- Email: [email protected]
On some characteristics of the Es-layer
Planet.
Space Ski.
Pergamon
Press 1959.
Vol. I, pp. 205-212.
ON SOME C~ARACTERrST~CS M. BOSSOLASCO
Printed
in Great
Britain.
OF THE l&LAYER
and A. ELENA
Istituto Geofisico e Geodetico, Universit& di Genova, Italy (Received
5 February
1959)
aspects of the dependence of the ES-ionization on the geomagnetic field are derived; further, an attempt is made to deduce the mean world-wide drift direction of E, in summer. using the time displacement that occurs for the daily maxima of the hourly median values Of fj?. Absbct-Some
1. E, AND THE GEOMAGNETIC
FIELD
It is generally assumed that some of the chief properties of the &layer are intimately related with the geomagnetic field, but this bond is still supported only on isolated results rather than on results collected and discussed on a worldwide basis. With the data gathered during the IGY a better knowledge of the same bond will be possible, allowing us to deduce the true nature of the ionization processes responsible for the G-layer. For some years it has been known, first for Huancayo and later for Koda~an~l,(l) that on the geomagnetic equator E, is generally so regular that the diurnal variation of its critical frequency follows roughly the cosx-law, with midday maxima mainly very much higher than the ~rr~~nding values occurring in other latitudes. The symmetry between the forenoon and the afternoon behaviour of fUEB is then characteristic for these equatorial stations. The same results, together with the seasonal variation of the fOEs suggest that the solar radiation is certainly the chief factor controlling the ionization of E, in the equatorial regions and, as we shall see, everywhere. Among the new ionospheric stations raised in the equatorial regions in the last few years, the most interesting ones are the African. Of these only Ibadan and Djibouti present a behaviour in the diurnal and seasonal variations of the foEs similar to those that are recorded in Huancayo and Kodaik~al, i.e. over these
places the &-layer is also prevailingly of the equatorial type. Ibadan and Djibouti have the geomagnetic latitude of lO%“N and 7*O”N, respectively, latitudes calculated in the assumption that the geomagnetic field is an approximation of an eccentric dipole. Therefore, these two places cannot be considered as properly lying on the magnetic equator of the eccentric dipole. Instead, Ibadan and Djibouti are nearly situated on the equator of 0” magnetic dip (of the present epoch), crossing the African continent ‘7-10” north of the geographic equator, while the geomagnetic dipole equator crosses from WSW to ENE the same geographic equator near Stanleyville. (2) Since in South America and in India the 0” dip equator coincides roughly with the dipole equator the importance of the former has been neglected. According to what RaweP) has pointed out, from the data now available it results that the 0” dip equator is effective in generating the equatorial type of E, only in a narrow band around the same 0” dip equator. Another proof, though indirect, that only the 0” dip equator is decisive in producing the io~zation of the equatorial Es--type, is given by the behaviour of E, resulting from the ionospheric stations located in the Belgian Congo. The Lwiro station (geomagnetic latitude 4.O”S), with the data of 1952-53, reveals diurnal and seasonal variations of normal type, as recorded
20.5
206
M. ‘BOSSOLASCO and A. ELENA
in middle latitudes. The same occurs at Bunia (Ruampara, geomagnetic latitude 0*3”S), a station which was active in the IGY 1957-58. We shall return later to the equatorial Es--type, that we consider as belonging only to the localities placed on the narrow band around the 0” dip equator. Now we shall examine the Es-data from observations made by ionospheric stations lying outside the band around the 0” dip-equator, and up to latitudes of nearly + 60”, where the polar type of Es that we are not considering here begins to appear. Since the critical frequency of a region is related to the maximum electronic density, N a fo2,it is clear that the highest ionization of the E#-layer does occur with the maximum of foEs. In the part of the earth we are considering, between the latitudes of - 60” and + 60”) these maxima always occur in the diurnal hours, i.e. when the sun is over the horizon, a result which confirms that the chief cause of the E,-ionization is due to the sun’s influence. Of course, isolated foEB maxima values are not significant from a statistical standpoint. Instead, we have considered the hourly median values of fOEs for each month and for each ionospheric station active in the IGY period (from which data have been received). Although the electrical characteristics of the sounding instruments used are often different, we have found that such medians are useful and therefore comparable. For nearly all ionospheric stations active during the IGY we have plotted the isopleth representation of such f,E, hourly medians (time in MLT or of the nearest n x 15” meridian as abscissa, versus months). This representation lets us immediately infer the chief features of the diurnal and seasonal variations of the median E,. Before discussing such variations, we shall examine the absdute maximum of the same hourly median values, that we call M (fOE8). Of course, this maximum appears in summer (generally in June-July in the northern hemisphere and December-January in the southern). Figure 1 represents these maxima M (f,EJ graphically, dependent on the geomagnetic latitude (thislast is sufficient in this respect).
In the majority of the stations used in the northern hemisphere we have indicated the two maxima, one relative to summer 1957 and the other to summer 1958, that generally are not coincident. The corresponding differences are also sometimes great, but, as we shall demonstrate, these differences should be ascribed generally to intensity-changes of the prevailing For the ionospheric winds in the Es-layer. southern hemisphere only one summer (December 1957-January 1958) was available, and therefore only one maxima is indicated. At the same geomagnetic latitude the least values of M (foEJ are more representative of the effective maximum ionization of the Eslayer, independent of the variation that might occur in wind intensity., Thus, in Fig. 1 we have drawn a dotted line giving the limits of the minimum values of M (fOEa)and this smoothed curve should approximately indicate the maxima ionization in the Es--layer, locally originated without or with little contribution of mass transport, that is, the change due to the mean magnetic field. Analysing the figure we can deduce:
(1) Although
the number of ionospheric stations in the southern hemisphere is considerably less than in the northern one, it seems that by deduction of wind effect the maxima of the mean E, ionization are sensibly greater in the southern hemisphere.
(2) The behaviour of the same maxima in function of the geomagnetic latitude reveals : (a) a minimum near the equator; (b) two maxima near + 15”-20”; (c) a sharp minimum at about -35” in the southern hemisphere, while the corresponding one in the northern hemisphere seems not at all so clear. Although for other ionospheric layers a dissymetry between the southern and the northern hemisphere has been exhibited, more stations in the southern hemisphere are needed to assure a general validity of the result. We should remember that the mean solar daily geomagnetic variation on quiet days (5,)
ON
SOME
CkIAFtACTERISTICS
Fig. 1.
Behaviour
of the
median hourlycritical on the from
geomagnetic
IGY
data).
The
OF ‘THE
absolute
frequency
E,-LAYER
maxima
M(foE,)
of the
dependent
latitude
(geperally
dotted
line at the
obtained left side
joins the minima values of M(fo
Es). the values that
are more
high speeds
Es- drift.
independent
of the
in the
207
208
M. BOSSOLA!XO
is generated by a current system in the ionosphere, one in the northern and the other in the southern hemisphere, each of them flowing around two foci, placed at latitude of +35-40”. In low latitudes the currents The two foci explain are both eastward. of M (f&), pa~ic~arly the burns evident in the southern hemisphere during the summer. This result is also confirmed by the corresponding mean seasonal variation of the same quantity expressed in function of the latitude, sin&we obtain a similar picture, as the right side of Fig. 1 shows. That the situation in the northern hemisphere is not so clear should be ascribed probably to more irregular induction effects of earth currents on the ionosphere. Nevertheless, it is proved that the mean characteristics of the &-layer are controlled by the normal current distribution responsible for the S,. It is known that in some equatorial places (Huancayo, Kodaikanal, etc.) lying on the 0” dip equator, the quiet day solar magnetic variation S, in H (horizontal magnetic intensity) is abnormally large. This fact can be explained by the daily rise and decrease of a concentrated eastward electric current, named electrojet, superposed on the normal current distribution responsible for the daily magnetic variation &.(Q Since the mean fJZ* in the same equatorial places has a similar abnormally large solar daily magnetic variation (S,), also the electrojet should be the cause of the equatorial type of E,. Indeed, if we suppose that the electrojet flows at a height less or little less than the Es-layer, since the geomagnetic field is horizontal (0” dip), by deflection an upward movement of electrons is originated and the process continues during the solar hours when the electrojet is active. Following McNicol and Gipps Is), E, might be formed due to the ionization of pm-excited atoms by the radiations in the visible part of the solar spectrum: and by the upward movement just mentioned this ionization increases chiefly vertically, causing accumulation that can explain the high or very high values recorded for f,,Es. As clearly pointed out by S. ChapmarP, “the
and
A. ELENA
abnormal band of eastward current superposed on normal current flow does not encircle the earth; . . . a few hours of longitude to the east and west of the focal meridian, 5, (H) has sunk almost to its night time value, indicating that there the abnormal eastward current is only just beginning to flow, or has just ceased. As the current must complete a circuit, there must also be a westward abnormal flow (presumably both to the north and south of the abnormal band of eastward current}, turning towards the equator in the forenoon to flow eastward, and away from the equator in the afternoon to flow westward. On the diagram of current intensity along the focal meridian, the return flow will correspond to depressions below the normal curve, to the left and right of the superposed abnormal peak, the combined curve being equal to the peak area above the normal curve.” (See Fig. 6 of the quoted papeP)). Therefore, where the 0” dip equator differs sensibly from the g~magnetic equator, on and near it during the sun hours we have a depression, i.e. the current intensity is below normal value. And if we simultaneously consider different stations at low latitudes (between - 10” and + 10” geomagnetic latitude), but not those on the 0” dip equator or in the band of &4” around them, the corresponding current intensity drawn in function of the geomagnetic latitude leads to a smoothed curve presenting a minimum near the geomagnetic equator. Because of the total electric current system resulting during the sunlight hours by the superposition of the electrojet on the normal system responsible for S,, we see that in low Iatitudes also the mean ~~~~~) as controlled by these currents should have a minimum near the geomagnetic equator. Since the ionospheric stations being considered here are mostly in Africa south of the geomagnetic equator, the ~orres~nding minimum of ~~~~~~ obtained by a smoothed curve going through a limited number of points, is placed somewhat south of the geomagnetic equator, but the result is qualitatively conlirmed. For the two maxima of the mean M @JZJ at the geomagnetic latitudes of nearly + 13” and - 22’) obtained from Fig. 1, the agreement with
ON SOME CHARACTERISTICS
the corresponding maxima of the total electric current flowing eastward during the sunlight hours should be regarded suthctent because of the very scarce data used.
OF THE Es-LAYER
209
flow occurs both to the north and south) should flow only to the south of the 0” dip equator, turning towards the magnetic equator in the forenoon to flow eastward and away from the equator in the afternoon to flow westward. Furthermore, considering the isopleth repreHerewith the westward compensating current of sentation of the hourly median values of foEo, it should be noted that some stations at low the electrojet transports the properties of this electrojet to the south part, i.e. including its latitudes reveal a somewhat irregular behaviour maximum seasonal activity (June), that will be in the seasonal variation of the same f,E,median. This occurs in Central Africa, south superposed on the normal current system. The proposed explanation seems satisfactory, partiof the 0” dip equator (Bunia, Lwiro, Leopoldcularly because of the dissymetry of the 0” dip ville, Elisabethville), as well as in Central equator and its bending over Africa and America, north of the same equator (Talara, Panama, Puerto Rico, Grand Bahama), where America. Similarly, over Central America the irregular seasonal types of foEs might be we have one of the following seasonal explained with the penetration of the abnormal behaviours (Fig. 2): westward electric current towards the north. (1) The seasonal variation is contrary to that expected; so too in Lwiro and Bunia, 2. THE ZONAL MOVEMENTS OF THE E, LAYER though to a lesser degree, the maximum For some years it was found-by the fading of foEs occurs in June (and not in December). Analogously in Talara the method or by “reflection” of radio waves at a seasonal maximum of foEo is recorded in frequency of 2OMc/s or more(@--that the ion clouds forming the Es-layer are submitted December (instead of in June). generally to translational movements; directions (2) In the stations at Leopoldville, Elisabethville, Grand Bahama, Puerto Rico and and speed values have been estimated, although Panama the seasonal variation of fOEIis they are not always in good agreement. Without attempting to investigate the true generally of mixed type, i.e. in these physical nature of these movements, more places the maxima of foEa occur mainly each six months (nearly at June and presumably generated by the electromagnetic forces causing the charged particles to move December) or are more or less irregularly relative to the neutral particles, we introduce distributed. Effectively, L4opoldville here a new method for obtaining the prevailing seems to have maxima in June somewhat more sharply than the other stations, and monthly direction of the zonal Es-drift, making use of the time-displacement (advance or delay) from this point of view it belongs more appropriately to the first group: while presented by the diurnal maxima of the f,,E,Puerto Rico is less irregular, having medians, as revealed by the isopleth representation. maxima generally in June. This procedure can clearly be justified conThe explanation of these results can be out- sidering, for instance, the isopleth representation lined as follows: We consider, for instance, of the f,E,-medians for Baguio (Philippine Central Africa, where the 0” dip-equator is Islands, Fig. 3). The figure shows that in situated several degrees north of the geographic summer at Baguio the maximum of the f,E,-and geomagnetic equators. Referring to the medians occurs very sharply in the forenoon, picture of the electrojet given inc4), this at about 7-10 a.m. (MLT), i.e. nearly abnormal eastward current flowing in the sun 5-2 hr before noon. A similar advance appears hours over the 0” dip band provokes a com- also, though not so obviously, in Formosa, pensating westward abnormal current that Okinawa and in Japan and is to be explained (contrary to what happens in the places where as an effect of very strong E-winds transporting the equators are not so distant, i.e. where the the ion clouds of the Es-layer at a speed nearly
210
M. ’ BOS!SOLA!SCO and A. ELENA
B
I
c
....
ON SOME CHARACTERISTICS
BAGUIO Fig.
6F
211
THE Es-LAYER
Time 12O.O’E 3.
hourly
lsopleth values of
representation
f. E, at
of
the
Baguio (Philippine
equal to the apparent speed of the sun and of the vertical growth of the same clouds. Indeed, as the greatest increase of ionization in the &layer occurs generally in the hours just after sunrise,* if we have E-winds transporting this effect with the apparent speed of the sun movement over Baguio, we obtain a concentration of the ionization in the same hours, i.e. a maximum of the f&,-medians a few hours after day break, as indicated in Fig. 3. Of course, the method, as based on median values, is appropriate only for deducing the prevailing zonal winds in the season when the subsequent
maxima of the f&-medians occur very clearly. As the example of Baguio shows, the agreement between the maxima relative to different months of the same season also reveals the stability of such zonal currents. An attempt to deduce a rough value of the __-* Contrary to what happens at a station on the 0" dip equator where a symmetric effect occurs around local noon (when only upward winds prevail), at low and middle latitudes the horizontal movements in the E,-layer, which during the day mix the ionization generated at sunrise, lead generally to a poorly defined effect of the sunset and, likewise, the nightly E,ionization does not diminish to very low values. The mixing seems mainly due to turbulence and recombination processes.
median Islands).
speed of these zorral winds can be made as follows. Denoting with v,, the speed of a point over the equator at 110 km height rotating with the earth, it is, approximately, ‘uO= 1696 km/hr and at latitude p the speed will be ‘u=vO cos 9. The sun’s rays first contact the 110 km height in advance of the corresponding points on the earth’s surface; if r is the time between this first contact of the sun’s rays and the local noon, we have, approximately”): y = 0” 10” 20” 30” 40” 45” TS6.7hr 7qOhr 7.5hr 7*9hr 8.5hr 9.0hr Now, denoting with a the number of hours between the maximum in advance of foEs and the local noon, it should be: ru=(7-u)(v+x) where x is the zonal wind speed acting in the sense of the earth rotation, the effect of which is just in advance of the ionization maxima in the forenoon. We have, therefore:
and numerically in km/h&. CO= a=lhr 2hr 3hr 4hr
I
10”
,
278 669 1286 2092
20”
30’
40”
245 569 1063 1771
209 490 918 1470
153 406 723 1183
45’
149 342 601 923
M. BOSSOLASCO and A. ELENA
212
Therefore,
assuming
a=3 hr for Baguio that in summer the zonal westward drift of the &-layer in the forenoon reaches mean values of the order of llOOkm/hr. This result is confirmed with nearly progressive decrease by the stations of Formosa, Okinawa and those of Japan, lying roughly along the same meridian. Obviously, because of longer sunshine in summer at higher latitudes, the meridional disequilibrium of the ionization processes is greater in the higher than in the lower latitudes, which also occurs in the mean intensity of the meridional winds. The very high speed of the mean zonal wind deduced for Baguio and the near stations, compared with the results obtained in other parts of the earth, reveals that in summer over the Far East the &-layer is presumably under the influence of this stronger drift*. In this regard it is to be remembered that for some Japanese stations (Le. Akita and TokyoKokubunji) a strong meridional wind has been found@) that should be logically coupled with a similar strong zonal &-movement. As pointed out by N. C. GersorP “the strongest evidence indicating that ~~-movements represent true wind motions comes from the fact that sporadic E-winds agree very well with those determined by other means.” Similarly, the validity of our method is indirectly proved by the good a~eement with the results gathered in stations or regions where other procedures have been applied. The first attempt undertaken with the maximadisplacement method was made using the median f&-values of the ionospheric stations active in the IGY. Generally for each of these it was possible to deduce the prevailing direction of the zonal current flowing during the day and mainly in the forenoon (as E-winds) in summer (June-July for the northern hemisphere and December-January for the southern hemisphere). The results are reported in Fig. 2, (y = 16” 25’) we deduce
* From another standpoint this anomalous behaviour of the &-occurrence over the Far East appears already in the Figs. II F-9 to F-14 of paper Worldwide Occurrence of Sporadic E by E. K, Smith, Jr. (National Bureau of Standards Circular 582, Washington D.C., 1957).
and from these it appears that in the n&hem ~e~~~~~~ the dead drift is generally prevailing during summer and in the forerwon. Only over Maui (Hawaii) a weak W-drift occurs in the afternoon. For North America (USA) this is in good agreement with the summer #ndi~ons deduced by N. C. Gerson@) using the “reflection” of radio waves at frequencies exceeding 20Mc/s. In Europe, if we suppose that the drift of E, involves the normal E-layer, it should be noted that by the fading method E. ~arni~~cher and K. Rawer(‘) have recently obtained the same westward movement with maxima of speed just in the forenoon. The mean intensity of such currents seems greater in Europe than over the USA, but far less than the mean speed of the zonal currents prevailing in summer over the Far East. Although in the southern hemisphere a smaller number of stations is available, Fig. 2 shows that the directions of the mean E,movement are grouped more irregularly. Our findings are in agreement with results in eastern Australia (Sydney), where the mean zo& drift in summer (December-January) occurs from E, as given by J. A. IIarvey”“? who used a system of spaced pulse transmitters and a central recorder. By our interpretation of the time displacement for the A4(f,,EJ we are led to assume that very high values of f$L. in non-~uatorial latitudes are generally generated by strong winds. REFERENCES 1. S. R~ARAJAN, J. Geophys. Res. 59, 239 (1954). 2. E. H. V~snmz et al., The geomagnetic field. Its description and analysis. Pubf. Carrteg. ante., 580 (1943. 3. K. RAWER,Geofk. puru appl. 32, 170 (1455). 4. S. (hFMAN, Arch. Met., Wien, 4(A), 368 (1951). 5. R. W. E. MCNICOL and G. DE V. GIPPS, J. Geophys. Res. 56, 17 (1951). 6. N. C. GEIZSON,I- Met. X2,74 (1955). 7. J. LUGEON, Tables crkpusculaires, Warszawa (1934). 8. S. MATSUSHITA, 3. Geomagn. Geoelect., Kyoto, 3, 119 (1951). 9. E. HAR~~~MA~~ and K. RARER, 3. Afmos. Terr. Phys. 13, 1 (19.58). 10. J. A. HARVEY, Amt. 1. Phys. 8, 523 (1955).
Space Ski.
Pergamon
Press 1959.
Vol. I, pp. 205-212.
ON SOME C~ARACTERrST~CS M. BOSSOLASCO
Printed
in Great
Britain.
OF THE l&LAYER
and A. ELENA
Istituto Geofisico e Geodetico, Universit& di Genova, Italy (Received
5 February
1959)
aspects of the dependence of the ES-ionization on the geomagnetic field are derived; further, an attempt is made to deduce the mean world-wide drift direction of E, in summer. using the time displacement that occurs for the daily maxima of the hourly median values Of fj?. Absbct-Some
1. E, AND THE GEOMAGNETIC
FIELD
It is generally assumed that some of the chief properties of the &layer are intimately related with the geomagnetic field, but this bond is still supported only on isolated results rather than on results collected and discussed on a worldwide basis. With the data gathered during the IGY a better knowledge of the same bond will be possible, allowing us to deduce the true nature of the ionization processes responsible for the G-layer. For some years it has been known, first for Huancayo and later for Koda~an~l,(l) that on the geomagnetic equator E, is generally so regular that the diurnal variation of its critical frequency follows roughly the cosx-law, with midday maxima mainly very much higher than the ~rr~~nding values occurring in other latitudes. The symmetry between the forenoon and the afternoon behaviour of fUEB is then characteristic for these equatorial stations. The same results, together with the seasonal variation of the fOEs suggest that the solar radiation is certainly the chief factor controlling the ionization of E, in the equatorial regions and, as we shall see, everywhere. Among the new ionospheric stations raised in the equatorial regions in the last few years, the most interesting ones are the African. Of these only Ibadan and Djibouti present a behaviour in the diurnal and seasonal variations of the foEs similar to those that are recorded in Huancayo and Kodaik~al, i.e. over these
places the &-layer is also prevailingly of the equatorial type. Ibadan and Djibouti have the geomagnetic latitude of lO%“N and 7*O”N, respectively, latitudes calculated in the assumption that the geomagnetic field is an approximation of an eccentric dipole. Therefore, these two places cannot be considered as properly lying on the magnetic equator of the eccentric dipole. Instead, Ibadan and Djibouti are nearly situated on the equator of 0” magnetic dip (of the present epoch), crossing the African continent ‘7-10” north of the geographic equator, while the geomagnetic dipole equator crosses from WSW to ENE the same geographic equator near Stanleyville. (2) Since in South America and in India the 0” dip equator coincides roughly with the dipole equator the importance of the former has been neglected. According to what RaweP) has pointed out, from the data now available it results that the 0” dip equator is effective in generating the equatorial type of E, only in a narrow band around the same 0” dip equator. Another proof, though indirect, that only the 0” dip equator is decisive in producing the io~zation of the equatorial Es--type, is given by the behaviour of E, resulting from the ionospheric stations located in the Belgian Congo. The Lwiro station (geomagnetic latitude 4.O”S), with the data of 1952-53, reveals diurnal and seasonal variations of normal type, as recorded
20.5
206
M. ‘BOSSOLASCO and A. ELENA
in middle latitudes. The same occurs at Bunia (Ruampara, geomagnetic latitude 0*3”S), a station which was active in the IGY 1957-58. We shall return later to the equatorial Es--type, that we consider as belonging only to the localities placed on the narrow band around the 0” dip equator. Now we shall examine the Es-data from observations made by ionospheric stations lying outside the band around the 0” dip-equator, and up to latitudes of nearly + 60”, where the polar type of Es that we are not considering here begins to appear. Since the critical frequency of a region is related to the maximum electronic density, N a fo2,it is clear that the highest ionization of the E#-layer does occur with the maximum of foEs. In the part of the earth we are considering, between the latitudes of - 60” and + 60”) these maxima always occur in the diurnal hours, i.e. when the sun is over the horizon, a result which confirms that the chief cause of the E,-ionization is due to the sun’s influence. Of course, isolated foEB maxima values are not significant from a statistical standpoint. Instead, we have considered the hourly median values of fOEs for each month and for each ionospheric station active in the IGY period (from which data have been received). Although the electrical characteristics of the sounding instruments used are often different, we have found that such medians are useful and therefore comparable. For nearly all ionospheric stations active during the IGY we have plotted the isopleth representation of such f,E, hourly medians (time in MLT or of the nearest n x 15” meridian as abscissa, versus months). This representation lets us immediately infer the chief features of the diurnal and seasonal variations of the median E,. Before discussing such variations, we shall examine the absdute maximum of the same hourly median values, that we call M (fOE8). Of course, this maximum appears in summer (generally in June-July in the northern hemisphere and December-January in the southern). Figure 1 represents these maxima M (f,EJ graphically, dependent on the geomagnetic latitude (thislast is sufficient in this respect).
In the majority of the stations used in the northern hemisphere we have indicated the two maxima, one relative to summer 1957 and the other to summer 1958, that generally are not coincident. The corresponding differences are also sometimes great, but, as we shall demonstrate, these differences should be ascribed generally to intensity-changes of the prevailing For the ionospheric winds in the Es-layer. southern hemisphere only one summer (December 1957-January 1958) was available, and therefore only one maxima is indicated. At the same geomagnetic latitude the least values of M (foEJ are more representative of the effective maximum ionization of the Eslayer, independent of the variation that might occur in wind intensity., Thus, in Fig. 1 we have drawn a dotted line giving the limits of the minimum values of M (fOEa)and this smoothed curve should approximately indicate the maxima ionization in the Es--layer, locally originated without or with little contribution of mass transport, that is, the change due to the mean magnetic field. Analysing the figure we can deduce:
(1) Although
the number of ionospheric stations in the southern hemisphere is considerably less than in the northern one, it seems that by deduction of wind effect the maxima of the mean E, ionization are sensibly greater in the southern hemisphere.
(2) The behaviour of the same maxima in function of the geomagnetic latitude reveals : (a) a minimum near the equator; (b) two maxima near + 15”-20”; (c) a sharp minimum at about -35” in the southern hemisphere, while the corresponding one in the northern hemisphere seems not at all so clear. Although for other ionospheric layers a dissymetry between the southern and the northern hemisphere has been exhibited, more stations in the southern hemisphere are needed to assure a general validity of the result. We should remember that the mean solar daily geomagnetic variation on quiet days (5,)
ON
SOME
CkIAFtACTERISTICS
Fig. 1.
Behaviour
of the
median hourlycritical on the from
geomagnetic
IGY
data).
The
OF ‘THE
absolute
frequency
E,-LAYER
maxima
M(foE,)
of the
dependent
latitude
(geperally
dotted
line at the
obtained left side
joins the minima values of M(fo
Es). the values that
are more
high speeds
Es- drift.
independent
of the
in the
207
208
M. BOSSOLA!XO
is generated by a current system in the ionosphere, one in the northern and the other in the southern hemisphere, each of them flowing around two foci, placed at latitude of +35-40”. In low latitudes the currents The two foci explain are both eastward. of M (f&), pa~ic~arly the burns evident in the southern hemisphere during the summer. This result is also confirmed by the corresponding mean seasonal variation of the same quantity expressed in function of the latitude, sin&we obtain a similar picture, as the right side of Fig. 1 shows. That the situation in the northern hemisphere is not so clear should be ascribed probably to more irregular induction effects of earth currents on the ionosphere. Nevertheless, it is proved that the mean characteristics of the &-layer are controlled by the normal current distribution responsible for the S,. It is known that in some equatorial places (Huancayo, Kodaikanal, etc.) lying on the 0” dip equator, the quiet day solar magnetic variation S, in H (horizontal magnetic intensity) is abnormally large. This fact can be explained by the daily rise and decrease of a concentrated eastward electric current, named electrojet, superposed on the normal current distribution responsible for the daily magnetic variation &.(Q Since the mean fJZ* in the same equatorial places has a similar abnormally large solar daily magnetic variation (S,), also the electrojet should be the cause of the equatorial type of E,. Indeed, if we suppose that the electrojet flows at a height less or little less than the Es-layer, since the geomagnetic field is horizontal (0” dip), by deflection an upward movement of electrons is originated and the process continues during the solar hours when the electrojet is active. Following McNicol and Gipps Is), E, might be formed due to the ionization of pm-excited atoms by the radiations in the visible part of the solar spectrum: and by the upward movement just mentioned this ionization increases chiefly vertically, causing accumulation that can explain the high or very high values recorded for f,,Es. As clearly pointed out by S. ChapmarP, “the
and
A. ELENA
abnormal band of eastward current superposed on normal current flow does not encircle the earth; . . . a few hours of longitude to the east and west of the focal meridian, 5, (H) has sunk almost to its night time value, indicating that there the abnormal eastward current is only just beginning to flow, or has just ceased. As the current must complete a circuit, there must also be a westward abnormal flow (presumably both to the north and south of the abnormal band of eastward current}, turning towards the equator in the forenoon to flow eastward, and away from the equator in the afternoon to flow westward. On the diagram of current intensity along the focal meridian, the return flow will correspond to depressions below the normal curve, to the left and right of the superposed abnormal peak, the combined curve being equal to the peak area above the normal curve.” (See Fig. 6 of the quoted papeP)). Therefore, where the 0” dip equator differs sensibly from the g~magnetic equator, on and near it during the sun hours we have a depression, i.e. the current intensity is below normal value. And if we simultaneously consider different stations at low latitudes (between - 10” and + 10” geomagnetic latitude), but not those on the 0” dip equator or in the band of &4” around them, the corresponding current intensity drawn in function of the geomagnetic latitude leads to a smoothed curve presenting a minimum near the geomagnetic equator. Because of the total electric current system resulting during the sunlight hours by the superposition of the electrojet on the normal system responsible for S,, we see that in low Iatitudes also the mean ~~~~~) as controlled by these currents should have a minimum near the geomagnetic equator. Since the ionospheric stations being considered here are mostly in Africa south of the geomagnetic equator, the ~orres~nding minimum of ~~~~~~ obtained by a smoothed curve going through a limited number of points, is placed somewhat south of the geomagnetic equator, but the result is qualitatively conlirmed. For the two maxima of the mean M @JZJ at the geomagnetic latitudes of nearly + 13” and - 22’) obtained from Fig. 1, the agreement with
ON SOME CHARACTERISTICS
the corresponding maxima of the total electric current flowing eastward during the sunlight hours should be regarded suthctent because of the very scarce data used.
OF THE Es-LAYER
209
flow occurs both to the north and south) should flow only to the south of the 0” dip equator, turning towards the magnetic equator in the forenoon to flow eastward and away from the equator in the afternoon to flow westward. Furthermore, considering the isopleth repreHerewith the westward compensating current of sentation of the hourly median values of foEo, it should be noted that some stations at low the electrojet transports the properties of this electrojet to the south part, i.e. including its latitudes reveal a somewhat irregular behaviour maximum seasonal activity (June), that will be in the seasonal variation of the same f,E,median. This occurs in Central Africa, south superposed on the normal current system. The proposed explanation seems satisfactory, partiof the 0” dip equator (Bunia, Lwiro, Leopoldcularly because of the dissymetry of the 0” dip ville, Elisabethville), as well as in Central equator and its bending over Africa and America, north of the same equator (Talara, Panama, Puerto Rico, Grand Bahama), where America. Similarly, over Central America the irregular seasonal types of foEs might be we have one of the following seasonal explained with the penetration of the abnormal behaviours (Fig. 2): westward electric current towards the north. (1) The seasonal variation is contrary to that expected; so too in Lwiro and Bunia, 2. THE ZONAL MOVEMENTS OF THE E, LAYER though to a lesser degree, the maximum For some years it was found-by the fading of foEs occurs in June (and not in December). Analogously in Talara the method or by “reflection” of radio waves at a seasonal maximum of foEo is recorded in frequency of 2OMc/s or more(@--that the ion clouds forming the Es-layer are submitted December (instead of in June). generally to translational movements; directions (2) In the stations at Leopoldville, Elisabethville, Grand Bahama, Puerto Rico and and speed values have been estimated, although Panama the seasonal variation of fOEIis they are not always in good agreement. Without attempting to investigate the true generally of mixed type, i.e. in these physical nature of these movements, more places the maxima of foEa occur mainly each six months (nearly at June and presumably generated by the electromagnetic forces causing the charged particles to move December) or are more or less irregularly relative to the neutral particles, we introduce distributed. Effectively, L4opoldville here a new method for obtaining the prevailing seems to have maxima in June somewhat more sharply than the other stations, and monthly direction of the zonal Es-drift, making use of the time-displacement (advance or delay) from this point of view it belongs more appropriately to the first group: while presented by the diurnal maxima of the f,,E,Puerto Rico is less irregular, having medians, as revealed by the isopleth representation. maxima generally in June. This procedure can clearly be justified conThe explanation of these results can be out- sidering, for instance, the isopleth representation lined as follows: We consider, for instance, of the f,E,-medians for Baguio (Philippine Central Africa, where the 0” dip-equator is Islands, Fig. 3). The figure shows that in situated several degrees north of the geographic summer at Baguio the maximum of the f,E,-and geomagnetic equators. Referring to the medians occurs very sharply in the forenoon, picture of the electrojet given inc4), this at about 7-10 a.m. (MLT), i.e. nearly abnormal eastward current flowing in the sun 5-2 hr before noon. A similar advance appears hours over the 0” dip band provokes a com- also, though not so obviously, in Formosa, pensating westward abnormal current that Okinawa and in Japan and is to be explained (contrary to what happens in the places where as an effect of very strong E-winds transporting the equators are not so distant, i.e. where the the ion clouds of the Es-layer at a speed nearly
210
M. ’ BOS!SOLA!SCO and A. ELENA
B
I
c
....
ON SOME CHARACTERISTICS
BAGUIO Fig.
6F
211
THE Es-LAYER
Time 12O.O’E 3.
hourly
lsopleth values of
representation
f. E, at
of
the
Baguio (Philippine
equal to the apparent speed of the sun and of the vertical growth of the same clouds. Indeed, as the greatest increase of ionization in the &layer occurs generally in the hours just after sunrise,* if we have E-winds transporting this effect with the apparent speed of the sun movement over Baguio, we obtain a concentration of the ionization in the same hours, i.e. a maximum of the f&,-medians a few hours after day break, as indicated in Fig. 3. Of course, the method, as based on median values, is appropriate only for deducing the prevailing zonal winds in the season when the subsequent
maxima of the f&-medians occur very clearly. As the example of Baguio shows, the agreement between the maxima relative to different months of the same season also reveals the stability of such zonal currents. An attempt to deduce a rough value of the __-* Contrary to what happens at a station on the 0" dip equator where a symmetric effect occurs around local noon (when only upward winds prevail), at low and middle latitudes the horizontal movements in the E,-layer, which during the day mix the ionization generated at sunrise, lead generally to a poorly defined effect of the sunset and, likewise, the nightly E,ionization does not diminish to very low values. The mixing seems mainly due to turbulence and recombination processes.
median Islands).
speed of these zorral winds can be made as follows. Denoting with v,, the speed of a point over the equator at 110 km height rotating with the earth, it is, approximately, ‘uO= 1696 km/hr and at latitude p the speed will be ‘u=vO cos 9. The sun’s rays first contact the 110 km height in advance of the corresponding points on the earth’s surface; if r is the time between this first contact of the sun’s rays and the local noon, we have, approximately”): y = 0” 10” 20” 30” 40” 45” TS6.7hr 7qOhr 7.5hr 7*9hr 8.5hr 9.0hr Now, denoting with a the number of hours between the maximum in advance of foEs and the local noon, it should be: ru=(7-u)(v+x) where x is the zonal wind speed acting in the sense of the earth rotation, the effect of which is just in advance of the ionization maxima in the forenoon. We have, therefore:
and numerically in km/h&. CO= a=lhr 2hr 3hr 4hr
I
10”
,
278 669 1286 2092
20”
30’
40”
245 569 1063 1771
209 490 918 1470
153 406 723 1183
45’
149 342 601 923
M. BOSSOLASCO and A. ELENA
212
Therefore,
assuming
a=3 hr for Baguio that in summer the zonal westward drift of the &-layer in the forenoon reaches mean values of the order of llOOkm/hr. This result is confirmed with nearly progressive decrease by the stations of Formosa, Okinawa and those of Japan, lying roughly along the same meridian. Obviously, because of longer sunshine in summer at higher latitudes, the meridional disequilibrium of the ionization processes is greater in the higher than in the lower latitudes, which also occurs in the mean intensity of the meridional winds. The very high speed of the mean zonal wind deduced for Baguio and the near stations, compared with the results obtained in other parts of the earth, reveals that in summer over the Far East the &-layer is presumably under the influence of this stronger drift*. In this regard it is to be remembered that for some Japanese stations (Le. Akita and TokyoKokubunji) a strong meridional wind has been found@) that should be logically coupled with a similar strong zonal &-movement. As pointed out by N. C. GersorP “the strongest evidence indicating that ~~-movements represent true wind motions comes from the fact that sporadic E-winds agree very well with those determined by other means.” Similarly, the validity of our method is indirectly proved by the good a~eement with the results gathered in stations or regions where other procedures have been applied. The first attempt undertaken with the maximadisplacement method was made using the median f&-values of the ionospheric stations active in the IGY. Generally for each of these it was possible to deduce the prevailing direction of the zonal current flowing during the day and mainly in the forenoon (as E-winds) in summer (June-July for the northern hemisphere and December-January for the southern hemisphere). The results are reported in Fig. 2, (y = 16” 25’) we deduce
* From another standpoint this anomalous behaviour of the &-occurrence over the Far East appears already in the Figs. II F-9 to F-14 of paper Worldwide Occurrence of Sporadic E by E. K, Smith, Jr. (National Bureau of Standards Circular 582, Washington D.C., 1957).
and from these it appears that in the n&hem ~e~~~~~~ the dead drift is generally prevailing during summer and in the forerwon. Only over Maui (Hawaii) a weak W-drift occurs in the afternoon. For North America (USA) this is in good agreement with the summer #ndi~ons deduced by N. C. Gerson@) using the “reflection” of radio waves at frequencies exceeding 20Mc/s. In Europe, if we suppose that the drift of E, involves the normal E-layer, it should be noted that by the fading method E. ~arni~~cher and K. Rawer(‘) have recently obtained the same westward movement with maxima of speed just in the forenoon. The mean intensity of such currents seems greater in Europe than over the USA, but far less than the mean speed of the zonal currents prevailing in summer over the Far East. Although in the southern hemisphere a smaller number of stations is available, Fig. 2 shows that the directions of the mean E,movement are grouped more irregularly. Our findings are in agreement with results in eastern Australia (Sydney), where the mean zo& drift in summer (December-January) occurs from E, as given by J. A. IIarvey”“? who used a system of spaced pulse transmitters and a central recorder. By our interpretation of the time displacement for the A4(f,,EJ we are led to assume that very high values of f$L. in non-~uatorial latitudes are generally generated by strong winds. REFERENCES 1. S. R~ARAJAN, J. Geophys. Res. 59, 239 (1954). 2. E. H. V~snmz et al., The geomagnetic field. Its description and analysis. Pubf. Carrteg. ante., 580 (1943. 3. K. RAWER,Geofk. puru appl. 32, 170 (1455). 4. S. (hFMAN, Arch. Met., Wien, 4(A), 368 (1951). 5. R. W. E. MCNICOL and G. DE V. GIPPS, J. Geophys. Res. 56, 17 (1951). 6. N. C. GEIZSON,I- Met. X2,74 (1955). 7. J. LUGEON, Tables crkpusculaires, Warszawa (1934). 8. S. MATSUSHITA, 3. Geomagn. Geoelect., Kyoto, 3, 119 (1951). 9. E. HAR~~~MA~~ and K. RARER, 3. Afmos. Terr. Phys. 13, 1 (19.58). 10. J. A. HARVEY, Amt. 1. Phys. 8, 523 (1955).