mentioned, the maximum rate of ion production decreases with increasing temperature. III.
THE SEASONAL
ALY
ANOM-
OF THEF~LAYEE
If we now assume that the average temperature of the upper atmosphere varies from season to season and, even during the day, is higher in summer than in winter (at noon) and higher at midday than in the morning, the results of the preceeding discussion lead to the conclusion that ionospheric layer characteristics which closely follow those of the ion production regions, will exhibit similar diurnal and seasonal temperature variations. In Figure 1 b are represented diurnal variations of the rate of ion producFig. 18. XorI’orI”/I, as & function of altitude for average temperatures X, 3 T/2 and 2X. tion, assuming that the temperature rises from llT at sunrise (x = 90°) to 312 T at noon (wit’h x = O’), at different levels A, B, C . . . corresponding to Figure la. It is quite obvious that at levels such as “D”, the ion production rate goes through a deep minimum at noon simply on account of a midday temperature increase. Ion production rates at other levels exhibit different types of diurnal variations, and at certain levels such as “C”Vhere is a rather sharp maximum of ion production just around noon. In spite of this fact, if one considers that the electron density at noon, at a given level of a layer, depends also on the “previous history” of it - i.e. on the time during which ion production took place in it, it becomes apparent 281
D. LEPECHINSKY that a layer formed in these circumstances, may well exhibit a lower maximum electron density at midday than usually expected, and even a double humped daily variation curve-so characteristic of the F, layer in summer. Other probable effects of temperature variation appearing in the F2 layer behaviour are : its formation at much higher levels in summer than in winter and its greater thickness and lower maximum electron density I in summer. r;; 0.6 Recordings of F2 characteristics obtained at Level-A LevelC Leve/-6 0.5Slough (England) yield the following median values 0.aq of gm and h, for January. March and June 1950 at 03 t
;
76 January . . 91 Narch . . . June. . . . 1 130
Zeni% uny/e *= 9P a0
o
0
JO’ 4’O
JO” L-2L-_T”
L eve/- II
0.4
Leve/-
Level-E
0.3
f
0.2
T “i’
i
0.1
+
Assuming that h,, i.e. t,he level of maximum electron density of the layer, is near the level h max. where the electron production rate is a maximum? we have: h
0 L!!!
@-!L
I
Fig. lb.
n
I/I, as a function of time (or zenith angle) at various levels A, B, C, etc. when the temperature rises from T at x= 90” to 3T/2 at x=0”.
265 320 350
max (Winter)
= H log A eOHsecXcs,, h =
aH
max (Summer) log A e. aWsecXC,s,,
where : xcwj and xCs,are the sun’s zenith angles at Slough at noon in winter and in summer respectively; a is the temperature increase coefficient. It follows that: a [l + (E&J log a set X(s)1 __ 350 h, (Summer) 265 ’ -hm (Winter) = ~ 1 _t (H/h,) log set x(w)
Taking for H/h,, the average value O-2 and secXCs+)= 3.3 ; secXCs, = 1.13, we find : a = 1.46. The temperature increase responsible for the summer elevation of the F, layer at Slough, at midday is thus about 46 % of the winter temperature of t,he region. 282
Effects of temperature
variations of the upper atmosphere on the formation
of ionospheric
layers
and A. P. MITRA [2] recently found by another method that region varied from 700” K in winter to 1200” K in summer (for the period of high solar activity) and from 500” K in winter to 900” K in summer (for the period of low solar activity); the corresponding percentages of increase thus being about 70% and 80% respectively, against the winter values. Some substantial objections existing in the literature [3] against such high percentages of temperature variations in the atmosphere under solar influence alone, the authors express the opinion that the total variation in temperature is not due entirely to seasonal increase and decrease of heating by the sun, but is partly an effect of the rising temperature gradient above the E level and of the elevation of the F region in summer. Our result regarding the mean midday temperatures of the whole region thus appears to be a conservative one. On the other hand, the daily variation of h,, which in the case of the F2 layer formation by the regular CHAPMAN process in a constant temperature, should exhibit a single minimum value around midday, and which instead, as it is well known, exhibits two minima especially marked on summer days (one in the morning, the other in the afternoon with a slight maximum around noon), may similarly be interpreted as due to a diurnal variation of the mean temperature. A rough calculation made on the same lines as the proceeding one, with June 1950 values obtained at Slough, shows that the temperature increase of the region between 6 a.m. and noon, responsible for the F2 layer height variation on a summer day, is of about 20 %. Turning now to the F2 layer seasonal anomaly in its electron densities (the highest midday maximum value being found in winter 134*104 ions cm3 in January against 63*104 ions cm3 in June at Slough 1950, i.e. with a much lower value of cosx than in summer), one finds that the summer maximum rate of electron production, is still about two times greater than its winter value at Slough, even when a summer temperature increase of 46% is considered. S. S. BARAL
F2 temperature
It can be shown [4] th&t in these circumstances if the electron disappearance function w(N) has the form [2] where A, and &, are the apparent recombination and attachment coefficients respectively, and if B,, is supposed to be strictly proportional to partial pressure, the above mentioned anomaly remains inexplicable by the classical theory and the temperature effect, unless A, is assumed to increase by a factor of 10 to 20 when the temperature rises by 46 %. The validity of this assumption will not be discussed here. We shall mention in this connection that according to LANGEVIN [6] the coefficient of recombination of a gas increases with its density and ,therefore with its pressure. IV. According ionization
SPLITTING OB REGION F INTO Fl AND F2 LAYERS.
to [a], h,, the height of maximum process, i8 a function of:
ion production
in a gas by the photo-
(a) The scale height H of the gas, (b) its absorption constant A, specific of its nature and of the ionization process involved and (c) of its density e. as measured at ground level. Now if there are several gases present in the upper atmosphere with individual exponential distributions with height, or even if a single gas is subjected to ionization by different absorption processes, equation [4] shows that distinct regions of ion production will be set up and distinct ionized layers will 283
D. LEPECAINSKY be formed. F, and Fl layers may thus be reasonably well regarded as due t.o photo ionization whether of two distinct gases (0 and M2 respectively according to BHAR, _MITRAand GHOSH) or of a single gas (*X2 at its two ionization potentials, according to WULF and DEMIKO ; 0 or 0 and N, according to D. R. BATES) by different processes. In these circumstances we can write in the general case of two distinct gases : h, = H IogA ,Q,,H, for the F, layer hi, = H’logA’&H’ The separation
h,-- hb of the two layers
for the Fl layer. is given
by :
where :
It. is thus easily seen that the higher the temperature the greater is the separation between the layers, the latter being practically proportional to H (H = kT,!mg and the ratio H’IH = r being less than or equal to unity, with the present assumptions regarding the gases involved). The theory is thus in accord with the well known facts about the Fl layer behaviour, as well during daytime, when its greatest separation from F, occurs at midday, as during the whole year (the greatest separation of t.he layers occuring on summer days), if temperature variat.ions of the upper atmosphere are taken into account. V.
BEHAVIOUR OF THE F REGIOK DURING ~IAGSETIC STORMS
It is well known that magnetic storms are usually accompanied or closely followed by the following phenomena in the F region of the ionosphere: (a) The critical frequency of the F, layer drops to an abnormally low value; (6) There is a substantial increase in the virtual height of the layer and in its h, value ; (c) An obvious splitting of the F region into Fl and F2 occurs, even on winter days, when both layers usually merge. The preceeding discussion leads to the conclusion that all the above phenomena may be due to a single cause: a general increase of the temperature of the This point of view is further supported by the gases subjected to photoionization. fact that during the days of critical frequencies depression, in or after magnetic storm periods, the F, layer daily evolution usually remains quite regular. *‘fo F2” reaching its reduced maximum value at midday; the F2 layer formation thus appears t.o be a normal one but with modified constants. It is interesting to not.e in this connection that some years ago already, S. CHAPMAX expressed the opinion [5] that “intense and concent,rated currents that flow along the aurora1 zone during magnetic storms may appreciably heat. the air there; but such heating is likely t,o be exceeded by more direct heating due to degradation into thermal energy of part of the kinetic energy of the incoming ionizing particles.” An estimate of the temperature increase of the F region during or just after a magnetic storm can be made on t,he basis of the F2 layer elevation a.bove its normal level. Thus, for instance, the severe magnetic storm of 24 and % January 1949
Effects of temperature rariat,ions of the upper atmosphere on the formation of ionoaphcric layera
(when oscillations on the H component exceeded 400 gammas) was followed by a period of marked depression in the “fo F2” values, accompanied by greatly increased h, values of the layer (4.9 Mc/sec and 400 km respectively on 26 January at 1300 hrs G.M.T., against 9.5 Mc/sec and 276 km respectively on 28 January, as recorded at the POITIERS station). The temperature increase on 26 January against normal, corresponding to the recorded h, values, when calculated by t.hc usual method, appears to have been of about 36%. On the other hand the maximum ion production rate drop corresponding to the above temperature increase is of about 26% only (l/a=O*74). This value appears to be rather insufficient alone to justify the important critical frequency drop recorded. It seems thus that a concurrent increase of apparent recombination and attachment coefficients has to be invoked.
VI.
coh.cLusIos
The preceeding discussion leads to the conclusion that temperature variations of the upper atmosphere are playing an import.ant role in the layer formation and morphology, and that the F, layer seasonal and diurnal behaviour is practically always in accord with CHAPMANB ion production theory, when temperature changes are taken into account,. Moreover, magnetic storms’ effects on this layer appear to be essentially due to an abnormal heating of the gases subjected to photo-ionization, by arriving corpuscules’kinetic energy dissipation and by current,s set up Other evidence of temperature variations in the upper atmosin the ionosphere. phere can be seen in the seasonal changes of the absorption of short waves which, as is well known, increases substantially in summer; indeed if temperature and thus parCal pressure of gases in the D region are higher in summer, the absorption coefficient,, which is a function of the pressure, should increase accordingly. REFEREXCES [I] Ler~car~slr~, D.; Note pr&ninairo So. 136 du Laboratoiro national de radioolectricitb (Bagneux France). [2] BARAI., S. S. and MITRA, A. P.; J. Atmosph. Terr. Phys. 1950 1 9.5. [3] CHAPMAX,S. and BARTELS, J.; Ceomagnetiam 1940 1 504. [4] LEPECHISSKY,D.; Note ptiliminaire No. 151 du Laboratoire national do radioolectricit~ (Bagneux, Prance). [5] CHUUK, 8.; Terr. Msg.- and AtmoFtph. Electr. 1937 43 355. [6] LANCEVIS, 31.; AM. Chim. Phys. 1903 26 433.
of Atmosphericand TerrestrialPhysics,185X,Vol. 1. pp.278.to 285. Pergamonpress Ltd., London
Effects of temperature variations of the upper atmosphere on the formation of ionospheric layers F2 LAYER
F2 LAYER
ANOMALY-l?2 BEHAVIOUR
AND
DURING
Fl
SEPARATLON-
MAGNETIC
STORMS
D. LEPECHINSKY f,aboratoire National de Rrtdioelectricit6 196, Rue de Paris, Bagneux (Seine) (Reeeiwd 16 May
1951)
ABSTRACT S. CHAPMANS well known theory of ion production by photo-ionization of a gas with an exponentially decretxsing pressure considers the c&se of a constant temperature. In the present paper a study is made of the effect of an increase of the mean temperature on the distribution of the rate of Ffonproduction a,t various heights. It is shown tbrtt at s given height, this rate is extremely sensitive to temperature changes and that it may go through a sharp minimum at midday if it is assumed that the temperature rises regularly from morning to midday. The two main effects of a temperature increase being a general elevation of the region where ions are produced and a general decrease of the rate of ion production, an estimete is made of the percentage of temperature increase which one has to assume to explain t.hn higher summer altitude of the F2 layer at midday as compared with its winter vdue, on the one hand, and its slight and gradual elevation between 6 a.m. and midday on a summer day, on the other. The percentages found (based on Slough 1950 records) are 46% and 21% respectively. It is further shown that the splitting of the F region into .Fr and F2 may be due to a general temperature increase of the upper atmosphere. The “summer structure” of this region and the critical frequency drop of the J?slayer during magnetic storms may similarly be considered as an effect of a temperature increase of the gas submitted to the photo-ioniz&tion process. Finally it is pointed out th& the seasonal variation of the absorption of short waves may also find an explanation in a seasonal temperature variation. I.
INTRODUCTION
In a recent work [l] a graphical method hes been proposed for the determination of the rate of ion production and of the apparent recombination and attachment coefficients governing the formation of a layer the hourly critical frequency and height of which are known experimentally. The assumptions made were: fa) That the layer’s density distribution with height. is almost, parabolic; (b) That the layer formation obeys the law: ax --=~--lyw)~ at
(1) where : N is the electron density q the rate of electron production y (N) a function of N only, when [l] applies to a constant height above ground. An analysis t&s made of the daily variation of the electron density of the Ft layer at various levels (based on data published for Slough, March 1948) led to the following results : function y(X) values obtained for various levels at (I ) The electron “disappearance” different
hours,
passing through
plotted against corresponding values of N, fit fairly well parabolas the origin of the coordinates, thus justifying its expression by:
@hen
YW)=A,N2$_B,N*
278
(2)
Effwtsoft.~mperaturavaria&ms of the upper atmosphere DKI the formation of ionospheric &WS
(2) The rates af electron~rod~et~~~l q,‘as expected, wcsre found to reach their highestdailg value at noon at Ibe Ievelsstudied; they ranged fram 150 per cm9, aec_J (at 350 km) Lo2f0 per em3 . see-l (at 275 km), When applied to summer months this m&hod of investigation leads to on apparently anomalous hehaviour of the rate of electron production: the latter exhibits at some le~eIs a marked minimum at noon. S. S.BARAL and A. P. Ildrrr~[2] found the same anomaly in the daily variation of the &c;tron production rate during summer months for the pa and even for the E la,yer, ushtg another method. They point out that it may be a possible effect of Lida motions. Tt seemed interesting to us to examine closely whether the &as&al S. &%APMA'pJS layer f~~a~on thecay [SJ can explain this anomaly, when it is as~n~~d that the mean t~rnp~ratur~ of the upper atm~s~hare ~adu~~ rises from morning to noon during the long summer days. A study of the effects of temperature variations on layer formation has thus been nnd~rtak~~*
INCRXASXON THE RATE OP ION PROIXTCTIOW AT VARIOUSHEICHZ~~ It is well known that in his theory of photo-ionisation of a gas with an ex~~e~~~ll~ decreasing density, S. CHAPMAN assumed t.he latter to be at constant ~rnpe~~tnre T and t,o have a constant “‘Scale height” H given by: 11.
EFFECT
OF A TENWCRATURE
Jj zzz.!!r m!?’ where T is the teln~erature in degrees R m is the mo~eeular mass of the gas g the acceleration of gravity k the BOLTZNJWNconstant. Xow, if we suppose that the t.emperatnre becomes T’===oT, where a>f, the height of max~mnm ion productions given by:
(3)
h,=HlogA~,H
(4 where A is the absorption constant and @a the ground level density of the gas, becomes : ~~~~~*~~H~~~~. (4’1 On the other hand, the maximum rate of ion ~rodnct~on~ given by: 1, E _&FE Ee )
(5)
where ~3is the ionization constant and 8, the intensity of the ionizing ~adi&tio~~ outside the atmosphere, becomes when the temperature is a times higher: & = Is__%? and thus, we can write: and :
2*
aLle ’
w
D. LEPECHINSKY
This means that when the average temperature of the gas rises, t.he whole region where ionization t.akes place moves up to higher levels and t,hat at the same time the maximum rate of ion production decreases. In order to get a still better idea of the effect of a temperature increase on ion production we shall draw curves representing t,he rate of ion production as a function of height, for different zenith angles x, when the temperature is T’ = a T and compare them to those corresponding to temperature T. This comparison can be done directly only if unit lengths chosen for the abscissae on the one hand and for the ordinates on the other are the same. 1st us take I, (the rate of maximum ion production in a temperature T) as unit length for the abscissae and H (the scale height in a temperature T) as unit length for the ordinates. The rate of ion production in a temperature 7’ being given by: I = I,cxp[l
-z-(secx)ecZ].
(8)
where : z=h-ho
(9)
n T’ = a T, a rate I’ given by:
wf: shall have in a temperature I’=
I;,exp[l
---2’ --(secx)e-Z’].
where : tl = h hb (1.II .
w (9’)
We thus see that, when expressed in units I,, which we have chosen t.he rate I’ has to be represented by a length II/I,; but since I; = I,/a, because of [i], we have: I’._ I, -
’
n
exp [ 1 -- 2’ .- (fjfxX) e- “1
at t.he same time the rate I, corresponding to temperature ding to [8] by a length: I = cxp [l ---Z ---(secx)ePL]. 10
(8”) T is of course given accor(8”‘)
The following rule can thus be used for plotting our curves: “The value of the exponential exp [I --z -(secX)e“1 for z = z, divided by a gives the value of F/I, at the level a Hz, (or az,, when expressed in units “H “) above hb. At the same time the value of this exponential for z = z1 gives the value of l/1,, at the level Hz, (or z,, when expressed in units “H”) above h,. It is clear in these circumst.ances that our two families of curves can be plotted and compared only if we know what are the values of h, and hi, or at least, what is the value of the difference (hb-h,), which: according to [6]. and expressed in is given by: unit,s “H” nh,. -__ ho R (a-l)+aloga. (6’) H In Figure la three families of curves have been represented, corresponding respect.ively to temperatures T, T’= 1.5 T and T” = 2 T with the assumption that the ratio h,/H in (6’) is equal to 4.5 (this corresponds to a value of h, ranging from 200 to 300 km with an H from 45 to 65 km). 280
Effects
of temperature
variations
of the upper akmosphere on the formation
of ionospheric layers
Careful inspection of these curves shows at once how sensitive to temperature variations is the rate of ion production at a given height. Thus for instance at level z=O, for x=O”, a 50% increase of the temperature reduces this rate more than 30 times. It is further seen that the higher the temperature, the higher is the region where ions are produced and the thicker it is. Finally, as already mentioned, the maximum rate of ion production decreases with increasing temperature. III.
THE SEASONAL
ALY
ANOM-
OF THEF~LAYEE
If we now assume that the average temperature of the upper atmosphere varies from season to season and, even during the day, is higher in summer than in winter (at noon) and higher at midday than in the morning, the results of the preceeding discussion lead to the conclusion that ionospheric layer characteristics which closely follow those of the ion production regions, will exhibit similar diurnal and seasonal temperature variations. In Figure 1 b are represented diurnal variations of the rate of ion producFig. 18. XorI’orI”/I, as & function of altitude for average temperatures X, 3 T/2 and 2X. tion, assuming that the temperature rises from llT at sunrise (x = 90°) to 312 T at noon (wit’h x = O’), at different levels A, B, C . . . corresponding to Figure la. It is quite obvious that at levels such as “D”, the ion production rate goes through a deep minimum at noon simply on account of a midday temperature increase. Ion production rates at other levels exhibit different types of diurnal variations, and at certain levels such as “C”Vhere is a rather sharp maximum of ion production just around noon. In spite of this fact, if one considers that the electron density at noon, at a given level of a layer, depends also on the “previous history” of it - i.e. on the time during which ion production took place in it, it becomes apparent 281
D. LEPECHINSKY that a layer formed in these circumstances, may well exhibit a lower maximum electron density at midday than usually expected, and even a double humped daily variation curve-so characteristic of the F, layer in summer. Other probable effects of temperature variation appearing in the F2 layer behaviour are : its formation at much higher levels in summer than in winter and its greater thickness and lower maximum electron density I in summer. r;; 0.6 Recordings of F2 characteristics obtained at Level-A LevelC Leve/-6 0.5Slough (England) yield the following median values 0.aq of gm and h, for January. March and June 1950 at 03 t
;
76 January . . 91 Narch . . . June. . . . 1 130
Zeni% uny/e *= 9P a0
o
0
JO’ 4’O
JO” L-2L-_T”
L eve/- II
0.4
Leve/-
Level-E
0.3
f
0.2
T “i’
i
0.1
+
Assuming that h,, i.e. t,he level of maximum electron density of the layer, is near the level h max. where the electron production rate is a maximum? we have: h
0 L!!!
@-!L
I
Fig. lb.
n
I/I, as a function of time (or zenith angle) at various levels A, B, C, etc. when the temperature rises from T at x= 90” to 3T/2 at x=0”.
265 320 350
max (Winter)
= H log A eOHsecXcs,, h =
aH
max (Summer) log A e. aWsecXC,s,,
where : xcwj and xCs,are the sun’s zenith angles at Slough at noon in winter and in summer respectively; a is the temperature increase coefficient. It follows that: a [l + (E&J log a set X(s)1 __ 350 h, (Summer) 265 ’ -hm (Winter) = ~ 1 _t (H/h,) log set x(w)
Taking for H/h,, the average value O-2 and secXCs+)= 3.3 ; secXCs, = 1.13, we find : a = 1.46. The temperature increase responsible for the summer elevation of the F, layer at Slough, at midday is thus about 46 % of the winter temperature of t,he region. 282
Effects of temperature
variations of the upper atmosphere on the formation
of ionospheric
layers
and A. P. MITRA [2] recently found by another method that region varied from 700” K in winter to 1200” K in summer (for the period of high solar activity) and from 500” K in winter to 900” K in summer (for the period of low solar activity); the corresponding percentages of increase thus being about 70% and 80% respectively, against the winter values. Some substantial objections existing in the literature [3] against such high percentages of temperature variations in the atmosphere under solar influence alone, the authors express the opinion that the total variation in temperature is not due entirely to seasonal increase and decrease of heating by the sun, but is partly an effect of the rising temperature gradient above the E level and of the elevation of the F region in summer. Our result regarding the mean midday temperatures of the whole region thus appears to be a conservative one. On the other hand, the daily variation of h,, which in the case of the F2 layer formation by the regular CHAPMAN process in a constant temperature, should exhibit a single minimum value around midday, and which instead, as it is well known, exhibits two minima especially marked on summer days (one in the morning, the other in the afternoon with a slight maximum around noon), may similarly be interpreted as due to a diurnal variation of the mean temperature. A rough calculation made on the same lines as the proceeding one, with June 1950 values obtained at Slough, shows that the temperature increase of the region between 6 a.m. and noon, responsible for the F2 layer height variation on a summer day, is of about 20 %. Turning now to the F2 layer seasonal anomaly in its electron densities (the highest midday maximum value being found in winter 134*104 ions cm3 in January against 63*104 ions cm3 in June at Slough 1950, i.e. with a much lower value of cosx than in summer), one finds that the summer maximum rate of electron production, is still about two times greater than its winter value at Slough, even when a summer temperature increase of 46% is considered. S. S. BARAL
F2 temperature
It can be shown [4] th&t in these circumstances if the electron disappearance function w(N) has the form [2] where A, and &, are the apparent recombination and attachment coefficients respectively, and if B,, is supposed to be strictly proportional to partial pressure, the above mentioned anomaly remains inexplicable by the classical theory and the temperature effect, unless A, is assumed to increase by a factor of 10 to 20 when the temperature rises by 46 %. The validity of this assumption will not be discussed here. We shall mention in this connection that according to LANGEVIN [6] the coefficient of recombination of a gas increases with its density and ,therefore with its pressure. IV. According ionization
SPLITTING OB REGION F INTO Fl AND F2 LAYERS.
to [a], h,, the height of maximum process, i8 a function of:
ion production
in a gas by the photo-
(a) The scale height H of the gas, (b) its absorption constant A, specific of its nature and of the ionization process involved and (c) of its density e. as measured at ground level. Now if there are several gases present in the upper atmosphere with individual exponential distributions with height, or even if a single gas is subjected to ionization by different absorption processes, equation [4] shows that distinct regions of ion production will be set up and distinct ionized layers will 283
D. LEPECAINSKY be formed. F, and Fl layers may thus be reasonably well regarded as due t.o photo ionization whether of two distinct gases (0 and M2 respectively according to BHAR, _MITRAand GHOSH) or of a single gas (*X2 at its two ionization potentials, according to WULF and DEMIKO ; 0 or 0 and N, according to D. R. BATES) by different processes. In these circumstances we can write in the general case of two distinct gases : h, = H IogA ,Q,,H, for the F, layer hi, = H’logA’&H’ The separation
h,-- hb of the two layers
for the Fl layer. is given
by :
where :
It. is thus easily seen that the higher the temperature the greater is the separation between the layers, the latter being practically proportional to H (H = kT,!mg and the ratio H’IH = r being less than or equal to unity, with the present assumptions regarding the gases involved). The theory is thus in accord with the well known facts about the Fl layer behaviour, as well during daytime, when its greatest separation from F, occurs at midday, as during the whole year (the greatest separation of t.he layers occuring on summer days), if temperature variat.ions of the upper atmosphere are taken into account. V.
BEHAVIOUR OF THE F REGIOK DURING ~IAGSETIC STORMS
It is well known that magnetic storms are usually accompanied or closely followed by the following phenomena in the F region of the ionosphere: (a) The critical frequency of the F, layer drops to an abnormally low value; (6) There is a substantial increase in the virtual height of the layer and in its h, value ; (c) An obvious splitting of the F region into Fl and F2 occurs, even on winter days, when both layers usually merge. The preceeding discussion leads to the conclusion that all the above phenomena may be due to a single cause: a general increase of the temperature of the This point of view is further supported by the gases subjected to photoionization. fact that during the days of critical frequencies depression, in or after magnetic storm periods, the F, layer daily evolution usually remains quite regular. *‘fo F2” reaching its reduced maximum value at midday; the F2 layer formation thus appears t.o be a normal one but with modified constants. It is interesting to not.e in this connection that some years ago already, S. CHAPMAX expressed the opinion [5] that “intense and concent,rated currents that flow along the aurora1 zone during magnetic storms may appreciably heat. the air there; but such heating is likely t,o be exceeded by more direct heating due to degradation into thermal energy of part of the kinetic energy of the incoming ionizing particles.” An estimate of the temperature increase of the F region during or just after a magnetic storm can be made on t,he basis of the F2 layer elevation a.bove its normal level. Thus, for instance, the severe magnetic storm of 24 and % January 1949
Effects of temperature rariat,ions of the upper atmosphere on the formation of ionoaphcric layera
(when oscillations on the H component exceeded 400 gammas) was followed by a period of marked depression in the “fo F2” values, accompanied by greatly increased h, values of the layer (4.9 Mc/sec and 400 km respectively on 26 January at 1300 hrs G.M.T., against 9.5 Mc/sec and 276 km respectively on 28 January, as recorded at the POITIERS station). The temperature increase on 26 January against normal, corresponding to the recorded h, values, when calculated by t.hc usual method, appears to have been of about 36%. On the other hand the maximum ion production rate drop corresponding to the above temperature increase is of about 26% only (l/a=O*74). This value appears to be rather insufficient alone to justify the important critical frequency drop recorded. It seems thus that a concurrent increase of apparent recombination and attachment coefficients has to be invoked.
VI.
coh.cLusIos
The preceeding discussion leads to the conclusion that temperature variations of the upper atmosphere are playing an import.ant role in the layer formation and morphology, and that the F, layer seasonal and diurnal behaviour is practically always in accord with CHAPMANB ion production theory, when temperature changes are taken into account,. Moreover, magnetic storms’ effects on this layer appear to be essentially due to an abnormal heating of the gases subjected to photo-ionization, by arriving corpuscules’kinetic energy dissipation and by current,s set up Other evidence of temperature variations in the upper atmosin the ionosphere. phere can be seen in the seasonal changes of the absorption of short waves which, as is well known, increases substantially in summer; indeed if temperature and thus parCal pressure of gases in the D region are higher in summer, the absorption coefficient,, which is a function of the pressure, should increase accordingly. REFEREXCES [I] Ler~car~slr~, D.; Note pr&ninairo So. 136 du Laboratoiro national de radioolectricitb (Bagneux France). [2] BARAI., S. S. and MITRA, A. P.; J. Atmosph. Terr. Phys. 1950 1 9.5. [3] CHAPMAX,S. and BARTELS, J.; Ceomagnetiam 1940 1 504. [4] LEPECHISSKY,D.; Note ptiliminaire No. 151 du Laboratoire national do radioolectricit~ (Bagneux, Prance). [5] CHUUK, 8.; Terr. Msg.- and AtmoFtph. Electr. 1937 43 355. [6] LANCEVIS, 31.; AM. Chim. Phys. 1903 26 433.